Ultrasound Insertion Loss of Rat Parietal Bone Appears to Be Proportional to Animal Mass at Submegahertz Frequencies

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Transcranial ultrasound therapy is an increasing area of research for noninvasive treatments in the brain including targeted drug delivery. Measurements of ultrasound transmission through rat parietal bone at five frequencies (0.268 MHz, 0.841 MHz, 1.409 MHz, 1.972 MHz and 2.53 MHz) were performed at 88 locations in 22 ex vivo rat skullcaps (Wistar) using a fiber-optic hydrophone system. At submegahertz frequencies, the skull insertion loss was found to be proportional to animal mass, while at higher frequencies this trend was lost. Maps of the transverse pressure profile of the transducer before and after skull insertion showed increased distortion effects at higher frequencies. Parietal bone thickness was measured and was found to increase with increasing body mass. Additional measurements were made through mouse and rabbit skulls at 2.53 MHz. At this frequency, aberration effects through mouse skull were negligible, while large distortions were observed through rat and rabbit skull. Preclinical transcranial ultrasound studies in rats may be improved by scaling applied powers according to body mass to produce more consistent in situ pressures.