Robust fully automated shimming of the human brain for high‐field ¹H spectroscopic imaging

Abstract

Although a variety of methods have been proposed to provide automated adjustment of shim homogeneity, these methods typically fail or require large numbers of iterations in vivo when applied to regions with poor homogeneity, such as the temporal lobe. These limitations are largely due to 1) the limited accuracy of single evolution time measurements when full B 0 mapping studies are used, and 2) inaccuracies arising from projectionbased methods when the projections pass through regions where the inhomogeneity exceeds the order of the fitted parameters. To overcome these limitations we developed a novel B 0 mapping method using multiple evolution times with a novel unwrapping scheme in combination with a user-defined ROI selection tool. We used these methods at 4T on 10 control subjects to obtain high-resolution spectroscopic images of glutamate from the bilateral hippocampi. A variety of rapid automated shim mapping methods for the human brain based on acquisition of a limited number of columnar projections using phase mapping have been described for small localized volumes (1,2). Shen and colleagues (3,4) demonstrated the need for alternate projection geometries when shimming planar volumes. Although these methods have been successfully applied in regions of good homogeneity, such as the occipital lobe for single volume measurements (1,2) and for slices above the corpus callosum (3), they can fail or require large numbers of iterations in vivo when applied to regions with very poor B 0 homogeneity adjacent to air-filled sinuses, such as the temporal lobes. Intrinsic to these maps is the assumption that the B 0 inhomogeneity present in the entire sample or region of interest (ROI) can be accurately represented by the limited number of columns acquired, and that the columns are sufficiently narrow for there to be negligible inhomogeneity across them. However, this assumption fails when the spatial order of the inhomogeneity present (i.e., the order of the spherical harmonic terms that characterize the spatial symmetry of the inhomogeneity) exceeds the spatial order that is used or measurable in the columnar analysis model, or when the inhomogeneity changes very rapidly in relationship to the cross-sectional width of the column (5). In this case lower-order corrections in localized regions are then used to attempt to compensate for higher-order corrections that do not have the same spatial symmetry. The calculated shim corrections can either over-or underestimate the size of the adjustments required to provide "optimal" homogeneity over the entire target volume given the available currents.Alternatively, shim methods that utilize complete maps (5-9) as opposed to columnar projections can provide accurate maps over large regions without assumptions regarding the order of the inhomogeneity present. In these cases the residual inhomogeneity is minimized by using information from sampling a much greater extent of the target volume. For B 0 mapping, typically a single, relatively short phase evolution time is used becau...

The neurochemical profile quantified by in vivo 1H NMR spectroscopy

“…Therefore, automatic adjustments of these shim terms or even higher-order shim terms (Gruetter, 1993;Gruetter and Boesch, 1992;Gruetter and Tkáč, 2000;Hetherington et al, 2006;Miyasaka et al, 2006;Schneider and Glover, 1991;Shen et al, 1999;Zhang et al, 2009) are necessary for studying the most common VOI in the rodent brain.…”

Proton Magnetic Resonance Spectroscopy of the Central Nervous System

“…This is why most devices come equipped with second or third order shimming by monitoring either the time domain or frequency domain of the 1 H-MRS signal [48]. Some times 4-order shimming might be necessary [49], especially in cases when field homogeneity should be reached in large volumes of interest during magnetic resonance spectroscopic imaging (MRSI).…”

“…Methods that have been developed for field mapping can be grouped in two categories: those which are based on 3D field mapping [49] and those which map the magnetic field along projections [50]. In both shimming methods, information about the magnetic field variation is calculated from phase differences acquired during the evolution of the magnetization in a non-homogeneous field.…”

Registered Bioimaging of Nanomaterials for Diagnostic and Therapeutic Monitoring

“…This is done by mapping the field distribution and calculation of a precisely determined spatial correction based on measured responses of a series of electromagnets or "shim coils" within the system designed to adjust the field homogeneity. Shimming is accomplished using a multi-gradient echo sequence and mapping software developed by Dr. Hetherington 13 . Regions of homogeneity are matched to the region examined by each individual imaging method.…”