Ultrafast Ultrasound Flashcards
How does ultrafast ultrasound work?
All elements are used to transmit an unfocussed plane wave
The backscattered waves are measured by all the elements at once
An algorithm is used to form the image (all the A-lines simultaneously)
How does the conventional B-mode ultrasound imaging work?
An image consists of A-lines
For each A-line, a subset of the available transducer elements is used to generate a focussed transmit beam
A subset of the elements is used in receive mode to form an A-line
How can any field e.g. image, acoustic pressure map be constructed?
As a weighted sum of plane waves (has the form of a Fourier transform)
How can images be obtained with one incident plane wave?
1.Measure backscattered time series data on array
- Find the amplitudes of the constituent propagating plane waves of the backscattered field
- Relate these to the amplitudes of the constituent plane waves of the scattering distribution
What is an advantage of ultrafast ultrasound?
An image can be formed from one transmit pulse, so the image frame rate can be high
How is the round trip time found?
round trip time = 2 x depth / speed
How is the maximum frame rate found?
1 / round trip time
What are the disadvantages of ultrafast ultrasound?
There is no transmit focussing, so the backscattered signal amplitude is lower (and hence lower signal-to-noise ratio, SNR)
- To ameliorate this, coherent angle-compounding is sometimes used (averaging unprocessed images from many plane wave transmit angles)
- Incoherent angle compounding can still be used for speckle reduction
- Any compounding reduces the frame rate
What needs to occur for a clear image to be produced using ultrafast ultrasound?
Multiple angles need to be used to produce clear image as features are less visible when single plane wave at one angle is used
How does ultrafast doppler work?
By sending a plane wave to acquire the full time samples for all image pixels simultaneously
The greyscale value is used to find doppler
What effect does ultrafast doppler have on power doppler?
Power Doppler information obtained at all pixels simultaneously at ~ 80 Hz (compared to 20 Hz with conventional systems)
Power doppler s-to-n ~ 50 x higher than conventional systems so finer details can be obtained
How does functional ultrasound work?
Relies on doppler images of the brain and the neurovascular coupling between blood flow and activated regions in brain (when neurons are activated with stimulus)
How does supersonic shear imaging work?
a sequential series of push pulses with an increasing focal depth is used to generate a single conical shear wave travelling across the entire field of view
Second, the field of view is imaged as the shear wave travels across it, deforming the tissue as it passes, and from these measurements the spatially-varying shear wave speed is determined and an image of the shear modulus is formed.
How are sequential push pulses used in supersonic shear imaging?
With focal regions of increasing depth along the same line
Where is the shear wave elasticity imaging (SWEI) time of flight assumptions valid?
Only over limited axial depth
