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Exponential Function Modeling of Radiation Dose Dependence On Subject Size in CT: Water Phantoms Versus Patient Organs

X Li*, K Yang , B Liu , Massachusetts General Hospital, Boston, MA


(Wednesday, 8/1/2018) 10:00 AM - 10:30 AM

Room: Exhibit Hall | Forum 1

Purpose: In the development of patient organ dose evaluation method in CT, the dose dependence on patient size is often modeled with exponential attenuation law. This work was to perform exponential regression analysis of the dose to water phantom with respect to its diameter, and to compare the regression parameters to those of patient models of eight previous studies.

Methods: A dose calculation algorithm and the published data by Li et al. [Med. Phys. 39, 5347-5352 (2012); 43, 5878-5888 (2016)] were used to calculate longitudinal dose distribution in 10- to 50-cm diameter water phantoms at seven scan lengths (15, 20, 25, 30, 40, 50, 70 cm) with constant mA. The dose versus diameter data were processed with regression analysis, y = α × exp(-β × diameter).

Results: This work obtained 84 sets of regression parameters for 3 water phantom axes (center, cross sectional average, periphery), 7 scan lengths, and 4 longitudinal locations (midpoint z=0, half-center point z=L/4, scan range average, 1cm to a scan range edge) in each scan range. Both α and β increased in the order of phantom periphery, planar average, and phantom center. Except near a scan range edge, α and β were relatively insensitive to scan length and the location inside scan range. The median values of α and β of water phantoms undergoing constant mA scans were approximately consistent with those of the exponential regression analyses of dose versus subject size data in eight previous studies with constant mA or tube current modulation.

Conclusion: Water phantom-based dose evaluation can reproduce the organ dose dependence on patient size. As the former may be much easier than the patient model-based simulations, it may be used for characterizing the organ dose dependence on tube current line-shape, patient size, scan length, and organ location across or beyond scan range.


CT, Dose, Modeling


IM- CT: Radiation dosimetry & risk

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