Power Doppler sonography: improving disease activity assessment in inflammatory musculoskeletal disease.

Abstract

Musculoskeletal ultrasonography (US) is a powerful tool for evaluating joint and soft tissue pathology and is fast becoming an integral part of routine diagnosis and management in rheumatology practice (1–6). This imaging technique is now being performed by rheumatologists, particularly in Europe, as part of their standard clinical assessment of patients. Increasing evidence supports the use of US in a variety of different locations with demonstrable advantage over standard clinical assessment, enabling more accurate patient diagnosis and facilitating the most appropriate management decisions (7). The trend toward earlier aggressive therapy for inflammatory musculoskeletal disease requires reliable initial diagnosis and optimal disease activity assessment. Interest has therefore been directed toward the use of US as an objective tool for the detection and monitoring of joint and soft tissue inflammation and bone damage (8–15) in early disease. US has a number of advantages over other imaging techniques. It is safe, noninvasive, and emits no ionizing radiation. The equipment can be situated in the rheumatology outpatient clinic, improving patient access and enabling rapid, “real-time” dynamic examinations of multiple joints in multiple planes at one sitting. In addition, both the capital and running costs of US are significantly lower than those of other imaging modalities, such as magnetic resonance imaging (MRI) and computed tomography (CT). Traditional gray-scale US has been successfully used for some time for the detection of joint and soft tissue inflammation (1–15). More recently, additional US techniques, including Doppler, have been introduced, offering the potential for improving the accuracy of a US assessment. Doppler US is a technique for making noninvasive measurements of blood flow and was developed from the principles first described by Austrian physicist Christian Doppler in 1842 (16). He was the first to observe the effect of motion on sound when he detected a change in the frequency of a sound wave as a result of movement of either its source or receiver. There are two main types of Doppler US, color flow Doppler (CFD) and power Doppler (PDS). Both produce a similar color spectral map superimposed onto the gray-scale image (the colors being related to the difference in frequency between the transmitted sound wave and that reflected from the moving interface [the Doppler frequency shift]), but they actually encode different information. CFD represents an estimate of the mean Doppler frequency shift and relates to velocity and direction of red blood cells, whereas PDS denotes the amplitude of the Doppler signal, which is determined by the volume of blood present. In this way, CFD is better suited for evaluating high-velocity flow in large vessels (e.g., carotids), whereas PDS is better suited for assessing low-velocity flow in small vessels (e.g., synovium). There are a number of particular advantages for using PDS in musculoskeletal assessment. Because PDS provides increased sensitivity to low-volume, lowvelocity blood flow at the microvascular level, it is particularly useful for measuring and detecting changes Dr. Brown is an Arthritis Research Campaign (ARC) Research Fellow. Dr. Emery is an ARC Professor of Rheumatology. Richard J. Wakefield, MRCP, Andrew K. Brown, MRCP, Paul Emery, MD, MA, MB, FRCP: University of Leeds, Leeds, UK; Philip J. O’Connor, FRCR: Leeds General Infirmary, Leeds, UK. Address correspondence and reprint requests to Paul Emery, MD, MA, MB, FRCP, Academic Unit of Musculoskeletal Disease, Department of Rheumatology, Leeds General Infirmary, Leeds LS1 3EX, UK. E-mail: p.emery@leeds.ac.uk. Submitted for publication November 5, 2002; accepted November 6, 2002. Arthritis & Rheumatism