Ultrasound is a type of sound wave with a frequency of greater than 20,000 cycles per second. Therapeutic US's frequency (Hz) is above audible sound (16 - 20,000hz). When waves are generated in rapid succession molecules are jostled to a fro by the alternating phases of the waves. The energy that these waves produce is passed through the patients tissues. US needs a medium for conduction, or must have matter to vibrate.
Transducer: the part of the US unit that contains the crystal (quartz) which converts electrical energy into sound.
Power; Amount of accoustic energy per unit time, measured in wats.
Intensity: Power per unit area of the sound head
Beam Nonuniformity Ratio (BNR); informs the clinician of what the highest intensity would be within the beam for any give transducer. (BNR greater than 6:1 has risk of giving "hotspots")
Continuous: the set intensity remains the same throughout the entire treatment session
Pulsed: The delivery is pulsed on and off throughout the treatment time
Sequence and Sensations of Ultrasound in Treatment Plan:
- US may be used first to take advantage of its pain relieving effects.
- Stretch should be done immediately after US, due to ability to increase tissue length - Optimal is within 8-10min of treatment or during treatment
- Do not use with ice, however you may use ice at the very end of treatment.
- During continuous US, gentle warmth should be felt within a few minutes.
- Units with high BNR, often get superficial heating, so pt may not feel sensation at all.
- Pulsed US is SUB Sensory = no feeling
- Abnormal sensations
- Deep ache from periosteal heating = lower the intensity
- Superficial burning, itching, prickling
Penetration of Ultrasound:
On average, skin, blood vessels, and muscle absorb relatively little US. Tissues with high cartilage absorb more US ie: Bone, Cartilage, and Tendon.
Biophysical Effects of Ultrasound:
- Tissue temperatures rise. This rise is caused by cavitation, or pulsation of bubbles in tissue fluid. This is more likely to occur with 1MHz US. Clinically dense tissues retain more heat than vascular tissues.
- Diminish pain perception
- Slows nerve conduction
- Raises metabolism rate
- Increases blood flow
- Assists in resolution of swelling
- Stimulates immune system
- Increases soft tissue extensibility
- Reaches deeper than superficial heat - 1MHz used to heat tissue up to 5cm deep; 3 MHz used to heat tissue 1-2cm deep.
- 3MHz US heats quicker than 1 MHz US; about 3 - 4 times greater increase thus you should use a lower intensity with application of 3 MHz.
- Changes in cell function, effects occur at cell membrane with micromassage, acoustic streaming (or fluid movement in sound field) and cavitation.
- Clinically facilitates tissue repair
- Increases cell membrane and vascular wall permeability
- Increases Cellular activity
- Increases capillary density and improves blood flow
- Increases protein synthesis for wound healing
- Stimulate histamine release, serotonin release from blood platelet, chemtatic agents and growth factors from macrophages, formation of new blood capillaries to accelerate healing.
- Direct realignment of collagen fibers to increase stretchability of tissues.
- Increases collagen content to increase tensile strength of tissues
- Increase motor and sensory nerve conduction to reduce pain.
When doing US to the back or neck, some clinicians pick up the transducer when crossing the midline. Actually, bone in a normal vertebra prevents sound from getting to the area of the spinal cord, except if the patient has had a laminectomy or spina bifida. The brain is similarly protected by bone, cannot be reached by US. Therefore, cross midline with transducer quickly or treat 1 side, then the other.
- Most common is burning with high intensity, continuous US
- Risk of burn increased in areas of impaired circulation or sensation. Thus, keep the sound head moving, reduce intensity over superficial bone. Use patients complaints of discomfort as a guide to reduce intensity.