= Chihiro Kasai
{c}
\b[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.<Real-Time Two-Dimensional Blood Flow Imaging Using an Autocorrelation Technique by Chihiro Kasai and others>[Printed pp. 458–459: full byline, affiliation and estimator principle]{ref}
The computational idea can be seen without reproducing the instrument. For an ideal complex echo sequence $z_n=Ae^{in\phi}$, multiplying by a <complex conjugate> gives
$$
z_n\overline{z_{n-1}}=|A|^2e^{i\phi}.
$$
Averaging 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 $\phi=\pi/4$, the corresponding <Doppler effect> frequency shift is $f_D=\phi/(2\pi T)=500\,\mathrm{Hz}$. 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.
Back to article page
