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

Fergus V. Coakley, MD University of California, San Francisco, CA

Dianna D. Cody, PhD The University of Texas MD Anderson Cancer Center, Houston, TX

Mahadevappa Mahesh, PhD, FACR Johns Hopkins University School of Medicine, Baltimore, MD

Overview

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

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:

Table1

Dose (mGy)
Low risk model
Intermediate risk model
High risk model
10
1 in 4545
1 in 3571
1 in 1667
20
1 in 2272
1 in 1786
1 in 834
30
1 in 1515
1 in 1190
1 in 556
40
1 in 1136
1 in 892
1 in 417
50
1 in 909
1 in 714
1 in 334

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.

Category
Measure
General
Provide patient information material Review CT protocols and indications
Before the test
Promote alternative non-ionizing studies Decision support software
During the test
Automatic tube current modulation Empower technologists to adjust protocol Improve reconstruction algorithms
After the test
Calculate radiation dose Report radiation dose

References

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.