Application of CPMG acquisition in Fast-Field-Cycling relaxometry

Abstract

The possibility and limits for applying the CPMG sequence during the acquisition period of Fast-Field-Cycling relaxometry measurements were investigated. Fluctuations of the acquisition field strength were quantified and related to the signal phase fluctuations they cause in CPMG measurements. Tests under different operating conditions of the relaxometer were performed, revealing a dependence of echo phase fluctuations on the magnet temperature. The reliability of T1 relaxation dispersion measurements using CPMG acquisition with accumulated echo signals was verified by comparison with reference measurements obtained from FID acquisition. Furthermore, the reliability and limits of CPMG-based T2 measurements in the FFC relaxometer were tested by comparison with a permanent magnet system. An application of field-cycled T1-T2 correlation measurements is shown, exploiting the T2 relaxation contrast at the acquisition field for distinguishing multiple relaxation components in a model system with water-filled pores of different sizes. Thereby different T1 relaxation components could be resolved despite the fact that they differed only by a factor of two in total at high fields. Herein we demonstrate that the application of the CPMG pulse sequence at the acquisition field of a commercial Fast-Field-Cycling relaxometer provides reliable results for the measurement of T1 and T2 relaxation times as well as for T1-T2 correlation spectra.