Novel MRI finding for diagnosis of invasive placenta praevia: evaluation of findings for 65 patients using clinical and histopathological correlations.

OBJECTIVE: To review established magnetic resonance (MR) criteria and describe a new MR finding for the diagnosis of invasive placenta praevia.
METHODS: A retrospective review of prenatal MRI examinations of 65 patients (median age: 35 years) who underwent MR for the screening of invasive placenta praevia. All MRIs were performed on a 1.5-T unit, including axial, coronal and sagittal T2-weighted half-Fourier single-shot turbo spin echo imaging. Fifteen patients were diagnosed with invasive placenta praevia. Two experienced radiologists reviewed the MR images and evaluated a total of six MRI features of the placenta, including our novel finding of the placental protrusion into the internal os (placental protrusion sign). Inter-rater reliability was assessed by using kappa statistics. Features with a kappa statistic >0.40 were evaluated using Fisher's two-sided exact test for comparison of their capabilities for placental invasion assessment.
RESULTS: Interobserver reliability was moderate or better for the intraplacental T2 dark band, intraplacental abnormal vascularity, uterine bulging, heterogeneous placenta and placental protrusion sign. Fisher's two-sided exact test results showed all these features were significantly associated with invasive placenta praevia.
CONCLUSION: The novel MRI finding of a placental protrusion sign is a useful addition to the established MRI findings for the diagnosis of invasive placenta praevia. KEY POINTS: • Prenatal diagnosis for an invasive placenta is essential for perinatal planning. • Magnetic resonance imaging provides useful information for the diagnosis of invasive placenta. • The placental protrusion sign is a useful novel MRI finding for predicting invasive placenta.