Room: Exhibit Hall | Forum 8
Purpose: To present and evaluate a long-T2 tissue suppression method in ultra-short TE(UTE) or zero-TE(ZTE) MRI obtained from a Look-Locker inversion recovery UTE sequence.
Methods: Instead of acquiring MR signal only at zero-crossing to achieve long-T2 suppression, this method utilized the inherent destructive addition of complex signal acquired before and after zero-crossing. Phantom images with fat, fluid (to simulate muscles), and rubber (to simulate short-T2) were acquired. A 3D stack-of-stars radial acquisition UTE MRI sequence was combined with Look-Locker IR method to generate multiple (~10 or more) 3D datasets with varying contrasts determined by different IR times. An adiabatic inversion recovery pulse, which fully inverted long-T2, but only partially inverted short-T2 tissues, was applied right before each 3D UTE gradient-echo readout train. A linear combination of these 3D complex images was employed to generate the final 3D image with any desired contrast. Firstly, the inversion recovery curves from different regions-of-interest (fat, fluid, rubber, and noise) were obtained. Secondly, a Powell minimization procedure was employed to determine the optimized linear combination coefficients modeled with a 5th-degree polynomial function, based on the inversion recovery curves obtained earlier. Finally, different solutions were found to synthesize UTE images with: 1) fat suppression; 2) lean tissue suppression; 3) long-T2 suppression.
Results: Almost perfect suppression of fat, muscle, or both were achieved with this approach. Generally, SNR of rubber was high when either fat or muscle was suppressed, and SNR lowered when both had to be suppressed. Optimization of MR imaging protocol parameters (primarily the inter-inversion duration, FA, and post-acquisition delay) based on T1 of specific tissues improved SNR when all long-T2 had to be suppressed.
Conclusion: Long-T2 tissue suppression can achieved by combining Look-Locker IR UTE MRI datasets. Future efforts will be the optimization of application-specific MRI protocols to achieve desired contrast with high SNR.