Role of MRI in assessment of placenta accreta

Abstract

Placenta accrete (PA) is a significant cause of maternal morbidity and mortality and is now the most common reason for emergent postpartum hysterectomy. Placenta previa and previous cesarean section are the two most important known risk factors for placenta accreta. Accurate prenatal identification of affected pregnancies allows optimal obstetric management. US has been the primary diagnostic tool for PA for many years and has been shown to help detect this disorder in 50%–80% of cases .Most patients will present for a US examination at 18–20 weeks gestation, which provides an ideal opportunity to screen for the disorder. Although Ultrasound is the first-rate method to diagnose placental invasion. However, it is usual to notice discrepancies between ultrasound diagnosis and surgical complexity, which make adequate therapeutic planning difficult.

The findings ultrasound described in the literature have always only described implantation anteriorly in the lower uterine segment of women with placenta Previa and a previous caesarean section, however ultrasound findings are not accurate in diagnosis of posterior and laterally located placenta accrete. In recent years, there has been increased interest in magnetic resonance (MR) imaging for the evaluation of Placenta accreta, since it can provide information on depth of invasion and more clearly detect posterior and lateral placentas. It can be used as a diagnostic and complementary imaging modality especially when ultrasound or Doppler findings are not accurate. 1.5-T MRI scanner is often used with T2-weighted half-Fourier RARE (rapid acquisition with relaxation enhancement) sequence (HASTE or half-Fourier single-shot fast spin-echo) acquired in the axial, sagittal, and coronal planes is the key technique employed for evaluation of placenta. Balanced steady-state free precession (True FISP) sequence in three orthogonal planes and T1-weighted gradient-echo sequence in any one plane are also acquired. Diffusion-weighted imaging (DWI) of MRI is a recently prevailing technique used to show tissue characteristics based on tissue-specific diffusion motion of water molecules. Normal endometrium has a high DWI signal intensity because of its high cellularity and abundant cytoplasm. Key imaging features of normal placentation are Homogeneous T2-intermediate signal intensity of placenta , Subtle thin, regularly spaced placental septi ,normal sub-placental vascularity , triple-layered sandwich appearance of myometrium and Pear-shape of normal gravid uterus with smooth contour. Various MR imaging features of Placenta accreta with differing sensitivities and specificities are described in literature. The cardinal signs are: placenta Previa, dark intraplacental bands on T2-weighted images. Heterogeneity within the placenta, Abnormal disorganized placental vascularity.