Investigation of hardware and software techniques to enhance the characteristics of focused ultrasound (FUS) spectra.

<i>Objective</i>. Microbubble cavitation generated by focused ultrasound (<i>FUS</i>) can induce safe blood-brain-barrier (<i>BBB</i>) 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 the<i>BBB.</i>Gibbs' ringing, peak intensity loss and peak width increase are well-known distortions evident when using the discrete Fourier transform (<i>DFT</i>) to transform data containing a few hundred points. We investigate overcoming the fact that<i>FUS</i>time signals (10 ms providing 312 500 points sampled at 32 ns intervals) can generate such sharp spectral peaks that variations in their<i>DFT-</i>related distortions can significantly impact the values of the key metrics used for cavitation characterization.<i>Approach</i>. 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 between<i>FUS</i>harmonic intensities. We investigate the enhanced<i>FUS</i>spectral stability and resolution obtained from a new technique, physical sparsification<i>(PH-SP),</i>customized to the<i>a-priori</i>information that all key<i>FUS</i>components are harmonically related. Results are compared with standard<i>DFT</i>optimizations involving time data windowing and Fourier interpolation.<i>Main results</i>. 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 the<i>FUS</i>hydrophone 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 with<i>PH-SP</i>.<i>Significance</i>. Special<i>FUS</i>characteristics mean starting transients and minor variations in experimental procedures lead to significant changes in the spectral metrics used to monitor cavitation levels. Customizing<i>PH-SP</i>to these characteristics led to sharper, more stable spectra with the potential to track the impact of microbubble environment changes.