k-Wave

1. HuiLIN
   

   Member
   Joined: Nov '17
   Posts: 7

   Dear Bradley,

   I have studied the examples in Photoacoustic Image Reconstruction and Diagnostic Ultrasound Simulation. I am wondering the differences between them?
   1) There are clear source frequency (i.e. tone_burst_freq) and pulse loop count ( tone_burst_cycles) in US examples, however there are no these in PR examples.
   2) There are FFT and TR Image Reconstruction processes in PR examples, but there are no these in US examples.

   I am wondering whether the first point means the laser (T ~ ns) in Photoacoustic is a continuous beam, whereas the microwave (T ~ us) in Ultrasound is pulse beam? Could the user change source frequency in Photoacoustic example? And how?
   The second question is, could we use FFT and TR Image Reconstruction processes in US examples? It seems that US image is pulse and dynamic, and it cannot use the smooth process required by Image Reconstruction. How to solve this problem?

   Excuse my stupid questions if there are some common senses. I am novice and aim to link the pulse beam with the image reconstruction.
   Thank your time a lot.

   Hui

   Posted 6 years ago #

2. Bradley Treeby
   

   Developer
   Joined: Mar '10
   Posts: 1,643

   Hi Hui,

   In photoacoustics, it is generally assumed that both stress confinement and thermal confinement are satisfied. This means that the wave propagation can be modelled as an initial value problem. In this case, the intrinsic bandwidth of the source will depend only on the source geometry. In an experiment, the measured bandwidth will also depend on acoustic absorption (which acts to proportionally reduce high-frequencies), and the bandwidth of the detector.

   In simulation, the bandwidth is also limited by resolution of the grid, where the maximum frequency is given by c/(2*dx). To avoid oscillations in the simulated pressure field due to the bandlimited interpolant, k-Wave spatially smooths the initial pressure distribution (see the Source Smoothing Example). Thus, numerically, the bandwidth will depend on:
   (1) the shape of the source,
   (2) the grid resolution,
   (3) the smoothing applied by k-Wave (which is related to 1), and
   (4) any acoustic absorption in the simulation.

   If stress confinement is not satisfied, i.e., if the acoustic wave starts to travel some appreciable distance while the optical energy is being deposited, then the bandwidth will also depend on the time profile of the laser energy. This can be modelled in k-Wave using a time-varying source, e.g., in the shape of a Gaussian. If you want your simulations to be consistent, the spatial smoothing can also be applied when using a time-varying source. This is explained in the Filtering A Delta Function Input Signal Example.

   The situation in diagnostic ultrasound is quite different as you mention, and depends on the bandwidth of the transducer and driving conditions, etc.

   Hope that helps,

   Brad.

   Posted 6 years ago #

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