Ultrasound Flashcards
rarefraction
low pressure and density
compression
high pressure and density
what is a wavelenght made up of
1 cycle = 1 compresion and 1 rarefraction
frequency
1 cycle per second = 1Hz
propagation
speed at which a wave moves through a medium
metres/second
in soft tissue - approx 1540 m/s
equation for propogation
propogation = frequency x wavelength
what does propgation depend on
density
stiffness
gases - lower propagation speed
solids - higher propagation speed
why can you not constantly put sound wave into tissue
will cause tissue to heat up
why does reflection occur
because of various densities and stiffness = impedence between tissues
similar impedance = weak echo
identical impedance = no echo
different impedance = stong echo
what is the range equation
t = 2d/c
t = echo time d = distance to reflective c = speed of sound
what is impedance
relates acoustic pressure and speed of particle vibration
impedance calculation
impedance = density of medium x propagation speed w
what is intenisty transmission coefficient T
the fraction of the sound which is transmitted
what is intensity reflection coefficient R
the fraction of the sound intensity which is reflected
how are the sound waves generated
the probe (transducer) converts electrical signal to sound waves and vice versa uses piezoelectric crystals
what material is used in ultrasound probes
man made ceramin material PLT
lead Zirconate titante
how do piezoelectric crystals work
electrodes on opposites sides
dioples aligned obliqely
change in current create pressure changes around the crystals causing them to rotate to create compression and rarefraction
what is a dipole
closely spaced pair of electric charges
describe the process of the transmitter
electrical drive signal
apply an electrical field
crystals change in physical dimesions
generates an ossiciating acoustic pressure
desribe the process of the reciever
osicilating acoustic pressure
change in physical dimesions of the crystal
generates an electrical field
electrical signal to display electrons
connections in the transducer
coacial cables deliver voltage pulses which drive the crystals
two faces of the crystlas are coated with thin conducting films which contact the cable terminations
front electrode is earthed to prevent patient from electric shock
what is the baking block
absorbs ultrasound from back face
limits crystal vibrations to allow short pulse duration
what is the role of gel
elimates layer of air between patient and probe to prevent reflection
prevents an impedance mismatch
requirements of an image test
define normal anantomy detect variations from normal charactise abnormaility stage of the disease guide interventions monitor effects of treatment screening risk vs benefit
