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Introduction ⬇

MRI is a diagnostic imaging technique that employs a superconducting magnet and radiofrequency (RF) signals to construct detailed, sectional images of the body. Unlike CT, no ionizing radiation is used.

Magnetic resonance angiography (MRA) is a type of MRI that provides images of the major blood vessels. It provides both anatomic and hemodynamic information in two-dimensional and three-dimensional representations (likened to noninvasive angiography). MRA is becoming more common; it is used to evaluate known vascular lesions and is finding greater utility in evaluating stroke.

Magnetic resonance spectroscopy (MRS), also known as nuclear magnetic resonance spectrography, provides information about the chemical composition of tissue and is commonly used to evaluate brain function. MRS is performed to detect chemicals in all body tissues to evaluate tumors, muscle disease, or ischemic heart disease; to differentiate causes of coma; to rule out Alzheimer disease; to monitor cancer treatment; to differentiate the diagnosis of multiple sclerosis, HIV infection, and adrenoleukodystrophy; to prepare for temporal lobe epilepsy surgery; and to assess the extent of stroke and head injury.

In general, the most common MRI tests include the following:

  1. MRI of the brain provides exquisite visualization of the soft tissue structures of the brain. Some laboratories perform neurofunctional imaging, which maps the brain's response to the stimuli. Although bony anatomy is seen using MRI, CT is the test of choice to evaluate bone lesions and fractures. A newer MRI technology referred to as whole-brain T1 mapping can detect damage to the white and gray matter of the brain in patients with multiple sclerosis. T1 mapping measures "proton relaxation" following exposure to a magnetic field and an RF pulse. Patients with multiple sclerosis show a reduction in brain volume, that is, brain atrophy. Another form of MRI, functional MRI (fMRI), uses MR to measure metabolic changes in the active parts of the brain. fMRI is used quite extensively to study the diseased or injured brain.

  2. MRI of the spine provides excellent views of the spinal cord and subarachnoid space without intrathecal contrast injection.

  3. MRI of the musculoskeletal system accurately demonstrates fat, muscles, tendons, ligaments, nerves, blood vessels, and bone marrow. If the anatomic region of interest is a small area, a surface coil, which produces the RF signal, is placed directly on the skin overlying the part to be examined. Dynamic studies of the joint in motion can be performed on open scanners.

  4. MRI of the heart (cardiac MRI) allows visualization of the structures of the heart, including valves and coronary vessels. Image acquisition is synchronized to the ECG—a process known as gating—to help eliminate motion artifacts. Functional studies can evaluate cardiac wall motion in response to exercise.

  5. MRI of the abdomen and pelvis visualizes soft tissue organs, particularly the liver, pancreas, spleen, adrenals, kidneys, blood vessels, and reproductive organs. This is the preferred method for staging uterine, cervical, and vulvar carcinoma as well as prostate cancer.

  6. MRI of the breast is a technique capable of producing exquisitely detailed analysis of complex breast lesions.

A contrast agent may be utilized for certain MRI tests. MR contrast agents have lower toxicity and fewer side effects than x-ray contrast agents. However, because these MR contrast agents are primarily excreted through the kidneys, kidney disease or failure is a contraindication for use. Other potential contraindications include pregnancy, allergies or asthma, anemia, hypotension, epilepsy, and sickle cell disease.

Clinical Alert

  1. Adverse effects of contrast agents, although rare, may include vomiting, sensations of local warmth or coldness, headache, dizziness, urticaria, paresthesias, unusual mouth sensations, and respiratory problems.

  2. MR contrast agents allow for better basic contrast and tissue signals; most abnormal tissues show regions of increased T1 and T2 (relaxation time, RF signals) regardless of the nature of tumors, edema, hemorrhage, inflammation, and necrosis.

  3. Some open MRI systems use only a fraction of the traditional high-field magnets, which results in a slimmer profile and much less intimidating appearance for the magnet. Although extremely appealing in certain instances, the open-design magnet is not always the best choice for MRI testing, and careful consideration to magnetic field strength should be given. Certain types of studies can only be performed with a high-field magnet. Some scans performed on an open-design, low-field magnet must be repeated.

Pediatric cautions related to MRI testing include the following considerations:

  1. Age, ability to understand and cooperate, physical condition, and reasons for testing.

  2. MRI body imaging: Most of the adult guidelines apply. Sedatives, tranquilizers, or modified restraints may be necessary if the child is uncooperative or fearful.

  3. MRI for blood flow studies in extremities: Simple restraints may be used to restrict motion of arms or legs. No tranquilizers or sedatives may be used because blood flow will be affected.

Procedure ⬆ ⬇

  1. Have the patient lie supine on a movable examination couch after a thorough medical history is obtained or MRI questionnaire is completed.

  2. Provide sedation, as ordered, if the patient is claustrophobic or restless. Provide earplugs with music and explain that a two-way communication system between the patient and the operator is available, which allows continual monitoring and vocal feedback to reduce the patient's sense of isolation. Many MRI laboratories routinely use a pulse oximeter to monitor the patient's arterial oxygenation during the study.

  3. For examining many superficial structures (e.g., knee, neck, shoulder, breast), apply a surface coil over the skin. Obtain improved images of the prostate or reproductive organs by using a transrectal coil.

  4. Once the patient is positioned and instructed to remain still, move the couch into the scanner.

  5. In some instances, inject a noniodinated contrast agent into a vein for better anatomic visualization. For abdominal or pelvic scans, administer glucagon to reduce bowel peristalsis.

  6. Examination time varies and averages between 30 and 90 minutes.

  7. Follow guidelines in Chapter 1 for safe, effective, informed intratest care.

Procedural Alert

Usually, no special dietary restrictions or preparations are necessary before MRI, unless conscious sedation is to be used. However, numerous safety factors must be considered:

  1. Absolute contraindications to MRI include the following conditions:

    1. Implanted devices, including pacemakers, automatic cardiac defibrillators, cochlear implants, certain prosthetic devices (consult with MR laboratory for specific information), implanted drug infusion pumps, neurostimulators, bone growth stimulators, cardioverters, certain intrauterine contraceptive devices, and metal artificial heart valves. Most new surgical implants are MRI compatible; however, most laboratories will not scan a patient with compatible implants until approximately 2 weeks after placement. The exact brand, style, and serial number of the device are needed in order for the MRI department to verify compatibility.

    2. Internal metallic objects such as bullets or shrapnel and certain surgical clips, pins, plates, screws, metal sutures, or wire mesh.

  2. MRI is generally not advised for pregnant patients (increase in amniotic fluid temperature may be harmful) or individuals with epilepsy. All patients having an MRI need to remove hearing aids, dentures, jewelry or piercings, hair pins, wigs, hairpieces, and other accessories.

  3. Patients unable to remain still and those who are claustrophobic may require IV conscious sedation before MRI.

  4. Certain types of eye makeup and permanent eye liners that contain metallic fragments sometimes cause discomfort during MRI. Assess for these cosmetic enhancements.

  5. A thorough patient history is mandatory before any MRI study. Many facilities require a specific questionnaire to be completed. Commonly, radiology services perform conventional x-ray imaging to confirm or rule out the presence of metallic fragments before MRI. This is particularly important for metal or foundry workers, who may have tiny metallic fragments in their eyes.

  6. Common metallic equipment (e.g., scissors, oxygen tanks, electronic devices) can become lethal projectiles when exposed to the strong magnetic fields. Therefore, a thorough screening of all patients, visitors, and staff before entering the scan room is mandatory.

  7. Local burns from ECG electrodes, leads, other wires, and surface coils have been reported. It is imperative that the patient describe any burning sensation to the technologist during the procedure.

  8. In the event of respiratory or cardiac arrest, the patient must be removed from the scanning room before resuscitation. Most general hospital equipment (e.g., oxygen tanks, IV pumps, monitors) is not permitted in the MRI suite.

Clinical Implications ⬆ ⬇

  1. MRI and MRS of the brain may identify the following conditions:

    1. White matter disease (e.g., multiple sclerosis)

    2. Infectious disorders affecting the brain (e.g., toxoplasmosis in AIDS, vasculitis, tuberculosis)

    3. Neoplasms (primary and metastatic brain tumors, pituitary adenomas)

    4. Ischemia, stroke

    5. Aneurysms, hemorrhage

    6. Hydrocephalus

    7. Vascular abnormalities (aneurysm, angiomas)

    8. Congenital central nervous system defects (Chiari malformation, Dandy-Walker syndrome)

    9. Alzheimer disease

  2. MRI and MRS of the spine may identify the following conditions:

    1. Disk herniation or degeneration

    2. Neoplasm (primary and metastases)

    3. Inflammatory disease

    4. Demyelinating disease

    5. Congenital abnormalities (e.g., tethered cord, spinal dysraphism)

  3. MRI of the heart may identify the following conditions:

    1. Abnormal chamber size or myocardial thickness

    2. Cardiac tumors

    3. Congenital heart disorders

    4. Pericarditis

    5. Graft patency

    6. Thrombotic disorders

    7. Aortic dissection or aneurysm

    8. Cardiac ischemia

    9. Anomalous pulmonary venous connection

  4. MRI and MRS of the limbs, joints, and soft tissue may identify the following conditions:

    1. Neoplasms of soft tissue and bone

    2. Ligament or tendon damage

    3. Osteonecrosis, occult fracture

    4. Bone marrow disorders

    5. Muscle fatigue

    6. Changes in blood flow:

      1. Atherosclerosis

      2. Aneurysm

      3. Thrombus

      4. Embolism

      5. Bypass grafts

      6. Endocarditis

      7. Shunt placement

  5. MRI of the abdomen and pelvis may identify the following conditions:

    1. Neoplasms (especially useful in staging tumors)

    2. Retroperitoneal structures

    3. Status of kidney transplants

  6. MRA may identify the following conditions:

    1. Aneurysms

    2. Stenosis or occlusions

    3. Graft patency

    4. Vascular malformations

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose, procedure, benefits, and risks.

  2. Assess for contraindications to testing. Obtain a relevant history regarding any implanted devices such as heart valves, surgical and aneurysm clips, plates, internal orthopedic screws and rods, and pacemakers, among other objects.

  3. Ensure that the following materials are removed before the procedure: removable dental bridges and oral appliances, credit cards, keys, hair clips, shoes, belts, jewelry, clothing with metal fasteners, wigs, hairpieces, and removable prostheses.

  4. Claustrophobic feelings can be avoided if the patient keeps the eyes closed during the test. Recommend that the patient not eat a large meal within 1 hour of testing to reduce physiologic demands and possible emesis while in the scanner.

  5. Encourage the patient to relax and remain as motionless as possible during testing. Reassure the patient that this is a painless procedure.

  6. Ask patients having blood flow testing to abstain from alcohol, nicotine, caffeine, and prescription drugs for iron. Tell the patient to fast for 2 hours before testing to avoid unexpected blood vessel vasoconstrictions or dilation. Explain that no smoking is permitted before the test. Promote rest in the supine position for 10 minutes before the test.

  7. Explain that fasting or drinking only clear liquids may be necessary for several hours before an abdominal pelvic MR.

  8. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed.

  2. Monitor the patient for side effects of the MR contrast agent. Common side effects include coldness at the injection site, dizziness, and headache. Treatment is usually not needed unless symptoms are bothersome or prolonged. Rare side effects include convulsions, irregular or rapid heart rate, itching and watery eyes, skin rash or hives, facial swelling, thickening of tongue, fatigue or weakness, wheezing, chest tightness, and difficulty breathing. Alert the healthcare provider if any of these occur and initiate treatment as indicated.

  3. Assess the contrast dye injection site for signs of inflammation, bruising, irritation, or infection.

  4. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆ ⬇

  1. Respiratory motion causes severe artifacts with abdominal and thoracic imaging.

  2. Persons with morbid obesity may not fit into the gantry opening or surface coil configurations.

Reference Values ⬆

Normal

  • Soft tissue structures: Normal brain, spinal cord, subarachnoid spaces, fat, muscles, tendons, ligaments, nerves, blood vessels, marrow of limbs and joints, heart, abdomen, and pelvis

  • Blood vessels: Normal size, anatomy, and hemodynamics