Movement is essential to life. Our brain is hard-wired to detect motion, and we respond to video much more than static pictures. Consider these two different depictions of a horse galloping:
Now, the video version:
Clearly, the video version is more appealing. Moreover, the video version contains important information that the still picture does not. This video, made by Eadweard Muybridge in 1878, was the first motion picture ever made. Muybridge was commissioned by Leland Stanford (California governor/ Stanford University) to answer a popularly debated question of this era— are all four of a horse's hooves ever off the ground at the same time while the horse is galloping? Muybridge's time-motion photography proved they were off the ground simultaneously. Video analysis had answered a scientific question for the first time in history.
Although cross-sectional imaging has revolutionized radiology, video representations of anatomic detail are the exception in MRI and CT. Volume-rendering techniques enable us to depict 3D data sets, but these techniques are diffusing into clinical practice in a sluggish fashion.
Almost everyone reads off PACS now, which is clearly superior to reading from film for many reasons. One advantage of PACS is the ability to rapidly scroll through a stack of images. This video-like representation of data sets will sometimes enable you to quickly recognize abnormalities that are subtle and easily missed on static images.
One example is the analysis of the anterior cruciate ligament (ACL), a key stabilizer of the knee. It is important to recognize tears of this structure, as the presence of an ACL tear will often change the treatment algorithm. There have been many excellent articles written about the MRI analysis of ACL tears, and these tears are usually easy to recognize on MRI.
The key word here is usually. These tears can be subtle in some patients. The ACL should be examined in the sagittal, axial, and coronal planes to maximize your accuracy. One common error is to rely solely on the sagittal plane to determine the status of the ACL. The axial and coronal planes will often yield additional information about this critical structure.
In the coronal plane, the normal ACL has an oblique course, arising from the lateral femoral condyle, and coursing anteriorly and medially before inserting on the anterior tibia. In the following figure, the green arrow depicts the normal oblique course of the ACL:
Any significant deviation from this normal oblique course of the ACL on coronal images is abnormal.
Here is a movie of a stack of coronal images, depicting a normal ACL. Note how the ACL tracks from the top left of the figure to the bottom right, following its normal oblique course:
Next, let's look at a sagittal image from a 37 year-old patient with knee pain:
The ACL is visualized (red arrow) throughout most of its course. The femoral origin (green arrow) is hazy , but on the adjacent image, this part of the ACL can be seen:
The ACL looks "funny", but it would not shock me if a busy reader passed this off as volume averaging, or a partial tear of the ACL.
A complete analysis of the ACL, however, requires scrutiny of the axial and coronal planes as well, particularly in complex cases. Here is a movie of a stack of coronal images from the same patient. The ACL (red arrow) no longer follows its normal oblique course through the intercondylar notch. Its path is now curvilinear, and the inferior aspect of the ACL is deviated laterally:
With these additional images, we can confidently diagnose a complete tear of the ACL, which was subsequently confirmed at arthroscopy.
One of the less-advertised benefits of PACS is its ability to give us video-like representations of the imaging data.