| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (VW) Class of 2021, Baylor College of Medicine, Houston, Texas |
| Transcript |
[Transcript of video] So, today we are going to be talking about DWI, diffusion weighted imaging. And it is exactly what it says. It is imaging that has been weighted on MRI towards the diffusion of water. And so under normal conditions, water is free to move. So this is the water, and it's able to diffuse. So, water, under normal conditions, is free, that is, unrestricted in its diffusion. And that's normally dictated just by Brownian motion. And, in a normal person who has an MRI, the DWI will show no restriction of the diffusion of water. However, under conditions of cytotoxic edema, for example in an acute infarct, ("cyto"- cell, "toxic"- death), so if the cell dies, the cells will swell up, and because they're all swollen up, this restricts the movement of water and causes restricted diffusion which will be bright on the DWI. And, you have to look at its paired study, the apparent diffusion coefficient (ADC). So it is the combination of the DWI and the ADC map that tells us whether we have true restricted diffusion or not. And in cytotoxic edema, the DWI will be bright and the ADC will show hypo-intensity corresponding with the restricted diffusion. In contrast with other types of edema that are not cytotoxic edema. And the predominant form that we're trying to differentiate is vasogenic ( "vaso"- vessel, "genesis" - coming from). So if the water is just coming from a leaky blood vessel, that just increases the amount of water without restricting its movement. And so under normal circumstances, a vasogenic form of edema does not restrict the diffusion of water and therefore the DWI would expect be unrestricted. However, sometimes the edema itself is so bright on the T2. So, edema increases the T2 signal and on the FLAIR (the fluid attenuation inversion recovery sequences), it will also be bright. And if the edema is so bright it can shine through the other sequences. And so vasogenic edema can shine through if it's sufficiently bright on T2. And the way to differentiate that is it'll be both bright on the DWI and bright on the ADC. And that is the marker that it's just shine-through edema from increased T2 signal. Because restricted diffusion from cytotoxic edema normally produces bright on DWI but dark on ADC. That is the marker of true restrictive diffusion. And there are other things other than ischemia that cause bright DWI and can actually produce restricted diffusion. But in most cases of neuro-ophthalmology, we're using it for identifying acute ischemia. So if we see restricted diffusion and that can perceive the structural imaging abnormality on MRI, we're really thinking about cytotoxic edema, a stroke. However vasogenic and cytotoxic edema can occur together. The main thing that we're trying to differentiate in patients that present acutely, for example, homonymous hemianopsia cortical visual loss, is posterior reversible encephalopathy syndrome, which is a vasogenic edema. And so, if a patient has PRES (posterior reversible encephalopathy syndrome), they are more likely to have a reversible and posterior form of edema. And that's usually vasogenic edema, and that means it might get better and the diffusion won't be restricted in the reversible vasogenic form. However, sometimes you have both vasogenic and cytotoxic. So, the bottom line is we use the diffusion weighted imaging to image based on the diffusion of water, which could be restricted or unrestricted. |
