High‐precision calibration of MRS thermometry using validated temperature standards: effects of ionic strength and protein content on the calibration

Abstract

Currently, there is very limited ability to measure the temperature of the brain, but a direct technique for its estimation in vivo could improve the detection of patients at risk of temperature-related brain damage, help in the diagnosis of stroke and tumour, and provide useful information on the mechanisms of thermoregulation of the brain. In this article, new calibrations in vitro of MRS thermometry using temperature-stabilised reference phantoms are reported. The phantoms comprise two concentric glass spheres: the inner sphere contains the phantom material to be measured by MRS, and the outer sphere contains a substance with a known temperature stable to within 0.2 °C. The substances were freezing organic fixed-point compounds (diphenyl ether and ethylene carbonate, freezing at 26.3 and 35.8 °C, respectively) or temperature-controlled circulating water. The phantom temperature was continuously monitored with a fluoroptic probe calibrated at the National Physical Laboratory with traceability to the International Temperature Scale 1990 (ITS-90). The MRS temperature calibration was obtained by measuring the chemical shift of water relative to N-acetylaspartate (NAA) in a single voxel as a function of temperature using a 1.5-T Philips Intera scanner. Measurements were made for several phantom materials to assess the effect of tissue composition on the water-NAA chemical shift against temperature calibration. The phantom mixtures contained 25 mm of NAA buffered to pH 6.5 or 7.5 and several ionic salts or bovine serum albumin (BSA). Spectra were acquired from 25 to 45 °C. The correlation between frequency differences and phantom temperature was very linear with small residuals. However, the linear fitting parameters varied with ionic composition and BSA concentration. The 'apparent' temperature (calibrated using the water-NAA frequency differences) decreased by approximately 1 °C for every 100 mm increase in ionic concentration and increased proportionally to the concentration of BSA.

Effects of tissue susceptibility on brain temperature mapping

“…In common with all MRS studies of brain temperature in human subjects the accuracy of the measurement remains unverified, as it is dependent upon the calibration factors used for which there is considerable variation in published values and a known dependence on acquisition and analysis methods. Of the published calibrations, similar values for the temperature dependence of Δ H2O-NAA have been reported in four studies (Kuroda et al, 2003; Prakash et al, 2014; Vescovo et al, 2013; Zhu et al, 2008) and therefore this study has used these values while modifying the intercept values to obtain the same assumed mean WM temperature from all three reference measurements.…”

“…Several reports have published calibration values based on measurements in phantoms (Cady et al, 1995; Corbett et al, 1995; Covaciu et al, 2010), rat brain (Zhu et al, 2008) and piglet brain (Cady et al, 1995; Cady et al, 2010), however, with little agreement on the calibration values. It has been demonstrated that calibrations in solutions depend on protein and ionic content (Vescovo et al, 2013) and that the spectral fitting (Zhu et al, 2008) and data acquisition (Childs et al, 2007; Prakash et al, 2014) methods can affect the results. The in vivo calibration measurements will additionally be affected by the magnetic susceptibility of the sampled tissue region, which has previously not been taken into account.…”

MRS thermometry calibration at 3 T: effects of protein, ionic concentration and magnetic field strength

“…Six pH normalized, pH ~ 7, solutions were studied, using N ‐acetylaspartate (NAA) (25 mM ) as the reference metabolite (see Table later), with four out of the six solutions identical to those used in the study by Vescovo et al to allow investigation of the effect of magnetic field strength on the calibration curve. A control solution and varied NaCl/KCl concentrations, used to approximate the ionic concentrations of neuronal and glial cells , facilitated the investigation of the ionic concentration effect on the temperature calibration (see Table later).…”

“…The fixed‐point organic compounds were ethylene carbonate and diphenyl ether, freezing at 35.8 °C and 26.3 °C, respectively. The procedure used to attain a stable temperature with the frozen organic compounds has been described in detail previously . In brief, the organic temperature reference phantoms were heated in a water bath (70 °C) for 1 h then transferred to a cool water bath (20 °C) and left to super‐cool (1 h).…”

Multi-parameter brain tissue microsensor and interface systems: Calibration, reliability and user experiences of pressure and temperature sensors in the setting of neurointensive care

“…Due to the difficulties of direct monitoring of temperature (or any other parameter) deep inside the head, our understanding of cerebral homeothermy of the healthy human brain has been rather limited. However, in recent years, noninvasive, magnetic resonance spectroscopy ( 1 HMRS) and imaging 1 HMRSI techniques have been developed, allowing internal temperature to be measured in healthy adults [3,4]. Results show that small differences only exist between the brain and deep body temperature [5,6].…”