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Cardiac CT Physics: Isotropic Imaging (Spatial Resolution)

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Isotropic imaging.

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It's key for coronary.

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So what exactly is it?

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The term basically means that the spatial

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resolution is the same in all three

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planes, three planes being X, Y, and Z.

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So spatial resolution is very important for coronary

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because coronary arteries tend to be very small.

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So we want to see the small diameter structures

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and be able to quantify the degree of stenosis.

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There are two types of spatial resolution.

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There's the in-plane.

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Which is the XY plane, which is the lower-lying

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fruit because all you have to do is increase

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the matrix size, narrow the field of view, and

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you can get very high spatial resolution.

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The more difficult part is getting a

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spatial resolution through the Z axis.

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So XY will create the pixel, Z will create

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the voxel, and that depends on detector width.

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Many of us now are used to CT scans with multiple

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detectors, with very thin, small detector sizes

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in detectors, but that wasn't always the case.

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And so as we approach isotropic

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imaging, because we're never quite there

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perfectly, it's really a

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testament to the progress of CT.

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To tell you how far we've gone,

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this is the first CT scan done.

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The first-generation scanner.

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CT of the head, which alongside the fact that it

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appears very grainy, took a long time to acquire.

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Parameters are 3x3x13mm.

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That's the in-plane spatial resolution is 3mm.

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Again, it wasn't very difficult to get

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3 millimeters in-plane, but through

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plane, slice thickness is 13 millimeters.

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Where are we today?

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Here's a curved MPR of the coronary artery, which,

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by the way, is something you can only do well.

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That is 3D manipulation and reformation

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because we have isotropic imaging.

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It's 0.4 by 0.4 by 0.5.

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46 00:02:10,835 --> 00:02:11,425

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So the degree of improvement

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is by 100, but not just that.

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It's the fact that we're able to deliver this with

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a reasonable radiation dose.

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One of the reasons why coronary CTA took off and

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coronary MR didn't is because of spatial resolution.

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Coronary MRA, actually the first papers for

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coronary MRA came before coronary CTA, but

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even now with the 3 Tesla, which is very much

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a powerful signal-to-noise magnet, you still

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struggle to get less than three millimeters.

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I mean, you can, but that might

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increase the scan time too much.

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MR, spatial resolution and scan time are related

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because to increase spatial resolution, you

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would need to increase the number of slices

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or partitions, as I said, and increase the

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phase matrix, increase the phase matrix.

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You increase the scan time.

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So you get a penalty.

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CT, spatial resolution and acquisition and

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temporalization are mostly unrelated, mostly.

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So this is what happens when

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you don't have isotropic voxels.

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You get what's known as a stair-step artifact,

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which is fine if you're looking at the femur.

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It's not so good if you're looking at the

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coronary artery where those stair steps

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themselves can be a source of confusion and

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artificially create stenosis when there isn't any.

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So to recap.

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Coronary arteries are tortuous.

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They don't travel along a single plane.

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They go in and out of plane.

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So in order to be able to see an

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artery, for example, at this point here and

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know exactly what the degree of stenosis

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is, you can't cut it axially, coronally, or sagittally.

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You have to go along the long axis

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of the artery and cut through it

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orthogonally.

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So it's a two-step thing, but you're only able to

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do that if it doesn't matter which plane you're in.

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So it doesn't matter if you're in

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axial, coronal, or off-axial plane.

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The fact that you have isotropic voxels

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means that there's no informational loss.

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And it's not just good having isotropic because you

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can have isotropic imaging with five millimeters

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by five millimeters by five millimeters.

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The point is you want to have isotropic

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and thin, not just isotropic, and both of

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which are necessary for coronary imaging.

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So how has the spatial resolution improved?

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And I think that this is a lecture in its own right.

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You can even write a book on it.

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Two things to appreciate.

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The first is that the detectors

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have become much more efficient.

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The power of the CT scan has increased.

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So we're delivering more iodine flux, but

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we're getting more out of that iodine flux.

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So we're not wasting radiation.

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So in a sense, things are more powerful,

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but also things are more efficient.

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So power and efficiency.

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Has resulted, along with multiple other things,

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in us being able to do isotropic imaging without

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hugely penalizing in terms of radiation dose.

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Thank you.

Report

Faculty

Saurabh Jha, MD

Co-Program Director, Cardiothoracic Imaging Fellowship, Associate Professor of Radiology

University of Pennsylvania

Tags

Vascular

Coronary arteries

Cardiac

CTA

CT

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