bluuebunny/crossref_metadata_2025 huggingface dataset:
Competitive Equilibria in Atom Transfer Radical Polymerization
Authors: Tsarevsky Nicolay V. , Braunecker Wade A. , Vacca Alberto , Gans Peter , Matyjaszewski Krzysztof
Published: March 2007

With the recent development of new initiation techniques in atom transfer radical polymerization (ATRP) that allow catalysts to be employed at unprecedented low concentrations (∼10 ppm), a thorough understanding of competitive equilibria that can affect catalyst performance is becoming increasingly important. Such mechanistic considerations are discussed herein, including i) factors affecting the position of the ATRP equilibrium; ii) dissociation of the ATRP catalyst at high dilution and loss of deactivator due to halide dissociation; iii) conditional stability constants as related to competitive monomer, solvent, and reducing agent complexation as well as ligand selection with respect to protonation in acidic media; and iv) competitive equilibria involving electron transfer reactions, including the radical oxidation to carbocations or reduction to carbanions, radical coordination to the metal catalyst, and disproportionation of the CuI‐based ATRP activator.
RAFT Polymerization in Bulk and Emulsion
Authors: Buttè Alessandro , Peklak A. David , Storti Giuseppe , Morbidelli Massimo
Published: March 2007

Detailed models of the RAFT polymerization in both non‐segregated (bulk) and segregated (seeded emulsion) systems are presented. It is shown that satisfactory agreements between experiments and models can be achieved, and that effects such as inhibition and retardation, or the polymerization behavior at high conversions can be readily explained. In all cases the model parameter fitting has been minimized, being mostly limited to the rate coefficients of the addition/fragmentation reactions in the RAFT polymerization. Therefore, such models are believed to be invaluable tools towards a deeper understanding of the main phenomena underlying RAFT polymerization.
Size‐Exclusion Effect and Protein Repellency of Concentrated Polymer Brushes Prepared by Surface‐Initiated Living Radical Polymerization
Authors: Yoshikawa Chiaki , Goto Atsushi , Ishizuka Norio , Nakanishi Kazuki , Kishida Akio , Tsujii Yoshinobu , Fukuda Takeshi
Published: March 2007

The adsorption of proteins on poly(2‐hydroxyethyl methacrylate) (PHEMA) brushes was systematically investigated from the viewpoint of the size‐exclusion effect of the concentrated brushes. By use of surface‐initiated atom transfer radical polymerization, well‐defined, concentrated PHEMA brushes were successfully grafted on the inner surface of the silica monolithic column with meso pores of ca. 80 nm as well as a silicon wafer and a quartz crystal microbalance (QCM) chip. By eluting low‐polydispersity pullulans with different molecular weight through the modified monolithic column, the concentrated PHEMA brush was characterized and demonstrated to sharply exclude solute molecules with the critical molecular size (size‐exclusion limit) comparable to the distance between the nearest‐neighboring graft points d. The elution behaviors of proteins with different sizes were studied with this PHEMA‐grafted column: the protein sufficiently larger than the critical size was perfectly excluded from the brush layer and separated only in the size‐exclusion mode by the meso pores without affinity interaction with the brush surface. Then, the irreversible adsorption of proteins on PHEMA brushes was investigated using QCM by varying graft densities (σ = 0.007, 0.06, and 0.7 chains/nm2) and protein sizes (effective diameter = 2–13 nm). A good correlation between the protein size and the graft density was observed: proteins larger than d caused no significant irreversible adsorption on the PHEMA brushes. Thus, we experimentally substantiated the postulated size‐exclusion effect of the concentrated brushes and confirmed that this effect plays an important role for suppressing protein adsorption.
Synthesis of Rod‐Coil Block Copolymers using Two Controlled Polymerization Techniques
Authors: Steig Simone , Cornelius Frauke , Heise Andreas , Knoop Rutger J. I. , Habraken Gijs J. M. , Koning Cor E. , Menzel Henning
Published: March 2007

A double‐headed initiator was synthesized yielding two functional groups for the initiation of the nickel mediated ring‐opening polymerization of γ‐benzyl‐L‐glutamate‐N‐carboxyanhydride and controlled radical polymerization of vinyl monomers via ATRP or NMP. Well‐defined block copolymers combining polypeptides and synthetic polymers were obtained.
Unusual Kinetics in Aqueous Heterophase Polymerizations
Authors: Tauer Klaus , Mukhamedjanova Muyassar , Holtze Christian , Nazaran Pantea , Lee Jeongwoo
Published: March 2007

The heterogeneous nature of aqueous heterophase polymerizations is the base for an easy route to unique block copolymers, for the development of new and more effective polymerization strategies, and the abilities to unique studies of radical polymerization kinetics. Thermo‐sensitive double hydrophilic block copolymers and micro‐ or nano‐gel particles of poly(N‐isopropyl acrylamide) as thermo‐responsible block and charged or uncharged hydrophilic polymers can easily be prepared if the polymerization of N‐isopropyl acrylamide is started with the corresponding polymeric radicals. The application of extremely fast microwave heating allows the development of highly effective pulsed thermal polymerization strategies and the production of polymers with desired molecular weight distributions over wide ranges. 2,2′‐azobisisobutyronitrile simultaneously initiates the polymerization in both the monomer and the aqueous phase and leads, even under surfactant‐free conditions, to stable latex particles.
Surface – Functionalized Inorganic Nanoparticles in Miniemulsion Polymerization
Authors: Töpfer Oliver , Schmidt‐Naake Gudrun
Published: March 2007

Two inorganic cores, consisting of silica and titania, have been prepared via basic Stoeber synthesis. Those cores have been functionalized, using trimethoxysilyl propylmethacrylate (MPTMS) and introduced into a miniemulsion copolymerization system. The miniemulsion consisted of styrene (S) and 2‐hydroxyethyl methacrylate (HEMA), styrene sulfonic acid (SSA) or aminoethyl methacrylate hydrochloride (AEMA) as comonomer in varying compositions. The morphology of the products has been investigated by SEM and dynamic light scattering (DLS) measurements. The composition of the products has been investigated by photoacoustic FTIR (PA‐FTIR) spectroscopy and elemental analysis. Thermal properties have been determined by TGA and DSC analysis.
A Study of Microstructures and Properties of High‐Carbon Si‐Cr Cast Steel Containing Rare Earth and Titanium
Authors: Fu H. , Miao Y. , Chen X. , Hu K. , Tian Z.
Published: March 2007

The microstructure, tensile and impact behaviour of high‐carbon Si‐Cr cast steel containing rare earth (RE) and titanium have been determined after austempering. The additions of RE and titanium refined the primary austenite grain size resulting in improving toughness. The addition of silicon handicapped the formation of carbide and carbide‐free bainitic ferrite and carbon enriched retained austenite could be obtained in the austempering structures of high‐carbon Si‐Cr cast steel, which had excellent mechanical properties and abrasion resistance. Moreover, the basic tendency of the mechanical properties of high‐carbon Si‐Cr cast steel influenced by the austempering temperature was that the hardness and tensile strength reduced and the impact toughness and fracture toughness increased with increasing temperature. The comprehensive properties were the best while austempering at 330oC.
Development of Wear and Corrosion Resistant Metal Matrix Composites with hard Particles formed in Situ
Authors: Weber S. , Berns H.
Published: March 2007

Metal matrix composites (MMC) were developed, consisting of hardenable steel matrices and embedded particles of titanium nitride (TiN) or vanadium nitride (VN) formed in situ during the consolidation by hot isostatic pressing (HIP). Designing wear and corrosion resistant materials was the objective of this development. The materials were produced by means of powder metallurgy and are composed of a ferrritic stainless steel powder X4CrMo15–1 and crushed particles of ferro titanium or ferro vanadium, respectively. These powder mixtures were first heat treated in an N2‐atmosphere for alloying with nitrogen and subsequently consolidated to wear resistant MMC by HIP. This publication describes the necessary processing steps, taking into account thermodynamic equilibrium calculations with Thermo‐Calc.
Eigenspannung im mit gasdruckunterstütztem Feingussverfahren hergestellten langfaserverstärkten Aluminium‐Matrix‐Verbundwerkstoff
Authors: Sun S. , Rosefort M. , Bäumer A. , Chauhan D. , Possel‐Dölken I. , Bührig‐Polaczek A.
Published: March 2007

Die Eigenspannungen zwischen Matrix und Faser in einer mit dem gasdruckunterstütztem Feingussverfahren hergestellten γ‐Al2O3‐langfaserverstärkten Aluminiumlegierung AlZn6Mg1Ag1 wurden mit der Röntgenbeugungsmethode experimentell gemessen und mit FEM numerisch simuliert. Die Ergebnisse haben wider Erwarten gezeigt, dass die durchschnittlichen Eigenspannungen sowohl in der Matrix als auch in der Faser nicht sehr groß sind. Allerdings sind die Eigenspannungen sehr ungleichmäßig verteilt. An der Grenzfläche in der Matrix gibt es in der Faserquer‐ und Faserlängsrichtung jeweils einen kleinen Bereich mit relativ hohen Zugeigenspannungen. Durch die Analyse der Eigenspannung in mit verschiedenen Faservolumenanteilen verstärkten Proben wurde der Einfluss des Faservolumenanteils im Verbundwerkstoff analysiert. Tendenziell steigt die Zugeigenspannung in der Matrix und sinkt die Druckeigenspannung in der Faser mit zunehmendem Faservolumenanteil. Wenn der Faservolumenanteil 65 % überschreitet, erreichen die endgültigen maximalen Zugeigenspannungen in der Matrix die Streckgrenze der Aluminiumlegierung und es entsteht lokale plastische Verformung in der Matrix.
Olfactory loss may be a first sign of idiopathic Parkinson's disease
Authors: Haehner Antje , Hummel Thomas , Hummel Cornelia , Sommer Ulrike , Junghanns Susann , Reichmann Heinz
Published: March 2007

Recent studies support the idea of olfactory dysfunction as a very early sign of idiopathic Parkinson's disease (IPD). Aim of the present study was to clinically follow‐up patients with idiopathic hyposmia to find out the percentage of patients developing IPD after 4 years time. At baseline, olfactory tests had been combined with transcranial sonography of the substantia nigra and 123I‐FP‐CIT SPECT imaging. At the present neurological examination, 7% of the individuals with idiopathic hyposmia had developed clinical IPD. Altogether, 13% presented with abnormalities of the motor system. Our data suggest that a combination of olfactory testing and other tests may constitute a screening tool for the risk to develop IPD. © 2007 Movement Disorder Society
Levodopa response in Parkinsonism with multiple mitochondrial DNA deletions
Authors: Wilcox Robert A. , Churchyard Andrew , Dahl Henrik H. , Hutchison Wendy M. , Kirby Denise M. , Thyagarajan Dominic
Published: March 2007

We report a patient with an autosomal dominant chronic progressive external ophthalmoplegia phenotype associated with multiple mtDNA deletions in muscle from a family in which linkage analysis excluded mutations in DNA polymerase γ (POLG), adenine nucleotide translocase (ANT‐1) or C10orf2 (Twinkle). She presented with prominent Parkinsonism characterized by prolonged benefit from levodopa (L‐dopa) and the later development of L‐dopa induced dyskinesias and motor fluctuations. Thus L‐dopa responsiveness, L‐dopa induced dyskinesias and motor fluctuations may also occur in atypical Parkinsonism of mitochondrial disease, just as they may in multiple system atrophy. © 2007 Movement Disorder Society
Sweating in Parkinsonian patients with wearing‐off
Authors: Pursiainen Ville , Haapaniemi Tarja H. , Korpelainen Juha T. , Sotaniemi Kyösti A. , Myllylä Vilho V.
Published: March 2007

Disorders of the autonomic nervous system are common in Parkinson's disease (PD). Earlier studies suggest that some nonmotor symptoms may fluctuate with the motor symptoms, but the possible interrelationship between motor phenomena and sweating has not been studied. The authors measured sweating by using an evaporimeter on three different areas of the body (right hand, left hand, chest) immediately before the morning PD medication (baseline), and thereafter hourly up to 4 hour, in 16 PD patients with wearing‐off type of motor fluctuations and in 15 patients without clinical motor fluctuations. The clinical state of the patients was evaluated using the Unified Parkinson's Disease Rating Scale motor score every hour. Sweating increased during the 4‐hour follow‐up, and reached its maximum level at the time of the highest Unified Parkinson's Disease Rating Scale motor score phase (off‐stage) in patients with wearing‐off (initially affected hand, P = 0.007; left hand, P = 0.004; right hand, P = 0.034), but in the patients without wearing‐off no changes in sweating were observed during the follow‐up. Sweating of the left hand (P < 0.001), right hand (P < 0.001), and initially affected hand (P = 0.008) during the whole observation period was significantly higher in patients with motor fluctuations than in those without. The present study shows that sweating fluctuates in conjunction with wearing‐off phenomenon. © 2007 Movement Disorder Society
Neurological deficits are associated with increased brain calcinosis, hypoperfusion, and hypometabolism in idiopathic basal ganglia calcification
Authors: Saiki Misuzu , Saiki Shinji , Sakai Koichiro , Matsunari Ichiro , Higashi Kotaro , Murata Ken‐ya , Hattori Nobutaka , Hirose Genjiro
Published: March 2007

We report two familial cases of idiopathic basal ganglia calcification. A 60‐year‐old proband with choreoathetosis, dysarthria, and cognitive decline showed more extensive brain calcinosis, hypoperfusion, and hypometabolism than did his asymptomatic 82‐year‐old mother. The mother had no frontal lobe calcinosis but basal ganglia and dentate nucleus depositions were detectable. Perfusion neuroimaging, however, was normal in the asymptomatic mother and abnormal in the clinically impaired proband. The presence of calcinosis cannot be used as an index of neurological impairment but the extent of calcinosis and reduction in perfusion and metabolism may be useful for separating symptomatic from asymptomatic subjects with IBGC. These findings suggest that an interruption of neuronal circuitry may cause neurological deficits. The degree of neurological deficits may correlate with the severity of calcinosis and the reduction of perfusion and metabolism. © 2007 Movement Disorder Society
Ciprofloxacin‐induced palatal tremor
Authors: Cheung Yuk‐Fai , Wong Winnie Wing‐Yin , Tang Kwok‐Wing , Chan John Hiu‐Ming , Li Patrick Chung‐Ki
Published: March 2007

We describe an 84‐year‐old man with an unusual clinical presentation of palatal tremor in association with ciprofloxacin treatment. The patient had rhythmical movements not only of the soft palate but also of the face and trunk. Complete resolution of the symptoms occurred 2 days after discontinuation of ciprofloxacin and administration of sodium valproate. This is the first reported case of palatal tremor secondary to the use of ciprofloxacin. © 2007 Movement Disorder Society
Detection of 13C‐labeled metabolites in the in vivo canine heart by B1 insensitive heteronuclear coherent polarization transfer and comparison of signal enhancement with NOE
Authors: Wei Haoran , Merkle Hellmut , Xu Ya , Ellermann Jutta , Sipprell Kevin , Uǧurbil Kǎmil
Published: March 2007

A recently developed adiabatic coherent polarization transfer enhancement technique [H. Merkle, H. Wei, M. Garwood, K. Uǧurbil. J. Magn. Reson. 99, 480–494 (1992)] was employed to perform 13C spectroscopy in the intact canine heart in vivo during [2‐13C]‐acetate infusion into the left descending coronary artery; the results were compared with 13C spectra obtained with conventionally employed nuclear Overhauser enhancement. The results demonstrate that both methods can be performed by using surface coils to obtain in vivo 13C spectra and that coherent polarization transfer provides better enhancement than NOE for [2‐13C]‐acetate but not for short T2 compounds.
Multi‐gradient echo with susceptibility inhomogeneity compensation (MGESIC): Demonstration of fMRI in the olfactory cortex at 3.0 T
Authors: Yang Qing X. , Dardzinski Bernard J. , Li Shizhe , Eslinger Paul J. , Smith Michael B.
Published: March 2007

Short image acquisition times and sensitivity to magnetic susceptibility favor the use of gradient echo imaging methods in functional MRI (fMRI). However, magnetic susceptibility effects attributed to air‐tissue interfaces also lead to severe signal loss in images of the large inferior frontal and lateral temporal cortices of the human brain, which renders these regions inaccessible to fMRI. The signal loss is caused by the local field gradients in the slice selection direction. A multigradient echo with magnetic susceptibility inhomogeneity compensation method (MGESIC) is proposed to overcome this problem. The MGESIC method effectively corrects the susceptibility artifacts and maintains the advantages of gradient echo methods to both BOLD sensitivity and fast image acquisition. The effectiveness of the MGESIC method is demonstrated by fMRI experimental results within the olfactory cortex.
Time‐resolved projection angiography after bolus injection of contrast agent
Authors: Hennig Jürgen , Scheffler Klaus , Laubenberger Jörg , Strecker Ralf
Published: March 2007

A method for MR angiography after bolus injection of a normal dose (0.1 mmol/kg) of contrast agent is presented. Projection angiograms are acquired with a non‐slice selective Snapshot FLASH sequence with a time resolution of 1 s per image or better. Typically 40 to 60 images are acquired consecutively after bolus injection of a contrast agent. The signal from vessels can be separated from background by postprocessing based on the observed temporal evolution of the signal intensities during bolus passage. The subsecond projection MR‐DSA is a reliable and robust technique to produce high resolution anatomical images of the vascular system avoiding the necessity of exact timing of the contrast agent bolus. It also supplies functional information about the hemodynamics in the observed region including perfusion.
Effect of temperature in focal ischemia of rat brain studied by 31P and 1H spectroscopic imaging
Authors: Kozlowski Piotr , Buchan Alastair M. , Tuor Ursula I. , Xue Dong , Huang Zhi‐Gao , Chaundy Karen E. , Saunders John K.
Published: March 2007

31P, 1H and lactate spectroscopic imaging was used to evaluate the effects of hypothermia on focal cerebral ischemia produced by middle cerebral artery occlusion. The effects on high energy phosphate metabolism, pH, lactate and NAA were investigated in 24 spontaneously hypertensive rats subjected to either permanent or transient ischemia. Under either normothermic (37.5°C) or hypothermic (32°C) conditions, with permanent 6‐h occlusion, there was little difference between groups in either the NMR measurements or the volume of infarction. In animals that underwent 3 h of ischemia followed by 12 h of reperfusion, the ischemic changes in lactate, pH, NAA, and high‐energy phosphate returned toward control values, and there was a protective effect of hypothermia (infarct volume of 211 ± 26 and 40 ± 14 mm3 in normothermic and hypothermic groups, respectively). Thus, hypothermia did not ameliorate the changes in lactate, pH, NAA, or high energy phosphate levels occurring during ischemia, however, during reperfusion there was an improvement in both the recovery of these metabolites and pathological outcome in hypothermic compared with normothermic animals.
13C editing of glutamate in human brain using J‐refocused coherence transfer spectroscopy at 4.1 T
Authors: Pan Jullie W. , Mason Graeme F. , Vaughan J. Thomas , Chu Wen‐Jang , Zhang Yantian , Hetherington Hoby P.
Published: March 2007

The method of single quantum 13C editing is analyzed and implemented with water suppressed J‐refocused coherence transfer spectroscopy. Analysis of the 13C inversion pulse demonstrates that it is optimally placed into the second echo of the J‐refocused sequence. We have used this method to acquire 13C‐edited spectra of glutamate from phantoms and in vivo. The turnover of 13C4‐labeled glutamate in human brain in vivo was observed in parasagittal gray matter using a volume head coil at 4.1 T with a time resolution of 5.3 min.
Temperature dependence and phase transition of proton relaxation of hydrated collagen in intact beef tendon specimens via cross‐relaxation spectroscopy
Authors: Tzou Der‐Lii , Lee Shu‐Ming , Yeung Hong N.
Published: March 2007

The role of water of hydration in proton relaxation in tissues as exemplified by hydrated collagen in beef tendon was studied as a function of temperature from −40° to 37°C by using cross‐relaxation spectroscopy. Experimental data were fitted to a simple binary spin‐bath model. The outcome of this procedure allows the construction of a semi‐quantitative depiction of proton relaxation in a heterogeneous system and its change as one of the water fractions freezes at about −10 to −20°C, a transition observed by NMR and confirmed independently by differential scanning calorimetry. Such physical depiction provides a crude but insightful interpretation of the role “bound” water plays in proton relaxation. This may be important in shedding light on the mechanism of tissue relaxation and its role in MRI diagnosis, particularly for those diseases such as liver cirrhosis where the water‐macromolecular interaction plays a prominent role.
Estimating NAA in cortical gray matter with applications for measuring changes due to aging
Authors: Lim Kelvin O. , Spielman Daniel M.
Published: March 2007

N‐acetylaspartic acid (NAA), a prominent peak in the proton spectrum, is an amino acid thought to be present almost exclusively in neurons and their dendritic and axonal extensions. While 1H MRS studies are showing promise in identifying NAA deficits in different patient groups, unwanted lipid signal from subcutaneous fat surrounding the skull, and necessarily large voxels have limited investigators' ability to assess NAA in cortical gray matter. Here we report a technique developed to derive estimates of NAA signal from cortical gray matter. This approach uses an inversion recovery imaging pulse sequence with a long TE to suppress lipid signal from the scalp and information from concurrently obtained structural MR images to determine the CSF, white and gray matter composition of each spectroscopic voxel. A regression analysis is then used to estimate what NAA levels would be in “pure” white and gray matter voxels. This technique has been applied to demonstrate reduced NAA gray/white levels in the brains of five healthy older compared with five healthy younger women.
Perfusion imaging by a flow‐sensitive alternating inversion recovery (Fair) technique: Application to functional brain imaging
Authors: Kim Seong‐Gi , Tsekos Nikolaos V.
Published: March 2007

Perfusion is a crucial physiological parameter for tissue function. To obtain perfusion‐weighted images and consequently to measure cerebral blood flow (CBF), a newly developed flow‐sensitive alternating inversion recovery (FAIR) technique was used. Dependency of FAIR signal on inversion times (TI) was examined; signal is predominantly located in large vessels at short TI, whereas it is diffused into gray matter areas at longer TI. CBF of gray matter areas in the human brain is 71 ± 15 SD ml/100 g/min (n = 6). In fMRI studies, micro‐ and macrovessel inflow contributions can be obtained by adjusting TIs. Signal changes in large vessel areas including the scalp were seen during finger opposition at a TI of 0.4 s; however, these were not observed at a longer TI of 1.4 s. To compare with commonly used BOLD and slice selective inversion recovery techniques, FAIR and BOLD images were acquired at the same time during unilateral finger opposition. Generally, activation sites determined by three techniques are consistent. However, activation of some areas can be detected only by FAIR, not by BOLD, suggesting that the oxygen consumption increase couples with the CBF change completely. Relative and absolute CBF changes in the contralateral motor cortex are 53 ± 17% SD (n = 9) and 27 ± 11 SD ml/100 g/min (n = 9), respectively.
Reducing flow artifacts in echo‐planar imaging
Authors: Pat Gerard T. Luk , Meyer Craig H. , Pauly John M. , Nishimura Dwight G.
Published: March 2007

Echo‐planar imaging (EPI) is very susceptible to flow artifacts. Two ways to improve its flow properties are presented. First, “partial flyback” is proposed to reduce artifacts arising from flow in the readout direction. Near the center of k‐space, only the even echoes of the EPI echo‐train are used. Partial flyback is shown to improve the readout‐flow properties at the expense of a slight worsening of the phase‐encode flow and off‐resonance properties. We recommend that the flyback region acquire 95% of the energy in k‐space. Second, “inside‐out” EPI is used to reduce artifacts arising from flow in the phase‐encode direction. Data collection begins at the center of k‐space, with separate interleaves to acquire the top and bottom halves of k‐space. Partial flyback is combined with partial‐Fourier EPI and inside‐out EPI. Partial‐flyback inside‐out EPI has worse off‐resonance properties than partial‐flyback partial‐Fourier EPI but demonstrates better flow properties and does not require partial k‐space reconstruction.
Improved intracranial lesion characterization by tissue segmentation based on a 3D feature map
Authors: Vinitski Simon , Gonzalez Carlos , Mohamed Feroze , Iwanaga Tad , Knobler Robert L. , Khalili Kamil , Mack John
Published: March 2007

Our aim was to develop an accurate multispectral tissue segmentation method based on 3D feature maps. We utilized proton density (PD), T2‐weighted fast spin‐echo (FSE), and T1‐weighted spin‐echo images as inputs for segmentation. Phantom constructs, cadaver brains, an animal brain tumor model and both normal human brains and those from patients with either multiple sclerosis (MS) or primary brain tumors were analyzed with this technique. Initially, misregistration, RF inhomogeneity and image noise problems were addressed. Next, a qualified observer identified samples representing the tissues of interest. Finally, k‐nearest neighbor algorithm (k‐NN) was utilized to create a stack of color‐coded segmented images. The inclusion of T1 based images, as a third input, produced significant improvement in the delineation of tissues. In MS, our 3D technique was found to be far superior to that based on any combination of 2D feature maps (P < 0.001). We identified at least two distinctly different classes of lesions within the same MS plaque, representing different stages of the disease process. Further, we obtained the regional distribution of MS lesion burden and followed its changes over time. Neuropsychological aberrations were the clinical counterpart of the structural changes detected in segmentation. We could also delineate the margins of benign brain tumors. In malignant tumors, up to four abnormal tissues were identified: 1) a solid tumor core, 2) a cystic component, 3) edema in the white matter, and 4) areas of necrosis and hemorrhage. Subsequent neurosurgical exploration confirmed the distribution of tissues as predicted by this analysis.
The nature of spatiotemporal changes in cerebral hemodynamics as manifested in functional magnetic resonance imaging
Authors: Mitra Partha P. , Ogawa Seiji , Hu Xiaoping , Uǧurbil Kǎmil
Published: March 2007

The nature of changes in rapidly acquired magnetic resonance images of the brain was studied by using a denoising method and spectral techniques optimally suited to short time series. It was found that the stimulus related changes have a complex spatiotemporal structure even for simple visual stimuli. Apart from a low frequency window, the non‐stimulusrelated changes were found to correspond in detail to either breathing or heartbeat, although these two sources show different patterns of spatial coherence in the image. At low frequencies, oscillations with frequencies around 0.1 Hz were observed, also with nontrivial space‐time structure, which are likely to be vasomotor in origin.
Hepatic hemosiderosis in non‐human primates: Quantification of liver iron using different field strengths
Authors: Bulte Jeff W. M. , Miller Georgina F. , Vymazal Josef , Brooks Rodney A. , Frank Joseph A.
Published: March 2007

Using a non‐human primate model of idiopathic hemochro‐matosis, hemosiderin‐induced T2 shortening of the liver was assessed at nine different field strengths over a range of 0.05 to 1.5 Tesla. The 1/T2 values increased linearly with field strength, with all specimens having approximately the same zero‐field intercept. The slope of the field increase, termed “field‐dependent T2 proton relaxation enhancement (PRE)”, appeared to be proportional to the chemically determined tissue iron content, viz. 10.8 s−1T−1(mg Fe/g wet tissue)−1. The correlation between iron content and field‐dependent T2 PRE (r = 0.94) was better than the correlation between iron content and 1/T2 values obtained at single field strengths. For livers containing ± 2 mg Fe/g wet weight, biexponential T2 relaxation behavior emerged at higher field strengths, with the short T2 component (intracellular water) exhibiting a linear dependence of 1/T2 on field, while T2 of the long component (extracellular/sinusoidal water) was nearly field‐independent. After maceration of the specimens, all T2 relaxation curves became monoexponential, including those for high iron content at high field strengths. The present data suggest that the use of double‐field MR imaging to assess the field‐dependent T2 PRE has potential for specific quantification of (liver) tissue iron stores.
Reduction of motion artifacts in cine MRI using variable‐density spiral trajectories
Authors: Liao Jan‐Ray , Pauly John M. , Brosnan Thomas J. , Pelc Norbert J.
Published: March 2007

Dynamic cardiac imaging in MRI is a very challenging task. To obtain high spatial resolution, temporal resolution, and signalto‐noise ratio (SNR), single‐shot imaging is not sufficient Use of multishot techniques resolves this problem but can cause motion artifacts because of data inconsistencies between views. Motion artifacts can be reduced by signal averaging at some cost in increased scan time. However, for the same increase in scan time, other techniques can be more effective than simple averaging in reducing the artifacts. If most of the energy of the inconsistencies is limited to a certain region of k‐space, increased sampling density (oversampling) in this region can be especially effective in reducing motion artifacts. In this work, several variable‐density spiral trajectories are designed and tested. Their efficiencies for artifact reduction are evaluated in computer simulations and in scans of normal volunteers. The SNR compromise of these trajectories is also investigated. The authors conclude that variable‐density spiral trajectories can effectively reduce motion artifacts with a small loss in SNR as compared with a uniform density counterpart.
Quantitative diffusion imaging in implanted human breast tumors
Authors: Maier Cynthia F. , Paran Yael , Bendel Peter , Rutt Brian K. , Degani Hadassa
Published: March 2007

Quantitative diffusion measurements were performed in tumors arising from inoculation of nude mice with two human breast cancer cell lines (MCF7 and T47D) to evaluate the specificity of this technique for characterizing solid tumors. ADC maps were compared to histology and correlated well with gross tumor morphology. Measured ADCs were highly specific for viable and necrotic tumor in the five T47D tumors included in this study (P < 0.02), while only two of the five MCF7 tumors studied developed distinguishable areas of necrosis. No statistically significant difference was observed in ADCs from viable tumor between the different cell lines (P > 0.05).
What is the optimum phased array coil design for cardiac and torso magnetic resonance?
Authors: Bottomley Paul A. , Olivieri Carlos H. Lugo , Giaquinto Randy
Published: March 2007

To determine the optimum configuration of a phased array MR coil system for human cardiac applications, the sensitivity of 10 flexible array designs operating under ideal conditions was calculated at 13 points circling the myocardium of a model torso whose geometry was determined from healthy volunteers. The array geometries that were evaluated included continuous strips of 2,4,6, and 10 circular coils of diameter equal to half the torso thickness wrapped laterally around the torso, 2 pairs of coils located on the left side of the chest and back, clusters of 3 coils in 2 orientations, clusters of 4 and 6 coils, and a hybrid cross of 6 coils. The 4‐, 6‐, and 10‐coil strip arrays out‐performed the other designs for a given number of coils, yielding average theoretical sensitivity improvements of 45%, 53%, and 55% relative to a single flexible coil positioned at the point closest to the anterior myocardium, compared with about 30% for 4‐ and 6‐coil clusters and the 2‐pair geometry (P < 0.02). A flexible 4‐coil strip array was constructed for a clinical 1.5 T scanner with 15‐cm diameter circular surface coils on flexible circuit board. The signal‐to‐noise ratio (SNR) of this coil at the 13 cardiac locations was measured in 15 normal volunteers and compared with the SNR measured in images acquired with standard commercial MR coils: a body coil, a flexible torso array, a general purpose flexible coil, and, in 4 subjects, a dual array coil. In the prone orientation, the average myocardial SNR improvement of the 4‐coil strip array was 650% relative to the whole body coil, compared with 310–340% for the other commercial coils (P < 0.00005). The twofold advantage over the commercial coils persisted in supine studies (P < 0.00005, n = 5). Thus, flexible circumferential phased arrays of strips of surface coils of diameter comparable with the depth of the heart generally out‐perform many other standard geometries for a given number of coils, and can yield dramatically improved SNR over coils available for general use in the torso.
A method to create an optimum current distribution and homogeneous B1 field for elliptical birdcage coils
Authors: Li Shizhe , Collins Christopher M. , Dardzinski Bernard J. , Chin Chih‐Liang , Smith Michael B.
Published: March 2007

For a birdcage coil with elliptical cross‐section, a sinusoidal current pattern does not provide a homogeneous B1 field. A simple theory was developed to create an optimized current distribution for elliptical birdcage coils. This optimized current pattern can create a perfectly homogeneous B1 field inside any elliptical shape. To verify the theory, a 16‐element high‐pass elliptical birdcage coil was built inside a circular RF shield. The current was optimized by using the inductance characteristics of the coil components to calculate the endring capacitances. The B1 field was theoretically calculated and experimentally mapped for the optimized elliptical birdcage coil and a nonoptimized coil. The results demonstrate that by optimizing the current distribution, a very homogeneous B1 field is produced. This method can be directly applied in design and construction of elliptical birdcage coils for imaging of the naturally occurring elliptical cross‐sectional geometries in the human body.
Oblique proton chemical shift imaging for the presurgical localization of mesial temporal epilepsy
Authors: Xue Min , Ng Thian C. , Majors Anthony , Modic Michael , Kolem Heiner
Published: March 2007

Precise localization of the epileptogenic zone in patients suffering from mesial temporal lobe epilepsies is a prerequisite for surgery. For this purpose, a scheme for selecting the volume of interest in an oblique plane covering the head, body, and tail of the hippocampal formation (HF) was implemented in a water‐suppressed proton chemical shift imaging (CSI) acquisition. This approach, evaluated by consecutively acquired CSI spectra of both temporal lobes, demonstrates that it can potentially be a routine technique for clinical diagnosis. Since the method produces localized spectral detail of the HF, it facilitates a convenient comparison of CSI lateralization to that assessed by HF atrophy (due to sclerosis) using the MRI‐volumetry technique.
A study of diffusion isotropy in single neurons by using NMR microscopy
Authors: Hsu Edward W. , Aiken Nanci R. , Blackband Stephen J.
Published: March 2007

The apparent diffusion coefficient (ADC) of water was measured in single Aplysia californica neurons by using NMR microscopy encoded in each of two perpendicular gradient directions. Comparisons of the mean ADCs of the gross nuclear and cytoplasmic compartments in five cells, and 50 subregions within these cells, showed no significant difference between the diffusion measurements in the majority of cases. Since anisotropic diffusion would make the ADC dependent on the encoding direction, the results indicate that the ADC in these single neurons is isotropic at the spatial and temporal resolutions used in these studies. Consequently, a single scalar ADC measurement is sufficient for characterizing the ADC in these cells, hence reducing the acquisition time and measurement complexity that would have been required had the ADC been anisotropic.
Limitations of temporal resolution in functional MRI
Authors: Kim Seong‐Gi , Richter Wolfgang , Uǧurbil Kǎmil
Published: March 2007

In fMRI, images can be collected in a very short time; therefore, high temporal resolution is possible in principle. However, the temporal resolution is limited by a blurred intrinsic hemodynamic response and a finite signal‐to‐noise ratio. To determine the upper limit of temporal resolution in a single area during repeated tasks, motor cortex activity was investigated during visually instructed finger movements. Without averaging, a sequence of four single‐finger movements with an execution time of approximately 2 s can be resolved when the delay time between consecutive sequences is at least 3 s. The hemodynamic response time is constant for each subject, but not among different subjects. The temporal resolution can be better when the signal from spatially distinct regions is examined. For a series of experiments involving a visually instructed delayed cued finger movement task with a well‐defined, independently determined, variable delay time, time courses in the motor area are distinct from each other in two experiments if the difference in delay time is as little as 2 s. The activation in the visual area due to the presentation of the task serves here as an internal time reference. By comparing a set of fMRI time courses in multiple distinct areas, serial neural processing may be investigated.
NMR “diffusion‐diffraction” of water revealing alignment of erythrocytes in a magnetic field and their dimensions and membrane transport characteristics
Authors: Kuchel Philip W. , Coy Andrew , Stilbs Peter
Published: March 2007

“Diffusion‐diffraction” experiments on water, yielding “q‐space” plots, were conducted on suspensions of oxygenated (diamagnetic) human erythrocytes. (i) These suspensions displayed diffusion‐diffraction of water; (ii) the shape of theq‐space plots depended on the direction along which the diffusion was measured, thus implying alignment of the cells in the magnetic field of the NMR spectrometer; (iii) the diffusion anisotropy was altered in a predictable way by converting the hemoglobin to a paramagnetic form; (iv) the shapes of theq‐space plots were altered in a predictable way by inhibiting water transport; (v) the pseudo‐first order rate constant characterizing the covalent inhibition of water transport, byp‐chloromercuribenzenesulfonate (p‐CMBS), was measured; and (vi) the cell diameter and intercellular spacing were measured from the positions of the interference minima and maxima in theq‐space plots. The relevance of these findings to NMR‐based histological characterization of tissues, and the implications, for magnetic resonance imaging (MRI), of eryth‐rocyte alignment in the small vessels of the brain in particular, are noted.
3D localized in vivo1H spectroscopy of human brain by using a hybrid of 1D‐hadamard with 2D‐chemical shift imaging
Authors: Gonen Oded , Arias‐Mendoza Fernando , Goelman Gadi
Published: March 2007

We report acquisition of 3D image‐guided localized proton spectroscopy (1H‐MRS) in the human brain on a standard clinical imager. 3D coverage is achieved with a hybrid of chemical shift imaging (CSI) and transverse Hadamard spectroscopic imaging (HSI). 16 × 16 × 4 arrays of 3.5 and 1 ml voxels were obtained in 27 min. The spatially selective HSI 90° pulses incorporate naturally into a PRESS double spin‐echo sequence to subdivide the VOI into four partitions along its short axis. 2D CSI (16 × 16) is performed along the other long axes. Because the hybrid excites the spins in the entire VOI, a √N signal‐to‐noise‐ratio (SNR) gain per given examination time is realized compared with sequentially interleaving N 2D slices. A two‐fold gain in sensitivity is demonstrated in the brain for N = 4.
Off‐resonance pulsed magnetization transfer in clinical MR imaging: Optimization by an analysis of transients
Authors: Listerud John
Published: March 2007

Significant degrees of magnetization transfer (MT) have been observed in the magnetic resonance imaging of biological materials by conventional clinical imaging sequences, as well as by sequences specifically designed to enhance MT image contrast. Two aspects of these procedures distinguish them from the classic spectroscopic MT experiments using either so‐called “hard” radiofrequency (RF) pulses of short duration and high power, or continuous wave RF irradiation of low power. First, clinical sequences must make exclusive use of “soft” pulses of intermediate length and power. Second, biological materials are modeled by a two‐spin system involving magnetization transfer between a narrow and a broad homogeneous spectral component. Such materials are a relatively restricted group within the larger family of materials studied with MT by spectroscopy. The current paper addresses these two issues with a theory that gives a new transient analysis of the off‐resonance pulsed MT problem for biological materials. It leads to predictions for optimal magnetization transfer in the context of medical imaging that have been verified by computer modeling.
Spatially filtering functional magnetic resonance imaging data
Authors: Lowe Mark J. , Sorenson James A.
Published: March 2007

When constructing MR images from acquired spatial frequency data, it can be beneficial to apply a low‐pass filter to remove high frequency noise from the resulting images. This amounts to attenuating high spatial frequency fluctuations that can affect detected MR signal. A study is presented of spatially filtering MR data and possible ramifications on detecting regionally specific activation signal. It is shown that absolute activation levels are strongly dependent on the parameters of the filter used in image construction and that significance of an activation signal can be enhanced through appropriate filter selection. A comparison is made between spatially filtering MR image data and applying a Gaussian convolution kernel to statistical parametric maps.
Biochemical heterogeneity in hysterectomized uterus measured by31P NMR using SLIM localization
Authors: Xu Su , Yang Yihong , Gregory Carl D. , Vary Jay C. , Liang Zhi‐Pei , Dawson M. Joan
Published: March 2007

Ultrasound and magnetic resonance imaging show contrast between the inner and outer myometrium, which is useful in the diagnosis of gynecological disorders. To determine whether the image contrast is associated with biochemical differences between these myometrial regions, phosphorus metabolite concentrations in the inner one third of the myometrium (the junctional zone; JZ) were compared with the outermost one third of the myometrium (OM) in hysterectomized uteri using31P spectral localization by imaging (SLIM). The technique was validated by comparing the results of SLIM with the results of standard Fourier‐encoded spectroscopic imaging (FSI) analysis using phantoms, and by nonlocalized spectroscopy on biopsies taken from the same hysterectomy specimens. As expected theoretically, SLIM yielded better localization than FSI, as judged by spectral intensity and leakage measurements on phantom compartments of known composition. SLIM localization revealed that the JZ has a higher intracellular phosphomonoester (PME) concentration than does the OM, which was confirmed by nonlocalized spectroscopy, and that there is very little NMR‐visible phosphorus in the cervix.
Common k‐space acquisition: A method to improve myocardial grid‐tag contrast
Authors: Doyle Mark , Walsh Edward G. , Foster Robert E. , Pohost Gerald M.
Published: March 2007

A current limitation of many myocardial tag acquisitions employing SPAMM encoding is the relatively rapid loss of tag contrast over the cardiac cycle. Acquisition schemes that apply line tags produce prolonged tag contrast compared with directly excited grid tags. However, a grid‐tag series can be generated by combining two orthogonal line‐tag series. To be time efficient, each line‐tag series can be acquired with the read gradient oriented perpendicular to the line‐tag direction. There are several disadvantages associated with this approach, including the requirement to avoid signal fold‐over and that fat shift artifacts appear in different directions in each line‐tag series. Presented here is a method of applying separate line tags that does not require interchanging the read and phase encoding gradients and does not extend the scan time compared with a conventional grid‐tag acquisition. Additionally, the means of generating grid tags results in a doubling of the tag contrast to noise ratio compared a line‐tag set. Computer simulations are presented along with phantom and volunteer scans.
MRI contrast enhanced study of cartilage proteoglycan degradation in the rabbit knee
Authors: Bacic Goran , Liu Ke Jian , Goda Fuminori , Hoopes P. Jack , Rosen Gerald M. , Swartz Harold M.
Published: March 2007

Early degeneration of cartilage is accompanied by a loss of proteoglycans and consequent changes in the content of water. Conventional magnetic resonance imaging (MRI) cannot reliably detect this change, since the relaxation properties of the cartilage are dominated by its collagen content. The applicability of a positively charged nitroxide as an MRI contrast agent in detection of the content of the negatively charged proteoglycans within the cartilage was investigated. The results from both MRI and electron paramagnetic resonance (EPR) spectroscopy indicate that the accumulation of the contrast agent reflects the amount of proteoglycans within the cartilage, presumably due to the electrostatic interactions between the negatively charged proteoglycans and the positively charged nitroxide. Such a contrast agent could be useful in the detection and study of early stages of the degeneration of joints.
Multifrequency interpolation for fast off‐resonance correction
Authors: Man Lai‐Chee , Pauly John M. , Macovski Albert
Published: March 2007

Field inhomogeneities or susceptibility variations produce blurring in images acquired using non‐2DFT k‐space readout trajectories. This problem is more pronounced for sequences with long readout times such as spiral imaging. Theoretical and practical correction methods based on an acquired field map have been reported in the past. This paper introduces a new correction method based on the existing concept of frequency segmented correction but which is faster and the‐oretically more accurate. It consists of reconstructing the data at several frequencies to form a set of base images that are then added together with spatially varying linear coefficients derived from the field map. The new algorithm is applied to phantom and in vivo images acquired with projection reconstruction and spiral sequences, yielding sharply focused images.