Spin-lock based functional magnetic resonance imaging (fMRI) has the potential for direct spatially-resolved detection of neuronal activity and thus may represent an important step for basic research in neuroscience. In this work, the corresponding fundamental effect of Rotary EXcitation (REX) is investigated both in simulations as well as in phantom and in vivo experiments. An empirical law for predicting optimal spin-lock pulse durations for maximum magnetic field sensitivity was found. Experimental conditions were established that allow robust detection of ultra-weak magnetic field oscillations with simultaneous compensation of static field inhomogeneities. Furthermore, this work presents a novel concept for the emulation of brain activity utilizing the built-in MRI gradient system, which allows REX sequences to be validated in vivo under controlled and reproducible conditions. Via transmission of Rotary EXcitation (tREX), we successfully detected magnetic field oscillations in the lower nano-Tesla range in brain tissue. Moreover, tREX paves the way for the quantification of biomagnetic fields.
Resonant absorption during spin-lock preparation can be used to measure tiny oscillating magnetic fields acting as direct evidence of electrical neuronal activity. Different spin-locking techniques were compared with respect to their sensitivity in magnetic field detection. As a specialty, the oscillating magnetic fields were generated by the built-in gradient system in an offcenter slice. The spin-lock time was identified as the crucial parameter for the performance of NEMO (neuro-electro-magnetic-oscillations) detection, since minima and maxima in the signal amplitude emerged in phantom and in vivo experiments. Affirmative, the experimental results show an excellent agreement with simulation results.
Spin-lock based absorption of magnetic oscillations offers potential for direct detection of electrical neuronal activity. We propose a novel versatile validation and calibration technique which paves the way for emulation and quantification of biomagnetic fields. Using ultra-weak gradient waveforms, the method mimics brain activity and thus projects artificial fields onto the tissue under investigation. The method applicable for sequence validation or signal calibration was tested in phantom and in vivo experiments with the built-in gradient system providing sinusoidal field modulations down to 1 nT. It proved to be reliable and reproducible and hence can potentially enable quantification of biomagnetic fields.
Contrary to the basic principle of CEST, the RACETE technique allows for the direct detection of positive chemical exchange contrast. This method, which was previously demonstrated only under ultra-high field conditions at 7T-17.5T, has now successfully been implemented on a clinical 3T scanner in initial phantom experiments. Furthermore, we present a novel dual-contrast RACETE-technique for simultaneous imaging of the positive RACETE and the negative CEST contrast.
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