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IMAGING TECHNIQUES IN ORTHOPAEDICS
Use of radiologic techniques differs in evaluating the presence, type, and extent of various bone, joint, and soft-tissue abnormalities. Therefore, the radiologist and orthopedic surgeon must know the indications for use of each technique, the limitations of a particular modality, and the appropriate imaging approaches for abnormalities at specific sites. The question, “What modality should I use for this particular problem?” is frequently asked by radiologists and orthopedic surgeons alike, and although numerous algorithms are available to evaluate various problems at different anatomic sites, the answer cannot always be clearly stated. The choice of techniques for imaging bone and soft-tissue abnormalities is dictated not only by clinical presentation but also by equipment availability,
expertise, and cost. Restrictions may also be imposed by the needs of individual patients. For example, allergy to ionic or nonionic iodinated contrast agents may preclude the use of arthrography; the presence of a pacemaker would preclude the use of magnetic resonance imaging (MRI); physiologic states, such as pregnancy, preclude the use of ionized radiation, favoring, for instance, ultrasound. Time and cost consideration should discourage redundant studies. No matter what ancillary technique is used, conventional radiograph should be available for comparison. Most of the time, the choice of imaging technique is dictated by the type of suspected abnormality. For instance, if osteonecrosis is suspected after obtaining conventional radiographs, the next examination should be MRI, which detects necrotic changes in bone long before radiographs, tomography, computed
tomography (CT), or scintigraphy become positive. In evaluation of internal derangement of the knee, conventional radiographs should be obtained first and, if the abnormality is not obvious, should again be followed-up by MRI, because this modality provides exquisite contrast resolution of the bone marrow, articular cartilage, ligaments, menisci, and soft tissues. MRI and arthrography are currently the most effective procedures for evaluation of rotator cuff abnormalities, particularly when a partial or complete tear is suspected. Although ultrasonography can also detect a rotator cuff tear, its low sensitivity (68%) and low specificity (75% to 84%) make it a less definitive diagnostic
procedure. In evaluating a painful wrist, conventional radiographs and trispiral tomography should precede use of more sophisticated techniques, such as arthrotomography or CT–arthrography. MRI may also be performed; however, its sensitivity and specificity in detecting abnormalities of
triangular fibrocartilage and various intercarpal ligaments is slightly lower than that of CT arthrotomography, particularly if a three-compartment injection is used. If carpal tunnel syndrome is suspected, MRI is preferred because it provides a high-contrast difference among muscles, tendons, ligaments, and nerves. Similarly, if osteonecrosis of carpal bones is suspected and the conventional radiographs are normal, MRI would be the method of choice to demonstrate this abnormality. In evaluation of fractures and fracture healing of carpal bones, trispiral tomography and CT are the procedures of choice, preferred over MRI, because of the high degree of spatial resolution. In diagnosing bone tumors, conventional radiography and tomography are still the gold standard for diagnostic purposes. However, to evaluate the intraosseous and soft-tissue extension of tumor, they should be followed by either CT scan or MRI, with the latter modality being more accurate. To evaluate the results of radiotherapy and chemotherapy of malignant tumors, dynamic MRI using gadopentetate dimeglumine (Gd-DTPA) as a contrast enhancement is far superior to scintigraphy,
CT, or even plain MRI.
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