Ultrasound therapy uses high frequency sound waves to create effects in the body. There are thermal and non-thermal effects. Thermal effects occur when ultrasound is used to heat tissues, potentially increasing flexibility. Non-thermal effects are thought to be from cavitation and acoustic streaming. Cavitation involves gas bubble formation, while acoustic streaming disturbs fluid flow. Together these effects can increase cell membrane permeability and metabolic activity. Proper ultrasound dosage considers frequency, intensity, pulse ratio, and treatment duration to target specific depths and achieve desired effects while avoiding harm.

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

2. INTRODUCTION
 Ultrasound refers to mechanical vibrations which are essentially the same as sound
waves but of a higher frequency (> 20.000 Hz)
 Sonic waves are a series of mechanical compressions and rarefactions in the direction of
wave’s travel, hence they are called Longitudinal waves
 Ultrasonic energy describes any vibrations at a frequency above the sound range
 A few Megahertz that are tipically used in physiotherapy 0.5 – 5 Mhz

5. ULTRASONIC WAVES

6. FRESNEL & FRAUNHOFFER ZONE

7. Length of Fresnel Zone = r2/ 𝜆

8.  A ) High-frequency transducer with long near-zone length and narrow beam width
 ( B ) Low-frequency transducer with short near-zone length and wide beam width
 ( C ) Focusing narrows beam width

9. ULTRASOUND TRANSMISSION THROUGH
THE TISSUES
 The greater the difference in impedance at a boundary, the greater the reflection that
will occur, and therefore, the smaller the amount of energy that will be transferred

10. As a matter of (clinical) interest, the US
treatment should be cleaned with an
alcohol based swab (not just wiped with
tissue) between treatments to minimise the
potential transmission of microbial agents
between patients

11. ULTRASOUND APPLICATION - THE CRITICAL
ANGLE
In addition to the reflection that occurs
at the boundary due to impedance,
there will also be some refraction if the
wave does not strike the boundary
surface at 90°

12. ULTRASOUND ABSORPTION AND
ATTENUATION
 As the US beam penetrates further into the tissues, a greater proportion of the energy
will have been absorbed and therefore there is less energy available to achieve
therapeutic effects
 The half value depth is often quoted in relation to US and it represents the depth in the
tissues at which half the surface energy is available
 These will be different for each tissue and also for different US frequencies

13. HALF VALUE DEPTH
average Half Value Depths are employed for each frequency: 3 MHz = 2 cm & 1 MHz = 4 cm

14. PULSED
 the pulse duration (the time during which the machine is on) was almost exclusively
2ms (2 thousandths of a second) with a variable off period
 The effects of pulsed US are well documented and this type of output is preferable
especially in the treatment of the more acute lesions

15. Some manufacturers describe their pulsing in terms of a percentage rather than a ratio

16. THERMAL EFFECT
 In thermal mode, US will be most effective in heating the dense collagenous tissues and
will require a relatively high intensity, preferably in continuous mode to achieve this
effect
 The oscilation of particles due to sonic energy converted into heat energy
 If heat dissipation equals heat generation there is no net rise
 If local temperature is raised to between 40 – 45 degree hyperaemia will result, to
achieve a usefull effect the tissue temperature has to be maintained between these
values for at least 5 minutes
 Heating ligaments, joint capsules, tendons may cause a temporary increase in their
extensibility and hence decrease in joint stiffness

17. NON THERMAL EFFECT
 The non-thermal effects of US are now attributed primarily to a combination of
CAVITATION and ACOUSTIC STREAMING
 CAVITATION in its simplest sense relates to the formation of gas filled voids within the
tissues & body fluids
 There are 2 types of cavitation - STABLE & UNSTABLE which have very different effects

18. CAVITATION
 STABLE CAVITATION does seem to occur at therapeutic doses of US
 This is the formation & growth of gas bubbles by accumulation of dissolved gas in the
medium
 They take aproximatelly 1000 cycles to reach their maximum size
 The `cavity' acts to enhance the acoustic streaming phenomena & as such would
appear to be beneficial

19. UNSTABLE CAVITATION
 UNSTABLE (TRANSIENT) CAVITATION is the formation of bubbles at the low pressure part of the
US cycle
 These bubbles then collapse very quickly releasing a large amount of energy which is detrimental
to tissue viability

20. ACCOUSTIC STREAMING
 ACOUSTIC STREAMING is described as a small scale eddying of fluids near a vibrating
structure such as cell membranes & the surface of stable cavitation gas bubble
 This phenomenon is known to affect diffusion rates & membrane permeability
 Sodium ion permeability is altered resulting in changes in the cell membrane potential
 Calcium ion transport is modified which in turn leads to an alteration in the enzyme
control mechanisms of various metabolic processes, especially concerning protein
synthesis & cellular secretions
 The result of the combined effects of stable cavitation and acoustic streaming is that the
cell membrane becomes ‘excited’ (up regulates), thus increasing the activity levels of the
whole cell

23. MICROMASSAGE
 The waves of compression and rarefaction may produce a form of micromassage which
could reduce oedema

24. ULTRASOUND DOSAGE
FREQUENCY
 Taking into account that the most frequently available treatment frequencies are 1 and
3MHz
 3MHz ultrasound is absorbed more rapidly in the tissues, and therefore is considered to
be most appropriate for superficial lesions, whilst the 1MHz energy is absorbed less
rapidly with progression through the tissues, and can therefore be more effective at
greater depth

25. FREQUENCY
 To identify the appropriate dose to set on the machine, determine
 The estimated depth of the lesion to be treated
 The intensity of ultrasound required at that depth to achieve the desired effect. The
intensity to set on the machine is that which is indicated where the two columns/rows
meet
 to achieve a 0.5W cm-2 intensity at 1cm tissue depth, select 3MHz treatment option
and set machine to 0.7 W cm-2

26. DUTY CYCLE/ PULSE RATIO

27. INTENSITY
 The intensity shown on the meter is usually the Space Average Temporal Peak (ISATP)
 For continues the SATP and SATA are the same
 For pulsed the SATP x Duty Factor = SATA
 Pulsed : SATP intensity of 2 W/cm2 at 1:4 pulse ratio is equivalent to SATA intensity
(continues) of 0.4 W/cm2
 For acute intensities in the range 0.1 – 0.25 W/cm2
 For chronic intensities in the region of 0.25 – 1 W/cm2

28. DURATION/ TIME
 CONTINUES MODE : TREATMENT AREA / ERA
 PULSED MODE : TREATMENT AREA / ERA X PULSE RATIO (to total period)
 Convinient guide is to give1-2 minutes of treatment for every 10 cm2
 Minimum treatment times are considered to be 1-2 minutes, maximum 10-15 minutes
and an everage would be 5 minutes