Chapter 10: Diagnostic Imaging

Question 1

A-mode imaging

Answer 1

The peaks in the scan line correspond to the position of acoustic reflectors within the tissue, while the amplitude of the peaks is related to the impedance mismatch of the reflecting surface or object from the surrounding medium.

Question 2

M-mode imaging (motion)

Answer 2

If several a-mode scan lines obtained at different times are then displayed, and shown how it changes in time

Question 3

B-mode imaging (brightness)

Answer 3

The elements of the transducer array are used in groups to transmit and recieve ultrasound pulses to form A-mode scan lines. The ultrasound beam is then stepped sideways within the scan plane by exciting a different subset of elements in the array.

Question 4

What are the steps of processing between the acquisition of raw signals and the display of a B-mode image?

Answer 4

For each channel:

• > ADC
• > Time delay (beamforming delays)
• > Weighting (apodisation weights)

Then all together:

• > TGC (Time gain compensation)
• > Filtering (noise removal)
• > Envelope detection (demodulate signal)
• > Log comp ( increase dynamic range)

Question 5

Analogue to digital conversion

Answer 5

The analogue voltage signals are generated by the piezoelectric elements due to the incident acoustic waves are first digitised using an ADC. This samples the analogue signal at 80 Hz and the sampled voltage values are quantised into the closest discrete value, where the quantisation precision is controlled by the number of bits used for ADC. Typically 16-bit precision is used for different discrete voltage values. (96db dynamic range)

Question 6

Dynamic receive focusing

Answer 6

when the ultrasound waves are transmitted, a single value for the time delay for each element is used to focus or steer the beam to a particular location.