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Chapter 3 Diagnostic Imaging Techniques

Manual of Rheumatology and Outpatient Orthopedic Disorders

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Chapter 3 Diagnostic Imaging Techniques

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Robert Schneider

Imaging techniques
	Plain roentgenography
	Fluoroscopy
	Tomography
	Radionuclide scanning
	Computed tomography (CT)
	Magnetic resonance imaging (MRI)
	Ultrasonography
Invasive imaging methods
	Arthrography

Numerous diagnostic imaging techniques may be used to supplement history, physical examination, and laboratory tests in the evaluation of bone and joint disease. Figure 3-1 illustrates some of these techniques as they were used in the workup of a patient with hip pain who was found to have septic arthritis. The decision regarding which imaging technique to use and in what sequence depends on the sensitivity and specificity of the technique for a particular problem and on the availability, cost, and risk of the technique and experience in its use. Providing clinical information when ordering an imaging examination will help the radiologist or technologist to tailor the examination to the problem under investigation. The goal is to make a confident diagnosis in the shortest time at the least cost and risk to the patient. For example, magnetic resonance imaging (MRI) has been shown to be the best method of detecting or ruling out hip fractures when radiographic findings are negative.

FIG. 3-1. Septic arthritis of the hip. A: Anteroposterior radiograph of the right hip shows marked narrowing of the joint space, indicating destruction of the articular cartilage. B: Dynamic flow scan shows increased vascularity in the right hip. C: Blood-pool scan shows increasing vascularity in the right hip. D: Delayed static image shows increased uptake in the right hip. E: Coronal T ₁ -weighted (TR/TE MSC 500/12) image shows a decreased signal in the right femoral head and neck and acetabulum. F: Coronal T ₂ -weighted (TR/TE MSCE 2000/80) image with fat suppression by chemical shift technique shows increased signal in the femoral head and neck and acetabulum caused by bone marrow edema, in the joint capsule caused by synovial effusion, and in the soft tissues caused by inflammation and edema. G: Hip arthrogram shows contrast material in the joint with irregularity of the joint capsule, indicating synovitis.

I. Imaging techniques

1. Plain roentgenography is usually the initial diagnostic imaging method in the evaluation of bone and joint pain. It provides excellent detail of bony anatomy and abnormalities. Structures other than bone, including cartilage, muscle, ligaments, tendons, and synovial fluid, all appear to have the same soft- tissue density on roentgenography, which makes evaluation of abnormalities of these tissues difficult unless fat or calcification is present. Cartilage destruction can be diagnosed if joint space narrowing is present (see Fig. 3-1). Synovitis may be detected in the knee, elbow , and ankle because of the displacement of adjacent fat pads, but it cannot be reliably detected in the hip and shoulder. Plain roentgenography is readily available and of relatively low cost. It is specific for the diagnosis of bony lesions, such as fractures, neoplasms, and osteomyelitis, but it is not as sensitive as other imaging techniques, such as radionuclide bone scanning and MRI, for the early diagnosis of these abnormalities.
2. Fluoroscopy may be used to determine position during surgical procedures (e.g., internal fixation of fractures and osteotomies); invasive radiologic procedures (e.g., myelography); injections of nerve root, facet, and epidural spine; percutaneous needle biopsy; diskography; and arthrography. Fluoroscopy can also be used for the evaluation of motion.

   Care must be taken to limit fluoroscopic time to avoid excessive radiation exposure. Video disk and videotape recording may help reduce fluoroscopic radiation exposure.

3. Tomography supplements plain roentgenograms and provides better detail by blurring out areas above and below the plane of interest. Tomography has been replaced in most instances by computed tomography (CT) and MRI. Tomography is still occasionally used for detecting subtle fractures and evaluating spine fusions for pseudoarthrosis.
4. Radionuclide scanning
   1. Bone scanning with use of technetium 99m phosphate complexes has been used most frequently in the evaluation of metastatic disease to the skeleton and has largely replaced routine roentgenographic skeletal surveys for this purpose. It is also used for the evaluation of benign bone disease, as abnormalities may be detected that are not visible on roentgenograms. Bone scanning detects physiologic changes in the bone, in comparison with the anatomic changes seen on roentgenograms. An increased uptake of radionuclide reflects increased blood flow to bone and increased osteoblastic activity associated with new bone formation. This can result from numerous causes, including infection, tumor, fractures, or synovitis. Thus, although bone scanning is sensitive in detecting abnormalities of the bones and joints, it is not specific. Bone scanning is indicated when bone or joint pain is present and roentgenographic findings are negative or inconclusive. It is useful in diagnosing early osteomyelitis, stress fractures, nondisplaced traumatic fractures, avascular necrosis, and metastatic disease as a cause of undiagnosed pain. Single-photon emission computed tomography (SPECT) may provide increased detail and can be helpful in diagnosing stress or traumatic spondylolysis and in detecting photopenic areas in avascular necrosis. Three-phase bone scanning, which includes blood flow and blood pool scans , as well as static images 2 to 4 hours or more after injection, should be ordered for the evaluation of localized bone or joint pain (see Fig. 3-1B, Fig. 3-1C, Fig. 3-1D). The early phases show increased vascularity, which may be helpful in diagnosing synovitis, infection, and soft-tissue abnormalities. The radiation exposure from a bone scan is similar to that from a roentgenographic series of the lumbar spine. MRI has a similar and in some cases better sensitivity than radionuclide bone scanning for early diagnosis of many bony and joint problems and in most cases has a better specificity. However, bone scanning is less expensive and has the advantage of being able to survey the entire skeleton during one examination.
   2. Radionuclide infection scanning
      1. 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.
      2. 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.
5. 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.
6. 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 ₁ -weighted [short echo time (TE), short repetition time (TR)] and T ₂ -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 T ₂ -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 T ₂ -weighted images (see Fig. 3-1E, Fig. 3-1F). 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:
      1. Evaluation of internal derangement of the knee (meniscal tears; cruciate, collateral , and quadriceps mechanism tears; bone contusion).
      2. Osteonecrosis.
      3. Rotator cuff tears and glenohumeral instability.
      4. Tendon, ligament, and muscle tears and other abnormalities.
      5. Back pain.
      6. Evaluation of brain and spinal cord.
      7. Evaluation of bone and soft-tissue tumors.
      8. 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 T ₁ 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: T ₁ -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: T ₁ -weighted axial image at the same level following contrast enhancement surrounding the right S-1 root, consistent with scar formation.

      
      
      
7. 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.

II. Invasive imaging methods.

1. Arthrography involves the intraarticular injection of contrast agent. The use of water-soluble contrast material with or without air allows the evaluation of structures in and around joints, such as cartilage, synovium, and ligaments. Injection of air alone may be used to detect loose bodies. Injection of contrast material confirms the intraarticular position of a needle placed in the joint for aspiration when no fluid is obtained. An allergic reaction to the contrast material used in arthrography can occur as the contrast material in the blood is absorbed by the synovium, but anaphylactic reaction is extremely rare. Infection after arthrography is also a rare complication. A vasovagal reaction may occur during arthrography and should not be mistaken for an allergic reaction. Knee and shoulder arthrography has been largely replaced by MRI but may be still used in patients for whom MRI is contraindicated or in patients who cannot tolerate MRI because of claustrophobia. Arthrography may be used in conjunction with MRI. Intraarticular injection of gadolinium may be used in some cases in the shoulder to evaluate the joint capsule and labrum in patients with instability, in the hip to detect labral tears, and in the wrist to detect ligament tears.
   1. Hip arthrography is used in children to evaluate the position of the cartilaginous femoral head with respect to the acetabulum in Legg-Perthes disease, developmental dysplasia of the hip, and coxa vara deformities. In adults, it is used to evaluate painful hip prostheses and the joint capsule after aspiration.

Bibliography

Berquist TH, ed. MRI of the musculoskeletal system, 3rd ed. New York: Raven Press, 1996.

Chan TW, Listerud J, Kressel HY. Combined chemical shift and phase selective imaging for fat suppression: theory and initial clinical experience. Radiology 1991;181(1):41.

Collier DB, Fogelman I, Rosenthall L, eds. Skeletal nuclear medicine. St. Louis: Mosby, 1996.

Formage B, ed. Musculoskeletal ultrasound. New York: Churchill Livingstone, 1995.

Glickstein MF, Sussman SK. Time-dependent scan enhancement in magnetic resonance imaging of the post- operative lumbar spine. Skeletal Radiol 1991;20(5):333.

Resnick D, ed. Diagnosis of bone and joint disorders, 3rd ed. Philadelphia: WB Saunders, 1995.

Shellock FG, Curtis JS. MR imaging and biomedical implants, materials, and devices: an updated review. Radiology 1991;180(2):541.

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Copyright 2000 by Lippincott Williams & Wilkins
Stephen A. Paget, M.D., Allan Gibofsky, M.D., J.D. and John F. Beary, III, M.D.
Manual of Rheumatology and Outpatient Orthopedic Disorders

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