Coded excitation in TCD ultrasound systems to improve axial resolution

The radiofrequency (RF) signal from a transcranial Doppler (TCD) ultrasound system allows us to track the motion of an embolus in a 2-D space of time and depth. The technique is limited by the narrow bandwidth (long duration) of the transmitted pulse because this provides poor axial resolution. This study aimed to assess whether implementing coded excitation and pulse compression in a TCD system would be a feasible means of increasing the bandwidth and hence improving the axial resolution, without the need for reducing the pulse length and increasing peak power levels. Embolic signals were collected in vitro from a flow phantom using a TCD system, which alternately transmitted coded and noncoded pulses. This allowed the same event to be investigated using the two different types of processing. Quantitative and qualitative measures were used to compare the two processing methods. The in vitro results were promising. They showed that the axial resolution could be improved, on average, by a factor of 6.6, using a pulse length of 13 micros and a chirp bandwidth of 0.8 MHz. This was reduced to a factor of 6 when a temporal bone sample was placed between the transducer and the flow phantom. Qualitatively the journey of an embolus was easier to track using the pulse compressed signal. Sonograms could be generated from smaller receive gates using the pulse compressed signal while still achieving an adequate signal-to-noise ratio. We found that it is both feasible and beneficial to implement coded excitation and pulse compression in a TCD system.