A sonohysterogram, also called saline infusion sonography, is an ultrasound-based imaging procedure used to evaluate the uterine cavity. It involves inserting sterile saline into the uterus through a thin catheter to expand the cavity and provide clearer images of the uterine lining. This test helps identify abnormalities such as polyps, fibroids, or adhesions and is often used in infertility assessments.