Ultrasound uses high-intensity sound waves in the range of 2-20 MHz to generate images of internal structures (Cosgrove et al. 2008). It is attractive in that it is portable and images can be achieved in real time. Further, this modality does not employ ionizing radiation or contrast agents, thus minimizing side effects and damage.
Sound waves are generated at the ultrasound transducer and are propagated through the tissue particles from one particle to the next much like marbles hitting one another. Some of the sound is absorbed and some is reflected, particularly at tissue boundaries (Cosgrove et al. 2008). By fixing the frequency of the transducer, the time required for sound waves to reflect back to the transducer can be related to depth; this is how ultrasound determines dimension. Wave reflection occurs at tissue boundaries, with the degree of reflection being related to the change in density of those two tissues. Thus ultrasound is very poor at imaging that for which radiographs are good: bone and air boundaries (Mettler 2005). Attempting to image the brain is near impossible as the sound waves do not traverse well past bone. Similarly, imaging lung is also difficult as the air boundary does not reflect sound and attenuates transmission. Contrast is achieved by the amount of reflection and absorption that returns to the transducer.
The utility of ultrasound in diagnosing pain conditions is quite limited. Because ultrasound is good for identifying tendons, it has a potential role in diagnosing tendonitis. However, the diagnosis of this condition is primarily clinical and does not require imaging to confirm or refute the diagnosis (Bogduk 2003). Ultrasound is of greater benefit therapeutically as it aids in the positioning of needles for nerve blocks. Such blocks may include aspiration and injection of joints or intercostals nerve blocks.
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