Scatter Coincidences
Annihilation radiations may undergo Compton scattering while passing
through the body tissue and, due to high energy of 511 keV, they are mostly
scattered forward without much loss of energy (Hoffman and Phelps, 1986).
Such scattering may also occur in the detector material itself. These scat-
tered photons may fall within the coincidence time window (Fig. 13.8C) and
be detected by the detector pair. One or both of the 511 keV photons from
the same annihilation event may be scattered. Note that coincident counts
of scattered photons from two separate annihilation events will be consid-
ered as random counts. Background of the image is increased by these radi-
ations with concomitant loss of image contrast. Scattering increases with
the density and depth of tissue, the density of the detector material, the
activity, and the pulse-height window.
Narrowing the pulse-height window reduces the scattered events signifi-
cantly. The use of septa in 2-D acquisition reduces the scatter events con-
siderably, but in 3-D acquisition, this becomes a problem. In modern PET
scanners, the scatter fraction is around 15% in 2-D acquisition, whereas it
can be as high as 40% in 3-D acquisition.
The practical methods of scatter correction in PET are essentially similar
to that in SPECT described earlier.
Dead Time
The effects of dead time and pulse pile-up have been discussed in Chapter
- The effects of high-count rates on the performance of gamma cameras
have been discussed in Chapter 10. These effects equally apply to the count-
ing of 511-keV annihilation photons in PET imaging. The correction for
dead time loss is made by measuring the observed count rates as a function
of increasing concentrations of activity. The dead time is calculated from
these data and then applied to actual data obtained in the patient’s scan-
ning. Uses of high-speed electronics, buffers, and pulse pile-up rejection cir-
cuits are some of the techniques that are employed to improve dead time
loss.
Radial Elongation
Radial elongation, also called the parallax error or radial astigmatism,
occurs from LORs that are off-centered. As shown in Figure 13.11, the two
511-keV photons originating along the actual LOR (solid line) may strike
the detectors tangentially at the back of the detector and form a coincident
event. But the X,Ypositioning of the detectors is defined by the dashed
line some distance daway from the actual LOR. This effect results in some
blurring of the image due to unknown depth of interactions, and worsens
with the LORs farther away from the center of the FOV and with a thicker
200 13. Positron Emission Tomography