Imaging of Joints: Techniques and Normal Anatomy

Imaging of Joints

TECHNIQUES AND NORMAL ANATOMY

Radiography

Conventional radiography is the most commonly used im-aging technique to evaluate the joints of the musculoskeletal system. This technique should always be the first imaging study performed in a patient suspected of having joint prob-lems. Radiography has the following important advantages: It is almost universally available, is relatively inexpensive compared to other imaging studies and delivers only a small radiation dose to the patient. When possible, orthogonal projections should be obtained, meaning two images of the joint that are perpendicular to each other (usually a frontal projection in either in the anteroposterior [AP] or pos-teroanterior [PA] directions and a lateral). In some instances oblique images may also be obtained, depending on the preferences of the referring physician or radiologist or the clinical situation. In certain instances it may also be impor-tant to obtain images of the joint proximal and distal to the injury. Examples of this include the forearm and lower leg (paired bones), as the joints proximal and distal are often in-jured. Because conventional radiography uses ionizing radi-ation, it should be used judiciously, especially in pediatric patients and pregnant women.

Historically, radiographic images were printed on film. However, with widespread adoption of PACS (picture archiv-ing and communications system), images can be electroni-cally processed and viewed on computer work screens. These images then can be transmitted anywhere electronically via the Internet.

Conventional Tomography

Conventional tomography is mentioned mainly for historical interest. High radiation dose, relatively poor image resolution, and imaging that is only possible in one plane were its major disadvantages. The technique has been almost totally re-placed by other imaging tests, especially computed tomo-graphic (CT) and magnetic resonance (MR) imaging. Orthopantograms are one of the few remaining vestiges of this imaging technique.

Arthrography

Arthrography is a technique in which contrast is injected into the joint using fluoroscopic guidance. The joint is then im-aged using radiography, CT, or MR imaging or a combina-tion of these techniques. The injected contrast may be an iodine-containing water-soluble compound (eg, Conray), subsequently imaged with radiography or CT (Figure 7-1). Alternatively, a paramagnetic compound (eg, gadolinium pentazocine) may be injected and imaged with MRI. MR arthrographic images of the joint may also be performed after intravenous injection of the paramagnetic contrast agent, although this technique does not distend the joint, and thereby is not used commonly today. MR arthrography is mainly used to evaluate the labrum of the hip or glenohumeral joint (Figure 7-2) but is also useful in the evaluation of the structures of the wrist and elbow joints. CT arthrography, and less commonly conventional arthrography can be useful in patients who cannot undergo, or have contraindications to, MR imaging (Figure 7-3).

Computed Tomography

Computed tomography (CT), a technique that makes indi-vidual axial (transverse) slices of the patient, uses the same ionizing radiation as in conventional radiography. CT technique has been vastly improved in the past decade. The development of spiral or helical CT has major advantages over earlier CT technology. With the spiral CT technique, axial (transverse) images are acquired much more rapidly with dramatic decreases in radiation dose. For instance, a CT of the chest, abdomen, and pelvis can be performed in about 16 seconds. CT data is stored in three-dimensional packets that can then be reconstructed and displayed in al-most any other plane. The most common images recon-structed from the axial plane are the sagittal and coronal planes (Figure 7-4).

Magnetic Resonance Imaging

MR imaging has revolutionized the imaging evaluation of almost all body areas, but particularly those of the central nervous system and musculoskeletal system. It has tremen-dous advantages over other imaging modalities in the eval-uation of joints because of its excellent soft-tissue contrast, high resolution, and ability to image in every plane. This technique may show pathophysiologic events even before they are seen on conventional radiographs or CT, for exam-ple, revealing the early changes of avascular necrosis (Figure 7-5). Because of its exquisite soft-tissue contrast, MR imag-ing allows radiologists to visualize subtle differences in soft tissues that had never before been seen with other imaging modalities. For example, the subtle contrast between fat and muscle seen on a conventional radiographs or CT is dra-matically highlighted with the use of MR imaging because of their very different chemical compositions (Figure 7-6). MR imaging can also depict subtle changes within the bone marrow cavity, an area difficult to evaluate with conven-tional radiography or CT. Therefore, MR imaging is a tremendous aid to preoperative evaluation of any patient who has unexplained joint pain or who has had joint trauma. One of the major disadvantages of MR imaging is that some patients with claustrophobia cannot tolerate the prolonged imaging time in the small bore of the magnet. In addition, patients with metallic foreign bodies or non-MR-compatible medical devices or hardware have contraindi-cations to MR imaging. Concerns include motion of the objects, abnormal electrical arcs resulting in burns, and de-vice malfunction.

Ultrasonography

Ultrasonography, first developed for use in World War II for detection of submarines, was adopted after the war for use in medical imaging. High-frequency transmission of sound can be used to evaluate the soft tissues, tendons, ligaments, and even the cartilage of the joint. The ultrasound waves cannot be transmitted through cortical bone, so the intramedullary cavity cannot be imaged with this technique. Ultrasound is used more extensively in Europe than in the United States; however, there is increasing interest in this modality within the United States. The main drawback of this modality is that it is highly user-dependent, relying heavily on the skill of the operator (Figure 7-7).

Radionuclide Imaging (Nuclear Medicine Bone Scans)

Radionuclide imaging uses radioactive materials that are injected intravenously and then localize in regions of ab-normally increased blood flow (hyperemia), increased os-teoblastic activity, or heightened metabolic activity. The major uses of bone scanning are in patients suspected of having metastatic disease or infection. This modality is very sensitive, but it has limited specificity, and often the findings must be correlated with other imaging modalities, especially radiography. Therefore, this technique is not usually used as a primary modality for the evaluation of joint disease.

Anatomy of the Normal Joint

The typical normal synovial joint consists of at least two ar-ticulating bones enclosed in a synovium-lined joint capsule. The apposing bony surfaces are covered by smooth articular cartilage (hyaline cartilage). On radiographs, the normal joint has a separation between the adjacent bones represent-ing the region occupied by the hyaline or articular cartilage, menisci, and joint fluid (the so-called articular space) de-pending on which joint is imaged. Because of the limited soft-tissue contrast of the technique, these structures are not normally depicted on radiographs unless they are calcified (Figure 7-8). However, MR imaging exquisitely shows the components of the normal joint (Figure 7-9).

Joint Disease

The clinical signs and symptoms of joint disease are mani-festations of abnormal function such as reduced mobility, hypermobility, and pain. Altered function may be due to pain, discomfort, apprehension, or instability. The wide range of joint abnormalities is summarized below, and many of these processes are discussed in the exercises. Any of these signs may occur in isolation or in combination with any others.

Radiographically, joint disease may be diagnosed by any of the following:

·      Incongruity of the articulating bone as is seen with dislo-cations, for example, traumatic dislocation or disloca-tions caused by arthropathies such as lupus arthritis or rheumatoid arthritis.

·      Irregularity of articulating bone surfaces and margins, as in erosions (eg, in psoriasis or gout).

·      Increased density or sclerosis of articulating bone surface (also called “eburnation”), as in osteoarthritis.

·              Bony outgrowths (proliferation) at bone ends, known as osteophytes.

·      Diffuse decrease of bone density adjacent to articular surfaces, described as juxtaarticular or periarticular os-teopenia (eg, rheumatoid arthritis, tuberculous arthritis).

·      Focal, well-defined, spherical lucencies within subchon-dral bone, known as subchondral cysts or geodes (eg, os-teoarthritis, rheumatoid arthritis).

·      Loss of articular joint space from articular cartilage de-struction (eg, septic arthritis, osteoarthritis).

·      Accumulation of excess joint fluid within the joint (joint effusion). Excess joint fluid is a common manifestation of joint disorders. The fluid may be synovial fluid, blood, or even pus, depending on the etiology of the joint disease.

·      Calcification of articular (hyaline) cartilage or fibrocarti-lage (chondrocalcinosis), or intraarticular soft-tissue cal-cification such as that seen in scleroderma or polymyositis or dermatomyositis.

·              Synovial proliferation or abnormal increase in the syn-ovial lining, such as that seen with pigmented villonodu-lar synovitis (PVNS).