Effect of Localizer Radiography Projection on Organ Dose at Chest CT with Automatic Tube Current Modulation

Abstract

Purpose To calculate the effect of localizer radiography projections to the total radiation dose, including both the dose from localizer radiography and that from subsequent chest computed tomography (CT) with tube current modulation (TCM). Materials and Methods An anthropomorphic phantom was scanned with 192-section CT without and with differently sized breast attachments. Chest CT with TCM was performed after one localizer radiographic examination with anteroposterior (AP) or posteroanterior (PA) projections. Dose distributions were obtained by means of Monte Carlo simulations based on acquired CT data. For Monte Carlo simulations of localizer radiography, the tube position was fixed at 0°and 180°; for chest CT, a spiral trajectory with TCM was used. The effect of tube start angles on dose distribution was investigated with Monte Carlo simulations by using TCM curves with fixed start angles (0°, 90°, and 180°). Total doses for lungs, heart, and breast were calculated as the sum of the dose from localizer radiography and CT. Image noise was defined as the standard deviation of attenuation measured in 14 circular regions of interest. The Wilcoxon signed rank test, paired t test, and Friedman analysis of variance were conducted to evaluate differences in noise, TCM curves, and organ doses, respectively. Results Organ doses from localizer radiography were lower when using a PA instead of an AP projection (P = .005). The use of a PA projection resulted in higher TCM values for chest CT (P < .001) owing to the higher attenuation (P < .001) and thus resulted in higher total organ doses for all investigated phantoms and protocols (P < .001). Noise in CT images was lower with PA localizer radiography than with AP localizer radiography (P = .03). The use of an AP projection allowed for total dose reductions of 16%, 15%, and 12% for lungs, breast, and heart, respectively. Differences in organ doses were not related to tube start angles (P = .17). Conclusion The total organ doses are higher when using PA projection localizer radiography owing to higher TCM values, whereas the organ doses from PA localizer radiography alone are lower. Thus, PA localizer radiography should be used in combination with reduced reference tube current at subsequent chest CT.

Vertical off‐centering affects organ dose in chest CT: Evidence from Monte Carlo simulations in anthropomorphic phantoms

“…Second, the phantoms were positioned with the axial center of the phantom in the gantry isocenter based on the two orthogonal LRs. However, the center of attenuation depends on the z‐location and therefore does not coincide with the isocenter for the entire scan range, which makes ideal patient positioning a challenging task . Finally, instead of using real female and overweight phantoms, we added bolus material to the standard anthropomorphic male phantom, while the rest of its habitus remaining unchanged.…”

“…The TCM technique is usually based on the width of the patient's body part and its attenuation obtained from the localizer radiograph (LR). Recent studies have suggested that inappropriate patient centering affects both image quality and patient dose when TCM is used . The reason behind this is that inaccurate centering of the patient in the CT gantry results in magnification of the acquired LR if the patient is positioned too close to the x‐ray source, leading to overestimation of the patient size and application of higher tube current than is actually necessary.…”

“…Recent studies have suggested that inappropriate patient centering affects both image quality and patient dose when TCM is used. 7,8 The reason behind this is that inaccurate centering of the patient in the CT gantry results in magnification of the acquired LR if the patient is positioned too close to the x-ray source, leading to overestimation of the patient size and application of higher tube current than is actually necessary. Thus, patient centering is crucial for optimal dose utility when the TCM function is applied.…”

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“…Previous researches reported the application of low-dose CT scan of chest, 19,20 in addition to study of low-dose CT scan of head and abdomen. 21,22 The study of low-dose CT scan of brain is limited, mainly due to the thicker skull and absorption.…”

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