Characterization of radiation dose from tube current modulated CT examinations with considerations of both patient size and variable tube current

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.

Advancing size-specific dose estimates in CT examinations: Dose estimates at longitudinal positions of scans

PURPOSE

American Association of Physicists in Medicine (AAPM) (Report 204) introduced the size-specific dose estimate (SSDE) for the average dose to the center of a fixed-mA scan. International standards establish a method that CT manufacturers and radiation dose index monitoring software may use to calculate SSDE(z) at longitudinal positions of scans with fixed mA or tube current modulation, and its scan range average SSDE ( z ) ¯ . We sought to test how accurate SSDE(z) is in tracking the average dose to the transverse slab of an axial image slice (D_slice ), evaluated with Monte Carlo calculation, in the chest and abdominopelvic examinations.

METHODS

We retrospectively identified 65 consecutive adult patients undergoing whole-body CT for transcatheter aortic valve implantation planning. Examination parameters (kV, mA, CTDI_vol ) were extracted from the DICOM headers. Patient water equivalent diameter D_W (z) was calculated at each image slice, excluding the patient table. A previously validated Monte Carlo simulation (Geant4) program was used to evaluate D_slice from the chest and abdominopelvic examinations. Alternatively, SSDE(z) was calculated at each slice. The results of the two methods were compared with descriptive statistical outcomes (R, version 4.0.2).

RESULTS

In chest and abdominopelvic CT examinations, D_slice largely changed with anatomic location and uniformly fell off toward scan range edges. Scan range averages SSDE ( z ) ¯ and D slice ¯ were consistent within 2.5%-3.1% (median) and 6.3%-10.4% (maximum) in two examinations. On individual image slices, SSDE(z) could be lower or higher than D_slice , with deviation ranging from -18.3% to 85% in two edges (2 × 5 cm) of scan range and from -35.2% to 18.7% in the remaining central region of the scan.

CONCLUSION

This study provides critical inputs for quality assurance programs. SSDE ( z ) ¯ is useful to track the average dose of all image slices, but further development may be useful for tracking patient dose at the level of individual image slices, especially near a scan range edge.

Radiation dose dependence on subject size in abdominal computed tomography: Water phantom and patient model comparison

PURPOSE

Development of patient organ dose evaluation method in computed tomography (CT) needs to model the correlation between organ dose and patient size, under various conditions of scan length, tube current lineshape, and organ location. To facilitate this task, this work was to perform a comprehensive study of the relationship between the dose to water phantom and its diameter under various settings of phantom axis, scan length, and the location across or beyond the scanned range.

METHODS

A dose calculation algorithm and the published data by Li et al. [Med. Phys. 39, 5347-5352 (2012); 40, 031903 (2013); 43, 5878-5888 (2016)] were used to calculate longitudinal dose distribution D_L (z) in 10- to 50-cm diameter water phantoms undergoing constant tube current scans. The relationship between dose and phantom diameter was examined on three phantom axes (center, cross-sectional average, periphery), at seven scan lengths from 15 to 70 cm, and at eight longitudinal locations within or beyond each scan range. The water phantom results were compared to those of patient models of eight previous studies.

RESULTS

For the water phantoms matching the abdominal perimeters (36.3-124.5 cm) of the GSF family of voxelized phantoms, the median and range of D_L (z)(water) across scan range were consistent with those of the organ doses from the GSF phantom abdominal scans of a previous study. In 41 water phantoms (diameters 10-50 cm), D_L (z)(water) at locations inside scan range decreased with increasing phantom diameters. Exponential regression analysis of the above trend yielded regression parameters approximately consistent with those of phantom or patient models of eight previous studies. However, the usual exponential function might not be optimal for modeling the dose dependence on subject size. Inside scan range, the log(dose) vs diameter curve was non-linear on a semilogarithmic graph. Outside of scan range, dose might increase with larger subject sizes, contradicting to the exponential attenuation law. In the CT examinations of a patient population, direct modeling of organ dose dependence on patient size would be more challenging due to varying scan lengths and changing organ distances to the scan range centers.

CONCLUSION

An efficient approach to take into account the abdominal organ dose dependences on other factors is to calculate D_L (z)(water) with the water equivalent diameter, scan length, and tube current lineshape from the patient examinations, and to evaluate the organ dose to D_L (z)(water) ratio, where z is at the organ's longitudinal location. The ratio may be used for abdominal organ dose evaluation in the patient examinations. How to make use of D_L (z)(water) for organ dose evaluation in other body regions may be explored in the future.