PERFIDI: Parametrically Enabled Relaxation Filters with Double Inversion

We present a novel approach to the nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) analysis of complex samples with a non-trivial distribution of longitudinal relaxation rates R1. The method, denominated PERFIDI, aims at separating signals arising from components with different R1's prior to actual data acquisition. Given any standard NMR / MRI pulse sequence, by itself insensitive to differences in R1's, it can be combined with a PERFIDI preamble which functions as a preliminary R1 filter and confers the original technique sensitivity to the R1 dimension. The article states the principles of the approach, including the way of accounting for instrumental imperfections, and shows how to build PERFIDI filters with specific filter profile functions. Using terms borrowed from electronics, these are classified as low-pass, high-pass and band-pass types. Also included are an experimental verification example and a discussion of potential applications of PERFIDI in various NMR areas.