Measurement of fibrin concentration by fast field-cycling NMR

The relaxation of 1H nuclei due to their interaction with quadrupolar 14N nuclei in gel structures is measured using fast field-cycling (FFC) NMR. This phenomenon called quadrupolar dips has been reported in different 1H-14N bond-rich species. In this study we have studied quadrupolar dips in fibrin, an insoluble protein that is the core matrix of thrombi. Fibrin was formed by the addition of thrombin to fibrinogen in 0.2 % agarose gel. T1-dispersion curves were measured using FFC NMR relaxometry, over the field range 1.5 - 3.5 MHz (proton Larmor frequency) and were analysed using a curve fitting algorithm. A linear increase of signal amplitude with increasing fibrin concentration was observed. This agrees with the current theory that predicts a linear relationship of signal amplitude with the concentration of contributing 14N spins in the sample. Interestingly, fibrin formation gave rise to the signal, regardless of cross-linking induced by the transglutaminase factor XIIIa. In order to investigate the effect of proteins that might be trapped in the thrombi in vivo, the plasma protein albumin was added to the fibrin gel and an increase in the quadrupolar signal amplitude was observed. This study can potentially be useful for thrombi classification by FFC-MRI techniques.