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INTRODUCTION

Since the mid-1990s, point-of-care ultrasound—that is, musculoskeletal ultrasound (MSKUS) performed and interpreted by the rheumatologist evaluating the patient—has become a tool of increasing importance to rheumatologists. MSKUS is a valuable imaging technique that enhances and expedites the diagnosis of inflammatory arthritides, musculoskeletal conditions, large-vessel vasculitis, polymyalgia rheumatica, Sjögren syndrome, and other diseases. The American College of Rheumatology (ACR) and the European League Against Rheumatism (EULAR) have recommended guidelines for MSKUS use (McAlindon et al, 2012; Möller et al, 2017), and rheumatology training programs around the world now include ultrasound curricula for trainees (American College of Rheumatology, 2020; Brown et al, 2004; Kissin et al, 2013; Naredo et al, 2010; Torralba et al, 2015; Torralba et al, 2017).

Technological advances in ultrasound equipment as this technique has evolved have allowed for smaller, smarter equipment. High-frequency transducers to provide gray scale (black and white images produced by sound waves) for assessment of joint, tendon, and soft-tissue structures. Power Doppler gray scale images evaluations permit the assessment of hyperemia or active inflammation. Advantages of ultrasound over other advanced imaging modalities include its portability, noninvasive nature, cost, and its lack of ionizing radiation. In addition, MSKUS permits dynamic evaluations of joints and tendon structures in motion, providing a significant edge over the more static technologies of plain radiography, magnetic resonance imaging (MRI), and computed tomography (CT).

Rheumatologists most commonly use ultrasound for detection of fluid collections and assessment of synovitis, tenosynovitis, and bony erosions. MSKUS is also useful for procedural guidance. Ultrasound has better spatial resolution than plain x-rays and even MRI, permitting the early detection of bony surface abnormalities or erosions. The high resolution of ultrasound also enhances the detection of crystalline deposition that is often too small to detect by radiograph or MRI. MSKUS studies have augmented the rheumatologist’s ability to diagnose crystalline disease even in intercritical periods. The ability to repeat examinations and correlate in-office image findings with the history and clinical examination also contributes to its usefulness in monitoring of treatment (Backhaus et al, 2001; Brown, 2009; Canella et al, 2014; Karim et al, 2001). For pediatric patients, use of ultrasound does not require sedation that may be necessary for MRI and other cross-sectional imaging (Roth, 2017). Extensive literature on the accuracy of ultrasound-guided procedures (aspiration, injection, biopsy) exists (D’Agostino, 2013; Epis, 2014; Gilliland, 2011; Raza, 2003; Robotti, 2013).

Although ultrasound has many advantages in evaluating the musculoskeletal system, limitations include its inability to penetrate bone. Because of this shortcoming, MSKUS imaging is limited to superficial structures or the surface of bone. Bone marrow edema cannot be assessed. Operator dependence has often been cited as a limitation and this is certainly true in poorly trained individuals. With appropriate training in ultrasound scanning technique and image interpretation, however, the ability of ultrasound to clarify articular disease from periarticular disease and to characterize structures precisely even when they are ambiguous on physical examination are ...

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