184 13. Positron Emission Tomography
photons arising from the interaction of g-rays in detectors to pulses, which
are used to determine the X-,Y-positions of the two detectors that detect
the two 511-keV photons (discussed later), and the PHA is used to check
if the pulse height is within the acceptable range, which is normally set at
350 keV to 650 keV for 511-keV annihilation photopeak in PET scanners.
PET Scanners
In the original PET scanners, many detectors (hundreds to thousands), each
connected to a photomultiplier tube, were arranged in multiple circular,
hexagonal, or orthogonal rings. The number of rings in current scanners
(18–32), and the number of detectors per ring vary with the manufacturer.
The number of rings and, hence, the width of the array of rings define the
axial field of view. Despite the advantage of good resolution with these
scanners, the use of many PM tubes and packing them with as many detec-
tors is very costly and impractical.
Block Detectors
In modern PET scanners, the block detectoris used, in which small detec-
tors are created by making partial cuts in a block of detector crystal and
each block detector is coupled to two to four PM tubes. A schematic block
detector is shown in Figure 13.1. Typically, each block is about 3-cm deep
and grooved into 6 ×8, 7 ×8, or 8 ×8 elements by partial cuts through the
crystal with a saw. The cuts are made at varying depths, with the deepest
cut at the edge of the block. The cuts are filled with opaque reflective mate-
rials to prevent spillover of light between elements. The size of each detec-
tor varies from 3 mm to 6.5 mm.
The block detectors are arranged in an array of full or partial rings with
a diameter of 80 to 90 cm. The full ring array may be in the form of a circu-
lar or hexagonal shape. Different arrangements of block detectors adopted
by manufacturers are shown in Figure 13.2. Siemens’ ECAT ACCEL has 24
rings and 64 detectors/block with a total of 9216 LSO detectors, coupled to
576 PM tubes. The ADVANCE Nxi of General Electric Healthcare has 18
rings and a total of 12,096 BGO detectors with 36 detectors/block, coupled
to 672 PM tubes. In the case of partial ring configurations, the blocks have
to be rotated around the patient to obtain 360° acquisition of data.
Coincidence Timing Window
In ideal coincidence counting in PET, the two 511-keV annihilation photons
should be detected by the two detectors exactly at the same time. In reality,
however, one photon may arrive at one detector earlier than the other in