Gated Study
The gated study was introduced in the mid-1970s to determine the ejection
fraction of the heart by acquiring two images, one at end diastole and the
other at end systole. It was later substituted by continuous acquisition of
data in multiple sequential images (multiple gated acquisition, MUGA) in
each cardiac cycle by gating between successive cycles.
In the MUGA study, the data are acquired in synchronization with the
R-wave of the cardiac cycle. The normal heart beat is about 1 beat/second,
and the R-R interval is therefore about 1 second, i.e., 1000 msec. First, the
R-R interval is divided into several segments or frames (16–32 segments)
depending on the number of frames one chooses to obtain. For example,
with a choice of 20 frames in the R-R interval, each frame will be 50 msec
long. In actual data collection, first the counts are acquired in frame 1 for
50 msec, followed by the collection of counts in frame 2 for another 50 msec,
and so on. After completion of counting in all 20 frames, a new R-wave is
detected, and the above sequence of counting continues until sufficient
counts have been accumulated in each frame. Assuming a count rate of
10,000 to 20,000 counts/s in a typical cardiac study, each 50 msec frame
would accumulate counts of the order of 500 to 1000. Normally, 64 ×64 or
larger matrices are used for the gated study.
The heart beat must be regular for the above method to work well. If the
heart beat is irregular such as in cardiac arrhythmia, the R-R interval is suf-
ficiently altered and the data become corrupted from R-wave to R-wave.
Modern acquisition programs have been devised to reject the bad heart
beat cycle. Using the list mode acquisition, bad heart beat data can be sorted
out and rejected in postacquisition reformatting.
SPECT is routinely used in nuclear medicine for various organ imaging,
particularly cardiac imaging. The gated SPECT study is also employed for
the cardiac studies using the typical 20 frames in each R-R interval.
Reconstruction of Images
In planar imaging, the acquired data are displayed in a two-dimensional
images without further processing. In tomographic imaging, data are
acquired in different angular projections around the patient. The data of
each projection are processed further using the methods described in
Chapter 12 to reconstruct the images at different depths of the patient’s
organ in 3-D directions. All reconstruction methods are accomplished by
the use of modern computers.
Superimposition and Subtraction of Images
It has been a common practice to superimpose image data from one modal-
ity onto another for better interpretation of the images. For example, com-
Application of Computers in Nuclear Medicine 147