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Medical Physics - INIR: Ultrasound Y2 > B-Mode Instrumentation > Flashcards

Flashcards in B-Mode Instrumentation Deck (13)
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What does the front end of the scanner control?

- Amplification
- Multiplexing
- Switching


What does the main hub of the scanner control?

- Controlling
- Beamforming
- Signal Processing


What does the back end of the scanner control?

- Scan conversion
- Pre and Post Processing


Why does multiplexing need to be used in modern scanners?

- Modern scanners have more elements than channels.
- Multiplexing allows the first n channels to drive the last n elements as well as the first.


What are the steps in B-Mode signal processing?

1. Amplify
2. Compensate -TCG
3. Digitise - ADC
4. Beamform
5. Demodulate - remove carrier frequency
6. Log-Compress - Dynamic Range
7. Reject


How is quadrature demodulation employed in B-Mode signal processing?

- RF data is convolved with an in-phase reference signal, and a 90deg out-of-phase signal separately.
- A band pass filter is then used to remove the carrier frequency.
- In-phase data represent the "real" component (I), and the quadrature data the imaginary component (Q), of the signal.
- I+iQ contains the amplitude and phase info.
- SQRT(I^2 + Q^2) is the envelope of the signal.


How does the point spread function obtained by the scanner?

- From a point object:
-- it is first convolved with the pulse shape to create a vertical profile
-- it is then convolved with the beam shape to get the horizontal profile
-- finally, the point spread function is obtained by demodulation.


How are data point filled between scan lines?

- Linear interpolation using a 2D triangular function.
- Secotr data mapped to pixel locations in digital memory.


What is the equation for the interpolated signal?

Z(r, theta) = SIgma_n{Sigma_m{S(r-ndr, theta-m.dtheta)*Z(ndr,m.dtheta)}}


What is the difference between read and write zoom?

- Write zoom increases the image size to fill the screen by increasing the line density in the selected region. This improves image quality and maintains frame rate.
- Read zoom increase the image size to fill the screen by increasing the pixel size.


Describe the basic scanner architecture.

1. Scanner - transmit beamformer produces signals.
2. Front end - signal passed through DACs, amplifiers and multiplexer, then to transducer elements.
3. Front end - received signals passed through the multiplexer, switches, amplifiers, TCG, and ADCs.
4. Scanner - signals passed to receive beamformer, which forms RF data lines, data lines then demodulated.
5. Back end - envelopes of data detected, compressed, and converted and post-processed (e.g. grey-mapped).
6. Back end- video signal generated and sent to display.


What are some common B-mode artefacts?

- Speckle.
- Reverberation
- Direct shadowing
- Post-cystic enhancement
- Edge shadowing
- Poor visualisation of boundaries.


Where are reverberation artefacts most likely to occur?

- Between strongly reflecting parallel interfaces (e.g. muscle fascia, fat-muscle interfaces, the probe itself)
- In liquid filled structures.