Ultrasound Imaging
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by applying an external alternating electric field. As a result, the medium made up
of these molecules changes shape in unison at the rhythm of the alternating current
through the medium.
Certain commonly used materials for transducers are ceramic, barium titanate,
lead-zirconate-titanate, quartz, polyvinylidene difluoride (PVDF). The most commonly used ceramic material is lead-zirconate-titanate. These crystals are sandwiched
between two electrodes that are able to apply a voltage across the crystal. The crystals
will expand and contract in a sinusoidal fashion under an applied alternating current
and will produce sound waves at levels appropriate for diagnostic use with only milliwatts
of power. The mechanical wave generation devices are called transducers.
Acoustic waves will partially reflect of interfaces separating two media with
different acoustical properties, while the remaining fraction will proceed in the initial direction. Both the reflected and the transmitted signal can be used for imaging
purposes. In detecting ultrasound wave, when an acoustic pressure wave reaches a
piezoelectric crystal (potentially the same crystal that generated the acoustic wave),
the mechanically induced pressure changes the shape of the crystal, which in turn
produces a voltage across that crystal. This voltage is detected by the electrodes
attached to the crystal. At this point, the pressure signal is converted into a voltage
spike whose amplitude is proportional to the mechanical pressure. This analog signal is then converted to a digital signal by an analog to digital converter. The resulting digital signal is then analyzed by the algorithms that extract information such as
signal intensity and time interval to form an image of the tissue under study.
In order to better understand both modalities, we need to review some principles
of ultrasound physics.
16.4 PHYSICAL AND PHYSIOLOGICAL PRINCIPLES
OF ULTRASOUND
In order to better understand the sound wave propagation formation, first, we review
some fundamental physics concepts of the ultrasound phenomenon. The concepts
reviewed here include the mechanics of sound waves, the frequency of the sound,
the frequency content of the emitted and detected ultrasound signal, and the acoustic
impedance. We also review the mathematical formulation of acoustic wave propagation and penetration. Knowing these concepts and mathematical formulations of
these concepts is essential for understanding and implementation of the tomographic
signal and image processing methods.
16.4.1 FUNDAMENTAL ULTRASOUND CONCEPTS
In wave theory, the period of a wave, T, describes the time between two consecutive
repetitions of an identical pattern in the time domain. The wavelength of the sound
wave describes the spatial repetition of pattern sequence. The wavelength λ is coupled
to the period of the wave as follows:
l = VT
(16.1)
311
by applying an external alternating electric field. As a result, the medium made up
of these molecules changes shape in unison at the rhythm of the alternating current
through the medium.
Certain commonly used materials for transducers are ceramic, barium titanate,
lead-zirconate-titanate, quartz, polyvinylidene difluoride (PVDF). The most commonly used ceramic material is lead-zirconate-titanate. These crystals are sandwiched
between two electrodes that are able to apply a voltage across the crystal. The crystals
will expand and contract in a sinusoidal fashion under an applied alternating current
and will produce sound waves at levels appropriate for diagnostic use with only milliwatts
of power. The mechanical wave generation devices are called transducers.
Acoustic waves will partially reflect of interfaces separating two media with
different acoustical properties, while the remaining fraction will proceed in the initial direction. Both the reflected and the transmitted signal can be used for imaging
purposes. In detecting ultrasound wave, when an acoustic pressure wave reaches a
piezoelectric crystal (potentially the same crystal that generated the acoustic wave),
the mechanically induced pressure changes the shape of the crystal, which in turn
produces a voltage across that crystal. This voltage is detected by the electrodes
attached to the crystal. At this point, the pressure signal is converted into a voltage
spike whose amplitude is proportional to the mechanical pressure. This analog signal is then converted to a digital signal by an analog to digital converter. The resulting digital signal is then analyzed by the algorithms that extract information such as
signal intensity and time interval to form an image of the tissue under study.
In order to better understand both modalities, we need to review some principles
of ultrasound physics.
16.4 PHYSICAL AND PHYSIOLOGICAL PRINCIPLES
OF ULTRASOUND
In order to better understand the sound wave propagation formation, first, we review
some fundamental physics concepts of the ultrasound phenomenon. The concepts
reviewed here include the mechanics of sound waves, the frequency of the sound,
the frequency content of the emitted and detected ultrasound signal, and the acoustic
impedance. We also review the mathematical formulation of acoustic wave propagation and penetration. Knowing these concepts and mathematical formulations of
these concepts is essential for understanding and implementation of the tomographic
signal and image processing methods.
16.4.1 FUNDAMENTAL ULTRASOUND CONCEPTS
In wave theory, the period of a wave, T, describes the time between two consecutive
repetitions of an identical pattern in the time domain. The wavelength of the sound
wave describes the spatial repetition of pattern sequence. The wavelength λ is coupled
to the period of the wave as follows:
l = VT
(16.1)