Interobserver agreement in MRI assessment of severity of placenta accreta spectrum disorders

Objective To evaluate the level of agreement in the prenatal magnetic resonance imaging (MRI) assessment of the presence and severity of placenta accreta spectrum (PAS) disorders between examiners with expertise in the diagnosis and management of these conditions. Methods This was a secondary analysis of a prospective study including women with placenta previa or low‐lying placenta and at least one prior Cesarean delivery or uterine surgery, who underwent MRI assessment at a regional referral center for PAS disorders in Italy, between 2007 and 2017. The MRI scans were retrieved from the hospital electronic database and assessed by four examiners, who are considered to be experts in the diagnosis and surgical management of PAS disorders. The examiners were blinded to the ultrasound diagnosis, histopathological findings and clinical data of the patients. Each examiner was asked to assess 20 features on the MRI scans, including the presence, depth and topography of placental invasion. Depth of invasion was defined as the degree of adhesion and invasion of the placenta into the myometrium and uterine serosa (placenta accreta, increta or percreta) and the histopathological examination of the removed uterus was considered the reference standard. Topography of the placental invasion was defined as the site of placental invasion within the uterus in relation to the posterior bladder wall (posterior upper bladder wall and uterine body, posterior lower bladder wall and lower uterine segment and cervix or no visible bladder invasion) and the site of invasion at surgery was considered the reference standard. The degree of interrater agreement (IRA) was evaluated by calculating both the percentage of observed agreement among raters and the Fleiss kappa (κ) value. Results Forty‐six women were included in the study. The median gestational age at MRI was 33.8 (interquartile range, 33.1–34.0) weeks. A final diagnosis of placenta accreta, increta and percreta was made in 15.2%, 17.4% and 50.0% patients, respectively. There was excellent agreement between the four examiners in the assessment of the overall presence of a PAS disorder (IRA, 92.1% (95% CI, 86.8–94.0%); κ, 0.90 (95% CI, 0.89–1.00)). However, there was significant heterogeneity in IRA when assessing the different MRI signs suggestive of a PAS disorder. There was excellent agreement between the examiners in the identification of the depth of placental invasion on MRI (IRA, 98.9% (95% CI, 96.8–100.0%); κ, 0.95 (95% CI, 0.89–1.00)). However, agreement in assessing the topography of placental invasion was only moderate (IRA, 72.8% (95% CI, 72.7–72.9%); κ, 0.56 (95% CI, 0.54–0.66)). More importantly, when assessing parametrial invasion, which is one of the most significant prognostic factors in women affected by PAS, the agreement was substantial and moderate in judging the presence of invasion in the coronal (IRA, 86.6% (95% CI, 86.5–86.7%); κ, 0.69 (95% CI, 0.59–0.71)) and axial (IRA, 78.6% (95% CI, 78.5–78.7%); κ, 0.56 (95% CI, 0.33–0.60)) planes, respectively. Likewise, interobserver agreement in judging the presence and the number of newly formed vessels in the parametrial tissue was moderate (IRA, 88.0% (95% CI, 88.0–88.1%); κ, 0.59 (95% CI, 0.45–0.68)) and fair (IRA, 66.7% (95% CI, 66.6–66.7%); κ, 0.22 (95% CI, 0.12–0.37)), respectively. Conclusions MRI has excellent interobserver agreement in detecting the presence and depth of placental invasion, while agreement between the examiners is lower when assessing the topography of invasion. The findings of this study highlight the need for a standardized MRI staging system for PAS disorders, in order to facilitate objective correlation between prenatal imaging, pregnancy outcome and surgical management of these patients