0:00

This is a 41-year-old man with ankle pain

0:03

and no other clinical symptomatology.

0:07

It is obvious we have a mass in

0:09

the distal aspect of the ankle.

0:10

On your left is a more proton density,

0:13

non-fat suppressed image, and that'll

0:15

come into play here in a minute.

0:18

In the middle, we've got a water-weighted

0:20

image with excellent fat suppression.

0:23

Here's our mass again.

0:24

And then finally, on the right,

0:26

we have a straight forward T1,

0:28

spin echo, fat-weighted image.

0:31

So, a couple of thoughts for you as you're

0:34

analyzing the case, especially if you're a

0:36

resident, a fellow, or a young attending.

0:39

The first thing you might notice

0:41

is the joint is pretty dry.

0:43

Let's scroll it here a little bit.

0:47

And I'll give you the fact that there's

0:48

a little bit of fluid here, but not much.

0:51

So the fact that the joint is pretty

0:53

dry moves us away from the diagnosis

0:58

of arthropathic cysts, geodes, cystic

1:03

erosions, and that whole family of lesions.

1:06

Another factor that is very noticeable

1:10

is the disease is pretty profound in the

1:14

tibia, but the talus is still standing.

1:17

The talus is unaffected.

1:19

Whatever this thing is has not crossed the joint.

1:22

It has not involved the other side of the joint.

1:24

So if you're going down the road of infection,

1:26

septic arthritis should not be your choice.

1:31

So then we, we look at the lesion and we

1:33

scroll about it and we see it's got one major

1:36

component and maybe one minor component.

1:39

And then our next job is to decide if

1:42

it has a sharp zone of transition or

1:44

not, and for the most part, it does.

1:47

One area of it that is very

1:50

concerning is right here.

1:55

Right there.

1:57

So it's eating the cortex

1:58

a little bit like Pac-Man.

2:00

We don't like that.

2:01

And, as you know, bone end lesions, you

2:05

know, there's a narrow differential diagnosis

2:07

for these, you know, giant cell tumor,

2:09

chondroblastoma, telangiectatic osteogenic

2:13

sarcoma, things related to arthropathy, but

2:15

another one would be intraosseous ganglia.

2:17

The intraosseous ganglia don't

2:19

produce this kind of edema.

2:22

I mean, look at what's going on

2:23

here on the T1 weighted image.

2:25

Gray, white, everywhere.

2:27

It goes all the way up the shaft.

2:29

And then, even on the proton density,

2:31

which is bereft of contrast signal in many

2:35

cases, we still see a little bit of edema.

2:38

So, proton density, not my favorite sequence,

2:42

but here's what it gives you in this case.

2:45

It tells you that this is not fluid.

2:48

Now the T1 does that as well.

2:49

The signal here is equal to muscle, but

2:53

the signal here is nowhere near what you

2:56

would expect simple fluid to look like.

2:58

And that can come in handy in cases

3:00

other than this one, where we already

3:02

know that this is not simple fluid.

3:06

So, is it blood?

3:07

Well, it's not blood, because there's no

3:09

methemoglobin staining, and there's no

3:11

deoxyhemoglobin or hemosiderin low signal

3:16

effect on the water weighted image.

3:18

And a gradient echo image might show

3:20

that to you a little bit better.

3:22

Even though we don't have any known

3:24

risk factors in this case, we're left

3:27

with the diagnosis of an abscess.

3:30

And then we have to decide, within our

3:32

abscess, do we have sequestered bone?

3:35

The answer is no.

3:36

Do we have a sinus tract that is

3:38

lurking towards the periphery?

3:39

Well, not yet, although this thing is pooching

3:42

towards the periphery and sometimes you can

3:45

get a little bit of periosteum and covering,

3:48

soft tissue covering of that sinus tract.

3:51

We don't have that here and it is wanting

3:55

to get into the joint, don't get me

3:56

wrong, it just hasn't quite made it yet.

3:59

Now there is another very useful sign

4:02

in this case that I use in the brain,

4:04

and in the lung, but especially in the brain.

4:07

And I do a fair amount of brain

4:08

imaging, and that is the rim.

4:10

So I'm going to draw some rims for you.

4:13

And so here's a rim, with

4:16

some signal in the middle.

4:18

And I'm going to give you the

4:19

signal in the middle, I'm going to

4:21

make the signal in the middle red.

4:24

And this is my rim, and I call that a thick rim.

4:28

And I'd also say that it's thinner

4:31

over here, and it's thicker over here.

4:33

So it, it's not uniformly the same thickness.

4:37

And if I was really being true to

4:39

form, I'd make this a little bit

4:41

more narrow, and this a little wider.

4:43

In fact, let's do that.

4:44

Let's make it thinner.

4:46

See, it's thinner here, and then all of a

4:48

sudden, it gets a bit thicker over here.

4:52

And abscesses don't do that.

4:55

Tumors do that.

4:56

So when you have variability in the thickness

4:59

of the wall, and some of it is awfully thick

5:02

with this really weird intervening tissue,

5:06

and maybe some high signal in the middle.

5:09

Which might be fluid, it might

5:11

be necrosis, it might be pus.

5:13

When you have this type of wall

5:16

inconsistency, it's usually not an abscess.

5:21

That is a very useful sign.

5:23

So, that would be something like a glioblastoma

5:26

multiforme with necrosis in the brain.

5:29

Whereas, when you look at an abscess,

5:32

you usually have a fairly thin wall.

5:35

And I'm gonna use a different color for my

5:37

abscess just so we can have a little bit of fun.

5:39

Let's go with aquamarine blue.

5:41

And that is about what you'd

5:44

expect to see with your abscess.

5:47

And if you look at the T1-weighted

5:48

image, that's exactly what you get.

5:54

So, in differentiating abscess from some

5:57

of these other cavitary lesions that you're

6:00

going to encounter, both in MSK and in

6:03

neuro, the fact that you have a homogeneous

6:07

thickness of the rim of the cavity is very

6:13

consistent with the diagnosis of abscess.

6:15

Now, I don't mind that there is extensive edema.

6:18

That's absolutely fine for

6:20

the diagnosis of an abscess.

6:23

Now, one other take-home point.

6:25

When you look inside these abscesses, and

6:28

I'm going to use the color brown, because

6:29

it's kind of dastardly finding, and that is,

6:33

you may get some signal inside your abscess.

6:37

And that happens not infrequently.

6:40

And it's not because it's bled.

6:42

It's because you have neutrophils, you have

6:46

phagocytic structures that are inside the

6:49

abscess, lots of them, and what are they making?

6:51

They're making peroxidases.

6:54

And what do peroxidases do?

6:56

They drive down the signal

6:58

intensity on pulsing sequences.

7:01

Especially T2, especially gradient echo,

7:05

and it gets a little bit confusing.

7:07

You say, well, it's pus, it's

7:09

fluid, it should be white.

7:10

Well, the answer is no, it's not always white

7:14

because you get the susceptibility phenomenon

7:17

from the collection of neutrophils that give you

7:20

peroxidases that produce low signal intensity

7:25

due to their contrast, relaxivity effect.

7:29

So the take-home message here

7:30

is, this is a homogeneous lesion.

7:33

This is a homogeneous abscess.

7:35

It's got that nice, consistent rim that

7:39

you see best on the T1-weighted image.

7:41

And the rim is thin all the way

7:44

around, which it usually is.

7:46

In tumors, the rim is thick.

7:48

And although it didn't occur in this

7:50

case, sometimes you get heterogeneity

7:53

inside the abscess for reasons mentioned,

7:57

especially those intense collections

7:59

of phagocytic cells like neutrophils.

8:03

Dr. P signing off.