Implementing the complete beam hardening effect of the bowtie filter versus scaling beam intensities: effects on dosimetric applications in computed tomography

Determination and verification of the x-ray spectrum of a CT scanner

The accuracy of Monte Carlo (MC) simulations in estimating the computed tomography radiation dose is highly dependent on the proprietary x-ray source information. To address this, this study develops a method to precisely estimate the x-ray spectrum and bowtie (BT) filter thickness of the x-ray source based on physical measurements and calculations. The static x-ray source of the CT localizer radiograph was assessed to measure the total filtration at the isocenter for the x-ray spectrum characterization and the BT profile (air-kerma values as a function of fan angle). With these values, the utilized BT filter in the localizer radiograph was assessed by integrating the measured air kerma in a full 360-deg cycle. The consistency observed between the integrated BT filter profiles and the directly measured profiles pointed to the similarity in the utilized BT filter in terms of thickness and material between the static and rotating x-ray geometries. Subsequently, the measured air kerma was used to calculate the BT filter thickness and was verified using MC simulations by comparing the calculated and measured air-kerma values, where a very good agreement was observed. This would allow a more accurate computed tomography simulation and facilitate the estimation of the dose delivered to the patients.

Do we need 3D tube current modulation information for accurate organ dosimetry in chest CT? Protocols dose comparisons

OBJECTIVES

To compare the lung and breast dose associated with three chest protocols: standard, organ-based tube current modulation (OBTCM) and fast-speed scanning; and to estimate the error associated with organ dose when modelling the longitudinal (z-) TCM versus the 3D-TCM in Monte Carlo simulations (MC) for these three protocols.

METHOD

Five adult and three paediatric cadavers with different BMI were scanned. The CTDI_vol of the OBTCM and the fast-speed protocols were matched to the patient-specific CTDI_vol of the standard protocol. Lung and breast doses were estimated using MC with both z- and 3D-TCM simulated and compared between protocols.

RESULTS

The fast-speed scanning protocol delivered the highest doses. A slight reduction for breast dose (up to 5.1%) was observed for two of the three female cadavers with the OBTCM in comparison to the standard. For both adult and paediatric, the implementation of the z-TCM data only for organ dose estimation resulted in 10.0% accuracy for the standard and fast-speed protocols, while relative dose differences were up to 15.3% for the OBTCM protocol.

CONCLUSION

At identical CTDI_vol values, the standard protocol delivered the lowest overall doses. Only for the OBTCM protocol is the 3D-TCM needed if an accurate (<10.0%) organ dosimetry is desired.

KEY POINTS

• The z-TCM information is sufficient for accurate dosimetry for standard protocols. • The z-TCM information is sufficient for accurate dosimetry for fast-speed scanning protocols. • For organ-based TCM schemes, the 3D-TCM information is necessary for accurate dosimetry. • At identical CTDI _vol , the fast-speed scanning protocol delivered the highest doses. • Lung dose was higher in XCare than standard protocol at identical CTDI _vol .

Comparing different methods for estimating radiation dose to the conceptus

PURPOSE

To compare different methods available in the literature for estimating radiation dose to the conceptus (D_conceptus) against a patient-specific Monte Carlo (MC) simulation and a commercial software package (CSP).

METHOD

Eight voxel models from abdominopelvic CT exams of pregnant patients were generated. D_conceptus was calculated with an MC framework including patient-specific longitudinal tube current modulation (TCM). For the same patients, dose to the uterus, D_uterus, was calculated as an alternative for D_conceptus, with a CSP that uses a standard-size, non-pregnant phantom and a generic TCM curve. The percentage error between D_uterus and D_conceptus was studied. Dose to the conceptus and percent error with respect to D_conceptus was also estimated for three methods in the literature.

RESULTS

The percentage error ranged from -15.9% to 40.0% when comparing MC to CSP. When comparing the TCM profiles with the generic TCM profile from the CSP, differences were observed due to patient habitus and conceptus position. For the other methods, the percentage error ranged from -30.1% to 13.5% but applicability was limited.

CONCLUSIONS

Estimating an accurate D_conceptus requires a patient-specific approach that the CSP investigated cannot provide. Available methods in the literature can provide a better estimation if applicable to patient-specific cases.

KEY POINTS

• A patient's internal anatomy affects the dose to the conceptus. • Conceptus position has an influence on its dose estimation. • Patient anatomy and specific TCM must be considered for accurate conceptus dosimetry. • D _uterus to a standard-size phantom should not be used as D _conceptus .