There is a widespread consensus that ultrasound is the clinical standard for the diagnosis of fetal anomalies, and a constellation of factors will ensure its central role into the foreseeable future.
Most importantly, both ultrasound technology and the expertise to perform and interpret it are now widely available. The technology also remains relatively inexpensive, compared with other modalities; its safety has been well established through both study and long-term experience; and it provides real-time visualization, as opposed to images acquired at a particular point in time. Overall, ultrasound should be the first technology employed in the evaluation of the fetal anomaly.
Still, there are well-recognized limitations to sonographic evaluation.
The ability to visualize structures—and thus, the accuracy of a diagnosis—is significantly compromised, for instance, in women who are obese. This is far from a trivial concern today, as the rate of obesity in the United States is high and climbing.
Sonographic evaluation also may be limited by fetal position. Even in an average-size woman, for instance, suboptimal fetal positioning can impair proper visualization of structures.
Another common limitation is the descent of the fetal head into the maternal pelvis. Transvaginal ultrasound is an alternative approach, but the physics of the transvaginal transducer often prevents us from seeing in as many planes as would normally be desirable.
Ultrasound tends to be optimal during midpregnancy. Beyond this point, calcification of the fetal bone structure intensifies. Cranial ossification, for example, can substantially obscure the visualization of intracranial structures.
Finally, effective ultrasound evaluation requires fluid around the fetus. With oligohydramnios, the quality of the sonographic images is significantly compromised.
All told, these limitations are not infrequent or inconsequential. Clinicians commonly encounter such situations during the course of their work.
MRI Technique and Safety
Fetal magnetic resonance imaging provides excellent tissue contrast and is not limited by maternal obesity, skull calcification, or fetal position. It can image the fetus in multiple planes and accomplish this with a large field of view.
MRI can therefore play a valuable role when the findings from ultrasound are unclear or incomplete, or when there is potential for other anomalies that cannot be sufficiently visualized with ultrasound.
MRI relies on the presence of the high water content of tissues, and on the magnetic qualities of the constituent hydrogen nuclei. When tissue is placed in the strong magnetic field of an MRI machine, the hydrogen nuclei or protons move into particular alignments with the applied magnetic field.
Once the protons are lined up, radio frequency pulses are applied, causing the protons to absorb additional energy and spin on their axes of alignment. When the radio frequency pulses are discontinued, the additional energy that the protons had previously absorbed is released. It is this released energy that is transformed into an image. The quantity of energy released will vary depending on the tissue characteristics, such as the relative water and fat content.
Unlike x-ray and CT scans, MRI does not use ionizing radiation. Numerous studies and reports, including studies of MRI technicians who become pregnant, have demonstrated the safety of MRI and the lack of adverse clinical effects. The American College of Radiology published a series of white papers from 1993 to 2004 outlining MRI's safety. Thus, although the safety of MRI continues to be studied, there is no evidence to date that MRI produces harmful effects on human embryos or fetuses.
To be exceedingly cautious, most authorities and practitioners of MRI advise that it not be done in the first trimester.
Even without this extra caution, however, MRI would likely be discouraged in the first trimester because the increased noise-to-signal ratio from imaging such a small structure limits its benefit. It isn't until later in the second trimester, with increased fetal size and fat content, that the quality and resolution of the images achieve a threshold that conveys clinical benefit.
MRI's Leading Indications
MRI is indicated when there is potential for significant change in diagnosis or in patient management beyond the initial ultrasound.
Several studies from both the United States and Europe have demonstrated the clear capability of MRI to significantly modify or alter diagnosis, patient counseling, and management.
In one study of 124 fetuses with central nervous system anomalies detected initially by ultrasound, Dr. Deborah Levine of Harvard Medical School and her colleagues showed that fetal MRI led to 49 major changes in diagnosis and 27 clear changes in management, compared with prior ultrasound.
Suspected central nervous system anomalies—particularly brain anomalies—are, in fact, the most common indication for fetal MRI. There is some literature to support benefits of fetal MRI for other anatomical defects, but the literature provides the strongest evidence of MRI's additional benefit for CNS anomalies. Beyond the CNS, the other two main clinical indications for fetal MRI are for evaluation of the fetal neck and chest.