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Practical Comments: Prior to Fetal MRI

Is the study indicated? A fetal MR examination may be ordered for an indication more appropriately answered by US. If the anatomy can be appropriately evaluated on US, then the MR examination is likely not needed.

Is there a high quality US for comparison? Quality of US varies among sites. It is helpful to have a sonogram performed by an individual experienced in detection and characterization of fetal anomalies. This will lessen the perceived impact of MRI on patient care (Fig. 1).

 
figure 1
 
Fig. 1. Coronal MR image of a fetus at 19 weeks referred for congenital diaphragmatic hernia. Confirmatory sonogram showed an anechoic cyst in the chest, absent stomach below the diaphragm, and no mediastinal shift. Due to the lack of mediastinal shift, the confirmatory sonographic diagnosis was felt to be a combination of foregut duplication cyst and esophageal atresia. MR showed the diaphragm to be intact. Without a confirmatory US this type of finding would suggest that MR showed increased information compared to US. Since a confirmatory sonogram was performed, we conclude that MR showed no new information and did not change patient care.
 
Is there a comparison to a RECENT US? Optimally, the US is performed immediately prior to the MR examination (Fig 2). Besides providing sonographic diagnoses for comparison with the MR examination, an US preceding the MR examination is helpful in directing placement of the surface coil to the area of interest in the fetus with respect to the maternal body (Fig 3).
 
figure 2
 
Fig. 2. US of fetus at 31 weeks gestation with a cystic brain lesion (calipers). This exam was performed 3 days prior to the MR examination. This was a fetus from a triamniotic dichorionic triplet pregnancy with demise of one of the monochorionic pair. A repeat US (not shown) performed immediately prior to the scheduled MR examination showed demise of this fetus, and therefore the MR examination was cancelled.
 
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Fig. 3. Three images of a fetus at 35 weeks gestational age with bladder exstrophy. Note the decreased signal in (B) and (C) relative to (A) in the maternal anterior abdominal wall and in the fetal structures as the images in (B) and (C) are obtained at the edge of the surface coil.
 
Patient positioning: We position the patient supine, with feet entering the magnet to minimize feeling of claustrophobia. A surface coil is centered over the region of interest (established on US performed immediately prior to the MR study). A pillow is placed below the patient's knees. If the patient is uncomfortable lying on her back for prolonged periods then the patient is imaged lying on her side (Fig. 4).
 
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Fig. 4. Two fetuses at 28 weeks gestational age. In (A) the patient was scanned in supine position and in (B) the patient was scanned in lateral decubitus position.