Objective. Microbubble cavitation generated by focused ultrasound (FUS) can induce safe blood-brain-barrier (BBB) opening allowing therapeutic drug passage. Spectral changes in the hydrophone sensor signal are currently used to distinguish stable cavitation from inertial cavitation that can damage theBBB.Gibbs' ringing, peak intensity loss and peak width increase are well-known distortions evident when using the discrete Fourier transform (DFT) to transform data containing a few hundred points. We investigate overcoming the fact thatFUStime signals (10 ms providing 312 500 points sampled at 32 ns intervals) can generate such sharp spectral peaks that variations in theirDFT-related distortions can significantly impact the values of the key metrics used for cavitation characterization.Approach. We introduce low-pass filter hardware to improve how the analogue to digital convertor handles high-frequency noise components and the orders of magnitude differences betweenFUSharmonic intensities. We investigate the enhancedFUSspectral stability and resolution obtained from a new technique, physical sparsification(PH-SP),customized to thea-prioriinformation that all keyFUScomponents are harmonically related. Results are compared with standardDFToptimizations involving time data windowing and Fourier interpolation.Main results. A new simulation model showed peak intensity, widths and metrics modified by small changes in the transformed signal's length when removing the noisy starting transient of theFUShydrophone signal or following minor excitation frequency or sampling rate adjustments. 25%-60% area-under-the-curve changes occurred in phantom studies at different pressure levels. Spectral peak sharpness was best optimized and stabilized withPH-SP.Significance. SpecialFUScharacteristics mean starting transients and minor variations in experimental procedures lead to significant changes in the spectral metrics used to monitor cavitation levels. CustomizingPH-SPto these characteristics led to sharper, more stable spectra with the potential to track the impact of microbubble environment changes.
Keywords: Gibbs’ artefact suppression; blood-brain barrier (BBB); focused ultrasound (FUS); physical sparsification (PH-SP); spectral resolution enhancement; therapeutic drug passage.
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