This document discusses key concepts in ultrasound therapy. It defines ultrasound as mechanical energy consisting of areas of compression and rarefaction. Frequency is the number of compression/rarefaction cycles per second. Propagation speed depends on the medium and determines wavelength. Accoustic impedance describes resistance to an ultrasound beam. Reflection and refraction occur at impedance boundaries. Attenuation reduces intensity as ultrasound propagates through tissue. Intensity is further defined as spatial peak, average, temporal peak and average.

1. ULTRASOUND THERAPY II
Aditya Johan .R, M.Fis

2. INTRODUCTION
 Ultrasound (US) is a form of
MECHANICAL energy, not
electrical energy
 Sound waves are longitudinal
waves consisting of areas of
compression and rarefaction

3. FREQUENCY
 Frequency - the number of times a particle experiences a complete
compression/rarefaction cycle in 1 second.
 1 MHz equal to 1000000 Hz

4. PROPAGATION SPEED
 Propagation speed/velocity is the maximum speed which an acoustic wave can move
through a medium (depens on mediums), unit (m/s)
 Propagation speed is greater in solids > liquids > gases
 the propagation speed is constant for a given medium, if the frequency increases, the
wavelength will decrease, viceversa
Propagation speed (c) = Frequency (f) . Wavelength (𝜆)

5. PROPAGATION SPEED

6. WAVELENGTH
 Wavelength is the distance between two equivalent
points on the waveform in the particular medium
 With a velocity or propagation speed (c) of 1,540 m/s,
the wavelength of 1 MHz is 1.54 mm, of 2 MHz is 0.77
mm, and of 3 MHz is 0.51 mm

7. ACCOUSTIC IMPEDANCE
 Accoustic Impedance describe how much resistance an ultrasound beam encounters as
it posses through a tissue
 Z (muscle) = 1075 kg/m3 x 1585 m/s = 1.7 x 106

9. REFRACTION
 At an acoustic impedance, the sound beam will be refracted, be reflected, or both
sin 𝐼
𝑐𝐼
=
𝑠𝑖𝑛𝑅
𝑐𝑅

10. TASK
 If incident wave strike to fat at 45 degree, what degree of refraction on a
muscle, knowing that fat has density 925 kg/m3 with acoustic impedance
at 1.34 x 106 and muscle has density 1075 kg/m3 with acoustic impedance
at 1.70 x 106 ?

11. REFLECTION
 At the boundary between media of different acoustic impedances, some of the wave
energy is reflected and some is transmitted
 The greater the difference in acoustic impedance between the two media, the greater
the reflection and the smaller the transmission
 The intensity reflection coefficient (IRC) is defined as the ratio of the intensity of the
reflected wave relative to the incident wave

12. TRANSMITION
 The Intensity Transmition Coefficient (ITC) is the amount or percent of
intensity which is transmitted at the interface
Intensity Transmission Coefficient = 1 - IRC

13. TASK
 Soundwave delivered at 1.5 W/cm2 through fat and muscle, knowing that
fat has density at 925 kg/m3 with propagation speed at 1450 m/s and
muscle has 1075 kg/m3 with propagation speed at 1590 m/s, how much
intensity will be reflected and transmitted?

14. ATTENUATION
 Attenuation is the reduction of the sound beam’s amplitude and intensity as it travels
through a medium
 Attenuation coefficient is the attenuation per unit length of sound wave travel
 For soft tissue, it is approximately one-half of the operating frequency of the transduce
 for every centimeter per MHz, there is approximately 0.5 dB of attenuation coefficient
Attenuation (dB) = Att coef x distance (d) x frequency (f)

15. EXAMPLE
 How much Intensity left for soundwave 3 MHz frequency with 1.5 W/cm2 at 4 cm?
Att = 0.5 x 4 cm x 3 MHz
Attenuation = - 6 dB
Value of dB = 10-6/10 = 0.251
Final Intensity = Incident Intensity x 0.251
= 1.5 W/cm2 x 0.251
= 0.38 W/cm2

16. POWER & INTENSITY
 The ultrasound power and the intensity of the ultrasound beam are not identical
 The ultrasound power is the rate of energy transferred
 The intensity is the power per unit area and represents the strength of the ultrasound
beam
 Intensities have both a peak value and an average value
 Spatial peak (SP) is intensity at the center of the beam
 Spatial average (SA) is intensity averaged throughout the beam

17. SPATIAL PEAK AND AVERAGE INTENSITY
 Spatial peak intensity is related to Spatial
avarege by the Beam Nonuniformity Ratio
(BNR)

18. SPATIAL AVERAGE TEMPORAL PEAK &
AVERAGE
 Spatial Average Temporal peak (SATP) is maximum
intensity in the pulse (measured when the pulse is
on)
 Spatial Average Temporal average (SATA) is
intensity averaged over one on-off beam cycle
(takes into account the intensity from the
beginning of one pulse to the beginning of next)
 SATP x duty cycle = SATA