Ultrasound Flashcards
what is ultrasound?
acoustic energy
not audible
>20,000 Hz, usually frequency in therapeutic range of .7-3.3 MHz
therapeutic function of ultrasound
deep heater of small areas in the body
How does ultrasound heat?
waves transmit energy by cycles of compressing and rarefacting material, these pulses propagate through tissue and produce heating
rarefaction
decreasing an item’s density, the opposite of compression
piezoelectricity
ability of some materials like crystals or ceramics/bone to generate an electric potential in response to applied mechanical stress
how does a piezoelectric crystal generate an electrical charge?
separation of electric charge across a crystal lattice
this charge creates a voltage across the material
crystal expands and contracts to create an ultrasound frequency
direct peizoelectric effect
production of electricity in a material when electricity is applied
reversible
converse peizoelectric effect
production of stress or strain when an electric field is applied
near field
convergent area of beam
where rays are aligned, more intense
far field
where beams diverge
less intensity
which frequency penetrates deeper, high or low frequency?
low frequency gets deeper tissue
higher frequency heats more shallow depths
when ultrasound meets tissue, it causes 5 effects:
pulse
scatter
absorption
reflection and transmission
refraction
waves bend slightly and change direction as they enter tissue instead of going straight down
reflection
return of energy, waves bounce back
waves move back in the opposite angle
increases at skin with poor ultrasound head contact
dangerous if bone reflects waves into soft tissue as it can create burns
absorption
conversion of the mechanical energy of an ultrasonic wave into heat
higher frequency increases absorption
wave goes into tissue
attenuation
absorption 50% + reflection and refraction 50%
increases with higher frequency
scatter
combination of refraction, diffraction, and reflection
wave moving in many/any direction other than the target
absorption coefficient
tissue and frequency dependent
highest for tissues with highest collagen content
higher coefficient means more heat is absorbed
attenuation coefficient
tissue and frequency specific
higher in tissues with a higher collagen content
which tissues respond best to ultrasound?
high absorption coefficient
high collagen content
poor response in those with high water content
heat behavior in tissues: which tissues have the greatest rise in temperature?
tendons rise up to 14-15 degrees
muscle rises around 5 degrees
how does scattering of ultrasound waves apply to multiple layers of tissue?
scatter effect occurs at each layer, and only waves that are absorbed transmit to the next layer where they are scattered again
this results in a loss of heat at greater depths
tissues best suited to ultrasound include:
tendons
ligaments
joint capsules
fascia
fat and ultrasound
fat can be overheated, need to be careful when applying ultrasound to area with overlying fat
is ultrasound effective at heating muscle?
Not well
low absorption coefficient
muscles are often too big and deep
half value depth
tissue depth at which 50% of the ultrasound delivered has been absorbed
half value depth of 3 mHz
2.5 cm
half value depth of 1 mHz
4 cm
power
amount of acoustic energy per unti time
watts
intensity
amount of power per area
W/cm2
frequency
number of cycles per unit time
Hz
cycles/s
spatial average intensity
average intensity of US input over the area of the transducer
spatial peak intensity
peak intensity of US output over area of transducer
greater in the center and lower in the periphery
effective radiating area
area of crystal from which US waves radiate
beam nonuniformity ratio
BNR is ratio of set intensity to max peak intensity
2:1, 3:1 is safe, 6:1 or higher is dangerous and can form hot spots
pulsed US
some on/off time
percentage of on time or ratio of on/off
duty cycle
on:off time in total cycle time
1:5 is 20% on, 2 s on 8 s off
non thermal effects of US
cavitation: sonically generated gas activity
acoustic streaming: circular flow of cellular fluids
microstreaming: eddying near vibrating object, gas bubbles oscillating, causing cellular effects
why do you need to move US head?
if it is held in the same place the same wave will be repeated over the same path, leading to extreme heating
does 3 or 1 MHz heat faster?
3 MHz, by factor of 3
1 Mhz heats at .2 C/min
changing which parameters leads to faster heating
increasing intensity, frequency
applying to higher protein tissue
getting more reflection
effects of heating on the body
increase metabolic rate/enzyme activity rate
vasodilation
increased collagen extensibility
decreased neural sensitvity
increased pain threshold
decreased m spasm
altered n conduction
non thermal effects: physiologic
increased:
membrane permeability
intracellular Ca
mast cell degranulation
Histamine release
proteinn synthesis rate
fibroblast stim
macrophage
indications for US
soft tissue shortening
pain control
dermal ulcers
surgical incision
tendon injury
resorption of Ca deposits
bone fracture
carpal tunnel syndrome
phonophoresis
plantar warts
herpes infection
How does US help ulcers?
nonthermal effects cause wound contraction, protein synthesis
accelerate healing of infected wounds
How does US help tendon healing?
faster recovery of tendon strength
phonophoresis
facilitate transdermal drug delivery with ultrasound for local and systemic drug delivery
appropriate size treatment area is:
2-4 times the size of the ERA
effective radiating area of head
application patterns of US
overlapping circles, completing 1 circle in 2 s
longitudinal strips overlapping and alternating directions creating a rectangle
pulsed vs continuous: thermal effect
100% max thermal effect
50% mod thermal effect
20% no thermal effect
contraindications to US
pregnancy - abd/low back
active epiphysis
cancer
TB
bleeding dx
impaired circulation
myositis ossificans
DVT
acute injury
radiated tissue
impaired sensation
implanted devices
repro organs
eyes
anterior neck
implants
regenerating nerves
