Musculoskeletal ultrasound (MSUS) imaging has been incorporated in the care of the rheumatologic patient for more than a decade in Europe. On the contrary, MSUS is a relatively recent addition in the diagnostic armamentarium of rheumatologists in the United States.
MSUS can significantly advance the diagnostic potential in rheumatology and improve the accuracy of musculoskeletal procedural interventions. The most experience with ultrasound stems from its applications in arthritic conditions. However, the diagnosis of a wide spectrum of rheumatologic conditions can be facilitated by its use.
Ultrasound machines consist of a processing unit and a transducer. Many models of different sizes and degrees of portability are available by various manufacturers. Ultrasound technology couples gray scale imaging and Doppler imaging to evaluate anatomic integrity and tissue inflammation.
Gray Scale Imaging — the Echo Phenomenon
The human ear can perceive sound waves in frequencies between 20 Hz and 20,000 Hz. Sound waves above the hearing frequency are called ultrasounds.
The acquisition of ultrasound images is based on the echo phenomenon. A transducer attached to the ultrasound machine transmits ultrasound waves that “travel” through the tissues. Each tissue has different acoustic properties based on their consistency, compressibility, and density. Different acoustic properties translate into differences in the velocity with which ultrasound waves are transmitted through each particular tissue. Reflection at tissue interfaces generates returning echoes that are collected by piezoelectric crystals in the transducer. The electric potentials generated at the piezoelectric crystals are then transformed to gray scale imaging displayed on a monitor.
The more echoes returned, the more “white” the tissues that generated the echoes will appear.
Structures without internal reflectors do not return any echoes, and this results in black areas in the ultrasound image. Such structures are fluid collections or tissues containing high concentrations of water such as the cartilage or joint effusions. These tissues are called hypoechoic in ultrasound terminology.
Structures that produce weaker ultrasound reflections generate low level echoes and correspond to dark gray areas in the ultrasound images. These structures are called isoechoic and include muscle, tendons, synovial tissue, and nerves (Figure 66–1).
A normal metacarpophalangeal joint. Key: 1, subcutaneous tissue; 2, extensor tendon; 3, metacarpal; *, metacarpal cartilage.
Lastly, tissues with strong internal reflectors return strong echoes and generate bright gray or white areas in the ultrasound image. These structures are called hyperechoic and include bone, calcifications, and foreign bodies.
Inflammation is associated with local hyperemia. Increased blood flow in synovitis (Figure 66–2 and Plate 50), tenosynovitis, and enthesitis (Figure 66–3) can be detected with the use of Doppler ultrasonography. The detection of blood flow locally is transformed to a color signal that ...