SU-E-I-42: Measurement of X-Ray Beam Width and Geometric Efficiency in MDCT Using Radiochromic Films

Area of Science:

• Medical Physics
• Radiological Imaging Science

Background:

• Multi-detector computed tomography (MDCT) scanners utilize 'over-beaming,' where the primary x-ray beam exceeds nominal collimation.
• This over-beaming increases patient radiation dose compared to single detector CT.
• Precise measurement of primary beam width and geometric efficiency (GE) is vital for accurate CT dose calculations using Monte Carlo or analytical methods.

Purpose of the Study:

• To measure the x-ray beam width and geometric efficiency (GE) of an MDCT scanner for various beam collimations.
• To assess the utility of radiochromic films for these measurements.
• To determine over-beaming factors for different collimations.

Main Methods:

• Radiochromic films (Gafchromic XR-QA2) were used to measure axial dose profiles in air for six different beam collimations.
• Measurements were performed on a Siemens SOMATOM Sensation 64 Scanner at 120 kVp.
• Films were calibrated against a PTW semiflex ionization chamber; geometric efficiency and full-width-at-half-maximum (FWHM) were determined.

Main Results:

• Geometric efficiency (GE) ranged from 58% to 85% for MDCT beam collimations from 7.2 mm to 28.8 mm.
• Over-beaming factors varied from 1.11 to 1.43.
• For smaller collimations (1x5 mm and 1x10 mm), GE was 77% and 84%, with over-beaming factors close to 1.

Conclusions:

• Radiochromic films provide accurate measurements of x-ray beam width and geometric efficiency due to their high spatial resolution.
• Both geometric efficiency and the over-beaming factor are strongly dependent on the selected beam collimation.
• These findings are essential for improving the accuracy of CT dose calculations.