Possibility of tissue heating and harm from loud noises seems unlikely but raises concerns

Some scientists are concerned that using MR imaging during pregnancy may harm the fetus, with fears focusing primarily on teratogenic effects and acoustic damage.

Several studies, however, show children who were exposed to 1.5T MR in utero do not demonstrate exposure-related negative outcomes at nine months or up to nine years old (Br J Radiol 2000;73:190-194, Magn Reson Imaging 2004;22:851-854).

“Possible mechanisms for apparent deleterious effects include the heating effect of magnetic resonance gradient changes and direct nonthermal interaction of the electromagnetic field with biological structures,” said Dr. Morie M. Chen, a clinical fellow in the radiology department at the University of California, San Francisco, and colleagues (Obstet Gynecol 2008;112:333-340).

Since tissue heating is greatest at the maternal body surface and approaches negligible levels near the body center, however, it is unlikely that thermal damage to the fetus is a serious risk, the researchers said. Even though teratogenic risks are unlikely, the Chen study advises physicians to take precautionary measures, especially during the first trimester, because several animal studies demonstrate that teratogenic effects are possible.

The 2007 American College of Radiology guidance document for safe MR practices does not differentiate among pregnancy trimesters and states that all pregnant patients may receive MR as long as the risk-benefit ratio warrants it.

“From a practical viewpoint, first trimester MRI usually will be performed for maternal rather than fetal indications, and in this context, MRI is still preferable to any imaging study involving ionizing radiation,” Chen said.

Another possible risk to the fetus is acoustic damage due to the loud noises generated by the MR scanner gradient coils, especially with echo-planar imaging, which is the noisiest sequence in current clinical use, according to Chen.

Several studies, however, show no significant risk of acoustic injury to the fetus during prenatal MR (Am J Obstet Gynecol 1994;170:32-33, Br J Radiol 1995;68:1090-1094).

In fact, acoustic damage to the fetus during pregnancy appears to be a theoretical concern rather than a real one, according to the Chen study.

-RM