Purpose: AAPM Task Group 111 proposed to measure the equilibrium dose-pitch product Deq for scan modes involving table translation and the midpoint dose DL(0) for stationary-table modes on the central and peripheral axes of sufficiently long (e.g., at least 40 cm) phantoms. This paper presents an alternative approach to calculate both metrics using the measurements of scanning the standard computed tomographic (CT) dosimetry phantoms on CT scanners.
Methods: Deq was calculated from CTDI100 and ε(CTDI100) (CTDI100 efficiency), and DL(0) was calculated from D̂eq and the approach to equilibrium function H(L) = DL(0)∕Deq, where Deq was the equilibrium dose. CTDI100 may be directly obtained from several sources (such as medical physicist's CT scanner performance evaluation or the IMPACT CT patient dosimetry calculator), or be derived from CTDIVol using the central to peripheral CTDI100 ratio (R100). The authors have provided the required ε(CTDI100) and H(L) data in two previous papers [X. Li, D. Zhang, and B. Liu, Med. Phys. 39, 901-905 (2012); and ibid. 40, 031903 (10pp.) (2013)]. R100 was assessed for a series of GE, Siemens, Philips, and Toshiba CT scanners with multiple settings of scan field of view, tube voltage, and bowtie filter.
Results: The calculated DL(0) and DL(0)/Deq in PMMA and water cylinders were consistent with the measurements on two GE CT scanners (LightSpeed 16 and VCT) by Dixon and Ballard [Med. Phys. 34, 3399-3413 (2007)], the measurements on a Siemens CT scanner (SOMATOM Spirit Power) by Descamps et al. [J. Appl. Clin. Med. Phys. 13, 293-302 (2012)], and the Monte Carlo simulations by Boone [Med. Phys. 36, 4547-4554 (2009)].
Conclusions: Deq and DL(0) can be calculated using the alternative approach. The authors have provided the required ε(CTDI100) and H(L) data in two previous papers. R100 is presented for a majority of multidetector CT scanners currently on the market, and can be easily assessed for other CT scanners or operating conditions not covered in this study. The central to peripheral Deq ratio is about 1.50 and 1.12 times of R100 for the 32- and 16-cm diameter PMMA phantom, respectively.