Types of magnetic resonance imaging systems

The conventional MRI uses a circumferential whole body scanner. The entire body moves into a narrow chamber that is open at both ends (Figure 66). With an open configuration scanner, the patient is not completely enclosed in the cylindrical chamber, but the images may not be as clear as with the conventional scanner and the procedure takes longer. The third type of MRI is the dedicated extremity MRI (E-MRI). This system uses a special scanner to image the affected extremity only. The imaging is for the middle and more distal joints of the arm or leg.

Figure 66. Magnetic resonance imaging. As the patient moves into the machine, the scanning process can provide images of neurologic, orthopedic, abdominal, and vascular tissues.

Safety concerns

Any ferromagnetic metal implant or other metal fragments within the patient’s body are contraindications for this imaging procedure. The magnetic forces of MRI are so great that they will twist, damage, or move the metallic object and cause injury. The metallic items include shrapnel, BBs, bullets, or other metal fragments that are imbedded in the body, particularly those located in or near the eyes or brain. Implants that are contraindications for the test include aneurysmal vascular clips, vascular stents, pacemakers, cochlear implants, joint implants, intrauterine devices, surgical screws, clips, staples, and other implanted therapeutic devices.

Dental appliances must be removed, but dental braces and a permanent dental bridge are acceptable. Although they are visible in the MRI images, they are not likely to interfere with the findings. During the imaging, the patient will feel vibration in the fillings of the teeth, but the vibrations will do no damage.

A strong magnetic force also exists around the outside of the scanner. The examination room must be kept clear of all extraneous metal objects such as oxygen tanks, wheelchairs, canes, crutches, and vacuum cleaners. No person can enter the examination room with coins, keys, stethoscopes, pens, credit cards, scissors, or hairpins. If these items are present, they become missiles that will be pulled into the scanner with force. The patient can be seriously injured by these objects.

Magnetic resonance angiography (MRA)

Vascular imaging by MRI is done to evaluate blood flow and the structure and location of the major blood vessels. Some studies require no contrast medium. When contrast is used, there is minimal occurrence of side effects and a lower toxicity than iodinated contrast (Frank, Long & Smith, 2007). In the head and neck, this procedure is used for imaging of the circle of Willis and the carotid arteries (Figure 67). In the evaluation of major arteries of the torso, the aorta and the renal, mesenteric, and iliac arteries can demonstrate an aneurysm, dissection, and coarctation. MRA is also used to visualize impaired blood flow in cases of peripheral vascular disease. In vascular applications, the procedure often is done before medical or surgical intervention. The data help to determine whether the problem can be corrected by endovascular stenting, angioplasty, or surgical repair with bypass or graft replacement of the blood vessel.

Figure 67. Magnetic resonance angiogram. The anterior view of the head shows the normal intracerebral vessels including the anterior cerebral artery (ACA) and the middle cerebral artery (MCA). No contrast medium was used for the imaging.

(From Mettler FA: Essentials of radiology, ed 2, Philadelphia, 2005, Saunders.)

Brain and spinal cord imaging

MRI is particularly valuable for assessment of the soft tissue of the brain and spinal cord. Although it cannot image the bones of the skull and vertebrae, it provides clear imaging of the organs and tissue contained within these bones. In the study of the brain, MRI provides images of tumors, cerebral edema, ischemia, multiple sclerosis, and other demyelinating diseases. In the study of the spinal cord, MRI provides images of disk degeneration, spinal cord tumor, epidural fat, and postoperative scar tissue that impinge on the cord or its nerve roots (Figure 68).

Figure 68. MRI image of the lumbar spine with a herniated disk. The spinal canal is filled with cerebrospinal fluid (F), except at the level of the linear nerve roots at the distal end of the spinal cord. Normal disks (N) remain in place between the vertebrae. At L 4-5, note the disk (D) that has herniated and protrudes into the spinal canal, pressing into the nerve roots. A slipped disk will cause acute radiating pain and neurologic symptoms in the lower back and legs.

(From Frank ED, Long BW, Smith BJ: Merrill’s atlas of radiographic positioning and procedures, ed 11, St Louis, 2007, Mosby.)

Musculoskeletal imaging

The soft tissue structures of the knee, hip, and shoulder joints are most often examined. MRI provides clear images of tears in the menisci, ligaments, tendons, and muscles that provide the structure for the knee or shoulder joint. The procedure can distinguish between an inflammatory problem that will heal with conservative therapy and a tear that will require surgical repair.