Proton magnetic resonance spectroscopy thermometry: Impact of separately acquired full water or partially suppressed water data on quantification and measurement error

Abstract

In proton magnetic resonance spectroscopy (¹ H MRS) thermometry, separately acquired full water and partially suppressed water are commonly used for measuring temperature. This paper compares these two approaches. Single-voxel ¹ H MRS data were collected on a 3-T GE scanner from 26 human subjects. Every subject underwent five continuous MRS sessions, each separated by a 2-min phase. Each MRS session lasted 13 min and consisted of two free induction decays (FIDs) without water suppression (with full water [FW or w]) and 64 FIDs with partial water suppression (with partially suppressed water [PW or w']). Frequency differences between the two FWs, the first two PWs, the second FW and the first PW (FW₂ , PW₁ ), or between averaged water ( $w_{\mathrm{av}}^{\text{'}}$ ) and N-acetylaspartate (NAA), were measured. Intrasubject and intersubject variations of the frequency differences were used as a metric for the error in temperature measurement. The intrasubject variations of frequency differences between FW₂ and PW₁$\left(f_{w_2}-f_{w_1^{\text{'}}}\right)$ , calculated from the five MRS sessions for each subject, were larger than those between the two FWs or between the first two PWs (p = 1.54 x 10^-4 and p = 1.72 x 10^-4 , respectively). The mean values of intrasubject variations of $\left(f_{w_2}-f_{w_1^{\text{'}}}\right)$ for all subjects were 4.7 and 4.5 times those of $\left(f_{w_2}-f_{w_1}\right)$ and $\left(f_{w_2^{\text{'}}}-f_{w_1^{\text{'}}}\right)$ , respectively. The intrasubject variations of the temperatures based on frequency differences, $\left(f_{w_2}-f_{\mathrm{NAA}}\right)$ or ( $f_{w_1^{\text{'}}}-f_{\mathrm{NAA}}$ ), were about 2.5 times greater than those based on averaged water and NAA frequencies $(f_{w_{\mathrm{av}}^{\text{'}}}-f_{\mathrm{NAA}}$ ). The mean temperature measured from $(f_{w_{\mathrm{av}}^{\text{'}}}-f_{\mathrm{NAA}}$ ) (n = 26) was 0.29°C lower than that measured from $\left(f_{w_2}-f_{\mathrm{NAA}}\right)$ and was 0.83°C higher than that from ( $f_{w_1^{\text{'}}}-f_{\mathrm{NAA}}$ ). It was concluded that the use of separately acquired unsuppressed or partially suppressed water signals may result in large errors in frequency and, consequently, temperature measurement.

Keywords: 1H MRS thermometry; error assessments; partially suppressed water; separately acquired water.