Consequently, the obtained spectra can differ substantially from what would be expected without the localization errors both in terms of quality and appearance. An even more problematic situation occurs for strongly coupled metabolites where only a subset of the spins in a metabolite experience the intended slice selective RF pulses, thus giving rise to unexpected or absent J-coupling effects and potentially signal loss. Correspondingly, the different peaks in a spectrum will originate from slightly different regions in space. The larger chemical shift separation, therefore, also results in a more problematic localization of the volume-of-interest (VOI) since its position in space depends on the resonance frequencies. PRESS, the bandwidth of the RF pulses is relatively low. However, for the pulse sequences typically used clinically, e.g. High magnetic field strength, 3 T for the clinical setting, is beneficial for MRS as it provides a stronger signal and a wider separation of chemical shifts of resonances in absolute numbers, thus enabling separation of a larger number of metabolites. The acquisition methods and their implementation have also varied between vendors and hospitals, making it hard to compare results. However, although MR spectroscopy has been available since the advent of clinical MR, it has not yet become a widespread clinical tool mainly since both data acquisition and analysis requires specially trained personnel. to detect and stage brain tumors, determine tumor treatment response, and to study neurodegeneration and psychiatric disorders. It can be used in a broad range of applications e.g. Magnetic resonance spectroscopy (MRS) is a unique technique in the sense that it can provide insight into the cell metabolism completely non-invasively. If computational time is a limiting factor, simple simulations with hard RF pulses can provide almost as accurate metabolite quantification as those that include the chemical-shift related displacement. The inclusion of the shaped RF pulses and magnetic field gradients in the simulation of basis sets for semi-LASER is only important for strongly coupled metabolites. Metabolite quantification using semi-LASER was thereby less dependent on the inclusion of gradients than PRESS, which was seen in both phantom and in vivo measurements.

The difference was larger for strongly coupled metabolites and at longer echo times. The effect of including gradients in the simulations was smaller for semi-LASER than for PRESS, however, still noticeable.

For comparison, simulations and measurements were performed with the PRESS pulse sequence. The influence on metabolite concentration quantification was assessed using both phantom and in vivo measurements. MRS basis sets where simulated at different echo times with hard RF pulses as well as with shaped RF pulses without or with magnetic field gradients included. The width of the peak is inversely proportionate to T2* relaxation time.To study the need for inclusion of shaped RF pulses and magnetic field gradients in simulations of basis sets for the analysis of proton MR spectra of single voxels of the brain acquired with a semi-LASER pulse sequence. The area below the peak curve will basically be determined by metabolite concentration. This axis is orientated from right to left. Tetramethylsilane is used as the axis origin reference, a molecule used in in-vitro NMR, which is conventionally at 0 ppm. as an abscissa: metabolite position according to chemical shift.The spacing between these peaks has a fixed frequency value (Hz), called the J-coupling constant, independent of magnetic field amplitude. Interaction between the atomic nuclei of neighboring chemical groups translates as each peak breaking down into a complex peak (doublet, triplet, multiplet): this is spin-spin coupling. In chemical shift, the difference in frequency compared to the reference molecule is proportional to the amplitude of the B0 magnetic field. ωref: resonance frequency of the reference molecule.ωm: resonance frequency of the studied molecule.The value of the chemical shift thus provides information about the molecular group carrying the hydrogen nuclei.įor a given molecule, the chemical shift in ppm versus the reference molecule is defined by the relationship: The resonance frequency difference is expressed as parts per million or ppm, a value that is independent of the amplitude of the magnetic field.

The presence of an electron cloud constitutes an electronic shield that slightly lowers the B0 magnetic field to which the nucleus would normally be subjected. Chemical shift corresponds to a change in the resonance frequency of the nuclei within the molecules, in function of their chemical bonds.

Inmr shifts and j couplings written out