Ultrasound application has evolved from what was once a rote "cookbook" approach to a clinical science. Determining if ultrasound is indicated and, if appropriate, the proper output parameters requires knowledge of the type of tissues involved, the depth of the trauma, the nature and inflammatory state of the injury, and consideration of the skin and tissues overlying the treatment area.
• Although some treatment parameters have been established, patient feedback and reevaluation of the response to prior treatments form the basis for adjusting the treatment parameters. Inform the patient about the expectations and sensations that are to be expected during the treatment and inform the patient to report any uncomfortable, unusual, or unexpected sensations such as pain or burning.
To ensure safe application of therapeutic ultrasound, units must be calibrated at least once a year (many manufacturers recommend that this be done twice a year). The U.S. Food and Drug Administration (FDA) requires that the output frequency, effective radiating area (ERA), and beam type must be indicated on the generator or the transducer10 (Fig. 8-1). The date of the last calibration must also be indicated somewhere on the unit. The reported ERA is the average for that make and model; the actual ERA may be significantly different from the reported value.11 The size of the transducer face may also be included if it is significantly different from the size of the ERA (see Fig. 7.1).1
FDA Labeling Requirements for Therapeutic Ultrasound. The sound head pictured here is capable of producing an output frequency of 1 or 3.3 MHz and has an effective radiating area of 4.0 cm2. Note that the BNR is listed as 5:1 for both outputs. The "Max." indicates that this was the maximum BNR found in a sample of sound heads.
Ultrasound can only increase tissue temperatures when the treatment area is approximately two times the size of the ERA.8 Note that the ERA is approximately half the size of the transducer face. Attempting to heat a larger area will significantly reduce the temperature increase. If the size of the target tissues is larger than three times the ERA, divide the area into two or more treatment zones (Fig. 8-2).
Treatment Zones. When treating an area more than twice the size of the sound head's ERA, divide the area into two or more "treatment zones." Use caution when treating overlapping areas. In this example treat the two outer zones and then the center zone. As one treatment zone is being heated, the prior treatment areas will cool, thus limiting the ability to effectively heat the muscle mass as a whole.