Diagnostic imaging plays a crucial role in confirming the diagnosis of musculoskeletal (MSK) infection and determining the severity and extent of disease. The clinical diagnosis may be challenging due to the nonspecific presentation of pain and swelling. There are certain features that are pathognomonic for infection. Pre-existing conditions an make diagnosing infection difficult. Prior surgery can create artifacts on advanced imaging modalities such as computed tomography and MRI, obscuring the tissues immediately around the hardware. Recent technological advances have improved physicians’ ability to diagnose MSK infection with greater sensitivity, specificity, and accuracy.
Key points
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Diagnostic imaging plays a crucial role in confirming the diagnosis of musculoskeletal infection and determining the severity and extent of disease.
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The clinical diagnosis may be challenging due to the nonspecific presentation of pain and swelling.
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On diagnostic imaging, there are certain features that are pathognomonic for infection; however, imaging diagnosis of infection can also be challenging, as there are many occasions where the imaging features are less specific and overlap with non-infectious etiologies.
Introduction
Diagnostic imaging plays a crucial role in confirming the diagnosis of musculoskeletal (MSK) infection and determining the severity and extent of disease. The clinical diagnosis may be challenging due to the nonspecific presentation of pain and swelling. On diagnostic imaging, there are certain features that are pathognomonic for infection; however, imaging diagnosis of infection can also be challenging, as there are many occasions where the imaging features are less specific and overlap with noninfectious etiologies. Pre-existing conditions such as inflammatory arthritis can make diagnosing infection difficult, as inflammation of any source, due to autoimmunity or a septic organism, can create similar abnormalities on imaging. Prior surgery, particularly joint replacements or indwelling metal, can create artifacts on advanced imaging modalities such as computed tomography (CT) and MRI, obscuring the tissues immediately around the hardware. Recent technological advances have improved physicians’ ability to diagnose MSK infection with greater sensitivity, specificity, and accuracy.
Introduction
Diagnostic imaging plays a crucial role in confirming the diagnosis of musculoskeletal (MSK) infection and determining the severity and extent of disease. The clinical diagnosis may be challenging due to the nonspecific presentation of pain and swelling. On diagnostic imaging, there are certain features that are pathognomonic for infection; however, imaging diagnosis of infection can also be challenging, as there are many occasions where the imaging features are less specific and overlap with noninfectious etiologies. Pre-existing conditions such as inflammatory arthritis can make diagnosing infection difficult, as inflammation of any source, due to autoimmunity or a septic organism, can create similar abnormalities on imaging. Prior surgery, particularly joint replacements or indwelling metal, can create artifacts on advanced imaging modalities such as computed tomography (CT) and MRI, obscuring the tissues immediately around the hardware. Recent technological advances have improved physicians’ ability to diagnose MSK infection with greater sensitivity, specificity, and accuracy.
Imaging modalities
It is important to approach the imaging of these patients with a focused strategy and algorithm, and knowledge of the role of the various imaging modalities, including the advantages and limitations of each, is essential to ensure an accurate and prompt diagnosis.
Conventional Radiographs
Radiographs are the first-line imaging study in the evaluation of suspected MSK infection, as they are easily accessible and cost-effective, and as there are no significant technical limitations associated with imaging artifacts as with more advanced modalities. Radiographs must be obtained in at least 2 orthogonal planes, and evaluation of the radiographs must include a thorough review of the classic ABCS: bony alignment, bones (mineralization, erosion, periostitis), cartilage (indirectly assessed by joint space), and soft tissues (swelling, gas, foreign body). Radiographs are often normal in patients with early infection, and these patients can be effectively imaged, with more advanced and more sensitive imaging modalities as the next step.
Nuclear Imaging
The most commonly utilized nuclear imaging studies for imaging infection have traditionally been the radionuclide triple-phase bone scan and the more infection-specific radiolabeled (indium −111) leukocyte scan or gallium scan. The radionuclide bone scan is highly sensitive for diagnosing osteomyelitis, with characteristic findings of increased uptake in the involved bone on the first (blood flow or perfusion), second (blood pool or soft tissue) and third (delayed) phases of the scan. Patients with soft tissue infection without osteomyelitis would exhibit increased uptake on the first 2 phases that is not present or dissipates on the third phase. Unfortunately, although the bone scan is highly sensitive, the specificity is poor, as these findings can also be seen in patients with inflammatory arthritis, fracture, or other conditions such as reflex sympathetic dystrophy. The radiolabeled leukocyte scan (or, less optimally, the gallium scan) can improve specificity; however, the anatomic resolution is not optimal. Furthermore, increased uptake can be seen on these scans with noninfectious conditions, particularly following joint arthroplasty placement, in which the specificity of indium-labeled leukocyte scan is approximately 50%. Supplementing the indium-labeled leukocyte scan with a bone marrow scan (technetium-labeled sulfur colloid) further improves the specificity; if the increased osseous indium uptake is due to infection, the corresponding uptake on the bone marrow scan will be diminished (secondary to inhibited uptake related to the intramedullary infectious process), resulting in a mismatch or incongruence when correlated with the bone scan. If the increased indium uptake is not caused by infection (eg, normal hyperplastic marrow elements such as marrow expansion following arthroplasty), the uptake on the marrow scan will not be impeded, and there will be matching or congruent uptake on the 2 scans. Unfortunately, these nuclear scans are costly and time-consuming, typically done over 2 days, and the findings are commonly equivocal.
Computed Tomography
CT provides cross-sectional images of the bones and soft tissue, and technological advancements have allowed rapid production of 2-dimensional reformatted images in any plane (typically coronal and sagittal). The advent of multidetector scanners has resulted in CT images with submillimeter spatial resolution and 3-dimensional volume datasets with isotropic voxels, allowing for manipulation of the data and viewing in any desired projection. Patients are placed in the supine or prone position on the CT table, and the body part of interest is scanned in the CT gantry. The gantry is wide, and the scans are completed in a matter of seconds, thus making CT an optimal alternative for patients who have claustrophobia or other contraindications to MRI. CT provides high-resolution cross-sectional visualization of cortical bone and soft tissues, and may be helpful for assessing the bony/cortical resorption, periosteal reaction, or soft tissue gas or fluid collections associated with infection. One drawback of CT is ionizing radiation, and the radiation dose ranges from 1 to 10 mSv, approximately 50 to 500 times more than a chest radiograph ( http://www.fda.gov/Radiation-EmittingProducts ).
MRI
MRI has revolutionized the ability to effectively and accurately image patients for a variety of clinical scenarios, including trauma, infection, and tumor imaging. In addition to providing high-resolution cross-sectional anatomic images in any plane, MRI also provides information regarding composition and water content of tissues. Many pathologic processes, including infection, result in increased water content in the involved tissue, and this can be effectively detected on MRI. MRI signal characteristics can differentiate between fluid, fat, and muscle, and the images are highly sensitive for detecting the increased water content/edema in bone marrow in patients with osteomyelitis or edema in the subcutaneous tissues or muscle in patients with soft tissue infection. The images are generated without the use of ionizing radiation, utilizing a powerful magnet (typically a high field strength superconducting magnet) that demagnetizes protons in the tissues and detects the signal as the protons return to their normal state. The patient is placed in the bore of the magnet, and a dedicated coil is applied to the involved body part, acting as an antenna to detect the signal. Contraindications to MRI include patients who have implanted devices that would malfunction when exposed to a strong magnetic field (older cardiac pacemakers, implantable cardiac defibrillators, cochlear implants) or ferromagnetic foreign bodies whose movement can cause injury (eg, orbital metallic foreign bodies).
Ultrasound
Ultrasound generates images as a function of differential reflection of sound beams through tissue. However, sound beams do not penetrate through bone; thus ultrasound cannot adequately assess bone marrow. However, ultrasound can assess the cortical surface of bone, and it is very useful for evaluating soft tissues around bone, including ligaments, muscles, and tendons. It is quick, easily accessible, and uses no ionizing radiation. It can accurately distinguish fluid from solid tissue, and at experienced centers, ultrasound is the primary modality used for image guidance in aspirations and biopsies. and differentiating solid tissue from fluid. In MSK infection, the primary utility of ultrasound is for diagnosis and localization of fluid, either in the soft tissues (abscess), or in the joint (joint effusion). Furthermore, ultrasound can be used for image guidance when performing an aspiration, drainage, or biopsy of a soft tissue fluid collection, joint effusion, or mass.
The advantages and limitations of each imaging modality are summarized in Table 1 .
| Advantages | Limitations | |
|---|---|---|
| Radiographs | Readily available, may be diagnostic | Ionizing radiation, not sensitive or specific |
| Nuclear Imaging | Sensitive, but not specific | Expensive, time consuming, low resolution |
| CT | Cross-sectional visualization, reformations | Ionizing radiation |
| MRI | High resolution, high sensitivity | Time-consuming, expensive, contraindications |
| Ultrasound | Good soft tissue detail, no ionizing radiation, allows guidance for procedures | Cannot penetrate through bone, operator dependent |
Imaging approach to specific clinical situations
Cellulitis—Rule Out Abscess
Cellulitis is readily diagnosed clinically; however, it is important to determine if there is an associated abscess or osteomyelitis, as this would affect management and treatment. Radiographic features of cellulitis are nonspecific, demonstrating varying degrees of superficial and/or deep soft tissue swelling and edema. The edema often results in poor definition or obliteration of the intermuscular fat planes. Similarly, MRI and CT reveal subcutaneous soft tissue edema in superficial cellulitis or intramuscular edema or swelling in cases of deep cellulitis. It may be difficult to discern an associated fluid collection in the presence of soft tissue edema, as fluid commonly has a similar soft tissue density on CT or a similar fluid signal intensity on MRI. When noncontrast studies are equivocal, additional imaging following intravenous contrast administration will demonstrate discrete fluid collections/abscesses if present; whether on CT or MRI, the surrounding inflammatory tissue enhances, while the fluid collection does not, making the abscess more conspicuous. Alternatively, ultrasound can effectively diagnose a soft tissue fluid collection, and can also be used for image-guided aspiration or drainage of the collection ( Fig. 1 ).
