Comprehensive Guide to Therapeutic Ultrasound in Physical Therapy Ultrasound

Background and History of Ultrasound

• Origin and Early Use: The technology originated in the 19th century and was utilized during World War II on submarines. Short pulses of ultrasound (US) were sent through water to detect the distance from various objects, such as the ocean floor, rocks, or other submarines.
• Environmental Impact: The use of high-power US in marine environments was eventually limited because findings indicated it was heating and injuring sea life.
• Modern Applications:
  • Clinical Diagnostics: Original uses persist today for taking fetal measurements in utero and varied diagnostic imaging.
  • Therapeutic Uses: Additional discovered uses include therapeutic applications such as thermal effects (effective in deep tissue with high collagen) and nonthermal effects (pulsed US and phonophoresis).

Physical Properties of Ultrasound

• Definition: Ultrasound is a type of sound characterized by low-frequency waves produced by the vibrations of material within the ultrasound head.
• Wave Type: It consists of longitudinal waves. These waves become alternately dense (compression) and rarer (rarefaction). This means molecules are essentially pushed together and pulled apart.
• Biological Interaction:
  • When applied to the body, US waves cause molecules in human tissue to oscillate back and forth, described as a "molecular massage."
  • Mechanical Energy: The movement of these molecules represents mechanical energy.
  • Heat Generation: As molecules move, they rub against each other, creating heat via friction within the tissues.
  • Tissue Target: This effect is most pronounced in collagen-rich tissues (tendons, ligaments, joint capsules, and scar tissue) because they absorb energy more efficiently than tissues with higher water content.
• Frequency Range:
  • Ultrasound is defined as having a frequency greater than $20,000\,Hz$.
  • Human hearing typically ranges from $20\,Hz$ to $20,000\,Hz$. Therefore, ultrasound is outside the limits of normal human hearing.

Understanding Attenuation

• Definition: Attenuation is the gradual decrease in the intensity of US waves as they penetrate deeper into the body. This is analogous to how audible sound gets quieter as the listener moves farther from the source.
• Primary Factors Influencing Attenuation:
  1. Tissue Type: The highest degree of attenuation occurs in tissues with the highest collagen levels.
  2. Wave Frequency: Higher-frequency waves have shorter wavelengths, which interact more with surrounding tissues, leading to greater absorption upon contact.
• Mechanisms of Attenuation:
  1. Absorption: Accounts for approximately $50\%$ of total attenuation.
  2. Reflection.
  3. Refraction.
• Tissue-Specific Attenuation Rates:
  • Blood: approximately $3\%$ attenuation for every $1\,cm$ of depth.
  • Bone: approximately $96\%$ attenuation for every $1\,cm$ of depth (due to high collagen content).
• Table: Ultrasound Half-Depths in Millimeters at $1$ and $3\,MHz$

• Key Takeaways on Attenuation:
  • Sound waves are more absorbed and better converted to energy at higher frequencies and in more collagen-rich tissue.
  • Because energy is absorbed more locally at high frequencies, less energy remains to be transferred to deeper tissues.
  • $3\,MHz$ Frequency: Absorbed at superficial depths (roughly $1\text{--}3\,cm$).
  • $1\,MHz$ Frequency: Penetrates to greater depths before absorption (up to $5\,cm$).

Ultrasound Generation and Application

• The Generator: Contains an electrical oscillator that produces high-frequency alternating current matching the intensity and frequency parameters of the applicator’s crystal.
• The Applicator (Sound Head): Contains a piezoelectric crystal.
  • Piezoelectric Properties: The crystal generates a charge in response to mechanical stimulation and, conversely, vibrates when a current is applied.
  • Vibration Frequency: Current sent to the crystal causes it to vibrate between $1\text{ million}$ and $3\text{ million}$ times per second ($1\,MHz$ or $3\,MHz$).
• Wave Delivery: The piezoelectric crystal transduces electrical energy into acoustic energy. The resulting pressure wave involves:
  • Compression: Areas of increased molecular concentration.
  • Rarefaction: Areas of decreased molecular concentration.

Essential Ultrasound Parameters

• Effective Radiating Area (ERA):
  • The area of the transducer head that produces sound waves at a significant intensity level.
  • It is a fixed property of the crystal, usually located toward the center of the head.
  • Higher ERA ensures more uniform wave emission and fewer "hot spots."
  • Clinical Rule: The treatment area should be $2\text{--}3\text{ times}$ the ERA of the sound head.
  • Example: If the unit has an ERA of $4\,cm^2$, the treatment area should be $8\text{--}12\,cm^2$.
• Beam Nonuniformity Ratio (BNR):
  • The ratio between the hottest spot in the beam (Spatial Peak Intensity or SPI) and the average intensity across the ERA (Spatial Average Intensity or SAI).
  • Low BNR: Indicates a more uniform beam, resulting in a safer/smoother treatment.
  • High BNR: Less uniform; increases risk of patient discomfort or periosteal pain.
  • Safety Standard: The FDA requires BNR to be $< 6:1$. High-quality units are $2:1$ or $3:1$. Standard clinic units are often $5:1$.
• Frequency:
  • $1\,MHz$: For deeper tissues (up to $5\,cm$).
  • $3\,MHz$: For superficial tissues ($1\text{--}2\,cm$).
• Intensity: Measured as Spatial Average Intensity (SAI) in $W/cm^2$. It is the ratio of total power (Watts) to ERA ($cm^2$).
  • $0.1\text{--}0.5\,W/cm^2$: Low intensity (non-thermal, pulsed).
  • $0.5\text{--}1.0\,W/cm^2$: Moderate intensity (subacute injuries, mild heating).
  • $1.0\text{--}2.0\,W/cm^2$: Higher intensity (thermal effects, continuous US).
  • $> 2.0\text{--}2.5\,W/cm^2$: Rarely used; high risk of periosteal pain.
• Mode and Duty Cycle:
  • Continuous: US is delivered at a constant energy level ($100\%$ duty cycle).
  • Pulsed: Energy flow is periodically ceased, reducing total energy delivered ($20\%$ or $50\%$ duty cycle).
  • Formula: $\text{Duty Cycle} = \frac{\text{Pulse Duration (On-Time)}}{\text{Pulse Period (On + Off Time)}} \times 100\%$.

Physical Effects: Thermal and Non-thermal

• Acoustic Streaming: Steady, one-directional flow of cellular fluids that promotes cellular activity and membrane permeability.
• Cavitation: The formation, growth, and vibration of microscopic gas bubbles; it creates currents that allow for acoustic streaming.
• Microstreaming: Tiny, localized fluid motion surrounding cavitation bubbles.
• Thermal Effects:
  • Temperature Change: Can increase tissue temperature by $4\text{--}5\,^{\circ}C$.
  • Rate: Soft tissue increases approx. $0.2\,^{\circ}C$ per minute at $1\,W/cm^2$ and $1\,MHz$.
  • Benefits: Increased metabolic rate (every $1\,^{\circ}C$ acceleration), pain control, muscle spasm reduction, vasodilation/increased circulation, and increased soft tissue extensibility.
  • Post-Treatment: Temperature returns to baseline quickly; stretching is most effective if applied during treatment.
• Non-thermal Effects:
  • Result from mechanical events (cavitation/streaming).
  • Cellular Impacts: Increased intracellular calcium, increased membrane permeability, and increased cell function (mast cells, fibroblasts, macrophages).

Clinical Indications and Guidelines

• Soft Tissue Shortening:
  • Parameters: $1\,MHz$ or $3\,MHz$; intensity $0.5\text{--}1.0\,W/cm^2$ ($3\,MHz$) or $1.5\text{--}2.5\,W/cm^2$ ($1\,MHz$); duration $5\text{--}10\,min$.
  • Application: Maintain stretch during US and for $5\text{--}10\,min$ after.
• Pain Control:
  • Can use thermal or non-thermal parameters based on the underlying cause of pain.
• Soft Tissue Healing (Ulcers):
  • Standard US: $3\,MHz$, $0.5\text{--}1.0\,W/cm^2$, $20\%$ duty cycle.
  • MIST Therapy: Non-contact US using saline mist at low frequency ($40\,kHz$) to promote angiogenesis and reduce bioburden.
• Bone Fractures:
  • LIPUS (Low-Intensity Pulsed US): Specific fixed parameters: $1.5\,MHz$, $0.15\,W/cm^2\,SATP$, $20\%$ duty cycle, $20\,min$. Available by prescription only.
• Phonophoresis:
  • Using US (non-thermal) for transcutaneous drug delivery. US increases the permeability of the stratum corneum via cavitation. By prescription only.

Contraindications and Precautions

• Contraindications:
  • Malignant Tumor: May increase growth or metastasis.
  • Pregnancy: Risk of maternal hyperthermia and fetal abnormalities; avoid abdomen, low back, and pelvis.
  • Sensitive Tissues: Eyes, heart, reproductive organs, or Central Nervous System (CNS) tissue (especially after laminectomy).
  • Electronic Devices: Pacemakers or internal defibrillators; avoid US within $6\,inches$ of the device.
  • Specific Implants: Methyl methacrylate cement or plastic (heats rapidly). Metal joints are less of a concern but require caution.
• Precautions:
  • Epiphyseal Plates: Avoid high-dose/thermal effects in children.
  • Acute Inflammation: Avoid thermal effects; pulsed US may be appropriate.
  • Fractures: High intensity is a precaution (except for specific LIPUS protocols).
  • Breast Implants: Avoid thermal effects due to risk of capsule heating and tissue damage.
  • Impaired Sensation/Cognition/Circulation: Risk of burns due to inability to feel or dissipate heat.

Clinical Application and Documentation

• Application Principles:
  • Movement: The sound head must keep moving to distribute heat and prevent standing waves.
  • Contact: The head must never be separated from the gel during treatment.
  • Area Limits: Do not treat areas smaller than $1.5 \times ERA$ (risk of overheating) or larger than $4 \times ERA$ (too much heat loss to achieve thermal effect).
  • Sequence: Do not use after interventions that alter sensation (like ice or heat packs).
• Documentation Essentials:
  • Body area, frequency, intensity, duty cycle, and duration.
  • Patient response (objective: pain level, ROM).
  • Crucial phrase: "No adverse reaction to treatment" (Avoid subjective phrases like "well tolerated").

Questions & Discussion

• Discussion Question 1: Refraction accounts for the majority of US wave attenuation. (True/False)
  • Answer: False. Absorption accounts for approximately $50\%$ of attenuation.
• Discussion Question 2: Ultrasound is what type of wave?
  • Answer: Longitudinal wave.
• Discussion Question 3: What is an important thing to do during the application of US to distribute the heat and minimize hot spots?
  • Answer: Keep the sound head moving.
• Discussion Question 4: A lower BNR is indicative of a more uniformly-shaped beam. (True/False)
  • Answer: True.
• Discussion Question 5: Pulsed US, with a duty cycle of $50\%$, is most commonly used to produce nonthermal benefits. (True/False)
  • Answer: False. A $20\%$ duty cycle is more common for purely non-thermal effects.
• Discussion Question 6: Advantages of phonophoresis vs. traditional drug routes?
  • Answer: Localized delivery, bypasses first-pass metabolism in the liver, and avoids GI irritation.
• Discussion Question 7: There is no concern for systemic effects with phonophoresis. (True/False)
  • Answer: False. Localized drugs can still enter the bloodstream and have systemic effects.
• Case Study 1: A PT is concerned about exacerbating inflammation. What is the MOST effective method to address this?
  • Option A: $1\,MHz$ frequency.
  • Option B: Limit time to $5\,min$.
  • Option C: Incorporate a pulsed $20\%$ duty cycle.
  • Option D: Intensity less than $1.5\,W/cm^2$.
  • Correct Strategy: Incorporate a pulsed $20\%$ duty cycle to target non-thermal effects.
• Case Study 2: Which general rule BEST determines the length of continuous US treatment?
  • Option A: $2\,min$ for an area $2\text{x}$ the ERA.
  • Option B: $5\,min$ for an area $2\text{x}$ the ERA.
  • Practice Recommendation: Typically $5\text{--}10\,min$ for an area twice the ERA is used to reach therapeutic temperatures.