Print Email Facebook Twitter Ultrafast ultrasound for superresolution imaging Title Ultrafast ultrasound for superresolution imaging: The effect of pulse inversion and harmonic or SVDfiltering on microbubble detection Author ter Wee, Annemiek (TU Delft Mechanical, Maritime and Materials Engineering) Contributor Deckers, Roel (mentor) Vos, Rik (graduation committee) Degree granting institution Delft University of Technology Programme Biomedical Engineering | Medical Physics Date 2021-03-29 Abstract Background/purpose: The mapping of microvasculature requires a highresolution imaging method. Superresolution ultrasoundimaging can provide such high resolution by detecting individual microbub bles (MBs) in a flow circuit. This method relies on the dynamic appearance of MBs, which appear as point scatterers on the US image, mixed with tissue signal. The detection of these MBs require good spatiotemporal resolution and suppression of the tissue signal. These requirements are met by using highframerate plane or diverging wave imaging combined with tissue suppression methods. In this project, the effect of pulse inversion (PI) and harmonic filtering or singular value decomposition (SVD) filtering on the MB detection in slow flow was evaluated. This effect was measured in a softtissue mimicking phantom containing a 400 휇m diameter wallless channel. Methods: The US acquisition was done with a Verasonics V1 system, using a convex (C52) probe. Two sequences for coherent compound divergingwave imaging were designed: the first for imaging in the fundamental (F) mode and the second for PI. The axial and lateral resolution were measured and the localization precision determined. Next, the effect of changing the number of compound angles, voltage, flow velocity, the receive mode (fundamental or harmonic), SVDfiltering and tissue motion on the MB detections was evaluated in four experiments. Results: The results of the system characterization showed an average lateral & axial resolution in F mode of 2.5 mm and 1 mm, respectively. In PImode the lateral & axial resolution was higher because of imaging at higher frequencies (2.0 mm and 0.8 mm, respectively). The MB localization precision was 20 휇m lateral and 6 휇m axial, from which we deduce that ultrasound localization microscopy improves spatial resolution with an average factor of 75. The results of the MB detection experiments showed a decrease of MB detections when the number of compounding angles increased. Possibly because of less false detections, since compound imaging results in improvement of contrast and a reduction in speckle and artifacts. No direct relation between the increase of transmit voltage and MB detection was found. In the SVDfiltered PImode data, the increase of flow velocity from 1 till 2 till 4 mm/s was accompanied by an increase in MB detections and a decrease in the false positive detections. At low flow velocities, the difference in spatial coherence between tissue and MBs was low which could have resulted in poor differentiation. Receiving in the harmonicmode, resulted in a small spatial shift of the location of the MB detections because of asymmetry of the used radiofrequency filter. Besides, in the harmonic receivemode the total number of MB detections dropped. A probable explanation is that less false MB detection were made because of suppression of flash artifacts. When tissue mo tion was induced, this led to higher MB detections in the nonfiltered PI mode than in the stationary case. However, these can also be motion artifacts which are falsely detected as MBs. SVDfiltering or harmonicfiltering of these acquisitions resulted in practically zero MB detections. Conclusion: We conclude that nonfiltered PI results in more truepositive MB detections than harmonic filtering and SVDfiltering when flow velocities are low. When tissue motion is negligible, PIimaging results in good SR images, with a possible disturbance of flash artefacts. Subject Ultrafast ultrasoundsuper-resolutionmicro-bubblesFiltering To reference this document use: http://resolver.tudelft.nl/uuid:5663ad01-92b2-40a6-9b24-f3b6a5f0216a Coordinates 52.0866, 5.1799 Part of collection Student theses Document type master thesis Rights © 2021 Annemiek ter Wee Files PDF MScThesis_MAterWee.pdf 3.65 MB Close viewer /islandora/object/uuid:5663ad01-92b2-40a6-9b24-f3b6a5f0216a/datastream/OBJ/view