Half Value Layers

The many years of x-ray equipment testing I’ve done has generated a lot of potentially useful data on the performance of imaging equipment that I lay hands on.

In the past, I’ve just measured the half value layer (the thickness of material required to reduce the x-ray beam intensity by one half) of the x-ray beam at one point: 80 kV_p. A couple years ago, I decided to add 60, 100 and 120 kV_p to satisfy my curiosity. I knew from testing mammography machines that HVL was pretty linear between a narrow range of kV (24-32). Turns out over the much wider range of 60-120 kV_p, HVL is still pretty linear. This is close to 3 years worth of HVL data from almost 40 radiographic units (fixed and portable).

Below is the mean HVL as a function of kV. Error bars represent the standard deviation at each kV. The linear fit equation is

HVL = -0.0254 + 0.03868*kV

The histograms below show the spread in HVLs measured at each kV.

There doesn’t seem to be any significant trend over time (which is what I would expect). Here I’m using the ID assigned to each survey as a surrogate for time.

A moderately interesting finding I think. One thing that might be interesting to see is how HVL changes with the type of machine (fixed room vs portable). I’d have to go back to my spreadsheets and recollect the data to add in the type of machine though. Not sure I’m curious enough about it to go through all that again, but perhaps.