Spoke wheel appearance | Radiology Signs

Answer:

A spoke wheel appearance is a radial arrangement of vessels or fibrous septa running from the centre of a lesion out to its periphery, like the spokes of a wheel around a hub. Two settings are classic. In renal oncocytoma the pattern corresponds to septa and vessels radiating from a central stellate scar, and the same pattern is described in salivary (parotid) oncocytoma, where colour Doppler shows centripetal vascular branches. In focal nodular hyperplasia of the liver the radiating arterial branches from a central feeding artery within the central scar produce the identical pattern on arterial-phase CT, MRI and contrast-enhanced ultrasound; there the combination with a T2-hyperintense central scar and retention of hepatobiliary contrast agent is diagnostic. The critical caveat is that in the kidney the sign is not reliable: it is inconsistent, infrequent, and overlaps substantially with chromophobe and clear cell renal cell carcinoma, so an oncocytoma cannot be diagnosed on imaging alone and tissue sampling or resection remains necessary.

Why is it called so?

The septa or vessels radiate from a central hub to the periphery like the spokes of a wheel.

Pathophysiology

In renal and salivary oncocytoma, the tumour grows as compact nests of oncocytes separated by a fibrovascular stroma that condenses centrally into a stellate scar; the feeding vessels and septa run outward from that scar along the planes of the stroma, which is what the imaging renders. In focal nodular hyperplasia the mechanism is different: a congenital arterial malformation produces a large central artery whose branches radiate through the fibrous septa of the hyperplastic parenchyma, and the resulting arterial pattern is directly visible on arterial-phase imaging. The shared geometry of a central hub with radiating stroma explains why lesions of quite different origin look alike.

Alternative names: Spoke wheel pattern, cartwheel appearance

Other associated named signs: Central scar of focal nodular hyperplasia and oncocytoma, and the segmental enhancement inversion described in oncocytoma

References

1. Zhao S, Shi J, Yang R, Zhang X, Zhao W, Sun Z. Ultrasonography findings for the diagnosis of renal oncocytoma. J Med Ultrason (2001). 2022;49(2):211-216. PMID: 35083534.