Model-Based Super-Resolution Reconstruction of T2 Maps

Purpose: High-resolution isotropic T2 mapping of the human brain with multi-echo spin-echo (MESE) acquisitions is challenging. When using a 2-D sequence, the resolution is limited by the slice thickness. If used as a 3-D acquisition, specific absorption rate (SAR) limits are easily exceeded due to the high power deposition of non-selective refocusing pulses. A method to reconstruct 1-mm3 isotropic T2 maps is proposed based on multiple 2-D MESE acquisitions. Data were undersampled (10-fold) to compensate for the prolonged scan time stemming from the super-resolution acquisition.
Method: The proposed method integrates a classical super-resolution with an iterative model-based approach to reconstruct quantitative maps from a set of undersampled low-resolution data. The method was tested on numerical and multipurpose phantoms, and in-vivo data. T2 values were assessed with a region-of-interest analysis using a single-slice spin-echo and a fully sampled MESE acquisition in a phantom, and a MESE acquisition in healthy volunteers.
Results: Numerical simulations showed that the best trade-off between acceleration and number of low-resolution datasets is 10-fold acceleration with four acquisitions (TA=18 mins). The proposed approach showed improved resolution over low-resolution images for both phantom and brain. Region-of-interest analysis of the phantom compartments revealed that at shorter T2, the proposed method was comparable with the fully sampled MESE. For the volunteer data, the T2 values found in the brain structures were consistent across subjects (8.5-13.1ms standard deviation).
Conclusion: The proposed method addresses the inherent limitations associated with high-resolution T2 mapping and enables the reconstruction of 1mm3 isotropic relaxation maps with a ten times faster acquisition.