16
Ultrasound Imaging
16.1 INTRODUCTION AND OVERVIEW
Ultrasound is a method that uses sound waves to interact with tissues and act as the
energy source for image formation. The ultrasound interaction in this manner determines specific characteristics of the tissue response to sound waves. Sound waves
are mechanical, longitudinal waves that travel through matter. In longitudinal waves,
the motion of the mechanism that forms the wave (e.g., particles and molecules) is
parallel to the direction of wave propagation.
The motion of sound waves is in sharp contrast to electromagnetic waves (e.g.,
light, x-ray, and radio waves). In electromagnetic waves, the electric field and the
magnetic field that provide the wave mechanism are perpendicular to each other
and perpendicular to the direction of propagation. These kinds of waves are called
transverse waves. Unlike electromagnetic waves, sound cannot travel in a vacuum;
its energy is propagated by the motion of the particles in the medium that it is traveling through. Ultrasound waves are represented by pressure waves; compression and
expansion form the crests and valleys, respectively, in the wave description.
One way to classify sound waves is based on the frequency of the waves. Because
sound waves less than 20 Hz cannot be heard by humans, they are referred to as
infrasound waves. Audible sound waves are between 20 and 20,000 Hz, whereas any
sound waves above the limit of human hearing are called ultrasound. For diagnostic
ultrasound, frequencies ranging from 1 up to 100 MHz are routinely used.
In this chapter, we first describe the physics of ultrasound and the interaction of
ultrasonic waves with biological tissues and then introduce some of the main medical ultrasound technologies. We also discuss in detail the signal processing methodologies used to create and analyze medical ultrasound imaging technologies.
16.2 WHY ULTRASOUND IMAGING?
We can ask ourselves why ultrasound is such a popular and invaluable diagnostic tool in medical disciplines such as cardiology, obstetrics, gynecology, surgery,
pediatrics, radiology, and neurology. One of the main advantages of ultrasound is
that this technology is relatively inexpensive, mainly due to the relatively “low-tech”
equipment needed in this modality. Ultrasound imaging produces relatively highresolution images that rival other relatively common imaging modalities such as
x-ray imaging, plus it provides soft tissue information. The axial resolution is in the
order of millimeters, while the radial resolution depends on the beam diameter.
Moreover, unlike technologies such as x-ray where the applied energy is ionizing
and therefore harmful to biological tissues, the sound waves used in ultrasound are
harmless. Almost all ultrasound systems can produce images in real time, which is a
309
Ultrasound Imaging
16.1 INTRODUCTION AND OVERVIEW
Ultrasound is a method that uses sound waves to interact with tissues and act as the
energy source for image formation. The ultrasound interaction in this manner determines specific characteristics of the tissue response to sound waves. Sound waves
are mechanical, longitudinal waves that travel through matter. In longitudinal waves,
the motion of the mechanism that forms the wave (e.g., particles and molecules) is
parallel to the direction of wave propagation.
The motion of sound waves is in sharp contrast to electromagnetic waves (e.g.,
light, x-ray, and radio waves). In electromagnetic waves, the electric field and the
magnetic field that provide the wave mechanism are perpendicular to each other
and perpendicular to the direction of propagation. These kinds of waves are called
transverse waves. Unlike electromagnetic waves, sound cannot travel in a vacuum;
its energy is propagated by the motion of the particles in the medium that it is traveling through. Ultrasound waves are represented by pressure waves; compression and
expansion form the crests and valleys, respectively, in the wave description.
One way to classify sound waves is based on the frequency of the waves. Because
sound waves less than 20 Hz cannot be heard by humans, they are referred to as
infrasound waves. Audible sound waves are between 20 and 20,000 Hz, whereas any
sound waves above the limit of human hearing are called ultrasound. For diagnostic
ultrasound, frequencies ranging from 1 up to 100 MHz are routinely used.
In this chapter, we first describe the physics of ultrasound and the interaction of
ultrasonic waves with biological tissues and then introduce some of the main medical ultrasound technologies. We also discuss in detail the signal processing methodologies used to create and analyze medical ultrasound imaging technologies.
16.2 WHY ULTRASOUND IMAGING?
We can ask ourselves why ultrasound is such a popular and invaluable diagnostic tool in medical disciplines such as cardiology, obstetrics, gynecology, surgery,
pediatrics, radiology, and neurology. One of the main advantages of ultrasound is
that this technology is relatively inexpensive, mainly due to the relatively “low-tech”
equipment needed in this modality. Ultrasound imaging produces relatively highresolution images that rival other relatively common imaging modalities such as
x-ray imaging, plus it provides soft tissue information. The axial resolution is in the
order of millimeters, while the radial resolution depends on the beam diameter.
Moreover, unlike technologies such as x-ray where the applied energy is ionizing
and therefore harmful to biological tissues, the sound waves used in ultrasound are
harmless. Almost all ultrasound systems can produce images in real time, which is a
309