# Measurement of CT radiation beam width with a pencil ionization chamber and radiopaque mask

## TL;DR

This study shows that a new method using an ionization chamber and radiopaque mask can accurately measure CT scanner beam widths.

## Contribution

The study introduces a new ionization chamber-radiopaque mask technique for measuring CT beam widths with high accuracy.

## Key findings

- The ionization chamber-mask method had an RMS difference of 0.31 mm compared to film measurements.
- The method showed an RMS difference of 0.28 mm compared to computed radiography measurements.
- The technique is accurate for beam widths ranging from 1 to 40 mm.

## Abstract

This study compares fan beam CT scanner radiation beam widths measured with a pencil ionization chamber‐radiopaque mask technique with commonly used film and computed radiography (CR) measurements.

For a given fan beam CT scanner x‐ray beam, the ionization chamber‐mask technique determines the radiation beam width by exposing a 100 mm pencil chamber with and without a radiopaque cylindrical mask of known width that is a fraction of the nominal beam width. Additional widths can then be measured using the same kV, mAs, and pre‐patient filtration without the mask. CT scanner radiation beam width measurements with the technique were compared with film and CR techniques.

Measurements from 20 different detector configurations/focal spot combinations on fan beam CT scanners from two manufacturers are presented. The root mean square (RMS) difference between the ionization chamber‐mask measured beam widths and film measurements was 0.31 mm, with a similar RMS difference of 0.28 mm with CR measurements. These results compare favorably with the RMS difference between film and CR measurements, which was 0.35 mm.

This study demonstrates that radiation beam widths of fan beam CT scanners can be measured using the ionization chamber‐radiopaque mask method with an RMS accuracy of better than 0.5 mm. We demonstrate the method is applicable to nominal beam widths ranging from 1 to 40 mm.