An MRI (or magnetic resonance imaging) scan is a radiology technique that uses magnetism, radio waves, and a computer to produce images of body structures. The MRI scanner is a tube surrounded by a giant circular magnet. The patient is placed on a moveable bed that is inserted into the magnet. The magnet creates a strong magnetic field that aligns the protons of hydrogen atoms, which are then exposed to a beam of radio waves. This spins the various protons of the body, and they produce a faint signal that is detected by the receiver portion of the MRI scanner. The receiver information is processed by a computer, and an image is produced.
Magnetic resonance imaging (MRI) provides a noninvasive, quantitative method of estimating parenchymal iron levels. In principle, MRI can be used to quantify iron stores wherever they exist in the body. In practice, MRI has been investigated in the assessment of hepatic, cardiac, and anterior pituitary iron stores.
MRI measures tissue iron concentration indirectly via the detection of the paramagnetic influences of storage iron (ferritin and hemosiderin) on the proton resonance behavior of tissue water (1). The longitudinal (R1) and transverse (R2) nuclear magnetic relaxation rates of nearby solvent water protons can then be calculated. Both R1 and R2 rates are increased when interacting with paramagnetic particles such as iron. R2 (or spin-echo imaging) is preferable to R1 for determining LIC, since ferritin enhances the relaxation of both R1 and R2, while hemosiderin only has a strong R2 relaxation accelerating effect. Gradient echo imaging produces images for calculating T2* and R2*, where R2* = 1000/T2*. A T2* of 20 ms is equivalent to an R2* of 50 Hz.
MRI detection of hepatic iron overload
MRI provides a non-invasive alternative to liver biopsy, and may actually be more accurate in patients with heterogeneous liver iron deposition (such as those with cirrhosis) since it measures iron in the whole organ. In addition, the pathologic status of the liver can also be assessed using MRI.
(i couldnt put the picture up)
MRI detection of hepatic iron overload
An MRI image (R2 map, inverse, false color) clearly displays iron-overloading in the liver.
MRI remains the only noninvasive modality in clinical use with the ability to detect cardiac iron deposition. T2* MRI is rapidly becoming the new standard for measuring cardiac iron levels. (cant put the picture)
A cardiac T2* MRI image shows myocardial iron stores. The lighter ventricle walls in the left image indicate heavy iron loading. Used with permission from Anderson, et al (2). © 2001 Oxford University Press. MRI detection of cardiac iron overload.
http://www.irontoxicity.com/hcp/diagnosis/imaging_studies/magnetic.jsp
Imaging Studies
· Skeletal survey and other imaging studies reveal classic changes of the bones that are usually encountered in patients who are not regularly transfused.
·
o The striking expansion of the erythroid marrow widens the marrow spaces, thinning the cortex and causing osteoporosis. These changes, which result from the expanding marrow spaces, usually disappear when marrow activity is halted by regular transfusions. Osteoporosis and osteopenia may cause fractures, even in patients whose conditions are well-controlled.
o In addition to the classic "hair on end" appearance of the skull, which results from widening of the diploic spaces and observed on plain radiographs , the maxilla may overgrow, which results in maxillary overbite, prominence of the upper incisors, and separation of the orbit. These changes contribute to the classic "chipmunk facies observed in patients with thalassemia major.
o Other bony structures, such as ribs, long bones, and flat bones, may also be sites of major deformities. Plain radiographs of the long bones may reveal a lacy trabecular pattern. Changes in the pelvis, skull, and spine become more evident during the second decade of life, when the marrow in the peripheral bones becomes inactive while more activity occurs in the central bones.
o Compression fractures and paravertebral expansion of extramedullary masses, which could behave clinically like tumors, more frequently occur during the second decade of life.
o MRI and CT scanning are usually used in diagnosing such complications.
· Chest radiography is used to evaluate cardiac size and shape.
· MRI and CT scanning can be used as noninvasive means to evaluate the amount of iron in the liver in patients receiving chelation therapy.
· A newer noninvasive procedure involves measuring the cardiac T2 with cardiac magnetic resonance (CMR). This procedure has shown decreased values in cardiac T2 due to iron deposit in the heart. Unlike liver MRI, which usually correlates very well with the iron concentration in the liver measured using percutaneous liver biopsy samples and the serum ferritin level, CMR does not correlate well with the ferritin level, the liver iron level, or echocardiography findings. This suggests that cardiac iron overload cannot be estimated with these surrogate measurements. This is also true in measuring the response to chelation therapy in patients with iron overload. The liver is clear of iron loading much earlier than the heart, which also suggests that deciding when to stop or reduce treatment based on liver iron levels is misleading.
http://www.medrat.com/info/thalassemia/doc019.php
Benefits
- MRI is a noninvasive imaging technique that does not involve exposure to radiation.
- MR images of the soft-tissue structures of the body—such as the heart, liver and many other organs— is more likely to identify and characterize abnormalities and focal lesions than other imaging methods. This detail makes MRI an invaluable tool in early diagnosis and evaluation of many focal lesions and tumors.
- MRI has proven valuable in diagnosing a broad range of conditions, including cancer, heart and vascular disease, and muscular and bone abnormalities.
- MRI enables the detection of abnormalities that might be obscured by bone with other imaging methods.
- MRI allows physicians to assess the biliary system noninvasively and without contrast injection.
- The contrast material used in MRI exams is less likely to produce an allergic reaction than the iodine-based materials used for conventional x-rays and CT scanning.
- MRI provides a fast, noninvasive alternative to x-ray angiography for diagnosing problems of the heart and blood vessels.
Risks
- The MRI examination poses almost no risk to the average patient when appropriate safety guidelines are followed.
- If sedation is used there are risks of excessive sedation. The technologist or nurse monitors your vital signs to minimize this risk.
- Although the strong magnetic field is not harmful in itself, medical devices that contain metal may malfunction or cause problems during an MRI exam.
- There is a very slight risk of an allergic reaction if contrast material is injected. Such reactions usually are mild and easily controlled by medication.
- Nephrogenic systemic fibrosis is currently a recognized, but rare, complication of MRI believed to be caused by the injection of high doses of MRI contrast material in patients with poor kidney function.
What are the limitations of MRI of the Body?
High-quality images are assured only if you are able to remain perfectly still while the images are being recorded. If you are anxious, confused or in severe pain, you may find it difficult to lie still during imaging.
A person who is very large may not fit into the opening of a conventional MRI machine.
The presence of an implant or other metallic object often makes it difficult to obtain clear images and patient movement can have the same effect.
Breathing may cause artifacts, or image distortions, during MRIs of the chest, abdomen and pelvis. Bowel motion is another source of motion artifacts in abdomen and pelvic MRI studies.
Although there is no reason to believe that magnetic resonance imaging harms the fetus, pregnant women usually are advised not to have an MRI exam unless medically necessary.
MRI may not always distinguish between cancer tissue and edema fluid.
MRI typically costs more and may take more time to perform than other imaging modalities
No comments:
Post a Comment