  Object detection networks are powerful in computer vision, but not
necessarily optimized for biomedical object detection. In this work, we propose
CircleNet, a simple anchor-free detection method with circle representation for
detection of the ball-shaped glomerulus. Different from the traditional
bounding box based detection method, the bounding circle (1) reduces the
degrees of freedom of detection representation, (2) is naturally rotation
invariant, (3) and optimized for ball-shaped objects. The key innovation to
enable this representation is the anchor-free framework with the circle
detection head. We evaluate CircleNet in the context of detection of
glomerulus. CircleNet increases average precision of the glomerulus detection
from 0.598 to 0.647. Another key advantage is that CircleNet achieves better
rotation consistency compared with bounding box representations.
