The purpose of this study was to correlate the in vivo endoanal MRI
          findings of the anal sphincter with the cross-sectional anatomy and
          histology. Fourteen patients with rectal tumours were examined with a
          rigid endoanal MR coil before undergoing abdominoperineal resection. In
          addition, 12 cadavers were used to obtain cross-sectional anatomical
          sections. The images were correlated with the histology and anatomy of the
          resected rectal specimens as well as with the cross-sectional anatomical
          sections of the 12 cadavers. The findings in 8 patients, 11 rectal
          preparations, and 10 cadavers, could be compared. In these cases, there
          was an excellent correlation between endoanal MRI and the cross-sectional
          cadaver anatomy and histology. With endoanal MRI, all muscle layers of the
          anal canal wall, comprising the internal anal sphincter, longitudinal
          muscle, the external anal sphincter and the puborectalis muscle wer

Entius, C.A.

Hollander, J.C. (Jan) den

Hussain, S.M. (Shahid)

Kuiper, J.-W. (Jan-Willem)

Lameris, J.S.

Stoker, J. (Jacob)

Zwamborn, A.W.

The purpose of this study was to correlate the in vivo endoanal MRI
          findings of the anal sphincter with the cross-sectional anatomy and
          histology. Fourteen patients with rectal tumours were examined with a
          rigid endoanal MR coil before undergoing abdominoperineal resection. In
          addition, 12 cadavers were used to obtain cross-sectional anatomical
          sections. The images were correlated with the histology and anatomy of the
          resected rectal specimens as well as with the cross-sectional anatomical
          sections of the 12 cadavers. The findings in 8 patients, 11 rectal
          preparations, and 10 cadavers, could be compared. In these cases, there
          was an excellent correlation between endoanal MRI and the cross-sectional
          cadaver anatomy and histology. With endoanal MRI, all muscle layers of the
          anal canal wall, comprising the internal anal sphincter, longitudinal
          muscle, the external anal sphincter and the puborectalis muscle were
          clearly visible. The levator ani muscle and ligamentous attachments were
          also well demonstrated. The perianal anatomical spaces, containing
          multiple septae, were clearly visible. In conclusion, endoanal MRI is
          excellent for visualising the anal sphincter complex and the findings show
          a good correlation with the cross-sectional anatomy and histology

Erasmus University Digital Repository

J. Anat. (1996) 189, pp. 677-682, with 2 figures Printed in Great BritainShort ReportEndoanal MRI of the anal sphincter complex: correlationwith cross-sectional anatomy and histologyS. M. HUSSAIN', J. STOKER', A. W. ZWAMBORN', J. C. DEN HOLLANDER2, J. W. KUIPER',C. A. C. ENTIUS3, AND J. S. LAMERISDepartments of ' Radiology, 2Pathology and I Anatomy, Erasmus University and University Hospital Rotterdam,The Netherlands(Accepted 16 July 1996)ABSTRACTThe purpose of this study was to correlate the in vivo endoanal MRI findings of the anal sphincter with thecross-sectional anatomy and histology. Fourteen patients with rectal tumours were examined with a rigidendoanal MR coil before undergoing abdominoperineal resection. In addition, 12 cadavers were used toobtain cross-sectional anatomical sections. The images were correlated with the histology and anatomy ofthe resected rectal specimens as well as with the cross-sectional anatomical sections of the 12 cadavers. Thefindings in 8 patients, 11 rectal preparations, and 10 cadavers, could be compared. In these cases, there wasan excellent correlation between endoanal MRI and the cross-sectional cadaver anatomy and histology.With endoanal MRI, all muscle layers of the anal canal wall, comprising the internal anal sphincter,longitudinal muscle, the external anal sphincter and the puborectalis muscle were clearly visible. The levatorani muscle and ligamentous attachments were also well demonstrated. The perianal anatomical spaces,containing multiple septae, were clearly visible. In conclusion, endoanal MRI is excellent for visualising theanal sphincter complex and the findings show a good correlation with the cross-sectional anatomy andhistology.Key words: Anorectum; magnetic resonance imaging.INTRODUCTIONIn the surgical literature, the external anal sphincterhas been described as consisting of 1, 2 or 3 parts(Dalley, 1987). According to Milligan & Morgan(1934) the external anal sphincter consists of sub-cutaneous, superficial and profundus components.Goligher et al. (1955), on the other hand, found nosuggestion of division of the external anal sphincterinto separate parts: the muscle was considered to be asingle continuous sheet. Oh & Kark (1972) proposeda 2 compartment organisation and later Shafik (1975)proposed a triple-loop system. More recently, Ayoub(1979) was unable to detect any planes of separationthat divided the sphincter into parts.These concepts were based on dissection studies,often strengthened by surgical experience (Dalley,1987). After the introduction of endoanal sonography(Law & Bartram, 1989) it became possible to visualisethe anal sphincters in vivo. The external anal sphincteris well demonstrated with this technique (Law &Bartram, 1989). In our experience, other structures,including the external anal sphincter and the perianalanatomical spaces, were either variable in echogenicityor not visible at all. Moreover endoanal sonography ismainly restricted to the axial plane and is operator-dependent. Such limitations caused diagnosticproblems in patients with faecal incontinence andfistula-in-ano.To overcome these problems, magnetic resonanceCorrespondence to Dr S. M. Hussain, Department of Radiology, University Hospital Rotterdam-Dijkzigt, Dr Molewaterplein 40, 3015 GDRotterdam, The Netherlands.677678 S. M. Hussain and othersFig. 1. MR-anatomical correlation. In the axial section (compare A with D), the muscle layers of the anal canal wall, comprising the internalanal sphincter (1), the conjoint longitudinal muscle (2), and the external sphincter (3) show an excellent correlation. In the coronal plane(compare B with E), the shape and the position of the external anal sphincter (3), the puborectalis muscle (4) and the levator ani muscle(5) should be noted. In the sagittal plane (compare C with F), note the anococcygeal ligament (arrow), which attaches the external analsphincter (3) to the coccyx (open arrow). a, intersphincteric space; b, ischioanal space; c, supralevator space; double arrow, bulbocavernosusmuscle.MRI of anal sphincter complexIN.~~~~jL4,~~~~~~~~~~~~~~~~Fig. 2. MR-histological correlation. The right lateral half of the anal canal in the coronal plane is represented by a histological specimen(A), an MR image (B) with the corresponding drawing (C). The muscles of the anal sphincter complex comprising the internal anal sphincter(1), the conjoint longitudinal muscle (2), the external anal sphincter (3), the puborectalis muscle (4), and the levator ani muscle (5), showan excellent MR-histological correlation. Note in A, the epithelium (arrow) and the anal muscularis mucosae (double arrow) which are notvisible on MRI (B).imaging (MRI) with a rigid endoanal coil wasintroduced (Hussain et al. 1994). The anatomicalconcept of the anal sphincter complex emerging fromthe endoanal MRI was in many aspects different fromthe current anatomical views (Hussain et al. 1995).The purpose of this study was to validate the recentMRI findings of the anal sphincter complex in vivo byobtaining a correlation between the MRI findings andthe cross-sectional anatomy and histology.MATERIALS AND METHODSFourteen patients (8 men and 6 women with a meanage of 58.5 y, range 27-81 y) with rectal tumours wereexamined with a rigid endoanal MRI coil beforeundergoing abdominoperineal resection. Informedconsent was obtained before examination. The re-sected rectal preparations were used to obtain ana-tomical and histological slices. In addition, 12cadavers (6 men and 6 women with a mean age of64.3 y, range 56-71 y) were used to obtain anatomicalsections. The cadavers belonged to the Department ofAnatomy for research purposes.MRI studyIn each patient MRI was performed at 0.5 Tesla(Gyroscan T5-II, Philips Medical Systems, Best, TheNetherlands). To reduce bowel motion, 1 ml ofbutylscopolamine bromide (Buscopan 20 mg/ml,Boehringer Ingelheim, Germany) was injected intra-muscularly before scanning. A recently developedrigid endoanal coil with a diameter of 19 mm (PhilipsMedical Systems, Best, The Netherlands) was used.The imaging was performed in the axial, coronaland sagittal planes. The axial T2-weighted 3-dimen-sional gradient-echo sequence (T2-weighted contrastenhanced fast field echo, acquisition time 6.5 min,imaging matrix 205 x 256, number of signal averages(NSA) 2, repetition time (TR) 30 ms, echo time (TE)13 ms, flip angle 600, field of view (FOV) 140 mm,slice thickness 2 mm), was placed perpendicular to thelong axis of the endoanal coil. Thirty-two contiguousslices were obtained. For the sagittal and the coronalscans, a T2-weighted turbo spin-echo was performed(acquisition time 5.0 min, imaging matrix 186 x 256,NSA 8, TR 2800, TE 120, turbo factor 10, FOV120 mm, slice thickness 4.0 mm with an interslice gapof 0.4 mm).Anatomical and histological studyThe rectal preparations of the patients were sliced inthe axial, coronal and sagittal directions. For his-tology, anatomical slices of 4 different rectal prep-arations were fixed in 4 % formalin for 2 d. After679680 S. M. Hussain and othersfixation, the slices were embedded in paraffin and10 gm sections obtained for histological examination.The sections were stained with Masson trichrome foreasy recognition of muscles, ligaments and col-lagenous fibrous tissue.For cross-sectional anatomy, in each cadaver arounded wooden stick, which served as a pseudocoil,was introduced at about the same angle as theendoanal coil in patients in vivo, and the cadaverswere frozen immediately for at least 3 d. Thepseudocoil had a diameter of 18 mm. The rationalefor using the pseudocoil was to simulate all changes inthe anal canal structures that would have been causedby the endoanal coil. In addition, the pseudocoil wasused as a reference during sectioning of the cadavers.The plane for the axial slices was perpendicular to thepseudocoil. The plane for the coronal slices wasparallel to the pseudocoil. The sagittal plane wasdetermined by drawing a line between the processusspinosi and the pseudocoil.The MRI scans and anatomicohistological sectionsof the rectal preparations and the cadavers werecompared side-by-side in order to correlate MRfindings with the anatomy and histology.RESULTSThe MRI scans from 2 patients were excluded becauseof excessive movement artifact. Of the 14 rectalresection preparations from the patients, 3 wereomitted because of histological evidence of invasionof the internal anal sphincter. Of the 12 cadavers, 2had to be excluded because of errors which occurredduring sectioning. The data from the remaining 8patients, 11 rectal preparations and 10 cadavers werecompared.In these cases, endoanal MRI correlated well withthe cross-sectional anatomy and histology in the axial,coronal and sagittal planes (Figs 1,2). In the axialplane, the layers of the anal canal wall correlated wellwith the anatomical sections (Fig. 1 A, D). In thecoronal plane, the muscle layers of the lower andupper parts of the anal canal differed (Fig. 1 B, E).The lower part of the anal canal was surrounded bythe internal anal sphincter, the longitudinal muscleand the external anal sphincter. The upper partcomprised the internal anal sphincter, the longitudinalmuscle and the sling of the puborectalis muscle (Fig.1 B, E). The males and females differed particularly inthe anterior part of the external anal sphincter. In thefemale, the anterior external anal sphincter waswhereas in the male the anterior part of the externalanal sphincter was connected to the bulbocavernousmuscle of the penis.Due to the compressive effect of the MRI coil, theepithelium and the subepithelial structures, such asthe anal musculus submucosae were difficult toidentify by MRI (Fig. 2). The muscle layers of the analcanal wall were well visualised. The internal analsphincter, which is the inner most muscle layer of theanal canal, showed an excellent MRI-anatomicalcorrelation (Figs 1,2). The longitudinal muscle, thelayer next to the internal anal sphincter, traversed thedistal part of the external anal sphincter (Figs 1 C, 2).The external anal sphincter surrounded only the lowerpart of the anal canal. The distal ends of the externalanal sphincter, particularly in the midcoronal plane,were folded inwards and upwards. This importantfinding correlated well with the cross-sectional anat-omy (Fig. 1 B, E) and histology (Fig. 2). Thepuborectalis muscle formed a sling around the upperpart of the anal canal. The levator ani muscle waseasily recognisable on MR images and in theanatomical sections (Fig. 1 B, E).Of the perianal anatomical spaces, the inter-sphincteric space, i.e. the space between the internaland the external anal sphincters and the puborectalis,was filled with connective tissue as well as fibres of thelongitudinal muscle (Fig. 2). The ischioanal space, i.e.the space around the anal canal, contained fatty tissuemixed with multiple fibrous septae (Figs 1, 2). Also thedeep postanal space of Courtney, i.e., the spaceposterior to the anal canal and above the anococcygeal ligament, was clearly identifiable (Fig. 1 C,F). The supralevator space was well identifiable abovethe levator ani muscle (Figs 1 B, 2).DISCUSSIONIn this study, MRI of the patients was correlated withthe resected rectal specimens of the same patients aswell as with cross-sectional cadaveric anatomy. Thecadaveric material was necessary because of a fre-quently incomplete removal of the anal sphinctercomplex during abdominoperineal resection. A directMR-anatomical correlation with the cadaveric ma-terial was not possible as the signal intensity of themuscles in the cadaveric MR images, probably due totissue changes during fixation, was very low.The basic anatomical concept of the anal sphinctercomplex has been controversial in the literature(Dalley, 1987). Endoanal MRI is a new techniqueshorter and had no muscular support anteriorly, which provides a different way of looking at theMRI of anal sphincter complex 681anatomy of the anal sphincter complex (Hussain et al.1995). Endoanal MRI findings show an excellentcorrelation with the cross-sectional anatomy andhistology.The use of the rigid MRI coil leads to the high-resolution thin MR images. The muscle layers of theanal canal were therefore well recognisable. A dis-advantage of the rigid coil, which has to be placedwithin the anal canal, is the compression of theepithelium and the subepithelial tissue, such as theanal musculus mucosae, of the anal canal. Thevisualisation of the muscle layers of anal canal and theperianal anatomical spaces is, however, clinicallymore important, for instance, in patients with perianalfistulae and faecal incontinence.The laminar concept of the external anal sphincter(Milligan & Morgan, 1934; Goligher et al. 1955; Oh& Kark, 1972; Shafik, 1975) has probably originatedfrom using different dissection techniques and bystudying slightly different parts of the sphincters. Thedifferences between these previous concepts and theendoanal MRI findings of the anal canal have beenstressed previously (Hussain et al. 1995).Soon after the introduction of endosonography(Law & Bartram, 1989), normal appearance of thesphincters was correlated with anatomy (Sultan et al.1993 a). This study showed a good correlation betweenendosonography and anatomy at certain levels of theanal canal in the axial plane, although, due to thelimitations of endosonography, the complex ana-tomical nature of this region was not fully understood.An additional, important observation during thecurrent study was that the anal sphincter complex iscontained within a small volume, not much largerthan about 6 cm3. Within this space, the muscleschanged their configuration as well as their relation-ship to the adjacent structures. For instance, in thecoronal plane, at the level of the anal canal, thepuborectalis muscle appears as a part of the externalsphincter, while more anteriorly, in the same plane, itbecomes a component of the levator ani muscle(Hussain et al. 1995). The puborectalis may, therefore,have been mistaken for the profundus part of theexternal anal sphincter (Milligan & Morgan, 1934;Goligher et al. 1955; Oh & Kark, 1972; Shafik, 1975).Currently the puborectalis muscle, its location in theupper part of the anal canal, was seen to contributefibres to the longitudinal muscle. This finding wasimportant in distinguishing the puborectalis musclefrom the external anal sphincter.With a combination of multiplanar capability andhigh soft tissue contrast of endoanal MRI, it is easierto understand the complexity of the sphincters. Theexact position of each MR slice is known. To identifya particular anatomical structure it is possible to goback and forth within a series of slices (Hussain et al.1995). The previous workers (Milligan & Morgan,1934; Goligher et al. 1955; Oh & Kark, 1972; Shafik,1975; Ayoub, 1979) lacked this facility during ana-tomical studies of the anal sphincter complex. Thismay also have played an important role in thedevelopment of many misconceptions about thesphincter anatomy existing in the literature (Dalley,1987). Despite using an angle of reference in thecurrent study, errors during sectioning were un-avoidable in 2 cadavers.Knowledge of the exact anatomy of the analsphincter complex is essential for management ofpathology in this area. In this respect, the findings ofthe current study can be of great clinical importance.For instance, in patients with anal fistulae, endoanalMRI could provide a road-map for surgical pro-cedures. This could help in reducing the recurrencerate of perianal fistulae and the occurrence ofpostoperative faecal incontinence. Due to the directvisualisation of the external anal sphincter byendoanal MRI, this modality could also be valuablein patients with faecal incontinence. Currently,endoanal sonography is the modality of choice inpatients with perianal fistulae and faecal incontinence(Choen et al. 1991; Sultan et al. 1993 b). To determinethe value of endoanal MRI in both groups of patients,comparative studies will be performed in our hospital.ACKNOWLEDGEMENTSWe thank Mrs J. J. M. van Loon-van Luijt forpreparation of the histological sections and T. Rijsdijkfor photographic assistance. This paper was presentedat the Rontgen Centenary Congress, Birmingham,UK, 1995.REFERENCESAYOUB SF (1979) Anatomy of the external anal sphincter in man.Acta Anatomica 105, 25-36.CHOEN S, BURNETT S, BARTRAM CI, NICHOLLS RJ (1991) Com-parison between anal endosonography and digital examination inthe evaluation of anal fistulae. British Journal of Surgery 78,445-447.DALLEY AF II (1987) The riddle of the sphincters. The morpho-physiology of the anorectal mechanism reviewed. AmericanSurgeon 53, 298-306.GOLIGHER JC, LEACOCK AG, BROSSY JJ (1955) The surgicalanatomy of the anal canal. British Journal of Surgery 43, 51-61.HUSSAIN SM, STOKER J, KUIPER JW, SCHOUTEN WR, DENHOLLANDER JC, LAMiRIs JS (1994) MR imaging of anal sphinctercomplex with an endoanal coil: normal anatomy and pathology.Radiology 193(P), 445.682 S. M. Hussain and othersHUSSAIN SM, STOKER J, LAMiRIS JS (1995) Anal sphincter complex:endoanal MR imaging of normal anatomy. Radiology 197,671-677.LAW PJ, BARTRAM CI (1989) Anal endosonography: technique andnormal anatomy. Gastrointestinal Radiology 14, 349-353.MILLIGAN ETC, MORGAN CN (1934) Surgical anatomy of the analcanal. Lancet i, 1150-1156.OH C, KARK AE (1972) Anatomy of the external anal sphincter.British Journal of Surgery 59, 717-723.SHARK A (1975) A new concept of the anatomy of the analsphincter mechanism and the physiology of defecation. Theexternal anal sphincter: a triple-loop system. InvestigativeUrology 12, 412-419.SULTAN AH, NICHOLLS RJ, KAMM MA, HUDSON CN, BEYNON J,BARTRAM CI (1993 a) Anal endosonography and correlation within vitro and in, vivo anatomy. British Journal of Surgery 80,508-511.SULTAN AH, KAMM MA, HuDsoN CN, THOMAS JM, BARTRAM CI(1993b) Anal-sphincter disruption during vaginal delivery. NewEngland Journal of Medicine 329, 1905-1916.

Endoanal MRI of the anal sphincter complex: correlation with cross-sectional anatomy and histology

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