Reading Ultrasound Imaging from Bjørn A.J. Angelsen (II)

My note will include the following content:

• Important concept.
• Factors affecting image quality.
• Pulse echo amplitude imaging VS. Doppler imaging and color flow imaging, requirement and application.
• Distinguishing and comparing PW/CW, LPRF/HPRF, different transducer arrays.
• Ultrasound scanline processing.
• Some problems according to the book and my confusing.

Important concept

refer to book. here if I used the concept I will explain right away or give links. But if there are sth. missing please find it yourself.

Factors affecting image quality

What is image quality?

1. Spatial resolution: Describes the spatial smearing in the image. Determined laterally by the width of the main lobe of the beam and radially by the length of the transmitted pulse.
2. Contrast resolution: Describes the ability to detect small variations between signal from very closed targets. Limited by signal generated noise (sidelobes effect and reverberations) and electronic noise.

Local dynamic range: is largerly determined by the side lobe value and is difficult to get above 50-60dB.

Factors

• side lobe
  effects: diffuse fog
  can be reduced by: apodization (trade off with spacial resolution since it makes the main lobe wider)
• inhomogeneities
1. geometric distortion (range error, geometric bending)
2. wave front aberration
3. reverberation (multiple relections)

wave front aberration and reberberation can be reduced by:

1. water path between transducer and tissue
2. convex shape transducer face
3. 2nd harmonic imaging——————I don’t understand here.
4. processing using mathematical modelling.
   some concept:phase conjugation, time reversal, etc.
   the main idea is modify the excitation elements in time and amplitude to compensate the aberrations.

Pulse echo amplitude imaging VS. Doppler imaging and color flow imaging, requirement and application

Thus there is something in practice:reference [6,11,16,21] in the book.

While for Doppler velocity and color flow imaging, the book point out the following ranges and modes in clinical practice:

• The maximum range of the Doppler instrument is set the same as the maximum range of the tissue image.
• The frequency range of the spectrum analyzer is set so that it covers the range of Doppler shifts present.
• If CW Doppler mode is selected, the velocities will be measured without aliasing, but with no range resolution.
• If LPRF Doppler mode is selected, only one range cell will be within our maximum range, but the Nyquist range of the Doppler instrument might be less than the range of the spectrum analyzer.——————I don’t understand here.
• If HPRF Doppler mode is selected, the Nyquist range of the Doppler instrument is the same as the range of the spectrum analyzer, but we might have several range cells within our maximum range.——————I don’t understand here.

(L/HPRF = low/high pulse repetation frequency, CW = continous wave)