(9.3)
wherek is a constant andk/Z is the amplifier gain. TheX andY pulses are
then projected on a CRT to depict the X,Y coordinates of the point of g-
ray interaction, which in turn corresponds to the coordinates of the loca-
tion in the field of view from which the g-ray originated. Similarly, these
pulses can be stored in the computer in a square matrix so that the data
can be processed later to reproduce an image. Details of data acquisition
and storage in the computer are given in Chapter 11. Or, they can be pro-
jected on an x-ray film. Nowadays, resistors and microprocessors have been
used in place of capacitors.
The larger the number of PM tubes, the better the accuracy of the X,Y
locations of all pulses on the image; that is, the better the spatial resolution
of the image (see Chapter 10).
Pulse-Height Analyzer
After theZ pulses are formed by the summing circuit, the PHA analyzes
their amplitude and selects only those of desired energy by the use of appro-
priate peak and window settings. In many gamma cameras, the energy
selection is made automatically by push-button–type isotope selectors
designated for different radionuclides such as 99mTc,^131 I, and so on. In
modern cameras, isotope peak and window settings are selected by the
mouse-driven menu on a computer monitor interfaced with the camera. In
some gamma cameras, two or three PHAs are used to select simultaneously
two or three g-rays of different energies. These types of cameras are useful
in imaging with^111 In and^67 Ga that possess two or three predominant g-rays.
The window settings are provided in percentages of the peak energy by a
control knob. For most studies, a 15% to 20% window centered symmetri-
cally on the photopeak is employed.
It should be noted thatX andY pulses are accepted if theZ pulse is
within the energy range selected by the PHA. If theZ pulse is outside this
range, thenX andY pulses are discarded.
Display and Storage
In a typical nuclear medicine study, data are collected normally for preset
counts (e.g., 500,000 counts) or a preset time (e.g., 10 min). Until the mid-
90s, image data were captured on x-ray film or Polaroid film or stored on
magnetic tapes, laser disks, and the like. Nowadays, all camera systems use
computer memories for storage of image data. The details of storage in com-
puters are given in Chapter 11. In older systems, images were mostly dis-
played on cathode ray tube (CRT) monitors and at present, all systems
commonly use LCD (liquid crystal display) video monitors for better
Y
k
Z=−()YY+−
114 9. Gamma Cameras