![]() Basis sets have historically been acquired experimentally where metabolite-specific phantoms were produced and measurements with the exact pulse sequence parameters were made. One way of doing this is to obtain a set of high-quality spectra for each metabolite separately to be used as a basis for the subsequent analysis. However, in more realistic cases of coupled and overlapping resonances, prior information must be incorporated into the analysis. In simplistic cases with well-separated metabolite resonances it may be possible to obtain these concentrations simply by calculating the area under a peak of interest in a given interval. Īnalysis of MRS data typically revolves around translating an MR spectrum into (relative) tissue concentrations of different metabolites. The most clinically relevant implementation is the semi-LASER pulse sequence, which can achieve echo times almost as short as PRESS, but with a substantially reduced localization error. PRESS, it has been suggested to use adiabatic RF pulses, especially for the refocusing pulses which typically have the lowest bandwidth and thereby give rise to the largest localization errors. To mitigate the problems related to localization errors with e.g. 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. I understand that the strong delocalisation in the aromatic ring may tend to "even out" the electron distribution across all the aromatic $\ce $.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. Clearly, the aromatic protons are in different chemical environments due to the fact that they are different distances away from the substituents from the benzene ring. Recently, I have started to learn about nuclear magnetic resonance (NMR) in school and something which I cannot seem to reconcile is the fact that all aromatic protons on any substituted benzene ring would give the same chemical shift.
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