Musculoskeletal sonography and MR imaging. A role for both imaging methods.

Musculoskeletal sonography has recently experienced an increase in popularity for several reasons. Advances in technology including the advent of high-frequency transducers have markedly improved image resolution. Additionally, the relative low expense of sonography compared with MR imaging has made this an attractive alternative imaging method for many indications. Before it gains universal acceptance in evaluation of the musculoskeletal system, however, sonography must be able to produce results similar to those of MR imaging. MR imaging is essentially the standard of care for the evaluation of the musculoskeletal system at most centers across the nation. Research has proved MR imaging to be effective for many indications. Advanced technology has resulted in improved image resolution and shortened imaging times. Images may be acquired at all times of the day at various physical sites. Interpretation of images can also be accomplished promptly with data transfer to computer workstations. Each of these points has allowed MR imaging to become widely accepted for evaluating the musculoskeletal system. There exist several advantages of MR imaging over sonography. The primary advantage is relative lack of operator dependence. This is achieved through the use of standardized MR imaging protocols. Another advantage is the ability of MR imaging to evaluate globally and thoroughly an anatomic area including deep soft tissues, bone marrow, and joint cartilage with high sensitivity. MR images are also familiar to both the radiologist and referring physician and can be interpreted in a time- and cost-efficient manner. The utilization of sonography, on the other hand, is quite variable. Outside of Europe, sonography as the primary diagnostic tool for musculoskeletal imaging is limited to a relatively small number of hospitals or academic institutions across the country. Many radiologists not trained in musculoskeletal sonography may have inadequate time, energy, or resources to acquire a new skill. Others may find it difficult to convince referring physicians of the utility of musculoskeletal sonography. Clinical studies, however, are demonstrating the potential of sonography for several indications and interest in this imaging method continues to grow. Sonography does have several potential advantages over MR imaing. This includes its relative low cost, portability, and in most cases improved accessibility. Evaluation of a soft tissue process near metal orthopedic hardware is possible with sonography without the artifact that limits MR imaging. Additionally, sonography can immediately guide percutaneous procedures when an abnormality, such as a joint effusion, is identified. Sonography also allows a dynamic evaluation of joints detecting abnormalities that may not be present during MR imaging positioning. Lastly, the improved resolution of superficial structures demonstrates subtle abnormalities that may be difficult to visualize with MR imaging. Current ultrasound technology produces in-plane resolutions of 200 to 450 μm and section thicknesses of 0.5 to 1 mm. 11a In fact, the spatial resolution of sonography exceeds that obtainable with MR imaging without the use of small surface coils and appropriate imaging parameters. 11a Controversy exists when sonography and MR imaging are compared. Unlike the research results using MR imaging, those pertaining to sonography are usually more variable. Although this can be partially explained by the inherent operator dependence of this imaging method, the reader is nonetheless uncertain of sonography's true effectiveness. Additionally, there are relatively few blinded research studies that directly compare sonography with MR imaging. Many sonographic studies are limited to small subject groups without a gold standard. Additional research is needed to determine sonography's true effectiveness in evaluating the musculoskeletal system relative to MR imaging. It is obvious that MR imaging will remain the most common advanced imaging method of the musculoskeletal system until research demonstrates that sonography can produce similar results. It is clear, however, that there are several areas where musculoskeletal sonography has been proved effective. The scope of this article is not to explore all possible imaging capabilities with musculoskeletal sonography. Instead, only the most accepted indications are reviewed that may produce results at least equal to MR imaging. This includes evaluation for (1) shoulder abnormalities, such as rotator cuff tear, calcium hydroxyapatite deposition, and biceps brachii tendon subluxation; (2) ankle tendon tear and Achilles xanthoma; (3) soft tissue infection and joint effusion; (4) soft tissue foreign bodies; (5) soft tissue masses, such as Baker's cyst and wrist ganglion cyst; (6) carpal tunnel syndrome; (7) developmental dysplasia of the hip; and (8) other indications when MR imaging is contraindicated. Each example includes sonographic images with correlative MR imaging wherever possible. The latter is included to familiarize the sonographic imaging results with the more familiar MR imaging. Additionally, the advantages and disadvantages of each imaging method may then become more apparent.