the collimator material (e.g., lead), andt is the length or thickness of the
collimator hole. This corrects for the penetration of the two corners of the
holes by the photons.
As seen from Eq. (10.2), the collimator resolution is improved by increas-
ing the length,t, of the collimator holes or by decreasing the diameter,d,
of the holes. Thus, long narrow holes provide best spatial resolution. Also,
the collimator resolution deteriorates with increasing source-to-collimator
distance,b, and is best at the collimator face. Therefore, in nuclear medi-
cine studies, patients should be placed as close to the collimator as possi-
ble to provide the best resolution.
The thickness of lead,a, between holes is called the septum.Septal pen-
etration of g-rays plays an important role in the collimator resolution and
depends on the g-ray energy. High-energy photons from outside the field of
view can cross the septum and yet interact in the detector, thus blurring the
image. Because of this,g-rays of only ~50–300 keV are suitable for com-
120 10. Performance Parameters of Gamma Cameras
Fig. 10.1. A parallel-hole collimator with thickness t, hole diameter d, septal thick-
ness a, and source-to-collimator distance b. The collimator is attached to a detector
whose midplane is at a distancec from the back surface of the collimator.Rgis the
collimator resolution.