How to read a lumbar spine MRI

The report is a list of levels, not a story

The lumbar spine has five vertebrae, numbered L1 at the top to L5 at the bottom, sitting on the sacrum. Between each pair is a disc, named for the two bones it separates: L4-L5 is the disc between the fourth and fifth lumbar vertebrae. A report works down that list and says something about every level, which is why it reads as a series of near-repeating paragraphs rather than as prose.

At each level four things are assessed: the disc itself, the central spinal canal behind it, the small exits on each side where nerve roots leave, and the pair of facet joints at the back. Once you know that structure, the report stops being a wall of terminology and becomes a table. Findings cluster at L4-L5 and L5-S1 in most people, because those two levels take the largest mechanical load.

What the images show

A standard lumbar study is mostly sagittal and axial images in two contrasts. On T2, water is bright: cerebrospinal fluid around the nerves glows white, and a healthy, well-hydrated disc has a bright centre. On T1, the anatomy is crisp and fat is bright, which makes it the better image for bone marrow and for seeing the fat that normally surrounds a nerve root in its exit canal.

That gives two quick visual checks that underlie much of the report. A disc that has darkened on T2 has lost water content, which is what degeneration looks like. And an exit canal in which the normal bright fat has been squeezed out is a narrowed one, whatever the measured dimensions say.

Bulge, protrusion, extrusion: what the words mean

These are the terms patients misread most, because they sound like a severity scale and are not. They are a shape vocabulary, standardised so that two radiologists describe the same disc the same way.

None of these words tell you whether a nerve is being compressed. That is a separate sentence in the report, and it is the one that matters clinically: a large-sounding extrusion pointing away from the nerves can cause less trouble than a small protrusion pointing straight at one.

Canal, foramina and what compresses what

• Central canal stenosis — the main tunnel carrying the nerve bundle has narrowed. Reports usually grade it mild, moderate or severe, and it classically causes symptoms that worsen with walking and ease when leaning forward.
• Foraminal or lateral recess narrowing — the side exits are narrowed where a single nerve root leaves. This is where a one-sided, one-leg pattern of symptoms typically comes from.
• Nerve root contact, abutment or displacement — a graded vocabulary from touching to pushing to compressing. Contact alone is not compression.
• Facet arthropathy — arthritic change in the small joints at the back of the spine, which can contribute to narrowing and to local pain.
• Spondylolisthesis — one vertebra has slipped forward relative to the one below, graded by how far.
• Modic changes — signal change in the bone next to a degenerated disc, described in numbered types; often mentioned, and their clinical significance is still debated.

Why almost everyone over forty has an abnormal lumbar MRI

Imaging studies of people with no back pain whatsoever find disc degeneration, bulges and annular fissures in a large and steadily rising share of them by decade. By the later decades of life, a completely normal-looking lumbar MRI is the exception rather than the rule. These findings are genuinely present; they are simply as ordinary as grey hair, and they are not by themselves an explanation for pain.

The practical consequence is that a lumbar MRI is much better at answering “is there something here that matches these specific symptoms” than at answering “why does my back hurt”. This is exactly why guidelines discourage scanning ordinary back pain early: the scan will find something in most people, and finding it does not improve the outcome. When the report says that findings are age-appropriate or degenerative, that is a meaningful statement and not a brush-off.

What a lumbar spine MRI does not show

This causes more confusion than any other aspect of the study, because pain in the lower back, buttock and hip region can come from several structures that a lumbar protocol does not properly image.

• The hip joints. They may be partly visible at the edge of the images, but they are not centred, not optimally imaged, and not formally assessed. Hip pathology is a well-known mimic of lumbar nerve pain, and it needs its own study.
• The sacroiliac joints. Usually only partly included, and inflammatory SI joint disease requires a dedicated protocol.
• The pelvis and abdominal organs. Outside the field of view or at its blurred margin.
• How the spine behaves under load. You are lying still; the study cannot show what happens when you stand, walk or bend.
• Nerve function. Compression on an image and a nerve that is not working are different statements, and the second is tested clinically or with nerve studies.

Frequently asked questions

Is a disc bulge serious?

On its own, usually not. A bulge is a generalised extension of the disc beyond its normal margin and is one of the most common findings in adults, including those with no symptoms. What matters is the separate sentence about whether anything is contacting or compressing a nerve, and whether that matches where your symptoms are.

What is the difference between a disc bulge and a herniation?

Extent and shape. A bulge involves most of the disc circumference and stays close to its normal outline. A herniation is focal: a protrusion is broad-based, an extrusion pushes out further than its base is wide, and a sequestered fragment has separated. The words describe geometry, not how much pain a disc causes.

Does a lumbar spine MRI show the hips?

Not properly. The hips may appear at the edge of the images, but a lumbar protocol is centred, angled and optimised for the spine, so the hip joints are not formally assessed and a report will not comment on them meaningfully. Hip disease is a common mimic of lumbar nerve pain and needs a dedicated hip study.

My MRI shows spinal stenosis. Do I need surgery?

Not automatically. Stenosis describes narrowing seen on images, graded mild to severe, and the finding is common with age. The decision rests on your symptoms, how disabling they are, how they have responded to non-surgical treatment, and whether the imaging matches them — not on the grade in the report by itself.

Why does my report describe every level even when nothing is wrong?

Because a radiology report is a complete inventory. Each level is described whether or not it is abnormal, so that a later scan can be compared against it and so that nothing is left ambiguous by omission. Levels reported as unremarkable or age-appropriate are the report saying they look fine.