Sunday, January 27, 2008

Magic Angle Effect


Ho, ho, ho it's magic....

Some of you may be old enough remember that lyric from the 1974 song "Magic", by the Scottish band Pilot.

A busy musculoskeletal radiologist could sing this lyric several times a day. Not talkin' about love, son, rather the magic angle effect.

The magic angle effect is a well-know phenomenon that results in artifactual increased signal in structures with ordered collagen, such as tendons, fibrocartilage, and hyaline cartilage.

Physicists tell us that this is because when collagen is oriented at 55 degrees to the main magnetic field of the magnet, dipole-dipole interactions go to zero, resulting in a prolongation of T2 relaxation time.

In English, that means that collagen-containing structures can exhibit increased signal when they are at certain angles with respect to the main magnetic field.

A picture is worth a thousand words. Here is a coronal gradient echo image showing magic angle artifact in the extensor pollicis longus (EPL) tendon (yellow arrows), which is oriented at 55 degrees to the main magnetic field:



Note that this tendon is perfectly normal; this increased signal is purely an artifact.

This phenomenon can be seen in multiple areas of the body. Common areas to see this artifact in high-field scanners include the posterior horn of the lateral meniscus, the patellar tendon, tendons in the ankle, and the glenoid labrum. This can confound accurate interpretation of these areas.

Finally, collagen fibers are oriented in various directions in tendons and cartilage. Thus, although magic angle effects tend to be greatest when the structure is at 55 degrees to the magnetic field, they can be seen at other angles as well.


4 comments:

comminuter said...
This comment has been removed by the author.
comminuter said...

fantastic blog! i am a pgy-4 orthopaedic surgery resident at Penn and just stumbled upon your blog while trying to review the magic effect (heard about it a while back--55 deg, etc--and couldn't remember the details). great explanation. your other entries are a fantastic imaging review for me as well. i have a strong interest in UE and your hand/elbow/shoulder entries were superb. i went through about 40 of your entries in the course of about an hour. well written, entertaining, definitely kept my attention. keep the entries coming. thanks alot!

Vic David MD said...

Thx for the kind comments...

Vic David

Anonymous said...

I am a humble patient with an orthopaedist who thinks he knows it all. He told me my EPL had ruptured after a Colles fracture when I lost the extension of my thumb. This arrogant physician did not listen when I told him that this rather sudden loss of thumb movement occurred after my first session of vigorous PT exercises [after the fracture]of flexing and extending my wrist, after which I had poked around in my palm to locate my trapezoid bone. I had learned that bone is what gives humans apposable thumbs, and since my thumb use was already slightly impaired by the fracture, I wanted to check it out. As I poked at a spot below the base of my forefinger down near my thumb, my thumb just dropped down and wouldn't be flexed!
But I've observed that the tendon is clearly there, NOT ruptured, and works partially; it's just not in the right place to do its proper work. My conclusion is that the tendon was dislocated.
Help! I don't know where/how to find a physician who will be able to properly diagnose and treat the problem - and not necessarily with surgery. I live in VT where there is apparently a shortage competent orthopedists. Thank you for your help.