Non-invasive Measurement of Fibrin Concentration by Fast Field-Cycling NMR Technique

Fast Field-Cycling (FFC) is a technique used in NMR since the 1970s [1], and more recently in MRI [2]. It involves changing the strength of the main magnetic field during the pulse sequence, typically during polarisation, evolution and detection. FFC NMR offers many possibilities since it allows using low evolution fields to investigate the dispersion of the relaxation rate (R1), and switching back to higher field during the detection phase to provide a high signal-to-noise ratio in the data. FFC NMR relaxometry has been used since the 1980s for the detection of the N-H quadrupolar relaxation effect [3], which can be seen in various biological samples as characteristic peaks in the R1 dispersion curve of water H, called the ‘quadrupolar peaks’. These peaks occur at well-defined magnetic fields (16 mT, 49 mT and 65 mT) at which the N nuclear quadrupole resonance frequency and the H NMR frequency coincide; the height of the peaks has previously been shown to be proportional to the concentration of immobilised protein [4]. The present work uses FFC NMR relaxometry to investigate the linear relationship suggested by the theory [5] between the amplitude of the quadrupolar peaks generated from cross-linked human fibrin (the main constituent of thrombus), with its concentration. The long-term aim of this study is to test the feasibility of thrombus detection and characterisation by FFC-MRI.