Comparison of high-frequency ultrasound transducers for microvascular localization microscopy in the mouse brain

Ultrasound localization microscopy is a super-resolution vascular imaging technique which has garnered substantial interest as a tool for small animal neuroimaging, neuroscience research, and the characterization of vascular pathologies. In the context of small animal neurovascular imaging, we posit that increasing the ultrasound imaging frequency is a straightforward approach to enable higher concentrations of microbubble contrast agents, thus increasing the likelihood of microvascular mapping and decreasing the imaging duration. To test this hypothesis, we compared ULM imaging resolution of mouse brain vasculature for three transducers with different center transmit frequencies (15 MHz, 23 MHz, and 31 MHz) under conditions of low and high MB concentration. We demonstrate that higher frequency imaging resulted in more efficient microbubble localization due to a smaller microbubble point-spread function that is easier to localize, and which can achieve a higher localizable concentration within the same unit volume of tissue. We found that increasing the imaging frequency had a minor impact on ULM spatial resolution, as measured by Fourier ring correlation, under the low MB concentration case, but a substantial impact in the high MB concentration case. High-frequency ULM yielded a spatial resolution of 6.9 μm, as measured by Fourier ring correlation, throughout the entire depth of the brain. This highlights the potential of this technology as a highly relevant tool for neuroimaging research, which has substantial implications for neuroscientists investigating microvascular function in disease states, regulation, and brain development.