MRI is the most sensitive of the structural imaging tests, meaning non-functional based, scans. MRI utilizes magnetic fields to image the brain, as opposed to the X-ray radiation of the CT or CAT scan. The patient lies within a magnetic tube, as opposed to the more open CT scanner.
When protons are placed in a magnetic field, as in the MRI, they absorb and transmit electromagnetic energy. The strength of the transmitted energy is relational to the number of protons in the organic tissue. Signal strength is differentiated by properties of each proton’s micro-environment. MR signal can be “weighted” to accentuate or differentiate some properties and not others.
Pros of MRI over CT:
- MRI has better resolution than CT scan.
- MRI is better for Parenchymal Lesions (smaller lesions).
- MRI has some value in post acute (after the initial days of trauma) evaluations.
- MRI has no risk of radiation poisoning, such as with a CT and X-ray.
- MRI images, particularly sagital images (side view) have greater resolution than CT.
Contras of MRI over CT:
- MRI is a structural test, so it won’t show change in function. A cadaver can have a normal MRI.
- MRI use is restricted if a patient has IV lines and ventilation equipment or pacemakers, because if these devices are metalic, they will react to the MRI’s magnetic field.
- Patients find MRI tube is a much more confined space increasing patient anxiety as a result of claustrophobia.
- MRI has a longer scanning time than CT.
Many of these limitations in MRI can be accommodated for, but they add cost to the evaluation process.
- Any non-acute scan should be done on a ake sure that the facility has a 3 Tesla Strength magnet in the MRI. Also SWI protocols should be considered as well.
- Consider anti-anxiety medications with claustrophobic patients.
- Never have a TBI patient scanned on an Open MRI – Resolution is not capable of identifying any but the most severe brain injury.
Magnetic Resonance Angiogram -
MRA can be a useful diagnostic tool if blood flow issues are suspected.