Polyacrylamide Gel Calibration Phantoms for Quantification in Sodium MRI

Quantitative sodium (²³Na) MRI utilises a signal calibration approach to derive maps of total sodium concentration (TSC). Agarose gel vials are often used as calibration phantoms, but as a naturally occurring substance, agarose may exhibit unfavourable qualities relating to instabilities, inconsistencies and heterogeneity. To contribute towards standardisation and methods harmonisation of quantitative ²³Na MRI, the objective of this study was to develop and test a novel, standardisable synthetic polymer calibration phantom for in vivo quantitative ²³Na MRI. Seven crosslinked polyacrylamide gel (PAG) samples were prepared, doped with sodium chloride (NaCl) at nominal concentrations of 10-150 mM. The sodium concentrations of all samples were estimated by volumetrics using high-precision mass measurements. Relaxation time constants ( $$ ) of all samples were measured at 3 T with a non-localised pulse-acquire sequence. $$ was measured longitudinally over 14 months to assess stability. Finally, in vivo TSC quantification with PAG phantoms was demonstrated in the human brain and calf muscle on different systems, with different imaging sequences. The measured sodium concentrations of phantoms were on average 5% lower than nominal ones, owing to the unknown volumetric contribution of the solid fraction. Hence, they were reported as apparent sodium concentrations, and the apparent TSC (aTSC) was quantified in vivo. Mean relaxation time constants of ²³Na in PAG were in the following ranges: $$  = 27-39 ms, $$  = 4.8-7.1 ms, $$  = 16.8-18.8 ms, short fraction $$  = 0.64-0.77. Over 14 months, relaxation time constants were stable within 10% (above sodium concentrations of 25 mM). In vivo aTSC measurements were in the expected ranges. PAG phantoms are well suited for quantification and standardisation in ²³Na MRI, offering tissue-matched relaxation time constants and the intrinsic benefits of a synthetic material.