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Welcome to MRI Online's section on renal masses.

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I'm here with my colleague,

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Dr. Pina Finazzo, an expert on body imaging.

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We're going to talk about how you use MRI to approach

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a lesion in the kidney and why you would use MRI.

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We're going to begin with a 69-year-old man.

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He's got a history of bladder carcinoma,

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and he's got a few renal masses which

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you would call "cysts."

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Now, just a little bit of background.

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Renal cell carcinoma

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is the most common malignant epithelial tumor.

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It accounts for 90 percent of all solid renal tumors.

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You think solid, CT, no big

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deal, I can figure that out.

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Why do I need an MRI?

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On the other hand, 70 percent of all renal

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cell carcinomas are clear cell carcinomas.

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And when you think clear, you think cytoplasm.

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When you think cytoplasm, you think water.

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And when you get close to water density,

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things get a little bit dicey on CT and MRI.

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So, why would you need an MRI?

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And let's illustrate it with this case.

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So, the most classic example of why we go to MRI is

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really to determine, number one, is there enhancement?

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And while that seems like such an easy task

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for us to do, it's sometimes very difficult.

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And we tend to start off with CT imaging and, uh, and

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so basically what are the criteria we use to determine

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whether something is purely a cyst and can be ignored?

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We start off with Hounsfield units.

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So, being able to put measurements and

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region of interest curves on a lesion can

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help us determine if it's a cyst or not.

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Anything less than 20, we can confidently ignore it.

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Anything greater than 90, we can say is a

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hyperdense cyst and we can confidently ignore it.

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One caveat, I didn't mean to interrupt you,

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but one caveat is most residents think,

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I need a Hounsfield unit of zero to say

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it's water, but 20 is the cutoff, right?

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44 00:01:52,775 --> 00:01:53,825 20 is the cutoff.

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But we're faced with a few dilemmas

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and pitfalls that we need to know.

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First of all, we need to classify that in the

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unenhanced CTs, and nowadays we're either doing

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base CT imaging, which is the non-contrast,

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or we're just doing post-contrast imaging.

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So when we get a lesion like this, if you see

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in 2017, the lesion looks like it's, you know,

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maybe about seven or eight millimeters, and now

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you look at it in 2019 and it's slightly larger.

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We did Hounsfield units on it

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and the Hounsfield units were 50.

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So the question here is, you

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know, it's an indeterminate lesion.

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It's grown, it's about a centimeter.

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Should we ignore it?

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Should we watch it?

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What do we do with it?

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Um, and we don't have a base unenhanced

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CT to determine if it enhanced or not.

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Uh, and that's one of the

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classic reasons why we're asked

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to evaluate somebody for MRI with,

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and that would've been helpful

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to have a base non-enhanced CT, right?

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I don't know that we do them in every single case, but

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it is helpful to have a pre and a post,

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especially when you're making that evaluation.

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Another caveat in this case is the patient in 2017

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had a round, smooth, close-to-water-density

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mass on the left side, and that one got smaller.

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And it's not uncommon for a cyst to shrink.

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Sometimes they grow a little bit.

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I've even seen them have a little bit

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of hemorrhage with a little hemocytin

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around them over a period of time.

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So they can change.

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So change alone is not a criterion

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necessarily to say something is malignant.

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So, you know, as we know, Bosniak criteria has held

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the test of time, and size of a lesion isn't a sign or a

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characteristic that changes a classification of a mass.

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It's the morphology.

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So the fact that a lesion gets

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bigger or smaller isn't a criterion.

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And like you said, it's the

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morphology that we're looking for.

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So that's one reason why we order MRIs,

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to look at enhancement versus non-enhancement.

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So is it easier

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on MRI to rule out or rule in enhancement?

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We

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have other tools we can use, which we'll go about

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in the next vignette, but some caveats

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we need to know about this when we're faced with

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these types of lesions is understanding that

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with the new multi-detector CT scanners, we're

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and there's increased Compton scatter, so because

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of that, we have a lot of pseudo enhancement.

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So we have to use criteria of what we're doing.

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When we see a lesion, what actually

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enhances, what's considered enhancement?

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An enhancement is greater than 15 Hounsfield

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units between the pre and the post.

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So when you're lucky enough to have a pre-contrast

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and a post-contrast, you take the post, you subtract the

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pre, and if it's more than 15%, then it's enhancing.

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And they even have on MRI, they've got

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something called the signal index, and we

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can talk about that a little bit later.

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But so just to summarize, we use MRI because

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it's better at seeing true enhancement.

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There's less artifact, especially when

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you subtract, which we'll talk about.

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So enhancement is an important criterion.

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Hounsfield unit measurements below 20 are great; above

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80, 90, or 70, getting a little nervous between in

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that intermediate no man's land, you need

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another tiebreaker to help yourself out.

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And then you've also got

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not size so much, but shape.

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You know, a nice, smooth, round lesion certainly

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pushes you in the direction of a benign lesion.

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Let's move on, shall we?

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Yes.