Pulsed Imaging Research Articles

THz vs NIR laser-assisted atom probe tomography of LaB6 samples

Terahertz (THz) radiation with low-energy photons (meV) is used in a wide range of applications, such as microscopy, sensing, and spectroscopy. However, recently, high amplitude THz pulses of MV/cm have been generated and used for electron emission and ion evaporation from field emitters, opening up the possibility of using high amplitude THz pulses for material imaging by THz-assisted atom probe tomography (APT). In this work, we compare the APT analyses of lanthanum hexaboride (LaB6) samples using a femtosecond near-infrared laser with those obtained using high-amplitude single-cycle THz pulses. The atomic-scale characterization of stoichiometric LaB6 is challenging in laser-assisted APT due to the detection losses of boron ions. Here, we show that the THz radiation reduces the emission of molecular ions and multiple detection events, and it increases the charge state of the emitted ions. All these effects result in an improvement in boron detection. Furthermore, the emission dynamics of boron and lanthanum ions differ in their evaporation times when using THz radiation. This work emphasizes the ability of high-amplitude, single-cycle THz pulses to well control material analysis in APT, leading to better results on chemical composition. It also paves the way for the use of this radiation for material manipulation.

An electroporation cytometry system for long-term, live cell cycle analysis.

Electric fields are used in biology to address a broad range of questions and through a variety of techniques, including electroporation, gene electrotransfer (GET), electrostimulation (ES), and electrochemotherapy. Each of these modalities requires specific conditions and has drastically different target outcomes on the cell. ES has demonstrated that non-pore forming electric fields alter cell cycle progression. However, pore forming electric fields such as with GET have not been as widely explored despite major clinical advancements. Additionally, the real-time visual analysis of electrical field effects on mammalian cell culture is currently lacking among most commercial systems. To facilitate investigations into these research areas, an electroporation cytometry system was developed including a custom chamber compatible with live cell imaging and exponential decay pulse generator for live cell analysis. The functionality of the system was demonstrated using a recombinant cell line using U-2 OS cells and FUCCI(CA)5 cell cycle reporter. The exposure of the cells to a 180 V pulse in both unsynchronized and synchronized populations revealed an effect on the cell cycle.

Abnormal cerebral blood flow in children with developmental stuttering.

Stuttering affects approximately 5% of children; however, its neurological basis remains unclear. Identifying imaging biomarkers could aid in early detection. Accordingly, we investigated resting-state cerebral blood flow (CBF) in children with developmental stuttering. Pulsed arterial spin labelling magnetic resonance imaging was utilised to quantify CBF in 35 children with developmental stuttering and 27 healthy controls. We compared normalised CBF between the two groups and evaluated the correlation between abnormal CBF and clinical indicators. Compared with healthy controls, the stuttering group exhibited decreased normalised CBF in the cerebellum lobule VI bilaterally, right cuneus, and left superior occipital gyrus and increased CBF in the right medial superior frontal gyrus, left rectus, and left dorsolateral superior frontal gyrus. Additionally, normalised CBF in the left cerebellum lobule VI and left superior occipital gyrus was positively correlated with stuttering severity. Children who stutter display decreased normalised CBF primarily in the cerebellum and occipital gyrus, with increased normalised CBF in the frontal gyrus. Additionally, the abnormal CBF in the left cerebellum lobule VI and leftsuperior occipital gyrus was associated with more severe symptoms, suggesting that decreased CBF in these areas may serve as a novel neuroimaging clue for stuttering. Stuttering occurs in 5% of children and often extends into adulthood, which may negatively affect quality of life. Early detection and treatment are essential. We used pulsed arterial spin labelling magnetic resonance imaging to visualise the resting-state cerebral blood flow (CBF) in children who stutter and healthy children. Normalised CBF was decreased in stutterers in the cerebellum and occipital gyrus and increased in the frontal gyrus. Stuttering severity was linked to abnormal normalised CBF in the left cerebellum lobule VI and left superior occipital gyrus, suggesting that CBF may serve as a novel neuroimaging clue for stuttering.

Accelerated mapping and robust parallel transmit pulse design for heart and prostate imaging at 7 T.

To investigate the impact of reduced k-space sampling on mapping and the resulting impact on phase shimming and dynamic/universal parallel transmit (pTx) RF pulse design. Channel-wise 3D maps were measured at 7 T in 35 and 23 healthy subjects for the heart and prostate region, respectively. With these maps, universal phase shims optimizing homogeneity and efficiency were designed for heart and prostate imaging. In addition, universal 4kT-point pulses were designed for the heart. Subsequently, individual phase shims and individual 4kT-pulses were designed based on maps with different acceleration factors and tested on the original maps. The performance of the pulses was compared by evaluating their coefficients of variation (CoV), efficiencies and specific energy doses (SED). Furthermore, validation measurements were carried out for one heart and one prostate subject. For both organs, the universal phase shims showed significantly higher efficiencies and lower CoVs compared to the vendor provided default shim, but could still be improved with individual phase shims based on accelerated maps (acquisition time = 30 s). In the heart, the universal 4kT-pulse achieved significantly lower CoVs than tailored phase shims. Tailored 4kT-pulses based on accelerated maps resulted in even further reduced CoVs or a 2.5-fold reduction in SED at the same CoVs as the universal 4kT-pulse. Accelerated maps can be used for the design of tailored pTx pulses for prostate and cardiac imaging at 7 T, which further improve homogeneity, efficiency, or SED compared to universal pulses.

High‐Resolution Carbon‐Based Tactile Sensor Array for Dynamic Pulse Imaging

AbstractWith the development of modern medicine, the importance of continuous and reliable pulse wave monitoring has increased significantly in physiological evaluation and disease diagnosis. Among them, the 3D reconstruction of the pulse wave is indispensable, and needs rely on ultra‐high resolution sensor arrays, that is, high spatial resolution, temporal resolution, and force resolution. Herein, a flexible high‐density 32 × 32 tactile sensor array based on pressure‐sensitive tunneling mechanism is develpoed. Conformal graphene nanowalls (GNWs) pattern arrays are deposited on micro‐pyramidal structural Si substrate via mask‐assisted plasma enhanced chemical vapor deposition (PECVD) method and are adopted as pressure‐sensitive electrode, exhibiting a spatial resolution of 64 dots/cm2, high sensitivity (222.36 kPa−1) and short response time (2 ms). More importantly, HfO2 tunneling layer can effectively suppress noise current, which made it sense weak pressure signals with 1/1000 force resolution and SNR of 36.32 dB. By leveraging its high‐resolution array, more holistic pulse signals are acquired and the 3D shape of the pulse wave are successfully replicated. This work shows high‐resolution sensors have significant promise for applications in remote intelligent diagnostics.

Yet more evidence that non-aqueous myelin lipids can be directly imaged with ultrashort echo time (UTE) MRI on a clinical 3T scanner: a lyophilized red blood cell membrane lipid study

Direct imaging of semi-solid lipids, such as myelin, is of great interest as a noninvasive biomarker of neurodegenerative diseases. Yet, the short T2 relaxation times of semi-solid lipid protons hamper direct detection through conventional magnetic resonance imaging (MRI) pulse sequences. In this study, we examined whether a three-dimensional ultrashort echo time (3D UTE) sequence can directly acquire signals from membrane lipids. Membrane lipids from red blood cells (RBC) were collected from commercially available blood as a general model of the myelin lipid bilayer and subjected to D2O exchange and freeze-drying for complete water removal. Sufficiently high MR signals were detected with the 3D UTE sequence, which showed an ultrashort T2* of ∼77–271 µs and a short T1 of ∼189 ms for semi-solid RBC membrane lipids. These measurements can guide designing UTE-based sequences for direct in vivo imaging of membrane lipids.

Evaluation of Carotid Stiffness in Metabolic Syndrome by Real-Time Shear Wave Elasticity Imaging and Ultrafast Pulse Wave Velocity

ObjectiveThis study utilized real-time shear wave elasticity imaging (SWE) and ultrafast pulse wave velocity (ufPWV) to assess carotid arterial stiffness, aiming to predict atherosclerosis risk in patients with metabolic syndrome (MS). MethodsIn this study, 181 patients with metabolic syndrome (MS group) were compared with 73 healthy adults. The MS group was divided into three groups: MS I group: CIMT was normal (CIMT < 1.0 mm, no plaque, n = 61); MS II group: CIMT thickening (1.0 mm ≤ CIMT<1.5 mm, no plaque, n = 39); MS III group: plaque group (CIMT ≥ 1.5 mm, plaque, n = 81). Concurrently, the group of 73 healthy individuals was designated as the control set (NC). Parameters assessed include carotid intima-media thickness (CIMT), elastic modulus values of the carotid artery's anterior and posterior walls (Mean, Max, Min), pulse wave velocity at systole's commencement (PWV-BS), and pulse wave velocity at systole's termination (PWV-ES). Differences, distribution characteristics, and correlations across these groups were analyzed. ResultsA significant association was found between PWV-BS, PWV-ES, and arteriosclerosis severity, with these factors gaining importance as arteriosclerosis progressed. Notably, PWV-ES differences were significant across the four groups (p < 0.05). Both MS III and MS II groups exhibited higher PWV-ES values compared to the MS I group and controls. Statistically significant differences were observed between MS III, MS II, and MS I groups relative to the control group (p < 0.05). Additionally, the Mean, Max, and Min values of the anterior and posterior carotid walls in the MS III group surpassed those of the other groups. ConclusionReal-time shear wave elasticity imaging and ultrafast pulse wave velocity are valuable tools for assessing atherosclerosis risk in MS patients. These non-invasive, safe, and reproducible imaging techniques can quantitatively evaluate the stiffness of the common carotid artery's wall, offering important insights into cardiovascular risk assessment.

Conduction system pacing preserves both electrical and mechanical interventricular synchrony, a UHF ECG validation study

Abstract Background Interventricular mechanical delay (IVMD) is an established echocardiographic risk factor for pacing-induced cardiomyopathy development. Ultra-high-frequency ECG (UHF-ECG) is a non-invasive tool visualizing the ventricular activation sequence. Purpose To compare UHF-ECG interventricular dyssynchrony with echocardiography and to establish interventricular dyssynchrony related to conductive system pacing (CSP) and right ventricular myocardial pacing (RVP). Methods 53 patients with advanced AV conduction disease, without organic heart disease, and preserved LV systolic function were prospectively included. Thirty-two had RVP, and twenty-one had CSP. CSP included both His bundle pacing (n=5) and left bundle branch area pacing (n=16). UHF-ECG and echocardiography were obtained at the baseline (during spontaneous rhythm) and after 1 year (during pacing). IVMD was manually calculated as a difference between the time from a distinct point of the QRS until the onset of the flow through LV and RV outflow tract using pulsed wave doppler imaging. Analysis was performed by two experienced and blinded cardiologists. Interventricular e-DYS was calculated automatically by a software as a time difference between activation in V8 (LV free wall) and V1 electrode (RV free wall). Results The average age of all included patients was 76 ± 6 years old and patients in both RVP and CSP group had similar clinical characteristics. The values of IVMD and interventricular e-DYS were similar in the whole study population both before the pacemaker implantation (-2 ± 21 ms for IVMD vs. 0 ± 20 ms for interventricular e-DYS; p=0.31) and after one year (14 ± 23 ms for IVMD vs. 14 ± 24 ms for UHF; p = 0.7). There were no significant differences in the baseline IVMD and interventricular e-DYS between those receiving CSP vs. RVP (IVMD -6 ± 22 ms for CSP vs 2 ± 20 ms for RVP, p=0.22) and interventricular e-DYS (e-DYS -5 ± 20 ms for CSP vs. 3 ± 20 ms for RVP, p=0.34). After one year of pacing, the patients with CSP preserved similar levels of both IVMD (mean change -2 ± 5 ms, p=0.74) and interventricular e-DYS V8-V1 (mean change 0 ± 4 ms, p = 0.95) – Figure 1. There was no difference between IVMD and interventricular e-DYS in CSP patients after one year (p=0.92). On the contrary, in the RVP group IVMD increased for 27 ± 5 ms (p &amp;lt;0.0001) and interventricular e-DYS for 24 ± 5 ms (p &amp;lt;0.0001) compared to the baseline - Figure 1. There was no difference between IVMD and interventricular e-DYS in RVP patients after one year (p=0.75). There was a moderate overall correlation between IVMD and e-DYS V8-V1 in all studied ventricular rhythms (R=0.73, p&amp;lt;0.0001) – Figure 2. Conclusions UHF-ECG expresses the interventricular dyssynchrony noninvasively by measuring the activation difference between V8-V1 chest leads. RV myocardial pacing increases interventricular dyssynchrony, while CSP preserves synchronous ventricular activation.CSP vs RVPcorrelation of dyssynchrony

The energy-resolved neutron imaging instrument at the China spallation neutron source

The energy-resolved neutron imaging instrument (ERNI) is built on the number 13 beamline at the China Spallation Neutron Source (CSNS), which is the first instrument dedicated to pulsed neutron imaging in China. ERNI is designed as a multi-scale, multi-dimensional, multi-modality characterization platform. The analytical techniques will include conventional neutron radiography and tomography, Bragg-edge neutron imaging, neutron grating imaging, neutron resonance imaging, neutron diffraction, and complementary X-ray tomography. The spatial resolution for conventional neutron imaging is about 15 μm; the maximum field of view could achieve 200 mm × 200 mm; the spectral resolution is better than 0.5%. ERNI is expected to serve the growing multidisciplinary imaging user community from materials science, engineering, energy, cultural heritage, biology, geosciences, industry, and so on.

Terahertz Nondestructive Evaluation of Corroding Multilayer Paint Stacks

This paper showcases terahertz nondestructive detection of corrosion buried underneath a multilayer paint stack. Periodically, during the accelerated corrosion protocol, samples are removed from the environmental chambers and characterized using terahertz pulse imaging. Analysis of the reflected waveforms indicates that corrosion leads to a decrease in the amplitude of the deconvoluted pulse, which reflects from the metallic layer. The decrease in amplitude results from a roughening of the metallic surface with corrosion. Surface roughness increases with corrosion, eventually leading to detachment of the multilayer paint stack from the substrate.

The effect of obesity and subsequent weight reduction on cardiac morphology and function in cats

BackgroundIn people, obesity is a risk factor for cardiovascular disease, associated with systemic hypertension, cardiac remodelling and systolic and diastolic dysfunction. Weight reduction can reverse myocardial remodelling and reduce risk of subsequent cardiovascular disease. In cats, far less is known regarding the effects of obesity and subsequent weight reduction on cardiovascular morphology and function. This prospective study aimed to assess cardiac morphology and function, heart rate variability, cardiac biomarkers and body composition before and after controlled weight reduction in cats with obesity. Body composition analysis (by dual energy x-ray absorptiometry, DEXA) and cardiovascular assessment (echocardiography, systemic arterial systolic blood pressure, electrocardiography, plasma cardiac biomarkers) were performed prior to weight management in twenty cats with obesity. These investigations were repeated in eleven cats that reached target weight.ResultsAt baseline, systemic hypertension was not documented, but the majority of cats with obesity (15 out of 19) showed echocardiographic evidence of diastolic dysfunction. Eleven of 20 cats had increased maximal end-diastolic septal or left ventricular free wall thickness (≥ 6.0 mm) at baseline. Median (interquartile range) percentage of weight lost in the cats reaching target weight was 26% (17–29%), with a median reduction in body fat mass of 45% (26–64%). Both the end-diastolic left ventricular free wall (median magnitude of change -0.85 mm, IQR -0.05 mm to -1.55 mm, P = 0.019; median percentage reduction 14.0%) and end-diastolic interventricular septum (median magnitude of change -0.5 mm, IQR -0.2 mm to -1.225 mm, P = 0.047; median percentage reduction 7.9%) thickness decreased after weight reduction. Following weight reduction, pulsed wave tissue Doppler imaging of the left ventricular free wall was consistent with improved diastolic function in 4 out of 8 cats, however there was no significant difference in overall diastolic function class. Further, there was no change in heart rate variability or cardiac biomarkers with weight reduction.ConclusionAn increase in left ventricular wall thickness and diastolic dysfunction were common echocardiographic features in cats with obesity within our study and may be reversible with successful weight and fat mass loss. Further studies are required to clarify the clinical consequences of these findings.

Time-Saving 3D MR Imaging Protocols with Millimeter and Submillimeter Isotropic Spatial Resolution for Face and Neck Imaging as Implemented at a Single-Site Major Referral Center.

MR imaging has become the routine technique for staging nasopharyngeal carcinoma, evaluating perineural tumor spread, and detecting cartilage invasion in laryngeal carcinoma. However, these protocols traditionally require in the range of 25 to 35 minutes of acquisition time. 3D sequences offer the potential advantage of time savings through the acquisition of 1-mm or submillimeter resolution isotropic data followed by multiplanar reformats that require no further imaging time. We have iteratively optimized vendor product 3D T1-weighted MR imaging sequences for morphologic face and neck imaging, reducing the average acquisition time of our 3T protocols by 9 minutes 57 seconds (40.9%) and of our 1.5T protocols by 9 minutes 5 seconds (37.0%), while simultaneously maintaining or improving spatial resolution. This clinical report describes our experience optimizing and implementing commercially available 3D T1-weighted MR imaging pulse sequence protocols for clinical face and neck MR imaging examinations using illustrative cases. We provide protocol details to allow others to replicate our implementations, and we report challenges we faced along with our solutions.

Combined Terahertz Pulsed Imaging and Optical Coherence Tomography Detection Method for Multiple Defects in Thermal Barrier Coatings

While thermal barrier coatings (TBCs) are being sprayed onto aero-engine turbine blades, or while the engine blade is working, high temperatures and strong impact forces will damage TBCs under thermal cycles, resulting in the coating peeling off from the blades. The current method of using ECT, IRT, or another method alone cannot achieve the real-time detection of coating defects with both high precision and high penetration power. Two detection methods, namely, terahertz pulsed imaging (TPI) and optical coherence tomography (OCT), were combined to evaluate typical defects observed in TBCs (including internal debonding cracks, surface high-temperature cracks, and surface etched cracks). The results showed that the OCT system successfully obtained the micron-level axial resolution, but the detection depth of the OCT system was limited. The TPI system achieved a higher penetration depth than OCT—hence, it can be used for the nondestructive detection and evaluation of the internal debonding defects in the sample—but its resolution needs to be improved. Following this conclusion, a method is proposed using TPI and OCT concurrently for the nondestructive testing and quantitative evaluation of TBCs on etched cracks, thus achieving progress both in terms of depth and resolution. In our experiment, defects with a depth of 519 μm and a width of 100 μm were measured. The proposed method is suitable for situations where multiple defects in TBC samples of blades need to be detected simultaneously during the working process. When there are defects deep inside the sample, more small cracks on the surface can be evaluated to achieve a combination of depth and accuracy.

Improvement of contrast ratio between cavitation bubbles and tissue by frequency filtering in triplet pulse ultrasound imaging

Bubble-enhanced high-intensity focused ultrasound treatment requires selective imaging of cavitation bubbles to ensure their localization to the target tissue. Previous studies have proposed the ultrasound imaging of cavitation bubbles with a filtering method, which makes bubble extraction by a triplet pulse sequence more selective. The envelope component as well as harmonic components are generated during the nonlinear propagation of an ultrasonic imaging pulse, but the triplet pulse sequence can significantly reduce only the harmonic components, and its selectivity is degraded by the residual envelope component. In this study, the effects of nonlinear propagation at various intensities of imaging pulse and the frequency filtering to obtain better selectivity are investigated for the triplet pulse sequence with a filtering method. The result shows that the imaging pulse intensity is an important parameter that determines the strength of the effect of nonlinear propagation and that the optimal frequency filtering changes according to it.

Two dimensional shear wave elastography—Basic principles and current applications

AbstractUltrasound‐based elastography techniques are increasingly available on modern ultrasound systems. The application of elastography extends the utility of ultrasound, yielding new information about the impact of disease on tissue biomechanics. By modifying the B‐mode imaging pulse, an Acoustic Radiation Force Impulse produces shear waves in selected tissue. Quantifying the speed of the resultant shear wave provides an indirect assessment of tissue stiffness, with stiffer tissue producing faster shear wave movement than softer tissue. Two main types of ultrasound shear wave elastography are currently in use. Point shear wave elastography uses one or two pulses to measure shear wave speeds over a small fixed area. Alternatively, two‐dimensional shear‐wave elastography provides a colour elastogram map over a larger area, allowing the operator to select specific regions of interest to measure shear wave speed. This article will discuss the essential physics principles, and clinical uses of two‐dimensional shear wave elastography, aiming to provide sonographers with a basic understanding of the technology and current applications.

Monte Carlo simulation of spatial resolution of lens-coupled LYSO scintillator for intense pulsed gamma-ray imaging system with large field of view

In this paper, we use a Monte Carlo (MC) simulation based on Geant4 to investigate the influence of four parameters on the spatial resolution of the lens-coupled lutetium yttrium orthosilicate (LYSO) scintillator, including the thickness of the LYSO scintillator, the F-number and minification factor of the lens, and the incident position of the gamma-rays. Simulation results show that when the gamma-rays are incident along the lens axis, the smaller the thickness, the larger the F-number, the larger the minification factor, the higher the spatial resolution, with an isotropic point spread function (PSF). As the incident position of the gamma-rays deviates from the lens axis, the spatial resolution decreases, and the PSF becomes anisotropic. In addition, by analyzing the whole physical process of the lens-coupled LYSO scintillator from gamma-rays to secondary electrons to fluorescence photons, we aim to provide a detailed analysis of the influence of each parameter on the spatial resolution. The results show that the PSF of the secondary electrons energy deposition is almost constant in the simulation, which determines the upper limit of the spatial resolution. Meanwhile, the dispersion process of the fluorescence photons can explain the reason why each parameter affects the spatial resolution.

Novel pore size-controlled, susceptibility matched, 3D-printed MRI phantoms.

We report the design concept and fabrication of MRI phantoms, containing blocks of aligned microcapillaires that can be stacked into larger arrays to construct diameter distribution phantoms or fractured, to create a "powder-averaged" emulsion of randomly oriented blocks for vetting or calibrating advanced MRI methods, that is, diffusion tensor imaging, AxCaliber MRI, MAP-MRI, and multiple pulsed field gradient or double diffusion-encoded microstructure imaging methods. The goal was to create a susceptibility-matched microscopically anisotropic but macroscopically isotropic phantom with a ground truth diameter that could be used to vet advanced diffusion methods for diameter determination in fibrous tissues. Two-photon polymerization, a novel three-dimensional printing method is used to fabricate blocks of capillaries. Double diffusion encoding methods were employed and analyzed to estimate the expected MRI diameter. Susceptibility-matched microcapillary blocks or modules that can be assembled into large-scale MRI phantoms have been fabricated and measured using advanced diffusion methods, resulting in microscopic anisotropy and random orientation. This phantom can vet and calibrate various advanced MRI methods and multiple pulsed field gradient or diffusion-encoded microstructure imaging methods. We demonstrated that two double diffusion encoding methods underestimated the ground truth diameter.

Increasing the sensor channels: a solution for the pressing offsets that cause the physiological parameter inaccuracy in radial artery pulse signal acquisition.

Introduction: In studies of pulse wave analysis, single-channel sensors only adopt single temporal pulse signals without spatial information to show pulse-feeling patterns. Multi-channel arterial pulse signals, also named as three-dimensional pulse images (3DPIs), provide the spatial and temporal characteristics of radial pulse signals. When involving single or few-channel sensors, pressing offsets have substantial impacts on obtaining inaccurate physiological parameters like tidal peak (P2). Methods: This study discovers the pressing offsets in multi-channel pulse signals and analyzes the relationship between the pressing offsets and time of P2 (T2) by qualifying the pressing offsets. First, we employ a data acquisition system to capture 3DPIs. Subsequently, the errorT2 is developed to qualify the pressing offsets. Results: The outcomes display a central low and peripheral high pattern. Additionally, the errorT2 increase as the distances from the artery increase, particularly at the radial ends of the blood flow direction. For every 1mm increase in distances between sensing elements and center sensing elements, the errorT2 in the radial direction escalates by 4.87%. When the distance is greater than 3.42mm, the errorT2 experiences a sudden increase. Discussion: The results show that increasing the sensor channels can overcome the pressing offsets in radial pulse signal acquisition.

4D Flow Cardiac MR in Primary Mitral Regurgitation.

Four-dimensional-flow cardiac MR (4DF-MR) offers advantages in primary mitral regurgitation. The relationship between 4DF-MR-derived mitral regurgitant volume (MR-Rvol) and the post-operative left ventricular (LV) reverse remodeling has not yet been established. To ascertain if the 4DF-MR-derived MR-Rvol correlates with the LV reverse remodeling in primary mitral regurgitation. Prospective, single-center, two arm, interventional vs. nonintervention observational study. Forty-four patients (male N = 30; median age 68 [59-75]) with at least moderate primary mitral regurgitation; either awaiting mitral valve surgery (repair [MVr], replacement [MVR]) or undergoing "watchful waiting" (WW). 5 T/Balanced steady-state free precession (bSSFP) sequence/Phase contrast imaging/Multishot echo-planar imaging pulse sequence (five shots). Patients underwent transthoracic echocardiography (TTE), phase-contrast MR (PMRI), 4DF-MR and 6-minute walk test (6MWT) at baseline, and a follow-up PMRI and 6MWT at 6 months. MR-Rvol was quantified by PMRI, 4DF-MR, and TTE by one observer. The pre-operative MR-Rvol was correlated with the post-operative decrease in the LV end-diastolic volume index (LVEDVi). Included Student t-test/Mann-Whitney test/Fisher's exact test, Bland-Altman plots, linear regression analysis and receiver operating characteristic curves. Statistical significance was defined as P < 0.05. While Bland-Altman plots demonstrated similar bias between all the modalities, the limits of agreement were narrower between 4DF-MR and PMRI (bias 15; limits of agreement -36 mL to 65 mL), than between 4DF-MR and TTE (bias -8; limits of agreement -106 mL to 90 mL) and PMRI and TTE (bias -23; limits of agreement -105 mL to 59 mL). Linear regression analysis demonstrated a significant association between the MR-Rvol and the post-operative decrease in the LVEDVi, when the MR-Rvol was quantified by PMRI and 4DF-MR, but not by TTE (P = 0.73). 4DF-MR demonstrated the best diagnostic performance for reduction in the post-operative LVEDVi with the largest area under the curve (4DF-MR 0.83; vs. PMRI 0.78; and TTE 0.51; P = 0.89). This study demonstrates the potential clinical utility of 4DF-MR in the assessment of primary mitral regurgitation. 2 TECHNICAL EFFICACY: Stage 5.

Optimization of flip angle and radiofrequency pulse phase to maximize steady-state magnetization in three-dimensional missing pulse steady-state free precession.

Missing pulse (MP) steady-state free precession (SSFP) is a magnetic resonance imaging (MRI) pulse sequence that is highly tolerant to the magnetic field inhomogeneity. In this study, optimal flip angle and radiofrequency (RF) phase scheduling in three-dimensional (3D) MP-SSFP is introduced to maximize the steady-state magnetization while keeping broadband excitation to cover widely distributed frequencies generated by inhomogeneous magnetic fields. Numerical optimization based on extended phase graph (EPG) simulation was performed to maximize the MP-SSFP steady-state magnetization. To limit the specific absorption rate (SAR) associated with the broadband excitation in 3D MP-SSFP, SAR constraint was introduced in the numerical optimization. Optimized flip angle and RF phase settings were experimentally tested by introducing a linear inhomogeneous magnetic field in a range of 10-20 mT/m and using a phantom with known T1/T2 relaxation and diffusion parameters at 3 T. The experimental results were validated through comparisons with EPG simulation. Image contrasts and molecular diffusion effects were investigated in in vivo human brain imaging with 3D MP-SSFP with the optimal flip angle and RF phase settings. In the phantom measurements, the optimal flip angle and RF phase settings improved the MP-SSFP steady-state magnetization/signal-to-noise ratio by up to 41% under the fixed SAR conditions, which matched well with EPG simulation results. In vivo brain imaging with the optimal RF pulse settings provided T2-like image contrasts. Diffusion effects were relatively minor with the linear inhomogeneous field of 10-20 mT/m for white and gray matter, but cerebrospinal fluid showed conspicuous signal intensity attenuation as the linear inhomogeneous field increased. Numerical optimization achieved significant improvement in the steady-state magnetization in MP-SSFP compared with the RF pulse settings used in previous studies. The proposed flip angle and RF phase optimization is promising to improve 3D MP-SSFP image quality for MRI in inhomogeneous magnetic fields.