A comprehensive Monte Carlo study of CT dose metrics proposed by the AAPM Reports 111 and 200

Implementation and evaluation of the AAPM TG 111 for radiation dosimetry in a state-of-the-art C-arm cone beam CT system

The aim of this study was to assess the dosimetric characteristics of a state-of-the-art C-arm cone beam computed tomography (CBCT) system using the methodology proposed by the American Association of Physicists in Medicine (AAPM) Task Group (TG) 111. The dose measurement methodology described in AAPM TG 111 for wide cone beam acquisitions without table translation was employed to estimate equilibrium beam length (αeq‾) and equilibrium dose (feq‾) in various interventional task-specific protocols with different tube arc projection geometries. Dose profiles were derived from point dose measurements in the centre and peripheral locations of the ICRU/AAPM and standard polymethyl methacrylate (PMMA) body phantom. Dose measurements were performed in phantom and free-in-air using a solid-state point detector. Monte Carlo (MC) based simulation dosimetry was performed to quantify the inhomogeneous dose patterns imparted in the phantoms. Estimatedαeq‾andfeq‾on the ICRU/AAPM phantom was up to 49.4 cm and 6.17 mGy/100 mAs, respectively. Corresponding values determined on the PMMA phantom were 139 cm and 8.8 mGy/100 mAs, respectively. Free-in-air dose measurement ranged from 1.43 mGy/100 mAs to 5.93 mGy/100 mAs. Per cent difference inαeq‾andfeq‾between MC simulation and solid-state point detector measurement methods in the ICRU/AAPM phantom were within 16% and 18%, respectively. Manufacturers can use the presented methodology to characterize the dosimetric properties of C-arm CBCT systems. Clinical medical physicists may follow this methodology to verify corresponding data provided by the manufacturer and check for C-arm CBCT system performance dosimetric consistency.

Visualization and comparison of CT dose distribution between axial and helical acquisitions

BACKGROUND

It is challenging to assess the accuracy of volume CT Dose Index (CTDIvol ) when the axial scan modes corresponding to a helical scan protocol are not available. An alternative approach was proposed to directly measureC T D I v o l H $CTDI_{vol}^H$ using helical acquisitions and relatively small differences (< 20%) from CTDIvol were observed.

PURPOSE

To visually demonstrate the 3D dose distribution for both axial and helical CT acquisitions and quantitively compareC T D I v o l H $CTDI_{vol}^H$ and CTDIvol .

METHODS

3D dose distribution within the standard CTDI phantoms (16 and 32 cm diameter) from a single CT projection, Dp (x,y,z) was first generated using Monte Carlo simulation (GEANT4) with 9×108 photons per combination of tube voltage (80-140 kV), collimation width (1-8 cm), and z-axis location of the central ray of the x-ray beam, with a spatial resolution of 1 mm3 . These dose distributions from one single projection were analytically ensembled to simulate 3D dose volumes DA (x,y,z) and DH (x,y,z) for axial and helical scans, respectively, with different helical pitches (0.3-2) and scan lengths (100-150 mm). 2D planar dose distributions were obtained by integrating the inside 100 mm of the dose volumes. CTDIvol andC T D I v o l H $CTDI_{vol}^H\;$ were calculated using the planar dose data at corresponding pencil chamber locations and the percentage differences (PD) were reported.

RESULTS

High spatial resolution 3D CT dose volumes were generated and visualized. PDs betweenC T D I v o l H $CTDI_{vol}^H$ and CTDIvol had strong dependency on scan length and peripheral chamber locations, with subtle dependency on collimation width and pitch. PDs were mostly within the range of ± 3% for a scan length of 150 mm with four peripheral chamber locations.

CONCLUSIONS

With a scan length covering the entire phantom length,C T D I v o l H $CTDI_{vol}^H$ directly measured from helical scans can serve as an alternative to CTDIvol only if all four peripheral locations were measured.

Organ dose in CT: Comparison between measurements and computational methods

PURPOSE

This study aims to compare two methods for the organ dose evaluation in computed tomography (CT) in the head- and thorax regions: an experimental method, using radiochromic films, and a computational one, using a commercial software.

METHODS

Gafchromic® XR-QA2 and EBT-3 were characterized in terms of energetic, angular, and irradiation configurations dependence. Two free-in-air irradiation calibration configurations were employed using a CT scanner: with the sensitive surface of the film orthogonal (OC) and parallel (PC) to the beam axis. Different dose-response curves were obtained by varying the irradiation configurations and the beam quality (BQ). Subsequently, films were irradiated within an anthropomorphic phantom using CT-thorax and -head protocols, and the organ dose values obtained were compared with those provided by the commercial software.

RESULTS

At different configurations, an unchanged dose response was achieved with EBT-3, while a dose response of 15% was obtained with XR-QA2. By varying BQ, XR-QA2 showed a different response below 10%, while EBT-3 showed a variation below 5% for dose values >20 mGy. For films irradiation angle equal to 90°, the normalized to 0° relative response was 41% for the XR-QA2 model and 83% for the EBT-3 one. Organ dose values obtained with EBT-3 for both configurations and with XR-QA2 for OC were in agreement with the DW values, showing percentage discrepancies of less than 25%.

CONCLUSIONS

The obtained results showed the potential of EBT-3 in CT patient dosimetry since the lower angular dependence, compared to XR-QA2, compensates for low sensitivity in the diagnostic dose range.

Calculation of scan length and size-specific dose at longitudinal positions of body CT scans using dose equilibrium function

BACKGROUND

Dose evaluation at longitudinal positions of body computed tomography (CT) scans is useful for CT quality assurance programs and patient organ dose evaluation. Accurate estimates depend on both patient size and scan length.

PURPOSE

To propose practical evaluation of the average dose to the transverse slab of an axial image slice for adult body CT examinations, considering not only patient size but also scan length, and to compare the results with those of Monte Carlo (Geant4) simulation [D_sim (z)] and size-specific dose estimates at longitudinal positions of scans [SSDE(z)] from international standards (IEC publication no. 62985).

METHODS

In a scan series, the total dose at each z-axis location was calculated using the input information identical to the SSDE(z) evaluation. Each axial image slice (slice thickness, 2.5 or 5 mm) was first considered independently. Its z-axis coverage and CTDI_vol (from the DICOM headers) were used to directly calculate a z-axis dose profile for the average dose over the cross-section of a water phantom, using the approach to equilibrium function. The phantom diameter was taken to be equal to the patient water equivalent diameter at that slice. The above was repeated at all slices and the dose at each z-axis location was accumulated from all profiles, referred to as D_calc (z). For validation, we considered a cohort of 65 patients, who underwent chest and abdominopelvic examinations. The resultant D_calc (z) was compared with D_sim (z) and SSDE(z), both available in a previous paper.

RESULTS

D_calc (z) evaluation could be used to accurately assess the scan range average dose, with an accuracy of 7.1%-8.7% for 65 patients in two examinations. On individual image slices, the maximum difference in magnitude between D_calc (z) and D_sim (z) [and between SSDE(z) and D_sim (z) in parentheses] was 37.5% (85%) [two edges (2 × 5 cm) of chest scan range], 17.8% (35.2%) (the remaining central region of chest scan), 26.8% (74.1%) [two edges (2 × 5 cm) of abdominopelvic scan range], and 14.2% (22.5%) (the remaining central region of abdominopelvic scan).

CONCLUSIONS

Identical input data are used for D_calc (z) and SSDE(z) evaluations. The latter is limited to the z-axis locations within scan range. At each image slice, SSDE(z) is equivalent to the midpoint dose of a fixed-mA scan of 15-30 cm (scan length). In contrast, D_calc (z) considers dose accumulation from varying scan length (from sub-centimeter to about 1 m) and tube current, and dose profile is also computed outside scan range. Besides greatly improving dose evaluation for individual image slices, D_calc (z) allows for evaluating dose accumulation from multiple series, which typically span different scan ranges. Our proposal may assist CT manufacturers and dose index monitoring software in assessing dose at longitudinal positions of body CT scans.