Broche, Lionel

Lurie, David John

Zampetoulas, Vasileios

The Open University

Calibration of a Fast Field-Cycling NMR Relaxometer for Measurements on Biological Samples that Extend to the Ultra-Low Field Region Vasileios Zampetoulas, Lionel M. Broche, and David J. Lurie Aberdeen Biomedical Imaging Centre, University of Aberdeen, AB25 2ZD, UK Introduction A graph of T1 versus the applied magnetic field (known as a T1-dispersion curve), obtained via Fast Field-Cycling (FFC) NMR and MRI techniques, can be developed into a new medical diagnostic tool thanks to the information about molecular motions that it provides1. For measurements at magnetic fields below 104 kHz (1H Larmor frequency), calibration is necessary so as to compensate for the unwanted environmental magnetic fields (including the earth’s field) acting on the instrument. This will extend the dispersion curves to the ultra-low field region that is not accessible with standard FFC techniques, and is expected to increase the potential of the method, allowing for the study of much slower motions than previously possible2, 3. Methods The calibration involves the application of FFC techniques along with varying correction magnetic fields. During this process, the frequency by which the magnetisation precesses around the resultant field Br (composed of the correction and environmental fields) is measured, and the magnitude of Br is estimated via the equation: ω = γBr. The acquired results are shown as graphs that plot Br as a function of the correction field, and the environmental fields are then measured by fitting a model to these data which expresses the relation of Br to all of its components (Equation 1). The cancellation of the environmental fields is finally achieved by adjusting appropriately opposing correction fields, based on the values estimated by the curve fitting.  Br = √(Bcl + Bel )2 + ((Bct ∙ sin(θ)) + (Bet ∙ sin(φ)))2 + ((Bct ∙ cos(θ)) + (Bet ∙ cos(φ)))2      (1) where Bcl and Bct are the longitudinal and transverse components of the correctionfields, Bel and Bet are the longitudinal and transverse components of theenvironmental fields, and θ and ϕ their azimuth angles. Results The correction fields found by the curve fitting shown in Figure 1 were Bcl: 500 Hz,and Bct: 66 Hz applied at θ: -43°. These led to dispersion curves that extended to100-500 Hz (equivalent to 2-10 µT). In the example shown in Figure 2, the calibrationrevealed a segment of different slope from the rest of the curve, indicating theoccurrence of slower motions.ESMRMB 33rd Annual Meeting – Vienna, Austria – September 2016 – abstract 41Conclusion FFC NMR techniques extended to the ultra-low field region can provide information on slow dynamic processes in tissues. The shape of the curves (at both low and high fields) needs further work to interpret, since they are likely to provide clinically relevant information and can form the basis of new types of contrast. References 1. Lurie D.J., Aime S., Baroni S., Booth N.A., Broche L.M., Choi C., Davies G.R.,Ismail S., Hogain D.O., Pine K.J. Fast field-cycling magnetic resonance imaging.Comptes Rendus Physique 2010;11(2):136-48.2. Anoardo E., Ferrante G.M.. Magnetic field compensation for field-cycling NMRrelaxometry in the ULF band. Applied Magnetic Resonance 2003;24(1):85-96.3. Kresse B., Privalov A.F., Fujara F. NMR field-cycling at ultralow magnetic fields.Solid State Nucl Magn Reson 2011;40(4):134-7.Figure 1. Graph of Br versus the longitudinal correction field. Figure 2. 1/T1-dispersion curve obtained from a sample of human cartilage.  ESMRMB 33rd Annual Meeting – Vienna, Austria – September 2016 – abstract 41

Calibration of a Fast Field-Cycling NMR Relaxometer for Measurements on Biological Samples that Extend to the Ultra-Low Field Region

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