A Method for Gradient Calibration in Diffusion Weighted Imaging
نویسندگان
چکیده
Introduction. Conventional gradient calibration procedures do not exclude significant bias in the diffusion-weighted (DW) signal during MR experiments. The error, arising from imperfections in the DW gradients, varies among the different gradient directions and exhibits different extent of offsets. We propose a calibration method for DW imaging using a homogeneous water phantom, presuming that its ADC profile is uniform and does not depends on the orientation of diffusion gradients. Our approach includes not only rescaling of the DW signal distributed among different diffusion encoding directions so as to minimise an angular error in ADC [1-3], but also an optimisation of balancing times for DW gradients among each diffusion direction. Such a technique corrects all possible diffusion gradient errors in a single protocol. The correction protocol was applied to a phantom and in vivo at 3T. Our results show the effectiveness of our approach and demonstrate that the described scheme of systematic error reduction is a valid method for quality control studies of gradient system performance for DW imaging. Methods and Results. In our study we used the twice-refocused spin echo sequence [4]. Our method involves high angular resolution DW imaging on a homogeneous water phantom and consists of two serial procedures: 1. First, the balancing times of the DW gradients for each diffusion gradient direction were optimised. This was achieved by measuring the DW signal for directions together with a variation of duration of one of the diffusion gradient lobes (See Figure 1: 4 n δ is a variable
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