bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural and electronic properties of boron‐doped lithium clusters: Ab initio and DFT studies
Authors: Li Ying , Wu Di , Li Zhi‐Ru , Sun Chia‐Chung
Published: March 2007

The lowest‐energy structures and electronic properties of the BLin (n = 1–7) clusters are reported using the B3LYP, MP2, and CCSD(T) methods with the aug‐cc‐pVDZ basis set. Though the results at the B3LYP level agree well with those at the CCSD(T) level, the MP2 method is rather unsatisfactory. The first three‐dimensional ground state in the BLin clusters occurs for BLi4, and the impurity B atom is seen to be trapped in a Li cage from the BLi6 cluster onwards. The evolution of the binding energies, vertical ionization potentials, and polarizability with size of cluster shows the BLi5 cluster to be most stable among the BLin clusters. Besides, the BLi5 cluster is also found to have the largest reaction enthalpy (49.8 kcal/mol) upon losing a Li atom, which is different from the previous prediction. The unique stability of the 8‐valence electron BLi5 can be understood from the cluster electronic shell model (CSM). However, in contradiction to the prediction of the CSM, the 2s level is filled prior to the 1d level in the BLin clusters. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007
Polarizable model potential function for nucleic acid bases
Authors: Nakagawa Setsuko
Published: March 2007

A polarizable model potential (PMP) function for adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U) is developed on the basis of ab initio molecular orbital calculations at the MP2/6‐31+G* level. The PMP function consists of Coulomb, van der Waals, and polarization terms. The permanent atomic charges of the Coulomb term are determined by using electrostatic potential (ESP) optimization. The multicenter polarizabilities of the polarization term are determined by using polarized one‐electron potential (POP) optimization in which the electron density changes induced by a test charge are target. Isotropic and anisotropic polarizabilities are adopted as the multicenter polarizabilities. In the PMP calculations using the optimized parameters, the interaction energies of Watson–Crick type A‐T and C‐G base pairs were −15.6 and −29.4 kcal/mol, respectively. The interaction energy of Hoogsteen type A‐T base pair was −17.8 kcal/mol. These results reproduce well the quantum chemistry calculations at the MP2/6‐311++G(3df,2pd) level within the differences of 0.6 kcal/mol. The stacking energies of A‐T and C‐G were −9.7 and −10.9 kcal/mol. These reproduce well the calculation results at the MP2/6‐311++G (2d,2p) level within the differences of 1.3 kcal/mol. The potential energy surfaces of the system in which a sodium ion or a chloride ion is adjacent to the nucleic acid base are calculated. The interaction energies of the PMP function reproduced well the calculation results at the MP2/6‐31+G* or MP2/6‐311++G(2d,2p) level. The reason why the PMP function reproduces well the high‐level quantum mechanical interaction energies is addressed from the viewpoint of each energy terms. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007
Protein structure prediction: Combining de novo modeling with sparse experimental data
Authors: Latek Dorota , Ekonomiuk Dariusz , Kolinski Andrzej
Published: March 2007

Routine structure prediction of new folds is still a challenging task for computational biology. The challenge is not only in the proper determination of overall fold but also in building models of acceptable resolution, useful for modeling the drug interactions and protein–protein complexes. In this work we propose and test a comprehensive approach to protein structure modeling supported by sparse, and relatively easy to obtain, experimental data. We focus on chemical shift‐based restraints from NMR, although other sparse restraints could be easily included. In particular, we demonstrate that combining the typical NMR software with artificial intelligence‐based prediction of secondary structure enhances significantly the accuracy of the restraints for molecular modeling. The computational procedure is based on the reduced representation approach implemented in the CABS modeling software, which proved to be a versatile tool for protein structure prediction during the CASP (CASP stands for critical assessment of techniques for protein structure prediction) experiments (see http://predictioncenter/CASP6/org). The method is successfully tested on a small set of representative globular proteins of different size and topology, including the two CASP6 targets, for which the required NMR data already exist. The method is implemented in a semi‐automated pipeline applicable to a large scale structural annotation of genomic data. Here, we limit the computations to relatively small set. This enabled, without a loss of generality, a detailed discussion of various factors determining accuracy of the proposed approach to the protein structure prediction. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007
Ultrafast shape recognition to search compound databases for similar molecular shapes
Authors: Ballester Pedro J. , Richards W. Graham
Published: March 2007

Finding a set of molecules, which closely resemble a given lead molecule, from a database containing potentially billions of chemical structures is an important but daunting problem. Similar molecular shapes are particularly important, given that in biology small organic molecules frequently act by binding into a defined and complex site on a macromolecule. Here, we present a new method for molecular shape comparison, named ultrafast shape recognition (USR), capable of screening billions of compounds for similar shapes using a single computer and without the need of aligning the molecules before testing for similarity. Despite its extremely fast comparison rate, USR will be shown to be highly accurate at describing, and hence comparing, molecular shapes. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007
Molecular dynamics simulation of single wall carbon nanotubes polymerization under compression
Authors: Braga S. F. , Galvão D. S.
Published: March 2007

Single wall carbon nanotubes (SWCNTs) often aggregate into bundles of hundreds of weakly interacting tubes. Their cross‐polymerization opens new possibilities for the creation of new super‐hard materials. New mechanical and electronic properties are expected from these condensed structures, as well as novel potential applications. Previous theoretical results presented geometric modifications involving changes in the radial section of the compressed tubes as the explanation to the experimental measurements of structural changes during tube compression. We report here results from molecular dynamics simulations of the SWCNTs polymerization for small diameter arm chair tubes under compression. Hydrostatic and piston‐type compression of SWCNTs have been simulated for different temperatures and rates of compression. Our results indicate that large diameter tubes (10,10) are unlike to polymerize while small diameter ones (around 5 Å) polymerize even at room temperature. Other interesting results are the observation of the appearance of spontaneous scroll‐like structures and also the so‐called tubulane motifs, which were predicted in the literature more than a decade ago. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007
Strategies to improve contrast in turboFLASH imaging: Reordered phase encoding and K‐space segmentation
Authors: Chien Daisy , Atkinson Dennis J. , Edelman Robert R.
Published: March 2007

TurboFLASH (fast low‐angle shot) sequences enable the acquisition of an image in a fraction of a second. However, unique to T1‐weighted ultra‐fast imaging, the magnetization variation during image acquisition can produce artifacts along the phase‐encoding direction. In this study, the signal behavior and nature of these artifacts were analyzed with various acquisition schemes to improve image contrast. The magnetization variation during image acquisition and its filtering effect on the image were simulated for three different approaches to T1‐weighted turboFLASH imaging: standard turboFLASH with (a) mono‐tonically ascending phase‐encoding steps, (b) reordered phase encoding, and (c) k‐space segmentation. Each of the modified data acquisition schemes has advantages. However, for sub‐second imaging, reordered phase encoding produced improved image contrast over that of standard turboFLASH, and segmented k‐space imaging gave superior tissue contrast compared with that of both standard and reordered turboFLASH, with imaging time that permits breath‐hold studies.
MR microscopy at 7.0 T: Effects of brain iron
Authors: Malisch Timothy W. , Hedlund Laurence W. , Suddarth Steve A. , Johnson G. Allan
Published: March 2007

The T2 of brain tissue is known to be field dependent, decreasing as B0 increases. Previous studies have attributed reduced T2 in the structures of the extrapyramidal motor system (EPMS) to high iron concentrations. The present study was designed to manipulate physiologic iron concentrations and study the effects on T2 and on the field dependence of T2 at 7.0 T in whole formalin‐fixed brains. A rat model was devised in which iron concentrations in the structures of interest were altered by diet manipulation. Cerebral structures with different iron content were imaged and T2 measured with MR microscopy at both 2.0 and 7.0 T. T2 of all tissues was shorter by 40%–60% at 7.0 T. Although some dependence of T2 on iron concentration was evident, it was less than expected. The strongest correlation was in the substantia nigra. The highest‐resolution studies, at 30 × 30 × 50 μm, show the myelin bundles in many of the EPMS structures but not in the substantia nigra. From these data, it appears that T2 at greater field strengths depends more on susceptibility‐induced spin dephasing imposed by diffusion through the tissue micro‐structure than on the presence of iron.
MR imaging of coronary artery flow in isolated and in vivo hearts
Authors: Burstein Deborah
Published: March 2007

Methods for imaging flow in coronary arteries with magnetic resonance (MR) imaging techniques are demonstrated in isolated heart preparations and live animal models. Coronary artery flow was first imaged with a flow‐compensated gradient‐echo pulse sequence in isovolumic and working perfused rat hearts and then in vivo. A bolus tracking technique was used to measure flow velocity in the coronary arteries. Ultrafast gradient‐echo imaging techniques were then applied, with high resolution obtained by combining the information from several cardiac cycles. A stimulated‐echo pulse sequence was demonstrated as a method for performing coronary angiography by flow tagging in isovolumic perfused hearts. This report describes the results of coronary flow MR imaging in isolated rat hearts and live mice and rats. The general approach has proved useful in evaluating new methods for coronary MR angiography and should permit wellcontrolled studies of pathologic conditions. This ability to image coronary flow in isolated hearts and in small animals should permit integrated MR studies of coronary flow, myocardial perfusion, myocardial metabolism, and cellular ionic status.
Measurement of abdominal fat with T1‐weighted MR images
Authors: Lancaster Jack L. , Ghiatas Abraham A. , Alyassin Abdalmajeid , Kilcoyne Ray F. , Bonora Enzo , Defronzo Ralph A.
Published: March 2007

The cross‐sectional area of visceral and subcutaneous fat in the abdomen was measured with T1 weighted spin‐echo images acquired with a 1.5‐T magnetic resonance (MR) imager. Four axial images centered on L‐4 were acquired in each patient. Outline regions of interest (ROIs) were drawn manually for subcutaneous and visceral fat. The subcutaneous fat cross‐sectional area was calculated from the ROIs drawn around the outer and inner margins of subcutaneous fat. Several adaptive processing methods were evaluated for measuring fat in the complex structure of the viscera. These methods were compared with an existing MR imaging measurement method for abdominal fat in 18 patients. The adaptive method that uses the valley between the fat and nonfat distributions in the average histogram curve was judged best for research evaluations because it reduces the effects of volume averaging while using a more natural division between fat and nonfat data. Another adaptive method that yielded comparable measurements was thought to be more suitable for clinical applications. Cross‐sectional area measurements of abdominal fat were compared in 18 nonobese and 17 obese women to illustrate the utility of these measurements.
Evaluation of functional positions of tongue and soft palate with MR imaging: Initial clinical results
Authors: Wein Berthold B. , Drobnitzky Matthias , Klajman Stanislaw , Angerstein Wolfgang
Published: March 2007

Magnetic resonance imaging was used to analyze functional positions of the tongue and soft palate in 10 patients and 10 healthy volunteers. The patients had velopharyngeal insufficiency and/or dysfunction of the tongue. With snapshot FLASH (fast low‐angle shot) sequences (data acquisition time, <1 second), malpositions of the articulatory apparatus were imaged and recorded for follow‐up documentation.
Right‐side aortic arch with aneurysm of aberrant left subclavian artery: MR imaging appearance
Authors: Benedikt Richard A. , Jelinek James S. , Schaefer Paul S. , Edwards Fred , Ghaed Victor
Published: March 2007

Reports of aneurysms of the subclavian artery in both normal and anomalous aortic arches have been rare. The authors describe a patient with a right‐side aortic arch and an aneurysm of the aberrant left subclavian artery, which, to the authors' knowledge, is a previously unreported association. At presentation, the aneurysm appeared as a calcified left superior mediastinal mass. Magnetic resonance imaging enabled preoperative diagnosis and guided surgical planning.
Interventricular differences in myocardial T2 measurements: Experimental and clinical studies
Authors: Scholz Thomas D. , Fisher David J. , Ehrhardt James C. , Skorton David J.
Published: March 2007

The goal of this study was to determine the accuracy, the reproducibility, and some of the tissue determinants of image‐based myocardial T2 measurements. Image‐based T2 calculations for the free walls of the right ventricle (RV) and left ventricle (LV), in vitro T2 determination (at 0.47 T), and water, fat, and collagen content analyses were performed in ex vivo hog hearts. T2 values of the RV and LV free walls were also determined from spin‐echo images of 14 healthy human subjects. Preliminary reproducibility studies were performed with 10 sets of images acquired from a single subject. For both in vitro and image‐based T2 values of hog hearts, RV T2 was significantly longer than LV T2. Water content was the only tissue factor to significantly correlate with in vitro and image‐based T2 values. For the 14 human subjects studied, image‐based T2 values calculated from the first‐ and third‐echo images demonstrated a significant difference between LV and RV. The difference was not significant when the first‐ and second‐echo images were used. Image‐based T2 measurements of a single subject showed a coefficient of variation of 6.8% for the LV and 9.1% for the RV. The authors conclude that image‐based T2 measurements of normal myocardium can be made with sufficient precision to identify differences of the magnitude of those found between RV and LV T2 values. Image‐based T2 values of myocardium may provide useful data to aid in patient treatment.
Multipoint dixon technique for water and fat proton and susceptibility imaging
Authors: Glover Gary H.
Published: March 2007

Extensions to a previously described three‐point Dixon magnetic resonance imaging technique are presented that use alternative water/fat phase‐encoding strategies. The technique is generalized to phase encoding of (−θ,0,θ) or (0,θ,2θ) radians, and the signal‐to‐noise ratio (S/N) performance is evaluated. It was found that a θ of 2π/3 radians has optimal S/N but that a θ of π radians is a good compromise and that phase encoding of (O,π, 2π) radians offer an advantage over the previous method, which used (−π, O, π) increments, in that a T2′ (intravoxel susceptibility dephasing) image may be obtained in addition to the usual water, fat, and B0 images. A new four‐point method with phase encoding of (O, π, 2π, 3π) radians that can also provide a measure of the spectral width of the fat resonance is suggested. The disadvantages of the method are the extra imaging time and low S/N efficiency.
Visualization of MR angiographic data with segmentation and volume‐rendering techniques
Authors: Hu Xiaoping , Alperin Noam , Levin David N. , Tan Kim K. , Mengeot Michel
Published: March 2007

Novel image processing and computer graphics techniques were developed to create three‐dimensional (3D) models of vasculature from magnetic resonance (MR) angiographic images of the head or neck. Region growing was used to produce a mask that isolated the vascular signal in the MR angiographic data. The masked images were subjected to gradient‐shaded volume rendering to create 3D views of the vasculature. The computer‐derived model of intracranial vasculature was then merged with a 3D model of brain parenchyma derived from a set of MR images. The combined display of vascular and gyral anatomy may be useful for neurosurgical planning.
Significance of the point of expansion in interpretation of gradient moments and motion sensitivity
Authors: Simonetti Orlando P. , Wendt Richard E. , Duerk Jeffrey L.
Published: March 2007

The relationship between magnetic field gradient waveform moments and the motion sensitivity of magnetic resonance imaging was explored analytically and by computer simulation. The analysis and simulations revealed several key points. In general, waveform time moments define sensitivity to the time derivatives of position of moving material only at a single time point: the time about which the moments are computed. A Taylor series description of instantaneous position is expanded about this same time point to compute the phase acquired due to specific derivatives of position. A moment is proportional to phase sensitivity to a particular derivative of position throughout the waveform only when sensitivity to all lower‐order derivatives is zero. Under restricted conditions of waveform symmetry and motion characteristics, the phase due to motion may be expressed in terms of the average value of a derivative of position over the duration of the waveform. The choice of the moment center, or point of expansion, adds a degree of freedom that may be used advantageously in the design of motion‐compensating and motion phase‐encoding gradient waveforms. These results facilitate a more complete understanding of the effects of motion through a magnetic field gradient.
Acoustic pressure wave generation within an MR imaging system: Potential medical applications
Authors: Jolesz Ferenc A. , Jakab Peter D.
Published: March 2007

The static magnetic field of a magnetic resonance (MR) imaging system was used as a component of an electromagnetic transducer for generating acoustic pressure waves. To permit limited focusing, the transducer was constructed from a conductive thin plate shaped like a section of a sphere. The plate was placed within the static field of the MR unit (B0 = 1.5 T), and current pulses with a rise time of 73 nsec and amplitude of 1.0 kA were applied to it. Hydrophonic recording demonstrated a shock wave with a peak pressure of 4.8 MPa at the approximated focal point. MR guidance of lithotripsy would be particularly useful to limit undesirable soft‐tissue damage. It is also suggested that the integration of an acoustic pressure wave generator with MR imaging and control provides a novel technology for the treatment of solid soft‐tissue tumors.
Serial P‐31 MR spectroscopy after fructose infusion in patients with chronic hepatitis
Authors: Sakuma Hajime , Itabashi Kenji , Takeda Kan , Hirano Tadanori , Kinosada Yasutomi , Nakagawa Tsuyoshi , Yamada Masao , Nakano Takeshi
Published: March 2007

Serial changes in phosphorus metabolites after intravenous administration of fructose were compared between five healthy volunteers and five patients with chronic hepatitis by means of phosphorus‐31 magnetic resonance (MR) spectroscopy. P‐31 spectra were obtained every 5 minutes after intravenous drip infusion of 20% fructose at a dose of 0.5 g/kg of body weight. In the healthy volunteers, phosphomonoesters (PME) increased to 338% ± 76% of the preadministration value at 15‐20 minutes. Inorganic phosphate (Pi) was depleted in the first 15 minutes, then rebounded to 260% ± 67% of the initial value. β‐adenosine triphosphate decreased to less than 50% of its initial value and then gradually recovered. In patients with chronic hopatitis, the increase of PME at 15‐20 minutes (151% ± 49% of the preadministration value) was significantly less than that in healthy volunteers (p <.05). In addition, the rebound of pi at 35‐40 minutes (126% ± 42%) was significantly less than that in healthy volunteers (P <.05). IN conclusion, P‐31 MR Spectroscopy with fructose administration is valuable in the fuctional evaluation of diffuse liver diseases.
Phase‐constrained data extrapolation method for reduction of truncation artifacts
Authors: Amartur Sundar , Liang Zhi‐Pei , Boada Fernando , Haacke E. Mark
Published: March 2007

The authors present an improvement to a sigma‐filter extrapolation method for the reconstruction of magnetic resonance (MR) images from symmetric discrete Fourier data. By making use of the phase information in the image data, the proposed method can overcome the data inconsistency problem of the original method for handling MR image data with large phase variations, such as those obtained in gradientecho pulse sequences. Reconstruction results show that its performance is comparable with that of the modified complex sigma‐filter method proposed previously to handle the inconsistency problem. However, the new approach has the advantage of reducing computation time by a factor of two with use of a sigma filter applied to real instead of complex images. It is expected that this method will be more practical for use in clinical MR imaging systems.
Ionic versus nonionic MR imaging contrast media: Operational definitions
Authors: Allen Chang C. , Sieving Paul F. , Watson Alan D. , Dewey Torin M. , Karpishin Timothy B. , Raymond Kenneth N.
Published: March 2007

An experimental rationale is provided to differentiate between the terms ionic and nonionic for magnetic resonance (MR) imaging contrast media such as gadodiamide and gadopentetate dimeglumine. Four independent types of physical measurements (electric conductivity, osmolality, elec‐trophoresis, and ion exchange) were performed on a range of test compounds, including D‐glucose, iohexol, gadopentetate dimeglumine, and gadodiamide. Iohexol, D‐glucose, and gadodiamide are shown to be nonionic species at physiologic pH (7.4), not measurably dissociating in solution. A range of gadopentetate salts behave as electrolytes, dissociating into constituent charged ions in aqueous media. Operational definitions for the terms ionic and nonionic are provided, and the terms neutral and net zero charge are compared with nonionic for accuracy. The nomenclature nonionic and ionic is deemed appropriate for differentiating MR imaging contrast media.
Temperature monitoring during MR imaging: Comparison of fluoroptic and standard thermistors
Authors: Taber Katherine H. , Hayman L. Anne
Published: March 2007

Temperature readings from standard wire lead thermistor probes and a fluoroptic temperature probe were compared before, during, and after exposure to a spin‐echo imaging sequence at 2.35 T. All the probes were placed in saline‐filled vials (1‐mL capacity) surrounded by dense foam rubber to isolate and preserve local heating effects. There were no consistent differences in the temperatures recorded by the various probes. The authors conclude that wire lead thermistor probes do not exhibit substantial heat deposition when exposed to a standard spin‐echo imaging sequence at 2.35 T.
Visualizing three‐dimensional flow with simulated streamlines and three‐dimensional phase‐contrast MR imaging
Authors: Napel Sandy , Lee Donald H. , Frayne Richard , Rutt Brian K.
Published: March 2007

Three‐dimensional (3D) velocity maps acquired with 3D phase‐contrast magnetic resonance (MR) imaging contain information regarding complex motions that occur during imaging. A technique called simulated streamlines, Which facilitates the display and comprehension of these velocity data, is presented. Single or multiple seed points may be identified within blood vessels of interest and tracked through the velocity field. The resulting trajectories are combined with a 3D MR angiogram and displayed with 3D volume visualization software. Mathematical analysis highlights potential applications and pitfalls of the technique, which was implemented both in phantoms and in vivo with excellent results. For example, single streamlines reveal helical flow patterns in aneurysms, and multiple streamlines seeded in the common carotid artery reveal branch filling‐time relationships and slow filling of the carotid bulb. The technique is helpful in understanding these complex flow patterns.
Dynamic signal intensity changes in liver with superparamagnetic MR contrast agents
Authors: Reimer Peter , Kwong Kenneth K. , Weisskoff Robert , Cohen Mark S. , Brady Thomas J. , Weissleder Ralph
Published: March 2007

The dynamic effects of three different superparamagnetic magnetic resonance (MR) contrast agents on liver signal were evaluated with an echo‐planar imaging technique. The contrast agents were (a) USPIO (ultrasmall superparamagnetic iron oxide), which has a long blood half‐life and was developed for MR imaging of lymph nodes and bone marrow; (b) AG (arabinogalactan)‐USPIO, an asialoglycoprotein receptor directed iron oxide with hepatocyte uptake; and (c) AMI‐25, a conventional reticuloendothelial iron oxide agent. Dynamic liver signal intensity (SI) curves reflect different uptake mechanisms for the different agents. Receptor blocking experiments indicate that intracellular redistribution or clustering of the AG‐USPIO (known to occur from electron microscopy studies) does not contribute to the decrease in liver SI. Monitoring liver SI over time with echo‐planar imaging may provide a better understanding of the kinetics of the growing number of MR contrast agents and allow optimization of imaging protocols to exploit peak enhancement.
In vivo relationship between marrow T2* and trabecular bone density determined with a chemical shift—selective asymmetric spin‐echo sequence
Authors: Majumdar Sharmila , Genant Harry K.
Published: March 2007

Maguetic field inhomogeneities due to differences in susceptibility between trabecular bone and bone marrow result in a reduction in T2*. The authors previously quantified the relationship between the relaxation rate enhancement per unit change in bone density, Δ*, using dried, excised vertebral bodies immersed in saline. In the present study, they investigated the precision and reproducibility of such measurements in vitro and found that the short‐term precision ranges from 2% to 11%, while the longterm precision error, which may be governed by the placement of the region of interest, can vary up to 50%. A chemical shift—selective asymmetric spin‐echo sequence was used to assess T2* changes in the saturated fat component of bone marrow in vivo. It was shown that the marrow fat relaxation rate increases as the surrounding trabecular bone density increases and that the Δ* of the marrow fat component was 0.20 sec −1/mg/cm3. The results also indicate that the distribution of T2* varies with image resolution. Both in vitro and in vivo, characteristics of the relaxation time distribution such as the mean, standard deviation, and skewness decrease as image resolution decreases, the degree of variation depending on the density of the surrounding trabecular network.
Policies, guidelines, and recommendations for MR imaging safety and patient management
Authors: Kanal Emanuel , Shellock Frank G. ,   SMRI Safety Committee
Published: March 2007

The following are policies, guidelines, and recommendations from the Safety Committee of the Society for Magnetic Resonance Imaging (SMRI) concerning various issues related to magnetic resonance (MR) imaging safety and patient management. These policies, guidelines, and recommendations were developed to provide standardized and consistent information for use by health practitioners involved in clinical MR imaging.
Analysis of flow effects in echo‐planar imaging
Authors: Butts Kim , Riederer Stephen J.
Published: March 2007

The effects on the phase of spins moving during echo‐planar imaging (EPI) acquisition were studied. Standard single‐shot and interleaved multishot blipped EPI acquisitions were considered, assuming either high gradient strength and slew rates or standard gradient strength and slew rates. A spiral k‐space trajectory was also considered. Flow components in the section‐select and phase‐ and frequency‐encoding directions were analyzed separately. While the effect of flow in the section‐select direction is identical to that in a standard two‐dimensional Fourier transform (2DFT) acquisition, flow in the phase‐ or frequency‐encoding directions can have substantial effects on the image, different from that in 2DFT imaging. The magnitude of these effects, which include displacement, distortion, and/or ghosting of vascular structures, is analyzed and predicted for a given velocity and direction of flow, the specific acquisition sequence, and the strength and slew rate of the gradients. For example, 50‐cm/sec flow along the phase‐encoding direction can cause a blurring of 1.25 cm full width at half maximum for blipped EPI with high‐strength gradients, assuming a 40‐cm field of view and 64 × 64 matrix.
Mitral and aortic valvular flow: Quantification with MR phase mapping
Authors: Sondergaard Lars , Thomsen Carsten , Stahlberg Freddy , Gymoese Erik , Lindvig Knud , Hildebrandt Per , Henriksen Ole
Published: March 2007

When magnetic resonance phase mapping is used to quantitate valvular blood flow, the presence of higher‐order‐motion terms may cause a loss of phase information. To overcome this problem, a sequence with reduced encoding for higher‐order motion was used, achieved by decreasing the duration of the flow‐encoding gradient to 2.2 msec. Tested on a flow phantom simulating a severe valvular stenosis, the sequence was found to be robust for higher‐order motion within the clinical velocity range. In eight healthy volunteers, mitral and aortic volume flow rates and peak velocities were quantified by means of phase mapping and compared with results of the indicator‐dilution technique and Dop‐pler echocardiography, respectively. Statistically significant correlations were found between phase mapping and the other two techniques. Similar studies in patients with valvular disease indicate that phase mapping is also valid for pathologic conditions. Phase mapping may be used as a noninva‐sive clinical tool for flow quantification in heart valve disease.
Pelvic imaging with phased‐array coils: Quantitative assessment of signal‐to‐noise ratio improvement
Authors: Hayes Cecil E. , Dietz Mark J. , King Bernard F. , Ehman Richard L.
Published: March 2007

The signal‐to‐noise ratios (S/Ns) of two different pelvic magnetic resonance (MR) imaging phased arrays were compared with that of the body coil. Each array consisted of two coils placed anteriorly and two posteriorly, oriented transversely in one array and longitudinally in the other. S/N measurements were obtained in an adjustable water‐filled phantom that simulated the shape and radio‐frequency loading effects of various‐size patients. Depending on the simulated anterior‐posterior thickness of the patient, the S/N produced by the longitudinal array ranged from 2.3 to 3.1 times higher than that of the body coil. The S/N of the transverse array was 3.1 to 3.4 times higher. The increased coil sensitivity permits imaging with shorter acquisition times, smaller fields of view, finer resolution, and/or thinner sections. Two examples in patients demonstrate the enhanced imaging capability of the phased arrays.
Centric phase‐encoding order in three‐dimensional MP‐RAGE sequences: Application to abdominal imaging
Authors: Bampton Anna E. H. , Riederer Stephen J. , Korin Hope W.
Published: March 2007

Three‐dimensional (3D) magnetization‐prepared rapid gradient‐echo imaging has been proposed as a method for improving signal‐to‐noise ratio (S/N) and contrast‐to‐noise ratio (C/N) in rapid abdominal imaging. Originally, a standard sequential phase‐encoding order was proposed. In the present study, two approaches to a 3D centric phase‐encoding order are presented: (a) application of the two‐dimensional (2D) centric order to one of the 3D encoding directions, and (b) an interleaved square spiral order, which is the segmented 3D analog of the 2D centric order. With use of simulation, phantom, and volunteer results, the proposed 3D centric methods are compared in terms of S/N, C/N, and artifacts to the 3D sequential method and 2D magnetization‐prepared methods. The second centric approach was found to be superior to the first; however, in general, the 3D technique was found to be inferior to the 2D technique for abdominal imaging because of motion artifact in the 3D image set caused by misregistration among the multiple breath holds required.
Noise and artifact comparison for Fourier and polynomial phase correction used with Fourier reconstruction of asymmetric data sets
Authors: Hua Jianmin , Hurst Gregory C.
Published: March 2007

Two processes of phase correction, Fourier phase mapping and second‐order polynomial phase fitting, are compared in combination with four different schemes for Fourier reconstruction of asymmetric data, using one‐dimensional simulations and two‐dimensional human head data. Polynomial phase correction provides systematically less image noise and is much less affected by localized phase differences caused by object edges and motion.
In vivo Na‐23 MR imaging and spectroscopy of rat brain during TmDOTP5− infusion
Authors: Bansal Navin , Germann Martha J. , Lazar Istvan , Malloy Craig R. , Sherry A. Dean
Published: March 2007

In vivo sodium‐23 and hydrogen‐1 magnetic resonance (MR) imaging and spectroscopy of the rat brain during infusion of the shift reagent thulium DOTP5− (1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetra[methylene phosphonate]) was performed to assign the various peaks observed during infusion and to evaluate the shift reagent in discriminating tissue compartments. Na‐23 spectra collected during the infusion showed two shifted peaks that were assigned to intravascular Na+ and extracellular muscle Na+, respectively, and one unshifted peak assigned to intra‐ and extracellular brain Na+ and cerebrospinal fluid Na+. These assignments were validated with H‐1 and Na‐23 MR imaging and Na‐23 chemical shift imaging (CSI). The H‐1 and Na‐23 images showed that a surface coil placed on a rat head can detect a substantial amount of signal from muscle surrounding the skull. Na‐23 CSI spectra from successive 1‐mm‐thick coronal sections indicated that the shift reagent did not cross the blood‐brain barrier. The study also showed that bulk susceptibility shifts are quite small with Tm‐DOTP5−. This reagent may be useful in determining compartmental Na+ concentrations and blood flow kinetics in brain and in examining the integrity of the blood‐brain barrier.
Comparison of velocity‐encoded MR imaging and fluid dynamic modeling of steady and disturbed flow
Authors: Sun Yi , Hearshen David O. , Rankin Gary W. , Haggar Allan M.
Published: March 2007

The contrast of flow‐encoded magnetic resonance (MR) images obtained in vivo and the accuracy of velocity measurements are complicated by the presence of complex flow states. The effects of complex flow states on MR flow‐encoded images were studied and quantitative flow information was obtained with an MR phase‐subtraction technique. Regions of complex flow, including flow stagnation and separation and laminar flow, could be clearly identified on the phase images. The MR imaging velocity measurements were validated by comparison with numerical simulation results for three‐dimensional velocity distributions. The velocity MR images and the profiles obtained from the simulation generally agreed well for flow rates of 660 and 1,680 mL/min. This agreement lends support to both the fluid dynamic model and the physical basis of the phase imaging technique and establishes the validity of flow‐encoded phase imaging as an in vivo flow quantization method, especially under low Reynolds number flow conditions.
Diagnosis of fatty liver with MR imaging
Authors: Kreft Burkhard P. , Tanimoto Akihiro , Baba Yuji , Zhao Longhai , Chen John , Middleton Michael S. , Compton Carolyn C. , Finn J. Paul , Stark David D.
Published: March 2007

The diagnosis of fatty liver with magnetic resonance (MR) imaging was evaluated in experimental rat models of simple fatty infiltration and fatty liver with hepatocellular injury. T1 and T2 were measured ex vivo and correlated with the histologic degree of fatty infiltration. Enhancement of fatty liver with four different cell‐specific contrast agents was studied with ex vivo relaxometry and in vivo MR imaging. Quantitative analysis of conventional and chemical shift MR images was correlated with biochemically determined fat content of the liver. Diet‐induced simple fatty infiltration of the liver caused a decrease in T1 of 15%, whereas the T1 of L‐ethionine‐induced fatty liver with hepatocellular injury increased by 12%. T2 showed a positive correlation with the degree of fatty infiltration in both models. Cell‐specific hepatobiliary contrast agents showed the same liver uptake and relaxation enhancement in fatty livers as in normal livers. Conventional T1‐weighted images and chemical shift images showed good correlation (r =.83 and.80, respectively) between signal intensity and the degree of fatty infiltration. However, only chemical shift imaging was reliable in the diagnosis of fatty liver.
Fast spin‐echo studies of contrast and small‐lesion definition in a liver‐metastasis phantom
Authors: Chien Daisy , Mulkern Robert V.
Published: March 2007

A liver‐metastasis model was used to study the ability of fast spin‐echo (FSE) imaging to show small lesions (1 pixel in diameter) relative to conventional spin‐echo imaging. FSE images of the liver‐metastasis phantom were acquired with various phase‐encode reordering schemes to manipulate T2 contrast. The imaging time for multisection acquisitions was 27 seconds for FSE imaging and 6 minutes 48 seconds for conventional spin‐echo imaging. Computer simulations were performed to determine how the point spread function varies with the different phase‐encoding orders in FSE imaging. Contrast‐to‐noise ratios and signal profiles of the lesions were measured as a function of the effective TE and lesion size. Experimental results and theoretical simulations showed that T2‐weighted FSE imaging provides high contrast and good edge definition even for small lesions. The results indicate that FSE imaging may become a powerful method for the early detection of liver metastases.
Cerebral lactate production and blood flow in acute stroke
Authors: Henriksen Ole , Gideon Peter , Sperling Bjørn , Olsen Tom Skyhøj , Jørgensen Henrik S. , Arlien‐Søborg Peter
Published: March 2007

Eight stroke patients were examined serially in the acute phase and 1 week and 2–4 weeks after stroke with water‐suppressed proton magnetic resonance spectroscopy. The time courses of lactate level and regional cerebral blood flow were studied. A high lactate level was found in the acute phase. The lactate content decreased to barely detectable levels during the following 3 weeks, while regional blood flow increased during this period. The inverse relationship between lactate level and cerebral blood flow suggests that lactate plays no substantial role in the vasodilatation underlying the hyperemia that follows reperfusion. The amount of lactate present in the acute phase reflects the severity of ischemia in the affected region. The lactate level was still above normal in the subacute phase with hyperemia, suggesting lactate production through aerobic glycolysis. Thus, the lactate level in the subacute phase probably does not reflect the degree of anaerobic glycolysis in hypoxic neuronal tissue.
Automated local maximum‐intensity projection with three‐dimensional vessel tracking
Authors: Lin Weili , Haacke E. Mark , Smith Alison S. , Masaryk Thomas J.
Published: March 2007

Despite the simplicity and widespread acceptance of the maximum‐intensity‐projection (MIP) technique for displaying three‐dimensional (3D) magnetic resonance angiographic data, several disadvantages are associated with MIP as it is applied to all of the 3D data. These include elevated noise level, reduced contrast between small vessels and background tissue, and the inability to distinguish arteries from veins. The authors have developed a 3D vessel tracking method combined with a local MIP around a given vessel to complement conventional MIP and alleviate these problems. The method is referred to as a traveling MIP (TMIP). TMIP was evaluated in both patients and healthy volunteers. The results indicate that TMIP provides better contrast between vessels and background tissue, better lumen definition, and better vascular visualization than either MIP or vessel tracking alone.
Why fat is bright in rare and fast spin‐echo imaging
Authors: Hardy Peter A. , Henkelman R. Mark , Bishop Jonathan E. , Poon E. Colin S. , Plewes Donald B.
Published: March 2007

Fast spin‐echo (FSE) sequences are becoming popular for T2‐weighted clinical imaging because they result in a severalfold reduction in imaging time and because they provide conventional spin‐echo contrast for most tissues. Fat, however, has been observed to have anomalously high signal intensity on FSE images. The present study shows that the brighter fat results from the multiple 180° refocusing pulses, which eliminate diffusion‐mediated susceptibility dephasing and suppress J‐coupling modulation of the echo train.
Practical T2 quantitation for clinical applications
Authors: Poon Colin S. , Henkelman R. Mark
Published: March 2007

Many studies have investigated the use of magnetic resonance relaxation times for tissue characterization. A number have been performed in vivo with clinical whole‐body imagers. Unfortunately, the results have yet to establish the role of quantitative tissue relaxation time measurements in clinical diagnosis. One of the major problems is that the techniques used in many of these studies are error prone, making the results inconclusive. In the present study, the problems associated with clinical T2 measurements were systematically evaluated in an attempt to obtain reliable in vivo quantitation. The authors demonstrate that spoiler gradients are the most effective technique for artifact suppression but that they render ineffective radio‐frequency phase schemes such as the Meiboom‐Gill modification to the Carr‐Purcell sequence to compensate experimental imperfections. The present study results in a more reliable multiecho sequence for T2 measurement. Preliminary clinical results in brain and cervix demonstrate the performance of the new technique.
Mucinous carcinomas involving the prostate: Atypical findings at MR imaging
Authors: Outwater Eric , Schiebler Mark L. , Schnall Mitchell D. , Kressel Herbert Y. , Tomaszewski John E.
Published: March 2007

The magnetic resonance (MR) imaging findings in four patients with mucinous tumors involving the prostate are presented. The MR appearance of these tumors differed from that of typical prostatic adenocarci‐nomas in that the signal intensity on T2‐weighted images approximated or exceeded that of the uninvolved peripheral zone. The unusual appearance of mucinous prostate tumors may present problems in diagnosis and staging with MR imaging.
Correlation of MR changes with doppler US measurements of blood flow in exercising normal muscle
Authors: Morvan Daniel , Vilgrain Valérie , Arrivé Lionel , Nahum Henri
Published: March 2007

Muscle data from phosphorus‐31 magnetic resonance (MR) spectroscopy and hydrogen‐1 MR imaging and popliteal artery data from duplex Doppler ultrasound were compared during an exercise test of the anterior compartment of the leg, in nine healthy volunteers. Significant variations (mean ± standard deviation) were observed at the end of exercise versus rest in intracellular pH (pHi) (6.32 ± 0.02 vs 7.02 ± 0.04, P < .001), T2 (38.2 msec ± 2.3 vs 29.5 msec ± 1.1, P < .001), and popliteal output (652 mL/min ± 232 vs 149 mL/ min ± 65, P < .001). These variables showed the following significant correlations at the end of exercise: T2 and pHi (r = −.784, P < .01), T2 and popliteal output (r = −.737, P < .03), and pHi and popliteal output (r = −.902, P < .001). However, during recovery, the T2 curve was significantly different from those of pHi and popliteal output. This suggests that even if circulatory conditions play a role in the maximum T2 variation during exercise, they do not directly explain T2 changes. Furthermore, the correlations involving pHi suggest the role of the metabolism of exercising muscle in transcapillary fluid movement.
Magnetization‐prepared MR angiography with fat suppression and venous saturation
Authors: Richardson Donna B. , Bampton Anna E. H. , Riederer Stephen J. , Macfall James R.
Published: March 2007

Magnetization‐prepared magnetic resonance (MR) angiography (MPMRA) is an inflow‐based two‐dimensional (2D) imaging sequence in which a preparation phase precedes rapid image acquisition. For maximal blood/tissue contrast, an inversion‐recovery preparation nulls signal from static tissue. If needed, a second inversion suppresses signal from fat. Fully magnetized blood flows in after the inversion pulse (s), providing high signal intensity. The centric phase‐encoding order, which ensures that the initial contrast is reflected in the image set, requires the use of a modified venous saturation technique. The sequence is described and its performance assessed with regard to (a) depiction of in‐plane flow, (b) fat suppression, and (c) venous saturation. Phantom and volunteer studies showed good performance in all three areas. MPMRA images, acquired in just 2–4 seconds per image, had a blood/tissue contrast‐to‐noise ratio nearly twice that of standard 2D time‐of‐flight MR angiograms, acquired in 5–7 seconds. The technique is promising for restless patients and in anatomic areas plagued by motion degradation.
MR imaging with spatially variable resolution
Authors: Cao Yue , Levin David N.
Published: March 2007

In some situations it may be advantageous to produce “locally focused” magnetic resonance images that have nonuniform spatial resolution matching the expected local rate of spatial variation in the object. Because such an image has fewer pixels than a conventional image with uniformly high resolution, it can be reconstructed from fewer signals, acquired in less time. This can be done by using a highly convergent representation of the image as a sum of orthonormal functions with slow (fast) spatial variation in relatively homogeneous (heterogeneous) parts of the object. Since this series is shorter than a conventional truncated Fourier series, its terms can be calculated from a subset of the usual array of phase‐encoded signals. The optimal choice of these phase encodings, which are usually scattered nonuniformly in k space, results in minimization of noise in the reconstructed image. The technique is illustrated by applying it to simulated data and to data from images of phantoms.
Does memory loss occur after MR imaging?
Authors: Brockway John P. , Bream Peter R.
Published: March 2007

In four separate studies, the existence of specific memory loss after magnetic resonance (MR) imaging at 1.5 T was assessed by means of recognition and recall tests for faces, common objects, lexical items, and by digit span, in a pre‐post paradigm. Although specific memory loss was demonstrated, it was shown that the loss was not due to the main magnetic field of the imager or to radio‐frequency pulse sequences, but rather to probable psychological (not physical) factors. No gross or subtle memory changes could be attributed to MR imaging, because control groups showed similar patterns of memory loss.