Magnetic Resonance Imaging (MRI)

Norm of Magnetic Resonance Imaging (MRI)

Description of normal tissue, structure, and blood flow.

 

Usage of Magnetic Resonance Imaging (MRI)

To detect abscesses, abnormalities in blood flow through coronary branches and through extremities, acute tubular necrosis, adenopathy, aortic and ventricular aneurysm, atrial and ventricular septal defects, avascular necrosis, blood clots, brain contusion, cancer and tumors (brain, bone, disk herniation, epidural hematoma on spine, extra-axial, head, intracardiac, hilar, mediastinal, neck, parenchymal, pericardiac, pituitary, pulmonary, renal, sarcoma, and spinal cord), cavernous hemangioma, cerebral infarction, congenital heart disease, cysts, dementia, demyelinating disease, edema, epilepsy, focal viral encephalitis, Gaucher's disease, glomerulonephritis, hemorrhage, hydronephrosis, hyperparathyroidism, infection, intervertebral disk abnormalities, knee abnormalities, Marfan syndrome, myocardial infarction (and afterward to detect scars, aneurysms, pseudoaneurysms, septal defects, mural thrombi and valvular regurgitations), multiple sclerosis, muscular disease, osteomyelitis, plaque formation, pulmonary atresia, renal transplants, renal vein thrombosis, seizures, shoulder abnormalities, skeletal abnormalities, soft-tissue infections, spinal cord compression or injuries, subarachnoid hemorrhage, subdural hematoma, temporomandibular joint abnormalities, tumor invasion (inferior vena cava and seminal vesicles), and tumor staging (cervix, large hydronephroma, prostate, urinary bladder, and uterus). MRI is superior to computed tomography and ultrasound for its sensitivity in detection of changes in soft tissue. MRI is the standard in the diagnosis of most abnormalities of the brain and spine (except trauma). The ability of MRI to support the diagnosis of multiple sclerosis declines with increasing age of the client. MRI eliminates the need for many knee arthroscopies and has virtually replaced arthrography. Unlike computerized axial tomographic (CAT) scans, MRI can evaluate cerebral infarction within hours of the event. MRI virtually eliminates the need for myelography. MRI is more effective than CT in identifying white matter brain disease, such as multiple sclerosis.

 

Description of Magnetic Resonance Imaging (MRI)

MRI is a noninvasive diagnostic tool that enables visualization of the body's tissues, structure, and blood flow. It uses a strong magnetic field in conjunction with radiofrequency waves to transmit signals from the body's cells to a computer that produces cross-sectional images. MRI actually stimulates the body to produce a signal that causes the cell's nuclei to react as tiny magnets in the presence of a strong external magnetic field (MRI). The signal density of the multiple body-plane images depends primarily on the tissue characteristics, pulse sequence, and timing parameters. Newer enhancements of MRI include the use of diffusion-weighted imaging, functional MRI, and fast MRI. In diffusion-weighted MRI, the intracellular and extracellular spaces are compared for the degree of diffusion of water molecules contained within them. Brighter areas indicate restricted diffusion, such as in ischemic cell damage or blockage by tumor. In functional MRI (fMRI), successive images are taken in rapid succession while the client follows commands. The images are compared for signal intensity and cerebral blood flow to help evaluate brain function in pathologic brain deterioration and psychiatric disorders, and can also be used to evaluate the auditory system. Research uses for fMRI include identifying patterns in brain images that help predict which clients are likely to respond to specific drugs, such as antidepressants. Fast MRI, which has become possible through software advances, allows shortened breath-holding timeframes, better resolution, and procedure completion in 30 minutes or less. Fast MRI is used for evaluation of fetal anatomy or pathology when ultrasound does not yield enough information, and is also showing promise for evaluation of heart failure and screening for metastases. In addition, the newest equipment, called “Dual Mode Imaging,” combines MRI with functional imaging modalities such as PET or SPECT for improved imaging results (see Dual modality imaging). MRI is painless and has no known side effects.

 

Professional Considerations of Magnetic Resonance Imaging (MRI)

Consent form IS required. 1%–2% of people refuse MRI because of claustrophobia.

Risks
Critical injury to the client could result from ferrous metal in the body (e.g., flecks of ferrous metal in eye could cause retinal hemorrhage).
Contraindications
Intraocular metal foreign bodies; heart valves manufactured before 1964 and middle ear prosthetics (these can be tested by obtainment of a duplicate, which is then placed into the bore, and if no torque is experienced, the test may be safely performed); nerve-stimulating devices may be a contraindication.
Precautions
Some older versions of aneurysm clips may not be ferromagnetic; verify this from manufacturer or hospital records. MRI uses non-ionizing radiation, and thus is considered the least risky of radiographic procedures during pregnancy, and there is no evidence of teratogenic or developmental abnormalities associated with this procedure. However, the literature recommends that pregnant women should not be scanned unless absolutely necessary. Radiologists and operators must be informed of the presence of cardiac pacemakers, implanted cardioverter-defibrillators and implanted venous access devices, and cochlear implants, though they are rarely a contraindication. Most stainless-steel orthopedic implants and prosthetic devices are not ferromagnetic and are not affected by MRI. Clients with tattooed eyeliner may experience skin irritation or swelling around the eyes caused by the MRI's effects on ferrous pigments in the tattoo. Use of sedatives during this test is contraindicated in clients with central nervous system depression.

 

Preparation

1. See Client and Family Teaching.
2. Screen the client for cardiac pacemaker, artificial heart valve, brain aneurysm clips or any type of surgical clip or staple, shunt, neurostimulation (TENS unit), implanted insulin pump, implanted venous access infusion devices, bone growth stimulator, internal electrodes, embolic spring coil, eye implant surgery (with staples), cochlear implant, hearing aid, foil or metallic medication patches, any orthopedic item(s) (such as pins, wires, rods, screws, clips, plates), artificial limb or joint, dental braces, any type of removable dental item, IUD, metallic eye makeup, metal fragments (in head, eye, skin), history of work in the machine tool industry, history of work with a metal lathe, or history of any accidents with metal or ferromagnetic objects (e.g., beebee [BB] guns, flecks of ferrous metal in eye). These items may be hazardous to the client's safety.
3. Screen the accompanying adult for the above items if the client undergoing the procedure is a pediatric client.
4. Remove any loose metal objects (such as hairpins, barrettes, watches, jewelry, pen clip, steel-toed shoes or clothing with metal snaps or zippers) because they can become projectile in the magnetic force.
5. Inform the physician if the client is using an IV controller pump or computerized equipment because the magnets in the MRI can disrupt the function of the machine (such as IV flow).
6. Determine if client has any problems with claustrophobia. Interventions to reduce claustrophobia include, as appropriate, the following:
   • a. Relaxation techniques or a sedative may be used.
   • b. Determine availability of an “open MRI.”
   • c. Interact with client in an unhurried, relaxed manner.
   • d. Provide a thorough explanation of the procedure, including methods to reduce anxiety, such as relaxation or controlled breathing techniques.
   • e. Suggest that the client keep his or her eyes closed throughout the procedure.
   • f. Offer a cool cloth to be placed over the eyes.
   • g. Point out that the ends of the scanner are always open.
   • h. Offer to have client remove his or her shoes and be covered with a light sheet.
   • Keep room temperature cool. Use compressed air through a cannula positioned to blow past the client's face during the procedure.
   • j. Arrange for family member to enter the scanner room to speak with the client between scans.
7. If the client is very young or unable to follow directions, sedation may be indicated to complete the scan.
8. Start an IV line, if contrast medium is to be given.
9. Just before beginning the procedure, take a “time out” to verify the correct client, procedure, and site.

 

Procedure

1. The client is positioned on a padded table and moved into the cylinder-shaped scanner (such as a magnet bore).
2. Contrast medium may be administered before the procedure if prescribed.
3. The technologist will operate controls determining image-signal density, pulse sequence, and timing parameters.
4. If blood flow is to be determined in an extremity, the arm or leg to be examined is placed into a cradle-like support. The technologist will mark reference sites to be imaged on the arm or leg. Then the extremity is moved into a flow cylinder.
5. If a functional MRI is being done, the client may be asked to watch a display of images and press a button when certain images or patterns of images are noted.
6. The MR image is interpreted by a specially trained radiologist.

 

Postprocedure Care

1. Continue the assessment of respiratory status after receiving sedation. If deep sedation was used, follow institutional protocol for post sedation monitoring. Typical monitoring includes continuous ECG monitoring and pulse oximetry, with continual assessments (every 5–15 minutes) of airway, vital signs, and neurologic status until the client is lying quietly awake, is breathing independently, and responds to commands spoken in a normal tone.
2. Remove the IV line if one was inserted for injection of the contrast medium.

 

Client and Family Teaching

1. For pelvic or abdominal scans, do not eat or drink for 6 hours before the procedure.
2. You will lie on a flat, narrow, padded surface and will be rolled into a cylinder-shaped scanner. The scanner will be around the area of the body that is being scanned.
3. You will hear various noises from the test, including a muffled drumbeat sound. You may bring in earplugs for the test or use the earplugs that are available. In an open MRI machine, inform the client that there are no loud noises.
4. You can communicate with MRI personnel, who will be in another room, by means of an intercom system.
5. It is important to remain completely still during the scan.
6. Remove jewelry, watches, hairpins, glasses, and any metal objects. The magnetic field can damage watches.
7. Do not approach the MRI unit if you have a cardiac pacemaker.
8. You will not be exposed to radiation during this procedure. The contrast medium that may be used is not an iodinated contrast.
9. The procedure may take 45–90 minutes to scan the head or chest area and approximately 15 minutes to scan an arm or leg. In fast MRI, the procedure will take 30 minutes or less.

 

Factors That Affect Results

1. The image will be distorted by movement during the procedure.
2. Metal, whether ferrous or nonferrous, may produce artifacts that degrade the images if the metal is in proximity to the area of the body that is being scanned.
3. Because of the possibility of loss of data contained on magnetic recording media, MRI systems are normally contained within a restricted magnetic range, from 15 to 50 gauss.
4. MRI does not use ionizing radiation; therefore there are none of the hazards found in x-rays.

 

Other Data

1. Intravenous gadolinium-DTPA contrast, which is a commercially available contrast medium, may be necessary for some examinations at the discretion of the radiologist. This contrast is chemically unrelated to the iodinated contrast, which is used in conventional radiography.
2. Magnetophosphenes (flickering lights in the visual field), which can occur with MRI, are completely reversible and have no known long-term health effects.
3. The Food and Drug Administration (FDA) has classified MRI devices into class II, which includes low-risk devices.