Abstract
Diffusion-weighted EPI has become an indispensable tool in body MRI. Geometric distortions due to field inhomogeneities are more prominent at large field–of–view and require correction for comparison with T2W TSE. Several known correction methods require acquisition of additional lengthy scans, which are difficult to apply in body imaging. We implement and evaluate a geometry correction method based on the already available non phase-encoded EPI reference data used for Nyquist ghost removal. The method is shown to provide accurate and robust global geometry correction in the absence of strong, local phase offsets. It does not require additional time for calibrations and is directly compatible with parallel imaging methods. The resulting images can serve as improved starting point for additional geometry correction methods relying on feature extraction and registration.
Highlights
Diffusion-weighted (DW) EPI provides improved specificity in oncologic body MRI beyond T2W TSE sequences [1]
To evaluate the efficacy of the proposed geometry correction method, we use a 1–dimensional numerical phantom consisting of 3 parts: a central part with 80% water and 20% fat (ΔfF = 440 Hz at 3T), two equal intensity outer parts representing 100% fat, and a narrow water-only structure representing a susceptibility artifact
EPI geometry correction using full characterization of the spatial frequency offset has been investigated by several original image corr image (a) original image fs corr image (b) original fatsat image fatsat fs corr image fatsat (c) no fat 20% fat
Summary
Diffusion-weighted (DW) EPI provides improved specificity in oncologic body MRI beyond T2W TSE sequences [1]. Global and local geometric distortions inherent to DW EPI often complicate diagnostic combination of DW and T2W data. These EPI artifacts result from static or dynamic field variations, which add a phase error relative to the slow phase evolution in the ky direction, i.e., along the blip gradients [2]. Diffusion gradients induce eddy currents from which phase is accumulated during the EPI acquisition, resulting in shear, stretch and compression, and displacements in the images [3]. Compensation is provided by hardware improvements or sequence modifications to reduce the eddy currents, or by image registration approaches; for an overview see [4, 5]. Given typical bandwidths along ky of 20–30 Hz per pixel, image distortions of 3–5 pixels (global) up to more than 10 pixels (local) are observed and require additional correction strategies
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