SU-E-I-49: Influence of Scanner Output Measurement Technique on KERMA Ratios in CT

Area of Science:

• Medical Physics
• Radiological Imaging
• Dosimetry

Background:

• KERMA ratios (RK) are crucial for estimating organ doses in CT scans.
• Accurate measurement of in-center CT scanner output (KCT) is essential for calculating RK.
• Variations in KCT measurement techniques can significantly affect dose calculations.

Purpose of the Study:

• To investigate the impact of different measurement methodologies on CT scanner output (KCT) values.
• To determine how extended z-axis measurements and scanning techniques influence KCT.
• To highlight the importance of consistent KCT measurement for accurate organ dose estimation.

Main Methods:

• Optical Stimulated Luminescence (OSL) dosimeters were used to measure KCT on a GE VCT scanner.
• Methodologies included single-point measurements, extended z-axis measurements (single rotation, contiguous axial, helical scans).
• Measurements were performed in air and on the scan table at 80 kV and 120 kV.

Main Results:

• Single-point and single-rotation extended measurements showed good consistency (CV 2.5%).
• Multiple contiguous axial scans resulted in higher KCT values (10.3-14.9%) due to z-axis over-beaming.
• Helical scan KCT values, corrected for pitch, were comparable to axial scan results. In-air measurements consistently exceeded on-table measurements (19.4-23%).

Conclusions:

• CT scanner KCT values must be measured using a defined methodology.
• It is imperative that KCT measurement techniques match those used for published KERMA ratio (RK) values.
• Inconsistent KCT measurement methodologies can introduce substantial errors into organ dose estimates.