Purpose: To enable whole‐brain quantitative CEST MRI at ultra‐high magnetic field strengths (B₀ ≥ 7T) within short acquisition times.Methods: Multiple interleaved mode saturation (MIMOSA) was combined with fast online‐customized (FOCUS) parallel transmission (pTx) excitation pulses and correction to achieve homogenous whole‐brain coverage. Examinations of 13 volunteers were performed on a 7T MRI system with 3 different types of pulse sequences: (1) saturation in circular polarized (CP) mode and CP mode readout, (2) MIMOSA and CP readout, and (3) MIMOSA and FOCUS readout. For comparison, the inverse magnetic transfer ratio metric for relayed nuclear Overhauser effect and amide proton transfer were calculated. To investigate the number of required acquisitions for a good correction, 4 volunteers were measured with 6 different B₁ amplitudes. Finally, time point repeatability was investigated for 6 volunteers.Results: MIMOSA FOCUS sequence using correction, with both single and multiple points, reduced inhomogeneity of the CEST contrasts around the occipital lobe and cerebellum. Results indicate that the most stable inter‐subject coefficient of variation was achieved using the MIMOSA FOCUS sequence. Time point repeatability of MIMOSA FOCUS with single‐point correction showed a maximum coefficient of variation below 8% for 3 measurements in a single volunteer.Conclusion: A combination of MIMOSA FOCUS with a single‐point correction can be used to achieve quantitative CEST measurements at ultra‐high magnetic field strengths. Compared to previous correction methods, acquisition time can be reduced as additional scans required for correction can be omitted.

Objective: To establish an image acquisition and post-processing workflow for the determination of the proton density fat fraction (PDFF) in calf muscle tissue at 7 T. Materials and methods: Echo times (TEs) of the applied vendor-provided multi-echo gradient echo sequence were optimized based on simulations of the effective number of signal averages (NSA*). The resulting parameters were validated by measurements in phantom and in healthy calf muscle tissue (n = 12). Additionally, methods to reduce phase errors arising at 7 T were evaluated. Finally, PDFF values measured at 7 T in calf muscle tissue of healthy subjects (n = 9) and patients with fatty replacement of muscle tissue (n = 3) were compared to 3 T results. Results: Simulations, phantom and in vivo measurements showed the importance of using optimized TEs for the fat–water separation at 7 T. Fat–water swaps could be mitigated using a phase demodulation with an additional B₀ map, or by shifting the TEs to longer values. Muscular PDFF values measured at 7 T were comparable to measurements at 3 T in both healthy subjects and patients with increased fatty replacement. Conclusion: PDFF determination in calf muscle tissue is feasible at 7 T using a chemical shift-based approach with optimized acquisition and post-processing parameters.

Purpose: CEST MRI is influenced by fat signal, which can reduce the apparent CEST contrast or lead to pseudo‐CEST effects. Our goal was to develop a fat artifact correction based on multi‐echo fat–water separation that functions stably for 7 T knee MRI data.Methods: Our proposed algorithm utilizes the full complex data and a phase demodulation with an off‐resonance map estimation based on the Z‐spectra prior to fat–water separation to achieve stable fat artifact correction. Our method was validated and compared to multi‐echo–based methods originally proposed for 3 T by Bloch–McConnell simulations and phantom measurements. Moreover, the method was applied to in vivo 7 T knee MRI examinations and compared to Gaussian fat saturation and a published single‐echo Z‐spectrum–based fat artifact correction method.Results: Phase demodulation prior to fat–water separation reduced the occurrence of fat–water swaps. Utilizing the complex signal data led to more stable correction results than working with magnitude data, as was proposed for 3 T. Our approach reduced pseudo‐nuclear Overhauser effects compared to the other correction methods. Thus, the mean asymmetry contrast at 3.5 ppm in cartilage over five volunteers increased from −9.2% (uncorrected) and −10.6% (Z‐spectrum–based) to −1.5%. Results showed higher spatial stability than with the fat saturation pulse.Conclusion: Our work demonstrates the feasibility of multi‐echo–based fat–water separation with an adaptive fat model for fat artifact correction for CEST MRI at 7 T. Our approach provided better fat artifact correction throughout the entire spectrum and image than the fat saturation pulse or Z‐spectrum–based correction method for both phantom and knee imaging results.

Purpose: To achieve a reliable fat/water separation (FWS) at 7 T using a new 3D radial sequence and reconstruction workflow with quasi‐continuous TE sampling. Methods: A 3D radial density‐adapted sequence with quasi‐continuous TE sampling was developed for a 7 T whole‐body MRI system. The reconstruction workflow included an off‐resonance correction to reduce chemical shift artifacts. Fat and water signals were separated using a graph cut algorithm, yielding proton density fat fraction maps. The sequence and reconstruction pipeline were tested and validated using phantom measurements and in vivo acquisitions of the lower leg of healthy volunteers. Results: The proposed sequence and reconstruction pipeline enabled sampling of the fat/water oscillation curve over a range from a minimal mean TE of 0.27 ms to a maximum mean TE of 10.13 ms, with an effective TE increment of 85 μs. The off‐resonant correction significantly reduced chemical shift artifacts in the fat signal. The data points of the fat/water oscillation showed good agreement with a multi‐peak fit function. The quantification of phantom fat percentages was verified with a mean absolute error of 1.5%. The proposed pipeline resulted in consistent interpretation of fat and water signals without any swaps, neither in phantom nor in in vivo measurements. Conclusion: This study successfully demonstrated the application of a 3D radial sequence with quasi‐continuous TEs for FWS at 7 T. Due to the high sampling rate (effective TE increment below 100 μs), the presented FWS workflow demonstrated high robustness against fat/water swaps, which are typical at ultra‐high field strengths.

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by cysts from different nephron segments, yet their origin remains inaccessible to in vivo imaging. Building on ex vivo studies that cyst sodium concentration ([Na⁺]) may reflect tubular origin, we developed a non-invasive ²³ Na magnetic resonance imaging (MRI) approach at 7 Tesla to phenotype ADPKD cysts in vivo based on cyst sodium. In nephrectomized ADPKD kidneys, cyst fluid [Na ⁺ ] closely matched ²³ Na MRI signal intensities, revealing two distinct cyst phenotypes: serum-like high-[Na ⁺ ] cysts and low-[Na ⁺ ] cysts. High-[Na ⁺ ] cysts expressed the proximal tubular marker sodium-glucose cotransporter-2, whereas low-[Na ⁺ ] cysts expressed markers of distal tubules and collecting ducts. The vasopressin V2 receptor (V2R), the therapeutic target of tolvaptan, was detected exclusively in low-[Na ⁺ ] cysts, linking sodium phenotype to therapeutic target expression. In vivo, ²³Na MRI enabled classification of 2,299 cysts in 20 ADPKD patients. High-[Na ⁺ ] cysts were 1.8-fold more frequent overall. Anatomical cyst localization did not predict sodium phenotype, underscoring the need for functional imaging rather than structural inference. Patients exhibited marked interindividual variation in cyst composition, with low-[Na ⁺ ] cysts comprising 4.6% to 83.9% of all cysts. Given the exclusive expression of V2R in low-[Na ⁺ ] cysts, this heterogeneity may influence disease progression and therapeutic responsiveness. These findings establish ²³Na MRI as a non-invasive method to phenotype ADPKD cysts in vivo and provide a functional imaging approach with potential relevance for stratifying patients and guiding future therapeutic interventions.