High Resolution Ultrasonography: A New Diagnostic Tool for Small Rodent Medicine
Despite recent rapid advances in ultrasound imaging technology, the application of ultrasound imaging as a diagnostic tool in small rodent medicine remains limited, especially in nondomestic rodents. The aim of this study was to establish a reliable noninvasive examination protocol for monitoring prenatal development and ovarian morphology during pregnancy in small rodents. Specifically, we wished to evaluate the utility of high-resolution ultrasonography for imaging reproductive-tract structures and pregnancy progress in a small Hystricognathi rodent species, wild cavy (Cavia aperea aperea).
We examined 19 adult female wild cavies from the breeding colony of the University of Bielefeld, which had been introduced to a male at different periods. The animals varied in age (127–896 days old) and weight (309–654 g). The entire reproductive tract of each animal in each group was imaged using a Diasis™ ultrasound machine (Dynamic Imaging, Livingston, Scotland, UK) two or three times during a four-week period. All animals were examined in dorsal recumbency under isoflurane anesthesia (Forene™, Abbott GmbH, Wiesbaden, Germany).
Three different ultrawide-band electronic linear-array-probe-types with frequencies from 8–16, 10–22, and 28 MHz were used. The miniaturized size of the transducer (active length: 26 mm) allowed small contact areas. Frequencies from 8–16 MHz were used to visualize ovaries, uterus, number of concepti, as well as embryonic and fetal development. Placental integrity and amniotic fluid quality were also evaluated. The application of higher frequencies (18–28 MHz) permitted imaging of structures down to 0.4 mm. This allowed reliable pregnancy diagnosis shortly after implantation, on day 8 after copulation. Ovarian functional structures (e.g., corpora lutea and follicles) could be distinguished and measured. Changes during pregnancy were followed. The 8–16 and 10–22 MHz transducers were used to identify fetal organs and to record precise measurements of crown-rump length, biparietal distance, cerebellum, ventricle, lens, orbita, long bones, and umbilical cord. Evidence of embryonic or fetal resorption was identified in four female guinea pigs, but no disturbance of general or reproductive health could be recognized.
With high-resolution frequencies up to 28 MHz, the applied ultrasound system was able to image even the slightest morphologic changes in reproductive tissue in small rodents. The use of these techniques could well replace the standard methods for pregnancy monitoring in small rodents (i.e., carcass dissection), and thereby provides a new option in small animal diagnostics.