Figure 1.
Best sets of 5 SNR′0 values for which the bias in κ and 1/λ is minimized, as determined from Monte Carlo simulations.
The results for different parameters λ = 1/90, κ = 1.4/1.8, SNR′0 ranges = 50–600/50–300 are presented as histograms of the top 5% best sets of SNR′0 in (A–D). Acq # orders the 5 acquisitions according to their SNR′0. It can be seen that these best sets sample low and high SNR’ values more densely. For the small SNR′0 range (C, D), high SNR′0 values are more strongly represented, in order to better estimate 1/λ.
Figure 2.
Validation of extended model fit for phantom data acquired at 3T using a 32-channel RF head receive coil.
Results for a single central slice through phantom data show maps of A) tSNR, B) SNR′0 C) κ, D) SNR0 E) SNR′0/κ F) SNR′0/(κ ·SNR0). The tSNR and SNR maps are shown for the EPI data acquired with the largest flip angle (90°). The κ map was estimated by fitting the extended model to each voxel of the tSNR and SNR′0 maps calculated from EPI time series acquired at 3 different flip angles (17°, 37° and 90°). In G, a plot of the mean and SD of tSNR against SNR′0 (red squares), SNR0 (green triangles) and SNR′0/κ (blue circles) within a 20×20 voxels ROI at the centre of the phantom is shown for all flip angle time series and the red solid line shows the fit of the extended model to the SNR′0 values. The black dashed line is the line of identity.
Figure 3.
Fit of extended model to human task-free fMRI data acquired at 7T with a 24-channel receive head coil.
A) Plot of tSNR against SNR′0 is shown for EPI time series acquired using 5 different flip angles (8°, 16°, 26°, 38° and 70°). Each data point (black squares) represents the mean and standard error over 5 subjects of mean tSNR and SNR′0 in the VC ROI. The solid red line represents the fit of the extended model (using equation 6) and the blue dashed line represents the fit of the original model (using equation 3). B) Plot of tSNR against the independently measured SNR0 for the same data as in (A). Black triangles represent the mean and standard error over 5 subjects, the solid green line represents the fit of the extended model (using equation 6) and the blue dashed line represents the fit of the original model (using equation 3).
Figure 4.
Subject specific maps of κ estimated from fit of extended model to human task-free fMRI data acquired at 7T with a 24-channel receive head coil (bottom row).
The top row shows the corresponding subject specific EPI images acquired at the 70° flip angle.
Table 1.
Parameters estimated from fit of extended model (equation 6) and original model (equation 3) to human task-free fMRI data acquired at 7T with a 24-channel receive head coil (mean ± standard deviation) and sum-of-squares errors (SSE) indicating goodness-of-fit of the models.