0:00

We are looking at an MRI of

0:03

the knee of an eight-month-old.

0:06

On the left, I have a fluid

0:08

sensitive fat-suppressed sequence.

0:09

On the right, I have a T1-weighted sequence.

0:11

We know it's T1 because

0:13

the fat is nice and bright.

0:14

The marrow, which has fat, the

0:16

metaphysis is bright, the epiphysis

0:18

that's ossified is also bright.

0:20

But there's a big chunk right over

0:22

here that is the epiphyseal cartilage

0:26

that is sort of gray in color.

0:28

Thank you.

0:28

You notice the difference in the

0:29

cartilage appearance between a T2

0:32

FATSAT or a STIR or a pool of

0:35

some sort of fat-suppressed image.

0:37

Look at how much variation

0:38

there is between cartilage here, the cartilage

0:41

in the big bulk of the epiphysis, and

0:44

cartilage on the articular surface.

0:46

Remember we talked about initially on

0:48

the vignettes that the composition of

0:50

cartilage is vastly different depending

0:52

on where in the epiphysis you look.

0:54

Which do you think is a better comparison

0:56

image for cartilage.

0:57

Is it this one, or is it this one?

1:00

If you said this one, you were wrong.

1:02

It's this one because here you

1:04

can actually see the difference in

1:05

what the cartilage looks like.

1:06

And again, what causes the difference?

1:09

It's the amount of free water that's available.

1:12

This tells me here that the articular

1:14

cartilage over here is very bright.

1:15

So the amount of water here is great.

1:18

Over here, the epiphyseal cartilage,

1:21

although you do have water, that water is

1:24

not free; it's bound to macromolecules.

1:27

That's why it doesn't have the same bright

1:29

appearance as the articular cartilage.

1:32

The physeal cartilage has cells, but

1:35

remember those cells are very big.

1:37

There are huge cartilage cells as opposed to

1:39

the cartilage cells over here, and those cells

1:41

have a lot of water in them that is unbound.

1:44

So that's why that also appears as bright.

1:46

Contrast that to what the cartilage looks like

1:49

here in the epiphysis on a T1-weighted sequence.

1:52

It's uniformly gray.

1:54

I can't tell where the articular cartilage

1:56

begins, and the physeal cartilage ends.

1:59

Vice versa, I can't tell where the

2:01

epiphyseal cartilage begins, and the

2:02

physeal cartilage here ends, okay?

2:05

So, it's really not a great way of

2:07

differentiating the different types.

2:09

Also, I mentioned to look for

2:10

the trilaminar appearance.

2:12

That trilaminar appearance is

2:14

really, really well seen on this fat

2:17

suppressed, fluid-sensitive sequence.

2:19

Can you see that here?

2:20

Not really.

2:21

It just looks like a single dark gray blob.

2:23

So, if you want to look at cartilage, you

2:26

should be focusing on a fluid-sensitive,

2:28

fat-suppressed sequence like over here.

2:30

And you can scroll.

2:31

You can see, you can look for that

2:32

trilaminar appearance throughout.

2:35

Okay, this is a beautiful image here.

2:36

You've got a nice trilaminar

2:38

appearance throughout the entire

2:39

visible portion of the distal femur.

2:42

There may be areas where you

2:43

don't quite see it that well.

2:45

It's just because of the slice thickness.

2:46

As long as there is some smooth continuity from

2:49

one slice to the other, I know that trilaminar

2:52

appearance and that, and that physeal cartilage

2:54

and the metaphysis are just doing just fine.

2:57

Contrast hyaline cartilage, what

2:59

we've been talking about,

3:01

to the fibrous cartilage that you see

3:04

in your meniscus.

3:05

Look how dark that meniscus is in the fibrous

3:07

cartilage, as opposed to the hyaline cartilage.

3:10

Again, it's dark here on the T1-weighted

3:12

sequence, but the difference in those

3:14

two structures is just not that great.

3:17

I'm going to bring you this sequence

3:19

over here, and this is called the Dual

3:22

Echo Steady State (DESS). You know,

3:25

it's basically a gradient sequence.

3:27

It's a gradient sequence.

3:29

We know that because the cartilage

3:30

is awfully, awfully bright, right?

3:33

And the bone is very, very dark.

3:36

Why is the bone so dark?

3:37

Because it's a gradient sequence

3:39

and there's trabecula in there.

3:40

The mineralization, the trabecula causes

3:43

what's called susceptibility artifact.

3:45

The susceptibility artifacts

3:47

lead to this dark signal.

3:49

So there's a beautiful contrast between

3:52

bone and cartilage, but there's not a lot of

3:56

difference between, again, epiphyseal cartilage,

4:00

physeal cartilage, and articular cartilage.

4:03

But it's a great way to look at the trilaminar

4:06

appearance in the sense that because you

4:08

have this dark band of zonal provisional

4:11

calcification, it does a nice job of separating

4:14

the brightness of the metaphyseal spongiosa,

4:17

remember that's the area where it's very

4:18

vascular, and it's where the blood vessels come

4:20

in and bring in nutrients and apoptotic factors

4:23

to cause chondrocytes to die and form bone. The

4:26

metaphyseal spongiosa layer, zonal provisional

4:29

calcification, and the physeal layer, which

4:33

is filled with those hypertrophic cartilage.

4:35

Again, because it's not a great differentiator

4:37

between epiphyseal and physeal cartilage

4:39

and articular cartilage, but it's a great

4:41

way to look at your trilaminar appearance.

4:44

And these gradient sequences are often

4:47

acquired in an isovolumetric thin slice.

4:50

What does that mean?

4:51

That means we can reconstruct

4:53

this in any plane that we want.

4:54

For example, here's that same image

4:58

that we've reconstructed in a

4:59

sagittal plane and we get a great

5:01

view of that trilaminar appearance.

5:04

Again, using the susceptibility artifacts

5:06

caused by the trabecular bone to our advantage.