PERFIDI: parametrically enabled relaxation filters with double and multiple inversion

We present a novel approach to the nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) analysis of complex samples with nontrivial distribution of longitudinal relaxation rate R1. Parametrically enabled relaxation filters with double and multiple inversion (PERFIDI) aim to separate signals arising from components with different R1 values prior to actual data acquisition. Given any standard NMR/MRI pulse sequence, which, by itself, is insensitive to differences in R1 values, it can be combined with a PERFIDI preamble, which functions as a preliminary R1 filter and confers on the original technique sensitivity to the dimension R1. This article states the principles of the approach, including the way to account for instrumental imperfections, and shows how PERFIDI with specific filter profile functions can be built. Using terms borrowed from electronics, these filters 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.