MRI of bone and bone marrow in osteoarthritis

The cortical layer and trabeculae of the bone contain few protons of hydrogen and a lot of calcium, which greatly reduces TR, and therefore do not produce any specific MP signal. On MP tomograms, they have an image of curves with no signal, i.e. Dark stripes. They create a silhouette of medium-intensity and high-intensity tissues, outlining them, such as bone marrow and adipose tissue.

Pathology of bone due to osteoarthritis, includes the formation of osteophytes, subchondral bone sclerosis, the formation of subchondral cysts and swelling of the bone marrow. MRI because of its multiplanar tomographic capabilities is more sensitive than radiographic or computed tomography to visualize most of these types of changes. Osteophytes are also better imaged with MRI than with conventional radiography — especially central osteophytes, which are particularly difficult to detect radiographically. The reasons for the formation of central osteophytes are somewhat different than the regional ones, and therefore have a different meaning. Bone sclerosis is also well detected with MRI and has a low signal intensity in all pulse sequences, due to calcification and fibrosis. MRI can also detect inflammation of the enthesis and periostitis. High-resolution MRI is also the main MP technology for studying trabecular microarchitecture. This can be useful for monitoring trabecular changes in the subchondral bone in order to determine their significance in the development and progression of osteoarthritis.

MRI is a unique opportunity to get a bone marrow image and is usually a very sensitive, although not very specific, technology for detecting osteonecrosis, osteomyelitis, primary infiltration and injuries, especially bone contusion and fractures without displacement. Signs of these diseases on radiographs are not detected until the cortical and / or trabecular sections of the bone are affected. In each of these cases, the free water content increases, which has the form of a low-intensity signal on T1-VI and a high-intensity signal on T2-VI, showing high contrast with normal bone fat, having a high-intensity signal on T1-VI and a low signal on T2 -IN AND. The exception is T2-VI FSE (fast spin echo), in which the images of fat and water have a high-intensity signal and require fat suppression in order to obtain a contrast between these components. GE sequences, at least with great field strength, are mostly insensitive to bone marrow pathology, because the magnetic effects are extinguished by bone. Edema of the subchondral bone marrow is often visible in joints with progressive osteoarthritis. Usually, these areas of local bone marrow edema in osteoarthrosis develop in places of loss of articular cartilage or chondromalacia. Histologically, these areas are typical fibrovascular infiltration. They may be due to mechanical damage to the subchondral bone, caused by a change in the points of contact of the joint in places of biomechanically weak cartilage and / or loss of stability of the joint, or possibly due to leakage of synovial fluid through a defect in the exposed subchondral bone. Sometimes the epiphyseal edema of the bone marrow is visible at some distance from the articular surface or enthesis. It remains unclear what extent and prevalence of these bone marrow changes contribute to the occurrence of local pain and weakness of the joint and when they are precursors to the progression of the disease.

[1], [2], [3], [4], [5], [6], [7]

MRI of the synovial membrane and synovial fluid

The normal synovial membrane is generally too thin for imaging with conventional MRI sequences and is difficult to distinguish from the adjacent articular fluid or cartilage. In most cases, in osteoarthritis, there may be a slight increase in simonitoring response to treatment in patients with osteoarthritis or to study the normal physiological functioning of the synovial fluid in the joint in vivo, this technique is very useful.

The non-hemorrhagic synovial fluid MP signal has low intensity on T1-weighted images and high on T2-weighted images due to the presence of free water. Hemorrhagic synovial fluid may contain methemoglobin, which has a short T1 and gives a high-intensity signal on T1-VI, and / or deoxyhemoglobin, which has the form of a low-intensity signal on T2-VI. In chronic recurrent hemarthrosis, hemosiderin is deposited in the synovial membrane, which gives a low-intensity signal to T1 and T2-VI. Hemorrhages often develop in the popliteal cysts, they are located between the gastrocnemius and the soleus muscles on the back of the leg. The outflow of synovial fluid from a damaged Baker cyst may resemble the shape of a pen when it is enhanced by gadolinium-containing contrast agents. When intravenous CA is injected, it is located along the surface of the fascia between the muscles posterior to the joint capsule of the knee joint.

The inflamed, edematous synovial membrane usually has a slow T2, reflecting the high content of interstitial fluid (it has a high intensity MP signal on T2-VI). On T1-VI, synovial tissue thickening has a low- or medium-intensity MR signal. However, thickened synovial tissue is difficult to distinguish from the nearby synovial fluid or cartilage. The deposition of hemosiderin or chronic fibrosis can reduce the intensity of the signal of hyperplastic synovial tissue in images with a long TE (T2-VI) and sometimes even in images with a short TE (T1-VI; images weighted in proton density; in all GE sequences).

As noted earlier, the spacecraft exerts a paramagnetic effect on adjacent protons of water, causing their faster T1 relaxation. Aqueous containing tissues that have accumulated spacecraft (containing Gd chelate) show an increase in signal intensity at T1-VI in proportion to the concentration of accumulated spacecraft in the tissue. When administered intravenously, CA is rapidly distributed through hypervascularized tissues, such as inflamed synovial membrane. The gadolinium chelate complex has relatively small molecules that quickly diffuse inwards even through normal capillaries and, as a disadvantage, over time into the synovial fluid nearby. Immediately after the bolus injection of the spacecraft, the synovial membrane of the joint can be seen separately from other structures, as it is intensively strengthened. Contrast imaging of high-intensity synovial membrane and adjacent fatty tissue can be increased by the method of suppressing fat. The speed with which the contrast enhancement of the synovial membrane occurs depends on a number of factors, including: the speed of blood flow in synovia, the volume of hyperplastic synovial tissue and indicates the activity of the process.

In addition, determining the number and distribution of inflammatory synovial membrane and fluid in the joints in arthritis (and osteoarthrosis) provides an opportunity to establish the severity of synovitis by monitoring the rate of synovial enhancement with Gd-containing KA during the patient observation period. The high rate of synovial enhancement and the rapid achievement of peak gain following a bolus injection of CA belong to active inflammation or hyperplasia, while slower gain corresponds to chronic fibrosis of the synovial membrane. Although it is difficult to control the subtle differences in the pharmacokinetics of Gd-containing CA during MRI studies in different periods of the disease of the same patient, the speed and peak of synovial amplification can serve as criteria for prescribing or canceling the corresponding anti-inflammatory therapy. High rates of these parameters are characteristic of histologically active synovitis.

[8], [9], [10], [11], [12], [13], [14], [15], [16], [17]