Technical note using linear and polynomial approximations to correct IEC CTDI measurements for a wide-beam CT Scanner.
Victor J. Weir, Ph.D.
Weir, V. J. and J. Zhang (2019). “Technical note using linear and polynomial approximations to correct IEC CTDI measurements for a wide-beam CT Scanner.” Med Phys Sep 4. [Epub ahead of print].
PURPOSE: Investigate a feasible correction to align the IEC CTDI measurement with other approaches for an accurate measure of radiation output. METHODS: Radiation dose measurements were performed in a GE 256-slice CT scanner using three methods. The first method used a 0.6cc Farmer chamber to measure peak dose and then to calculate dose length integral (DLI). The second method integrated dose profiles with a pencil chamber over 600mm for both body and head phantoms. Both methods achieved scatter equilibrium using a 600mm long body and head phantom. The third method followed IEC recommendations by adjusting traditional CTDI with beam width. We performed measurements using polymethyl methacrylate (PMMA) 32cm diameter body and 16cm diameter head phantoms, combining with various available bowtie filters and at different kV settings. Correction factors using linear or polynomial functions were developed based on these measurements. RESULTS: CTDI measurements using the DLI method and direct integration (DLP) method align with each other with an error of less than 6.7% for the body phantom, and 6.9% for head phantom respectively. The IEC method underestimates radiation dose for body and head phantoms relative to the DLI, with an error range from 8.9% to 19.4%, depending on the phantom and bowtie filter. A correction factor of 0.15 (15%) could be used for body and head phantom measurements with large body, head and pediatric head bowtie filters. While for body phantom with medium filter and head phantom with small body filter which are not routinely used for CTDI measurements, a correction factor of 0.30 (30%) could be used. The proposed correction factors are validated using various kV and filter combinations. Compared to a linear approximation, a polynomial correction is better at adjusting the IEC measurements, with an error of 5.2%. We found that the a1 coefficient of the polynomial correction is approximately equal to Aeq obtained from DLI measurements for all cases studied, with an average percent difference of 6.7%. CONCLUSION: Both linear and polynomial approximations can be used to correct the IEC measurements, aligning them with the direct integration of dose profiles or the point detector method of CT dosimetry on a 256 slice GE Revolution scanner. Using a polynomial correction may potentially bypass the need for an elongated phantom in the DLI method since the a1 coefficients are approximately equal to Aeq obtained from the DLI method.