tion in spatial resolution. Manufacturers provide detailed methods of deter-
mining COR alignment for SPECT systems, which should be included in
the routine quality control procedure. Currently, manufacturers include the
COR alignment in their maintenance services.
Sampling
It is understood that the larger the number of projections (i.e., small angular
increment), the less is the star or streaking effect and hence the image
quality is better. However, this requires a longer acquisition time. Ideally,
for accurate reconstruction, the number of angular projections should be
at least equal to the size of the acquisition matrix (e.g., 64 angular projec-
tions for a 64 ×64 matrix or 128 projections for a 128 ×128 matrix). A fewer
number of projections may not erase the streaking effect. How many
angular projections should be taken over 180° or 360° to reconstruct
the images accurately depends on the spatial resolution of the camera.
As a general rule, 120 to 128 projections (using a 128 ×128 matrix) are
needed for large organs such as lungs and liver, whereas 60 to 64 projec-
tions (using a 64 ×64 matrix) are sufficient for smaller organs such as head
and heart. Typically, angular sampling at 6° intervals for 360° acquisition or
at 3° intervals for 180° acquisition are commonly used for most SPECT
systems.
Scattering
Radiations are scattered in patients, and the scattered photons, depending
on the energy and angle of scattering, may strike the detector. Scattering
may occur in the detector itself, and also within or outside the FOV. Nor-
mally, most of these scattered photons fall outside the photopeak window
and are rejected. However, a fraction whose photon energy falls within the
photopeak window will be counted, but their (X,Y) positions remain uncer-
tain causing degradation of the image resolution. In SPECT imaging, more
than 95% of the 140 keV photons of 99 mTc are scattered in the patient and
pose a serious problem. Scatter correction should be applied to improve the
spatial resolution of images.
There are a few methods of scatter correction, of which the most common
method is the use of two windows: a scatter window and a photopeak
window. The scatter window is set at a lower energy than the photopeak
window, and it is assumed that scatter in the photopeak window is the same
as that in the scatter window. The scatter counts in the scatter window
are subtracted from the photopeak counts for each projection to obtain
the scatter-corrected projections, which are then used for reconstruction.
The scatter spectrum is variable in energy; therefore, to have more accu-
rate scatter corrections, multiple scatter windows can be used. Scatter cor-
rections are made prior to attenuation correction, because the former are
amplified during the latter operation.
176 12. Single Photon Emission Computed Tomography