Effects of photon attenuation on increased counts in fanbeam collimation

This study examined the effects of photon attenuation on the increased sensitivity (gain of counts) produced by a fanbeam collimator with imaging geometry similar to clinical practice.

The number of counts was measured in experiments and by computer simulations for two cylindrical phantoms of different sizes using a fanbeam and a parallel-hole (parallel-beam) collimator with the same spatial resolution. The phantoms were located either in air (simulating the brain) or in a cubic water tank (simulating a source in the body). The ratio of the counts from the fanbeam collimator versus the parallel-beam collimator was obtained as a function of the distance between the phantom and detector.

Even with attenuation inside the phantom, the count ratio for the phantom located in air remains almost the same as that theoretically predicted without attenuation. But the count ratio obtained in the cubic water tank was decreased compared to the predicted values. The amount of decrease (reduced sensitivity) depended on the distance from the detector, the size of the source and the depth of the source.

For brain studies, attenuation caused little change in the gain of counts provided by a fanbeam collimator. When imaging a structure located deep within the body, however, the gain of counts (sensitivity) may be appreciably reduced due to a longer path of photons within the body which results in more photon attenuation in fanbeam geometry.