Reflection, Refraction, Scattering and Attenuation

Reflection can be categorized as either specular or diffuse.  Specular reflectors are large, smooth surfaces, such as bone, where the sound wave is reflected back in a singular direction.  The greater the acoustic impedance between the two tissue surfaces, the greater the reflection and the brighter the echo will appear on ultrasound.  Conversely, soft tissue is classified as a diffuse reflector, where adjoining cells create an uneven surface causing the reflections to return in various directions in relation to the transmitted beam.  However, because of the numerous surfaces, sound is able to get back to the transducer in a relatively uniform manner.

 

The tibia, (yellow arrows) is a good example of a specular reflector.  The large smooth surface of the bone causes a uniform reflection because of the significant difference in the acoustic impedance between it and the adjoining soft tissue.

 

 

The pectoris major muscle (PM) located between the white arrows is an example of diffuse reflection.  The different accoustic impedances of the structures located within the muscle result in the various shades of grey seen on the BMode image.

 

 

Refraction is governed by Snell’s Law and describes reflection where sound strikes the boundary of two tissues at an oblique angle.  The reflections generated do not return directly back to the transducer.  The angle of refraction is dependent on two things; the angle the sound wave strikes the boundary between the two tissues and the difference in their propagation velocities.  If the propagation velocity is greater in the first medium, refraction occurs towards the center, or perpendicular (A).  If the velocity is greater in the second medium, refraction occurs away from the originating beam (B).  Because sound is not reflected directly back to the transducer, the image being depicted may not be clear, or potentially altered, “confusing” the ultrasound system since it assumes that sound travels in a straight line.

In the video below, a needle is inserted into a phantom at an acute angle causing refraction of the sound beam back to the transducer at an oblique angle,   resulting in a weaker “second” needle to be seen to the right of the actual needle.

Rayleigh scattering occurs at interfaces involving structures of small dimensions.  This is common with red blood cells (RBC), where the average diameter of an RBC is 7μm, and an ultrasound wavelength may be 300μm (5 MHz).  When the sound wave is greater than the structure it comes in contact with, it creates a uniform amplitude in all directions with little or no reflection returning to the transducer.

 

 

 

In the image below of the left saphenous vein (SV), common femoral vein (CFV), superficial femoral (SFA) and profunda femoris (PFA) arteries, Rayleigh scattering is present within each of the blood vessels.  Scattering is dependent for four different factors: the dimension of the scatterer, the number of scatterers present, the extent to which the scatterer differs from surrounding material, and the ultrasound frequency.

The culminating effect of tissue on sound as it travels through the body is attenuation.  Attenuation is the decreasing intensity of a sound wave as it passes through a medium.  It is the result of energy absorption of tissue, as well as reflection and scattering that occurs between the boundaries of tissue with different densities.   The attenuation coefficient of tissues is the relation of attenuation to distance, and depends on the tissues traversed and the frequency of the ultrasound wave.  In general, a reduction of 3dB equals diminution of the original wave intensity by half.  To compensate for attenuation, returning signals can be amplified by the ultrasound system, known as gain.   Attenuation is high in muscle and skin, and low in fluid-filled structures.  Higher frequency waves are subject to greater attenuation than lower frequency ones.

Attenuation Coefficient

Attenuation coefficients for different mediums.


References

Aldrich J E. Basic physics of ultrasound imaging. Crit Care Med. 2007;35(5 Suppl):S131-S137.

Zagzebski JA. Physics and instrumentation in Doppler and B-mode ultrasonography. In: Zweibel WJ. Introduction to Vascular Ultrasonography. 4th ed. Philadelphia, PA: W.B. Saunders Company; 2000:17-43.

Marhofer P, Frickey N. Ultrasonographic guidance in pediatric regional anesthesia part 1: Theoretical background. Paed Anaesth. 2006;16(10):1008-1018.

Sites B D, Brull R, Chan V W, et al. Artifacts and pitfall errors associated with ultrasound-guided regional anesthesia. part I: understanding the basic principles of ultrasound physics and machine operations. Reg Anesth Pain Med 2007;32(5):412-418.

Falyar CR. Ultrasound in anesthesia: applying scientific principles to clinical practice. AANA J. 2010 Aug; 78(4):332-40.

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