220 ULTRASOUND BIOMICROSCOPY: A NONINVASIVE APPROACH FOR STUDYING THE DEVELOPMENT OF SMALL FOLLICLES IN THE BOVINE OVARY

Abstract

Ultrasonography has revolutionized our understanding of the dynamics of antral follicles >3 mm; however, very little is known about the growth patterns of small antral follicles (0.2-2 mm). Ultrasound biomicroscopy (UBM) permits in vivo imaging of tissues with spatial resolution of 50 μm or more, although the depth of penetration is limited to about 2.5 cm. Our objectives were to (1) evaluate the feasibility of UBM for imaging cow v. heifer ovaries in vivo for study of small antral follicles; (2) compare transvaginal v. transrectal imaging approaches; and (3) compare the echotextures in images acquired by UBM and conventional ultrasonography of the wall and antrum (follicular fluid) from follicles >3 mm. Mature cows (n = 5) and prepubertal heifers (11-13 months; n = 5) were examined once irrespective of ovarian status using conventional ultrasonography (Aloka 900, Tokyo, Japan) with a 7.5-MHz transducer via a transrectal approach, and with a 5-MHz transducer via a transvaginal approach. A second series of examinations was performed using an ultrasound biomicroscope (Visualsonics Vevo 660, Toronto, Ontario, Canada) equipped with either a 40-MHz probe (transvaginal) or a 30-MHz probe (transrectal). All examinations were recorded digitally in real time. Spot-analyses of images of the antrum and line-analyses of images of the wall of follicles >3 mm were performed using a custom-developed software program (Synergyne 2 version 2.8, Saskatoon, Saskatchewan, Canada). Data were analyzed by 2-sample t-test or two-way ANOVA. Using the transvaginal approach, more follicles were detected by UBM than by conventional ultrasonography in heifers (40.4 ± 17.4 v. 14.6 ± 5.6; P = 0.01) but not in cows (38.3 ± 16.4 v. 21.7 ± 6.2; P = 0.20). Using the transrectal approach, however, more follicles were detected by conventional ultrasonography than by UBM in both heifers (17.6 ± 4.9 v. 8.6 ± 5.6; P = 0.02) and cows (20.3 ± 7 v. 5.3 ± 6.1; P = 0.04). More small follicles (<3 mm) were detected using the transvaginal approach with UBM than by using conventional ultrasonography in both heifers (32.4 ± 4.24 v. 7.2 ± 1.4; P < 0.0001) and cows (35.0 ± 13.8 v. 10.7 ± 7.5; P = 0.0013). The number of medium (3-5 mm) and large (> 5 mm) follicles detected using the transvaginal approach was similar between UBM and conventional ultrasonography in both heifers and cows (P > 0.90). For transrectal UBM imaging, both distance between the scanhead and the ovary and signal attenuation due to intervening tissues resulted in poor image quality. Lower signal attenuation caused by thinner vaginal walls in heifers than in cows resulted in better quality of UBM imaging in the transvaginal approach. Mean pixel values and heterogeneity of images of the follicle antrum were higher in UBM images than in conventional ultrasonography images. In conclusion, UBM using a transvaginal approach may be used for the in vivo assessment of small ovarian follicles in cattle. However, sequential monitoring of follicular development still needs to be tested and some limitations of the technique, such as the approach, need to be considered.

This study was supported by NSERC and CIHR, Canada. Luiz Pfeifer was supported by CAPES foundation, Brazil.