Estimating size specific dose estimate from computed tomography radiograph localizer with radiation risk assessment

Abstract

Background: Quantifying radiation burden is necessary for optimizing imaging protocols. The normalized dose coefficient (NDC) is determined from the water-equivalent diameter (WED) and is used to scale the CTDIvol based on body habitus to determine the size specific dose estimate (SSDE). In this study we determine the SSDE prior to the CT scan and how sensitive the SSDE from WED is to the lifetime attributable risk (LAR) from BEIR VII.

Method: For calibration, phantom images are used to relate the mean pixel values along a profile ( $\overline{\mathrm{PPV}}$ ) of the CT localizer to the water-equivalent area (A_W ) of the CT axial scan at the same z-location. Images of the CTDIvol phantoms (32 cm, 16 cm, and ∼1 cm) and ACR phantom (Gammex 464) were acquired on four scanners. The relationship between the A_W and $\overline{\mathrm{PPV}}$ was used to calculate the WED from the CT localizer for patient scans. A total of 790 CT examinations of the chest and abdominopelvic regions were used in this study. The effective diameter (ED) was calculated from the CT localizer. The LAR was calculated based on the patient chest and abdomen using the National Cancer Institute Dosimetry System for Computed Tomography (NCICT). The radiation sensitivity index (RSI) and risk differentiability index (RDI) were calculated for SSDE and CTDIvol.

Results: The WED from CT localizers and CT axials scans show good correlation (R² = 0.96) with the maximum percentage difference being 13.45%. The NDC from WED correlates poorly with LAR for lungs (R² = 0.18) and stomach (R² = 0.19), however that is the best correlation.

Conclusion: The SSDE can be determined within 20% as recommended by the report of AAPM TG 220. The CTDIvol and SSDE are not good surrogates for radiation risk, however the sensitivity for SSDE improves when using WED instead of ED.

Keywords: CT; SSDE; localizer; water-equivalent diameter.

FIGURE 1

Flow chart for the calibration method on the phantom scans and how the calibration method is applied to the patient scan per scanner. Figure 2 and Equations (2), (3), and 4 are referenced in this flowchart.

FIGURE 2

shows (a) the correlation between the CT localizer‐based WED as a function of CT axial‐based WED (R ² = 0.96, 95% confidence interval of 16 mm) with VEREOS (R ² = 0.99), iQon (R ² = 0.99), Siemens (R ² = 0.96), and GE Revolution (R ² = 0.96) and (b) the correlation between the CT localizer‐based NDC as a function of CT axial‐based NDC for all scanners.

FIGURE 3

Example of calibration curves for (a) GE Revolution with CT axial scans of phantoms acquired at 120 kV and CT localizers acquired at 70, 80, 100, and 120 kV with R ² > 0.99, (b) SIEMENS Intevo with CT axial scans acquired at 110 kV and CT localizers acquired at 80, 110, and 130 kV with R ² > 0.99, (c) Philips iQon with CT axial scans at 120 kV and CT localizers acquired at 80, 100, 120, and 140 kV, and (d) Philips VEREOS PET/CT with CT axial scans at 120 kV and CT localizers acquired at 80, 100, 120, and 140 kV.

FIGURE 4

(a) Water‐equivalent diameter (WED) estimated from CT localizer as a function of CT axial‐based water‐equivalent diameter for GE Revolution, SIEMENS Intevo, Philips iQon, and VEREOS PET/CT. (b) Normalized dose coefficient (NDC) calculated from CT localizer WED as a function of NDC calculated from CT axial‐based WED for GE Revolution, SIEMENS Intevo, Philips iQon, and VEREOS PET/CT. The line of best fit and 95% confidence interval (CI) are plotted for all graphs, including the results from the report of TG 220. The R ² value represents the correlation for the entire data set.

FIGURE 5

(a and b) shows two examples of the water‐equivalent diameter and effective diameter as a function of slice location for CT localizers on a patient scan.