Arthritis and Cartilage

How to Image Arthritis and Cartilage

Magnetic resonance imaging ( MRI ) remains the ideal imaging technique for evaluating articular cartilage, as it allows for direct visualization of the cartilage and subchondral bone, and its multiplanar capability and exquisite soft tissue contrast provide unparalleled information about structures in and around the joint space. Although the diagnosis of the type of arthritis is made with conventional radiographs, evaluation with MRI can demonstrate changes in the articular cartilage, assess for loose bodies within the joint space, and determine synovial involvement, thereby aiding in treatment planning.

• Coils and patient position: The joint being imaged determines which coil and which position are used. In the knee, the standard extremity coil is used in the same manner as imaging for a torn meniscus. The same would hold for the wrist, elbow, and so forth.

• Image orientation: Joints imaged for arthritis and for cartilage are best seen with the standard planes of imaging discussed in other chapters. In the knee, three planes (axial, coronal, and sagittal) should be used to evaluate the cartilage adequately.

• Pulse sequences and regions of interest: For imaging a joint for arthritis, it is recommended that T1-weighted (T1W) and some type of T2-weighted (T2W) sequence be used in each plane of imaging. Cartilage-sensitive sequences are discussed in greater detail later in this chapter.

• Contrast: Contrast is often utilized for the diagnosis of synovial involvement, as well as assessing for treatment response because it markedly increases the conspicuity of hypertrophied synovium (pannus).

Most joint abnormalities are discussed in the chapters addressing specific joints (e.g., avascular necrosis in the marrow or hip chapters). This short chapter discusses a few additional abnormalities that can affect any joint, such as pigmented villonodular synovitis (PVNS), synovial chondromatosis, and a few common arthritides; it also provides an overview of cartilage imaging.

Current trends in rheumatology suggest that MRI is a valuable tool for assessing synovitis and allowing for aggressive and earlier treatment. Awareness of findings associated with the arthropathies is important, as the changes encountered can be seen in patients undergoing imaging for other reasons.

Rheumatoid Arthritis

Early diagnosis and treatment have been recognized as essential for improving clinical outcomes in patients with rheumatoid arthritis (RA). The erosive changes in RA can be detected earlier with MRI than conventional radiographs, allowing for earlier diagnosis and treatment ( Fig. 6.1 ). It has been recognized that bone marrow edema, a precursor to the development of erosions, can be readily seen on MRI and may be a sign of active inflammation. The location of the bone marrow edema is important, because inflammation at tendon and/or ligamentous insertions may signify a peripheral spondyloarthropathy rather than RA.

Careful evaluation of T1W images of a joint affected by RA may reveal slightly higher signal intensity within pannus relative to joint fluid, allowing for identification without the need for intravenous contrast administration. In many cases, however, pannus cannot be reliably differentiated from synovium and joint fluid using standard imaging sequences ( Fig. 6.2 ). After the intravenous administration of gadolinium, the hypervascular pannus can be easily identified because of its intense enhancement. It has also been reported that contrast-enhanced T1W images identify more periarticular bone abnormalities than fat-suppressed T2W images. Previously, treatment of RA was not predicated on the amount of pannus present, but rheumatologists have become more aggressive in initiating treatment, and MRI is often done to evaluate pannus and help in the design of a treatment plan.

Occasionally, a swollen joint in a patient with RA will contain multiple small, loose bodies, called rice bodies ( Fig. 6.3 ). These occur in certain chronic synovial conditions such as RA or indolent infections when the synovial villi hypertrophy to such an extent that they outgrow their blood supply, necrose, and slough off into the joint. They are called rice bodies because of their resemblance at surgery to white rice. On MRI, rice bodies can mimic multiple loose bodies or synovial chondromatosis, but typically rice bodies are much smaller than the bodies of synovial chondromatosis and remain low signal on T2W images. Most, but not all, patients already carry a diagnosis of RA, so the entity is easily recognized as rice bodies if the radiologist is familiar with this process. Rice bodies can be removed easily by a surgeon if they cause mechanical symptoms, but otherwise the treatment is the same as for any joint involved with RA.

Ankylosing Spondylitis

Generally, the changes of ankylosing spondylitis can be appreciated with conventional x-ray. The early changes such as enthesopathy producing inflammation and marrow edema at tendon or ligament insertion sites can be appreciated much earlier on MRI than with conventional x-ray and as such, MRI is useful in the early diagnosis of the disease. In addition to the enthesopathic changes, the presence of bone marrow edema at the sacroiliac joints as well as the vertebral body corners (“shiny corners”) will aid in early diagnosis and treatment and are evident earlier on MRI than on x-ray ( Fig. 6.4 ). The administration of intravenous contrast to assess the degree of enhancement may also assist with treatment planning.

Gout

As with RA, the radiographic findings in gout are typically sufficient for diagnosis, and MRI has little to offer in this disease. It is important to appreciate, however, that gouty tophi occasionally are seen in patients not known to have gout, in which case they can cause diagnostic confusion. Gouty tophi can occur in almost any soft tissue location and may erode bones or begin within a bone (intraosseous tophus). In cases in which the tophus is large and the diagnosis of gout is unknown, the tophus can be misdiagnosed as a tumor with resultant biopsy. Tophi are typically low in signal on T1W and T2W images ( Fig. 6.5 ), which distinguishes them from most other types of joint pathology and from most tumors (with the exception of fibrous tumors, PVNS, and amyloid). Tophi occasionally demonstrate increased signal intensity on MR images ( Fig. 6.6 ) and may show uniform contrast enhancement or have a nonenhancing center. MRI can also reveal cortical erosion related to an adjacent tophus due to the associated bone marrow edema.

Because of the nonspecific features of gout on MRI, supporting features include the site of involvement and distribution of the findings. The lack of an overlying or contiguous ulcer can distinguish gout in the foot from osteomyelitis. The popliteus and patellar tendons in the knee have a predilection for gout involvement ( Fig. 6.7 ). Clinically, the patient may present with a mass. If not familiar with the appearance and location, this finding may be misconstrued as a soft tissue malignancy.

Calcium Pyrophosphate Dihydrate Deposition

MRI has little to offer in the diagnosis of calcium pyrophosphate dihydrate deposition (CPPD), or pseudogout. The appearance of chondrocalcinosis in the menisci of the knee has been reported to have linear high signal that can mimic a meniscal tear, but this has not been a significant pitfall in our experience. One might intuitively think that calcification would produce low signal on MR images; however, in some cases, calcification paradoxically causes intermediate to high signal on T1W images. The reason for this has not been determined, but several theories have been discussed in the literature. Chondrocalcinosis also can appear as linear or punctate areas of low signal in hyaline cartilage, which are particularly noticeable on T2* sequences because of the associated blooming artifact ( Fig. 6.8 ). A utility of MRI in the evaluation of CPPD is to assess the degree of cartilage loss associated with the arthropathy, which can be more accurately assessed with MRI than with radiographs so that appropriate treatment planning can be instituted in a timelier fashion.