How to read a knee MRI

Three planes, two kinds of image

A knee MRI images the joint in three directions, and each one exists because certain structures are only assessable from a particular angle. Sagittal slices run front to back and are where the cruciate ligaments and the body of each meniscus are evaluated. Coronal slices run side to side and show the collateral ligaments, the joint line and the cartilage surfaces. Axial slices are cross-sections and are used chiefly for the kneecap, its cartilage and how it sits in its groove.

Cutting across those planes are the sequences. T1 and proton-density images give crisp anatomy: fat is bright, fluid is not, and the shapes are easy to follow. Fluid-sensitive sequences with fat suppression — T2 fat-sat, STIR — do the opposite, turning fat dark so that anything wet lights up against a black background. Almost all acute injury shows on the second kind, which is why an unremarkable-looking T1 next to a dramatic-looking STIR is a normal appearance for a real injury rather than a contradiction.

What normal looks like

The organising principle of musculoskeletal MRI is that tightly organised, water-poor tissue produces almost no signal. Ligaments, tendons and menisci are therefore uniformly black when healthy — a normal meniscus on a sagittal slice through the outer joint is a solid dark bow-tie, and a normal anterior cruciate ligament is a continuous dark band running at a consistent angle across the notch.

• Cartilage is a smooth, even layer of intermediate signal covering the ends of the bones, with no thinning, cracks or defects reaching bone.
• Bone marrow is uniform and bright on T1, and stays dark on fat-suppressed images. A bright patch on fat-sat is oedema.
• A small amount of joint fluid is normal. A thin bright rim is not an effusion.
• The extensor mechanism — quadriceps tendon, patella, patellar tendon — is dark, continuous and correctly aligned.

The structure-by-structure checklist

1. 1.Menisci

   Each meniscus is followed slice by slice in two planes. What matters is not whether there is internal signal but whether that signal reaches the articular surface: degenerative change inside the substance is common with age, and a tear is called when the bright line extends to the edge.

2. 2.Anterior and posterior cruciate ligaments

   Assessed on sagittal images for continuity, thickness and angle. A torn ACL may be discontinuous, wavy, or replaced by a swollen bright mass; secondary signs such as a characteristic pattern of bone bruising support the call.

3. 3.Collateral ligaments

   The medial and lateral supporting ligaments are checked on coronal images. Sprains show as bright fluid signal around or within an otherwise intact dark band; a complete tear shows discontinuity.

4. 4.Cartilage

   Graded by how deep the loss goes, from surface softening and irregularity through to a full-thickness defect exposing bone. Reports often mention the compartment — medial, lateral or behind the kneecap.

5. 5.Bone marrow

   Fat-suppressed images reveal oedema invisible on T1. A bruise pattern can indicate the mechanism of injury, and marrow oedema is also seen in stress injury, arthritis and after surgery.

6. 6.Joint fluid, cysts and the rest

   Effusion volume, a Baker’s cyst behind the knee, plica, loose bodies, bursae, and the tendons around the joint. Many of these are common findings that carry no symptoms.

Why the report distinguishes signal from a tear

Radiologists commonly grade signal within a meniscus on a three-point scale. A rounded or linear focus that stays inside the substance of the meniscus reflects degeneration, which accumulates with age and with use. Only when that signal clearly extends to the surface of the meniscus is a tear diagnosed, because that is the point at which the structure is mechanically breached rather than merely worn.

This is why a report can describe intrasubstance signal at length and still conclude that no tear is present, which reads as a contradiction if you have only skimmed the findings section. It is not. The findings describe what is visible; the impression states the conclusion.

The most important caveat in the whole report

Studies that scan people with no knee complaints at all repeatedly find meniscal tears, cartilage loss and degenerative change in a substantial share of them, and the proportion climbs steadily with age. The finding is real; it is simply not the cause of anything. That mismatch is the single most important thing to carry into an appointment, because a knee MRI can only tell you what the joint looks like, never what hurts.

It follows that the interesting question is rarely “what did the MRI find”. It is whether what it found matches where the pain is, what makes it worse, and what an examination shows. Two people with identical images can need completely different treatment, and neither answer can be read off the pictures.

This is also why surgical decisions are made on the combination of imaging, examination and how the knee behaves over time — not on the report alone.

Frequently asked questions

What does a torn meniscus look like on an MRI?

A normal meniscus is uniformly black and wedge-shaped. A tear appears as a bright line within it that reaches the articular surface. Bright signal that stays inside the substance without reaching a surface is usually described as degenerative change rather than a tear, which is why a report can mention signal and still conclude no tear is present.

Why do some of my knee MRI images look completely different from others?

Because they are different sequences of the same knee. On T1 and proton-density images fat is bright and the anatomy is crisp; on fat-suppressed fluid-sensitive images fat is turned dark so that any excess fluid glows. Acute injury and bone bruising are usually obvious on the second kind and easy to miss on the first.

What does bone marrow oedema mean in a knee MRI report?

A bright area within bone on fat-suppressed images, meaning increased fluid in the marrow. It is seen after impact as a bone bruise, in stress injury, in arthritis and after surgery. It is a sign that bone has been stressed or is inflamed, not a fracture in itself, and it typically resolves over weeks to months.

My knee MRI found several things. Do I need surgery?

The images cannot answer that. Degenerative tears and cartilage loss are found in many people who have no pain at all, so a finding only matters if it explains your symptoms and examination. Whether an operation helps depends on which structure, the pattern of the tear, your age and activity, and how the knee has behaved over time.

Was a knee MRI without contrast enough?

For most knee questions, yes. Contrast is not routine for menisci, cruciate ligaments or cartilage, because the fluid-sensitive sequences already show them well. It is added for specific reasons — assessing a previously operated meniscus, infection, or a mass — and the technique section of your report states what was done.