5.24 Ultrasonic Dosimetry
175
biological cell walls. The effect is enhanced if the membranes are in resonance
with the sound.
5.24.2 Effects of High-Level Ultrasonics
Very high levels of ultrasonic waves can be disruptive and destructive to tissue.
We have already given a general list of the mechanisms for sound attenuation in
Sect. 5.11. We will now consider the most important ones for ultrasonics in medical
applications.
Heat Production
If there is insufficient time for the heat generated in tissue from a pulse of sound to
diffuse away, then the rise in temperature T can be found from the definition of
the specific heat of the tissue, c h , namely
Q = c h δM MT ,
(5.56)
where δM is the mass of a segment of the tissue. We will use δx to represent the
small length through which the sound has passed. Now the intensity of the sound
will drop as it passes through, according to I = I o exp (−α L x) with α L being
the sound attenuation coefficient, the relative drop in the sound intensity per unit
distance. (See Eq. (5.43).) We will assume that the energy loss in the sound passing
through a segment of tissue of area A and thickness δx will go into heat Q. This
means, after a time t,
Q = −(dI /dx)δx A At .
(5.57)
Writing the mass of the segment of tissue, δM, as density ρ times its volume δV =
Aδx, we will have
Q
t
= α L I δV = c h ρ δV
T
t
(5.58)
As a result,
T =
2 α L I us
ρ C h
t
(5.59)
where I us is the time-averaged ultrasonic (US) sound intensity. Given that some heat
will dissipate, the expression for T should be an upper limit. To get an estimate in
a practical case, we will take the specific heat of wet tissue and its density close to
175
biological cell walls. The effect is enhanced if the membranes are in resonance
with the sound.
5.24.2 Effects of High-Level Ultrasonics
Very high levels of ultrasonic waves can be disruptive and destructive to tissue.
We have already given a general list of the mechanisms for sound attenuation in
Sect. 5.11. We will now consider the most important ones for ultrasonics in medical
applications.
Heat Production
If there is insufficient time for the heat generated in tissue from a pulse of sound to
diffuse away, then the rise in temperature T can be found from the definition of
the specific heat of the tissue, c h , namely
Q = c h δM MT ,
(5.56)
where δM is the mass of a segment of the tissue. We will use δx to represent the
small length through which the sound has passed. Now the intensity of the sound
will drop as it passes through, according to I = I o exp (−α L x) with α L being
the sound attenuation coefficient, the relative drop in the sound intensity per unit
distance. (See Eq. (5.43).) We will assume that the energy loss in the sound passing
through a segment of tissue of area A and thickness δx will go into heat Q. This
means, after a time t,
Q = −(dI /dx)δx A At .
(5.57)
Writing the mass of the segment of tissue, δM, as density ρ times its volume δV =
Aδx, we will have
Q
t
= α L I δV = c h ρ δV
T
t
(5.58)
As a result,
T =
2 α L I us
ρ C h
t
(5.59)
where I us is the time-averaged ultrasonic (US) sound intensity. Given that some heat
will dissipate, the expression for T should be an upper limit. To get an estimate in
a practical case, we will take the specific heat of wet tissue and its density close to