The synovial fluid is a viscid fluid secreted by the synovium. The knee synovial fluid is commonly depicted on MR imaging, but a definitive criterion that enables to define the quantity of the effusion as normal or pathological has not been clearly established. Clinically, knee effusions of less than 6–8 mL cannot be appreciated. However, on MR imaging, a volume of 1 mL of effusion may be visible adjacent to the femoral condyles [13]. Some authors demonstrated that a volume of 4 mL of fluid is depicted on plain radiography and, on midline sagittal MR images, leads to an anteroposterior diameter of the midline suprapatellar recess of 4 mm [13, 14]. This volume of effusion was considered to be clinically important [15]. With increasing volumes of effusion, the fluid first collects in the suprapatellar bursa and subsequently in the posterior recesses and popliteal tendon sheath [13].

Although the knee effusion may be the only finding on MR imaging, most commonly the synovial fluid is the result of a different underlying pathology (e.g., inflammatory diseases, trauma, degenerative changes, tumors). On MR imaging, the joint fluid appears homogeneously hypointense or of intermediate signal intensity on T1-weighted images and hyperintense on T2-weighted images.

Hemarthrosis is the result of the hemorrhage within the joint due to ligamentous injury, bone fracture, patellar dislocation, or other diseases including pigmented villonodular synovitis, hemophilia, articular tumors, neuroarthropathy, gout, and anticoagulant therapy [12, 16, 17]. The MRI appearance of hemarthrosis depends on the stage of the hemorrhage. In the early stage, the hemorrhage has a double-layer fluid level appearance with a superior layer of blood serum floating on an inferior layer of cellular debris of intermediate signal intensity on T1- and T2-weighted images. Gradient-echo images are especially useful for the identification of blood products due to the “blooming” effect.

Lipohemarthrosis, the presence of blood and fat within the joint, is a very strong indicator for intraarticular fracture. Most commonly, lipohemarthrosis is seen on MR imaging as a three-layer fluid level that appears approximately 3 h after trauma (Fig. 8.17) [18]. The superior layer with fat signal intensity (hyperintense on T1- and T2-weighted images and hypointense on fat-suppressed images) represents the floating fat. The intermediate or the central layer appears as a normal joint fluid and represents the blood serum, and the inferior layer of cellular debris is seen as intermediate signal intensity on T1- and T2-weighted images. In the very early stage, lipohemarthrosis may show a double-layer appearance with entrapment of globules of fat [18].

In rheumatological disorders (rheumatoid arthritis, psoriatic arthritis, and ankylosing spondylitis), the synovium is affected first, followed by involvement of the cartilage and bones. MR imaging enables the diagnosis of all intra-articular pathological changes including synovitis and joint effusion, bone marrow edema, and subchondral erosions (Fig. 8.20). There are different MRI techniques used in the evaluation of the synovial proliferation that are used either for qualitative or quantitative evaluation. Synovitis is best identified on T2-weighted images and postcontrast T1-weighted images as uniform or irregular thickening of the synovial membrane. Different quantitative evaluations of synovitis have been described including the synovial volume measurement and the rate of contrast enhancement. Dynamic contrast-enhanced T1-weighted sequences may differentiate between active and chronic phases of the disease with a rapid synovial enhancement (30–60 s) after contrast administration in active or acute phases [23, 24]. Nevertheless, synovial volume or enhancement rate measurements require post-processing work, which is time consuming in clinical practice and thus rarely used [12]. Apart from demonstrating the synovial proliferation, MRI demonstrates periarticular inflammation and tendon and ligament inflammation as well as complications such as ruptures of the tendons, presence of “rice-bodies” bursitis, osteonecrosis, and stress fractures [25, 26].

Although rheumatoid arthritis and the seronegative spondyloarthropathies share similar pathological changes, several findings can narrow the differential diagnosis [12]. In rheumatoid arthritis, there are symmetrical abnormalities, and the extent of subchondral erosions and the degree of synovial inflammation are more prominent. However, intra-articular rheumatoid nodules are rarely seen [27]. The rheumatoid nodules are solitary or multiple nodules that appear as inhomogeneous isointense or hypointense lesions on T1-weighted images and inhomogeneous isointense or hyperintense on T2-weighted images compared with muscles (Fig. 8.24). They may enhance homogeneously or heterogeneous predominantly in the periphery. In ankylosing spondylitis and other seronegative spondyloarthropathies, the knee joint is involved in 30 % of the patients with long-standing disease, and the bone marrow edema has a perientheseal distribution that is not commonly seen in patients with rheumatoid arthritis [12, 28]. The osseous fusion is also more common in seronegative spondyloarthropathies. Overall, however, differential diagnosis remains difficult on MR images, and in the clinical routine, one might not be disappointed when a final diagnosis cannot be made with 100 % confidence.

Synovial proliferation and joint effusion in osteoarthritis is the result of ligament injuries, loose bodies, cartilage deterioration, and meniscal damage (Fig. 8.25) [29, 30]. Synovitis is present even in the early phase of osteoarthritis, and there is evidence that synovial proliferation is not only a secondary phenomenon but that it also plays a role in progression of cartilage loss [29]. Synovial inflammation is believed to contribute to pain in patients with knee osteoarthritis, and several studies have indeed shown that high-grade synovitis in patients with knee osteoarthritis is statistically associated with pain [29, 31].

The imaging findings in infectious synovitis are nonspecific, and MRI cannot differentiate septic from nonseptic arthritis. The clinical history is key for differential diagnosis. Early detection of infectious synovitis is crucial in the treatment management and in preventing bone deformities. Thus, if there is clinical suspicion for infection, MR should be performed immediately and any destructive monoarticular process should be regarded as infection until proven otherwise [12]. There are several MRI findings that suggest an infectious etiology. These include synovial thickening, soft tissue edema, and extensive bone marrow edema with enhancement involving both sides of articulation and bone erosions (Fig. 8.26) [32, 33]. Inhomogeneous joint effusion with septa and debris or the presence of synovial or bone abscess is highly correlated with septic arthritis (Fig. 8.26). The evolution and the variation of the imaging findings can help narrowing down the differential diagnosis. In bacterial arthritis, there is a rapid evolution with extensive erosions, whereas in tuberculosis, the evolution is slow and the granulation inflammatory tissue extends in the subchondral bone (pannus) [12].