Collimation

Collimation, or specifically collimation refers to beam size alterations. Gamma ray collimation employs a brute force method of beam size reduction, namely; lead in the way of the beam (at least part of it).

I have noted that gamma ray collimation is a variable in all three dimensions: x, y and z. Z, in this case, is determined by thickness of the absorbing material, typically lead, while X and Y are basically designed as beam stoppers.

A few texts suggest that beam stopping should be in the vicinity of 99% efficient. By this it is meant that virtually all gamma rays employ the absorber. This is quite impossible to achieve regardless of expectations and or specifications. Gamma ray absorption is a probability factor and not an absorption value. Typically, a useful unit of measure is the half value layer. This means that the gamma ray beam at 100 kev will be reduced to half at 0.12 mm of lead. For purposes of these assumptions we will consider our typical gamma ray energy to approximate 100 kev.

At 100 kev the beam thickness in lead is 0.12 mm. This means that 50% of the beam has been absorbed. That also means that 50% of the beam passes straight on through.

0.12mm lead reduces the beam to 50% value, therefore, an additional 50% is reduced by an additional 0.12 mm lead. The original beam of X-rays is now down to 25% beam strength.

What this means is that all body parts outside of the collimated field will receive a radiation dose equal to 25% of the raw beam. That seems to be a somewhat careless determination.

By measurement of transmitted beam it has been determined that some collimators in clinical use produce one half layer of beam absorption or deliver a radiation dose at half that of raw beam. Others that have been tested reduce that be another factor of two with 25% of original beam intensity passing through the patient.

I suggest that collimation should be improved to reduce the incident beam strength to approximately 12.5%. Beyond that it seems unnecessary to increase absorption.

I intend to look into collimation improvements to that 12.5% level.