Biomedical Engineering Reference
In-Depth Information
and power. This section addresses the key definitions and mea-
surement techniques of these parameters and introduces the
concept of quality assurance (QA), a system by which the user
can be confident of the consistency in performance of his or her
equipment over time.
sinusoidal with a rectangular modulation, this would be given
by Equation 5.24:
2
p
Z
I
SPPA =
(5.24)
2
where p and Z are defined in Equation 5.19. The factor of 2 in the
denominator comes from the averaging of a sine-squared wave.
However, in a realistic situation, the frequency and modulation are
not so well defined. Instead, the pulse intensity integral is calcu-
lated and averaged over the pulse duration. Standards published by
the American Institute of Ultrasound in Medicine (AIUM 1992)
state that the pulse duration is 1.25 times the time interval between
the 10% and the 90% levels of the pulse intensity integral.
Where pulsed beams are used, the intensity values defined thus
far assume the sound wave is on continuously, and thus overes-
timate the effect on tissue. To account for this, the spatial peak,
temporal average intensity ( I SPTA ) is often quoted. This is defined as
5.5.2 Field parameters and their Definitions
5.5.2.1 pressure
Both positive and negative acoustic pressures occur during the
oscillation of an acoustic wave. In plane wave linear oscillation,
the peak positive and negative values are of equal amplitude.
However, nonlinear propagation causes a change in the shape of
the pressure waveform as described in Section 5.3, resulting in a
greater change in peak positive pressure than in the peak nega-
tive. As such, both are important parameters in the description of
an acoustic field. Negative pressures are responsible for cavitation
activity, and the relationship between negative and positive pres-
sures can help to indicate the potential heating effects through the
concept of intensity, as described in the following sections.
PD
PRP
I
=
I
(5.25)
SPTA
SPPA
where PD is the pulse duration, and PRP is the pulse repetition
period. This gives the average intensity over the time between
the start of one pulse and the start of the next, to give a bet-
ter impression of the overall effect on tissue over time. Finally,
physiotherapy systems will often be defined by a spatial average,
temporal average intensity ( I SATA ) as they are commonly apply-
ing heat to a larger area. This is calculated simply by averaging
I SPTA over the effective radiating area of the emitting device.
5.5.2.2 Intensity
For a plane wave, the instantaneous intensity at any given point
is described by Equation 5.19 (Section 5.4). This can be used, for
example, to determine the spatial peak, temporal peak intensity
within the field from the spatial peak pressure. In order to relate
this to the biological effects of real treatments, both spatial and
temporal averaging are used. Figure 5.9 helps to illustrate the
different definitions of intensity. Although most commonly used
in relation to the safety of diagnostic ultrasound systems, they
are equally applicable to the output of therapeutic devices.
Spatial peak, pulse average intensity ( I SPPA ) describes the aver-
age intensity value over a single pulse, defined as the ratio of the
pulse intensity integral to the pulse duration, at the point in the
field at which the value is a maximum. If the pulse were exactly
5.5.2.3 power
As an ultrasonic field propagates, it exerts a radiation force ( F rad )
on the traversed medium through which it passes, as described
previously. The power in the beam is closely related to this quan-
tity and is expressed by
WFc
= rad
(5.26)
I SPTP
I SPPA
I SPTA
FIGURE 5.9 Intensity pulses (∝ pressure 2 ) showing the definitions of spatial peak, temporal peak (SPTP), spatial peak, pulse average (SPPA), and
spatial peak, temporal average (SPTA) intensity.
 
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