link to lecture transcript - UT-H GSBS Medical Physics Class Site

What about the dependence of the Compton coefficient on atomic number Z? The model we are using is a free
electron gas. We are looking at the probability of an interaction per electron. We are not talking about the
nucleus. Nor are we talking about the size of the nucleus or the number of electrons in the atom. Consequently,
the electronic coefficient for Compton scatter is essentially independent of Z, and, as we have seen previously, it
is also roughly independent of photon energy.
So what is it dependent on? About the only quantity the linear attenuation coefficient is going to be dependent
on is density. The mass attenuation coefficient is going to be essentially independent of everything.
Consequently, when Compton scatter is the predominant interaction, the linear attenuation is going to be linearly
dependent on the density.
AAnother th way of f saying i this thi is i “Equal “E l masses attenuate tt t equal l amounts.” t ” That’s Th t’ actually t ll a very good d thing; thi and d
the reason why that’s good results from the fact that radiation dose is defined to be energy absorbed per unit
mass. If we look at the absorption of energy in one gram of bone versus one gram of soft tissue with the same
incident photon fluence, we find that one gram of bone absorbs approximately the same amount of energy as one
gram of soft tissue. Equal masses absorb equal amounts.
How is that different from a situation in which the photoelectric effect is the predominant interaction?
Remember that for the photoelectric effect, absorption is going to be very strongly dependent upon atomic
number. So high-Z materials absorb a greater fraction of photons than do low-Z materials. This difference in
absorption gives rise to contrast in diagnostic imaging and an enhancement of dose to bone in low-energy
radiation therapy therapy. This was a problem in the pre-megavoltage pre megavoltage era when we were treating patients with 100 kVp
or 250 kVp x-rays. Because there was so much photoelectric interaction, for a given amount of radiation, bone
would receive a much higher dose than soft tissue.
When we’re in the Compton regime, bone receives the same dose as soft tissue, and that’s good.
Now, from the point of view of imaging, Compton interaction is not good because we don’t have as much
contrast. Recall that equal masses absorb equal amounts, so that because bone is a little denser than soft tissue,
we get a little bit of contrast between bone and soft tissue.
From the point of view of shielding, the Compton interaction is very good because equal masses absorb equal
amounts. So an equal mass of lead is as good a shield as an equal mass of concrete. Now if you were a builder
what would you rather work with, lead or concrete? Concrete is a standard building material; builders know
how to pour concrete. They need to make sure there aren’t any air bubbles or anything like that. It is easy to
pour four feet of concrete. Consequently when we shield a radiation therapy facility we are going to be looking
at conventional building materials to do our shielding; we just use thick walls. Several inches of lead shielding
may be just as effective a shield as a few feet of concrete, but the lead is much harder to work with and builders
don’t like to use it.
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