The purpose of this work is to propose a tier‐based formalism for safety assessment of custom‐built radio‐frequency (RF) coils that balances validation effort with the effort put in determinating the ...safety factor. The formalism has three tier levels. Higher tiers require increased effort when validating electromagnetic simulation results but allow for less conservative safety factors. In addition, we propose a new method to calculate modeling uncertainty between simulations and measurements and a new method to propagate uncertainties in the simulation into a safety factor that minimizes the risk of underestimating the peak specific absorption rate (SAR). The new safety assessment procedure was completed for all tier levels for an eight‐channel dipole array for prostate imaging at 7 T and an eight‐channel dipole array for head imaging at 10.5 T, using data from two different research sites. For the 7 T body array, the validation procedure resulted in a modeling uncertainty of 77% between measured and simulated local SAR distributions. For a situation where RF shimming is performed on the prostate, average power limits of 2.4 and 4.5 W/channel were found for tiers 2 and 3, respectively. When the worst‐case peak SAR among all phase settings was calculated, power limits of 1.4 and 2.7 W/channel were found for tiers 2 and 3, respectively. For the 10.5 T head array, a modeling uncertainty of 21% was found based on B1+ mapping. For the tier 2 validation, a power limit of 2.6 W/channel was calculated. The demonstrated tier system provides a strategy for evaluating modeling inaccuracy, allowing for the rapid translation of novel coil designs with conservative safety factors and the implementation of less conservative safety factors for frequently used coil arrays at the expense of increased validation effort.
We propose a tier‐based formalism that stratifies the amount of effort put in validating EM simulations for custom built transmit coils. For increasing tier levels, more validation effort is required, which provides higher confidence in the EM simulations and allows for less conservative power limits.
Phosphorus MRS offers a non‐invasive tool for monitoring cell energy and phospholipid metabolism and can be of additional value in diagnosing cancer and monitoring cancer therapy. In this study, we ...determined the transverse relaxation times of a number of phosphorous metabolites in a group of breast cancer patients by adiabatic multi‐echo spectroscopic imaging at 7 T. The transverse relaxation times of phosphoethanolamine, phosphocholine, inorganic phosphate (Pi), glycerophosphocholine and glycerophosphatidylcholine were 184 ± 8 ms, 203 ± 17 ms, 87 ± 8 ms, 240 ± 56 ms and 20 ± 10 ms, respectively. The transverse relaxation time of Pi in breast cancer tissue was less than half that of healthy fibroglandular tissue. This effect is most likely caused by an up‐regulation of glycolysis in breast cancer tissue that leads to interaction of Pi with the GAPDH enzyme, which forms part of the reversible pathway of exchange of Pi with gamma‐adenosine tri‐phosphate, thus shortening its apparent transverse relaxation time. As healthy breast tissue shows very little glycolytic activity, the apparent T2 shortening of Pi due to malignant transformation could possibly be used as a biomarker for cancer.
Transverse relaxation times of phosphorous metabolites were determined in breast cancer tissue at 7 T. The apparent T2 of Pi was less than half that of healthy fibroglandular tissue. This effect is most likely caused by an up‐regulation of glycolysis in breast cancer tissue that leads to increased interaction of Pi with the GADPH enzyme, thus shortening its apparent T2. As healthy breast tissue shows very little glycolytic activity, the observed effect may serve as a biomarker for cancer.
Patient‐derived cancer cells cultured in vitro are a cornerstone of cancer metabolism research. More recently, the introduction of organoids has provided the research community with a more versatile ...model system. Physiological structure and organization of the cell source tissue are maintained in organoids, representing a closer link to in vivo tumor models. High‐resolution magic angle spinning magnetic resonance spectroscopy (HR MAS MRS) is a commonly applied analytical approach for metabolic profiling of intact tissue, but its use has not been reported for organoids. The aim of the current work was to compare the performance of HR MAS MRS and extraction‐based nuclear magnetic resonance (NMR) in metabolic profiling of wild‐type and tumor progression organoids (TPOs) from human colon cancer, and further to investigate how the sequentially increased genetic alterations of the TPOs affect the metabolic profile. Sixteen metabolites were reliably identified and quantified both in spectra based on NMR of extracts and HR MAS MRS of intact organoids. The metabolite concentrations from the two approaches were highly correlated (r = 0.94), and both approaches were able to capture the systematic changes in metabolic features introduced by the genetic alterations characteristic of colorectal cancer progression (e.g., increased levels of lactate and decreased levels of myo‐inositol and phosphocholine with an increasing number of mutations). The current work highlights that HR MAS MRS is a well‐suited method for metabolic profiling of intact organoids, with the additional benefit that the nondestructive nature of HR MAS enables subsequent recovery of the organoids for further analyses based on nucleic acids or proteins.
Tumor progression organoids (TPOs) representing the genetic alterations in colorectal cancer were analyzed by two methods: extraction NMR and high‐resolution magic angle spinning (HR MAS) MRS. Correlation analysis and principal component analysis from MR spectra show that the two methods can be useful in the metabolic profiling of organoids. NMR analysis of the TPOs indicates that metabolic changes occur at an early stage during tumor progression.
Purpose
Single‐voxel MRS (SV MRS) requires robust volume localization as well as optimized crusher and phase‐cycling schemes to reduce artifacts arising from signal outside the volume of interest. ...However, due to local magnetic field gradients (B0 inhomogeneities), signal that was dephased by the crusher gradients during acquisition might rephase, leading to artifacts in the spectrum. Here, we analyzed this mechanism, aiming to identify the source of signals arising from unwanted coherence pathways (spurious signals) in SV MRS from a B0 map.
Methods
We investigated all possible coherence pathways associated with imperfect localization in a semi‐localized by adiabatic selective refocusing (semi‐LASER) sequence for potential rephasing of signals arising from unwanted coherence pathways by a local magnetic field gradient. We searched for locations in the B0 map where the signal dephasing due to external (crusher) and internal (B0) field gradients canceled out. To confirm the mechanism, SV‐MR spectra (TE = 31 ms) and 3D‐CSI data with the same volume localization as the SV experiments were acquired from a phantom and 2 healthy volunteers.
Results
Our analysis revealed that potential sources of spurious signals were scattered over multiple locations throughout the brain. This was confirmed by 3D‐CSI data. Moreover, we showed that the number of potential locations where spurious signals could originate from monotonically decreases with crusher strength.
Conclusion
We proposed a method to identify the source of spurious signals in SV 1H MRS using a B0 map. This can facilitate MRS sequence design to be less sensitive to experimental artifacts.
Single-voxel MRS (SV MRS) requires robust volume localization as well as optimized crusher and phase-cycling schemes to reduce artifacts arising from signal outside the volume of interest. However, ...due to local magnetic field gradients (B
inhomogeneities), signal that was dephased by the crusher gradients during acquisition might rephase, leading to artifacts in the spectrum. Here, we analyzed this mechanism, aiming to identify the source of signals arising from unwanted coherence pathways (spurious signals) in SV MRS from a B
map.
We investigated all possible coherence pathways associated with imperfect localization in a semi-localized by adiabatic selective refocusing (semi-LASER) sequence for potential rephasing of signals arising from unwanted coherence pathways by a local magnetic field gradient. We searched for locations in the B
map where the signal dephasing due to external (crusher) and internal (B
) field gradients canceled out. To confirm the mechanism, SV-MR spectra (TE = 31 ms) and 3D-CSI data with the same volume localization as the SV experiments were acquired from a phantom and 2 healthy volunteers.
Our analysis revealed that potential sources of spurious signals were scattered over multiple locations throughout the brain. This was confirmed by 3D-CSI data. Moreover, we showed that the number of potential locations where spurious signals could originate from monotonically decreases with crusher strength.
We proposed a method to identify the source of spurious signals in SV
H MRS using a B
map. This can facilitate MRS sequence design to be less sensitive to experimental artifacts.
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