Sequences and Field of View
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I'd like to talk about wrist imaging technique,
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and I have actually picked a series that
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I normally would not use for the wrist.
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A long TR 4317, a short TE, a proton
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density image, which gives exquisite
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anatomy but very poor contrast resolution.
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So I'm really showing it for the anatomy, which by
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the way, I'm not here to talk about right this moment.
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I'm here to talk about the other ancillary
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aspects of technique that are shown magnificently.
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In this particular image that looks somewhat
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T1 weighted, but it's not really T1 weighted.
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And what are those aspects?
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First, look to the left.
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The patient's in a rigid coil.
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The patient's hand is in pronation.
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So, it's important, it's incumbent upon
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you to realize how your technologist
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has positioned the hand for your exam.
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Is it over the head?
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Is it at the side?
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Is it in pronation?
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Is it neutral?
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Is it in supination?
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Because this will alter the tissue relationships.
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But even more important than that
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is the overall field of view.
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In other words, the circular area that you have
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prescribed on your MR scanner to cover, and the
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smaller that area is, the higher the resolution.
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Now the field of view for a wrist
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should be approximately 8 or less,
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depending upon what you're chasing.
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Ten is acceptable.
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Twelve is borderline.
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Anything above twelve, not acceptable.
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The field of view in this case was eight.
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The other area that I want to
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concentrate on is the slice thickness.
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For a high-quality wrist with a sequence that has
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robust signal to noise, lots of signal, I like two
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millimeters or less, down to about 0.5 millimeters.
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I'll accept
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two to three millimeters.
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Much beyond three millimeters, probably not optimal.
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And then one more caveat, the matrix.
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The matrix also helps determine
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the spatial detail and resolution.
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We've got field of view, we've got
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slice thickness, and now we have matrix.
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I like, in the wrist, to have
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a matrix that is asymmetric.
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In other words, in the Y or phase-encoded direction,
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you see we have this number here, 384, and in
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the X direction, 512, because we don't have a
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lot of time penalty when we use 384, and the
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wrist really isn't a square structure anyway.
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So we might put the 512 matrix this way, and the
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384 matrix this way, and we might end up with some
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pretty square voxels, saving ourselves a little time,
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but still, the matrix, 384 by 512, is a high matrix.
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Now how high will I go?
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I'll go up to 1024.
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I'll go up to 512 by 1024, or sometimes even
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1024 by 1024, if I've got to have exquisite,
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in-plane, pixel-by-pixel resolution.
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So let's summarize.
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Field of view, I like 8.
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Slice thickness, I like 2 or less.
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Matrix, I like somewhere around 256.
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To 384 by 512 at the least, and I will go up to
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1024 by 1024, but 512 by 1024 is still pretty good.
Stephen J Pomeranz, MD
Chief Medical Officer, ProScan Imaging. Founder, MRI Online
ProScan Imaging
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