bluuebunny/crossref_metadata_2025 huggingface dataset:
Computer modeling of surface coil sensitivity
Authors: Lawry T. J. , Weiner M. W. , Matson G. B.
Published: March 2007

A simple model is presented for the calculation of relative signal‐to‐noise (S/N) ratios of coils of different sizes and configurations when applied to in vivo MRS. Axial symmetry is assumed, which enables rather simple expressions to be used for the calculation of coil loading by the tissue. The model is calibrated to experiments through measurement of the loaded and unloaded coil Q's. Applications of the model demonstrate that for small, superficial regions of interest (ROI), small surface coils can provide a S/N much improved over that of a larger coil. However, for very deep ROIs, larger coils or coils producing S/N. © 1990 Academic Press, Inc.
PC‐based system for retrospective cardiac and respiratory gating of NMR data
Authors: Bohning Daryl E. , Carter Bruce , Liu Shinshang , Pohost Gerald M.
Published: March 2007

A method and a means for retrospectively clustering NMR k‐space measurement profiles with respect to both cardiac and respiratory phases were developed to explore strategies for (1) reducing cardiovascular and respiratory flow/motion image artifacts and (2) improving T1 and T2 characterization of the heart. The image data are collected at a uniform rate so that echo (TE) and repetition (TR) times are independent of the varying cardiac cycle R‐R interval and/or respiratory motions. Cardiac (C) time, respiratory (R) time or diaphragm position, and NMR data acquisition (A) cycle time are collected by microcomputer in parallel with free running (untriggered) image collection on a standard magnetic resonance imager. After the raw data equivalent of multiple images are collected, the C‐A‐R phase timing data are uploaded from the microcomputer to the scanner's minicomputer for use in a normalized C‐R phase plane clustering of the image raw data. Each profile's position in the C‐R phase plane is determined and then clustered into a new set of data, one image being equivalent for each desired C‐R phase combination. These raw data are then zero‐filled and (optionally) filtered to compensate for the nonuniform k‐space sampling and, finally, reconstructed. Cardiac “cines” made from these retrospectively gated images are comparable to similarly phased triggered images. When high time resolution is required, retrospective gating can be expected to show improvements over triggering, especially toward the critical latter part of the cardiac cycle, where coronary artery filling occurs. The system described can readily be assembled from generally available components. © 1990 Academic Press, Inc.
The importance of phase‐encoding order in ultra‐short TR snapshot MR imaging
Authors: Holsinger Anna E. , Riederer Stephen J.
Published: March 2007

We show the importance of phase‐encoding order in ultra‐short TR (< 10 ms) snapshot MR imaging. Contrast is more heavily dictated by the contrast preparation phase early in the measurement phase than later. When the low spatial frequencies are measured first, the resultant image contrast more closely resembles that established by the preparation phase. The transient nature of the data acquisition causes a k‐space filter to be applied to the data. The effects of this filter can be minimized by appropriate choice of scan parameters. © 1990 Academic Press, Inc.
Passive magnetic screening
Authors: Andrew E. R.
Published: March 2007

It is shown that a passive magnetic shield for a 1,5‐T whole‐body magnet requires about 20 tons of iron. Moreover, to first order, the amount of shielding material is independent of the radius of the shield. The choice between a thick shield fitting tightly round the magnet and a thinner shield of larger radius is determined by considerations of available space and the need for the highest uniformity of field in the bore. Very high permeability materials such as mu‐metal are useful only in special circumstances. Multiple shields are valuable if a high degree of shielding is required, but the spacing between the shields needs careful attention. Although exact reciprocity of internal and external shielding is not found in the general case, the degree of shielding will be of the same order in both cases. The complete behavior of cylindrical shields around superconducting magnets can be determined by analytical solution of Maxwell's equations; for less regular shapes, solutions may be determined numerically by computer. © 1991 Academic Press, Inc.
Saturation effects in phosphorus‐31 magnetic resonance spectra of the human liver
Authors: Cox I. J. , Coutts G. A. , Gadian D. G. , Ghosh P. , Sargentoni J. , Young I. R.
Published: March 2007

Phosphorus‐31 liver spectra were recorded from 6 controls and 12 patients with liver disease using TR values of 0.5 and 5 s and a pulse angle of 45°. One of the control subjects was also examined at seven TR values ranging from 0.5 to 20 s. Spectra from one additional patient were collected at TRs of 0.5, 1, and 2 s only. There was a significant increase in the mean ratio of peak areas phosphomonoesters (PME)/β‐ATP in 9 of the 13 patients and a decrease in phosphodiesters (PDE)/β‐ATP in 3 of the patients, compared with controls, at the longer TR values. The saturation factors for PME and PDE were greater than those for Pi and β‐ATP, and spectral abnormalities in disease were often more evident as the TR value was increased from 0.5 to 5 s. Acquisition parameters need to be chosen with a knowledge of the impact that saturation effects have on metabolite quantification and Spectral Contrast. © 1991 Academic Press, Inc.
31P NMR spectroscopy in chronic adriamycin cardiotoxicity
Authors: Bittner Vera , Reeves Russell C. , Digerness Stanley B. , Caulfield James B. , Pohost Gerald M.
Published: March 2007

Abnormal cardiac energy metabolism has been postulated as a mechanism for adriamycin induced cardiotoxicity. This study was designed to determine high energy phosphate stores at rest and with hemodynamic stress in perfused rat hearts after animals had been chronically exposed to adriamycin (2 mg/kg weekly for 14 weeks). Morphologic and hemodynamic changes were mild in this model. Phosphorus‐31 NMR determined intracellular pH and levels of inorganic phosphate (Pi) and ATP were comparable in treated and control hearts. Phosphocreatine (PCr) levels were markedly decreased in treated hearts (0.89 ± 0.07 units/g versus 1.7 ± 0.13 units/g, p < 0.001). The PCr/Pi ratio decreased in both groups during hemodynamic stress. It recovered earlier in controls and there was a marked overshoot after cessation of rapid pacing in this group which was not present in adriamycin treated hearts. These results suggest that metabolic regulation in response to hemodynamic stress is impaired after chronic adriamycin exposure. PCr depletion and delayed metabolic recovery after hemodynamic stress appear to be potentially useful markers for the effect of adriamycin on the heart. © 1991 Academic Press, Inc.
Site‐specific water proton relaxation enhancement of iron (III) chelates noncovalently bound to human serum albumin
Authors: Jenkins Bruce G. , Armstrong Eileen , Lauffer Randall B.
Published: March 2007

Binding of potential blood pool and hepatobiliary paramagnetic iron(III) contrast agents, rac‐ and Meso‐Fe(5‐Br‐EHPG)− (iron(III) N,N′‐ethylenebis[(5‐bromo‐2‐hydroxyphenyl)glycinate]) and Fe(5‐Br‐HBED)− (iron(III) N,N′‐bis‐(5‐bromo‐2‐hydroxybenzyl)ethylenediaminediacetic acid) to human serum albumin (HSA) has been studied using the proton relaxation enhancement (PRE) effect on solvent protons. These chelates bind avidly to multiple sites on HSA with binding constants on the order of 104 to 105 M−1. Interestingly, binding results in a decrease in the diamagnetic component of the water relaxivity due to HSA, while the expected enhancement of the paramagnetic component of water proton relaxation rates occurs due to the increase in the rotational correlation times of the protein‐bound agents. These relaxation enhancements are variable, depending upon the site on the protein to which these chelates are bound, and can be as high as ∼7 mM−1 s−1 at 5°C and ∼5 mM−1 s−1 at 37°C at 20 MHz (enhancements of ∼2−5). Change of temperature from 5 to 37°C also appears to switch the relative affinities of these chelates for their primary and secondary binding sites. It is found that the important HSA binding site for the heme breakdown product, bilirubin‐IXα, is a target for these agents and is the site of highest relaxivity for all the agents. © 1991 Academic Press, Inc.
GdDTPA‐enhanced magnetic resonance imaging of the aqueous flow in the rabbit eye
Authors: Cheng Hong‐Ming , Kwong Kenneth K. , Xiong Jing , Chang Chen
Published: March 2007

Magnetic resonance imaging with gadolinium‐diethylenetriaminepentaacetic acid complex (GdDTPA) as the contrast agent was used to image the aqueous chamber of the eye. This method, in addition to providing spatial information, permits quantitative study of the aqueous flow. GdDTPA solution was applied either topically or intravenously, entering the anterior chamber via different pathways. The wash‐in and wash‐out of GdDTPA follow a two‐compartment model which enables determination of the aqueous flow rate by multiplying the aqueous chamber volume by the wash‐out rate constant. Rabbit eyes showed a flow rate of 1.5−2 μ1/min which was retarded by the systemic administration of acetazolamide ( Diamox ). © 1991 Academic Press, Inc.
NMR determination of myocardial pH in vivu: Separation of tissue inorganic phosphate from blood 2, 3‐DPG
Authors: Zahler Raphael , Majumdar S. , Frederick Blaise , Laughlin Maren , Barrett Eugene , Gore John C.
Published: March 2007

Phosphorus NMR can measure myocardial tissue pH from the chemical shift of inorganic phosphate (Pi) in isolated buffer‐perfused hearts, but in vivo the Pi peak originating from the myocardium is obscured by the resonance of 2, 3‐diphosphoglycerate (DPG) in the blood, making pH difficult to determine. Taking advantage of the fact that most of the interfering DPG is within the cardiac chambers and is rapidly flowing out of the sensitive volume of our coil, we developed a pulse sequence which would separate myocardial Pi signal from interfering DPG. We tested this method on a flow phantom and in living rat heart, using exogenous glycerol phosphate as a blood‐pool marker. The results indicated that signal from moving and nonmoving substances could be separated, and derived values for myocardial pH and PCr/Pi ratio were consistent with previous estimates. This method should be useful for studying myocardial acid‐base physiology with NMR. © 1991 Academic Press, Inc.
Fast MR cardiac profiling with two‐dimensional selective pulses
Authors: Cline Harvey E. , Hardy Christopher J. , Pearlman Justin D.
Published: March 2007

A rapid‐profiling NMR pulse sequence has been designed to provide an interactive, real‐time cardiac probe analogous to M‐mode ultrasound. The pulse sequence employs a two‐dimensional (2D) selective NMR pulse to excite a narrow (nominally 1 ‐cm‐diameter) cylinder of magnetization intersecting the heart. This procedure is followed by a readout gradient applied along the length of the cylinder, or “beam,” to yield an M‐mode type profile with a one‐dimensional Fourier transform reconstruction. k‐space techniques were used to design 2D pulses which excite cylinders characterized by either Gaussian or square radial excitation profiles. Images of phantoms acquired at 1.5 T confirm the predictions of the k‐space analysis. The cylinder can be displaced interactively by modulating the rf excitation and the beam axis can be reoriented to any oblique direction by changing the relative mixing of the gradient waveforms. Flow compensation using bipolar gradient waveforms inverts the contrast of flowing blood and suppresses flow artifacts. A gated cardiac image is acquired as a reference to locate the excitation axis. A series of cardiac experiments was performed on several healthy volunteers. As the beam is moved and rotated to probe the myocardium, the profile plots resemble an M‐mode echocardiogram. Unlike in M‐mode echocardiography, however, the axis of interrogation is not limited to specific windows, and there is distinct flexibility of contrast. However, the temporal resolution is currently less than that achieved by ultrasound. NMR M‐mode profiling provides a direct, fast method of measuring heart motion to assess cardiac function as part of an MR cardiac exam. © 1991 Academic Press, Inc.
A study of nephrotoxin‐induced acute tubular necrosis with 31P magnetic resonance spectroscopy
Authors: Schutz Thomas , González‐Méndez Ricardo , Nabseth Donald C. , Tune Bruce M. , Jardetzky Oleg , Stevenson David K.
Published: March 2007

Phosphorus magnetic resonance spectroscopy (31P MRS) was used to obtain in vivo spectra from rat kidneys undergoing acute tubular necrosis induced by a nephrotoxic dose of cephaloridine (CLD). Spectra were obtained 0, 24, and 48 h after injection of CLD (experimental group, n = 6) or saline vehicle (control group, n = 6). The nephrotoxicity of CLD was demonstrated by severely increased serum creatinine levels and the development of extensive proximal tubular necrosis in the CLD‐injected rats, and the lack of such changes in the controls. 31P MRS showed an increase in the inorganic phosphate region signal (P1, p = 0.004) and a decrease in the phosphodiester region signal (PDE, p = 0.01) in the experimental group by 48 h, whereas these parameters did not vary significantly in the control group during the experiment. Significant correlations were found between serum creatinine and the same two 31P MRS parameters. In summary, rat kidneys which have developed severe CLDinduced proximal tubular necrosis exhibit changes in the 31P spectrum 48 h after administration of the drug. The causes of these changes were not determined. © 1991 Academic Press. Inc.
The phenomenon of separate intra‐ and extracellular resonances of difluorophosphate in 31P and 19F NMR spectra of erythrocytes
Authors: Xu Arron S.‐L. , Potts Jennifer R. , Kuchel Philip W.
Published: March 2007

Trifluoroacetate and trifluoroacetamide, when added to a suspension of human red blood cells, give rise to separate 19F NMR signals from the intra‐ and extracellular species. This phenomenon has recently been exploited for measuring the membrane potential of erythrocytes. However, the separation of the peaks was incorrectly ascribed to a difference in magnetic susceptibility between the intra‐ and extracellular environments. Previously, we have reported well‐resolved resonances in 31P NMR spectra for the intra‐ and extracellular populations of some phosphoryl compounds; in these cases, however, the intracellular peak is shifted to low frequency which is the opposite to the situation with the fluorinated compounds. By using difluorophosphate, which rapidly equilibrates across the membrane of human erythrocytes and which has both the phosphoryl and fluorine functional groups, we observed the separate intra‐ and extracellular resonances. But, the intracellular resonance was shifted to high frequency of the extracellular resonance in the 19F spectra and to low frequency in the 31P spectra. The basis for the phenomenon in both cases is thought to be the reduced hydrogen bonding inside the cells between the solvent water and the phosphoryl oxygen or fluorine atoms. © 1991 Academic Press. Inc.
Image reconstruction from Fourier domain data sampled along a zig‐zag trajectory
Authors: Yan Hong , Braun Michael
Published: March 2007

When the conventional Fourier transform (FT) algorithm is applied to reconstruct a magnetic resonance (MR) image from data sampled along a zig‐zag trajectory in the Fourier space, the nonuniform sampling in the spatial frequency direction may give rise to artifacts. In this paper the nature of the artifacts is analyzed and an alternative reconstruction algorithm is developed to produce artifact‐free images. Methods for reducing noise level in the reconstructed image are discussed. Our approach is compared with another method based on the interlace sampling theorem. © 1991 Academic Press. Inc.
A general formalism for the manipulation of multiple‐echo data sets, and application to chemical shift editing
Authors: Webb A. G. , Hall L. D.
Published: March 2007

A general data processing formalism is presented which can be applied to the signal intensities of successive echoes of a multiple‐echo data set in order to null those species for which the signal decay function is exponential. If asymmetric echo times are introduced into the multiple‐echo sequence then chemical shift images akin to those obtained from the magnitude Dixon method can be obtained in half the data acquisition time. © 1991 Academic Press. Inc.
Nonperturbing test for cytotoxicity in isolated cells and spheroids, using electron paramagnetic resonance
Authors: Dobrucki Jerzy W. , Sutherland Robert M. , Swartz Harold M.
Published: March 2007

Electron paramagnetic resonance imaging (EPRI) and EPR spectroscopy of nitroxides have been applied to detect and quantify cytotoxic effects in mammalian cells in vitro Images depicting microscopic viable and nonviable areas in the same multicellular spheroid (a model of solid tumors) have been acquired prior to and following the treatment with an antitumor drug, Adriamycin. The loss of viability in the inner region of the viable rim was detected. Such mapping of the time‐course changes of the localization of viable and nonviable cells in the same intact biological object is not possible with routinely used methods to measure viability, such as histological examination. The experimental conditions required for high accuracy and sensitivity of the EPRI of spheroids have been evaluated and directions for further development of this approach are indicated. © 1991 Academic Press, Inc.
Comparative evaluation of two membrane‐based liposomal MRI contrast agents
Authors: Karlik Stephen , Florio Eugene , Grant Chris W. M.
Published: March 2007

Two phospholipids—one bearing a nitroxide free radical covalently attached to its polar headgroup, and the other bearing a similarly attached chelating agent with bound gadolinium—were compared in vivo as liposomal contrast agents for MR imaging. In each case the phospholipid contrast agent was incorporated into the membranes of sonicated unilamellar vesicles. The agent with bound gadolinium proved to be considerably more potent at highlighting regions of liposome biodistribution than did its spin‐labelled analogue. Injected intramuscularly into rats, the spin‐label liposomes produced local tissue contrast that persisted for 1 h; while under similar conditions, the liposomal gadolinium persisted for over 24 h. By comparison, water‐soluble, nonliposomal DTPA‐Gd3+ (dimeglumine), was rapidly cleared from the same intramuscular sites—appearing in kidney and bladder within 15 min of injection. When delivered intravenously, maximum effect from both liposomal agents was observed in liver and spleen within 1–2 h, although the spin‐label agent produced only marginal contrast. The concomitant use of fat suppression proved a valuable adjunct to liposomal contrast for imaging organs of the reticuloendothelial system. © 1991 Academic Press, Inc.
Simultaneous multinuclear magnetic resonance imaging and spectroscopy
Authors: Moore Gregory J. , Hrovat Mirko I. , González R. Gilberto
Published: March 2007

A technique has been developed to perform simultaneous multinuclear magnetic resonance imaging and spatially localized spectroscopy. It is inherently superior in terms of time efficiency over current approaches which use sequential or interleaved methods. The pulse sequence uses a parallel excitation and acquisition scheme to acquire multislice proton images concurrently with phosphorus‐31 spectroscopic images. Because the phosphorus signal is necessarily collected in the presence of a gradient, an essential element of the technique is an algorithm to extract pure chemical‐shift information. © 1991 Academic Press, Inc.
In vivo NMR, biochemical, and histologic evaluation of alcohol‐induced fatty liver in rat and a comparison with CCl4 hepatotoxicity
Authors: Hazle John D. , Narayana Ponnada A. , Dunsford Harold A.
Published: March 2007

Magnetic resonance imaging (MRI) and spectroscopy (MRS) were used to follow the time course of ethanol‐induced fatty liver in a group of 10 rats fed a diet containing 12% alcohol (ethanol) over a 5‐week period. The MR data consisted of T1‐weighted images, in vivo 1H spectra, and in vivo T1, relaxation measurements. Changes in short TR images as a result of fatty accumulation were noted only as a slight increase in liver intensity relative to surrounding muscle. A poorly correlated (r = 0.54) increase in water T1 with time was observed. No statistically significant changes in lipid T1 were found. MRS derived lipid content was compared with biochemically derived total lipids and histology. MRS determined liver lipids were found to increase linearly with time (r = 0.91). Biochemically derived lipid content also increased with prolonged exposure to ethanol (r = 0.96). The averages of MRS derived lipid content agreed well with the average changes in biochemically determined total lipid concentration. Histologic examination revealed slight to moderate changes in fatty accumulation with significant variation in the group at the end of the study. On an individual basis the MRS and histologic evaluation were highly correlated (r = 0.94). © 1991 Academic Press, Inc.
Dynamic changes in 13C NMR spectra of intact hearts under conditions of varied metabolite enrichment
Authors: Lewandowski E. Douglas , Hulbert Chrys
Published: March 2007

Dynamic changes in 13C NMR signal from enriched glutamate pools within hearts have been examined under varied conditions of metabolite pool size and fractional enrichment. Relative signal intensities of 13C‐enriched glutamate isotope isomers were similar within spectra from both intact hearts and corresponding in vitro samples. The parameters used to assess metabolic activity with 13C NMR proved independent of fractional enrichment and pool size. The data show the importance of acknowledging unlabeled, 13C NMR invisible metabolites. © 1991 Academic Press, Inc.
Sucrose polyester: A new oral contrast agent for MRI
Authors: Ballinger Ray , Magin Richard L. , Webb Andrew G.
Published: March 2007

Sucrose polyester (a dietary fat substitute) opacifies the entire gastrointestinal tract of the rat with oral administration in magnetic resonance imaging (MRI). These results suggest the potential use of this material as an oral contrast agent for human abdominal MRI studies. © 1991 Academic Press, Inc.
Applications of the intravoxel coherent motion technique to the central nervous system
Authors: Young I. R. , Doran M. D. , Hajnal J. V.
Published: March 2007

Phase difference images, obtained by comparing data from motion‐sensitized and ‐unsensitized spin‐echo sequences, can be used to study intravoxel coherent motions. The resulting images, which have been routinely used to study fast flowing blood, may potentially yield information about perfused flows and bulk tissue motion. The likely limitations and potential of this technique applied to the central nervous system are reviewed with reference to parallel studies of intravoxel incoherent motion. © 1991 Academic Press, Inc.
Diffusion/microcirculation MRI in the rat brain
Authors: Macfall J. R. , Maki J. H. , Johnson G. A. , Hedlund L. , Benveniste H. , Copher G.
Published: March 2007

The CO2 fraction of an anesthetized rat's breathing mixture was changed (from 0 to 10%) to attempt to change the brain microcirculation and observe these changes in diffusion measurements of the neural tissue. Brain apparent diffusion coefficients were measured to be (0.71 ± 0.01) × 10−1 mm2/s before sacrifice and (0.39 ± 0.01) × 10−3 mm2/s after sacrifice. Multiple diffusion components were observed, consistent with flowing material, but the extra components did not increase with increased CO2. It is proposed that the additional components may be due to extracellular, extravascular water such as CSF. © 1991 Academic Press, Inc.
A dual‐frequency circularly polarizing whole‐body MR antenna for 69/170 MHz
Authors: Dürr W. , Rauch S.
Published: March 2007

The design concept of a dual‐frequency whole‐body MR antenna for circular polarization is presented. First, the principle of the capacitively shortened transmission line antenna, which is the basis of the design, is discussed. Emphasis is given to the antenna design at high frequencies. Two variants of possible antenna constructions are described and the RF efficiency is calculated for both frequencies. The antenna performance is discussed for the example of 69 and 170 MHz, the nuclear magnetic resonance frequencies of the nuclei 31P and 1H at 4 T. For 31P experiments, a whole‐body antenna is useful only for transmitting and, thus, the antenna performance can be optimized for 1H. The design principle is not limited to two frequencies; however, the RF efficiency decreases with an increasing number of resonance frequencies. Results of a laboratory version and 1H images at 4 T are presented. © 1991 Academic Press, Inc.
Multiecho, spin‐echo sequence to eliminate unwanted echoes
Authors: Zur Y. , Zou X. , Neuringerc L. J.
Published: March 2007

Two types of artifacts—a mirror‐reversed image about the phase‐encode direction and a wave‐like intensity variation across the image—may appear when using a conventional multiecho spin‐echo sequence. We describe and demonstrate a new method, which eliminates such artifacts, using only one excitation per phase‐encoding step and without the need for spoiler gradients. © 1991 Academic Press, Inc.
Longitudinal spin‐order‐based pulse sequence for lactate editing
Authors: Reddy Ravinder , Subramanian V. H. , Clark B. J. , Leigh John S.
Published: March 2007

A new pulse sequence which edits proton spectra of lactate with full signal return and gives good suppression of water and fat signals is described. This sequence exploits longitudinal spin‐order from lactate to edit lactate from fat. Experimental results from phantoms and excised pig heart are presented. © 1991 Academic Press, Inc.
Rapid, fully automatic, arbitrary‐volume in vivo shimming
Authors: Webb Peter , Macovski Albert
Published: March 2007

MR spectroscopy and many MR imaging methods benefit from a well‐shimmed magnet. We have developed a pulse sequence which enables fast and accurate measurement of three‐dimensional field maps in vivo, and a data analysis package that allows calculation of shim currents to optimally shim arbitrary selected volumes. A data link to the shim power supply allows automatic update of currents. No intervention by the operator is required. Typical in vivo shimming time is less than 5 min. Performance analysis, phantom, and in vivo results are presented. © 1991 Academic Press, Inc.
On the origin of paramagnetic inhomogeneity effects in whole blood
Authors: Matwiyoff N. A. , Gasparovic C. , Mazurchuk R. , Matwiyoff G.
Published: March 2007

Asymmetric 7 T proton NMR signals of water in RBC suspensions containing intracellular deoxyhemoglobin are composites of chemically shifted extracellular and intracellular resonances broadened by gradient diffusion and modulated by transmembrane water exchange. This allows assessment of field dependences of acute hematoma intensities in proton MRIs at lower field strengths ( <1.5 T). © 1991 Academic Press, Inc.
31P MRS of myocardial inorganic phosphate using radiofrequency gradient echoes
Authors: Gober J. R. , Schwartz G. G. , Schaefer S. , Massie B. M. , Matson G. B. , Weiner M. W. , Karczmar G. S.
Published: March 2007

Determination of the chemical shift and integral of the myocardial intracellula inorganic phosphate (Pi) resonance by 31P magnetic resonance spectroscopy (MRS) is often precluded due to a large overlapping signal from 2,3‐diphosphoglycerate (2,3‐DPG) from chamber and myocardial blood. This report demonstrates the use of radiofrequency (RF) magnetic field gradient echoes (RFGE) to eliminate signals from 2,3‐DPG in flowing blood, while retaining signals from intracellular myocardial Pi, ATP, and phosphocreatine (PCr). The ECG‐triggered 31P spectra were acquired from the myocardium of open chest pigs using a Philips Gyroscan 2‐T magnetic resonance spectrometer. A 2.5‐cm‐diameter surface coil attached to the myocardium was used to provide the RF gradient as well as for excitation and detection of signals. Optimal performance of the RFGE pulse sequence was obtained when the RF gradient pulses were centered at peak diastole or peak systole. Under these conditions, 2, 3‐DPG signals were completely suppressed, and sensitivity was usually sufficient to allow detection of a well‐resolved Pi signal. Myocardial pH determined from RFGE experiments was 7.16 ± 0.10, and the ratio of the integrals of the Pi and ATP resonances (Pi/ATP) was 0.24. The mean signal‐to‐noise ratio (S/N) for PCr in control spectra acquired in 4 min was 19/1, while the mean S/N for PCr in RFGE‐edited spectra acquired in 15 min was 11/1, demonstrating that the present implementation of the RFGE method results in significant loss in sensitivity. These experiments demonstrate that RFGE‐editing allows accurate determination of the chemical shift and integral of the Pi resonance in blood‐perfused myocardium in situ. © 1991 Academic Press, Inc.
Localized in vivo proton spectroscopy of the human kidney
Authors: Shah N. J. , Carpenter T. A. , Wilkinson I. D. , Hall L. D. , Dixon A. K. , Freer C. E. L. , Prosser K. , Evans D. B.
Published: March 2007

In vivo 1H spectroscopy using the STEAM sequence for localization has been applied to the human kidney in normal volunteers and subjects with succesful renal transplants. We show that, within the resolution of our measurements, trimethylamines are present in the spectra from some of the subjects and absent from others, the prominent peak seen at 5.8 ppm in the spectrum is identified as that from urea and not lipid, as previously suggested. © 1991 Academic Press, Inc.
Measurements of pertinent concentrations of oxygen in vivo
Authors: Swartz H. M. , Boyer S. , Gast P. , Glockner J. F. , Hu H. , Liu K. J. , Moussavi M. , Norby S. W. , Vahidi N. , Walczak T. , Wu M. , Clarkson R. B.
Published: March 2007

A new method able to measure the concentration of oxygen in complex biological systems, including in vivo, has been developed using low‐frequency EPR and newly characterized free radicals that are very sensitive to the concentration of oxygen. The free radicals (fusinite and lithium phthalocyanine) are very stable in tissues (for at least 150 days), apparently nontoxic, and can reflect oxygen concentrations that are less than the Km of cytochrome oxidase (0.1 μM or lower). Their biological stability is indicated by the fact that repeated measurements with fusinite of the concentration of oxygen in skeletal muscle have been made in the same animal for more than 150 days without any change in sensitivity or signs of toxicity. © 1991 Academic Press, Inc.
Correction for chemical‐shift artifacts in 19F imaging of PFOB: Simultaneous multislice imaging
Authors: Lee H. K. , Nalcioglu O. , Buxton R. B.
Published: March 2007

One of the difficulties encountered in 19F NMR imaging of fluorinated blood substitutes is that these compounds often exhibit complex multipeak spectra. These peaks result in chemical‐shift artifacts along the readout direction and blurred images. In addition, each peak excites a different slice (mis‐selection) when a slice selection gradient is applied during the selective rf pulse. A simultaneous multislice imaging method has been developed to solve the inherent problem of mis‐selection. The essence of this method is to use the two strongest peaks of the spectrum to excite controlled different multiple slices simultaneously, with or without a slice gap. The images corresponding to the two spectral lines are then separated from in‐and out‐of‐phase images (Dixon method). This method corrects the problem of mis‐selection and either improves the SNR or increases the number of slices over spectrally selective methods which image only one peak. © 1991 Academic Press, Inc.
Magnetic resonance imaging study of the rat cerebral ventricular system utilizing intracerebrally administered contrast agents
Authors: Wan Xiaoming , Fu Tsu‐Ching , Smith Paul H. , Brainard James R. , London Robert E.
Published: March 2007

Magnetic resonance imaging (MRI) was employed to study the rat brain in conjunction with intracerebral (ic) injection of three contrast agents: GdHAM, GdDPTA, and MnCl2, The results demonstrate several advantages of ic administration of MRI contrast agents over the other routes of injection in examining CSF dynamics and brain ventricular structure. Apparent affinity of the luminal ventricular wall of the brain for positively charged GdHAM and Mn2+ ions is observed, presumably reflecting the presence of negatively charged wall components. Respiratory distress caused by (intravenous) injection of GdHAM was found to be minimized in the case of ic injections. Time‐dependent changes in observed contrast indicate that diffusive processes rather than flow of CSF play a dominant role in distributing the contrast agents. Possible applications of this approach in brain research are discussed. © 1991 Academic Press, Inc.
Quantitative determination of the partial oxygen pressure in the vitrectomized rabbit eye in Vivo using 19F NMR
Authors: Berkowitz Bruce A. , Wilson Charles A. , Hatchell Diane L. , London Robert E.
Published: March 2007

We have quantitatively determined the PO2 within the vitreous space of the vitrectomized rabbit eye in vivo using 19F NMR and perfluorotributylamine (FTBA). The results of the present work are in good agreement with previous measurements of PO2, within the nonvitrectimized rabbit eye obtained using oxygen microelectrodes. In this study, good precision and accuracy were achieved by: (1) having minimal flow effects present, (2) optimizing the signal‐to‐noise through the use of neat FTBA and surface coil transmission and detection, and (3) performing an inversion‐recovery pulse sequence, with adiabatic pulses, to optimize the dynamic range of the T1 experiment. Possible deleterious effects on the measured T1, due to the vitreal temperature gradient are discussed. To the best of our knowledge the results of this study demonstrate, for the first time, a quantitative determination of intraocular PO2 in vivo using 19F NMR and FTBA. © 1991 Academic Press, Inc.
Tumor bioenergetics and blood flow in RIF‐1 murine tumors treated with 5‐fluorouracil
Authors: Li Shi‐Jiang , Wehrle Janna P. , Glickson Jerry D. , Kumar Nirmal , Braunschweiger Paul G.
Published: March 2007

Treatment of RIF‐1 solid tumors with 5‐fluorouracil (5‐FU, 100 or 200 mg/kg, ip) caused substantial regression of the tumors, with regrowth initiated on Day 6 (100 mg/kg) or Day 9 (200 mg/kg). Blood perfusion in the tumor, estimated by uptake of 86Rb+, was significantly increased after treatment with 5‐FU, while Rb+ uptake in normal tissues (skin, muscle) was unaffected. The increase in tumor perfusion during the first few days following treatment was significantly greater in animals treated with the higher dose of 5‐FU. Perfusion‐dependent 86Rb+ uptake returned to control levels by the 9th day after treatment with 100 mg/kg of 5‐FU, but remained elevated on Days 9–12 after the higher dose. By the lst day following treatment with 5‐FU, in vivo 31P NMR spectra of treated tumors indicated significantly higher ratios of phosphocreatine to Pi, higher pH, and lower ratios of Pi to nucleoside triphophates compared to untreated age‐matched controls. These changes persisted for 9 days following the lower 5‐FU dose and for at least 12 days following the higher dose. Treatment with 5‐FU induces profound, dose‐dependent changes in tumor bioenergetics, which may result, at least in part, from changes in tumor perfusion after cytoreduction. © 1991 Academic Press, Inc.
Quantitative study of the susceptibility difference between trabecular bone and bone marrow: Computer simulations
Authors: Majumdar S.
Published: March 2007

Inherent differences in tissue magnetic susceptibility produce inhomogeneities in the static magnetic field which give rise to an additional dephasing of the transverse magnetization in gradient‐echo images. The enhanced dephasing of the signal results in an increase of the apparent relaxation rate 1/T and a corresponding decrease in signal intensity. These effects have been used to explain the regional loss of marrow signal intensity in the appendicular skeleton, where in the presence of trabecular bone in the proximal tibia there is an enhanced loss of signal compared to the tibial shaft where there is no trabeculation. It has been postulated that differences in tissue magnetic susceptibility arising due to the marrow‐trabeculae interface give rise to magnetic field inhomogeneities and a reduced T. In this study computer simulations are used to determine whether susceptibility differences comparable to that between trabecular bone and tissue relate to the reduction of tissue T and whether the reduction in T is also related to the concentration and magnitude of susceptibility differences. In addition the effects of the spatial distribution of these particulate discontinuities in susceptibility on the measured relaxation time T are also estimated. This model demonstrates that 1/T increases as the number density and magnitude of such susceptibility differences increase. In a pixel of linear dimension L consisting of material simulating tissue water, the presence of circular point susceptibility differences of dimension 0.001 L with magnetic susceptibility equivalent to trabecular bone, 1/T, increases at a rate of 1.60 × 10−2 s−1 /N for N ranging from 25–2500. Differences in magnetic susceptibility that are less than that between soft tissue and trabecular bone are also modeled and the simulations demonstrate that differences in magnetic susceptibility, much lower than that between trabecular bone and tissue equivalent interfaces, also produce a relaxation rate enhancement in gradient‐echo images. © 1991 Academic Press, Inc.
Water–proton nuclear magnetic relaxation in heterogeneous systems: Hydrated lysozyme results
Authors: Lester Cathy Coolbaugh , Bryant Robert G.
Published: March 2007

Spin‐lattice relaxation rates of water protons in hydrated immobilized lysozyme are measured as a function of magnetic field strength. The dependence of water relaxation versus hydration is presented from 35 to 55% by weight water content. The water‐proton relaxation is directly coupled to that of the protein and the coupling exists in the absence of chemical exchange. A model is applied where relaxation within the two proton phases is coupled through a dipolar cross‐relaxation mechanism as well as chemical exchange. The observed amplitudes of the water‐proton relaxation profiles scale with the ratio of protein to water protons as well as the protein‐proton relaxation rate. The field dependence of the protein‐proton spin‐lattice relaxation is presented in the presence of D2O where a cross‐relaxation coupling is absent. The coupled relaxation model accounts well for the NMR relaxation data as a function of magnetic field strength which is similar to measurements on other heterogeneous systems such as tissues. © 1991 Academic Press, Inc.
Paramagnetic iron (III) MRI contrast agents
Authors: White David L.
Published: March 2007

The advantages and disadvantages of a number of paramagnetic Fe(III)‐containing MRI contrast agents are discussed. © 1991 Academic Press, Inc.
Targeted relaxation enhancement agents for MRI
Authors: Lauffer Randall B.
Published: March 2007

The design, relative merits, and initial applications of targeted relaxation enhancement agents, a new form of MRI contrast media, are briefly discussed. These agents are designed to bind reversibly to target macromolecules in a tissue of interest, thus enhancing water proton relaxation efficiency and MRI contrast. Initial applications for targeted agents include blood pool and hepatobiliary imaging. © 1991 Academic Press, Inc.
A new method for the evaluation of the liver injury by 19F MRS of 5‐fluorouracil
Authors: Harada Masafumi , Koga Keiko , Miura Iwao , Nishitani Hiromu
Published: March 2007

An investigation was made of 5‐fluorouracil metabolism in rat liver by 19F MRS. Fluoronucleotide synthesis increased markedly with liver injury and accelerated with uracil treatment, indicating that 19F MRS may serve as a new method for evaluating liver viability. © 1991 Academic Press, Inc.
“One‐shot” velocity microscopy: NMR imaging of motion using a single phase‐encoding step
Authors: Xia Yang , Callaghan Paul T.
Published: March 2007

The use of the pulsed gradient spin‐echo sequence in NMR microscopy enables the measurement of molecular translational motion and simultaneous construction of velocity and self‐diffusion images, a technique that has been termed dynamic NMR microscopy. In this method the PGSE contrast gradient is stepped in a fourth dimension (q space) and so is inherently inefficient. Provided that one is prepared to sacrifice some of the additional information provided by the multiple PGSE gradient approach, it is possible to construct a velocity image alone by means of a single PGSE phase‐encoding step. We illustrate applications of this method in which a signal from the stationary spins is nulled by the use of both gradient phase cycling and a final “z‐storage” rf pulse. The limits to velocity resolution are around 10μm s−1 in free water but can be considerably smaller for molecules with a low self‐diffusion coefficient. We demonstrate this method in a study of water capillary flow at 12 μm transverse pixel resolution, extending the velocity range by employing a four‐quadrant analysis method. This method is also used to measure vascular transport in a living plant and find a flow rate of around 45μm s−1. © 1992 Academic Press, Inc.
Formulation of radiographically detectable gastrointestinal contrast agents for magnetic resonance imaging: Effects of a barium sulfate additive on MR contrast agent effectiveness
Authors: Rubin Daniel L. , Muller Holde H. , Young Stuart W.
Published: March 2007

Complete and homogeneous distribution of gastrointestinal (GI) contrast media are important factors for their effective use in computed tomography as well as in magnetic resonance (MR) imaging. A radiographic method (using fluoroscopy or spot films) could be effective for monitoring intestinal filling with GI contrast agents for MR imaging (GICMR), but it would require the addition of a radiopaque agent to most GICMR. This study was conducted to determine the minimum amount of barium additive necessary to be radiographically visible and to evaluate whether this additive influences the signal characteristics of the GICMR. A variety of barium sulfate preparations (3‐12% wt/vol) were tested in dogs to determine the minimum quantity needed to make the administered agent visible during fluoroscopy and on abdominal radiographs. Solutions of 10 different potential GI contrast agents (Gd‐DTPA, ferric ammonium citrate, Mn‐DPDP, chromium‐EDTA, gadolinium‐oxalate, ferrite particles, water, mineral oil, lipid emulsion, and methylcellulose) were prepared without (“nondoped”) and with (“doped”) the barium sulfate additive. MR images of the solutions in tubes were obtained at 0.38 T using 10 different spin‐echo pulse sequences. Region of interest (ROI) measurements of contrast agent signal intensity (SI) were made. In addition, for the paramagnetic contrast media, the longitudinal and transverse relaxivity (R1 and R2) were measured. A 6% wt/vol suspension of barium was the smallest concentration yielding adequate radiopacity in the GI tract. Except for gadolinium‐oxalate, there was no statistically significant difference in SI for doped and nondoped solutions with most pulse sequences used. In addition, the doped and nondoped solutions yielded R1 and R2 values which were comparable. We conclude that barium sulfate 6% wt/vol added to MR contrast agents produces a suspension with sufficient radiodensity to be viewed radiographically, and it does not cause significant alteration in the MR signal appearance of most GICMR. These formulations can be useful for achieving optimal filling of the gastrointestinal tract prior to MRI. © 1992 Academic Press, Inc.
Reduction of RF penetration effects in high field imaging
Authors: Foo Thomas K. F. , Hayes Cecil E. , Kang Yoon‐Won
Published: March 2007

A method of correcting for the RF inhomogeneity in the body by dielectric loading of the coil‐to‐shield space in an RF resonator (coil and shield assembly) is described. The presence of the RF coil and RF shield have significant effects on RF homogeneity. Based on theoretical calculations, a method for adjusting the RF homogeneity by manipulating the axial propagation constant, kz, is proposed. This is accomplished by loading the coil‐to‐shield space with dielectric material of suitable relative permittivity so as to increase kz and decrease the radial propagation constant, kp. In this manner, the radial wavelength (λp, = 2π/kp) can be increased relative to the body dimensions, and the field amplitude variations in the axial plane minimized. Theoretical calculations indicate that a value of between 30 and 40 for the relative permittivity of the dielectric material in the coil‐to‐shield space would reduce the RF field inhomogeneity from ±15% to about ±3% over a central 30‐cm‐diameter region of a homogeneous 40‐cm‐diameter body at both 64 and 170 MHz. The theoretical model was verified in laboratory measurements of the |B1|, field generated in a test coil at 170 MHz which was scaled to correspond to a body at 64 MHz. However, the improved RF field homogeneity would be accompanied by increased RF power requirements and reduced coil sensitivity. © 1992 Academic Press, Inc.
Image reconstruction for echo planar imaging with nonequidistant k‐space sampling
Authors: Bruder H. , Fischer H. , Reinfelder H.‐E. , Schmitt F.
Published: March 2007

Echo planar imaging is characterized by scanning the 2D k‐space after a single excitation. Different sampling patterns have been proposed. A technically feasible method uses a sinusoidal readout gradient resulting in measured data that does not sample k‐space in an equidistant manner. In order to employ a conventional 2D‐FFT image reconstruction, the data have to be converted to a Cartesian grid. This can be done either by interpolation or alternatively by a generalized transformation. Filtering methods are described to minimize ghosting artifact that is typical in echo planar imaging. Results both from computer simulation and from experiments will be presented. Experimental images were obtained using a 2‐T whole‐body research system. © 1992 Academic Press, Inc.
The longitudinal relaxation time (T1) of the intracellular 23Na NMR signal in the isolated perfused rat heart during hypoxia and reoxygenation
Authors: Prince Lawrence S. , Miller Sandra K. , Pohost Gerald M. , Elgavish Gabriel A.
Published: March 2007

Each of six perfused rat hearts was subjected to 30 min of hypoxia followed by 60 min of reoxygenation. Inversion‐recovery data on the intracellular Na NMR signal, differentiated by a shift reagent, 6 mM Dy(PPP)2, were obtained every 5 min, and T1 values were calculated. The T1 of the intracellular Na signal did not show any significant change either during hypoxia or upon reoxygenation, although the level of Nai increased about 50%. Such an increase of total Nai, is expected to reduce the observed relaxation rate by diluting the fraction of Nai, ions that interact with intracellular polyelectrolytes. The observed constancy of T1 in our study is explained on the basis of the typical values of the dissociation constants of sodium ions, in aqueous solutions, in interaction with polyelectrolytes. Although the constancy of intracellular sodium T1 during hypoxia may preclude the utilization of T1, weighting for the monitoring of pathology, its determination could be important for setting optimal acquisition times in high time‐resolution experiments. © 1992 Academic Press, Inc.