Quantitative magnetic resonance spectroscopy at 3T based on the principle of reciprocity

Abstract

Quantification of magnetic resonance spectroscopy signals using the phantom replacement method requires an adequate correction of differences between the acquisition of the reference signal in the phantom and the measurement in vivo. Applying the principle of reciprocity, sensitivity differences can be corrected at low field strength by measuring the RF transmitter gain needed to obtain a certain flip angle in the measured volume. However, at higher field strength the transmit sensitivity may vary from the reception sensitivity, which leads to wrongly estimated concentrations. To address this issue, a quantification approach based on the principle of reciprocity for use at 3T is proposed and validated thoroughly. In this approach, the RF transmitter gain is determined automatically using a volume-selective power optimization and complemented with information from relative reception sensitivity maps derived from contrast-minimized images to correct differences in transmission and reception sensitivity. In this way, a reliable measure of the local sensitivity was obtained. The proposed method is used to derive in vivo concentrations of brain metabolites and tissue water in two studies with different coil sets in a total of 40 healthy volunteers. Resulting molar concentrations are compared with results using internal water referencing (IWR) and Electric REference To access In vivo Concentrations (ERETIC). With the proposed method, changes in coil loading and regional sensitivity due to B inhomogeneities are successfully corrected, as demonstrated in phantom and in vivo measurements. For the tissue water content, coefficients of variation between 2% and 3.5% were obtained (0.6-1.4% in a single subject). The coefficients of variation of the three major metabolites ranged from 3.4-14.5%. In general, the derived concentrations agree well with values estimated with IWR. Hence, the presented method is a valuable alternative for IWR, without the need for additional hardware such as ERETIC and with potential advantages in diseased tissue.

T2 relaxation times of macromolecules and metabolites in the human brain at 9.4 T

“…To derive absolute concentrations of metabolites in MRS, several different approaches exist . In one method, the unsuppressed internal water signal is used as a reference .…”

In situ postmortem ethanol quantification in the cerebrospinal fluid by non‐water‐suppressed proton MRS

“…This was done to assure an optimal flip angle and for data calibration using a reference signal acquired in a phantom. This method, in this investigation referred to as the drive scale method, is based on the principle of reciprocity and uses the voltage needed to obtain a certain flip angle in the measured voxel as a measure for the sensitivity and coil loading, and therefore allows the correction of the differences in these quantities between the in situ and phantom measurements …”

“…In the first step, the water signal was measured ( T E / T R = 35/7000 ms, eight averages), since the water signal obtained in the MC scans may not be valid for quantification. For data calibration with the drive scale method, one additional scan of the water signal with the body coil as the transceiver was performed with the same settings as above . The water scans were followed by three non‐water‐suppressed, inner‐volume saturated, single‐voxel, point‐resolved spectroscopy (PRESS) proton MRS scans using the MC technique ( T E / T R 35/3000 ms, readout duration 1024 ms, bandwidth 2000 Hz).…”

“…Ethanol concentrations estimated based on a reference signal S ref,cal acquired in a phantom were calculated using the drive scale method as described in detail in Reference . The scaling factor DS is proportional to the output voltage of the RF transmitter needed to obtain a certain flip angle and was determined with a volume‐selective power optimization with the body coil as the transceiver.…”

“…Compared with the blood‐alcohol concentration (BAC), the ethanol concentration in the CSF is expected to be greater due to the difference in the relative water contents. Calibration of the MRS data was performed using the internal water as a reference with subject‐specific measured water relaxation times, and for comparison also based on a reference signal acquired in a phantom . The latter does not depend on a correct estimation of the fully relaxed water signal.…”