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THERAPEUTIC ULTRASOUND

Therapeutic ultrasound produces acoustic vibrations of high frequency (not detectable by human ear) that produce thermal and/or nonthermal effects in the body’s tissues.

Unique advantages over other nonacoustic heating modalities:

Soundwaves selectively heat tissues high in collagen (e.g., tendons, ligaments, muscles, joint capsules, menisci, nerve roots, periosteum, and cortical bone) without causing a significant tissue temperature increase in skin or fat.
Provides therapeutic benefits by stimulating normal body physiological functions to perform better for the primary purposes of enhanced soft tissue injury repair and pain relief.
Greater tissue heating depth penetration: For example: a 4-minute whirlpool @ 105ºF (40.6°C) produces a temperature increase at 1 cm below the fat tissue of about 2.0°F (1.1ºC). At the same 1 cm depth, 3 MHz ultrasound treatment increases temperature over 10.8°F (6°C).

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THE ULTRASOUND UNIT: 4 MAIN PARTS

Generator: a high frequency electric generator linked through an oscillator circuit and a transformer. Control panel: on-off, power, time, intensity, duty cycle, continuous or pulsed modes, 1 or 3 MHz frequency, and automatic shut-off in case the crystal overheats.
Crystal: a thin synthetic ceramic crystal that is a transducer that converts electric energy to acoustic energy through mechanical deformation (expands & contracts) of the crystal that causes the soundhead to vibrate resulting in the mechanical production of high-frequency sound waves.

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THE ULTRASOUND UNIT: 4 MAIN PARTS�CONT . . . .

Soundhead: a ceramic, aluminum, or stainless steel plate attached to the crystal that is a transmitter that transfers the acoustic sound waves from the crystal to the tissues where it causes the tissues to vibrate.
Applicator: this is the housing for the crystal and the soundhead. A handheld device that facilitates the application of the ultrasound to patients. It should be ergonomically designed so that when held the wrist is able to be kept in a neutral position where it is functional strong to withstand the constant motion required to apply the therapy without succumbing to a repetitive motion injury.

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HOW DOES ULTRASOUND WORK – THE PHYSICS

Sound

Sound is created when a force acts upon a molecule causing it to vibrate back and forth which then sets in motion a cascade of molecular vibrations producing a mechanical wave (sound wave).

Ultrasound is a mechanical wave in which energy is passed by the vibrations of the molecules of a biological medium through which the wave is traveling, concentrating the energy in a targeted or limited area.

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HOW DOES ULTRASOUND WORK – �THE PHYSICS, CONT . . . .

Types of sound waves

Longitudinal waves: these waves travel with vibration (molecular displacement) in alignment or longitudinally through soft tissues.
Transverse waves: these waves vibrate in a direction perpendicular to the longitudinal waves, such as when the sound wave rebounds after hitting bone.

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HOW DOES ULTRASOUND WORK – �THE PHYSICS, CONT . . . .

Frequency of sound wave transmission

In body tissues, the higher the frequency of soundwave, the greater the amount of ultrasound energy that is absorbed in the tissues, resulting in less energy that is transmitted to deeper tissues.

3 MHz frequency – these sound waves are absorbed superficially and are only transmitted to a depth of 2-3 cm.
1 MHz frequency – these sound waves are transmitted deeper before being absorbed to a depth of approx. 6 cm.

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HOW DOES ULTRASOUND WORK – �THE PHYSICS, CONT . . . .

Attenuation

Attenuation is a decrease in energy as a sound wave passes through body tissues as some of the energy is absorbed in the tissues.

Fat tissue: minimal energy absorption
Muscle tissue: has a high rate of energy transmission
Dense tissue high in protein: energy is well absorbed in this type of tissue
Tissue high in water content: the energy penetrates easily to a deeper level
Superficial bone: energy is easily absorbed
Peripheral nerves: energy is absorbed at a rate twice that of muscle

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HOW DOES ULTRASOUND WORK – �THE PHYSICS, CONT . . . .

Characteristics of crystal – also impacts how well ultrasound will penetrate tissues.

The quality of a crystal can affect how well the ultrasound wave penetrates tissues. Crystal quality is partly determined by:

Effective Radiating Area (ERA): The surface area of a soundhead that transmits a sound wave from the crystal to the tissues.
Beam Nonuniformity Ratio (BNR): A measure of the uniformity of the ultrasound beam.

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EFFECTIVE RADIATING AREA (ERA)

ERA is the surface area of a soundhead (faceplate) that transmits a sound wave from the crystal to the tissues. Important considerations include:

The ERA is always smaller than the crystal and sometimes significantly smaller than the soundhead.
The entire surface of the soundhead does not radiate ultrasound output.
It is recommended that the total size of the crystal and soundhead surface match as closely as possible for ease of application in order to maintain the most effective contact with the targeted tissue area.

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BEAM NONUNIFORMITY RATIO (BNR)

The BNR is an indicator as to variability of intensity within an ultrasound beam as they are not completely uniform – with some point higher and other points of lower intensity. The BNR is the ratio of the spatial peak intensity (highest intensity in the beam), to the spatial average intensity (average intensity across the beam).

A good BNR is between 2 to 5. The lower the BNR, the more uniform the ultrasound output will be and the less likelihood of developing hot spots.
Hot Spot: An area at tissue interfaces that may become overheated from too much concentrated energy in one area, resulting in discomfort and potential tissue damage.
The higher the BNR ratio, the more important it is to use lower intensities or move the soundhead faster during treatment to avoid hot spots.

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ULTRASOUND TREATMENT PARAMETERS

The “therapeutic dose” of ultrasound is determined by these parameters:

Mode
Frequency
Intensity
Treatment Length
Treatment Area Size

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ULTRASOUND PARAMETER - MODE

Ultrasound devices produce “continuous” or “pulsed” mode ultrasound.

Continuous Mode: the ultrasound energy remains constant (100%) throughout the treatment.
Pulsed Mode: the ultrasound energy is periodically interrupted so that no ultrasound energy is being produced during the “off” period.
Duty Cycle: the percentage of time that ultrasound is generated over one pulse period.

Pulse Duration (on time) x 100 / Pulse Period (on-time + off-time)
Example: pulse duration of 10ms & total pulse period of 40ms. The duty cycle would be 25% = the total amount of energy delivered to the tissues would be only 25% (10 x 100 = 1,000/40 = 25). This means that the total amount of energy delivered to the tissues would be only 25% of the energy delivered if a continuous mode was used.

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ULTRASOUND PARAMETER - FREQUENCY

Therapeutic ultrasound typically has a frequency range of 0.75 to 3.0 MHz.

Ultrasound frequency determines the depth of tissue penetration and the rate of tissue heating.

3 MHz frequency soundwaves are absorbed more superficially (2cm-3cm depth) and therefore are best for treating conditions that are more superficial. For example, medial and lateral epicondylitis, patellar tendinitis, plantar fasciitis, Achilles tendinitis, radial or ulnar nerve entrapment, hand/finger and foot/toe joints, carpal tunnel syndrome, muscles close to the body surface, etc.

At 3 MHz the rate of absorption/attenuation is not only absorbed more superficially but is also absorbed 3 times faster than at 1 MHz and therefore heats the tissue 3 times faster.

1 MHz frequency soundwaves are transmitted through the more superficial tissues and absorbed primarily in deeper tissues (3cm to 6cm depth). Because of its depth of penetration, it is recommended to use this frequency on individuals with a high percentage of body fat and when deeper tissues need to be heated. For example, deeper muscles such as piriformis, soleus, hip internal rotators, supraspinatus, or larger joints, etc.

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ULTRASOUND PARAMETER - INTENSITY

Intensity is the rate at which energy is being delivered per unit area and is a function of both the pulse width and pulse frequency, measured in watts (W). Also referred to as spatial average intensity (SAI). SAI is the intensity of the ultrasound beam averaged over the area of the soundhead.

SAI = Total watts (W) / Effective radiating area (cm²) = W/cm².
There are no definitive guidelines for intensity for selecting specific ultrasound intensities during treatment. A safe range of intensity for a moving soundhead is 0.5 to 3 W/cm².
Thermal ultrasound treatments should feel slightly warm and adjusted to patient tolerance. If there is a significant adjustment in intensity it should be countered with an adjustment in the treatment time. If intensity is increased, treatment time is reduced. If intensity is reduced, treatment time is increased.

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ULTRASOUND PARAMETER – �TREATMENT LENGTH

The length of treatment time for any individual should be based on:

Size of the area to be treated should only be about 2x to 3x the size of the sound head, otherwise the energy will be distributed too broadly, and therapeutic temperature will not be reached
Ultrasound frequency (remember, ultrasound at 3 MHz heats tissues 3x faster than 1 MHz)
Intensity in W/cm² (the lower the intensity the longer the treatment time)
Desired temperature increase

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ULTRASOUND PARAMETER – �TREATMENT LENGTH, CONT . . . .

TEMPERATURE INCREASES & ASSOCIATED TISSUE EFFECT

1.8°F (1°C) = (↑ metabolism, ↓ mild inflammation)
3.6 °F to 5.4°F (2°C-3°C) = (↓ pain & muscle spasm, ↑ blood flow)
7.2°F (4°C) = (↑ ROM & tissue extensibility)

PER MINUTE RATE OF MUSCLE HEATING USING ULTRASOUND

Intensity Frequency Frequency

(W/cm²) (1 MHz) (3 MHz)

0.5 0.07°F (0.04°C) 0.54°F (0.3°C)

1.0 0.36°F (0.2°C) 1.08°F (0.6°C)

1.5 0.54°F (0.3°C) 1.62°F (0.9°C)

2.0 0.72°F (0.4°C) 2.52°F (1.4°C)

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DETERMINING ULTRASOUND TREATMENT LENGTH – THE “GUESSTIMATE” METHOD

Scenario: Patient is experiencing medial epicondylitis. In preparation for massage and eccentric resistance exercises, you want to heat the origin of the wrist flexor muscle group. An appropriate approach would be to:
Heat the muscle tendon junction using 3 MHz frequency at an intensity of 1 W/cm². This would take approx. 7 minutes to reach the 7.2°F (4°C) temperature.

1 W = 1.08°F (0.6°C)/min x 7 minutes = 7.6°F or 4.2°C

Note: If at some point the patient complains that the temperature is too hot and uncomfortable, simply decrease the intensity dose and add some additional treatment time.

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TREATMENT AREA SIZE & SOUNDHEAD MOTIONS

Treatment Area Size: Ultrasound is a very effective tool for heating deep tissues. However, the treatment should be limited to an area that is two or three times the size of the ERA of the crystal, or twice the size of the soundhead surface to ensure proper heating depth.
Soundhead Motions: Ultrasound head can be moved in back-and-forth linear strokes or small overlapping circular motions at a rate of 3-4 cm/second.

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ULTRASOUND BEAM TRANSMISSION – THE MEDIUM OF “GEL”

Ultrasound beams must be transmitted from the soundhead to the patient through a coupling medium, either a gel or emersion in water. The coupling gel serves as a lubricant and fills incongruities between the soundhead and skin, helping to ensure that the ultrasonic energy enters the target tissue at the desired intensity. Important considerations include:

The application of gel to the body surface treatment area should be sufficient in amount to maintain good contact and lubrication, but not allow air pockets to form from the movement of the soundhead.
When administering the treatment with a gel medium it is essential that the soundhead’s surface remain in complete contact with the body surface at all times. Lifting or tilting the soundhead at an angle of even 15° will result in a significant reflection loss of energy and reduced treatment effects.

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ULTRASOUND BEAM TRANSMISSION – “IMMERSION OF THE SOUNDHEAD IN WATER”

The immersion in water of the body part to be treated and the soundhead to deliver the ultrasonic energy is used for areas of the body like hands/wrist and feet/ankle where irregular surfaces and body prominences exist. Important considerations include:

The container of the water should NOT be metal as this will reflect some of the ultrasound.
The water used for immersion should be at least body core temperature, not room temperature, to ensure the desired “thermal” effects.
The soundhead should move parallel to the body treatment surface area at a distance of less than 1cm.
If air bubbles form on the soundhead or treatment area, wipe them off immediately.

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“THERMAL” EFFECTS OF THERAPEUTIC ULTRASOUND

Increased local blood flow – delivering oxygen and nutrition to healing tissues
Increased extensibility of connective tissues (tendons, joint capsule, adhesions, scar tissue) – enhancing the effectiveness of stretching
Increased metabolism in tissues in which heat is absorbed
Diminished pain perception – increased blood flow that washes away metabolic waste products/chemical irritants that cause pain

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THERAPEUTIC ULTRASOUND – AS AN AID TO “STRETCHING CONNECTIVE TISSUE”

Just as plastic is hard and not pliable when cool, connective tissues have a somewhat similar characteristic. However, heat up the plastic object and it becomes pliable and can be stretched. Heat up connective tissues and they become more pliable and extensible and can more effectively be stretched/elongated. Therefore, to enhance the effectiveness of stretching, joint mobilizations, friction massage, muscle warm-up, you should stretch these tissues when they are still warmed, preferably immediately after ultrasound treatment is terminated. The window of opportunity for this therapeutic benefit is around 5 to 10 minutes.

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“NONTHERMAL” EFFECTS OF THERAPEUTIC ULTRASOUND

Tissue regeneration
Facilitating inflammation and wound healing
Cell membrane alteration – facilitating diffusion of metabolites and promoting protein and collagen synthesis
Increased phagocytic activity of macrophages

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“INDICATIONS” FOR USE OF THERAPEUTIC ULTRASOUND

Soft Tissue Shortening – restoring ROM lost from contracture and/or scare tissue – combined with stretching when tissue is still warmed
Soft tissue healing and repair
Diminish pain – increased blood flow that washes away metabolic waste products/chemical irritants that cause pain

“itis” conditions – tendinitis, bursitis, fasciitis, epicondylitis, arthritis
Musculoskeletal syndromes/conditions (carpal tunnel, nerve entrapment, TMJ dysfunction)
Ligament sprains, muscle/tendon strains
Trigger points – desensitization
Muscle spasms – heated tissues have increased blood flow → eliminates chemical irritants, alters nerve conduction speed and relaxes muscle tension (e.g., IT Band tightness)

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“CONTRAINDICATIONS” FOR USE OF THERAPEUTIC ULTRASOUND

Acute and post-acute conditions with continued bleeding
Over reproductive organs (a gravid uterus) – could potentially damage developing fetus
Cancer – a local exposure may contribute to metastasizing from increased blood flow
Over eyes – could cause cavitation in the fluid compartments
Impaired arterial circulation – sufficiently healthy to meet the increased metabolic demand
Over spinal cord after laminectomy – cavitation may occur in the cerebrospinal fluid
Over pacemaker or carotid sinus – could affect normal pacing of heart or baroreceptors
Thrombophlebitis – potential to cause an emboli
Presence of infection – heat could accelerate infection

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“PRECAUTIONS” FOR USE OF THERAPEUTIC ULTRASOUND

Over epiphyses of bones of young children – in high doses may disrupt growth by affecting early closure
Over metal implants - potential to heat implant
Over areas of impaired sensory – could hinder patient feedback on excessive heating
Excessive exposure to bony prominences – to avoid concentration on periosteum
Only use intensities considered therapeutically safe – to avoid cavitation and the potential for tissue damage

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ULTRASOUND & ELECTROTHERAPY

The combination of ultrasound and electrotherapy (combination therapy) involves the stimulation of muscle contraction as the ultrasound head serves as an electrode in delivering the electrotherapy. The benefits of combination therapy are:

Pain reduction and muscle contraction stimulation
Effective in the treatment of myofascial trigger points
Effective in reducing the pain-spasm-pain cycle

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INSTRUCTIONS FOR USE OF ULTRASOUND – PREPARATION FOR APPLICATION

Application of ultrasound should be performed using only calibrated and properly working ultrasound unit
Identify specific location for treatment (may consider marking area so stay within 2x to 3x ERA) and evaluate for any sensory deficit in the skin, for open wounds or rashes
Explain to patient the procedure – gel medium, soundwaves generate gentle warmth, report if any feeling of hot, total Tx time, etc.
Prepare patient – positioned and draped for comfort, modesty, and accessibility to treatment site, including positioning for access to deeper tissues
Use appropriate soundhead size
Apply appropriate amount of gel medium (or immerse part in tepid water)

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INSTRUCTIONS FOR USE OF ULTRASOUND –SET APPLICATION DOSAGE

Frequency: 1MHz for mod to deep tissue penetration “or” 3 MHz for superficial tissue heating

Mode:

Continuous: the ultrasound energy remains constant (100%) throughout the treatment
Pulsed: the ultrasound energy is periodically interrupted so that no ultrasound energy is being produced during the “off” period.

Duty Cycle: the percentage of time that ultrasound is generated over one pulse period

Intensity: The lower the intensity the longer the treatment time. Also, adjust the intensity to the patient’s heat perception. If it gets too hot for the patient, move the applicator faster as you turn down the intensity.

Treatment Duration:

1 MHz: 10 to 15 minutes for vigorous heating
3 MHz: 3 to 7 minutes for vigorous heating

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INSTRUCTIONS FOR USE OF ULTRASOUND –OTHER CONSIDERATIONS

Frequency of therapy applications: 1x to 2x daily
Number of treatments: approximately 14 sessions
If goal is to increase joint ROM, place body part in stretch position during latter part of ultrasound application and stretch the tissue immediately after termination of ultrasound

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POST THERAPY PROCEDURES

Clean the soundhead of gel and disinfect
Wipe any remaining gel from the patient’s skin
Perform any stretching or mobilization technique of the patient while the tissue is still warm and pliable
Documentation of ultrasound treatment dosages: Frequency, intensity, mode and duty cycle if pulsed, size and location of treatment, treatment length, patient response to treatment, etc.

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REFERENCES

Draper, D.O., Jutte, L.S., Knight, K.L., Therapeutic Modalities: The Art and Science, 3rd Edition
Cameron, M. H., Physical Agents in Rehabilitation: From Research to Practice, 4th Edition