Identifying the source of spurious signals caused by B0 inhomogeneities in single-voxel 1 H MRS

Abstract

Purpose: Single-voxel MRS (SV MRS) requires robust volume localization as well as optimized crusher and phase-cycling schemes to reduce artifacts arising from signal outside the volume of interest. However, due to local magnetic field gradients (B₀ inhomogeneities), signal that was dephased by the crusher gradients during acquisition might rephase, leading to artifacts in the spectrum. Here, we analyzed this mechanism, aiming to identify the source of signals arising from unwanted coherence pathways (spurious signals) in SV MRS from a B₀ map.

Methods: We investigated all possible coherence pathways associated with imperfect localization in a semi-localized by adiabatic selective refocusing (semi-LASER) sequence for potential rephasing of signals arising from unwanted coherence pathways by a local magnetic field gradient. We searched for locations in the B₀ map where the signal dephasing due to external (crusher) and internal (B₀ ) field gradients canceled out. To confirm the mechanism, SV-MR spectra (TE = 31 ms) and 3D-CSI data with the same volume localization as the SV experiments were acquired from a phantom and 2 healthy volunteers.

Results: Our analysis revealed that potential sources of spurious signals were scattered over multiple locations throughout the brain. This was confirmed by 3D-CSI data. Moreover, we showed that the number of potential locations where spurious signals could originate from monotonically decreases with crusher strength.

Conclusion: We proposed a method to identify the source of spurious signals in SV ¹ H MRS using a B₀ map. This can facilitate MRS sequence design to be less sensitive to experimental artifacts.

Keywords: 1H MRS; B0 inhomogeneity gradients; coherence pathways; spurious signals.

FIGURE 1

A, Schematic of excitation and refocusing slices and volume of interest (VOI; blue box) in an imperfect localization. B, Twenty‐five spatial regions are determined by the imperfect refocusing pulses. Yellow bands represent transition areas of the 180° pulses. C, Sequence diagram of the semi‐LASER sequence with optimized crusher scheme, assuming that the relatively long slice‐selective gradients act as crushers as well. AP, anterior–posterior; FH, feet–head; RL, right–left

FIGURE 2

A, Spectra from a phantom containing a small air bubble, using single‐voxel (SV) semi‐LASER (no phase cycling) and CSI with the same VOI localization as the single voxel. Compared with the VOI of the CSI, SV MRS exhibits some spurious signals indicated by red arrow. B, The MRS voxel localized on the shimmed B₀ map (PB‐2nd VOI shim). The possible origins of spurious signals were overlaid on representative slices of the B₀ map and color‐coded according to the number of pathways associated with these regions. These slices are within the CSI grid (shown in [C]) and visualize locations where the signal dephasing due to crusher gradients and B₀ field gradients canceled out, for those pathways. C, The CSI data confirmed the same locations of artifact (air bubble and the edge of the phantom) as the B₀ map analysis. The spectra were scaled to the SD of the noise

FIGURE 3

Resulting spectra and source of spurious signals (SoSS) maps of the phantom without air bubble from the same SV and CSI experiments as in Figure 2. The CSI and SoSS maps show no such artifacts as observed in the phantom with an air bubble. The spectra were scaled to the SD of the noise

FIGURE 4

Representative sagittal slices of the B₀ map (PB‐2nd VOI shim) inside and outside of the MRS voxel (white square). The voxels in red show the locations where the magnetic‐field inhomogeneity gradient potentially causes a spurious signal (Eq. 4) by canceling out the effect of external gradients through pathway [0 0 −1 −1 −1] for the crusher scheme in Figure 1C (A) and when the gradient polarity was inverted along the FH axis (B)

FIGURE 5

The SoSS maps of all possible pathways overlaid on the B₀ map (PB‐2nd VOI shim) for the optimized crusher scheme in Figure 1C (top row) and (the same scheme with inverted polarity of the gradients along FH direction bottom row). The SoSS maps show the spurious signal origins and the number of pathways (color‐coded) associated with them. At these voxels, ${\mathit{\varphi }}_{\text{crusher},i}$ satisfied Eq. 4 ($\Vert {\mathit{\varphi }}_{\text{inhom}.\mathrm{gr},i}(x,y,z;\mathrm{TE})\Vert \le \Vert {\mathit{\varphi }}_{\text{crusher},i}\Vert \le \Vert {\mathit{\varphi }}_{\text{inhom}.\mathrm{gr},i}(x,y,z;\mathrm{TE}+100\mathrm{ms})\Vert$). The red arrows indicate the locations where the associated number of pathways that could lead to potential spurious signals is greater than three (see Supporting Information Figure S2 for more example slices). The MRS voxel is shown as a black box

FIGURE 6

Spectrum acquired with the semi‐LASER scheme in Figure 1C with PB‐2nd VOI shim setting. A, Time and frequency‐domain signals from a voxel of 2 × 2 × 2 cm³. The resulting FID and spectrum after gradients polarity reversal are shown on top. B, Spectra from 3D CSI (blue box; number of signal averages [NSA] = 1, matrix size = 7 [AP] × 7 [RL] × 6 [FH]) with the same localization as SV MRS (green box; semi‐LASER, TE = 31 ms). Compared with SV MRS, the VOI spectrum exhibits less amount of artifacts. The red boxes depict the locations in CSI data from where possible spurious signals originate (Figure 4). Cho, choline; Cr, creatine; Glu, glutamate; NAA, N‐acetylaspartate

FIGURE 7

The effect of increasing the crushing area (scheme in Figure 1C) on the number of voxels being detected as potential sources of spurious signals based on PB‐2nd VOI shim and second‐order whole‐brain shim. The x‐axis values represent the factor by which the crushing area was increased. Crushing factors of 1 and 1.25 (first and second point in the graph) indicate the current total crusher area and current total crusher area × 1.25, respectively. For the crusher scheme in Figure 1C with TE = 31 ms, only the first four crushing factors would be practical