1.1 Background Information
Ultrasound (US) is an oscillating sound pressure wave with a frequency greater than the upper limit of the human hearing range. Ultrasonic imaging (ultrasonography) is used in both veterinary and human medicine (Novelline, 1997). Today, US is one of the most widely used imaging technologies in medicine. It is portable, readily available, free of radiation risk, and relatively inexpensive when compared with other imaging devices, such as magnetic resonance and computedtomography. Furthermore, US images are tomographic, i.e., offering a “cross-sectional”view of anatomical structures (Edler and Lindstrom, 2004).
The frequencies used in medical imaging is between 2 and 10 megahertz (MHz), where 1 MHz is 1 million cycles per second, 50 times greater than the maximum frequency of audible sound by the human ear; hence the name ultrasound(Luc DesCoteaux, 2010). US imaging is based on the 'pulse-echo' principle in which a short burst of ultrasound is emitted from a transducer and directed into tissue (Burns, 2011).
Reflection of sound waves produces the echoes that result in the echographic image. Reflection occurs when the ultrasound wave meets an acoustic interface, which is the interface between two media with different acoustic impedances (Luc DesCoteaux, 2010). Reproductive ultrasonography has increased the knowledge of the changes during early pregnancy in different animals (Ginther and Pierson, 1984; Curran, 1986).With the use of a real-time, B-mode ultrasonography and 5 MHz transducer, pregnancy can be detected as early as 9thto 12thdays post artificial insemination into gestation (Ginther and Pierson, 1984; Curran, 1986).