ULTRASOUND

MUTHUUKARUPPAN M.

Objectives 1. Understand the physics and properties of

ultrasonic waves2. Explain the production of ultrasonic waves3. Enumerate the Physiological effects of

Ultrasound4. Enumerate the therapeutic uses of ultrasound5. Evaluate the Indications and contraindications

for applying ultrasound6. Understand the precautions for applying

ultrasound7. Select appropriate methods of ultrasound

application to produce desired therapeutic benefits

8. Choose and use the appropriate treatment parameters for the safe application of ultrasound

9. Describe phonophoresis10. Identify the molecules used for phonophoresis,

indications and contraindications for phonophoresis

INTRODUCTION• Sound is defined as the periodic

mechanical disturbance of an elastic medium such as air.

• Sound requires a medium for its transmission and cannot cross a vacuum.

• Ultrasound refers to mechanical vibrations, which are essentially the same as sound waves but of a higher frequency. Such waves are beyond the range of human hearing and can therefore be called ultrasonic.

INTRODUCTION

• Vibration merges with sound at frequencies around 20 Hz; vibration below this frequency is often called infrasound or infrasonic. ▫ Audible sound – 20 to 20000 Hz ▫ Ultrasound – Greater then 20000 Hz ▫ Infrasound – Less than 20 Hz ▫ Therapeutic ultrasound – 0.5 to 5 MHz – 1 to 3 MHz

INTRODUCTION• Wavelength • Frequency • Velocity, and varies depending upon the

physical nature of the medium.

INTRODUCTION• Sonic waves are series of mechanical

compressions and rarefactions in the direction of travel of the wave, hence they are called longitudinal waves. • They can occur in solids, liquids, and gases and

are due to regular compression and separation of molecules.

• Passage of these waves of compression through matter is invisible because it is the molecules that vibrate about their average position as a result of the sonic wave. It is energy that travels and not the matter.

• As sound waves pass through any material their energy is dissipated or attenuated.

• All the energy is absorbed at once; sound wave passes with almost no loss.

• The molecules of all matter are in constant random motion; the amount of molecular agitation is what is measured as heat – the greater the molecular movement, the greater the heat. • Sound waves will pass more rapidly through

material in which the molecules are close together, thus their velocity is higher in solids and liquids than in gases

Material Velocity m/sec

Granite & Iron 6000

Lead 2100

Bone 3445

Tendon 1750

Cartilage 1665

Muscle 1552

Blood 1566

Fat 1478

Air at 20 C⁰ 343

Air at 0 C⁰ 331

Other uses of ultrasound:-• In industry low-frequency ultrasound is used

for cleaning and mixing processes since efficient vibration of very small particles is achieved. • It can also be used for cutting and engraving as

well as detecting cracks in metal such as welding defects. • The other major medical uses of ultrasound are

in body imaging (6-18 MHz) and dental drills / descalers. These latter usually operate at between 20 to 60 kHz.

Production of Therapeutic Ultrasound

• Piezo-electric effect: The production of a small electro motive force(e.m.f.) across certain substances on being subjected to external pressure. Such substances are known as piezo-electric substances

• Reverse piezo-electric effect: Production of mechanical waves or vibrations due to the application of e.m.f.

• Many types of crystal can be used but the most favored are quartz, which occurs naturally, and some synthetic ceramic materials such as barium titanate and lead zirconate titanate (PZT).

• These crystals deform when subjected to a varying potential difference – a piezo-electric effect • In order to apply the electric charges, metal

electrodes must be fixed to the crystal.

• If a suitable metal plate is fixed to one surface of the crystal while the opposite surface is in air, then almost all the vibrational energy is transmitted from the crystal to the plate and hence to any solid or liquid to which it is applied.

• The other essential parts of a therapeutic ultrasound generator are a circuit to produce oscillating voltages to drive the transducer, which can turn the oscillator on and off to give a pulsed output.

• A suitable circuit can maintain a constantly oscillating electric charge to cause the piezoelectric crystal to change shape at the same frequency • So drive the metal plate backwards and forwards

also at the same frequency in any medium with which it is in contact. • This amplitude is referred to as the intensity and

is the energy crossing unit area in unit time perpendicular to the sonic beam. It is therefore measured in watts per square centimeter.

• Current supplied to the oscillator circuit can be automatically switched on and off to produce a pulsed output, typically giving ratios 1:1 or 1:4. • A meter is often included which measures the

electrical oscillations applied to the crystal but not the vibration of the crystal

BOUNDARIES BETWEEN MEDIUM• Sonic waves involve vibratory motion of molecules so

that there is a characteristic velocity of wave progression for each particular medium.

• It depends on the density and elasticity of the medium and together these specify the acoustic impedance of the medium.

• Acoustic impedance = density of medium x velocity of wave

• Some of the energy is reflected back. The amount of

the energy reflected is proportional to the difference in

acoustic impedance between the two media. – Water / Glass – 63% of energy is reflected – Water / Soft tissue – 0.2% of energy is reflected

• Refraction also occurs with sonic waves due to the difference in acoustic impedance.• The beam of sonic energy that passes through the

second medium does not continue in a straight line but changes direction at the boundary because of the different velocities in the two media. • If the acoustic impedances are closely matched

little refraction will occur.

Absorption of Sonic waves• Kinetic energy is converted to heat energy as

it passes through the material. • The energy will decrease exponentially with

distance from the source because a fixed proportion of it is absorbed at each unit distance so that the remaining amount will become a smaller and smaller percentage of the initial energy

• The conversion of sonic energy to heat is due to increased molecular motion

• Half value depth: depth of tissue at which the US intensity is half its initial intensity• Absorption of sonic energy is greatest in

tissues with largest amounts of structural protein and lowest water content. • Blood – least protein content and least

absorption• Bone - greatest protein content and greatest

absorption

Attenuation of Ultrasound in the Tissues: • The loss of energy from the ultrasound beam in

the tissues is called attenuation and depends on both absorption and scattering• Absorption accounts for some 60 – 80% of the

energy lost from the beam. The scattered energy may also be absorbed other than in the region to which the ultrasound beam is applied. • Scattering is caused by reflections and

refractions, which occur at interfaces throughout the tissues. This is particularly apparent where there is a large difference in acoustic impedance.

• Transducer: ultrasound unit that contains the crystal

• Power: amount of acoustic energy per unit time (watts)

• Intensity: power per unit area of the ultrasound head (watts/cm2)

• Spatial average intensity: Average intensity of the US output over the area of the transducer

• Spatial peak intensity: Peak intensity of the ultrasound output over the area of the transducer. The intensity is usually great in the centre of the beam and lowest at the edges of the beam.

• Beam non-uniformity ratio (BNR): Ratio between peak intensity and average intensity in the beam. The lower the BNR the more uniform the beam • With BNR 5:1, when the

spatial average intensity is 1W/cm2, the spatial peak intensity would be 5W/cm2

• Continuous ultrasound: continuous delivery of US through out the treatment period

• Pulsed ultrasound: delivering US only during a portion of the treatment period. Pulsing reduces the thermal effects

• Duty cycle: proportion of the total treatment time that the US is on. This can be expressed in percentage or a ration

• 20% or 1:5 duty cycle, is on for 20% of the time and off for the 80% of time.

• Spatial average temporal peak intensity: spatial average intensity of the US during the on time

• Clinically US displays SATP intensity and duty cycle

• Spatial average temporal average intensity: The spatial average intensity of the US averaged over both the on time and the off time

• SATP X duty cycle = SATA• SATA is frequently used in

research and non clinical literatures

• Frequency: number of compression-rarefraction cycles per unit of time, usually expressed in cycles per second (Hertz)

• Increasing the frequency of US causes a decrease in its depth of penetration and concentration of the US energy in the superficial tissues.

• Effective radiating area (ERA): The area of the transducer from which the US energy radiates. Since the crystal doesn’t vibrate uniformly , the ERA is always smaller than the area of the treatment head.

• Some waves cancel out, others reinforce so that the net result is a very irregular pattern of the sonic waves in the region close to the transducer face, called the near field or Fresnel zone.

• Beyond this, the far field or Fraunhofer zone, the sonic field spreads out somewhat and becomes much more regular because of the differing path lengths from points on the transducer.

• The length of the near field depends directly on the square of the radius of the transducer face and inversely proportional to the wavelength of the sonic waves.

• Length of Fresnel zone = r2 / λ

• For practical purposes therapeutic ultrasound utilizes the near field. The relatively more energy on average is carried in the central part of the cross-section of the beam. • The irregularity of the near field can be ‘ironed

out’ to some extent by continuous movement of the treatment head during the therapy.

Propotional heating of 1 and 3 MHz ultrasound through tissues

• Shear waves can be formed which transmit energy along the periosteal surface at right angles to the ultrasound beam.

• Due to the fact that this reflection is quite large (almost 25%) and that sonic energy is absorbed almost immediately in bone, there is marked heating at the bone surface.

• This is considered to account for the periosteal pain that can arise with excessive doses of therapeutic ultrasound.

Heating in the tissues due to the Ultrasound:-• The important factor for heating in the tissue due to

ultrasound is the rate of tissue heating, which is, influenced both by the blood flow, which constantly carries heat away, and by heat conduction. • In highly vascular tissues such as muscle it is likely

that heat would be rapidly dissipated preventing any large temperature rise; on the other hand, less vascular tissue, such as dense connective tissue in the form of tendon or ligament, may experience a relatively greater temperature rise.

Moving the transducer head during the treatment is important because of following effects :-• To smooth out the irregularities of the near field • It reduces the irregularities of absorption that might

occur due to reflection at interfaces, standing waves, refraction, and differences in tissue thermal conduction or blood flow • It also reduces shear wave formation and thereby

reduces chances of periosteal pain • Thus resulting heating pattern is likely to be much more

evenly distributed. It has been estimated that for an output of 1 W/cm2 there is a temperature rise of 0.8°C/min if vascular cooling effects are ignored

• The effect is not the same because with pulsed treatment there is time for heat to be dissipated by conduction in the tissues and in the circulating blood. Therefore, higher intensities can be safely used in a pulsed treatment because the average heating is reduced. • Ultrasound application can increase rates of ion

diffusion across cell membranes; this could be due to increased particle movement on either side of the membrane and possibly, increased motion of the phospholipids and proteins that form the membrane.

Physical & Physiological effects:• As oscillation or sonic energy is passed through

the body tissue, it causes transfer of heat energy in the body tissues. If this energy is not dissipated by normal physiological response, then there is local rise in temperature, which accounts for thermal effects. • If heat dissipation equals heat generation there is

no net rise in temperature and any effects are said to be non-thermal.• Using low intensities or pulsing the output

achieves non-thermal effects.

Thermal effects:• The advantage of using ultrasound to achieve

heating is due to the preferential heating of collagen tissue and to the effective penetration of this energy to deeply placed structures. • Heating fibrous tissue structures such as joint

capsules, ligaments, tendons, and scar tissue may cause a temporary increase in their extensibility, and hence a decrease in joint stiffness. • Mild heating can also have the effect of reducing

pain and muscle spasm and promoting healing processes.

Non thermal effects:-Cavitation: • Cavitation is the formation of tiny gas bubbles

in the tissues as a result of ultrasound vibration. These bubbles, generally of a micron (10-6 m) diameter. • These can be of two types, namely stable

cavitation or transient(non-stable) cavitation. • Stable cavitation occurs when the bubbles

oscillate to and fro within the ultrasound pressure waves but remain intact.

• Transient (or collapse) cavitation occurs when the volume of the bubble changes rapidly and then collapses causing high pressure and temperature changes and resulting in gross damage to tissues. • Stable cavitation associated with acoustic

streaming, is considered to have therapeutic value but the transient cavitation, which is only likely to occur at high intensities, can be damaging.

In practice the danger of tissue damage due to cavitation is minimized by the following measures: • Using space-averaged intensities below 4W/cm2 • Using a pulsed source of ultrasound • Moving the treatment head during insonation Acoustic streaming:• Acoustic streaming is a steady circulatory flow due

to radiation torque. • Additionally, as a result of either type of cavitation

there is a localized, unidirectional fluid movement around the vibrating bubble. • These very small fluid movements also occur

around cells, tissue fibres, and other boundaries, which is known as microstreaming.

• Microstreaming exerts stress on the cell membrane and thus may increase membrane permeability. • This may alter the rate of ion diffusion causing

therapeutically useful changes, which includes increased secretion from mast cells, increased calcium uptake, and production of macrophages. • All these effects could account for the

acceleration of repair following ultrasound therapy.

Standing waves:-• Standing waves are due to reflected waves

being superimposed on the incident waves. • The result is a set of standing or stationary

waves with peaks of high pressure (antinodes).• Gas bubbles collect at the antinodes, and cells

collect at the nodes. • This pressure pattern causes stasis of cells in

blood vessels.

• The endothelium of the blood vessels exposed to standing waves can also be damaged leading to thrombus formation. • There is also the possibility of marked local

heating where the amplitude of the combined waves is high. • If transducer head is moved during the

treatment, then standing waves are unlikely to form.

Micromassage:-• The micromassage effect of ultrasound occurs

at a cellular level where the cells are alternately compressed and then pulled further apart. • The waves of compression and rarefaction may

produce a form of micromassage, which could reduce oedema. • Ultrasound has been found to be effective at

reducing recent traumatic oedema and chronic indurated oedema.

Acute stage:-• Stable cavitation and acoustic streaming

increases calcium ion diffusion across the cell membrane, which works as a cellular ‘secondary messenger’, and thereby increases the production and release of wound-healing factors. • These include the release of histamine from

mast cells and growth factors released from macrophages. • In this way, ultrasound has the potential to

accelerate normal resolution of inflammation providing that the inflammatory stimulus is removed.

• This acceleration could also be due to the gentle agitation of the tissue fluid, which may increase the rate of phagocytosis and movement of particles and cells. • Thus, ultrasound has a pro-inflammatory, not

an anti-inflammatory action.

Proliferative (Granulation) stage:-• This begins approximately 3 days after injury

and is the stage at which the connective tissue framework is laid down by fibroblasts for the new blood vessels. • During repair, fibroblasts may be stimulated to

produce more collagen; ultrasound can promote collagen synthesis by increasing cell membrane permeability, which allows the entry of calcium ions, which control cellular activity.

• Not only is more collagen formed but it is also of greater tensile strength after ultrasound treatment. • Ultrasound encourages the growth of new

capillaries in chronic ischaemic tissue and the same could happen during repair of soft tissues after injury. • The enhanced release of growth factors from

macrophages following exposure to therapeutic ultrasound may cause proliferation of fibroblasts.

• It has been suggested that ultrasound treatment given during the first 2 weeks after injury accelerates bony union, but, if given to an unstable fracture during the phase of cartilage formation, it may result in the proliferation of the cartilage and consequently delay of bony union.

Remodelling Stage:-• This stage last months or years until the new

tissue is as near in structure as possible to the original tissue. • Ultrasound is considered to improve the

extensibility of mature collagen such as is found in scar tissue, which occur by promoting the reorientation of the fibres (remodelling), which leads to greater elasticity without loss of strength.

Therapeutic Uses:-Varicose Ulcers: • Ultrasound promotes healing of varicose ulcers

and pressure sores (decubital ulcer). • Varicose Ulcer: Ulcer (circumscribed depressed

lesion on the skin or mucous membrane of any internal organ following sloughing of necrotic inflammation) in the leg associated with varicose veins is known as varicose ulcer.

• Pressure Sore: A bed sore; a decubital ulcer appearing on dependent sites usually on lumbosacral region, most commonly in bed-ridden elderly persons is known as pressure sore.

Pain relief: • Ultrasound is used in herpes zoster, low

backache, prolapsed intervertebral disc (PIV) and many other conditions. • Herpes Zoster: Shingles (band-like involvement

of neurocutaneous tissues) caused by varicellazoster virus.

• It involves posterior root ganglia and presents with severe continuous pain in the distribution of the affected nerve • Prolapsed Intervertebral Disc: Abnormal

descent of intervertebral disc between the vertebra is known as prolapsed intervertebral disc

Acute tissue injury:-• Ultrasound is used in soft tissue and sport

injuries, in occupational injuries and post-natal injuries. It is used for perineal post-natal pain, for painful shoulders and for both neurogenic & chronic pain.

Scar Tissue:-• Ultrasound improves quality of scar tissue and

excessive fibrous tissue. It is used in conditions like Dupuytren’s contracture and plantar fasciitis.

• Dupuytren’s contracture: Thickening and contracture of palmar fascia, typically affects the ring finger and may involve years later incompletely little finger is called Dupuytren’s contracture.

• Plantar fasciitis: Tenderness under the heel from plantar fibromatosis or tear of plantar fascia is called plantar fasciitis.

Bone injury:-• Ultrasound therapy in the first 2 weeks after

bony injury can increase bony union, but, given to an unstable fracture during the phase of cartilage proliferation, it may result in the proliferation of cartilage and therefore decrease bony union. Ultrasound has also been used in the early diagnosis of stress fractures.

Chronic Indurated Oedema: • The mechanical effect of ultrasound has an

effect on chronic oedema and helps in its treatment. It also breaks down adhesions formed between adjacent structures.

Contraindications • Tumors – it might encourage neoplastic

growth and provoke metastases or over precancerous tissue should be avoided

• Pregnant Uterus – avoid applying ultrasound over a pregnant uterus, probable risk to the rapidly dividing and differentiating cells of the embryo and fetus

• Epiphyseal plates – avoid giving ultrasound over epiphyseal plates as growth of the bone is impeded

Spread of Infection - Bacterial or viral infection could be spread by ultrasound, presumably by facilitating microorganism movement across membranes and through the tissues. The low-grade infections of venous ulcers, or similar, would seem to be safe to treat.

Tuberculosis - Due to the possible risk of reactivating encapsulated lesions tuberculous regions should not be treated.

Vascular Problems- • Circumstances in which hemorrhage might

provoke should not be treated. For example, where bleeding is still occurring or has only recently been controlled, such as an enlarging haemarthrosis or haematoma or uncontrollable haemophilia.

• Severely ischaemic tissues should be avoided because of the poor heat transfer and possible greater risk of arterial thrombosis due to stasis and endothelial damage.

• Treatment over recent venous thrombosis might extend the thrombus or disrupt its attachment to the vein wall forming an embolus. Areas of atherosclerosis are best avoided for the same reason

• Haemarthrosis: Bleeding into the joint usually from an injury, which results in a swelling of the joint, is known as haemarthrosis.

• Haematoma: A collection of blood inside the body, caused by bleeding from an injured vessel is called haematoma.

• Haemophilia: An inherited coagulation defect characterized by a permanent tendency to hemorrhages due to a defect in the coagulation of blood is known as haemophilia.

• Atherosclerosis: A condition caused by intramural deposition of Low Density Lipoprotein (LDL), secondary to exposure of smooth muscles to lipid, resulting in platelet induced smooth muscle proliferation, formation of fibrotic plaques and calcification is known as atherosclerosis

Radiotherapy - Areas that have received radiotherapy in the last few months should not be treated because of the risk of encouraging pre-cancerous changes.

Nervous System - Where nerve tissue is exposed, e.g. over a spina bifida or after a laminectomy, ultrasound should be avoided. Treatment over the cervical ganglia or vagus nerve might be dangerous in cardiac disease.

Specialized Tissue - The fluid-filled eye offers exceptionally good ultrasound transmission and retinal damage could occur. Treatment over the gonads is not recommended.

Implants - Smaller and superficial implants, like metal bone-fixing pins subcutaneously placed; as a precaution, low doses should be used in these circumstances.

• Treatment over implanted cardiac pacemakers should not be given because the sonic vibration may interfere with the pacemaker’s stimulating frequency

Anaesthetic areas - High doses should not be given over anaesthetic areas.

Dangers of Ultrasound: • There are very less evidences of dangers of

ultrasound but it may occur in some conditions only. –Burns could occur if the heat generated exceeded

the physiological ability to dissipate it. – Tissue destruction would result from transient

cavitation. –Blood cell stasis and endothelial damage may

occur if there is standing wave formation. • These dangers would be more likely with

high-intensity continuous output with a stationary head or over bony prominences

Precautions:

• Acute inflammation• Epiphyseal plates• Fractures• Breast Implants

References:

• Agents in Rehabilitation, From research to practice; Michelle H. Cameron, 2nd Edition

• Electrotherapy Explained, Low, J. & Reed, A. (1990).