Physics and Radiobiology of Nuclear Medicine

resenting four directional signals,X+,X−,Y+, and Y−. The capacitance values
are assigned in direct proportion to the location of the PM tube relative to
the four signals. Suppose a g-ray interacts at a location (*) near tube 7. The
largest amount of light is received by tube 7, and other tubes receive light
in proportion to their distances from the point of interaction. The output
signals of PM tubes are weighted by the appropriate capacitance values and
then summed to form each of the X+,X−,Y+, and Y−signals individually. In
this case,X−will be greater than X+, and Y+will be greater than Y−, because
the interaction occurred in the upper left quadrant. The X-,Y-designating
pulses,X and Y, and theZ pulse are then obtained as follows:

Z=X++X−+Y++Y− (9.1)

X (9.2)

k

Z

=−()XX+−

Gamma Cameras 113

Fig. 9.4. Arrangement of seven photomultiplier (PM) tubes to produceX and
Y pulses for the X,Y location of the g-ray interaction in the detector.X+,X−,Y+,
and Y−pulses are obtained by summing the output of all PM tubes weighted by
capacitors for the location of each PM tube in relation to the site of g-ray interac-
tion. (Adapted from Anger HO. Scintillation camera. Rev Sci Instr. 1958; 29:27.)