Magnetic Resonance of Hyper-polarised Helium-3 at Low Magnetic Fields

In this work signal to noise ratio issues (SNR) of MRI at ultra-low field have been studied and potential applications using hyper-polarised 3He gas have been explored. Two detection bandwidth broadening techniques have been considered: active feedback and coil coupling scheme. Active feedback was shown to be a more appropriate solution and has been implemented. Systematic diffusion-induced attenuation measurements in cylindrical 3He gas samples have been performed over a very wide range of conditions using multiple spin-echo and gradient-echo schemes. Low gradient strengths and long gradient durations can be used at low fields, only limited by the inhomogeneity of the applied field and not by the susceptibility problem met at high fields. Our measurements provide a wide body of data for direct comparison with theoretical models and other data. A new imaging sequence (Slow Low Angle Shot, SLASH) is proposed for imaging at low fields. Series of 2D imaging experiments in phantoms proved that it provides a better image resolution and SNR than a FLASH sequence. Besides that, ADC maps in preserved lungs were created from 2D and 3D images. The obtained ADC histograms are consistent with the results of global CPMG diffusion measurements in the same preserved lungs and are similar to published in-vivo results, which makes this convenient inflatable porous system a relevant lung phantom for methodological studies. Since low-field ADC measurements in lungs are immune to susceptibility gradients, they can be performed over a wide range of diffusion times. (author)