display of images. The computer manipulation of image contrast on LCD
monitors provides a better view of images leading to more accurate diag-
nosis of diseases. The details of display and storage are given in Chapter 11.
Digital Cameras
It is seen from the above description that the X- and Y-pulses are obtained
in analog form and are projected on different display and recording systems.
Such analog processing inherently includes instability in pulse formation
and results in image nonlinearity and nonuniformity. These are caused by
fluctuations in PM tube output due to high voltage (HV) variations, drift in
preamplifier output, and variations in PH and X-,Y-positioning analyses.
To correct for these effects and also for the manipulation of data at a later
time, analog data are digitized to be stored in a matrix map in a computer.
Digitization of the analog signal is performed by an electronic circuit,
called the analog-to-digital converter (ADC). The digitized data are later
retrieved for further processing to display on video monitors.
In modern cameras each PM tube output is digitized by the ADC before
PH and X-,Y-positioning analyses. These cameras are called “all-digital”
cameras. In these cameras, the gains of all PM tubes are initially optimized
by placing a narrow beam of a radioactive source in front of each PM tube
and determining the center of the photopeak by adjusting the high voltage
of the PM tube with a digital computer. Next, the camera is calibrated, in
which a source of interest is positioned in front of each PM tube, and output
from each PM tube is sampled, integrated, and digitized by a high-speed
ADC in the computer. Each signal is then normalized by dividing it with
the sum of all digital signals arising from the same scintillation event. In a
two-dimensional array of PM tubes, the normalized digital output Zi(X,Y)
corresponds to the X,Y location of the PM tube i. To determine the loca-
tion of each signal Zi, a weighting factor is calculated from the inverse of
the uncertainties ofX andY positions, i.e., 1/DXand 1/DY, that are related
to the spatial distribution of Zivalues around the center of the PM tube.
The X,Ylocations and weighting factors are mapped and stored in refer-
ence tables as functions of Zivalues for all PM tubes for positional and
Z-pulse analyses of a scintillation event in later imaging studies.
In subsequent patient imaging studies, the output signal of each PM tube
from a scintillation event is sampled, integrated, digitized and finally nor-
malized to give Zi. The location (X,Y) of the scintillation event is then cal-
culated by using the appropriate values of locations and weighting factors
in the reference tables in the memory. The digitized Zi(X,Y) is stored in
the X,Y location of the image matrix, if the pulse discrimination does not
reject the signal. Since the location of each event is determined by digitiz-
ing and analyzing the individual signal from each PM tube, the accuracy of
positioning of the signals is greatly improved. For these reasons, the digital
Digital Cameras 115