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Acquisition Parameters (acquisition + parameter)
Selected AbstractsHow accurately can parameters from exponential models be estimated?CONCEPTS IN MAGNETIC RESONANCE, Issue 2 2005A Bayesian view Abstract Estimating the amplitudes and decay rate constants of exponentially decaying signals is an important problem in NMR. Understanding how the uncertainty in the parameter estimates depends on the data acquisition parameters and on the "true" but unknown values of the exponential signal parameters is an important step in designing experiments and determining the amount and quality of the data that must be gathered to make good parameter estimates. In this article, Bayesian probability theory is applied to this problem. Explicit relationships between the data acquisition parameters and the "true" but unknown exponential signal parameters are derived for the cases of data containing one and two exponential signal components. Because uniform prior probabilities are purposely employed, the results are broadly applicable to experimental parameter estimation. © 2005 Wiley Periodicals, Inc. Concepts Magn Reson Part A 27A: 73,83, 2005 [source] Assessment of the increase in variability when combining volumetric data from different scannersHUMAN BRAIN MAPPING, Issue 2 2009Santiago Reig Abstract In multicenter MRI studies, pooling of volumetric data requires a prior evaluation of compatibility between the different machines used. We tested the compatibility of five different scanners (2 General Electric Signa, 2 Siemens Symphony, and a Philips Gyroscan) at five different sites by repeating the scans of five volunteers at each of the sites. Using a semiautomatic method based on the Talairach atlas, and SPM algorithms for tissue segmentation (multimodal T1 and T2, or T1-only), we obtained volume measurements of the main brain lobes (frontal, parietal, occipital, temporal) and for each tissue type. Our results suggest that pooling of multisite data adds small error for whole brain measurements, intersite coefficient of variation (CV) ranging from 1.8 to 5.2%, respectively, for GM and CSF. However, in the occipital lobe, intersite CV can be as high as 11.7% for WM and 17.3% for CSF. Compared with the intersite, intrasite CV values were always much lower. Whenever possible, T1 and T2 tissue segmentation methods should be used because they yield more consistent volume measurements between sites than T1-only, especially when some of the scans were obtained with different sequence parameters and pixel size from those of the other sites. Our study shows that highest compatibility among scanners would be obtained using equipments of the same manufacturer and also image acquisition parameters as similar as possible. After validation, data from a specific ROI or scanner showing values markedly different from the other sites might be excluded from the analysis. Hum Brain Mapp, 2009. © 2007 Wiley-Liss, Inc. [source] Simultaneous myocardial and fat suppression in magnetic resonance myocardial delayed enhancement imaging,JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 4 2007Thomas K.F. Foo PhD Abstract Purpose To develop a method for fat suppression in myocardial delayed enhancement (MDE) studies that achieves effective signal intensity reduction in fat but does not perturb myocardial signal suppression. Materials and Methods A new approach to fat suppression that uses a spectrally-selective inversion-recovery (SPEC-IR) tip-up radio frequency (RF) pulse following the conventional nonselective IR RF pulse together with a second SPEC-IR RF pulse is proposed. The tip-up pulse restores the fat longitudinal magnetization after the nonselective IR pulse and allows the fat magnetization to recover more fully toward its equilibrium value, providing for better fat suppression by the second SPEC-IR RF pulse. This new approach was validated in phantom studies and in five patients. Results Effective fat suppression was achieved using the proposed technique with minimal impact on normal myocardial signal suppression. Mean fat suppression achieved using this approach was 67% ± 8%, as measured in the chest wall immediately opposite the heart. Conclusion The results indicate this modular-type approach optimizes fat suppression in myocardial delayed enhancement studies but does not perturb the basic IR pulse sequence or change basic acquisition parameters. J. Magn. Reson. Imaging 2007;26:927,933. © 2007 Wiley-Liss, Inc. [source] Routine clinical brain MRI sequences for use at 3.0 TeslaJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2005Hanzhang Lu PhD Abstract Purpose To establish image parameters for some routine clinical brain MRI pulse sequences at 3.0 T with the goal of maintaining, as much as possible, the well-characterized 1.5-T image contrast characteristics for daily clinical diagnosis, while benefiting from the increased signal to noise at higher field. Materials and Methods A total of 10 healthy subjects were scanned on 1.5-T and 3.0-T systems for T1 and T2 relaxation time measurements of major gray and white matter structures. The relaxation times were subsequently used to determine 3.0-T acquisition parameters for spin-echo (SE), T1 -weighted, fast spin echo (FSE) or turbo spin echo (TSE), T2 -weighted, and fluid-attenuated inversion recovery (FLAIR) pulse sequences that give image characteristics comparable to 1.5 T, to facilitate routine clinical diagnostics. Application of the routine clinical sequences was performed in 10 subjects, five normal subjects and five patients with various pathologies. Results T1 and T2 relaxation times were, respectively, 14% to 30% longer and 12% to 19% shorter at 3.0 T when compared to the values at 1.5 T, depending on the region evaluated. When using appropriate parameters, routine clinical images acquired at 3.0 T showed similar image characteristics to those obtained at 1.5 T, but with higher signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR), which can be used to reduce the number of averages and scan times. Recommended imaging parameters for these sequences are provided. Conclusion When parameters are adjusted for changes in relaxation rates, routine clinical scans at 3.0 T can provide similar image appearance as 1.5 T, but with superior image quality and/or increased speed. J. Magn. Reson. Imaging 2005;22:13,22. © 2005 Wiley-Liss, Inc. [source] 31P NMR spectroscopy in the quality control and authentication of extra-virgin olive oil: A review of recent progressMAGNETIC RESONANCE IN CHEMISTRY, Issue 5 2007Photis Dais Abstract This review is a brief account on the application of a novel methodology to the quality control and authentication of extra-virgin olive oil. This methodology is based on the derivatization of the labile hydrogens of functional groups, such as hydroxyl and carboxyl groups, of olive oil constituents with the phosphorus reagent 2-chloro-4,4,5,5-tetramethyldioxaphospholane, and the use of the 31P chemical shifts to identify the phosphitylated compounds. Various experimental aspects such as pertinent instrumentation, sample preparation, acquisition parameters and properties of the phosphorus reagent are reviewed. The strategy to assign the 31P signals of the phosphitylated model compounds and olive oil constituents by employing 1D and 2D NMR experiments is presented. Finally, the capability of this technique to assess the quality and the genuineness of extra-virgin olive oil and to detect fraud is discussed. Copyright © 2007 John Wiley & Sons, Ltd. [source] Enhancing the precision of quantitative water content mapping by optimizing sequence parametersMAGNETIC RESONANCE IN MEDICINE, Issue 1 2006H. Neeb Abstract This article describes a methodology for determining the precision of a recently published method for quantitative water content mapping using MRI. A framework for calculating the random error component as a function of sequence parameters is provided and predictions are compared with experimental results obtained in a phantom. The experimental and theoretical results are in good agreement and demonstrate that the precision of quantitative water content mapping can be significantly increased by a proper and objective choice of acquisition parameters. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source] Robust automated shimming technique using arbitrary mapping acquisition parameters (RASTAMAP)MAGNETIC RESONANCE IN MEDICINE, Issue 5 2004L. Martyn Klassen Abstract Quantitative MRI techniques as well as methods such as blood oxygen level-dependent (BOLD) imaging and in vivo spectroscopy require stringent optimization of magnetic field homogeneity, particularly when using high main magnetic fields. Automated shimming approaches require a method of measuring the main magnetic field, B0, followed by adjusting the currents in resistive shim coils to maximize homogeneity. A robust automated shimming technique using arbitrary mapping acquisition parameters (RASTAMAP) using a 3D multiecho gradient echo sequence that measures B0 with high precision was developed. Inherent compensation and postprocessing methods enable removal of artifacts due to hardware timing errors, gradient propagation delays, gradient amplifier asymmetry, and eddy currents. This allows field maps to be generated for any field of view, bandwidth, resolution, or acquisition orientation without custom tuning of sequence parameters. Field maps of an aqueous phantom show ± 1 Hz variation with altered acquisition orientations and bandwidths. Subsequent fitting of measured shim coil field maps allows calculation of shim currents to produce optimum field homogeneity. Magn Reson Med 51:881,887, 2004. © 2004 Wiley-Liss, Inc. [source] Time of flight mass spectrometry applied to the liquid chromatographic analysis of pesticides in water and foodMASS SPECTROMETRY REVIEWS, Issue 6 2006Sílvia Lacorte Abstract Liquid chromatography coupled to mass spectrometry (LC-MS) is an excellent technique to determine trace levels of polar and thermolabile pesticides and their degradation products in complex matrices. LC-MS can be equipped with several mass analyzers, each of which provides unique features capable to identify, quantify, and resolve ambiguities by selecting appropriate ionization and acquisition parameters. We discuss in this review the use of LC coupled to (quadrupole) time-of-flight mass spectrometry (LC-(Q)ToF-MS) to determine the presence of target and non-target pesticides in water and food. This technique is characterized by operating at a resolving power of 10,000 or more. Therefore, it gives accurate masses for both parent and fragment ions and enables the measurement of the elemental formula of a compound achieving compound identification. In addition, the combination of quadrupole-ToF permits tandem mass spectrometry, provides more structural information, and enhances selectivity. The purpose of this article is to provide an overview on the state of art and applicability of liquid chromatography time-of-flight mass spectrometry (LC-ToF-MS), and liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QToF-MS) for the analysis of pesticides in environmental matrices and food. The performance of such techniques is depicted in terms of accurate mass measurement, fragmentation, and selectivity. The final section is devoted to describing the applicability of LC-(Q)ToF-MS to routine analysis of pesticides in food matrices, indicating those operational conditions and criteria used to screen, quantify, and identify target and "suspected" pesticides and their degradation products in water, fruits, and vegetables. The potential and future trends as well as limitations of LC-(Q)ToF-MS for pesticide monitoring are highlighted. © 2006 Wiley Periodicals, Inc. [source] Optimized protocol of a frequency domain fluorescence lifetime imaging microscope for FRET measurementsMICROSCOPY RESEARCH AND TECHNIQUE, Issue 5 2009Aymeric Leray Abstract Frequency-domain fluorescence lifetime imaging microscopy (FLIM) has become a commonly used technique to measure lifetimes in biological systems. However, lifetime measurements are strongly dependent on numerous experimental parameters. Here, we describe a complete calibration and characterization of a FLIM system and suggest parameter optimization for minimizing measurement errors during acquisition. We used standard fluorescent molecules and reference biological samples, exhibiting both single and multiple lifetime components, to calibrate and evaluate our frequency domain FLIM system. We identify several sources of lifetime precision degradation that may occur in FLIM measurements. Following a rigorous calibration of the system and a careful optimization of the acquisition parameters, we demonstrate fluorescence lifetime measurements accuracy and reliability. In addition, we show its potential on living cells by visualizing FRET in CHO cells. The proposed calibration and optimization protocol is suitable for the measurement of multiple lifetime components sample and is applicable to any frequency domain FLIM system. Using this method on our FLIM microscope enabled us to obtain the best fluorescence lifetime precision accessible with such a system. Microsc. Res. Tech., 2009. © 2008 Wiley-Liss, Inc. [source] Investigation and correction of the gene-derived sequence of glutenin subunit 1Dx2 by matrix-assisted laser desorption/ionisation mass spectrometryRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 20 2002Vincenzo Cunsolo Direct matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOFMS) analysis of a mixture of tryptic peptides was used to verify the gene-derived amino acid sequence of the high molecular weight (HMW) subunit 1Dx2 of bread wheat. Analysis of the digest was performed by recording several MALDI mass spectra of the mixture at low, medium and high mass ranges, and optimising the matrix and the acquisition parameters for each mass range. This resulted in coverage of the whole sequence except for a short fragment T3 (3 amino acids), which was not detected. It also allowed the insertion of a Pro residue in position 59 to be identified. The results obtained provide evidence for the lack of a substantial level of glycosylation or other post-translational modifications of subunit 1Dx2, and demonstrate that MALDI-MS is the most useful method presently available for the direct verification of the gene-derived sequences of HMW glutenin subunits and similar proteins. Copyright © 2002 John Wiley & Sons, Ltd. [source] Three-dimensional inversion of automatic resistivity profiling dataARCHAEOLOGICAL PROSPECTION, Issue 4 2009Nikos G. Papadopoulos Abstract Geophysical investigations through mobile multi-electrode systems, such as the automatic resistivity profiling (ARP) method, can increase the size of the surveyed areas without jeopardizing the spatial resolution of the survey. The representation of the apparent resistivity data in maps corresponding to the different measuring dipoles is sufficient in most routine applications for outlining the buried archaeological structures. In specific cases where a more quantitative interpretation of the apparent resistivity data is demanded, a three-dimensional resistivity inversion can provide the necessary tool for this purpose. This work investigates the possibilities and limitations of the three-dimensional resistivity inversion in processing the ARP data. A three-dimensional finite element smoothness-constrained inversion algorithm was used. The active constraint balancing (ACB) method was also applied in order to enhance the stability and the resolving power of the inversion procedure. Resistivity models that are commonly encountered in archaeological exploration were used to generate synthetic apparent resistivity data using a three-dimensional finite element forward modelling program. Inversion of the synthetic data showed that the maximum investigation depth of the ARP method is comparable to the length of the larger receiving dipole and cannot exceed the 2,2.5,m for the particular ARP device tested in this work. Archaeological structures buried within this depth range can be effectively mapped, while the resolution of the subsurface structures is related to the data acquisition parameters. The inversion algorithm was also used to reconstruct the three-dimensional resistivity distribution from the ARP data set collected from the Andilly archaeological site in France. The results effectively showed that the three-dimensional inversion can act as a complementary tool in acquiring a more quantitative interpretation model of the buried archaeological features. Copyright © 2009 John Wiley & Sons, Ltd. [source] | |||||||||||||