Shimming is an iterative adjustment process to improve magnetic field homogeneity within a spectroscopic volume. First order shimming is now entirely automated on clinical scanners, but occasionally second order (manual) shimming may be needed. As shown left, poor shimming can be recognized by spectral lines that are too wide, short, and poorly separated.
Incomplete water suppression
Because metabolites of interest exist in such low concentrations in tissues, suppression of water signal is an essential component of nearly all ¹H spectroscopic examination. When water suppression fails or proves inadequate, the metabolite spectra may be inapparent or lost in background noise.
The time domain NMR signal in a spectroscopy experiment is typically considered to be an exponentially decaying sine wave or FID (Free Induction Decay). After Fourier transformation, the FID becomes a complex-valued function in the frequency domain known as a Lorentzian. Lorentzian functions have both "real" and "imaginary" components in a fixed 90° (quadrature) relationship to one another. The relative values of these two components, however, depend on the initial (and generally unknown) phase offset (φ) of the FID. In the ideal case where φ = 0, the real component of the Lorentzian has the desired shape of a narrow, upright spectral peak known as the absorption mode lineshape. The imaginary component becomes an generally undesirable biphasic waveform known as the dispersion mode lineshape.
Chemical shift displacement artifacts between metabolites in MRS are identical in nature to familiar fat-water artifacts seen on conventional MRI. Consider, for example, a brain voxel containing lactate (Lac) and myo-inositol (mI). Their chemical shift difference is 2.3 ppm, which translates into a frequency difference of about 300 Hz at 3.0T. If the transmit RF-bandwidth defining the voxel were 1500 Hz, then the two metabolites would be spatially mismapped about 20% (300/1500) relative to one another. Because this displacement would occur in all 3 directions, only (1−0.20)³ = 51% overlap would occur in the entire voxel, even though the two resonances should coincide exactly. Chemical shift displacement artifacts are most problematic with single voxel spectra at higher magnetic fields. The general solution is to employ larger bandwidth RF pulses and stronger imaging gradients.
Advanced Discussion (show/hide)»
The chemical shift displacement artifact not only affects the relative positions of metabolites, but also their position relative to the localizer grid (which is placed on the relevant anatomy based on the water image.) Accordingly, the frequency of the slice-selective RF pulses is typically offset from the center (water) frequency by about −2 ppm, which is a mid-value for the range of ¹H metabolites measured.
Spectral contamination artifacts are made worse by the use of elliptical, centrally weighted, or partial k-space sampling commonly employed to reduce imaging times in 2D- or 3D-CSI studies.
One method to reduce spectral contamination is to multiply the time-domain (FID) data by an apodization function prior to Fourier transformation. Apodization, from the Greek, literally means "removing the feet" (i.e. tails of the signal). An apodization function causes the data at the edge of a measurement volume to decay more gradually and symmetrically. Although many different apodization functions exist, the Hamming filter is the most frequently used for MR spectroscopy.
Bottomley PA. The trouble with spectroscopy papers. Radiology 1991; 181:344-350. (An old but classic reference by one of the fathers of clinical MRS. Many of the points he makes are still valid 25 years later.)
Cianfoni A, Law M, Re TJ, et al. Clinical pitfalls related to short and long echo times in cerebral MR spectroscopy. J Neuroradiol 2011; 38:69-75. (pitfalls in interpretation of brain spectra, not really artifacts, but worth a look)
Juchem C, de Graaf R. Bo magnetic field homogeneity and shimming for in vivo magnetic resonance spectroscopy. Anal Biochem 2016; 1-13.
Kreis R. Issues of spectral quality in clinical 1H-magnetic resonance spectroscopy and a gallery of artifacts. NMR Biomed 2004;17:361-381. (excellent review of artifacts, also discussion of SNR)
Murali N. Fourier Transform. Notes for Lecture 3, Chem 542, NMR spectroscopy: Principles and applications. Spring, 2010. Available at this link. (an excellent description of absorption and dispersion spectra and mathematical derivation of the Lorentzian).
What is a Gibbs artifact?
What is a chemical-shift artifact?
Why and how do you suppress water signal in MRS?
What is shimming and why is it needed?