bluuebunny/crossref_metadata_2025_split huggingface dataset:
Migration and biodegradation of free silicone from silicone gel‐filled implants after long‐term implantation
Authors: Pfleiderer Bettina , Ackerman Jerome L. , Garrido Leoncio 
Published: March 2007

AbstractIn vivo1H NMR chemical shift imaging (CSI), 1H NMR localized spectroscopy (STEAM) and multinuclear NMR spectroscopy 29sl, 13C, 1H) were used to characterize the aging process of silicone gel‐filled implants in a rat model after long‐term implantation. Although no significant changes could be observed in the implants or surrounding tissue by in vivo 1H chemical shift imaging, in vivo 1H localized spectroscopy of the livers from the longer term population revealed the presence of silicone. Ex vivo 29Sl spectroscopy of the liver, spleen, and the capsule formed around the 9 and 12 month implants clearly demonstrated and confirmed for the first time that a significant amount of free silicone migrates from silicone gel‐filled implants. Also, these results show that silicones are not metabolically inert, and their biodegradation in tissue and within the implant can be monitored after 9 and 12 months by changes in the 29Sl chemical shifts seen in corresponding ex vivo spectra. The NMR findings are supported by those obtained by atomic absorption spectroscopy. Silicone aging changes not only the chemical composition of the gel, but also its proton T2 relaxation times, which increase with long implantation times. The three dimensional structure of the gel disintegrates (i.e., polymer chain rupture), increasing the molecular mobility of the polymer and, consequently, its protons T2 values. The relaxation data we obtained reflect this in vivo degradation, especially in the case of implant rupture. Additionally, small concentrations of fat in the silicone gel were found within the implants. The presence of these lipophilic substances also might increase the T2 values (plasticizer effect). These findings may assist in evaluating the implant integrity and disease symptoms related to their presence in humans.
The use of dietary loading of 133Cs as a potassium substitute in NMR studies of tissues
Authors: Philip Shehan B. , Mark Wellard R. , Adam William R. , Craik David J. 
Published: March 2007

Abstract133Cs NMR chemical shifts and relaxation times have been measured for tissue samples in vitro and in vivo from rats which have been fed on a high cesium, low potassium diet, which leads to a predominantly intracellular distribution of this ion, similar to that of K
+. The high sensitivity, large chemical shift range, and narrow linewidths of 133Cs, compared with 39K, allow chemical shift differences to be observed between tissues, and in subcellular organelles such as mitochondria. For example, in vitro tissue chemical shifts, relative to 150 mM CsCl, are 1.06 ± 0.11 ppm for liver, 0.02 ± 0.05 ppm for brain, 1.76 ± 0.20 ppm for erythrocytes, and −0.13 ± 0.02 ppm for plasma. T1 and spin‐echo T2 values range from 1.26 ± 0.05 s (T2), and 0.028 ± 0.006 s (T2) for liver, to 6.49 ± 0.19 s and 1.12 ± 0.03 s for plasma. 133Cs relaxation times show the same relative trends between tissues as are observed in 39K tissue Studies.
A molecular theory of relaxation and magnetization transfer: Application to cross‐linked BSA, a model for tissue
Authors: Koenig Seymour H. , Brown III Rodney D. 
Published: March 2007

AbstractHomogeneous soft tissue, as regards its magnetic relaxation properties, is well‐modeled by solutions of cross‐linked protein (see Koenig and Brown, Prog. NMR Specfr. 22,487 (1991)). Interactions at the solute‐solvent interface alter the hydrodynamics of solvent water, and also couple the solute and solvent proton Zeeman energy reservoirs, giving hydrodynamic and cross‐relaxation contributions to water proton relaxation that respond differently to deuteration of solvent. We report measurements of the magnetic field dependence of 1/T1, of water protons in cross‐linked bovine serum albumin (BSA), for partially deuterated solvent and, in order to separate these two contributions, of 1/T1, of deuterons. The major experimental finding is that, in addition to recently identified water‐binding sites on protein (covering ∼0.2% of the surface) with water lifetimes of about 1 μs, there is another group of sites with lifetimes of about 23 ns, covering ∼2% of the surface, which are evident in both proton and deuteron data. In addition, we have formulated a theory of interfacial proton‐proton magnetic interactions which—with these four parameters, plus two that quantify the protein‐water coupling at each site—can account for all the proton and deuteron data, in both native and cross‐linked BSA.
Dendrimer‐based metal chelates: A new class of magnetic resonance imaging contrast agents
Authors: Wiener Erik , Brechbiel M. W. , Brothers H. , Magin R. L. , Gansow O. A. , Tomalia D. A. , Lauterbur P. C. 
Published: March 2007

AbstractWe have developed a new class of magnetic resonance imaging contrast agents with large proton relaxation enhancements and high molecular relaxivities. The reagents are built from the polyamidoamine form of Starburs™ dendrimers in which free amines have been conjugated to the chelator 2‐(4‐isothiocyanatobnenzyl)‐6‐methyl‐diethylenetriaminepenta‐acetic acid. The dendrimer gadolinium poly‐chelates have enhancement factors, i.e., the ratlo of the relaxivity per Gd(III) ion to that of Gd(III)‐diethylenetriaminepentaacetic acid, of up to 6. These factors are more than twice those observed for analogous metal‐chelate conjugates formed with serum albumins, polylysine, or dextran. One of the dendrimer‐metal chelate conjugates has 170 gadolinium ions bound, which greatly exceeds the number bound to other macromolecular agents reported In the literature, and has a molecular relaxivity of 5,800 (mM · s)−1, at 25 MHz, 20°C, and pH of 7.4. We observed that these dendrimer‐based agents enhance conventional MR images and 3D time of flight MR angiograms, and that those with molecular weights of 8,508 and 139,000 g/mole have enhancement half lives of 40 ± 10 and 200 ± 100 min, much longer than the 24 ± 4 min measured for Gd(III)‐diethylenetriaminepentaacetic acid. Our results suggest that this new and powerful class of contrast agents have the potential for diverse and extensive application in MR imaging.
Effect of vitreous fluidity on the measurement of blood‐retinal barrier permeability using contrast‐enhanced MRI
Authors: Berkowitz Bruce A. , Wilson Charles A. , Tofts Paul S. , Peshock Ronald M. 
Published: March 2007

AbstractBreakdown of the blood‐retinal barrier (BRB), frequently an early clinical sign in retinopathy, can be accurately determined using contrast‐enhanced MRI. However, increased vitreous fluidity with age and disease may affect the accuracy of the MRI method. We compared the permeability surface area product per area of leaky retina in eyes with normal vitreous (5.42 ± 0.48 × 10−4 cm/min, mean ± SEM, n = 5) to the contralateral gas‐compressed vitrectomized eyes (5.41 ± 0.54 × 10−4 cm/min, n = 5). The effect of vitrectomy was not significant (P = 0.325) using a Wilcoxon matched pairs signed rank test on the signed differences of the PS' values.
BOLD MRI monitoring of changes in cerebral perfusion induced by acetazolamide and hypercarbia in the rat
Authors: Graham G. D. , Zhong J. , Petroff O. A. C. , Constable R. T. , Prichard J. W. , Gore J. C. 
Published: March 2007

AbstractTo evaluate MRI methods for estimating cerebrovascular reserve, we computed changes in the R2* and R2, transverse relaxation rate and apparent diffusion coefficient (ADC) at 2.0 Tesla in five rats after administration of 30 mg of acetazolamide and in four rats during inhalation of 20% carbon dioxide gas. Significant decreases in R2*, corresponding to increases in gradient echo MRI signals, occurred in both the acetazolamide (average change −8.3%, P = 0.005) and the carbon dioxide (−2.7%, P = 0.009) treated animals. The computed values for R2 and ADC were unchanged. The magnitude of the gradient echo MRI changes observed should permit anatomic mapping of blood flow reactivity patterns in normal human subjects and in patients at risk for cerebrovascular disease.
Method for the quantitative assessment of contrast agent uptake in dynamic contrast‐enhanced MRI
Authors: Hittmair Karl , Gomiscek Gregor , Langenberger Karl , Recht Michael , Imhof Herwig , Kramer Josef 
Published: March 2007

AbstractIn previous papers relative signal intensity increase was used as a quantitative assessment parameter for contrast uptake in contrastenhanced MRI. However, relative signal intensity increase does not only reflect contrast uptake but depends also on tissue parameters (native T1 relaxation time) and sequence parameters (repetition time and flip angle); thus, the contrast uptake cannot be assessed accurately using relative signal intensity increase. Based on an analysis of the contrast behavior of spoiled gradient echo sequences, a method is described in this paper that overcomes the limitations of relative signal intensity increase measurement. A parameter, called “enhancement factor” (EF) is introduced that approximates differential T1 relaxation rate. The enhancement factor scales linearly with contrast uptake and is independent of tissue and sequence parameters. The additional measurement time involved in determining the enhancement factor is less than 1 min and computation is straightforward. The practicality of the new method was confirmed by phantom measurements using T1‐weighted and proton density‐weighted spoiled gradient echo sequences (FLASH‐2D). Enhancing tissues were simulated by water phantoms doped with increasing concentrations of Gd‐DTPA.
Validation of 13C NMR measurements of liver glycogen in vivo
Authors: Gruetter Rolf , Magnusson Inger , Rothman Douglas L. , Avison Malcolm J. , Shulman Robert G. , Shulman Gerald I. 
Published: March 2007

AbstractThe natural abundance 13C NMR intensity of the glycogen C1 resonance was measured in the surgically exposed liver of rabbits in vivo (n = 17) by integration from 98 to 104 ppm and compared double blindedly to the subsequent biochemical measurement. Coil loading was measured each time from a reference sphere at the coil center and the NMR Intensity was normalized accordingly. For quantification, the normalized NMR intensity was calibrated using aqueous glycogen solutions ranging from 110 to 1100 μmol glucosyl units/g (n = 14). An in vivo range from 110 to 800 pmol glucosyl unit/g wet weight was measured with a highly linear correlation with concentration (r = 0.85, P < 0.001). The in vivo NMR concentration was 0.95 ± 0.05 (mean ± standard error, n = 17) of the concomitant enzymatic measurement of glycogen content. We conclude that the 13C NMR signal of liver glycogen C1 is essentially 100% visible in vivo and that natural abundance 13C NMR spectroscopy can provide reliable noninvasive estimates of in vivo glycogen content over the physiological range of liver glycogen concentrations when using adequate localization and Integration procedures.
Short TE phosphorus spectroscopy using a spin‐echo pulse
Authors: Lim Kelvin O. , Pauly John , Webb Peter , Hurd Ralph , Macovski Albert 
Published: March 2007

AbstractIn vivo phosphorus spectroscopy requires very short acquisition delays in order to capture the signal from components with short transverse relaxation times (T2). The echo time typical of standard slice selective spin‐echo pulses are too long for this application, so hard pulse, free induction decay (FID) acquisitions have frequently been used instead. With FID, however, there is an interval between the time of coherence and data acquisition (acquisition delay), with resulting baseline distortions. In this paper we describe the design of a new short TE, slice‐selective, composite spin‐echo pulse with echo times as short as 2.5 ms. With a long TR, fully relaxed, multislice spectra can be collected. This technique will be useful for assessing, in vivo, changes in brain phospholipid activity associated with psychiatric and neurological diseases.
Multipoint K‐space point mapping (KPM) technique for NMR microscopy
Authors: Cho Zang H. , Ro Yong M. 
Published: March 2007

AbstractAn extended version of point mapping in k‐space for microscopic imaging applications is analyzed and described. Because the method can offer a number of advantages over the other conventional techniques due to the short echo time (TE), the technique is ideally suited for microscopic imaging where field in homogeneity dependent signal degradation is one of the main causes of image resolution degradation (1–3). Another application area is the case of imaging in a highly inhomogeneous situation such as the fringe field imaging (Z.H. Cho, E. Wong, US. Patent) #5023554 (1990). The first original point mapping technique described by Nauert et al. (1) and Emid and Creyghton (2) is analyzed as a k‐space point mapping technique, and the original technique is extended to a multipoint k‐space point mapping (MKPM) technique. With the extended MKPM technique, much faster microscopic imaging that is free of susceptibility and diffusion effects can be performed (4–6). To examine this idea, computer simulations are performed and their results are given.
Dynamically adaptive MRI with encoding by singular value decomposition
Authors: Zientara Gary P. , Panych Lawrence P. , Jolesz Ferenc A. 
Published: March 2007

AbstractA new MRI spatial encoding method based upon the singular value decomposition (SVD) and using spatially selective RF excitation is described. This encoding technique is particularly; applicable to dynamic adaptive MRI, because it provides a near minimal set of spatial encoding profiles computed using an image estimate that is determined from a previously obtained image. Experimental results are presented for two cases, which exemplify its potential use in different dynamic imaging tasks. SVD‐encoded MRI has demonstrated to be a highly efficient encoding scheme.
Relaxivity corrected response modulated excitation (RME): A T2‐corrected technique achieving specified magnetization patterns from an RF pulse and a time‐varying magnetic field
Authors: Pearlman Justin D. , Wieczorek Tad J. 
Published: March 2007

AbstractWe have reworked the theory of RF excitation to enable correction for relaxivity while designing response‐modulated excitation (RME) to achieve specifled magnetization targets. This results in a significant improvement in the ability to achieve a specified target magnetization, especially if excitation time is long or T2 is short. The methods presented may also be used to improve the quality of spatial‐spectral pulses as well as localized spectroscopy, real‐time imaging, real‐time localized velocity, and noninvasive pressure measurement.
Metabolic loss of deuterium from isotopically labeled glucose
Authors: Ben‐Yoseph Oded , Kingsley Peter B. , Ross Brian D. 
Published: March 2007

AbstractThe isotopically substituted molecule (6‐13C, 1, 6, 6‐2H3)glucose was evaluated to determine whether metabolic 2H loss would prevent its use in quantitating pentose phosphate pathway (PPP) activity. PPP activity causes the C1 of glucose to be lost as CO2, while C6 can appear in lactate. 2H NMR analysis of the lactate produced from this glucose can distinguish (3‐2H)‐lactate (from C1 of glucose) from (3‐13C, 3, 3‐2HZ)lactate (from C6 of glucose). 2H NMR spectroscopic analysis of medium containing (4‐13C, 1,6,6−2H3) glucose after incubation with cultured rat 9L glioma cells suggested a 30.8 ± 2.1% PPP activity as compared with 6.0 ± 0.8% from separate, parallel incubations with (1‐13C)glucose and (6‐13C)glucose. Subsequent experiments with other isotopically labeled glucose molecules suggest that this discrepancy is due to selective loss of 2H from the C1 position of glucose, catalyzed by phosphoman‐nose isomerase. Failure to consider 2H exchange from the C1 and C6 positions of glucose can lead to incorrect conclusions in metabolic studies utilizing this and other deuterated or tritiated glucose molecules.
The effect of high performance gradients on fast gradient echo imaging
Authors: Reeder Scott B. , McVeigh Elliot R. 
Published: March 2007

AbstractThe effect of gradient system performance on segmented k‐space gradient echo imaging is presented. Three cases were investigated. First, an ideal system that has infinite slew rates and unlimited maximum gradient strengths was considered. Second, a “high speed” imaging system (2.3 (G/cm), 23 (G/cm)/ms) was considered. These two cases were compared with a “conventional” imaging system (1(G/cm), 1.67 (G/cm)/ms). It was found that substantial increases in SNR can be achieved (≈︁ 45%) by using high speed versus a conventional gradient system, for a TR of 6 ms. For trapezoidal gradient waveforms, there exists an optimum maximum gradient strength for a given slew rate, and any increase in gradient strength above this optimum will not be utilized by an optimized sequence. These studies have shown that increasing TR without decreasing the bandwidth is not a good way to increase SNR for constant scan time.
Noninvasive MRI Thermometry with the Proton Resonance Frequency (PRF) Method: In Vivo Results in Human Muscle
Authors: Poorter John De , Wagter Carlos De , Deene Yves De , Thomsen Carsten , Ståhlberg Freddy , Achten Eric 
Published: March 2007

AbstractThe noninvasive thermometry method is based on the temperature dependence of the proton resonance frequency (PRF). High‐quality temperature images can be obtained from phase information of standard gradient‐echo sequences with an accuracy of 0.2°C in phantoms. This work was focused on the in vivo capabilities of this method. An experimental setup was designed that allows a qualitative in vivo verification. The lower‐leg muscles of a volunteer were cooled and afterwards reheated with an external water bolus. The temperature of the bolus water varied between 17°C and 37°C. The in vivo temperature images can be used to extract the temperature in muscle tissue. The data in the fat tissue are difficult to interpret because of the predominance of susceptibility effects. The results confirm the method's potential for hyperthermia control.
Compensation of Diffusion Effects in T2 Measurements
Authors: Deichmann Ralf , Adolf Holger , Kuchenbrod Erwin , Nöth Ulrike , Schwarzbauer Christian , Haase Axel 
Published: March 2007

AbstractMeasurements of the transverse relaxation time T2 are usually conducted with the Carr Purcell Meiboom Gill (CPMG) pulse sequence, which causes T2‐weighted magnetization. Diffusion effects are a common source of error in measurements of this kind, because the incoherent motion of spins in external magnetic field gradients distorts T2 weighting of the transverse magnetization. As a result, inaccurate T2‐values are obtained. In this work, we present a method which completely compensates for the effect of diffusion.
Water‐Macromolecular Proton Magnetization Transfer in Infarcted Myocardium: A Method to Enhance Magnetic Resonance Image Contrast
Authors: Scholz Thomas D. , Hoyt Robert F. , Deleonardis John R. , Ceckler Toni L. , Balaban Robert S. 
Published: March 2007

AbstractWater proton nuclear magnetic resonance relaxation times and magnetization transfer (MT) parameters of rat hearts were studied 24 h or 4 weeks after ligation of the left coronary artery or sham operation. Compared with sham‐operated controls, measured relaxation times (T1sat' and T2) of both acute and chronic myocardial infarction increased. The MT effect significantly decreased in the infarcted myocardium. The changes in relaxation times and MT effect were significantly greater in chronic infarcts compared with acute infarcts. Improvements in calculated image contrast between normal and infarcted tissue were supported by images of ex vivo hearts with chronic infarction. Image contrast was increased at short echo times in the presence of macromolecular proton pool irradiation. Exploiting changes in tissue MT following myocardial infarction to enhance contrast between normal and infarcted tissue should allow improved identification and characterization of infarcted myocardium.
In vivo Oximetry Using EPR and India Ink
Authors: Goda Fuminori , Liu Ke Jian , Walczak Ted , O'Hara Julia A. , Jiang Jinjie , Swartz Harold M. 
Published: March 2007

AbstractRecent advances in electron paramagnetic resonance (EPR) oximetry have established the use of the particulate materials fusinite and lithium phthalocyanine (LiPc) as probes for measurement of oxygen tensions in tissues in vivo with a sensitivity and accuracy that is relevant for studying pathophysiological processes. India ink, another potentially very useful new probe for EPR oximetry, shares the critical properties of fusinite and LiPc and has the additional advantage of already having been widely used clinically with no apparent toxicity. The line width is particularly sensitive to changes in oxygen tension of less than 30 mmHg; in this range the line broadening/unit of pO2 is much greater than that of fusinite and LiPc. Over the range of biological conditions that can occur in vivo, the response of the EPR spectrum of India ink to pO2 is independent of pH, oxidants, reductants, and the nature of the medium. In this paper we describe the relevant properties of India ink and its use to measure pO2 in vivo in experimental animals and a human subject.
Rotating Frame RF Current Density Imaging
Authors: Scott Greig C. , Joy Michael L. G. , Armstrong Robin L. , Mark Henkelman R. 
Published: March 2007

AbstractRF current density imaging (RF‐CDI) is a new MRI technique for imaging the Larmor frequency current density parallel to B0 in electrolytic media. To extend the use of RF‐CDI to biological tissue for generating conductivity contrast, the sensitivity must be increased and the data requirements reduced. A rotating frame approach, in which a large B1 field is applied simultaneously as a rotary echo with RF current, is proposed to meet these requirements. Rotating frame magnetic fields are encoded in the phase of an MRI image. Trials have now been performed with this sequence in a three‐compartment cylindrical phantom containing doped water or mineral oil for detecting displacement, conduction and fringe field currents. In a postmortem rat study, 85.56 MHz RF currents injected by implanted electrodes created tissue dependent contrast because of the electrical properties of tissue. A sensitivity and artifact analysis was also performed. The sensitivity of this method is determined by the maximum RF pulse duration. SAR limits pose an upper bound on this time and B1, whereas the avoidance of phase artifacts imposes a lower bound on B1.
Phase Constrained Encoding (PACE): A Technique for MRI in Large Static Field Inhomogeneities
Authors: Kim Jae K. , Plewes Donald B. , Mark Henkelman R. 
Published: March 2007

AbstractIn spin echo imaging, magnetization is assigned to a location defined by its frequency of rotation. In the presence of a static magnetic field inhomogeneity, however, this location does not correspond to the true location of the magnetization. This paper describes a magnetic resonance imaging technique called phase constrained encoding (PACE) that assigns magnetization to its true location through the use of a spin echotrain and alternating readout gradients. Small artifactual sidebands occur in the point spread function but can be minimized or eliminated using higher gradient strengths, more echoes, and/or additional acquisitions. Implementation of a simple version of this technique confirms simulations.