Photograph (a) and MR images of spherical homogeneity phantom for MFH measurement method acquired in MRI scanner in axial (b), sagittal (c), and coronal (d) orientations. Position shift is calculated by subtracting by the distances between the corresponding points across isocenter (arrows) in two different BW images from various DSV.
The diagram of the custom-designed program. First, image from MR scan was converted to BMP format, pixel value threshold was set as the cutoff value (from 0 to 255) and image view (the orientations of the images) was chosen by the user, Then, a new image with inverse color drawn by the software was shown. Artifacts and the pixels below the threshold value from the image were removed. If the user decides all the landmarks are visible, the coordinate of each landmark signed with a corresponding number was identified and the pixel distance was calculated. If not, the loop will go back to the point of setting the pixel value threshold until all the landmarks are visible. Finally, measured pixel distances and actual distance according to its matrix size were output into a Microsoft Excel spreadsheet.
The MFH was determined in a Philips MRI scanner using the method (diamonds) with a gradient-echo sequence compared with the data that manufacturer provided at the time that the system was reinstalled. The manufacturer determined MFH by measuring the frequency differences of the 144 locations at a DSV (circle). MFH values at 10, 20, 30, and were then calculated (squares). Error bars represent standard error of the mean (SEM).
The linewidth (triangles), (circle), and (squares) data were compared by using the MFH spherical phantom in a Siemens scanner. MFH (ppm) by linewidth method for from DSV, DSV, DSV– DSV, and using method from DSV, DSV– DSV compared to the method from DSV, DSV, DSV, DSV, DSV– DSV. Error bars represent .
MFH measurements from a Siemens MRI system obtained using the method and performed with the shim intentionally misadjusted with 100, 50, and off from central transmitter frequency using the -, -, and -gradient offsets. The six DSVs are at DSV, DSV, DSV, DSV, , and DSV. Error bars represent .
Phase shift (ppm) due to material susceptibility effect measured from phase shift map produced by a MRI system.
Scanning parameters used for the MFH measurements on various makes and models of MRI systems. FOV on all scans was .
MFH measurements by resonance frequency mapping, FWHM, , and method in different sites with different magnetic strength and manufacturers’ models.
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