Ultrasound therapy ii

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