Ultrasound Flashcards
Define the following
- sound
- rarefaction
- compression
- sound - series of compression and rarefactions
- rarefraction - LOW pressure and density
- compression - HIGH pressure and density
what do you mean by acoustic variables? and what are they in sound prorogation?
- acoustic variables are the properties that vary in a sound wave
- these include - variations in pressure and density + particle vibration
Define the following
- amplitude
- frequency
- wavelength
- proportion speed
- amplitude is the magnitude of a compression or rarefaction
- frequency - cycles per second (Hz)
- wavelength - length of space over which one cycle occurs (mm)
- prorogation speed - speed at which waves travel through a medium (m/s)
what is audible range for humans and animals?
- humans = 20Hz - 20k Hz
2. animals = >20kHz e.g. dogs, bats
what is the range of sound used in clinical ultrasound?
2-20MHz
what is the relationship b/w wavelength and frequency?
- they are inversely proportional to each other
how does the wavelength change if
- frequency increases
- frequency decreases
- frequency increases - wavelength decreases
2. frequency decreases - wavelength increases
what does the propagation speed depends on?
depends on the buck modulus of the medium i.e.
- density of the medium
- stiffness - resistance to compression
how does the prorogation speed vary across mediums?
- Solids have highest prorogation speed due to high density and stiffness
- Gases have lower propagation speed due to low density and is hard to compress air
what is the propagation speed in the following subjects?
- bone
- soft tissue
- fat
- air
- bone: 3000-5000 m/s
- soft tissue: 1440 - 1640m/s
- fat: 1440m/s
- air: 330m/s
what is pulsed ultrasound?
is a series of few cycles of ultrasound followed by gap
how is an ultrasound image formed?
- propogation of sound wave from the ultrasound probe on skin to the object
- reflection of sound wave from the object to the ultrasound probe
- measure the time taken for ultrasound echo to return to probe
- calculate the distance of object from probe
- average propagation speed in soft tissue is 1540m/s or 1.54mm/microsecond
what causes the reflection of sound in ultrasound?
- tissue interface b/w organ and tissue interfaces
- due to differences in impedance b/w tissues
- similar impedance - weak echo
- identical impedance = no echo
- very different impedance = strong echo
what is impedance (p)?
- it is the product of density of medium and prorogation speed
- relates to the acoustic pressure and speed of particle vibration
what is transmission coefficient T and R?
- intensity transmission coefficient T - the fraction of sound intensity which is transmitted (p2c2)
- intensity transmission coefficient R - the fraction of sound intensity which is reflected (p1c1)
and T + R = 1
what happens when an ultrasound pulse is incident on an interface?
- when the pulse is incident on the interface, the impedance changes (pc)
- some of the pulse is transmitted and some is reflected (making an echo)
what is the function of probe/transducer?
- converts electrical signal to sound wave and vice versa
- produces the ultrasound pulse
- detects the returning ultrasound echo
how does probe/transducer produce ultrasound wave/pulse?
it uses the piezoelectric effect
what is the piezoelectric effect?
A transducer with piezoelectric crystals is used to produce the ultrasound beam
how does the transducer act as both receiver and transmitter
- This is a material in which mechanical energy is converted into electrical energy and vice versa.
- This means that transmitting an electric voltage through the material will cause it to vibrate, producing a sound wave.
3 Similarly, the returning echo sound wave vibrates the crystals producing an electric voltage that can be measured.
briefly outline how electric field is generated in the piezo crystals?
- electric field applied
- that changes the material’s physical dimensions
- the physical dimensions are then changed/altered
- the alteration generates an electric electric field
name a natural occurring and man made crystal used as piezo crystals?
natural - quartz crystal
man-made - ceramic material PZT (lead zirconate titanate)
what does the piezoelectric crystal consist of?
- PZT (lead zirconate titanate)
- electrodes (conducting film) on opposite faces
- electric dipoles
what is the effect of electric field on PZT?
- normal state - electric dipoles aligned obliquely during manufacturing
- compression - electric dipoles aligned vertically
- expansion - aligned horizontally
name the components of transducer
- matching layer
- grounded electrode
- peizoelectric systole
- live electrode
- backing block
- acoustic insulator
- housing
- coaxial cable
what are the properties of piezoelectric crystal?
- resonant response - works at a particular response
2. resonate
what is the function of coaxial cable?
coaxial cable delivers voltage pulses which drives the crystal
why is the front electrode earthed/grounded?
to protect patient from electric shock
what is the function of backing block?
- absorbs ultrasound from back face
2. limits crystal vibration to allow short pulse duration
what is an acoustic insulator made of and what’s its role?
- made of cork/rubber/synthetic
2. prevents ultrasound probation into the plastic housing and operators hand - u/s emitted only by the from surface
why do we need a matching layer?
- impedance mismatch hinders the transmission of sound
2. there is 20:1 mismatch b/w PZT and tissue
what is the function of matching layer?
it is thin intermediate layer that can improve transmission of u/s pulse into the body
why do we need a coupling/coupling medium?
- impedance mismatch hinders the transmission of sound
2. there is 4000:1 mismatch b/w tissue and air
name a coupling/coupling medium and its function?
- ultrasound gel - aqueous gel
2. eliminates the layer of air b/w probe and patient tissue to prevent reflection
what is doppler ultrasound?
doppler ultrasound utilises Doppler effect to evaluate the moment of blood through vessel
how and why are contrast agents used? give one example
contrast agent - micro bubbles
HOW -
1. administered intravenously
2. very delicate so must reduce power output from scanner
WHY -
1. highly echogenic so reflections are greater from areas with micro-bubbles than without
what are the advantages of ultrasound?
- No radiation exposure
- Non-invasive
- Fast
- Inexpensive
- Real-time imaging
- Can measure velocity e.g. of blood
- Cross-sectional imaging
what are the disadvantages of ultrasound?
- Can’t image through bone or gas
2. Very dependent on operator skill, training and experience
