79.- Magnetic resonance imaging of articular abnormalities
This presentation has dealt with the analysis of a variety of articular abnormalities utilizing magnetic resonance imaging. Anatomic and technical considerations were evaluated, and then a discussion followed regarding specific abnormalities and specific articular diseases.
This presentation will be divided into three parts. First, we will deal briefly with some important anatomic considerations; second, we will deal with some specific abnormalities that involve synovium-lined articulations; and third, we will address some specific diseases in which magnetic resonance imaging may be helpful.
A typical synovium-lined articulation possesses two bones separated by a joint cavity. A fibrous capsule surrounds the joint cavil y continuous with the periosteal membrane that cloaks the tubular bones. The ends of the bones are covered by articular cartilage, usually hyaline cartilage. Marginal regions exist in the joint at which synovium is applied directly to bone without protective cartilage.
In some synovium-lined articulations, a fibrocaltilaginous disc divides the joint cavity partially or completely i nto two parts. Joints containing such a disc include the Knee, wrist, and temporomandibular, acromioclavicular, sternoclavicular and costovertebral articulations. In the knee, the meniscus serves as a cushion, protecting adjacent cartilage.
Various technical considerations involved in assessing joints utilizing magnetic resonance will be addressed in this presentation. However, at the outset, it should be recognized that a variety of pulse sequences are available for use. These include spin-echo, gradient echo, STIR, fat suppression, and volume acquisition techniques. Spin-echo acquisition has been used most frequently and is particularly good at demonstrating abnormalities within the bone marrow, cortical violation (in a indirect manner), intra-articular menisci, and intra-articular and periarticular ligaments and tendons. However, spin-echo techniques generally are not useful in the evaluation of articular cartilage.
We may utilize a paramagnetic contrast agent, gadolinium, when applying magnetic resonance imaging. The gadolinium can be delivered to the human body either by an intravenous route or an intra-articular route. Intravenous gadoliniumis particularly useful in the assessment of synovial inflammation in patients with rheumatoid arthritis or other related diseases. It allows differentiation of pannus from joint fluid, a differentiation that otherwise is difficult. Intra-articular gadolinium should be applied only in limited circumstances. Such circumstances include the evaluation of partial tears of the rotator cuff and the evaluation of the menisci following previous meniscal surgery.
A joint effusion represents a non-specific response to a variety of articular insults. Routine radiography is relatively insensitive in the detection of early effusions. For example in the knee, it requires approximately 5 ml before routine radiographs become positive. Magnetic resonance imaging allows us to assess small amounts of fluid in the joint, and in some locations, collections of less than 1 ml can be appreciated.
A hemarthrosis represents blood in the joint. Causes of hemarthroses include injury, hemophilia and other bleeding disorders, pigmented villonodular synovitis, synovial hemangioma and other neoplasms, and neuroarthropathy. With routine radiography, there are no specific findings of a hemarthrosis. With magnetic resonance imaging, however, hemosiderin deposition may be seen.
A lipohemarthrosis represents a combination of fat and blood within the joint. With routine radiography, cross table technique allows detection of a characteristic fat-fluid level. Similarly, magnetic resonance imaging may allow evaluation of a lipohemarthrosis, commonly demonstrating more than one fluid-fluid level. A lipohemarthrosis is a common finding following an injury in which a fracture has taken place.
A synovial cyst represents a collection of fluid adjacent to a joint, general y present owing to an increase of intra-articular pressure. Ultrasonography, arthrography, and magnetic resonance imaging have been employed to evaluate synovialcysts. Of these, magnetic resonance imaging appears to be best. Furthermore, a variety of para-articular cysts have been identified with magnetic resonance. These include meniscal cysts about the knee, non communicating synovial cysts, and ganglia. Two important sites of intra-articular ganglia are the region about the cruciate ligaments of the knee, and the suprascapular notch of the shoulder.
Synovial plica represent remnants of folds normally present during development that persist and may even enlarge. In the knee, there are three plica and the medial plica are encountered frequently. Both of these may become symptomatic. Both are well demonstrated with magnetic resonance.
Bursitis and tenosynovitis can be recognized very easily utilizing magnetic resonance. Examples include rheumatoid involvement of the retrocalcaneal bursa, prepatellar bursa, and subacromial-subdeltoid bursa.
The evaluation of articular cartilage has been attempted utilizing a variety of magnetic resonance sequences. In general, most of these are inadequate. Part of the problem relates to the complexity of articular cartilage, which is not a uniform structure with regard to its cellularity or collagen fiber composition. Previous descriptions utilizing magnetic resonance have indicated that the articular cartilage may have a homogeneous appearance, or contain two or there different layers.
Spin-echo technique generally is inadequate in the evaluation of articular cartilage. Although there are some advantages of T1-weighted images (including anatomic detail and high contrast between cartilage and bone and T2-weighted images (positive arthrogram like effect and the ability to see internal signal within abnormal cartilage), both are in adequate in the evaluation of articular cartilage. Spin-echo technique combined with fat suppression has the added advantage of improving contrast between cartilage and fluid. However, it too is not adequate.
Gradient echo sequences have been utilized to study articular cartilage. These include both steady-state and spoiled gradient echo techniques. Many of the gradient echo techniques have the advantage or providing an arthrogram-like effect with high contrast between cartilage and fluid. However, there are certain problems related to evaluation of articular cartilage with gradient Spin-echo technique generally is inadequate in the evaluation of articular cartilage. Although there are some advantages of T1-weighted images (including anatomic detail and high contrast between cartilage and bone and T2-weighted images (positive arthrogram like effect and the ability to see interna) signal within abnormal cartilage), both are in adequate in the evaluation of articular cartilage. Spin-echo technique combined with fat suppression of the added advantage of improving contrast between cartilage and fluid. However, it too is not adequate.
Gradient echo sequences have been utilized to study articular cartilage. These include both steady-state and spoiled gradient echo techniques. Many of the gradient echo techniques have the advantage or providing an arthrogram-like effect with high contrast between cartilge and fluid. However, there are certain problems related to evaluation of articular cartilage with gradient echo sequences, and these sequences demonstrate in creased sensitivity to artifacts, increased magnetic susceptibility, and increased signal of flowing blood. Gradient echo images combined with volumetric acquisition have the added advantage of increased spatial resolution owing to the ability to obtain thin contiguous slices. In addition, one can reformat the images in multiple planes.
Currently, the best available sequence appears to be a spoiled gradient echo sequence utilizing fat suppression and three dimensional acquisition. With such a sequence, spatial resolution in high, and there is high contrast to noise ratios between cartilage and fluid and between cartilage and bone. With this sequence, three layers within cartilage can be seen as well as a very deep layer of low signal consisting of calcified cartilage and subchondral bone plate. This SPGR technique can be used to evaluate cartilage degeneration, demonstrating loss of signal in a superficial bright layer and varying degrees of loss of signal in intermediate and deep layers within the cartilage.
MR arthrography is an invasive technique, but can be used effectively to study the surface of articular cartilage.
Osteochondritis dissecans represents an injury to the articular surface. It is frequently observed in the knee, particularly in the medial femoral condyle, in the talus, in the capitellum of the elbow, and in the posterior surface of the patella. Magnetic resonance imaging can be utilized to determine the extent of the lesion and, in some case, its stability. The latter depends upon defining the degree of cartilage irregularity as well as the presence of fluid between the lesion and the parent bone.
Intra-articular bodies can relate to a variety of articular insults. These bodies may consist of cartilage alone, cartilage and bone together, or rarely bone alone. Routine radiography and conventional tomography as well its computed tomography are useful techniques in the assessment of intra-articular bodies. Magnetic resonance imaging lacks sensitivity in this particular diagnosis.
Magnetic resonance imaging can be utilized to study a variety of subchondral bone abnormalities including edema, ichemia, fracture, tumor and infection.
Rheumatoid arthritis produces characteristic abnormalities that can be detected with magnetic resonance imaging. These include erosions of bone, subchondral cystic lesion, para-articular synovial cysts, and malalignment or subluxations, particularly in the cervical spine.
There are three basic crystal induced articular diseases, gout, pseudogout, andhidroxyapatite deposition disease. Of interest, in some cases, urate deposits lead to low signal intensity on T1-weighted spin-echo images and areas of low and high signal intensity on T2-weighted spin-echo images. Hydroxyapatite crystal deposition disease produces calcific tendonitis that is best recognized with routine radiography. Magnetic resonance imaging may overlook small areas of calcification.
A variety of tumors and tumors-like diseases occur in and around joints. These include pigmented villonodular synovitis, idiopathic synovia losteochondromatosis, lipoma, lipoma arborescens, chondroma, and chondrosarcoma. Each produces characteristic magnetic resonance imaging findings, which will be illustrated in this presentation■