The Pregnant Patient: Alternatives to CT and Dose-Saving Modifications to CT Technique

Overview

Four key points should be remembered about performing CT in pregnant patients:

• The primary risk to the irradiated fetus is carcinogenesis (i.e., increased risk of childhood cancer) and not teratogenesis. The radiation dose from a single diagnostic CT is highly unlikely to exceed the estimated threshold dose of 100 mGy for the induction of malformations.
• The relative risk of childhood cancer may be as much as doubled in an irradiated fetus, but this should be offset by the risks of failing to diagnose serious disease in the mother and by realizing the absolute risk remains very small.
• Iodinated contrast (unlike gadolinium) appears safe to use in pregnancy, and arguably it is better to administer any potentially useful contrast (intravenous, oral, or rectal) prior to scanning rather than having to rescan because such contrast was not administered from the start.
• The fetus is exposed to a significant dose during a CT exam only when the fetus is within the primary beam. The scattered dose to the fetus from a chest or head CT for example is usually negligible, unless the lower cuts of a chest CT include the fetus in the primary beam (which might occur in late gestation).

Dose calculation and risk

The radiation dose to the fetus from a typical CT study of the maternal pelvis is variable and depends on the gestational age and scanning parameters, but typically ranges from about 10 to 50 mGy (1-3).

The fetal dose for an average-size patient can be estimated from the technique used to scan the pregnant uterus using dose conversion factor of 10.8 mGy/100 effective mAs, for 120 kV abdominal exams (4). (Effective mAs is defined as tube current in mA multiplied by rotation time in seconds divided by pitch.) For example, an effective mAs 222 for a CT scan of the pelvis would result in a fetal dose of 24 mGy (10.8 x 2.22). The baseline risk of childhood cancer is about 1.0 to 2.5 per 1000 (5). Estimates for the extra risk of childhood cancer from a fetal radiation dose of 1000 mGy range from 0.022 (Oxford Survey Childhood Cancer) through 0.028 (Life Span Study of atomic bomb survivors) to 0.060 (expert statistical review) (4, 6). After classifying these risk values as low, intermediate, and high, the excess risk of cancer for a variety of fetal doses would then be as follows:

Table 1

The lowest achievable dose is zero! That is, non-ionizing options are always preferable to any test with ionizing radiation in pregnant patients. In particular, most pregnant patients with pelvic pain should initially be scanned with ultrasound. When the diagnosis with ultrasound is not clear, imaging can be performed with MRI, in particular when appendicitis is suspected (7). Other strategies for CT dose reduction, which are equally applicable to CT in pregnant patients, have been described elsewhere (8) and are summarized in Table 2. More information can be obtained by referring to the ACR Practice guideline for imaging pregnant patients (9), which lists ultrasound as the initial imaging modality of choice for right lower quadrant pain in pregnancy, left lower quadrant pain in women of reproductive years, flank pain in pregnancy, and acute pelvic pain in the reproductive age group.

Table 2: Potential strategies and measures to reduce CT radiation dose and to address patient concerns regarding radiation risk.

References

• Damilakis J., Perisinakis K., Voloudaki A., Gourtsoyiannis N. “Estimation of Fetal Radiation Dose From Computed Tomography Scanning in Late Pregnancy: Depth-Dose Data From Routine Examinations,” Investigative Radiology, September 2000;35:527–33.
• McCoullough C.H., Schueler B.A., Atwell T.D., Braun N.N., Regner D.M., Brown D.L., LeRoy A.J.. “Radiation Exposure and Pregnancy: When Should We Be Concerned?,” RadioGraphics, July 2007;27:909 –917.
• Angel E., Wellnitz C.V., Goodsitt M.M., Yaghmai N., DeMarco J.J., Cagnon C.H., Sayre J.W., Cody D.D., Stevens D.M., Primak A.N., McCollough C.H., PhD and McNitt-Gray M.F.. “Radiation dose to the fetus for pregnant patients undergoing multidetector CT imaging: Monte Carlo simulations estimating fetal dose for a range of gestational age and patient size,” Radiology, October 2008;249:220–7.
• Doll R., Wakeford R. “Risk of childhood cancer from fetal irradiation,” British Journal of Radiology, February 1997;70:130-139.
• Stiller C.A., Parkin D.M. “Geographic and ethnic variations in the incidence of childhood cancer,” British Medical Bulletin, 1996;52:682–703.
• Ratnapalan S., Bentur Y., Koren G. “Doctor, will that x-ray harm my unborn child?” Canadian Medical Association Journal. December 2008;179:1293–1296.
• Pedrosa I, Levine D, Eyvazzadeh AD, Siewert B, Ngo L, Rofsky NM. MR imaging evaluation of acute appendicitis in pregnancy. Radiology 2006; 238: 891–9.
• Coakley F.V., Gould, R., Yeh B.M., Arenson R.L.. “CT radiation dose: What can you do right now in your practice?,” American Journal of Roentgenology, (In press).
• ACR Practice Guideline for imaging pregnant or potentially pregnant adolescents and women with ionizing radiation. 2008.

Useful links and resources

Chen M.M., Coakley F.V., Kaimal A., Laros R.K. Jr. “Guidelines for computed tomography and magnetic resonance imaging use during pregnancy and lactation," Obstetrics & Gynecology, August 2008; 112(2 Pt 1): 333-40.