Positron Emission Tomography
351
the PET images with other scanning techniques that will provide better anatomical details of the biological tissues. In this case, external markers are often needed.
These makers are used as tie points to register the two images with each other.
In Figure 17.8, a whole-body PET scan in 3-D rendering shows the anatomical
details that can be revealed by PET imaging; note, however, the unevenness in the
body surface as acquired by PET imaging.
17.7 PROCESSING AND FEATURE EXTRACTION OF PET IMAGES
Now that we are familiar with the physics of PET and its applications in biomedical
diagnostics, we briefly review the computational methods specialized for processing
of PET images.
17.7.1 SOURCES OF NOISE AND BLURRING IN PET
The fact that two gamma quanta need to be detected simultaneously and the gamma
quanta have distinct energy content makes the detection method very precise. There is
no significant noise in this energy spectrum, nor is there any background radiation that
fits the criteria for detection. Emission from outside the slice of interest is also negligible
since such an emission is never registered by both detectors in the plane of observation.
The uptake of FDG, and thus 18 F, is enhanced during any type of inflammatory process,
which can result in false-positive recordings. This reduces the overall accuracy of PET in
certain types of diagnostic applications.
While external sources of noise are practically nonexistent in PET, three main sources
of blurring contribute in the visible blurring of the PET image. The main source of misreading the location of the source of the positron emission is attributed to the fact that the
positron will travel some distance before it interacts with an electron and is annihilated.
The two photons that are emitted will be recorded as being displaced from the true
location of the isotope by less than 1 mm.
A second source of blurring roots in a 0.5° spread in both the positive and the negative arc from the 180° dual gamma quanta emission. This is another systematic cause of
blurring that typically cannot be overcome with the current state of the PET technology.
The third cause of blurring and loss of details is a machine artifact that is a direct
result of the spacing of the detectors in the tomographic ring. This last issue also
introduces an axial point-spread resulting from the width of the detector, or the
thickness of the ring of detectors. The pixel size is the total area of the detector.
While none of these blurring effects can be practically avoided, image processing
filters can be used to somehow sharpen the images before processing. All filtering
techniques described in Chapter 4, especially the high-boost filter, can be used to
address the blurring nature of the PET images.
17.7.2 IMAGE REGISTRATION WITH PET
Since the resolution of PET is very low, it is often required that before any specific image
classification decision can be made, the PET image must be registered and superimposed
by CT or MR images of the same entity. Combining x-ray tomography with PET is
351
the PET images with other scanning techniques that will provide better anatomical details of the biological tissues. In this case, external markers are often needed.
These makers are used as tie points to register the two images with each other.
In Figure 17.8, a whole-body PET scan in 3-D rendering shows the anatomical
details that can be revealed by PET imaging; note, however, the unevenness in the
body surface as acquired by PET imaging.
17.7 PROCESSING AND FEATURE EXTRACTION OF PET IMAGES
Now that we are familiar with the physics of PET and its applications in biomedical
diagnostics, we briefly review the computational methods specialized for processing
of PET images.
17.7.1 SOURCES OF NOISE AND BLURRING IN PET
The fact that two gamma quanta need to be detected simultaneously and the gamma
quanta have distinct energy content makes the detection method very precise. There is
no significant noise in this energy spectrum, nor is there any background radiation that
fits the criteria for detection. Emission from outside the slice of interest is also negligible
since such an emission is never registered by both detectors in the plane of observation.
The uptake of FDG, and thus 18 F, is enhanced during any type of inflammatory process,
which can result in false-positive recordings. This reduces the overall accuracy of PET in
certain types of diagnostic applications.
While external sources of noise are practically nonexistent in PET, three main sources
of blurring contribute in the visible blurring of the PET image. The main source of misreading the location of the source of the positron emission is attributed to the fact that the
positron will travel some distance before it interacts with an electron and is annihilated.
The two photons that are emitted will be recorded as being displaced from the true
location of the isotope by less than 1 mm.
A second source of blurring roots in a 0.5° spread in both the positive and the negative arc from the 180° dual gamma quanta emission. This is another systematic cause of
blurring that typically cannot be overcome with the current state of the PET technology.
The third cause of blurring and loss of details is a machine artifact that is a direct
result of the spacing of the detectors in the tomographic ring. This last issue also
introduces an axial point-spread resulting from the width of the detector, or the
thickness of the ring of detectors. The pixel size is the total area of the detector.
While none of these blurring effects can be practically avoided, image processing
filters can be used to somehow sharpen the images before processing. All filtering
techniques described in Chapter 4, especially the high-boost filter, can be used to
address the blurring nature of the PET images.
17.7.2 IMAGE REGISTRATION WITH PET
Since the resolution of PET is very low, it is often required that before any specific image
classification decision can be made, the PET image must be registered and superimposed
by CT or MR images of the same entity. Combining x-ray tomography with PET is