ELSEVIER l Original Contribution Magnetic Resonance Imaging, Vol. 14, No. 2, pp. 151-156, 1996 Copyright 0 1996 Elsevier Science Inc. Printed in the USA. All rights reserved 0730-725x/96 $15.00 + .OO 0730-725X( 95)02056-X MRI OF THE ANAL CANAL: CORRELATION WITH HISTOLOGIC EXAMINATION B.E. VAN BEERS, * A. KARTHEUSER,? M.A. DELOS,~ C. GRANDIN, * R. DETRY,~ J. JAMART,~ and J. PRLNGOT* Departments of *Radiology and tsurgery, University of Louvain, St-Luc University Hospital, Brussels, Belgium, and SDepartment of Pathology and §Center of Biostatistics and Medical Documentation, Mont-Godinne University Hospital, Yvoir, Belgium The purpose of this study was to correlate the MRI features of the anal canal with histologic findings. T,- and T2-weighted MR images of nine anal canals were obtained after fixation in 10% formalin. In three specimens, imaging was repeated after removal of histologic layers with a dissecting microscope. Corre- sponding histologic slices were stained with hematoxylin-eosin, Masson trichrome, and periodic acid8chiff. Four layers were visualized on T,-weighted images. An inner layer of high signal intensity and a second layer of low signal intensity corresponded to the mucosa as well as mucous secretions and to the submucosa. The high signal intensity layer vanished at the distal part of the anal canal in accordance with the lack of mucus-secreting epithelimn below the level of the dentate line. A third layer of intermediate signal intensity corresponded to the internal sphincter. A fourth layer of low signal intensity corresponded to the longitudinal muscle and external sphincter. T,-weighted MRI is capable of showing the internal architecture of the wall of the anal canal. In particular, the internal sphincter can be differentiated from the external sphincter and longitudinal muscle. Keywords: Anus; Magnetic resonance; Tissue characterization. INTRODUCTION The anal canal is a complex structure composed of mucosa, submucosa, internal sphincter, longitudinal muscle, and external sphincter. On T,-weighted MR images of the anal canal, several layers of various sig- nal intensity are routinely seen on examinations per- formed with a body coil in a clinical setting’ (Fig. 1). On the contrary, the layers of the rectal wall are only seen when an endorectal coil is used.’ This difference is explained by the marked increase in thickness of the muscular layers in the anus. The histologic counter- parts of the anal layers seen at MRI remain unclear. In particular, the internal sphincter has been reported to be included in the peripheral hypointense layer’ or to correspond to a more central hyperintense layer.3 Understanding the normal anatomy of the anal canal at MRI is important to accurately assess the extent of anal fistulae4 and tnmors.5 The purpose of this study was to correlate the MRI features of the wall of the anal canal with histologic findings. MATERIALS AND METHODS Nine anal canals were dissected less than 24 h post- mortem. MRI could not be performed immediately after dissection, owing to time constraints. Thus, the specimens were fixed in 10% formalin to prevent autol- ysis before imaging. Fixation influences the signal characteristics on MRI, but it has been reported that the relative signal intensity of the different layers of the gastrointestinal wall does not significantly change after fixation.6*7 One specimen was fixed without divid- ing the anal canal. The other specimens were imaged after dividing the anal canal anteriorly. MRI was performed with a 1.5 T MR system (Gy- RECEIVED 3130195; ACCEPTED 915195. of Radiology, St-Luc University Hospital, Avenue Hippo- Address correspondence to B.E. Van Beers, Department crate 10, B-1200 Brussels, Belgium. 151