New Universal NMR Sequences: PERDIDI and LAPSR

This work presents two new families of sequences developed within two different projects at the University of Bologna. The two families are related by a common feature, which is the use of 180o inversion pulses as a preamble to a classical NMR technique. The exclusive use of inversion pulses confers them a remarkable insensitivity to offset-related artifacts, a property particularly appreciated in NMR relaxometry of large, complex samples and in MR imaging. The final aims of the two families, however, are somewhat different: PERFIDI (Parametrically Enabled Relaxation Filters with Double and multiple Inversion) is a family of relaxation filters applicable as a preamble to almost any NMR pulse sequence. As such, it will find a wide range of applications in all branches of NMR, including spectroscopy, relaxometry and imaging. It has been designed keeping in mind complex, chemically and/or physically heterogeneous systems such as untreated body fluids, biological tissues and whole organs, porous media, etc. The patented sequences (patent BO2005A000445, University of Bologna), comprised of n inversion pulses (n = 2,3,4,...) are crafted to provide (borrowing electronics terminology) high-pass, low-pass and band-pass T1-filters of various shapes and cut-off sharpness. LAPSR (Logarithmically distributed A-Periodic Saturation Recovery) is one of a family of SMS (Sample Magnetization Suppression) sequences whose aim is to suppress as fast as possible the nuclear magnetization of all components of a sample. A particular attention is paid to samples with wide distributions of relaxation times (e.g., from 0.3 ms to 3 s), offsets, and nutation angles (B1 inhomogeneity). The development of SMS sequences started from the observation that: (i) Classical methods of NMR relaxometry such as inversion recovery (IR) are rather slow because they require reaching the equilibrium magnetization before every scan. In addition, they fail in situations where sample complexity combines with severe and unavoidable instrumental imperfections (ex-situ NMR, large samples and coils, severe B1 inhomogeneity, insufficient transmitter power, etc). (ii) The alternative is to use the saturation recovery sequence (SR) or the APSR sequence (one composed of 90o pulses with linearly decreasing delays), possibly in combination with gradient pulses. The goal is to achieve the zero-magnetization starting state and do so in a fast and reproducible way. The results are in fact often better than using IR but, nevertheless, still far from being free of artifacts. These observation prompted us to start an extensive series of theoretical simulations trying to answer the question of how fast and how well can one suppress the magnetization of complex samples using standard pulse sequences. The theoretical results, by themselves very interesting, were then compared with experiments. It turns out that the best sequences in this category are composed of a large number (15-20) of inversion pulses (nominally 180o) with logarithmically decreasing delays. Relaxation curves can be obtained about 3 times faster than using IR and they remain meaningful even in experimental conditions which, from the NMR point of view, appear quite unsatisfactory.