Comparison of Parallel Image Reconstruction Methods to Acquire High Resolution ADC-Tensor Images of 3He in Human Lungs.

Introduction Many lung diseases, such as emphysema, leads to severe destruction of the lung’s alveoli walls. Determining the Apparent Diffusion Coefficient (ADC) of hyperpolarized He-gas in lungs is a proven method of non-invasive probing the integrity of the lung’s microstructure [Morb1]. Due to sophisticated configuration of small airways the diffusion in lungs is anisotropic and can be characterized by ADC-tensor. The characteristics of the ADC-tensor symmetry may provide valuable additional information on the pathological changes of lung microstructure integrity [Schr1, Mor1]. The essential problem of the efficiency of He ADC evaluation of lungs is the poor localisation of integrity defects caused by typically low spatial resolution of ADC-maps. The ADC-tensor symmetry characteristics i.e. different combinations of tensor main values are even more sensitive to the low SNR and artefacts. The low resolution of ADC-images originates from (i) breath hold time restriction limiting the amount of phase encodings and (ii) limited SNR of diffusion encoded image. The solution of both problems can be achieved by using multi-channel phased arrays parallel imaging acquisition. The principal advantage of using phased arrays for hyperpolarized He is the possibility to compensate the unavoidable for conventional MRI parallel acquisition SNR penalty by increasing the magnetisation flip angle. The purpose of current work is to demonstrate the possibilities, which provides the phased array parallel acquisition for improving the efficiency of ADC-tensor measurements with hyperpolarized He as well as to make comparison of different methods of reconstruction of the imaging datasets to get optimal quality ADC-image.