Edge Packing
Edge packing is seen around the edge of an image as a bright ring and
results in nonuniformity of the image. This results from the fact that more
light photons are reflected near the edge of the detector to the PM tube.
Normally a 5-cm wide lead ring is attached around the edge of the colli-
mator to mask this effect. In modern cameras, electronic masking is
employed.
Gamma Camera Tuning
Nonlinearity and pulse-height correction factors are generated with prop-
erly balanced PM tubes, and for them to work accurately, the PM tubes
must be maintained properly balanced or “tuned.” Changes can occur over
time in the gain of the PM tubes and other electronic circuits that can
invalidate these factors. Most modern cameras have circuits to monitor the
PM tube gains and automatically can make minor adjustments using uni-
formity corrections. Still they need routine maintenance of tuning monthly
or quarterly.
The methods of tuning of the gamma camera vary from manufacturer to
manufacturer. One method of tuning involves the use of a 99mTc source in
front of the camera and then obtaining the pulse-height spectrum for each
PM tube. If a photopeak shift in a PM tube is seen from the original value
in a PM tube, the preamplifier gain is adjusted. This technique was
employed in earlier models of the gamma camera.
Some manufacturers use a split-window technique for each PM tube with
two narrow tuning windows set on the high side of the photopeak to min-
imize the scattered radiation. The source of radiation can be either the radi-
ation from the patient or an external radioactive source, for example,99mTc.
Counts are collected in the two windows for each PM tube and the ratios
are calculated, which are stored in look-up tables and should be constant
over time for each well-tuned PM tube. Deviation of the ratio from the con-
stant value in subsequent patient studies indicates a drift in the gain of the
PM tube, which is then adjusted electronically.
In another method, once a week each PM tube of the camera is tuned
up with a radioactive source followed by illumination of the crystal with
a light-emitting diode (LED) attached to each PM tube. The LED is
pulsed at a high frequency and the response of each PM tube is recorded
as a reference value. Subsequently, the illumination of the detector is
carried out daily by the LED optical system, and the PM tube response
is measured and compared with the reference value. If there is any drift,
the gain is adjusted. Tuning should be performed separately for each
radionuclide.
Gamma Camera Tuning 131