Chapter 7 Rehabilitation Physical Modalities 147sound energy to target tissues (Steiss & Adams,
1999) and should therefore be clipped in the
treatment area.
Different tissues also heat at different rates.
For example, the rate of heating in tendon is
about three times faster than that of muscle
(Draper, 2014). There is also evidence that
patients with a higher body mass index may
experience less effective outcomes due to poor
transmission of TUS energy through adipose
tissue (Muftic & Miladinovic, 2013).
Sound energy also does not transmit well
through air; a coupling medium is required to
deliver ultrasound energy into tissues. There
are direct and indirect coupling methods. The
technique selected is often based on the size
and contours of the body part being treated.
The direct method is used when the treatment
area is relatively smooth and larger than the
transducer head. Some TUS units are equipped
with several sizes of transducer heads
(Figure 7.11). A water‐soluble gel is usually
used as the coupling medium and is applied on
the treatment area. The transducer head then
maintains contact with the gel throughout
treatment.
Indirect coupling methods may be selected
when the treatment area is irregularly shaped,
is smaller than the transducer head, or if the
pressure of direct coupling will cause discomfort.
The most commonly used indirect method is
submersion. The body part is placed into a
container of water and the transducer head
is held approximately 1–2 cm from the skin
surface. It is important to note that with the
submersion technique, heating is significantly
less than with direct coupling techniques
(Forrest & Rosen, 1989; Draper et al., 1993).
To ensure adequate energy delivery, the treat
ment area should not exceed two to three times
the size of the transducer head. Treatment dura
tion of 5 to 10 minutes is generally recom
mended, but according to Draper, the depth of
the target tissue, the intensity, and the treatment
goal need to be considered to determine an
appropriate duration. There are several meth
ods mentioned in the literature for determining
TUS dosage. Draper has established the rate of
tissue heating per minute for both 1 MHz and
3 MHz frequencies using four different intensi
ties. Based on this, proper treatment duration
can be established when tissue heating is a goal
(Draper, 2014). Another technique has been pro
posed by Watson in which the tissue depth,
stage of healing, and the size of the treatment
area are considered to help determine frequency,
intensity, duty cycle, and duration (Watson,
2009). Regardless of how the therapist deter
mines the treatment dosage, the patient’s com
fort must be a primary concern and parameters
should be adjusted accordingly.
The speed of transducer head movement
should also be considered. As the distribution
of energy is not uniform under the transducer,
maintaining it in one location can lead to devel
opment of hot spots and possible tissue dam
age. Moving the transducer head slowly and
evenly (approximately 4 cm/s) leads to a better
energy distribution. The need to move the
transducer head more rapidly to avoid patient
discomfort is often an indicator of a lower
quality TUS unit. Ensuring the TUS unit has a
low beam nonuniformity ratio (BNR) reduces
the risk of development of hotspots under the
transducer.
Finally, the transducer should be positioned
directly over the target tissue and perpendic
ular to the skin. If the angle of delivery moves
beyond 75°, the beam will travel along the
skin rather than into the target tissue, render
ing the treatment ineffective (Michlovitz &
Nolan, 2005).Figure 7.11 Example of a therapeutic ultrasound (TUS)
unit with various sized transducer heads.