**2. Ultrasound machine basics**

Ultrasound uses probes that act as transducers for the ultrasound waves. Each transducer uses a piezoelectric crystal material that converts electricity to ultrasound waves, and then converts the returning waves to an electrical signal that is interpreted by the machine to create the two-dimensional image we recognize as the internal structures [11, 12]. The interchangeable probes vary by the ultrasound wave frequency emitted, which provides an array of image acquisition capabilities that favor resolution in different areas. The curvilinear probe emits a lower frequency of 2–5 MHz, providing greater lateral resolution and better penetration for deeper structures. The linear transducer emits higher frequencies of 7–13 MHz, providing much higher resolution for superficial structures. The phased array probe lies between these two, emitting a frequency of 2.5–5 MHz and providing moderate resolution for superficial structures and good penetration for deep structures [9, 11].

If the ultrasound pulse encounters a structure that reflects most waves, it will appear bright white and is termed hyperechoic. If the pulse encounters a structure that does not reflect much or any of the waves, it appears darker or black and is termed hypoechoic or anechoic, respectively [13]. There are also various ultrasound modes that allow us to address specific questions related to movement or flow. "B" mode (brightness mode) is primarily used for diagnostic imaging with two-dimensional displays in gray-scale based on the tissues echogenicity. "M" (motion) mode depicts the motion through time of structures along a single vertical line within the B-mode image. This mode is frequently used to look for subtle movement in images or better characterize the degree of movement experienced by a structure like as a heart valve [12]. Color flow mode depicts direction and flow velocity of fluids, such as blood within the heart or vessels. Power Doppler mode emits short bursts of waves, allowing more accurate localization of echo sources and is more sensitive in measuring flow velocity in low-flow states [12, 13].
