 Welcome to MRI Onlines, section on renal masses. I'm here with my colleague, Dr. Pina Fennazo, an expert on body imaging. We're going to talk about how you use MRI to approach lesion in the kidney and why you would use MRI. We're going to begin with a 69-year-old man. He's got a history of bladder carcinoma, and he's got a few renal masses which you would call quote, unquote, cis. Now just a little bit of background. Renal cell carcinoma is the most common malignant epithelial tumor. It accounts for 90% of all solid renal tumors. So you think solid, CT, no big deal. I can figure that out. Why do I need an MRI? On the other hand, 70% of all renal cell carcinomas are clear cell carcinomas. And when you think clear, you think cytoplasm. When you think cytoplasm, you think water. When you get close to water density, things get a little bit dicey on CT and MRI. So why would you need an MRI? And let's illustrate it with this case. So the most classic example of why we go to MRI is really to determine, number one, is their enhancement. And while that seems like such an easy task for us to do, it's sometimes very difficult. And we tend to start off with CT imaging. And so basically, what are the criteria we use to determine whether something is purely a cyst and can be ignored? We start off with Hounfield units. So being able to put measurements and region of interest curves on a lesion can help us determine if it's a cyst or not. Anything less than 20, we can confidently ignore it. Anything greater than 90, we can say is a hyperdense cyst and we can confidently ignore it. And one caveat, I mean, I interrupt you. But one caveat is most residents think I need a Hounfield unit of zero to say it's water. But 20 is the cutoff, right? 20 is the cutoff. But we're faced with a few dilemmas and pitfalls that we need to know. First of all, we need to classify that in the unenhanced CTs. And nowadays, we're either doing base CT imaging, which is the non-contrast, or we're just doing post-contrast imaging. So when we get a lesion like this, if you see in 2017, the lesion looks like it's maybe about seven or eight millimeters, and now you look at it in 2019 and it's slightly larger. We did Hounfield units on it, and the Hounfield units was 50. So the question here is, it's an indeterminate lesion, it's grown, it's about a centimeter. Should we ignore it? Should we watch it? What do we do with it? And we don't have a base unenhanced CT to determine if it enhanced or not. And that's one of the classic reasons why we're asked to evaluate somebody for MRI. And that would have been helpful to have a base non-enhanced CT. I don't know that we do them in every single case, but it is helpful to have a pre and a post, especially when you're making that evaluation. Another caveat in this case is the patient in 2017 had a round, smooth, close to water density mass on the left side, and that one got smaller. And it's not uncommon for cysts to shrink. Sometimes they grow a little bit. I've even seen them have a little bit of hemorrhage with a little hemosteterin around them over a period of time, so they can change. So change alone is not a criteria necessarily to say something as malignant. So as we know Bosnia criteria has held the test of time and size of a lesion isn't a sign or a characteristic that changes the classification of mass. It's the morphology. So the fact that a lesion gets bigger or smaller isn't a criteria. And like you said, it's the morphology that we're looking for. So that's one reason why we order MRIs to look at enhancement versus non enhancement. So is it easier on MRI to see, to rule out, rule in enhancement? We have other tools we can use, which we'll go about in the next vignette, but some caveats we need to know about this when we're faced with these types of lesions is understanding that with the new multi-detector CT scanners, we're, and there's an increased count in scatter. So because of that, we have a lot of pseudo enhancement. So we have to use criterias of when we see a lesion what's considered enhancement. And enhancement is greater than 15 hound field units between the pre and the post. So when you're lucky enough to have a pre-contrast and a post-contrast, you take the post, you minus the pre, and if it's more than 15%, then it's enhancing. And they even have on MRI, they've got something called the signal index, and we can talk about that a little bit later. But so just to summarize, we use MRI because it's better at seeing true enhancement. There's less artifact, especially when you subtract, which we'll talk about. So enhancement is an important criteria. Hounds field unit measurements, below 20, great. Above 80, 90, 70, getting a little nervous. Between, in that intermediate no man's land, you need another tie breaker to help yourself out. And then you've also got not size so much, but shape. A nice smooth round lesion certainly pushes you in the direction of a nine lesion. Let's move on, shall we? Yes. Okay.