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!
