Visualization of the Anal Dimple by Fetal MRI: Is It Feasible?

The aim of this study was to determine the feasibility of fetal MRI in identifying the normal anal dimple (AD) and compare it with prenatal ultrasound (US). Retrospective review of 130 patients with both fetal MRI and US. The gestational age (GA) was stratified into four groups: (1) 16 to 21 weeks-6 days; (2) 22 to 27 weeks-6 days; (3) 28 to 33 weeks-6 days; and (4) 34 weeks and beyond. Steady-state free precession (SSFP) and single shot fast spin echo (SSFSE) axial T2 MRI and transverse US images of the fetal perineum were analyzed, and visualization of the AD was determined. Clinical indication, gender, single versus multiple gestation, best MRI sequence where it was seen, and postnatal AD information were recorded. The AD was visualized in 125/130 fetal MRIs, and visualization was independent of GA (p 0.230). US visualized the AD in 67/130 cases, and the best GA for visualization was in group 3 (p < 0.001). There was no difference in AD visualization between SSFSE and SSFP sequences (p 0.167). Prenatal visualization of the AD by MRI is feasible and superior to US, independent of GA. Adding AD visualization to routine screening prenatal US and MRI may increase recognition of anorectal malformation.

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