 Okay, so the next part we're going to talk about is anatomy and so the first question in terms of anatomy is what defines the rectum on an MR? And the definition of rectum on an MR is slightly deviant from the traditional anatomic definition in that MR follows the endoscopic perspective of the rectum. And if you ask the endoscopic what constitutes the rectum, they'll tell you that it is the most distilled part of the GI tract that extends 15 centimeters from the anal verge. In other words, you start at the anal verge, you go along the axis of the bowel lumen for 15 centimeters and that's what identifies the rectum. So this anatomic definition is a little bit unique in a sense that you are including the distilled two to three centimeters of the anal canal as part of the rectum. You are not distinguishing the rectum and the anal canal separately but in fact are including the anal canal as part of the rectum. So that's the first important thing is what constitutes the rectum is the 15 centimeter distal end of the GI tract extending all the way from the anal verge. That begs the next question is what defines or what constitutes the anal verge and how do you define the anal verge on MR? Looking at the sagittal weighted image you can see that in within the anal canal there are two components or there are two structures. The little bit brighter inner structure is the internal sphincter and then surrounding the internal sphincter, this darker structure which has the same signal intensity as the skeletal muscle is the external sphincter. And typically when you look at the anal canal the external sphincter goes or extends a little bit lower than the internal sphincter. And so there are some institutions that will take the anal verge as where the external sphincter ends or the most distal part of the GI tract. And there are some institutions that will take the anal verge to be the point where the internal sphincter ends which is right around here. Now we are talking of a difference of a few millimeters but it's important to have a dialogue with your surgeon or your oncology team you know who typically take care of rectal cancer patients to find out which definition would they want you to use. We use at our institution we use the most distal aspect of the anal canal. We do not take the most distal aspect of the internal sphincter but there are other institutions that do that. So that's sort of something else to keep in mind in terms of anatomy depiction. Following that the next part of the anatomy is so we have laid out the extent of the rectum we have laid out where you start measuring. Now as far as the rectum goes there are a couple of things. So in this patient you can see this young patient that has doesn't have a lot of fat that surrounds around the rectum so we'll switch gears a little bit to a different patient who has a little more fat so that we lay out the anatomy a little bit better. So in this instance you're looking at the rectum again you can nicely see the anal verge here where from where you start measuring. And if you look at the the axial or the oblique axial images the rectal wall is composed of two parts. You have the outer dark ring which is the muscularis and the inner bright ring which is a combination of the mucosa and submucosa. We cannot make a distinction between the mucosa and the submucosa it's sort of combined into this bright ring and then the structure that surrounds that the outer ring is the darker muscularis. So those are the two important structures that you always need to identify because that governs the staging of the cancer. The next important structure that we need to talk about is the what is referred to as a mesorectal fascia. And so typically whether it's a male or a female pelvis you have the endopelvic fascia which supports organs in the pelvis so that you know it supports the organs to stay in their location. And typically the endopelvic fascia has two components. It has the visceral component and the parietal component. The visceral component or the visceral layer covers the pelvic organs and the parietal layer typically covers the muscle the ligaments etc. So in case of the rectum the visceral layer is a thin transparent layer that surrounds the the outline of the rectum and it's referred to as a mesorectal fascia. So this is the outline that we nicely see of the mesorectal fascia. It's this dark line that you can outline on MR very nicely and basically the mesorectal fascia surrounds not only the rectum but the mesorectal fat as well as well as the vessels and small lymph nodes that are located in that fat. So this is a very important structure to identify on the MR because that has implications on staging. It also has implications on prognosis if there is involvement of this layer by the tumor. Posteriorly the mesorectal fascia is separated by the parietal layer which is referred to as a pre-sacral fascia and that's an important point to keep in mind because when they are doing surgery they have to make sure they don't violate the pre-sacral fascia because you can see that a lot of flow voids in that location. So there's a lot of vessels sitting right on top of the sacrum and if you violate that space and get into this pre-sacral space you're going to get a lot of bleeding. So what is the extent of the mesorectal fascia? So superiorly as we come up higher it ends at the level of the rectosigmoid junction where it blends with the connective tissue of the sigmoid mesentry which is right around here and if you extend down lower it sort of gets closely applied to the wall of the rectum and it typically is attached to the pubo rectalis and the levator muscle. So it's sort of like a inverted or it's like a teardrop shape where it's narrow at the top and narrow at the bottom but it's quite capacious in the center. So that's one point to keep in mind in terms of the mesorectal fascia. Now the rectum itself in its lower aspect is entirely extroperitoneal but as you come up superiorly and let me just bring up the sagittal to go along with this. As you come up superiorly in the rectum you can see that in this is a male patient you can see here is a prostate this is a seminal vesicle and extending posteriorly from the tip of the seminal vesicle is this thin black line and that is a line of the peritoneal reflection. On the axil it is seen as this gullwing right here you can see very subtle right insertion right here anteriorly and so that's another important point to keep in mind is that below the level of this peritoneal insertion the entire rectum is extroperitoneal but above the level of this peritoneal insertion the anterior part of the proximal mid rectum and the upper rectum has peritoneal lining anteriorly and that is again an important structure to identify because tumor can spread into the peritoneum not very likely even but you need to pay close attention to that if it does happen because that affects the staging and the other important thing is in terms of pea staging you know we have to pay attention to the peritoneal insertion because that can lead to slight changes in the way you stage and what you put in the report so that's the other important anatomic structure you need to identify. So we spoke about extent of the rectum we spoke about rectal wall we spoke about mesorectal fascia we spoke about the peritoneal insertion. Now another important anatomic landmark that you need to be cognizant of and sort of keep in mind is especially in low and low rectal cancers is the levator muscle and so if you look on the coronal images by the way here again you can nicely see the mesorectal fascia that is surrounding the rectum and this is the insertion of the peritoneum that you see. So when you come down into the pelvis you see this sort of fan shaped or curvilinear muscle on either side this is the iliocoxidious part of the levator in eye muscle. So the levator in eye muscle is composed of three distinct components you have the iliocoxidious which is seen right here on the coronal and then the second component is the pubo rectalis which is this u-shaped muscle that surrounds the rectum. The third component which is not seen very well on MRN so we don't need to worry about that but it's important to pay attention to the iliocoxidious and the pubo rectalis. So these are the two components that you need to identify and pay attention to because again that influences the way we stage and look at low rectal cancers. The last anatomy structure that you also need to pay attention to are these nice thin wispy venous plexuses that arise from the rectal wall and sort of extend superiorly which are essentially tributaries of the inferior miscentric vein branches and the reason you want to identify and learn and sort of pay attention to this in the normal cases is because many a times you can have tumor extent into the vessels and when that happens you have to learn how to identify that and also call attention to that because that means worse prognosis. So those are sort of the key anatomic points that we need to pay attention to.