With active compensation, the dreMR imaging system is capable of 0.22T field shifts within a clinical 1.5T MRI with no significant residual eddy-current fields.
This study quantified the spin-lattice relaxation rate (R 1 ) dispersion of murine tissues from 0.24 mT to 3 T. A combination of ex vivo and in vivo spin-lattice relaxation rate measurements were acquired for murine tissue. Selected brain, liver, kidney, muscle, and fat tissues were excised and R 1 dispersion profiles were acquired from 0.24 mT to 1.0 T at 37°C, using a fast field-cycling MR (FFC-MR) relaxometer. In vivo R 1 dispersion profiles of mice were acquired from 1.26 T to 1.74 T at 37°C, using FFC-MRI on a 1.5 T scanner outfitted with a field-cycling insert electromagnet to dynamically control B 0 prior to imaging. Images at five field strengths (1.26, 1.39, 1.5, 1.61, 1.74 T) were acquired using a field-cycling pulse sequence, where B 0 was modulated for varying relaxation durations prior to imaging. R 1 maps and R 1 dispersion (ΔR 1 /ΔB 0 ) were calculated at 1.5 T on a pixel-by-pixel basis. In addition, in vivo R 1 maps of mice were acquired at 3 T. At fields less than 1 T, a large R 1 magnetic field dependence was observed for tissues. ROI analysis of the tissues showed little relaxation dispersion for magnetic fields from 1.26 T to 3 T. Our tissue measurements show strong R 1 dispersion at field strengths less than 1 T and limited R 1 dispersion at field strengths greater than 1 T. These findings emphasize the inherent weak R 1 magnetic field dependence of healthy tissues at clinical field strengths. This characteristic of tissues can be exploited by a combination of FFC-MRI and T 1 contrast agents that exhibit strong relaxivity magnetic field dependences (inherent or by binding to a protein), thereby increasing the agents' specificity and sensitivity. This development can provide potential insights into protein-based biomarkers using FFC-MRI to assess early changes in tumour development, which are not easily measureable with conventional MRI. MRI has a history of constant technical progress towards higher magnetic field strengths for imaging. 1 The progressive increases in magnetic field strength have been spurred by significant gains in the signal-to-noise ratio (SNR), and spectral and spatial resolution exploited in imaging of anatomy and brain function. [2][3][4][5] While high and ultra-high magnetic field strengths are attractive for producing exquisite images of internal structures with unprecedented detail that aid in tissue characterization, imaging at these magnetic field strengths results in very little difference in the relaxation times of different tissues, thereby losing the variability of an important intrinsic tissue property that leads to image contrast. 6,7 Differences in Abbreviations used: B P , polarization magnetic field; B R , relaxation magnetic field; dreMR, delta relaxation enhanced MR; FFC-MR, fast field-cycling MR; FFC-MRI, fast field-cycling MRI; FID, free induction decay; FSE, fast spin-echo; NMRD, nuclear magnetic relaxation dispersion; NP, non-polarized; PP, pre-polarized; ROI, region of interest; SNR, signal-to-noise ratio
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