B-Mode Instrumentation

Question 1

What does the front end of the scanner control?

Answer 1

• Amplification
• Multiplexing
• Switching

Question 2

What does the main hub of the scanner control?

Answer 2

• Controlling
• Beamforming
• Signal Processing

Question 3

What does the back end of the scanner control?

Answer 3

• Scan conversion

- Pre and Post Processing

Question 4

Why does multiplexing need to be used in modern scanners?

Answer 4

• Modern scanners have more elements than channels.

- Multiplexing allows the first n channels to drive the last n elements as well as the first.

Question 5

What are the steps in B-Mode signal processing?

Answer 5

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

Question 6

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

Answer 6

• 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.

Question 7

How does the point spread function obtained by the scanner?

Answer 7

• 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.

Question 8

How are data point filled between scan lines?

Answer 8

• Linear interpolation using a 2D triangular function.

- Secotr data mapped to pixel locations in digital memory.

Question 9

What is the equation for the interpolated signal?

Answer 9

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

Question 10

What is the difference between read and write zoom?

Answer 10

• 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.

Question 11

Describe the basic scanner architecture.

Answer 11

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.

Question 12

What are some common B-mode artefacts?

Answer 12

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

Question 13

Where are reverberation artefacts most likely to occur?

Answer 13

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