Hybrid Gamma Cameras
Conventional dual-head and triple-head gamma cameras (Fig. 12.2) can be
utilized as PET cameras by connecting the appropriate heads with a coin-
cidence circuitry. The typical time window is ~12 ns for dual-head and
~10 ns for triple-head cameras. In SPECT mode, the cameras are used with
collimators, whereas in PET mode, collimators are removed, and there-
fore they can be used in either mode as needed. These cameras are called
hybrid gamma cameras and are very attractive to community hospitals
because of their low cost with the scope of PET imaging.
The hybrid cameras suffer from a disadvantage of low sensitivity due to
low detection efficiency of NaI(Tl) crystal for 511-keV photons. To improve
sensitivity, thicker crystals of sizes 1.6 cm to 2.5 cm have been employed in
some cameras, with a resultant increase in coincidence photopeak efficiency
of only 3 to 4%, with concomitant degradation of spatial resolution. There
is a significant camera dead time loss and pulse pile-up of counts in PET
mode in the absence of a collimator. Overall, the spatial resolution of a
hybrid camera is poorer than that of a dedicated PET scanner.
Data Acquisition
In PET imaging, two 511-keV annihilation photons are detected in coinci-
dence by two opposite detectors along a straight line, called the line of
response(LOR). In a full ring system, data are collected in 360° simulta-
neously, whereas in the partial ring system, the rings are rotated around the
patient for 360° data acquisition. There are three steps in PET data acqui-
sition. First, the location of the detector pair in the ring is determined for
each coincident event. Next, the pulses are analyzed by PHA to check if
they are within the energy window set for 511 keV. Finally, the position of
the LOR is determined in polar coordinates to store the data in computer
memory.
Because each detector is connected to many opposite detectors in coin-
cidence, which detector pair detected a coincidence event must be deter-
mined. As in gamma cameras, the position X, Yof each detector in the ring
is determined by
(13.1)
(13.2)
where A, B, C, D are the pulses from the four PM tubes attached to the
block, as shown in Figure 13.1.
Y
AD BC
ABC D
=
()+ −+()
+++
X
CD AB
ABC D
=
()+ −+()
+++
Data Acquisition 191