Leanne L. Seeger
There are several modalities available for the imaging evaluation of sports injuries. The strengths and weaknesses of each modality, along with their specific indications are discussed in this chapter.
The choice of the imaging modality depends on several factors, including the anatomic location of interest, chronicity of the symptoms, suspected pathology, and potential treatment alternatives. Guidelines for “what to use when” can be found on the American College of Radiology Web site “ACR Appropriateness Criteria” under Musculoskeletal Imaging at: www.acr.org/SecondaryMainMenuCategories/quality_safety/app_criteria.aspx.
Imaging tools that are commonly available are plain radiography (with or without applied stress), conventional arthrography, magnetic resonance imaging (MRI), which may be combined with arthrography, computed tomography (CT), which may be combined with arthrography, ultrasonography (US), and radionuclide bone scans. Over the past several years, US has gained substantial popularity in sports medicine imaging. Radionuclide bone scans have diminished in popularity, having been largely replaced by MRI.
MODALITY STRENGTHS AND WEAKNESSES
Plain radiography is widely available, relatively inexpensive, and provides excellent detail of bony structures and soft tissue calcifications. Although resolution has improved with digital radiography, the ability of radiography to depict soft tissue pathology remains far inferior to cross-sectional imaging (MRI, CT, and US).
Stress radiography (e.g., varus or valgus) reveals abnormal laxity of joints and can indirectly diagnose soft tissue injury. Stress must be applied by the referring physician. Disadvantages include availability of the physician, radiation exposure, and subjectivity of the amount of stress needed. In some cases, it may exacerbate underlying pathology. For subtle cases, comparison with the contralateral side may be needed. Consequently, indirect evaluation of soft tissue injury with stress radiography has been replaced by MRI and US.
Arthrography delineates the synovial space and intraarticular structures by joint distention. Although invasive, there are few inherent risks. Arthrography requires patient preparation and cooperation, informed consent, and the availability of a radiologist. Prior to the advent of cross-sectional imaging (MRI and CT), conventional arthrography was popular, consisting of a fluoroscopically guided injection of iodinated contrast and/or air, followed by spot imaging during provocative maneuvering. This procedure is now rarely done in isolation, having been supplanted with injection of dilute gadolinium for magnetic resonance arthrography or, less commonly, dilute iodinated contrast for CT arthrography when there is a contraindication to MRI (e.g., cardiac pacemaker). For both of these procedures, coordination is needed for scheduling scanner time to immediately follow the procedure.
MRI provides unparalleled soft tissue and bone marrow contrast. Soft tissue, marrow, and/or even periosteal edema is readily seen, even when subtle. MRI will demonstrate an early stress reaction of bone when plain radiographs and CT are normal. Structural abnormalities depicted with MRI include ligament and tendon tears, as well as the amount of retraction. There are several relative and absolute contraindications to MRI, including claustrophobia, cardiac pacemakers, certain kinds of neurosurgical aneurysmal clips, and inner ear implants. Although there is a plethora of pulse sequences that can be used with MRI, four are the mainstay of musculoskeletal imaging; T1, proton density (PD), T2, and inversion recovery (IR). T1, PD, and T2 sequences fall under the category of “spin echo” imaging. T1-weighted imaging displays anatomy. Fat (including fatty bone marrow) appears with high signal intensity, or bright; muscles appear with intermediate signal intensity; and cortex, tendons, and ligaments appear with low signal intensity, or dark. T2-weighted (usually undertaken with fat suppression) and IR imaging highlights tissues with increased water content, displaying them as bright. By suppressing fat signal (turning it dark),
T2 and IR imaging obscures normal anatomy but highlights edema and tears of muscles, tendons, and ligaments. Hematomas and most tumors also appear primarily bright, while surrounding normal tissue is dark. PD imaging takes advantage of both of these techniques by demonstrating anatomy but highlighting certain types of pathology. A primary role for PD scans in musculoskeletal imaging is for detection of meniscal pathology. Another technique called gradient recalled echo imaging is useful for demonstrating blood products and has been shown useful in imaging glenoid and acetabular labral pathology. Diffusion tensor imaging (DTI) is a newer technique that is commonly used in neuroimaging. In combination with the previously mentioned methods, DTI is proving to be an effective tool for assessing the severity of cerebral concussion, including minor head injury.
CT is superior to other modalities for fine bone detail and is an important tool for depicting the anatomy of complex fractures such as around the knee, hip, elbow, and shoulder. Newer generation multislice scanners have significantly diminished acquisition time and artifact and are capable of submillimeter slice thickness. CT is often used as a surrogate for MRI, in cases where MRI is contraindicated. Volumetric image acquisition enables rapid data reformation into not only standard axial, sagittal, and coronal planes, but also any obliquity desired to optimize depiction of anatomy. Reconstruction artifact commonly encountered with older scanners is far less of a problem with newer generation scanners. Disadvantages of CT are limited soft tissue contrast and radiation dose. To minimize radiation, scanning should be coned as tightly as possible to the area of interest.
Although popular in Europe and Asia for over three decades, musculoskeletal US was slow to be widely accepted in the United States. However, that has dramatically changed. Portable US equipment is now common at sporting events to provide a rapid assessment of injury severity and clearly competes with MRI in evaluation of muscle, tendon, and ligament injuries. One of the strengths of US lies in its ability to acquire dynamic images, depicting soft tissue structures while in motion. A normal control is readily available by acquiring images from the contralateral side. There is direct patient contact of the sonographer, facilitating immediate customization of the exam to the patient’s symptoms. The major disadvantage is that US is strongly operator dependent, requiring intense training with extensive hands-on experience for competency. US does not provide adequate resolution of intraarticular structures.
Radionuclide bone scanning with technetium-99m methylene diphosphonate (Tc-99m MDP) is extremely sensitive for detecting areas of increased bone turnover. However, it is nonspecific, and traumatic lesions cannot be differentiated from inflammation or neoplasia. Correlation with plain films is usually needed. Bone scans also have poor spatial resolution, which may be improved by either obtaining oblique projections or using single photon emission computed tomography (SPECT). SPECT imaging produces multiplanar tomographic slices similar to CT and MRI, allowing precise localization of foci of abnormal tracer activity in complex structures such as the spine. The triple-phase (three-phase) bone scan includes an angiogram acquired at the time of tracer injection, a blood-pool phase within 5 minutes of injection, and 3-hour delayed (static) images. This technique assists in differentiating soft tissue from bone pathology. Soft tissue abnormalities generally show preferential uptake on the first two phases, whereas bone pathology should show increased activity on all three phases. For single- or triple-phase bone scans, pinhole collimation should be used for small parts such as feet and hands to provide magnification and increased spatial resolution. Over the past years, MRI has largely replaced radionuclide bone scans.
SPECIFIC USES OF DIFFERENT MODALITIES
Radiography should usually be used for the initial assessment of an acute traumatic event to assess for fracture and/or alignment abnormality. In the case of chronic disorders, radiographs can eliminate alternate diagnoses, such as arthritis or neoplasia. Radiography is also the standard method for following fracture healing and alignment abnormalities (subluxation or dislocation).
Stress radiography may find its niche in cases of chronic trauma with instability and suspected soft tissue injury. It has, however, been widely replaced by MRI and/or US.
Arthrography is used when joint distention is required for lesion detection. This may include cases of cartilage injury, labral or meniscal pathology, capsular tears, and intraarticular loose bodies.
MRI is used for suspected bone or soft tissue injury, especially when plain radiographs are normal. Indications for magnetic resonance arthrography include glenoid or acetabular labral tears, low-grade superior labrum anterior-posterior
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