345
Positron Emission Tomography
Each detector will operate in multiple coincidence sampling mode, interacting with
multiple detectors at a range of angles. As discussed in Chapter 11, this method of
detector configuration is called fan-beam response.
Another mode of operation collects at perpendicular incidence only and is
called coincidence sampling in linear mode. In this configuration, the detectors
collect parallel to each other. The parallel mode of detection is illustrated in
Figure 17.5. Each mode of operation has its own advantages and disadvantages.
For example, the maximum error in location detection often occurs under fanbeam detection, while the crossover differences are relatively limited under parallel
ray detection.
Before discussing the significance of the PET modality in medical diagnostics,
we briefly review an alternative method of radioactive detection in PET called scintigraphy. In scintigraphy, instead of converting the gamma photons to visible light
detection, the high-energy gamma radiation is used to directly detect the gamma
photon. The gamma radiation excites a medium in a container with a window that is
transparent to gamma radiation. The excited medium (solid, liquid, or gas) degenerates back to its ground state and emits a photon in the visible spectrum in the process. The induced fluorescence is then used to induce a photoelectric effect in the
metal wall of the detector to produce a corresponding electric current. This method
constitutes an alternative to the CCD mentioned earlier.
β+
β+
β+
β+
β+
β+
e
–
e
–
e
–
e
–
e
–
e
–
θ 1
θ 2
r
FIGURE 17.5 Coincidence detection; schematic diagram of parallel beam detection.
Positron Emission Tomography
Each detector will operate in multiple coincidence sampling mode, interacting with
multiple detectors at a range of angles. As discussed in Chapter 11, this method of
detector configuration is called fan-beam response.
Another mode of operation collects at perpendicular incidence only and is
called coincidence sampling in linear mode. In this configuration, the detectors
collect parallel to each other. The parallel mode of detection is illustrated in
Figure 17.5. Each mode of operation has its own advantages and disadvantages.
For example, the maximum error in location detection often occurs under fanbeam detection, while the crossover differences are relatively limited under parallel
ray detection.
Before discussing the significance of the PET modality in medical diagnostics,
we briefly review an alternative method of radioactive detection in PET called scintigraphy. In scintigraphy, instead of converting the gamma photons to visible light
detection, the high-energy gamma radiation is used to directly detect the gamma
photon. The gamma radiation excites a medium in a container with a window that is
transparent to gamma radiation. The excited medium (solid, liquid, or gas) degenerates back to its ground state and emits a photon in the visible spectrum in the process. The induced fluorescence is then used to induce a photoelectric effect in the
metal wall of the detector to produce a corresponding electric current. This method
constitutes an alternative to the CCD mentioned earlier.
β+
β+
β+
β+
β+
β+
e
–
e
–
e
–
e
–
e
–
e
–
θ 1
θ 2
r
FIGURE 17.5 Coincidence detection; schematic diagram of parallel beam detection.