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

Purpose: 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. Methods: 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. Results: 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. Conclusion: 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.