Purpose To study the effects of magnetization transfer (MT, in which a semi‐solid spin pool interacts with the free pool), in the context of magnetic resonance fingerprinting (MRF). Methods Simulations and phantom experiments were performed to study the impact of MT on the MRF signal and its potential influence on T1 and T2 estimation. Subsequently, an MRF sequence implementing off‐resonance MT pulses and a dictionary with an MT dimension, generated by incorporating a two‐pool model, were used to estimate the fractional pool size in addition to the B1+, T1, and T2 values. The proposed method was evaluated in the human brain. Results Simulations and phantom experiments showed that an MRF signal obtained from a cross‐linked bovine serum sample is influenced by MT. Using a dictionary based on an MT model, a better match between simulations and acquired MR signals can be obtained (NRMSE 1.3% vs. 4.7%). Adding off‐resonance MT pulses can improve the differentiation of MT from T1 and T2. In vivo results showed that MT affects the MRF signals from white matter (fractional pool‐size ~16%) and gray matter (fractional pool‐size ~10%). Furthermore, longer T1 (~1060 ms vs. ~860 ms) and T2 values (~47 ms vs. ~35 ms) can be observed in white matter if MT is accounted for. Conclusion Our experiments demonstrated a potential influence of MT on the quantification of T1 and T2 with MRF. A model that encompasses MT effects can improve the accuracy of estimated relaxation parameters and allows quantification of the fractional pool size.
Our experimental results showed that different kinds of motion have distinct effects on the precision and effective resolution of the parametric maps measured with MRF. Although MRF-based acquisitions can be relatively robust to motion effects occurring at the beginning or end of the sequence, relying on redundancy in the data alone is not sufficient to assure the accuracy of the multi-parametric maps in all cases.
Purpose The goal of this work is to demonstrate a method for the simultaneous acquisition of proton multiparametric maps (T1, T2, and proton density) and sodium density images in 1 single scan. We hope that the development of such capabilities will help to ease the implementation of sodium MRI in clinical trials and provide more opportunities for researchers to investigate metabolism through sodium MRI. Methods We developed a sequence based on magnetic resonance fingerprinting (MRF), which contains interleaved proton (1H) and sodium (23Na) excitations followed by a simultaneous center‐out radial readout for both nuclei. The receive chain of a 7T scanner was modified to enable simultaneous acquisition of 1H and 23Na signal. The obtained signal‐to‐noise ratio (SNR) was evaluated, and the accuracy of both proton T1, T2, and B1+ and sodium density maps were verified in phantoms. Finally, the method was demonstrated in 2 healthy subjects. Results The SNR obtained using the simultaneous measurement was almost identical to single‐nucleus measurements (<1% change). Similarly, the proton T1 and T2 maps remained stable (normalized root mean square error in T1 ≈ 2.2%, in T2 ≈ 1.4%, and B1+ ≈ 5.4%), which indicates that the proposed sequence and hardware have no significant effects on the signal from either nucleus. In vivo measurements corroborated these results and demonstrated the feasibility of our method for in vivo application. Conclusions With the proposed approach, we were able to simultaneously acquire sodium density images in addition to proton T1, T2, and B1+ maps as well as proton density images.
To develop a 3D MR technique to simultaneously acquire proton multiparametric maps (T 1 , T 2 , and proton density) and sodium density weighted images over the whole brain. Methods:We implemented a 3D stack-of-stars MR pulse sequence which consists of interleaved proton ( 1 H) and sodium ( 23 Na) excitations, tailored slice encoding gradients that can encode the same slice for both nuclei, and simultaneous readout with different radial trajectories ( 1 H, full-radial; 23 Na, center-out radial).The receive chain of our 7T scanner was modified to enable simultaneous acquisition of 1 H and 23 Na signal. A heuristically optimized flip angle train was implemented for proton MR fingerprinting (MRF). The SNR and the accuracy of proton T 1 and T 2 were evaluated in phantoms. Finally, in vivo application of the method was demonstrated in five healthy subjects. Results:The SNR for the simultaneous measurement was almost identical to that for the single-nucleus measurements (<2% change). The proton T 1 and T 2 maps remained similar to the results from a reference 2D MRF technique (normalized RMS error in T 1 ≈ 4.2% and T 2 ≈ 11.3%). Measurements in healthy subjects corroborated these results and demonstrated the feasibility of our method for in vivo application. The in vivo T 1 values measured using our method were lower than the results measured by other conventional techniques.Conclusions: With the 3D simultaneous implementation, we were able to acquire sodium and proton density weighted images in addition to proton T 1 , T 2 , and B + 1 from 1 H MRF that covers the whole brain volume within 21 min.
Purpose Illustrate potential for high permittivity materials to be used in decreasing peak local SAR associated with implants when the imaging region is far from the implant. Methods Numerical simulations of a human subject with a pacemaker in a body-sized birdcage coil driven at 128MHz with and without a thin (5mm) shell of material of high electric permittivity around the head were performed. Results For a shell with relative permittivity of 600, the maximum Specific energy Absorption Rate (SAR) averaged over any 1g of tissue near the pacemaker was reduced by 73.5% for a given B1 field strength at the center of brain. Conclusion While significant further work is required, initial simulations indicate that strategic use of high permittivity materials may broaden the conditions under which patients containing some implants can be imaged safely.
The objective of the current study was to design and build a dual-tuned coil array for simultaneous 23 Na/ 1 H MRI of the human brain at 7 T. Quality factor, experimental B 1 + measurements, and electromagnetic simulations in prototypes showed that setups consisting of geometrically interleaved 1 H and 23 Na loops performed better than or similar to 1 H or 23 Na loops in isolation. Based on these preliminary findings, we built a transmit/receive eight-channel 23 Na loop array that was geometrically interleaved with a transmit/receive eight-channel 1 H loop array. We assessed the performance of the manufactured array with mononuclear signal-to-noise ratio (SNR)and B 1 + measurements, along with multinuclear magnetic resonance fingerprinting maps and images. The 23 Na array within the developed dual-tuned device provided more than 50% gain in peripheral SNR and similar B 1 + uniformity and coverage as a reference birdcage coil of similar size. The 1 H array provided good B 1 + uniformity in the brain, excluding the cerebellum and brain stem. The integrated 23 Na and 1 H arrays were used to demonstrate truly simultaneous quantitative 1 H mapping and 23 Na imaging. K E Y W O R D S magnetic resonance fingerprinting, simultaneous multinuclear MRI, sodium ( 23 Na) MRI, ultrahigh field MRI 1 | INTRODUCTION Multinuclear proton ( 1 H) and sodium ( 23 Na) MRI can provide complementary anatomical and biochemical information to characterize various soft-tissue neuropathologies, such as traumatic brain injury, cancer, and multiple sclerosis, where sodium metabolic dysfunction is believed to play a key role. Two challenges in 23 Na MRI are its low NMR sensitivity (9.2% that of proton) and the low concentration of 23 Na ions in vivo
Proton MRI can provide detailed morphological images, but it reveals little information about cell homeostasis. On the other hand, sodium MRI can provide metabolic information but cannot resolve fine structures. The complementary nature of proton and sodium MRI raises the prospect of their combined use in a single experiment. In this work, we assessed the repeatability of normalized proton density (PD), T1, T2, and normalized sodium density-weighted quantification measured with simultaneous 3D 1H MRF/23Na MRI in the brain at 7 T, from ten healthy volunteers who were scanned three times each. The coefficients of variation (CV) and the intra-class correlation (ICC) were calculated for the mean and standard deviation (SD) of these 4 parameters in grey matter, white matter, and cerebrospinal fluid. As result, the CVs were lower than 3.3% for the mean values and lower than 6.9% for the SD values. The ICCs were higher than 0.61 in all 24 measurements. We conclude that the measurements of normalized PD, T1, T2, and normalized sodium density-weighted from simultaneous 3D 1H MRF/23Na MRI in the brain at 7 T showed high repeatability. We estimate that changes > 6.6% (> 2 CVs) in mean values of both 1H and 23Na metrics could be detectable with this method.
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