Interactions of ultrasound with matter

Various basic interactions of ultrasound with matter i.e., Reflection, Refraction, Absorption and Scattering

1. INTERACTION OF ULTRASOUND WITH MATTER Swapnil Shetty 201142002 MSc.MIT MCHP MAHE

2. ULTRASOUND Ultrasound is the sound of frequencies above 20,000 Hz. Frequencies of 1–30 megahertz (MHz) are typical for diagnostic ultrasound. Different ranges of frequency are used for examination of different parts of the body: 3–5 MHz for abdominal 5–10 MHz for superficial parts 10–30 MHz for the skin, eyes

3. INTRACTIONS OF ULTRASOUND WITH MATTER Ultrasound interactions are determined mostly by the acoustic properties of matter Interactions that occur include: Reflection Refraction Absorption Scattering

4. REFLECTION The most important single interaction process for purposes of generating an ultrasound image is reflection. Best reflection occurs from a smooth surface and is called specular reflection. Reflection depends on Acoustic Impedance Angle Of Incidence

5. ACOUSTIC IMPEDANCE The ratio of the pressure over an imaginary surface in a sound wave to the rate of particle flow across the surface. It’s the fundamental properties of matter. Z = ρ v Z = acoustic impedance ρ = density V = velocity of sound

6. More difference in acoustic impedance value between two materials, more will be reflected echo MATERIAL ACOUSTIC IMPEDANCE Air 0.0004 Fat 1.38 WATER (50 C) 1.54 BRAIN 1.58 BLOOD 1.61 KIDNEY 1.62 LIVER 1.65 MUSCLE 1.70 SKULL (BONE) 7.8

7. Coupling agent ( gel) is used as the coupling medium is to facilitate transmission of the ultrasound energy from the machine head to the tissues.

8. ANGLE OF INCIDENCE The amount of reflection is determined by the angle between the sound beam and reflecting surface (Angle of incidence) Higher the angle of incidence , less the amount reflection

9. Positions within tissue where the values of acoustic impedance change are very important in ultrasound interactions. These positions are called acoustic boundaries, or tissue interfaces. For example, urine in the bladder will have an acoustic impedance value which differs from that of the bladder wall, hence their common interface constitutes an acoustic boundary.

10. Percentage of beam reflected is given by R = (Z 2 -Z 1 )/Z 2 +Z 1 ) 2 *100 R = Percentage of beam reflected Z 1 = Acoustic impedance of medium 1 Z 2 = Acoustic impedance of medium 2

11. Percentage of beam transmitted is given by T = 4Z 1 Z 2 /(Z 1 +Z 2 ) 2 *100 T = Percentage of beam transmitted

12. 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.

13. With diffuse reflection, the ultrasound wave is reflected in multiple directions upon striking a particle in the medium

14. REFRACTION Change in the direction due to change in medium is referred to as Refraction When sound passes from one medium to another its frequency remains constant but its wavelength change to accommodate a new velocity in the second medium. Refraction can cause artifacts Distortion and loss of resolution in image

15. The angle of refraction is determined by the change in the speed of sound that occurs at the boundary, and is related to the angle of incidence by Snell's law: Өi = incidence angle Өt = transmitted angle c1= velocity of sound for incident medium c2= velocity of sound for transmitting medium

16. ABSORPTION Absorption is the main form of attenuation. Absorption refers to the conversion of ultrasonic to thermal energy. Absorption happens as sound travels through soft tissue, the particles that transmit the waves vibrate and cause friction and a loss of sound energy occurs and heat is produced Three factors determine the amount of absorption. Frequency of the sound Velocity of the conducting medium Relaxation time

17. Relaxation time : is the time take for a molecule to return to its original position after it has been displaced Hence Tissue with longer relaxation absorbs more ultrasound energy Absorption happens as sound travels through soft tissue, the particles that transmit the waves vibrate and cause friction and a loss of sound energy occurs and heat is produced. Absorption causes heating in the tissues and does not has any role in image formation .

18. SCATTER When the reflecting surface is irregular in surface, and its dimensions are smaller than the diameter of ultrasound beam, the incident beam is reflected in many directions. This is known as non- specular reflection or scattering

19. SCATTERING Scattered echoes are much weaker than specularly reflected echoes he beam in different directions. Though scattering of the beam decreases the quality of the image obtained is grainy due to scatter of sound wave but Within the organs, there are many structures which have dimensions of less than 1 mm, and so scattered ultrasound provides much useful information about the internal texture of organs.

20. DIFFRACTION Diffraction is the uniform spreading of an ultrasound beam as it propagates from the source. It is an additional type of scattering The smaller the source of the sound , the higher the diffraction of the beam. Hence More diffracted ,more attenuated.

21. Significance of reflection Acoustic impedance is the resistance to propagation of ultrasound waves through tissues. Each tissue has a unique acoustic impedance. As density of tissue increases, impedance also increases. Its effects are noticeable at interface between different tissue types. Larger the difference, sound is reflected.

22. Significance of refraction Refraction occurs when the ultrasound signal is deflected from a straight path and the angle of deflection is away from the transducer. Ultrasound waves are only refracted at a different medium interface of different acoustic impedance. 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. Refraction causes double-image artefact (Ghosting artefact).

23. Effects of absorption Due to the law of the conservation of energy, all of the ultrasound attenuated by tissues must be converted to other forms of energy. The majority of this is turned into heat. As such, it is possible for ultrasound to raise tissue temperature by up to 1.5°C. - BIOEFFECT

24. Effects of scattering Most echoes from ultrasound imaging arise from scattering, rather than the reflection from specular reflectors. The speckle arising from this scatter results in the grainy appearance of the parenchyma of organs and also the signal in doppler ultrasound.

25. REFERENCES Christensen’s Physics of Diagnostic Radiology – Thomas S. Curry & Robert C . Murrey https://www.vaultrasound.com/educational-resources/ultrasound-physics/reflection-refraction/

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