How To Read An MRI of the Lumbar Spine Introduction
Reading an MRI lumbar spine is quite easy if you know where to look. Just like x-ray films and three dimensional x-rays called computed tomography (CT), MRIs are a great addition in the field of radiology.
An exciting new healthcare modality in the 1980s called magnetic resonance imaging (MRI) unleashed a new era in medicine. High tech MRI uses radio waves to look for lesions and abnormalities in bones and soft tissues of the spine. This imaging system opens a whole new way to look for abnormalities that may be the cause of low back pain, sciatica or leg pain. However, MRIs cannot assess spinal ligaments or, most importantly pinpoint the exact cause of pain.
Reading an MRI lumbar spine is very easy to understand if you know where to look and radiologists who do this for a living make it look very easy. The following is a very simple guide to help you understand the nomenclature of your MRI.
MRI cannot pinpoint the exact cause of pain.
Two Basic MRI of the Lumbar Spine Images (T1 AND T2 Images)
There are two basic types of MRI images which differ by the timing of the radiofrequency pulses, named T1-weighted images and T2-weighted images. T1 images highlight FATty tissue. T2 images highlight FAT AND WATER within tissues. The contrast between these tissues highlight amazing details.
Basic Anatomy of How to Read an MRI Lumbar Spine
The length-wise sagittal views, or what I call lengthwise baguette views, are the easiest to recognize. Axial or cross-section views are what I call the sliced bread views which are best for highlighting the intervertebral discs. The most important anatomical parts of the sagittal and axial images of a normal MRI lumbar spine are pictured below. The spinous processes are the only part of your spine that y0u can feel posteriorly in the middle of your back.
The Aging Process
With age, the spine stiffens as the intervertebral discs dehydrate and slowly degenerate. MRIs demonstrate this with progressively darkening discs that lose vertical height. As they shrink, the discs begin to bulge into the central spinal canal.
A Lumbar Disc Herniation May Disappear
Disc herniations can be large and appear permanent. However, the immune system breaks down the disc material slowly over time. If the disc herniation material remains viscous, it may disappear as seen in the picture below.
How to Read an MRI Lumbar Spine in 8 Steps
1. The Central Canal in the MRI Lumbar Spine
Begin with the images of the lengthwise spine, also known as the sagittal images. Identify the middle of the central canal where the spinal cord and nerves float in the protective cerebral spinal fluid (CSF). This area highlights the junction between the intervertebral discs, bones, ligaments and nerves of the spine. Anything that interrupts the smooth linear border of the central canal may be a potential problem. A normal image is noted below on the left compared with the figure on the right which demonstrates severe spinal stenosis.
2. Vertebral body
In the lengthwise spine views, identify the five vertebral bodies that form the lumbar spine. Disc levels are numbered top down from one to five. Their function is to support and protect the delicate nerves that pass through to the pelvis and lower legs. They should be rectangular and the bone marrow within should be a uniform grey color. However, if the vertebral bodies look triangular, it indicates a fracture (inset).
In step 3, look at the alignment of the posterior borders of the five vertebral bodies (red line shown below). The picture on the left demonstrates a normal spine with a smooth line connecting the posterior borders. Disruption of that smooth line by the vertebral bodies is a result of wear-in-tear of the bone, ligament or discs. Images of spinal instability are demonstrated in the pictures on right. Further misalignment of the spine causes a condition called spondylolisthesis. If instability increases, a pars defect results as the lamina fractures and spondylolysis is a consequence. Spondylolysis describes an unstable spine which ultimately increases the likelihood of surgery. Instability places greater stress on the discs and the outcome is a greater risk of disc herniation, nerve irritation, arthritis and persistent pain.
4. Intervertebral Discs
In addition, identify the space between the vertebral bodies for the intervertebral discs. The disc levels are numbered from L1-2 to L5-S1 based on their location in between the vertebral bodies. Discs have a great deal of fluid within them which allow them to act as shock absorbers. Unfortunately, over time the discs dry out and degenerate. With a fracture of the end plate, degenerative disc disease occurs as the fluid within the disc leaks out.
5. Disc herniation and degeneration
A normal intervertebral disc has significant white signal internally (figure A) which represents normal fluid. As the intervertebral disc degenerates, the normal white signal darkens (figure B) and narrows (figure C). With loss of the interior supporting fluid, the thick collagen walls of the disc begin to bulge outwards into the central canal from the excessive pressure (figure D). Extrusions described a tear in the disc wall in which the fluid squeezes out into the central canal (figure E). If a little material squeezes out it can result in a mild disc herniation. If a great deal of fluid squeezes out it is called a disc extrusion which can migrate (figure F). There is a greater chance of severe pain, weakness, numbness, and tingling as more disc material is squeezed into the central canal.
In contrast to the solid structures of the spine, foramen are narrow keyhole-shaped canals located on either side of the spinal column. These foraminal canals allow individual nerve roots to exit from the spine. Borders of the foramen are hard bony pedicles and intervertebral discs shrouded in a thin membrane called the thecal sac which keeps the CSF from leaking out.
Patients are at risk of a pinched or irritated nerve with a disc herniation in the narrow foramen compared with the larger central canal. Nerve root compression occurs in the adjacent area called the lateral recess. Facet joints or associated synovial cysts posteriorly in the foramenal canal compress the nerve roots. The cauda equina (horses tail) is a bundle of nerve fibers found at the bottom of the spine.
7. Axial views
The axial images or sliced bread views provide a clearer picture of a specific intervertebral disc and the adjacent nerves. A normal disc (figure A) provides ample room for the nerves to pass through. Any type of disc herniation (figure B) narrows the normally roomy canals causing the transiting nerves to become irritated or compressed which results in symptoms. A small disc herniation (figure C) may not produce symptoms. A large disc herniation (figure D) may result in severe pain, weakness, numbness or tingling. Over time, a lumbar disc degenerates by first developing a disc bulge or bulging disc and/or an annular tear. The stages of disc degeneration are a herniated disc, protrusion of the disc material (nucleus pulposus), extrusion, and disc fragment sequestration.
8. Spinal stenosis
The normal central canal is usually quite big (figure A) and houses and protects the descending nerves. Spinal stenosis describes narrowing of the central canal (figure B). The condition occurs with extensive wear-in-tear of the intervertebral disc (figure C), ligamentum flavum (figure D) and facet joints (figure E). As they degenerate, they expand into and narrow the central canal which results in nerve compression. The resulting nerve compression can cause progressive pain, weakness and numbness.