Original Article
Effect of bismuth breast shielding on radiation dose and image quality in coronary CT angiography

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Abstract

Background

Coronary computed tomographic angiography (CCTA) is associated with high radiation dose to the female breasts. Bismuth breast shielding offers the potential to significantly reduce dose to the breasts and nearby organs, but the magnitude of this reduction and its impact on image quality and radiation dose have not been evaluated.

Methods

Radiation doses from CCTA to critical organs were determined using metal-oxide-semiconductor field-effect transistors positioned in a customized anthropomorphic whole-body dosimetry verification phantom. Image noise and signal were measured in regions of interest (ROIs) including the coronary arteries.

Results

With bismuth shielding, breast radiation dose was reduced 46%-57% depending on breast size and scanning technique, with more moderate dose reduction to the heart, lungs, and esophagus. However, shielding significantly decreased image signal (by 14.6 HU) and contrast (by 28.4 HU), modestly but significantly increased image noise in ROIs in locations of coronary arteries, and decreased contrast-to-noise ratio by 20.9%.

Conclusions

While bismuth breast shielding can significantly decrease radiation dose to critical organs, it is associated with an increase in image noise, decrease in contrast-to-noise, and changes tissue attenuation characteristics in the location of the coronary arteries.

Introduction

Coronary computed tomography angiography (CCTA) is increasingly utilized for the non-invasive assessment of coronary artery disease due its ability to exclude or diagnose coronary artery disease with high accuracy.1,2 However, CCTA can be associated with high radiation doses to organs, such as the breasts and lungs, which lie in the path of the x-ray beam. When helical technique is used, absorbed doses to these critical organs have been observed to range between 42 and 91 mGy, and the attributable risks of breast and lung cancer can each exceed 0.2%.3

As with all medical procedures utilizing ionizing radiation, it is incumbent upon practitioners to keep radiation exposure from CCTA as low as reasonably achievable (ALARA), while still ensuring diagnostically adequate image quality. Several scanner-based approaches have been employed to significantly decrease radiation doses from CCTA, including modulating the x-ray’s tube current depending on the portion of the cardiac cycle,4 step-and-shoot,5,6 volume,7 and high-pitch helical8 scan modes, decreasing tube potential,9 and iterative reconstruction of images.10 Many of these dose reduction features, however, are only implemented on more recent 64+ slice CT scanners and are thus not available for a sizable portion of the installation base of CCTA-capable scanners. An alternative or additional approach to these scanner-based methods for reducing patient radiation exposure is to employ patient-based shielding.

Of particular potential appeal is the use of in-plane bismuth shielding. In-plane bismuth shielding over radiosensitive superficial organs, e.g., eyes, thyroid, and breasts, has been successfully employed for other types of CT scans. Bismuth breast shielding has been demonstrated in multiple studies to reduce radiation doses to the breasts by as much as 57% during CT examinations.11., 12., 13., 14., 15., 16., 17., 18., 19., 20., 21., 22., 23. One recent study noted bismuth shielding to result in a reduction in breast dose of 38.4% in 64-slice and 46.8% in 320-slice CCTA.24

Thus, breast shielding is an attractive possibility for CCTA due the significant reduction in radiation doses achieved in other contexts and its ease of use. Nevertheless, a barrier to the widespread adoption of bismuth shielding in CCTA protocols is the potential increase in image noise and streak artifacts and loss of signal that may create sub-optimal examinations. Recent studies have shown mixed effects of bismuth breast shielding in CT examinations for types of examinations other than CCTA. Reflecting this discrepant data, some authors have recommended the use of shielding in non-cardiac CT11,13 and others have recommended against it.15,16 To the best of our knowledge, no previous publication has studied the effect of breast shielding on image quality in CCTA.

The purpose of this study was to determine the reduction in radiation dose to critical organs, namely, the breasts, lungs, esophagus, and heart using bismuth breast shielding, and to determine how shielding affects image quality during CCTA. In addition, the effects of breast size and scanning technique (helical vs axial step-and-shoot) on radiation dose and image quality were investigated.

Section snippets

Study Design

Doses to critical organs were measured using solid-state metal-oxide-semiconductor field-effect transistor (MOSFET) radiation detectors placed in a whole-body anthropomorphic phantom. Image signal, contrast, and noise in coronary arteries were calculated using the same phantom with MOSFETs removed. Doses and image signal, contrast, and noise were determined both with and without a commercially available bismuth breast shield, using both helical and axial step-and-shoot scan protocols, and for

Radiation Doses

Organ absorbed doses from CCTA are summarized in Table 1. Dose-length product was 1424.5 mGy × cm for helical scanning and 351.3 mGy × cm for axial scanning. Bismuth shielding significantly decreased organ doses (P < 0.001); a significant difference was found in both medium- and large-breast phantoms, and for both helical and axial scanning (P < 0.02 for each). Breast dose was decreased by 46%-57% with shielding, depending on the scan protocol and breast size. Doses to the lungs, heart, and

Discussion

Bismuth shielding has been shown in multiple studies to decrease radiation doses from CT examinations to radiosensitive superficial organs including the breast. Here, we observed bismuth breast shielding to reduce breast dose from CCTA by 46%-57%, depending on breast size and scanning method, comparable to dose reductions reported previously. Absorbed dose to the heart, esophagus, and lungs was more modestly decreased with bismuth shielding, accounting to the breast’s superficial location and

Disclosures

Dr Wolff serves as a consultant to GE Healthcare and is an owner of NeoSoft, LLC and NeoCoil, LLC. Dr Einstein has received research grants from GE Healthcare and Spectrum Dynamics.

Funding

Supported by NIH Grants 1R01 HL109711-01 and 5KL2 RR024157, a Nuclear Cardiology Foundation/Covidien Research Award, and an Irving Institute for Clinical and Translational Research/Columbia University Clinical Trials Office Pilot Study Award (UL1 RR024156).

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