Abstract
PurposePhosphorus spectroscopy can differentiate among liver disease stages and types. To quantify absolute concentrations of phosphorus metabolites, sensitivity calibration and transmit field (B1+) correction are required. The trend toward ultrahigh fields (7 T) and the use of multichannel RF coils makes this ever more challenging. We investigated the constraints on reference phantoms, and implemented techniques for the absolute quantification of human liver phosphorus spectra acquired using a 10‐cm loop and a 16‐channel array at 7 T.MethodsThe effect of phantom conductivity was assessed at 25.8 MHz (1.5 T), 49.9 MHz (3 T), and 120.3 MHz (7 T) by electromagnetic modeling. Radiofrequency field maps (B1±) were measured in phosphate phantoms (18 mM and 40 mM) at 7 T. These maps were used to assess the correction of 4 phantom 3D‐CSI data sets using 3 techniques: phantom replacement, explicit normalization, and simplified normalization. In vivo liver spectra acquired with a 10‐cm loop were corrected with all 3 methods. Simplified normalization was applied to in vivo 16‐channel array data sets.ResultsSimulations show that quantification errors of less than 3% are achievable using a uniform electrolyte phantom with a conductivity of 0.23‐0.86 S.m−1 at 1.5 T, 0.39‐0.58 S.m−1 at 3 T, and 0.34‐0.42 S.m−1 (16‐19 mM KH2PO4(aq)) at 7 T. The mean γ‐ATP concentration quantified in vivo at 7 T was 1.39 ± 0.30 mmol.L−1 to 1.71 ± 0.35 mmol.L−1 wet tissue for the 10‐cm loop and 1.88 ± 0.25 mmol.L−1 wet tissue for the array.ConclusionIt is essential to select a calibration phantom with appropriate conductivity for quantitative phosphorus spectroscopy at 7 T. Using an 18‐mM phosphate phantom and simplified normalization, human liver phosphate metabolite concentrations were successfully quantified at 7 T.
Highlights
Magnetic resonance spectroscopy is a useful tool for investigating in vivo metabolism, as it gives insight into the concentrations of various metabolites that are otherwise inaccessible using noninvasive methods.[1]
We investigated the constraints on reference phantoms, and implemented techniques for the absolute quantification of human liver phosphorus spectra acquired using a 10‐cm loop and a 16‐channel array at 7 T
For non‐proton‐MRS data, we previously showed that whitened singular value decomposition (WSVD) often gives a better SNR in the combined spectrum than using measured field maps.[11]
Summary
Magnetic resonance spectroscopy is a useful tool for investigating in vivo metabolism, as it gives insight into the concentrations of various metabolites that are otherwise inaccessible using noninvasive methods.[1] Metabolite concentrations often change in diseased tissue. Precise and accurate concentration measurements can differentiate different pathologies, stage the progress of disease, or monitor its treatment. For phosphorus (31P) MRS of the liver, disease targets include viral and alcoholic liver disease, cirrhosis, nonalcoholic fatty liver disease, and type 2 diabetes.[2]. Metabolite concentrations are calculated by normalizing the acquired signal and calibrating it to a reference of known concentration. The calibration reference can be either internal or external. An internal reference simplifies normalization, as several factors can be assumed to be the same for both the reference and the metabolite of interest
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