Wheel within a wheel | Radiology Signs

Answer:

The wheel within a wheel is a small target lesion on ultrasound consisting of a central echogenic nidus, surrounded by a hypoechoic zone, surrounded again by an echogenic rim. It is the classic early appearance of hepatosplenic candidiasis, also called chronic disseminated candidiasis, in a patient recovering from neutropenia after treatment for acute leukaemia or after stem cell transplantation, who typically presents with persistent fever and a rising alkaline phosphatase despite antibacterial therapy. The appearance evolves: the wheel within a wheel is seen early, a bull’s eye lesion with an echogenic centre appears as neutrophils return, a uniformly hypoechoic nodule is the commonest chronic appearance, and healed lesions become echogenic and may calcify. On CT the lesions are small, low-attenuation and best seen in the portal venous phase, and MRI is the most sensitive modality, showing multiple small T2-hyperintense foci with rim enhancement, and it is used to monitor response, which takes months. The differential includes microabscesses of other organisms, sarcoidosis, lymphoma and metastases. Recognition matters because the appearance frequently emerges only after neutrophil recovery, so it should be sought at that point rather than during the neutropenic phase, and because treatment is prolonged antifungal therapy, sometimes with corticosteroids for the immune reconstitution component.

Why is it called so?

The concentric alternating echogenic and hypoechoic layers look like a small wheel inside a larger wheel.

Pathophysiology

Candida species disseminate haematogenously from the gut during profound neutropenia and seed the liver and spleen through the portal and arterial circulations. While neutrophils are absent, the host response is minimal and lesions may be invisible on imaging. When the neutrophil count recovers, an intense inflammatory reaction forms around the fungal foci, and it is this response that makes the lesions detectable. The layers correspond histologically to a central focus of fungal elements and necrosis, a surrounding zone of inflammatory cells and a peripheral rim of fibrosis, which is why the appearance changes as the lesion matures and why the imaging findings paradoxically worsen as the patient’s marrow recovers.

Alternative names: Bull’s eye lesion; target lesion of hepatosplenic candidiasis

Other associated named signs: Bull’s eye sign at a later stage and the target lesions of hepatic metastases and microabscesses

References

1. Pastakia B, Shawker TH, Thaler M, O’Leary T, Pizzo PA. Hepatosplenic candidiasis: wheels within wheels. Radiology. 1988;166(2):417-21. PMID: 3275982.