A time-frequency analysis, which represents the time change of a signal, is significant in all fields. Wignerdistribution, a short time Fourier transform (STFT), a kernel method, and a characteristic function method etc. are knownas a time-frequency analyzing method, and those methods have merits and demerits. STFT method has been used asultrasound blood-flow imaging for a long time, because it is suitable for a non-stationary signal analysis. In this paper, Iinvestigated how to control a time-frequency resolution of STFT, and evaluated the image quality of non-stationarysignals using a point-spread function (PSF). The time-frequency resolution of an image corresponds to the aspect ratio ofa pixel. Because of the uncertainty principle of time and frequency, to control the aspect ratio is not easy. The PSF ischanged by the parameters, such as a frequency-range, a frequency-resolution, a time-range, a window function, asampling frequency, etc. I propose the control method that keeps the aspect ratio of PSF constant with expansion andcontraction of an image.
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