window or adjusts the pulse height to compensate for the variations. This
is performed in real-time during the data collection in patient studies and
should be carried out for each radionuclide.
Nonlinearity
The spatial nonlinearities are systematic errors in the positioning of X-,
Y-coordinates of pulses in the image and result from local count compres-
sion or expansion. For example, when a radioactive source is moved across
from the edge to the center of a PM tube, more counts are found at the
center (pincushion distortion) giving a hot spot. Counts are less toward the
edges of the PM tubes, causing a cold spot (barrel distortion) in the area.
The spatial distortions due to nonlinearity are corrected by micro-
processors built into modern cameras that use correction matrices. Nonlin-
earity correction factors are generated by calculating the spatial shift of the
observed position of an event from its actual position. An intrinsic image
(without collimator) is taken using a line or orthogonal hole test pattern
and a source of radioactivity of interest placed at a distance five times the
detector’s FOV. The test pattern is placed directly on the detector, and an
image is stored in a 128 ×128 matrix. The actual location (X,Y) of each
pixel is known and the corresponding location on the image is measured.
These variations in X, Yfor all pixels are calculated as correction factors
and are normally supplied by the manufacturer in the form of a look-up
table. These correction factors are subsequently applied in real-time to each
detected event to move it to the actual position during patient imaging. This
is performed for all events throughout the field.
As stated above, modern cameras include two look-up matrices, the
pulse-height correction and the linearity correction, to compensate for vari-
ations in the overall uniformity of images. As the camera slowly drifts over
time, the correction tables have to be updated for proper correction of the
patient scan. The exact frequency of reacquiring the correction tables
depends on stability of the camera and varies with the manufacturer. The
pulse-height correction tables require more frequent acquisitions, whereas
the linearity correction tables are typically performed by a service engineer.
Different manufacturers recommend monthly to quarterly acquisition of
these correction factor maps.
It should be pointed out that uniformity corrections are mainly for minor
variations in PM tube response across the field of view and are not a
replacement for proper tuning of the gamma camera. Even though the uni-
formity corrections at times can correct for large nonuniformities, frequent
retuning of the gamma camera is essential as these corrections affect lin-
earity, resolution, and overall sensitivity of the camera. However, in modern
cameras, uniformity has improved considerably with better attachment of
the PM tubes to the detector and the use of PM tubes with higher quantum
efficiency.
130 10. Performance Parameters of Gamma Cameras