The cogwheel sign refers to the pelvic imaging appearance of dilated loops of fallopian tube seen in cross-section. It represents infolding projections (sometimes looking like nodules) into the fallopian tube lumen which are likened to that of a cogwheel. The sign is typically described on ultrasound and is suggestive of tubal inflammation in pelvic inflammatory disease with a pyosalpinx or a hydrosalpinx.