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 Sound waves: series of mechanical pressure waves that require a medium to travel through.

 US waves undergo attenuation, reflection, refraction, and scattering as they travel through tissue.

 Acoustic impedance: resistance of tissue to passage of US waves.

 Degree of reflection is determined by difference in acoustic impedance of two tissues at interface.

 US image is formed from reflected echoes.

Table 4.1 Features of ultrasound physics.

Body tissue	Acoustic impedance	Degree of reflection
Air	Very low	High
Liver, blood, kidney	‘Average’ soft tissue	Low
Bone	Very high	High

Table 4.2 Probe types.

Linear array probe	Phased array probe	Large curvilinear probe
Commonly referred to as vascular probe High frequency (typically 5–10 MHz)Large footprintExcellent image resolution of superficial structures at expense of tissue penetrationUse: vascular, lung (specifically pleura)	Commonly referred to as cardiac probe Small footprint; sound waves originate from single point and fan outwardLow frequency (typically 1–5 MHz)Excellent tissue penetration at expense of image resolutionUse: cardiac, lung, abdomen	Large footprint; sound waves originate from large area and fan outwardLow frequency (typically 2–5 MHz)Excellent tissue penetration at expense of image resolutionUse: abdomen