In this lecture, the most common pre-scan adjustments for MRS and MRSI will be discussed, as well as the most recent and upcoming developments.
For MRS studies, the pre-scan adjustments are an important factor for achieving best quality and reproducability.
Failing of even one of the adjustments can lead to severe loss in spectral quality.To perform a correct pre-scan adjustment, the user is required to monitor the different preparation steps before starting the actual experiment. Correct preparation of an MRS acquisition requires knowledge of the following steps:
As a first step, the B0 field optimization or shimming is typically performed to homogenize the background static magnetic field. This is needed to improve the linewidth in the MRS spectra.
The background B0 field gets disturbed by the variations in susceptibility in the tissue, and is therefore performed for every subject and scan location. Many different sequences exist for performing this adjustment. The most commonly used and most robust are a measurement of the B0 field through projections1 and measurement of a 3D field map. From this data, the correction terms for the shim coils are calculated to perform an appropriate compensation of the field.
During an experiment, the B0 field can also show temporal variations, due to for example heating/cooling of the scanner or due to physiological sources. This can be corrected for with real-time updating based on MR measurements2 or external monitoring equipment3.
Another pre-acquisition adjustment is the frequency adjustment (F0). This is a quick measurement of the water resonance frequency after the B0 shimming in a reference volume. For single-voxel MRS, this reference volume is typically the MRS voxel, or a slightly larger area that includes that. For MRSI, the frequency reference should also be taken inside the region of interest.
For some sequence types, for example MEGA-editing, special care should be taken to accurately determine the resonance frequency, as already a small offset for the reference frequency can have a large effect on the editing effiency.
With MRS of other nuclei than 1H, the signal intensity is too low to do a fast reference measurement. Here also the 1H water frequency is typically used to determine the resonance frequency through the known ratio of the gamma for 1H and the nucleus of interest.
B1 field shimming/flip angle calibration
In the pre-acquisition adjustment it is important to set the correct flip angle for the localization, water suppression and possible saturation RF pulses. This is performed as a pre-scan calibration where the flip angle is measured and adjusted, preferably in the same volume as the MRS voxel4.
When multi-channel transmit coils are used, a more elaborate calibration of the transmit field is required, where B1 field maps5-7 are acquired, and the correct drive for the channels is determined. This can be either a single setting for the whole scan8,9, a setting per pulse10, or even a variation within the RF pulses11.
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11. Ma J, Wismans C, Zhipeng C, Klomp DWJ, Wijnen JP, Grissom WA. Tailored spiral in‐out spectral‐spatial water suppression pulses for magnetic resonance spectroscopic imaging. Magn Reson Med. 2018 Jan;79(1):31-40