Investigation of ultrasound phase shifts caused by the skull bone using low-frequency reflection data

A potential noninvasive means for obtaining the value of phase shifts caused by the skull is examined. Knowledge of these shifts could be used to restore the focus from an ultrasound array after transcranial propagation. A pulsed signal is emitted from a single element of a therapeutic ultrasound transducer. After reflecting off the skull, the steady-state amplitude of the response is recorded. The data are examined over the bandwidth of the transducer, between 0.6 MHz and 1.7 MHz, producing amplitude data as a function of frequency. This measurement methodology is similar to that previously used by Hakim, et. al. [1997 Proc. IEEE Ultrason. Symp. p1153]. After deconvolution of the transducer´s frequency response, a periodic appearance of local maxima and minima is observed in the data as a function of frequency. We hypothesize the amplitude is primarily determined by the superposition of reflections between the interfaces at the inner and outer surfaces of the skull and between the interior interfaces of trabecular and cortical bone. A homogeneous layer model is used to predict the forward-propagated phase using the reflection data. Good correlation is found between the numeric calculation and phases measured after propagation through a three-layer plastic phantom. The method is next tested on three excised human skulls using CT images of the skulls to provide the thickness of the bone layers. The procedure could eventually be applied toward phasing multi-element arrays. Such an application could have implications in both therapeutic and diagnostic brain procedures