Rheumatoid Diseases

General characteristics

Rheumatoid diseases include a variety of conditions with different clinical and radiological characteristics. Proliferation of the synovial membrane is a common feature of all. Clinical, serological, and immunological examinations play an important role in diagnosis and classification.

MRI may contribute substantially in the early diagnosis since unambiguous radiographic findings are frequently absent in the early stages of these disease entities. New treatment options, such as tumor necrosis factor (TNF)- alpha inhibitors, require sensitive assessment of response and MRI might be useful for such follow-up.

Rheumatoid Arthritis

Irreversible damage of the joints may take place in the early phase of the disease. Therefore, diagnosis needs to be established at an early stage so that effective therapy can be instituted. Sugimoto et al. (1996) showed that contrast-enhanced T1-w fat-suppressed imaging of the hands and wrists in patients without any findings in radiographs is very sensitive and specific for the diagnosis of early rheumatoid arthritis (RA), and has a higher sensitivity compared to clinical exams according to the revised criteria of the American Rheumatism Association of 1987. These criteria include: - morning stiffness; - soft-tissue swelling; - swelling of the proximal interphalangeal, metacarpophalangeal, or wrist joints; - symmetric swelling; - rheumatoid nodules; - presence of rheumatoid factor; and - radiographic erosions and/or periarticular osteopenia.

Clinically, RA is defined by the presence of four or more criteria.

Imaging

MRI allows monitoring of therapeutic effects, assessment of surgical indications, and planning of synovectomies. The earliest recognizable pathology in rheumatoid arthritis is acute synovitis. Synovial proliferations present as villous, nodular, laminar, or even solid structures.

Synovial proliferations can be directly visualized by MRI. On T1-weighted images, they exhibit low signal intensity, which makes differentiation from joint effusion difficult. On T2-weighted images, they manifest with variable signal intensities; low signal intensity is due to fibrous, collagenous connective tissue or hemosiderin. Foci of high signal intensity are due to inflammatory tissue with edema. After intravenous administration of contrast agents, a strong signal increase is found within the synovial proliferations, reaching its maximum after 50 s and continuing until approximately 200 s (Adam 1991; Reiser 1988).

On contrast-enhanced T1-weighted fat-suppressed images, synovial proliferations are clearly highlighted. Joint effusion experiences a delayed enhancement, so that high contrast of joint effusion and synovial proliferations is obtained. Approximately 6 min after i.v. contrast administration, signal enhancement is found in the joint effusions, starting in the periphery and gradually progressing to the center. After approximately 24 min, a complete filling of the effusion is visualized (Yamamoto 1993). MRI also allows differentiation between active and inactive pannus, since active hypervascularized pannus shows a significantly higher contrast uptake than inactive fibrous pannus.

MRI allows for visualization of synovial proliferations in tendon sheaths. Inflammatory lesions and ruptures of tendons and ligaments are also clearly visualized. When using sequences adequate for cartilage imaging (e.g., 3D FLASH, DESS, trueFISP) structural damage and thinning of cartilage can be assessed. On radiographs, joint-related osteopenia is detected as a typical sign of rheumatoid arthritis; this cannot be seen on conventional MRI. Cysts and erosions, on the other hand, are detected with higher sensitivity with MRI.

In addition to the small joints of the hand and foot, the wrist, the knee, the elbow, the glenohumeral and acromioclavicular joints, the cervical spine, and the atlanto- occipital junction are frequently involved in rheumatoid arthritis. Synovial proliferations, pseudobasilar impression, and atlantodental subluxation may result in neurological complications; all can be clearly delineated in MRI. Anterior atlantodental, vertical (also known as atlantoaxial impaction), lateral and posterior subluxations are found in 40% to 85% of patients with RA.

With sagittal MRI, the foramen magnum and the distance to the medulla oblongata and the superior cervical cord can be readily appreciated. On T2- and PD-weighted images, myelomalacia due to pressure on the cord and medulla oblongata, respectively, manifests with signal increase.

Seronegative Spondyloarthropathies

The three major types of seronegative spondyloarthropathy include ankylosing spondylitis, psoriatic arthritis, and Reiter’s disease. Like in RA, there is involvement of synovial joints, but with a different morphology and distribution of articular lesions. The key finding in spondyloarthropathies is sacroiliitis. Radiographs and CT are frequently negative in the early phase of the disease. As transarticular bone bridges, and ankylosis are found. MRI allows detection of early stages of sacroiliitis and assessment of disease activity.

On STIR images, bone marrow edema in the subchondral zone of the iliac and sacral bones is found in active inflammation, and is associated with increased contrast enhancement. Active erosions are hyperintense on STIR and T2-weighted images and exhibit pronounced contrast enhancement. Histologically they represent invasive-destructive pannus at the cartilage-bone junction. On T1-weighted images, band-like hypointense signal is found in both active inflammation and sclerosis.

Moreover, increased contrast enhancement is also found in periarticular bone marrow as well as in the anterior and posterior capsules of the sacroiliac joint. Another common finding in spondyloarthropathies is enthesitis, such as in the pelvis, the femoral trochanter, the iliac crest, the ischial tuberosity, the patella and the calcaneus. Bone marrow edema and contrast enhancement as well as inflammation of tendons, ligaments, and muscles may be found at these sites.