The objectives of this lesson is to understand transmission of ultrasound through tissues, the interactions of ultrasound wave with body tissues. The ability to see differences between tissues is determined by the variation in acoustic impedance of the tissues, and depends on the mass density in different tissues. The ultrasound wave is reflected (echo) at the boundary of two media with different acoustic impedances. Ultrasound waves interact with the tissues in three main ways: reflection, absorption and scattering. The depth of echo in tissues is determined by the measurement of time of flight.