You don't necessarily need to rule out CT scans of pregnant patients if appropriate precautions are taken, according to a study in the May issue of the American Journal of Roentgenology. The study found that fetal radiation dose is generally low and has been dropping with newer generations of scanners.
That said, the authors warned that the type of scan and the gestational age of the fetus can have a statistically significant effect on radiation dose.
"These risks are strongly dependent on the gestational age of the fetus at the time of exposure and, consequently, the stage of development of the organs at risk, the magnitude of the dose, and the biologic sensitivity of the tissue and their repair mechanism," wrote Dr. Anthony Gilet and colleagues from Stony Brook University Medical Center (AJR, May 2011, Vol. 196:5, pp. 1133-1137).
Nevertheless, doses for CT scans of pregnant women are low and safe when scans are properly performed, they concluded -- which is fortunate considering that the volume of such scans has increased by 107% over the past decade.
Gilet and his team evaluated fetal doses for two common exams for which CT is difficult to replace in pregnant women: pulmonary CT angiography studies and abdominal and pelvic CT exams. The doses were evaluated on several generations of MDCT scanners (LightSpeed 4, LightSpeed 16, and LightSpeed 64 VCT, GE Healthcare), with the results measured in anthropomorphic phantoms simulating a pregnant patient.
The scans were performed for women in early pregnancy and subsequently at 10, 18, and 36 weeks. The investigators measured gestational age, fetal dose, and entrance skin exposure using the phantoms.
Phantoms were equipped with five standard 3 x 3 x 0.8-mm lithium fluoride thermoluminescent dosimeters (TLDs) positioned at the uterine fundus and modified to represent an average pregnant patient, based on a large cohort of pregnant patients examined with ultrasound.
"By changing both the maternal size and the fetal depth, the most accurate fetal dose estimate should be obtained," they wrote.
For the pulmonary CT angiograms, the patient's abdomen was shielded using two 0.5-mm-thick lead-equivalent aprons. The simulated CT examination of the chest covered the region from the lung apices to the diaphragmatic dome. The CT scan of the abdomen and pelvis covered the region from the lung bases to the pubic symphysis, Gilet and colleagues explained.
The results showed that with each scanner generation, fetal dose decreased for CT pulmonary angiography: The mean fetal dose was 0.77, 0.54, and 0.33 mGy for 4-, 16-, and 64-MDCT scanners, respectively.
When constant parameters were used for pulmonary CT angiography, the fetal radiation dose was not significantly associated with gestational age.
"These results are similar to those of previous studies and are well below the level at which deterministic adverse effects on the fetus would be seen," the authors wrote. "Using one current estimate for the stochastic result of cancer of approximately 0.006% per milligray, this would equate to a maximum increased risk of 0.0046%," they wrote.
Abdominal studies produce higher doses
For a single-phase study of the abdomen and pelvis, as expected, the fetal dose was far higher than with pulmonary CT angiography because the fetus is directly within the scan beam.
With the exception of the 64-MDCT scanner during the third trimester, the fetal dose was approximately 15 mGy for all pregnancy stages and all scanners.
"The exception to this rule is the 64-MDCT scanner at 32 weeks' gestation, where the fetal dose increased to 20.5 mGy, an approximately 33% increase," they wrote.
The higher dose resulted from the use of automatic tube current modulation (ATCM) on the 64-detector-row scanner, wherein tube current is increased to maintain constant signal-to-noise ratios through greater volumes of soft tissue in the gravid abdomen.
Second- and third-trimester scans were repeated on the 64-detector-row scanner with the addition of a 0.035-mm-thick lead-equivalent abdominal shield, which reduced the fetal dose by an average of 10%.
Appropriate utilization of pulmonary CT angiography will result in "negligible radiation dose to the fetus," the authors wrote. "Newer-generation scanners with greater detector number will actually confer a lower dose to the fetus, possibly because of improved efficiency of collimation."
The authors routinely use abdominal shielding during chest CT exams, and it was also used in the study to reduce fetal dose. The 10% dose decrease with the use of thin lead shielding for abdominal and pelvic CT on a 64-detector-row scanner should be further studied over a wide range of scan parameters to determine the maximum dose savings without loss of diagnostic ability, they wrote.
The authors cited several study limitations, mainly related to the difficulty of extrapolating the phantom readings to patients. The phantom assumes tissue homogeneity that may not exist in vivo, for example. Moreover, the phantom is designed to simulate the "average" patient, so differences in body habitus alter the fetal radiation dose, they wrote.
But they concluded that radiation dose to the fetus from CT pulmonary angiograms is low enough that it shouldn't prevent the performance of the exam when clinically appropriate. For CT of the abdomen and pelvis, scans are reasonable when nonradiation-based modalities are unavailable, such as in trauma settings. Scan parameters should be altered, however, especially during the third trimester, such as by using a maximum of 450 mAs.
