Adaptive focusing for transcranial ultrasound imaging using dual arrays.

Ultrasonic brain imaging remains difficult and limited because of the strong aberrating effects of the skull (absorption, diffusion and refraction of ultrasounds): high resolution transcranial imaging would require adaptive focusing techniques in order to correct the defocusing effect of the skull. In this paper, a noninvasive brain imaging device is presented. It is made of two identical linear arrays of 128 transducers located on each side of the skull. It is possible to separate the respective influence of the two bone windows on the path of an ultrasonic wave propagating from one array to the other, and thus estimate at each frequency the attenuation and phase shift locally induced by each bone window. The information obtained on attenuation and phase is used to correct the wave fronts that have to be sent through the skull in order to obtain a good focusing inside the skull. Compared to uncorrected wave fronts, the spatial shift of the focal spot is corrected, the width of the focal spot is reduced, and the sidelobes level is decreased up to 17 dB. Transcranial images of a phantom are presented and exhibit the improvement in image quality provided by this new noninvasive adaptive focusing method.

[1]  J. Campbell,et al.  Effect of the skull in degrading the display of echoencephalographic B and C scans. , 1968, The Journal of the Acoustical Society of America.

[2]  Jian Shen,et al.  An optimal image operator design technique for coded excitation ultrasound imaging system , 1994, 1994 Proceedings of IEEE Ultrasonics Symposium.

[3]  M. Fink.  Time reversed acoustics , 1997 .

[4]  J.-L. Thomas,et al.  Ultrasonic beam focusing through tissue inhomogeneities with a time reversal mirror: application to transskull therapy , 1996, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control.

[5]  J L Thomas,et al.  Optimal focusing by spatio-temporal inverse filter. II. Experiments. Application to focusing through absorbing and reverberating media. , 2001, The Journal of the Acoustical Society of America.

[6]  C.A. Cain,et al.  Multiple-focus ultrasound phased-array pattern synthesis: optimal driving-signal distributions for hyperthermia , 1989, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control.

[7]  F. L. Thurstone,et al.  Sampled Aperture Techniques Applied to B-Mode Echoencephalography , 1975 .

[8]  I. Witten.  On transferring fundamental‐frequency contours from one utterance to another , 1979 .

[9]  Mathias Fink,et al.  Real time inverse filter focusing through iterative time reversal. , 2004, The Journal of the Acoustical Society of America.

[10]  A new SVD-based optimal inverse filter design for ultrasonic applications , 1993 .

[11]  P. VanBaren,et al.  Dynamic focusing in ultrasound hyperthermia treatments using implantable hydrophone arrays , 1994, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control.

[12]  J L Thomas,et al.  Optimal focusing by spatio-temporal inverse filter. I. Basic principles. , 2001, The Journal of the Acoustical Society of America.

[13]  J. L. Thomas,et al.  Focusing and steering through absorbing and aberrating layers: application to ultrasonic propagation through the skull. , 1998, The Journal of the Acoustical Society of America.