05 Aug 2016
MRI scans: where abnormal findings are normal
Magnetic Resonance Imaging (MRI) is a tool that is used to visualize the inner workings of the human body. The widespread availability of MRI scanners has turned them into “answer machines”: What’s wrong with the patient? Don’t know? Get an MRI scan! MRI uses a very powerful magnetic field to magnetize the atoms in the human body and then a special computer detector is able to reconstruct an 3-dimensional image of the anatomic structures anywhere in the body with incredible clarity, focus, and contrast. This allows us to visualize the human body in ways that were completely unimaginable a generation ago.
The crux of the MRI scan is the interpretation. The images produced by the MRI scan are interpreted by a radiologist. In the process of determining what’s normal and what’s abnormal the images are compared in the mind’s eye of the radiologist to a theoretically normal “young” human with no wear and tear. Here is an analogy that I use often. Look at these two pictures of human skin. If the skin of the person in the second picture was biopsied and viewed under a microscope, the pathologist would probably find evidence of advanced solar elastosis, pre-malignant cells, severe degenerative changes, and a long list of terrible medical conditions which would be described as VERY ABNORMAL. But this is only because this skin is seen in the context of the first picture which is being used to define the NORMAL state. Looked at another way, the skin of the second person is perfectly normal as a function of age. It may not be as appealing, but similar changes are going to happen to all of us.
Now lets examine the images from MRI scans of the lumbar spine. This first image is called a sagittal slice of the lumbar spine. It allows us to clearly see the bones of the spinal column (the vertebral bodies), the intervertebral disks, the spinal cord, and the nerve roots. The lumbar spine is composed of 5 vertebral bodies: L1, L2, L3, L4, and L5. There are 5 intervertebral disks which are named according to the vertebral bodies that they separate. These five disks are called: L1/2, L2/3, L3/4, L4/5, and L5/S1. The center of the intervertebral disk is a soft squishy gel-like substance made of proteins that is surrounded by a tough ligamentous wrap called the annulus fibrosis.
This is a normal MRI scan from a healthy 19 year old patient. The first image is of a “sagittal slice” of the lumbar spine which shows the spine as if we used a laser beam cut the spine in half lengthwise. In this image the head is to the top, the back to the right, and the feet to the bottom of the image. It is oriented as if the patient were standing up, facing to the our left. On this image, the spine is normally aligned, the center of the disks show up as bright white because they are well hydrated and have a high water content, and there is no evidence of compression of the nerve roots. On the second image we see an “axial slice” which is a horizontal cut through the spine at the level of the horizontal reference line seen on the first image. In the 3rd and 4th images, I have annotated the anatomic landmarks so that you can see where the spinal canal, neuroforamen, and facet joints are located.
This is an MRI scan from a 22 year old woman with low back pain. In this image, we see a tiny white dot in the midst of the fibers of the annulus fibrosis at the back of the intervertebral disk. This is called a “High Intensity Posterior Zone Lesion” and it is often described by the radiologist as an “annular tear”. The finding is a tiny white dot seen in the posterior fibers of the disk just underneath the yellow reference line.
This MRI scan is from a 50 year old male patient with moderate back pain. The most noticeable change in this image is the change in color of the center of the disks — from white to dark. This is a normal part of aging that occurs because the proteins in the center of the disks lose their water content, become brittle and stiff, and the blood supply to the center of the disk naturally shuts down relatively early in life. While this is a change that will occur to every human spine, it is described as an abnormal finding by the radiologist as “disc desiccation” and “degenerative disk disease.”
This is an MRI scan from a 26 year old patient. In these two images of the L4.5 and L5.S1 disks, the disks are bulging slightly into the spinal canal. While this is basically a normal finding, this is described by the radiologist as “multi-level degenerative disk disease with broad based disk bulging and focal disk extrusions at L4.5 and L5.S1”
Recently, it has become much more common to give the patient a copy of the MRI and report from the radiologist before it is interpreted for them by someone who is knowledgeable about the natural history of low back pain and MRI findings. Unfortunately, this often creates a lot of confusion that can be difficult to rectify. By the time that a patient sees me, they usually have had the opportunity to read their own MRI report which typically sounds something like: “disc dessication is prominent at L5/S1. There is broad based disk bulging at L4/5 and L5/S1 and degenerative disk disease at these two levels.” This patient arrives in my office CONVINCED that there is something dramatically wrong with their back.
I begin by explaining that these findings are normal and that they frequently occur in patients without back pain or sciatica. For example, in a study published in a medical journal called Radiology, 36 volunteers with no history of recent back pain had MRI scans of the lumbar spine. In this group of “NORMAL” patients, bulging disks were seen 81% of the time, annular tears were seen 56% of the time, and small disk herniations were seen 33% of the time. Click here for this reference.
Unfortunately, after I have reviewed the scan with the patient and explained that the MRI scan could be described as normal for the patient’s age, the patient is usually unconvinced. An MRI scan is high-tech piece of equipment, the process of being scanned is frequently quite an experience for the patient, and the ability to see inside the human body with this tool is dramatic. I believe that the patient remains unconvinced because they just had an expensive high-tech test, the radiologist who looked at the MRI scan found lots of abnormal findings, and they certainly have back pain. Now they have a spine surgeon telling them that their MRI scan is normal!
While there is a healthy debate about what constitutes an abnormal versus a normal MRI scan of the spine, there are a few key findings that reveal a problem that we know will require surgery. For example, here are some images of problems in the spine that are more likely to require surgical treatment….
Large disk herniations with nerve root pain
These are the images of a large disk herniation at the L4/5 level that is causing severe low back and leg pain, numbness in the L5 distribution, and weakness in the calf. This patient has had pain for several months, is unable to work, sit, or ride in a car, and ultimately she underwent a microscopic discectomy with excellent relief. In the following image I have outlined a similarly large disk herniation at the L5/S1 level that also required a microscopic discectomy. The herniation is outlined in white dots and is completely filling the neuroforamen, putting severe pressure on the nerve root.
These images are of someone with an instability of the lumbar spine called a spondylolisthesis. In the following images there is a slippage between the L4 and L5 vertebral bodies which results in a narrow of the spinal canal and severe pressure on the nerve roots. This causes severe low back pain, bilateral leg pain, and typically the patient is not able to stand for long periods without feeling like their legs are going to collapse. Ultimately, this patient underwent an operation in which the spine was stabilized and fused at this level. In the first two images we can see the amount of narrowing at the L4/5 level is severe compared to the space available for the nerve roots just above the level of the slip. I have outlined the limited amount of space available for the nerve roots with white dots and also the misalignment between L4 and L5 by drawing a series of dots along the posterior aspect of L4/5 and L3/4.
These images are of someone with severe stenosis of the lumbar spine. This is caused by severe arthritis and degeneration of the spine and this patient has low back and bilateral leg pain and can walk less than 100 feet without having to find a place to sit down and flex forward. This patient experienced profound improvement in their symptoms after a microscopic lumbar laminectomy done via a minimally invasive approach. On the first image we can see the severe narrowing of the spinal canal and neuroforamen at the L4/5 level and on the second image we can see how much space should be available for the nerve roots.
Take home message #1: If your MRI scan does not show evidence of severe stenosis, spondylolisthesis, neuroforaminal narrowing, or a large extruded disk hernation, chances are you will not be a candidate for surgical treatment.