H. Wayne Slone, MD
Magnetic resonance imaging (MRI) uses a powerful magnetic field and radiofrequency waves to produce images. No ionizing radiation is involved. Most MRI units in clinical use are 1.5 Tesla units with 3 Tesla units are becoming more readily available.
Depending on imaging parameters, different pulse sequences can be obtained producing images yielding different information. Traditional imaging involves T1-weighted (T1W) and T2-weighted (T2W) spin echo imaging. Gradient echo imaging allows for faster imaging. T1W images are obtained after administration of IV contrast, which does not affect T2W images very much. Fat suppression images help to identify fatty lesions or lesions obscured by fat, such as those in the orbits. Proton density and fluid attenuated inversion recovery (FLAIR) images are useful in evaluation of white matter disease. Diffusion and perfusion imaging is invaluable in the evaluation of stroke. MR angiography (MRA) and MR venography (MRV) are noninvasive means of evaluation of the vasculature of the head and neck. Functional imaging is increasingly used in preoperative evaluation, especially of brain tumors, helping to noninvasively identify speech and motor centers. Spectroscopy can provide a measure of brain chemistry by identifying spectra that can help in the differential diagnosis of tumors, demyelination, and radiation necrosis to mention a few. Diffusion tensor imaging (DTI) can noninvasively identify white matter tracts and obtain information on connectivity of the brain. Susceptibility-weighted imaging is very sensitive in the evaluation of hemorrhage and calcifications.
- Stroke—diffusion-weighted imaging is very sensitive for acute stroke.
- Tumor
- Infection—encephalitis, abscess, empyema
- Demyelinating disorders—MS
- Seizures
- Neurodegenerative Disorders—Alzheimer’s, Parkinsonism
- Granulomatous disease—e.g., sarcoidosis
- Intracranial hemorrhage
- Negative Ct with continuing neurologic deficits
INDICATIONS FOR BRAIN MRI WITH CONTRAST
- Tumor
- Infection
- Demyelinating disorders—helpful to monitor disease activity
- Seizures—new onset to R/O tumor
- Granulomatous disease
- Tumor—primary spinal cord, metastatic to bone, R/O cord compression, leptomeningeal carcinomatosis, and R/O drop metastases
- Infection—osteomyelitis, discitis, and epidural abscess
- Trauma—cord or ligamentous injury with short tau inversion recovery (STIR) images
- Degenerative disc disease—R/O disc herniation and spinal or foraminal stenosis
- Cord abnormalities—tumor, demyelination, infarction, extremity weakness, incontinence, and paralysis
INDICATIONS FOR SPINE MRI WITH CONTRAST
- Tumor
- NOT necessary for vertebral body metastases
- Primary spinal cord or nerve root tumors, leptomeningeal carcinomatosis, and epidural tumor
- Infection—osteomyelitis, discitis, and epidural abscess
- Demyelination
- Granulomatous disease
- Any cord lesion
- Prior lumbar spine surgery—R/O epidural scar; not an issue in the cervical or thoracic region
- Superior soft-tissue contrast/resolution
- Direct multiplanar imaging
- No ionizing radiation
- No beam-hardening artifacts related to bone
- Length of exam—at least 20 minutes
- Sensitivity to motion
- Cost
- Difficulty in monitoring critically ill patients—ECGs work poorly during scanning even with MR-compatible equipment; pulse oximeter monitoring should be available; MR-compatible ventilator is required
- Difficulty in obtaining STAT off hours—technologist availability limited at many centers
- Metal artifacts
- Cortical bone, although MR is excellent for evaluation of bone marrow
Many contraindications are relative and depend on magnetic field strength relating to heating issues. Need documentation of implants (make and model number). It is best to consult the MR facility for local policy and constantly changing policies. Do not just assume that something is contraindicated. Electrically, magnetically, or mechanically activated implants are generally contraindicated.
- Pacemakers, although a new FDA-approved pacemaker by Medtronics is MR compatible, but as yet with limitations
- Defibrillators
- Neurostimulators
- Bone growth stimulators
- Cochlear implants
- Ocular metallic foreign bodies
- Swan–Ganz catheters
- Allergy to IV gadolinium (MR) contrast for contrast study
- Aneurysm clips—most clips used currently are MR compatible; however, safety concerns exist and many facilities consider them absolute contraindications
- Heart valves—current valves are not contraindicated; old Starr Edwards (pre-6000 series) are contraindicated
- Inferior venacaval (IVC) filters—current filters mostly are MR compatible, although recommendations are to wait 2–6 weeks after insertion prior to imaging
- Inner ear implants—cochlear implants contraindicated, some stapes implants
- Drug-infusion pumps—generally not contraindicated, although MR may stop infusion and necessitate pump reprogramming
- Bullets, pellets, shrapnel—must use judgment; duration, proximity to vessel? Most bullets are not contraindicated.
- Stents—most are not contraindicated but many aortic stent grafts are
Hemostatic clips, wire sutures, plates, pins, screws, nails, dental devices (e.g., braces, bridges despite artifacts), orthopedic implants (joint replacements, spinal rods), ocular implants, ventricular shunts.
PREPARATIONS/SPECIAL INSTRUCTIONS FOR PATIENTS
- Patients must remove all jewelry and metal items from their body and clothing, including glasses, dentures and all other removable dental work, wigs, and hairpins.
- An extensive history and screening form, designed to ensure that there are no contraindications and safety of the exam, is filled out.
- Claustrophobia is a problem in 5–10% of patients. Often, this is transient and eliminated by reassurance from the technologist. Most are able to get through the exam with mild sedation, usually 5 mg PO of Valium. Approximately 1% will require heavy sedation in order to complete the exam in a closed MR. Open MRIs accommodate such patients at the cost of reduced image quality. Newer, wide-bore MRIs are also an option.
- MR has not been proven safe in pregnancy, but is not believed hazardous. It is indicated in pregnancy if it will provide information critical to the patient's well-being or because the patient would otherwise require exposure to ionizing radiation.
- Contrast is generally contraindicated in pregnancy.
MRI is, generally, the preferred exam in the evaluation of the brain and spine due to the lack of ionizing radiation, but sedation is usually required in young children.
- Can be challenging in patients with dementia who forget the need to not move, thus necessitating sedation
- In the imaging of demyelinating disorders, higher field strength (i.e., 3 Tesla) magnets may increase the yield for T2– and proton–density-weighted lesions.