Multiple-echo Acquisition Research Articles

Data-driven synthetic MRI FLAIR artifact correction via deep neural network.

FLAIR (fluid attenuated inversion recovery) imaging via synthetic MRI methods leads to artifacts in the brain, which can cause diagnostic limitations. The main sources of the artifacts are attributed to the partial volume effect and flow, which are difficult to correct by analytical modeling. In this study, a deep learning (DL)-based synthetic FLAIR method was developed, which does not require analytical modeling of the signal. To correct artifacts in synthetic FLAIR using a DL method. Retrospective. A total of 80 subjects with clinical indications (60.6 ± 16.7 years, 38 males, 42 females) were divided into three groups: a training set (56 subjects, 62.1 ± 14.8 years, 25 males, 31 females), a validation set (1 subject, 62 years, male), and the testing set (23 subjects, 57.3 ± 20.4 years, 13 males, 10 females). 3 T MRI using a multiple-dynamic multiple-echo acquisition (MDME) sequence for synthetic MRI and a conventional FLAIR sequence. Normalized root mean square (NRMSE) and structural similarity (SSIM) were computed for uncorrected synthetic FLAIR and DL-corrected FLAIR. In addition, three neuroradiologists scored the three FLAIR datasets blindly, evaluating image quality and artifacts for sulci/periventricular and intraventricular/cistern space regions. Pairwise Student's t-tests and a Wilcoxon test were performed. For quantitative assessment, NRMSE improved from 4.2% to 2.9% (P < 0.0001) and SSIM improved from 0.85 to 0.93 (P < 0.0001). Additionally, NRMSE values significantly improved from 1.58% to 1.26% (P < 0.001), 3.1% to 1.5% (P < 0.0001), and 2.7% to 1.4% (P < 0.0001) in white matter, gray matter, and cerebral spinal fluid (CSF) regions, respectively, when using DL-corrected FLAIR. For qualitative assessment, DL correction achieved improved overall quality, fewer artifacts in sulci and periventricular regions, and in intraventricular and cistern space regions. The DL approach provides a promising method to correct artifacts in synthetic FLAIR. 4 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;50:1413-1423.

One- and Two-Dimensional High-Resolution NMR from Flat Surfaces.

Determining atomic-level characteristics of molecules on two-dimensional surfaces is one of the fundamental challenges in chemistry. High-resolution nuclear magnetic resonance (NMR) could deliver rich structural information, but its application to two-dimensional materials has been prevented by intrinsically low sensitivity. Here we obtain high-resolution one- and two-dimensional 31P NMR spectra from as little as 160 picomoles of oligonucleotide functionalities deposited onto silicate glass and sapphire wafers. This is enabled by a factor >105 improvement in sensitivity compared to typical NMR approaches from combining dynamic nuclear polarization methods, multiple-echo acquisition, and optimized sample formulation. We demonstrate that, with this ultrahigh NMR sensitivity, 31P NMR can be used to observe DNA bound to miRNA, to sense conformational changes due to ion binding, and to follow photochemical degradation reactions.

True phase quantitative susceptibility mapping using continuous single-point imaging: a feasibility study.

In this study, we explore the feasibility of a new imaging scheme for quantitative susceptibility mapping (QSM): continuous single-point imaging (CSPI), which uses a pure phase encoding strategy to achieve true phase imaging and improve QSM accuracy. The proposed CSPI is a modification of conventional SPI to allow acquisition of multiple echoes in a single scan. Immediately following a phase encoding gradient, the free induction decay is continuously sampled with extremely high temporal resolution to obtain k-space data at a fixed spatial frequency (i.e., at a fixed k-space coordinate). By having near-0 readout duration, CSPI results in a true snapshot of the transverse magnetization at each TE. Additionally, parallel imaging with autocalibration is utilized to reduce scan time, and an optional temporal averaging strategy is presented to improve signal-to-noise ratio for objects with low proton density or short T2* decay. The reconstructed CSPI images were input to a QSM framework based on morphology enabled dipole inversion. In an experiment performed using iron phantoms, susceptibility estimated using CSPI showed high linearity (R2 = 0.9948) with iron concentration. Additionally, reconstructed CSPI phase images showed much reduced ringing artifact compared with phase images obtained using a frequency encoding strategy. In an ex vivo experiment performed using human tibia samples, estimated susceptibilities ranged from -1.6 to -2.1 ppm, in agreement with values reported in the literature (ranging from -1.2 to -2.2 ppm). We have demonstrated the feasibility of using CSPI to obtain true phase images for QSM.

Fast REDOR with CPMG multiple-echo acquisition

Rotational-Echo Double Resonance (REDOR) is a widely used experiment for distance measurements in solids. The conventional REDOR experiment measures the signal dephasing from hetero-nuclear recoupling under magic-angle spinning (MAS) in a point by point manner. A modified Carr–Purcell Meiboom–Gill (CPMG) multiple-echo scheme is introduced for fast REDOR measurement. REDOR curves are measured from the CPMG echo amplitude modulation under dipolar recoupling. The real time CPMG–REDOR experiment can speed up the measurement by an order of magnitude. The effects from hetero-nuclear recoupling, the Bloch–Siegert shift and echo truncation to the signal acquisition are discussed and demonstrated.

Broadband adiabatic inversion pulses for cross polarization in wideline solid-state NMR spectroscopy

Efficient acquisition of ultra-wideline solid-state NMR powder patterns is a continuing challenge. In particular, when the breadth of the powder pattern is much larger than the cross-polarization (CP) excitation bandwidth, transfer efficiencies suffer and experimental times are greatly increased. Presented herein is a CP pulse sequence with an excitation bandwidth that is up to ten times greater than that available from a conventional spin-locked CP pulse sequence. The pulse sequence, broadband adiabatic inversion CP (BRAIN-CP), makes use of the broad, uniformly large frequency profiles of chirped inversion pulses, to provide these same characteristics to the polarization transfer process. A detailed theoretical analysis is given, providing insight into the polarization transfer process involved in BRAIN-CP. Experiments on spin-1/2 nuclei including 119Sn, 199Hg and 195Pt nuclei are presented, and the large bandwidth improvements possible with BRAIN-CP are demonstrated. Furthermore, it is shown that BRAIN-CP can be combined with broadband frequency-swept versions of the Carr–Purcell–Meiboom–Gill experiment (for instance with WURST–CPMG, or WCPMG for brevity); the combined BRAIN-CP/WCPMG experiment then provides multiplicative signal enhancements of both CP and multiple-echo acquisition over a broad frequency region.

MATPASS/CPMG: A sensitivity enhanced magic-angle spinning sideband separation experiment for disordered solids

A Carr–Purcell Meiboom–Gill (CPMG) sensitivity-enhanced spinning sideband separation experiment is presented. The experiment combines the idea of magic-angle turning and phase-adjusted sideband separation (MATPASS), allowing for isotropic/anisotropic chemical shift separation of disordered solids with line widths far greater than the magic-angle spinning frequency. The use of CPMG enhances the sensitivity of the wide-line spectra by an order of magnitude via multiple-echo acquisition. The MATPASS/CPMG protocol involves acquisition of time-domain data using a MAT/CPMG pulse sequence followed by f1 shearing during data processing to arrive at the PASS representation. Such a protocol has √2 higher sensitivity than the conventional PASS method because all CPMG echo signals are used for the final PASS spectrum. Application of this method is demonstrated using a GeSe4 glass sample with both 77Se isotropic line widths and chemical shift anisotropy that far exceed the spinning frequency. The sideband separation allows for the measurement of chemical shift anisotropy of the disordered solids.

Solid state deuteron relaxation time anisotropy measured with multiple echo acquisition

The signal to noise ratio of solid state deuteron NMR line shapes can be significantly improved by recording multiple echoes, generated either by a quadrupole Carr-Purcell-Meiboom-Gill pulse train (QCPMG) or by magic angle spinning (MAS). It is shown in this article, theoretically and experimentally, that when these techniques are used to record partially relaxed spectra, the relaxation times of Zeeman order, T(1Z), and quadrupole order, T(1Q), measured for individual side bands in QCPMG experiments preserve relaxation time anisotropy, while rotational side bands in MAS spectra do not. The relaxation times of individual QCPMG sidebands are not identical to those measured at the same frequencies on partially relaxed quadrupole echo powder patterns, and must be computed by explicit simulation.

Voxel-Based Iterative Sensitivity (VBIS) analysis: Methods and a validation of intensity scaling for T2-weighted imaging of hippocampal sclerosis

Abnormalities in the brain generally manifest on MRI as changes in shape (morphometry) or changes in the nature of the tissue (signal intensity). Voxel Based Morphometry (VBM) is a whole brain quantitative way of assessing morphometric changes. Voxel Based Relaxometry (VBR) directly assesses signal intensity changes in quantitative maps of T2 relaxation time, but this requires specialised multiple-echo acquisition sequences that are not usually available at clinical sites. This paper introduces and assesses an objective voxel-based statistical method for evaluation of signal intensity in groups of routinely acquired qualitative images. We call the method Voxel-Based Iterative Sensitivity (VBIS) analysis. It adaptively optimises the relative global scaling of images to maximise sensitivity to regional effects. We apply and validate the method of analysis for T2-weighted images of the human brain. To validate the method, it was directly compared with VBR by extracting T2-weighted images of a single echo from multi-echo T2 relaxometry acquisitions from a group of 24 patients with left hemisphere hippocampal sclerosis and 97 healthy controls. Expected signal abnormalities in the patients were detectable with VBIS-T2, confirming the feasibility of the technique. This opens the door to the use of a voxel-based analysis approach on the vast amount of T2-weighted image data that has been and is being acquired on MRI scanners. When a quantitative modality is not available, VBIS can be an effective way to quantify differences between groups. We expect the method could also assist quantitative analysis of other qualitative modalities such as T1-weighted MRI, SPECT and CT.

Improved time efficiency and accuracy in diffusion tensor microimaging with multiple-echo acquisition

In high-field NMR microscopy rapid single-shot imaging methods, for example, echo planar imaging, cannot be used for determination of the apparent diffusion tensor (ADT) due to large magnetic susceptibility effects. We propose a pulse sequence in which a diffusion-weighted spin-echo is followed by multiple gradient-echoes with additional diffusion weighting. These additional echoes can be used to calculate the ADT and T 2 ∗ maps. We show here that this results in modest but consistent improvements in the accuracy of ADT determination within a given total data acquisition time. The method is tested on excised, chemically fixed rat spinal cords.

A New Method for Fast Multislice T1 Mapping

A sequence for T1 relaxation-time mapping which enables high-resolution, multislice imaging in short acquisition times is presented. The sequence is based on the Look-Locker method and employs a magnetization-preparation module prior to data acquisition with a banded k-space data collection scheme. The method was implemented on a standard clinical scanner and the accuracy of the T1 results was evaluated against spectroscopic measurements. The accuracy of the T1 maps validated by phantom imaging measurements is <3% for slow-relaxing compartments (T1 ≅ 2000 ms) and is around 1% for faster-relaxing species (T1 ≤ 1200 ms). Additionally, the inherent multislice, multipoint capability of the method is demonstrated. Multislice, multipoint in vivo results of the human brain obtained using this method are presented. An acquisition time of approximately 8 min was achieved for a T1 map, which, in principle, can provide whole-brain coverage with 25 slices, a matrix size of 256 × 256, and 12 time points. The speed of the sequence is derived through optimized interleaving of slices and time points, together with the acquisition of multiple echoes, which are used to fill a 3-segment k-space.

True water and fat MR imaging with use of multiple-echo acquisition.

The original Dixon method for chemical shift imaging of a two-component system is extended to a multiple-echo technique. Four echoes are used to gain all the information required to remove non-chemical shift-dependent phase changes and produce true fat and water images. Applications of the method both to a phantom and to human tissue are shown. This technique results in information comparable to that obtained from the Dixon method in half the imaging time and also gives an equivalent, if not slightly improved, signal-to-noise ratio.

Generation of coupled spin-only images using multiple-echo acquisition

Generation of coupled spin-only images using multiple-echo acquisition

Natural abundance silicon-29 imaging incorporating polarization transfer and multiple-echo acquisition

Natural abundance silicon-29 imaging incorporating polarization transfer and multiple-echo acquisition

Natural abundance 13C imaging incorporating multiple-echo acquisition

Natural abundance 13C imaging incorporating multiple-echo acquisition