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Magnetic Resonance Imaging of Musculoskeletal Infections

Posted Oct 31, 2012 Updated Apr 10, 2023
By Theodoros Soldatos MD PhD , Daniel J. Dur MD , Ty K. Subhawong MD , John A. Carrino MD MPH , Avneesh Chhabra MD DNB
12 min read

Prompt diagnosis and treatment are essential in preventing the complications of musculoskeletal infection. In this context, imaging is often used to confirm clinically suspected diagnoses, define the extent of infection, and ensure appropriate management. Because of its superior soft-tissue contrast resolution, magnetic resonance imaging (MRI) is the modality of choice for evaluating musculoskeletal infections. This article describes the MRI features along the full spectrum of musculoskeletal infections and provides several illustrative case examples.

Musculoskeletal infections are common in medical practice and represent a unique diagnostic and therapeutic challenge . Early diagnosis and treatment of these infections are crucial in order to prevent disabling sequelae . Although clinical information often reveals the body part involved and may point toward certain pathogens, even an expert physical exam is not sufficient to define the extent and nature of the inflammatory process beneath the skin surface. Therefore, imaging is often employed to provide additional information essential to management . Because of its inherent high spatial and contrast resolution, magnetic resonance imaging (MRI) provides exquisite anatomic information, including the ability to evaluate both bone and adjacent soft tissue, and has been established as the imaging modality of choice for the evaluation of soft-tissue abnormalities throughout the musculoskeletal system, including infection . Although imaging findings are not pathogen-specific, MRI is a highly sensitive examination allowing clinicians to confirm or exclude the clinical diagnosis of musculoskeletal infection and define the anatomical distribution and pathophysiology . This article systematically reviews the spectrum of musculoskeletal infections organized by fascial and anatomic compartments and describes the MRI features of various entities while providing relevant case examples.

Imaging technique

Although multiple pulse sequences may be employed for the evaluation of musculoskeletal infections, a suggested imaging protocol preferred at the authors’ institution consists of T1-weighted (T1W) and fluid-sensitive (short-tau inversion recovery, STIR or fat-suppressed T2-weighted, fsT2W) images in two or three planes. Because of superior fat suppression, especially along extremity curvatures, STIR images are inherently more sensitive than any other fat-suppressed sequence for the detection of soft-tissue and bone marrow edema. Compared to fsT2W images however, STIR images demonstrate lower signal-to-noise ratio (SNR). Therefore, echo time should be kept below 30 to 40 ms to maintain high SNR. Although phlegmon and infectious/inflammatory synovitis enhance more intensely compared to reactive edema and normal synovium, respectively, it should be noted that the role of contrast is not to confirm infection, but to define the extent of the latter, outline abscesses or sinus tracts, and detect non-enhancing devitalized soft tissue or sequestrum. In some cases, because of underlying comorbidities such as renal insufficiency, contrast should be administered at a lower dose or withheld altogether. In the authors’ institute, routine non-contrast imaging suffices in most circumstances. In addition, diffusion-weighted images are being exploited as an experimental tool for cases of suspected soft-tissue or intraosseous abscess.

Classification and Imaging Features of Musculoskeletal Infections

Musculoskeletal infections can be classified into superficial lesions, which include cellulitis, infectious bursitis, and tenosynovitis; and deep lesions, which include necrotizing fasciitis, pyomyositis, osteitis, osteomyelitis, and septic arthritis. Further discussion will address the pathophysiology and MRI appearances of each of these entities. It should be noted that both superficial and deep infections may evolve into a phlegmon that, with increasing necrosis, may organize into an abscess .

Cellulitis

Cellulitis refers to a non-necrotizing infection, which is restricted to the skin and subcutaneous tissues, without involvement of the underlying fascia or muscles. It is typically caused by the residual flora or exogenous contamination, with Staphylococcus aureus and Streptococcus pyogenes being the most common offending agents. In the majority of cases, the latter gain access to the dermis through a break in the skin resulting from a penetrating injury. Though far less common, inoculation may also occur via hematogenous seeding or spread from a contiguous focus of osteomyelitis, especially in immunocompromised individuals. Cellulitis presents with local erythema, warmth, swelling, and tenderness and is frequently accompanied by systemic symptoms such as fever, malaise, chills, and lymphadenopathy. Although a purely clinical diagnosis is usually sufficient, an MRI study may be obtained in cases of rapidly advancing cellulitis or in cases with severe systemic manifestation, either of which may suggest an underlying deep abscess . On MRI, cellulitis is seen as skin thickening along with focal or diffuse areas of low and high signal intensity of the subcutaneous tissues on T1W and STIR/fsT2W images, respectively . There is also moderate contrast enhancement, differentiating the entity from non-inflammatory causes of soft-tissue edema, such as congestive heart failure, diabetic vascular insufficiency, or lymphatic obstruction, which do not show enhancement ( ; Fig 1 ). Treatment is conservative, consisting of antibiotics and supportive measures, whereas surgery may be required in progressive cases, particularly if necrosis develops .

Figure 1, Cellulitis of the forefoot. Axial STIR (a) and noncontrast T1-weighted (b) images demonstrate edema restricted to the superficial soft tissues ( arrow ). In the respective postcontrast fat-suppressed T1-weighted image (c) , there is enhancement of the involved tissues.

Septic Bursitis

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Figure 2, Infectious intermetatarsal bursitis complicated with metatarsals osteomyelitis. Axial T1-weighted (T1W) (a) , T2-weighted (T2W) (b) , and postcontrast fat-suppressed T1W (c) images of the forefoot show a distended intermetatarsal bursa with thick enhancing wall ( arrow ). Also note evidence of diffuse cellulitis as well as loss of the normal bone marrow signal in the third metatarsal, along with cortical erosion, in keeping with osteomyelitis. Notice preserved T1 signal and cortical integrity in the second and fourth metatarsals, compatible with reactive edema.

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Infectious Tenosynovitis

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Figure 3, Infectious tenosynovitis in the setting of Dupuytren's disease. On coronal (a) and axial (b) T2-weighted images, the common tendon sheath of the flexor digitorum superficialis and profundus muscles is distended by hypeintense fluid ( arrow ), compatible with tenosynovitis, whereas intermediate signal within the respective tendons is indicative of tendinitis. There is also patchy reticulation of the subcutaneous tissues, which are hypeintense on the respective STIR (c,d) images, signifying edema. Culture of tenosynovial fluid revealed Mycobacterium fortuitum .

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Necrotizing Fasciitis

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Figure 4, Eoshinophilic fasciitis. Axial STIR image (a) of the thighs shows diffuse edema of the deep fascia bilaterally. Steroid treatment was initiated, and the respective image (b) from a follow-up examination demonstrates partial regression of the findings.

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Abscess

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Figure 5, Soft-tissue abscesses complicating tuberculous sacroilitis. Axial T1-weighted (T1W) (a) , T2-weighted (b) , and postcontrast fat-suppressed T1W (c) images through the pelvis a patient with infectious sacroilitis demonstrate two well-defined rim-enhacing fluid collections ( arrows ), with erosion of the right sacroiliac joint, in this case of tuberculosis.

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Infectious Myositis (Pyomyositis)

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Figure 6, Pyomyositis. Sequential axial fat-suppressed proton density-weighted images (a,b) of the knee in a patient with fever and prior tick bite reveals edema of the popliteus muscle ( arrow ). In (a) , a well-defined intramuscular cavity ( arrowhead ) corresponds to an abscess from Lyme's pyomyositis.

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Septic Arthritis

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Figure 7, Osteomyelitis complicated with intraosseous abscess and septic arthritis. Coronal fat-suppressed T2-weighted images (a,b) demonstrate edema of the distal femoral and proximal tibial metaphysis, extending to the respective epiphysis. A well-defined fluid collection in the femoral metaphysis ( arrow ) corresponds to an abscess. The respective axial (c) and coronal (d) contrast-enhanced fat-suppressed T1-weighted images exhibit rim enhancement of the abscess ( arrow ), along with synovial thickening and enhancement of the suprapatellar bursa ( arrowhead ), which is indicative of septic arthritis.

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Osteomyelitis

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Figure 8, Coronal T1-weighted (T1W) image (a) of the lower extremities shows loss of the normal bone marrow signal in the proximal half of the right tibia. In the corresponding fat-suppressed T2-weighted (b) , and contrast-enhanced T1W (c) images, the involved marrow is edematous and shows patchy enhancement with loss of the normal bone marrow signal, in keeping with osteomyelitis.

Figure 9, Neuropathic joint. Sagittal T1-weighted (a) and fat-suppressed T2-weighted (b) images of the foot exhibit a neuropathic ankle joint along with reactive marrow edema of the adjacent bones.

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Figure 10, Brodie's abscess. Sagittal T1-weighted (a) , T2-weighted (b) , and STIR (c) images of the ankle demonstrate relatively well-defined fluid-filled cavities with a multilayered configuration in the distal tibia, along with diffuse marrow edema in this case of Brodie's abscesses.

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Spinal Infections

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Figure 11, Spondylodiskitis. Mid-sagittal T1-weighted (a) , T2-weighted (b) , and STIR (c) images of the lumbar spine demonstrate reduction of the L5-S1 disk height, along with erosions of the adjacent endplates end marrow edema of the vertebral bodies.

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Important Diagnostic Key Points

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Figure 12, Axial T1-weighted (T1W) (a) , STIR (b) , and constrast-enhanced fat-suppressed T1W (c) images of the forefoot in a patient with diabetic foot and a sinus tract on the plantar surface demonstrate T1 hypointensity, T2 hyperintensity, and contrast enhancement of the marrow of the third metatarsal head along with cortical irregularity, in keeping with osteomyelitis. Findings of cellulitis are evident in the soft tissues.

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Conclusion

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Academic Radiology, Volume 19, Issue 11
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