bluuebunny/crossref_metadata_2025_split huggingface dataset:
Limits to neural stimulation in echo‐planar imaging
Authors: Mansfield P. , Harvey P. R. 
Published: March 2007

AbstractSimple electrical circuits are used to model neural stimulation. The Stimulation current is evaluated for a variety of magnetically induced waveforms and for sinusoidal stimulation as a function of frequency. Experimental results obtained using a small scale magnetic stimulator show that the transient response rather than the steady‐state behavior determines the stimulation characteristics thus suggesting a nonlinear model. We show that a nonlinear circuit changes a steady‐state current solution into a repeated transient solution and thereby better explains our experimental results. Our model also shows that for very short times and high frequencies, contrary to the current widely held view, neural stimulation is independent of the magnetic field switching rate dBldt, but depends on the final magnetic field value, Bm. At lower frequencies and rise‐times, there may be a small advantage in using square wave over sine wave gradient modulation for whole body echo‐planar imaging (EPI). It is found that the peak stimulation fields, Bm, for both sinusoidal and trapezoidal waveforms are equal for equal risetimes. This means that for a given image array size and imaging time, trapezoidal modulation EPI ultimately can be made to operate below the neural triggering threshold while the equivalent sine modulation EPI version transcends the threshold.
Automatic, localized in Vivo adjustment of all first‐and second‐order shim coils
Authors: Gruetter Rolf 
Published: March 2007

AbstractThe implementation of a “fast, automatic shimming technique by mapping along projections” (FASTMAP) on a 2.1‐Tesla whole‐body system is described. The method provides a localized adjustment of all first‐ (X, Y, Z) and second‐order (Z2, ZX, ZY, X2‐Y2, 2XY) coils in 2 min. The time savings are achieved by mapping the magnetic field along six projections rather than from whole‐imaging data sets. An analysis of noise error propagation suggests that in 64‐ml volumes the residual linewidths obtained with the method are negligible when the signal‐to‐rms‐noise ratio is above 30. The initial application of the method to localized 13C, 31P, and 1H spectroscopy of the human brain resulted in linewidths of ∼2 Hz for 13C (144‐ml volume), ∼2.5 Hz for 31P (36‐ml volume) and ∼4 Hz for 1H (36‐ml volume) with symmetric and reproducible lineshapes.
Shift reagent enhanced concurrent 23Na and 1H magnetic resonance spectroscopic studies of transcellular sodium distribution in the dog brain in vivo
Authors: Eleff Scott M. , McLennan Ian J. , Hart Graeme K. , Maruki Yuichi , Traystman Richard J”︁ , Koehler Raymond C”︁ 
Published: March 2007

AbstractThe intracellular to extracellular sodium distribution is one of the primary determinants of action potentials necessary for the electrical function of organs such as brain, heart and skeletal muscle. The ability of shift reagent enhanced 23Na MRS to directly measure the intracellular and extracellular sodium distribution in brain is controversial and centers on the relative contributions of bulk magnetic susceptibility and hyper‐fine interactions to the observed chemical shifts. In this study, infusion of dysprosium (111) triethylenetetraminehexacetate (Dy(TTHA)−3), resulted in a 23Na MRS spectrum of dog brain with two well resolved peaks at 9 and 0.4 ppm. The 9 ppm peak corresponded to the resonance seen in aspirated blood. After disruption of the blood brain barrier, the single peak at 0.4 ppm split into two peaks at 3 and 0 ppm. The ability of Dy‐(lTHIA)−3 enhanced 23Na MRS to follow global changes in brain sodium distribution was tested during cardiac arrest. The expected rapid Na influx into the intracellular space produced a marked decrease in the 3 ppm signal and a parallel increase in the 0 ppm peak. This is consistent with the assignment of the 3 ppm peak as interstitial sodium and the 0 ppm peak as intracellular sodium.
Magnetic resonance‐guided thermal surgery
Authors: Cline H. E. , Schenck J. F. , Watkins R. D. , Hynynen K. , Jolesz F. A. 
Published: March 2007

AbstractA demonstration of MR guided thermal surgery involved experiments with imaging of focused ultrasound in an MRI system, measurements of the thermal transients and a thermal analysis of the resulting images. Both the heat distribution and the creation of focused ultrasound lesions in gel phantoms, in vitro bovine muscle and in vivo rabbit muscle were monitored with magnetic resonance imaging. Thermal surgical procedures were modeled by an elongated gaussian heat source where heat flow is controlled by tissue thermal properties and tissue perfusion. Temperature profiles were measured with thermocouples or calculated from magnetic resonance imaging in agreement with the model. A 2‐s T1‐weighted gradient‐refocused acquisition provided thermal profiles needed to localize the heat distribution produced by a 4‐s focused ultrasound pulse. Thermal analysis of the images give an effective thermal diffusion coefficient of 0.0015 cm2/s in (gel and 0.0033 cm2/s in muscle. The lesions were detected using a T2‐weighted spin‐echo or fast spin‐echo pulse sequence in agreement with muscle tissue sections. Potential thermal surgery applications are in the prostate, liver, kidney, bladder, breast, eye and brain.
Study of ischemic myocardial buffering capacity in perfused rat heart through rapidly interleaved 1H and 31P MRS measurements
Authors: Schussheim Adam E. , Radda George K. 
Published: March 2007

AbstractWe present a method for the direct correlation of quantitative changes in lactate concentration and intracellular pH during global ischemia in the perfused rat heart using rapidly interleaved 31P MRS and water‐suppressed 1H MRS acquisitions. The ischemic myocardial buffering capacity was determined and found to be consistent with previous experimentation.
In Vivo31P NMR measurement of glucose‐6‐phosphate in the rat muscle after exercise
Authors: Bloch G. , Chase J. R. , Avison M. J. , Shulman R. G. 
Published: March 2007

AbstractComparison of 31P NMR spectra of the rat gastrocnemius, obtained in vivo and from PCA extracts, after electrically induced contractions, demonstrates that glucose‐6‐phosphate (G6P) is the major metabolite in the low‐field part of the PME spectral region. In vivo 31P NMR can thus be used to measure the muscle G6P concentration after exercise.
Magnetic resonance imaging of perfusion in the isolated rat heart using spin inversion of arterial water
Authors: Williams Donald S. , Grandis Donald J. , Zhang Weiguo , Koretsky Alan P. 
Published: March 2007

AbstractMeasurement of regional myocardial perfusion is important for the diagnosis and treatment of coronary artery disease. Currently used methods for the measurement of myocardial tissue perfusion are either invasive or not quantitative. Here, we demonstrate a technique for the measurement of myocardial perfusion using magnetic resonance imaging (MRI) with spin tagging of arterial water. In addition, it is shown that changes in perfusion can be quantitated by measuring changes in tissue T1. Perfusion images are obtained in Lan‐gondorff‐perfused, isolated rat hearts for perfusion rates ranging from 5 to 22 ml/g/min. The MRI‐determined perfusion rates are in excellent agreement with perfusion rates determined from measurement of bulk perfusate flow (r = 0.98). The predicted linear dependence of the measured T1 (T1app) on per‐fusion is also demonstrated. The ability of perfusion imaging to measure regional variations in flow is demonstrated with hearts in which perfusion defects were created by ligation of a (coronary artery. These results indicate that MRI of perfusion using spin inversion of arterial water gives quantitative maps of cardiac perfusion.
Quantitative proton spectroscopy and histology of a canine brain tumor model
Authors: Barker P. B. , Blackband S. J. , Chatham J. C. , Soher B. J. , Samphilipo M. A. , Magee C. A. , Hilton J. D. , Strandberg J. D. , Anderson J. H. 
Published: March 2007

AbstractQuaintitative, single voxel proton nuclear magnetic resonance (NMR) spectroscopy and histological analysis was performed in eight dogs implanted with the transplantable canine glioma model of Wodinsky (Proc. Am. Assoc. Cancer Res. 10, 99 (1969). Signals from choline, creatine, N‐Acetyl Aspartate (NAA) and lactate were converted to molar concentration units and correlated with the quantitative analysis of histologically determined tissue types within the localized volume selected for NMR spectroscopy. In general, compared with normal brain, the lesions were associated with reductions in all metabolite concentrations, with the exception of lactate, which was increased. NAA and creatine decreases were most significantly correlated with the total lesion volume (P < 0.01), suggesting that these compounds are present in normal brain only. Changes in choline levels did not correlate strongly with any particular tissue type. Lactate was found to increase with increasing total lesion volume (P < 0.01), but not with increasing percent tumor, suggesting that it accumulates in abnormal tissue other than the tumor. The spectra reported were similar to those observed in human glioblastomas, with the exception that elevations of choline were not observed. The transplant‐able canine gliosarcoma system appears to be a suitable tumor model for evaluation by clinical radiological techniques such as magnetic resonance imaging (MRI) and proton NMR spectroscopy.
23Na‐NMR detects hypoxic injury in intact kidney: Increases in sodium inhibited by DMSO and DMTU
Authors: Cross M. , Endre Z. H. , Stewart‐Richardson P. , Cowin G. J. , Westhuyzen J. , Duggleby R. G. , Fleming S. J. 
Published: March 2007

AbstractHypoxic injury in the isolated perfused rat kidney (IPRK) was monitored using 23Na‐NMR in the presence or absence of 1.5 and 15 mM dimethylthiourea (DMTU) or 15 mM dimethylsulphoxide (DMSO) before and after inducing hypoxia. Hypoxia induced a prompt exponential increase in total renal 23Na+, renal vascular resistance, and sodium excretion and decreased inulin clearance and adenine nucleotides and reduced glutathione concentrations. Lipid peroxide metabolites were unaltered. The increase in 23Na+ was significantly reduced (P < 0.001) by both DMTU and DMSO although hypoxic perturbations of function and biochemical parameters were not. Posthypoxic increases in renal 23Na+ include approximately 10% from the intratubular compartment, but principally reflect the intracellular and interstitial compartments. The results demonstrate that 23Na‐NMR is a sensitive indicator of hypoxic renal injury in intact kidney and suggest that DMTU and1 DMSO protect against hypoxic injury by a mechanism independent of free radical‐binding.
Complete elimination of the extracellular 23Na NMR signal in triple quantum filtered spectra of rat hearts in the presence of shift reagents
Authors: Navon Gil 
Published: March 2007

AbstractA method is suggested whereby the shifted extracellular triple quantum filtered 23Na signal of an isolated organ is completely eliminated. The method is based on the long relaxation time of the triple quantum coherence and on its fast evolution rate. When the carrier frequency is set on top of the intracellular sodium signal and the time interval between the last two pullses to (12 Δv)‐l (Δv is the frequency difference between the intiracellular and the extracellular signals), a complete elimination of the extracellular 23Na signal is achieved. The method is demonstrated for isolated rat hearts and the quantification of intracellular sodium using triple quantum filtered spectroscopy is discussed.
MR angiography using velocity‐selective preparation pulses and segmented gradient‐echo acquisition
Authors: Korosec F. R. , Grist T. M. , Polzin J. A. , Weber D. M. , Mistretta C. A. 
Published: March 2007

AbstractWe describe a cardiac‐gated MR angiographic imaging method that employs velocity‐selective preparation (VSP) pulses in conjunction with segmented gradient‐echo acquisitioin and subtraction to produce images that, ideally, contain no signal from stationary tissues and display vessels with a signal intensity that is dependent on the velocity of the blood in the vessels. The novel features of this method are a) it acquires several phase‐encoding valueslapplication of a single VSP pulse, b) it uses subtraction to eliminate signal that is not sufficiently suppressed by the VSP pulses, and c) it uses VSP pulses that are synchronized with the cardiac cycle so it can be used to produce ghost‐free images of pulsatile blood. An advantage of this sequence is that it detects a signal that, after preparation, is relatively unaffected by changes in blood velocity. This leads to a large signal‐to‐noise ratio for all the phase‐encoding values, a reduction of ghosting artifacts, and the ability to visualize blood that is in motion for only a short time during the cardiac cycle. Because the signal is prepared during peak flow, venous signal can be suppressed by making the sequence sensitive to high velocities. An additional advantage of this sequence is that it permits sampling with a short TE because the velocity‐encoding gradient can be applied in a preparatory interval. Signal loss that results from dephasing during the longer TE preparation interval can be reduced or eliminated by allowing the dephased spins to flow out of the region of complex flow, and perhaps out of the field‐of‐view, by introducing a delay between the finish of the VSP pulse and the beginning of data acquisition.
Evaluation of radiation injury by 1H and 31P NMR of human urine
Authors: Yushmanov Victor E. 
Published: March 2007

Abstract1H and 31P NMR techniques were applied to study the changes in metabolite profiles in human urine resulting from radiation exposure following the Chemobyl reactor accident. In cases of acute leukemia and different accumulated doses of external radiation (from 0.20 to 4.00 Sv), the proton spectra were classified on the basis of the peaks due to N‐trimethyl groups, creatinine, citrate, glycine, and hippurate. Unidentified resonances were observed between 15.9 and 21.4 ppm in six phosphorus spectra of patients with preleukemia and acute leukemia. Characteristic spectral changes were similar for external radiation and Incorporation‐induced internal irradiation. The spectral patterns described may serve as a criterion of radiation injury.
pH control in rat skeletal muscle during exercise, recovery from exercise, and acute respiratory acidosis
Authors: Kemp G. J. , Thompson C. H. , Sanderson A. L. , Radda G. K. 
Published: March 2007

AbstractWe used 31P magnetic resonance spectroscopy to compare the response of rat skeletal muscle to three kinds of proton load. During exercise (tetanic sciatic nerve stimulation), protons from lactic acid were buffered passively and consumed by net hydrolysis of phosphocreatine (PCr). During recovery from exercise, the pH‐dependent efflux of protons produced by PCr resynthesis could be partially inhibited by amiloride or 4,4′‐diisothiocyanostilbene‐2,2′‐disulphonate (DIDS), implicating both sodiudproton and bicarbonatelchloride exchange, but was not inhibited by simultaneous respiratory acidosis. In early recovery, up to 30% of proton efflux was mediated by lactatelproton cotransport. During acute respiratory acidosis at rest, the eventual change in muscle pH was consistent with passive buffering and was unaffected by amiloride or DIDS, implying no significant contribution of proton fluxes.
Magnetic characterization of iron oxides for magnetic resonance imaging
Authors: Sjøgren Carl E. , Briley‐Sæbø Karen , Hanson Maj , Johansson Christer 
Published: March 2007

AbstractThe OMP particle and four additional iron oxide samples were investigated in terms of zero field cooled magnetization (ZFCM) curves and hysteresis loops. The observation of superparamagnetic blocking and the absence of magnetic remanence demonstrated that the samples are superparamagnetic at room temperature. The magnitude of the ZFCM peak temperatures are in qualitative agreement with the iron oxide crystal size. One sample deviated from the remaining four in that it had a significantly lower magnetic moment and an irregular ZFCM curve showing the presence of various less magnetic phases. The similar results obtained with the OMP particles and the individual OMP crystallites without the polymer support show that the superparamagnetic properties of the individual OMP crystallites are retained on the OMP particle. Depending on the application, the ZFCM experiment may be viewed as an alternative or a supplement to Mössbauer spectroscopy in studies of the superaparamagnetic blocking of iron oxides.
Diffusion and relaxation mapping of cartilage‐bone plugs and excised disks using microscopic magnetic resonance imaging
Authors: Xia Yang , Farquhar Tony , Burton‐Wurster Nancy , Ray Edward , Jelinski Lynn W. 
Published: March 2007

AbstractSpatially resolved maps of proton self‐diffusion coefficients (D) and relaxation times (T1andT2) were obtained on cartilage‐bone plug samples and on excised disks of canine cartilage at a transverse resolution of 30 μm, using microscopic magnetic resonance imaging (micro‐MRI). Results are compared for excised disks of cartilage and intact cartilage‐bone plugs. Correlations between the absolute water concentration, the self‐diffusion coefficient and theT1relaxation are reported. The diffusion coefficient is not a linear function of water concentration. The thickness of the disks is 600 μm, compared with the ca. 900 μm observed for the cartilage‐bone plugs, presumably due to the absence of the interfacial or tidemark layer of interdigitated cartilage and bone in the former samples. Our results suggest that excised disks of cartilage are excellent models for the articular surface and the first 500 or so microns of tissue. The molecular parameters of spin‐spin and spin‐lattice relaxation times, as well as the water self‐diffusion coefficient, are virtually identical in the two types of samples. However, the cartilage‐bone plugs have the additional feature of permitting the study of the tidemark region, a region that likely plays a major role in the transmission of mechanical force.
Experimental study of the effects of “Fractional” gating on flow measurements
Authors: Buonocore Michael H. , Gao Lisheng 
Published: March 2007

AbstractVelocity encoded phase imaging is subject to errors from phase and amplitude variations of the k‐space data caused by beat‐to‐beat variations of the flow. Fractional cardiac gating is defined as asynchronous gating with each phase encode step occupying a fixed fraction of the RR interval. The gating fraction is the inverse of the number of phase encode steps taken per RR interval. Studies in normal subjects show that deviations and standard errors of ascending and descending aorta flow measurements are significantly greater with decreased gating fraction. Significant errors occur when gating does not separate systolic and diastolic data. The studies establish a graded trade‐off between flow measurement accuracy and precision with imaging time, and show that standard nongated phase contrast measurements of strongly pulsatile flow are unreliable.
adaptive fourier threshold filtering: A method to reduce noise and incoherent artifacts in high resolution cardiac images
Authors: Doyle M. , Chapman B. L. W. , Blackwell G. , Walsh E. G. , Pohost G. M. 
Published: March 2007

AbstractWe introduce an image processing method which reduces white noise and random artifacts in sets of high resolution, time resolved images. At each pixel, the processing consists of: (1) the isolation of a time intensity curve (TIC), (2) Fourier transformation of each TIC, (3) application of a threshold to remove low intensity coefficients, (4) inverse transformation to generate noise reduced TICS which are recombined to form images with improved signal‐to‐noise ratio (SNR). Noise filtering by Fourier thresholding is demonstrated on a set of cardiac images, resulting in a reduction of the noise energy by approximately 90%.
Cerebral blood oxygenation in rat brain during hypoxic hypoxia. Quantitative MRI of effective transverse relaxation rates
Authors: Prielmeier Franz , Nagatomo Yasushi , Frahm Jens 
Published: March 2007

AbstractOxygenation‐sensitive MRI of respiratory challenges in the brain of experimental animals will considerably benefit from a quantitative relationship between cerebral blood oxygenation and MRI parameters. Here, a multi‐echo gradient‐echo MRI technique was used to determine effective transverse relaxation rates R = 1/T of rat brain in vivo during short periods of hypoxia and interleaved normoxic phases. The differential contribution ΔR observed during hypoxia was found to increase linearly with arterial blood deoxygenation for mild to moderate conditions. Severe deoxygenation resulted in a plateau most likely due to enhanced cerebral blood flow.
Functional NMR imaging using fast spin echo at 1.5 T
Authors: Constable R. Todd , Kennan Richard P. , Puce Aina , McCarthy Gregory , Gore John C. 
Published: March 2007

AbstractFunctional NMR imaging of the brains response to a simple visual task has been performed using a fast spin echo (FSE) imaging sequence at 1.5 T. The FSE method refocuses dephasing effects induced by large‐scale susceptibility variations, and permits imaging in regions where macroscopic field gradients produce artifacts in gradient echo sequences. At 1.5 T, gradient echo (GRE) sequences are sensitive to the effects of brain activation, but relatively large effects may arise from large vessels and veins, and these may dominate the effects produced by smaller capillaries. Spin echo (SE) sequences with short echo times are relatively immune to large vessel effects and emphasize the susceptibility induced losses from small capillaries, but the imaging time for these sequences is prohibitive for most functional brain studies. We demonstrate that multislice functional brain imaging may be performed in reasonable imaging times at 1.5 T using an FSE imaging sequence. The FSE sequence with short echo spacing but long effective TE is sensitive to susceptibility induced effects at the capillary level. It is not sensitive to larger scale in homogeneities such as those found in veins and can be used in regions near tissuelair boundaries. Results are shown comparing conventional GRE and FSE images in activation of the visual cortex and these are supported by theoretical calculations and phantom experiments.
Construction of a 28‐mm 1H/13C probe and actively shielded Z‐gradient set for operation in a 9.4 T/89 mm magnet
Authors: Pruski Jan , Ahmad Asif , Sun Ling , Robitaille Pierre‐Marie L. 
Published: March 2007

AbstractIn this work, we present a new RF and gradient assembly for operation in a 9.4 Tesla 189 mm magnet. This assembly was designed in order to enable 1H‐NMR perfusion studies that are based on proton‐observed carbon‐edited approaches or gradient selected double quantum coherence. The RF portion of this probe assembly is comprised of a modified Alderman‐Grant coil and a saddle coil operating at 400 and 100 MHz, respectively. These coils are surrounded by an actively shielded Z gradient, which also allows for the use of gradient‐based water suppression without the need for carbon selection. We demonstrate that this probe can be used to implement gradient selected double quantum coherence experiments resulting in a high degree of water suppression.
MRI scanner transport system
Authors: Phillips Jason R. , Falconer James C. , Botzer Shabtai , Narayana Ponnada A. 
Published: March 2007

AbstractA computer‐controlled transport system has been designed and tested for small bore magnetic resonance scanners for animal research. The transport system allows remote animal positioning based on scout images using a graphic interface. The design utilized commercially available components suitable for use in the scanner's strong magnetic field. Tests indicate positioning accuracy of the transport system to be within 0.13 mm.