310
Biomedical Signal and Image Processing
major advantage for medical imaging. Furthermore, due to the portability of most of
ultrasound units, they can easily be used for various purposes in different locations.
Additionally, ultrasound imaging can provide important physiological data such
as flow magnitude and direction by applying the Doppler principle, as discussed
later in this chapter. A quantitative description of blood flow derived from ultrasound
measurements provides vital information to physicians about the local flow characteristics. The flow characteristics will provide indirect information on the tissue
metabolism and functionality. This unique capability of ultrasound imaging makes
it very suitable for cardiovascular measurements.
Some other unique properties of ultrasound can be better understood when it
is compared with x-ray. Ultrasound imaging has reasonable similarities to x-ray
imaging as far as the methodology is concerned. Both techniques rely on the
assumption of rectilinear propagation for the image formation purposes. Even
though ultrasonic waves undergo considerably more diffraction than x-ray, for
imaging purposes, the sound waves are often assumed to travel along a straight
line in the first-order approximation. On the other hand, ultrasound imaging has
considerable differences with x-ray imaging. Ultrasound, as mentioned earlier, has
no reported epidemiological side effects on biological tissues under the conditions
currently used for imaging. X-ray, on the other hand has been shown to have ionizing effects on biological tissues and can be mutagenic. In many medical diagnostic
procedures, such as monitoring of a fetus in the womb, the use of x-ray is strictly
prohibited due to the potential mutagenic effects that can permanently alter the
genetic makeup of the fetus.
Moreover, while electromagnetic waves only show a relatively negligible different
speed of propagation across most biological tissues, the speed of an ultrasound wave
is considerably different in different tissues. Informally speaking, wave spends more
time passing though one type of tissue than another. These rather small but detectable variations in the speed of sound provide the means to create detailed structural
information about the tissue. In addition, the speed of ultrasonic waves in soft tissue
is much less than the speed of electromagnetic waves (including x-ray and light),
i.e., the typical speed of sound in biological tissues is V = 1540 m/s, while the speed
of electromagnetic waves is C = 2.9979 × 10 8 m/s. The fact that the speed of sound is
relatively small is heavily used in the determination of the depth of an echo caused
by specific tissue inside the body, as explained later in this chapter.
16.3 GENERATION AND DETECTION OF ULTRASOUND WAVES
Ultrasound waves can be produced in two distinctly different modes of operation.
One of the mechanisms that can be used for ultrasound generation is magnetorestrictive based. A magnetic material can be made to change its shape under the influence
of an external magnetic field. When this magnetic field is a periodically changing
field with a constant period, the medium oscillates with the same identical frequency
as the driving magnetic field.
The second and most often used mechanism to generate ultrasound is using
piezoelectric materials. In piezoelectric ultrasound generation, a class of molecules with an unequal distribution of electric charges can be driven to oscillation
Biomedical Signal and Image Processing
major advantage for medical imaging. Furthermore, due to the portability of most of
ultrasound units, they can easily be used for various purposes in different locations.
Additionally, ultrasound imaging can provide important physiological data such
as flow magnitude and direction by applying the Doppler principle, as discussed
later in this chapter. A quantitative description of blood flow derived from ultrasound
measurements provides vital information to physicians about the local flow characteristics. The flow characteristics will provide indirect information on the tissue
metabolism and functionality. This unique capability of ultrasound imaging makes
it very suitable for cardiovascular measurements.
Some other unique properties of ultrasound can be better understood when it
is compared with x-ray. Ultrasound imaging has reasonable similarities to x-ray
imaging as far as the methodology is concerned. Both techniques rely on the
assumption of rectilinear propagation for the image formation purposes. Even
though ultrasonic waves undergo considerably more diffraction than x-ray, for
imaging purposes, the sound waves are often assumed to travel along a straight
line in the first-order approximation. On the other hand, ultrasound imaging has
considerable differences with x-ray imaging. Ultrasound, as mentioned earlier, has
no reported epidemiological side effects on biological tissues under the conditions
currently used for imaging. X-ray, on the other hand has been shown to have ionizing effects on biological tissues and can be mutagenic. In many medical diagnostic
procedures, such as monitoring of a fetus in the womb, the use of x-ray is strictly
prohibited due to the potential mutagenic effects that can permanently alter the
genetic makeup of the fetus.
Moreover, while electromagnetic waves only show a relatively negligible different
speed of propagation across most biological tissues, the speed of an ultrasound wave
is considerably different in different tissues. Informally speaking, wave spends more
time passing though one type of tissue than another. These rather small but detectable variations in the speed of sound provide the means to create detailed structural
information about the tissue. In addition, the speed of ultrasonic waves in soft tissue
is much less than the speed of electromagnetic waves (including x-ray and light),
i.e., the typical speed of sound in biological tissues is V = 1540 m/s, while the speed
of electromagnetic waves is C = 2.9979 × 10 8 m/s. The fact that the speed of sound is
relatively small is heavily used in the determination of the depth of an echo caused
by specific tissue inside the body, as explained later in this chapter.
16.3 GENERATION AND DETECTION OF ULTRASOUND WAVES
Ultrasound waves can be produced in two distinctly different modes of operation.
One of the mechanisms that can be used for ultrasound generation is magnetorestrictive based. A magnetic material can be made to change its shape under the influence
of an external magnetic field. When this magnetic field is a periodically changing
field with a constant period, the medium oscillates with the same identical frequency
as the driving magnetic field.
The second and most often used mechanism to generate ultrasound is using
piezoelectric materials. In piezoelectric ultrasound generation, a class of molecules with an unequal distribution of electric charges can be driven to oscillation