Nuclear magnetic resonance - cranial; Magnetic resonance imaging - cranial; Head MRI scan; MRI - cranial; NMR - cranial; Cranial MRI
An MRI of the head is a non-invasive procedure that uses powerful magnets and radio waves to construct pictures of the clear, detailed pictures of brain tissues.
Unlike conventional radiography and computed tomography (CT), which make use of potentially harmful X-rays passing through a patient to generate images, MRI imaging is based on the magnetic properties of atoms. A powerful magnet generates a magnetic field roughly 10,000 times stronger than the Earth’s. A very small percentage of hydrogen atoms within the body will align with this field.
The “nuclear” in the original name (now seldom used) refers to the proton in the nucleus of the hydrogen atom and does not imply radioactivity.
When focused short radio wave pulses are broadcast towards the aligned hydrogen atoms in tissues of interest, they will return a signal of their own. The subtle differences in that signal from dissimilar tissues enables the MRI to differentiate between various organs, and potentially, provide contrast between benign and malignant (cancerous) tissue.
Any imaging plane, or “slice”, can be projected, and then stored in a computer or printed on film. MRI can easily be performed through clothing and bones. However, certain types of metal in or around the area of interest can cause significant errors in
the reconstructed images. (The errors are called artifacts.)
How the test is performed
Since MRI makes use of radio waves very close in frequency to those of ordinary FM radio stations, the scanner must be located within a specially shielded room to avoid outside interference.
The patient will be asked to lie on a narrow table which slides into a large tunnel-like tube within the scanner. In addition, a small device may be placed around the head to be studied. This is special body coil which send and receive the radio wave pulses, and is designed to improve the quality of the images.
If contrast is to be administered, intravenous access will be placed, usually in a small vein of the hand or forearm.
A technologist will operate the machine and observe you during the entire study from an adjacent room. Several sets of images are usually required, each taking from 2 to 15 minutes. A complete scan may take up to one hour or more. Newer scanners with more powerful magnets utilizing updated software and advanced sequences may complete the process in less time.
How to prepare for the test
No preparatory tests, diets, or medications are usually needed. You may be asked to fast for 4 to 6 hours prior to the scan. You must sign a consent form before the scan.
Because of the strong magnets, no metallic objects are allowed into the room. Items such as jewelry, watches, credit cards, and hearing aids, can be damaged by the strong magnets. Pins, hairpins, metal zippers, and similar metallic items can distort the images. Frequent exposure to ferrous metals (such as a sheet metal worker has) may make MRI difficult.
A hospital gown may be recommended, or you may be allowed to wear “sweats” or similar clothing without metal fasteners. Take out removable dental work just prior to the scan.
For infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child’s age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child’s age:
- Infant test or procedure preparation (birth to 1 year)
- Toddler test or procedure preparation (1 to 3 years)
- Preschooler test or procedure preparation (3 to 6 years)
- School age test or procedure preparation (6 to 12 years)
- Adolescent test or procedure preparation (12 to 18 years)
How the test will feel
There is no pain. The magnetic field and radio waves are not felt. The primary discomfort is the claustrophobic feeling that some people experience from being inside the scanner. The table may be hard or cold, but you can request a blanket or pillow.
The machine produces loud thumping and humming noises. Ear plugs are usually given to reduce the noise.
A technologist observes you during the entire procedure and may enter the room to speak to you or may speak with you through an intercom in the scanner.
Excessive movement can blur MRI images. If you have difficulty lying still or are very anxious, you may be given a sedative. The viewing of some brain lesions may require use of a dye injected into the veins. There is no recovery required (unless you have been sedated). After an MRI scan, you can usually resume normal diet, activity, and medications.
Why the test is performed
MRI provides detailed pictures of the brain and nerve tissues from multiple planes without obstruction by overlying bone. In fact, about 90% of all MRI scans are for brain or spine disorders.
MRI is the procedure of choice for most brain disorders. MRI is particularly useful in brain and neurologic disorders, because it can clearly show various types of nerve tissue. It provides clear pictures of the brainstem and posterior brain, which are difficult to view on CT scan. It is also useful for the diagnosis of demyelinating disorders (disorders such as multiple sclerosis that cause destruction of the myelin sheath of the nerve).
MRI is a noninvasive procedure that can evaluate blood flow and the flow of cerebrospinal fluid (CSF). MRI can distinguish tumors or other lesions from normal tissues. MRI is sometimes used to avoid the dangers of angiography or of repeated exposure to radiation.
What abnormal results mean
The sensitivity of an MRI depends, in part, on the experience of the radiologist.
An MRI of the head may reveal disorders including:
- primary brain tumors
- metastatic brain tumors
- structural abnormality of the brain, ventricles, pituitary gland
- pituitary masses
- lesions or masses (any location)
- acoustic neuroma
- optic glioma
- Arteriovenous malformations of the head
- brain aneurysms
- damage to basal ganglia
- subdural hematoma, blood clots
- intracranial hemorrhage (more than 48 hours old)
- radiation damage to the brain
- brain swelling
- demyelinating diseases
- infarction or other tissue destruction
- brain abscess
- abnormalities of blood flow (such as carotid artery stenosis)
Additional conditions under which the test may be performed:
- amyotrophic lateral sclerosis
- throat cancer
- chronic subdural hematoma
- Cushing’s syndrome
- deep intracerebral hemorrhage
- dementia due to metabolic causes
- diabetes insipidus; central
- Hemorrhagic stroke
- Huntington’s disease
- hypertensive intracerebral hemorrhage
- intracerebral hemorrhage
- lobar intracerebral hemorrhage
- melanoma of the eye
- Meniere’s disease
- multi-infarct dementia
- multiple endocrine neoplasia (MEN) I
- multiple sclerosis
- normal pressure hydrocephalus (NPH)
- partial (focal) seizure
- partial complex seizure
- petit mal seizure
- pituitary Cushing’s (Cushing’s disease)
- Reye’s syndrome
- senile cerebral amyloid angiopathy
- senile dementia/Alzheimer’s type
- acute sinusitis
- Sinusitis; chronic
- stroke secondary to atherosclerosis
- stroke secondary to cardiogenic embolism
- stroke secondary to FMD
- stroke secondary to syphilis
- temporomandibular joint (TMJ) syndrome
- transient ischemic attack (TIA)
- Wernicke-Korsakoff syndrome
- Wilson’s disease
What the risks are
There is no ionizing radiation involved in MRI, and there have been no documented significant side effects of the magnetic fields and radio waves used on the human body to date.
However, people have been harmed in MRI machines when they did not remove metal objects from their clothes or when metal objects were left in the room by others.
Because the effects of strong magnetic fields on a fetus are not well documented at this time, pregnant women are usually advised to avoid MRI scans.
The most common MR intravenous contrast agent, gadolinium, is very safe, and although there have been documented allergic reactions to it, it is an extremely rare occurrence.
If sedation is used, there are associated risks of over-sedation. The technologist monitors the patient’s vital signs, including heart rate and respiration as needed.
MRI is superior to computed tomography (CT) in most cases where differentiation of soft tissues is necessary. It can view organs without obstruction by bone and foreign bodies. It is capable of showing the tissues from multiple viewpoints and is a noninvasive way to evaluate blood flow.
A CT scan may be preferred for:
- acute trauma of the head and face
- acute (less than 72 hours) neurologic dysfunction
- early symptoms of stroke
- subarachnoid or intracranial hemorrhage (within the first 24 to 48 hours)
- skull bone disorders, disorders involving the bones of the ear
by Dave R. Roger, M.D.
All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.