174 ULTRASOUND BIOMICROSCOPIC IMAGE ATTRIBUTES OF OVARIAN FOLLICLES IN HEIFERS

Abstract

High-resolution ultrasound biomicroscopy (UBM) with high-frequency transducers (20 to 50 MHz) has enabled imaging of small antral follicles (<1 mm) and acoustic characterisation of the follicular antrum and wall. Quantitative echotextural analysis of the images generated by UBM provides detailed functional information about the granulosa and theca layers of follicle walls. The objectives of our study were to compare biomicroscope image echotextures between dominant and largest subordinate follicles during the first follicular wave and to characterise UBM attributes of the ovulatory follicle. Hereford crossbred heifers (14 to 16 months old, n = 12) were used. Ovarian examinations were performed transvaginally from Day –4 or –3 (Day 0 = ovulation) to Day 10 using a UBM imaging instrument (Vevo 660; Visual Sonics Inc., Toronto, Canada) equipped with a 25-MHz end-fire transducer. Echotextural assessment was performed by spot metrics of the follicle antrum and follicle wall by using a series of custom-developed computer algorithms optimized for ultrasonography (Synergyne^©, Version 2.8, WHIRL, Saskatoon, SK, Canada). Although subordinate and dominant follicles development were tracked from Day –2 (around 1 mm in diameter) to Day 10, the ovulatory follicle was tracked only from Day –4 to ovulation. All data were analysed by analysis of variance for repeated measures using the Mixed procedure (Littell et al. 1998) in the Statistical Analysis System software package (SAS version 8.2 for MS Windows; SAS Institute Inc., Cary, NC, USA). No differences were detected between dominant and largest subordinate follicle in the mean pixel value (MPV) or pixel heterogeneity (PH) of the antrum. However, the MPV of the wall of dominant follicles was greater and PH tended to be greater than that of the largest subordinate follicles (P = 0.05 and P = 0.1, respectively). This result could be evidence that the dominant follicle would be healthier. The PH of the wall the ovulatory follicle decreased (P = 0.03) from Day –4 to Day –1; however, no changes in MPV were observed. Similarly, no changes were detected in MPV or PH of the follicular antrum. The loss of granulosa cells from the follicular wall can explain, at least in part, the decrease in mean pixel value and pixel heterogeneity of the follicular wall detected in ovulatory follicles. Moreover, morphologic changes indicative of atresia are typically represented by high values of number of pixel values. In conclusion, although no differences in antral characteristics of dominant v. subordinate follicles were detected, mean pixel value and pixel heterogeneity of the follicular wall were higher in dominant follicles, and pixel heterogeneity of follicular wall of ovulatory follicles decreased over days. Quantitative pixel analysis from follicular wall images using UBM could be applied to evaluate follicles in live animals. However, more studies using this technique are necessary.

Research was supported by NSERC and CIHR. Luiz Pfeifer was supported by CAPES from Ministry of Education, Brazil.