Combining prospective motion correction and distortion correction for EPI: towards a comprehensive correction of motion and susceptibility-induced artifacts

Object  

State-of-the-art MR techniques that rely on echo planar imaging (EPI), such as real-time fMRI, are limited in their applicability by both subject motion and B₀ field inhomogeneities. The goal of this work is to demonstrate that in principle it is possible to accurately predict the B₀ field inhomogeneities that occur during echo planar imaging in the presence of large scale head motion and apply this knowledge for distortion correction.     

Cardiac magnetic resonance T1 and extracellular volume mapping with motion correction and co-registration based on fast elastic image registration

Objective

Our aim was to investigate the technical feasibility of a novel motion compensation method for cardiac magntic resonance (MR) T1 and extracellular volume fraction (ECV) mapping.

Materials and methods

Native and post-contrast T1 maps were obtained using modified look-locker inversion recovery (MOLLI) pulse sequences with acquisition scheme defined in seconds. A nonrigid, nonparametric, fast elastic registration method was applied to generate motion-corrected T1 maps and subsequently ECV maps. Qualitative rating was performed based on T1 fitting-error maps and overlay images. Local deformation vector fields were produced for quantitative assessment. Intra- and inter-observer reproducibility were compared with and without motion compensation.

Results

Eighty-two T1 and 39 ECV maps were obtained in 21 patients with diverse myocardial diseases. Approximately 60% demonstrated clear quality improvement after motion correction for T1 mapping, particularly for the poor-rating cases (23% before vs 2% after). Approximately 67% showed further improvement with co-registration in ECV mapping. Although T1 and ECV values were not clinically significantly different before and after motion compensation, there was improved intra- and inter-observer reproducibility after motion compensation.

Conclusions

Automated motion correction and co-registration improved the qualitative assessment and reproducibility of cardiac MR T1 and ECV measurements, allowing for more reliable ECV mapping.

     

Correction to: 3D SASHA myocardial T1 mapping with high accuracy and improved precision

Purpose

To improve the precision of a free-breathing 3D saturation-recovery-based myocardial T1 mapping sequence using a post-processing 3D denoising technique.

Methods

A T1 phantom and 15 healthy subjects were scanned on a 1.5 T MRI scanner using 3D saturation-recovery single-shot acquisition (SASHA) for myocardial T1 mapping. A 3D denoising technique was applied to the native T1-weighted images before pixel-wise T1 fitting. The denoising technique imposes edge-preserving regularity and exploits the co-occurrence of 3D spatial gradients in the native T1-weighted images by incorporating a multi-contrast Beltrami regularization. Additionally, 2D modified Look-Locker inversion recovery (MOLLI) acquisitions were performed for comparison purposes. Accuracy and precision were measured in the myocardial septum of 2D MOLLI and 3D SASHA T1 maps and then compared. Furthermore, the accuracy and precision of the proposed approach were evaluated in a standardized phantom in comparison to an inversion-recovery spin-echo sequence (IRSE).

Results

For the phantom study, Bland–Altman plots showed good agreement in terms of accuracy between IRSE and 3D SASHA, both on non-denoised and denoised T1 maps (mean difference −1.4 ± 18.9 ms and −4.4 ± 21.2 ms, respectively), while 2D MOLLI generally underestimated the T1 values (69.4 ± 48.4 ms). For the in vivo study, there was a statistical difference between the precision measured on 2D MOLLI and on non-denoised 3D SASHA T1 maps (P = 0.005), while there was no statistical difference after denoising (P = 0.95).

Conclusion

The precision of 3D SASHA myocardial T1 mapping was substantially improved using a 3D Beltrami regularization based denoising technique and was similar to that of 2D MOLLI T1 mapping, while preserving the higher accuracy and whole-heart coverage of 3D SASHA.

     

Combination of tagging and tissue phase mapping to accelerate myocardial motion measurements in three directions

Object

Until now, a three-directional velocity field has mostly been obtained by velocity encoding in three directions, which is very time-consuming and hence not usually used in clinical routine. We show the feasibility of combining in-plane tagging with through-plane tissue phase mapping (TPM) to encode a three-directional velocity field at 3 T with reduced overall acquisition time.

Materials and methods

Assessment of a three-directional velocity field was performed for 10 healthy volunteers. The motion patterns obtained by use of five different sequences including three-directional TPM, TPM in the through-plane direction, TPM in the through-plane direction with horizontal or vertical tagging lines, and TPM in the through-plane direction combined with a tagging grid were evaluated and compared.

Results

A three-dimensional velocity field can be obtained in approximately half the acquisition time by combining through-plane TPM with in-plane tagging. Although the velocity information is derived by different means, differences between the information obtained by three-directional TPM encoding and the suggested technique are only minor.

Conclusion

The combination of tagging and TPM enables assessment of the three-directional velocity field in nearly half the time taken when the conventional three-directional TPM sequence is used.     

Online motion correction for diffusion-weighted segmented-EPI and FLASH imaging

This paper explores the application of online motion correction using navigator echoes to the segmented-EPI and FLASH techniques. In segmented EPI this has the advantage over post-acquisition correction that the position in k-space of each segment is no longer subject to arbitrary shifts caused by rotation. In diffusion-weighted FLASH it has the advantage that the full magnetisation can be utilised in comparison to other methods of eliminating the sensitivity to bulk motion, in which the sensitivity is halved. Healthy subjects were investigated on a 3 T whole-body system in which the hardware has been modified so that navigator echoes can be recorded on a personal computer which generates the necessary magnetic field gradient correction pulses and shifts in the Larmor frequency within 800 s. ECG triggering was used to avoid the period of non-rigid-body brain motion. Two orthogonal navigator echoes were employed. For segmented EPI it was found essential to minimise the T2* weighting of the navigator echoes to about 10 ms to obtain reliable results. High quality images were obtained for both methods examined. Online motion correction brings direct benefits to both the diffusion-weighted segmented-EPI and FLASH techniques.     

An improved PSF mapping method for EPI distortion correction in human brain at ultra high field (7T)

Object  

To further improve the quality and robustness of the point-spread function (PSF) mapping method for fully automatic and accurate correction of geometric distortions in EPI at ultra high field such as 7 Tesla with high fidelity.     

前馈控制算法校正相移微动台非线性运动

压电陶瓷具有推力大,响应速度快,分辨率高和能耗低等优点,但其形变量小,所以需要位移放大机构对其形变量进行放大,实现长行程高分辨率位移。针对压电陶瓷作为驱动器,柔性铰链位移放大装置作为导向机构的相移微动台运动存在非线性问题,采用多项式模型对其位移特性进行建模,通过求解逆多项式获得使其线性运动非线性电压,利用实验对非线性运动校正结果进行验证。实验结果表明,所采用前馈控制算法可以很好的校正相移微动台非线性运动,校正后相移台在x,y方向产生的最大相移误差为4°与3.2°,满足高精度相移干涉测量对相移台相移误差要求。     

Correction of <Emphasis Type="Italic">B</Emphasis><Subscript>0</Subscript>-induced geometric distortion variations in prospective motion correction for 7T MRI

Objective

Prospective motion correction can effectively fix the imaging volume of interest. For large motion, this can lead to relative motion of coil sensitivities, distortions associated with imaging gradients and B ₀ field variations. This work accounts for the B ₀ field change due to subject movement, and proposes a method for correcting tissue magnetic susceptibility-related distortion in prospective motion correction.

Materials and methods

The B ₀ field shifts at the different head orientations were characterized. A volunteer performed large motion with prospective motion correction enabled. The acquired data were divided into multiple groups according to the object positions. The correction of B ₀-related distortion was applied to each group of data individually via augmented sensitivity encoding with additionally integrated gradient nonlinearity correction.

Results

The relative motion of the gradients, B ₀ field and coil sensitivities in prospective motion correction results in residual spatial distortion, blurring, and coil artifacts. These errors can be mitigated by the proposed method. Moreover, iterative conjugate gradient optimization with regularization provided superior results with smaller RMSE in comparison to standard conjugate gradient.

Conclusion

The combined correction of B ₀-related distortion and gradient nonlinearity leads to a reduction of residual motion artifacts in prospective motion correction data.

     

Navigator motion correction of diffusion weighted 3D SSFP imaging

Diffusion weighted (DW) 3D steady state MR (SSFP) head imaging technique using navigator echo’s motion correction is presented. This new scheme enables acquisition of DW images even at regions where severe susceptibility is present. Another advantage is the moderate gradient performance requirements. DW imaging methods are sensitive to any kind of motion, thus, most of these methods might suffer from bulk motion artifacts. The common solution to avoid motion artifacts in a 2D DW SSFP acquisition is multi averaging. To avoid the time consuming multi averaging, the new scheme, described here, utilizes navigator echo’s motion correction to remove respiratory bulk motion artifacts. At some brain regions, where the motion is governed by blood or CSF pulsation, the navigator motion correction fails. At these regions the correction is an interpolation of corrections from regions where the motion is particularly of the respiratory type. The combination of a 3D sequence with a navigator echo motion correction, enables acquisition of 10 DW slices within a time of 0:50-2:30 min.     

Fast and accurate compensation of signal offset for T2 mapping

Magnetic Resonance Materials in Physics, Biology and Medicine - T2 maps are more vendor independent than other MRI protocols. Multi-echo spin-echo signal decays to a non-zero offset due to...     

Pseudo-T2 mapping for normalization of T2-weighted prostate MRI

Magnetic Resonance Materials in Physics, Biology and Medicine - Signal intensity normalization is necessary to reduce heterogeneity in T2-weighted (T2W) magnetic resonance imaging (MRI) for...     

Fast T1 and T2 mapping methods: the zoomed U-FLARE sequence compared with EPI and snapshot-FLASH for abdominal imaging at 11.7 Tesla

Objective

A newly adapted zoomed ultrafast low-angle RARE (U-FLARE) sequence is described for abdominal imaging applications at 11.7 Tesla and compared with the standard echo-plannar imaging (EPI) and snapshot fast low angle shot (FLASH) methods.

Materials and methods

Ultrafast EPI and snapshot-FLASH protocols were evaluated to determine relaxation times in phantoms and in the mouse kidney in vivo. Owing to their apparent shortcomings, imaging artefacts, signal-to-noise ratio (SNR), and variability in the determination of relaxation times, these methods are compared with the newly implemented zoomed U-FLARE sequence.

Results

Snapshot-FLASH has a lower SNR when compared with the zoomed U-FLARE sequence and EPI. The variability in the measurement of relaxation times is higher in the Look–Locker sequences than in inversion recovery experiments. Respectively, the average T1 and T2 values at 11.7 Tesla are as follows: kidney cortex, 1810 and 29 ms; kidney medulla, 2100 and 25 ms; subcutaneous tumour, 2365 and 28 ms.

Conclusion

This study demonstrates that the zoomed U-FLARE sequence yields single-shot single-slice images with good anatomical resolution and high SNR at 11.7 Tesla. Thus, it offers a viable alternative to standard protocols for mapping very fast parameters, such as T1 and T2, or dynamic processes in vivo at high field.

     

Influence of spatial resolution and contrast agent dosage on myocardial T1 relaxation times

Objective

Our aim was to study the influence of small variations in spatial resolution and contrast agent dosage on myocardial T1 relaxation time.

Materials and methods

Twenty-nine healthy volunteers underwent cardiovascular magnetic resonance at 3T twice, including a modified look-locker inversion recovery (MOLLI) technique—3(3)3(3)5—for T1 mapping. Native T1 was assessed in three spatial resolutions (voxel size 1.4 × 1.4 × 6, 1.6 × 1.6 × 6, 1.7 × 1.7 × 6 mm³), and postcontrast T1 after 0.1 and 0.2 mmol/kg gadobutrol. Partition coefficient was calculated based on myocardial and blood T1. T1 analysis was done per segment, per slice, and for the whole heart.

Results

Native T1 values did not differ with varying spatial resolution per segment (p = 0.116–0.980), per slice (basal: p = 0.772; middle: p = 0.639; apex: p = 0.276), and globally (p = 0.191). Postcontrast T1 values were significantly lower with higher contrast agent dosage (p < 0.001). The global partition coefficient was 0.43 ± 0.3 for 0.2 and 0.1 mmol gadobutrol (p = 0.079).

Conclusion

Related to the tested MOLLI technique at 3T, very small variations in spatial resolution (voxel sizes between 1.4 × 1.4 × 6 and 1.7 × 1.7 × 6 mm³) remained without effect on the native T1 relaxation times. Postcontrast T1 values were naturally shorter with higher contrast agent dosage while the partition coefficient remained constant. Further studies are necessary to test whether these conclusions hold true for larger matrix sizes and in larger cohorts.

     

Volumetric assessment of myocardial viability in rats using 3D double contrast enhanced T1 and T2-weighted MRI

Objective: Volumetric evaluation of the myocardial viability post-infarction in rats using 3D in vivo MR imaging at 7 T using injection of an extracellular paramagnetic contrast agent and intravascular superparamagnetic iron oxide nanoparticles in the same imaging session. Materials and methods: Five hours after induction of permanent myocardial infarction in rats (n=6), 3D in vivo T1- and T2-weighted MR Imaging was performed prior to and after Gd-DOTA injection (0.2 mmol/kg) and prior to and after nanoparticle injection (5 mg Fe/kg) to assess infarct size and myocardial viability. Results: 3D MR Imaging using a successive contrast agent injection showed a difference of infarct size after Gd-DOTA injection on T1-weighted images compared to the one measured on T2-weighted images after Gd-DOTA and nanoparticle injection. Conclusion: The use of 3D T1- and T2-weighted MR Imaging using a double contrast agents protocol made possible the accurate characterization of myocardial infarction volume and allowed the detection of myocardial viability post-infarction in rats     

Influence of motion correction on the visual analysis of cardiac magnetic resonance stress perfusion imaging

Objective

Image post-processing corrects for cardiac and respiratory motion (MoCo) during cardiovascular magnetic resonance (CMR) stress perfusion. The study analyzed its influence on visual image evaluation.

Materials and methods

Sixty-two patients with (suspected) coronary artery disease underwent a standard CMR stress perfusion exam during free-breathing. Image post-processing was performed without (non-MoCo) and with MoCo (image intensity normalization; motion extraction with iterative non-rigid registration; motion warping with the combined displacement field). Images were evaluated regarding the perfusion pattern (perfusion deficit, dark rim artifact, uncertain signal loss, and normal perfusion), the general image quality (non-diagnostic, imperfect, good, and excellent), and the reader’s subjective confidence to assess the images (not confident, confident, very confident).

Results

Fifty-three (non-MoCo) and 52 (MoCo) myocardial segments were rated as ‘perfusion deficit’, 113 vs. 109 as ‘dark rim artifacts’, 9 vs. 7 as ‘uncertain signal loss’, and 817 vs. 824 as ‘normal’. Agreement between non-MoCo and MoCo was high with no diagnostic difference per-patient. The image quality of MoCo was rated more often as ‘good’ or ‘excellent’ (92 vs. 63%), and the diagnostic confidence more often as “very confident” (71 vs. 45%) compared to non-MoCo.

Conclusions

The comparison of perfusion images acquired during free-breathing and post-processed with and without motion correction demonstrated that both methods led to a consistent evaluation of the perfusion pattern, while the image quality and the reader’s subjective confidence to assess the images were rated more favorably for MoCo.

     

Native T1 mapping of the breast in MRI to differentiate fibroadenomas from benign phyllodes tumors: a preliminary study

Magnetic Resonance Materials in Physics, Biology and Medicine - Both fibroadenomas (FAs) and phyllodes tumors (PTs) are classified as fibroepithelial lesions. PTs are rare fibroepithelial neoplasms...     

7 Tesla quantitative hip MRI: a comparison between TESS and CPMG for T2 mapping

Objectives

We aimed to evaluate the feasibility of triple-echo steady state (TESS) T2 mapping as an alternative to conventional multi-echo-spin-echo (CPMG) T2 mapping for the quantitative assessment of hip joint cartilage at 7 T.

Materials and methods

A total of eight healthy volunteers and three patients were included. Reproducibility of both techniques was evaluated in five volunteers in five scans each. T2 relaxation times were measured by manually drawing regions of interest in multiple regions of the hip joint. Data from both methods were compared using Pearson correlation coefficient, intra-class correlation coefficient, and coefficient of repeatability. The overall image quality and presence of artifacts was assessed.

Results

Cartilage transplant and surrounding fluid were well depicted by both methods. Compared to CPMG, TESS provided systematically reduced T2 values (43.3 ± 7.3 vs. 19.2 ± 5.5 ms for acetabular cartilage, and 41.4 ± 5.6 vs. 21.7 ± 5.2 ms for femoral cartilage), in line with previously reported values. No correlation between both methods was found. TESS yielded a slightly better reproducibility than CPMG, while CPMG showed pronounced sensitivity to B1 inhomogeneities.

Conclusion

TESS seems to be an attractive alternative to CPMG for improvements in quantitative hip joint imaging at 7 T, allowing shortening of the total acquisition time paired with insensitivity to B1, while rendering comparable image quality with good repeatability.

     

采空区1 000 kV特高压试验线路设计建议

鉴于特高压输电线路在系统中的重要性,为确保线路安全、稳定运行,从地质理论、运行经验出发,提出了特高压输电线路通过采空区的建议,并给出具体设计方案.     

国产化配电GIS示范工程的开发和应用

介绍了厦门电业局国产化配电地理信息系统(GIS)的开发、主要功能以及应用情况,总结了技术的关键和创新点,如采用国产化GIS开发平台GeoStar、应用三维全景技术、采用正向和反向物理隔离装置的物理隔离方案等。文中还介绍了系统开发及应用体会。