Ultrasound energy impacts body tissues with mechanical vibration that exceeds human hearing range, creating sounds at frequencies well beyond human audibility. Therapeutic ultrasound machines typically use frequencies between 1 MHz and 3 MHz while longwave devices produce 40-50kHz frequencies – well outside what could be audible noise limits.
Different frequencies vary the rate at which energy is absorbed by tissue, leading to different heating effects.
1. Intensity
Ultrasound therapy treatments vary in intensity depending on how much energy is transmitted from the ultrasound soundhead to tissue interface. Frequency used and depth penetration all affect intensity.
Ultrasound is an acoustic mechanical energy wave beyond human hearing range (16Hz-20kHz). When applied to soft tissues, ultrasound causes micro-vibrations within it which stimulate blood flow towards that area and can aid with oxygen and chemical delivery into promote healing processes.
Different tissues absorb ultrasound waves differently; generally speaking, high collagen structures such as ligaments, tendons and fascia absorb more of its energy while more dense tissues such as muscle and bone don’t. 2.
Therapeutic ultrasound requires some sort of conductor in order to deliver its acoustic mechanical energy into tissue interface. This could take the form of ultrasound gel, gel pads, mineral oil or lotions or water immersion (via plastic tub).
These factors all impact how much energy is transferred into tissues and absorbed by them, creating multiple factors to consider when selecting an ultrasound frequency frequency for your patient.
At its core, ultrasound wave intensity matters most when treating tissues; typically 1-MHz treatments heat intramuscular tissues up to 2.5 cm while 3-MHz treatments heat intermuscular tissues up to 1.6 cm.
The difference in penetration depth between the two frequencies can also be explained by differences in their acoustic impedance, which describes how much resistance a material offers against transmission of ultrasound waves 2.
Higher frequency frequencies tend to have greater acoustic impedance, making them more likely to transmit through soft tissues than dense tissues, so lower ultrasound frequencies should be chosen for deep-tissue work and higher ones for superficial tissue work.
2. Duty Cycle
The amount of acoustic energy absorbed by tissues depends on several factors, including frequency, dose (W/cm2), duty cycle and duration of treatment. Tissues with higher protein density such as tendon, ligament or muscle absorb more sound energy than tissues with greater water content such as skin or adipose; while distance from sound head to tissue surface influences how quickly energy is absorbed.
Ultrasound waves vibrate tissue, heating it up. This process is known as cavitation; by increasing tissue temperature it may help alleviate pain, increase blood flow to an area and accelerate transport of chemicals that aid healing.
Dependent upon the frequency of an ultrasound machine, you have two modes for use: continuous or pulsed. Continuous mode involves sending sound waves all of the time while pulsed mode includes periods when sound energy does not travel through tissue. You can set your duty cycle between 10%-20-50%.
Studies have demonstrated that 120kHz UET’s lytic efficacy increases with increasing duty cycle in an in vitro human clot model. While cavitation or other mechanisms may contribute to this increased lysis, optimizing ultrasound parameters is still crucial when treating complex thrombolysis cases.
Before using ultrasound, it’s essential that a conductor be used to transfer sound energy into tissue. Conductors include gel pads, mineral oil, different lotions or even water immersion – some conductors provide thermal effects while others only mechanical ones; in either case allowing tissue “cool down” after each transmission of ultrasound wave transmission.
3. Pulsed Ratios
Ultrasound units used for therapeutic application transmit energy that lies within the acoustic spectrum, typically between 1 MHz and 3 MHz, providing clinicians with flexibility in selecting which operating frequency best meets each application. Most high quality machines allow clinicians to choose which operating frequency best matches each use case.
Ultrasound machines emit sound waves known as pulsed waves. This sound wave is created by a vibrating crystal which alternately compresses and refracts as it hits tissue interface (see figure 12.1), creating an asymmetrical compression/refraction process which results in an irregular, pulsatile wave – this may lead to cavitation (also known as the Acoustic Streaming Effect or ASEA; see figure 13.3) which may cause local heating. Although this does not occur with all pulsed modes such as 1:4 mode or continuous output mode output mode (see figure 13.3) this local heating can occur with some such as 1:4 mode or continuous output mode (CW or continuous output mode).
For therapeutic purposes, it’s crucial that we understand how ultrasound wave frequency affects its therapeutic effects. As ultrasound frequency increases, more energy is absorbed by deeper tissues instead of superficial ones – hence why higher frequencies are often employed when targeting tendons and fascia with lower BNR values.
With lower frequencies, more of the ultrasound energy reflected back from tissues or refracted back towards the surface, enabling clinicians to use lower frequency ultrasound for tissue that does not absorb its full energy absorption potential. This allows more superficial tissues such as muscle or fat layers to benefit from being treated at this frequency level.
Before starting therapy with ultrasound, it is crucial that a Physical Therapist Assistant communicate with their client and discuss why ultrasound might be an appropriate modality as well as screen for potential precautions or contraindications to this therapy modality.
As part of their treatment regimen, therapists should monitor patients during each ultrasound treatment to make sure that they don’t overdo or tolerating it too strongly. An overzealous individual could quickly cause pain in the treated area as well as overexposure to potentially harmful ultrasound rays.
4. Time
As ultrasound waves pass through tissue, part of them reflect off of interfaces between tissues; while others are absorbed and cause temperatures in that tissue to increase – with frequency increase coming faster temperatures can rise further.
Thus a 3 MHz emission can produce faster temperature rise in tissues than 1 MHz emission; however, energy available is still limited by what can pass through tissue at any given point – known as penetration depth; half value depth (or half power penetration) indicates where half of available energy has been absorbed into tissues.
At The Physio Place, physiotherapists utilize therapeutic (not diagnostic) ultrasound technology to treat muscles, joints and soft tissues of the body. As ultrasound is an electromagnetic form of energy it falls under Electro Physical Agents category.
Ultrasound therapy is a safe, noninvasive solution with many applications and indications. It can be combined with other therapies to boost their healing effects and can even serve as a preventative measure by keeping muscles and connective tissues warm through friction massage, tissue mobilisations or stretching techniques. Ultrasound has also proven useful as a painless and cost-effective means of treating injuries such as muscle strains, tendonitis or soft tissue pathologies without incurring further injury to soft tissue pathologies in patients.
Therapeutic ultrasound in your practice can be done simply and painlessly by placing a transducer onto the skin with hypoallergenic contact gel and connecting it to a computerised generator that transmits sound waves at predetermined frequencies through its probe. Ultrasound imaging has become an indispensable tool in obstetrics for monitoring fetal heart rates, joint injections and arterial line placements. Other uses for ultrasound include lithotripsy, atrial fibrillation diagnosis and fracture healing. Cosmetic applications of LED therapy also include the reduction of fat, cellulite and adipose tissue. Furthermore, the technology’s use has quickly expanded due to its many clinical advantages such as increasing hip range of motion and knee range of motion; strengthening weak muscles; and decreasing foot and leg edema.
