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Background
According to the 2016 WHO data, an estimated 422 million adults have diabetes mellitus (DM). Diabetes-related foot complications, such as soft-tissue infection, osteomyelitis (OM), and charcot neuropathic osteoarthropathy (CN), account for up to 20% of all diabetic-related North American hospital admissions. Almost all OM of the diabetic foot represent areas of contiguous infection from adjacent skin ulcerations and not hematogenous seeding. The pathogenic mechanisms of CN are still unclear, although there is consensus that the cause is multifactorial (vascular, neuropathic, and traumatic). OM and CN can present with similar clinical features (redness, swelling, warmth, pain) and radiographic findings. MRI is the imaging modality of choice for diabetic foot evaluation, with high sensitivity to bone marrow abnormalities.

Educational Goals / Teaching Points
Recognize the MRI characteristics of osteomyelitis. Recognize the MRI characteristics of charcot osteoarthropathy. Determine which characteristics in the image allow us to differentiate osteomyelitis from charcot osteoarthropathy. Normal marrow signal reliably excludes OM and indicates that acute CN is unlikely. The most common location for OM is the pressure points of the forefoot (metatarsal heads, interphalangeal joints) and in the hindfoot at the plantar aspect of the posterior calcaneus (1–2 bones compromised). CN is primarily an articular disease (several midfoot bones compromised), which is most commonly located in the midfoot.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The findings in OM are bone marrow edema, elevation of the periosteum with edema of adjacent soft tissues, cortical destruction, and cloaca or intraosseous fistula that can evolve into an abscess or sinus tract. In chronic stages (> 6 weeks), sequestra or necrosis and significant sclerosis (reactive bone) manifested with periosteal and endosteal thickening, thickened and disorganized trabeculae may appear. The NC findings are acute: clinical signs of inflammation and MRI shows subchondral bone marrow edema, with enhancement in the subchondral bone; or chronic: this stage no longer shows warm or red foot, and edema usually persists. Clinical findings are the result of extensive destruction of bone and cartilage. Joint deformity, subluxation, and dislocation of the metatarsals lead to a rocker-bottom type deformity in which the cuboid becomes a weight-bearing structure. On MRI, edema and enhancement is less prominent or absent, and subchondral cysts and bone proliferation are more striking, and in advanced cases, bone may appear necrotic and collapsed or resorbed. The “ghost” sign is indicative of CN with superimposed OM and refers to poor definition of the margins of a bone on T1-weighted images, which become clear after contrast administration.

Conclusion
This review highlights the difficulty and importance of the differential diagnosis between CN and OM. Delaying and misdiagnosing these pathologies can lead to devastating health outcomes for the patient, with higher rate of unnecessary amputation, higher hospital costs, and a longer hospital stay.