Therapy and Imaging With Ultrasound

Therapeutic ultrasound is the use of an ultrasound transducer that produces electrical oscillations at a specified frequency, which causes the transducer in the applicator, probe, or treatment head to generate sound waves.

The resulting ultrasonic radiation is transmitted from the ultrasound transducer through a coupling medium—ultrasound gel—to the patient’s tissue. The physiological effect of the ultrasound therapy depends upon the frequency of the ultrasound signal. The lower frequency (1 MHz) penetrates deeper than does the higher frequency (3 MHz), and the practitioner decides which frequency to use depending upon the patient’s condition. Ultrasound therapy is primarily used in the treatment of sports-related injuries, and most health care facilities have ultrasound devices in their physical therapy departments. Another common use is the treatment of circulatory disorders and rheumatic diseases of the musculoskeletal system and peripheral nerves. Ultrasound has been found to be extremely effective in treating areas with a great deal of scar tissue. The deep-heating capability of ultrasound reaches into the muscles to increase collagen elasticity, reduce muscle spasm and pain, and improve blood flow. Ultrasound has also been shown to stimulate protein synthesis, tissue regeneration, and bone healing. The mechanical effects are best described as micromassage, a deep stirring action within the tissue. The benefit of this action is increased circulation to the damaged tissue. In addition, ultrasound is capable of separating collagen fibers and of changing the tensile strength of tendons, thereby increasing their extensibility.

Regular accuracy tests on a therapeutic ultrasound machines help guarantee the proper output energy of the device. Because a vibration or oscillation is present, the clinician may not be aware that the output energy is not at its set level. This can result in an insufficient or excessive amount of energy being released. Transducer replacement and minor calibration have resolved the majority of problems I have encountered while working on these devices. Since some types of ultrasound applicators can be damaged by extreme temperature changes, avoiding excessive temperature changes over a short period of time can increase the device’s life expectancy.

Diagnostic ultrasound, on the other hand, uses reflections of high-frequency sound waves that are displayed on a monitor to view anatomy. An ultrasound image is formed by many discrete lines of echo information that are generated one at a time in rapid succession. A pulse of ultrasound energy is transmitted into the body along the axis of each line by the transducer. Echoes are created when the sound wave bounces off the boundary between tissues of dissimilar acoustic impedance. After the ultrasound pulse is transmitted, the transducer “listens" for echoes from points along each line. The quality of the image is determined by the precision with which the ultrasound beam is focused in both transmit and receive modes and the sensitivity to reflected signals.

Imaging ultrasound preventive maintenance procedures include—with occasional variation—cleaning the inside of the machine, checking to see if the printed circuit boards are aligned properly, testing for dead zones in the probe head, and verifying the voltages of the power supply and other test-point outputs. Probe failure and distortion are problems routinely encountered with diagnostic ultrasound devices and can be a source of huge expense. Probes can have a variety of problems—ranging from broken cables to dead zones in the probe head. Many times, however, the probes with dead zones are used with no complaints from the technologists.

Tim Hooks, CRES, CBET, is East Coast field engineer for Medstone Technologies in Aliso Viejo, Calif.