A novel phase and frequency navigator for proton magnetic resonance spectroscopy using water-suppression cycling

Abstract

Magnetic resonance spectroscopy is sensitive to movements, in part, because of motion-induced phase and frequency variations that lead to incoherent averaging. For in vivo proton magnetic resonance spectroscopy, the unsuppressed or under-suppressed water signal can be used to restore coherent averaging; however, this approach results in baseline distortions due to the large water peak. Therefore, a novel water-suppression cycling scheme was developed that alternates between positive and negative residual water signal. Using the residual water signal, the method allows for shot-to-shot phase and frequency correction of individual free induction decays and restoration of signal losses due to incoherent averaging, yet near-complete elimination of residual water. It is demonstrated that the residual water signal can be used to restore metabolite peaks in a brain spectrum from a subject who performed intentional head movements. The ability to correct phase and frequency fluctuations during subject motion is vital for use with adaptive motion correction approaches that ensure proper voxel positioning during head movements.

Figure 1

A) Pulse sequence for cycling of water suppression cycling. The basic sequence (yielding positive residual water) consists of two 90° and two 180° RF pulses, with a fixed inter-pulse delay and crusher gradients. The first of the two 180° RF pulses is omitted to obtain negative residual water signal. B) and C) show the evolution of the z-magnetization for both cases.

Figure 2

In vivo spectra from frontal gray matter acquired with positive (left) and negative residual water signal (TE/TR = 30/3000ms, 16 averages per condition). Summation of the two spectra results in a spectrum with little residual water and a well-defined baseline (right).

Figure 3

Effect of varying the B1-field strength of water suppression pulses on the positive and negative residual water signal. The black line shows the calculated effect based on evolution of z-magnetization. The colored lines represent in vivo measurements in the frontal cortex of 3 volunteers. While the amplitude of the residual water signals shows some dependency on the RF power, the summed water signal is attenuated over 100-fold over a range of ± 25% (± 2dB) relative to the unsuppressed water.

Figure 4

Calculated effect of the T1-value on the positive and negative residual water signals, based on evolution of z-magnetization. While the amplitude of the residual water signals shows some dependency on the T1 value, the summed water signal is attenuated over 100-fold for T1-values greater than 700ms.

Figure 5

Frontal gray matter spectra without subject motion (left), motion (simulated tremor) without phase correction (center), and motion with phase correction (right). Each spectrum was acquired at short echo time (TE/TR 30/3000ms, 32 averages).