Ultrasound

PHTY8102 Fundamentals of Physiotherapy Practice Week 10 Lecture: Therapeutic Ultrasound

Acknowledgement

• Dr. Joel Fuller

• A/Professor Julia Hush

Learning Outcomes for Therapeutic Ultrasound

• Understanding of principles of EPA use in physiotherapy treatment

  • Proposed mechanisms and physiological effects of ultrasound

  • Clinical indications for ultrasound use

  • Selection of appropriate treatment parameters

  • Identification of contraindications and precautions

  • Summary of current evidence

What is Ultrasound?

• Definition: Ultrasound refers to a type of mechanical vibration, fundamentally similar to sound waves, but with frequencies exceeding the human hearing range. It is a form of acoustic energy transmitted through oscillations of particles in a medium.

Categories of Sound Waves

• Infrasound: Frequencies below 20 Hz (e.g., sounds heard by elephants, moles). These low-frequency waves have long wavelengths and can travel great distances.

• Audible Sound: Frequencies range between 20 Hz to 20 kHz (within the human auditory field). This is the range of sounds detectable by the human ear.

• Ultrasound: Frequencies exceeding 20 kHz (e.g., heard by cats, dogs, bats, dolphins). Therapeutic ultrasound typically uses frequencies in the MHz range, far above human perception.

Wave Properties

• Longitudinal Waves: Ultrasound propagates as longitudinal waves, meaning the particles in the medium oscillate parallel to the direction of wave propagation. This creates areas of mechanical compressions (higher pressure, particles closer together) and rarefactions (lower pressure, particles further apart) that travel through the tissue, separated by a fixed distance known as wavelength.

• Medium Requirement: Unlike electromagnetic radiation, ultrasound requires a physical medium (solids, liquids, gases) to propagate. It cannot travel through a vacuum, as it relies on the vibration of particles to transmit energy. Coupling agents like gel are essential to eliminate air between the transducer and skin, ensuring efficient transmission into the body.

Therapeutic Ultrasound Components

• Equipment Components:

  • Gating Circuit: Controls the pulsed output of ultrasound, determining the duty cycle.

  • Power Supply: Provides electrical energy to the ultrasound unit.

  • 1 MHz Sinewave Generator and Amplifier: Generates and amplifies the alternating current at the desired frequency (1 MHz or 3 MHz).

  • Piezo-electric Crystal: Mounted in a handheld probe (transducer). This crystal converts electrical energy into mechanical sound waves (and vice-versa) due to the piezo-electric effect, making it critical for generating ultrasound.

  • Metal End-plate and Housing: Forms the structure of the treatment head, protecting the crystal and directing the ultrasound beam.

Ultrasound Wave Production

• Mechanism: Electrical energy from the generator is applied to the piezo-electric crystal, causing it to rapidly expand and contract. This oscillation of the crystal, and consequently the metal plate of the treatment head, emits a continuous or pulsed stream of compression and rarefaction waves, forming the ultrasound beam that enters the tissues.

• Frequency Range: Emission frequencies typically between 1 MHz and 3 MHz.

  • 1 MHz is used for deeper tissues (up to approximately $6$ cm, e.g., muscle, deep joint capsules) as it has greater penetration due to less absorption.

  • 3 MHz is used for more superficial structures (up to approximately $3$ cm, e.g., tendons, ligaments, superficial muscles) as its energy is absorbed more rapidly and superficially.

• Energy Transmission: The efficiency of energy transmission depends significantly on tissue properties and acoustic impedance. Acoustic impedance is the resistance a medium offers to the passage of sound waves. A larger difference in acoustic impedance between two media results in greater reflection of ultrasound energy at their boundary.

Ultrasound Absorption

• Energy Dissipation: As ultrasound travels through biological materials, its energy is attenuated (reduced). This occurs primarily through absorption, where the kinetic energy of the vibrating particles is converted into heat energy due to molecular friction and viscous drag within the tissues. Tissues with high protein content, like bone and muscle, absorb ultrasound more efficiently.

• Half-Value Depth: This measure demonstrates the depth at which the ultrasound intensity is reduced by half. It illustrates the penetration depth in various tissues and highlights the difference between frequencies:

  • 1 MHz:

    • Fat Tissue: $50$ mm

    • Muscle Tissue: $12$ mm

    • Bone Tissue: $1.5$ mm

    This frequency is chosen when targeting deeper structures because it penetrates further before being significantly absorbed.

  • 3 MHz:

    • Fat Tissue: $17$ mm

    • Muscle Tissue: $4$ mm

    • Bone Tissue: $0.5$ mm

    This frequency is preferred for superficial tissues due to its rapid absorption and more concentrated heating effect close to the surface.

Ultrasound Reflection at Tissue Boundaries

• Energy Interaction: When an ultrasound wave encounters a boundary between two different tissue types (e.g., muscle to bone), a portion of the energy reflects back, while the remaining portion refracts (bends) into the new medium. The amount of reflection is proportional to the difference in acoustic impedance between the two media. Air has a very high acoustic impedance mismatch with skin, which is why a coupling gel is crucial to eliminate air and ensure good transmission into the body.

Proposed Physiological Effects of Ultrasound

1. Thermal Effects: Deep heating occurs when ultrasound energy is absorbed by tissues and converted into heat. This increase in tissue temperature can lead to:

   • Increased metabolic rate and enzymatic activity.

   • Increased blood flow (vasodilation) to promote healing and reduce ischemia.

   • Increased extensibility of collagen-rich tissues (e.g., scar tissue, joint capsules), facilitating stretching and reducing stiffness.

   • Elevated pain threshold due to nerve conduction velocity changes.

2. Non-Thermal Effects: These mechanical effects occur due to the physical forces of the sound waves and are present even at low intensities or with pulsed modes:

   • Cavitation: The formation and oscillation of minute gas bubbles within the tissue's interstitial fluid due to pressure changes.

     • Stable cavitation involves bubbles that oscillate in size without collapsing, leading to microstreaming around the bubbles.

     • Unstable (transient) cavitation involves bubbles that grow and then violently collapse, releasing high amounts of energy. Therapeutic ultrasound aims for stable cavitation, which can alter cell membrane permeability, facilitating ion and nutrient exchange to promote cellular healing without causing tissue damage.

   • Standing Waves: Occur when incident and reflected waves superimpose, creating fixed zones of high and low pressure. This can potentially lead to localized hot spots, periosteal pain, or even cellular damage if the transducer is kept stationary or moved too slowly, concentrating energy in one area.

   • Acoustic Streaming: Refers to the bulk flow of fluids in the ultrasound field, particularly around oscillating cells and bubbles (micro-streaming). This steady, circular flow of interstitial fluids can facilitate the movement of ions, small molecules, and cellular components, influencing cell membrane permeability, protein synthesis, and cellular activity essential for tissue repair and inflammation resolution.

   • Micromassage: The direct mechanical pressure changes from the ultrasound waves create a subtle internal massage effect on cells and tissues. This mechanical agitation can stimulate cellular activity, increase fibroblast activity, and promote fluid movement.

Pulsed Ultrasound

• Function: Pulsed ultrasound delivers the ultrasound energy in intermittent bursts rather than continuously. This significantly reduces the average intensity of the energy delivered, thereby minimizing the thermal (heating) effects while still allowing the non-thermal mechanical effects to occur. It is often preferred in acute inflammatory conditions or when heating is undesirable.

• Duty Cycle: The duty cycle represents the percentage of time the ultrasound beam is 'on' during each pulse period. Options include 10%, 20%, 50%, and 100% (continuous).

  • Pulse Ratio Example: A 1:1 ratio (e.g., 50% duty cycle, meaning the ultrasound is on for as long as it is off) reduces the overall dosage by 50% compared to continuous mode, necessitating an extended treatment time to deliver the equivalent total energy for non-thermal effects.

Therapeutic Benefits of Ultrasound

• Potential benefits include:

  • Increased healing of chronic pressure ulcers: Non-thermal effects may stimulate cellular activity, blood flow, and collagen production.

  • Enhanced healing of soft tissue injuries (e.g., sprains, strains): Both thermal (in subacute/chronic) and non-thermal effects contribute to inflammation resolution, tissue repair, and pain relief.

  • Improved extensibility of scar tissue: Thermal effects can increase collagen extensibility, making scar tissue more amenable to stretching and remodeling.

  • Facilitation of fracture healing (requires use of low-intensity pulsed ultrasound): Specific parameters can stimulate osteogenesis without significant heating.

• Research: Though robust high-quality clinical evidence is limited, anecdotal reports and in vitro (laboratory) studies continue to suggest potential therapeutic benefits, particularly for non-thermal effects on cellular processes.

Healing Stages and Ultrasound Applications

• Stages of Healing: Ultrasound application can be tailored to different phases of the healing process:

  • Inflammatory Phase: $0 - 72$ hours. Pulsed ultrasound (low intensity, 20-50% duty cycle) is typically indicated to promote non-thermal effects, minimize inflammation, reduce pain, and facilitate cellular events without increasing heat or exacerbating inflammation.

  • Proliferation Phase: $48$ hours - $6$ weeks. Both pulsed and continuous (low to moderate intensity) ultrasound can be used to enhance fibroblast activity, collagen synthesis, angiogenesis, and tissue regeneration.

  • Remodelling Phase: $3$ weeks - $12$ months. Continuous ultrasound (moderate to high intensity) can be applied to increase tissue extensibility, facilitate collagen alignment, and reduce scar tissue contractures, especially in dense, fibrotic tissues.

Safety Considerations for Ultrasound Use

• Common Risks:

  • Burns: Can occur due to improper technique such as insufficient coupling gel, maintaining a stationary treatment head, inadequate contact with the skin, or treating over bony prominences where reflection can concentrate energy. Burns are a primary risk if thermal effects are not properly monitored or controlled.

  • Tissue Damage: High intensities, prolonged application, or inappropriate duty cycles can lead to cellular damage, necrosis, or structural disruption, especially in sensitive tissues.

  • Periosteal Pain: Experienced when treating too close to bony prominences. Bone has a high acoustic impedance and absorbs ultrasound energy very efficiently, leading to rapid temperature increases at the bone-soft tissue interface. Reflection from bone can also create standing waves. Recommended to use low intensity ( < 0.5 W/cm²) and increase head movement speed to disperse energy.

Contraindications for Ultrasound

• Situations where ultrasound should be avoided to prevent adverse effects:

  • Proximity to cardiac pacemaker or any implantable stimulator: Risk of electromagnetic interference with device function.

  • Conditions with circulatory insufficiency: Impaired blood flow may prevent adequate heat dissipation, increasing burn risk.

  • Worsening existing issues (e.g., acute infections, recent radiotherapy): Ultrasound may exacerbate inflammation or tissue fragility.

  • Risk of dissemination (e.g., acute infections, tumors): Ultrasound could spread infectious agents or metastasize malignant cells.

  • Application to sensitive locations (e.g., eyes, testes, pregnant uterus): High risk of damage to developing tissues, gonads, or the fetus.

  • Patients who are unable to communicate or have sensory loss affecting temperature or pain discrimination: Risk of burns or tissue damage goes undetected.

Skin Safety Tests

• Two necessary tests to ensure patient safety before applying continuous ultrasound (thermal effects):

  • Thermal Sensitivity Test: Assesses the patient's ability to perceive hot and cold stimuli. This is crucial for detecting excessive heat during treatment.

  • Sharp/Blunt Discrimination Test: Uses a toothpick, paperclip, or sharp pen for testing. Assesses the patient's tactile discrimination and pain perception, which is important as discomfort can indicate a problem.

• Contraindication: Inability to accurately distinguish between hot/cold or sharp/blunt stimuli (sensory impairment) contraindicates thermal ultrasound application.

Machine Check

• Importance of conducting a machine check prior to each use to assess the power output, calibration, and physical condition of the ultrasonic crystal in the transducer head. This ensures the machine is functioning safely and effectively.

Consent and Warning for Patients

• Guidelines for patient consent prior to using ultrasound:

  • Patients should typically feel little to no discomfort, only applicator pressure or minimal warmth (in continuous mode). They should be reassured that the procedure is generally painless.

  • If discomfort (e.g., sharp pain, aching) or excessive heat is experienced, they must inform the physiotherapist immediately. This immediate feedback helps prevent the risk of burns or other tissue damage.

Ultrasound Parameters

• Methods:

  • Application: Direct contact with a coupling gel applied to the skin (most common). Alternative methods like water baths are used for irregular body parts (e.g., hands, feet) or when direct contact is difficult.

  • Mode: Either continuous mode (for thermal effects, e.g., muscle spasm, chronic inflammation, scar tissue) or pulsed mode (for non-thermal, mechanical effects, e.g., acute injuries, promoting cellular healing without heat).

  • Frequency: Options of 1 MHz (for deeper tissues, approximately $6$ cm, due to lower absorption) or 3 MHz (for more superficial structures, approximately $3$ cm, due to higher, more concentrated absorption).

Duration of Treatment

• Generally calculated based on the size of the treatment area and the effective radiating area (ERA) of the transducer head:

  • Duration = $1$ min $imes$ Number of applicator heads fitting over the treatment area (typically equates to $5-10$ min). A common guideline is to treat an area approximately $2-3$ times the size of the ERA for about $5-10$ minutes. This ensures adequate energy delivery to the target tissue.

  • Must adjust duration based on pulse ratios if pulsed ultrasound is used (e.g., 50% duty cycle would require double the treatment time to deliver the same total energy for non-thermal effects).

Intensity Settings

• Varied by phase of inflammation, tissue depth, and desired effect:

  • Acute & Subacute Conditions/Non-thermal effects: Low intensities of $0.1 \text{ – } 0.5$ W/cm² are used when heating is undesirable or for conditions like acute inflammation or pain modulation.

  • Chronic Conditions/Thermal effects: Moderate to high intensities of $0.3 \text{ – } 1$ W/cm² are typically used to achieve thermal effects, such as increasing tissue extensibility or reducing chronic pain. Some studies highlight use up to $3$ W/cm² for specific deep, highly absorptive tissues.

Ultrasound Application Process

• Strict steps for effective and safe application:

  • Perform a machine check to ensure power output, calibration, and integrity of the transducer head.

  • Clean the treatment area thoroughly and apply a generous layer of aqueous gel/couplant to the skin. This eliminates air and facilitates efficient ultrasound transmission.

  • Position treatment head flat and firmly on the skin before turning on the output. Output should only be on during contact and continuous movement.

  • Maintain full contact with the skin and move the transducer head continuously at approximately $4$ cm/s (a slow, steady pace). This prevents standing waves, localized heating, and ensures even distribution of energy.

  • Switch off the machine output while the transducer head is still in contact and moving, before lifting it off the skin. This minimizes the risk of damage to the piezo-electric crystal (transducer) from overheating or mechanical shock when operating in air.

Additional Considerations for Water Bath Application

• Water Bath Specifics: Used for irregular body parts where direct contact is challenging.

  • Use a non-metal container for the process, as metal can reflect ultrasound waves causing uneven distribution or hot spots.

  • Maintain a distance of $1 \text{ – } 2$ cm between the sound head and the skin. This allows the ultrasound beam to form properly before reaching the tissue.

  • Ensure uniform distance throughout application by consistently moving the transducer parallel to the skin surface.

  • Eliminate air bubbles from the skin and sound head before and during treatment, as air significantly impedes ultrasound transmission.

  • Intensity Adjustment: Due to some energy loss in the water medium, the intensity needs to be increased by $30\% \text{ – } 55\%$ (or $0.5$ W/cm²) for effective treatment to compensate for attenuation in the water.

Evidence of Ultrasound Effectiveness

• Cochrane Review Statement: There is no high-quality evidence supporting the use of ultrasound for improving pain or quality of life in patients with non-specific chronic low back pain (Ebadi et al., 2014). This highlights the need for caution and evidence-based practice.

• CTS Treatment Findings: Limited evidence suggesting therapeutic ultrasound may be marginally more effective than placebo for symptom improvement in carpal tunnel syndrome (Page et al., 2013). This indicates some potential but not a definitively strong effect.

Low-Intensity Pulsed Ultrasound Benefits

• Potential benefits include:

  • Facilitation of bone healing using lower intensity than traditional methods, promoting osteogenesis and fracture repair.

  • Typical parameters: Delivery at $1.5$ MHz, $0.03$ W/cm², and a $20\%$ duty cycle, applied for $20$ mins daily over $4-6$ weeks. These specific parameters are chosen to maximize non-thermal mechanical effects for bone growth while minimizing thermal effects.

  • Low heating impact due to reduced intensity and pulsed delivery, making it safe for daily, prolonged application without thermal risks (Tajali et al., 2012; Gan et al., 2014).

Class Participation Notice

• Students are required to wear enclosed shoes during all classes. Entry will be denied to those not in compliance.

References

• Ebadi S, et al. Therapeutic ultrasound for chronic low-back pain. Cochrane Database Syst Rev. 2014;3:CD009169.

• Gan TY, et al. Low-intensity pulsed ultrasound in lower limb bone stress injuries: a randomized controlled trial. Clin J Sports Med. 2014;24:457-60.

• Page M, et al. Therapeutic ultrasound for carpal tunnel syndrome. Cochrane Database Syst Rev. 2013;3:CD009601.

• Robertson VJ, Chipchase LS, Laakso EL, Whelan KM, McKenna LJ. Guidelines for the clinical use of electrophysical agents. Australian Physiotherapy Association; 2001.

• Robertson V, Ward A, Low J, Reed A. Electrotherapy explained: principles and practice. 4th ed. London: Butterworth-Heinemann: 2006.

• Tajali SB, et al. Effects of low-intensity pulsed ultrasound therapy on fracture healing: a systematic review and meta-analysis. Am J Phys Med Rehabil. 2012;91:349-67.