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Prostate Anatomy on MRI in the Axial Projection

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Let's look at some prostate anatomy

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and the axial projection on MRI.

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This is a 62-year-old, and I think we'll

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start someplace easy, right in the middle.

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So this is a sagittal view,

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just to get you oriented.

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There's the coccyx in the back.

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Here's the front of the patient.

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Heads up here.

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Feet are down here.

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Side view of the prostate.

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Let's get to the midline.

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So that's the midline.

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Now let's see where our axial projection is.

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Almost smack dab in the middle of the prostate.

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And that is the easiest spot.

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We've got two major zones of the prostate.

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We've got a central gland, which is made

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up of a central zone and transitional zone.

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So, for all intents and purposes, I'll use

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those terms interchangeably, even though

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central gland consists of two components.

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Now, this component, unfortunately,

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as you get older, it gets bigger.

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And it makes more PSA, Prostate-Specific Antigen.

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So as you get older, your PSA is going to go up.

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How fast it goes up depends on how fast this

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gets bigger, and also what kind of tissue it is.

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So if it gets bigger and it's benign tissue,

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it's going to go up at a slower rate.

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If it gets bigger and it's malignant tissue,

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it's going to go up at a faster rate,

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but no matter what, it's going to go up.

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So as you get older, this portion of the prostate,

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the central area or central zone of the prostate,

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Properly said, the central gland is going to

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enlarge, and it'll press on this outer part,

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which is the peripheral zone of the prostate.

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Now, when you're young, this may make

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up as little as 5 percent or as much

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as 20 or 25 percent of the gland.

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But when you're older, that number goes up,

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30, 35, 40, as much as 60 percent of the entire

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gland as it presses against the peripheral zone.

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Now, where do the cancers usually occur?

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They usually occur in this area, this bright area

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out here on this axial, water-weighted T2 image.

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What percent?

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About 70%.

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And what are you looking for?

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You're looking for nodules.

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Now, as they say in the Wizard of Oz,

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pay no attention to the man behind the curtain.

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Well, pay no attention to these little tiny septa.

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Things that are radially oriented from the

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center to the outside are almost always benign.

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If they're nodular or mass

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like, then you've got to worry.

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Now, a couple of other

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housekeeping items at this level.

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In the front, we've got some fat.

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And that is known as the pre-fat.

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Prostatic Space of Retzius.

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Now also, don't pay too much attention

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to this little bump right here, which is

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some capsular swelling of the pubic bone,

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completely unrelated to the prostate.

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We have on the far left a T1-weighted image.

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And at this level, we've got these small little

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nubbins of signal surrounded by high signal fat.

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So these two little grey dots with

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fat around them on each side, at the

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7 o'clock and 5 o'clock position.

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These are the neurovascular bundles.

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You want to see a clean, fatty,

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bright contour on the T1-weighted image all the way

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around these dots, like that one right there.

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That tells you there is no

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neurovascular bundle invasion.

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Now staying simple at the mid-level

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of the gland, we've also got two capsules.

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We've got a surgical capsule, between

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the central zone and the peripheral zone.

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And then we have the anatomic capsule.

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Now it's the anatomic capsule, when it's crossed,

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that may preclude a patient from having a surgical

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prostatectomy, because then it may be too late.

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So some patients with borderline

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capsular invasion, they may get surgery.

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But this is a very important landmark.

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The surgical capsule is valuable in another way.

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Now tumors that arise in the

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central zone are less common.

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You're looking for something that looks like,

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say, cream cheese on a bagel, kind of smears.

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It's also called the charcoal sign.

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When that charcoal sign crosses from

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the central zone to the peripheral zone,

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when it goes across this black band

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right here, you've got big trouble.

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Now you've got a malignancy.

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You're crossing a critical boundary.

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So let's stay with some other

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adjacent structures at this level.

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We've already discussed the periprostatic fat.

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There's also some bright signal

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intensity in the neurovascular bundle

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region and around the prostate.

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It's subtle, but that is the

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periprostatic venous plexus.

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And at certain levels, it'll be

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more prominent than at other levels.

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But on the T2-weighted image, it's bright.

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Between the prostate and the

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rectum is the prostatorectal space.

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And volume-averaged on this axial with the

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capsule, the reason the capsule looks a bit

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thicker right there, it's because you're volume

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averaging the Denonvilliers fascia between the

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rectum and the prostate and the capsule together.

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So that's another important crossing

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landmark that you don't want to see violated,

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especially the midline of the capsule.

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Now, the posterior portion of the capsule indents

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a little bit anteriorly, so it makes a

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little bit of an inverted V, if you will.

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Now let's go down.

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So we're going to follow

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the prostate more caudally.

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And as we get more caudal, there is less of the

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central gland, which consists, again, of the

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transitional zone and central zone, and more

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of the encircling, wrapping, peripheral zone.

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Look how far down the peripheral zone goes.

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Let's keep going, shall we?

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The peripheral zone's a little more hyperintense.

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It is enveloped by the levator sling,

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a subject we talk about in detail.

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In our pelvic floor assessment vignette.

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And in the back, we have the rectum right here.

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So let's keep going down further.

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And as we go down further, things are converging.

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The transitional and central zone are converging.

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And then all the way down, they have converged.

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This little nubbin of low signal

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intensity, without any urine in

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it, in the urethra, is the urethra.

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Let's keep going down, and as we get down really

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low, we start to run into some darker tissue.

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Which consists of fibromuscular

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stromal tissue anteriorly.

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This is also known as the AS zone.

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Now, this patient doesn't have a very

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prominent anterior fibromuscular zone.

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You can see a little bit of it here,

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but it's going to be lentiform.

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It'll be a little bit indented in the front.

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And it's going to be lower in signal

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intensity than the rest of the prostate gland.

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In some patients, you may even see a

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little raffae, right there,

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running down the middle of the prostate.

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Now, it's not uncommon in adults.

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What's an adult?

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Well, anybody over age 10 is an adult.

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Not really.

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But anybody over age 35 or 40 will

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have some nodules in the prostate

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because the prostate is getting bigger.

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So what kind of nodules?

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They can be nodules that contain, um, mucinous

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material, or secretory material, like this one.

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They can be bright nodules.

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We used to biopsy these back in the old days.

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No more.

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Bright signal intensity is a good sign.

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Or they can be round, gray nodules.

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I don't see any here right now.

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Here's another bright nodule.

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But round, gray nodules don't bother me.

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Unless they really tightly diffusion restrict.

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They're markedly hypervascular with a washout,

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but especially diffusion restriction,

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or the nodule has a margin of irregularity,

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and it starts to smear the adjacent tissues.

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So a smooth, round, gray nodule, or even a

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smooth, round, darker nodule in the central

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portion of the gland doesn't bother me at all.

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Now let's work our way proximal.

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We're going north.

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Let's go up a bit towards the prostate base.

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And at the prostate base,

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we see the seminal vesicles.

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And a little bit of secretory

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material in the left ductus deferens.

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Let's talk a little bit

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about the seminal vesicles.

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There's a fair amount of variability

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in the signal of the seminal vesicles

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as we get to the prostate base.

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And that's because the secretions

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may be inspissated or dried out.

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So if they're inspissated or dried out,

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then they're going to be grayer or darker.

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Typically in younger individuals, you'll

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have a fair amount of secretory material.

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You'll have an acinar gland pattern,

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so that each portion of the gland looks like

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this little irregular, hexagon

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right there, and then they kind of fit

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together one right next to the other.

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So the younger the patient, the more likely

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you are to have homogeneous bilateral high

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signal intensity in the seminal vesicle.

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Now let's go back to the base.

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At the base of the prostate,

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we've got quite a bit.

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of central gland material, and only

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when we come down off the base do

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we see much of the peripheral zone.

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So as we come down, the peripheral

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zone assumes greater prominence.

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As we go up, the hyper intense peripheral zone

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assumes a little bit less, less prominence.

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That's not to say you don't get cancers

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here, because you certainly do, and they're

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more easily seen in the coronal projection.

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Let's take on a few additional

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anatomic structures.

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For instance, next door we've got the obturator

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internist canal, anterior, and in fact,

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anterosuperior on the sagittal, but anterior

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and superior on the axial is the bladder.

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You have to assess the bladder size,

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the thickness of the wall, whether it's ated.

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In other words, these small

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nubbins of high signal intensity.

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that penetrate the inner lumen of

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the bladder, but don't go through it.

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So we call this bladder

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hypertrophy with cellulation.

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If one of these penetrates through and fills

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with urine, then we call that a diverticulum.

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Now while you're in the neighborhood, you've

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got a lot of other structures hanging out.

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You've got your hips on either side.

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Don't forget to look at those

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in the axial projection.

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On the film edge, this patient happens

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to have a cyst arising from the left hip,

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and has some arthrosis in the right hip.

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Both hips in the back, we've got the

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rectum and the pararectal space.

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We're also gonna look at the

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character of the rectal wall.

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And when we get down low on the T2

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weighted image, we're gonna look for fissures

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in anal sinus tracts and microabscesses.

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So that concludes.

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Our view of the prostate and the

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axial material, and the axial projection.

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We're not going to talk about the erectile

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tissue, because we're focusing purely on

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the prostate right now, and you've seen the

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difference between the apex, the inferior

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portion of the gland, the mid portion

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of the gland, and the base of the gland.

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Let's move on to another projection.

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Let's try coronal, shall we?

Report

Editorial Note

Faculty

Stephen J Pomeranz, MD

Chief Medical Officer, ProScan Imaging. Founder, MRI Online

ProScan Imaging

John F. Feller, MD

Chief Medical Officer, HALO Diagnostics. Medical Director & Founder, Desert Medical Imaging. Chief of Radiology, American Medical Center, Shanghai, China.

HALO Diagnostics

Tags

Prostate/seminal vesicles

Neoplastic

MRI

Genitourinary (GU)

Body

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