using a low-energy high-resolution collimator. If the peak shift is more than
10%, the camera must be tuned. Tuning is performed by the computer
program by repeaking of the camera with a 99mTc source placed at least
30 cm away from the detector and without a collimator (intrinsic method).
The same method is applied for other radionuclides.
Uniformity
The uniformity of the detector response is checked daily by using a^57 Co
flood source. The flood source is placed on the detector with a low-energy
high-resolution collimator attached (extrinsic) and an energy window of
20% is used. An image is acquired containing about 1 million counts (Fig.
10.9) and then assessed for uniformity, linearity (visibility of tube pattern),
integrity of PM tubes (energy calibration), artifacts, and so on by visual
inspection. For SPECT camera heads, 3–5 million counts are acquired.
Nonuniformity exceeding 5 to 10% is detectable by human eyes. Visual
inspection of the image should reveal any nonuniformity that warrants the
tuning of the camera, particularly the PM tubes.
According to the NEMA protocol, the intrinsic method is used with a
100 to 200mCi (3.7 to 7.4 MBq) 99mTc source, and two parameters are eval-
uated for uniformity, which are defined as follows.
where Cmaxand Cminare the maximum and minimum count rates across the
field of view in a nine-point smoothed image.
integral uniformity=−
+
×
CC
CC
max min
max min100
Quality Control Tests for Gamma Cameras 135(a) (b)Fig. 10.9. Images of a^57 Co flood source showing the uniformity (a) and nonunifor-
mity (b) of the response of a gamma camera.