This artifact can be observed when using the algebraic resoconstruction technique.
It does not show the correct reconstructed phantom but rather a shaded circle that becomes edgy with increasing iteration. Its appearance reminds of a flower.

The ART algorithm iterates the sinogram with specific angles. If the angle increases too much in each iteration the geometry does not hold and instead of the correctly reconstructed image we get these flowers. This occurs for example when the angle is not correctly converted from degrees to radians. After correction, the phantom is correctly reconstructed as shown on the right.