Surabhi Sood^{1}, David C Reutens^{1}, Shrinath Kadmangudi^{1}, Markus Barth^{1}, and Viktor Vegh^{1}

Quantitative susceptibility mapping is an MRI tool for mapping anatomical variations. The region specific echo time dependence of frequency shift curves computed from gradient recalled echo MRI data are likely due to variations in tissue microstructure, arrangement and packing. However, the effect of field strength on frequency shift curves has not been established to date. We investigated how frequency shift curves vary with field strength (3T versus 7T) and assessed how changes in the quantitative susceptibility mapping pipeline change the result. 7T data leads to less variability in frequency shift curves and, non-linear trends are present irrespective of methodological differences.

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Fig 1. The pipeline used to
compute quantitative susceptibility maps. Individual channel data were processed
using STI Suite and combined into a single image at the very end.
Susceptibility was then converted to a frequency shift value.

Fig 2. Depiction of brain
regions-of-interest used to compare frequency shifts as a function of echo
time.

Fig 3. Shown are
the signal magnitudes at 3T and 7T plotted over a log scale. Inset table shows signal
magnitude at time zero (S0),
the relaxation time (T2*) and quality-of-fit measure (r2)
using a single compartment mono-exponential model for both the 3T and 7T data. At each field
strength we also computed the average value and a measure of variation through
the coefficient of variation (CoV) metric. A larger variation across brain
regions can be seen with higher field strength.

Fig 4. Frequency shift values plotted for the corpus callosum, cerebrospinal
fluid (CSF) and thalamus using the Laplacian and path-based reconstruction
pipelines. The mean (solid line) and standard deviation (corresponding shaded
area) across the six participants are shown as well.

Fig 5. Frequency shift values plotted for the pallidum, caudate and putamen
using the Laplacian and path-based reconstruction pipelines. The mean (solid
line) and standard deviation (corresponding shaded area) across the six
participants are shown as well.