MRI
From Medical-Wiki
The MRI is a sensitive and revolutionary imaging test in the treatment and diagnoses of medical illnesses. Since the first MRI image was taken on 1977, the detection of neurological, musculoskeletal, cardiovascular, and oncological pathologies has been greatly enhanced due to outstanding clarity.
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How the MRI Works
Like the CT, the MRI provides detailed images of the body. However, unlike the CT which uses radiation, the MRI uses a powerful magnetic field to align protons in the body. As the field arranges the hydrogen atoms in the body, the emitted radio waves provide energy to these protons which bounce back differently depending on the type of tissue. The MRI can provide very non-invasive, meticulous images depending on the realignment of the protons. These slices are combined to form a three-dimensional image of the studied area. [1]
Like CT and plain films, contrast can be given to the patient in order to provide greater clarity. Unlike the CT and plain film, however, there is no ionizing radiation distributed in the body. Contrasts used include water, gadolinium (which is a paramagnetic contrast agent), barium sulfate (a dimagnetic agent), and iron oxide nanoparticles (a superparamagnetic contrast agent). Gadolinium has lately been implicated in the causation of Nephrogenic Systemic Fibrosis, a kidney disease that causes hardening of the joints and skin.
The Magnetic Resonance Angiography scan (MRA) has proven especially beneficial in the focus of Neuroradiology. It evaluates blood vessels and can detect narrowing or obstruction in the vessels, advantageous in the discovery of carotid or vertebral stenosis.
MRI Uses in Trauma
MRI use in acute trauma is controversial due to the necessity of attaining patient history before performing the MRI, including placement of any foreign objects such as metal plates. Therefore, there is an inherent risk in placing a comatose patient in the MRI. In addition, due to the longer scanning time, MRI is not usually the first test in an emergency. However, after a patient has become stabilized, an MRI could reveal the existence of lesions not previously detected by CT.
The MRI has not been as heavily tested as the CT or X-ray on the USMLE but an adequate information base regarding its utilization will prove valuable in clinical rounds.
MRI General Diagnostic Uses and Advantages
Advantages
•Better resolution than CT
•No ionizing radiation, unlike the CT and x ray.
•No known side effects
•Can eliminate the need for biopsy or exploratory surgery
•Earlier detection of disease [2]
The MRI has been specifically effective in the diagnosis of the following pathologies:
•Multiple Sclerosis
•Bone tumors
•CNS Infections
•Strokes in their earliest stages
•Soft tissue masses
•Tumors of the pituitary gland and brain [pictured on right]
•Ligaments in the wrist, knee and ankle
MRI Disadvantages
•Expensive
•Difficult to keep the patient still during duration of the exam
•Risk of harm to patients with pacemakers or IV lines due to the strong magnetic field (25,000 times stronger than the magnetic field of the earth).
•Difficult to scan obese or claustrophobic patients
•Loud noise during the scan due to the strong electrical current in the magnets
•Distortion of images due to hardware in the body such as artificial joints.
Despite the disadvantages, experts agree that the benefits of MRI usually far outweigh the risks.
References
[1]RadiologyInfo. MRI of the Body. http://www.radiologyinfo.org/en/info.cfm?pg=bodymr
[2]Gould,Todd, RRT. How MRI Works. http://health.howstuffworks.com/mri10.htm
