Anal sphincter complex: endoanal MR imaging of normal anatomy

Comparative study of female pelvic floor among undeformed high-resolution thin-sectional anatomical (visible human) images and MRI and ultrasound images

PURPOSE

Using visible human, MRI and ultrasound images, we aim to provide an anatomical basis for the identification and diagnosis of pelvic floor structure and disease by ultrasound imaging.

METHODS

One Chinese visible human (CVH) image, one American visible human image, 9 MRI images of normal volunteers, and 40 ultrasound images of normal volunteers or pelvic organ prolapse patients were used. Pelvic organs, pelvic floor muscles, and the connective tissue in CVH, VHP, MRI, and ultrasound images were selected for comparative study.

RESULTS

We successfully identified the boundary of the anal sphincter complex, including the subcutaneous, superficial, and deep parts of the external anal sphincter, conjoined longitudinal muscles and internal anal sphincter; the levator ani muscle (LAM), including the internal and external parts of the pubovisceral muscle and the superficial and deep parts of the puborectal muscle; the urethral sphincter complex, including the urethral sphincter proper and the urethral compressor; and the perineal body, the rectoperineal muscle and superficial transverse perineal muscle.

CONCLUSIONS

We successfully recognized and studied the location, subdivisions, 2D morphology and spatial relationships of the LAM, anal sphincter complex, urethral sphincter complex and perineal body in ultrasound images, thereby helping sonologists or clinicians accurately identify pelvic floor muscles and supporting structures in ultrasound images.

Anorectal Anatomy and Function

Anatomy of pelvic floor muscles has long been controversial. Novel imaging modalities, such as three-dimensional transperineal ultrasound imaging, MRI, and diffusion tensor imaging, have revealed unique myoarchitecture of the external anal sphincter and puborectalis muscle. High-resolution anal manometry, high-definition anal manometry, and functional luminal imaging probe are important new tools to assess anal sphincter and puborectalis muscle function. Increased understanding of the structure and function of anal sphincter complex/pelvic floor muscle has improved the ability to diagnose patients with pelvic floor disorders. New therapeutic modalities to treat anal/fecal incontinence and other pelvic floor disorders will emerge in the near future.

Assessing Anorectal Function in Constipation and Fecal Incontinence

Constipation and fecal incontinence are commonly encountered complaints in the gastrointestinal clinic. Assessment of anorectal function includes comprehensive history, rectal examination, and prospective stool diary or electronic App diary that accurately captures bowel symptoms, evaluation of severity, and quality of life of measure. Evaluation of a suspected patient with dyssynergic constipation includes anorectal manometry, balloon expulsion test, and defecography. Investigation of a suspected patient with fecal incontinence includes high-resolution anorectal manometry; anal ultrasound or MRI; and neurophysiology tests, such as translumbosacral anorectal magnetic stimulation or pudendal nerve latency. This article provides an approach to the assessment of anorectal function.

Three-Dimensional Endoanal Ultrasound Features of the Anal Sphincter in Asian Primigravidae

OBJECTIVES

The normal parameters of 3-dimensional endoanal ultrasound (3DEAUS) of the anal sphincter have not been reported for primigravidae or pregnant women at present. 3DEAUS parameters in Asian primigravidae were assessed in this study.

METHODS

We analyzed 3DEAUS data of 101 consecutives Asian primigravidae, assessed in the early third trimester. The assessment was performed with a rigid ultrasonic probe (Olympus^® RU 12M-R1 probe and EU-ME1 ultrasound system (Olympus Corp., Shinjuku, Japan). The Wilcoxon signed-rank test was used to detect the differences in pressure in different quadrants.

RESULTS

The participants had a mean age of 24.7 (standard deviation [SD], 5.1) years. The Cleveland Clinic Incontinence Score was normal in all participants. The anal sphincter complex had 3 characteristic segments that were identifiable: upper, middle and lower. The puborectalis muscle was identified as a striated "V"-shaped sling, and its mean thickness was 7.44 (SD, 1.41) mm. The mean thickness of internal (IAS) and external (EAS) sphincters at the mid-sphincter level were 1.78 (SD, 0.59) and 5.49 (SD, 1.21) mm, respectively. The EAS measured 6.02 (SD, 1.07) mm at the lower sphincter level. The statistically significant differences seen in the in quadrants were: the IAS was thicker anteriorly (Z = -2.642; P = .008), the EAS at both midsphincter level (Z = -3.70; P < .001) and lower sphincter level (Z = -7.712; P < .001) was thicker posteriorly, and the IAS was thicker at the 9 o'clock position (Z = -2.081; P = .037). Good symmetry at all 3 levels was seen in the EAS (including the puborectalis muscle).

CONCLUSIONS

Normal values of 3DEAUS for primigravidae have been identified and may serve as reference values for other laboratories.

MR-FLIP: a new method that combines a functional lumen imaging probe with anatomical information for spatial compliance assessment of the anal sphincter muscles

AIM

Continence results from a complex interplay between anal canal (AC) muscles and sensorimotor feedback mechanisms. The passive ability of the AC to withstand opening pressure - its compliance - has recently been shown to correlate with continence. A functional lumen imaging probe (FLIP) is used to assess AC compliance, although it provides no anatomical information. Therefore, assessment of the compliance specific anatomical structures has not been possible, and the anatomical position of critical functional zones remains unknown. In addition, the FLIP technique assumes a circular orifice cross-section, which has not been shown for the AC. To address these shortcomings, a technique combining FLIP with a medical imaging modality is needed.

METHOD

We implemented a new research method (MR-FLIP) that combines FLIP with MR imaging. Twenty healthy volunteers underwent MR-FLIP and conventional FLIP assessment. MR-FLIP was validated by comparison with FLIP results. Anatomical markers were identified, and the cross-sectional shape of the orifice was investigated.

RESULTS

MR-FLIP provides compliance measurements identical to those obtained by conventional FLIP. Anatomical analysis revealed that the least compliant AC zone was located at the proximal end of the external anal sphincter (EAS). The cross-sectional shape of the AC was found to deviate only slightly from circularity in healthy volunteers.

CONCLUSION

The proposed method is equivalent to classical FLIP. It establishes for the first time direct mapping between local tissue compliance and anatomical structure, which is key to gaining novel insights into (in)continence. In addition, MR-FLIP provides a tool for better understanding conventional FLIP measurements in the AC by quantifying its limitations and assumptions.

Anorectal Cancer: Critical Anatomic and Staging Distinctions That Affect Use of Radiation Therapy

Although rectal and anal cancers are anatomically close, they are distinct entities with different histologic features, risk factors, staging systems, and treatment pathways. Imaging is at the core of initial clinical staging of these cancers and most commonly includes magnetic resonance imaging for local-regional staging and computed tomography for evaluation of metastatic disease. The details of the primary tumor and involvement of regional lymph nodes are crucial in determining if and how radiation therapy should be used in treatment of these cancers. Unfortunately, available imaging modalities have been shown to have imperfect accuracy for identification of nodal metastases and imaging features other than size. Staging of nonmetastatic rectal cancers is dependent on the depth of invasion (T stage) and the number of involved regional lymph nodes (N stage). Staging of nonmetastatic anal cancers is determined according to the size of the primary mass and the combination of regional nodal sites involved; the number of positive nodes at each site is not a consideration for staging. Patients with T3 rectal tumors and/or involvement of perirectal, mesenteric, and internal iliac lymph nodes receive radiation therapy. Almost all anal cancers warrant use of radiation therapy, but the extent and dose of the radiation fields is altered on the basis of both the size of the primary lesion and the presence and extent of nodal involvement. The radiologist must recognize and report these critical anatomic and staging distinctions, which affect use of radiation therapy in patients with anal and rectal cancers.

Clinical value of endoluminal ultrasonography in the diagnosis of rectovaginal fistula

Background

Rectovaginal fistula (RVF) refers to a pathological passage between the rectum and vagina, which is a public health challenge. This study was aimed to explore the clinical value of endoluminal biplane ultrasonography in the diagnosis of rectovaginal fistula (RVF).

Methods

Thirty inpatients and outpatients with suspected RVF from January 2006 to June 2013 were included in the study, among whom 28 underwent surgical repair. All 28 patients underwent preoperative endoluminal ultrasonography, and the obtained diagnostic results were compared with the corresponding surgical results.

Results

All of the internal openings located at the anal canal and rectum of the 28 patients and confirmed during surgery were revealed by preoperative endosonography, which showed a positive predictive value of 100 %. Regarding the 30 internal openings located in the vagina during surgery, the positive predictive value of preoperative endosonography was 93 %. The six cases of simple fistulas confirmed during surgery were revealed by endosonography; for the 22 cases of complex fistula confirmed during surgery, the positive predictive value of endosonography was 90 %. Surgery confirmed 14 cases of anal fistula and 14 cases of RVF, whereas preoperative endoluminal ultrasonography suggested 16 cases of anal fistula and 12 cases of RVF, resulting in positive predictive values of 92.3 and 93 %, respectively.

Conclusion

The use of endoluminal biplane ultrasonography in the diagnosis of RVF can accurately determine the internal openings in the rectum or vagina and can relatively accurately identify concomitant branches and abscesses located in the rectovaginal septum. Thus, it is a good imaging tool for examining internal and external anal sphincter injuries and provides useful information for preoperative preparation and postoperative evaluation.

The anococcygeal ligaments: Cadaveric study with application to our understanding of incontinence in the elderly

The term "anococcygeal ligament (ACL)" has been used to refer to two distinct structures: a superficial fibrous band originating from the myosepta of the external anal sphincter (EAS) and running upwards to the coccyx (the superficial ACL); and a deep fibrous band originating from the periosteum of the coccyx, merging with the thick presacral fascia and attaching to the superior end of the EAS (the deep ACL). In the present work, elastic fiber histology and muscle immunohistochemistry of sagittal sections obtained from 15 donated elderly male cadavers showed that superficial ACL, corresponding to a superficial fascia or skin ligament, was composed of very tortuous elastic fibers, with a fine elastic fiber mesh at their coccygeal attachment; whereas the deep ACL was composed of almost straight collagen and elastic fibers, intermingled with the coccygeal periosteum. Due to the weak insertion into the coccyx and the wavy course, the superficial ACL is unlikely to provide, even in association with contraction of the longitudinal anal muscle, a stable mechanical support to maintain the configuration of the EAS. Being similar to the suspensory ligament of breast, tissue repair of the skin ligament would not have a mechanical role. In contrast, the deep ACL, in association with the thick presacral fascia, likely plays a role in maintaining a suitable positioning of the anorectum to the coccyx. However, their relative lack of smooth muscles compared with rich elastic fibers indicates that both ACLs may become permanently overextended under conditions of long-term mechanical stress.

3D Topography of the Young Adult Anal Sphincter Complex Reconstructed from Undeformed Serial Anatomical Sections

Background

Pelvic-floor anatomy is usually studied by artifact-prone dissection or imaging, which requires prior anatomical knowledge. We used the serial-section approach to settle contentious issues and an interactive 3D-pdf to make the results widely accessible.

Method

3D reconstructions of undeformed thin serial anatomical sections of 4 females and 2 males (21–35y) of the Chinese Visible Human database.

Findings

Based on tendinous septa and muscle-fiber orientation as segmentation guides, the anal-sphincter complex (ASC) comprised the subcutaneous external anal sphincter (EAS) and the U-shaped puborectal muscle, a part of the levator ani muscle (LAM). The anococcygeal ligament fixed the EAS to the coccygeal bone. The puborectal-muscle loops, which define the levator hiatus, passed around the anorectal junction and inserted anteriorly on the perineal body and pubic bone. The LAM had a common anterior attachment to the pubic bone, but separated posteriorly into puborectal and “pubovisceral” muscles. This pubovisceral muscle was bilayered: its internal layer attached to the conjoint longitudinal muscle of the rectum and the rectococcygeal fascia, while its outer, patchy layer reinforced the inner layer. ASC contraction makes the ano-rectal bend more acute and lifts the pelvic floor. Extensions of the rectal longitudinal smooth muscle to the coccygeal bone (rectococcygeal muscle), perineal body (rectoperineal muscle), and endopelvic fascia (conjoint longitudinal and pubovisceral muscles) formed a “diaphragm” at the inferior boundary of the mesorectum that suspended the anorectal junction. Its contraction should straighten the anorectal bend.

Conclusion

The serial-section approach settled contentious topographic issues of the pelvic floor. We propose that the ASC is involved in continence and the rectal diaphragm in defecation.

Purse-string morphology of external anal sphincter revealed by novel imaging techniques

The external anal sphincter (EAS) may be injured in 25-35% of women during the first and subsequent vaginal childbirths and is likely the most common cause of anal incontinence. Since its first description almost 300 years ago, the EAS was believed to be a circular or a "donut-shaped" structure. Using three-dimensional transperineal ultrasound imaging, MRI, diffusion tensor imaging, and muscle fiber tracking, we delineated various components of the EAS and their muscle fiber directions. These novel imaging techniques suggest "purse-string" morphology, with "EAS muscles" crossing contralaterally in the perineal body to the contralateral transverse perineal (TP) and bulbospongiosus (BS) muscles, thus attaching the EAS to the pubic rami. Spin-tag MRI demonstrated purse-string action of the EAS muscle. Electromyography of TP/BS and EAS muscles revealed their simultaneous contraction and relaxation. Lidocaine injection into the TP/BS muscle significantly reduced anal canal pressure. These studies support purse-string morphology of the EAS to constrict/close the anal canal opening. Our findings have implications for the effect of episiotomy on anal closure function and the currently used surgical technique (overlapping sphincteroplasty) for EAS reconstructive surgery to treat anal incontinence.

Functional morphology of anal sphincter complex unveiled by high definition anal manometery and three dimensional ultrasound imaging

BACKGROUND

Anal sphincter complex consists of anatomically overlapping internal anal sphincter (IAS), external anal sphincter (EAS) and puborectalis muscle (PRM). We determined the functional morphology of anal sphincter muscles using high definition anal manometery (HDAM), three dimensional (3D)-ultrasound (US) and Magnetic resonance (MR) imaging.

METHODS

We studied 15 nulliparous women. High definition anal manometery probe equipped with 256 pressure transducers was used to measure the anal canal pressures at rest and squeeze. Lengths of IAS, PRM, and EAS were determined from the 3D-US images and superimposed on the HDAM plots. Movements of anorectal angle with squeeze were determined from the dynamic MR images.

KEY RESULTS

High definition anal manometery plots reveal that anal canal pressures are highly asymmetric in the axial and circumferential direction. Anal canal length determined by the 3D-US images is slightly smaller than that measured by HDAM. The EAS (1.9 ± 0.5 cm long) and PRM (1.7 ± 0.4 cm long) surround distal and proximal parts of the anal canal, respectively. With voluntary contraction, anal canal pressures increase in the proximal (PRM) and distal (EAS zone) parts of anal canal. Posterior peak pressure in the anal canal moves cranially in relation to the anterior peak pressure, with squeeze. Similar to the movement of peak posterior pressure, MR images show cranial movement of anorectal angle with squeeze.

CONCLUSIONS & INFERENCES

Our study proves that the PRM is responsible for the closure of the cranial part of anal canal. HDAM, in addition to measuring constrictor function can also record the elevator function of levator ani/pelvic floor muscles.

Perineal body anatomy in living women: 3-dimensional analysis using thin-slice magnetic resonance imaging

OBJECTIVE

The objective of the study was to describe a framework for visualizing the perineal body's complex anatomy using thin-slice magnetic resonance (MR) imaging.

STUDY DESIGN

Two millimeter thick MR images were acquired in 11 women with normal pelvic support and no incontinence/prolapse symptoms. Anatomic structures were analyzed in axial, sagittal, and coronal slices. Three-dimensional (3-D) models were generated from these images.

RESULTS

Three distinct perineal body regions are visible on MR imaging: (1) a superficial region at the level of the vestibular bulb, (2) a midregion at the proximal end of the superficial transverse perineal muscle, and (3) a deep region at the level of the midurethra and puborectalis muscle. Structures are best visualized on axial scans, whereas craniocaudal relationships are appreciated on sagittal scans. The 3-D model further clarifies interrelationships.

CONCLUSION

Advances in MR technology allow visualization of perineal body anatomy in living women and development of 3-D models that enhance our understanding of its 3 different regions: superficial, mid, and deep.

Intraoperative pelvic nerve stimulation performed under continuous electromyography of the internal anal sphincter

PURPOSE

The aim of this animal study was to investigate the effect of intraoperative pelvic nerve stimulation on internal anal sphincter electromyographic signals in order to evaluate its possible use for neuromonitoring during nerve-sparing pelvic surgery.

METHODS

Eight pigs underwent low anterior rectal resection. The intersphincteric space was exposed, and the internal (IAS) and external anal sphincter (EAS) were identified. Electromyography of both sphincters was performed with bipolar needle electrodes. Intermittent bipolar electric stimulation of the inferior hypogastric plexus and the pelvic splanchnic nerves was carried out bilaterally. The recorded signals were analyzed in its frequency spectrum.

RESULTS

In all animals, electromyographic recordings of IAS and EAS were successful. Intraoperative nerve stimulation resulted in a sudden amplitude increase in the time-based electromyographic signals of IAS (1.0 (0.5-9.0) μV vs. 4.0 (1.0-113.0) μV) and EAS (p < 0.001). The frequency spectrum of IAS in the resting state ranged from 0.15 to 5 Hz with highest activity in median at 0.77 Hz (46 cycles/min). Pelvic nerve stimulation resulted in an extended spectrum ranging from 0.15 to 20 Hz. EAS signals showed higher frequencies mainly in a range of 50 to 350 Hz. However, after muscle relaxation with pancuronium bromide, only the low frequency spectrum of the IAS signals was still present.

CONCLUSIONS

Intraoperative verification of IAS function by stimulation of pelvic autonomic nerves is possible. The IAS electromyographic response could be used to monitor pelvic autonomic nerve preservation.

Anorectal and pelvic floor anatomy

The anorectum and pelvic floor are crucial in maintaining continence, facilitating evacuation, providing pelvic organ support while in females the pelvic floor is part of the birth canal. The anal sphincter is a multilayered cylindrical structure, including the smooth muscle internal sphincter and the outer striated muscle layer. The latter comprises the external sphincter as lower outer half and puborectalis as upper outer half of the sphincter. The external sphincter is continuous with the rectum at the anorectal junction. The pelvic floor constitutes four principal layers: endopelvic fascia, the muscular pelvic diaphragm (commonly referred to as levator plate), the perineal membrane (urogenital diaphragm) and the superficial transverse perineii. Anorectum and pelvic floor have multiple interconnections by fascia and ligaments as well as multiple indirect connections to the bony pelvis. Other structures as perineal body and a fibro-elastic network add to this support.

Correlation between gross anatomical topography, sectional sheet plastination, microscopic anatomy and endoanal sonography of the anal sphincter complex in human males

This study elucidates the structure of the anal sphincter complex (ASC) and correlates the individual layers, namely the external anal sphincter (EAS), conjoint longitudinal muscle (CLM) and internal anal sphincter (IAS), with their ultrasonographic images. Eighteen male cadavers, with an average age of 72 years (range 62-82 years), were used in this study. Multiple methods were used including gross dissection, coronal and axial sheet plastination, different histological staining techniques and endoanal sonography. The EAS was a continuous layer but with different relations, an upper part (corresponding to the deep and superficial parts in the traditional description) and a lower (subcutaneous) part that was located distal to the IAS, and was the only muscle encircling the anal orifice below the IAS. The CLM was a fibro-fatty-muscular layer occupying the intersphincteric space and was continuous superiorly with the longitudinal muscle layer of the rectum. In its middle and lower parts it consisted of collagen and elastic fibres with fatty tissue filling the spaces between the fibrous septa. The IAS was a markedly thickened extension of the terminal circular smooth muscle layer of the rectum and it terminated proximal to the lower part of the EAS. On endoanal sonography, the EAS appeared as an irregular hyperechoic band; CLM was poorly represented by a thin irregular hyperechoic line and IAS was represented by a hypoechoic band. Data on the measurements of the thickness of the ASC layers are presented and vary between dissection and sonographic imaging. The layers of the ASC were precisely identified in situ, in sections, in isolated dissected specimens and the same structures were correlated with their sonographic appearance. The results of the measurements of ASC components in this study on male cadavers were variable, suggesting that these should be used with caution in diagnostic and management settings.

Posterior compartment anatomy as seen in magnetic resonance imaging and 3-dimensional reconstruction from asymptomatic nulliparas

OBJECTIVE

The objective of the study was to identify characteristic anatomical features of the posterior compartment using magnetic resonance (MR) cross-sectional anatomy and 3-dimensional (3-D) modeling.

STUDY DESIGN

Supine, static proton-density MR images of 20 nulliparas were analyzed. MR images were used to create models in a selected exemplar.

RESULTS

The compartment's upper, mid, and lower segments are best seen in the axial plane. It is bounded inferiorly by the perineal body, ventrally by the posterior vaginal wall, and dorsally by the levator ani muscles and coccyx. In the upper portion, the compartment is bordered laterally by the uterosacral ligaments, whereas in the middle portion, there is more direct contact with the lateral levator ani muscles. In the lower portion, the contact becomes obliterated because the vagina and levator ani muscles become fused to each another and to the perineal body.

CONCLUSION

The posterior compartment has characteristic anatomic features in MR cross-sectional anatomy that can be further elucidated and integrated with 3-D anatomy.

Three-dimensional transperineal ultrasonography of the pelvic floor: improving visualization for new clinical applications and better functional assessment

OBJECTIVE

With increasing awareness of the scope of pelvic floor disorders has come development and introduction into clinical practice of new imaging techniques, with increasing importance of ultrasonography. Complex pelvic floor anatomy, the conceptual difficulty in the basics of some pelvic floor disorders, and the uneven standardization of ultrasonographic techniques were the impetuses of this review. The purpose of this study was to review the basic anatomy of the pelvic floor and the transperineal ultrasonographic evaluation technique and to provide an overview of the current clinical use of 3-dimensional transperineal ultrasonography in the evaluation of the anterior and posterior pelvic floor compartments.

METHODS

A literature review illustrated with index cases from our center was conducted.

RESULTS

Ultrasonography has been widely applied to evaluation of the anterior and posterior compartments of the pelvic floor. Three-dimensional ultrasonography has a role in improving pelvic floor assessment.

CONCLUSIONS

Three-dimensional transperineal ultrasonography has been applied to evaluation of normal and pathologic pelvic floor anatomy. Practical application, through well-designed and sufficiently powered clinical studies, will establish the association between the clinical presentations of dysfunction with ultrasonographic findings.

Three-dimensional transperineal sonographic characteristics of the anal sphincter complex in nulliparous women

OBJECTIVE

To explore the morphological characteristics and normal biometry of the anal sphincter complex in nulliparous Chinese women using three-dimensional (3D) transperineal ultrasound.

METHODS

3D sonographic data from 55 nulliparous Chinese women (aged 19-38 years) who had no pelvic organ prolapse and no symptoms of pelvic floor dysfunction were retrieved from an image dataset and analyzed by offline post-processing. The morphological characteristics of the external and internal anal sphincters, puborectalis muscle and perineal body were assessed in the sagittal, coronal and axial views.

RESULTS

The external anal sphincter had three sonographic components: the circular main body, a subcutaneous part and an extension portion. It was significantly thinner at 12 o'clock than at the 3, 6 and 9 o'clock positions. The internal anal sphincter was seen as dark echolucent strips of equal thickness. In the mid-sagittal view, it started from the anal verge and ended at the anorectal junction. The perineal body was an ovoid structure covering the upper margin of the external sphincter, while the puborectalis muscle was banana-shaped in the sagittal view and was located behind the anorectal junction, extending downward along the inferior margin of the posterior external sphincter extension. Sonographic characteristics of the anal sphincter complex did not vary with age, weight, height or body mass index.

CONCLUSIONS

3D transperineal ultrasound clearly demonstrates the spatial relationships of each component of the anal sphincter complex. This should allow standardized measurement of the complex for investigations of its function.

Utility of magnetic resonance imaging in anorectal disease

Imaging of both benign and malignant anorectal diseases has traditionally posed a challenge to clinicians, and as a result history and physical exam have been relied on heavily. CT scanning and endorectal ultrasound have become popular in assessment of anatomy and staging of tumors, but have limitations. Magnetic resonance imaging (MRI) has the capability to fill in the gaps left open by more conventional imaging modalities and continues to be promising as the definitive imaging technique in the pelvis, especially with advancement of emerging technologies in this field. A comprehensive review of this topic has been undertaken. Anorectal disease is divided into three broad categories: cancer, fistula/abscess, and pelvic floor disorders. A review of the literature is performed to evaluate the use of MRI and other imaging modalities in these three areas. Preoperative imaging is useful in the evaluation of all three areas of anorectal disease. MRI is an effective tool in delineating anatomy and, when correlating with the specific clinical scenario, is an effective adjunct in clinical decision-making in order to optimize outcome. MRI continues to be a promising and novel approach to imaging various afflictions of the anorectum and the pelvic floor. Its role is more well-established in some areas than in others, and there are still significant limitations. As technology advances, MRI will shed more light on a complex anatomical area.

Magnetic resonance fistulography for the demonstration of anovaginal fistula: an alternative imaging technique?

Anovaginal fistulae (AVF) are frequently seen in patients with inflammatory bowel disease, especially in Crohn disease with active colonic inflammation. Herein, we report a 21-year-old woman with Crohn disease suffering from vaginal discharge and anal pain. Although clinical presentation was very suggestive of AVF, physical examination and colonoscopy were inconclusive. We used an alternative technique and performed magnetic resonance fistulography by applying rectal contrast for the demonstration of AVF.

Magnetic resonance imaging and 3-dimensional analysis of external anal sphincter anatomy

OBJECTIVE

To use magnetic resonance images of living women and 3-dimensional modeling software to identify the component parts and characteristic features of the external anal sphincter (EAS) that have visible separation or varying origins and insertions.

METHODS

Detailed structural analysis of anal sphincter anatomy was performed on 3 pelvic magnetic resonance imaging (MRI) data sets selected for image clarity from ongoing studies involving nulliparous women. The relationships of anal sphincter structures seen in axial, sagittal, and coronal planes were examined using the 3-D Slicer 2.1b1 software program. The following were requirements for sphincter elements to be considered separate: 1) a clear and consistently visible separation or 2) a different origin or insertion. The characteristic features identified in this way were then evaluated in images from an additional 50 nulliparas for the frequency of feature visibility.

RESULTS

There were 3 components of the EAS that met criteria as being "separate" structures. The main body (EAS-M) is separated from the subcutaneous external anal sphincter (SQ-EAS) by a clear division that could be observed in all (100%) of the MRI scans reviewed. The wing-shaped end (EAS-W) has fibers that do not cross the midline ventrally, but have lateral origins near the ischiopubic ramus. This EAS-W component was visible in 76% of the nulliparas reviewed.

CONCLUSION

Three distinct external anal sphincter components can be identified by MRI in the majority of nulliparous women.

Endosonographic imaging of anorectal diseases

OBJECTIVE

The normal sonographic anatomy of the anorectum, sonographic findings of anorectal diseases, and indications and limitations of endosonography compared with magnetic resonance imaging are reviewed. Methods. Endosonographic imaging was performed with a Siemens (Erlangen, Germany) FI 400 ultrasound scanner with an end-fire 7.5-MHz biplane endorectal probe and a B-K Medical (Sandhoften, Denmark) scanner with an 1850 axial-type side-fire 5.0- to 10.0-MHz rotating endoscopic probe.

RESULTS

Rectal carcinoma appears on endorectal sonography as a low-echogenicity lesion that abruptly interrupts the normal sequence of layers. The internal anal sphincter is seen very clearly on endoanal sonography, and it is easy to appreciate atrophy and small tears of this sphincter. Endoanal sonography cannot accurately show thinning of the external anal sphincter. Peroxide-enhanced endoanal sonography is especially useful for patients with recurrent perianal fistulas in whom scarring should be distinguished from recurrent fistulas and detection of the internal opening. However, sonography does not provide an adequate deep and global display of all adjacent pelvic and perineal spaces.

CONCLUSIONS

Endosonography can accurately stage primary rectal tumors and assess the internal anal sphincter. Peroxide-enhanced 3-dimensional imaging can increase the utility of endoanal sonography in detection and characterization of perianal fistulas and planning of optimal therapy. However, magnetic resonance imaging can be used a complementary modality to endosonography, especially for evaluation of external anal sphincter atrophy and deep pelvic inflammation.

Can three-dimensional endoanal ultrasonography detect external anal sphincter atrophy? A comparison with endoanal magnetic resonance imaging

PURPOSE

Anal sphincter atrophy is associated with a poor clinical outcome of sphincter repair in patients with faecal incontinence. Preoperative assessment of the sphincters is therefore relevant. External anal sphincter (EAS) atrophy can be detected by endoanal magnetic resonance imaging (MRI), but not by conventional endoanal ultrasonography (EUS). Three-dimensional EUS allows multiplanar imaging of the anal sphincters and thus enables more reliable anal sphincter measurements. The aim of the present study was to establish whether 3D EUS measurements can be used to detect EAS atrophy. For this purpose 3D EUS measurements were compared with endoanal MRI measurements.

METHODS

Patients with symptoms of faecal incontinence underwent 3D EUS and endoanal MRI. Internal anal sphincter (IAS) and EAS defects were assessed on 3D EUS and endoanal MRI. EAS atrophy was determined on endoanal MRI. The following measurements were performed: EAS length, thickness and area. Furthermore, EAS volume was determined on 3D EUS and compared with EAS thickness and area measured on endoanal MRI.

RESULTS

Eighteen parous women (median age 56 years, range 32-80) with symptoms of faecal incontinence were included. Agreement between 3D EUS and endoanal MRI was 61% for IAS defects and 88% for EAS defects. EAS atrophy was seen in all patients on endoanal MRI. Correlation between the two methods for EAS thickness, length and area was poor. In addition, correlation was also poor for EAS volume determined on 3D EUS, and EAS thickness and area measured on endoanal MRI.

CONCLUSION

Three-dimensional EUS and endoanal MRI are comparable for detecting EAS defects. However, correlation between the two methods for EAS thickness, length and area is poor. This is also the case for EAS volume determined on 3D EUS and EAS thickness and area measured on endoanal MRI. Three-dimensional EUS can be used for detecting EAS defects, but no 3D EUS measurements are suitable parameters for assessing EAS atrophy.

Volume measurements of the anal sphincter complex in healthy controls and fecal-incontinent patients with a three-dimensional reconstruction of endoanal ultrasonography images

OBJECTIVES

The aim of this study was to determine sphincter volume, length, and external anal sphincter thickness in healthy controls and fecal incontinent patients by use of a three-dimensional reconstruction of endoanal ultrasonography images.

METHODS

Forty-four controls (15 males, 15 females, and 14 parous females) and 28 incontinent parous females (with and without a sphincter defect) were studied. Internal anal sphincter, external anal sphincter and puborectalis volume, sphincter length, and external anal sphincter thickness were measured. Intraobserver and interobserver variability were assessed. Anal pressure profile was also determined.

RESULTS

Internal anal sphincter and external anal sphincter volumes were larger in males than in females (P = 0.001 and P = 0.04), and external anal sphincter volume was smaller in parous females but this was not significant (P = 0.084). Anterior sphincter length was longer in males (P = 0.004) and shorter in parous females (P = 0.06). Males had a larger anterior external anal sphincter thickness (P = 0.018); parity made no difference. Sphincter volumes were not smaller in incontinent females. Incontinent females with a sphincter defect had a shorter anterior sphincter length than that of continent (P = 0.001) and incontinent females without a sphincter defect (P < 0.001). Anterior external anal sphincter thickness was smaller in incontinent females with a sphincter defect (P = 0.006), and posterior and right external anal sphincter thickness was smaller in incontinent females without a sphincter defect (P = 0.02 and P = 0.03). Intraobserver variability was seen for internal anal sphincter volume and sphincter length, but there was no interobserver variability. Correlation between anal pressures and endoanal ultrasonography measurements was poor.

CONCLUSIONS

Differences in anal sphincter volumes are seen for gender but not for parity. Fecal incontinence is not associated with loss of sphincter volume. However, anterior sphincter length and external anal sphincter thickness are smaller.

Magnetic resonance imaging anatomy of the anal canal

The anatomy of the anal canal is complex but well demonstrated by MRI. Understanding the anatomy is a prerequisite for determining the true site and the extent of pathology, especially for surgical workup. In this article, the MRI anatomy of the anal canal has been displayed using highlighted MRI images and line diagrams.

Management and imaging of low rectal carcinoma

Large variations in recurrence rates have been reported with the best results following total mesorectal excision (TME) surgery for low and middle rectal cancers. However, the low rectal cancers still have higher rates of local recurrence (up to 30%) whether operated by low anterior resection or abdominoperineal excision (APE) due to high rates of circumferential margin involvement. The treatment of choice for low rectal cancers that encroach upon the potential circumferential resection margin is surgery combined with preoperative neoadjuvant treatment. Preoperative chemotherapy combined with long-term radiotherapy reduces recurrence rates and preoperative loco-regional staging can help to select the patients more likely to benefit from neo-adjuvant therapy. Surface coil MRI is the most promising modality for patient selection, which can provide good views of the circumferential resection margin especially the presence or absence of tumour encroaching the intersphincteric plane.

Morphologies of the interfaces between the levator ani muscle and pelvic viscera, with special reference to muscle insertion into the anorectum in elderly Japanese

A specific, smooth muscle-mediated interface between the levator ani muscle (LA) and the pelvic viscera has been reported. Using 110 sagittally trimmed anorectal tissue strips (80 lateral, 15 anterior and 15 dorsal specimens) obtained from the donated cadavers of 46 elderly subjects, we examined variations in the interface between the LA and the rectal muscularis propria, including the so-called conjoined longitudinal muscles. In type A (9/46), little or no tissue connected the LA to the external rectal muscularis propria, but the LA and external sphincteric mass formed a definite complex. In type B (26/46), the covering fascia of the LA changed abruptly into smooth muscles, which merged into the external rectal muscularis propria. In type C (11/46), most of the LA-associated connective tissues, composed of smooth muscles, were tightly connected to the internal and external rectal muscularis propria. These variations seemed to depend on the morphology of the recto-urethralis, the lateral extension of which reinforced the LA-associated smooth muscles sufficiently to form type B and C insertions. We also demonstrated differences in the interfacial tissues between the LA and other pelvic viscera. We hypothesize that, to avoid injury of the LA and its interfaces during strong movement of the pelvic viscera, for example during childbirth, coitus or squeezed evacuation, the pelvic connective tissue-like smooth muscles play an important role as an autonomic buffer and/or modulator of pelvic floor function. Digital examination and transrectal or transvaginal sonography may be useful for evaluating interindividual variation in these interfacial tissues in elderly patients.

Anovaginal Fistulas: Evaluation with Endoanal MR Imaging

PURPOSE

To evaluate endoanal magnetic resonance (MR) imaging in the assessment of anovaginal fistulas and associated findings.

MATERIALS AND METHODS

In a retrospective descriptive study, two radiologists systematically reviewed MR findings in 20 patients with a clinically proved anovaginal fistula and looked for the main fistula tract, the internal opening in the anal canal and/or vagina, secondary fistula tracts, abscesses within the rectovaginal septum, and sphincter damage. Interobserver variability was calculated, and clinical records were searched for possible underlying causes that could explain the complexity of anovaginal fistulas. The kappa value was calculated. Patients with or without a complex anovaginal fistula were compared in regard to the presence of any underlying disease or condition. Statistical significance was calculated with the Fisher exact test.

RESULTS

In all 20 patients, anovaginal fistulas were identified on T2-weighted MR images as predominantly high-signal-intensity linear abnormalities extending between the anal canal and the vagina. In all patients, the fistulas were typically located in the sagittal plane, and the mean distance from the anal verge to the fistula was 25.0 mm (range, 13-32 mm). The internal opening in the anal canal was detected in all patients. The internal opening in the vagina was detected in 19 (95%) patients. In seven (35%) patients, an anovaginal fistula with an additional abnormality was found and included an abscess within the rectovaginal septum (n = 1), a perianal fistula (n = 3), and a perianal fistula in combination with an abscess (n = 3). Defects of the external anal sphincter were present in three (15%) patients. There was complete agreement between observers for all items on endoanal MR images, except for the presence of secondary fistula extensions (agreement, 90%; kappa, 0.74). History of obstetric trauma, pelvic floor surgery, or Crohn disease was present in 10 (50%) patients. Of these patients, six (60%) had a complex anovaginal fistula and four (40%) had a simple anovaginal fistula. In the remaining 10 patients without relevant medical history, one (10%) had a complex anovaginal fistula. This difference tended toward statistical significance (P =.057).

CONCLUSION

Endoanal MR imaging allows evaluation of anovaginal fistulas and additional abnormalities, such as abscesses within the rectovaginal septum, secondary perianal fistula tracts, and sphincter damage.

Differentiation of Anal Sphincters With High-Resolution Magnetic Resonance Imaging Using Contrast-Enhanced Fast Low-Angle Shot 3-Dimensional Sequences

OBJECTIVE

The imaging of the anal apparatus is becoming more and more important in the management of patients who suffer from anorectal disease. The exact differentiation of the sphincter muscles is a major requirement for the detection of disorders of the anal canal. The purpose of this study was to evaluate a new magnetic resonance (MR) imaging protocol using contrast-enhanced, high-resolution, fast low-angle shot, 3-dimensional (3D) sequences and image subtraction regarding the visualization and differentiation of the internal and external sphincter muscles.

METHODS

High-resolution pelvic MR imaging (1.5 T) was performed in 85 patients (42 male, 43 female; age range: 12-81 years) with a phased-array body coil. For an anatomic overview of the pelvic region, a short tau inversion recovery sequence in the coronal plane was carried out, followed by a 3D, high-resolution, fat-saturated, T1-weighted, gradient echo sequence before and after intravenous administration of a contrast agent (gadobenic acid, 0.15 mmol/kg). To optimize the visualization of the sphincter muscles, subtraction of the unenhanced from the contrast-enhanced sequences was routinely performed. The signal intensities of the internal and external sphincter muscles were measured in the axial plane on the subtracted images.

RESULTS

The distribution of the mean signal intensities of the internal and external sphincter muscles as well as the difference between both revealed a normal deviation. The confidence interval on a 95% significance level ranged between 1.6941 and 1.9393, with a mean of 1.81. In the whole study group, the signal intensity of the internal sphincter muscle was significantly higher than that of the external sphincter, thus facilitating the identification and differentiation of the 2 components of the anal sphincter complex.

CONCLUSION

The presented MR imaging protocol is robust, provides a high image quality, and is well accepted by patients because of its noninvasiveness.

Outcome measures for fecal incontinence: anorectal structure and function

Fecal incontinence is a symptom attributable to a variety of disorders affecting one or more factors that maintain continence. Objective assessments should complement symptom assessments as outcome measures in therapeutic trials; conceivably, these assessments may also predict the response to therapy. Consistent with existing trends, most therapeutic trials should incorporate anal sphincter pressures and rectal sensation as outcome variables, paying meticulous attention to techniques. Rectal sensation is increased after pelvic floor retraining by biofeedback therapy in fecal incontinence; however, the predictive value of improved anal pressures after biofeedback has not been clearly established. Other factors maintaining continence can be assessed by newer approaches. In addition to assessing rectal sensation, a barostat also measures rectal compliance; alterations in rectal compliance modulate rectal perception. Particularly appropriate end points for trials involving surgical repair are sphincter integrity, assessed by endoanal ultrasound or magnetic resonance imaging (MRI), and puborectalis and pelvic floor motion, assessed by dynamic MRI. Despite disagreement about which technique is superior for evaluating the internal sphincter, MRI performs the same or better than ultrasound for assessing the external sphincter. The utility of measuring pudendal nerve latencies as a marker of pudendal nerve injury is limited; needle electromyography provides a sensitive measure of denervation and can usually identify myopathic damage, neurogenic damage, or mixed injury. These standardized, reproducible assessments of the multifaceted mechanisms maintaining fecal incontinence should be incorporated as outcome variables in therapeutic trials of fecal incontinence.

Imaging fecal incontinence

Fecal incontinence is the inability to defer release of gas or stool from the anus and rectum by mechanisms of voluntary control. It is an important medical disorder affecting the quality of life of up to 20% of the population above 65 years. The most common contributing factors include previous vaginal deliveries, pelvic or perineal trauma, previous anorectal surgery, and rectal prolapse. Many physicians lack experience and knowledge related to pelvic floor incontinence disorders, but advancing technology has improved this knowledge. Increased experience with endoanal ultrasound and endoanal magnetic resonance imaging have given us a better understanding not only of the anatomy of the anal canal but also of the underlying morphological defects in fecal incontinence. Current imaging methods are emphasized and recent literature is reviewed.

Topographic anatomy of the male perineal structures with special reference to perineal approaches for radical prostatectomy

AIM

Although perineal approaches for radical prostatectomy have recently gained renewed attention as excellent methods for minimally invasive surgery, the most commonly used techniques, Belt's and Young's approaches, have inadequacies regarding the topographical relationship between the rectourethral and levator ani muscles.

METHODS

Using macroscopic observations of sagittal slices of 27 male pelvises and smooth muscle immunohistochemical staining of semiserial sections of another eight pelvises, we investigated the topographical anatomy of the perineal structures and their interindividual variations in elderly Japanese men.

RESULTS

The inferomedial edge of the levator ani was located 5-15 mm lateral to the midsagittal plane in an area between the urethra and the rectum. The rectourethral smooth muscle had a superoinferior thickness of 5-10 mm and occupied a space between the right and left levator slings. The levator was adjacent to, or continuous with, the striated anal sphincters. A thick connective tissue septum, composed of smooth muscle, was evident between the rectal smooth muscle and the anal sphincter-levator ani complex.

CONCLUSION

Because the connective tissue septum guides the surgeon's finger upwards towards the rectoprostatic space, Belt's approach appears relatively easy; however, rectal injury can sometimes occur if the surgeon loses this guidance. In contrast, if the levator edge is identified as the first step in Young's approach, the rectourethral muscle can be precisely divided, leaving a 3-5-mm margin from the rectum and sphincter-levator complex. Clinical investigations are now required to modify Young's approach based on the present results.

Dynamic MR imaging of female organ prolapse

MR imaging is a viable technique for evaluating pelvic organ prolapse. Dynamic imaging with adequate increase in abdominal pressure is necessary for a successful study. Rectal contrast for patient defecation is easy to administer and also allows for the diagnosis of rectal abnormalities. Additional work is needed on the soft tissue changes of prolapse and grading criteria for MR.

Magnetic resonance imaging of anatomic and dynamic defects of the pelvic floor in defecatory disorders

OBJECTIVE

Endoanal ultrasound identifies anal sphincter anatomy, and evacuation proctography visualizes pelvic floor motion during simulated defecation. These complementary techniques can evaluate obstructed defecation and fecal incontinence. Our aim was to develop a single, nonionizing, minimally invasive modality to image global pelvic floor anatomy and motion.

METHODS

We studied six patients with fecal incontinence and seven patients with obstructed defecation. The anal sphincters were imaged with an endoanal magnetic resonance imaging (MRI) coil and endoanal ultrasound (five patients). MR fluoroscopy acquired images every 1.4-2 s, using a modified real-time, T2-weighted, single-shot, fast-spin echo sequence, recording motion as patients squeezed pelvic floor muscles and expelled ultrasound gel; no contrast was added to other pelvic organs. Six patients also had scintigraphic defecography.

RESULTS

Endoanal ultrasound and MRI were comparable for imaging defects of the internal and external sphincters. Only MRI revealed puborectalis and/or external sphincter atrophy; four of these patients had fecal incontinence. MR fluoroscopy recorded pelvic floor contraction during squeeze and recorded relaxation during simulated defecation. Corresponding comparisons for angle change and perineal descent during defecation were not significant; only MRI, but not scintigraphy, identified excessive perineal descent in two patients.

CONCLUSIONS

Pelvic MRI is a promising single, comprehensive, nonradioactive modality to measure structural and functional pelvic floor disturbances in defecatory disorders. This method may provide insights into mechanisms of normal and disordered pelvic floor function in health and disease.

Three-dimensional reconstruction of the anorectal continence organ in a 14-week-old fetus

BACKGROUND/PURPOSE

The fetal development and anatomy of the muscular structures of the anorectal continence system are unclear. To the pediatric surgeon, these structures are of clinical relevance in reconstructive surgery. The aim of this study was to investigate the fetal development of the anorectal continence organ.

METHODS

A male fetus (14 weeks postconceptionem) of 114-mm crown-rump length was sectioned serially at 18-micrometer intervals. The sections were stained, and relevant contours of the sections were transferred onto paper using a Zeiss Axioskop drawing apparatus. The drawings then were scanned and digitized.

RESULTS

Three-dimensional images were created (and animated in a video). These have permitted the demonstration of isolated anatomic structures, the disassembling and reassembling of compound structures, as well as the visualization of structures from different angles.

CONCLUSIONS

Further studies are now undertaken of older fetal stages through to birth, as well as during postnatal stages. Comparative studies in animals and animations of isolated muscles also are required to show functional capacities. Such studies may lead eventually to an improvement of contemporary surgical techniques.

The anatomy of the perineal membrane: its relationship to injury in childbirth and episiotomy

BACKGROUND

Episiotomy during childbirth, intended to protect the anal sphincter, may fail to do so. Furthermore damage to the anal sphincter complex may occur without complete perineal tear. We hypothesise that these particular injuries may occur due to posterior displacement of the anus leading to distraction of the anal sphincter complex from an anterior attachment to the perineal membrane. However, the anatomical basis for this has not been well defined.

OBJECTIVE

To investigate the relationship between the anal sphincter and the perineal membrane.

MATERIALS AND METHODS

High-resolution MRI scans of a female cadaver perineum were performed. The imaging findings were correlated with the anatomical structure identified on dissection and histological examination.

RESULTS

The perineal membrane was easily identified on MR imaging. Fibres from the perineal membrane could be seen to attach to the anal sphincter complex at the apex of the perineal body This was confirmed on histological examination and was a deeper layer than that of the decussation of the superficial transverse perineal muscle with the superficial part of the external anal sphincter.

CONCLUSION

The upper ano-rectal canal and apex of the perineal body have demonstrable attachment to the free margin of the perineal membrane postero-lateral to the lower vagina. This attachment would resist posterior displacement of the anal canal.

Anatomy of the anal canal and perianal structures as defined by phased-array magnetic resonance imaging

BACKGROUND

The anatomy of the anal canal and perianal structures has been imaged using endoluminal magnetic resonance imaging (MRI). Phased-array MRI avoids the use of an endoluminal coil that may distort anatomy. The aim of this study was to describe the anatomy of the anal canal and perianal structures using phased-array MRI.

METHODS

Imaging was performed in 14 men and 19 nulliparous women. The dimensions of the anal canal, puborectalis, external anal sphincter, perineal body, superficial transverse perineal muscle, bulbospongiosus, ischiocavernosus and anococcygeal body were measured in different planes, and sex differences were calculated.

RESULTS

The lateral canal was significantly longer than its anterior and posterior part (P < 0.001). The anterior external anal sphincter was shorter in women than in men (P = 0.01) and occupied, respectively, 30 and 38 per cent of the anal canal length (P = 0.001). The caudal ends of the external anal sphincter formed a double layer. The perineal body was thicker in women than in men (P < 0.001) and easier to define. The superficial transverse muscles had a lateral and caudal extension to the ischiopubic bones. The bulbospongiosus was thicker in men than in women (P < 0.001). The ischiocavernosus and anococcygeal body had the same dimensions in both sexes.

CONCLUSION

Phased-array MRI is a non-invasive technique that allows an accurate description of the normal anatomy of the anal canal and perianal structures.

Measurement of anal sphincter muscles: endoanal US, endoanal MR imaging, or phased-array MR imaging? A study with healthy volunteers

PURPOSE

To compare endoanal ultrasonography (US), endoanal magnetic resonance (MR) imaging, and phased-array MR imaging for anal sphincter muscle measurement.

MATERIALS AND METHODS

Sixty healthy volunteers underwent 1.5-T phased-array MR, endoanal MR, and endoanal US examinations. Sphincter muscle thicknesses were measured. Measurement reliability was analyzed, and correlations among the imaging methods were calculated. Multivariate analysis was performed to assess the influence of age, weight, height, sex, parity, and obstetric trauma on sphincter dimensions.

RESULTS

Both MR methods had good reliability for measurements of all sphincter components, whereas endoanal US was reliable for internal sphincter measurement only. There was little correlation between the techniques, except between the two MR techniques, with a strong correlation for total sphincter and perineal body thickness. The internal sphincter thickened significantly (P =.002) with age at endoanal US and endoanal MR imaging but not at phased-array MR imaging. There were small sex-based differences in sphincter muscle measurements at phased-array MR imaging only.

CONCLUSION

Endoanal US enables reliable measurement of only internal sphincter thickness, whereas both MR imaging methods enable reliable measurement of all sphincter components. Sphincter measurement with phased-array MR imaging is as reliable as that with endoanal MR imaging.

Multiplanar anal endosonography--normal anal canal anatomy

INTRODUCTION

Volume acquisition during anal ultrasound enables multiplanar imaging of the anal canal. The normal ultrasonic multiplanar appearance of the anal canal is described and gender differences in canal anatomy are highlighted.

METHODS

Ten male and 12 female normal volunteers (mean ages 31.5 years (s.d. 5.9) and 31.2 (s.d. 6.7)) had three-dimensional anal endosonography (3-D AES). Each volume dataset was seeded in the axial plane facilitating multiplanar identification of known anatomical structures.

RESULTS

The anterior external anal sphincter (EAS) was significantly longer in men than women 30.1 mm (3.9) vs 16.9 mm (7.4) (P < 0.001). There was no difference in the length of the puborectalis 24.7 mm (6.4) vs 24 mm (5) (P=0.78) in men compared with women. The cranial extent of the anterior EAS was tilted forward in both sexes. The angle formed by the anterior EAS and the longitudinal axis of the anal canal was more acute in men than in women (11.1 degrees vs 18.6 degrees; P=0.007). Dataset volume seeding of familiar structures in the axial plane allowed the multiplanar endosonographic anatomy to be described.

CONCLUSIONS

Multiplanar AES has enabled detailed longitudinal measurement of the components of the anal canal and has revealed important gender differences. The multiplanar ultrasonic appearance of the normal anal canal has been described for the first time.