mounted on a stand. The head can be moved up or down and rotated with
electrical switches to position it in the field of view on the patient. The X-,
Y-positioning circuits, PHA, and some recording devices are mounted on a
console. In the past, the cameras were operated by switches and dials on
the console. Currently, much of the operation of the camera is performed
by a computer built in it. The computer is run by appropriate software in
conjunction with a keyboard, a mouse, and a video monitor. High voltage,
window, and photopeaks are all set by the operator’s choice of parameters.
Acquisition of the data and processing of the data are carried out by the
computer. Whereas stationary cameras are permanently installed at desired
locations, portable gamma cameras are mounted on wheels for use in situ-
ations requiring movement of the camera from room to room, such as to
the patient’s bedside. Mobile cameras are installed in wheeled vans such
that they can be moved to places where gamma cameras are not available
for nuclear medicine studies.
The operational principles of a gamma camera are identical to those of
solid scintillation counters described in Chapter 8. Basically,g-rays from a
source interact with the NaI(Tl) detector, and light photons are emitted.
The latter strike the photocathode of PM tubes, and a pulse is generated,
which is then amplified by an amplifier and sorted out by a PHA. Finally,
the pulse is positioned by an X-,Y-positioning circuit on the recording
device or stored in the computer, corresponding to the location of g-ray
interaction in the detector.
The functions of PM tubes, preamplifier, amplifier, PHA, and recording
devices are the same as described in Chapter 8, and therefore only essen-
tial features pertaining to the use of gamma cameras will be highlighted.
Detector
Detectors used in gamma cameras are typically circular NaI(Tl) detectors,
which have dimensions of 25–50 cm in diameter and 0.64–1.84 cm in thick-
ness. The most common thickness is 0.95 cm. The 0.64-cm thick detectors
are used in mobile gamma cameras and are useful for^201 Tl,99mTc, and^123 I
radionuclides. Larger detectors (>40 cm in diameter) are used in large field
of view (LFOV) cameras. Rectangular NaI(Tl) detectors (38.7 ×61 cm or
45 ×66 cm) are also available in gamma cameras.
Increasing the thickness of a detector increases the probability of com-
plete absorption of g-rays and hence the sensitivity (defined in Chapter 10)
of the camera. However, the probability of multiple Compton scattering
also increases in thicker detectors, and therefore the X,Y coordinates of
the point of g-ray interaction can be misplaced. This results in poor re-
solution of the image of the area of interest. For this reason, thin NaI(Tl)
detectors are used in gamma cameras, but this decreases the sensitivity of
the camera, because many g-rays may escape from the detector without
interaction.
110 9. Gamma Cameras