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Obstetric Imaging |
1 From the Departments of Radiology (F.V.C., H.H., R.A.F., A.J.B.) and Surgery, Fetal Treatment Center (M.R.H.), University of California, San Francisco, 505 Parnassus Ave, San Francisco, CA 94143-0628. From the 1998 RSNA scientific assembly. Received January 11, 1999; revision requested February 18; revision received March 10; accepted July 1. Address reprint requests to F.V.C. (e-mail: fergus.coakley@radiology.ucsf.edu).
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Abstract |
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MATERIALS AND METHODS: MR imaging of the fetus was performed in 25 consecutive pregnant patients referred because of possible complex fetal disorders suspected on the basis of ultrasonographic (US) findings. Spoiled gradient-echo and single-shot rapid acquisition with relaxation enhancement MR imaging were performed in multiple planes anatomic to the fetus during maternal breath holding.
RESULTS: In the fetuses in 24 of 25 women, MR studies were technically satisfactory. MR imaging directly influenced fetal care in four (17%) of 24 cases by demonstrating congenital high airway obstruction syndrome, congenital hemochromatosis, unilateral cerebellar deficiency in association with congenital diaphragmatic hernia, and severe facial disfigurement due to a giant anterior neck mass. In eight (33%) cases, MR imaging provided supplementary findings, but did not affect fetal care. In 12 (50%) cases, MR imaging results confirmed US findings.
CONCLUSION: In cases of complex fetal disorders, MR imaging results can be used to supplement or confirm US findings and may directly affect management.
Index terms: Brain, hydrocephalus, 10.145 • Fetus, central nervous system, 153.141, 856.8744 • Fetus, gastrointestinal tract, 761.594, 856.14, 856.8754 • Fetus, MR, 856.121411, 856.121412 • Fetus, respiratory system, 67.141, 856.8759 • Fetus, US, 856.12981, 856.12983 • Hemochromatosis, 761.659, 856.8769 • Hernia, diaphragmatic, 856.8754 • Lung, congenital malformation, 67.141, 856.8759
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Introduction |
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The use of magnetic resonance (MR) imaging in pregnancy was described in 1983 (3). Initially, the major obstetric applications of MR imaging were in the evaluation of maternal and placental diseases (1,4). In those early days, image degradation by means of fetal motion was a major problem, and MR imaging of the fetus was largely confined to volumetric measurements (5–7). Attempts to eliminate fetal motion artifact have included the administration of muscle relaxants directly into the umbilical vein (8).
The recent development of single-shot rapid acquisition with relaxation enhancement MR imaging has been a major advance in the evolution of fetal MR imaging (9). This sequence, which is commercially available as "single-shot fast spin-echo" and "half-Fourier acquisition turbo spin-echo," is a very rapid T2-weighted sequence with a section acquisition time of less than 1 second. This rapid acquisition time effectively "freezes" fetal motion and makes possible the routine acquisition of high-quality T2-weighted MR images of the fetus during maternal breath holding (10). The sequences are commercially available as upgrade options from major manufacturers and can be installed in medium- to high-field-strength MR imaging units with high-performance gradient capabilities. An additional impetus to fetal MR imaging has been the emergence of fetal medicine as a recognized specialty (11).
Despite the technologic innovations that have led to the emergence of fetal MR imaging, to our knowledge the incremental clinical benefit has not been evaluated. We undertook this study to determine the effect of MR imaging on management in complex fetal disorders.
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MATERIALS AND METHODS |
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A total of 1,160 detailed obstetric US examinations were performed during the study period. There were 23 singleton and two twin pregnancies in the study group. The mean maternal age was 30 years (range, 20–42 years). The mean gestational age was 25 weeks (range, 20–35 weeks). All patients underwent detailed obstetric US before MR imaging. US and MR imaging were performed on the same day in 22 patients. In the remaining three patients, MR imaging was performed 2, 4, and 14 days after the US examination.
Reasons for referral for MR imaging were as follows:
1. To further evaluate an intracranial abnormality seen at US (n = 13), which included isolated lateral ventriculomegaly (n = 6); intraventricular hemorrhage (n = 3), including one patient with a history of recurrent fetal demise due to an undiagnosed fetal condition characterized by multiple intracerebral hemorrhages; hydrocephalus (n = 3); and subependymal nodularity (n = 1).
2. To determine suitability for fetal surgery (n = 10). The conditions under consideration for fetal surgery were CDH (n = 6), myelomeningocele (n = 1), congenital high airway obstruction syndrome (CHAOS) (n = 1), amniotic band syndrome (n = 1), and giant anterior neck mass (n = 1). Unilateral cerebellar deficiency was suspected on the basis of US findings in one fetus with CDH, and this patient was primarily referred for assessment of the fetal cerebellum and cerebrum rather than for assessment of CDH. This is because a major coexistent structural abnormality is considered to be a contraindication to fetal surgery for CDH (12).
3. To evaluate for possible congenital hemochromatosis (n = 1). This patient had a history of recurrent fetal demise due to congenital hemochromatosis. In both previous unsuccessful pregnancies, fetal demise was preceded by oligohydramnios. In the current pregnancy, serial US surveillance demonstrated oligohydramnios early in the 3rd trimester. At US, the fetal liver was unremarkable. Hemochromatosis was strongly suspected clinically, and MR imaging was requested.
4. To evaluate for possible ischemic brain damage (n = l). The fetus in question was a living twin who sustained a profound bradycardic episode after division of the umbilical cord to the other twin. The results of karyotype analysis confirmed the diagnosis of Turner syndrome in the latter twin, who had diffuse lymphangiectasia and hydrops seen at US. The umbilical cord to the twin with Turner syndrome was divided to prevent twin-twin embolization syndrome. The twins were monochorionic and diamniotic. Prenatal cranial US of the living twin was unremarkable both immediately and 2 weeks after the procedure. MR imaging was requested 2 weeks after the procedure to evaluate for ischemic brain injury that was occult at US.
US and MR Imaging Techniques
US was performed with state-of-the-art equipment (Sequoia; Acuson, Mountain View, Calif) and 3.5–5.0-MHz transducers. The examinations were combined gray-scale and color Doppler studies. All US studies were reviewed and reported by attending radiologists with extensive experience in prenatal US.
MR imaging was performed with a 1.5-T superconducting magnet (Signa; GE Medical Systems, Milwaukee, Wis) and a two- or four-element phased-array surface coil (GE Medical Systems). The choice of coil was determined on the basis patient size: Smaller patients underwent imaging with the two-element coil, and larger patients underwent imaging with the four-element coil. T1-weighted MR images were obtained by using a breath-hold spoiled gradient-echo sequence (100–140/4.2 [repetition time msec/echo time msec], 70°–90° flip angle, 256 x 160–256 matrix, one signal acquired). T2-weighted images were obtained using a single-shot rapid acquisition with relaxation enhancement sequence (
/100–120, 256 x 160–256 matrix). A variable bandwidth was used for all sequences. Sequence acquisition times were all less than 30 seconds. The section thickness was 4–6 mm, and the intersection gap was 0–1 mm. The field of view, number of sections, section thickness, and intersection gap were optimized for each patient by the supervising radiologist (F.V.C.). For brain studies, T1- and T2-weighted images were obtained in the axial, coronal, and sagittal planes relative to the fetus. For other studies, sequence choice and plane of section were chosen as appropriate for the clinical context. In the case of possible congenital hemochromatosis, a T2*-weighted gradient-echo sequence was performed (130/20, 20° flip angle). These parameters (long repetition and echo times and small flip angle) were chosen because they have been shown (13) to optimize MR quantification of iron overload in adult hemochromatosis.
Image Interpretation
The MR imaging studies were interpreted by attending radiologists who were experienced in MR imaging and who had a special interest in genitourinary radiology (F.V.C., H.H.). Images in neurologic cases were interpreted in consultation with an attending pediatric neuroradiologist (A.J.B.). Clinical and previous imaging results, including US findings, were available to the radiologists who interpreted the MR studies. Findings were interpreted by means of consensus. Image quality was rated as diagnostically satisfactory or unsatisfactory.
Assessment of Effect on Management
The impression in the original MR imaging report was used for assessment of the effect on management. The effect of MR imaging findings on management was assessed by means of direct consultation with the referring physician and with clinical follow-up results. Cases referred by the Fetal Treatment Center (n = 14) were presented at the weekly fetal treatment meeting. The emphasis of this meeting is the formulation of a management plan, so the role of radiology in planning patient care can be assessed with particular immediacy. MR imaging findings were considered to have affected management if subsequent treatment was consistent with and positively influenced by the MR imaging diagnosis and if the referring physician concurred. All other studies were considered to have had no effect on management. The study was performed retrospectively, and referring physicians were not asked to formulate pre– and post–MR imaging treatment plans.
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RESULTS |
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In the single technically unsatisfactory study, MR imaging was limited due to fetal motion artifact. No gross abnormality was seen. The fetus was at 24 weeks gestation. An initial US study obtained at another institution was reported as showing a small intraventricular hemorrhage. The US study obtained at our institution before MR imaging was interpreted as normal.
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DISCUSSION |
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These results demonstrate that fetal MR imaging can be used as a complementary modality to US in those rare cases of fetal abnormality in which US findings are indeterminate or equivocal. In such cases, when critical management decisions are dependent on the radiologic diagnosis, supplementary or corroborative MR imaging should be performed. We do not suggest that MR imaging be used as an alternative to US for routine prenatal screening. It should be noted that the MR imaging examinations in this study were individually tailored to address specific questions raised by the US findings and were not comprehensive "head-to-toe" screening studies of the fetus.
The prenatal diagnoses of CHAOS and congenital hemochromatosis are good examples of how fetal MR imaging can contribute to management in rare and complex anomalies. CHAOS is a rare condition characterized by developmental obstruction of the upper airway and resulting in retention of bronchial secretions and pulmonary distention (17). Overinflation of the lungs is believed to impair venous return to the heart, leading to fetal hydrops and ascites. Confirmation of the diagnosis at MR imaging was followed by successful in utero tracheostomy. To our knowledge, postnatal survival has not previously been described for this condition. Congenital (perinatal, neonatal) hemochromatosis is characterized clinically by severe neonatal liver failure and histologically by prominent stainable iron in the parenchymal cells of the liver and other viscera (18). Oligohydramnios may occur (19). A specific cause has not been identified (18), but excessive transfer of iron from the mother to the fetus across the placenta may be the underlying abnormality (20). We are aware of only one previous case report (21) in which the prenatal MR imaging diagnosis of congenital hemochromatosis was described.
There are four potential criticisms of our study. First, the study population (n = 25) was relatively small, particularly when compared with the large number of detailed obstetric US examinations performed during the study period (n = 1,160). However, this reflects the adequacy of clinical and US evaluation in the majority of fetal abnormalities. It also may be due to limited awareness of fetal MR imaging, financial constraints on the use of expensive high-technology investigations, and the currently limited potential for fetal intervention. The small number of patients is indicative of a selection bias in our study population, because only patients with findings that were clinically complex or inconclusive at US or who were under consideration for fetal intervention were referred for MR imaging. Nonetheless, we specifically wished to investigate cases of such complex abnormalities.
Second, the study was performed retrospectively and was based on the original reports, which were generated by radiologists with knowledge of clinical and US results. This method may also have introduced bias in our study, but it had the advantage of producing results that represented the real-life incremental benefit of MR imaging.
Third, we determined the effect of MR imaging on management by means of a retrospective review of patient outcome and by consulting with the referring physician. We judged MR imaging findings to have influenced management if subsequent treatment was consistent with and positively influenced by the MR imaging diagnosis and if the referring physician concurred. This method is not scientifically rigorous, but we believe it is a fair and reasonable approach in the context of a preliminary retrospective clinical review. If anything, this method may have resulted in underestimation of the true effect of MR imaging, because it did not allow inclusion of cases in which MR imaging findings may have increased physician confidence in the choice of management or helped in parental counseling.
Fourth, pathologic confirmation of the presumptive imaging diagnosis was not obtained. This is a general problem in prenatal imaging research, because a tissue diagnosis rarely is available. Postnatal imaging can be used as a standard of reference, but this remains a radiologic rather than a pathologic standard and may not accurately reflect the pathologic conditions that existed at the time of prenatal imaging. The effect on management may be a more appropriate end point for prenatal imaging outcomes research.
In conclusion, our preliminary results indicate that MR imaging of complex fetal disorders can provide incremental information that may directly affect management in a substantial proportion of cases; in other cases, MR imaging findings may help supplement or confirm indeterminate or equivocal US findings. These results have important implications for clinicians and radiologists who may have difficulty reaching critical management decisions that are based purely on prenatal US results. In such cases, fetal MR imaging should be considered. We believe fetal MR imaging will remain a niche application with a limited but definite role as a problem-solving modality when prenatal US results are indeterminate or inconclusive, and supplementary or corroborative diagnostic assessment is needed. The degree of confidence required for prenatal diagnosis is clearly higher in cases where fetal intervention is a consideration, and fetal MR imaging may become more widely used as fetal surgery develops.
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Footnotes |
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Author contributions: Guarantor of integrity of entire study, F.V.C.; study concepts and design, F.V.C., H.H.; definition of intellectual content, F.V.C., H.H.; literature research, F.V.C.; clinical studies, F.V.C., H.H., R.A.F.; data acquisition, F.V.C., A.J.B.; data analysis, F.V.C.; manuscript preparation, F.V.C.; manuscript editing, F.V.C., H.H.; manuscript review, R.A.F., A.J.B., M.R.H.
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