Objectives The gold standard of postpartum anal sphincter imaging has been the 3D endoanal ultrasound (EAUS). Development of magnetic resonance imaging (MRI) has allowed anal sphincter evaluation without the use of endoanal coils. The aim of this study is to compare these two modalities in diagnosing residual sphincter lesions post obstetric anal sphincter injury (OASI). Methods Forty women were followed up after primary repair of OASI with both 3D EAUS and external phased array MRI. Details of the anal sphincter injury and sphincter musculature were gathered and analysed. Results There was a moderate interrater reliability ( κ = 0.510) between the two imaging modalities in detecting sphincter lesions, with more lesions detected by MRI. There was a moderate intraclass correlation (ICC) between the circumference of the tear ( κ = 0.506) and a fair ICC between the external anal sphincter thickness measurements at locations 3 and 9 on the proctologic clock face ( κ = 0.320) and ( κ = 0.336). Conclusions The results of our study indicate that the use of external phased array MRI is feasible for detecting obstetric anal sphincter lesions postpartum. This allows for imaging of the sphincter defects in centres where EAUS imaging is not available. Key Points • A two centre prospective study that showed external phased array MRI to be a valid imaging modality for diagnosing obstetric anal sphincter injuries. Electronic supplementary material The online version of this article (10.1007/s00330-019-06125-8) contains supplementary material, which is available to authorized users.

Background Three-dimensional endoanal ultrasound (3D EAUS) has been the gold standard for detecting anal sphincter lesions in patients with a history of obstetric anal sphincter injury (OASI). Advances in imaging technologies have facilitated the detection of these lesions with external phased-array magnetic resonance imaging (MRI), which could offer an alternative imaging modality for the diagnosis of residual OASI (ROASI) in centers where 3D EAUS imaging is not available. Purpose To compare two diagnostic modalities: the 3D EAUS and 3T external phased-array MRI in the detection of residual anal sphincter lesions. Material and Methods A total of 24 women with a history of OASI were imaged with both 3D EAUS and 3T external phased-array MRI after primary repair of the injury. Intraclass correlation (ICC) and interrater reliability (IRR) values were calculated for the grade and circumference of the sphincter lesion. Sphincter lesions were graded according to the Sultan classification. Results There was an almost perfect agreement between 3D EAUS and 3T external phased-array MRI in determining the extent of the sphincter lesions according to the Sultan classification (κ = 0.881; P < 0.001) and the circumference of the external anal sphincter defects, measured in degrees (κ = 0.896; P < 0.001). Conclusion The results of this study indicate that 3T external phased-array MRI and 3D EAUS yield comparable results in the diagnosis of ROASI. These findings suggest that 3T external phased-array MRI could serve as an alternative diagnostic modality to 3D EAUS in the diagnosis of ROASI.