Biomedical Engineering Reference
In-Depth Information
where c is the speed of sound in the medium. It is also described
by the integral of temporal average intensity over the beam area.
Each has its own advantages. A needle hydrophone has a small
active element at its very tip, which is oriented to point toward
the ultrasound transducer. Small needle diameters are desirable
to avoid excessive disturbance of the acoustic field during mea-
surement; however, reducing the element size results in a low-
ered sensitivity of the hydrophone. A membrane hydrophone is
typically comprised of a piezoelectric PVDF membrane with an
active element in the center, stretched over a circular frame and
polarized to allow the incident compressions and rarefactions to
register as a change in voltage across the element. Membranes are
typically positioned in the plane perpendicular to the direction
of ultrasound propagation and are large compared to the size of
the field, so that the outer frame securing the membrane does
not significantly disturb the acoustic field. The membrane is suf-
ficiently thin to be essentially transparent to the incident ultra-
sound beam. Membrane hydrophones cannot be implanted into
tissue, whereas this is possible with the needle geometry. More
recently, fiber-optic systems have been developed (Koch 1996,
Lim et al. 1999, Takahashi et al. 2000, Beard and Mills 1997,
Morris et al. 2009), typically with high sensitivity and narrow
(~ μm) sensitive tips, allowing high spatial resolution field plots
to be acquired. Due to their small geometry, these sensors also
lend themselves well to making pressure measurements in situ,
through insertion into a medium. This has been taken further
into an in vivo situation (Coleman et al. 1998), thus removing
the inaccuracies introduced by correcting for the attenuation
of sound as it passes through tissue. This process of derating is
discussed later.
5.5.3 Measurement Methods and
Quality assurance
With all medical ultrasound equipment, both diagnostic and
therapeutic, there is a requirement for a complete understand-
ing of the transducer output, not only for the manufacturers
but also the end user in order to ensure its consistency and
the safety of patients. Two different approaches to ultrasound
device characterization and acoustic measurement are possible.
The first requires a full understanding of the entire ultrasound
field. This involves lengthy measurements over large areas and
computationally intensive calculations and is commonly the
domain of manufacturers and physicists. The second is based on
the needs of the end user to make a regular and relatively quick
measurement to check consistency. Both fall under the umbrella
of quality assurance (QA). In the following, a brief discussion of
acoustic measurement equipment and techniques is given, with
reference to the appropriate standards for further information.
5.5.3.1 Hydrophone
Hydrophones are the key devices used in ultrasound measure-
ment. Their mode of action is based on the inverse piezoelec-
tric effect. The most commonly used hydrophones take the
form of membranes or needles, shown in Figures 5.10a and b,
respectively.
(a)
Active element
(b)
FIGURE 5.10
(a) Membrane and (b) needle hydrophones for measuring oscillating pressure waves in an acoustic field.
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