On a normal knee MRI, bones appear bright white, tendons and ligaments look like dark bands or cords, cartilage shows as a smooth gray layer over bone surfaces, and fluid fills the joint space with a bright signal. Each tissue has a predictable appearance depending on the type of MRI sequence used, and understanding these basics can help you make sense of your own images.
How Different Tissues Look on MRI
MRI works by detecting water and fat content in tissues, which is why different structures light up differently. On the most common sequences, here’s what you’ll see:
- Bone marrow: Appears bright on T1-weighted images (the sequence that shows anatomy clearly) because adult marrow contains a lot of fat. It’s brighter than the surrounding muscle.
- Muscle: Shows up as a medium gray, darker than bone marrow and fat.
- Fat: Bright on almost every sequence unless the radiologist specifically suppresses it. The fat pad behind the kneecap (Hoffa’s fat pad) is one of the most obvious bright structures.
- Fluid: Dark on T1-weighted images but lights up bright white on T2-weighted or fluid-sensitive sequences. A small amount of joint fluid is normal.
- Tendons and ligaments: Uniformly dark (low signal) on all sequences because their tightly packed collagen fibers contain very little water.
Radiologists typically capture images in three planes: sagittal (side view, slicing the knee left to right), coronal (front view, slicing front to back), and axial (cross-sectional, slicing top to bottom). Each plane highlights different structures. The sagittal view is best for the cruciate ligaments and cartilage surfaces. The coronal view shows the collateral ligaments and meniscus bodies clearly. The axial view is ideal for evaluating the kneecap’s position in the trochlear groove.
Normal Meniscus Appearance
The menisci are two crescent-shaped pads of fibrocartilage that cushion the space between your thighbone and shinbone. On MRI, a healthy meniscus appears dark (black or charcoal gray) on every sequence because fibrocartilage contains very little water. When you look at a coronal or sagittal slice, each section of the meniscus has a neat triangular shape, almost like a wedge or a bowtie when viewed from the side.
The medial meniscus (inner side of the knee) and lateral meniscus (outer side) look slightly different from each other. The body of the medial meniscus has an equilateral triangle shape, while the lateral meniscus body and both horns (the front and back tips) look more like elongated isosceles triangles. The posterior horn of the medial meniscus is normally larger than its anterior horn.
One thing that can look alarming but is often harmless: some people have small areas of increased signal (lighter patches) within the meniscus. These represent internal mucinous degeneration, a common age-related change. As long as the signal doesn’t extend all the way to the surface of the meniscus, it’s not considered a tear.
Normal Cruciate Ligaments
The anterior cruciate ligament (ACL) runs diagonally from the outer part of the thighbone (lateral femoral condyle) down and forward to attach just in front of the tibial spine on the shinbone. On a normal MRI, the ACL appears dark on all sequences but has a characteristic striated look, almost like a bundle of parallel fibers rather than one solid cord. You can see it on sagittal images running at a roughly 45-degree angle.
The posterior cruciate ligament (PCL) runs from the inner side of the thighbone down and backward to the back of the shinbone. It’s also dark on all sequences, but unlike the ACL, it appears more homogeneous, like a smooth, uniform band rather than a striated bundle. On sagittal images, a normal PCL has a gentle curved shape, sometimes described as resembling a hockey stick. Both ligaments should look continuous from one attachment to the other with no gaps or areas of bright signal breaking through them.
Normal Cartilage and Bone Surfaces
Articular cartilage is the smooth, glassy coating on the ends of bones where they meet inside the joint. It’s thin, reaching a maximum thickness of about 4 millimeters in the thickest areas of the knee (typically the back of the kneecap). On proton density or specialized cartilage-sensitive sequences, healthy cartilage appears as a smooth, intermediate-signal gray layer with even thickness and no irregularities along its surface. On T1-weighted images, cartilage looks darker (hypointense).
Beneath the cartilage sits a thin layer of calcified tissue called the tidemark, where collagen fibers anchor into the subchondral bone. On a normal MRI, the subchondral bone should appear as a smooth, dark line right beneath the cartilage, with bright marrow signal behind it. There should be no patchy bright areas in the bone on fluid-sensitive sequences, which would suggest swelling or stress reactions.
Normal Collateral Ligaments and Tendons
The medial collateral ligament (MCL) runs along the inner side of the knee, best seen on coronal and axial images. It appears as a thin, dark, ribbon-like structure lying flat against the bone. The lateral (fibular) collateral ligament runs along the outer side and looks like a thin, dark cord. Both should be continuous and uniformly low signal from top to bottom.
The patellar tendon connects the bottom of the kneecap to the top of the shinbone. On MRI, it’s a straight, dark band. One normal variant worth knowing: a small triangle of slightly lighter signal sometimes appears where the tendon attaches to the kneecap. This is caused by a phenomenon related to how fibers are oriented relative to the MRI magnet (called the magic angle effect) and is not a sign of damage, provided no other abnormalities are present.
Normal Kneecap Tracking
Axial images through the kneecap let radiologists evaluate whether the patella sits properly in the trochlear groove, the shallow channel on the front of the thighbone. On a normal MRI, the inner edge of the kneecap should be no more than about 2 millimeters away from the front of the inner femoral condyle. The angle between the kneecap and the groove normally measures greater than 8 degrees and opens toward the outer side of the knee.
The trochlear groove itself should have a well-defined V-shape. Radiologists measure the sulcus angle (how steep or shallow the groove is), and a normal groove has an angle of less than 144 degrees with a depth of at least 5 millimeters. A flatter groove can indicate trochlear dysplasia, which predisposes the kneecap to dislocate.
How a Child’s Knee MRI Looks Different
If you’re looking at a child’s knee MRI, the most striking difference is the growth plates (physes). These are bands of cartilage near the ends of the thighbone and shinbone that allow bones to lengthen during growth. On MRI, an open growth plate appears as a bright stripe of cartilage signal running horizontally between the rounded end of the bone (epiphysis) and the shaft (metaphysis). In younger children, this stripe is thick (greater than 1.5 millimeters) and has a layered, multilaminar appearance.
As a child approaches skeletal maturity, the growth plate gradually narrows and eventually fuses completely. In a fully mature knee, the growth plate disappears entirely, sometimes leaving behind a faint line called an epiphyseal scar. Seeing open growth plates on a child’s MRI is completely normal and should not be confused with a fracture, which would show irregular edges, surrounding bone swelling, and bright signal on fluid-sensitive sequences rather than the smooth, organized stripe of a healthy physis.
Common Normal Variants That Can Look Abnormal
Several findings on knee MRI look suspicious but are actually harmless. Internal signal within the meniscus that doesn’t reach the surface is degenerative change, not a tear. The patellar tendon’s triangular bright spot at its upper attachment is a normal artifact. Small amounts of fluid behind the knee (in the popliteal region) can represent a normal bursa rather than a pathological cyst.
Red marrow reconversion, where bone marrow in certain areas appears slightly less bright on T1 images, can occur in athletes, anemic patients, or smokers and mimics more serious marrow conditions. The key distinction is that normal or reconverted marrow still appears brighter than muscle on T1 images. If marrow signal drops below that of surrounding muscle, it warrants further evaluation.