| 摘要 |
The effect of noise in the displayed information obtained from transcranial Doppler measurements is reduced by estimating the spectra of the returned Doppler signal as the average of Fourier spectra computed using multiple windows for the data being analyzed. The windows are chosen so that the spectra from different windows are approximately statistically independent, thereby reducing the variance of the spectrum by the averaging process. The Doppler data may be obtained by applying repeated pulses of ultrasonic energy to the head of a subject and isolating the Doppler frequency components of the return signal to obtain a set of data from a selected number of pulses. The data set so obtained has the multiple window functions applied thereto to obtain multiple windowed data sets which are operated on, as in a digital signal processor, to obtain multiple discrete Fourier transforms of the windowed data sets. A power spectrum is estimated as the averaged sum of the squares of the magnitudes of each of the discrete Fourier transforms. The spectra obtained from data sets gathered in sequence may be displayed to an operator to show the spectral content of the return signal over time, which corresponds to the velocity of blood flow at a position within the head of the subject as a function of time. |
