bluuebunny/crossref_metadata_2025_split huggingface dataset:
Human auditory system response to pulsed radiofrequency energy in RF coils for magnetic resonance at 2.4 to 170 MHz
Authors: Röschmann Peter 
Published: March 2007

AbstractThe threshold conditions for an auditory perception of pulsed radiofrequency (RF) energy absorption in the human head have been studied on six volunteers with RF coils for magnetic resonance (MR) imaging. For homogeneous RF exposure with MR head coils in the 2.4‐to 170‐MHz range and pulse widths 3 μs, ⩽Tp< 100 μs, the auditory thresholds were observed at 16 ± 4 mJ pulse energy. Localized RF exposure with optimized surface coils positioned flush with the ear lowers the auditory threshold to only 3 ± 0.6 mJ. The hearing threshold of RF pulses with Tp> 200 μs occurs at more or less constant peak power levels of typically 150 ± 50 W for head coils and as low as 20 W for surface coils. The results from this study confirm theoretical predictions from a thermoeleastic expansion model and compare well with reported thresholds from near field antenna measurements at 425 to 3000 MHz. Details of the threshold dependence on RF pulse length reveal primary sites of RF to acoustic energy conversion at the mastoid and temporal bone region and the outer layer of the brain from where thermoelastically generated pressure transients excite audible pressure waves at the resonance modes of the skull around 1.7 kHz and of the brain around 11 kHz. If not masked by usually dominating noise from switched gradients, the conditions for hearing RF pulses, as applied to head coils in MR studies with flip angle α at main field Bo, is given byTp/ms ⩽ 0.4 (α/π)Bo/[T]. At peak power levels up to 15 kW presently available in clinical MR systems, there is no evidence known for detrimental health effects arising from the RF auditory phenomenon which is a secondary cause associated with primary RF to thermal energy conversion in body tissues. To avoid the RF‐evoked sound pressure levels in the head rising above the discomfort threshold at 110 dB SPL, an upper limit of 30 kW applied peak pulse power is suggested for head coils and 6 kW for surface coils. © 1991 Academic Press, Inc.
Toward fully automatic estimation of in Vivo31P spectra
Authors: Burger Cyrill , McKinnon Graeme , Buchli Reto , Boesiger Peter 
Published: March 2007

AbstractFitting a model to an experimental spectrum is a difficult nonlinear estimation problem. The solution presented here is to start an iterative search procedure sufficiently close to the optimal model parameter set. This is achieved by providing tissue‐dependent a priori peak information and by a novel correlation method to get good primary estimates of the resonance and phase offset parameters. The resulting estimation procedure is fully automatic and has proven to be robust for 31P data. © 1991 Academic Press, Inc.
Pulsed NMR relaxometry of striated muscle fibers
Authors: English A. E. , Joy M. L. G. , Henkelman R. M. 
Published: March 2007

AbstractThe longitudinal (T1,) and transverse (T2,) proton (1H) nuclear magnetic resonance (NMR) relaxation of fast‐and slow‐twitch muscle fibers are examined using rat muscle tissues in which one fiber type predominates. Both continuum and discrete exponential component fits are made to Carr‐Purcell‐Meiboom‐Gill (CPMG) and inversion recovery pulse sequence measurements. In addition, experiments which illustrate the large sources of variability that have led to apparent conflicts in the literature are presented. Based on the results of this study, unique NMR features that distinguish fast‐ and slow‐twitch muscle fibers are presented. The feasibility of differentiating fast‐and slow‐twitch muscle fibers using magnetic resonance (MR) imaging is briefly discussed. © 1991 Academic Press, Inc.
Localized in vivo proton spectroscopy in the brain of patients with multiple sclerosis
Authors: Larsson H. B. W. , Christiansen P. , Jensen M. , Frederiksen J. , Heltberg A. , Olesen J. , Henriksen O. 
Published: March 2007

AbstractLocalized proton spectroscopy was performed in 15 patients with acute or chronic multiple sclerosis (MS). Some of the patients were investigated serially, being given a total of 22 spectroscopic investigations. Resonances corresponding to free lipids were observed in six plaques. This was distinctly seen in two plaques at Days 70 and 85 after the occurrence of the plaques. A lesser content of lipids in plaques was observed as early as Day 10 and as late as nearly 1 year after occurrence. The relative concentration of N‐acetyl asparate (NAA) was significantly lower in patients than in controls, and the relative concentration of choline (Cho) was significantly higher in patients than in controls. These differences were most pronounced in older plaques. MR spectroscopic demonstration of lipids in a MS plaque probably reflects disintegration of myelin, and a decreased NAA/Cho ratio may be related either to gliosis or to axonal degeneration, which sometimes occurs in longstanding MS. © 1991 Academic Press, Inc.
Noninvasive determination of regional cerebral blood flow in rats using dynamic imaging with Gd(DTPA)
Authors: Rudin Markus , Sauter Andre 
Published: March 2007

AbstractA method for the noninvasive, quantitative determination of regional cerebral blood flow (rCBF) in small laboratory animals, such as the rat, is described. The change in magnetic susceptibility in the vicinity of brain capillaries evoked by iv infusion of Gd(DTPA), a paramagnetic agent not crossing the blood‐brain barrier, is detected as an attenuation of the 1H signal in T2‐weighted MRI sequences. The reduction of the MR signal intensity after 1 or 2 min of Gd(DTPA) infusion correlates well (r > 0.95) with the rCBF values obtained using the well‐established [14C] iodoantipyrine method. At a slice thickness of 3 mm, the pixel size typically is 0.15 × 0.5 mm2. The clearance rate of Gd(DTPA) from blood allows repeated measurements of CBF to be made using this technique approximately every 30 min. The experimental results obtained in normal and ischemic rat brain have been rationalized within the frame of a two‐compartment model, allowing the definition of optimal experimental conditions for a given CBF: for normal and reduced CBF values (<150 ml/100 g/min) the reduction in MR signal intensity, measured after 1 or 2 min of Gd(DTPA) infusion, correlates closely with the CBF. However, for CBF values<150 ml/100g/min, a more reliable parameter would be the time constant of the MR signal attenuation during the infusion. © 1991 Academic Press, Inc.
Quantitation of the susceptibility difference between trabecular bone and bone marrow: Experimental studies
Authors: Majumdar S. , Thomasson D. , Shimakawa A. , Genant H. K. 
Published: March 2007

AbstractIn this study we quantify the effects of different relaxation mechanisms on the signal intensity in gradient‐echo images of tissue such as bone marrow in the presence of trabecular bone. The susceptibility difference between trabecular bone and soft tissue produces distortions in the magnetic lines of force which induce strong inhomogeneities in the static magnetic field. Diffusion of tissue protons in such magnetic field gradients produce a shortening of the transverse relaxation time T2, while the dephasing of the transverse magnetization due to susceptibility differences produces a shortening of the apparent relaxation time T as demonstrated in gradient‐echo images. We have used specimens of dried human vertebrae with different bone densities immersed in either saline to simulate tissue water or an emulsion of oil and water to simulate bone marrow to quantify these relaxation mechanisms in vitro. We have measured the MR relaxation times T1, T2, and T of protons within the trabecular spaces and correlated their variations with trabecular bone density. We have found that in vitro, at 1.5 T, the relaxation times T1 and T2 do not show significant variations with bone density and there are no significant contributions to the transverse relaxation rate due to the diffusion of tissue water in the magnetic field gradients. However, the relaxation rate, 1/T, of saline in the presence of trabecular bone increases at a rate of 0.2 s−1 mg/cc due to the dephasing of the transverse magnetization in the magnetic field inhomogeneities. Similar bone density‐related T variations were observed for fat protons within the trabeculae where the chemical‐shift‐induced modulations of signal intensity in an oil‐water emulsion have been separated from the susceptibility‐induced relaxation effects. In addition, we have verified these effects in vivo and quantified in vivo variations in fat and water relaxation rates of bone marrow in the epiphysis and diaphysis in the appendicular skeleton of normal volunteers and found that both fat and water T are shorter in the epiphysis compared to the diaphysis, which correlates well with previous observations. © 1991 Academic Press, Inc.
Pitfalls of contrast‐enhanced imaging in the nervous system
Authors: Brant‐Zawadzki Michael 
Published: March 2007

AbstractThis brief review covers the evolving clinical role of paramagnetic contrast agents in magnetic resonance imaging of the central nervous system, with particular emphasis on potential pitfalls. The safety and efficacy of Gd‐DTPA is now well‐established. Approximately 35–40% of all scans performed in this country now utilize the agent. The major pitfalls center on overinterpretation of the characterization of disease by either enhancement or lack thereof in any particular process. © 1991 Academic Press, Inc.
Alkalosis monitored by 31P NMR in a human glioma cell line exposed to the anti‐tumor drug 1, 3‐bis (2‐chloroethyl)‐1‐nitrosourea
Authors: Commodari Fernando , Sanctuary Bryan C. , Feindel William , Shoubridge Eric A. 
Published: March 2007

AbstractA transient alkalosis of similar magnitude to that observed in vivo has been observed using 31P NMR and 2‐deoxy‐D‐glucose‐6‐phosphate as a pH marker in a human glioma cell line, SKI‐1, with demonstrated sensitivity to 1, 3‐bis (2‐chloroethyl)‐1‐nitrosourea. At an effective dose of 5 ± 1 × 10 μg/ml, an increase of 0.13 ± 0.05 pH units was observed within 4 ± 1 × 10 min of introducing the drug into the perfusion chamber. Although the in vitro response is of a time course much faster than that in vivo, these results suggest that this immediate pH change could be an indicator of the cytotoxic action of the drug. © 1991 Academic Press, Inc.
31P spectroscopy of the human prostate gland in vivo using a transrectal probe
Authors: Kurhanewicz John , Thomas Albert , Jajodia Prahalad , Weiner Michael W. , James Thomas L. , Vigneron Daniel B. , Narayan Perinchery 
Published: March 2007

AbstractUsing a transrectal probe, good quality 31P magnetic resonance spectroscopy of the human prostate was performed safely, consistently, and in a reasonable amount of time (average of 60 min). Initial results indicate that transrectal 31P MRS has the ability to characterize the phosphorylated metabolites of normal, hyperplastic, and malignant prostates. This study demonstrated that malignant prostates are characterized by significantly decreased levels of phosphocreatine (PCr) and increased levels of phosphomonesters (PME) as compared to healthy prostates. © 1991 Academic Press, Inc.
Practical Implementation and Optimization of One‐shot T1 imaging
Authors: Kay I. , Henkelman R. M. 
Published: March 2007

AbstractLongitudinal relaxation times (T1) can be measured rapidly in an imaging context using a “one‐shot” method based on the pulse sequence originally proposed by D. C. Look and D. R. Locker (Rev. Sci. Instrum. 41, 250 1970). This sequence is significantly faster than either repeated inversion recovery or repeated saturation recovery methods. The method uses a 180° inversion pulse followed by multiple small‐angle α pulses that sample the longitudinal magnetization during its recovery. Choices of inversion pulse, tip angle, and time intervals are discussed for optimal clinical use. We can produce 29 images sampling the full T1 recovery curve with a 256 × 256 resolution in about 10 min. From this data, T1 images can be calculated with a precision of 10%. © 1991 Academic Press, Inc.
Measurement of water movement in the rabbit eye in vivo using H217O
Authors: Kwong Kenneth K. , Xiong Jing , Kuan Wei‐Peng , Cheng Hong‐Ming 
Published: March 2007

AbstractTopically applied (eyedrop) H2 17O (10% enrichment) was used in magnetic resonance studies to quantify intraocular water dispersion in vivo. The rate of H2 17O removed from the aqueous chamber is in the order of 0.1 ml/min/ml of tissue. Similar results are obtained with either proton imaging or 17O spectroscopy. Parallel experiments of direct injection of H217O into the aqueous chamber supported the topical data. Proton MR imaging of H217O dissipation from the aqueous chamber with clinical imagers, coupled with eyedrop application. suggerts clinical possibilites. © 1991 Academic Press, Inc.
Inhomogeneity correction for in vivo spectroscopy by high‐resolution water referencing
Authors: Webb Peter , Spielman Daniel , Macovski Albert 
Published: March 2007

AbstractOne of the most common sources of distortion in in vivo spectroscopy is the inhomogeneity of the main magnetic field. This effect is particularly problematic when performing spectroscopic imaging, as the shim cannot be simultaneously optimized for all voxels. In this paper we present a technique to measure inhomogeneity rapidly, then show how to use the measurement to improve the analysis of the spectrum. This technique can be applied in conjunction with any spectroscopic localization method and any spectral quantitation algorithm. We present results from spectroscopic imaging of phantoms, then show application to a single‐voxel water‐suppressed proton brain study. We find that the quantitation of the in vivo spectrum is made immune to inhomogeneous line broadening. © 1992 Academic Press, Inc.
13CNMR studies of glucose metabolism in human leukemic CEM‐C7 and CEM‐C1 cells
Authors: Post Jan F. M. , Baum Erna , Ezell Edward L. 
Published: March 2007

AbstractGlucose metabolism of human leukemic cell lines CEM‐C7 and CEM‐C1 was investigated in vivo by 13C NMR using 13C‐labeled glucose. Exact knowledge of glucose concentration, cell count, and cell viability of the cell suspensions made it possible to analyze glucose metabolism in detail. In both cell lines aerobic glycolysis accounts for virtually all glucose consumption. The use of D‐[13C2] glucose provided a simple method to measure the glucose flux through the pentose phosphate pathway as 9% (CEM‐C1) and 11% (CEM‐C7) of glucose channeled into glycolysis. The dexamethasone‐sensitive CEM‐C7 cells consume glucose at a rate about 50% higher than the dexamethasone‐resistant CEM‐CI cells. It is shown that this higher consumption correlates with a larger size of the CEM‐C7 cells. Therefore in CEM cells the development of drug resistance does not seem to involve related changes in cell energetics. © 1992 Academic Press, Inc.
Iron–dextran as a magnetic susceptibility contrast agent: Flow‐related contrast effects in the T2‐weighted spin‐echo MRI of normal rat and cat brain
Authors: White David L. , Aicher Klaus P. , Tzika A. Aria , Kucharczyk John , Engelstad Barry L. , Moseley Michael E. 
Published: March 2007

AbstractIron–dextran (1 mmol Fe/kg) was used as an intravascular, paramagnetic contrast agent in rat and cat brain in conventional spin‐echo T2‐weighted (TR 2800/TE 100)1H magnetic resonance imaging. The resulting images displayed differential decreases (30–50%) in intensity whose pattern was similar to that obtained with the superparamagnetic particulate iron oxide AMI‐25 (0.18 mmol Fe/kg). Postcontrast images displayed improved anatomic detail, and contrast effects were observed to be greater in cortical and subcortical gray matter than in adjacent white matter. Intravenous injection of acetazolamide after administration of iron‐dextran caused a small additional decrease in image intensity. Measurement of whole blood and plasma at 5 min postinjection of either contrast agent revealed significant increases in their volume magnetic susceptibilities. The contrast effect appears to be related to magnetic susceptibility changes brought about by the iron–dextran it has both blood volume and blood flow components. The static model of magnetic susceptibility effects in brain capillaries is modified to include bolus flow of erythrocytes, providing a mechanism for the observed flow effects. © 1992 Academic Press, Inc.
Separation of diffusion and slow flow effects by use of flow rephasing and dephasing
Authors: Fujita N. , Harada K. , Sakurai K. , Akai Y. , Kozuka T. 
Published: March 2007

AbstractSpin‐echo signal intensity alterations due to diffusion and slow flow are investigated in connection with modified Stejskal‐Tanner pulse sequences for 2D Fourier MR imaging, one for flow rephasing and the other for flow dephasing gradient waveforms. The theoretical considerations and experimental results concerning the diffusion coefficient measurements of slowly flowing material by these sequences are summarized as follows: (a) By using the flow dephasing sequence with different diffusion and flow‐sensitive gradients, slow flow effects can be distinguished from diffusion effects based on the quantitative difference between the diffusion‐and flow‐sensitive gradient amplitude dependences of these processes. (b) By using the combination of the flow rephasing and dephasing sequences with the same diffusion sensitivity, slow flow effects can be distinguished from diffusion effects directly based on their qualitative difference. By use of the latter approach, several in vivo images are also presented, which mainly represent macroscopic motions of spins, including slow flow such as tissue perfusion. © 1992 Academic Press, Inc.
Wavelet‐encoded MR imaging
Authors: Weaver John B. , Xu Yansun , Healy Dennis M. , Driscoll James R. 
Published: March 2007

AbstractWavelet encoding is presented and compared to phase encoding. In wavelet encoding a distribution of spins is excited by a slice selective RF pulse; for each repetition time the distribution excited has the profile of a wavelet at different scale and translation. The spin density can be reconstructed with an inverse wavelet transform. Wavelet encoding has three advantages over phase encoding: (1) there is no Gibblquot's ringing from partial volume effects, (2) the effective repetition time can be 36 times the repetition time for a 256 × 256 image, and (3) motion artifacts are local and dramatically reduced. Using wavelet encoding, a 256 × 256 T2‐weighted projection image can be acquired in 33s. © 1992 Academic Press, Inc.
Perfusion and diffusion MR imaging
Authors: King M. D. , Van Bruggen N. , Busza A. L. , Houseman J. , Williams S. R. , Gadian D. G. 
Published: March 2007

AbstractDiffusion‐weighted images of the rat brain were obtained using the pulsed‐gradient spinecho method. An attempt was made to extract perfusion‐related parameters from signal intensity data taken from the caudate‐putamen region of the images, by using a nonlinear least‐squares calculation to fit the Le Bihan biexponential expression (Le Bihan et al., Radiology, 168, 497 (1988)) to the data. The perfusion‐related parameters could not be obtained with sufficient accuracy to be useful, although the perfusion‐weighted images appear to contain meaningful qualitative information. An analysis of the perfusion model is presented and shows why the Le Bihan pseudo‐diffusion coefficient is particularly difficult to measure with reasonable accuracy. © 1992 Academic Press, Inc.
Two‐dimensional selective adiabatic pulses
Authors: Conolly Steven , Pauly John , Nishimura Dwight , Macovsiu Albert 
Published: March 2007

AbstractUsing the technique of separable k‐space excitation, we have designed a two‐dimensional selective adiabatic pulse that inverts magnetization from a square region in the xy plane with insensitivity to RF variations. We also have designed a two‐dimensional adiabatic pulse that inverts selectively in frequency and in one spatial dimension. The pulses should be useful for both MR imaging and spectroscopy. We present experimental results to demonstrate that the two‐dimensional adiabatic pulses are feasible on commercial MR imaging systems. © 1992 Academic Press, Inc.
The analytic theory, optimization, and performance of transparent pulses
Authors: Buonocore Michael H. 
Published: March 2007

AbstractTransparent pulses are defined by the property of having no net effect on stationary spins, while selectively nutating and dephasing flowing spins. They are derived from the inverse scattering transform, a nonlinear extension of the inverse Fourier transform. They can be used as presaturation pulses to suppress selectively the signal from flowing spins. Their intended imaging application is suppression of blood flow artifacts arising from the heart and major vessels. Diastolic images of these areas have significant artifacts that hinder accurate delineation of vessel and chamber boundaries. Transparent pulses significantly improve delineation of these boundaries and therefore could be used to improve the accuracy and precision of cross‐sectional area measurements. In this paper, the analytic theory, numerical optimization, and performance characteristics are described. Experimental evidence of flow signal suppression is provided in diastolic imaging of the heart in normal subjects. The long duration and power requirements of the present generation of transparent pulses currently limits their general Clinical use. © 1992 Academic Press, Inc.
Concurrent quantification of tissue metabolism and blood flow via 2h/31P NMR in Viva iii. alterations of muscle blood flow and metabolism during sepsis
Authors: Song Sheng‐Kwei , Hotchkiss Richard S. , Karl Irene E. , Ackerman Joseph J. H. 
Published: March 2007

AbstractIn the conclusion of this series of reports, the application of 31P/2H NMR to investigate the pathophysiology of sepsis in rat hindlimb muscle is demonstrated. Sepsis decreased muscle [PCr] by 18%, 18 ± 4 SD vs 22 ± 4 SD mmol/kg tissue wet wt (P = 0.01) in control rats but [ATP] was unchanged, 6 mmol/kg tissue wet wt (P = 0.2). The derived free cytosolic [ADP] in the two groups was similar, [ADP]septic = 0.023 ± 0.004 SD and [ADP]control, = 0.021 ± 0.003 SD mmol/kg tissue wet wt, and not statistically different (P =0.14). Likewise [Pi] in the septic and control groups was not statistically different, [Pi]septic = 1.1 ± 0.5 SD and [Pi]control, = 1.2 ± 0.4 SD mmol/kg tissue wet wt (P = 0.2). Septic rats presented the symptom of respiratory alkalosis evidenced by elevated blood pH. Sepsis decreased muscle blood flow by 33%, P = 0.003, but examination of individual subjects did not demonstrate a correlation with the reduction in [PCr]. Thus, a metabolic energy deficit caused by cellular ischemia/hypoxia is not a likely cause of cellular abnormality in rat hindlimb muscle during sepsis.
Twisting radial lines with application to robust magnetic resonance imaging of irregular flow
Authors: Jackson John I. , Nishimura Dwight G. , Macovski Albert 
Published: March 2007

AbstractA problem with magnetic resonance angiograms of vessels containing irregular flow isthat flow‐induced dephasing can result in voids in the image. These void regionsare susceptible to misinterpretation as regions of stenosis or other vessel pathology. Flow‐induced dephasing can be minimized by using radial lines to cover k space. However, radial lines provide a very nonuniform, and hence, inefficient, coverage of k space. By twisting the outer portions of the radial trajectories, undistorted images of very rapid and turbulent flow can be obtained with a reasonable number of RF excitations. o 1992 Academic Press, Inc.