Crosssectional imaging

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Computed tomography

CT provides images with high spatial and contrast resolution of bone and surrounding soft tissue, and exceptional cortical bony detail. It can provide a good definition of cortical bone destruction, periosteal reaction (Fig. 12), and soft tissue changes. postcontrast images are more useful for soft tissue abnormalities than for bony changes. Increased density of the medullary cavity can be seen replacing the normal low-density fatty marrow, but this finding is nonspecific and may be seen not only in infections but also in neoplasms, hemorrhage, fractures, or irradiation [68]. It is the best method of detection of small foci of gas within the medullary canal, an infrequent but reliable diagnostic sign of osteomyelitis [69]; areas of cortical erosion or destruction; tiny foreign bodies serving as a nidus for infection; and involucrum and sequestration formation [69-72].

The primary applications of CT to the evaluation of infections of the musculoskeletal system are the delineation of the osseous and soft tissue extent of the disease process, especially in areas of complex anatomy, such as the vertebral column, and the guidance of interventional procedures (biopsies and aspirations), particularly of the spine (Fig. 13) and sacroiliac joints.

Fig. 12. Infective periostitis. Transaxial CT scan of the tibia and fibula shows a well-defined shell of lifted periosteum surrounding an area of abnormal medullary bone. (Courtesy of Sergio Fernandez Tapia, MD, Tampico, Mexico.)

In chronic osteomyelitis, CT demonstrates abnormal thickening of the affected cortical bone, with sclerotic changes, encroachment of the medullary cavity, and the abnormal chronic draining sinus. CT provides excellent mul-tiplanar reconstructions of the axial images allowing delineation of even the subtlest osseous changes. The CT is superior to MRI in the detection of sequestra (see Fig. 3) and identifies even small devitalized bony fragments. Because resection of necrotic bone with thorough debridement of infected bone and excision of soft tissue fistulae are two major aims of surgical treatment, CT has considerable importance in determining operative therapy [73]. One limitation of CT is in the assessment of body parts with metallic implants because of beam-hardening artifact [35]. The sensitivity and specificity of CT for diagnosis of osteomyelitis has not been established clearly, but it is known to be lower than the sensitivity of MRI [69]. Its use in clinical practice should be limited to specific circumstances and it should not be used as part of the regular osteomyelitis imaging. CT is not routinely useful for diagnosis in prosthetic joints, because artifacts diminish image resolution [11].

Craig Biopsy

Fig. 13. Craig needle vertebral body biopsy. With the use of fluoroscopic guidance a T10-T11 bone biopsy was obtained. A diagnosis of S aureus osteomyelitis was established.

Ultrasound

US is a useful tool in musculoskeletal infections; it can be helpful to differentiate acute or chronic infection from tumors or noninfective inflammatory conditions with similar clinical presentation, to localize the site and extent of infection, and to identify precipitating factors as foreign bodies or fistulae. Moreover, sonography can provide guidance for diagnostic or therapeutic aspiration, drainage, or biopsy (Fig. 14) [74].US may detect features of osteomyelitis several days earlier than conventional radiographs [75], predominately in children [48,76,77].

In osteomyelitis, typically US usefulness was limited because of its restricted visualization of outer cortical and juxtacortical tissues [23]. Nevertheless, new high-resolution transducers allow visualization of soft tissues close to the bone and bone surface itself, particularly in the extremities [75]. Until now, however, evaluation of osseous involvement has required additional imaging (MRI, CT, nuclear medicine), and a normal US does not exclude bone infection [78].

The earliest sign of acute osteomyelitis on US is juxtacortical soft-tissue swelling associated with early periosteal elevation or thickening. This abnormality is followed by increase in periosteal reaction, and in up to two thirds of cases it is accompanied by a layer of subperiosteal exudates and, rarely, abscess formation. Periosteal abscess should be suspected if a hypo- to hy-perechogenic change contiguous to bone surface with adjacent structure displacement is demonstrated (Fig. 15) [75]; in the appropriate clinical settings, these collections confirm the clinical diagnosis of osteomyelitis [79]. Finally, cortical erosions can become apparent on US [80].

US assessment is likely to be more sensitive in children who have suspected acute osteomyelitis. In the immature skeleton, periosteal reaction is greatest, mainly in tubular bones, because of its relatively weak attachment to the subjacent bone [81].

In chronic osteomyelitis, US is not only valuable in making the diagnosis but also can be used to assess involvement of the adjacent soft tissues and

Lupus Breast Ultrasound

Fig. 14. US-guided needle aspiration of an abscess. Longitudinal scans of a soft tissue mass located on the upper left arm. (A) Well-defined heterogeneous fluid collection within the soft tissues. (B) Needle aspiration (arrow) yielded 30 mL of purulent material. (C) Postprocedure scan demonstrated marked reduction in the abscess size.

Fig. 14. US-guided needle aspiration of an abscess. Longitudinal scans of a soft tissue mass located on the upper left arm. (A) Well-defined heterogeneous fluid collection within the soft tissues. (B) Needle aspiration (arrow) yielded 30 mL of purulent material. (C) Postprocedure scan demonstrated marked reduction in the abscess size.

Hyperemic Needle

Fig. 15. S aureus periosteal abscess. (A) Longitudinal US scan. (B) Power Doppler US. (C, D) Transverse scans during needle aspiration. US demonstrated a hyperechogenic area contiguous to the tibial shaft with soft tissues displacement. Power Doppler shows a hyperemic area consistent with an active periosteal abscess.

Fig. 15. S aureus periosteal abscess. (A) Longitudinal US scan. (B) Power Doppler US. (C, D) Transverse scans during needle aspiration. US demonstrated a hyperechogenic area contiguous to the tibial shaft with soft tissues displacement. Power Doppler shows a hyperemic area consistent with an active periosteal abscess.

guide placement of a percutaneous drainage catheter. Soft tissue abscess related to chronic osteomyelitis is identified as a hypo- or anechoic fluid collection. After US localization of the abscess, a needle can be placed into the collection under sonographic guidance. The collection is then aspirated (Fig 14).

There are several limitations in the radiologic diagnosis of osteomyelitis reactivation because of the concurrent presence of bone changes induced by infection and by bone remodeling. In this regard, US can help in the evaluation of a chronic osteomyelitis reactivation, which may be associated with soft tissue abscess, fistula, or sinus tract formation [80]. Fluid collection adjacent to the cortex with its extension into the marrow cavity through a cortical break is almost pathognomonic of reactivated chronic osteomyelitis [82]. Collections in contact with the cortex in absence of soft tissue or muscle intervening are suggestive of active disease [83].

Special conditions

US may be useful in differentiating septic arthritis from osteomyelitis, especially in the case of septic hip and its differences with proximal femur or pelvis osteomyelitis [84]. US is a reliable noninvasive technique that can be helpful for the diagnosis of reactivation of posttraumatic chronic osteomyelitis in adults [82].

This technique also is helpful for evaluating musculoskeletal soft tissues next to orthopedic hardware and in cases of osteomyelitis complicating metal fixation in an extremity, without significant degradation by metallic artifact [82]. It can demonstrate loosening of metallic hardware and fluid collection or sinus tracts in the soft tissues of the involved extremity [78]. Ultrasonography is a relatively inexpensive imaging method for osteomyelitis evaluation; in the future, higher resolution equipment and color Doppler and power Dopp-ler sonography may increase the usefulness and accuracy of this method.

In patients who have sickle cell disease with osteomyelitis suspicion, US should be the initial imaging investigation. in children who have sickle cell disease, subperiosteal fluid concentrations of 4 mm or more in depth seem to be diagnostic [85]; however, less pronounced concentrations (<4 mm) should be distinguished from acute osteomyelitis and can occur in association with medullary infarction [75]. US also may be used to distinguish infection from infarction in these patients [2].

Magnetic resonance imaging

MRI has been used widely for evaluation of musculoskeletal infections; spatial resolution makes it useful in differentiating between bone and soft tissue infection, which is often a problem with radionuclide scans [86]. MRI is highly sensitive for detecting osteomyelitis as early as 3 to 5 days [2,84,87] with satisfactory specificity [88], because of the excellent contrast provided between the abnormal areas and the normal bone marrow. It has been demonstrated in several studies that MRI sensitivity is greater than that of plain films and CT, and similar to that of radionuclide studies [86,89-92]. MRI sensitivity for the diagnosis of osteomyelitis generally has been reported between 82% and 100%, and specificity between 75% and 96%.

MRI is the modality of choice in cases of well-established osteomyelitis and in determining the extent of infection, especially of eventual epidural abscess or phlegmon, the intramedullary involvement, and consecutive neural compression [88]. In addition it is a useful tool in the evaluation of the presence of intraosseous abscess [78], except when multiple lesions are suspected [11]. Furthermore, MRI helps the orthopedic surgeon to plan the optimal surgical management [93,94], allowing better planning for open or percutaneous drainage of fluid collections and surgical debridement [95,96]. MRI also contributes to the assessment of the extent of devitalized tissue and to the definition of the critical adjacent structures (spine, physes, joint spaces, and so forth) that require modified management to avoid morbidity and complications [97-99].

Initial MRI screening usually includes T1-weighted and T2-weighted spin-echo pulse sequences. Different pulse sequences and imaging protocols can be used in the evaluation of the musculoskeletal system; depending on the pulse sequences used, major differences can be noted on the signal intensity and appearance of normal and abnormal tissues.

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