On the Temporal Fidelity of Nonlinear Inverse Reconstructions for Real- Time MRI – The Motion Challenge

Jens Frahm,Shuo Zhang,Joachim Lotz,Dirk Voit,Markus Untenberger,Jan M. Sohns,Sebastian Schätz,K. Dietmar Merboldt,Martin Uecker

doi:10.2174/1874347101408010001

Abstract

Purpose: To evaluate the temporal accuracy of a self-consistent nonlinear inverse reconstruction method (NLINV) for real-time MRI using highly undersampled radial gradient-echo sequences and to present an open source framework for the motion assessment of real-time MRI methods. Methods: Serial image reconstructions by NLINV combine a joint estimation of individual frames and corresponding coil sensitivities with temporal regularization to a preceding frame. The temporal fidelity of the method was determined with a phantom consisting of water-filled tubes rotating at defined angular velocity. The conditions tested correspond to real-time cardiac MRI using SSFP contrast at 1.5 T (40 ms resolution) and T1 contrast at 3.0 T (33 ms and 18 ms resolution). In addition, the performance of a post-processing temporal median filter was evaluated. Results: NLINV reconstructions without temporal filtering yield accurate estimations as long as the speed of a small moving object corresponds to a spatial displacement during the acquisition of a single frame which is smaller than the object itself. Faster movements may lead to geometric distortions. For small objects moving at high velocity, a median filter may severely compromise the spatiotemporal accuracy. Conclusion: NLINV reconstructions offer excellent temporal fidelity as long as the image acquisition time is short enough to adequately sample (“freeze”) the object movement. Temporal filtering should be applied with caution. The motion framework emerges as a valuable tool for the evaluation of real-time MRI methods.

Highlights

While conventional magnetic resonance images represent direct reconstructions of the acquired data by inverse Fourier transformation, iterative reconstruction techniques only result in optimized estimations of the true spin-density distribution of an object
The results were obtained at 1.5 T using 40 ms temporal resolution (SSFP contrast) as well as at 3.0 T using 33 and 18 ms resolution (T1 contrast)
The use of a temporal median filter, which extends over 5 consecutive frames and in human cardiac studies helped to remove residual streaking artifacts, partly compromises the correct geometrical structure. This applies to high velocities (7.5 and 10 cm s-1) at Temporal fidelity of real-time MRI

Summary

Introduction

While conventional magnetic resonance images represent direct reconstructions of the acquired data by inverse Fourier transformation, iterative reconstruction techniques only result in optimized estimations of the true spin-density distribution of an object. In recent years, such mathematical approaches found increasing use for MRI reconstructions from undersampled datasets with applications ranging from parallel imaging, e.g. Rather than detailing the general mathematical properties of regularized iterative optimization problems – which for MRI applications are often ill-conditioned and not even convex – the rationale of this work is to provide experimental evidence for conditions that correspond to a realistic clinical imaging scenario, namely cardiovascular MRI in real time

Methods

Results

Conclusion