This document discusses the interaction of ultrasound with matter. It explains that ultrasound reflections, refractions, absorptions, and scatterings are determined by the acoustic properties of tissues. Reflection is the most important interaction for generating ultrasound images. Reflection depends on the acoustic impedance at tissue interfaces, which is determined by density and sound velocity. Differences in acoustic impedance between tissues result in more reflection. Absorption converts ultrasound to heat as it passes through tissues. Scattering results in weaker, diffuse reflections that degrade image quality. Refraction bends ultrasound beams at tissue boundaries based on changes in sound speed. The effects of these interactions are important for ultrasound imaging.

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

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

5. 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

6. 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

7. 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
0
C) 1.54
BRAIN 1.58
BLOOD 1.61
KIDNEY 1.62
LIVER 1.65
MUSCLE 1.70
SKULL (BONE) 7.8

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

9. ANGLE OF
INCIDENCE
•The amount of
reflection is
determined by the
angle between the
sound beam and
reflecting surface
(Angle of incidence)

10. • 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

11. Percentage of beam reflected is given by
R = (Z2-
Z1)/Z2+Z1)2 *100
R = Percentage of beam reflected
Z1 = Acoustic impedance of medium 1
Z2 = Acoustic impedance of medium 2

12. Percentage of beam transmitted is given
by
T = 4Z1Z2/(Z1+Z2)2
*100
T = Percentage of beam
transmitted

14. • 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

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

16. 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

19. • 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

20. 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

21. • 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

22. SCATTER
•When the reflecting
surface is irregular
in surface, and its
dimensions are
smaller than the
diameter of
ultrasound beam,

24. 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

26. 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.

28. 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.

29. 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

30. 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

31. 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.

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

33. THANK YOU