27.6 Ultrasound Flashcards
What is ultrasound
Why can’t we hear
Type of wave?
Just sound wage nothing special = thus longtidunal
But frequency is GREATER than 20kHz
Therefore we can’t hear it as this exceeds humans audible range
What are ultrasound used for here
Imaging baby, and also detecting cracks in buildings etc
Why is ultrasound advantage ti image a baby
NO ionising radiation
NON INVASIVE
Quick, inexpensive and Easy to use
How does an ultrasound transducer work
It not only EMITS ultrasound waves, but RECEIVES the reflected ones, and processes the, into electrical signals which can then be computerised to make an image
It produces and interprets ultrasound waves by using the PIEZOELECTRIC EFFECT
What is piezoelectric effect?
Effect that some crystals such as quartz
- can produce an emf when they are stretched / compressed , interpreting ultrasound into alternating emf which computer can process
- OR when emf is applied across the crystal can stretch and compress accordingly , producing ultrasound as a result
The actual ultrasound produced is at a certain frequency, but it is EMITTED at a certain frequency too… why?
This MUST BE PULSED, so that you can actually tell what reflected waves came from when, otherwise it’s CONSTSNT reflection and this information is lost
How does general ultrasound work?
Transducer emits pulsed of ultrasound
- they are reflected at different boundaries based on properties of the boundaries
- the time of flight is used to locate the distance of the boundary and create an image based on depth , in REAL TIME
When does a reflection of ultrasound occur?
Always between the entrance to A DIFFERENT MEDIA
So how would a typical ultrasound voltage graph look for like an eye, and we want to find distance of retina
So 1st reflection is always due to the ultrasound ENTERING the thing
- then reflections after that are all due to difference in media, such as going in to the lend, going out, hitting back of the eye
We want the time taken for the FURTHEST reflection ( the back of the eye) , so that we can use speed = d/ t to work out the distance
Why when piezoelectric effect is Used to generate ultrasound waves the alternating emf is of the same frequency as the natural frequency of the crystal?
This so resonance can happen = maximum trsndfer of energy = maximum amplitude of wave produced for ultrasound
What happens to the ultrasound wave when it meets a boundary
1) partially transmitted/ partially reflected
Therefore intensity of reflected wave must be less, as it has lost energy = attenuated
There can be some interference of signal due to many waves reflecting forever and arriving at transducers in a blur. How is this taken care of
Taken care of as the computer ALGORITHM knows which ones to block out and which ones to go through to find the distance
What happens in an ultrasound A scan?
What are A scans typically used to do?
- a SINGLE TRANSDUCER is used to image along a STRAIGHT LINE
- ultrasound sent in a single direction snd time taken for reflections recorded
2) typically used to measure retina eye, distance of bone, or cracks
What happens in an ultrasound B SCAN?
This forms a 2D IMAGE
- transducer moved over patient skin
- every time there is reflection at a boundary, a computer plots dots here.
- the more INTENSE the reflected wave, the more brighter the dot
At the end a 2d image can be made due to the difference of boundary
So difference between A and B scan
A scan = single transducer , along one line of axis, emits ultrasound in one direction only , and is used to measure distance. DOES NOT PRODUCE AN IMAGE
B scan = transducer is moved across patient and a series of dots is plotted based on distances worked out from reflections of boundaries. The more intense reflection = brighter the dot, which really differentiated boundsried well .
This is for modelling 2d Images of babies!
Main difference A vs B scan?
B scan produces 2d image, A just information about reflections
How to justify why the frewuency of emission of ultrasound (not scc frequency of ultrasound ) is good based on speed
You want all your waves to be able to reach the max depth before the second wave emitted!
So find time period and multiply by speed
If it can reach max distance before next wave released it’s good
What is acoustic impedance?
Z = p x c
Acoustic Impedance = density x speed of ULTRASOUND in the object
Acoustic impedances give us a measure of how much something might reflect intensity wise for ULTRASOUND SPECIFICALLY
How to find out PERCENTAGE OF ORIGINAL INTENSITY THAT WILL BE REFLECTED
IR/I0
= (z1-z2)/(z1+z2) ALL SQUARED
So PERCENTAGE of how much is reflected = this
Why does the more different the impedances are the more Is reflected
Why when impedance is the same (so going through same thing ) is no thing reflected
More different = higher number in equation
If same then z1 -z2 =0 so nothing is reflected back!
How to make it so something becomes soundproof essentially?
You make it have a GRADUAL CHANGE IN IMPEDANCES as it goes
This means it won’t really reflect back STRAIGHT AWAY, but small reflections as it moves on
Esch time it small reflects it loses energy by the time it reaches the end it won’t have much energy at all
- as a result the sound has been proofed without all of it reflecting back at you at once! ( because that’s the only other way they get reflected )
How much is transmitted in terms of I0 and Ir ?
I0 - IR is let through
What happens when a normal transducer is placed against skin
Why is the bad
Placed Agaisnt skin casually the ultrasound is fired through the AIR between the transducer and the skin which then has to go THROUGH THE SKIN
The air skin boundary, acoustic Impedance is SO different that about 99% of intensity is REFLECTED BACK
Thus we can never produce imaging B scan here
How to fix air skin boundary problem
We add some COUPLING GEL
- this is gel that is of similar acoustic impedance AS the skin
- applying this between skin and transducer, now ultrasound is fired through the coupling gel, and when it meets the skin boundary barely is reflected.
- it makes its way trough skin amd is reflected at baby tissue as normal as this the first time the acoustic impedance is chsnged
What is the form of putting something so impedances are similar and it doesn’t reflect straight away called?
Acoustic matching , impedance marching
How can Doppler shift be used with trailing blood away and close to you
If ultrasound is fired toward blood travelling towards you, then the frequency of ultrasound REFLECTED will be MORE due to the relative motion , (closer = smaller wavelength = higher frequency )
If it’s going away then frewuency will decrease by doppler shift (apparent frequency)
What does the ultrasound actually get reflected by in the blood
Iron rich blood cells
So what is equation for doppler shift
Change in f = f x 2 x (speed of blood) x cos theta/ speed of ultrasound in object
Where theta is angle between beam and blood!
Similarities with Doppler shift equation? - still change of f / f = ratio of v/speed of thing in object
But the speed calculation is done for us
Why when we induce normal to blood flow we get NO speed of blood?
This because cos theta would be cos 90 = 0!
But you won’t be able to detect anything it just goes back up
