Therapeutic Ultrasound PDF

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12/05/2020

Week 9, Lecture 1 Sue Barrs

If you have any questions… To contact Sue Barrs via e-mail: [email protected]

Student consultation times: By Appointment (booked via email or phone) Building 43, Room 113

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Background  Part of clinical practice since sometime back in the

1950's  Popular and evidenced intervention for a range of

clinical problems  There are myriad therapy ultrasound machines

available, from the small, portable devices, through to the multimodal machines which include ultrasound as one of the available options

Ultrasound Energy  Mechanical Energy  Frequencies used are

typically 1.0MHz and 3.0MHz  Sound waves are LONGITUDINAL waves consisting of areas of COMPRESSION and RAREFACTION

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Ultrasound Energy  Non thermal effects too.  As the US wave passes through a material (the

tissues), the energy levels within the wave will diminish as energy is transferred to the material.  Frequency  Wavelength  Velocity  The above three factors are related but not constant

for all types of tissue

Ultrasound Beam Near & Far Fields  Not uniform  Near / Interference Field nearest the treatment head  Size of Near Field: radius of transducer squared and

divided by the US wavelength (r²/λ) e.g. for 1MHZ with diameter of head = 25mm; 12.5 ÷ 1.5mm = 10cm

 Far Field US beam more uniform and gently divergent  The ‘hot spots’ noted in the near field are not significant.

For the purposes of therapeutic applications, the far field is effectively out of reach.

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Beam Nonuniformity Ratio (BNR)  Indication of near field interference  Describes numerically the ratio of the intensity peaks

to the mean intensity  For most machines usually BNR is 4 – 6 i.e. the peak intensity could be 4 – 6 times higher than the mean intensity  Therefore keep transducer/treatment head

moving to avoid build up of hot spots and unstable cavitation

Ultrasound Transmission Through Tissues  All tissues will present an

impedance to the passage of sound waves  Specific impedance of a tissue will be determined by its density and elasticity  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.

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Ultrasound Transmission and Coupling Mediums  Water based Gel  Water Bath  Gel pads no more then 0.5cm thick  There is no realistic (clinical) difference between the

gels in common clinical use (Poltawski and Watson 2007).  The addition of active agents (e.g. anti-inflammatory

drugs) to the gel is widely practiced, but remains incompletely researched

Ultrasound Transmission  Refraction if the wave does not strike the boundary

surface at 90°  Direction of beam will alter through the second medium i.e. skin  Critical angle for US at the skin interface = 15°  If hold treatment/transducer head at an angle of 15° or more to the plane of the skin surface, the majority of the US beam will travel parallel to the skin surface through the dermal tissues

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Ultrasound Absorption & Attenuation  Exponential pattern

Table to show average half value depths for therapeutic ultrasound

 Will be a point at which the

energy has no therapeutic effect  As penetrates further into the tissues a greater proportion of the energy is absorbed and thus less available for therapeutic effects

1MHz

3MHz

Muscle

9.0mm

3.0mm

Fat

50.0mm 16.5mm

Tendon

6.2mm

2.0mm Watson 2013

Ultrasound Half Value Depths  Average half value

depths are employed for each frequency: 3MHz = 2.0cm 1MHz = 4.0cm  Some research (as yet unpublished) suggests that in the clinical environment, they may be significantly lower.

Depth 3MHz (cm) 2 50%

1MHz

4 6

50%

8

25%

25% Watson 2013

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Absorption Characteristics of US Watson 2013

Pulsed Ultrasound  Typical pulse ratios are 1:1

Illustration of Effect of Varying the Pulse Ratio

and 1:4  In 1:1 mode, the machine offers an output for 2ms followed by 2ms rest  In 1:4 mode, the 2ms output is followed by an 8ms rest period.  Pulse Frequency - the 2ms pulse time is 'normal'

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Thermal Effects  US less effective for thermal component – to raise

temperature by about 3-4°C to be of therapeutic value far too inefficient.  Use other EPAs e.g. PSWT or a hot water bottle!

 US far more effective for non-thermal effects

Non-Thermal Effects 1. Cavitation – Stable & unstable 2. Acoustic Streaming  The non-thermal effects are attributed primarily

to a combination of the 1 and 2 above.

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Cavitation  Formation of gas filled voids within the tissues &

body fluids  STABLE CAVITATION does seem to occur at therapeutic doses of US. It is the formation & growth of gas bubbles by accumulation of dissolved gas in the medium.  UNSTABLE (TRANSIENT) CAVITATION is the formation of bubbles at the low pressure part of the US cycle

Acoustic Streaming  Small scale eddying of fluids near a vibrating

structure such as cell membranes & the surface of stable cavitation gas bubble  Affects diffusion rates and membrane permeability  Cell membrane transport mechanism

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Non Thermal Effects of US

US and Tissue Repair  The process of tissue

repair is a complex series of cascaded, chemically mediated events that lead to the production of scar tissue that constitutes an effective material to restore the continuity of the damaged tissue (Watson, 2013).

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Inflammation and US

Proliferation and US

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Remodelling and US

US and Healing  The application of ultrasound during the

inflammatory, proliferative and repair phases is not of value because it changes the normal sequence of events, but because it has the capacity to stimulate or enhance these normal events and thus increase the efficiency of the repair phases (ter Haar 1999, Watson 2007, 2008, Watson & Young, 2008 cited in Watson, 2013)

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Ultrasound Dose Calculation

Ultrasound Treatment  Ultrasound treatment principle – 1 minutes worth of

ultrasound per treatment head area  Therefore longer if PULSED and longer for LARGER TREATMENT AREAS  To determine the PULSE FACTOR, add the two

components of the ratio together e.g. - Pulsed at 1:4 adds up to 5,multiply by 5. Pulsed at 1:1, adds up to 2, multiply by 2 etc.

 Treatment time = 1 x (no of times

treatment head fits onto tissue to treat) x (pulse factor)

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Other Ultrasound Applications  Low Intensity Pulsed Ultrasound (LIPUS) - Fracture

repair  High Intensity Focussed Ultrasound (HIFU)  Low Frequency (Longwave) Ultrasound  Therapy Ultrasound as a Diagnostic Tool for Stress Fractures  Ultrasound Therapy for Wound Healing  US at trigger points

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Longwave (kilohertz) Ultrasound  40-50Hz  Suggested that due to lower frequency and therefore

greater wavelength the energy penetrates further into the tissues  Lack of specific research into longwave or kilohertz

ultrasound BSB

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Longwave (kilohertz) Ultrasound  The penetration depth of kilohertz U/S is expected to

be in excess of 20x greater than MHz U/s.  Near and far fields  Ward & Robinson (1996) estimated the energy content of kHz U/S will be 50% reduced at 0.5cm and decreased to 10% at 2cm from the surface.

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Longwave (kilohertz) Ultrasound  Effects – very little evidence – anecdotal evidence but

no ‘hard‘ evidence  Little or no heating beyond 1-2cm depth (Robertson & Ward, 1996).  Meakins & Watson (2006) compared thermal effects of kHz U/S and heat (hot water bottle) – X-over design; both sig. Increased ankle mobility

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Summary for Longwave U/S  Longwave U/S has some different characteristics to

MHz U/S  In theory will penetrate further into tissues  A very high percentage of energy absorbed in very superficial tissues  Limited research evidence of effect, although anecdotally suggested ‘better’ than MHz U/ S

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Low Intensity Pulsed Ultrasound  Warden et al (1996) published a review paper

concluding that U/S could accelerate the rate of fracture repair by a factor of 1.6 using a low intensity (0.03Wcm²) at 1.5MHz pulsed at 1:4 for 20 mins. Daily.  No sig. Increase in tissue temperature  Mechanisms for U/S to be effective are the nitric oxide (NO) pathways and prostaglandin (PGE2)

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Shockwave Essentials Defined as: A largeamplitude compression wave, as that produced by an explosion or by supersonic motion of a body in a medium.

Shockwave  Effectively a controlled explosion which is reflected,

refracted, transmitted and dissipated in the tissues.  As with US consists of high pressure phase and a low pressure phase  Shock waves were initially employed as a non invasive treatment for kidney stones  Still a relatively new technology for musculoskeletal intervention

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Shockwave Therapy  Four ways to produce the

acoustic wave:

1. 2. 3. 4. 

Piezoelectric Spark Discharge Electromagnetic Electrohydraulic/Pneumatic The wave generated will vary in its energy content and will have different penetration characteristics in human tissue.

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Shockwave Therapy Characteristics  Peak pressure 50-80mPa (Ogden et al.,2001) 35-

120MPa (Speed, 2004).  Fast pressure rise (less than 10ns)  Short duration (10 microseconds)  Narrow effective beam (2-8mm diameter)  Cavitation occurs during low pressure phase (as with U/S). The collapse of the cavitation responsible in part for the efficacy of the therapy

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Shockwave Treatment Divided according to energy content:  LOW – up to 0.08mJ/mm²  Medium – up to 0.28mJ/mm²  HIGH – over 0.6mJ/mm²

Physiological Effects and Mechanisms of Action The following are the most strongly established treatment effects at therapy level  Mechanical stimulation  Increased blood flow  Increase in cellular activity  Transient analgesic effect on afferent nerves  Break down calcific deposits (primarily tendon)

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Proposed Mechanisms hierarchy

Clinical Application Treatment Dose:  Number of shocks – usually between 1000 and 1500 (used in clinical trials with most significant outcomes)  Number of treatment session repetitions – most research 3-5 sessions at low energy with time between to allow tissue reaction to ‘subside’  The weight of the evidence is more supportive of the

intervention than not, with the anecdotal evidence being even stronger

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Shockwave Therapy Contraindications, Precautions and Dangers  Lung Tissue  Epiphysis  Haemophilia/anticoagulant therapy  Malignancy  Metal implants ok; but implanted cardiac stents and

implanted heart valves awaiting further investigation  Infection  Joint replacements

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REFERENCES  Kitchen, S. (2002) Electrotherapy Evidence –Based Practice

Churchill Livingstone, London.

 Knight, K. L. & Draper, D. O. (2013) Therapeutic

Modalities: The Art and Science (2nd edition) Lippincott Williams & Wilkins: London

 Low & Reed, (2000) Electrotherapy Explained Principles And

Practice Butterworth Heinmann, Oxford  Walsh, (1997) TENS Clinical Applications and Related Theory

Churchill Livingstone, London  Watson (2008) Electrotherapy: Evidence-Based Practice (12th edition) Churchill Livingstone: London  Watson, T. (2013) www.electrotherapy.org

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