Osteomyelitis is a bone infection caused by infectious agents (most commonly Staphylococcus aureus). It develops through hematogenous spread (metaphysis in children), direct inoculation (trauma/surgery), or contiguous spread (diabetic foot). Brodie abscess (encapsulated abscess in chronic form of subacute osteomyelitis), sequestrum (dead bone), involucrum (new periosteal bone), and cloaca (drainage channel) are classic radiological findings.
Age Range
5-80
Peak Age
50
Gender
Male predominant
Prevalence
Common
Osteomyelitis begins when pathogenic microorganisms (most commonly S. aureus) reach bone tissue; in children, the metaphyseal vascular architecture predisposes to bacterial colonization because metaphyseal vessels form slow-flow sinusoidal structures as terminal arteries. Infection initiates an inflammatory cascade in bone marrow, intramedullary pressure increases, and bone tissue necrosis (sequestrum) develops — visible on CT as a high-density dead bone fragment separate from viable bone. The periosteum reactively forms new bone (involucrum), which appears on MRI as thickened periosteum with low T1 signal and prominent post-contrast enhancement. Drainage channels (cloaca) open through the involucrum forming fistula tracts. In subacute/chronic osteomyelitis, infection becomes localized and Brodie abscess develops; on MRI post-contrast T1 series, the 'penumbra sign' — a hyperintense granulation tissue rim around the abscess — is pathognomonic for this condition and distinguishes pyogenic abscess from tumor.
Hyperintense rim of hypervascularized granulation tissue surrounding intraosseous Brodie abscess on post-contrast fat-suppressed T1 series. While the central pus does not enhance, the outer rim enhances intensely. This three-layered structure (central low + rim high + outer edema) is pathognomonic for subacute osteomyelitis and the most important finding distinguishing it from tumor.
On post-contrast T1 series, a thin hyperintense rim (penumbra) is seen around the intraosseous abscess. This rim represents hypervascularized granulation tissue. Inside is non-enhancing pus/necrotic tissue (central hypointense), outside is reactive bone marrow edema. The penumbra sign is pathognomonic for Brodie abscess in subacute/chronic osteomyelitis.
Report Sentence
On fat-suppressed post-contrast T1 series, a central non-enhancing component with surrounding hyperintense rim (penumbra sign) is demonstrated in the metaphyseal lesion, consistent with Brodie abscess.
Within a lytic lesion, a high-density devitalized bone fragment (sequestrum) separate from surrounding viable bone. Low-density granulation tissue/pus surrounds the sequestrum. CT is the most sensitive modality for sequestrum detection. Sequestrum does not enhance on contrast studies as it has lost its vascularity.
Report Sentence
On CT, a high-density devitalized bone fragment (sequestrum) surrounded by granulation tissue is identified in the center of the lytic area within the tibial lesion.
On T1-weighted series, infected bone marrow demonstrates hypointense signal with loss of normal fat signal. The abscess cavity shows low-to-intermediate T1 signal (proteinaceous pus). As the precontrast component of the penumbra sign, a slightly hyperintense rim (granulation tissue) around the abscess may also be visible on pre-contrast T1. Cortical and periosteal thickening is noted.
Report Sentence
On T1-weighted series, a hypointense area with loss of normal marrow signal and central abscess cavity is demonstrated in the metaphysis.
On T2-weighted series, infected bone marrow demonstrates markedly hyperintense signal (edema + pus). The abscess cavity shows homogeneous high T2 signal. Surrounding bone marrow edema extends beyond the true lesion margins. If soft tissue extension is present, periosteal collections and subperiosteal abscess also appear T2 hyperintense. Fistula tracts may be seen as linear high T2 signal structures.
Report Sentence
On T2-weighted series, diffuse bone marrow edema throughout the metaphysis and diaphysis with a central high-signal abscess cavity is demonstrated.
On STIR sequence, fat suppression makes bone marrow edema, soft tissue edema, and collections conspicuous. The full extent of osteomyelitis spread (intramedullary + subperiosteal + soft tissue) is best assessed with STIR. STIR plays a critical role in mapping abscess, fistula tracts, and soft tissue extension for surgical planning.
Report Sentence
On STIR sequence, diffuse bone marrow edema in the tibial metaphysis and diaphysis, periosteal collection, and medial soft tissue edema/abscess are demonstrated.
In chronic osteomyelitis, reactive new bone formation (involucrum) is seen around the infected bone. Involucrum appears as irregular, thickened periosteal bone. Cloaca is a drainage opening through the involucrum, identified on CT as a cortical defect. Fistula tracts draining from the cloaca to soft tissue may be visible. Involucrum and cloaca together confirm the diagnosis of chronic osteomyelitis.
Report Sentence
On CT, involucrum surrounding the infected bone and cloaca (drainage channel) in the lateral cortex are identified, consistent with chronic osteomyelitis.
On diffusion-weighted imaging, the abscess cavity demonstrates marked restricted diffusion (high DWI signal, low ADC value: 0.3-0.8 × 10⁻³ mm²/s). This finding reflects viscous pus and dense inflammatory cell content. DWI is valuable in differentiating abscess from cystic/necrotic tumor (tumor necrosis generally shows higher ADC).
Report Sentence
On diffusion-weighted series, marked restricted diffusion is noted in the abscess cavity (ADC: ~0.5 × 10⁻³ mm²/s), consistent with viscous pus content.
On PET-CT, increased FDG uptake is noted at the infection site (SUVmax typically 2-8). FDG is taken up by activated inflammatory cells and granulation tissue. PET-CT is used particularly in chronic osteomyelitis to distinguish active infection from healing/inactive areas and for treatment response assessment. However, tumor versus infection differentiation by FDG alone is difficult.
Report Sentence
On PET-CT, increased FDG uptake is noted at the infection site (SUVmax: X), consistent with active inflammatory/infectious process.
Criteria
Metaphyseal location in children (<18 years), hematogenous spread, <2 weeks symptom duration, fever + leukocytosis + elevated CRP
Distinct Features
On MRI, bone marrow edema is early and extensive. Sequestrum and involucrum have not yet formed. Subperiosteal abscess can develop rapidly. Radiographs may be normal in first 48 hours; MRI is the earliest diagnostic modality. Urgent antibiotic therapy and surgical drainage may be needed.
Criteria
2-6 weeks symptom duration, low-virulence organism or partial treatment, localized intraosseous abscess, mild/moderate clinical symptoms
Distinct Features
Classic Brodie abscess: metaphyseal localized lytic lesion, positive penumbra sign, sclerotic margin. Can be confused with tumor (Ewing sarcoma, osteoid osteoma). Penumbra sign on post-contrast T1 is diagnostic. Long-term antibiotic therapy ± curettage required.
Criteria
>6 weeks symptom duration, sequestrum + involucrum + cloaca + fistula tracts, recurrent flare-ups, surgical history or inadequate treatment
Distinct Features
CT best demonstrates sequestrum, involucrum, and cloaca. MRI is superior in distinguishing active/inactive areas (enhancement = active). Squamous cell carcinoma can develop in fistula tracts (Marjolin ulcer). Sequestrectomy + debridement + prolonged antibiotics required.
Distinguishing Feature
Ewing sarcoma typically presents at age 10-20, diaphyseal location, with large soft tissue mass. Unlike osteomyelitis, it does not show penumbra sign. 'Onion-skin' periosteal reaction is more common in Ewing, and post-contrast series show solid tumor enhancement (different from abscess cavity). Clinically, fever and leukocytosis can occur in both — biopsy may be needed.
Distinguishing Feature
LCH shows beveled edge skull defect, button sequestrum (viable bone) and vertebra plana. Sequestrum in osteomyelitis is dead bone and does not enhance. LCH shows more sharply marginated geographic lytic lesion, while permeative/moth-eaten pattern is more common in osteomyelitis. Infection markers (fever, leukocytosis, CRP) are absent or mild in LCH.
Distinguishing Feature
Osteoid osteoma shows <2cm nidus + surrounding reactive sclerosis. Nidus appears on CT as low-density center + sclerotic halo. In osteomyelitis (Brodie abscess), the lesion is usually larger and penumbra sign is present. Night pain in osteoid osteoma responds to aspirin — this finding is absent in osteomyelitis.
Distinguishing Feature
Osteosarcoma demonstrates aggressive bone formation (osteoid matrix, tumor bone) — mineralized matrix and 'sunburst' periosteal reaction on CT. Unlike osteomyelitis, penumbra sign is absent, and post-contrast series show solid tumor enhancement. On DWI, osteosarcoma shows restricted diffusion in solid component unlike abscess cavity pattern.
Urgency
urgentManagement
Akut osteomyelit: acil IV antibiyotik (empirik anti-stafilokokal) ± cerrahi drenaj. Subakut (Brodie apsesi): uzun süreli oral antibiyotik ± küretaj. Kronik: sequestrektomi + debridman + antibiyotik impregne spacer + uzun süreli antibiyotik.Biopsy
NeededFollow-up
Akut: 2-4 hafta sonra MRG (tedavi yanıtı). Kronik: 3-6 aylık MRG + inflamatuar belirteçler. Tedavi bitiminde kontrastlı MRG (rezidü enfeksiyon).Osteomyelitis diagnosis is made by combination of clinical (fever, pain, swelling), laboratory (leukocytosis, elevated CRP/ESR), and imaging findings. MRI is the most sensitive early diagnostic modality (90-100% sensitivity). Blood culture positivity is 50-60%, while bone aspirate culture is the gold standard. Diabetic foot osteomyelitis is a special situation — positive 'probe to bone' test is diagnostic and confirmed by bone marrow edema on MRI.
Osteomyelitis is an emergency requiring early diagnosis and treatment. Acute osteomyelitis is treated with IV antibiotics (4-6 weeks). Chronic osteomyelitis may require surgical debridement. MRI is the most sensitive imaging modality. Diabetes and peripheral vascular disease are important risk factors.