Brain MR Imaging- Indications

By Ana Cajade-Law, M.D.

Many clinicians think that MRI is the study of choice for all pathology. Although MRI is an extremely sensitive imaging study there are limitations to this exam. There are both absolute and relative contraindications to MRI:

ABSOLUTE

1) Pacemaker

2) Aneurysm clips that have not been tested in a magnet by manufacturers. The clinician must obtain details on the aneurysm clip and provide that information to the MRI technologist.

3) Cochlear implants

4) Fragments of metal within the body (especially if near the orbits or spine)

5) Infusion pumps

RELATIVE

1) Ventilated patient (Patients needs to be placed on a MRI compatible ventilator)

2) Recent hardware placement (unless absolutely emergent it is recommended to wait 6 weeks to allow scar formation around implants)

3) Penile and valve prosthesis (Most of these prosthesis are MRI compatible. The physician should be aware of the type of prosthesis the patient has had and provide this information to the technologist who will then do some research to see if it is safe.

4) Pregnancy: We try to avoid scanning in the first trimester since we are not sure if there are any adverse effects to MRI. Pregnant women never receive the contrast agent gadolinium.

5) Claustrophobic or anxious patients can not tolerate the MRI scanner.

6) Obese patients may not fit into the small opening of the scanner. In addition the maximum weight that the MRI table can sustain is 350 lbs.

Aside from the above generic constraints on MRI the clinicians should be aware that an average MRI of the head takes 20 to 30 minutes to perform. As a result, it is not the study of choice in emergent situations or in extremely unstable patients. Also MRI is very motion sensitive. If the patient is moving around, the study will be degraded and essentially will be unreadable.

MRI with its increased sensitivity to water content is a very sensitive tool when looking for pathology but it is very nonspecific. All pathologic processes, whether it is Neoplastic, inflammatory, infectious or metabolic will result in increased water content and be bright on a T2 weighted image.

Let us look at different clinical situations:

TRAUMA

Most trauma patients are usually too unstable to undergo MR imaging. In addition MRI takes too long to get information and it is a very insensitive to cortical bony pathology. As a result fractures are easily missed on MRI. Also if one is concerned about subarachnoid blood either from trauma or an aneurysm a CT scan is still the study of choice. MRI is extremely poor at evaluating SAH (subarachnoid hemorrhage). There are certain sequences that are slightly better to evaluate SAH but since CT is so fast and the gold standard there should never be a reason to order a MRI first.

MRI is the best test if we are evaluating for diffuse axonal injury (DAI). However this is usually not the question in the acute setting and the MRI can be performed once the patient is stabilized.

INFECTIOUS / NEOPLASTIC / INFLAMMATORY

Patients present with variable history that make the physician suspicious of possible meningitis, intraparenchymal abscess or tumor. Imaging studies for meningitis are mostly normal. Rarely we see hydrocephalus from obstruction and even rarer do we see meningeal enhancement. Both of these findings can be seen on CT although meningeal enhancement maybe slightly better seen on an MRI. An abscess can be seen equally as well on CT as on MRI although MRI may have the sensitivity to see an earlier stage on infection such as cerebritis or evaluate for smaller foci that may be missed on CT scans. However most abscess arise from sinuses or mastoid air cells and as stated above bony architecture is not seen on MRI so a CT scan is a better test.

The situations in which MRI is a better study of choice is for evaluation of 1) subdural and epidural abscess where we are looking for subtle extraaxial fluid collections and in the evaluation of 2) Herpes encephalitis or other forms of 3) cerebritis.

In regards to malignancy, both CT and MRI can demonstrate lesions and surrounding edema. MRI because of its multiplanar capability, and its thinner axial sections may demonstrate a few more lesions. As a result MRI is probably a better study if evaluating for metastasis. However, when a new patient with mental status changes arrives in the ER we should start with a CT scan since it is faster cheaper and better to evaluate fro blood. If there are areas of low density or midline shift we should evaluate this further with contrast. This should now be your next question: Do I need a MRI. If you answered yes then do not waste time and money in giving CT contrast. However if there is a reason why the patient can not get a MRI then go ahead and give contrast on CT.

Inflammatory processes such as multiple sclerosis, ADEM (acute disseminated encephalomyelitis) or sarcoid are disease processes which are better seen on MRI but can be seen on CT as well. This is especially true if we want to evaluate the spinal cord in the same patient. All of these process require contrast

There are certain clinical situations where MRI is a better study of choice and should be done instead of a CT scan:

1) Evaluation for acoustic schwannoma

2) Sellar / parasellar or Hypothalamic pathology i.e. pituitary or hypothalamic tumor or cavernous sinus mass

3) Pons and medullary pathology i.e. infarcts, encephalitis, demyelination

4) Optic neuritis

5) Myeolopathic symptoms

6) Encephalitis / Leptomenigeal carcinomatosis

Lastly I just want to briefly mention sinus thrombosis. It is our belief at this institution that the study of choice is a CT scan without and with contrast. MRI because of different flow related artifacts can create a confusing picture as to whether the venous sinuses are open or not.

On CT scan sinus thrombosis will be simpler to interpret. The sinus will appear as a hyperdensity prior to contrast and there will be no enhancement after contrast is administered since it is thrombosed.

In summary MR imaging of the brain can provide important information. Many of the findings can also be seen on CT scans and MRI should never replace CT scans when evaluating for SAH or cortical bone. However, MRI is extremely good for evaluation of certain areas such as the posterior fossa, parasellar region, or when we are evaluating for encephalitis /optic neuritis.