Dr. Aryan (Anish Dhakal), profile picture
Uploaded byDr. Aryan (Anish Dhakal)
PPTX, PDF404 views

CT Artifacts: Physics, Understanding and Minimization

CT artifacts are image distortions or errors that can mimic or obscure pathology, arising from patient factors, physics, or hardware limitations. In the amazing world of radiology, we recognize that understanding artifacts is not just about identifying them—it is about safeguarding patients from misdiagnosis. Whether it is beam hardening on CT, motion artifacts on MRI, reverberation on ultrasound, or scatter on X-ray, every modality has its own pitfalls. Knowledge of these artifacts enables us to distinguish true disease from false appearances, optimize image quality, and ensure accurate reporting. Mastering artifacts, to us, feels like learning the “hidden language” of radiology—achieving accurate interpretation despite the noise.

Embed presentation

Download to read offline
1 / 109
2 / 109
Most read
3 / 109
4 / 109
5 / 109
Most read
6 / 109
7 / 109
8 / 109
9 / 109
10 / 109
11 / 109
12 / 109
13 / 109
14 / 109
15 / 109
16 / 109
17 / 109
18 / 109
19 / 109
20 / 109
21 / 109
22 / 109
23 / 109
24 / 109
25 / 109
26 / 109
27 / 109
28 / 109
29 / 109
30 / 109
31 / 109
32 / 109
33 / 109
34 / 109
35 / 109
36 / 109
37 / 109
38 / 109
39 / 109
40 / 109
41 / 109
42 / 109
43 / 109
44 / 109
45 / 109
46 / 109
47 / 109
48 / 109
49 / 109
50 / 109
51 / 109
52 / 109
53 / 109
54 / 109
55 / 109
56 / 109
57 / 109
58 / 109
59 / 109
60 / 109
61 / 109
62 / 109
63 / 109
64 / 109
65 / 109
66 / 109
67 / 109
68 / 109
69 / 109
70 / 109
71 / 109
72 / 109
73 / 109
74 / 109
75 / 109
76 / 109
77 / 109
78 / 109
79 / 109
80 / 109
81 / 109
82 / 109
83 / 109
84 / 109
85 / 109
86 / 109
87 / 109
88 / 109
89 / 109
90 / 109
91 / 109
92 / 109
93 / 109
94 / 109
95 / 109
96 / 109
97 / 109
98 / 109
99 / 109
100 / 109
101 / 109
102 / 109
103 / 109
104 / 109
105 / 109
106 / 109
107 / 109
108 / 109
109 / 109
CT ARTIFACTS: PHYSICS, UNDERSTANDING AND MINIMIZATION Presented by: Dr. Anish Dhakal Resident MD Radiodiagnosis, KUSMS 24th September, 2025
What is an artifact? Literal meaning in medical world: “Any unintended pattern, distortion, or flaw in an image that does not represent the actual anatomy or pathology of the patient.”
The term artifact is applied to any systematic discrepancy between the CT numbers in the reconstructed image and the true attenuation coefficients of the object. CT images are inherently more prone to artifacts than conventional radiographs because the image is reconstructed from something on the order of a million independent detector measurements. The reconstruction technique assumes that all these measurements are consistent, so any error of measurement will usually reflect itself as an error in the reconstructed image. Artifacts can seriously degrade the quality of CT images, sometimes to the point of making them diagnostically unusable.
The types of artifact that can occur are as follows: (a) Streaking, which is generally due to an inconsistency in a single measurement (b) Shading, which is due to a group of channels or views deviating gradually from the true measurement (c) Rings, which are due to errors in an individual detector calibration (d) Distortion, which is due to helical reconstruction.
Contd.. Streaks Shading Rings/Bands
Based on origin of artifacts: (a) Physics-based artifacts, which result from the physical processes involved in the acquisition of CT data (b) Patient-based artifacts, which are caused by such factors as patient movement or the presence of metallic materials in or on the patient (c) Scanner- based artifacts, which result from imperfections in scanner function (d) Helical and multisection artifacts, which are produced by the image reconstruction process.
Physics-based Artifacts
A. Beam hardening artifact B. Partial volume effect artifact C. Photon starvation artifact D.Undersampling artifact E. Edge gradient artifact F. Blooming artifact
A. Beam Hardening An x-ray beam producing the CT image is not a monochromatic beam. The x-ray beam is composed of individual photons with a range of energies.
As a heterogeneous x-ray beam passes through the patient, the low energy protons are rapidly absorbed. This means the x-ray beam exiting the patient contains a lower percentage of energy photons than the beam had when it entered the patient. This effect is called "beam hardening." The linear attenuation coefficient of a tissue is directly related to the average energy of the x-ray beam. Reconstruction programs anticipate and correct for variation in linear attenuation coefficients caused by beam hardening, but such corrections are not precise.
As the beam passes through an object, it becomes “harder,” that is to say its mean energy increases, because the lower energy photons are absorbed more rapidly than the higher-energy photons. This focally increased mean beam energy is interpreted as being due to it passing through a less attenuating material relative to the surroundings and so a lower Hounsfield unit is assigned and the image will be represented as more black.
Two types of artifact can result from this effect: so-called cupping artifacts and the appearance of dark bands or streaks between dense objects in the image.
1. Cupping Artifacts. X rays passing through the middle portion of a uniform cylindrical phantom are hardened more than those passing though the edges because they are passing though more material. As the beam becomes harder, the rate at which it is attenuated decreases, so the beam is more intense when it reaches the detectors than would be expected if it had not been hardened. Therefore, the resultant attenuation profile differs from the ideal profile that would be obtained without beam hardening. A profile of the CT numbers across the phantom displays a characteristic cupped shape
Beam Hardening Artifacts
2. Streaks and Dark Bands. In very heterogeneous cross sections, dark bands or streaks can appear between two dense objects in an image. They occur because the portion of the beam that passes through one of the objects at certain tube positions is hardened less than when it passes through both objects at other tube positions. This type of artifact can occur both in bony regions of the body and in scans where a contrast medium has been used.
Dark and bright streaks radiating from and between high-density objects, such as dental amalgam. Alternating dark and bright streaks in a narrow band extending across the posterior fossa
Methods to reduce Beam Hardening I. Built-in Features for Minimizing Beam Hardening 1. Filtration: A flat piece of attenuating, usually metallic material is used to “pre-harden” the beam by filtering out the lower-energy components before it passes through the patient. An additional “bow tie” filter further hardens the edges of the beam, which will pass through the thinner parts of the patient.
2. Calibration correction: Manufacturers calibrate their scanners using phantoms in a range of sizes. This allows the detectors to be calibrated with compensation tailored for the beam hardening effects of different parts of the patient. Since patient anatomy never exactly matches a cylindrical calibration phantom, in clinical practice there may be either a slight residual cupping artifact or a slight “capping” artifact, with a higher central CT value due to overcorrection.
3. Beam hardening correction software:  An iterative correction algorithm may be applied when images of bony regions are being reconstructed. This helps minimize blurring of the bone–soft tissue interface in brain scans and also reduces the appearance of dark bands in nonhomogeneous cross sections.
II. Avoidance of Beam Hardening by the Operator. It is sometimes possible to avoid scanning bony regions, either by means of patient positioning or by tilting the gantry. It is important to select the appropriate scan field of view to ensure that the scanner uses the correct calibration and beam hardening correction data.
III. Dual energy CT: •DECT can measure attenuation at two energies that allows estimation of effective monochromatic attenuation and mathematical correction for beam hardening. •Low- and high-energy datasets are combined to calculate: virtual monochromatic images, where the beam behaves as if all photons have the same energy. •These reduce the effect of differential absorption of low-energy photons.
B. Partial Volume Artifact These artifacts are a separate problem from partial volume averaging, which yields a CT number representative of the average attenuation of the materials within a voxel. Partial volume artifact occurs when a dense object lying off-center protrudes partway into the width of the x-ray beam. The off-axis object can be within the beam, and therefore “seen” by the detectors, when the tube is pointing in one direction but outside the beam, and therefore not seen by the detectors, when the tube is pointing in another The inconsistencies between the views cause shading artifacts to appear in the image
Non linear partial volume averaging Nonlinear partial volume averaging is an advanced form of partial volume averaging (PVA), a phenomenon in medical imaging (like CT and MRI) where a single image voxel contains multiple types of tissue, leading to blurring and image artifacts. Unlike simple linear averaging, nonlinear PVA considers that the relationship between tissue types and their resulting signal or attenuation is not a simple average, but more complex, often appearing as shading or streaking artifacts, especially in helical scans.
From a strict standpoint of physics, linear partial volume averaging is not a reconstruction artifact, it’s just a blurring/pseudo-density effect that can cause misinterpretation hence clinically important.
Physics behind partial volume averaging Pattern 1 (Partial Volume Effect): A dense object lying off- centre protrudes partially into the width of an x-ray beam . This results in divergence of the beam and manifests as shading artifacts adjacent to said object [non-linear PVA] Pattern 2: (CT voxels are 3D cubes): If you have a dense thing taking up half the cube, and a sparse ( low attenuating ) thing in the other half of the cube, the machine will average the two together giving something that has intermediate density. The classic location is the skull base averaging with CSF or brain to look like blood [linear PVA]
Arise essentially from reconstructing low resolution images, typically thick slice images. It produces CT numbers as an average of all types of tissues. It will appear as bands or streaks.
Appearance: The left upper lobe segmental arterial branch appears to have a low-attenuating filling defect (arrow) on the CT image obtained with 3-mm section thickness (a), which resolves on the CT image obtained with 1.5-mm section thickness (b). Volume Averaging Artifact 3 mm 1.5 mm a b
Methods to reduce partial volume artifacts Partial volume artifacts can best be avoided by using a thin acquisition section width. If the noise is a problem, acquiring thin slices then generating thicker slices by adding them together. This is necessary when imaging any part of the body where the anatomy is changing rapidly in the z direction, for example in the posterior fossa.
C. Photon starvation artifact A potential source of serious streaking artifacts is photon starvation, which can occur in highly attenuating areas such as the shoulders. When the x-ray beam is traveling horizontally, the attenuation is greatest and insufficient photons reach the detectors. The result is that very noisy projections are produced at these tube angulations. The reconstruction process has the effect of greatly magnifying the noise, resulting in horizontal streaks in the image.
Appearance: Sagittal CT image (a) of the cervical spine shows significant noise and decreased contrast discrimination (oval) through the lower cervical and upper thoracic vertebral bodies. A similar appearance is depicted on the axial CT image (b) of the chest, secondary to lateral body soft-tissue thickness. This is secondary to photons traversing a dense structure, limiting the number of photons that strike the detector, which results in increased noise. Photon Starvation Artifact a b
If the tube current is increased for the duration of the scan, the problem of photon starvation will be overcome, but the patient will receive an unnecessary dose when the beam is passing through less attenuating parts.
Methods to reduce photon starvation 1. Automatic Tube Current Modulation.  On some scanner models, the tube current is automatically varied during the course of each rotation, a process known as milliamperage modulation. This allows sufficient photons to pass through the widest parts of the patient without unnecessary dose to the narrower parts.
2. Adaptive Filtration This software correction smooths the attenuation profile in areas of high attenuation before the image is reconstructed.
A multi-dimensional adaptive filtration technique is used on multi-section scanners. For the small proportion of projection data that exceed a selected attenuation threshold, smoothing is carried out between adjacent in-plane detectors and between successive projection angles while the z filter used in helical reconstruction is broadened for high-attenuation projection angles to allow more photons to contribute to the reconstruction
D. Undersampling artifact/Aliasing artifact The number of projections used to reconstruct a CT image is one of the determining factors in image quality. An insufficient number of projections used to reconstruct the CT can diminish quality, and result in mis- registration artifacts. An insufficient number of projections used to reconstruct the CT can diminish quality, and result in mis- registration artifacts Too large an interval between projections (undersampling) can result in misregistration by the computer of information relating to sharp edges and small objects.
View Aliasing: This is when you have under sampling between projections. You see line stripes radiating from the edge (but at a distance from) a dense object. This is fixed by acquiring the largest possible number of projects per rotation - slowing the rotation speed. Ray Aliasing: This is when you have under sampling within a projection. You see strips appearing close to the structure . This is fixed by using specialized high resolution techniques - manufacturer employed.
Method to reduce undersampling View aliasing can be minimized by acquiring the largest possible number of projections per rotation. On some scanners, this can be achieved only by using a slower rotation speed, while on others the number of projections is independent of rotation speed. Using high resolution technique like 1. Flying focal spot, and 2. Quarter detector shift
•Instead of aligning each scan exactly over the previous • detector positions, the detector array is shifted by ¼ of a detector width for successive rotations. •Example: if detector width = 1 mm: •Rotation 1: detectors at positions 0,1,2,… mm •Rotation 2: detectors at positions 0.25,1.25,2.25,… mm •Rotation 3: 0.5,1.5,2.5,… mm •Rotation 4: 0.75,1.75,2.75,… mm This results in:  Improves sampling density: effectively gives 4× more sampling points.  Reduces undersampling artifacts: smoother images, fewer aliasing streaks.  Better resolution in z-direction (longitudinal) without reducing detector size.
E. Edge Gradient Artifact Arise from irregularly shaped object that have a pronounced difference in density from surrounding structure Occurs at sharp density transitions, where the CT beam passes from one tissue type to another (e.g., bone–soft tissue interface). Results in streak artifact or shading To minimize: Using thinner slices Using a low HU- value oral contrast Change in patient’s position
Appearance: Small highly dense structures such as calcifications and stents appear larger than they truly are, as depicted on the accompanying CT images. The circumferential coronary artery calcification (arrow) appears progressively smaller from left to right, with the changing window width (W) and level (L). Cause: Very high CT numbers of the structure cause pixel saturation when using typical lookup table (LUT) windows, causing the structure to appear larger than it is. Also, using a smoothing filter kernel makes small bright objects appear larger. Minimization: 1. Use a LUT window that is wide enough so that the displayed pixels are not saturated. 2. Use a standard or sharper filter kernel with an iterative noise reduction algorithm. 3. Reduce the display FOV to improve the spatial resolution. 4. Use a high kiloelectron-volt (keV) monoenergetic reconstruction (dual-energy acquisitions only). 5. Use small section thicknesses. W/L: 800/350 HU W/L: 1000/500 HU W/L: 1650/750 HU F. Blooming Artifact
Patient-based Artifacts
A. Metallic materials artifact B. Patient motion artifact C. Incomplete projection artifact
A. Metallic materials artifact The presence of metal objects in the scan field can lead to severe streaking artifacts. They occur because the density of the metal is beyond the normal range that can be handled by the computer, resulting in incomplete attenuation profiles. Additional artifacts due to beam hardening, partial volume, and aliasing are likely to compound the problem when scanning very dense objects. Metals with high Z (Iron , Platinum) tend to have more artifacts than those with lower Z (Titanium)
Methods to reduce metal artifacts I. Avoidance of Metal Artifacts by the Operator Patients are normally asked to take off removable metal objects such as jewelry before scanning commences. For nonremovable items, such as dental fillings, prosthetic devices, and surgical clips, it is sometimes possible to use gantry angulation to exclude the metal inserts from scans of nearby anatomy. When it is impossible to scan the required anatomy without including metal objects, increasing technique, especially kilovoltage, may help penetrate some objects, and using thin sections will reduce the contribution due to partial volume artifact.
II. Software Corrections for metal artifacts Streaking caused by overranging can be greatly reduced by means of special software corrections.  Manufacturers use a variety of interpolation techniques to substitute the overrange values in attenuation profiles. The usefulness of metal artifact reduction software is sometimes limited because, although streaking distant from the metal implants is removed, there still remains a loss of detail around the metal-tissue interface, which is often the main area of diagnostic interest. Beam hardening correction software should also be used when scanning metal objects to minimize the additional artifacts due to beam hardening.
MAR (Metal artifact reduction) technique:- Acquisition and storage of the raw data Reconstruction of CT image Identification of the implant Automatic definition of the boundaries of the implant within the projection data. For each projection, the implant boundaries are automatically defined within the given ROI by the use of given threshold values Iterative reconstruction of the missing projection data Reconstruction of the artifact-reduced image from the newly computed projection data
B. Patient Motion Short scan times have diminished but not eliminated motion artifacts. Patient motion can cause misregistration artifacts, which usually appear as shading or streaking in the reconstructed image. Steps can be taken to prevent voluntary motion, but some involuntary motion may be unavoidable during body scanning.
Why motion artifacts occur? The reconstructed image will display an object in motion as a streak in the direction of motion. On reconstruction the scanner will average the density of the pixels covering the motion area. In some fashion, the intensity of the streak artifact will depend on the density of the object in motion. Motion of objects that have densities much different from their surroundings produces more intense artifacts. Thus, motion of metallic or gas-containing structures produce striking artifacts.
Produces “ghosting” effect (high-contrast structures appear duplicated or smeared in adjacent slices or projections. Image appears– as if it is composed of superimposed images.
Appearance: An apparent coronary artery filling defect and/or occlusion (arrow in a; oval in b) is depicted in these prospectively gated cardiac CT angiograms. The defect is related to respiratory motion, which is best appreciated on CT images obtained in the lung window (c). Distortion of anatomy by dark and/or bright bands and loss of image resolution secondary to respiratory motion between axial acquisitions causes this artifact. Respiratory Motion Artifact a b c
Appearance: Apparent coronary artery filling defect and/or occlusion (arrow in a) resolved on the CT image (b) after the vendor- provided intelligence correction was applied. Cause: Distortion of anatomy by dark and/or bright bands and loss of image resolution secondary to misregistration related to motion Cardiac Motion Artifact References 4, 5. a b
Methods to reduce motion artifacts I. Avoidance of Motion Artifacts by the Operator The use of positioning aids is sufficient to prevent voluntary movement in most patients. In some cases (eg, pediatric patients), it may be necessary to immobilize the patient by means of sedation. Using as short a scan time as possible helps minimize artifacts when scanning regions prone to movement. Respiratory motion can be minimized if patients are able to hold their breath for the duration of the scan. The sensitivity of the image to motion artifacts depends on the orientation of the motion. Therefore, it is preferable if the start and end position of the tube is aligned with the primary direction of motion, for example, vertically above or below a patient undergoing a chest scan. Specifying body scan mode, as opposed to head scan mode, may automatically incorporate some motion artifact reduction in the reconstruction.
II. Built-in Features for Minimizing Motion Artifacts 1. Overscan and underscan modes: The maximum discrepancy in detector readings occurs between views obtained toward the beginning and end of a 360° scan. Some scanner models use overscan mode for axial body scans, whereby an extra 10% or so is added to the standard 360° rotation. The repeated projections are averaged, which helps reduce the severity of motion artifacts. The use of partial scan mode can also reduce motion artifacts, but this may be at the expense of poorer resolution.
2. Software correction: Most scanners, when used in body scan mode, automatically apply reduced weighting to the beginning and end views to suppress their contribution to the final image. However, this may lead to more noise in the vertical direction of the resultant image, depending on the shape of the patient.
3. Cardiac gating: The rapid motion of the heart can lead to severe artifacts in images of the heart and to artifacts that can mimic disease in associated structures, for example, dissected aorta. To overcome these difficulties, techniques have been developed to produce images by using data from just a fraction of the cardiac cycle, when there is least cardiac motion. This is achieved by combining electrocardiographic gating techniques with specialized methods of image reconstruction.
C. Incomplete projections If any portion of the patient lies outside field of view, the computer will have incomplete information relating to this portion and streaking or shading artifacts are likely to be generated. For example a patient is scanned with the arms down instead of being raised out of the way of the scan. As the arms are outside the scan field, they are not present in the image, but their presence in some views during scanning can led to such severe artifacts throughout the image as to significantly degrade its usefulness. Similar effects can be caused by dense objects such as an intravenous tube containing contrast medium lying outside the scan field.
Axial CT image through the abdomen truncation artifact with increased curvilinear attenuation along the edges of the image. Occurs when the anatomy of interest is outside the scan field of view (FOV) (eg, a large patient, or patient’s arms resting at their side), which results in the disruption of the accurate attenuation measurement along image edges.
Methods to reduce incomplete projections To avoid artifacts due to incomplete projections, it is essential to position the patient so that no parts lie outside the scan field Scanners designed specifically for radiation therapy planning have wider bores and larger scan fields of view than standard scanners and permit greater versatility in patient positioning. They also allow scanning of exceptionally large patients who would not fit within the field of view of standard scanners. Taping patient tissue and raising patients arms above their head on the scan of chest and abdomen also helps
Scanner-based Artifacts
A. Ring artifacts B. Line in topogram C. Tube arcing
A. Ring Artifacts If one of the detectors is out of calibration on a third- generation (rotating x-ray tube and detector assembly) scanner, the detector will give a consistently erroneous reading at each angular position, resulting in a circular artifact. A scanner with solid-state detectors, where all the detectors are separate entities, is in principle more susceptible to ring artifacts than a scanner with gas detectors, in which the detector array consists of a single xenon-filled chamber subdivided by electrodes. They can impair the diagnostic quality of an image, and this is particularly likely when central detectors are affected, creating a dark smudge at the center of the image.
Ring artifacts are the result of miscalibration of one detector in a rotate-rotate geometry scanner. Detector failure would also produce a ring artifact If a detector is miscalibrated, it will record incorrect data in every projection. This misinformation is reconstructed as a ring in the image, with the radius of the ring determined by the position of the faulty detector in the detector array. Faulty detectors in rotate-fixed units also record false information. However, this information is not visible in the reconstructed image because the faulty detector collects data from many angles (rings may appear in rotate-fixed geometry if the x-ray tube is not aligned correctly).
Appearance: Concentric circular structures expanding from the scanner isocenter (arrows) are depicted on axial CT images. Cause: Related to miscalibrated detector (a), insufficient dose (b), or contrast material on the polyester film window that covers the detector (c). As the assembly rotates, the detector gives an incorrect value at each location, causing the ring artifact. Ring Artifact a b c
Methods to reduce ring artifacts The presence of circular artifacts in an image is an indication that the detector gain needs recalibration or may need repair services. Selecting the correct scan field of view may reduce the artifact by using calibration data that fit more closely to the patient anatomy. All modern scanners use solid-state detectors, but their potential for ring artifacts is reduced by software that characterizes and corrects detector variations.
B. Line in topogram Due to faulty detectors Remedy Detector replacement
Tube arcing Occurs when there is a short circuit within the tube, typically from cathode to tube envelope. Tungsten vapor from anode and cathode intercepts the projectile electrons intended for collisions with the target. Causes momentary loss of x-ray output. Remedy: Tube Replacement.
Helical and Multisection CT Artifacts
A. Helical Artifacts in the Axial Plane: Single-Section Scanning B. Helical Artifacts In Multisection scanning C. Artifacts In Multiplanar and Three dimensional Reformation
A. Helical Artifacts in the Axial Plane: Single-Section Scanning Artifacts related to helical scanning is due to the helical interpolation and reconstruction process. The artifacts occur when anatomic structures change rapidly in the z direction (eg, at the top of the skull) and are worse for higher pitches. If a helical scan is performed of a cone-shaped phantom lying along the z axis of the scanner, the resultant axial images should appear circular. In fact, their shape is distorted because of the weighting function used in the helical interpolation algorithm. For some projection angles, the image is influenced more by contributions from wider parts of the cone in front of the scan plane; for other projection angles, contributions from narrower parts of the cone behind the scan plane.
Methods to reduce helical artifacts To keep helical artifacts to a minimum, steps must be taken to reduce the effects of variation along the z axis. This means using, where possible, 1. Low pitch, 2. A 180° rather than 360° helical interpolator if there is a choice, and thin acquisition sections rather than thick. Sometimes, it is still preferable to use axial rather than helical imaging to avoid helical artifacts (eg, in brain scanning).
B. Helical Artifacts In Multisection scanning 1. Windmill artifact: The typical windmill-like appearance of such artifacts is due to the fact that several rows of detectors intersect the plane of reconstruction during the course of each rotation. As helical pitch increases, the number of detector rows intersecting the image plane per rotation increases and the number of “vanes” in the windmill artifact increases. More complicated form of axial image distortion. Seen in thin slice images reconstructed from high pitch helical multislice CT images. Type of aliasing artifact The term windmill comes from the spiral appearance of shading artifact.
Regular equidistant streaks radiating from high-density foci scattered throughout the soft tissues are depicted on the axial CT image. The streak pattern rotates and continues from one image to another. This is caused by inadequate sampling in the z direction (aliasing). This artifact occurs when multiple rows of detectors intersect the plane of reconstruction during a rotation. The number of pairs in the windmill correspond to the number of detectors intersecting the reconstructed image plane.
2. Cone Beam Effect: As the number of sections acquired per rotation increases, a wider collimation is required and the x-ray beam becomes cone-shaped rather than fan shaped. As the tube and detectors rotate around the patient the data collected by each detector correspond to a volume contained between two cones, instead of the ideal flat plane. Small structure, such as piece of bone is detected by beam from one direction but is missed by opposing beam resulting inconsistency , leads to streak artifact This leads to artifacts similar to those caused by partial volume around off-axis objects. The artifacts are more pronounced for the outer detector rows than for the inner ones where the data collected correspond more closely to a plane. 
Reconstruction techniques like axial multiplanar reconstruction (AMPR) are used that account for the cone beam angle thereby reducing cone beam artifact Cone beam Algorithms- FDK (Feldkamp Davis and Kress) algorithms Rebinning technique (transforming and reorganizing 3D projection data into a 2D format to allow for faster and more efficient reconstruction using existing 2D reconstruction algorithms)
Cone beam effects get worse for increasing numbers of detector rows. Thus, 16-section scanners should potentially be more badly affected by artifacts than four-section scanners. However, manufacturers have addressed the problem by employing various forms of cone beam reconstruction instead of the standard reconstruction techniques used on older scanners.
C. Artifacts In Multiplanar and Three dimensional Reformation 1. Stair Step Artifacts. Stair step artifacts appear around the edges of structures in multiplanar and three-dimensional reformatted images when wide collimations and nonoverlapping reconstruction intervals are used. They are less severe with helical scanning, which permits reconstruction of overlapping sections without the extra dose to the patient that would occur if overlapping axial scans were obtained.
Physics behind staircase artifacts Results from insufficient sampling along the z-axis during helical acquisition. In helical CT, the patient table moves continuously while the X-ray gantry rotates, generating discrete slices along the longitudinal (z) axis. When reconstructing oblique or curved structures, the software interpolates between these slices to generate the desired plane. If the slice thickness is large or the pitch is high, the interpolation cannot accurately follow the true contour of the structure.
This mismatch between actual anatomy and interpolated data produces a “stepped” or staircase appearance along edges, rather than smooth contours. The artifact is more pronounced with curved or angled structures and can be reduced by using thin slices, lower pitch, overlapping reconstruction, or iterative reconstruction algorithms.
Minimization: Thin slice use 50% overlap on recon slice incrementation. Stair step artifacts are virtually eliminated in multiplanar and three-dimensional reformatted images from thin-section data obtained with today’s multisection scanners.
Appearance: (a) Jagged margins along high-attenuating borders of the right axillary vessels (oval) and right hemidiaphragm (arrow) on a reconstructed coronal image obtained with 5-mm section thickness axial acquisitions. (b) This resolved on image b, which was reconstructed from 2- mm section thickness axial acquisitions. Stair-step Artifact a b
2. Zebra Artifacts. Faint stripes may be apparent in multiplanar and three- dimensional reformatted images from helical data because the helical interpolation process gives rise to a degree of noise inhomogeneity along the z axis. This “zebra” effect becomes more pronounced away from the axis of rotation because the noise inhomogeneity is worse off-axis. Less severe with the helical scans .
Physics behind Zebra artifact In helical CT, the table moves continuously while the gantry rotates. •The reconstruction interpolates slices between measured data. •If slice thickness is too large or pitch is high, interpolation cannot accurately follow curved surfaces. •Result → Alternating high and low-density bands along the curve → “zebra” appearance. Reduction: Thinner slices Lower pitch Overlapping reconstruction, Iterative algorithms.
References:  Triche, B. L., Nelson, J. T., McGill, N. S., Porter, K. K., Sanyal, R., Tessl er, F. N., McConathy, J. E., Gauntt, D. M., Yester, M. V., & Singh, S. P. ( 2019). Recognizing and minimizing artifacts at CT, MRI, US, and m olecular imaging. Radiographics , 39(4), 1017–1018. https://doi.org/10.1148/rg.2019180022  Christensen's Physics of Diagnostic Radiology, 4th edition  Barrett, J. F., & Keat, N. Artifacts in CT: recognition and avoidance. Radiographics , 24(6), 1679–1691. https://doi.org/10.1148/rg.246045065  Radiologic Science For Technologists Physics Biology And Protectio
THANK YOU

More Related Content

PPTX
CT ARTIFACTS.pptx
bydypradio
 
PPTX
ct artefacts final part 1.pptx good ppt on ct artefacts
bysatarudramohanty55
 
PPTX
Ct artiact
byYosefAlemayehu3
 
PPTX
Ct artefacts_urooj Mushtaq Bhat 021.pptx
byUrooj Mushtaq Bhat
 
PPTX
6 CT ARTIFACTS-Rohit.pptx
byRohit Bansal
 
PPTX
Ct image artifacts
byCharleneMutambanengw
 
PDF
An overview of CT Detectors and Artefacts.pdf
bydypradio
 
PPT
ct 7.ppt computed tomography chapter six
byLinaShweiki
 
CT ARTIFACTS.pptx
bydypradio
 
ct artefacts final part 1.pptx good ppt on ct artefacts
bysatarudramohanty55
 
Ct artiact
byYosefAlemayehu3
 
Ct artefacts_urooj Mushtaq Bhat 021.pptx
byUrooj Mushtaq Bhat
 
6 CT ARTIFACTS-Rohit.pptx
byRohit Bansal
 
Ct image artifacts
byCharleneMutambanengw
 
An overview of CT Detectors and Artefacts.pdf
bydypradio
 
ct 7.ppt computed tomography chapter six
byLinaShweiki
 

Similar to CT Artifacts: Physics, Understanding and Minimization

PPTX
DIFFERENT TYPES OF RADIOGRAPHY CT_ARTIFACTS
byMichaelAkortsu1
 
PPTX
artifacts in computed tomography and MRI by remi.pptx
bymumunimomoh2018
 
PPTX
CT-Scan Artifacts and their Types .pptx
byANKITA KUMARI
 
PPTX
IMAGE QUALITY AND IMAGE ARTIFACTS .pptx
bylavanyarangaswamy31
 
PPTX
CT Artifacts
byKyle Rousseau
 
PPTX
CT PHysics for the medical gradutaes in india
byRejoJohn2
 
PPTX
Noise and artifacts in CT
byAngelineSP
 
PPTX
Physics_of_CT , CT machine and it’s parts, ct generations
byNeetuMeena56
 
PPTX
ARTIFACTS IN IMAGING MODALITIES. power ptx
byAbubakar Hammadama
 
PPTX
CT artifact
byGanesan Yogananthem
 
PPTX
SOME COMMON ARIFACTS IN COMPUTED TOMOGRAPHY.pptx
bystarrgazer0000
 
PPTX
artifacts copy copy presenatation o.pptx
byShafiq38
 
PPTX
Ct physics – II
byArchana Koshy
 
PDF
sure poster
byLydia Jim
 
PPTX
Back projection geometry in cbct
byDrGayatriMehrotra
 
PPTX
Cone beam ct
bynivedhanive8
 
PPTX
Contrast based artifacts
byshajitha khan
 
PPTX
4 computed tomography Dr. Muhammad Bin Zulfiqar
byDr. Muhammad Bin Zulfiqar
 
PPTX
MRI Artifacts (introduction, classification, types)
bysudhanracki
 
PPTX
Artifacts
byInosRagan
 
DIFFERENT TYPES OF RADIOGRAPHY CT_ARTIFACTS
byMichaelAkortsu1
 
artifacts in computed tomography and MRI by remi.pptx
bymumunimomoh2018
 
CT-Scan Artifacts and their Types .pptx
byANKITA KUMARI
 
IMAGE QUALITY AND IMAGE ARTIFACTS .pptx
bylavanyarangaswamy31
 
CT Artifacts
byKyle Rousseau
 
CT PHysics for the medical gradutaes in india
byRejoJohn2
 
Noise and artifacts in CT
byAngelineSP
 
Physics_of_CT , CT machine and it’s parts, ct generations
byNeetuMeena56
 
ARTIFACTS IN IMAGING MODALITIES. power ptx
byAbubakar Hammadama
 
CT artifact
byGanesan Yogananthem
 
SOME COMMON ARIFACTS IN COMPUTED TOMOGRAPHY.pptx
bystarrgazer0000
 
artifacts copy copy presenatation o.pptx
byShafiq38
 
Ct physics – II
byArchana Koshy
 
sure poster
byLydia Jim
 
Back projection geometry in cbct
byDrGayatriMehrotra
 
Cone beam ct
bynivedhanive8
 
Contrast based artifacts
byshajitha khan
 
4 computed tomography Dr. Muhammad Bin Zulfiqar
byDr. Muhammad Bin Zulfiqar
 
MRI Artifacts (introduction, classification, types)
bysudhanracki
 
Artifacts
byInosRagan
 

More from Dr. Aryan (Anish Dhakal)

PPTX
Imaging approach to Tumors of the Meninges
byDr. Aryan (Anish Dhakal)
 
PPTX
Radiological approach to Neurodegenerative and Dementia disorders
byDr. Aryan (Anish Dhakal)
 
PPTX
Imaging approach to Demyelinating diseases
byDr. Aryan (Anish Dhakal)
 
PPTX
Computed Tomography: Hardware and Instrumentation
byDr. Aryan (Anish Dhakal)
 
PPTX
Reading between the Rings: Imaging in Brain Infections
byDr. Aryan (Anish Dhakal)
 
PPT
Imaging in Cerebrovascular Malformations
byDr. Aryan (Anish Dhakal)
 
PPTX
Stroke Imaging: Fundamental principles and concepts
byDr. Aryan (Anish Dhakal)
 
PPTX
Errata and Book reviews: PGMEE in a Nutshell
byDr. Aryan (Anish Dhakal)
 
PPTX
Neuroimaging in Traumatic Brain Injury: Primary effects of CNS Trauma
byDr. Aryan (Anish Dhakal)
 
PPTX
Radiological approach to Lung Carcinoma: A 2025 update
byDr. Aryan (Anish Dhakal)
 
PPT
Physics of X-ray production by Dr. Aryan
byDr. Aryan (Anish Dhakal)
 
PPTX
X-ray interactions with matter by Dr. Aryan
byDr. Aryan (Anish Dhakal)
 
PPTX
Introduction to Ultrasound Transducers and Knobology
byDr. Aryan (Anish Dhakal)
 
PPTX
Radiological anatomy of Pancreas and Spleen
byDr. Aryan (Anish Dhakal)
 
PPTX
X-ray Generators: Basic concepts and Application
byDr. Aryan (Anish Dhakal)
 
PPTX
X-ray film: Physical characterstics and Film processing
byDr. Aryan (Anish Dhakal)
 
PPTX
Sonological evaluation of the Thyroid Gland
byDr. Aryan (Anish Dhakal)
 
PPTX
Solitary Pulmonary Nodule: Diagnosis and Management (Fleischner Society Update)
byDr. Aryan (Anish Dhakal)
 
PPTX
Radiological Anatomy of the Skull Base by Dr. Aryan
byDr. Aryan (Anish Dhakal)
 
PPTX
Pulmonary tuberculosis: A Radiology Perspective
byDr. Aryan (Anish Dhakal)
 
Imaging approach to Tumors of the Meninges
byDr. Aryan (Anish Dhakal)
 
Radiological approach to Neurodegenerative and Dementia disorders
byDr. Aryan (Anish Dhakal)
 
Imaging approach to Demyelinating diseases
byDr. Aryan (Anish Dhakal)
 
Computed Tomography: Hardware and Instrumentation
byDr. Aryan (Anish Dhakal)
 
Reading between the Rings: Imaging in Brain Infections
byDr. Aryan (Anish Dhakal)
 
Imaging in Cerebrovascular Malformations
byDr. Aryan (Anish Dhakal)
 
Stroke Imaging: Fundamental principles and concepts
byDr. Aryan (Anish Dhakal)
 
Errata and Book reviews: PGMEE in a Nutshell
byDr. Aryan (Anish Dhakal)
 
Neuroimaging in Traumatic Brain Injury: Primary effects of CNS Trauma
byDr. Aryan (Anish Dhakal)
 
Radiological approach to Lung Carcinoma: A 2025 update
byDr. Aryan (Anish Dhakal)
 
Physics of X-ray production by Dr. Aryan
byDr. Aryan (Anish Dhakal)
 
X-ray interactions with matter by Dr. Aryan
byDr. Aryan (Anish Dhakal)
 
Introduction to Ultrasound Transducers and Knobology
byDr. Aryan (Anish Dhakal)
 
Radiological anatomy of Pancreas and Spleen
byDr. Aryan (Anish Dhakal)
 
X-ray Generators: Basic concepts and Application
byDr. Aryan (Anish Dhakal)
 
X-ray film: Physical characterstics and Film processing
byDr. Aryan (Anish Dhakal)
 
Sonological evaluation of the Thyroid Gland
byDr. Aryan (Anish Dhakal)
 
Solitary Pulmonary Nodule: Diagnosis and Management (Fleischner Society Update)
byDr. Aryan (Anish Dhakal)
 
Radiological Anatomy of the Skull Base by Dr. Aryan
byDr. Aryan (Anish Dhakal)
 
Pulmonary tuberculosis: A Radiology Perspective
byDr. Aryan (Anish Dhakal)
 

Recently uploaded

PPTX
Beyond Tissue Damage: How Nociplastic Pain is Reshaping Our Understanding of ...
byDaradia: The Pain Clinic
 
PDF
Minerals Characterization of Antibodies Medicinal Plants Using Atomic Absorpt...
byDr. Juber Hawaldar
 
PPTX
ANTENATAL MOTHER NUTRITIOUS DIETARY PLANNING
byKrantiKhobragade
 
PDF
Definitive Innovation in Resectable Head and Neck Cancer: New Immunotherapy A...
byPVI, PeerView Institute for Medical Education
 
PPTX
Placebo And Acupuncture Friend or Foe.pptx
byAntonio Moreira
 
PDF
Pòster "Virtual Reality in Pain Management"
byBadalona Serveis Assistencials
 
PPTX
Biostatistics_Unit_II_Research Methodology And Biostatistics.pptx
byDHANASHREE KOLHEKAR
 
PPTX
Functional Anatomy and Hormonal Regulation of the Pituitary Gland
by2025msyt016
 
PPTX
Oncology Physiotherapy_ A Key Piece in Cancer Care Puzzle.pptx
byasmitashresthapt
 
PPTX
PERCEPTION.pptx I SEMESTER STUDENTS BY SHILPA HOTAKAR
bySHILPA HOTAKAR
 
PPTX
THYROID GLAND ANATOMY PHYSIOLOGY AND DISORDER
byKrantiKhobragade
 
PDF
TMJ Surg.Anat.&.Fn - Final.pdf. Dr. Sanjab
byabdihakeemahmed951
 
PPTX
SHOCK, TYPES AND MANAGEMENT - SUSHIL GYAWALI.pptx
byDr Sushil Gyawali
 
PPTX
4.Carpal Tunnel Syndrome (CTS)compression of the median nerve. General orthop...
byBolan University of Medical and Health Sciences ,Quetta
 
PPTX
ADRENAL GLAND DISORDERS: A BRIEF OVERVIEW
byhelenmorish
 
PPTX
DEC 2025ONCOLOGY CARTOONS BY DR KANHU CHARAN PATRO
byKanhu Charan
 
PDF
Concept of Drug: Foundations of General Pharmacology
byBaasir Umair Khattak
 
PPTX
Herbs As Health Food....pptx ( Pharmacognosy )
byBushraDeshpande
 
PPTX
Understanding Diabetes Mellitus - A comprehensive guide for nurses and Eviden...
byViresh Mahajani
 
PPTX
Leadership , teamwork and ethics in disaster nursing (1).pptx
byViresh Mahajani
 
Beyond Tissue Damage: How Nociplastic Pain is Reshaping Our Understanding of ...
byDaradia: The Pain Clinic
 
Minerals Characterization of Antibodies Medicinal Plants Using Atomic Absorpt...
byDr. Juber Hawaldar
 
ANTENATAL MOTHER NUTRITIOUS DIETARY PLANNING
byKrantiKhobragade
 
Definitive Innovation in Resectable Head and Neck Cancer: New Immunotherapy A...
byPVI, PeerView Institute for Medical Education
 
Placebo And Acupuncture Friend or Foe.pptx
byAntonio Moreira
 
Pòster "Virtual Reality in Pain Management"
byBadalona Serveis Assistencials
 
Biostatistics_Unit_II_Research Methodology And Biostatistics.pptx
byDHANASHREE KOLHEKAR
 
Functional Anatomy and Hormonal Regulation of the Pituitary Gland
by2025msyt016
 
Oncology Physiotherapy_ A Key Piece in Cancer Care Puzzle.pptx
byasmitashresthapt
 
PERCEPTION.pptx I SEMESTER STUDENTS BY SHILPA HOTAKAR
bySHILPA HOTAKAR
 
THYROID GLAND ANATOMY PHYSIOLOGY AND DISORDER
byKrantiKhobragade
 
TMJ Surg.Anat.&.Fn - Final.pdf. Dr. Sanjab
byabdihakeemahmed951
 
SHOCK, TYPES AND MANAGEMENT - SUSHIL GYAWALI.pptx
byDr Sushil Gyawali
 
4.Carpal Tunnel Syndrome (CTS)compression of the median nerve. General orthop...
byBolan University of Medical and Health Sciences ,Quetta
 
ADRENAL GLAND DISORDERS: A BRIEF OVERVIEW
byhelenmorish
 
DEC 2025ONCOLOGY CARTOONS BY DR KANHU CHARAN PATRO
byKanhu Charan
 
Concept of Drug: Foundations of General Pharmacology
byBaasir Umair Khattak
 
Herbs As Health Food....pptx ( Pharmacognosy )
byBushraDeshpande
 
Understanding Diabetes Mellitus - A comprehensive guide for nurses and Eviden...
byViresh Mahajani
 
Leadership , teamwork and ethics in disaster nursing (1).pptx
byViresh Mahajani
 

CT Artifacts: Physics, Understanding and Minimization

  • 1.
    CT ARTIFACTS: PHYSICS, UNDERSTANDINGAND MINIMIZATION Presented by: Dr. Anish Dhakal Resident MD Radiodiagnosis, KUSMS 24th September, 2025
  • 2.
    What is anartifact? Literal meaning in medical world: “Any unintended pattern, distortion, or flaw in an image that does not represent the actual anatomy or pathology of the patient.”
  • 3.
    The term artifactis applied to any systematic discrepancy between the CT numbers in the reconstructed image and the true attenuation coefficients of the object. CT images are inherently more prone to artifacts than conventional radiographs because the image is reconstructed from something on the order of a million independent detector measurements. The reconstruction technique assumes that all these measurements are consistent, so any error of measurement will usually reflect itself as an error in the reconstructed image. Artifacts can seriously degrade the quality of CT images, sometimes to the point of making them diagnostically unusable.
  • 4.
    The types ofartifact that can occur are as follows: (a) Streaking, which is generally due to an inconsistency in a single measurement (b) Shading, which is due to a group of channels or views deviating gradually from the true measurement (c) Rings, which are due to errors in an individual detector calibration (d) Distortion, which is due to helical reconstruction.
  • 5.
  • 6.
    Based on originof artifacts: (a) Physics-based artifacts, which result from the physical processes involved in the acquisition of CT data (b) Patient-based artifacts, which are caused by such factors as patient movement or the presence of metallic materials in or on the patient (c) Scanner- based artifacts, which result from imperfections in scanner function (d) Helical and multisection artifacts, which are produced by the image reconstruction process.
  • 7.
  • 8.
    A. Beam hardeningartifact B. Partial volume effect artifact C. Photon starvation artifact D.Undersampling artifact E. Edge gradient artifact F. Blooming artifact
  • 9.
    A. Beam Hardening Anx-ray beam producing the CT image is not a monochromatic beam. The x-ray beam is composed of individual photons with a range of energies.
  • 10.
    As a heterogeneousx-ray beam passes through the patient, the low energy protons are rapidly absorbed. This means the x-ray beam exiting the patient contains a lower percentage of energy photons than the beam had when it entered the patient. This effect is called "beam hardening." The linear attenuation coefficient of a tissue is directly related to the average energy of the x-ray beam. Reconstruction programs anticipate and correct for variation in linear attenuation coefficients caused by beam hardening, but such corrections are not precise.
  • 11.
    As the beampasses through an object, it becomes “harder,” that is to say its mean energy increases, because the lower energy photons are absorbed more rapidly than the higher-energy photons. This focally increased mean beam energy is interpreted as being due to it passing through a less attenuating material relative to the surroundings and so a lower Hounsfield unit is assigned and the image will be represented as more black.
  • 13.
    Two types ofartifact can result from this effect: so-called cupping artifacts and the appearance of dark bands or streaks between dense objects in the image.
  • 14.
    1. Cupping Artifacts. Xrays passing through the middle portion of a uniform cylindrical phantom are hardened more than those passing though the edges because they are passing though more material. As the beam becomes harder, the rate at which it is attenuated decreases, so the beam is more intense when it reaches the detectors than would be expected if it had not been hardened. Therefore, the resultant attenuation profile differs from the ideal profile that would be obtained without beam hardening. A profile of the CT numbers across the phantom displays a characteristic cupped shape
  • 16.
  • 18.
    2. Streaks andDark Bands. In very heterogeneous cross sections, dark bands or streaks can appear between two dense objects in an image. They occur because the portion of the beam that passes through one of the objects at certain tube positions is hardened less than when it passes through both objects at other tube positions. This type of artifact can occur both in bony regions of the body and in scans where a contrast medium has been used.
  • 21.
    Dark and brightstreaks radiating from and between high-density objects, such as dental amalgam. Alternating dark and bright streaks in a narrow band extending across the posterior fossa
  • 22.
    Methods to reduceBeam Hardening I. Built-in Features for Minimizing Beam Hardening 1. Filtration: A flat piece of attenuating, usually metallic material is used to “pre-harden” the beam by filtering out the lower-energy components before it passes through the patient. An additional “bow tie” filter further hardens the edges of the beam, which will pass through the thinner parts of the patient.
  • 23.
    2. Calibration correction: Manufacturerscalibrate their scanners using phantoms in a range of sizes. This allows the detectors to be calibrated with compensation tailored for the beam hardening effects of different parts of the patient. Since patient anatomy never exactly matches a cylindrical calibration phantom, in clinical practice there may be either a slight residual cupping artifact or a slight “capping” artifact, with a higher central CT value due to overcorrection.
  • 24.
    3. Beam hardeningcorrection software:  An iterative correction algorithm may be applied when images of bony regions are being reconstructed. This helps minimize blurring of the bone–soft tissue interface in brain scans and also reduces the appearance of dark bands in nonhomogeneous cross sections.
  • 26.
    II. Avoidance ofBeam Hardening by the Operator. It is sometimes possible to avoid scanning bony regions, either by means of patient positioning or by tilting the gantry. It is important to select the appropriate scan field of view to ensure that the scanner uses the correct calibration and beam hardening correction data.
  • 27.
    III. Dual energyCT: •DECT can measure attenuation at two energies that allows estimation of effective monochromatic attenuation and mathematical correction for beam hardening. •Low- and high-energy datasets are combined to calculate: virtual monochromatic images, where the beam behaves as if all photons have the same energy. •These reduce the effect of differential absorption of low-energy photons.
  • 28.
    B. Partial VolumeArtifact These artifacts are a separate problem from partial volume averaging, which yields a CT number representative of the average attenuation of the materials within a voxel. Partial volume artifact occurs when a dense object lying off-center protrudes partway into the width of the x-ray beam. The off-axis object can be within the beam, and therefore “seen” by the detectors, when the tube is pointing in one direction but outside the beam, and therefore not seen by the detectors, when the tube is pointing in another The inconsistencies between the views cause shading artifacts to appear in the image
  • 29.
    Non linear partialvolume averaging Nonlinear partial volume averaging is an advanced form of partial volume averaging (PVA), a phenomenon in medical imaging (like CT and MRI) where a single image voxel contains multiple types of tissue, leading to blurring and image artifacts. Unlike simple linear averaging, nonlinear PVA considers that the relationship between tissue types and their resulting signal or attenuation is not a simple average, but more complex, often appearing as shading or streaking artifacts, especially in helical scans.
  • 30.
    From a strictstandpoint of physics, linear partial volume averaging is not a reconstruction artifact, it’s just a blurring/pseudo-density effect that can cause misinterpretation hence clinically important.
  • 31.
    Physics behind partialvolume averaging Pattern 1 (Partial Volume Effect): A dense object lying off- centre protrudes partially into the width of an x-ray beam . This results in divergence of the beam and manifests as shading artifacts adjacent to said object [non-linear PVA] Pattern 2: (CT voxels are 3D cubes): If you have a dense thing taking up half the cube, and a sparse ( low attenuating ) thing in the other half of the cube, the machine will average the two together giving something that has intermediate density. The classic location is the skull base averaging with CSF or brain to look like blood [linear PVA]
  • 34.
    Arise essentially from reconstructinglow resolution images, typically thick slice images. It produces CT numbers as an average of all types of tissues. It will appear as bands or streaks.
  • 35.
    Appearance: The left upperlobe segmental arterial branch appears to have a low-attenuating filling defect (arrow) on the CT image obtained with 3-mm section thickness (a), which resolves on the CT image obtained with 1.5-mm section thickness (b). Volume Averaging Artifact 3 mm 1.5 mm a b
  • 36.
    Methods to reducepartial volume artifacts Partial volume artifacts can best be avoided by using a thin acquisition section width. If the noise is a problem, acquiring thin slices then generating thicker slices by adding them together. This is necessary when imaging any part of the body where the anatomy is changing rapidly in the z direction, for example in the posterior fossa.
  • 37.
    C. Photon starvationartifact A potential source of serious streaking artifacts is photon starvation, which can occur in highly attenuating areas such as the shoulders. When the x-ray beam is traveling horizontally, the attenuation is greatest and insufficient photons reach the detectors. The result is that very noisy projections are produced at these tube angulations. The reconstruction process has the effect of greatly magnifying the noise, resulting in horizontal streaks in the image.
  • 39.
    Appearance: Sagittal CT image(a) of the cervical spine shows significant noise and decreased contrast discrimination (oval) through the lower cervical and upper thoracic vertebral bodies. A similar appearance is depicted on the axial CT image (b) of the chest, secondary to lateral body soft-tissue thickness. This is secondary to photons traversing a dense structure, limiting the number of photons that strike the detector, which results in increased noise. Photon Starvation Artifact a b
  • 40.
    If the tubecurrent is increased for the duration of the scan, the problem of photon starvation will be overcome, but the patient will receive an unnecessary dose when the beam is passing through less attenuating parts.
  • 41.
    Methods to reducephoton starvation 1. Automatic Tube Current Modulation.  On some scanner models, the tube current is automatically varied during the course of each rotation, a process known as milliamperage modulation. This allows sufficient photons to pass through the widest parts of the patient without unnecessary dose to the narrower parts.
  • 42.
    2. Adaptive Filtration Thissoftware correction smooths the attenuation profile in areas of high attenuation before the image is reconstructed.
  • 43.
    A multi-dimensional adaptivefiltration technique is used on multi-section scanners. For the small proportion of projection data that exceed a selected attenuation threshold, smoothing is carried out between adjacent in-plane detectors and between successive projection angles while the z filter used in helical reconstruction is broadened for high-attenuation projection angles to allow more photons to contribute to the reconstruction
  • 46.
    D. Undersampling artifact/Aliasingartifact The number of projections used to reconstruct a CT image is one of the determining factors in image quality. An insufficient number of projections used to reconstruct the CT can diminish quality, and result in mis- registration artifacts. An insufficient number of projections used to reconstruct the CT can diminish quality, and result in mis- registration artifacts Too large an interval between projections (undersampling) can result in misregistration by the computer of information relating to sharp edges and small objects.
  • 47.
    View Aliasing: Thisis when you have under sampling between projections. You see line stripes radiating from the edge (but at a distance from) a dense object. This is fixed by acquiring the largest possible number of projects per rotation - slowing the rotation speed. Ray Aliasing: This is when you have under sampling within a projection. You see strips appearing close to the structure . This is fixed by using specialized high resolution techniques - manufacturer employed.
  • 49.
    Method to reduceundersampling View aliasing can be minimized by acquiring the largest possible number of projections per rotation. On some scanners, this can be achieved only by using a slower rotation speed, while on others the number of projections is independent of rotation speed. Using high resolution technique like 1. Flying focal spot, and 2. Quarter detector shift
  • 50.
    •Instead of aligningeach scan exactly over the previous • detector positions, the detector array is shifted by ¼ of a detector width for successive rotations. •Example: if detector width = 1 mm: •Rotation 1: detectors at positions 0,1,2,… mm •Rotation 2: detectors at positions 0.25,1.25,2.25,… mm •Rotation 3: 0.5,1.5,2.5,… mm •Rotation 4: 0.75,1.75,2.75,… mm This results in:  Improves sampling density: effectively gives 4× more sampling points.  Reduces undersampling artifacts: smoother images, fewer aliasing streaks.  Better resolution in z-direction (longitudinal) without reducing detector size.
  • 51.
    E. Edge GradientArtifact Arise from irregularly shaped object that have a pronounced difference in density from surrounding structure Occurs at sharp density transitions, where the CT beam passes from one tissue type to another (e.g., bone–soft tissue interface). Results in streak artifact or shading To minimize: Using thinner slices Using a low HU- value oral contrast Change in patient’s position
  • 52.
    Appearance: Small highly densestructures such as calcifications and stents appear larger than they truly are, as depicted on the accompanying CT images. The circumferential coronary artery calcification (arrow) appears progressively smaller from left to right, with the changing window width (W) and level (L). Cause: Very high CT numbers of the structure cause pixel saturation when using typical lookup table (LUT) windows, causing the structure to appear larger than it is. Also, using a smoothing filter kernel makes small bright objects appear larger. Minimization: 1. Use a LUT window that is wide enough so that the displayed pixels are not saturated. 2. Use a standard or sharper filter kernel with an iterative noise reduction algorithm. 3. Reduce the display FOV to improve the spatial resolution. 4. Use a high kiloelectron-volt (keV) monoenergetic reconstruction (dual-energy acquisitions only). 5. Use small section thicknesses. W/L: 800/350 HU W/L: 1000/500 HU W/L: 1650/750 HU F. Blooming Artifact
  • 53.
  • 54.
    A. Metallic materialsartifact B. Patient motion artifact C. Incomplete projection artifact
  • 55.
    A. Metallic materialsartifact The presence of metal objects in the scan field can lead to severe streaking artifacts. They occur because the density of the metal is beyond the normal range that can be handled by the computer, resulting in incomplete attenuation profiles. Additional artifacts due to beam hardening, partial volume, and aliasing are likely to compound the problem when scanning very dense objects. Metals with high Z (Iron , Platinum) tend to have more artifacts than those with lower Z (Titanium)
  • 56.
    Methods to reducemetal artifacts I. Avoidance of Metal Artifacts by the Operator Patients are normally asked to take off removable metal objects such as jewelry before scanning commences. For nonremovable items, such as dental fillings, prosthetic devices, and surgical clips, it is sometimes possible to use gantry angulation to exclude the metal inserts from scans of nearby anatomy. When it is impossible to scan the required anatomy without including metal objects, increasing technique, especially kilovoltage, may help penetrate some objects, and using thin sections will reduce the contribution due to partial volume artifact.
  • 58.
    II. Software Correctionsfor metal artifacts Streaking caused by overranging can be greatly reduced by means of special software corrections.  Manufacturers use a variety of interpolation techniques to substitute the overrange values in attenuation profiles. The usefulness of metal artifact reduction software is sometimes limited because, although streaking distant from the metal implants is removed, there still remains a loss of detail around the metal-tissue interface, which is often the main area of diagnostic interest. Beam hardening correction software should also be used when scanning metal objects to minimize the additional artifacts due to beam hardening.
  • 59.
    MAR (Metal artifactreduction) technique:- Acquisition and storage of the raw data Reconstruction of CT image Identification of the implant Automatic definition of the boundaries of the implant within the projection data. For each projection, the implant boundaries are automatically defined within the given ROI by the use of given threshold values Iterative reconstruction of the missing projection data Reconstruction of the artifact-reduced image from the newly computed projection data
  • 62.
    B. Patient Motion Shortscan times have diminished but not eliminated motion artifacts. Patient motion can cause misregistration artifacts, which usually appear as shading or streaking in the reconstructed image. Steps can be taken to prevent voluntary motion, but some involuntary motion may be unavoidable during body scanning.
  • 63.
    Why motion artifactsoccur? The reconstructed image will display an object in motion as a streak in the direction of motion. On reconstruction the scanner will average the density of the pixels covering the motion area. In some fashion, the intensity of the streak artifact will depend on the density of the object in motion. Motion of objects that have densities much different from their surroundings produces more intense artifacts. Thus, motion of metallic or gas-containing structures produce striking artifacts.
  • 64.
    Produces “ghosting” effect(high-contrast structures appear duplicated or smeared in adjacent slices or projections. Image appears– as if it is composed of superimposed images.
  • 66.
    Appearance: An apparent coronaryartery filling defect and/or occlusion (arrow in a; oval in b) is depicted in these prospectively gated cardiac CT angiograms. The defect is related to respiratory motion, which is best appreciated on CT images obtained in the lung window (c). Distortion of anatomy by dark and/or bright bands and loss of image resolution secondary to respiratory motion between axial acquisitions causes this artifact. Respiratory Motion Artifact a b c
  • 67.
    Appearance: Apparent coronary arteryfilling defect and/or occlusion (arrow in a) resolved on the CT image (b) after the vendor- provided intelligence correction was applied. Cause: Distortion of anatomy by dark and/or bright bands and loss of image resolution secondary to misregistration related to motion Cardiac Motion Artifact References 4, 5. a b
  • 68.
    Methods to reducemotion artifacts I. Avoidance of Motion Artifacts by the Operator The use of positioning aids is sufficient to prevent voluntary movement in most patients. In some cases (eg, pediatric patients), it may be necessary to immobilize the patient by means of sedation. Using as short a scan time as possible helps minimize artifacts when scanning regions prone to movement. Respiratory motion can be minimized if patients are able to hold their breath for the duration of the scan. The sensitivity of the image to motion artifacts depends on the orientation of the motion. Therefore, it is preferable if the start and end position of the tube is aligned with the primary direction of motion, for example, vertically above or below a patient undergoing a chest scan. Specifying body scan mode, as opposed to head scan mode, may automatically incorporate some motion artifact reduction in the reconstruction.
  • 69.
    II. Built-in Featuresfor Minimizing Motion Artifacts 1. Overscan and underscan modes: The maximum discrepancy in detector readings occurs between views obtained toward the beginning and end of a 360° scan. Some scanner models use overscan mode for axial body scans, whereby an extra 10% or so is added to the standard 360° rotation. The repeated projections are averaged, which helps reduce the severity of motion artifacts. The use of partial scan mode can also reduce motion artifacts, but this may be at the expense of poorer resolution.
  • 70.
    2. Software correction: Mostscanners, when used in body scan mode, automatically apply reduced weighting to the beginning and end views to suppress their contribution to the final image. However, this may lead to more noise in the vertical direction of the resultant image, depending on the shape of the patient.
  • 71.
    3. Cardiac gating: Therapid motion of the heart can lead to severe artifacts in images of the heart and to artifacts that can mimic disease in associated structures, for example, dissected aorta. To overcome these difficulties, techniques have been developed to produce images by using data from just a fraction of the cardiac cycle, when there is least cardiac motion. This is achieved by combining electrocardiographic gating techniques with specialized methods of image reconstruction.
  • 73.
    C. Incomplete projections Ifany portion of the patient lies outside field of view, the computer will have incomplete information relating to this portion and streaking or shading artifacts are likely to be generated. For example a patient is scanned with the arms down instead of being raised out of the way of the scan. As the arms are outside the scan field, they are not present in the image, but their presence in some views during scanning can led to such severe artifacts throughout the image as to significantly degrade its usefulness. Similar effects can be caused by dense objects such as an intravenous tube containing contrast medium lying outside the scan field.
  • 74.
    Axial CT imagethrough the abdomen truncation artifact with increased curvilinear attenuation along the edges of the image. Occurs when the anatomy of interest is outside the scan field of view (FOV) (eg, a large patient, or patient’s arms resting at their side), which results in the disruption of the accurate attenuation measurement along image edges.
  • 75.
    Methods to reduceincomplete projections To avoid artifacts due to incomplete projections, it is essential to position the patient so that no parts lie outside the scan field Scanners designed specifically for radiation therapy planning have wider bores and larger scan fields of view than standard scanners and permit greater versatility in patient positioning. They also allow scanning of exceptionally large patients who would not fit within the field of view of standard scanners. Taping patient tissue and raising patients arms above their head on the scan of chest and abdomen also helps
  • 76.
  • 77.
    A. Ring artifacts B.Line in topogram C. Tube arcing
  • 78.
    A. Ring Artifacts Ifone of the detectors is out of calibration on a third- generation (rotating x-ray tube and detector assembly) scanner, the detector will give a consistently erroneous reading at each angular position, resulting in a circular artifact. A scanner with solid-state detectors, where all the detectors are separate entities, is in principle more susceptible to ring artifacts than a scanner with gas detectors, in which the detector array consists of a single xenon-filled chamber subdivided by electrodes. They can impair the diagnostic quality of an image, and this is particularly likely when central detectors are affected, creating a dark smudge at the center of the image.
  • 80.
    Ring artifacts arethe result of miscalibration of one detector in a rotate-rotate geometry scanner. Detector failure would also produce a ring artifact If a detector is miscalibrated, it will record incorrect data in every projection. This misinformation is reconstructed as a ring in the image, with the radius of the ring determined by the position of the faulty detector in the detector array. Faulty detectors in rotate-fixed units also record false information. However, this information is not visible in the reconstructed image because the faulty detector collects data from many angles (rings may appear in rotate-fixed geometry if the x-ray tube is not aligned correctly).
  • 82.
    Appearance: Concentric circular structuresexpanding from the scanner isocenter (arrows) are depicted on axial CT images. Cause: Related to miscalibrated detector (a), insufficient dose (b), or contrast material on the polyester film window that covers the detector (c). As the assembly rotates, the detector gives an incorrect value at each location, causing the ring artifact. Ring Artifact a b c
  • 83.
    Methods to reducering artifacts The presence of circular artifacts in an image is an indication that the detector gain needs recalibration or may need repair services. Selecting the correct scan field of view may reduce the artifact by using calibration data that fit more closely to the patient anatomy. All modern scanners use solid-state detectors, but their potential for ring artifacts is reduced by software that characterizes and corrects detector variations.
  • 84.
    B. Line intopogram Due to faulty detectors Remedy Detector replacement
  • 85.
    Tube arcing Occurs whenthere is a short circuit within the tube, typically from cathode to tube envelope. Tungsten vapor from anode and cathode intercepts the projectile electrons intended for collisions with the target. Causes momentary loss of x-ray output. Remedy: Tube Replacement.
  • 86.
  • 87.
    A. Helical Artifactsin the Axial Plane: Single-Section Scanning B. Helical Artifacts In Multisection scanning C. Artifacts In Multiplanar and Three dimensional Reformation
  • 88.
    A. Helical Artifactsin the Axial Plane: Single-Section Scanning Artifacts related to helical scanning is due to the helical interpolation and reconstruction process. The artifacts occur when anatomic structures change rapidly in the z direction (eg, at the top of the skull) and are worse for higher pitches. If a helical scan is performed of a cone-shaped phantom lying along the z axis of the scanner, the resultant axial images should appear circular. In fact, their shape is distorted because of the weighting function used in the helical interpolation algorithm. For some projection angles, the image is influenced more by contributions from wider parts of the cone in front of the scan plane; for other projection angles, contributions from narrower parts of the cone behind the scan plane.
  • 91.
    Methods to reducehelical artifacts To keep helical artifacts to a minimum, steps must be taken to reduce the effects of variation along the z axis. This means using, where possible, 1. Low pitch, 2. A 180° rather than 360° helical interpolator if there is a choice, and thin acquisition sections rather than thick. Sometimes, it is still preferable to use axial rather than helical imaging to avoid helical artifacts (eg, in brain scanning).
  • 92.
    B. Helical ArtifactsIn Multisection scanning 1. Windmill artifact: The typical windmill-like appearance of such artifacts is due to the fact that several rows of detectors intersect the plane of reconstruction during the course of each rotation. As helical pitch increases, the number of detector rows intersecting the image plane per rotation increases and the number of “vanes” in the windmill artifact increases. More complicated form of axial image distortion. Seen in thin slice images reconstructed from high pitch helical multislice CT images. Type of aliasing artifact The term windmill comes from the spiral appearance of shading artifact.
  • 93.
    Regular equidistant streaksradiating from high-density foci scattered throughout the soft tissues are depicted on the axial CT image. The streak pattern rotates and continues from one image to another. This is caused by inadequate sampling in the z direction (aliasing). This artifact occurs when multiple rows of detectors intersect the plane of reconstruction during a rotation. The number of pairs in the windmill correspond to the number of detectors intersecting the reconstructed image plane.
  • 94.
    2. Cone BeamEffect: As the number of sections acquired per rotation increases, a wider collimation is required and the x-ray beam becomes cone-shaped rather than fan shaped. As the tube and detectors rotate around the patient the data collected by each detector correspond to a volume contained between two cones, instead of the ideal flat plane. Small structure, such as piece of bone is detected by beam from one direction but is missed by opposing beam resulting inconsistency , leads to streak artifact This leads to artifacts similar to those caused by partial volume around off-axis objects. The artifacts are more pronounced for the outer detector rows than for the inner ones where the data collected correspond more closely to a plane. 
  • 95.
    Reconstruction techniques likeaxial multiplanar reconstruction (AMPR) are used that account for the cone beam angle thereby reducing cone beam artifact Cone beam Algorithms- FDK (Feldkamp Davis and Kress) algorithms Rebinning technique (transforming and reorganizing 3D projection data into a 2D format to allow for faster and more efficient reconstruction using existing 2D reconstruction algorithms)
  • 98.
    Cone beam effectsget worse for increasing numbers of detector rows. Thus, 16-section scanners should potentially be more badly affected by artifacts than four-section scanners. However, manufacturers have addressed the problem by employing various forms of cone beam reconstruction instead of the standard reconstruction techniques used on older scanners.
  • 100.
    C. Artifacts InMultiplanar and Three dimensional Reformation 1. Stair Step Artifacts. Stair step artifacts appear around the edges of structures in multiplanar and three-dimensional reformatted images when wide collimations and nonoverlapping reconstruction intervals are used. They are less severe with helical scanning, which permits reconstruction of overlapping sections without the extra dose to the patient that would occur if overlapping axial scans were obtained.
  • 101.
    Physics behind staircaseartifacts Results from insufficient sampling along the z-axis during helical acquisition. In helical CT, the patient table moves continuously while the X-ray gantry rotates, generating discrete slices along the longitudinal (z) axis. When reconstructing oblique or curved structures, the software interpolates between these slices to generate the desired plane. If the slice thickness is large or the pitch is high, the interpolation cannot accurately follow the true contour of the structure.
  • 102.
    This mismatch betweenactual anatomy and interpolated data produces a “stepped” or staircase appearance along edges, rather than smooth contours. The artifact is more pronounced with curved or angled structures and can be reduced by using thin slices, lower pitch, overlapping reconstruction, or iterative reconstruction algorithms.
  • 103.
    Minimization: Thin slice use 50%overlap on recon slice incrementation. Stair step artifacts are virtually eliminated in multiplanar and three-dimensional reformatted images from thin-section data obtained with today’s multisection scanners.
  • 104.
    Appearance: (a) Jagged marginsalong high-attenuating borders of the right axillary vessels (oval) and right hemidiaphragm (arrow) on a reconstructed coronal image obtained with 5-mm section thickness axial acquisitions. (b) This resolved on image b, which was reconstructed from 2- mm section thickness axial acquisitions. Stair-step Artifact a b
  • 105.
    2. Zebra Artifacts. Faintstripes may be apparent in multiplanar and three- dimensional reformatted images from helical data because the helical interpolation process gives rise to a degree of noise inhomogeneity along the z axis. This “zebra” effect becomes more pronounced away from the axis of rotation because the noise inhomogeneity is worse off-axis. Less severe with the helical scans .
  • 106.
    Physics behind Zebraartifact In helical CT, the table moves continuously while the gantry rotates. •The reconstruction interpolates slices between measured data. •If slice thickness is too large or pitch is high, interpolation cannot accurately follow curved surfaces. •Result → Alternating high and low-density bands along the curve → “zebra” appearance. Reduction: Thinner slices Lower pitch Overlapping reconstruction, Iterative algorithms.
  • 108.
    References:  Triche, B.L., Nelson, J. T., McGill, N. S., Porter, K. K., Sanyal, R., Tessl er, F. N., McConathy, J. E., Gauntt, D. M., Yester, M. V., & Singh, S. P. ( 2019). Recognizing and minimizing artifacts at CT, MRI, US, and m olecular imaging. Radiographics , 39(4), 1017–1018. https://doi.org/10.1148/rg.2019180022  Christensen's Physics of Diagnostic Radiology, 4th edition  Barrett, J. F., & Keat, N. Artifacts in CT: recognition and avoidance. Radiographics , 24(6), 1679–1691. https://doi.org/10.1148/rg.246045065  Radiologic Science For Technologists Physics Biology And Protectio
  • 109.