Reducing distortions in echo‐planar breast imaging at ultrahigh field with high‐resolution off‐resonance maps

Abstract

Purpose DWI is a promising modality in breast MRI, but its clinical acceptance is slow. Analysis of DWI is hampered by geometric distortion artifacts, which are caused by off‐resonant spins in combination with the low phase‐encoding bandwidth of the EPI sequence used. Existing correction methods assume smooth off‐resonance fields, which we show to be invalid in the human breast, where high discontinuities arise at tissue interfaces. Methods We developed a distortion correction method that incorporates high‐resolution off‐resonance maps to better solve for severe distortions at tissue interfaces. The method was evaluated quantitatively both ex vivo in a porcine tissue phantom and in vivo in 5 healthy volunteers. The added value of high‐resolution off‐resonance maps was tested using a Wilcoxon signed rank test comparing the quantitative results obtained with a low‐resolution off‐resonance map with those obtained with a high‐resolution map. Results Distortion correction using low‐resolution off‐resonance maps corrected most of the distortions, as expected. Still, all quantitative comparison metrics showed increased conformity between the corrected EPI images and a high‐bandwidth reference scan for both the ex vivo and in vivo experiments. All metrics showed a significant improvement when a high‐resolution off‐resonance map was used ( P < 0.05), in particular at tissue boundaries. Conclusion The use of off‐resonance maps of a resolution higher than EPI scans significantly improves upon existing distortion correction techniques, specifically by superior correction at glandular tissue boundaries.

Analysis and correction of off‐resonance artifacts in echo‐planar cardiac diffusion tensor imaging

“…18,30 A challenge of in vivo off-resonance correction relates to a lack of ground truth. Although various strategies have been evaluated, such as image registration to an undistorted reference 25 or assessing alignment of hand-drawn contours, 49 these methods have limitations themselves. Using the MRXCAT numerical phantom, we were able to evaluate the reconstruction method with a phantom under realistic measurement conditions and compare the outcome to a ground truth reference.…”

“…The occurrence of localized field gradients in the myocardium poses another challenge for off‐resonance correction, namely that an accurate representation of the field map during data acquisition is required. Because the source of object stretching or compression relates to intravoxel field gradients, the spatial resolution of the field map needs to be higher than that of the acquired data 25,26 . To this end, various strategies, which have been proposed to estimate the field map from the actual data by altering the EPI blip direction (down‐up or up‐down), 27,28 would not be suitable.…”

“…Because the source of object stretching or compression relates to intravoxel field gradients, the spatial resolution of the field map needs to be higher than that of the acquired data. 25,26 To this end, various strategies, which have been proposed to estimate the field map from the actual data by altering the EPI blip direction (down-up or up-down), 27,28 would not be suitable. If a high-resolution (HR) field map is obtained separately, SNR and spatial misalignment between field map and cDTI data must be considered during off-resonance correction.…”

Qualitative and quantitative comparison of image quality between single-shot echo-planar and interleaved multi-shot echo-planar diffusion-weighted imaging in female pelvis

“…Since the reconstruction with phase correction is basically based on parallel imaging principles, extra navigator is usually needed for high shot numbers such as 6 shots in order to maintain a reliable performance. The multi-shot DWI techniques, including interleaved EPI or readout-segmented EPI [11], have been applied in brain and other body organs with promising results [12][13][14][15][16].…”

Properties of polycrystalline nanoparticles with uniaxial and cubic types of magnetic anisotropy of individual grains

“…Meanwhile, the formation of polycrystalline particles in the course of chemical reactions is quite likely [6], if the growth of large nanoparticles occurs as a result of the coalescence of single-crystal nuclei. It has been found recently, that monodisperse iron oxide nanoparticles of sufficiently large sizes prepared by thermal decomposition of organometallic precursors [7] or other methods [8][9][10][11] may have complicated crystalline structure. Multi-core nanoparticles [12,13] consisting of monocrystalline magnetic grains can also be considered as polycrystalline nanoparticles if there is exchange interaction of appreciable value between the constituting grains.…”