Sunday, January 16, 2011

Magic Angle and the Meniscus

Photo by Brother O'Mara

The way MRI images structures deep inside the body can seem like magic. Ironically, there is also an imaging artifact in MRI with the tag "magic" attached to it— the magic angle artifact, also know as the magic angle phenomenon or effect.

In an earlier post, I discussed the magic angle phenomenon. If you recall, the magic angle effect results in artifactual increased signal in structures with ordered collagen, such as tendons, fibrocartilage, and hyaline cartilage.

One well-known structure that contains ordered collagen is the meniscus of the knee. Magic angle effects can lead to artifactual increased signal in the meniscus, and can be confused with meniscal degeneration or a meniscal tear.

Knees are often imaged on both closed and open MRI scanners, and the magic angle effect has different manifestations, depending on what type of MRI scanner is used.

Here is a conventional, high-field closed MRI scanner:

The main magnetic field is horizontal in nature, and runs directly through the center of the magnet (red arrow). In the following diagram, we are looking down the bore of the magnet, with the "X" the back of an arrow denoting the direction of the main magnetic field:

Open MRI scanners have two parallel "plates", with the patient lying between the plates. The main magnetic field is vertical in nature; this can be diagrammed in the following way:

Now let's do an experiment to see how an ordered, collagenous structure behaves in these two environments. Take a beef hamstring tendon, and fold it into a C-shape, to mimic the shape of the normal knee meniscus. Once we do that, place it into a water bath, and perform a T2-weighted image, using a TE (echo time) of 50 ms:

As expected, the ordered collagen in this tendon is completely black, except for some fatty tissue at the inner margin of the tissue (yellow arrow).
Now let's take our model meniscus, and perform a scan in an open MRI, using a TE of 15 ms:

At this TE, magic angle artifacts will become evident. Note the bright signal (red arrows) at the edges of the meniscus, where the collagen fibers are close to the magic angle (55 degrees with respect to the main magnetic field, which is straight up and down in this open magnet). There is also bright signal at the inner margin of the specimen from the fatty tissue, which you can ignore for this illustration.

OK, now let's take our model meniscus, and move it to a closed MRI, and repeat the scan, using the same TE (15 ms).

Note that the artifactual signal has completely disappeared. The main magnetic field is perpendicular to the plane of the picture (going into the plane of the picture), and none of the collagen fibers are at the magic angle, so there is no artifactual signal.

From this example, it should be apparent that in open, vertical field magnets, magic angle artifact will be found in two locations: 1) junction of anterior horn and body of meniscus and 2) junction of posterior horn and body of meniscus. Here is one such example:

Ah, but only if it were that simple! Magic angle artifact can also occur in closed MRI scanners, but it will occur at different locations. Remember that in closed scanners, the main magnetic field runs from head to foot. The upsloping posterior horn of the lateral meniscus is often close to 55 degrees to the main magnetic field, and one will often see magic angle artifact in this region, as in this example:

The inner edge of the posterior horn of the lateral meniscus is the classic location for magic angle artifact, due to the upsloping nature of this area of the meniscus in many patients. Although less frequent, the posterior horn of the medial meniscus can also be upsloping, and magic angle can be encountered in medial meniscus as well. In rare cases, magic angle artifact can also be associated with the anterior horns of the menisci.

If you made it this far, congratulations, you now have a good grasp of some situations where magic angle artifact can confound the interpretation of meniscal pathology in both open and closed MRI.

Vic David MD