(Radiographics. 2002;22:580-582.)
© RSNA, 2002
Invited Commentary
Fergus V. Coakley, MD
Department of Radiology, University of California, San Francisco, San Francisco, California
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Commentary
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The preceding article by Nagayama et al (1) is a timely and comprehensive review of the maternal (as distinct from fetal) applications of modern fast MR imaging in obstetrics. Safety concerns, technical issues, and specific indications are discussed and reviewed. The incremental clinical benefits of properly performed MR imaging studies are highlighted, with illustrated examples. A number of observations merit specific emphasis and comment.
The safety of MR imaging during pregnancy is of paramount concern to both physicians and parents. Although the absolute safety of MR imaging during pregnancy has not been definitively established, it is equally correct to state that no studies have demonstrated a clear risk to unborn human fetuses from the procedure. However, animal studies have raised the question of possible harmful effects early in development (2), and a cautious approach to MR imaging in the first trimester is appropriate. This does not mean that first-trimester pregnancy is an absolute contraindication to MR imaging; rather, such cases should be considered on their merits after appropriate discussion of the risks and benefits and alternative imaging procedures. For example, the risks associated with a CT scan of the pelvis in a pregnant patient are not trivial, given that the subsequent incidence of childhood cancer in the exposed fetus is increased by a factor of two to four (3–6).
Gadolinium is teratogenic in animal studies, albeit in high doses, and therefore pregnancy represents a strong contraindication. A number of studies have evaluated the administration of gadolinium contrast material during pregnancy and reported no obvious harmful effects (7,8). However, the results of these small studies do not exclude the possibility of such effects. Administration of gadolinium contrast material to a pregnant patient, particularly in the first trimester when teratogenesis would be of greatest concern, should be considered only under exceptional circumstances. From a practical standpoint, con-trast material is of limited usefulness in most obstetric applications and would more likely be required for an extraabdominal indication such as a suspected maternal brain tumor. In such a situation, contrast-enhanced CT may represent the safer course of action. The only obstetric indication for gadolinium contrast material that has been suggested is the evaluation of placenta accreta (9), which is discussed further later in this commentary.
Readers should be aware that the clinical applications of obstetric MR imaging are still evolving and that many of the indications described are primarily based on anecdotal experience or small case series. More generally, the incremental benefit of MR imaging can be difficult to assess systematically because it is not always easy to determine whether imaging findings have truly changed management. Furthermore, determining the appropriateness of such management change can be even more problematic in pregnant patients, since a contemporaneous standard of reference is frequently lacking and the gold standard becomes clinical outcome or the results of postnatal imaging. Another potential pitfall in such studies is the use of comparison with US to demonstrate a beneficial role for MR imaging. This is a valid comparison only if the MR imaging and US studies are of similar quality and the results are interpreted by experienced observers. Studies in which MR imaging is compared with US performed at a variety of community hospitals (10) tend to exaggerate the apparent advantages of MR imaging in comparison with studies in which US is performed at tertiary referral centers and the results are interpreted by experienced prenatal sonologists (11).
The usefulness of MR urography in symptomatic hydronephrosis during pregnancy remains controversial. Such patients are likely to be treated with internal or external decompression as merited by clinical severity, irrespective of the underlying cause. Therefore, the role of imaging, other than US to confirm the presence of hydronephrosis, is uncertain. Furthermore, the assertion that MR urography allows reliable identification of obstructing stones runs contrary to our institutional experience and appears to be based on the results of small studies, which may not be representative of the performance of MR urography in a broader context (12,13).
In the generic sense, the term placenta accreta refers to any degree of abnormally deep placental attachment. In the more narrow use of the term, placenta accreta refers only to superficial myometrial invasion, whereas the terms placenta increta and placenta percreta are used to refer to deep myometrial invasion and extrauterine extension, respectively. Invasion of the superficial myometrium is the commonest form (70%), whereas deep myometrial invasion and extrauterine extension account for only 20% and 10% of cases, respectively. Placenta accreta is a rare condition, affecting one in 7,000 pregnancies, but is clinically of great importance because the condition results in life-threatening hemorrhage at delivery and may require a cesarean hysterectomy for treatment. The condition is often suspected before delivery because of the presence of one or more risk factors, particularly placenta previa and a history of cesarean section. Twenty-four percent or more of such patients will have placenta accreta.
The reported MR imaging findings of placenta accreta are thinning or irregularity of the subplacental myometrium, transmural extension of placental signal intensity, and extrauterine invasion of local structures (14). Note that the latter finding is a sign of placenta percreta and will be seen only in a minority of cases. Therefore, most cases can be recognized only by identification of abnormality at the interface between the placenta and myometrium. The largest study to date of the role of MR imaging in diagnosis of placenta accreta examined 18 high-risk patients, six of whom had proved placenta accreta (15). Two independent readers examined US images alone followed by US and MR images to assess the likelihood of placenta accreta. For one reader, the addition of MR imaging improved the number of true positives from five to six; for the second reader, the addition of MR imaging left the number of true positives unchanged at four. These results do not suggest a compelling role for MR imaging in diagnosis of placenta accreta. In practice, we have found placenta accreta a difficult diagnosis to confidently include or exclude. It is our practice to inform clinicians of these limitations prior to performing such studies, in order to avoid unrealistic expectations.
Obstetric MR imaging continues to grow and evolve. The current applications are predominantly problem solving, and frequently arise after a US examination with indeterminate or inconclusive results. With appropriate case selection and preprocedural consultation, the benefits of obstetric MR imaging should outweigh any small potential risks. Although US remains the primary modality for obstetric imaging, MR imaging is a useful adjunct that can provide useful and clinically important incremental information in appro-priately selected cases. The review of maternal applications of obstetric MR imaging by Nagayama et al (1) represents a topical and inclusive review of maternal applications in obstetric MR imaging.
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References
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