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MD Consult: Books: Goldman: Cecil Medicine: MAGNETIC RESONANCE IMAGING

Goldman: Cecil Medicine, 23rd ed.

Copyright © 2007 Saunders, An Imprint of Elsevier

MAGNETIC RESONANCE IMAGING

The demands of imaging are increased in this age of effective therapy to halt the progression of arthropathy. Over the last two decades, MRI has become the imaging modality of choice for evaluation of joint soft tissue, marrow structures, and muscle pathology, because it provides high-contrast resolution that enables the differentiation of various soft tissues from each other. Abnormal joint structures usually display different signal intensities than their normal counterparts. Morphologic changes resulting from destruction of tissue by the arthropathy and muscle inflammation are also identified.

MRI applies a strong magnetic field with radiofrequency pulses that pass through the various body tissues to produce different signal intensities. Each soft tissue structure contains unique properties that give it a characteristic signal intensity based on the proton density, T1 relaxation time, and spin density of the tissue. Various pulse sequences can be used to evaluate the joint; discussion of these pulse sequences is beyond the scope of this chapter.

Many factors can influence spatial resolution and thus the quality of the MRI. These include field strength and coil selection. Most imaging is performed on high-field superconducting magnets. Permanent or resistive magnets are also in use at lower field strengths. Field strength is measured in tesla (T). In general, the higher the field strength, the better the signal from the tissue and the shorter the imaging time.

Five types of magnets are used for clinical imaging of joints: ultra-high-field (3.0 T), high-field (1.0 to 2.0 T), midfield (0.3 to 1.0 T), low-field (less than 0.1 to 0.2 T), and ultra-low-field (less than 0.1 T). Most scanners operate between 0.3 and 1.5 T. Open scanners are less confining for claustrophobic patients but usually are not available at the high field range (above 1.0 T) that provides increased resolution. At the current time, 3.0 T scanners are proliferating in clinical and hospital settings. These ultra-high-field scanners offer the highest spatial resolution and are being increasingly used for imaging of small joints and cartilage.

Coils, electrical devices that can generate or detect a magnetic field, are also important for proper joint imaging. They are placed over the body region of interest, keeping the field of view as low as possible, and improve spatial resolution. Many types of coils are available from various manufacturers for different joint sizes and configurations, including shoulder, extremity, wrist, and flex coils. Phased array coils use several coils in one component, allowing for a larger field of view; these are often used in spine imaging.

The widely used contrast agent for magnetic resonance (MR) studies is a neutral, hydrophilic salt of the gadolinium chelate, gadolinium diethylenetriamine-penta acetate (Gd-DTPA). Gadolinium can be injected intravenously or directly into the joint. Intravenous injection (indirect MR arthrography) carries the contrast agent in the vascular system to areas of hyperemia and inflammation. It is recommended for assessment of synovial activity for inflammatory joint processes. Gadolinium is taken up by inflamed synovium and is able to demonstrate thickened pannus. The slope of the curve of time versus signal intensity is useful for showing inflammatory activity and is commonly employed in research and clinical trials. Injection of dilute gadolinium into the joint (direct MR arthrography) is helpful for outlining structures to determine if there is morphologic damage. This technique is particularly effective for visualization of small structures, such as the labrum of the hip or shoulder, if there is no joint effusion. It is also helpful for demonstrating breakdown of soft tissue structures that normally prevent communication between joint compartments, such as the rotator cuff, triangular fibrocartilage of the wrist, and ligaments in the various joints.

With special regard to rheumatic disease, MRI demonstrates joint effusion, intra-articular masses, synovitis, damage to hyaline and fibrocartilage, tendons, ligaments, bone, muscle, and surrounding soft tissue ( Fig. 279-2 ). MRI is sensitive for marrow abnormalities such as reactive edema and erosion from arthritis, although it can be nonspecific, and infectious processes can have a similar appearance. Hemosiderin has a characteristic low signal intensity on MRI, which allows for characterization of arthropathies such as pigmented villonodular synovitis and hemophilia. MRI is useful for early detection of osteonecrosis, particularly when radiographs are normal. It is also used to evaluate the spine for disc disease, nerve root compression, and inflammatory changes.

FIGURE 279-2  Sausage digit in psoriatic arthritis demonstrated by radiography, scintigraphy, and magnetic resonance imaging. A, Anteroposterior radiograph of the forefoot demonstrates soft tissue swelling of the second toe without obvious bone involvement (arrow). B, There is increased tracer uptake of Tc99m-MDP in the entire second toe, from the metacarpophalangeal joint to the distal phalanx on the bone scan, suggesting osteitis and soft tissue inflammation. C, A coronal fat-suppressed T1-weighted magnetic resonance image of the second toe obtained after intravenous gadolinium administration reveals high-signal-intensity uptake in the bone and surrounding soft tissues, consistent with osteitis and soft tissue inflammation.

Disadvantages of MRI include the facts that it is more costly than CT and other imaging modalities and that it can be claustrophobic. Calcification is not easily identified, and radiographs or CT scans are often complementary in this regard. MRI is contraindicated in patients who have certain devices implanted into their tissues. These include cochlear implants, some heart valves, cardiac pacemakers, and spinal implants. Metal implants, such as joint prostheses and spinal hardware, are not contraindicated for MRI. Although metallic devices can cause some distortion of the image, newer materials such as cobalt-chrome cause less artifact. Various imaging parameters can also be adjusted to improve the visibility of tissues surrounding metallic devices, allowing for diagnostic interpretation.

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