On the design of filters for fourier and oSVD-based deconvolution in bolus tracking perfusion MRI

Object  

Bolus tracking perfusion evaluation relies on the deconvolution of a tracers concentration time-courses in an arterial and a tissue voxel following the tracer kinetic model. The object of this work is to propose a method to design a data-driven Tikhonov regularization filter in the Fourier domain and to compare it to the singular value decomposition (SVD)–based approaches using the mathematical equivalence of Fourier and circular SVD (oSVD).     

3D magnetic resonance fingerprinting on a low-field 50 mT point-of-care system prototype: evaluation of muscle and lipid relaxation time mapping and comparison with standard techniques

Objective

To implement magnetic resonance fingerprinting (MRF) on a permanent magnet 50 mT low-field system deployable as a future point-of-care (POC) unit and explore the quality of the parameter maps.

Materials and methods

3D MRF was implemented on a custom-built Halbach array using a slab-selective spoiled steady-state free precession sequence with 3D Cartesian readout. Undersampled scans were acquired with different MRF flip angle patterns and reconstructed using matrix completion and matched to the simulated dictionary, taking excitation profile and coil ringing into account. MRF relaxation times were compared to that of inversion recovery (IR) and multi-echo spin echo (MESE) experiments in phantom and in vivo. Furthermore, B₀ inhomogeneities were encoded in the MRF sequence using an alternating TE pattern, and the estimated map was used to correct for image distortions in the MRF images using a model-based reconstruction.

Results

Phantom relaxation times measured with an optimized MRF sequence for low field were in better agreement with reference techniques than for a standard MRF sequence. In vivo muscle relaxation times measured with MRF were longer than those obtained with an IR sequence (T₁: 182 ± 21.5 vs 168 ± 9.89 ms) and with an MESE sequence (T₂: 69.8 ± 19.7 vs 46.1 ± 9.65 ms). In vivo lipid MRF relaxation times were also longer compared with IR (T₁: 165 ± 15.1 ms vs 127 ± 8.28 ms) and with MESE (T₂: 160 ± 15.0 ms vs 124 ± 4.27 ms). Integrated ΔB₀ estimation and correction resulted in parameter maps with reduced distortions.

Discussion

It is possible to measure volumetric relaxation times with MRF at 2.5 × 2.5 × 3.0 mm³ resolution in a 13 min scan time on a 50 mT permanent magnet system. The measured MRF relaxation times are longer compared to those measured with reference techniques, especially for T₂. This discrepancy can potentially be addressed by hardware, reconstruction and sequence design, but long-term reproducibility needs to be further improved.

     

Fast multiecho balanced SSFP metabolite mapping of 1H and hyperpolarized 13C compounds

Object  To investigate the feasibility of multiecho balanced steady-state free precession (bSSFP)-based fast chemical shift mapping hyperpolarized ¹³C metabolites. The overall goal was to reduce total imaging time and to increase spatial resolution compared to common chemical shift imaging (CSI). Materials and methods  A multiecho bSSFP sequence in combination with an iterative reconstruction algorithm was implemented. ¹H experiments were performed on phantoms and on a human volunteer in order to investigate the feasibility of the method on a system with metabolite maps that are known beforehand. ¹³C experiments were performed in vivo on pigs, where CSI images were acquired also for comparison. Results  Chemical shift images of three and four distinct ¹H resonance frequencies as well as chemical shift images of up to five hyperpolarized ¹³C metabolites were successfully obtained. Conclusion  Fast metabolite mapping based on multiecho balanced SSFP in combination with an iterative reconstruction approach could successfully separate several ¹H resonances and hyperpolarized ¹³C metabolites.     

Self-consistent 3-D numerical modeling of a uniformly doped nanoscale FinFET using interpolating wavelets

A three-dimensional numerical modeling of a uniformly doped nanoscale FinFET including quantum-mechanical effects has been developed. A self-consistent solution of 3D Poisson-Schr?dinger equation has been obtained using multiresolution approach to achieve adaptively refined mesh that can be used to get a solution with the same level of accuracy of a reference, but with a considerable lower number of points. To the best of our knowledge, this is the first approach for the self-consistent solution to surface potential computations of nanoscale FinFET device using interpolating wavelets. It performs an efficient computation by dynamically adjusting the computational mesh in order to obtain surface potential variations during simulation. This method allows non-uniform grids and scales the CPU time linearly with the number of mesh points. The exact potential profile, subthreshold swing (S) and threshold voltage (V _th) rolloff are estimated. The accuracy of the model has been verified with finite difference, finite element and experimental results. This method provides more accurate results than other existing methods.     

Measurement of T1 and T2 relaxation times of the pancreas at 7 T using a multi-transmit system

Objective

To determine T₁ and T₂ relaxation times of healthy pancreas parenchyma at 7 T using a multi-transmit system.

Materials and methods

Twenty-six healthy subjects were scanned with a 7 T MR system using eight parallel transceiver antennas, each with two additional receive loops. A Look-Locker sequence was used to obtain images for T₁ determination, while T₂ was obtained from spin-echo images and magnetic resonance spectroscopy measurements with different echo times. T₁ and T₂ times were calculated using a mono-exponential fit of the average magnitude signal from a region of interest in the pancreas and were tested for correlation with age.

Results

The age range of the included subjects was 21–72 years. Average T₁ and T₂ relaxation times in healthy pancreas were 896 ± 149 ms, and 26.7 ± 5.3 ms, respectively. No correlation with age was found.

Conclusion

T₁ and T₂ relaxation times of the healthy pancreas were reported for 7 T, which can be used for image acquisition optimization. No significant correlations were found between age and T₁ or T₂ relaxation times of the pancreas. Considering their low standard deviation and no observable age dependence, these values may be used as a baseline to study potentially pancreatic tissue affected by disease.

     

Digital architectures realizing piecewise‐linear multivariate functions: Two FPGA implementations

Digital architectures for the circuit realization of multivariate piecewise‐linear (PWL) functions are reviewed and compared. The output of the circuits is a digital word representing the value of the PWL function at the n‐dimensional input. In particular, we propose two architectures with different levels of parallelism/complexity. PWL functions with n = 3 inputs are implemented on an FPGA and experimental results are shown. The accuracy in the representation of PWL functions is tested through three benchmark examples, two concerning three‐variate static functions and one concerning a dynamical control system defined by a bi‐variate PWL function. Copyright © 2009 John Wiley & Sons, Ltd.     

Removing the Effects of CSF Partial Voluming on Fitted CBF and Arterial Transit Times Using FAIR, a Pulsed Arterial Spin Labelling Technique

FAIR, an arterial spin labelling technique, provides non-invasive, quantitative CBF values and arterial transit times δt. This paper focuses on the negative impact of CSF partial voluming on FAIR results. To understand and solve this problem, we performed a theoretical analysis and a range of simulations. We then acquired FAIR data from a volunteer to illustrate our findings. We found that the determinant effect of CSF is a delayed zero-crossing during inversion recovery. The subtraction of magnitude inversion recovery data in FAIR generates erroneous negative data and distorted fit results: we simulated that for CSF percentages of 0–40%, CBF and δt will be progressively overestimated by up to 50%. For higher CSF percentages the errors were found to increase steeply. We explored a straightforward solution: taking the magnitude of the FAIR data before fitting. This provided a remarkably strong antidote against the effects of CSF partial voluming: for CSF percentages of 0–40%, simulations now gave CBF values accurate within 1%, and δt within 5%. The fit remained robust for high CSF fractions. Our analysis and simulations demonstrate that using magnitude FAIR data minimises the detrimental effects of CSF partial voluming. Data from a healthy volunteer illustrate these results.     

Rank-constrained semidefinite program for unit commitment

Unit Commitment (UC) is a combinatorial optimization problem that can be posed as minimizing a quadratic objective function under quadratic constraints. This paper presents a solution to UC based on Semidefinite Programming (SDP). In particular, it shows that an approximate solution can be obtained by using Shor’s semidefinite relaxation scheme together with a rank constraint enforced via convex iteration. The approximate solution has the majority of Boolean variables set by the SDP solver to either 0 or 1; it is modified by a simple heuristic to yield a feasible schedule. The proposed SDP formulation employs 3 × 3 semidefinite matrices and therefore requires computational effort that increases only moderately with problem size. Numerical results on test systems with up to 100 units dispatched over a period of 24 h show that the method is robust and produces schedules that are comparable with those from previous techniques.     

Pre-clinical results with Clariscan™ (NC100150 Injection); experience from different disease models

A superparamagnetic nanoparticle (NC100150 Injection) was investigated in two different animal models; renal perfusion in pigs and tumour imaging in mice. In the pig model, qualitative first-pass perfusion maps following a bolus injection of NC100150 Injection enabled good visualisation of hypoperfused regions of the renal cortex following partial ligation of the renal artery. High temporal resolution was found to be essential to accurately capture the first passage of the contrast agent through the kidney due to the very rapid blood flow in normal renal cortex. In the tumour model (LS174T cells implanted in nude mice), NC100150 Injection was found to cause a gradual (over 60 min) signal increase on Tl-w images in part of the tumours which was attributed to contrast agent leakage from the vascular space to the extravascular space in areas of increased capillary permeability. This observation is consistent with previous reports on the molecular cut-off size for vascular extraction for this tumour cell line. The specific enhancement of tumour tissue suggest potential utility of NC100150 Injection as an angiogenesis marker.     

23Na chemical shift imaging in the living rat brain using a chemical shift agent,Tm[DOTP]5–

Objective

It is well known that the use of shift reagents (SRs) in nuclear magnetic resonance (NMR) studies is substantially limited by an intact blood–brain barrier (BBB). The current study aims to develop a method enabling chemical shift imaging in the living rat brain under physiological conditions using an SR, Tm[DOTP]⁵⁻.

Materials and methods

Hyperosmotic mannitol bolus injection followed by 60 min infusion of a Tm[DOTP]⁵⁻ containing solution was administered via a catheter inserted into an internal carotid artery. We monitored the homeostasis of physiological parameters, and we measured the thulium content in brain tissue post mortem using total reflection fluorescence spectroscopy (T-XRF). The alterations of the ²³Na resonance spectrum were followed in a 9.4T small animal scanner.

Results

Based on the T-XRF measurements, the thulium concentration was estimated at 2.3 ± 1.8 mM in the brain interstitial space. Spectroscopic imaging showed a split of the ²³Na resonance peak which became visible 20 min after starting the infusion. Chemical shift imaging revealed a significant decrease of the initial intensity level to 0.915 ± 0.058 at the end of infusion.

Conclusion

Our novel protocol showed bulk accumulation of Tm[DOTP]⁵⁻ thus enabling separation of the extra-/intracellular ²³Na signal components in the living rat brain while maintaining physiological homeostasis.

     

基于差分进化算法的 ICP-AES 谱线重叠干扰校正方法研究

在电感耦合等离子体原子发射光谱法(ICP-AES)测量系统中,光谱的叠加现象会导致大部分谱线受到不同程度的重叠 干扰,引起后续的定量分析误差。 根据光谱叠加产生机理,重点阐述谱线重叠干扰校正思路,建立干扰校正评价函数;基于有限 差分法计算重叠谱线的近似二阶导数,根据其极小值分布确定重叠谱线中子峰谱线特征波长的最小取值区间,并以此作为差分 进化算法的初始条件之一,然后利用差分进化算法求解评价函数的最优解,作为重叠谱线的最佳特征向量,进而解析出重叠谱 线中的干扰谱线和目标谱线。 通过构造的模拟重叠谱线数据,研究差分进化算法的种群数量和进化代数对算法性能的影响,并 通过实测重叠谱线数据验证了提出的谱线重叠干扰校正方法在工程领域中应用的可行性。 实验结果表明,差分进化算法能够 较为准确地计算出评价函数的最佳特征向量,并解析出重叠谱线中的目标谱线和干扰谱线,从而实现谱线重叠干扰校正,为待 测溶液中元素含量的定量分析奠定了基础。     

Hybrid spatial‐spectral MoM analysis of microstrip structures

In this paper, a new hybrid method formulation to resolve the mixed potential integral equation in an efficient and a fast approach, which is based on a simultaneous formulation in both spatial and spectral domains, is proposed. The entries of the method of moments matrix are then given by the sum of two integrals. The first one is expressed in the spatial domain. This part is analytically evaluated after a development in Taylor series of the exponential terms in the function to integrate. The integrals expressed in the spectral domain have a finite range, and they are calculated using numerical integration. Then the convergence problem is avoided in this approach. A good agreement between the simulated and measured results has been achieved. Copyright © 2015 John Wiley & Sons, Ltd.     

Relaxation studies on power equipment

This paper describes a method of studying and modeling the dielectric relaxation for stationary power apparatus in an interconnected network to get the necessary aging indicators. The polarization phenomenon was studied using Recovery Voltage (RVM) and Polarization Current (PCM) Measurements on individual power apparatus and on network with other connected apparatus. Because of dielectric absorption in composite insulation, all RVM responses showed a single peak response and the response was linear with charging voltage. PCM decreased monotonically with a sudden drop in the initial period and after 100 s, it decreased very slowly and the response was linear with charging voltage. The responses were analyzed by modeling the dielectric function as an exponential function and as two-time dependent fractional power law function to determine suitable aging parameters. Exponential function model was found to fit both responses to get an equivalent electrical circuit of the system. Interrelation of RVM and PCM was established in most of the cases. Predicted dielectric response function for generator was more linear with time than for transformers and cable. In the frequency plane, it was found to have distinct regions of relaxation spectrum. Studies indicated that the parameters of fit for RVM response with each charging period differed from all RVM responses.     

Functional magnetic resonance imaging of swine brain during change in thiopental anesthesia into EEG burst-suppression level--a preliminary study

Deepening anesthesia produces well known changes in electroencephalogram (EEG) and evoked potentials, differing in pathological and normal brain. Yet, it is not known how the T2*-weighted signal changes in the healthy brain during deepening anesthesia. We studied the effect of thiopental bolus on functional magnetic resonance imaging (fMRI) in the healthy brain using porcine model. In five pigs (2–3 months, 20–25 kg), the control bolus prior to fMRI resulted in a change into burst-suppression. After the recovery of continuous EEG, fMRI (4 min) was performed with a single bolus of thiopental (11.4-17.1 mg/kg) administered 1 min after the onset of imaging. This was repeated in four of five pigs. Positive (6-8%) or negative (-3 to -8%) signal intensity changes correlated to the thiopental bolus injection were seen in the group average fMRI response. Positive response was 1.6% and negative response 2.3% of the total brain region of interest (ROI) voxels. Responding voxels were distributed more prominently in the thalamic ROI (4.5%) than in the cortical ROI (2.2%). The group average of unthresholded voxel time courses showed that the net effect of thiopental bolus was a small (0.5%) but a clear positive change in the thalamic region, while variance changed in the global level. In conclusion, this study is the first to show that significant signal intensity changes occur in fMRI response during the sudden deepening of thiopental anesthesia. However, these responses are neither anatomically constant nor global in the healthy swine brain.     

Magnetic field dependence of the distribution of NMR relaxation times in the living human brain

OBJECTIVE: This study investigates the field dependence of the distribution of in vivo, whole-brain T1 values, and its usefulness for white matter/grey matter segmentation. Results on T1 values are presented on 12 healthy volunteers. T2 and T2* distributions and their field dependence have been measured on the same cohort of volunteers. In this paper, however, only the T2 and T2* results on a single volunteer are presented. The reported field dependence of T2 and T2* values should, therefore, be given less weight than that of T1 times. MATERIALS AND METHODS: Relaxation times were measured in vivo on 12 healthy volunteers, using three nearly identical whole-body scanners, operating at field strengths of 1.5, 3, and 4 T and employing nearly identical software platforms and very similar hardware. T1 mapping was performed using TAPIR, a sequence based on the Look-Locker method. T2* mapping was performed with a multi-slice, multi-echo, gradient echo sequence. A multi-slice, multi-echo T2 mapping sequence based on the Carr-Purcell-Meiboom-Gill (CPMG) method was used to map T2. For each volunteer, the global distribution of T1 relaxation times was described as the superposition of three Gaussian distributions. The field and age-dependence of the centroids and widths of the three Gaussians was investigated. The segmentation of the brain in white and grey matter was performed separately for each field strength. Using the T1 segmentation and the fact that all maps were coregistered, we investigated the distribution of T2 and [Formula: see text] values separately for the white and grey matter and described them with a Gaussian distribution in each case. RESULTS: Multi-slice quantitative maps were produced for the relaxation parameters T1 (near whole-brain coverage with 41 slices), T2* (whole-brain coverage, 55 slices), and T2 (27 slices). A clear age dependence was identified for grey matter T1 values and correlated with similar behaviour observed in a separate study of the brain water content. The increase with field strength of the bulk white and grey matter T1 values was well reproduced by both Bottomley's [1] and Fischer's [2] formulae, with parameters taken from the literature. The separation between the centroids was, however, either overestimated or underestimated by the two formulae. The width of the T1 distributions was found to increase with increasing field. CONCLUSIONS: The study of the field dependence of the NMR relaxation times is expected to allow for better differentiation between regions which are structurally different, provide a better insight into the microscopic structure of the brain and the molecular substrate of its function.     

Relaxation measurements of an MRI system phantom at low magnetic field strengths

Objective

Temperature controlled T₁ and T₂ relaxation times are measured on NiCl₂ and MnCl₂ solutions from the ISMRM/NIST system phantom at low magnetic field strengths of 6.5 mT, 64 mT and 550 mT.

Materials and methods

The T₁ and T₂ were measured of five samples with increasing concentrations of NiCl₂ and five samples with increasing concentrations of MnCl₂. All samples were scanned at 6.5 mT, 64 mT and 550 mT, at sample temperatures ranging from 10 °C to 37 °C.

Results

The NiCl₂ solutions showed little change in T₁ and T₂ with magnetic field strength, and both relaxation times decreased with increasing temperature. The MnCl₂ solutions showed an increase in T₁ and a decrease in T₂ with increasing magnetic field strength, and both T₁ and T₂ increased with increasing temperature.

Discussion

The low field relaxation rates of the NiCl₂ and MnCl₂ arrays in the ISMRM/NIST system phantom are investigated and compared to results from clinical field strengths of 1.5 T and 3.0 T. The measurements can be used as a benchmark for MRI system functionality and stability, especially when MRI systems are taken out of the radiology suite or laboratory and into less traditional environments.

     

Wide‐band dual‐resonance capacitive cross‐coupled divide‐by‐5 injection‐locked frequency divider

A wide locking range divide‐by‐5 injection‐locked frequency divider (ILFD) is proposed and was implemented in the TSMC 0.18‐μm 1P6M CMOS process. Conventional divide‐by‐5 ILFD has limited locking range. The proposed divide‐by‐5 ILFD is based on a capacitive cross‐coupled voltage‐controlled oscillator (VCO) with a dual‐resonance resonator, which is implemented in the divide‐by‐5 ILFD to obtain a wide overlapped locking range. At the drain‐source bias V_DD of 0.9 V and at the incident power of 0 dBm, the measured locking range of the divide‐by‐5 ILFD is 3.2 GHz, from the incident frequency 9.4 to 12.6 GHz, the percentage is 29.09%. The core power consumption is 2.98 mW. The die area is 0.987 × 1.096 mm².     

Focal ischemia in cat brain as studied by diffusion-weighted and dynamic susceptibility-contrast magnetic resonance imaging

Diffusion-weighted and susceptibility-contrast-enhanced magnetic resonance imaging were used to monitor the development of focal ischemia in cat brain. Diffusion-sensitized imaging was used to assess early ischemic tissue damage which was confirmed for the latest time point (12 h) with postmortem histological analysis.T*₂-sensitized FLASH was used to measure the first passage of a bolus of FeO particles. Gamma function fitting of R*₂-time curves resulted in 2D maps of relative hemodynamic parameters, including cerebral blood volume and flow. The present data provide indications for cerebral blood flow thresholds for acute as well as for delayed ischemic tissue damage.