bluuebunny/crossref_metadata_2025 huggingface dataset:
A general kinetic model for quantitative perfusion imaging with arterial spin labeling
Authors: Buxton Richard B. , Frank Lawrence R. , Wong Eric C. , Siewert Bettina , Warach Steven , Edelman Robert R.
Published: March 2007

Recently, several implementations of arterial spin labeling (ASL) techniques have been developed for producing MRI images sensitive to local tissue perfusion. For quantitation of perfusion, both pulsed and continuous labeling methods potentially suffer from a number of systematic errors. In this study, a general kinetic model for the ASL signal is described that can be used to assess these errors. With appropriate assumptions, the general model reduces to models that have been used previously to analyze ASL data, but the general model also provides a way to analyze the errors that result if these assumptions are not accurate. The model was used for an initial assessment of systematic errors due to the effects of variable transit delays from the tagging band to the imaging voxel, the effects of capillary/tissue exchange of water on the relaxation of the tag, and the effects of incomplete water extraction. In preliminary experiments with a human subject, the model provided a good description of pulsed ASL data during a simple sensorimotor activation task.
Induction of apoptosis in two mammalian cell lines results in increased levels of fructose‐1,6‐Bisphosphate and CDP‐choline as determined by 31P MRS
Authors: Williams Shane N. O. , Anthony Maria L. , Brindle Kevin M.
Published: March 2007

Programmed cell death or apoptosis was induced in human promyelocytic leukemia (HL‐60) and Chinese hamster ovary (CHO‐K1) cells using several cytotoxic drugs that have different modes of action, including camptothecin, ceramide, chelerythrine, etoposide, farnesol, geranyl geraniol, and hexadecylphosphocholine. The consequent changes in cellular metabolism were monitored using 31P MRS measurements on intact cells and cell extracts. Cells undergoing programmed cell death exhibited characteristic changes in the levels of glycolytic and phospholipid metabolites. The most significant changes were increases in the concentration of the glycolytic intermediate, fructose‐l,6‐bisphosphate and in the concentration of CDP‐choline, which is an intermediate in phosphatidylcholine biosynthesis. In HL‐60 cells, the increase in fructose‐l,6‐bisphosphate levels could be explained by depletion of cellular NAD(H) levels. All of the agents used to induce apoptosis caused the accumulation of CDP‐choline. Since the resonances of this compound occur in a relatively well resolved region of tissue spectra, it could provide a marker for apoptosis that would allow the noninvasive detection of the process in vivo using 31P MRS measurements.
A programmable pre‐emphasis system
Authors: Gach H. Michael , Lowe Irving J. , Madio David P. , Caprihan Arvind , Altobelli Stephen A. , Kuethe Dean O. , Fukushima Eiichi
Published: March 2007

MRI systems often use magnetic field gradient and shim pulse‐shaping networks (pre‐emphasis) to correct for magnetic field distortions caused by eddy currents. A pre‐emphasis system that uses up to 16 fixed resistor‐capacitor (RC) time constants per channel with programmable amplitude coefficients is described. The magnetic fields induced by the pre‐emphasis RC time constants serve as a set of basis functions for compensating eddy‐current fields induced by the gradient set. The resultant time‐varying magnetic field gradient accurately reflects the gradient specified by the pulse programmer. Reductions in eddy‐current fields are demonstrated for actively shielded and unshielded gradient sets.
Asymmetric spin‐echo imaging of magnetically inhomogeneous systems: Theory, experiment, and numerical studies
Authors: Stables Lara A. , Kennan Richard P. , Gore John C.
Published: March 2007

The ability of the asymmetric spin‐echo (ASE) pulse sequence to provide different degrees of spin‐echo (SE)‐type and gradient‐echo (GE)‐type contrast when imaging media containing magnetic inhomogeneities is investigated. The dependence of the ASE signal on the size of magnetic field perturbers is examined using theory, computer simulations, and experiment. A theoretical prediction of the ASE signal is obtained using the Anderson‐Weiss mean field theory, the results of which are qualitatively supported by computer simulations and experimental studies. It is shown that the ASE sequence can be used to tune the range of perturber sizes that provide the largest contributions to susceptibility contrast effects.
Ex vivo tissue‐type independence in proton‐resonance frequency shift MR thermometry
Authors: Peters Rober T. D. , Hinks R. Scott , Henkelman R. Mark
Published: March 2007

The temperature sensitivity of the proton‐resonance frequency (PRF) has proven valuable for the monitoring of MR image‐guided thermal coagulation therapy. However, there is significant inconsistency in reported values of the PRF‐thermal coefficient, as measured from experiments encompassing a range of in vivo and ex vivo tissue types and experimental conditions. A method of calibrating the temperature dependence of the PRF is described and results are presented that indicate a tissue‐type independence. To this end, other possible mechanisms for variations in the PRF‐thermal coefficient are suggested, including physiological perturbations and volume magnetic susceptibility effects from geometry and orientation.
In vivo perfusion measurements in the human placenta using echo planar imaging at 0.5 T
Authors: Gowland Penny A. , Francis Susan T. , Duncan Keith R. , Freeman Alan J. , Issa Bashar , Moore Rachel J. , Bowtell Richard W. , Baker Philip N. , Johnson Ian R. , Worthington Brian S.
Published: March 2007

This paper presents the first in vivo measurements of perfusion in the human placenta from 20 weeks gestational age until term, using the non‐selective/selective inversion recovery echo‐planar imaging sequence, in which data is alternately acquired following a selective and non‐selective inversion pulse. Twenty pairs of images were collected, two each at the following inversion times: 20, 310, 610, 910, 1110, 1410, 1910, 2810, 3310, and 4510 ms with the sequence being repeated with a repetition time (TR) of 10 s. The results of these measurements were used to suggest the optimum sequence for future work in terms of the signal to noise ratio in the measured perfusion rate in a given measurement time. The sequence was also analyzed to determine the expected variability in the measurements. In normal pregnancies the average value of perfusion rate was found to be 176 (standard error = ±24) ml/100 mg/min. (n = 16, standard deviation = 96 ml/100 mg/min). The expected variability in the measured parameters due to signal to noise ratio considerations alone was calculated to be 71%. For a maximum scanning time of 400 s, the optimum sequence for measuring placental perfusion was found to require 8 repetitions at each of 10 inversion times which were geometrically spaced (given by a0 a0r0 a0r2 a0r3,…), with a0 = 850 ms, r = 1.073 and TR = 5 s, giving a pixel variability of 38%. Other timing schemes are recommended for measuring perfusion in other anatomical regions with different values of perfusion rate and longitudinal relaxation time.
Reduced circular field‐of‐view imaging
Authors: Scheffler Klaus , Hennig Jürgen
Published: March 2007

A rapid dynamic imaging technique based on polar k‐space sampling is presented. A gain in temporal resolution is achieved by angular undersampling. A detailed analysis of the point spread function of angular undersampled polar imaging reveals a reduced diameter of the corresponding circular field of view. Under the assumption that dynamic changes are restricted to a local circular field of view, angular under‐sampled dynamic imaging allows the recording of rapid changes at high temporal and spatial resolution. The theoretical and experimental details of the technique are presented.
ESR spectroscopy for analysis of hippocampal elimination of a nitroxide radical during kainic acid‐induced seizure in rats
Authors: Ueda Yuto , Yokoyama Hidekatsu , Ohya‐Nishiguchi Hiroaki , Kamada Hitoshi
Published: March 2007

To analyze the balance between free radical production and elimination during seizure, techniques were developed to monitor the sequential changes in the level of an exogenous free radical by ESR in the brains of freely moving rats given kainic acid (KA) to induce seizures (n = 6) and control rats given saline (n = 6). The hippocampus of each rat was perfused with a nitroxide radical solution for 120 min by in vivo microdialysis. Then an intraperitoneal injection of KA or the physiological saline was given to each rat, and the perfusate was changed to Ringer's solution. ESR analysis of sequential samples of dialysate taken after injection revealed an exponential decay in signal amplitude. The median half‐life of the nitroxide radical was significantly longer in the KA‐treated group than in the control group (P < 0.01). This finding demonstrates a decreased capacity of the hippocampus to eliminate the nitroxide radical from the extracellular space during seizure and may help clarify the mechanism of neuronal cell vulnerability to free radicals during insult or degeneration.
Partial volume effects on vascular T2 measurements
Authors: Stainsby Jeff A. , Wright Graham A.
Published: March 2007

Quantitative, vascular T2 measurements are of interest for applications such as MR oximetry. In the situation of a vessel with long T2 relaxation times embedded in tissue with relatively short T2 values, contamination of the blood signal from the surrounding tissue can bias T2 measurements. Limited data sampling and vessels running obliquely through the imaging slice can cause significant signal contamination. Using a model of these effects to predict the behavior of T2 measurements under a range of conditions, a set of parameters that provides the best combination of measurement accuracy with a signal‐to‐noise ratio as high as possible is proposed.
Quantitative regional blood volume studies in rat myocardium in Vivo
Authors: Kahler Elke , Waller Christiane , Rommel Eberhard , Hiller Karl‐Heinz , Voll Sabine , Broich Andrea , Hu Kai , Schnackerz Klaus D. , Bauer Wolfgang R. , Ertl Greoug , Haase Axel
Published: March 2007

Many pathophysiological processes in the myocardium are in close relation to changes of the regional blood volume and regional myocardial blood flow or perfusion. Only few methods exist to obtain quantitative values for these parameters. Quantitative regional blood volume (RBV) studies in rat myocardium are presented using snapshot fast low angle shot (FLASH) inversion recovery T1 measurements with two different blood pool contrast agents, gadolinium diethylenetriamin‐opentaacetic acid (Gd‐DTPA) albumin and Gd‐DTPA polyly‐sine. In contrast to previous attempts, each snapshot FLASH image acquisition was ECG‐triggered under breathhold conditions. To measure relaxation times shorter than a heart cycle, each T1 sequence was repeated two times with different delays between inversion pulse and first image acquisition. The experiments were performed on a Bruker Biospec 70/21 using a homogeneous transmitter coil and a circularly polarized surface receiver coil, a special ECG trigger unit, and a respirator that is controlled by the pulse program. Based on a fast exchange model RBVm maps were calculated from the relaxation time maps for different concentrations of the two blood pool contrast agents. A significant dependence of the RBV, values on blood T1 was found. This is in accordance with a model that has been developed recently relating the dependence of RBVm on T1 of blood to perfusion. For Gd‐DTPA albumin, the application of the model to the experimental data yields realistic values for RBV and perfusion. The values, which are in accordance with literature data, were obtained at highest contrast agent concentrations i.e., lowest relaxation times of blood (ca. 200 ms).
Quantification of the extraction fraction for gadopentetate across breast cancer capillaries
Authors: Daldrup Heike E. , Shames David M. , Husseini Waleed , Wendland Michael F. , Okuhata Yoshitaka , Brasch Robert C.
Published: March 2007

To quantify the extraction fraction, E, for gadopentetate across tumor capillaries, R3230 adenocarcinomas were implanted in the mammary fat pads of seven rats. The value of E was determined by using a two‐compartment tissue model in which the endothelial transfer coefficient, KPS (ml. min −1. cc−1 of tissue), was estimated from the model fitted to changes in R1 relaxation time (ΔR1; s−1) measured by dynamic three‐dimensional spoiled gradient recalled magnetic resonance imaging after injection of 0.1 mmol. kg−1 of gadopentetate dimeglumine. The plasma flow rate through the tumor capillaries, Fp, (ml. min−1·g−1 of tissue), was independently measured with fluorescent microspheres. E could be calculated by the relationship, E = KPS/Fp.

The mean E for gadopentetate in the R3230 tumor was 0.197 ± 0.118 with a range of 0.123–0.454. The relatively small mean value of E for gadopentetate allows a fair approximation of the permeability surface area product by KPS in this R3230 tumor model.
39K NMR measurement of intracellular potassium during ischemia in the perfused guinea pig heart
Authors: Radford Nina B. , Babcock Evelyn E. , Richman Angela , Szczepaniak Lidia , Malloy Craig R. , Sherry A. Dean
Published: March 2007

The hyperfine shift reagent, TmDOTP5−, was used to resolve the 39K NMR resonances of intra‐ (Ki+) and extracellular (Ke+) potassium in isolated, perfused guinea pig hearts. [Ki+] as measured by 39K NMR was 25.9 ± 10.3 mM, compared with 114.4 ± 10.8 mM as measured by atomic absorption spectros‐copy (AAS) using TmD0TP5‐ as a marker of extracellular space. Thus, only approximately 23% of intracellular potassium was detected by 39K NMR using our experimental conditions. The area of the Ki+ signal increased during early ischemia then returned to baseline levels during reperfusion. In an effort to learn more about the Ki+ not detected by 39K NMR, hearts were perfused with a Rb+‐enriched, K+ ‐depleted buffer for an extended period. This resulted in loss of the entire 39K NMR signal, and Ki+, as measured by AAS, decreased from ∼60 to ∼6 to 7 μmol/g wet weight. When K+‐depleted hearts were subjected to global ischemia, a small 39K NMR signal reappeared, suggesting that at least a portion of the nonexchangeable Ki+ becomes detectable by NMR during ischemia. This newly visible K+ signal subsequently dissipated during reperfusion of ischemic hearts. We conclude that ischemia induces changes in the NMR visibility of 39K in perfused guinea pig hearts.
Characterization of flow emerging from a stenosis using MRI
Authors: Gach H. Michael , Lowe Irving J.
Published: March 2007

MRI ultra‐fast imaging techniques are used to characterize flow emerging from streamlined and abrupt stenoses inside cylindrical channels. Reattachment lengths of the shear boundary to the channel wall are measured using rotating ultra‐fast imaging sequence (RUFIS) in‐flow imaging. Velocity profiles of flow are created using velocity (sine and cosine)‐encoded RUFIS sequences. The sine‐encoded images permit one to identify reverse flow (i.e., eddies) that arise within the region of flow reattachment. The ratios of peak velocities (downstreamhpstream of the stenosis) derived from the cosine‐encoded images are used to identify the transition from the laminar to the turbulent regimen. Based on these experiments, the transition from the laminar to turbulent regimen occurs at a stenotic Reynolds Number of 350, whereas fully developed turbulence occurs at a stenotic Reynolds Number of 2600. These results are compared with the results from invasive studies.
3D Time‐resolved contrast‐enhanced MR DSA: Advantages and tradeoffs
Authors: Mistretta Charles A. , Grist Thomas M. , Korosec Frank R. , Frayne Richard , Peters Dana C. , Mazaheri Yousef , Carroll Timothy J.
Published: March 2007

The 3D TRICKS method for contrast‐enhanced, time‐resolved MR DSA has been recently described. In this paper, computer simulations are used to investigate the relative frame rate, temporal window, artery‐vein temporal separation, contrast‐to‐noise ratio, and spatial resolution of TRICKS and conventional scans for breath‐hold and non‐breath‐hold applications. For non‐breath‐hold applications, TRICKS can be coofigured to provide increased CNR or spatial resolution at an increased frame rate, but with a longer temporal window when compared with a series of conventional scans in which the central portion of k‐space is sampled at the same rate as for the TRICKS scans. For breath‐hold applications, TRICKS typically provides three images with 75% of the conventional single acquisition spatial resolution and is more tolerant of variations in contrast curve shape within the field of view.
Artifacts induced by concomitant magnetic field in fast spin‐echo imaging
Authors: Zhou Xiaohong Joe , Tan Steve G. , Bernstein Matt A.
Published: March 2007

It has been observed that fast spin‐echo (FSE) images with a large field of view (>40 cm) in certain directions exhibit unusual ghosting artifacts that cannot be eliminated with existing ghost removal methods. These artifacts have been related to a higher‐order magnetic field perturbation (known as the concomitant field, or Maxwell field) concomitant to the linear i‐maging gradient, in accordance with the Maxwell equations Several methods have been developed to eliminate or minimize the effects of the concomitant magnetic field by redesigning the FSE pulse sequences. In the slice‐selection direction, the gradient waveforms are made symmetrical about the refocusing RF pulses wherever possible. Surrounding the first refocusing pulse, such symmetry cannot be achieved due to the slice‐refocusing gradient, which is often combined with the left crusher. In this case, it is shown how crusher gradients can be reshaped to nullify the phase due to the concomitant field. In the phase‐encoding direction, the gradient amplitude is reduced and its duration is prolonged. Artifacts due to the readout gradient are eliminated by reshaping the prephasing lobe, while keeping its area fixed. In all the three directions, the gradient waveforms are adjusted so that they have minimal overlap. Selected methods have been implemented on a clinical scanner, and typically reduce the ghost intensities in phantom and human images by a factor of 3.
Functional burst imaging
Authors: Jakob Peter M. , Schlaug Gottfried , Griswold Mark , Lovblad Karl O. , Thomas Robert , Ives John R. , Matheson Jean K. , Edelman Robert R.
Published: March 2007

A quiet magnetic resonance (MR) imaging technique for detecting changes in cerebral activity functions is presented. This single‐shot method, functional Burst imaging (FBI), combines elements of Burst imaging with an offset technique known as asymmetric spin echo (ASE). The FBI sequence has the unique feature of being nearly silent, because of the low number of gradient switching steps involved. Furthermore, this approach has the key advantage that it can be implemented on conventional MR systems. Established auditory and visual paradigms were used to evaluate whether FBI can detect changes in cerebral activity using a 1.5 Tesla MR system. In a second set of experiments, the FBI technique was used to evaluate cerebral activity changes during different sleep stages in humans. The results obtained demonstrate that the FBI sequence provides an alternative approach for functional imaging of brain activity in primary and secondary sensory areas of the human brain. Furthermore, in using this quiet MR technique, it was possible to scan continuously during different stages of human sleep without acoustic noise perturbation.
Single‐shot diffusion‐weighted trace imaging on a clinical scanner
Authors: Chun Terry , Uluğ Aziz M. , van Zijl Peter C. M.
Published: March 2007

The single‐shot trace‐weighted acquisition scheme for isotro‐pic diffusion imaging suggested by Mori and van Zijl Magn. Reson. Med. 33, 41–52 (1995) was implemented on a clinical scanner in combination with echo‐planar imaging. The results on phantoms and seven volunteers show that good‐quality multiplanar isotropic diffusion‐weighted images can be obtained within seconds and absolute Dav images within 10–40 s, depending on the number of gradient strengths used. However, care has to be taken in using this type of multi‐bipolar‐gradient sequence to avoid measurement of artifactual diffusion constants due to background gradient interference. It is outlined how to test for this interference and how to optimize for the correct experiment.
Strategy for lipid suppression in lactate imaging using STIR‐DQCT: A study of hypoxic‐ischemic brain injury
Authors: Nakai Toshiharu , Rhine William D. , Okada Tomohisa , Stevenson David K. , Spielman Daniel M.
Published: March 2007

In vivo lactate detection using gradient enhanced double quantum coherence transfer (DQCT) was significantly improved by addition of short‐time‐inversion‐recovery (STIR). Phantom studies demonstrated lipid suppression down to the background noise level with 33% loss of lactate signal. In vivo studies using a rabbit model of hypoxic and unilateral‐ischemic brain injury showed reduction down to 29 ± 11% in lipids with inversion times between 140 and 170 ms. Lactate signals on the ischemic side were 51 ± 53% higher than the nonischemic side at the peak of hypoxia. STIR‐DQCT can be a useful robust method of obtaining metabolic maps of lactate in vivo.
A technique for determining the spatial relationship between the rotator cuff and the subacromial space in arm abduction using MRI and 3D image processing
Authors: Graichen Heiko , Bonel Harald , Stammberger Tobias , Heuck Andreas , Englmeier Karl‐Hans , Reiser Maximilian , Eckstein Felix
Published: March 2007

An MR imaging‐based technique for three‐dimensional determination of subacromial space width in relation to the rotator cuff in arm abduction is presented. Five volunteers were examined in an open MRI in seven arm positions, and coronal images were obtained with a gradient‐echo sequence. 30 reconstruction of the bones and the supraspinatus was performed, and the minimal spatial distances between acromion, clavicle, and humerus were calculated. The closest contact between the supraspinatus and the anterior ioferior part of the acromion occurred at 90° abduction in internal rotation. The technique presented allows investigation of the morphological basis of the impingement syndrome.
Coronary flow and flow reserve in canines using MR phase difference and complex difference processing
Authors: Wedding Kristin L. , Grist Thomas M. , Folts John D. , Maalej Nabil , Vigen Karl K. , Peters Dana C. , Osrnan Hashirn , Mistretta Charles A.
Published: March 2007

Coronary artery disease continues to be the leading cause of death for adults in the United States. Magnetic resonance imaging (MR) has the potential to dramatically impact the diagnosis of heart disease by noninvasively providing a wide range of anatomic and physiologic information. Previous re search has shown that coronary flow, one component of a complete examination, can be accurately measured in the left anterior descending artery in viva The current work validates MR flow measurements in canine circumflex arteries using transit time ultrasound as a standard. The circumflex artery experiences greater in‐plane motion and is a more stringent test for flow measurement accuracy. This work also com pares two methods of processing MR velocity data, phase difference and complex difference techniques, and examines the sources of error present in the animal validation model. Phase difference processing with a 30% magnitude threshold best matched the mean ultrasound flow values (30% PD = 1.04 × US + 1.49, r = 0.94), but it was very sensitive to vessel boundary identification. The complex difference process was less sensitive to vessel boundary identification and correlated well with the transit time ultrasound despite systematic un‐derestimations. The reasons for the discrepancies are shown to stem from a number of possible sources including variabil ity of the ultrasound standard, low signal‐to‐noise ratios in the MR images, sensitivity of the MR technique to vessel bound ary identification, and motion artifacts in the images.
Continuous assessment of perfusion by tagging including volume and water extraction (CAPTIVE): A steady‐state contrast agent technique for measuring blood flow, relative blood volume fraction, and the water extraction fraction
Authors: Zaharchuk Greg , Bogdanov Alexei A. , Marota John J. A. , Shimizu‐Sasamata Masao , Weisskoff Robert M. , Kwong Kenneth K. , Jenkins Bruce G. , Weissleder Ralph , Rosen Bruce R.
Published: March 2007

A new technique, CAPTIVE, that is a synthesis of arterial spin labeling (ASL) blood flow and steady‐state susceptibility con trast relative blood volume imaging is described. Using a single injection of a novel, long half‐life intravascular magnetopharma‐ceutical with a high tissue:blood susceptibility difference (Δ χ) to ΔR1 ratio, changes in tissue transverse relaxivity (ΔR2 or ΔR2*) that arise from changes in blood volume were measured, while preserving the ability to measure blood flow using traditional T1 ‐based ASL techniques. This modification permits the contin uous measurement of both blood flow and blood volume. Also, because the contrast agent can be used to remove the signal from intravascular spins, it is possible to measure the first‐pass water extraction fraction. Contrast‐to‐noise is easily traded off with repetition rate, allowing the use of non‐EPI scanners and more flexible imaging paradigms. The basic theory of these measurements, several experimental scenarios, and validating results are presented. Specifically, the PaCO2‐reactivity of mi‐crovascular and total relative cerebral blood volume (rCBV), ce rebral blood flow (CBF), and the water extraction‐flow product (EF) in rats with the new contrast agent MPEG‐PL‐DyDTPA is measured, and the values are concordant with those of previous literature. As an example of one possible application, continuous flow and volume measurements during transient focal ischemia are presented. It is believed that CAPTIVE imaging will yield a more complete picture of the hemodynamic state of an organ, and has further application for understanding the origins of the BOLD effect.
Changes of intracellular sodium T2 relaxation times during ischemia and reperfusion in isolated rat hearts
Authors: van Emous Jan G. , van Echteld Cees J. A.
Published: March 2007

The effect of ischemia and reperfusion on transverse relax ation (T2) of intracellular Na+ (Na+i) was measured with 5‐min time resolution in isolated rat hearts. Nai T2 relaxation was biexponential with 28 ± 7% fast (T2f)and 72 ± 7% slow (T2s) decay. This ratio was constant throughout the protocol. Dur ing 20 min of ischemia, Nai T2s increased from 18.9 ± 2.7 ms to 26.4 ± 1.1 ms (P < 0.001), whereas T2f did not change significantly (3.1 ± 1.8 versus 2.3 ± 1.6 ms during control), and Na+i increased from 9.0 ± 1.0 to 19.5 ± 1.0 mmol/liter (P < 0.001). T2s and Na+i declined again during reperfusion. Changes in T2s relaxation correlated significantly (r = 0.73, P < 0.001) with the time course of Na+i.
Total brain N‐acetylaspartate concentration in normal, age‐grouped females: Quantitation with non‐echo proton NMR spectroscopy
Authors: Gonen Oded , Viswanathan A. Kasi , Catalaa Isabelle , Babb James , Udupa Jayaram , Grossman Robert I.
Published: March 2007

The intra‐individual and inter‐individual variations of the global N‐acetylaspartate (NAA) concentration were measured in a cohort of five 42 ± 5 year‐old normal females. The total NAA signal from the whole head was obtained with non‐localized non‐echo proton spectroscopy (1H‐MRS) and con verted into absolute mole amounts using phantom replace ment. Since NAA is assumed to be present only in neurons, its concentration was obtained by dividing these mole amounts with the brains' volume, calculated from high resolution MRI. The key feature of the procedure is its near‐complete sup pression of the intense subcutaneous and bone marrow lipids' signals, whose chemical shifts neighbor and underlay the NAA. This was achieved by exploiting the lipids' much shorter T1s, compared to that of NAA, for destructive interference of their signals in co‐addition following alternating, nonselective 180° inversions. The average global, inter‐individual NAA concentration in that group was found to be 10.63 mM with a 95% confidence interval of 10.43–10.82 mM.
Adiabatic water suppression using frequency selective excitation
Authors: de Graaf Robin A. , Nicolay Klaas
Published: March 2007

A new method for B1‐insensitive water suppression using adiabatic RF pulses is described. The transition zone of the inversion profile of adiabatic full passage (AFP) pulses is used for frequency‐selective excitation followed by dephasing of the excited water with magnetic field gradients. Several im provements of AFP pulses, which also have implications for other applications, are described. The technique was evalu ated by simulations based on the Bloch equations (including relaxation), in vitro experiments and an in vivo verification on neonatal and adult rat brain.
3D MR coronary artery segmentation
Authors: Cline Harvey E. , Thedens Daniel R. , Irarrazaval Pablo , Meyer Craig H. , Hu Bob S. , Nishimura Dwight G. , Ludke Siegwalt
Published: March 2007

Coronary arteries are segmented from the blood pool using mathematical morphology operations from a 3D magnetic resonance spiral acquisition on a continuously breathing healthy volunteer. The segmented volume is maximal intensity projected at different views to yield coronary angiograms showing the left anterior descending artery (LAD), right coro nary artery (RCA), and left circumflex artery (LCX). Magnetic resonance coronary angiography provides a retrospective ro tating view of the coronary artery tree that complements oblique reformatted sections.
FAIR excluding radiation damping (FAIRER)
Authors: Zhou Jinyuan , Mori Susumu , van Zijl Peter C. M.
Published: March 2007

It is shown that the flow‐sensitive alternating inversion recov ery (FAIR) technique is complicated by the effect of radiation damping, leading to problems in calibrating this method on phantoms and to inaccuracies in measured flows. A modified scheme called FAIRER (FAIR excluding radiation damping) is proposed, which suppresses the damping effects by employ ing very weak magnetic field gradients (0.06 G/cm) during the inversion recovery, spin‐echo, and predelay periods. Results on phantoms and in vivo on cat brain are presented that demonstrate that FAIRER effectively solves these problems.
Changes of myocardial high‐energy phosphates with the cardiac cycle during acute or chronic myocardial stress
Authors: Illing Barbara , Horn Michael , Urban Bert , Strömer Hinrik , Schnackerz Klaus , de Groot Mark , Haase Axel , Hu Kai , Ertl Georg , Neubauer Stefan
Published: March 2007

Whether changes of cardiac high‐energy phosphate concen trations occur over the cardiac cycle remains controversial. The hypothesis was that such cyclical changes are accentu ated during acute or chronic myocardial stress. Isolated rat hearts were perfused under four conditions: (1) control, (2) inotropic stimulation by doubling of perfusate [Ca2+], (3) acute hypoxia (buffer Po2 ∼ 150 torr), and (4) failing, chronically infarcted hearts. 31P‐MR spectra were obtained at seven time points of the cardiac cycle. Under control conditions, cyclical changes (“cycling”) of ATP (11 ± 3%*, *P < 0.05) and phos‐phocreatine (9 ± 2%*) were detected, inorganic phosphate cycling did not reach statistical significance. At high [Ca2+] perfusion, cycling of phosphocreatine (9 ± 5%*) was not accentuated, cycling of ATP and inorganic phosphate did not reach significance. During acute hypoxia, cycling of ATP (10 ± 4%*) and inorganic phosphate (11 ± 4%*) occurred, but cyclical changes of phosphocreatine were not significant. In chronically infarcted hearts, the extent of cyclical changes of ATP, phosphocreatine, and inorganic phosphate was not accentuated. Thus, in perfused rat heart, small oscillations of high‐energy phosphates during the cardiac cycle are detect able, but such changes are not accentuated during acute or chronic stress. The concentrations of high‐energy phosphates over the cardiac cycle are tightly regulated.
In vivo neuronal tract tracing using manganese‐enhanced magnetic resonance imaging
Authors: Pautler Robia G. , Silva Afonso C. , Koretsky Alan P.
Published: March 2007

Development of efficient imaging techniques to trace neuro nal connections would be very useful. Manganese ion (Mn2+) is an excellent T1 contrast agent for magnetic resonance imaging (MRI). Four reports utilizing radioactive Mn2+ in fish and rat brain indicate that Mn2+ may be useful for tracing neuronal connections. Therefore, the purpose of this work was to determine if Mn2+ can be used as an in vivo MRI neuronal tract tracer. The results indicate that topical admin istration of MnCI2 solution to the naris of mice as well as to the retinal ganglion cells via intravitreal injection leads to en hancement of contrast along the respective pathways. There fore, application of Mn2+ to neurons allows the use of MRI to visualize neuronal connections.
Age‐related changes in Brain T1 are correlated with iron concentration
Authors: Ogg Robert J. , Steen R. Grant
Published: March 2007

Age‐related changes in brain T1 from 115 healthy subjects (range, 4.5–71.9 yr) were analyzed in relation to published regional brain iron concentration in cortex, caudate, putamen, and frontal white matter. The relaxation rate in these struc tures was linear with respect to iron concentration (P < 0.001). The iron relaxivity, k1 (s−1/mg iron/g wet weight), was much higher in cortex (5.5) and white matter (6.1) than in caudate (1.7) and putamen (1.0). These results are consistent with evidence that iron is an important factor in determining the relaxation properties of brain tissue. Iron relaxivity may reflect regional differences in the physical state of brain iron or in the interaction of brain iron with tissue water.
Effects of acute normovolemic hemodilution onT2* ‐ weighted images of rat brain
Authors: Lin Weili , Paczynski Richard P. , Celik Azim , Hsu Chung Y. , Powers William J.
Published: March 2007

Acute normovolemic hernodilution (HD) was induced in anesthetized rats to assess the effect of changes in hematocrit (Hct) on signal intensity inT2*‐weighted magnetic resonance (MR) images. Other relevant physiological parameters were maintained invariant. Two degrees of HD were induced: mild (Hct reduced from 42.6 ± 2.2% to 33.4 ± 2.1%) and moderate (Hct reduced from 44.6 ± 2.7% to 26.2 ± 1.7%). A two‐dimensional gradient‐echo sequence was used to monitor signal changes with high temporal resolution before, during, and after HD protocols. The time course of signal intensity change was closely related to that of changes in Hct. Corresponding changes in R2* (ΔR2*) with respect to the pre‐HD state were calculated for the brain parenchyma. Average ΔR2* values of −0.24 ± 0.06 s−1and −0.40 ± 0.07 s−1were obtained for the mild and moderate HD groups, respectively, during the final 2 min of MR imaging (proximal to correlative measurements of Hct). MR measured ΔR2* values were in close agreement with the expected changes inR2* predicted from theory when the measured changes in Hct were used as independent variables. These data are in good agreement with the current understanding of the effects of changes in the intravascular concentration of deoxyhemoglobin on induced magnetic susceptibility and hold promise for quantitative measurement of brain oxygenationin vivo.
Quantitative magnetic resonance imaging in the assessment of degenerative disc disease
Authors: Antoniou John , Pike G. Bruce , Steffen Thomas , Baramki Hani , Poole A. Robin , Aebi Max , Alini Mauro
Published: March 2007

Understanding degenerative intervertebral disc diseases hinges on the ability to objectively and noninvasively assess the disc matrix composition and integrity. The potential of quantitative magnetic resonance imaging to meet these demands was evaluated. Analyzing the T1 and T2 signal patterns in the nucleus pulposus with increasing Thompson morphological grade revealed a significant reduction related to grade. This significant decrease in T1 and T2 in the nucleus pulposus with grade 4 degeneration and the corresponding low correlation coefficients with respect to the content of individual matrix molecules suggest that matrix integrity plays an important and distinct role in determining T1 and T2 signal. Similarly, the significant increase in magnetization transfer in the nucleus pulposus observed in grade 4 degeneration did not correlate with the changes in molecular content in these highly degenerated discs. Again, this lack of correlation clearly indicates that the tissue integrity and matrix composition independently contribute to the magnetization transfer signal. This study presents the first clear evidence that quantitative magnetic resonance analysis reflects not only the disc matrix composition, but also the structural integrity of the matrix of the disc.
Spatial localization combining projection presaturation with a two‐dimensional excitation pulse
Authors: Lawrence Keith St. , Lee Ting‐Yim , Henkelman Mark
Published: March 2007

Single‐shot spatial localization of short T1 nuclei was achieved by outer volume suppression with projection presaturation followed by selective excitation of the desired volume with a two‐dimensional (2D) pulse. After the projection, presaturation with a 2D pulse avoided signal contamination from magnetization regrowth in the outer volume, whereas preceding the 2D pulse with projection presaturation reduced any unwanted excitation in the outer volume caused by the 2D pulse. The improvement in outer volume suppression achieved by combining these two techniques was demonstrated by images collected after the application of projection presaturation alone, a 2D pulse alone, and the two combined.
Variational h‐adaption in finite deformation elasticity and plasticity
Authors: Mosler J. , Ortiz M.
Published: March 2007

We ropose a variational h‐adaption strategy in which the evolution of the mesh is driven directly by the governing minimum principle. This minimum principle is the principle of minimum potential energy in the case of elastostatics; and a minimum principle for the incremental static problem of elasto‐viscoplasticity. In particular, the mesh is refined locally when the resulting energy or incremental pseudo‐energy released exceeds a certain threshold value. In order to avoid global recomputes, we estimate the local energy released by mesh refinement by means of a lower bound obtained by relaxing a local patch of elements. This bound can be computed locally, which reduces the complexity of the refinement algorithm to O(N). We also demonstrate how variational h‐refinement can be combined with variational r‐refinement to obtain a variational hr‐refinement algorithm. Because of the strict variational nature of the h‐refinement algorithm, the resulting meshes are anisotropic and outperform other refinement strategies based on aspect ratio or other purely geometrical measures of mesh quality. The versatility and rate of convergence of the resulting approach are illustrated by means of selected numerical examples. Copyright © 2007 John Wiley & Sons, Ltd.
Accurate eight‐node hexahedral element
Authors: Fredriksson Magnus , Ottosen Niels Saabye
Published: March 2007

Based on the assumed strain method, an eight‐node hexahedral element is proposed. Consistent choice of the fundamental element stiffness guarantees convergence and fulfillment of the patch test a priori. In conjunction with a γ‐projection operator, the higher order strain field becomes orthogonal to rigid body and linear displacement fields. The higher order strain field in question is carefully selected to preserve correct rank for the element stiffness matrix, also for distorted elements. Volumetric locking is also removed effectively. By considerations of the bending energy, improved accuracy is obtained even for coarse element meshes. The choice of local co‐ordinate system aligned with the principal axes of inertia makes it possible to improve the performance even for distorted elements. The strain‐driven format obtained is well suited for materials with non‐linear stress–strain relations. Several numerical examples are presented where the excellent performance of the proposed eight‐node hexahedral is verified. Copyright © 2007 John Wiley & Sons, Ltd.
Mesh deformation based on fully stressed design: the method and 2‐D examples
Authors: Hsu Su‐Yuen , Chang Chau‐Lyan
Published: March 2007

Mesh deformation in response to redefined boundary geometry is a frequently encountered task in shape optimization and analysis of fluid–structure interaction. We propose a simple and concise method for deforming meshes defined with three‐node triangular or four‐node tetrahedral elements. The mesh deformation method is suitable for large boundary movement. The approach requires two consecutive linear elastic finite element analyses of an isotropic continuum using a prescribed displacement at the mesh boundaries. The first analysis is performed with homogeneous elastic property and the second with inhomogeneous elastic property. The fully stressed design is employed with a vanishing Poisson ratio and a proposed form of equivalent strain (modified Tresca equivalent strain) to calculate, from the strain result of the first analysis, an element‐specific Young's modulus for the second analysis. The theoretical aspect of the proposed method, its convenient numerical implementation using a typical linear elastic finite element code in conjunction with very minor extra coding for data processing, and results for examples of large deformation of 2‐D meshes are presented in this paper. Copyright © 2007 John Wiley & Sons, Ltd.
Two‐scale method for shear bands: thermal effects and variable bandwidth
Authors: Areias Pedro M. A. , Belytschko Ted
Published: March 2007

A method for the analysis of shear bands using local partition of unity is developed in the framework of the extended finite element method (XFEM). Enrichments are introduced for both the displacement field and the thermal field. The shear band width is determined by minimizing the plastic work. A coupled finite strain thermo‐elastoplastic constitutive law is used. The enrichment is injected into the mesh when the material law becomes unstable. The criterion based on a complete stability analysis for materials in the finite strain regime including heat conduction, strain hardening, strain rate hardening and thermal softening is presented. A mixed continuous quadrilateral element is employed. The method is applied to the Nesterenko experiments, which exhibit multiple propagating shear bands and other problems. Copyright © 2007 John Wiley & Sons, Ltd.
Locking‐free finite elements for shear deformable orthotropic thin‐walled beams
Authors: Minghini F. , Tullini N. , Laudiero F.
Published: March 2007

Numerical models for finite element analyses of assemblages of thin‐walled open‐section profiles are presented. The assumed kinematical model is based on Timoshenko–Reissner theory so as to take shear strain effects of non‐uniform bending and torsion into account. Hence, strain elastic‐energy coupling terms arise between bending in the two principal planes and between bending and torsion. The adopted model holds for both isotropic and orthotropic beams. Several displacement interpolation fields are compared with the available numerical examples. In particular, some shape functions are obtained from ‘modified’ Hermitian polynomials that produce a locking‐free Timoshenko beam element. Analogously, numerical interpolation for torsional rotation and cross‐section warping are proposed resorting to one Hermitian and six Lagrangian formulation. Analyses of beams with mono‐symmetric and non‐symmetric cross‐sections are performed to verify convergence rate and accuracy of the proposed formulations, especially in the presence of coupling terms due to shear deformations, pointing out the decay length of end effects. Profiles made of both isotropic and fibre‐reinforced plastic materials are considered. The presented beam models are compared with results given by plate‐shell models. Copyright © 2007 John Wiley & Sons, Ltd.
Boundary recovery after 3D Delaunay tetrahedralization without adding extra nodes
Authors: Liu Jianfei , Chen Bin , Chen Yongqiang
Published: March 2007

In this paper, we investigate boundary recovery, the problem that has troubled researchers ever since Delaunay‐based methods were applied to generate mesh. There are a number of algorithms for boundary recovery already and most of them depend heavily on adding extra nodes. In this paper, we make an effort to seek a method to recover boundaries without using extra nodes.

It was noted that some previous algorithms imposed artificial boundary constraints on a meshing problem at the recovering stage; we first try to discard these artificial constraints and thus make things easier. Then a new method is proposed by which the boundaries can be recovered by means of two operations: (1) creating a segment in the mesh and (2) removing a segment from the mesh. Both operations are special cases of a general local transformation called small polyhedron reconnection operation. The method works well when coupled with the sphere‐packing method proposed by the first author. If the mesh sizing function is suitable, a good configuration of nodes will be created accordingly by the sphere‐packing method and the boundary can be recovered by the local transformation presented here without inserting extra nodes. Copyright © 2007 John Wiley & Sons, Ltd.
High quality baseline for high efficiency, Cu(In1−x,Gax)Se2 solar cells
Authors: Jackson Philip , Würz Roland , Rau Uwe , Mattheis Julian , Kurth Matthias , Schlötzer Thomas , Bilger Gerhard , Werner Jürgen H.
Published: March 2007

We report on the progress that we have made in the quality of our baseline process for the production of high efficiency soda lime glass/Mo/Cu(In,Ga)Se2 (CIGS)/CdS/i‐ZnO/ZnO:Al/MgF2 solar cells. The enhancement of the average performance level has enabled us to reach conversion efficiencies of up to 19·3% (internal measurement). The new quality initiative uses process control, optical and electrical modelling, and the critical revision of all process steps as tools for the attainment of the 19% efficiency level. Our experiments show that the compositional process window for CIGS solar cells that have an efficiency of η ≈ 19% is very wide. Accordingly, we suggest that an efficiency of 19·0–19·5% is achievable in the following compositional process windows: 0·69 ≤ Cu/(Ga + In) ≤ 0·98 and 0·21 ≤ Ga/(Ga + In) ≤ 0·38. In addition, our results show that large CIGS grains are not a necessary requirement for high efficiencies up to 19%. These findings and the partly lacking ability to correlate certain aspects of our progress with experimental parameters lead us to the conclusion that there are still some important process variables undiscovered. From this conclusion and from the evaluation of the available data we infer that there is a potential for the enhancement of CIGS solar cell efficiencies beyond 20%. Copyright © 2007 John Wiley & Sons, Ltd.
Nebulisers: their effectiveness, indications and limitations
Authors: Johns Robin , Roberts C Michael
Published: March 2007

The authors describe the indications and adverse effects of nebulisers and show how careful assess ment can identify those patients who will benefit. Copyright © 2007 Wiley Interface Ltd
Current management of nutritional deficiencies in older people
Authors: Lehmann Anthea
Published: March 2007

Our series Prescribing in older people gives practical advice for successful management of the special problems experienced by this patient group. Here, Dr Lehmann discusses some of the common nutritional problems that arise and looks at developments in management. Copyright © 2007 Wiley Interface Ltd