 Hello everyone, I am Dr. Priya Ghosh. I work as a consultant radiologist at Tata Medical Center in Kolkata. This short tutorial deals with the MRI anatomy of rectum. MRI rectum is a very commonly performed investigation especially at oncology centers. The primary indication for which it is performed is rectal cancer. There are various steps during the management of rectal cancer where rectal MRI is done. In primary staging MRI assists in selecting patients with a locally advanced rectal cancer for the treatment with neoadjuvant chemo radiation and also for guiding surgical planning. Following neoadjuvant therapy MRI is done again for restaging. Sometimes in postoperative rectal cancers MRI may help in early diagnosis of local recurrence. Reporting MRI for rectal cancer requires knowledge of the therapy as well as normal anatomy. This is an MRI reporting template and I have highlighted the points where knowledge about anatomical landmarks is relevant. This tutorial would focus on these areas of rectal anatomy that are relevant for MRI reporting of rectal cancer. So how do we define the rectum in the first place? It is the distal most segment of the digestive system part of the hindgut in the post between the sigmoid colon and the anal canal. For MR reporting of rectal adenocarcinomas usually rectum and the anal canal are considered together. The upper limit of rectum is the rectosigmoid junction. The location and the landmarks for the rectosigmoid junction are controversial. It is different for the anatomists, for the surgeons and for the radiologists. There are differing opinions even among the radiologists. According to anatomists the rectosigmoid junction is the point where the sigmoid mesocolon ends, the appendices epiploisia are lost and the tiniacoli gradually converge and then disappear. These features may be seen over a considerable length and not at a single point. Also these features are not visible to the radiologist or internally by the endoscopist hence not applicable in fractal setting. The bony landmarks which have been used by anatomists are several. Most commonly S3 vertebra is considered as the upper limit of rectum by the anatomists. Sacral promontory has also been used as a landmark for Swedish rectal cancer trial and several other guidelines. Even the second sacral segment has been used as a landmark in Japan and Holland. Several features defining the rectosigmoid junction or transition are variously accepted by different radiologists. The most commonly used landmark for radiologists is 15 centimetre from the anal verge. This is a sagittal detuated MRI and I have marked the anal verge by a white dashed line. This is the point where the anal mucosa gives way to the perianal skin. The rectum is not in a single vertical plane. It has several curves along its length and it in general follows the sacral concavity. 15 centimetre from the anal verge is calculated along this curved long axis of the rectum. Two recent concepts of the rectosigmoid junction are the sigmoid takeoff or the sweep and the change in vascularity between the sigmoid and the rectum. These might help in locating the rectosigmoid junction in MRI. The meso rectum which is the fat surrounding the rectum is fixed to the presexual fascia while the sigmoid colon is mobile owing to its sigmoid mesocolon. There is a point at which this fixed rectum gives way to a horizontally lying sigmoid colon which sweeps away from the sacrum. This may be seen here in this actual detuated MRI as the point between the fixed rectum and a mobile horizontally lying sigmoid. This point may be taken as a rectosigmoid junction. Usually at this point of sweep a change in vascularity also occurs. This is a sagittal detuated MRI showing the sigmoid colon and the spidering branches of sigmoid artery supplying the sigmoid colon. This is above the level of the sweep. Below the level of the sweep the rectum is supplied by superior rectal vessels as shown here. If a cancer lies on both sides of this point the differentiation between rectal and sigmoid cancer is done by the location of the epicenter of the tumour. This was about the upper limit of rectum. Coming to the lower limit of rectum the anal canal is the inferior continuation of the rectum which opens to the exterior at the anal verge. The anal rectal junction is taken to be the point at which the long axis of rectum changes from anterior inferior to posterior and this has been marked by yellow dashed line in this sagittal detuated MRI. This point also corresponds to the upper limit of the puborectalis muscle but it fuses with the levator muscle. The change in axis of anal canal and rectum is well visualized in sagittal detuated MRI whereas puborectalis is better seen in coronal detuated MRI. The dentate line is another landmark which is taken by the anatomists to be the point for anal rectal junction but this is not relevant clinically or religiologically as it is not seen. Now that we have defined the upper and the lower limits of rectum we come to the divisions of rectum. As I have stated before for imaging and surgical purposes the rectum and anal canal are considered together in cases of rectal adenocarcinoma. This is not applicable to cases of squamous adenocarcinoma of the anal canal as management differs. Surgically and radiologically rectum is divided into thirds the lower third the middle third and the upper third. Most commonly this division is according to the distance or length from the anal verge. The lower third of rectum is 0 to 5 centimeter middle third is 5 to 10 and upper third is 10 to 15 centimeter from the anal verge. The axis of the rectum changes at multiple points and measurements are made along this curved long axis as shown in this image. The axis of the rectum changes from posterior inferior to vertical to anterior inferior and again to posterior inferior at the level of anal rectal junction. Another relevant point to mention here is that ancient MR images should be taken perpendicular to the changing long axis of rectum. The lower rectum is sometimes separately defined as surgical management and considerations of lower rectal cancers differs from that of mid and high rectal cancers. Some radiologists define the lower rectum as 6 centimeter or less from the anal verge mid rectum as 7 to 11 centimeter and upper rectum as 12 to 15 centimeters. Another way of defining lower rectum is to draw a line connecting the proximal origins of the levators at the pelvic side wall in coronal titulated MRI as shown here. So, this is the rectum these are the levators and this is the line joining the proximal origins of the levators at the pelvic side walls. The part of the rectum line distant to this line may be defined as lower rectal cancer. This technician has been provided by LOREC or the English National Low Rectal Cancer Development Program. Now, we are going to discuss the axial anatomy of rectum. This is a diagram of the cross section of rectum and this is the corresponding axial titulated MR image. So, from inside out towards the layers of the rectum are the mucosa, submucosa, muscularis propria and partially serosine the upper rectum. The mucosa of the rectum and anal canal up to the end of the line is simple column epithelium and it gradually changes as it goes out of the anal canal up to the perianal skin. Mucosa in titulated MRI is seen as a thin high-pointed layer. Deep to the epithelium lies the laminar propria and muscularis mucosa but these layers are not seen in imaging and deep to the mucosa lies the submucosa containing fat, interspersed lymphatics and vessels and this layer is seen as hyper intense in titulated MRI. Deep to the submucosa again lies the muscularis propria. It is seen as a high-pointed layer deep to the submucosa and consists of internal continuous circular muscle fibres and external discontinuous longitudinal muscle fibres. These two layers are separated by magnetic neural plexus which is not seen in imaging. The serosa is seen only in upper rectum and derailing whereas adventitial layer is seen in the mid and lower rectum. The t-staging of rectal cancer depends on the depth of invasion of rectal cancer and the layers involved. Now what is miso rectum? This is the same axial titulated MRI image of rectum showing the layers of rectum and this hyper intense fat layer surrounding the rectum is the miso rectum. It appears hyper intense in titulated MRI and it has interspersed miso rectal lymph nodes and lymphatics as well as blood vessels. The lymphatics and blood vessels are seen as hyper intense or intermediate signal intensity linear structures and I'll show the miso rectal lymph nodes later on. The miso rectal fascia is a thin layer of fascia covering the miso rectal fat on all sides circumferential. It is seen as a thin hyper intense layer in titulated MRI as shown by the white arrowheads here. The miso rectal fascia separates the miso rectal fat from the extra miso rectal tissues of the pelvis. There's a relatively avascular plane of adiolate tissue between the miso rectal fascia and the parietal pelvic fascias. This facilitates the surgery along this plane. The miso rectal fascia forms the basis of the concept of circumferential resection margin and it is the boundary of the surgical excision plane in TME or total miso rectal excision which is the standard surgical for rectal cancers at present. Now assessment of the structures within miso rectum is very important for staging and prognostication purposes. Miso rectal node involvement is N disease. Human extension within miso rectal vessels is known as EMVI or extramural vascular invasion and is thought to be associated with higher chances of vascular dissemination and distant metastasis. Now I come to the dispositions of the miso rectal fascia. Anteriorly the miso rectal fascia fuses with the remnant of urogenital septum to form fascia of denonvilia in males and rectovaginal septum in females. This septum can be followed inferiorly to insert in the midline onto the perineal body and terosuperiorly the miso rectal fascia ends at the level of anterior reflection of the peritoneum. Posteriorly almost entire extent of miso rectal fascia is anchored to the sacral fun cavity but at its superior extent it is no longer anchored and it is surrounded by the mobile sigmoid mesentery containing sigmoid branches of inferior mesentery and vein. Inferiorly and laterally the miso rectal fascia extends up to the distant levitas. Now coming to the peritoneal covering of the rectum. The mid and lower rectum are not peritonealized and they are in the pelvic retroperitoneal plane. The upper rectum is partially and variably peritonealized. Usually the upper rectum is anteriorly covered by peritoneum. The peritoneal covering gradually widened superiorly till it is covered by peritoneum on all sides at the level of sigmoid colon. The exact level of peritoneal reflection varies between individuals and is quite often seen in axial and sagittal MRI. This is a sagittal-detuated MRI and this is an axial-detuated MRI showing the anterior peritoneal reflection. In sagittal MRI the peritoneum is seen as a thin hypointense layer reflecting from the anterior rectal wall to the uterus infimense and urinary tatter immense. In axial-detuated MRI a thin hypointense V-shaped attachment is seen to the anterior rectal wall at the level of upper rectum. This represents the anterior peritoneal reflection and is known as the seagull sign. In patients with low rectal cancers the radiologists play a pivotal role in preoperative evaluation. Accurate staging is required to determine the need for new adjuvant chemo-radiation or more extensive surgery and to provide the surgeon with a guide for planes of excision. Conventional staging is insufficient in low rectal cancers. This is because of two reasons. The tumor in the lower rectum are in close proximity to the anal sphincter complex and the involvement of the sphincters decide the type of the surgery. Also, mesorectum is narrowed at this location and tumors are more likely to invade the mesorectal fascia and adjacent organs with a high rate of positive surgical margins. Knowledge about the sphincters is also necessary for imaging of perineal fistula. Now coming to the sphincter anatomy in MR images. This is a coronal MR image deduated and this is an axial deduated MR image at the level of low rectum. The internal anal sphincter shown here, it consists of condensed circular muscle fibers of the lower third of the rectum. The external anal sphincter is the downward extension of the pleborectalis muscle and the inter sphincteric plane lies in between the internal and external anal sphincter here. In the coronal image again this is the puborectalis muscle and the continuation inferiority is the external anal sphincter. This is the internal anal sphincter and the thin hyper intense plane is the inter sphincteric plane here. The mesorectum papers at the level of anorectal junction and during rectal surgery downward extension of the surgical dissection around the mesorectal fascia passes within the puborectalis line into the inter sphincteric space. The upper part of the external anal sphincter forms a circular ring of fibers while the lower end curves inward to lie below the lower end of the internal anal sphincter. Pelvic floor muscle anatomy is very important in rectal cancer and also in reporting of pelvic MR done for other indications. I will discuss it here in brief. The main muscle of the pelvic floor is the levator ni. It has caps in the midline for the pelvic fissera to pass through and support them. In females there are compartments for the bladder and the urethra anteriorly, vagina in the middle and rectum posteriorly. The two most important components of the levator ni are the ideocoxiges and the puborectalis. The ideocoxiges starts as the same fibers of the external anal sphincter and then fans out laterally as a sheet to insert at the pelvic side wall and the tendinous arch. Posteriorly these fibers fuse together to form the levator wrapping. The pubocoxiges and the puborectalis are considered together as the pubo visceralis muscle. These insert lateral to the symphysis pubis anteriorly and forms a sling around the rectum pulling it anteriorly and forming the anorectal junction curve. Components of the levator ni may be identified in T2 weighted MR images. Coronal images are particularly important to assist the involvement of the levators. In males the compartments are slightly different. Anteriorly the compartment contains the bladder, urethra, prostate, seminal vesicles and the posterior compartment contains the rectum. This is a coronal T2 weighted MR image showing the rectum in the midline, the left and the right levator, the ideocoxiges component and the puborectalis sling is seen here. An important imaging line in relation to the pelvic floor is the pubocoxigial line. It is a line connecting the inferior margin of the symphysis pubis to the last coxigial joint. This represents the level of the pelvic floor and it is important in imaging of prolapse. The H line is drawn from the inferior aspect of the symphysis pubis to the posterior wall of the rectum at the level of anorectal junction. This represents the levator hiatus anterior posterior width and the upper limit of normal is 5 centimeter. The M line is the vertical descent of the levator hiatus drawn as a perpendicular line dropped from the pubocoxigial line to the posterior most aspect of the H line. An upper limit of normal of M line is 2 centimeter. In order to understand the pathways of disease spread in rectal cancer and perirectal inflammation, it is important to understand the various fascial reflections of the pelvis. I have already discussed the mesorectal fascia covering the rectum and mesorectal fat. During rectal surgery, the plane between the mesorectal fascia and the other pelvic fascia is the surgical plane. Osteria to the mesorectal fascia lies the presecral fascia. This is a thin layer in front of the sacrum and presecral space is the area between the presecral fascia and the sacrum. This contains the presecral venous flexors and the hypogastric nerves. The space between the mesorectal fascia and the presecral fascia is variably known as the retro rectal space, retro sacral space or pelvic sacral space. The perirectal pelvic fascia is the lateral continuation of this presecral fascia covering the lateral pelvic wall and in case of pelvic visceral nerves which course forward from the presecral space to the pelvic organs. The recto sacral fascia or the wall-dress fascia called as the recto sacral ligament by the anatomists is a thickening arising from the presecral fascia and running forward to meet the mesorectal fascia. This divides the recto sacral space into a superior and inferior compartment which communicate with each other. In males anteriorly between the rectum and the prostate cement vesicles lies the rectochrostatic fascia or denonvelius fascia. It is difficult to distinguish from the closely related mesorectal fascia. In females anterior to the mesorectal fascia lies the retro vaginal septum. The lateral rectal ligaments are controversial structures as they are not visualized in imaging and may not carry important structures. However in some patients the middle rectal artery may run through them and accompanying lymphatics may provide a pathway between the mesorectal and extremisorectal lymph nodes. Lastly I would discuss the lymphatics of the rectum briefly. The major group of lymph nodes training the rectum are the mesorectal nodes. This is an axiom-tituated MRI showing the rectum mesorectal fat containing the normal appearing mesorectal nodes here. The rectum and proximal anal canal along with the internal anal sphincter and longitudinal muscle cord drain into the mesorectal nodes. Lymphatics from the mesorectal nodes then follow the superior rectal artery into the superior rectal nodes and from there to the inferior mesentric nodes. Anal canal below the dendrit line and the external anal sphincter drain into the superficial inguinal nodes. The other group of pelvic nodes are the extremisorectal nodes which consist of the internal iliac, the obturated and the external iliac medial group mainly. Lymphatic vessels from the rectum travel along the median sacral artery and may drain the fibrorectalis muscle finally before joining the internal iliac nodes. Eventually the internal iliac obturated and the external iliac group of lymph nodes drain into the distant pre- and pariotic lymph nodes. This slide takes me to the end of this tutorial. I hope this has been helpful in understanding the anatomy of rectum and its relevance in reporting. I would like to end with a few pictures of the wonderful department and team as well as the hospital at Tata Medical Centre that I work at. Thank you.