photon emission computed tomography (SPECT) studies. In this mode, a
matrix is chosen that approximates the entire area of the detector so that
a position (X,Y) in the detector corresponds to a pixel position in the
matrix. Digitized signals (X,Y) are stored in the corresponding (X,Y) posi-
tions (pixel) of the matrix of choice in the computer. Every time a new X,
Ysignal arrives, it is added to the (X,Y) pixel (Fig. 11.2A). In this mode,
one must specify the size and depth of the matrix, the number of frames
per study, and the time of collection of data per frame or total counts to be
collected. Data acquisition continues until a preselected time or total count
is reached. This mode provides instant images for storage and display.
In the list mode, digitized X- and Y-signals are coded with “time marks”
as they are received in sequence in time, and are stored as individual events
in the order they occur (Fig. 11.2B). After the data acquisition is completed,
the data can be sorted to form images in a variety of ways to suit a specific
need. Data can be manipulated by changing the matrix size and the time of
acquisition per frame. Also, physiologic markers, such as the start of a
cardiac cycle (e.g., the start of the R-wave) in the gated cardiac studies, can
be incorporated in the list mode acquisition. Since the data are listed
sequentially without overlapping each other, the bad signals from an
arrhythmic cardiac cycle can be discarded, as found appropriate, in the
Application of Computers in Nuclear Medicine 145ABFig. 11.2. Data acquisition in the frame mode and the list mode.