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Technique for foot and ankle imaging, focusing

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on the ankle, low field, not a dirty word.

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Low field is particularly well-suited

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for the ankle, because you can put

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the ankle right into the center of the

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magnet bore where the sweet spot is.

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And most low field scanners are open,

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so it's particularly comfortable.

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But the fact that you can get

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the foot centered is critical.

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Second point, the spatial resolution at

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low field is not going to be as good,

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or as pretty as it is at high field.

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The images may appear a little noisier,

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but on the flip side, the T1, this is

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a T1 spin echo on the left, the T1 spin

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echo contrast characteristics are actually

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better at 3 tesla than they are at 3 tesla.

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In other words, the contrast

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resolution is better, but the spatial

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resolution not so much, not as good.

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Now on the far right, the I have the uber

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water-weighted image, known as the STIR.

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Not the PD SPR, not the SPECIAL,

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not the SPARE, the STIR.

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Which is a non-frequency-selective type

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of inversion recovery imaging that gives

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you enormous water-weighted imaging.

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So here too, we have superior contrast

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resolution, even though the image is fuzzy,

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The whites are whiter and the darks are

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darker, kind of like Tide, the detergent.

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In the middle, we've got the qualifying

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T2 weighted image once again,

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which is absolutely respectable.

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At 0.3 Tesla, and we use these sequentially to evaluate.

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38 00:01:50,649 --> 00:01:53,390 Long structures like tendons in this projection.

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We'll also use it to evaluate, say,

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the anterior talofibular ligament and the

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higher anterior tibial fibular ligaments.

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Now these images were acquired just like with

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high field, with the ankle in plantar flexion.

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And then the axials are perpendicular

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to the long axis of our tendons.

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So let's look at the posterior tibial tendon.

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And there is the orientation of our axis.

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Let me actually draw on it to

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make it a little more clear.

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I'm gonna use my drawing

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tool, if I can get it to work.

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And I can.

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So we're gonna be perpendicular.

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And that is how our axials are

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obtained, if we're interested in the

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long tendons, which we often are.

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Now, plantar flexion, not quite as good as we said

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in the past for collaterals, but good enough.

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For an expert or master-level reader.

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So you really don't have to dorsiflex the

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foot if you're more, a more advanced reader.

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So then, now let's move, let's

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get out of our drawing tool.

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And move over to our sagittal projection.

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And I, I took the same projection,

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I put it in the center.

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And I've only got one sagittal

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projection this time around.

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And it is the uber water-weighted image.

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And it's often the one I start with.

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In fact, I'm going to highlight it so

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that it's the only image on there.

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I'm going to make it a little smaller,

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so you can see it a little better.

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And the beauty of starting with this

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is you just scroll back and forth.

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It's so water-sensitive, even at 3T, that all

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you're doing is you're looking for hotspots.

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So this is the search and destroy bone

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scan sequence of low field imaging.

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I could have a T1-weighted sagittal image with

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it if I wanted to, but I really don't need it.

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Now there's one more sequence

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that I want to share with you.

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And that is the three-dimensional sequence.

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So let me put that up.

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The three-dimensional sequence can

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be acquired with one of two different

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types of pulsing sequences at low field.

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It can be acquired with a T1-

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appearing gradient echo image,

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and that has all kinds of names which

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I'm not going to go through right now.

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One of the most common names you've

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heard is Spoiled Gradient Echo.

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Or, you can perform Steady

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State Free Precession, SSFP.

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And a common name that you'll hear

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associated with this is called SARGE.

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These two sequences are inordinately

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strong signal-to-noise sequences.

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So even though this is a 1 mm

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image, even though it's 0.3 Tesla, look at it.

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You are seeing the deep fibers of the

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deltoid beautifully, the superficial fibers

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of the deltoid beautifully, the subtalar

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ligaments absolutely, positively, beautifully.

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It has such great spatial detail,

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and such good contrast resolution for certain

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structures, like cartilage, not bone,

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but cartilage, and tendons, that it is an exquisite

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supplement to evaluate these structures.

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It's also an exquisite supplement to evaluate

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thin ligaments because of its spatial detail.

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Now if I click a button,

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I can produce an axial sequence.

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I can produce a sagittal sequence.

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It's a reconstruction.

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Now granted, it's not as pretty, but it still

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allows me to cross-reference areas of interest.

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For instance, if I was interested in the

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peroneus tendon, I'd go for my sagittal,

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I put a marker here, then to my coronal, I put a

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marker in the same spot, then to my axial and

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cross-reference to see if an abnormality is

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really there and then where it is spatially.

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But the gold standard for this particular

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sequence is the original plane of acquisition.

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A 1 or a 1.2 millimeter acquisition

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with a 50 percent overlap.

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It's shocking that you could get something that

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thin, but with 3D imaging at low field with

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the right kind of scanner, you absolutely can.

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And it is an integral part of foot and ankle

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imaging, and you'll see later on, without it,

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you can't do plantar plate imaging at low field.

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That's the story for low field.