Chihiro Kasai

Chihiro Kasai is the first named author of the 1985 paper on real-time blood flow imaging with autocorrelation, coauthored with Koroku Namekawa, Akira Koyano and Ryozo Omoto. The original paper identifies Kasai with Aloka and presents an estimator suited to repeated ultrasound echoes.^[ref]

The computational idea can be seen without reproducing the instrument. For an ideal complex echo sequence $z_n=A{e}^{in\varphi }$, multiplying by a complex conjugate givesAveraging such products estimates the phase increment rather than requiring a complete spectrum at every image location. This is Lag-one Doppler phase estimation. With stipulated pulse spacing $T=0.25\mathrm{ms}$ and increment $\varphi =\pi /4$, the corresponding Doppler effect frequency shift is $f_D=\varphi /(2\pi T)=500\mathrm{Hz}$. The convention assumes increasing complex phase for a positive shift.

Only the phase modulo $2\pi$ is observed. The same sequence results from $f_D+m/T$ for any integer $m$, so the calculation alone cannot select the physical frequency outside a chosen unambiguous interval. This is aliasing, not evidence that the scatterer has reversed direction. The example exposes a measurement limit of the phase estimator independently of any diagnostic interpretation.