Ultrasound Imaging
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dimension will result in a higher divergence of the beam. The divergence of the
beam causes greater problems than the initial size of the beam as the returning beam
will continue to expand due to diffraction and scattering. Such a broad beam will
excite a very large number of transducers upon returning to the ultrasound probe.
The lateral resolution will then be lost entirely. In some practical systems, in order
to improve the resolution, instead of a single large transducer, an array of medium
size transducers is used.
Now we are ready to discuss the main tomographic ultrasound imaging methods
used in medical applications.
16.6 ULTRASOUND IMAGING MODALITIES
Three main types of ultrasound imaging can be distinguished: attenuation tomography,
reflection tomography, and time-of-flight (TOF) tomography.
One major medical ultrasound imaging system, reflection tomography, relies on
the fact that sound waves will be reflected from the border of two different tissues.
In reflection tomography, detecting and analyzing the echoes that are reflected or
scattered from the different tissues in the biological medium creates an image of the
tissue. The intensity of each echo is related to the difference in the acoustic impedances of the respective tissues at that specific interface.
In early ultrasound systems, the transducer would determine whether or not there
was an echo, but was not accurate in measuring the intensity of the echoes. These
early brightness mode (B-mode) devices could tell that there were interfaces and would
display a bright spot on the image that would correspond with that interface. New
ultrasound systems can accurately determine the intensity of each echo and therefore
determine the characteristics of the interface that has reflected that ultrasound wave.
The imaging systems then translate these tissue characteristic specifications into gray
scales that can be used in rendering the image. Informally speaking, an intense echo is
going to be received by the transducer if there is a large difference in acoustic impedance; therefore, the image for this interface will be brighter than the image of an
interface with a lesser difference in acoustic impedance. This technique allows for
not only the location of structures but also the characterization of certain material
properties of these structures. Since the acoustic impedance is proportional to both
the density and the elastic modulus of the medium, these parameters can be derived
through inverse solution of the acquired signals.
Attenuation tomography, as another ultrasound imaging modality, uses the fact
that the transmitted sound waves will be attenuated to different degrees as they pass
through tissues with different properties. This is essentially the same principle used
in x-ray attenuation tomography. In attenuation tomography, the attenuation of the
sound waves passing through the tissues is utilized to form an image.
The formulation of the TOF tomography is very much similar to those of attenuation and reflection tomography and will be discussed coupled with attenuation
tomography. The principle idea of the TOF tomography is based on the observation
that the time it takes the sound wave to travel through different tissues varies from
one tissue to another.
319
dimension will result in a higher divergence of the beam. The divergence of the
beam causes greater problems than the initial size of the beam as the returning beam
will continue to expand due to diffraction and scattering. Such a broad beam will
excite a very large number of transducers upon returning to the ultrasound probe.
The lateral resolution will then be lost entirely. In some practical systems, in order
to improve the resolution, instead of a single large transducer, an array of medium
size transducers is used.
Now we are ready to discuss the main tomographic ultrasound imaging methods
used in medical applications.
16.6 ULTRASOUND IMAGING MODALITIES
Three main types of ultrasound imaging can be distinguished: attenuation tomography,
reflection tomography, and time-of-flight (TOF) tomography.
One major medical ultrasound imaging system, reflection tomography, relies on
the fact that sound waves will be reflected from the border of two different tissues.
In reflection tomography, detecting and analyzing the echoes that are reflected or
scattered from the different tissues in the biological medium creates an image of the
tissue. The intensity of each echo is related to the difference in the acoustic impedances of the respective tissues at that specific interface.
In early ultrasound systems, the transducer would determine whether or not there
was an echo, but was not accurate in measuring the intensity of the echoes. These
early brightness mode (B-mode) devices could tell that there were interfaces and would
display a bright spot on the image that would correspond with that interface. New
ultrasound systems can accurately determine the intensity of each echo and therefore
determine the characteristics of the interface that has reflected that ultrasound wave.
The imaging systems then translate these tissue characteristic specifications into gray
scales that can be used in rendering the image. Informally speaking, an intense echo is
going to be received by the transducer if there is a large difference in acoustic impedance; therefore, the image for this interface will be brighter than the image of an
interface with a lesser difference in acoustic impedance. This technique allows for
not only the location of structures but also the characterization of certain material
properties of these structures. Since the acoustic impedance is proportional to both
the density and the elastic modulus of the medium, these parameters can be derived
through inverse solution of the acquired signals.
Attenuation tomography, as another ultrasound imaging modality, uses the fact
that the transmitted sound waves will be attenuated to different degrees as they pass
through tissues with different properties. This is essentially the same principle used
in x-ray attenuation tomography. In attenuation tomography, the attenuation of the
sound waves passing through the tissues is utilized to form an image.
The formulation of the TOF tomography is very much similar to those of attenuation and reflection tomography and will be discussed coupled with attenuation
tomography. The principle idea of the TOF tomography is based on the observation
that the time it takes the sound wave to travel through different tissues varies from
one tissue to another.