postacquisition reformatting. Although the list mode acquisition provides
wide flexibility, its major disadvantages are larger memory space and longer
processing time required and unavailability of images during or immedi-
ately after the completion of the study.
Static Study
A static study is the collection of data in one view of a region of interest in
an object for a preset time or preset total counts. Data are acquired in the
frame mode, and normally the matrix size is specified prior to starting the
study. The choice of a matrix size depends on the field of view of the imaging
system and the pixel size to give desired image resolution. For all practical
purposes, a pixel size of 2 to 3 mm is considered appropriate for good image
resolution. Thus, for large FOV scintillation cameras (>400 mm), one would
need a 256 ×256 matrix to obtain the above pixel size. Because of the high
count densities in static views, data acquisition in byte mode may overflow
in individual pixels and, therefore, the word mode is usually employed.
Digital images essentially represent the count density in regions of inter-
est in an object. How many counts should be acquired in an image? It
depends on how small a region in an image is to be identified and its appar-
ent contrast with the surrounding background. Large and high-contrast
objects are easily detectable at low count densities, whereas small and low-
contrast objects are difficult to delineate from the statistical noise. Count
density should be optimum for desirable contrast.
Dynamic Study
In dynamic studies, a series of images are collected and each image (frame)
is acquired over a certain period of time selected by the operator. While
the patient’s position cannot be changed during the image acquisition, the
matrix size and the frame rate (time of acquisition) can be changed. The
frame rate can vary from many frames per second to a single frame per
hour. The acquisition of image data is buffered such that while one frame
is being collected, the previous frame is stored in the external storage device
(e.g., disk). Data in dynamic studies can be collected in a sequence of several
phases, e.g., 1 frame per second for 1 minute, then 1 frame/minute for 5
minutes, followed by 1 frame/10 minutes for 2 hours. The choice of frame
rate for a given study depends on the kinetics of the radiotracer through
the organ of interest.
The common matrix size used in dynamic studies is 64 ×64 or 128 ×128,
although some loss of spatial resolution is expected with these matrices.
However, 256 ×256 or larger matrices require larger memory size. Since
counts collected per frame are low in number, the data are collected in byte
mode, which obviates the need for a large memory space, and normally does
not allow pixel counts to exceed 255 with little chance of counts overflow.
146 11. Digital Computers in Nuclear Medicine