Hexagonal undersampling for faster MRI near metallic implants

Abstract

Purpose Slice encoding for metal artifact correction (SEMAC) acquires a 3D image of each excited slice with view-angle tilting to reduce slice and readout direction artifacts respectively, but requires additional imaging time. The purpose of this study is to provide a technique for faster imaging around metallic implants by undersampling k-space. Methods Assuming that areas of slice distortion are localized, hexagonal sampling can reduce imaging time by 50% compared with conventional scans. This work demonstrates this technique by comparisons of fully sampled images with undersampled images, either from simulations from fully acquired data or from data actually undersampled during acquisition, in patients and phantoms. Hexagonal sampling is also shown to be compatible with parallel imaging and partial Fourier acquisitions. Image quality was evaluated using a structural similarity index (SSIM). Results Images acquired with hexagonal undersampling had no visible difference in artifact suppression from fully sampled images. The SSIM index indicated high similarity to fully sampled images in all cases. Conclusion The study demonstrates the ability to reduce scan time by undersampling without compromising image quality.

Accelerating fully phase‐encoded MRI near metal using multiband radiofrequency excitation

“…Field‐of‐view reduction strategies near metal that were not implemented in the present study may prove helpful, such as adaptive phase encoding , slab selection , and/or changing slab selection gradients between excitation and refocusing . Recently proposed reconstruction and sampling strategies tailored for 3D‐MSI acquisitions may allow higher acceleration factors: hexagonal undersampling of the ACR combined with a masking operation to eliminate the aliased signal , exploiting the unique spatial distribution of signal encoded for each RF offset in a manner analogous to parallel imaging , and an ultrawide excitation bandwidth calibration scan that allows externally calibrated parallel imaging, eliminating the need for self‐calibration (and therefore a fully sampled k ‐space region) at each RF offset .…”

Externally calibrated parallel imaging for 3D multispectral imaging near metallic implants using broadband ultrashort echo time imaging

“…Optimization of pulse‐sequence design, such as strategies to reduce the FOV and optimization of multiband excitations , are complementary methods that could be used to further accelerate acquisitions. Furthermore, improvements in reconstruction techniques through the use of compressed sensing , hexagonal undersampling , and/or off‐resonance encoding could also accelerate acquisitions.…”

“…Optimization of pulse-sequence design, such as strategies to reduce the FOV (8,23) and optimization of multiband excitations (16), are complementary methods that could be used to further accelerate acquisitions. Furthermore, improvements in reconstruction techniques through the use of compressed sensing (12,24), hexagonal undersampling (21), and/or off-resonance encoding (14) could also accelerate acquisitions. Acquisition of a single high-resolution fully phaseencoded UTE acquisition rather than a low-resolution calibration and a 3D-MSI acquisition may also be a viable option for in vivo imaging near metal (25).…”

Accelerated three‐dimensional multispectral MRI with robust principal component analysis for separation of on‐ and off‐resonance signals

“…Analogous to the original CLEAR method, Equation [9] is shown to be the product of a full-rank matrix and a low-rank matrix R b diagðcos ð/X 0 ÞÞS diagðsin ð/X 0 ÞÞS ½ , which can be viewed as 2N C real-valued channel sensitivities. A modifed regularizer is…”