Ask Dr. C-Episode 12-Understanding Static vs. Dynamic Imaging
I do love looking at the questions everyone submits. Today we’ll focus on a critical difference in imaging that many physicians don’t understand, but that’s critical for the average patient to understand. We’ll dive into static versus dynamic imaging.
When looking at a cervical spine MRI, how can you tell whether any of the ligaments are injured?
I like this question because it will allow me to dive deep into the difference behind static and moving images and how each plays a roll in making a diagnosis.
The Problem with Imaging
Imaging like MRI is a two-edged sword in medicine. On the one hand, it can help identify hard to diagnose issues and on the other, it can also show the doctor red herring findings that should never be treated and aren’t causing the symptoms. For example, if you read this blog, you know that meniscus tears in middle-aged and elderly people are about as common and significant as wrinkles. Meaning that many people without knee pain have them and they should generally not be treated. However, in this country, we have hundreds of thousands of unnecessary meniscus surgeries that happen in this age group merely because someone’s knee hurts and there was a meniscus tear found on MRI.
Static means not moving. This is the way that most x-rays and MRIs are taken. However, this approach has its positives and negatives.
First, static imaging, like MRI is good at looking for structures. Hence, if your goal is to find a tumor, it’s a fantastic tool. Or if you’re looking for a bone that could be fractured and the x-ray is inconclusive, MRI can show you swelling in the bone that you just can’t see on x-ray, nailing the diagnosis of a small fracture. However, the two areas where MRI falls apart are in determining if something hurts and looking for instability.
Does It Hurt?
The number one reason that someone gets an MRI these days is not looking for a tumor or a mass, but to determine why they have pain. However, the irony is that this is where MRI is least effective. For example, since meniscus tears are common in older knees that don’t hurt, taking an MRI of someone’s knee when there is pain and pinning that pain to a meniscus tear is almost impossible. The same holds true for back and neck pain, where there is often pathology on MRI when patients don’t have much pain.
How can it be that there are be structural problems on MRI and the patient has no pain? Pain is generated by chemicals and electrical activity in nerves and MRI can’t really image either of those well. Hence, we see structural problems all the time that shouldn’t be acted on as they’re not causing the patient any discomfort.
Another area where static MRI is awful is looking for instability. That means looking for joints that move too much in the wrong directions due to damaged ligaments.
Why? Well, this one is easier to understand. If your wheel is misaligned and loose and wobbles when you get on the highway, what are the odds that taking a cell phone picture of the car will allow your mechanic to diagnose what’s wrong? Very low.
The same holds true for ligaments. While MRI can get a reasonable handle on the shape of the ligament and its density, it generally can’t tell if that ligament is capable of doing its job. The one exception is when the ligament is torn and snapped back like a rubber band.
Dynamic imaging moves the joint to see if it moves too much in certain directions that it shouldn’t. Just like your mechanic checks to see if the tire is loose. This motion is then tied back to which ligament is injured. Here are some examples of dynamic imaging:
- Stress x-rays of the ankle
- Dynamic ultrasound imaging of the knee
- Digital motion x-ray (DMX) of the neck (see video below)
Getting Back to Our Question
So when looking at a neck MRI, how can we tell if the neck ligaments are injured? The short answer is that we usually can’t. This is why dynamic imaging like DMX or a dynamic moving MRI is usually ordered. However, physicians have developed a few measurements that can be applied to a static neck MRI that may help.
First, in the neck, it’s hard to see many of the ligaments like the supraspinous ligaments that hold things together in the back part of the neck. Other major ligaments like the anterior and posterior longitudinal ligaments can also be hard to see. The ligaments that hold the head on can be easier to see with a specialized MRI. It’s this last set of ligaments that we’ll focus on.
The ligaments that hold your head on consist of the alar, transverse, and accessory among others. These can be seen with a specialized neck MRI, but they are not usually seen on a general neck MRI. We can look at their density and from there, this may help to see if they are injured (darker is better here and lighter is worse). We can also see if they still connect point A to B, but realize that very often they are still present without being retracted back like a rubber band.
The doctor can also take various measurements to get a sense of whether cranial cervical instability may be present. These include measurements like Grabb-Oakes, Clivo-axial Angle, Powers Ratio, and others. See my videos below to learn more.
The upshot? While MRI imaging of ligaments can be helpful, realize that static imaging has its positives and negatives. Hence, dynamic imaging of ligaments is often a better way to go.