Bismuth Shields for CT Dose Reduction: Do They Help or Hurt?

Bismuth shield affecting CT image quality and radiation dose in adjacent and distant zones relative to shielding surface: A phantom study

BACKGROUND

To quantify image quality and radiation doses in regions adjacent to and distant from bismuth shields in computed tomography (CT).

METHODS

An American College of Radiology accreditation phantom with four solid rods embedded in a water-like background was scanned to verify CT number (CTN) accuracy when using bismuth shields. CTNs, image noise, and contrast-to-noise ratios (CNRs) were determined in the phantom at 80-140 kVp. Image quality was investigated on image portions in the zones adjacent (A zone) to and distant (D zone) from a bismuth shield. Surface radiation doses were measured using thermoluminescent dosimeters. Streak artefacts were graded on a 3-point-scale.

RESULTS

Changes in CTN caused by a bismuth shield resulted in changes in X-ray spectra. CTN changes were more apparent in the A zone than in the D zone, particularly for a low tube voltage. The degrees of CTN changes and image noise were proportional to the thickness of the bismuth shields. A 1-ply bismuth shield reduced surface radiation doses by 7.2%-15.5%. The overall CNRs were slightly degraded, and streak artefacts were acceptable.

CONCLUSIONS

Using a bismuth shield could result in significant CTN changes and perceivable artefacts, particularly for a superficial organ close to the shield, and is not recommended for quantification CT examinations or follow-up CT examinations.

Use of bismuth shield for protection of superficial radiosensitive organs in patients undergoing computed tomography: a literature review and meta-analysis

The study aimed to assess the effect of bismuth (Bi) shielding on dose reduction and image quality in computed tomography (CT) through a literature review. A search was conducted in the following databases: Web of Science, PubMed, Google Scholar, and Scopus. Studies that reported estimated dose reduction with bismuth shielding during imaging of the eye, thyroid, and breast were included, and a meta-regression analysis was used to examine the influence of the CT scanner type on the dose reduction. The studies included a total of 237 patients and 34 pediatric and adult anthropomorphic phantoms for whom the radiation dose was reported. Bismuth shielding was recommended in 88.89% of the studies based on the maintenance of appropriate image quality under shielding. Noise associated with Bi shielding was 7.5%, 263%, and 23.5% for the eye, thyroid, and breast, respectively. The fixed-effects pooled estimate of dose reduction was 34% (95% CI: 13-55; p < 0.001) for the eye, 37% (95% CI 14-61; p < 0.001) for the thyroid, and 36% (95% CI 36-55; p < 0.001) for the breast. The image quality, usage of foams, CT scanner type, beam energies, and backscatter radiation were important factors that directly affected the efficacy of Bi shielding to reduce the radiation dose at the superficial radiosensitive organs.

Reducing Radiation Doses in Female Breast and Lung during CT Examinations of Thorax: A new Technique in two Scanners

BACKGROUND

Chest CT is a commonly used examination for the diagnosis of lung diseases, but a breast within the scanned field is nearly never the organ of interest.

OBJECTIVE

The purpose of this study is to compare the female breast and lung doses using split and standard protocols in chest CT scanning.

MATERIALS AND METHODS

The sliced chest and breast female phantoms were used. CT exams were performed using a single-slice (SS)- and a 16 multi-slice (MS)- CT scanner at 100 kVp and 120 kVp. Two different protocols, including standard and split protocols, were selected for scanning. The breast and lung doses were measured using thermo-luminescence dosimeters which were inserted into different layers of the chest and breast phantoms. The differences in breast and lung radiation doses in two protocols were studied in two scanners, analyzed by SPSS software and compared by t-test.

RESULTS

Breast dose by split scanning technique reduced 11% and 31% in SS- and MS- CT. Also, the radiation dose of lung tissue in this method decreased 18% and 54% in SS- and MS- CT, respectively. Moreover, there was a significant difference (p< 0.0001) in the breast and lung radiation doses between standard and split scanning protocols.

CONCLUSION

The application of a split scan technique instead of standard protocol has a considerable potential to reduce breast and lung doses in SS- and MS- CT scanners. If split scanning protocol is associated with an optimum kV and MSCT, the maximum dose decline will be provided.

Hospital-level factors associated with use of pediatric radiation dose-reduction protocols for head CT: results from a national survey

OBJECTIVES

To examine hospital-level factors associated with the use of a dedicated pediatric dose-reduction protocol and protective shielding for head CT in a national sample of hospitals.

METHODS

A mixed-mode (online and paper) survey was administered to a stratified random sample of US community hospitals (N = 751). Respondents provided information on pediatric head CT scanning practices, including use of a dose-reduction protocol. Modified Poisson regression analyses describe the relative risk (RR) of not reporting the use of a pediatric dose-reduction protocol or protective shielding; multivariable analyses adjust for census region, trauma level, children's hospital status, and bed size.

RESULTS

Of hospitals that were contacted, 38 were ineligible (no CT scanner, hospital closed, do not scan infants), 1 refused, and 253 responded (35.5% response rate). Across all hospitals, 92.6% reported using a pediatric dose-reduction protocol. Modified Poisson regression showed that small hospitals (0-50 beds) were 20% less likely to report using a protocol than large hospitals (>150 beds) (RR: 0.80, 95% confidence interval [CI]: 0.65-0.99; adjusted for covariates). Teaching hospitals were more likely to report using a protocol (RR: 1.10, 95% CI: 1.02-1.19; adjusted for covariates). After adjusting for covariates, children's hospitals were significantly less likely to report using protective shielding than nonchildren's hospitals (RR: 0.64, 95% CI: 0.56-0.73), though this may be due to more advanced scanner type.

CONCLUSION

Results from this study provide guidance for tailored educational campaigns and quality improvement interventions to increase the adoption of pediatric dose-reduction efforts.

Generation of the additional fluorescence radiation in the elastomeric shields used in computer tomography (CT)

Two commercially available (EP, Z) and eight new elastomeric composites (M1-M4, G1-G4, of thickness ≈1 mm) containing mixtures of differing proportions of heavy metal additives (Bi, W, Gd and Sb) have been synthesised and examined as protective shields. The intensity of the X-ray fluorescence radiation generated in the typical elastomeric shields for CT, containing Bi and other heavy metal additives influence on the practical shielding properties. A method for assessing the radiation shielding properties of elastomeric composites used in CT examination procedures via X-ray spectrometry has been proposed. To measure the radiation reduction ability of the protective shields, the dose reduction factor (DRF) has been determined. The lead equivalents for the examined composites were within the ranges of 0.046-0.128 and 0.048-0.130 mm for 122.1 and 136.5 keV photons, respectively. The proposed method, unlike to the common approach, includes a dose contribution from the induced X-ray fluorescence radiation of the heavy metal elements in the protective shields. The results clearly indicate that among the examined compositions, the highest values DRF have been achieved with preparations containing Bi+W, Bi+W+Gd and Bi+W+Sb mixtures with gradually decreasing content of heavy metal additives in the following order: Bi, W, Gd and Sb. The respective values of DRF obtained for the investigated composites were 21, 28 and 27 % dose reduction for a 1 mm thick shield and 39 and ~50 % for a 2 mm thick layer (M1-M4).

Comparison of coronary CT angiography image quality with and without breast shields

OBJECTIVE

The purpose of this study is to compare the image quality of coronary CT angiography performed with and without breast shields.

MATERIALS AND METHODS

This study involved a retrospective cohort of 72 women with possible angina who underwent 64-MDCT retrospective ECG-gated coronary CT angiography at a single academic tertiary medical center. Images of 36 women scanned while wearing bismuth-coated latex breast shields and 36 control subjects scanned without shields, matched by heart rate and body mass index, were graded on a standardized Likert scale for image quality, stenosis, and plaque by two independent board-certified readers blinded to breast shields.

RESULTS

Seventy-two patients (mean [± SD] age, 53 ± 9 years) were included. The pre scan heart rate, body mass index, and Agatston score did not differ between groups. The median estimated radiation dose was 13.4 versus 16.1 mSv for those with and without breast shields (p = 0.003). For shielded versus unshielded scans, 86% versus 83% of coronary segments were rated excellent or above average (p = 0.4), median image quality was 2.0 for both groups, mean signal was 474 ± 75 and 452 ± 91 HU (p = 0.27), mean noise was 33.9 ± 8.5 and 29.8 ± 8.3 HU (p = 0.04), and median signal-to-noise ratio was 14.4 and 14.7 (p = 0.56), respectively.

CONCLUSION

Breast shields for women undergoing coronary CT angiography slightly increased noise but did not negatively affect signal, signal-to-noise ratio, quality, or interpretability. Breast shield use warrants further study.