Lead versus bismuth-antimony shield for fetal dose reduction at different gestational ages at CT pulmonary angiography.

PURPOSE

To compare the effective fetal dose reduction at different stages of gestation during maternal computed tomographic (CT) pulmonary angiography by using traditional lead apron and bismuth-antimony shields combined with limited z-axis and tube current.

MATERIALS AND METHODS

Phantom with gravid prosthesis, 0.5-mm lead, and two grades of bismuth-antimony shield was used. Thermoluminescent dosimeters (TLDs) measured radiation in the first- to third-trimester uterus. Fetal dose was determined for each gestation by using 100 kVp to the costophrenic angles (CPAs) with and without shielding for a total of 12 scans. Eight third-trimester scans were used to compare shields using 120 kVp to CPAs versus those using 100 kVp to the diaphragm.

RESULTS

Average fetal dose increased with gestation with use of 100 kVp to CPAs, from 0.11 mGy in first trimester to 0.50 mGy in third trimester. Average third-trimester unshielded fetal dose was reduced from 0.82 mGy by using 120 kVp to CPAs to 0.17 mGy (79%, P < .001) by using 100 kVp to the diaphragm. Lead apron reduced dose more than either of the bismuth-antimony shields (72%-79% vs 57%-81%) with use of 100 kVp to CPAs. Shields reduced the dose by 73% (lead), 62% (90% attenuation bismuth-antimony), and 72% (95% attenuation bismuth-antimony) (P < .01) at 120 kVp to CPAs. No significant difference between shields was demonstrated with 100 kVp to the diaphragm (P < .01). Maternal dose was 8.13 mSv at 120 kVp to CPAs, 4.90 mSv at 100 kVp to CPAs, and 4.02 mSv at 100 kVp to the diaphragm.

CONCLUSION

Reducing voltage and limiting z-axis is more effective than shields at reducing fetal dose. Shielding improves reduction with no significant difference between lead and bismuth-antimony shields when conservative scanning parameters are observed.