Modalities for the optimization of ultrasound transmission through the human skull
Open Access
- Author:
- White, Phillip Jason
- Graduate Program:
- Acoustics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 22, 2005
- Committee Members:
- Nadine B Smith, Committee Chair/Co-Chair
Ryan Clement, Committee Member
Thomas B Gabrielson, Committee Member
Victor Ward Sparrow, Committee Member
Gregory J Clement, Committee Member - Keywords:
- ultrasound
skull
therapy - Abstract:
- This thesis explores three mechanisms for ultrasound propagation through the human skull mainly for, but not limited to, noninvasive transcranial therapy and surgery. Hypotheses were proposed for the examination of these mechanisms based on previous studies in ultrasound propagation in the skull bone (Fry, F.J. Ultrasound Med. Biol. 1977) and recent developments in transducer technology (Clement, G.T. et al. Phys. Med. Biol. 2000). The state-of-the-art has progressed to the point where noninvasive surgery through the skull in conjunction with alternate surgical monitoring modalities has been demonstrated to be feasible (Hynynen, K. et al. Med. Phys. 1993). This has been most recently been shown in the case of large-aperture segmented transducer arrays producing lesions within in vivo animal brain tissues through intact human skulls (Hynynen, K. et al. Magn. Resonance Med. 2003). But these studies have also identified a need for better energy transmission through bone layers to achieve an effective and noninvasive procedure. Through a series of computer simulations and cadaveric experiments, it was demonstrated that modalities of enhancing the transcranial transmission of ultrasound energy do exist. First, the controlled modification of ultrasound amplitude distribution across a sonicated skull surface was shown to produce changes in the acoustic pressure field at the surgical site (White, P.J. et al. IEEE Trans. on Ultrasonics Ferroelectrics and Freq. Cont. 2005). Using this method, it was observed that the homogenizing of amplitude across a spherically focused wavefront has a less significant effect at the focus than had been previously proposed (Thomas, J. and Fink, M.A. IEEE Trans. Ultrason. Ferroelectr. Freq. Contr. 1996) (Aubry, J.F. et al. J. Acoust. Soc. Am. 2003). Second, the frequency dependence of transmission at local portions of the skull was determined with backscattered signals. These spectra were then used to examine the optimization of total energy transmission by the adaptive adjustment of sonicating frequency. And finally, a shear mode of vibration in the skull bone was shown to be better matched in characteristic impedance to surrounding media. This mechanism was then determined to have a higher attenuation index than the longitudinal mode of vibration in the skull bone, reducing its applicability for high-power applications. These findings have revealed some fundamental aspects of ultrasound interaction with the structure of the human skull, opening up possibilities for clinical implementation. But they have also given impetus into finding other potential pathways for the transcranial delivery of energy into the brain parenchyma. With continued research, these discoveries will eventually integrate into a complete and effective system for noninvasive surgery within the brain.