Practical dose point‐based methods to characterize dose distribution in a stationary elliptical body phantom for a cone‐beam C‐arm CT system

Abstract

Purpose: To propose new dose point measurement-based metrics to characterize the dose distributions and the mean dose from a single partial rotation of an automatic exposure control-enabled, C-armbased, wide cone angle computed tomography system over a stationary, large, body-shaped phantom. Methods: A small 0.6 cm 3 ion chamber (IC) was used to measure the radiation dose in an elliptical body-shaped phantom made of tissue-equivalent material. The IC was placed at 23 well-distributed holes in the central and peripheral regions of the phantom and dose was recorded for six acquisition protocols with different combinations of minimum kVp (109 and 125 kVp) and z-collimator aperture (full: 22.2 cm; medium: 14.0 cm; small: 8.4 cm). Monte Carlo (MC) simulations were carried out to generate complete 2D dose distributions in the central plane (z = 0). The MC model was validated at the 23 dose points against IC experimental data. The planar dose distributions were then estimated using subsets of the point dose measurements using two proposed methods: (1) the proximity-based weighting method (method 1) and (2) the dose point surface fitting method (method 2). Twenty-eight different dose point distributions with six different point number cases (4,5,6,7,14, and 23 dose points) were evaluated to determine the optimal number of dose points and their placement in the phantom. The performances of the methods were determined by comparing their results with those of the validated MC simulations. The performances of the methods in the presence of measurement uncertainties were evaluated. Results: The 5-, 6-, and 7-point cases had differences below 2%, ranging from 1.0% to 1.7% for both methods, which is a performance comparable to that of the methods with a relatively large number of points, i.e., the 14-and 23-point cases. However, with the 4-point case, the performances of the two methods decreased sharply. Among the 4-, 5-, 6-, and 7-point cases, the 7-point case (1.0% [±0.6%] difference) and the 6-point case (0.7% [±0.6%] difference) performed best for method 1 and method 2, respectively. Moreover, method 2 demonstrated high-fidelity surface reconstruction with as few as 5 points, showing pixelwise absolute differences of 3.80 mGy (±0.32 mGy). Although the performance was shown to be sensitive to the phantom displacement from the isocenter, the performance changed by less than 2% for shifts up to 2 cm in the x-and y-axes in the central phantom plane. Conclusions: With as few as five points, method 1 and method 2 were able to compute the mean dose with reasonable accuracy, demonstrating differences of 1.7% (±1.2%) and 1.3% (±1.0%), respectively. A larger number of points do not necessarily guarantee better performance of the methods; optimal choice of point placement is necessary. The performance of the methods is sensitive to the alignment of the center of the body phantom relative to the isocenter. In body applications where dose distributions are important, method 2 is a better choice than method 1, as it reconstructs the do...

AAPM Task Group Report 238: 3D C‐arms with volumetric imaging capability*

“…[92][93][94][95][96] Choi et al have recently reported that a proximity-based weighting method and a 2D surface fitting method can accurately estimate the planar average dose in an elliptical phantom using as few as 5 point dose measurements. 91 The measurement of f (0) in a single 16-cm CTDI phantom can also be limited by the fact that the longitudinal extent of a single phantom is not sufficient to capture the entire contribution of dose from scatter at the edges of the longitudinal extent of the beam, especially if the longitudinal extent of the beam at isocenter is greater than the extent of the phantom. In the case of the beam exceeding the longitudinal extent of the measurement phantom, the beam may be collimated to a limited range for the C-arm CBCT acquisition, or two or more phantoms may be stacked in series to create a longer phantom to capture the entire contribution to the central plane dose from scatter (Figure 19a).…”

“…Following the conventions introduced in the AAPM Report 111, 88 the dose profile along the z‐axis of a CT acquisition is $$f( z )$$, and a measurement of the dose at the center of the scan range is defined as f (0). The distribution of dose in a plane for a C‐arm CBCT acquisition is not radially or angularly symmetric 91–95 . Accordingly, measurements of the dose profile or the dose at the central extent of the scan range must be acquired at different angular positions and may be defined as $${f}_{position}( z )$$ or $${f}_{position}( 0 )$$ where the position is the angular position of the placement of the dosimeter in the measurement phantom, with 0 degrees defined to be along the positive y‐axis of a cross‐sectional view of the phantom and 90 degrees is along the positive x‐axis, and so on.…”

“…Choi et al. have recently reported that a proximity‐based weighting method and a 2D surface fitting method can accurately estimate the planar average dose in an elliptical phantom using as few as 5 point dose measurements 91 …”

3D Non-Rigid Alignment of Low-Dose Scans Allows to Correct for Saturation in Lower Extremity Cone-Beam CT

“…As depicted in Fig. 4 , during the AEC scan the tube outputs (i.e., the tube voltage and tube current-time product per projection) were modulated by the AEC system as a function of the projection number [ 28 ], [ 29 ]. While the gantry of the C-arm CT system rotated around the two knees together, the AEC elevated the tube outputs along the lateral direction at the beginning and end of the scan, and reduced the output in the anterior-posterior direction where the legs do not overlap.…”

Dose Estimation by Geant4-Based Simulations for Cone-Beam CT Applications: A Systematic Review

“…J.H. Choi et al propose new dose point measurement-based method to characterize the dose distributions and the mean dose from a single partial rotation of a CBCT system over a stationary, large, body-shaped phantom validated by a Geant4 simulation [112].…”