Ultrasound Better than MRI for Placenta Accreta Diagnosis

Ultrasound remains most sensitive imaging modality when diagnosing placenta accreta.

Ultrasonography, when performed by obstetricians or radiologists experienced in abnormal adherent placenta, was more accurate than MRI for the diagnosis of placenta accreta, according to the results of a French study published recently in PLOS One.

MRI appeared to be complementary to diagnosis with ultrasound, especially in cases where the ultrasound yielded few signs of the condition.

“Placenta accreta is a significant cause of maternal morbidity and mortality, and its prevalence has risen 10-fold in the United States over the past 50 years due to the rising number of cesarean deliveries,” said Anne-Sophie Riteau, MD, of the Hôpital Antoine Béclère, Clamart, France. “What’s more, the prenatal diagnosis of placenta accreta still remains difficult; however, accurate prenatal identification allows optimal obstetric management and reduces maternal risks.”

Riteau and colleagues compared the two imaging modalities’ accuracy among 42 pregnant women who had undergone both ultrasound and MRI when suspected of having placenta accreta. The women were identified from the medical records of all patients referred for investigation of suspected placenta accreta at two hospitals between 2001 and 2012. Forty of these women had cesarean delivery.

Images from the investigation were reassessed by two raters with more than five years of experience diagnosing placentation disorders and scored for features that help to diagnose abnormal placental invasion.

Twenty-six women were identified as having placenta accreta. Ultrasound successfully diagnosed 100 percent of the cases. However, in 10 of the 16 women found to have a normal placenta, ultrasound had indicated adherent placenta. In contrast, MRI diagnosed 20 of 26 cases of placenta accreta, and gave false positives in eight of 16 women eventually found to have normal placenta.

The sensitivity of ultrasound was 100 percent compared with 76.9 percent for MRI (P=0.03) and the specificity of ultrasound was 37.5 percent compared with 50 percent for MRI (P=0.6).

The researchers also identified features present with each imaging modality that helped predict abnormal placental invasion. According to Riteau, the best predictive ultrasound features were found to be disappearance of retroplacental hypoechoic clear zone and vascularization perpendicular to the myometrium. The best MRI features were uterine bulging and the presence of dark intraplacental bands associated with thinning or disappearance of the myometrium.

“Ultrasonography remains the most sensitive and commonly used imaging modality for the diagnosis of placenta accreta, because it is accurate, inexpensive, non-invasive and time-saving,” Riteau told Diagnostic Imaging. “If there is a strong suspicion of placenta accreta or percreta at Doppler ultrasound, with several signs present with good positive predictive value, the result of the MRI exam should not alter the obstetric management and the placenta should be considered as accreta when organizing the course of delivery."

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