bluuebunny/crossref_metadata_2025_split huggingface dataset:
Polar sampling in k‐space: Reconstruction effects
Authors: Lauzon M. Louis , Rutt Brian K. 
Published: March 2007

AbstractMagnetic resonance images are most commonly computed by taking the inverse Fourier transform of the k‐space data. This transformation can potentially create artifacts in the im age, depending on the reconstruction algorithm used. For equally spaced radial and azimuthal k‐space polar sampling, both gridding and convolution backprojection are applicable. However, these algorithms potentially can yield different res olution, signal‐to‐noise ratio, and aliasing characteristics in the reconstructed image. Here, these effects are analyzed and their tradeoffs are discussed. It is shown that, provided the modulation transfer function and the signal‐to‐noise ratio are considered together, these algorithms perform similarly. In contrast, their aliasing behavior is different, since their re spective point spread functions (PSF) differ. In gridding, the PSF is composed of the mainlobe and ringlobes that lead to aliasing. Conversely, there are no ringlobes in the convolution backprojection PSF, thus radial aliasing effects are mini mized. Also, a hybrid gridding and convolution backprojection reconstruction is presented for radially nonequidistant k‐space polar sampling.
In vivo lactate and β‐hydroxybutyrate editing using a pure‐phase refocusing pulse train
Authors: Shen Jun , Novotny Edward J. , Rothman Douglas L. 
Published: March 2007

AbstractA refocusing pulse train consisting of a semiselective refo cusing pulse and a selective inversion pulse to obtain a pure‐phase refocusing at the frequency of maximal excitation of the semiselective refocusing pulse is proposed and applied to in vivo lactate and β‐hydroxybutyrate editing using difference spectroscopy. It is shown, using both rotation matrix theory and phantom experiments, that the soft inversion pulse has to be halved to flank the semiselective pulse to obtain perfect refocusing and cancellation of interfering resonances. The editing method is used to obtain lactate and β‐hydroxybu‐tyrate spectra from the occipital cortex of juvenile epilepsy patients before and after ketogenic diet treatment.
In vivo gradient echo microimaging of rodent spinal cord at 7 T
Authors: Meyerand Mary E. , Cremillieux Yannick , Wadghiri Youssef Zaim , Azzawi Ali , Hoopes Paul J. , Dunn Jeffrey F. 
Published: March 2007

AbstractAn optimization scheme was developed for gradient echo imaging using a half‐birdcage RF coil at 7 T to obtain maximal contrast between gray and white matter in the spinal cord of rodents. This optimization was combined with microimaging techniques to obtain in vivo pixel sizes of 78 × 78 × 700 μm. These techniques can be implemented in an in vivo study to investigate the myelin structure within the white matter of the rodent spinal cord.
NMR imaging of changes in vascular morphology due to tumor angiogenesis
Authors: Dennie Joelle , Mandeville Joseph B. , Boxerman Jerrold L. , Packard Scott D. , Rosen Bruce R. , Weisskoff Robert M. 
Published: March 2007

AbstractTumor‐sprouted vessels are greater in both number and diameter in comparison to their healthy counterparts. A novel technique based on magnetic susceptibility contrast mechanisms that are sensitive to varying sizes of blood vessels is presented to measure differences between the relaxation rates (1/T2 and 1/T) in a rat glioma model and normal cerebral cortex. ΔR2 and ΔR2*, the differences between relaxation rates precontrast and postcontrast agent injection, were measured for an intravascular equilibrium contrast agent (MION) at various echo times. Since ΔR2*/ΔR2 increases as vessel size increases, this ratio can be used as a measure of the average vessel size within an ROI or a voxel. The stability and longevity of the contrast agent within the vasculature were verified (n = 2 trials), and the ratio of ΔR2*/ΔR2 between the tumor and normal cortex was measured to be 1.9 ± 0.2 (n = 4, echo time = 20 ms, and susceptibility difference (Δχ) ≈︁ 10−6). This ratio compared favorably to a predicted ratio determined using histologically determined vessel sizes and theoretical Monte Carlo modeling results (1.9 ± 0.1). Maps of the ratio of ΔR2*/ΔR2 were also made on a pixel‐by‐pixel basis. These techniques support the hypothesis that susceptibility contrast MRI can provide useful quantitative metrics of in vivo tumor vascular morphology.
EPISTAR MRI: Multislice mapping of cerebral blood flow
Authors: Edelman Robert R. , Chen Qun 
Published: March 2007

AbstractA method is described for multislice EPISTAR that perfectly compensates magnetization transfer effects. lnflowing arterial spins are labeled with a 360° adiabatic pulse. Two control tags are applied sequentially at the same location as the labeling pulse, each with a 180° adiabatic pulse so the total RF irradiation, frequency shift, and bandwidth of the labeling and control pulses are identical. Therefore, magnetization transfer effects are the same as for the labeling pulse and cancel with image subtraction for all slices. The method also eliminates tagging of venous spins and concern about asymmetric magnetization transfer effects.
In vivo topical EPR spectroscopy and imaging of nitroxide free radicals and polynitroxyl‐albumin
Authors: Kuppusamy Periannan , Wang Penghai , Shankar Ravi A. , Ma Li , Trimble Charles E. , Hsia Carleton J. C. , Zweier Jay L. 
Published: March 2007

AbstractPiperidine nitroxides have considerable clinical potential, both as antioxidant therapeutic compounds and contrast agents in magnetic resonance imaging. However, their development has thus far been limited by their rapid bioreduction in vivo. Recently, it was reported that polynitroxyl albumin (PNA) can reverse the bioreduction of the reduced 4‐hydroxy‐2,2,6,6‐tetramethylpiperidine‐N‐oxyl (Tempol) in the rat heart, enabling the performance of high resolution EPR imaging for prolonged time (Kuppusamy et al., Biochemistry 35, 7051–7057 (1996)). In this report, the efficacy of PNA in maintaining Tempol concentrations in vivo in mice was demonstrated, using L‐band (1.25 GHz) EPR spectroscopy and imaging. The EPR signal of intravenous Tempol had a half‐life of 1.0 ± 0.2 min and became undetectable within 6 min. Subcutaneous Tempol, however, decayed at a slower rate (half‐life, 5.0 ± 0.5 min) suggesting that Tempol had been bioreduced to the corresponding hydroxylamine form, Tempol‐H. Subcutane‐ously injected PNA restored 20% of the Tempol signal in the vicinity of the PNA deposit. In vivo topical EPR imaging demonstrated that the Tempol signal was restored at the site of PNA injection, but not at locations remote from the PNA injection site. The ability of PNA to maintain Tempol in its paramagnetic state in vivo should enable a wide range of therapeutic and diagnostic applications of piperidinyl nitroxides.
Automated spectral analysis I: Formation of a priori information by spectral simulation
Authors: Young Karl , Govindaraju Varanavasi , Soher Brian J. , Maudsley Andrew A. 
Published: March 2007

AbstractA spectral simulation method is described for generating a priori information for use in parametric spectral analysis. The method makes use of GAMMA (S. A. Smith, T. O. Levante, B. H. Meier, R. R. Ernst, J. Magn. Reson., 106A, 75–105, 1994), a programming environment that facilitates simulation of magnetic resonance phenomena. The input parameters consist of the chemical shifts and scalar spin‐coupling constants for the compounds to be analyzed, the acquisition pulse sequence, and the field strength used. The resultant spectral information consists of the relative amplitude, frequency, and phase of all resonances, which are stored in a spectral database. This procedure can be rapidly and conveniently modified to reflect different acquisition parameters and data analysis requirements.
Automated spectral analysis III: Application to in Vivo proton MR Spectroscopy and spectroscopic imaging
Authors: Soher Brian J. , Young Karl , Govindaraju Varanavasi , Maudsley Andrew A. 
Published: March 2007

AbstractAn automated method for analysis of in vivo proton magnetic resonance (MR) spectra and reconstruction of metabolite distributions from MR spectroscopic imaging (MRSI) data is described. A parametric spectral model using acquisition specific, a priori information is combined with a wavelet‐based, nonparametric characterization of baseline signals. For image reconstruction, the initial fit estimates were additionally modified according to a priori spatial constraints. The automated fitting procedure was applied to four different examples of MRS data obtained at 1.5 T and 4.1 T. For analysis of major metabolites at medium TE values, the method was shown to perform reliably even in the presence of large baseline signals and relatively poor signal‐to‐noise ratios typical of in vivo proton MRSI. identification of additional metabolites was also demonstrated for short TE data. Automated formation of metabolite images will greatly facilitate and expand the clinical applications of MR spectroscopic imaging.
In vivo1H MR spectroscopic imaging and difusion weighted mri in experimental hydrocephalus
Authors: Braun Kees P. J. , de Graaf Robin A. , Vandertop W. Peter , Gooskens Rob H. J. M. , Tulleken Kees A. F. , Nicolay Klaas 
Published: March 2007

AbstractThe severity and progression of ventricular enlargement, the occurrence of cerebral edema, and the localization of isch‐emic metabolic changes were investigated in a rat model of hydrocephalus, using in vivo 1H MR spectroscopic imaging (SI) and diffusion weighted MRI (DW MRI). Hydrocephalic rats were studied 1, 2, 4, and 8 weeks after injection of kaolin into the cisterna magna. Parametric images of the apparent diffusion coefficient (ADC) revealed a varying degree of ventricu‐lomegaly in all rats, with different time courses of ventricular explansion. Extracellular white matter edema was observed during the early stages of hydrocephalus, most extensively in cases of progressive ventriculomegaly. In gray matter regions, ADC values were not changed, compared with controls. In case of fatal hydrocephalus, high lactate levels were observed throughout the whole brain. In all other rats, at all time points after kaolin injection, lactate was detected only in voxels containing cerebrospinal fluid. This suggests accumulation of lactate in the ventricles, and/or an ongoing peri ventricular production of lactate as a consequence of cerebral ischemia in experimental hydrocephalus.
Physiological basis for BOLD MR signal changes due to neuronal stimulation: Separation of blood volume and magnetic susceptibility effects
Authors: Kennan Richard P. , Scanley B. Ellen , Innis Robert B. , Gore John C. 
Published: March 2007

AbstractAn NMR method is applied for separating blood volume and magnetic susceptibility effects in response to neuronal stimulation in a rat model. The method uses high susceptibility contrast agents to enhance blood volume induced signal changes. In the absence of exogenous agent, the dominant source of signal change on neuronal activation is associated with the signal increase from the blood oxygen level dependent (BOLD) effect. The relative negative contribution of blood volume changes to BOLD changes is maximally estimated to be 34%. The blood volume changes associated with median nerve stimulation (7 Hz) in the motor cortex are 26 ± 7% and the corresponding blood susceptibility changes are 0.021 ± 0.006 ppm. These methods can be applied to enhance the sensitivity of fMRl signal response and provide accurate quantitative measures of blood volume response to stimulation.
SAR and B1 field distributions in a heterogeneous human head model within a birdcage coil
Authors: Collins Christopher M. , Li Shizhe , Smith Michael B. 
Published: March 2007

AbstractCalculations of radiofrequency magnetic (B1) field and specific energy absorption rate (SAR) distributions in a sphere of tissue and a multi‐tissue human head model in a 12‐element birdcage coil are presented. The coil model is driven in linear and quadrature modes at 63, 175, 200, and 300 MHz. Plots of B, field magnitude and SAR distributions, average SAR, maximum local SAR, and measures of B1 field homogeneity and signal‐to‐noise ratio are given. SAR levels for arbitrary pulse sequences can be estimated from the calculated data. Maximum local SAR levels are lower at lower frequencies, in quadrature rather than in linear coils, and in linear fields oriented posterior‐to‐anterior rather than left‐to‐right in the head. It should be possible to perform many experiments in the head at frequencies up to 300 MHz without exceeding standard limits for local or average SAR levels.
Catheter‐tracking FOV MR fluoroscopy
Authors: Atalar Ergin , Kraitchman Dara L. , Carkhuff Bliss , Lesho Jeffery , Ocali Ogan , Solaiyappan Meiyappan , Guttman Michael A. , Charles Harry K. 
Published: March 2007

AbstractRecent improvements in intravascular magnetic resonance imaging techniques mandate an accurate method of monitoring the introduction of MR catheter probes into the vessel of interest. For this purpose, a novel imaging protocol and a display method have been designed. First, a roadmap 3D image data set with standard pulse sequences is obtained using an external imaging coil. Subsequently, using very narrow rectangular‐FOV fast‐spoiled gradient recalled (SPGR), a movie of the percutaneous placement procedure of an MR catheter probe is acquired at a rate of 7.3 frames/second. In this protocol, the probe is used to transmit RF pulses and receive MR signal. A computer program was written for image unwrapping and for displaying the unwrapped movie frames on the roadmap image. In an alternative protocol, the movie frames in two projection angles were acquired in an interleaved fashion. Frames were unwrapped and combined with a 3D roadmap and displayed on a Silicon Graphics workstation equipped with stereovision goggles. Using these methods, percutaneous catheter placement in a phantom and a dog was examined. In conclusion, a new visualization technique for MR catheter placement is proposed. Combining this technique with high resolution intravascular MRI techniques may result in a very useful diagnostic tool for the evaluation of atherosclerosis and other vessel diseases.
Optimized visualization of vessels in contrast enhanced intracranial MR angiography
Authors: Parker Dennis L. , Goodrich K. Craig , Alexander Andrew L. , Buswell Henry R. , Blatter Duane D. , Tsuruda Jay S. 
Published: March 2007

AbstractIn this study, the problem of small vessel visualization in magnetic resonance angiography is addressed. The loss of vessel contrast due to slow flow‐related signal saturation can be compensated by the T1 reduction obtained from the use of an MR contrast agent, such as Gd‐DTPA. The vesselfbackground signal‐difference‐to‐noise ratio (SDNR) is shown to strongly depend on the imaging parameters, as well as on the time course of the blood T1 values obtained from the contrast injection. Specifically, it was found that vessel SDNR increases almost linearly with TR, if the sampling bandwidth is reduced proportionately.
Two‐dimensional multishot echo‐planar coronary MR angiography
Authors: Slavin Glenn S. , Riederer Stephen J. , Ehman Richard L. 
Published: March 2007

AbstractThis work presents a two‐dimensional (2D) multishot echo‐planar imaging (EPI) technique for magnetic resonance angiography (MRA) of individual coronary arteries in a 17‐heart‐beat breath‐hold. Conventional 2D and 3D segmented gradient‐echo (GRE) coronary MRA requires repetitive excitation of the same slice or slab within each cardiac cycle, which can result in reduced blood signal and in motion artifacts. Two‐dimensional multishot EPI can address these limitations by eliminating multiple excitations per cardiac cycle, using large flip‐angle excitations, markedly reducing the data acquisition window, and performing oblique multislice 2D imaging. The goal of this study was to assess the feasibility of breath‐hold 20 multishot EPI for multislice coronary MRA and to demonstrate its reliability by consistently acquiring high‐quality images of the coronary arteries in a series of 16 volunteers.
Design of practical T2‐selective RF excitation (TELEX) pulses
Authors: Sussman Marshall S. , Pauly John M. , Wright Graham A. 
Published: March 2007

AbstractTraditional T2‐based imaging techniques are geared toward imaging long‐T2, species. Traditional techniques are, therefore, not optimal in clinical situations where the information of interest lies in the short‐T2 species. T2‐selective RF excitation (TELEX) is a technique for obtaining a T2‐based contrast that highlights short‐T2 values while suppressing long‐T2 values‐opposite to traditional T2 contrast. Previously, TELEX has been demonstrated qualitatively to highlight only very short‐T2 values (T2 = 0.001 s). When applied to longer T2 values (T2 ≈︁ 0.01 s), TELEX becomes sensitive to ΔBo non‐uniformities. This restricts its application to problems in which the T2 of interest is very short. In this study, TELEX is characterized quantitatively. Furthermore, a bandwidth broadening scheme is developed that reduces the ΔBo sensitivity of TELEX. This permits the technique to be applied to longer T2 values. The capabilities and limitations of a practical implementation of TELEX are discussed.
Integrated and interactive position tracking and imaging of interventional tools and internal devices using small fiducial receiver coils
Authors: Coutts Glyn A. , Gilderdale David J. , Chui Ming , Kasuboski Larry , Desouza Nandita M. 
Published: March 2007

AbstractA method is described of tracking the position of a rigid device within an MR scanner and imaging with the image slice position determined by the current position of the device, such that the manipulation of the device can be achieved in a robust, interactive manner. The device can be either external or internal to the patient. The position tracking is performed by means of two or three small MR receiver coils attached to individual receiver channels. Each coil contains a small sample that acts as a fiducial (MR‐visible marker point). The imaging is performed by any suitable receiver coil attached to a further receive channel. This method has a large number of applications, both in the field of MR‐guided procedures and in MR imaging using endocavitary coils. In particular, two devices are described, the first being a device for MR‐guided biopsies and the second being one for dynamic endorectal imaging of the anal sphincter.
The MR tomograph as a sound generator: fMRI tool for the investigation of the auditory cortex
Authors: Bilecen Deuiz , Radü Ernst W. , Scheffler Klaus 
Published: March 2007

AbstractAn automatic fMRl protocol for the investigation of the human auditory cortex has been developed. For the activation of the auditory system, the scanner noise itself is the source of sound. Rapid switching of 6, gradients allows the generation of tones of defined frequencies and amplitude on a conventional scanner without hardware change. No additional device for sound generation or sound delivety is needed. The proposed technique allows functional MRI of the auditory cortex in a clinical routine session.
Perfusion imaging using FOCI RF pulses
Authors: Yongbi Martin N. , Branch Craig A. , Helpern Joseph A. 
Published: March 2007

AbstractPulsed arterial spin‐tagging techniques for perfusion measurements (e.g., echo planar MR imaging and signal targeting with alternating radiofrequency (EPISTAR), flow‐sensitive alternating inversion recovery (FAIR), quantitative imaging of perfusion using a single subtraction (QUIPPS), uninverted FAIR (UNFAIR)) generally use hyperbolic secant (HS) pulses for spin inversion. The performance of these techniques depends on the inversion efficiency, as well as the sharpness of the slice profiles. Frequency offset corrected inversion (FOCI) pulses, a recently proposed HS variant, can provide slice profiles with edges that can be up to 10 times sharper than those obtained with conventional HS pulses. In this communication, the implementation and application of the C–shape FOCI pulse for perfusion imaging in rat brain with the FAIR technique is summarized. Despite providing a more rectangular slice profile than a conventional HS pulse, it is demonstrated both theoretically and experimentally that the FAIR perfusion signal is not increased by using a FOCI tagging pulse. However, the use of a FOCI inversion pulse is shown to significantly minimize static signal subtraction errors that are common with conventional HS pulses. Finally, the suitability of the pulse for perfusion studies is demonstrated, in vivo, on rat brain.
Dynamic refinement and boundary contact forces in SPH with applications in fluid flow problems
Authors: Feldman J. , Bonet J. 
Published: March 2007

AbstractThis paper presents a general method for dynamic particle refinement in smoothed particle hydrodynamics (SPH). Candidate particles are split into several ‘daughter’ particles according to a given refinement pattern centred about the original particle. Through the solution of a non‐linear minimization problem the optimal mass distribution of the daughter particles is obtained so as to reduce the errors introduced to the underlying density field. This procedure necessarily conserves the mass of the system. Conservation of energy and momentum results are also discussed. Copyright © 2007 John Wiley & Sons, Ltd.
A substructured FE‐shell/XFE‐3D method for crack analysis in thin‐walled structures
Authors: Wyart E. , Coulon D. , Duflot M. , Pardoen T. , Remacle J.‐F. , Lani F. 
Published: March 2007

AbstractStress intensity factors in thin‐walled cracked structures are computed using a mixed‐dimensional finite element (FE) shell/extended finite element (XFE) 3D formulation. The present approach belongs to the family of ‘substructuring’ methods. The domain is decomposed into cracked and safe subdomains which are solved by the XFE‐code and the FE‐software, respectively. The XFE‐3D domain may contain cracks that do not necessarily join the top and bottom faces of the plate. The interface problem is solved using the finite element tearing and interconnecting method. Several validations are provided. Copyright © 2007 John Wiley & Sons, Ltd.
Magnetic properties of Ce,Dy‐substituted yttrium iron garnet ferrite powders fabricated using a sol–gel method
Authors: Xu Haitao , Yang Hua 
Published: March 2007

AbstractY3–x –y Cex Dyy Fe5O12 (x = 0–0.1, y = 0–1.0) nanoparticles were fabricated by a sol–gel method. The crystalline structures and magnetic properties of the samples were investigated by X‐ray diffraction (XRD), scanning probe microscopy (SPM), X‐ray photoelectron spectroscopy (XPS), and vibrating sample magnetometry (VSM). The XRD patterns of Y3–x –y Cex Dyy Fe5O12 have only peaks of the garnet structure and the mean size range from 42 to 60 nm. The particles' sizes observed for all the samples analyzed by SPM are in good agreement with these data from XRD. Results of VSM show that the saturation magnetization increases with increasing Ce content (x) and decreases with increasing Dy content (y) in a linear manner. The saturation magnetization increases with increasing particle size. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)