Chemical Shift Misregistration and JModulation Artefacts

The spatial location of a signal is usually encoded by a volume-selective excitation. With three consecutive excitation pulses, the signal is first selectively excited in a slice, with a second pulse focused on a row inside the slice and finally selectively excited inside the volume of interest with a last, third pulse. The excitation performed by 90° and 180° excitation pulses used in a PRESS or STEAM sequence suffers from a frequency-dependent spatial misregistration known as the chemical shift error. While ideally all spins of every metabolite inside a VOI would be excited by the same pulse angle, the excitation varies across the metabolites and their localization. As a compromise NAA is often used as the reference for the VOI, so that all NAA protons are fully excited inside the selected VOI, while, for instance, the excited volume of metabolites right of the NAA would be shifted by a few millimetres to the right and upwards (Fig. 6.7). The size and location of chemical shift errors are influenced by a number of factors, including the field strength [35]. At higher magnetic fields, the selective pulses used for MRS volume localization go along with increased volume misregistration. This problem can be minimized by several techniques, where the use of outer volume suppression techniques with very selective saturation pulses has shown to be very promising [36].

J-Modulation anomalies for homonuclear-coupled resonances represent another difficulty more frequently encountered at higher magnetic fields. For molecules such as lactate, the extreme separation of the coupled resonances (a doublet at 1.33 ppm due to methyl protons and a quartet at 4.1 due to methine protons) will result in incomplete inversion of the coupled spin over a large portion of the selected volume, resulting in anomalous intensity losses, which are a function of position. This effect can be overcome by some methods, each presenting advantages and disadvantages [37].

Fig. 6.7. Voxel misregistration due to chemical shift error caused by spatially varying, frequency-dependent differences of excitation. The different metabolites are only in a subset of the whole excitation volume (red plus green area) and equally excited (green area), while certain metabolites are or are not excited in- or outside the ROI, which can have a significant impact on the achieved qualitative and quantitative results

Fig. 6.7. Voxel misregistration due to chemical shift error caused by spatially varying, frequency-dependent differences of excitation. The different metabolites are only in a subset of the whole excitation volume (red plus green area) and equally excited (green area), while certain metabolites are or are not excited in- or outside the ROI, which can have a significant impact on the achieved qualitative and quantitative results

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