Determination of potential uterine (conceptus) doses from axial and helical CT scans

Embryo dose estimates in body CT

OBJECTIVE

The purpose of this article is to develop a method for estimating embryo doses in CT.

MATERIALS AND METHODS

Absorbed doses to the uterus (embryo) of a 70-kg woman were estimated using the ImPACT CT Patient Dosimetry Calculator. For a particular CT scan length, relative uterus doses and normalized plateau uterus doses were determined for a range of commercial CT scanners. Patient size characteristics were obtained from cross-sectional axial images of 100 consecutive patients (healthy women undergoing unenhanced pelvic CT examinations). For each patient, the diameter of a water cylinder with the same mass as the patient's pelvis was computed. Relative dose values were generated for cylinder diameters ranging from 16 to 36 cm at x-ray tube voltages between 80 and 140 kV.

RESULTS

Values of relative uterus dose increased monotonically with increasing scan length, independently of scanner model, and reached a plateau for scan lengths greater than approximately 50 cm. The average normalized plateau uterus dose for all scanners was approximately 1.4 and showed interscanner differences of less than 10% for modern scanners operated at 120 kV. Normalized plateau doses show little dependence on the x-ray tube voltage used to perform the CT examination. Our results show that the uterus dose estimate in an abdominal or pelvis CT examination performed on a 70-kg patient is about 40% higher than the reported value of the volume CT dose index (CTDI(vol)). The pelvis of a 70-kg patient may be modeled as a water cylinder with a diameter of 28 cm and has an average anteroposterior dimension of 22 cm. For constant CT technique factors, embryo dose estimates for a 45-kg patient would be approximately 18% higher than those for a 70-kg patient, whereas the corresponding dose estimates in a 120-kg patient would be approximately 37% lower.

CONCLUSION

Embryo doses can be estimated using relative uterus doses, normalized plateau uterus doses, and CTDI(vol) data with correction factors for patient size.

Operative fixation of acetabular fractures in the pregnant patient

OBJECTIVE

To describe in utero radiation exposures in pregnant patients undergoing acetabular fracture repair.

DESIGN

Retrospective case series.

SETTING

University-affiliated regional trauma center.

PATIENT/PARTICIPANTS

Eight pregnant patients with acetabular fractures treated over a 6-year period. There were an additional 518 acetabular fractures in nonpregnant patients treated during the same time period.

INTERVENTION

Open reduction and internal fixation of the acetabulum fracture.

OUTCOME MEASUREMENTS

None. This is a descriptive series reporting fetal radiation doses, fetal fluoroscopy exposure times, and fetal viability after treatment.

RESULTS

The gestational age of the fetuses at presentation ranged from 5 to 26 weeks. Infant delivery averaged 27 weeks from the time of surgery and all pregnancies reached 36 weeks. Apgar scores were normal each child including 1 twin delivery. There were 4 posterior wall fractures, 3 transverse or posterior wall fractures, and 1 posterior column fracture. Intraoperative pelvic fluoroscopy averaged 39 seconds. There were no operative complications and fracture reductions were anatomic in 7 patients. Computed tomography scan of the pelvis conferred the greatest exposure risk to the fetus and fluoroscopy conferred the least. In each case that required a computed tomography scan of the pelvis, the calculated radiation exposure dose to the fetus was greater than 5 cGy.

CONCLUSIONS

The results of this study demonstrate that with a team approach and the judicious use of radiographic imaging during the surgical care of a displaced acetabular fracture in the pregnant patient, minimal risk to the baby can be achieved in pursuit of acceptable articular reductions.