0:01

So the next part we're going to talk about is anatomy.

0:04

And so the first question in terms of

0:07

anatomy is what defines the rectum on an MR?

0:10

And the definition of rectum on an MR

0:13

is slightly deviant from the traditional

0:15

anatomic definition in that MR follows

0:19

the endoscopist's perspective of the rectum.

0:24

And if you ask the endoscopist what constitutes

0:26

the rectum, they'll tell you that it is

0:28

the most distal part of the GI tract that

0:31

extends 15 centimeters from the anal verge.

0:34

So in other words, you start at the anal verge, you

0:37

go along the axis of the bowel lumen for 15

0:41

centimeters, and that's what identifies the rectum.

0:45

So this anatomic definition is a little bit unique in

0:49

a sense that you are including the distal two to three

0:53

centimeters of the anal canal as part of the rectum.

0:56

You are not distinguishing the rectum and

0:59

the anal canal separately, but in fact are

1:01

including the anal canal as part of the rectum.

1:04

So that's the first important thing is what constitutes

1:07

the rectum is the 15 centimeter distal end of the

1:11

GI tract extending all the way from the anal verge.

1:14

That begs the next question is what defines

1:17

or what constitutes the anal verge?

1:19

How do you define the anal verge on MR?

1:22

Looking at the sagittal weighted image, you

1:23

can see that within the anal canal, there are

1:26

two components, or there are two structures.

1:29

The little bit brighter inner structure is

1:32

the internal sphincter, and then surrounding

1:35

the internal sphincter, this darker structure,

1:37

which has the same signal intensity as the

1:40

skeletal muscle, is the external sphincter.

1:42

And typically, when you look at the anal

1:45

canal, the external sphincter extends

1:48

a little bit lower than the internal sphincter.

1:51

And so there are some institutions that will take

1:54

the anal verge as where the external sphincter

1:57

ends or the most distal part of the GI tract.

2:01

And there are some institutions that will take

2:03

the anal verge to be the point where the internal

2:05

sphincter ends, which is right around here.

2:08

Now we are talking about a difference of a few

2:09

millimeters, but it's important to have a dialogue

2:12

with your surgeon or your oncology team, you know,

2:15

who typically take care of rectal cancer patients to

2:18

find out which definition they would want you to use.

2:21

We use, at our institution, we use the

2:23

most distal aspect of the anal canal.

2:27

We do not take the most distal aspect

2:29

of the internal sphincter, but there

2:31

are other institutions that do that.

2:32

So that's something else to

2:35

keep in mind in terms of anatomic depiction.

2:38

Following that, the next part of the anatomy is,

2:41

so we have laid out the extent of the rectum,

2:43

we have laid out where you start measuring.

2:46

Now, as far as the rectum goes,

2:48

there are a couple of things.

2:49

So in this patient, you can see this

2:51

young patient and has, doesn’t have a lot

2:53

of fat that surrounds the rectum.

2:55

So we'll switch gears a little bit to a different

2:57

patient who has a little more fat so that

2:59

we lay out the anatomy a little bit better.

3:02

So in this instance, you're looking at the

3:04

rectum again, you can nicely see the anal

3:06

verge here, from where you start measuring.

3:09

And if you look at the axial or the oblique axial

3:12

images, the rectal wall is composed of two parts.

3:16

You have the outer dark ring, which is the

3:19

muscularis, and the inner bright ring, which

3:22

is a combination of the mucosa and submucosa.

3:25

We cannot make a distinction between the mucosa and the

3:28

submucosa; it's sort of combined into this bright ring.

3:31

And then the structure that surrounds that

3:33

the outer ring is the darker muscularis.

3:36

So those are the two important structures

3:38

that you always need to identify because

3:40

that governs the staging of the cancer.

3:43

The next important structure that we need to talk about

3:46

is what is referred to as the mesorectal fascia.

3:50

And so typically, whether it's a male or a female

3:53

pelvis, you have the endopelvic fascia, which

3:58

supports organs in the pelvis, so that, you know,

4:00

it supports the organs to stay in their location.

4:04

And typically, the endopelvic

4:05

fascia has two components.

4:07

It has the visceral component

4:08

and the parietal component.

4:10

The visceral component, or the visceral

4:12

layer, covers the pelvic organs.

4:14

And the parietal layer typically covers

4:16

the muscles, the ligaments, etc.

4:19

So in the case of the rectum, the visceral layer is a

4:22

thin transparent layer that surrounds the, um, the

4:26

outline of the rectum and its mesorectal fascia.

4:29

So this is the outline that we

4:30

nicely see of the mesorectal fascia.

4:32

It's this dark line

4:34

that you can outline on MR very nicely.

4:37

And basically, the mesorectal fascia surrounds

4:39

not only the rectum but the mesorectal fat

4:42

as well, as well as the vessels and small

4:44

lymph nodes that are located in that fat.

4:47

So this is a very important structure to identify

4:50

on the MR because that has implications on staging.

4:54

It also has implications on prognosis if there

4:57

is involvement of this layer by the tumor.

5:00

Posteriorly, the mesorectal fascia is

5:03

separated by the parietal layer, which

5:05

is referred to as the pre-sacral fascia.

5:07

And that's an important point to keep in mind because

5:10

when they are doing surgery, they have to make sure

5:13

they don't violate the pre-sacral fascia because you

5:16

can see that a lot of flow voids in that location.

5:18

So there's a lot of vessels

5:20

sitting right on top of the sacrum.

5:22

And if you violate that space and get into this pre-

5:24

sacral space, you're going to get a lot of bleeding.

5:27

So what is the extent of the mesorectal fascia?

5:29

So, superiorly, as we come up higher,

5:32

it ends at the level of the rectosigmoid junction

5:35

where it blends with the connective tissue of

5:37

the sigmoid mesentery, which is right around here.

5:41

And if you extend down lower, it sort of gets closely

5:45

applied to the wall of the rectum and it typically is

5:48

attached to the puborectalis and the levator muscle.

5:52

So it's sort of like an inverted, or it's like a

5:54

teardrop shape where it's narrow at the top and narrow

5:57

at the bottom, but it's quite capacious in the center.

6:00

So that's one point to keep in mind

6:02

in terms of the mesorectal fascia.

6:04

Now, the rectum itself in its lower aspect is entirely

6:09

extraperitoneal, but as you come up superiorly.

6:12

And let me just bring up the

6:13

sagittal to go along with this.

6:16

As you come up superiorly in the rectum, you

6:19

can see that, and this is a male patient, you

6:21

can see there is a prostate, this is a seminal

6:23

vesicle, and extending posteriorly from the tip

6:25

of the seminal vesicle is this thin black line.

6:28

And that is a line of the peritoneal reflection.

6:31

On the axial, it is seen as this gulving right here,

6:34

you can see a very subtle insertion right here and here.

6:39

And so that's another important point to keep in

6:41

mind is that below the level of this peritoneal

6:45

insertion, the entire rectum is extraperitoneal.

6:48

But above the level of this peritoneal insertion,

6:51

the anterior part of the proximal mid-rectum and

6:55

the upper rectum has peritoneal lining anterior.

6:57

And that is again an important structure to identify

7:02

because tumor can spread into the peritoneum.

7:06

Not a very likely event, but you need to

7:08

pay close attention to that if it does

7:10

happen because that affects the staging.

7:13

And the other important thing is in terms of T staging,

7:16

you know, we have to pay attention to the peritoneal

7:18

insertion because that can lead to slight changes

7:22

in the way you stage and what you put in the report.

7:24

So that's the other important anatomic

7:26

structure you need to identify.

7:28

So we spoke about the extent of the rectum, we spoke

7:30

about the rectal wall, we spoke about mesorectal fascia.

7:35

We spoke about the peritoneal insertion.

7:38

Now, another important anatomic landmark

7:41

that you need to be cognizant of and sort

7:44

of keep in mind is, especially in low and

7:47

low rectal cancers, is the levator muscle.

7:51

And so if you look on the coronal images, by the

7:53

way, here again, you can nicely see the mesorectal

7:55

fascia that is surrounding the rectum, and this

7:58

is the insertion of the peritoneum that you see.

8:02

So when you come down into the pelvis, you see this.

8:06

A sort of fan-shaped or

8:08

curvilinear muscle on either side.

8:10

This is the iliococcygeus part

8:13

of the levator ani muscle.

8:14

So the levator ani muscle is composed

8:16

of three distinct components.

8:18

You have the iliococcygeus, which is seen

8:20

right here on the coronal, and then the second

8:23

component is the puborectalis, which is this

8:26

U-shaped muscle that surrounds the rectum.

8:29

The third component, which is not seen very well

8:31

on MR, so we don't need to worry about that.

8:34

But it's important to pay attention to the

8:35

iliococcygeus and the puborectalis.

8:39

So these are the two components that you need

8:41

to identify and pay attention to because

8:44

again, that influences the way we stage an MRI.

8:47

Look at low rectal cancers.

8:48

The last anatomy structure that you also need to

8:50

pay attention to are these nice, thin, wispy,

8:54

venous plexuses that arise from the rectal wall

8:57

and sort of extend superiorly, which are essentially

9:00

tributaries of the inferior mesenteric vein branches.

9:04

And the reason you want to identify and learn

9:06

and sort of pay attention to this in the normal

9:08

cases is because many times you can have tumor

9:12

extend into the vessels and when that happens you

9:15

have to learn how to identify that and also call

9:19

attention to that because that means worse prognosis.

9:23

So those are sort of the key anatomic

9:26

points that we need to pay attention to.