New work in application of computer-assisted image analysis is demonstrating that ultrasound images have the potential to aid in the identification of healthy versus atretic follicles in natural and ovarian stimulation cycles (22,43,44) (Fig. 2). Physiologically dominant ovarian follicles are identifiable by ultrasonography at approximately day 7 post-menstruation in unstimu-lated cycles (22), and ovulatory and non-ovulatory follicles are identifiable in ovulation induction cycles (43). The image attributes of ultrasonographic images of normal preovulatory follicles include thick, low-amplitude walls and a gradual transformation zone at the fluid-follicle interface. The walls of preovulatory follicles are characterized by increased heterogeneity, increased wall breadth, and a more gradual transformation at the fluid-follicle wall interface. Atresia is characterized by thin walls, high numerical pixel value (bright) signals, and highly variable signals from the follicular fluid. Evaluation of the acoustic characteristics indicative of viability and atresia is an active area of research that has profound implications for development of safer and more effective ovarian stimulation protocols.
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