0:01

So having said that, we'll first dive into

0:03

the technique and talk about some of the key

0:06

points that you need to keep in mind when

0:08

you're looking at a patient's rectal cancer.

0:10

So as far as the MR goes, the first thing to, to

0:13

understand is to look at the area of coverage.

0:17

And so this is the triplane gradient echo,

0:20

um, localizer that most vendors will offer

0:23

you in terms of trying to figure out.

0:25

which anatomy you're trying to look at.

0:28

And typically for rectal cancer, you want

0:30

to be below the level of the anal verge

0:32

and go as high as the aortic bifurcation.

0:37

And the reason you want to do that is, uh, if you

0:39

are at the level of the aortic bifurcation, you

0:42

are sort of at the level of L5 or mid part of

0:46

L5 and that essentially covers the entire aspect

0:50

of the rectum and the reason you want to be below

0:52

the level of the anal verge is that if it's a

0:55

low-lying rectal cancer you have included the

0:57

relevant anatomy in terms of staging purposes.

1:00

So, that's sort of the extent of

1:02

what you're trying to look at.

1:05

Then comes the, uh, the sequences

1:07

that you're trying to assess.

1:09

And the most important, uh, sequence that

1:13

you have in your toolbox for looking at the

1:16

rectal cancer is a T2 weighted sequence.

1:21

And then the question is, which

1:22

type of T2 weighted sequence?

1:23

It's the TurboSpinECHO or

1:25

FastSpinECHO T2 weighted sequence.

1:28

because that gives you the best contrast

1:30

noise that you can achieve in terms of

1:33

looking at the various components of the

1:35

rectal wall and also looking at cancer.

1:38

You don't want to use this sequence, which is a

1:42

single shot or a single shot fast spin echo or

1:45

a HASTE sequence, although this sequence is very

1:48

rapid and quick and fast and can freeze motion.

1:52

It is not an ideal sequence for looking at the

1:55

rectum because it just does not have the contrast

1:58

to noise and just to show you side by side.

2:00

On your left is the, um, single shot fast spin

2:03

echo and on the right is the conventional

2:05

fast spin echo or turbo spin echo.

2:08

And if you look at the rectum, you can see

2:10

that there is a lot more contrast to noise.

2:13

You can see the wall of the rectum, you can

2:15

actually identify the cancer, which may be very

2:17

difficult to do on the single shot fast spin echo.

2:21

So that's sort of an important point to emphasize.

2:24

And so this is sort of the money sequence, if you

2:26

will, uh, in terms of staging the cancer, detecting

2:30

the cancer, and also looking at all the relevant points

2:33

within the staging system that we pay attention to.

2:36

And so it is important to perform this

2:38

T2 weighted sequence in three planes.

2:40

So this is the sagittal plane.

2:42

That's the first one you do.

2:44

Uh, and there is a reason behind why you

2:46

acquire the sagittal image first.

2:48

The first reason why you want to do that is you want

2:51

to make sure that the anatomic coverage is adequate.

2:55

Now, before we talk about the other planes of T2

2:58

weighted sequence, there are a couple of other

3:00

pointers in terms of, or options that you can avail of.

3:04

There is a question that we get asked

3:06

very often is, do we need to distend the

3:08

rectum with some kind of gel or saline?

3:12

At our institution, we don't do that, and the reason

3:14

for that is it's just a logistical challenge.

3:17

You know, we have scanners that span multiple

3:20

places, and it's not possible for us to do that.

3:23

There are other institutions which

3:25

do instill rectal gel, and opacifying

3:28

the lumen certainly has its benefits.

3:31

You can better see the tumor delineation.

3:35

And so that's something that if you are doing

3:37

or if you plan to do and have the capability

3:39

of doing, maybe a good exercise to do.

3:43

Some institutions have the patients

3:46

instill a micro enema, which is a small

3:48

enema kit right before the procedure.

3:51

And the reason for that is to empty out the rectum.

3:54

And again, that's been shown and proven in literature

3:56

to be of benefit in terms of having an empty

3:59

rectum which can give you a better image

4:02

quality as well as reduce the artifacts that

4:04

you can see on the diffusion-weighted sequences.

4:08

And again, that's something

4:09

if you're doing or plan to do.

4:11

As long as you have, you know, good control

4:13

over it and patients follow the instructions,

4:15

that would be a useful exercise to do.

4:19

And then the last point is in terms

4:21

of giving spasmolytics or things like

4:23

glucagon to freeze the motion of the bowel.

4:26

From a logistical perspective, it can be time

4:28

consuming because you have to screen the patient.

4:30

It also adds cost because glucagon is not cheap.

4:34

And the question is, when do you administer it?

4:36

So, because of all those reasons, we don't typically

4:38

give it, but if you have the means and the capability

4:42

of giving glucagon and adequate means of controlling when

4:45

and how to give it, then certainly that would be a

4:48

useful adjunct because you will freeze peristalsis

4:51

in the bowel and get much better signal delineation.

4:54

So having said that regarding the technique,

4:56

then moving on to the other planes.

4:57

So, as I said, we start with the sagittal plane.

5:00

And then following the sagittal, we acquire

5:03

a true coronal, as you are seeing in this instance.

5:06

And after the true coronal, you acquire a true axial.

5:09

And the reason for doing these three planes,

5:13

the sagittal, as I said, is primarily to get

5:15

a sense of whether there's adequate coverage.

5:18

The other two planes are useful for staging

5:21

the tumor as well as getting a sense of anatomy.

5:24

of what the structures that surround the

5:26

rectum look like and what some of the other

5:29

anatomic landmarks that we need to identify,

5:32

and we'll be talking about that in a bit.

5:34

Then comes the next most important sequence, and

5:37

that is referred to as the oblique axial plane.

5:41

And what essentially the oblique axial plane is

5:43

on the sagittal, you need to find out where the

5:46

cancer is, and then what you need to do is go

5:49

perpendicular to the plane of the tumor.

5:52

And the reason for doing that is it gives

5:55

much better delineation of the lumen, gives

5:58

better delineation of the tumor, leads to much

6:01

better staging, and also provides the necessary

6:05

information that you need to put in the report.

6:07

It also delineates the outline of the mesorectal

6:10

fascia, which we'll be talking about in a bit.

6:12

One of the key points to remember from

6:14

a technique perspective is you need to

6:16

do this oblique perpendicular plane.

6:18

Now here, the angle is a little

6:19

bit more than what I would have

6:22

planned, but it's still in the right thing.

6:24

Ideally, it should be something like this,

6:26

which is exactly 90 degrees or perpendicular to

6:29

the plane of the tumor or the axis of the tumor.

6:32

So that's the other

6:35

important point to keep in mind.

6:37

One other important plane to keep in mind

6:40

is what is referred to as the oblique coronal.

6:43

And now if you look at this sagittal plane,

6:45

you will see that the rectum sort of

6:48

makes a gentle curve along the sacral arch.

6:52

And as you get into the anal canal, which is

6:54

pointing posterior right here, and if you have a

6:58

rectal cancer that is low in the lumen of the rectum

7:01

and invades the anal canal in terms of staging,

7:04

it becomes important for providing the

7:07

plane for assessing the anal canal.

7:10

And in that case, you do an oblique coronal which

7:12

is going along the axis of the anal canal like so.

7:16

And just to kind of show you as I am scrolling

7:19

through, you will see that this is the oblique

7:22

coronal plane through the anal canal and that lays

7:24

out the anatomy of the anal canal very nicely.

7:27

So you're going along the plane of the

7:28

or along the axis of the anal canal.

7:31

So that's the other important point to

7:33

keep in mind in terms of acquisition.

7:35

So the take-home point here is the oblique axial

7:39

is the most important sequence for tumor staging.

7:43

It's the most important sequence for identifying

7:46

and accurately staging the cancer.

7:49

If you have a rectal cancer that involves the

7:52

lower part of the rectum or the anal canal,

7:55

make sure you get an oblique coronal to lay

7:59

out the anatomy of the anal canal nicely.

8:01

Following those key sequences, we acquire a diffusion

8:04

and this is typically acquired with a low B value and

8:08

then a higher B value and typically the higher B values

8:13

are in the 800 to 1200 range, typically around 1000 and

8:18

that suffices to null out signal from fluid and you

8:21

can nicely see the tumor showing restricted diffusion.

8:25

This is a good complement to the T2 weighted

8:27

sequences to identify where the cancer is.

8:30

It also is a good technique for doing,

8:32

especially if the patient has undergone

8:34

neoadjuvant therapy prior to surgery.

8:37

Because once the patient gets neoadjuvant therapy,

8:39

it becomes very difficult to identify the tumor.

8:42

And in those cases, these

8:44

diffusions can bail you out and help you.

8:47

And then the last part is doing T1 Fatsat.

8:52

And again, this is done as part of a Dixon.

8:54

So there is homogeneous fat suppression.

8:56

So this is the water-only image from the Dixon

8:58

showing nice homogeneous fat suppression.

9:01

And then at our institution, we

9:03

typically end up giving gadolinium.

9:05

There is a little bit of controversy in the

9:07

literature on the utility of giving gadolinium.

9:10

I can tell you because we do not give

9:13

spasmolytics because we do not instill

9:16

any kind of rectal gel for the exam.

9:19

This can be a useful adjunct.

9:21

Or complement to the T2 weighted

9:23

sequence for you to kind of localize

9:25

and look at what the tumor is doing.

9:27

It is also a very good sequence to complement

9:30

the diffusion in those patients that undergo

9:32

neoadjuvant chemoradiation to see

9:34

whether there is residual tumor present or not.

9:38

And so that's sort of the importance and we

9:40

typically and routinely acquire post-gadolinium

9:43

that it's not important to do the sequence as a

9:46

dynamic sequence as long as you acquire a post-

9:49

enhanced image. And typically we acquire them about

9:53

70 seconds to a minute after the injection. And we

9:56

acquire them axially and in the coronal plane again.

9:59

The idea is to just get a better

10:01

look at what the tumor is doing.

10:03

And then since we're giving gadolinium and acquiring

10:06

a post-gadolinium, it is also incumbent in that

10:08

instance to do subtractions because that again lays

10:11

out the area of enhancement a little bit better.

10:15

The other advantage of the gadolinium-enhanced

10:17

sequences is very often in these patients who are

10:20

undergoing staging for rectal cancer,

10:24

you can get incidental findings in the pelvis

10:26

and this can sometimes help you out if you truly

10:29

have incidental findings that are present.

10:33

And so that's sort of in a nutshell in terms

10:36

of the technique of what needs to be done and

10:38

how we typically acquire the sequence.

10:42

Now the next part that we're

10:43

going to talk about is anatomy.