Anterior Cruciate Ligament on MRI: Sagittal Views
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Knee anatomy.
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Cruciates on MRI sagittal projection. On the far left,
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oh, let's make it a little bigger.
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The proton density fat suppression image or the water-weighted image.
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In the middle, the T1,
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and on the far right, the T2.
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So this, the detection sequence.
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This, the morphology sequence.
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And this, the clean up sequence, the tweaker sequence.
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Approximately, we see the ACL looks a little bit pinced,
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it looks a little narrower than it really is.
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This is the over the top position.
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So where the ACL actually penetrates, the femur is not here,
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it's actually over here, about 15mm, anterior to this specific bump,
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which we articulated in one of our earlier vignettes.
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Then the ACL comes down and both
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on the T1-weighted image and on the PD spur water weighted image,
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the fanning begins about mid to distal third.
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The more fanning there is, the greater the ligament and that is normal.
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Between the interstices of our celery stalk ligament are these slightly brighter
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areas which some have attributed to fat, others have attributed to synovium,
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even though this is an intraarticular extra-synovial structure.
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So perhaps synovial interdigitation.
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But the cruciate, as it fans out,
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gets a little bit lighter on the water weighted image, and that's normal.
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It's not uncommon to see a little bit of fluid sitting just anterior to the ACL
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and behind the transverse ligament and or ligamentum mucosum.
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And I allow about a one to one and a half
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centimeter collection of fluid in that position.
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Now, in the T2-weighted image,
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the fanning and gray appearance is not nearly as apparent.
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And that can be helpful when you have
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a massive injury with tremendous amounts of blood and fluid in the area.
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For the actual cut, the transverse cut in the ACL will stand
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out while the other areas of swelling will often fade away.
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So in the acute setting, the T2-weighted image is invaluable.
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Now, let's scroll a little bit because
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the ACL, we said it attaches in front of the intercondylar eminents
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of the tibia, but it also blends with the anterior horn of the lateral meniscus.
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There's that blending right there.
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Here's the anterior horn of the lateral meniscus.
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That's the lateral meniscus root.
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There's the blending of a few wispy fibers of the ACL.
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Ooh, that's subtle, but it's real.
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Let's go back again and scroll it.
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ACL right there,
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blending with the anterior horn of the lateral meniscus.
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So the ACL with about ten degrees of flexion, should be straight.
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You want the knee to be externally rotated no more than ten degrees.
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And if you get an ACL view that's not
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optimal, then it's time to turn to the coronal and the axial projection.
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I love the sagittal projection with these three sequences
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for my mid-substance tears. It's pretty good for my distal tears.
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For the very far proximal tears, I like the axial.
Stephen J Pomeranz, MD
Chief Medical Officer, ProScan Imaging. Founder, MRI Online
ProScan Imaging
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