Abstract
To the Editor:
In susceptibility-weighted imaging (SWI), the contrast in magnitude gradient echo images is enhanced by multiplication with a mask generated from the phase information [1], [2]. The technique has proven useful for imaging of veins and venules in virtue of the local magnetic field changes in the presence of paramagnetic elements that affect the measured MR phase [3]. The actual value of the phase will not only depend on factors related to the geometry of the brain and vessel structures and their orientation with respect to the external static magnetic field B0 or the imaging parameters (such as TE, sampling bandwidth and voxel aspect ratio as described in Refs. [4], [5]), but will also be affected by the sum effect of hardware differences in different vendor systems, like the phase relationship between transmission and receiver lines, the sign relation between the direct and the delayed line in the demodulator, as well as the sign of the actual gradient applied during read out, besides possible postprocessing of the incoming signal before or after Fourier transform. The final result is that a system is going to be one of two ways in regard to the phase sign convention. The aim of this letter was to show how all these factors related to different parts of the MR system can be summed up in two frameworks for describing spin behavior: the rotating frame or complex plane descriptions. Moreover, we highlight that the desired improvement in SWI contrast can only be achieved if the vendor setting has been taken into account.
