bluuebunny/crossref_metadata_2025_split huggingface dataset:
Signal‐to‐noise measurements in magnitude images from NMR phased arrays
Authors: Constantinides Chris D. , Atalar Ergin , McVeigh Elliot R. 
Published: March 2007

AbstractA method is proposed to estimate signal‐to‐noise ratio (SNR) values in phased array magnitude images, based on a region‐of‐interest (ROI) analysis. It is shown that the SNR can be found by correcting the measured signal intensity for the noise bias effects and by evaluating the noise variance as the mean square value of all the pixel intensities in a chosen background ROI, divided by twice the number of receivers used. Estimated SNR values are shown to vary spatially within a bound of 20% with respect to the true SNR values as a result of noise correlations between receivers.
Planar gradient coil design by scaling the spatial frequencies of minimum‐inductance current density
Authors: Lee Soo Y. , Park Bu S. , Yi Jeong H. , Yi Wan 
Published: March 2007

AbstractGradient coil inductance has been remarkably reduced by the minimum‐inductance design technique, which minimizes the magnetic energy stored by the gradient coil. The planar gradient coil designed by this technique, however, often has poor magnetic field linearity. Scaling the spatial frequencies of the current density function derived by this method, the magnetic field linearity of the planar gradient coil can be greatly improved with a small sacrifice of gradient coil inductance. A figure of merit of the planar gradient coil has been found to be improved by scaling the spatial frequencies.
Functional MRI bold signal coincides with electrical activity in the rat whisker barrels
Authors: Yang Xiaojin , Hyder Fahmeed , Shulman Robert G. 
Published: March 2007

AbstractFunctional MRI (fMRI) provides a noninvasive method for mapping brain functional activity based on blood oxygenation level dependent (BOLD) image contrast that is primarily due to localized increases in perfusion. Recently, Malonek and Grinvald (Science 272:551–554, 1996) suggested that during sustained functional activation, the increases in perfusion were spread over a much larger area than the localized electrical activity. In this study, it is demonstrated that the spatial distribution of the BOLD fMRI signal during sustained stimulation of rat whiskers has the same spatial pattern and dimension as that of neuronal electrical activity in the rat whisker barrels.
Energetics of 3.5 s neural activation in humans: A 31P MR spectroscopy study
Authors: Rango Mario , Castelli Alberto , Scarlato Guglielmo 
Published: March 2007

AbstractNo direct information on brain energetics and energy‐related compounds in the first seconds of physiological activation has been reported to date. In this study visual cortex high energy phosphate changes were monitored in 11 normal subjects during 3.5 s activation and the following 23.5 s by a simple 31P magnetic resonance spectroscopic method. An intraactivation decrease of phosphocreatine (PCr) was observed in all subjects, with changes in pH in three, one of them also presenting a change in adenosine triphosphate (ATP). In the subgroup of eight subjects without changes in pH, the mean rate of mean PCr decrease (DPCr) was 7.24 ± 0.78 %/s, and the postactivation mean rate of mean PCr recovery was <1/2 DPCr. Short phasic neural activity requires a large amount of energy, i.e., at least three times basal consumption, in agreement with theoretical calculations. Additional energy demands in the visual cortex are several times those measured by positron emission tomography during prolonged stimulation studies, implying that mean energy requirements decrease with increases in duration of stimulation. During short activation, the vascular responses as detected by brainmapping techniques (BMT) are preceded by an important reduction of the intracellular high‐energy phosphate content, which returns to resting values during an interval that corresponds to the poststimulation return of BMT signals to baseline.
Two‐point Dixon technique for water‐fat signal decomposition with B0 inhomogeneity correction
Authors: Coombs Bernard D. , Szumowski Jerzy , Coshow William 
Published: March 2007

AbstractTo separate water and lipid resonance signals by phase‐sensitive MRI, a two‐point Dixon (2PD) reconstruction is presented in which phase‐unwrapping is used to obtain an inhomogeneity map based on only in‐phase and out‐of‐phase image data. Two relaxation‐weighted images, a “water image” and a “fat image,” representing a two‐resonance peak model of proton density, are output. The method is designed for T1‐ or density‐weighted spin‐echo imaging; a double‐echo scheme is more appropriate for T2‐weighted spin‐echo imaging. The technique is more time‐efficient for clinical fat‐water imaging than 3PD schemes, while still correcting for field inhomogeneity.
Dynamic 13C NMR analysis of pyruvate and lactate oxidation in the in vivo canine myocardium: Evidence of reduced utilization with increased work
Authors: Rath Dipti P. , Zhu Hui , Tong Xiaoxin , Jiang Zongcheng , Hamlin Robert L. , Robitaille Pierre‐Marie L. 
Published: March 2007

AbstractIn this work, substrate selection was monitored in the left ventricle of the canine myocardium by following pyruvate and lactate oxidation under in vivo conditions at basal and elevated workloads. These studies were conducted in the open chest model using dynamic 13C NMR techniques in the presence and absence of dichloroacetic acid (DCA), a well‐known activator of pyruvate dehydrogenase (PDH). Following the infusion of (3‐13C) pyruvate or (3‐13C) lactate into the left anterior descending artery, highly variable 13C enrichments of glutamate, alanine, aspartate, and citrate were noted under low (RPP <14,500 mmHg/min), intermediate (RPP = 15,000–25,000 mmHg/min), and high (RPP > 25,500 mmHg/min) rate pressure products (RPP). At low workloads, the myocardium typically oxidized the infused (3‐13C) pyruvate or (3‐13C) lactate and incorporated the labeled carbon into the glutamate pool as expected. However, in a few notable instances (n = 3), 13C‐enriched pyruvate and lactate were unable to label the glutamate pool under in vivo conditions even at the lowest RPPs, indicating a lack of selection for these substrates by the tricarboxylic acid (TCA) cycle. Nonetheless, the levels of glutamate C4 enrichment observed at low workloads could usually be enhanced by infusion of DCA. Importantly, 13C NMR extract analysis revealed that (3‐13C) pyruvate or (3‐13C) lactate labeling of the glutamate pool was reduced (<20%) at high workloads in spite of increased DCA concentrations.
19F NMR monitoring of in vivo tumor metabolism after biochemical modulation of 5‐fluorouracil by the uridine phosphorylase inhibitor 5‐benzylacyclouridine
Authors: Holland Scott K. , Bergman Andries M. , Zhao Yiming , Adams Earle R. , Pizzorno Giuseppe 
Published: March 2007

AbstractA uridine phosphorylase inhibitor, 5‐benzylacyclouridine (BAU), has been utilized as biochemical modulator of 5‐fluorouracil (5‐FU) anti‐tumor activity in a murine tumor model. The effect of BAU on 5‐FU metabolism has been evaluated using in vitro and in vivo 19F NMR spectroscopy. The analysis of the NMR data revealed an increased formation and retention of fluorouracil nucleotides and fluorouridine in colon 38 tumors treated with the regimen containing BAU and a reduction in 5‐FU catabolites (α‐fluoro‐β‐ureidopropionic acid and α‐fluoro‐β‐alanine). In the normal tissues evaluated, the presence of BAU did not significantly alter the metabolism and presence of fluoropyrimidine species, indicating a more selective effect on tumor tissues. Therapy experiments on C57/BL6 mice bearing colon 38 tumor showed that the administration of 120 mg/kg BAU 30 min before 5‐FU at 85 mg/kg, on a weekly basis, resulted in an increased antineoplastic effect compared to the same dose of 5‐FU alone. A smaller dose of 5‐FU (60 mg/kg) also administered 30 min after 120 mg/kg BAU caused a reduction in tumor growth similar to 5‐FU alone. The addition of BAU to 5‐FU (85 mg/kg) resulted in a slight increase, although statistically nonsignificant, in host toxicity without causing any toxic death during the chemotherapeutic treatment. 19F NMR spectroscopy is here shown to be a powerful technique to evaluate changes in the metabolism of fluoropyrimidines after the use of biochemical modulator and to allow a correlation between improved therapeutic response with the biochemical effects generated in tissues.
The effect of perfusion on T1 after slice‐selective spin inversion in the isolated cardioplegic rat heart: Measurement of a lower bound of intracapillary‐extravascular water proton exchange rate
Authors: Bauer Wolfgang R. , Roder Fridtjof , Hiller Karl‐Heinz , Han Hong , Fröhlich Susanne , Rommel Eberhard , Haase Axel , Ertl Georg 
Published: March 2007

AbstractMany NMR measurements of cardiac microcirculation (perfusion, intramyocardial blood volume) depend on some kind of assumption of intracapillary‐extravascular water exchange rate, e.g., fast exchange. The magnitude of this water exchange rate, however, is still unknown. The intention of this study was to determine a lower limit for this exchange rate by investigating the effect of perfusion on relaxation time. Studies were performed in the isolated perfused cardioplegic rat heart. After slice‐selective inversion, the spin lattice relaxation rate of myocardium within the slice was studied as a function of perfusion and compared with a mathematical model which predicts relaxation rate as a function of perfusion and intracapillary‐extravascular exchange rate. A linear relationship was found between relaxation rate T−1 and perfusion P normalized by perfusate/tissue partition coefficient of water, λ: ΔT−1 = m · ΔP/λ with 0.82 ≤ m ≤ 1.06. Insertion of experimental data in the model revealed that a lower bound of the exchange rate from intra‐to extravascular space is 6.6 s−1 (4.5 s−1, P < 0.05), i.e., the intracapillary lifetime of a water molecule is less than 150 ms (222 ms, P < 0.05). Based on this finding, the T1 mapping after slice‐selective inversion could become a valuable noncontrast NMR method to measure variations of perfusion.
Homocarnosine and the measurement of neuronal pH in patients with epilepsy
Authors: Rothman Douglas L. , Behar Kevin L. , Prichard James W. , Petroff Ognen A. C. 
Published: March 2007

AbstractHomocarnosine is a dipeptide of gamma‐aminobutyric acid (GABA) and histidine found uniquely in the brain, most likely in a subclass of GABAergic neurons. By comparison of spectra from the occipital lobe of patients receiving a homocarnosine elevation drug to normal subjects we have assigned two elevated resonances in the short TE 1H MRS spectrum to homocarnosine. These resonances are partially resolved at 7.05 and 8.02 ppm in a short TE spectrum at 2.1 T when macromolecule resonances are removed by subtraction of a spectrum in which the metabolite resonances are nulled by inversion recovery. The chemical shift of both of these resonances is sensitive to pHi. By comparison with a titration curve the pHi was calculated from the downfield resonance to be 7.06 in the patient group which is similar to values reported using the Pi resonance. Based on the in vivo results and theoretical considerations the potential sensitivity for using nonelevated homocarnosine to measure pH is similar to that of Pi under physiological conditions.
Localizedq‐space imaging of the mouse brain
Authors: King Martin D. , Houseman J. , Gadian D. G. , Connelly A. 
Published: March 2007

AbstractLocalizedq‐space imaging was used to obtain water displacement profiles from mouse brain. These profiles take the form of unidirectional diffusive displacement probability distributions. Two groups of mice were studied, a normal group and a group in which surgery had been performed to produce a unilateral reduction in the supply of blood to the forebrain.q‐Space measurements were made bothin vivoand postmortem. The displacement profiles were characterized using the summary parameter prob[d< 10], which is the proportion of water molecules that undergo a net diffusive displacement that is less than ±10 μm, during the diffusion period (50 ms). The range of prob[d< 10] values in the normal group was 0.71 to 0.77in vivocompared with 0.78 to 0.87 in the impaired hemisphere of the surgically treated group. An increase in prob[d< 10] occurred postmortem to yield values in the range 0.79 to 0.81 and 0.80 to 0.89 in the normal and surgically treated group, respectively. These observations are consistent with the diffusion‐weighted image intensity changes that occur after a period of ischemia.
On the SAR and field inhomogeneity of birdcage coils loaded with the human head
Authors: Jin Jianming , Chen Ji 
Published: March 2007

AbstractBirdcage coils are widely used as a radiofrequency (RF) resonator in magnetic resonance imaging (MRI) because of their capability to Produce a highly homogeneous B1 field Over a large volume within the coil. When they are employed for high‐frequency MRI, the interaction between the electromagnetic field and the object to be imaged deteriorates the B1‐field homogeneity and increases the specific absorption rate (SAR) in the object. To investigate this problem, a finite‐element method (FEM) is developed to analyze the SAR and the B1 field in a two‐dimensional (2D) model of a birdcage coil loaded with a 2D model of a human head. The electric field, magnetic field, and SAR distributions are shown, and a comprehensive study is carried out for both linear and quadrature birdcage coils at 64, 128, 171, and 256 MHz. It is that to generate the same value of the B1 field, the SAR is increased significantly with the frequency, and for the same imaging method the SAR produced by a quadrature coil is significantly lower than that of a linear coil. It is also shown that the B1‐field inhomogeneity is increased significantly with the frequency.
Non‐fourier encoding with multiple spin echoes
Authors: Panych Lawrence P. , Mulkern Robert V. , Saiviroonporn Pairash , Zientara Gary P. , Jolesz Ferenc A. 
Published: March 2007

AbstractThe advantages and limitations of multiple spin‐echo sequences for non‐Fourier encoding are investigated. Complications caused by improper encoding of alternate magnetization pathways due to imperfect refocusing pulses are analyzed. It is shown that mirror image ghosts result if the encoding RF pulse matrix is real‐valued. These ghosts can be avoided as long as the rows of the RF pulse matrix are conjugate symmetric, which implies that spatial profiles are real valued. Non‐Fourier encoding using bases derived from wavelet, Hadamard, and other real‐valued orthogonal functions does not result in a mirror ghost artifact. A RARE sequence for non‐Fourier encoding has been implemented on a clinical imaging system and successfully applied for brain imaging.
Magnetization transfer imaging in vivo of the rat brain at 4.7 T: Interpretation using a binary spin‐bath model with a superlorentzian lineshape
Authors: Quesson Bruno , Thiaudière Eric , Delalande Christophe , Dousset Vincent , Chateil Jean‐François , Canioni Paul 
Published: March 2007

AbstractProton magnetization transfer contrast (MTC) imaging, using continuous wave off‐resonance irradiation, was performed on the rat brain in vivo at 4.7 Tesla. The observed MTC was studied in three different brain regions: the corpus callosum, the basal ganglia, and the temporal lobe. By systematically varying the offset frequency and the amplitude of the RF irradiation, the observed signal intensities for each region of interest were modeled using a system including free water and a pool of protons with restricted motions (R. M. Henkelman, X. Huang, Q. Xiang, G. J. Stanisz, SD Swanson, M. J. Bronskill, Magn. Res. Med. 29, 759 (1993)). Most of the relaxation parameters of both proton pools remained fairly constant for the three regions of interest, with a T2 value of about 9 μs for the immobilized protons, whereas the rate of exchange increased significantly from the temporal lobe to the corpus callosum. The optimal acquisition parameters for the improved MTC under steady‐state saturation were found to be 2–10 kHz offset frequency and 500–800 Hz RF irradiation amplitude. Conversely, an irradiation amplitude of 3 kHz at an offset frequency of 12 kHz is required to minimize the direct effect of off‐resonance irradiation. Such an approach could be extended to human brain imaging with the aim of characterizing tissue‐specific disease.
MRI quantitative myocardial perfusion with compartmental analysis: A rest and stress study
Authors: Vallée Jean‐Paul , Sostman H. Dirk , Macfall James R. , Wheeler Ted , Hedlund Larry W. , Spritzer Charles E. , Coleman R. Edward 
Published: March 2007

AbstractK1 (first‐order transfer constant from arterial plasma to myocardium for Gd‐DTPA) and Vd (distribution volume of Gd‐DTPA in myocardium) were measured in vivo in a canine model (n = 5) using MRI‐derived myocardial perfusion curves and a compartmental model. Perfusion curves were obtained after a bolus injection of Gd‐DTPA (0.04 mM/kg) with an inversion‐prepared fast gradient echo sequence. Myocardium and blood signal intensity were converted to a concentration of Gd‐DTPA, according to a model appropriate for short (<1 s) interimage intervals characteristic of cardiac‐triggered acquisitions. Before dipyridamole‐induced stress, K1 and Vd, obtained from the fit of the MRI‐derived perfusion curves, were 6.2 ± 1.4 (mHz) and 17.5 ± 4.2%, respectively. After dipyridamole infusion, a K1 increase of a factor of 2.82 ± 0.72 was measured (P = 0.003). No change was observed in Vd (P = 0.98). These results suggest that the K1 increase after dipyridamole reflects a flow‐related effect that can be useful to quantity the MRI‐derived perfusion curves.
Spectrally weighted twisted projection imaging: Reducing T2 signal attenuation effects in fast three‐dimensional sodium imaging
Authors: Boada Fernando E. , Shen Gary X. , Chang Sam Y. , Thulborn Keith R. 
Published: March 2007

AbstractA scheme for the reduction of T2 signal attenuation effects in three‐dimensional twisted projection imaging is presented. By purposely reducing the sample density at the high spatial frequencies, a considerable reduction in readout time is achieved. The reduction in readout time leads to decreased T2 signal attenuation which translates into improved signal‐to‐noise ratio (SNR). The SNR improvement is achieved without decreasing the image's resolution since the point spread function depends on the sample weighting as well as the T2 attenuation. The results indicate that SNR improvements of up to 40% can be achieved using the proposed scheme.
k‐space detection and correction of physiological artifacts in fMRI
Authors: Wowk Brian , McIntyre Michael C. , Saunders John K. 
Published: March 2007

AbstractSignal phase variations caused by physiology are a major source of instability in fMRI images produced by multiple RF pulses. k‐Space phase variation maps show cyclic phase variations at the frequency of respiration combined with a cardiac variation of lower amplitude. The amplitude of the variation increases with gradient echo time and proximity to the chest, suggesting that the dominant cause of the phase variation is a B0 shift (∼0.01 ppm) produced by movement of organs in the chest. A simple k‐space phase correction method is proposed and demonstrated for FLASH fMRI. The retrospective method requires no pulse sequence modification, and is more effective than navigator echo correction. Physiological noise is dramatically reduced, especially at inferior slice locations.
Navigated single‐voxel proton spectroscopy of the human liver
Authors: Tyszka J. Michael , Silverman Jeffrey M. 
Published: March 2007

AbstractThe addition of a navigator echo acquisition to a 1H spectroscopy sequence allows retrospective motion correction using acceptance criteria based on measured displacement of the diaphragm. This technique has been applied to the acquisition of short echo‐time 1H spectroscopy of the liver in seven normal subjects. Navigation correction significantly improved overall spectral quality as measured by the phase variance, linewidth and suppression ratio of the water resonance. The hepatic choline resonance was consistently observed in all navigated spectra. Retrospective navigation makes close to optimal use of data acquisition time and, unlike external respiratory transducers, allows direct measurement of diaphragm displacement. The reconstruction of multiple voxel spectra from a single data set was also demonstrated.
A fast spin echo technique with circular sampling
Authors: Zhou Xiaohong , Liang Zhi‐Pei , Gewalt Sally L. , Gofer Gary P. , Lauterbur Paul C. , Johnson G. Allan 
Published: March 2007

AbstractThis paper presents a fast spin echo (FSE) imaging method that employs circular sampling of Jr‐space. The technique has been implemented on a 2 Tesla imaging system and validated on both phantoms and living animals. Experimental studies have shown that circular sampling can produce artifact‐free FSE images without the need of phase correction. Although not fully explored, preliminary results also show that circular sampling may have advantages over the conventional rectilinear FSE in signal‐to‐noise ratio and imaging efficiency. A major disadvantage is the increased sensitivity to off‐resonance effects. The authors expect that the FSE technique with circular sampling will find its applications in magnetic resonance microscopy, neuro‐functional imaging, and real‐time dynamic studies.
Regional metabolite concentrations in human brain as determined by quantitative localized proton MRS
Authors: Pouwels Petra J. W. , Frahm Jens 
Published: March 2007

AbstractThe regional distribution of brain metabolites was studied in several cortical white and gray matter areas, cerebellum, and thalamus of young adults with use of quantitative single‐voxel proton MRS at 2.0 T. Whereas the neuronal compound N‐acetylaspartate is distributed homogeneously throughout the brain, N‐acetylaspartylglutamate increases caudally and exhibits higher concentrations in white matter than in gray matter. Creatine, myo‐inositol, glutamate, and glutamine are less concentrated in cortical white matter than in gray matter. The highest creatine levels are found in cerebellum, parallel to the distribution of creatine kinase and energy‐requiring processes in the brain. Also myo‐inositol has highest concentrations in the cerebellum. Choline‐containing compounds exhibit a marked regional variability with again highest concentrations in cerebellum and lowest levels and a strong caudally decreasing gradient in gray matter. The present findings neither support a metabolic gender difference (except for a 1.3‐fold higher myo‐inositol level in parietal white matter of female subjects) nor a metabolic hemispheric asymmetry.