Keywords

History

A 73-year-old diabetic female with right foot soft tissue swelling and chronic ulceration over the midtarsal dorsal foot (Fig. 13.1).

Fig. 13.1
figure 1

T1 TSE sagittal (a), T2 TSE with fat suppression sagittal (b), PET/MR T1 TSE sagittal fusion (c), and PET sagittal (d)

Full size image

Diagnosis

Cellulitis

Findings

  • T1-weighted images show low signal intensity, thickened subcutaneous tissue along the dorsal aspect of the foot over the midtarsal region.

  • T2-weighted images reveal intense signal along the dorsal aspect of the foot representing edema.

  • PET/MR fusion demonstrates intense hypermetabolic activity within the subcutaneous tissues of the dorsal foot representing soft tissue infection (arrow).

  • No evidence of marrow replacement on MRI or PET images to suggest osteomyelitis.

  • T1-weighted images show a thin linear hypointense line in the medial cuneiform (arrowhead) compatible with a subchondral insufficiency fracture.

  • Note the excessive fat and muscle edema in the intrinsic plantar muscles on MR, with mild FDG uptake. This is a common pattern in diabetic muscle disease.

Discussion

Cellulitis is a bacterial infection of the dermis and subcutaneous tissues that quickly spreads. Symptoms include edema, erythema, pain, and warmth. The most common causative organisms are Staphylococcus aureus and streptococci. Patients with peripheral vascular disease and diabetes are in particular at risk for cellulitis. Proper clinical examination and imaging are important to distinguish cellulitis from more serious infections, such as osteomyelitis and most importantly necrotizing fasciitis . The latter is a rare progressive infection characterized by extensive tissue necrosis and severe systemic toxicity.

MR imaging is particularly helpful in distinguishing cellulitis from necrotizing osteomyelitis and fasciitis. Cellulitis characteristically manifests as superficial soft tissue thickening with increased signal on T2-weighted sequences and decreased signal on T1-weighted images, often in a honeycomb pattern. It often enhances following gadolinium administration. Pseudo-fluid collections are not infrequently seen adjacent to the deep fascia. When the patient is turned, these “collections” will move, confirming that even when they rim enhance, they are not abscesses.

In necrotizing fasciitis, deeper myofascial planes are involved with high T2-weighted signal and post-contrast enhancement. Also necrotizing fasciitis shows deeper muscle edema, thickened intermuscular septae on T2-weighted images, and only subtle contrast enhancement. Osteomyelitis will have marrow replacement appearing as low signal on T1-weighted images along with high T2-weighted signal from bone marrow edema. FDG has both high sensitivity and high specificity in identifying and localizing osteomyelitis, which is often the clinical concern. PET/MR imaging can provide excellent assessment of the deeper myofascial tissues and notice secondary sites of infection.