How to Read a Knee MRI: A Patient's Guide

MRI Basics: Views and Sequences

A knee MRI is typically acquired in three planes: sagittal (side view, dividing left from right), coronal (front view, dividing front from back), and axial (top-down slices). Each plane is best for evaluating specific structures. The sagittal view is ideal for the ACL, PCL, and menisci. The coronal view best shows the collateral ligaments and meniscal body. The axial view is used for the patella, patellar tendon, and cartilage under the kneecap.

Two main sequence types appear in most knee MRIs: T1-weighted images show anatomy clearly with fat appearing bright and fluid appearing dark. T2-weighted (or proton density fat-saturated) images highlight pathology because fluid and inflammation appear bright white. Understanding this basic principle — that bright signal on T2 within a normally dark structure suggests injury — is the key to reading knee MRI.

The Anterior Cruciate Ligament (ACL)

The ACL is best seen on sagittal images as a dark, taut band running from the posterior femur to the anterior tibia. A normal ACL has uniform low signal (dark) with clearly defined parallel fibers. Signs of an ACL tear include complete absence of the ligament on expected slices, a wavy or lax appearance instead of a taut band, bright signal within the ligament substance indicating edema or hemorrhage, and abnormal orientation.

Secondary signs of ACL injury include bone bruising on the lateral femoral condyle and posterolateral tibial plateau, anterior tibial translation, and a deep lateral femoral notch sign. For a detailed breakdown of injury severity, see our guide on ACL tear grades.

Meniscus Evaluation

The menisci appear as dark triangular or bowtie-shaped structures on MRI. The medial meniscus is C-shaped and the lateral meniscus is more circular. On sagittal images, you see the anterior horn, body (which has a characteristic bowtie shape on two consecutive slices), and posterior horn. A normal meniscus is uniformly dark (low signal) on all sequences.

Meniscal tears are diagnosed when abnormal bright signal extends to the articular surface — that is, the signal touches at least one edge of the meniscus. Internal signal that does not reach the surface (grades 1 and 2) represents degeneration, not a true tear. A grade 3 signal that touches the surface is a definitive tear. For a complete overview of tear types and classifications, see our article on meniscus tear types.

Cartilage, Bone, and Fluid

Bone marrow normally appears bright on T1 images (due to fat content). Bone marrow edema — bright signal on T2/STIR sequences and dark on T1 — indicates stress, contusion, or early pathology. Bone bruises are extremely common after ligament injuries and typically resolve over 6-12 weeks. Joint effusion (excess fluid) appears as bright signal within the joint capsule on T2 images. A small amount of fluid is normal, but a large effusion suggests acute injury, inflammation, or arthritis. To understand when MRI versus X-ray is the right choice, see our comparison guide on MRI vs X-ray.

If you are using AI to understand report wording, keep the distinction clear: an AI explanation can help you prepare questions, while a radiology second opinion is a licensed medical interpretation. See the AI MRI explanation vs second opinion guide.

Key Takeaways

• Knee MRI uses three planes (sagittal, coronal, axial) and two main sequences (T1 and T2)
• Normal ligaments and menisci appear dark; bright signal within them suggests injury
• Meniscal tears require abnormal signal to reach the articular surface — internal signal alone is not a tear
• Many MRI findings (mild degeneration, small effusions) are normal with aging and may not require treatment
• Bone marrow edema (bright on T2, dark on T1) indicates stress or contusion
• Always correlate MRI findings with symptoms — not every abnormality on MRI is clinically significant

Frequently Asked Questions