Purpose To develop an inversion pulse-based, CEST-like method for detection of 31P magnetization exchanges among all NMR visible metabolites suitable for providing an integrated kinetic analysis of phosphorus exchange reactions in vivo. Methods The EKIT sequence (Exchange Kinetics by Inversion Transfer) includes application of a frequency-selective inversion pulse arrayed over the range of relevant 31P frequencies, followed by a constant delay and a hard readout pulse. A series of EKIT spectra, each given by a plot of Z-magnetization for each metabolite of interest versus frequency of the inversion pulse, can be generated from this single data set. Results EKIT spectra reflect chemical exchange due to known biochemical reactions, cross-relaxation effects, and relayed magnetization transfers due to both processes. The rate constants derived from EKIT data collected on resting human skeletal muscle were: ATP synthesis via ATP synthase (0.050 ± 0.016 s-1), ATP synthesis via creatine kinase (0.264 ± 0.023 s-1), and cross-relaxation between neighboring spin pairs within ATP (0.164 ± 0.022 s-1). Conclusion EKIT provides a simple, alternative method to detect chemical exchange, cross relaxation and relayed magnetization transfer effects in human skeletal muscle at 7T.
Purpose To introduce a novel reconstruction method for simultaneous multi‐slice (SMS)‐accelerated multi‐shot diffusion weighted imaging (ms‐DWI). Methods SMS acceleration using blipped‐CAIPI schemes have been proposed to speed up the acquisition of ms‐DWIs. The reconstruction of the data requires (a) phase compensation to combine data from different shots and (b) slice unfolding to separate the data of different slices. The traditional approaches first estimate the phase maps corresponding to each shot and slice which are then employed to iteratively recover the slice unfolded DWIs without phase artifacts. In contrast, the proposed reconstruction directly recovers the slice‐unfolded k‐space data of the multiple shots for each slice in a single‐step recovery scheme. The proposed method is enabled by the low‐rank property inherent in the k‐space samples of ms‐DW acquisition. This enabled to formulate a joint recovery scheme that simultaneously (a) unfolds the k‐space data of each slice using a SENSE‐based scheme and (b) recover the missing k‐space samples in each slice of the multi‐shot acquisition employing a structured low‐rank matrix completion. Additional smoothness regularization is also utilized for higher acceleration factors. The proposed joint recovery is tested on simulated and in vivo data and compared to similar un‐navigated methods. Results Our experiments show effective slice unfolding and successful recovery of DWIs with minimal phase artifacts using the proposed method. The performance is comparable to existing methods at low acceleration factors and better than existing methods for higher acceleration factors. Conclusions For the slice accelerations considered in this study, the proposed method can successfully recover DWIs from SMS‐accelerated ms‐DWI acquisitions.
Background and Purpose: Ultra-high-field 7T promises more than doubling the signal-tonoise ratio (SNR) of 3T for MRI, particularly for MRI of magnetic susceptibility effects induced by B 0 . Quantitative susceptibility mapping (QSM) is based on deconvolving the induced phase (or field) and would therefore benefit substantially from 7T. The purpose of this work was to compare QSM performance at 7T versus 3T in an intra-scanner test-retest experiment with varying echo numbers (5 and 10 echoes).Methods: A prospective study in N=10 healthy subjects was carried out at both 3T and 7T field strengths. Gradient echo data using 5 and 10 echoes was acquired twice in each subject. Test-retest reproducibility was assessed using Bland-Altman and regression analysis of region of interest measurements. Image quality was scored by an experienced neuroradiologist.Results: Intra-scanner bias was below 3.6ppb with correlation R 2 >0.85. Inter-scanner bias was below 10.9ppb with correlation R 2 >0.8. The image quality score for the 3T 10 echo protocol was not different from the 7T 5 echo protocol (p=0.65). Conclusion:Excellent image quality and good reproducibility was observed. 7T allows equivalent image quality of 3T in half of the scan time.
Purpose: Typical quantitative susceptibility mapping (QSM) reconstruction steps consist of first estimating the magnetization field from the gradient-echo images, and then reconstructing the susceptibility map from the estimated field. The errors from the field-estimation steps may propagate into the final QSM map, and the noise in the estimated field map may no longer be zero-mean Gaussian noise, thus, causing streaking artifacts in the resulting QSM. A multiecho complex total field inversion (mcTFI) method was developed to compute the susceptibility map directly from the multiecho gradient echo images using an improved signal model that retains the Gaussian noise property in the complex domain. It showed improvements in QSM reconstruction over the conventional field-to-source inversion. Methods: The proposed mcTFI method was compared with the nonlinear total field inversion (nTFI) method in a numerical brain with hemorrhage and calcification, the numerical brains provided by the QSM Challenge 2.0, 18 brains with intracerebral hemorrhage scanned at 3T, and 6 healthy brains scanned at 7T. Results: Compared with nTFI, the proposed mcTFI showed more accurate QSM reconstruction around the lesions in the numerical simulations. The mcTFI reconstructed QSM also showed the best image quality with the least artifacts in the brains with intracerebral hemorrhage scanned at 3T and healthy brains scanned at 7T. Conclusion:The proposed multiecho complex total field inversion improved QSM reconstruction over traditional field-to-source inversion through better signal modeling. K E Y W O R D Sbrain imaging, nonlinear total field inversion, quantitative susceptibility mapping
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