INTERPRETATION OF FLOW ENCODING AND QUANTIFICATION IN MRI - TIME-DOMAIN VERSUS FREQUENCY-DOMAIN

The traditional approach to flow effects in MRI is based on the gradie nt moment expansion, Recently, we have presented an alternative descri ption by using linear response theory: the distortions of the velocity waveform induced by the gradient waveforms were analyzed in the frequ ency domain on the basis of the transfer function. In the present pape r, we perform an analysis of flow encoding and quantification in the t ime domain, on the basis of the impulse response, The analysis shows t hat flow encoding should be interpreted as a weighted averaging proces s, Instantaneous flow encoding is determined by the centroid of the im pulse response, but care should be taken regarding the physical meanin g of the instant of encoding. The relationship of this approach to the frequency domain and gradient moment expansion approaches is clarifie d, By way of example, some interesting applications are investigated: asymmetrical phase encoding gradients to minimize misregistration and oscillating read-out gradients for flow quantification. A variety of n ew applications are expected to derive from the combination of both th e time and frequency domains.