While analog signals are continuous in time, digital signals consist of
a fixed number of bits produced by the ADC by sampling a selected
number of time points in the analog signal. ADCs are available as 8-, 10-,
12-, or 16-bit depending on the number of bits they produce in the digital
signal from the analog signal. While the analog signals can be distorted by
the electronic noise, there is some inherent loss of signal information as a
result of digitization, i.e., due to different time-point selections during the
analog-to-digital conversion. This arises from the fact that there is a likeli-
hood of a small fraction of the signal being lost during the conversion of a
continuous analog signal to discrete digital values. However, higher bit
ADCs minimize this loss by producing a large number of bits from each
analog signal. The faster ADCs can handle higher count rates. The slower
ADCs increase the dead time of the system and hence are good for low
count rates only.
Digital-to-Analog Conversion
For video displays, data must be in the analog form, and therefore digitized
data must be converted back to analog data. This is performed by units,
called the digital-to-analog converters (DACs), similar to the ADCs. DACs
are connected to the computer via video interface cards, and the speed of
digital-to-analog conversion depends on the speed of various electrical
components included in its operation.
Digital Images
Digital images are characterized by two quantities: matrix size and pixel
depth. The computer memory approximates the area of the detector in a
gamma camera as a square matrix of a definite size that can range from
32 ×32 to 1024 ×1024 with 1024 (1 K) to 1,048,576 (1 M) picture elements,
called pixels, respectively. The size of a matrix is selected by the operator,
depending on the type of task to be performed and is approximated to the
field of view (FOV). Each pixel corresponds to a specific location in the
detector. As discussed in Chapter 9, the X- and Y-pulses are obtained in
the analog form from the photomultiplier (PM) tube, which originate from
the interaction of g-rays in the detector. The X- and Y-analog pulses are
digitized by the ADC and stored in the appropriate pixel of the matrix. How
many counts can be stored in a pixel depends on the depth of the pixel,
which is represented by a byte or a word. Thus, a 1-byte pixel could record
up to 2^8 , or 256, events, whereas a 1-word pixel could store up to 2^16 ,or
65,536, events.
The pixel size, which depends on the choice of the matrix size for a study,
is an important factor that affects the spatial resolution of a digital image.
The field of view is approximated to the matrix size; therefore the pixel size
is calculated by dividing the FOV by the number of pixels across the matrix.
Basics of a Computer 143