Dr. Peter Kranz at the 2023 Cedars-Sinai Intracranial Hypotension Conference
Dr. Peter Kranz, Associate Professor, Department of Radiology, Duke University Medical Center in Durham, NC, presented this talk titled “Spinal Imaging: Diagnostic Algorithm II, CT-Based” at the 2023 Cedars-Sinai Intracranial Hypotension Conference on July 8, 2023. The conference was hosted by Cedars-Sinai with generous support from the Spinal CSF Leak Foundation in Kohala Coast, Hawaii.
Slides from the talk
View a PDF of Dr. Kranz’s slides here
Transcript
Dr. Kranz discusses CT-based spinal imaging diagnostic algorithms:
[00:00:09] Thank you very much. Thank you to the organizers of the meeting for hosting me. Thank you to Marcel for letting me go after you so that our little tête-à-tête here about CT and DSM can end with the the correct information that I’m gonna—I’m only kidding. What I’m going to present today is the way that we work up spinal imaging using the CT based approach.
[00:00:42] And for all the for all the jabs that, that we’d like to throw at each other. I think, you know, it really is true that whether you use fluoro based techniques or CT based techniques, they’re tools and you can be very good with the tools that you have, and it’s probably impossible to answer which modality is better. It’s more important to be very good with the tools that you use. So this is, I’m going to show you some of the things that I like CT based evaluation for. And if you happen to have CT available to you at your institution and this is the way you use them, hopefully you’ll find this useful.
[00:01:25] So, I think for a lot of places now, spine MRI is sort of the first line method of imaging that places would use to, to look at the spine in patients with suspected CSF leaks. And based on that spine MRI, the goal of that spine MRI is not necessarily to be able to localize leaks, but it’s to be able to stratify patients into one of two conditions, which is that they either have epidural fluid, in which case they have an epidural leak, which can be one of those ventral leaks or one of those lateral dural tears that Dr. Schievink talked about; or you see no fluid. And if you know the patient has SIH, that’s going to be a presumed occult CSF venous fistula. But once you’ve done this spine MRI, you’ve only sort of done part of the evaluation, if you really want to get down to, you know, what is the exact cause of the leak.
[00:02:26] And from there you need a problem solving study. And so, for patients who have epidural fluid. What you need is a problem solving study that has high temporal resolution. And that can include ultra fast CTM, which I’m going to talk about, or it can include fluoroscopic evaluations like DSM and dynamic myelography, which I’m not going to talk about because Dr. Maya just gave a great talk on that. And then if you see no epidural fluid and you know the patient has SIH, you’re talking about a presumed occult CSF venous fistula. And again, you have choices. There are fluoro based choices like DSM or dynamic myelography, or CT based choices where you can do decubitus CT myelography, which is my preferred technique, or what’s called a dynamic CTM, or ultra fast CTM, which some other folks may like to use. So, I’m going to talk first about CT for epidural leak. So these are patients with epidural fluid collections, and the term that was originally sort of applied to the process of getting multiple very quick acquisitions as contrast is moving up the spine, was called ultra fast CTM, and this is a technique that was really pioneered by the folks at the Mayo Clinic.
[00:03:50] And so what is ultra fast CTM? Well, basically, you’re scanning the patient as contrast is being injected, and the patient is positioned in such a way that contrast is going to move up the spine. And so what you wind up with is multiple phases. Okay, so this is very early. And then this is very subtly slightly later.
[00:04:13] And then this is the next image after that. And you can see that the contrast opacification of the thecal sac progressively increases with each of those phases. And these phases are not minutes apart, they’re seconds apart. So they’re very close in time. And typically most places will do either three or four phases.
[00:04:32] And the goal, just like with digital subtraction myelography, as Dr. Maya talked about, is to catch that first moment where the CSF begins to leak out. Sometimes this will be called a dynamic CT myelography. I know the terminology is very complicated, but basically I think of dynamic CT myelography is anything where the contrast is moving during the course of the exam.
[00:04:58] And so again, the goal is to sort of catch, catch the CSF leakage at the first moment that it comes out. Here’s the equipment that you need, which is a CT scanner, of course, and that’s the most expensive bit. And then you need something to elevate the patient so that when you inject contrast, it will run with gravity downhill because I can tell you, CT scanners don’t tilt very easily, and your engineering department will get very upset at you if you try to tilt them.
[00:05:25] So you need something to tilt the patient instead. And and something like a foam wedge or something like that is typically what is used. And so this is what the patient looks like. This is a scout image with the patient on that wedge, and you can see the wedge is this radiolucent structure beneath them, and it causes them to have a slope in their spine so that if you could imagine dropping a marble on the back of their vertebral column, it would roll all the way down towards their head, and that’s what will happen when you inject contrast.
[00:05:56] And so, it’s very technique dependent. You have to make sure that the patient is sloped downwards, otherwise the contrast will just pool in the lower part of the spine. But if you do it right, you’ll get an image that looks like this, which is very reminiscent of the images that you see with digital subtraction myelography, where there’s a pool of intrathecal contrast, and then there’s a fork in the road at the spot where the epidural leak occurs here due to this calcified thoracic disc where contrast pours into the epidural space and then you’ve localized the site of the leak with a very high degree of accuracy. So, DSM is another way of doing this, and DSM also has great temporal resolution. So why do this with CT?
[00:06:42] Well, one of the problems that that I faced when trying to do these with DSM is what I sort of called a shoulder problem, which is that many of the leaks, the ventral leaks, occur in the upper thoracic spine, T1-2, T2-3, that’s probably about 80, I don’t know, 80, 85 percent of the leaks.
[00:07:03] And It’s very difficult to get an x ray beam to penetrate laterally through that area because you’ve got the patient’s shoulders in the area. So here’s an example of a patient who has a clear epidural fluid collection on their MRI scan. And then this is what the patient looks like under fluoroscopy, where you just can’t see anything in here at all.
[00:07:26] Now there are some tricks you can do in this case. We put the patient in what’s called the swimmer’s view, where one arm is up, and now you can see a little bit better that there is a split in the contrast here, which helped localize the site of the leak. But in many patients, it’s very difficult to make this imaging look very good.
[00:07:44] And so, why ultrafast CTM? Again, because most of these ventral leaks will occur up here, and that happens to be where the shoulders are. So here’s just another example of one of those ventral leaks. And so, at least for me, if the leak is below the T3 level, I think fluoro techniques are really great.
[00:08:04] I love the convenience and the speed that fluoro brings there. I think you can also do that evaluation with ultrafast CTM above that line because of the shoulder issue. I prefer the ultrafast CT technique. But again that’s what I’m comfortable with. Now you can also use these techniques for the lateral dural tears, which tend to occur a little bit lower down in the thoracic spine, so you don’t have to deal with the shoulder issues down here.
[00:08:29] So this is a typical appearance of what one of those lateral leaks look like. It is typically broad base against the lateral dura, typically in the axilla of the exiting nerve root. And oftentimes I’ll do this under fluoro, because I think in this situation CT, to be fair, doesn’t add a whole lot. So you put the patient on their side and the decubitus position and you gradually tilt them downward, and you’ll get a series of images that looks like this where you have contrast that is gradually creeping up the side of the thecal sac, and then you’ll start to see it pooling in this big lateral meningeal diverticulum that’s caused by the arachnoid poaching out through that dural tear. And then you’ll see it start to spread in the epidural space here, and you’ve localized your leak. So it’s a very nice technique. You can do this with or without digital subtraction, it really doesn’t matter.
[00:09:26] But you can also do this under CT. So just to show you in principle, here’s how you do it. This is a patient who has epidural fluid on their MR scan, had an outside CT myelo which showed some epidural fluid. I put the patient in lateral decubitus here knowing that the leak was going to be in the lower aspect of the thoracic spine, and we get multiple phases.
[00:09:47] Now, these are the coronal sort of reformats of those phases, so you can see in phase one, you’ve got the contrast dye that’s starting to trickle along the thecal sac, and you see this beginning filling of this focal outpouching with very early leakage of contrast into the epidural space. This is a little bit later on phase two, and then on phase three, you can start to recognize more of the basic anatomy of the thecal sac. Here’s that lateral diverticulum. There’s the leaked epidural contrast, both above and below that diverticulum. And so you’ve localized that leak very effectively with with this technique.
[00:10:25] And again, you can see this is a lateral dural tear right beneath the axilla of the nerve root hanging off their typical appearance. What about CT for CSF venous fistulas? So, again, you’re going to have the patient in the lateral decubitus position. The wedge is going to be underneath, well, in this case the wedge is underneath the patient’s hips.
[00:10:45] And there’s two ways of doing this. You can do this as a dynamic scan where you’re sort of capturing multiple phases as the contrast goes by. That to me is a little bit technically challenging. I like to be simpler. So what I do is I get the patient’s head up on three pillows, and what happens is you create this trough, a trough where contrast will accumulate in the in the thoracic spine, and that’s where most of these venous fistulas come from.
[00:11:12] Now, if you’re going to do this, The other way and you’re going to do this, in ultra fast you don’t want that trough because you want the contrast to continuously move down the spine. Again, some places will do it that way, we don’t do it that way. But the goal is to get high density contrast along the lateral aspect of the thecal sac. So this is the localizer image, and you can see this sort of white line here, which is high density contrast layering along the side of the thecal sac.
[00:11:37] It’s very important to get the head up so that contrast doesn’t go up into the head, because it’s not going to be useful there, you want it to be in the spine. And so in this particular case we’re going to get, I can look at this scout image on the CT scan and know that we’re going to get a very good evaluation of this area that I’m showing in orange because I can see that the contrast is going to be very high density.
[00:11:58] And then you’re going to get something that looks like this, where you get very high density contrast that’s layering dependently in the thecal sac. One of the nice things about CT is that you can, as long as you keep it out of the head, you can reposition the patient and move that contrast up and down at will.
[00:12:16] So in this particular case, this was the first set of images. I could see that it was going to be very dense in the upper thoracic spine up here, but I wanted it to be a little bit lower. So I repositioned the patient. Now we’ve got very dense contrast. We’re going to get a good evaluation here. We took a couple pictures in this position, but then I wanted to get a little bit lower so I could just reposition the patient again.
[00:12:39] Now we’ve got dense contrast down here. So you, by looking at the localizer images, you can know where you’re going to get a good evaluation. So it’s really important to keep the contrast out of the head. I use three pillows underneath the head. And early scanning I think is important, but at least in my opinion contrast density is much more important.
[00:12:59] And if you can get very high contrast density, that really is superior to getting very —fast images are important in DSM, because you have to have the subtraction of the two images, right, and so the patient has to be still during that period of time. There’s a limit to the amount of time that the patient can be still they have to hold their breath.
[00:13:17] But in CT, the very fast acquisition of images is less important because there’s not a mask that has to be subtracted. There’s nothing that needs to be subtracted. And so some places are describing now multiple phases of scanning. I’m not sure whether or not that’s going to be valuable.
[00:13:35] I think it’s interesting. I’m glad people are looking at that. But to me, it’s a little less important. So why do I like CTM for CSF venous fistulas? Well, as I mentioned before, first of all, I have access to the equipment. Okay. And at your institution, if you’re a radiologist, you may or may not have access to the equipment.
[00:13:53] I also like that I can do both sides in one day. And so this was a patient who had left side down, decubitus CT myelo. We didn’t see anything. We turned the patient over to the other side. You can see contrast is dependently layering on the other side. And then this is the movie of that. I’ll play that again. And what you’ll see is the CSF venous fistula coming out here in this paraspinal vein.
[00:14:19] So you can do this all in one day, and we very frequently find the fistula on the opposite side than the one where we started. So it’s one day, one needle, and no sedation. Now for me, I have higher confidence with CT for small or subtle fistulas. And this may be because I don’t DSM is not my preferred way of doing it.
[00:14:42] I do some DSMs, but in this particular case there’s a venous fistula on this image. There’s nothing that you could see on the subtracted images. On the unsubtracted images, it was very difficult to see, but on the CT images, what you can see is a little bit of contrast here in the internal epidural venous plexus.
[00:15:04] You can see some contrast coming out in some muscular branches. And then on the coronal reformats, you can see it again in the internal epidural venous plexus. For me, that’s just a little easier to see than looking at this digital subtraction myelogram. Here are some other cases where these are gonna be unsubtracted images to show very subtle venous fistulas like this one here. I personally feel a little more comfortable looking at that on CT. Here’s another one. You can see it on CT. It’s kind of subtle on the on the unsubtracted image, although visible for sure. Here’s another one super hard to see on the fluoroscopic images, I think.
[00:15:46] Another one there, but I just personally find them to be a little bit easier. Here’s a case where there’s a digital subtraction myelo. And again, these are two stations. Both of these encompass the level where the venous fistula is. Here’s the CT myelo of the same patient. You can see this really extensive venous fistula out lateral to the vertebral body.
[00:16:18] This is what it looked like on the coronal static image, just to show you, it’s this level on the DSM. I think that’s, I’m not sure that I see it there, but, you know, we all have cases where you see it on one modality or not the other modality. So ultimately it comes down to what your preference is and what you like.
[00:16:36] I thought this was interesting. This is a paper out of the Mayo Clinic looking at the incremental diagnostic yield of a lateral decubitus CT myelogram immediately following a negative DSM. So there may be a role for both in some cases. So if you do a DSM, which I think is fine. If you do a DSM and you don’t see it, sometimes doing a CT can be helpful, because in this study they found a 39 percent additional incremental diagnostic yield.
[00:17:05] So of the ones that were negative, they found a fistula in 39 percent of those cases. And then finally, you know, we’re learning some interesting things about respiratory phases. This is from a paper that we just was just published, I think, earlier this week in the AJNR looking at the effect of different respiratory phases.
[00:17:27] So this is the fistula on maximum inspiration. This is using a technique called resisted inspiration, where they suck in through a straw. Essentially, this was during Valsalva, and you can see that the appearance of this fistula changes with these respiratory phases. And so, you know, this is something that you know, if you’re doing subtracted images, it’s very difficult to get. Even if you’re doing dynamic CT, it’s very difficult to get multiple different phases.
[00:17:54] So, I’m not sure where this will take us, but I think there’s some potentially interesting information that we can get from different respiratory phases. So, in summary, this is the way I do it. You know, I like to look at a spine MR: if there’s epidural fluid, and it’s ventral predominant I’ll get an ultrafast CT myelo.
[00:18:14] If it’s lateral, I’ll do either CT or fluoro based evaluation of that. If we see no fluid on that initial spine MR, I’m going to go to bilateral decubitus CT myelo.