Dedicated extremity MRI with use of a significantly smaller 0.2 Tesla magnet has many obvious advantages over its high-field counterpart. The cost is reduced significantly, and the machine may be positioned conveniently in a suitable clinic room rather than in the purpose-built home that conventional MRI requires. The machine consists of a “cradle” in which the peripheral joints of interest, such as the MCP joints of the dominant hand, are placed. This avoids difficulties of conventional MRI use, such as claustrophobia and the positioning of arthritic limbs. Disadvantages include smaller fields of view with longer imaging times and, because magnet strength is reduced, some reduction in image quality.

Some studies have investigated whether such reduction affects the ability to detect synovitis and erosions significantly. Taouli and coworkers²³ compared conventional high-field–strength 1.5 Tesla MRI with 0.2 Tesla low-field dedicated extremity MRI and radiography in their ability to detect and grade bone erosions, joint-space narrowing, and synovitis in the hands and wrists of patients with RA (Figure 1). The results from use of the 2 field strengths were similar, although motion artefact limited the value of a few low-field studies. Lindegaard and colleagues²⁴ compared low-field MRI with clinical examination. The results of low field MRI were significantly better; it could identify synovial hypertrophy in joints in patients who did not have clinical signs of joint inflammation (eg, swelling and tenderness).

Ultrasonography

Musculoskeletal ultrasonography is extremely versatile; it allows the examiner to image many joints at the same sitting in a safe environment without x-ray exposure. The machines may be positioned in rheumatology outpatient clinics, which allows for immediate access as clinically indicated. In addition, ultrasonography is less expensive than CT and MRI.

Ultrasonography was shown to be more sensitive than conventional radiography in the detection of erosions.²⁵ In the same study, sonographic erosions that were not visible on radiography corresponded by site to MRI bone abnormalities.

Hau and coworkers²⁶ showed that ultrasonography provides better results than clinical examination alone. Wakefield and associates²⁵ demonstrated its superiority to plain radiography. However, disadvantages of ultrasonography include lack of standardization and reliability; the modality’s interobserver and intraobserver variability are of particular concern.

Kraan and colleagues²⁷ showed that ultrasonography and MRI both can detect subclinical synovitis with corroborative macroscopic and microscopic data from arthroscopy in clinically normal knees of patients with RA. Karim and coworkers²⁸ compared ultrasonography of the knee with the “gold standard” of arthroscopy, as well as clinical examination, to validate ultrasonographic images in terms of accurate representation of the pathology present in the joint. They concluded that ultrasonography was valid and reproducible, as well as superior to clinical examination for detecting knee synovitis.