PurposeTo develop SPARCQ (Signal Profile Asymmetries for Rapid Compartment Quantification), a novel approach to quantify fat fraction (FF) using asymmetries in the phase‐cycled balanced SSFP (bSSFP) profile.MethodsSPARCQ uses phase‐cycling to obtain bSSFP frequency profiles, which display asymmetries in the presence of fat and water at certain TRs. For each voxel, the measured signal profile is decomposed into a weighted sum of simulated profiles via multi‐compartment dictionary matching. Each dictionary entry represents a single‐compartment bSSFP profile with a specific off‐resonance frequency and relaxation time ratio. Using the results of dictionary matching, the fractions of the different off‐resonance components are extracted for each voxel, generating quantitative maps of water and FF and banding‐artifact‐free images for the entire image volume. SPARCQ was validated using simulations, experiments in a water‐fat phantom and in knees of healthy volunteers. Experimental results were compared with reference proton density FFs obtained with 1H‐MRS (phantoms) and with multiecho gradient‐echo MRI (phantoms and volunteers). SPARCQ repeatability was evaluated in six scan‐rescan experiments.ResultsSimulations showed that FF quantification is accurate and robust for SNRs greater than 20. Phantom experiments demonstrated good agreement between SPARCQ and gold standard FFs. In volunteers, banding‐artifact‐free quantitative maps and water‐fat‐separated images obtained with SPARCQ and ME‐GRE demonstrated the expected contrast between fatty and non‐fatty tissues. The coefficient of repeatability of SPARCQ FF was 0.0512.ConclusionSPARCQ demonstrates potential for fat quantification using asymmetries in bSSFP profiles and may be a promising alternative to conventional FF quantification techniques.
It was recently demonstrated that nonpersistent radicals can be generated in frozen solutions of metabolites such as pyruvate by irradiation with UV light, enabling radical-free dissolution dynamic nuclear polarization. Although pyruvate is endogenous, the presence of pyruvate may interfere with metabolic processes or the detection of pyruvate as a metabolic product, making it potentially unsuitable as a polarizing agent. Therefore, the aim of the current study was to characterize solutions containing endogenously occurring alternatives to pyruvate as UV-induced nonpersistent radical precursors for in vivo hyperpolarized MRI. The metabolites alphaketovalerate (αkV) and alpha-ketobutyrate (αkB) are analogues of pyruvate and were chosen as potential radical precursors. Sample formulations containing αkV and αkB were studied with UV-visible spectroscopy, irradiated with UV light, and their nonpersistent radical yields were quantified with electron spin resonance and compared with pyruvate. The addition of 13 C-labeled substrates to the sample matrix altered the radical yield of the precursors. Using αkB increased the 13 C-labeled glucose liquid-state polarization to 16.3% ± 1.3% compared with 13.3% ± 1.5% obtained with pyruvate, and 8.9% ± 2.1% with αkV. For [1-13 C]butyric acid, polarization levels of 12.1% ± 1.1% for αkV, 12.9% ± 1.7% for αkB, 1.5% ± 0.2% for OX063 and 18.7% ± 0.7% for Finland trityl, were achieved. Hyperpolarized [1-13 C]butyrate metabolism in the heart revealed label incorporation into [1-13 C]acetylcarnitine, [1- 13 C] acetoacetate, [1-13 C]butyrylcarnitine, [5-13 C]glutamate and [5-13 C]citrate. This study demonstrates the potential of αkV and αkB as endogenous polarizing agents for in vivo radical-free hyperpolarized MRI. UV-induced, nonpersistent radicals generated in endogenous metabolites enable high polarization without requiring radical filtration, thus simplifying the quality-control tests in clinical applications.
Purpose: To develop a free-running 3D radial whole-heart multiecho gradient echo (ME-GRE) framework for cardiac-and respiratory-motion-resolved fat fraction (FF) quantification. Methods: (N TE = 8) readouts optimized for water-fat separation and quantification were integrated within a continuous non-electrocardiogram-triggered free-breathing 3D radial GRE acquisition. Motion resolution was achieved with pilot tone (PT) navigation, and the extracted cardiac and respiratory signals were compared to those obtained with self-gating (SG). After extra-dimensional golden-angle radial sparse parallel-based image reconstruction, FF, R 2 *, and B 0 maps, as well as fat and water images were generated with a maximum-likelihood fitting algorithm. The framework was tested in a fat-water phantom and in 10 healthy volunteers at 1.5 T using N TE = 4 and N TE = 8 echoes. The separated images and maps were compared with a standard free-breathing electrocardiogram (ECG)-triggered acquisition. Results:The method was validated in vivo, and physiological motion was resolved over all collected echoes. Across volunteers, PT provided respiratory and cardiac signals in agreement (r = 0.91 and r = 0.72) with SG of the first echo, and a higher correlation to the ECG (0.1% of missed triggers for PT vs. 5.9% for SG). The framework enabled pericardial fat imaging and quantification throughout the cardiac cycle, revealing a decrease in FF at end-systole by 11.4% ± 3.1% across volunteers (p < 0.0001). Motion-resolved end-diastolic 3D FF maps showed good correlation with ECG-triggered measurements (FF bias of −1.06%). A significant difference in free-running FF measured with N TE = 4 and N TE = 8 was found (p < 0.0001 in sub-cutaneous fat and p < 0.01 in pericardial fat). Conclusion:Free-running fat fraction mapping was validated at 1.5 T, enabling ME-GRE-based fat quantification with N TE = 8 echoes in 6:15 min.
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