Whole‐brain quantitative CEST MRI at 7T using parallel transmission methods and correction

Self Cite

To evaluate the benefits and challenges of dynamic parallel transmit (pTx) pulses for fat saturation (FS) and water-excitation (WE), in the context of CEST MRI. Methods: "Universal" k T -points (for FS) and spiral non-selective (for WE) trajectories were optimized offline for flip angle (FA) homogeneity. Routines to optimize the pulse shape online, based on the subject's fields maps, were implemented (target FA of 110 • /0 • for FS, 0 • /5 • for WE at fat/water frequencies). The pulses were inserted in a CEST sequence with a pTx readout. The different fat suppression schemes and their effects on CEST contrasts were compared in 12 volunteers at 7T. Results: With a 25%-shorter pulse duration, pTx FS largely improved the FA homogeneity (root-mean-square-error (RMSE) = 12.3 • vs. 53.4 • with circularly-polarized mode, at the fat frequency). However, the spectral selectivity was degraded mainly in the cerebellum and close to the sinuses (RMSE = 5.8 • vs. 0.2 • at the water frequency). Similarly, pTx WE showed a trade-off between FA homogeneity and spectral selectivity compared to pTx non-selective pulses (RMSE = 0.9 • and 1.1 • at the fat and water frequencies, vs. 4.6 • and 0.5 • ). In the brain, CEST metrics were reduced by up to 31.9% at −3.3 ppm with pTx FS, suggesting a mitigated lipid-induced bias. Conclusion: This clinically compatible implementation of dynamic pTx pulses improved the fat suppression homogeneity at 7T taking into account the subject-specific B 0 heterogeneities online. This study highlights the lipid-induced biases on the CEST z-spectrum. The results are promising for body applications where B 0 heterogeneities and fat are more substantial.

Section: Pulse Designmentioning

confidence: 99%

Clinically compatible subject‐specific dynamic parallel transmit pulse design for homogeneous fat saturation and water‐excitation at 7T: Proof‐of‐concept for CEST MRI of the brain

Levy

Herrler

Liebert

et al. 2022

Self Cite

“…However, before NOE can be implemented in larger clinical studies, the reproducibility of the measurement in healthy subjects must be established. There are several studies in the literature that investigated the reproducibility of different metrics of NOE MRI with varying degrees of reproducibility 37–40 …”

Section: Introductionmentioning

confidence: 99%

In vivo reproducibility of 3D relayed NOE in the healthy human brain at 7 T

Benyard

Nanga

Wilson

et al. 2023

Purpose Nuclear Overhauser effect (NOE) is based on dipolar cross‐relaxation mechanism that enables the indirect detection of aliphatic protons via the water proton signal. This work focuses on determining the reproducibility of NOE magnetization transfer ratio (NOEMTR) and isolated or relayed NOE (rNOE) contributions to the NOE MRI of the healthy human brain at 7 Tesla (T). Methods We optimized the B1+$$ {\mathrm{B}}_1^{+} $$ amplitude and length of the saturation pulse by acquiring NOE images with different B1+$$ {\mathrm{B}}_1^{+} $$ values with multiple saturation lengths. Repeated NOE MRI measurements were made on five healthy volunteers by using optimized saturation pulse parameters including correction of B0 and B1+$$ {\mathrm{B}}_1^{+} $$ inhomogeneities. To isolate the individual contributions from z‐spectra, we have fit the NOE z‐spectra using multiple Lorentzians and calculated the total contribution from each pool contributing to the overall NOEMTR contrast. Results We found that a saturation amplitude of 0.72 μT and a length of 3 s provided the highest contrast. We found that the mean NOEMTR value in gray matter (GM) was 26%, and in white matter (WM) was 33.3% across the 3D slab of the brain. The mean rNOE contributions from GM and WM values were 8.9% and 9.6%, which were ∼10% of the corresponding total NOEMTR signal. The intersubject coefficient of variations (CoVs) of NOEMTR from GM and WM were 4.5% and 6.5%, respectively, whereas the CoVs of rNOE were 4.8% and 5.6%, respectively. The intrasubject CoVs of the NOEMTR range was 2.1%–4.2%, and rNOE range was 2.9%–10.5%. Conclusion This work has demonstrated an excellent reproducibility of both inter‐ and intrasubject NOEMTR and rNOE metrics in healthy human brains at 7 T.

“…Parallel transmission (pTx) is increasingly used in human high field MRI. It not only allows to prospectively cope with the high radiofrequency (RF)‐Field (

{\mathrm{B}}_1^{+}

)inhomogeneities inherent to high MRI field strengths, 1‐3 but also provides a way to shorten local excitation RF pulses 4 . For many brain imaging applications at clinical field strengths such as 3T,

{\mathrm{B}}_1^{+}

inhomogeneities are unproblematic.…”

Section: Introductionmentioning

confidence: 99%

“…inhomogeneities inherent to high MRI field strengths, [1][2][3] but also provides a way to shorten local excitation RF pulses. 4 For many brain imaging applications at clinical field strengths such as 3T, B + 1 inhomogeneities are unproblematic.…”

Section: Introductionmentioning

confidence: 99%

Optimized ultrahigh field parallel transmission workflow using rapid presaturated TurboFLASH transmit field mapping with a three‐dimensional centric single‐shot readout

Bosch

Bause

Geldschläger

et al. 2022

To evaluate the usage of three-dimensional (3D) presaturated Tur-boFLASH (satTFL) for B + 1 and B 0 mapping on single channel and parallel transmission (pTx) systems. Methods: B +1 maps recorded with 3D satTFL were compared to maps from three other 3D B + 1 mapping sequences in an agar phantom. Furthermore, individual-channel B + 1 maps of 18 human subjects were recorded with 3D sat-TFL using B + 1 interferometry. A neural network was trained for masking of the maps.Results: Out of the sequences compared satTFL was the only one with a mapping range exceeding well over 90 • . In regions with lower flip angles there was high correspondence between satTFL and AFI. DREAM and double angle method also showed high qualitative similarity, however the magnitude differed from the other two measurements. The individual-channel B + 1 maps were successfully used for pTx pulse calculation in a separate study. Conclusion: 3D satTFL can record high-quality B +1 maps with a high dynamic range in a short time. Correspondence with AFI maps is high, while measurement duration is reduced drastically.