Friday, May 8, 2009

Shoulder MR Arthrography and Butterflies

Insects often uses camouflage to hide from predators. Can you find the butterfly in this picture?

Photo by plj.johnny
Click on image to enlarge

Although arthrography (the placement of contrast within a joint) generally increases the accuracy of MRI, on occasion the contrast can camouflage important findings.

In this case, a patient with chronic shoulder pain was sent for an MR arthrogram. Here is a T1-weighted image with fat saturation:

Here is the corresponding T2-weighted image with fat saturation:

Do you see anything important?

Here are the two images next to each other:

(A) T1-weighted image with fat saturation and (B) T2-weighted image with fat saturation reveal a lobulated paralabral cyst (red arrows) adjacent to the anterosuperior labrum. Note how the hyperintense fluid within the joint on makes this lesion more difficult to perceive on the T2-weighted image. As expected, the paralabral cyst is hypointense on the T1-weighted image.

I have seen several cases where paralabral cysts are missed, because they blend in with the hyperintense joint fluid on T2-weighted images. This is unfortunate, as the presence of a paralabral cyst in the shoulder is highly predictive of a labral tear.

Axial image from the same patient identifies the paralabral cyst (red arrow) and the adjacent tear of the anterosuperior labrum (yellow arrow):

Thus, on MR arthrograms, be careful not to miss paralabral cysts on T2-weighted images, due to the presence of adjacent bright joint fluid.

Did you manage to find the butterfly? Here it is (yellow arrows):

Vic David MD

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