How to read a knee MRI, the images and the report

What you are looking at

A knee MRI is not one picture. It is several series — stacks of thin slices taken in different directions and with different settings — and the radiologist reads them all together. If you open the study yourself, the first thing to understand is which series you have in front of you.

Each series has a short description, and it usually tells you two things:

• The plane. SAG (sagittal) cuts the knee from the side, which is the classic view of the cruciate ligaments and of the front and back of each meniscus. COR (coronal) cuts it from the front, which shows the collateral ligaments and both menisci side by side. AX (axial) cuts it from above, which is where the kneecap and the cartilage behind it are seen best.
• The sequence. T1 is good for anatomy and for bone marrow. PD (proton density) shows the menisci and ligaments in detail. FS, FAT SAT or STIR mean the fat signal has been suppressed, so fluid shows up bright: joint fluid, a bone bruise, a torn ligament filling with fluid.

So SAG PD FS is a side view in which fluid is bright. That is the series most people want first.

What normal looks like

Knowing what is normal is most of the work, because the abnormal is defined against it.

• Menisci are uniformly dark on every sequence. On the outermost side slices they look like a bow tie; nearer the centre, like two dark triangles, one at the front and one at the back.
• The anterior cruciate ligament (ACL) runs diagonally from the back of the femur to the front of the tibia, and is seen on a couple of sagittal slices as a dark, slightly striped band. The posterior cruciate (PCL) is a thicker, darker curve behind it.
• The collateral ligaments, on the inner and outer side of the knee, are thin dark lines on the coronal images.
• Cartilage is a thin grey layer covering the ends of the bones; bone marrow is bright on T1 because it contains fat.
• A little joint fluid is normal. Every knee has some.

Looking at the images helps you follow the conversation with your doctor. It does not let you diagnose yourself: the same grey patch can be a normal variant, an artefact from movement or a real finding, and telling them apart is exactly what a radiologist is trained to do.

The words in the report

Knee reports go structure by structure, usually in a fixed order: menisci, ligaments, cartilage, bone, fluid and soft tissues. These are the terms that come up most:

• Meniscal tear — a line of brighter signal inside the meniscus that reaches its surface. The report usually says where (the posterior horn of the medial meniscus is the most common place) and the shape: horizontal, radial, vertical, complex, bucket-handle (a fragment flipped out of place). Radiologists generally want to see it on more than one image before calling it a tear.
• Intrasubstance degeneration or signal not reaching the articular surface — brighter signal that stays inside the meniscus. It is a wear change, common with age, and not a tear.
• Sprain or partial tear of a ligament — the ligament is intact but thickened or surrounded by fluid. Complete tear or full-thickness tear — the fibres are interrupted.
• Bone marrow oedema or bone bruise — fluid in the bone after an impact, without a fracture. After a twisting injury, a bruise on the outer side of the femur and the back of the tibia is a classic companion of an ACL tear, which is why the report may mention both.
• Chondromalacia or chondral defect — the cartilage is softened, thinned or missing in a patch. It is often graded from 1 to 4, from signal change to loss down to the bone.
• Joint effusion — more fluid than usual in the joint. Small, moderate or large.
• Baker's cyst (popliteal cyst) — a pocket of joint fluid at the back of the knee. Very common, and usually a consequence of something else in the joint rather than a problem on its own.

The number worth knowing

Meniscal tears are much more common than knee pain.

A study published in the New England Journal of Medicine in 2008 scanned the knees of 991 adults aged 50 to 90, chosen from the general population of Framingham, Massachusetts — not people who had gone to a doctor with knee trouble. Depending on age and sex, between 19% and 56% had a meniscal tear, and 61% of the people with a tear had had no pain, aching or stiffness in that knee during the previous month.

That does not make your tear irrelevant, especially after a clear injury in a younger knee. It means that the image alone cannot say whether the tear is what hurts, and that "degenerative meniscal tear" in an older knee is often a description of ageing.

Questions worth taking to your appointment

• Does what the report describes match where and how my knee hurts?
• Is the tear described as degenerative or as the result of an injury?
• Is anything out of place — a displaced fragment, a bucket-handle tear — or is it stable?
• Does this change what we do, or is it something to note and watch?

If the report as a whole is hard to follow, start with how a radiology report is structured: the impression at the end is where the radiologist says which of these findings matter. And if you have the disc but cannot open it, here is what to do with the hospital CD.

Sources

• Englund M. et al. Incidental meniscal findings on knee MRI in middle-aged and elderly persons. New England Journal of Medicine, 2008. The figures for the 991 adults from Framingham come from here.

This explains vocabulary, not your knee. Whether a finding explains your symptoms needs someone who can examine you and who knows your history.

What ExplainMyScan does is the step before that: you upload your report, say what worries you, and get it explained in plain language, so you walk into the appointment understanding what you are holding. It does not diagnose and it does not replace your doctor.