B-mode ultrasound Flashcards
Explain the pulse-echo sequence? (3 marks)
Transducer uses a piezoelectric crystal with an AC current applied across it to send out a pulse.
Transducer then enters receive mode where it waits for the pulse to return, after a time, t. Assuming a speed of 1540 m/s, we can estimate the depth of the object or interface.
What is A-mode, M-mode and B-mode and when are they used? (6 marks)
Amplitude mode used in ophthalmology, just like a line trace with amplitudes on it.
Motion mode is used for cardiology to see wall motion. That is like A mode but shows the motion of the object with time.
Brightness mode is most commonly used, which is lots of A modes stitched together and displayed on an image with the image intensity reflecting the strength of the signal reflected.
Sketch and explain why TCG is used? (4 marks)
Time gain compensation is used to increase the magnitude of the signal which is received from deeper depths to make it visible in comparison with the more shallow reflections. This is required due to the logarithmic decrease in the signal with depth. Otherwise the deeper reflections and details would not be visible.
Draw a plot of the signal exponentially decreasing and gain graph which counteracts this (looks like it is inverted).
What is demodulation and why is log compression used? (2 marks)
Demodulation = removing the high frequency carrier signal, results in a smoother signal…
Log compression is used due to the logarithmic nature of the decreasing signal intensity with depth. Allows signal to be displayed on 256 grey levels. The Demodulated signal range is too large to be displayed as it is.
For a PAT what is the resolution limited by? (6 marks)
There are three components to resolution:
Axial res which is limited by the pulse duration.
Lateral res which is limited by the beam width.
Slice thickness which is limited by the elevation direction.
What determines the pulse duration? (2 marks)
The number of cycles in a pulse (which is limited by how well damped the transducer is) and the frequency (or the time taken for each wave to be sent)..
What is meant by a well damped transducer? (1 mark)
A transducer which can send out pulses with a short number of cycles. Well damped and high frequency give out the shortest pulses and offer the best axial res.
How is pulse duration and bandwidth related? (4 marks)
The longer the pulse duration the narrower the bandwidth. An infinitely long pulse, i.e. only needs one frequency to transmit that pulse. A short pulse will have a broad bandwidth.
What is the axial resolution equation? (2 marks)
axial res = # cycles x wavelength / 2
How is the frame rate calculated for B-mode imaging? (1 mark)
Frame rate = N x (2d/c)
N is the number of lines in the array sent out. (2d/c) is the time take for one line.
