Fetal MRI's leading indications, limitations.(OBSTETRICS)

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.