Ultrasound therapy devices (Fig. 1 designed to produce clinically significant tissue heating operate at 0.75–3.0MHz, the lower frequencies mechanical waves being more penetrative and affecting deeper tissues than the higher frequency waves.The ultrasound (US) is produced by the reverse piezoelectric effect when a high frequency alternating current is applied to a ceramic crystal causing it to vibrate at the same frequency. The crystal is located inside an applicator fitted with a metal cap into which these vibrations pass.
Figure 1.Portable ultrasound therapy equipment. The ultrasound transducer is housed in the head of the applicator shown on the right.
A coupling medium such as either water or a gel with a high water content transmits the ultrasonic vibration into the tissues. US is transmitted readily by water and by tissue fluids. As US is more readily absorbed by protein than by fat, it can be used to heat muscle and collagenous tissue selectively without heating subcutaneous fat to a clinically significant level. Heating occurs where the US energy is absorbed. If the US enters the tissue at an angle other than 908, it is refracted; the transducer should therefore be held perpendicular to the surface being treated.
Absorption, and therefore heating, is greatest at higher frequencies. As a consequence, it is generally accepted that higher frequency US (e.g., 3 MHz) penetrates less deeply into the body than lower frequency US (e.g., 1 MHz). However, in some circumstances, 3MHz may heat deeper tissues than originally theorized, when for example, it is transmitted into muscle over a bony prominence via a gel pad coated on both sides with an ultrasound transmitting gel (43).
In this clinical investigation, it was found that 3MHz ultrasound applied over the lateral malleolus of the ankle heated the musculotendinous tissue deep to the peroneal groove 0.5 cm deeper than suggested by others.(portable ultrasound scanner)This interesting but surprising observation may be an artifact due to variations in coupling of the US transducers to the skin. The authors point out that air trapped at the gel/pad interfaces might result not only in nonuniform heating but also in hot spots on the US transducer faceplate and possibly the skin.