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Assessment of Measured, Simulated and Displayed Values of CTDI Volume

G Miller

G Miller1*, Y Hu2, C Ruan3, (1) Washington Hospital Center, Washington, DC, (2) University of Mississippi, Jackson, MS, (3) University of Oklahoma Health Science Center, Oklahoma City, OK

SU-E-I-50 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Computed tomography dose index (CTDI) is the most commonly used dose descriptor for imaging protocol optimization and patient dose estimation. The purpose of this research was to examine the CTDI Volume (CTDIVOL) variability by comparing measured values of CTDIVOL against simulated and machine displayed values.

Methods: A GE LightSpeed VCT scanner was used in this study. Measured values of CTDIVOL were determined using a 100 mm pencil ionization chamber in standard CT PMMA head and body phantoms. A Monte Carlo (MC) simulation of the CT scanner was developed using the MCNPX software package. The CT phantoms were modeled, and the resulting simulated dose was used to calculate CTDIVOL. The machine displayed values, which are based on patient scan protocols, were recorded directly from the CT scanner. The value presented by the scanner is actually the dose to air within a CT phantom which is about 12% higher than the dose to the phantom material. The machine displayed values have been corrected in order to make valid comparisons.

Results: The machine displayed values of CTDIVOL and the results from the MC simulation were compared against the measured values. The displayed values of CTDIVOL matched the measured values within 5-6% for both the head and body CT phantoms. The values from the MC simulation of CTDIVOL were 7-8% higher for the head phantom and 5-9% lower for the body phantom.

Conclusion: This study demonstrated the variations among the measured, simulated and displayed values of CTDIVOL. The displayed values were recently included in patient dose reports, so they should be calibrated periodically to avoid under or over estimating patient specific dose. The discrepancies between the simulated and measured values of CTDIVOL indicate the CT simulation model should be machine specific so it can be used for reliable patient organ dose evaluation.

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