Improving the reproducibility of proton magnetic resonance spectroscopy brain thermometry: Theoretical and empirical approaches

Abstract

In proton magnetic resonance spectroscopy (1H MRS)‐based thermometry of brain, averaging temperatures measured from more than one reference peak offers several advantages, including improving the reproducibility (i.e., precision) of the measurement. This paper proposes theoretically and empirically optimal weighting factors to improve the weighted average of temperatures measured from three references. We first proposed concepts of equivalent noise and equivalent signal‐to‐noise ratio in terms of frequency measurement and a concept of relative frequency that allows the combination of different peaks in a spectrum for improving the precision of frequency measurement. Based on these, we then derived a theoretically optimal weighting factor and proposed an empirical weighting factor, both involving equivalent noise levels, for a weighted average of temperatures measured from three references (i.e., the singlets of NAA, Cr, and Ch in the 1H MR spectrum). We assessed these two weighting factors by comparing their errors in measurement of temperatures with the errors of temperatures measured from individual references; we also compared these two new weighting factors with two previously proposed weighting factors. These errors were defined as the standard deviations in repeated measurements or in Monte Carlo studies. Both the proposed theoretical and empirical weighting factors outperformed the two previously proposed weighting factors as well as the three individual references in all phantom and in vivo experiments. In phantom experiments with 4‐ or 10‐Hz line broadening, the theoretical weighting factor outperformed the empirical one, but the latter was superior in all other repeated and Monte Carlo tests performed on phantom and in vivo data. The proposed weighting factors are superior to the two previously proposed weighting factors and can improve the reproducibility of temperature measurement using 1H MRS‐based thermometry.

Autonomous animal heating and cooling system for temperature‐regulated magnetic resonance experiments

“…Upon stabilization at each temperature, shimming was conducted to correct for temperature‐related susceptibility changes, 26,32 followed by a proton Point‐RESolved Spectroscopy (PRESS) sequence with the following parameters: TE = 40 ms, TR = 2500 ms, flip angle = 90°, averages = 150, voxel size = 3 × 3 × 3 mm 3 , acquisition bandwidth = 6010 Hz, and acquisition time = 375 s. Proton MR spectroscopic thermometry was used because it noninvasively measures temperature based on frequency differences between temperature‐dependent water and temperature‐independent metabolites. The three frequencies of the N ‐acetyl‐aspartate (NAA), creatine (Cr), and choline (Ch) singlets at 2.01, 3.03, and 3.19 ppm were assumed to be temperature independent and fixed, and were used as references to improve the reproducibility of the temperature measurement 58 . Upon completion of the acquisitions, the data were exported to TopSpin 4.2 software (Bruker), where the frequency difference between water and the metabolites was reconstructed and the average temperature was computed according to 58 where , and are the frequencies of the water, Cr, Ch, and NAA peaks, respectively, in units of ppm.…”

“…The three frequencies of the N ‐acetyl‐aspartate (NAA), creatine (Cr), and choline (Ch) singlets at 2.01, 3.03, and 3.19 ppm were assumed to be temperature independent and fixed, and were used as references to improve the reproducibility of the temperature measurement 58 . Upon completion of the acquisitions, the data were exported to TopSpin 4.2 software (Bruker), where the frequency difference between water and the metabolites was reconstructed and the average temperature was computed according to 58 where , and are the frequencies of the water, Cr, Ch, and NAA peaks, respectively, in units of ppm. Peak frequencies were identified by recording the frequency at the maximum of the peak magnitude.…”

“…The three frequencies of the N-acetyl-aspartate (NAA), creatine (Cr), and choline (Ch) singlets at 2.01, 3.03, and 3.19 ppm were assumed to be temperature independent and fixed, and were used as references to improve the reproducibility of the temperature measurement. 58 Upon completion of the acquisitions, the data were exported to TopSpin 4.2 software (Bruker), where the frequency difference between water and the metabolites was reconstructed and the average temperature was computed according to 58 T NAA ¼ 315:6 À 103:…”