The unit of intensity when using ultrasound is the watt.
Space averaged intensity where the average intensity over a specified area is given, e.g. watts per square centimeter (Wcm-2).
Time-averaged/space averaged intensity can be used when the ultrasound is being applied in a pulsed mode and gives the average intensity over the whole treatment time (per second) for a specified area Wcm-2).
Pulsed mark: space ratio
When ultrasound is applied in its pulsed mode, the ratio of the time on to time off should be expressed. This is the mark: space ratio, the mark being the time ultrasound is on space being the silence, both being measured in milliseconds eg 2: 8, 1: 7.
Reflection of ultrasound
Air does not transmit ultrasonic waves, so in ultrasonic treatment great care is taken to avoid leaving air between the treatment head and the patient, to minimize reflection. Though there will always be some reflection at each interface that the ultrasound beam encounters. This gives rise to acoustic impedance. When the acoustic impedance is low, the transmission is high, and vice versa.
Transmission of ultrasound
If the ultrasound beam encounters an interface between two media and is transmitted, it may be refracted. As refraction does not occur when the incident waves travel along the normal, treatment should be given with the majority of waves traveling along with the normal, whenever possible.
Attenuation of ultrasound
Attenuation is the term used for the gradual reduction in the intensity of the ultrasonic beam once it has left the treatment head. Two major factors contribute to attenuation.
Ultrasound is absorbed by the tissues and converted to heat at that point. This constitutes the thermal effect of ultrasound.
This occurs when the normally cylindrical ultrasonic beam is deflected from its path by reflection at interfaces, bubbles or particles in its path.
The overall effect of these two is such that the ultrasonic beam is reduced in intensity the deeper it passes. Therefore, when treating deep structures consideration needs to be given to the frequency and intensity of ultrasound chosen.
Depth of penetration and intensity of the ultrasonic beam is the division of the beam into a near and a far-field. The near and far-fields arise because the wavefronts from different parts of the source have to travel different distances and consequently there is interference between adjacent fronts. At some points, the interference is constructive at other points the interference is destructive. The extent of the near field is of significance in that it is more intense than the far-field and may have a more profound effect in the treatment of certain conditions. The frequency of the ultrasound and radius of the transducer may need to be considered when treating tissues at a depth greater than 6.5cm.
Ultrasonic waves are not transmitted by air, thus some couplant that does transmit then must be imposed between the treatment head (transducer) and the patient's skin. No couplant affords perfect transmission and only a percentage of the original intensity is transmitted to the patient. Air reflects the ultrasonic beam back into the treatment head and thus produces standing waves which might damage the crystal. The treatment head is never left switched on when not in contact with a transmission medium.