a. Scanning with gallium citrate 67 shows increased uptake at sites of infection in the bones or soft tissues. It has a high sensitivity for infection but is nonspecific, as it may show increased uptake associated with other causes of increased bone turnover, including fractures or tumors, and also shows increased uptake in noninfectious inflammatory conditions, such as inflammatory arthritis. The specificity of a gallium scan for infection may be increased if it is compared with a bone scan. If the gallium scan shows more intense uptake than the bone scan at the affected site or if the uptake of gallium is not congruent with the uptake on the bone scan, then infection is likely. However, only one-third or fewer of bone infections meet these criteria. False-negative gallium scans may be seen in chronic infection or if the patient is treated with antibiotics before the scan is performed.
b. Scanning with indium 111- or technetium 99m-labeled leukocytes can detect bone or joint infection and is more specific than bone scanning or gallium scanning. However, uptake may also be seen in noninfectious conditions. Comparison with bone scans or scans performed with radiolabeled colloids can increase the specificity for infection.
E. Computed tomography (CT) provides better soft-tissue contrast than does roentgenography, allowing the evaluation of soft-tissue abnormalities that cannot be visualized on roentgenograms. CT provides axial sections for visualization of cross-sectional anatomy, which often facilitates the evaluation of abnormalities in the pelvis and spine. This is especially valuable in evaluating pelvic fractures and localizing osteoid osteomas. Sagittal and coronal images can be obtained by reformatting thin axial sections or images obtained by helical scanning. CT is useful in evaluating the extent of bony and soft-tissue tumors. It can be used to diagnose intervertebral disk herniation and spinal stenosis. CT performed after the injection of contrast material (e.g., after myelography, diskography, or arthrography) provides additional information in these studies. F. Magnetic resonance imaging (MRI) has the advantage of not using ionizing radiation. It provides multiplanar imaging capabilities without sacrificing image resolution.
1. Spin echo technique. The most commonly utilized imaging sequence is a multislice, multiecho spin echo technique, used with both T -,-weighted [short echo time (TE), short repetition time (TR)] and T2-weighted (long TR/TE) images (see Fig 3-1E, Fig 3-1F). Superior soft-tissue contrast is achieved by virtue of differential tissue relaxation times. Normal fatty bone marrow exhibits a bright signal intensity on T 1-weighted sequences, with a slightly less bright signal on T2-weighted sequences. Conversely, pathologic processes (infiltrative disease, infection, bone marrow edema) will exhibit a low signal on T 1-weighted sequences (see Fig, 3-.1E.). Both cortical bone and fibrous tissue (including normal ligaments and tendons) maintain a low signal intensity on all pulse sequences. Fluid (synovial fluid, edema, cysts) exhibits a low signal intensity on T 1-weighted sequences, and a markedly bright signal intensity on
T2-weighted images (see Fig 3-1E, F.i9..,. 3z1.F). Intermediate-weighted or proton density images (long TR/short TE) reduce the differences in contrast between different tissues but provide a higher resolution for the evaluation of morphology.
2. Gradient echo techniques. Additional soft-tissue contrast is achieved through variation in pulse sequences. Gradient echo imaging provides rapid image acquisition with improved soft-tissue contrast. This is extremely useful in the evaluation of articular cartilage. Gradient echo techniques are also advantageous in spine imaging; volumetrically acquired techniques allow for thin (>3 mm) slice acquisition within a relatively short time period. Such thin slices are essential in diagnosing subtle cervical disk disease.
3. Fat-suppressed techniques. Although visualized on spin echo images, bone marrow edema may be seen better with fat-suppressed techniques, such as short tau inversion recovery (STIR) and chemical shift techniques.
4. Indications for MRI include the following:
a. Evaluation of internal derangement of the knee (meniscal tears; cruciate, collateral, and quadriceps mechanism tears; bone contusion).
c. Rotator cuff tears and glenohumeral instability.
d. Tendon, ligament, and muscle tears and other abnormalities.
f. Evaluation of brain and spinal cord.
g. Evaluation of bone and soft-tissue tumors.
h. Assessment of occult fracture.
In many cases, MRI obviates the need for the more invasive arthrogram or myelogram.
5. Contraindications to MRI include the presence of pacemakers, aneurysm clips, some prosthetic otologic and ocular implants, and some bullet fragments. Clinical concern is increased when the metallic object is anatomically close to a vital vascular or neural structure. Most prosthetic heart valves are felt to be safe for MRI. In addition, most orthopedic materials and devices are considered safe, including stainless steel screws and wires. However, ferromagnetic metallic implants will cause image artifact, with large areas of signal void and adjacent high signal ("flare" response), which may interfere with accurate image interpretation. Prior knowledge of the specific type (manufacturer, material) of metallic implant is essential before the patient is exposed to a strong magnetic field.
6. Gadolinium diethylenetriamine pentaacetic acid (GD-DTPA) is an MRI contrast agent that, when used in typical doses (0.1 mmol/kg of body weight), acts primarily to shorten T1 relaxation times. Thus, regions that readily enhance with contrast will appear bright on T 1-weighted images. In the evaluation of the postoperative spine, contrast may help to distinguish scar from recurrent disk herniation ( Fig..3:2). Postoperative scar is felt to enhance with contrast by virtue of the rich vascularity of epidural granulation tissue. Conversely, the avascular adult disk will not demonstrate similar signal enhancement. A contrast-enhanced MRI examination performed long after surgery may not prove as reliable, as scar tissue may become progressively fibrotic, with less discernible contrast enhancement. Relative contraindications to GD-DTPA administration include hemolytic anemia, as the agent may promote extravascular hemolysis. Because GD-DTPA is cleared via glomerular filtration, caution should be utilized in patients with impaired renal function. The most common reported adverse reaction is mild headache (<10% of patients).
FIG. 3-2. A: T1-weighted (TR/TE MSCE 1000/12) axial section through the L5-S1 disk demonstrating right-sided laminectomy defect and abnormal intermediate signal intensity surrounding right S-1 root ( arrow). B: Trweighted axial image at the same level following contrast enhancement surrounding the right S-1 root, consistent with scar formation.
G. Ultrasonography may be used to evaluate soft-tissue masses and characterize them as either cystic or solid. Popliteal cysts can easily be detected. Tendons are more echogenic than muscle and can be evaluated for continuity and inflammation. Tenosynovitis can be detected as fluid in the tendon sheath. Ultrasonography has been used in the shoulder for evaluation of the rotator cuff tendons. Complete and partial tears and tendinopathy can be diagnosed. Tendons in most other parts of the body can be evaluated in a similar manner. Plantar fasciitis can be diagnosed by evaluating the thickness and appearance of the plantar fascia. Calcific tendinitis can be detected as focal areas of high echogenicity. Aspiration and injection of soft-tissue ganglia, calcific deposits, and tendon sheaths can be performed under ultrasound guidance. Foreign bodies in the soft tissue can be localized. Ultrasound is used for the evaluation of developmental dysplasia of the hip in infants to determine the position of the nonossified femoral head with respect to the acetabulum.
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