Magnetic Resonance Imaging (MRI) uses strong magnets and radio waves to create detailed pictures of the body’s internal structures. For the knee, it provides exceptional visualization of soft tissues like ligaments, tendons, and cartilage, which often do not show up clearly on standard X-rays. Understanding how these images are generated and organized can help demystify the information presented in an MRI report and the accompanying scans. This knowledge is intended only to supplement a discussion with a healthcare provider, whose professional diagnosis remains the final word.
Decoding the MRI Views and Sequences
An MRI scan of the knee is a series of images, or “slices,” taken from different angles using various settings known as sequences. These sequences rely on the magnetic properties of water and fat within the body to create contrast. The fundamental image types are T1-weighted and T2-weighted sequences, which serve distinct purposes in analysis.
T1-weighted images are used for anatomical clarity; they make fat appear bright, while water and fluid appear dark. This provides sharp contrast for structures surrounded by fat, such as bone marrow. Conversely, T2-weighted images are sensitive to fluid and are preferred for identifying pathology, as water, inflammation, and edema appear bright. A bright signal on T2 indicates a buildup of water, which is a common sign of injury or inflammation.
The images are organized into three primary viewing planes, or orientations, to provide a complete, three-dimensional look at the knee.
Viewing Planes
- The sagittal plane slices the knee from side to side, offering a profile view useful for assessing the cruciate ligaments and menisci.
- The coronal plane slices the knee from front to back, helpful for examining the collateral ligaments and the alignment between the femur and tibia.
- The axial, or transverse, plane provides a cross-sectional view, valuable for evaluating the patella and the quadriceps and patellar tendons.
Radiologists interpret these images using signal intensity. Cortical bone appears dark on both T1 and T2 sequences because it generates very little signal. Tissues that are normally taut and dense, like healthy ligaments and menisci, also exhibit a low signal intensity, appearing uniformly dark.
Identifying the Major Knee Structures
Identifying the appearance of healthy, intact structures is the starting point for viewing the MRI. The main bony structures—the femur, tibia, and patella—all appear with a uniformly low signal, or dark outline, on both T1 and T2 sequences. The bone marrow inside these bones contains fat cells, causing it to appear bright on T1-weighted images.
The menisci, the C-shaped wedges of fibrocartilage between the femur and tibia, appear as distinct, uniformly dark triangular shapes in the sagittal and coronal views. Their low signal intensity reflects their dense collagen structure, which contains little water. Any deviation from this uniform darkness suggests tissue alteration.
The primary stabilizing ligaments also appear as taut, continuous dark bands across all sequences. The cruciate ligaments (ACL and PCL) cross within the center of the joint and are best evaluated on sagittal views. The collateral ligaments (MCL and LCL) are best visualized in the coronal plane, appearing as continuous dark lines.
Articular cartilage, the smooth tissue covering the ends of the femur and tibia and the back of the patella, is visible as a smooth, uniform gray layer. This cartilage is slightly brighter than the ligaments and menisci but darker than the fat in the bone marrow on T1 images. Its consistency and thickness should be inspected closely where the bones articulate.
Recognizing Signs of Injury and Pathology
The presence of a bright signal, or hyperintensity, in a structure that should normally be dark is the primary indicator of injury or pathology. This is especially true on T2-weighted or fluid-sensitive sequences. This brightness is caused by edema, the accumulation of fluid and inflammatory cells at the site of tissue damage.
A meniscal tear is identified by a bright signal that extends from the normally dark meniscus to at least one of its articular surfaces (a Grade 3 tear). Common tear patterns, such as a horizontal or complex tear, are described in the report based on the morphology of this bright line. A severe longitudinal tear may displace a fragment of the meniscus into the joint center, creating a “bucket-handle” appearance.
Ligamentous injury, or a sprain, is characterized by a bright signal within the normally dark, taut band of tissue. A partial tear (Grade I or II) shows a bright, hazy signal surrounding the ligament or within its substance. A complete tear (Grade III) shows discontinuity of the ligament’s fibers, meaning the dark band is completely broken, with a large fluid signal filling the gap.
Injuries to the bone often manifest as a bone bruise, or subchondral bone marrow edema. This appears as an ill-defined, cloud-like area of high signal intensity within the bone marrow on T2-weighted images, caused by microfractures and bleeding. A bone contusion is frequently associated with an ACL tear, often appearing in the lateral femoral condyle and the posterolateral tibial plateau.
Other common findings relate to joint fluid. An effusion is an abnormal collection of fluid within the joint capsule, appearing as a bright signal filling the joint space on T2-weighted scans. Degenerative changes, common in conditions like osteoarthritis, are noted by thinning and irregularity of the articular cartilage.