The detector head rotates around the long axis of the patient at small angle
increments (3° to 10°) for collection of data over 180° or 360°. The data are
collected in the form of pulses at each angular position and normally stored
in a 64 ×64 or 128 ×128 matrix in the computer for later reconstruction of
the images of the planes of interest. Note that the pulses are formed by the
PM tubes from the light photons produced by the interaction of g-ray
photons from the object, which are then amplified, verified by X,Yposi-
tion and PHA analyses, and finally stored. Transverse (short axis), sagittal
(vertical long axis), and coronal (horizontal long axis) images can be gen-
erated from the collected data. Multihead gamma cameras collect data in
several projections simultaneously and thus reduce the time of imaging. For
example, a three-head camera collects a set of data in about one third of
the time required by a single-head camera for 360° data acquisition.
Data Acquisition
The details of data collection and storage such as digitization of pulses, use
of frame mode or list mode, choice of matrix size, etc., have been given in
Chapter 11.
Data are acquired by rotating the detector head around the long axis of
the patient over 180° or 360°. Although 180° data collection is commonly
used (particularly in cardiac studies), 360° data acquisition is preferred by
some investigators, because it minimizes the effects of attenuation and vari-
ation of resolution with depth. In 180° acquisition using a dual-head camera
with heads mounted in opposition (i.e., 180°), only one detector head is
needed for data collection and the data acquired by the other head essen-
tially can be discarded. In some situations, the arithmetic mean (A 1 +A 2 )/2
or the geometric mean (A 1 ×A 2 )1/2of the counts, A 1 and A 2 , of the two heads
are calculated to correct for attenuation of photons in tissue. However, in
180° collection, a dual-head camera with heads mounted at 90° angles to
each other has the advantage of shortening the imaging time required to
sample 180° by half (Table 12.1). Dual-head cameras with heads mounted
at 90° or 180° angles to each other and triple-head cameras with heads ori-
ented at 120° to each other are commonly used for 360° data acquisition
and offer shorter imaging time than a one-head camera for this type of
angular sampling.
The sensitivity of a multihead system increases with the number of
heads depending on the orientation of the heads and whether 180° or 360°
acquisition is made. Table 12.1 illustrates the relationship among sensitiv-
ity, time of imaging, and acquisition arc (180° or 360°) for different camera
head configurations.
Older cameras were initially designed to rotate in circular orbits around
the body. Such cameras are satisfactory for SPECT imaging of symmetric
organs such as the brain, but because the body contour is not uniform, such
a circular orbit places the camera heads far from the other parts of the body
Single Photon Emission Computed Tomography 155