High resolution phase gradient mapping as a tool for the detection and analysis of local field disturbances

Introduction Since the early days of MRI phase mapping has been used to study object induced perturbations of the phase of the MR signal. Usually these phase perturbations result in signal loss due to signal dephasing. In the context of the detection of, for example, paramagnetic or diamagnetic contrast agents there is much interest in selectively turning this dark contrast into a positive contrast, as well as in quantifying the susceptibility effect. Special sequences have been developed to selectively obtain positive contrast [1, 2]. These sequences, however, in general need information on the local field to achieve the desired positive contrast [1, 2]. In previous work of other groups the echo shift phenomenon was used to obtain positive contrast on local field distortions, without the need of a special sequence or prior knowledge [3]. Disadvantages of this technique include a reduction of the resolution and absence of error measures, other than obtained values for the contrast to noise. The technique presented in this abstract does not use the echo shift artifact, but the signal itself to determine the phase gradient [4]. The proposed technique does not need phase unwrapping, maintains resolution and provides simple error measures. Experimental validation of the method is done with a coaxial cylinder phantom with a known susceptibility distribution. We further show the applicability of the technique in depicting and analyzing the distribution of holmium-loaded microspheres [5] in an ex-vivo rabbit liver.