Field homogeneity, SNR and spectrum quality

Analysis of the differences in metabolite resonance frequency can only be performed in the presence of a highly homogenous magnetic field. A heterogeneous magnetic field leads to resonance frequency dispersion, spreading out the peaks or even causing them to disappear into the background noise.
Prior to any MRS acquisition, the magnetic field is homogenized (shimming) in the region of interest. The bigger the region, the harder it is to homogenize the magnetic field throughout. Close to the bone, calcifications or hemorrhagic zones, spectroscopic quality will be poorer due to perturbations in the field generated by the differences in magnetic susceptibility compared to soft tissue. The precession frequency of the water must be optimized to adequately suppress the water peak, using selective frequency pulses and dephasing gradients.
The other problem with MRS concerns the weak signal-to-noise ratio. This entails multiplying the number of measurements (NSA) and limits spatial resolution (voxel of a minimum volume of roughly 3.5 cm3 i.e. of dimensions 1.5 x 1.5 x 1.5 cm).

 

Spectrum quality is evaluated according to two main criteria:

• signal-to-noise ratio (height of metabolite peaks in relation to background noise).
• spectral resolution (peak width, which determines whether the different metabolites can be separated).

Spectral resolution will depend on the homogeneity of the magnetic field B0 and on digital resolution, i.e. the precision with which the signal is sampled, determined according to sampling time (Te = 1/Fe) and the total number of points measured.