k-Wave

1. wcvogt
   

   Member
   Joined: Apr '14
   Posts: 1

   Hello,

   I am trying to use the FFT method kspaceLineRecon to reconstruct images from a 128-element linear array with 4-11 MHz bandwidth, sampled at 31.25 MHz. However, absorptive objects in reconstructed images appear not as expected point targets but as multiple bands or arcs with oscillating intensity between positive and negative values, especially along the axial (time-associated) direction.

   My two hypotheses as to where this artifact comes from are:
   1) Insufficient grid density. Is there a way to adjust the grid density for the FFT methods? The grid seems hard-coded based on the dataset size.

   2) The oscillations appear similar to the temporal impulse response of our array as measured by directly illuminating it. Is impulse deconvolution critical for PAT? What signal processing is necessary before inputting data to the FFT method?

   Thanks,

   -Bill Vogt, Ph.D
   Postdoctoral Fellow
   US Food and Drug Administration
   Silver Spring MD

   Posted 10 years ago #

2. Bradley Treeby
   

   Developer
   Joined: Mar '10
   Posts: 1,643

   Hi Bill,

   What you are seeing is the combination of two classical problems when using linear piezoelectric arrays for photoacoustic imaging.

   The first is the limited aperture problem. This arises because the finite size of the imaging aperture means you will not detect the acoustic waves travelling in all directions from your photoacoustic source. This will result in arc-like artefacts in the reconstructed image, which you can see in the 2D FFT Reconstruction For A Line Sensor Example. A more detailed explanation of the limited view problem is given here.

   The second problem is the use of band-limited detectors. As photoacoustic waves are inherently broadband, you will lose some high and low frequency information when using narrowband piezoelectric detectors. This results in positive and negative oscillations in the reconstructed image, which you can see in the Image Reconstruction With Bandlimited Sensors Example. Deconvolution might help, but you can't correct for frequencies that simply haven't been recorded. You'll also need to be careful you don't amplify noise.

   Regarding increasing the resolution of your image, you could use nearest neighbour interpolation to spatially upsample your data before calling kspaceLineRecon.

   Hope that helps,

   Brad.

   Posted 10 years ago #

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