Spatiotemporal encoding MRI in a portable low-field system.

PURPOSE: Demonstrate the potential of spatiotemporal encoding (SPEN) MRI to deliver largely undistorted 2D, 3D, and diffusion weighted images on a 110 mT portable system. METHODS: SPEN's quadratic phase modulation was used to subsample the low-bandwidth dimension of echo planar acquisitions, delivering alias-free images with an enhanced immunity to image distortions in a laboratory-built, low-field, portable MRI system lacking multiple receivers. RESULTS: Healthy brain images with different SPEN time-bandwidth products and subsampling factors were collected. These compared favorably to EPI acquisitions including topup corrections. Robust 3D and diffusion weighted SPEN images of diagnostic value were demonstrated, with 2.5 mm isotropic resolutions achieved in 3 min scans. This performance took advantage of the low specific absorption rate and relative long TEs associated with low-field MRI. CONCLUSION: SPEN MRI provides a robust and advantageous fast acquisition approach to obtain faithful 3D images and DWI data in low-cost, portable, low-field systems without parallel acceleration.

Understanding aliasing effects and their removal in SPEN MRI: A k-space perspective.

PURPOSE: To characterize the mechanism of formation and the removal of aliasing artifacts and edge ghosts in spatiotemporally encoded (SPEN) MRI within a k-space theoretical framework. METHODS: SPEN's quadratic phase modulation can be described in k-space by a convolution matrix whose coefficients derive from Fourier relations. This k-space model allows us to pose SPEN's reconstruction as a deconvolution process from which aliasing and edge ghost artifacts can be quantified by estimating the difference between a full sampling and reconstructions resulting from undersampled SPEN data. RESULTS: Aliasing artifacts in SPEN MRI reconstructions can be traced to image contributions corresponding to high-frequency k-space signals. The k-space picture provides the spatial displacements, phase offsets, and linear amplitude modulations associated to these artifacts, as well as routes to removing these from the reconstruction results. These new ways to estimate the artifact priors were applied to reduce SPEN reconstruction artifacts on simulated, phantom, and human brain MRI data. CONCLUSION: A k-space description of SPEN's reconstruction helps to better understand the signal characteristics of this MRI technique, and to improve the quality of its resulting images.

Pseudo multishot echo-planar imaging for geometric distortion improvement.

Conventional echo-planar imaging (EPI) uses a radiofrequency pulse for excitation and a prolonged echo train to sample k space, while off-resonance and T2 * decay effects caused by magnetic susceptibility variation accumulate within each echo, leading to geometric distortion. Multishot EPI methods, which divide k space into segments, can shorten the effective echo spacing and reduce the distortion on EPI images. But multiple shots cost longer scan time and render susceptibility to motion. In this study, we propose a new "multishot" EPI method termed pseudo multishot EPI (pmsEPI), in which phase-encoding lines are segmented as in multishot EPI but are collected within a single shot. With the magnetization divided into different pathways via interleaved excitation instead of refocusing in a single long echo train, the total phase error accumulation is reduced in each segmented acquisition, thereby improving distortion of the resultant EPI image. The performance of the pmsEPI method is demonstrated by phantom and in vivo brain experiments on a 3-T scanner. The experimental results show that the distortion displacements of pmsEPI acquisition compared with conventional EPI decrease by 50% with two pseudo shots and 66% with three pseudo shots, validating the ability of the method to obtain images with reduced distortion in a single shot, although magnetization splitting may induce more than 40% SNR loss and minor artifacts. Specifically, the ability of pmsEPI in diffusion-weighted imaging with different trajectory options is highlighted, and the flexibility is demonstrated in a single-shot blip up and down acquisition.

High-resolution multi-shot diffusion-weighted MRI combining markerless prospective motion correction and locally low-rank constrained reconstruction.

PURPOSE: Subject head motion is a major challenge in DWI, leading to image blurring, signal losses, and biases in the estimated diffusion parameters. Here, we investigate a combined application of prospective motion correction and spatial-angular locally low-rank constrained reconstruction to obtain robust, multi-shot, high-resolution diffusion-weighted MRI under substantial motion. METHODS: Single-shot EPI with retrospective motion correction can mitigate motion artifacts and resolve any mismatching of gradient encoding orientations; however, it is limited by low spatial resolution and image distortions. Multi-shot acquisition strategies could achieve higher resolution and image fidelity but increase the vulnerability to motion artifacts and phase variations related to cardiac pulsations from shot to shot. We use prospective motion correction with optical markerless motion tracking to remove artifacts and reduce image blurring due to bulk motion, combined with locally low-rank regularization to correct for remaining artifacts due to shot-to-shot phase variations. RESULTS: The approach was evaluated on healthy adult volunteers at 3 Tesla under different motion patterns. In multi-shot DWI, image blurring due to motion with 20 mm translations and 30° rotations was successfully removed by prospective motion correction, and aliasing artifacts caused by shot-to-shot phase variations were addressed by locally low-rank regularization. The ability of prospective motion correction to preserve the orientational information in DTI without requiring a reorientation of the b-matrix is highlighted. CONCLUSION: The described technique is proved to hold valuable potential for mapping brain diffusivity and connectivity at high resolution for studies in subjects/cohorts where motion is common, including neonates, pediatrics, and patients with neurological disorders.

Renal response and its predictive factors of lupus nephritis: a 2-year real-world study of 56 hospital-based patients.

OBJECTIVES: Our aim was to evaluate the renal response rate of patients with lupus nephritis (LN) undergoing standard treatment during a 2-year follow-up and to investigate its predictive factors. METHODS: A prospective cohort study that enrolled 56 clinically diagnosed LN patients with urinary protein positivity was carried out. All patients underwent standard treatment. All patients were followed up at 6-month intervals for 2 years. Data on renal response and clinical characteristics were collected and analyzed. RESULTS: Among 56 patients, 27 (48.2%) and 13 (23.2%) patients achieved complete renal response (CR) and partial renal response (PR) at 6 months after induction therapy, respectively, and 42 (75.0%) and 4 (7.1%) patients developed CR and PR at 2 years. Among patients who achieved PR at 6 months, 90.0% achieved CR at 24 months, while only 37.5% of the patients who were unresponsive at 6 months achieved CR. In the multivariable Cox proportional-hazards model, female (OR 6.51, 95% CI 1.23-34.52, p = 0.028), disease duration (OR 0.84, 95% CI 0.73-0.98, p = 0.021), achieving PR within 6 months (OR 8.09, 95% CI 2.06-31.73, p = 0.003), and urine protein/creatinine ratio (UPCR) (OR 0.998, 95% CI 0.996-1.000, p = 0.025) were found to be predictive factors of CR. CONCLUSION: A total of 48.2% of patients achieved CR at 6 months of induction therapy, and the response rates gradually increased to 60.7%, 64.3%, and 75.0% at 12, 18, and 24 months. Besides, female, disease duration, partial response within 6 months, and UPCR were predictive factors for a complete renal response. Key Points • We evaluate the renal response rates in Chinese patients with lupus nephritis in the real world for 2 years. • A total of 48.2% and 75.0% of patients achieved a complete response after standard treatment for 6 months and 2 years. • Female, disease duration, partial response within 6 months, and UPCR are predictors of complete renal response.