Normal knee MRI
The knee joint is a complex joint that connects three bones; the femur, tibia and patella. The arrangement of the bones in the knee joint, along with its many ligaments, provide it with the arthrokinematics that allows for great stability, combined with great mobility. Being arguably the most stressed and exposed joint of the body, the knee joint is predisposed to various injuries and degenerative disorders. Both the pronounced accuracy of the MRI and the high prevalence of knee disorders, makes the knee MRI the most frequently ordered imaging procedure of the musculoskeletal system.
Naturally, in order to assess pathologic knee imaging, it is necessary to know the appearance of a normal knee MRI. This article will provide a simple guide on the basics of the MRI and how to recognize and assess the most important structures on a normal knee MRI, including its bones, cartilage and ligaments. Then to wrap it up, we will give you a basic description of how to look out for any pathologic changes in these structures.
|MRI definition||Medical imaging technique used to examine the bones and soft tissue structures of the knee|
|Mechanism||Emission of magnetic fields that trigger the protons of tissues to produce a signal measured by the MRI and converted into a gray-scale image|
T1- weighted image: fats and bone marrow produce high signal (white); ligaments, cartilage, fluid produce low signal (black)
T2- weighted image: ligaments, cartilage and fluid produce high signal (white); bone marrow produces a low signal (black).
Proton density image: enhances tissues with high proton density (fatty bone marrow, hyaline cartilage, muscles)
Coronal: makes slices through the knee from the front to back
Sagittal: makes slices through the knee from medial to lateral
Axial: makes cross-sections of the shoulder from top to bottom.
- How to read a normal knee MRI
- Bones and cartilage
- Ligaments and menisci
- Extensor mechanism
How to read a normal knee MRI
The MRI has many advantages over other imaging techniques, one of them being its superior ability to discriminate soft tissue structures. Without going into too much detail, the MRI scanner does this by triggering the protons in the tissues to produce a signal that is measured by receivers in the MRI machine and transformed into an image. Since different tissues have different density of protons, the signal can vary in intensity, which allows the MRI scanner to discriminate one tissue from another. For example, the bones have a higher density in protons and therefore emit a high signal, appearing hyperintense (white), while fluid has a low density and emits a low signal, appearing hypointense (black) on an MRI.
To complicate matters a bit more, the intensity of tissue on a final MRI image also depends on the sequence technique being used. The most commonly used are T1 and T2- weighted sequences. In addition, proton density (PD) images are also used as a contrast enhancement technique. We can switch between these modalities depending on the tissue we want to observe:
- T1-weighted images are useful to visualize the tissues that predominantly contain fat (bone marrow) since they produce a high T1 signal and appear white, which makes them easier to analyze. The tissues that contain more water (ligaments, cartilage, synovial fluid) produce a lower T1 signal and appear darker.
- T2-weighted images are useful when we want to evaluate tissues that contain more water, or we want to detect pathological changes that are followed by fluid accumulation. Therefore tissues such as ligaments, cartilage and fluids produce a high T2 signal and appear white in this sequence, while bone marrow produces a low T2 signal and appears black. This can be remembered by the mnemonic WW2, which stands for “water is white on T2”
- Proton density (PD) images minimise the contribution of both T1 and T2 contrast, in order to create a better contrast. PD images, as the name suggests, enhance structures with a higher proton density. Thus, fatty bone marrows and hyaline cartilage produce a high signal intensity and appear white or gray, muscles produce an intermediate signal and appear gray, whereas ligaments produce a low signal and appear black.
Another important property of the MRI is its ability to produce images in multiple planes, which allows us to visualize the knee from different angles. Usually, the images are taken in three planes; coronal, sagittal and axial plane. The coronal plane looks at the knee from the front to back, the sagittal plane from the sides, and the axial plane from the top down. Ultimately, the image produced by the MRI is a thin slice through the knee in one of these three planes.
As an example, shown here is an overview of the knee on an axial PD image on a slice through the femoral condyles. In this modality, fat and hyaline cartilage show as white, bones as white to gray, muscles as gray, and tendons and ligaments as black.
If this is still a bit confusing to you, have a look at our article to learn more about the fundamentals of the MRI.
Since we know that the knee is a complex joint, it is important that we film the knee in all three spatial planes and in both T1- and T2-weighted sequences. We will now go through the important knee structures and give you a description of how they are seen in different planes on the MRI (coronal, sagittal and axial).
Bones and cartilage
When we look at the knee in the coronal plane, the first thing that we see is the patella as the most anterior structure. The patella is a triangular shaped sesamoid bone that is embedded in the quadriceps tendon. With its posterior surface, it articulates with the anterior surface of the femur, forming the patellofemoral joint.
When examining the patella, it is important to evaluate its contours, bone marrow and articular cartilage. On T1, the patella appears as a homogenous intense white triangular signal. We want to make sure that there are no signs of fractures or cyst formation in the bone, which would be seen as hypointense lines or circles.
Next, we should evaluate the thickness and homogeneity of the patellar cartilage by looking at images taken in the sagittal and axial planes. On T2, the patellar cartilage has a uniformly homogeneous white signal. It should be noted that the thickness of the patellar cartilage is greater in the mid and lower portions, while being thinner in the upper portion. Any abnormal signal and irregularities in the articular cartilage, such as fissures, ulcerations, or osteochondral fragments, may point towards chondromalacia of the patellofemoral joint.
We also need to inspect the cavity of the patellofemoral joint for any signs of fluid collection, which on T2 would appear as a hypointense signal between the patellar and femoral articular cartilages.
As we go deeper into the knee joint in the coronal plane, we can observe the medial and lateral femoral condyles superiorly articulating with the tibial plateau inferiorly, forming the tibiofemoral joint. If at this point you’re unsure about which side is medial and which is lateral, you can simply scroll deeper posteriorly until you find the fibula, which indicates the lateral side. While examining the tibiofemoral joint, we need to look at the contours of the bone, bone marrow and articular cartilage.
While evaluating the bone marrow, we should look for any sign of edema, contusion, tumor or fracture. On T1, a normal bone marrow is higher in intensity, appearing bright, while on T2 it is low in intensity and appears dark. The normal bone marrow shows a mainly homogenous intensity, with only some contrasting linear signals that correspond to blood vessels. Bone marrow edema on T1 typically appears as an increased, white signal, while fractures are suspected with the appearance of larger dark lines through the bone.
Next, we will look at the articular cartilage of the tibiofemoral joint. We should assess the thickness and intensity of their articular cartilages which are best seen in sagittal and coronal planes. On T2, normal articular cartilage appears as a thick homogenous white signal that covers the articular surfaces of the lateral and medial femoral condyle and the tibial plateaus.
Lastly, we want to look at the cavity of the tibiofemoral joint for any signs of fluid collection, which would be seen as a hypointense signal between the articular cartilages.
Ligaments and menisci
There are many ligaments within the knee joint that connect the bony structures in the joint and hold them in place, providing stability, and preventing dislocation. We will go through the most important ones with you and describe how they can be recognized and evaluated on an MRI image.
We will now dive into the center of the knee joint to look at the cruciate ligaments. The anterior and posterior cruciate ligaments are very important ligaments both in terms of their function, and their susceptibility to injuries. The cruciate ligaments stabilize the knee during dynamic motion and prevent rolling and displacement of the femoral condyle, as well as hyperextension and hyperflexion of the knee joint.
On a slice through the posterior compartment of the knee in the coronal plane, we can see the origin of the anterior cruciate ligament (ACL), arising from the intercondylar notch of the tibia. From here, and we can follow it by scrolling anteriorly towards its insertion on the medial surface of the lateral femoral condyle. The ACL is best seen on T1, appearing as two sets of black, low signal bands, often showing some linear striations with a higher signal near their tibial attachment.
It is important to check the ACL in the axial plane as well, where it is best visualized on a slice roughly through the intercondylar notch. On this level, the ACL is seen on the lateral femoral condyle as a black band about 1,5 cm in the anteroposterior diameter. Upon scrolling deeper towards the tibia, we can see that the ACL fans out and crosses over to the medial femoral condyle.
The posterior cruciate ligament (PCL) also contains two bands that arise from the posterior intercondylar area of the tibia and extend anteromedially to attach on the medial femoral condyle. Unlike the ACL, the PCL has a characteristic homogeneous low signal on a sagittal T1 and its two bands are not distinctly seen on the MRI. In the axial plane, the PCL is clearly seen on a slice through the tibial articular cartilage, appearing on the posterior aspect of the intercondylar area as a black ovoid signal. It can be traced by scrolling upwards as it ascends to attach onto the medial femoral condyle.
T2-weighted sequences are the gold standard for diagnosing tears of the ACL, while tears in the PCL are often not visible on this sequence. A tear in the ACL would be seen as a high signal intensity within the substance of the ligament. The assessment of pathological changes in the cruciate ligaments is best performed in the sagittal plane, with the axial and coronal planes being complementary.
As we scroll a bit deeper in the coronal view from the central part of the knee into the posterior compartment, we can identify the menisci. These are fibrocartilaginous C-shaped plates found between the articular surfaces of the femur and tibia that provide congruence and shock absorption to the knee joint.The menisci are best examined on T1 coronal and sagittal images, where it is important to assess their contours, position and intensity.
On a T1 sagittal image, we can see that the menisci are uniformly low in signal, normally appearing as a black bow tie-shaped structure in the joint cavity. On an axial image however, we see the medial and lateral meniscus as black semicircular structures just underneath the medial and lateral femoral condyles, respectively. We should look out for any increased intensity of the signal in the menisci. A hyperintense signal that reaches the signal of the articular cartilage can point us towards a meniscal tear or degeneration.
The medial collateral ligament (MCL) is best examined on a T2 coronal image, where it is seen as a long, thin uniformly low-intensity band on the medial side of the joint capsule. It originates from the medial femoral epicondyle and inserts into the medial metaphysis of the tibia.
On a slice through the intercondylar notch in the axial plane, we can find the origin point of the MCL, appearing as a short low signal band near the medial side of the femoral condyle. By scrolling downwards, we can follow the MCL to its attachment onto the tibia.
Like the medial collateral ligament, the lateral collateral ligament (LCL) is best seen on a T2 coronal image, appearing as a homogenous low-intensity structure on the lateral side of the joint capsule. The LCL originates from the lateral femoral epicondyle and inserts onto the fibular head together with the biceps femoris tendon.
If we look at the LCL in the axial plane on a cut through the intercondylar notch, we can see its origin as an ovoid low signal near the posterior aspect of the femoral condyle. By scrolling downwards, we can follow the LCL as it inserts onto the anterolateral aspect of the fibular head.
If you want to see more MRI images of the menisci, check out the following study units.
Last but not least, we will assess an important compound called the extensor mechanism. The extensor mechanism is made up of the major knee structures that cross the joint anteriorly which are the quadriceps tendon, patellar ligament and medial and lateral patellar retinacula. These tissues act passively when the quadriceps muscle contracts during extension of the knee.
As mentioned earlier, the patella is tightly embedded in the quadriceps tendon, which is a thick tendon that extends from the quadriceps femoris muscle. The quadriceps tendon is best visualized in the sagittal plane as a uniform low signal intensity band that contains 3-4 layers, with intervening higher signals of fat tissue that separate each layer.
The patellar ligament is a distal continuation of the quadriceps tendon, that begins on the patellar apex and inserts on the tibial tuberosity. We can see the patellar ligament on a slice through the tibial plateaus in the axial plane, appearing as a dark homogeneous low signal band covering the patella. A large collection of adipose tissue, called the infrapatellar fat pad, can also be seen on this level, found deep to the patellar ligament in the anterior part of the knee joint.
The medial and lateral patellar retinacula are best visualized in the axial plane on a cut through the patellofemoral joint. The medial and lateral patellar retinaculum are extensions of the vastus medialis and lateralis muscles, respectively, that connect them to the margins of the patella.
The quadriceps and patellar ligaments need to be examined for signs of tendinopathy and tears, which would appear as an increased white signal or discontinuity in the substance of the ligaments.