Victor J. Weir Ph.D.

Weir, V.J. and Zhang, J. (2020). “A polynomial approximation to an exponential growth function for calculating equilibrium dose in CT.” Phys Med Biol 65(20): 20nt01.

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We propose a polynomial approach to approximate equilibrium dose [Formula: see text] reported in AAPM TG111 method of wide beam CT dosimetry. A formula for [Formula: see text] was derived by expanding the exponential growth function in a Taylor series and comparing the resulting function to a polynomial. The formula incorporates coefficients of polynomial fits up to 3rd order. The polynomial coefficients were obtained as fits of the point dose data and used to calculate the length constant β and [Formula: see text] The length constant could also be made available to users by the vendors of various makes and models of CT scanners. We evaluated our polynomial approximation formula for [Formula: see text] by comparing with [Formula: see text] obtained from measured data in a 256 slice GE revolution CT scanner. To that end, point dose data was collected in 600 mm body and head phantoms with a Farmer chamber for beam widths from 40 to 160 mm. A table of [Formula: see text] and length constants β, and plots of fits for various filters (pediatric head, adult head, large body, medium body and small body bowtie filters) were presented. For the 256 slice GE revolution CT scanner, a length constant of [Formula: see text] can be used for pediatric head, adult head, body (large filter), body (medium filter), and body (small filter) at 120 kV when growth function fit is used. The estimated [Formula: see text] using the proposed polynomial based method is within 86.79% (83.14%-90.38%) of [Formula: see text] obtained from fitting the growth function for beam widths from 40 to 160 mm. The proposed polynomial based estimation to the equilibrium dose, [Formula: see text] can be readily implemented in practice for point dose measurements of wide beam CT scanners.

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].

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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.