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MRI CONTRAST AGENT.pdf
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- 2. INTRODUCTION • MRI providesexcellent soft tissue contrast. • However, enhancement with contrast agents substantially improves the sensitivity and specificity of lesions. • Contrast agents are pharmaceuticals that enhance the contrast between the lesions and normal structures. • Enhancement of the image contrast between normal and diseased tissue increases the diagnostic accuracy. • Contrast agents are commonly used in clinical practice for a broad range of indications.
- 3. IDEAL CONTRAST The idealcontrast agent should have the following properties: 1. The contrast agent must be efficient at low concentrations 2. The contrast agent should possesses tissue specificity to enable higher concentration in specific tissue 3. The contrast agent must be substantially chased from the targeted tissue. 4. It must have low viscosity 5. The contrast agent must possesses a suitable shelf life for storage purposes 6. It should be nontoxic
- 4. HISTORY • Nearly allMR contrast agents constitute paramagnetic compounds. • The most commonly used paramagnetic ion is the gadolinium (Gd) ion. • This ion attaches with various ligands (chemical compounds) such as diethylenetriamine pentaacetic acid (DTPA) that act as chelating agents. • The reaction of Gd ions with DTPA forms a stable chelate complex. • The Gd ion of the lanthanide metal group has a high spin quantum number (7/2), making this ion a desirable contrast agent.
- 5. • The firstand foremost MRI contrast agent on the world market was Schering Magnevist, the dimeglumine salt of gadolinium–diethylenetriamine penta acetic acid (Gd DTPA). • Presently three gadolinium based contrast agents are approved by the US Food and Drug Administration (FDA). • Nonionic Gd-DTPA-BMA (Gadodiamide or omniscan), • Ionic Gd-DTPA(gadopentetate dimeglumine or magnevist) and • Nonionic Gd-HP-DO3A (gadoteridol or prohance)
- 7. MECHANISM OF ACTION •MR contrast agents alter tissue contrast. Signal will increase when hydrogen proton increases, T1 decreases and T2 increases. • Signal will be decreased when hydrogen protons decrease, T1 increases or T2 decreases. • Paramagnetic materials like Gd, Fe and Mn decrease both T1 and T2 thereby altering the signal. • Super paramagnetic particles reduceT2 thereby decreasing the signal. • Diamagnetic substances have negative induced magnetization and are used in situations where they can displace or mix with normal tissue. • Water, fat, perfluorocarbon and CO2 are used as gastrointestinal contrast agents by the displacement of normal bowel contrast.
- 8. Classification of CM •Contrast agents have been broadly classified as positive relaxation agents (T1 contrast agents) and negative relaxation agents (T2 contrast agents).
- 9. Positive Relaxation Agents •The contrast agent that affects T1 relaxation is referred as positive relaxation agent. • These contrast agents reduce T1 relaxation times and shows increased signal intensity on T1 weighted images. • The gadolinium chelates, namely paramagnetic agents, are examples of positive relaxation agents and have wide clinical application.
- 10. Negative Relaxation Agent •The contrast agent that affects T2 relaxation is referred to as negative relaxation agent. • These contrast agents reduceT2 relaxation times resulting in decreased signal intensity on T2 weighted images. • Gd chelates in high concentration can also be used as negative relaxation agents, provided fast imaging sequences are used. • Negative relaxation agents have a limited role in clinical practice.
- 11. • In postcontrast images, the contrast enhancement is increased by various imaging parameters other than contrast media. • Such parameters include magnetization transfer, fat suppression techniques, etc
- 12. Magnetization Transfer (MT) •The application of magnetization transfer (MT) in spin echo imaging can improve the enhancement effect produced by a gadolinium chelate in the brain. • MT pulses preferentially suppress the signal from background tissue, usually improving the conspicuity of gadolinium-enhanced regions. • This can lead to improvement in the visualization of contrast enhancement at standard dose. • Hence MT is advisable in all post contrast sequences in brain.
- 13. Fat Suppression • Inareas of the body with abundant fat, T1 weighting with fat suppression (short T1 inversion recovery–STIR) provide improved depiction of contrast enhancement. • Fat suppression is indicated in all fat rich areas like breast, retroorbital and bone marrow, etc
- 14. Perfusion Imaging • Forperfusion imaging, the contrast medium is injected as a bolus using MRI compatible power injector. Images are acquired very rapidly during and immediately post injection. • This helps to study the first pass of the contrast agent through the brain. • In brain perfusion imaging studies T2 weighted scans are used. • These scans provide the required high temporal resolution and are also quite sensitive to the vascular bed.
- 15. • Perfusion imagingcan detect brain ischemia far sooner than standard T2 weighted scans. • It also detects the tissue at risk
- 16. Administration of ContrastAgent • Administration of a gadolinium chelate can substantially improve lesion identification and characterization. • After injection the MR contrast agent is distributed into the blood pool and extracellular fluid compartment of the body. • In the brain lesion, enhancement occurs as a result of disruption of the blood brain barrier (BBB) • MR contrast agents do not cross the normal blood brain barrier. • The standard dose of gadolinium is 0.1 mmol/kg, which is equivalent to 0.2 ml/kg. • The contrast is generally available in 10 or 20 ml vials. It is injected through the intravenous route and is rapidly excreted by glomerular filtration through the kidneys. • Contrast agents have half-lives between 1 and 2 hours
- 17. Adverse Reactions • Theremay be occasionally pain at the injection site. • Nausea and vomiting are the two most common mild reactions encountered. • Headache, paresthesia, dizziness, focal convulsions, skin reaction, flush are other adverse effects noted. • There is no major change of incidence in various contrast agents regarding severe anaphylactoid reaction. • Patients with asthma, multiple allergies, or known drug sensitivity (including to iodinated contrast media) are at increased risk.
- 18. • The incidenceof patients with adverse effects after IV injection of Gd-DTPA was found to be approximately 1 to 2%. • In a post marketing survey of 5 million applications of Gd- DTPA, possible drug related death was reported in one patient. • Gd contrast media are safe to be administered in children. During pregnancy use of contrast is not recommended as it crosses the placental barrier. • Lactating mothers should stop breastfeeding their babies, as contrast is excreted in breast milk. • All emergency drugs should be made available while injecting contrast
- 19. Oral Contrast Agents •Complete bowel opacification is difficult and side effects are high due to the rapid transit of the agent through the bowel. • The majority of the agents are eliminated rectally within the first few hours’ administration and are not absorbed by the bowel. • Either positive or negative relaxation agents can be used in bowel imaging. • Positive contrast agents, increasing the signal intensity of the bowel are GdDTPA with a volume expander or ferric ammonium citrate (geritol).
- 20. Conclusion • The gadoliniumchelates play a major role as contrast agents in the evaluation of the patients by MRI. • They improve sensitivity and specificity of the lesion detection. • Lesion delineation, assessment of lesion activity, and narrowing of differential diagnosis are improved with contrast study.