1.1 Introduction

Sound is pressure changes or vibrations in the frequency range that are emitted in a certain environment and can be detected by the human ear. The upper limit of the sound that the ear can detect is 20 kHz. Any sound above the audible frequency level is defined as ultrasound (ultrasonic sound wave). Ultrasound imaging (USG) is based on the piezoelectric (pressure-electric) effect that was discovered in 1880 by the brothers Pierre and Jacques Curie. The piezoelectric effect working with the expansion of crystals when electrical energy is given, and thus turn electricity into sound waves. Likewise, the sound waves that were returned back after reflection by the organs converted into electrical energy with the same method. The energy converting materials are called transducers. In USG devices, the transducer is also used as a receiver. Briefly, high sound waves are sent to organs through the skin and each organ reflects these sound waves differently. The reflected sounds are then collected again with the help of the transducer and following a live image is obtained [1]. The transducer in USG imaging is called a probe. The probe produces and emits the ultrasonic waves and transmits to the tissues where the reflected sound waves from the tissue are detected and converted into electrical signals to generate images. High-frequency sound waves lose their energy due to absorption and reflection as they pass through different tissues. Tissue depth is determined depending on the time required for the sound wave to leave and return to the probe [2, 3]. Images are generated according to tissue specifications, depth as well as the amplitude of the echoes returning from the tissues [2, 4]. The rapid vibration, which is transmitted to the patient through a conductive gel, propagates longitudinally into the body as a short, brief series of compressions (high pressure) and rarefactions (low pressure). Each ultrasound wave is characterized by a specific wavelength (distance between pressure peaks) and frequency (number of pressure peaks per second). The propagation velocity of a sound wave (i.e., acoustic velocity) is fairly constant.

in the human body (c) and is approximately 1540 meters per second. The process of transmission and reception can be repeated over 7000 times a second and, when coupled to computer processing, will result in the generation of a real-time two-dimensional image that appears seamless. The degree to which the ultrasound waves reflect off a structure and return to the probe will determine the signal intensity on an arbitrary grayscale. Structures that strongly reflect ultrasound generate large signal intensities and appear whiter or hyperechoic. In contrast, hypoechoic structures weakly reflect ultrasound and appear darker [5].

Dense and homogenous structures have little attenuation; waves pass through them and do not lose amplitude. For example, fluid-containing structures—such as vessels, distended bladder, and cysts—have minimal wave attenuation due to the fluid’s homogeneity and appear hypoechoic (dark gray) to anechoic (black). In contrast, muscle and visceral organs have higher attenuation coefficients as a result of their heterogeneity and appear hyperechoic (light gray to white). The properties of the various media in human tissue and their respective ultrasound appearances are summarized in Table 1.1. [6].

Table 1.1

Acoustic impedance and attenuation coefficients for various media and their resultant ultrasound appearance (derived from Bakhru and Schweickert, 2013)

Medium	Acoustic impedance (106kg/[s × m2])	Attenuation coefficient (dB/m at 1 MHz)	Ultrasound appearance
Air	0.0004	4500	Hypoechoic (high scatter)
Fat	1.3	60	Hypoechoic
Fluid	1.5	6	Very hypoechoic
Blood	1.7	9	Very hypoechoic
Liver/kidney	1.7	90	Echogenic
Muscle	1.7	350	Echogenic
Bone	7.8	870	Hyperechoic surface with anechoic posterior shadow

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