Category: 2. Acoustic Impedance

REFRACTION occurs with ultrasound waves when two adjacent tissues have Slightly Different Impedance Values.

So when ultrasound waves travel through tissue and meet another tissue with slightly different impedance values, the speed changes somewhat and cause the ultrasound waves to change in direction. This change in direction is called Refraction!

The degree of how much refraction occurs is dependent on what angle the ultrasound wave encounters the second medium and how much of a change in speed there is in the second medium. This is seen mostly in situations at the rounded interfaces between a fluid-filled circular structure and the adjacent soft tissue. This is what gives rise to the edge artifact seen in ultrasound with black lines arising from the edge of fluid-filled structures such as the gallbladder, cyst, vessels, and bladder.

The importance of Impedance in ultrasound becomes apparent at the interface of two tissue types with significantly different impedance values. Ultrasound waves will reflect when this situation occurs. The proportion of ultrasound waves reflected back is proportional to the difference in impedance (or density) of two tissue types

REFLECTION occurs with ultrasound waves when two adjacent tissues have Significantly Different Impedance Values.

This is why bone and air appear as bright lines on ultrasound and also why you get the reflected “A-Lines” with pulmonary ultrasound. There is such a large difference between impedance of tissue and bone/air that they will cause almost all of the ultrasound waves to reflect back instead of penetrating through. What is interesting is that the impedance values of Air (extremely low at 0.0004) and bone (very high at 12), both cause reflection because of its drastic difference from the impedance of soft tissue (approximately 1.6).

Acoustic Impedance is the Resistance to Ultrasound Propagation as it Passes Through a Tissue

Acoustic Impedance is probably one of the most confusing terms when trying to learn ultrasound physics.

Acoustic Impedance (Z) is actually a physical property of a medium or tissue. It is dependent on the tissue density and the speed of sound through that tissue.

Impedance = Density x Propagation Speed of Sound Wave

So if the density of a tissue increases, the impedance (resistance) will increase as well. Refer to the ultrasound physics table again:

Tissue or Material	Speed of Sound (m/s)	Acoustic Impedance (kg/[s m2]) × 10^6	Density (g/cm3)	Attenuation (dB/cm/MHz)
Air	330	0.0004	0.0012	12
Fat	1450	1.38	0.95	0.63
Blood	1575	1.66	1.055	0.18
Liver	1570	1.69	1.06	0.94
Bone	4080	7.75	1.9	15