Spinning dancer (Bistable perception)

Bistable perception: a kinetic, spontaneous perceptual alternations between two equally compelling interpretations.
The Spinning Dancer, also known as the silhouette illusion, is a kinetic, bistable optical illusion resembling a pirouetting female dancer. The illusion, created in 2003 by web designer Nobuyuki Kayahara, involves the apparent direction of motion of the figure. Some observers initially see the figure as spinning clockwise (viewed from above) and some anti-clockwise. Additionally, some may see the figure suddenly spin in the opposite direction.
There are other optical illusions that depend on the same or a similar kind of visual ambiguity known as multistable, in that case bistable, perception. (Wikipedia)

Spinning dancer

The illusion involves the apparent direction of motion of the figure. Some observers initially see the figure as spinning clockwise (viewed from above) and some anti-clockwise. Additionally, some may see the figure suddenly spin in the opposite direction.

If the foot touching the ground is perceived to be the left foot, the dancer appears to be spinning clockwise (if seen from above); if it is taken to be the right foot, then she appears to be spinning anti-clockwise. (Wikipedia)

Spinning cancer

Maximum Intensity Projection (MIP) of a whole body positron emission tomography (PET) acquisition of a 79 kg (170 lb) weighting female after intravenous injection of 371 MBq of 18F-FDG (one hour prior measurement). The investigation has been performed as part of a tumor diagnosis prior to applying a radiotherapy (tumor staging step). Besides normal accumulation of the tracer in the heart, bladder, kidneys and brain, liver metastases of a colorectal tumor are clearly visible within the abdominal region of the image. (Wikipedia)

This positron emission tomography scan of a woman has a similar effect when viewed spinning.