Lecture 3: Ultrasound Flashcards
describe the use and misuse of US
Less people are using now than previously
many studies show US no better than placebo
BUT problem may be in the study design and not US itself (want to use in a small area not large, pt variation, etc)
what is therapeutic US
high frequency mechanical waves (acoustic)
frequency of 1-3.3 million vibrations per sec
advantage of US
depth of tissue penetration
heats 2-5 cm deep
superior heating for structures with high collagen content
thermal and non-thermal effects
what does the piezoelectric crystal in the transducer head do
2-3 mm thick
made from synthetic plumbium zirconium titanate
high frequency AS current applied to crystal
converts electric energy to acoustic
crystal = mechanically deformed
what is piezoelectric
means it has ability to change shape in response to electric current
expands and contracts at same frequency as applied current
when it expands the molecules in front of it are compressed
when crystal compresses the molecules are rarefied
this then travels as a longitudinal wave
what size area is appropriate for US
you dont want to cover an area more than 2 times the sound head surface for 1 treatment (2-3 times the effective radiating area or ERA)
if a larger area is desired, 2 treatments should be completed or else it will lose effectiveness
what are the 2 components of beam nonuniformity ratio
spatial peak intensity and spatial average intensity
BNR = peak/average
what is spatial peak intensity
peak intensity of the US output over the area of the transducer
usually greatest in the center of the beam and lowest at the edges
what is spatial average intensity
average intensity of the US output averaged over the on/off time of the pulse
what should be the max BNR of a transducer
5:1 or 6:1
what is the relevance of wave transmission frequency
body tissues absorption of US increases with wave frequency
less energy goes deeper
how does a wave transmission of 3 MHz work
US waves absorped at 2-3 cm depth
absorption occurs 3-4x faster
how does wave transmission of 1 MHz work
effective treatment depth up to 6 cm
absorption occurs slower
what is attenuation
as US enters body it gradually decreases i intensity due to attenuation
dependent of frequency and tissue dependent
increases with collagen content of tissue and with higher frequencies
occurs due to absorption, reflection, and refraction of the wave
heat produced by US is dependent on what
energy delivered and the absorption coefficient
energy delivered depends on what
intensity and duty cycle of US
the absorption coefficient depends on what
collagen content and US frequency
what is responsible for 1/2 of the attenuation that occurs with US
absorption
the remainder of US attenuation (not due to absorption) is due to what
reflection and refraction
what is reflection
US beam is redirected from a surface angle equal and opposite of the original beam
where does energy absorption, reflection, and refraction usually occur and why is this important
usually at tissue interfaces where there is an impedence to sound changes
air has a high impedance and skin is low; this is why we need US gel
where does more heat accumulate with US and why
between bone and soft tissue because that is where most reflection occurs
thermal effects of US
increased metabolic rate
decreased pain/muscle spasm
change of nerve conduction velocity
improved circulation
increased soft tissue extensibility
US is good for heating what
tendons
ligaments
joint capsule
fascia
scar tissue (more collagen)
why is US not ideal for muscles
muscles have a low absorption coefficient and a large surface area so US not ideal
what is cavitation
alternating compression and expansion of gas bubbles in tissue fluids caused by mechanical pressure
makes cell membranes more permeable
what is microstreaming
eddying that occurs around a vibrating object
what is acoustic streaming
movement of fluids along boundaries of cell membranes resulting from mechanical pressure waves
produces alterations in call membrane activity, increased cell wall permeability, and increased intracellular calcium
increased macrophage response and increased protein synthesis
non thermal US effects
increased intracellular calcium levels
increased skin and cell membrane permeability
mast cell degeneration
promotes macrophages responsiveness
good for the inflammatory phase of tissue repair
promotes protein synthesis
accelerates tissue healing/reduces time of inflammatory process
when is thermal used and what should you be cautious of
continuous frequency
be cautious of hot spots
used before stretching and/or scar tissue mobilization as well as for pain control
when is non-thermal used and what is the advantage
pulsed frequency (20% duty cycle)
no concern for hot spots
used for alternating membrane permeability to accelerate tissue healing
clinical use of US
soft tissue shortening
tendon/ligament healing
bone fx
joint inflammation
phonophoresis
