Page 19 - Imaging of Osteoarthritis and Rheumatoid Arthritis in Hand Joints
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 Ultrasound 1 Ultrasound is an imaging technique that also does not use ionizing radiation,
but uses soundwaves above the threshold of human hearing. For hand joint
imaging, typically waves of 8-17 Mhz are used. A transducer containing piezo-
electric crystals is placed onto the anatomy of interest. These crystals generate ultrasound waves, which are sent into the patient. Depending on the properties of the underlying tissues some ultrasound waves travel through some of these tissues, while other soundwaves are bounced back to the transducer. These bounced back soundwaves are measured by the same piezo-electric crystals and multiple reflected soundwaves are used to compute the images. The travel speed of sound waves vary between different tissues. Sound waves are especially reflected back as the sound travels from one tissue to another tissue with a different sound speed. The travel speed of sound waves is approximately the same in most human tissues (1450-1580 m/sec) allowing the visualization of all these tissues at once. However, as the travel speed in bone is vastly different (4080 m/sec), all soundwaves are reflected at the bone cortex, and medical ultrasound can therefore not be used to look through bone. Images are generated very quickly, allowing for movement of the patient during examination. While ultrasound cannot look through the bone, views from different positions make it possible to look at the finger joint from a multitude of angles in a short time. However, the complex anatomy of the carpal bones makes this region harder to visualize with ultrasound.
In patients with RA and OA, ultrasound is mainly used to detect swelling of the joint and hypertrophy of the synovium (fig 5). It can be used to detect defects in the cortex of the bones. In addition Doppler ultrasonography can be used to detect movement within a scanned image. A moving object creates echoes with a slightly lower or higher velocity, which can be visualized within the image. In hand joint imaging this is mainly used to detect (increased) blood flow to the synovium in active synovitis.
General introduction

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