• Adequate heating of the deeper tissues without excessive heating of the superficial layers is one of the important problems in physical medicine. In their effects, ultrasonic vibration and microwave diathermy are closely related, although the former consists of longitudinal mechanical vibrations while the latter consists of transverse electromagnetic waves. Ultrasound provides highly effective depth of penetration in deep tissues having a high water content; microwaves are preferable in the case of bone structures.
Dosage cannot be read simply from meters, because tissues vary in their absorption coefficients and reflection occurs at interfaces. In the case of ultrasound the dosage must be closely watched because the margin between effective and injurious doses is narrow. The transducers available at present for ultrasound give a small radiation field that limits its usefulness to very local application.
Schwan HP. THE BIOPHYSICAL BASIS OF PHYSICAL MEDICINE. JAMA. 1956;160(3):191–197. doi:10.1001/jama.1956.02960380039010