Biomedical Engineering Reference
In-Depth Information
an ultrasound wave must be learned. Once the normal structure is recognized, the
diagnosis of pathology becomes apparent.
8.5.4.1 Detectors
The instrumentation of ultrasonic imaging consists of the transducer, a beam for-
mer, and a signal analysis and display component.
When a small handheld probe
is pressed against the skin, transducers located in the probe (Figure 8.11) convert
the electrical energy to acoustic energy, and vice versa. The conversion of electrical
pulses to mechanical vibrations and the conversion of returned mechanical vibra-
tions back into electrical energy is the basis for ultrasonic imaging. The position of
the probe and its anatomical location is important. As the probe is moved back and
forth over the skin, transducers produce sound waves at a known frequency based
on the input electrical signals. Since the ultrasound produced cannot travel through
the air and then into the body, the transducer must be in direct contact with the
skin, so a transmission jelly is used to insure a complete union. Ultrasound waves
are sent into the tissues and reflected signature waves are converted back to electri-
cal signals from which images are reconstructed. The signal processing involved in
forming images is based largely on linear models for sound transmission and reflec-
tion in tissue.
In order to get the best performance out of an ultrasound imaging system, the
transducer must not introduce distortion or have any negative influence on the
ultrasound signals. Sensitivity, pulse length, and beam width are the major pa-
rameters, which directly depend on the design and performance of the transducer.
The transducer is basically a piece of polarized material (i.e., some parts of the
molecule are positively charged, while other parts of the molecule are negatively
charged) with electrodes attached to two of its opposite faces. When an electric
field is applied across the material, the polarized molecules will align themselves
with the electric field, resulting in induced dipoles within the molecular or crystal
Figure 8.11
Dual element transducers contain two independently operated elements in a single
housing. One of the elements transmits and the other receives the ultrasonic signal.
