B-mode Ultrasound Imaging Flashcards
What is A-mode (amplitude mode) imaging?
A group of piezoelectric elements is used to transmit a focused ultrasound pulse into the tissue using the inverse piezoelectric effect.
Pulse scatters due to changes in acoustic impedance and it scatters.
Transducer array where they are detected using the same elements via the direct piezoelectric effect
What is an A-line/ A-mode?
The processed signal from A-mode imaging
The detected signals at each element are combined to give a single radiofrequency (RF) signal which is made positive, demodulated, and displayed as a function of time or distance. The conversion from time to distance is performed using the known speed of sound in tissue
In what ways can the incident plane be transmitted in the incident field?
Transmit a single plane wave pulse parallel to array (used in ultrafast ultrasound)
Transmit a series of pulses beamformed into lines perpendicular to array (used in B-mode imaging)
How does B-mode imaging work?
The elements of the transducer array are used in groups to transmit and receive ultrasound pulses to form A-mode scan lines
Beam is then stepped sideways within the scan plane by exciting a different subset of elements in the array.
In B-mode imaging what are the A-mode scan lines converted to?
Scan lines recorded at each position converted to grayscale and displayed as a continuous 2D image
What are the processing steps of images before they are displayed?
for each receive channel:
ADC -> time delay -> weighting
+
TGC -> filtering -> envelope detection -> log comp
What happens at the Analogue to digital conversion (ADC) step?
Analogue voltage signal is sampled at regular intervals in time
Voltages are quantised into closest discrete values (controlled by number of bits used)
What is the sampling rate for clinical scanner and the typical bit size used for precision?
80 MHz
16-bit precision -> 65,536 values -> 96 dB dynamic range
What is beam forming?
After ADC, the signals from each element are time delayed, weighted, and then summed together to give a single A-mode scan line
What is the equation of focal length (F) for dynamic receive focusing?
F = 1/2 c_0 t
t = time delays
What is dynamic receive focusing?
The time delays for each element are updated dynamically so the focus depth matches the depth where the echoes originated from
What is the beam width for a given aperture is proportional to and what does this mean for dynamic receive focusing?
The focal depth
When using dynamic receive focusing, the image resolution will degrade at larger depths
What is aperture expansion?
The receive aperture is expanded as the focus depth is increased to maintain a constant beam width (after dynamic receive focusing)
What is multi-zone transmit?
Image resolution can also be improved by using multiple transmit pulses
(multi-zone focusing)
and each is focused at a different imaging depth, and the resulting scan lines (or images) later combined
What is a draw back of multi-zone transmit?
It reduces the imaging frame rate, as a larger number of transmit-receive events are needed to form the image.
What is apodisation?
To reduce side lobes, the signals from the individual elements can be weighted such that the elements in the middle contribute more to the summation (increases main lobe width)
What does applying time gain compensation (TGC) do?
amplifies the signals proportional to the distance travelled (e^ax)
Why is TGC applied?
the acoustic waves generated by the ultrasound transducer lose energy as they propagate through the tissue due to acoustic attenuation
So reflections from deeper tissue features will appear weaker in the recorded signals
What happens at greater depths?
TGC will increase noise (dependent on frequency and SNR)
so if TGC is used at double gain it will reduce quality of image (so sweet spot needs to be found)
How is noise reduced in signal?
Signal is band-pass filtered to reduce noise outside the frequencies interested, obtains the harmonic signal for tissue harmonic imaging
What is the process of envelope detection?
Signal is demodulated to extract envelope of amplitude variations
This removes the oscillations at transmit frequency, leaving only information about the amplitude of the reflections
Why can’t all the reflections from tissue interfaces be displayed on computer monitor?
Reflections have range of order of 60 dB whereas the grey scale range of computer is only 20 - 30dB
What is the process of log compression?
to improve the differentiation of weaker signals, a nonlinear amplifier is used, where the output is proportional to the log of the input signal
What happens after the individual A-mode scan lines have been acquired?
they are combined together and converted to the correct resolution for display on screen
For steered beams, this includes converting from polar coordinates to Cartesian coordinates using interpolation
What is the common processing technique used for image acquisition and signal processing in modern ultrasound machines?
Image compounding
This is used to reduce the effect of speckle and noise on the image by averaging several ultrasound images taken under different conditions
What happens in M-mode imaging?
The peaks from each A-mode scan line are converted to pixels/dots with brightness proportional to echo peaks.
Multiple scan lines are then displayed side-by-side to reveal changes over time (used in echocardiography)
(stacks A-mode scan lines temporally)
How is each A-line obtained?
Using transmit and receive beam forming
