Characterization of follicle and CL development in beef heifers using high resolution three-dimensional ultrasonography

The aim was to characterize dominant follicle (DF) and CL development through the estrous cycle of cattle using three-dimensional (3D) ultrasonography while making a comparison with conventional two-dimensional (2D) B-mode ultrasound (US) and to relate the measures taken to systemic concentrations of steroid hormones and gonadotropins. After synchronization of estrus, the ovaries of crossbred beef heifers (N = 5) were assessed using daily US with a GE Voluson i US scanner until the end of the first follicle wave, then every other day until emergence of the final (ovulatory) wave, when daily US resumed until ovulation. Follicle and CL growth were recorded and mapped. Measures of diameter (2D) and volume (3D) of the DF from the first and ovulatory waves of the cycles; and CL development were captured and stored for further analysis. Blood flow to the DF and CL were assessed using 3D power Doppler US measuring vascularization index (VI; %), vascularization flow index (0/100) and flow index (0/100). Jugular blood samples were collected every 24 hours for progesterone from the first estrus until the second ovulation. Concentrations of estradiol (E2) and follicle stimulating hormone (FSH) were measured every 8 hours from estrus to second follicle wave emergence; then, E2 only was measured from final follicle wave emergence until ovulation. Data were analyzed using PROC MIXED and PROC REG in SAS. Dominant follicle blood flow tended to decrease during follicle wave emergence and DF VI increased (P < 0.05) 24 hours before ovulation after peak E2. Measures of the DF and CL volume (3D) were highly predictive of 2D diameter measures throughout the cycle (P < 0.0001). Predictive values (r²) for day of wave emergence and day from ovulation were similar for 2D and 3D measures; however, 2D measures had higher repeatability when compared with 3D measures. There was no relationship between CL VI and progesterone early in the cycle (r² = 0.12; P = 0.1); however, there was a strong positive relationship approaching ovulation (r² = 0.77; P < 0.0001). In conclusion, 3D power Doppler measures of blood flow appears to be representative of vascular changes in the DF and CL throughout the estrous cycle. However, the extra time required to acquire and analyze a 3D image and the relatively little additional information obtained over that achievable with 2D imaging in terms of follicle and CL development might preclude its widespread use other than for detailed research purposes.