Amide proton transfer (APT) imaging of brain tumors at 7 T: The role of tissue water T 1 -Relaxation properties

Section: Discussionsupporting

confidence: 87%

Chemical exchange saturation transfer for predicting response to stereotactic radiosurgery in human brain metastasis

Desmond

Mehrabian

Chavez

et al. 2016

“…For the next step, this acquisition protocol will be used to comprehensively assess the actual clinical relevance of relaxation compensation in CEST‐MRI for various neuro‐oncological clinical questions. In particular, the translation to 3 T enables investigation of the controversially discussed overcompensation of T 1 at this magnetic field strength using the AREX approach (Equation ) . The unravelling of the different CEST signals and compensation for competing relaxation effects by the presented acquisition protocol can also enable obtaining more detailed information on the origin of CEST contrast in vivo.…”

Section: Discussionmentioning

confidence: 99%

“…In particular, the translation to 3 T enables investigation of the controversially discussed overcompensation of T 1 at this magnetic field strength using the AREX approach (Equation 3). [56][57][58] The unravelling of the different CEST signals and compensation for competing relaxation effects by the presented acquisition protocol can also enable obtaining more detailed information on the origin of CEST contrast in vivo. For general recommendations regarding acquisition parameters for CEST-MRI at 3 T, we refer to the APTweighted approach of Zhou and van Zijl et al, [59][60][61] as they have collected a sizeable amount of clinical evidence up until now.…”

Section: Discussionmentioning

confidence: 99%

Relaxation‐compensated APT and rNOE CEST‐MRI of human brain tumors at 3 T

Goerke

Soehngen

Deshmane

et al. 2019

111

Purpose Relaxation‐compensated CEST‐MRI (i.e., the inverse metrics magnetization transfer ratio and apparent exchange‐dependent relaxation) has already been shown to provide valuable information for brain tumor diagnosis at ultrahigh magnetic field strengths. This study aims at translating the established acquisition protocol at 7 T to a clinically relevant magnetic field strength of 3 T. Methods Protein model solutions were analyzed at multiple magnetic field strengths to assess the spectral widths of the amide proton transfer and relayed nuclear Overhauser effect (rNOE) signals at 3 T. This prior knowledge of the spectral range of CEST signals enabled a reliable and stable Lorentzian‐fitting also at 3 T where distinct peaks are no longer resolved in the Z‐spectrum. In comparison to the established acquisition protocol at 7 T, also the image readout was extended to three dimensions. Results The observed spectral range of CEST signals at 3 T was approximately ±15 ppm. Final relaxation‐compensated amide proton transfer and relayed nuclear Overhauser effect contrasts were in line with previous results at 7 T. Examination of a patient with glioblastoma demonstrated the applicability of this acquisition protocol in a clinical setting. Conclusion The presented acquisition protocol allows relaxation‐compensated CEST‐MRI at 3 T with a 3D coverage of the human brain. Translation to a clinically relevant magnetic field strength of 3 T opens the door to trials with a large number of participants, thus enabling a comprehensive assessment of the clinical relevance of relaxation compensation in CEST‐MRI.

“…Fitting regressors for fMRI data is expected to further increase this pH sensitivity, but the precise estimation is difficult. Although BSA is a good model protein, care has to be taken when directly translating the phantom results to in vivo studies, because of differences in water T 1 relaxation times (52)(53)(54)(55), protein concentration, protein conformation, and catalytic microenvironment (56), among others.…”

Section: Discussionmentioning

confidence: 99%

Establishing upper limits on neuronal activity–evoked pH changes with APT‐CEST MRI at 7 T

Khlebnikov

Siero

Bhogal

et al. 2017

Self Cite

PurposeTo detect neuronal activity–evoked pH changes by amide proton transfer–chemical exchange saturation transfer (APT‐CEST) MRI at 7 T.MethodsThree healthy subjects participated in the study. A low‐power 3‐dimensional APT‐CEST sequence was optimized through the Bloch‐McConnell equations. pH sensitivity of the sequence was estimated both in phantoms and in vivo. The feasibility of pH–functional MRI was tested in Bloch‐McConnell‐simulated data using the optimized sequence. In healthy subjects, the visual stimuli were used to evoke transient pH changes in the visual cortex, and a 3‐dimensional APT‐CEST volume was acquired at the pH‐sensitive frequency offset of 3.5 ppm every 12.6 s.ResultsIn theory, a three‐component general linear model was capable of separating the effects of blood oxygenation level–dependent contrast and pH. The Bloch‐McConnell equations indicated that a change in pH of 0.03 should be measurable at the experimentally determined temporal signal‐to‐noise ratio of 108. However, only a blood oxygenation level–dependent effect in the visual cortex could be discerned during the visual stimuli experiments performed in the healthy subjects.ConclusionsThe results of this study suggest that if indeed there are any transient brain pH changes in response to visual stimuli, those are under 0.03 units pH change, which is extremely difficult to detect using the existent techniques. Magn Reson Med 80:126–136, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.