Physics and Radiobiology of Nuclear Medicine

Collimator Efficiency

Collimator efficiency, or geometric efficiency (Eg), is defined as the number
of g-ray photons passing through the collimator holes per unit activity
present in a source. For parallel-hole collimators (see Fig. 10.1), it is given
by

(10.7)

whered is the hole diameter,teis the effective length of the collimator hole
defined before, and ais the septal thickness. The constantK is a function
of the shape and arrangement of holes in the collimator and varies between
0.24 for round holes in a hexagonal array to 0.28 for square holes in a square
array.
The collimator efficiency for parallel-hole collimators increases with
increasing diameter of the collimator holes and decreases with increasing
collimator thickness (t) and septal thickness (a), which is quite opposite to

EK

d

tda

g

e

=⋅

()+

4

2 2

128 10. Performance Parameters of Gamma Cameras

Fig. 10.8. Variation of geometric efficiency with source-to-collimator distance for
various collimators. (A) High sensitivity parallel hole. (B) Converging. (C) All
purpose parallel hole. (D) Diverging. (E) High resolution parallel hole. (F) Pinhole.
(From Rollo FD, Harris CC. Factors affecting image formation. In: Rollo FD, ed.
Nuclear Medicine Physics. Instrumentation and Agents. St. Louis: Mosby; 1977:407.
Modified from Moyer RA. J Nucl Med 1974, 15:59.)