bluuebunny/crossref_metadata_2025_split huggingface dataset:
T2 Accuracy on a whole‐body imager
Authors: Foltz Warren D. , Stainsby Jeffery A. , Wright Graham A. 
Published: March 2007

AbstractMR oximetry requires a T2 measurement that is accurate within 5% in vivo. Simple methods are susceptible to signal loss and tend to underestimate T2. Current methods utilize RF pulses or RF cycling patterns that prevent signal loss at each data acquisition. However, using these methods with imperfect pulses, T2 tends to be overestimated due to temporary storage of the magnetization along the longitudinal axis where it decays more slowly with a time constant T1T2. To reduce the T1 dependence while preventing signal loss, we utilize simple 90x180y90x composite pulses and good RF cycling patterns. These trains are critical for T2 accuracy over typical ranges of RF and static field inhomogeneities and refocusing intervals. T1 signal decay during each 90x180y90x pulse must be accounted for to yield accuracy within 5% when the pulse‐width is 10% or more of the refocusing interval. A simple correction scheme compensates for this T1‐related error effectively.
Study of the metabolism of choline and phosphatidylcholine in tumors in vivo using phosphonium‐choline
Authors: Street James C. , Szwergold Benjamin S. , Matei Cornelia , Kappler Francis , Mahmood Umar , Brown Truman R. , Koutcher Jason A. 
Published: March 2007

AbstractThe results of an initial study on the feasibility of using the phosphonium analog of choline to follow the metabolism of phosphatidylcholine in tumors in vivo using 31P NMR are reported. C3H/He mice bearing a mammary carcinoma tumor on the foot pad were fed a choline‐free diet supplemented with the phosphonium analog of choline. Metabolites of this compound, including the phosphonium analogs of phosphatidylcholine, phosphocholine, glycerophosphocholine, and betaine were observed noninvasively in vivo in tumors by 31P NMR after 2–3 weeks of feeding. Clearance of these phosphonium‐labeled metabolites from tumors was measured after a change to a choline‐containing diet. Significant decreases were seen in the levels of the analogs of betaine (P < 0.003) and phosphatidylcholine (P < 0.004) by Day 4. A significant increase in the level of authentic phosphocholine (P < 0.003) occurred over the same time period.
Performance of an elliptical centric view order for signal enhancement and motion artifact suppression in breath‐hold three‐dimensional gradient echo imaging
Authors: Wilman Alan H. , Riederer Stephen J. 
Published: March 2007

AbstractAn elliptical centric phase‐encoding (PE) order is applied to steady‐state 3DFT imaging as performed during a single breath‐hold or following contrast agent administration. In a set of simulation, phantom, and in vivo experiments, this truly centric PE order is shown to be both more resistant to breathing artifact and more capable of suppressing undesirable venous signals that can arise following peak arterial enhancement than a number of other centric PE techniques that are presently in use. Unlike other PE orders, the elliptical centric ordering changes with the relative dimensions of the two PE fields of view and is optimal based on increasing k‐space radius. It thus creates a temporally diminishing importance to the PE order. The specific advantages of such an acquisition are demonstrated.
Evaluation of triple quantum‐filtered 23Na NMR spectroscopy in the in situ rat liver
Authors: Seshan Viswanathan , Sherry A. Dean , Bansal Navin 
Published: March 2007

AbstractTriple quantum (TQ)‐filtered 23Na NMR spectroscopy and the shift reagent, TmDOTP5‐, have been used to evaluate the contributions of intra‐ (Nai+) and extracellular (Nae+) sodium to the TQ‐filtered signal in the rat liver, in situ. Nae+ contributed significantly to the total TQ‐filtered signal in live animals, and the intensity of this signal did not change postmortem. The TQ‐filtered Nai+ signal increased by approximately 380% over a period of 1 h postmortem, whereas the single quantum (SQ) Nai+ increased by 90%. The constancy of the TQ‐filtered Nae+ signal indicates that changes in total TQ‐filtered 23Na signal intensity in liver (without a shift reagent) may accurately reflect changes in TQ‐filtered Nai+ signal intensity. The large percent increase in the TQ‐filtered Nai+ signal as compared to the SQ signal suggests that the fraction of Nai+ interacting with macromolecules increases after death.
Temperature Mapping using the water proton chemical shift: A chemical shift selective phase mapping method
Authors: Kuroda Kagayaki , Oshio Koichi , Chung Andrew H. , Hynynen Kullervo , Jolesz Ferenc A. 
Published: March 2007

AbstractA proton‐chemical‐shift‐based temperature imaging method, called chemical shift selective phase mapping, is proposed. The technique uses frequency‐selective suppression to provide frequency selectivity to the phase mapping method. Separate imaging of the phase distributions of the water and nonwater signals reduced the error due to the presence of a nonwater signal in measuring the water proton chemical shift change in two‐component samples. Imaging of the phase difference between water and oil yielded an internally referenced water proton chemical shift measurement to visualize the temperature change distribution, which was unaffected by motion‐induced susceptibility changes.
T1ρ‐relaxation in articular cartilage: Effects of enzymatic degradation
Authors: Duvvuri Umamaheswar , Reddy Ravinder , Patel Saurabh D. , Kaufman Jonathan H. , Kneeland J. Bruce , Leigh John S. 
Published: March 2007

AbstractSpin‐lattice relaxation in the rotating frame (T1ρ) dispersion spectroscopy and imaging were used to study normal and enzymatically degraded bovine articular cartilage. Normal specimens demonstrate significant T1ρ “dispersion” (∼60 to ∼130 ms) in the 100 Hz to 9 kHz frequency range. Proteoglycan‐degraded specimens have 33% greater T1ρ values than collagen‐degraded or normal samples. T1ρ‐weighted images reveal structure not found in conventional T1‐or T2‐weighted images. Our results suggest that T1ρ measurements are selectively sensitive to proteoglycan content. The potential of this method in distinguishing the early degenerative changes in cartilage associated with osteoarthritis is discussed.
A multiple echo pulse sequence for diffusion tensor imaging and its application in excised rat spinal cords
Authors: Gulani Vikas , Iwamoto Gary A. , Jiang Hong , Shimony Joshua S. , Webb Andrew G. , Lauterbur Paul C. 
Published: March 2007

AbstractA new imaging sequence for rapid determination of the apparent self‐diffusion tensor of water was developed and tested on fixed excised rat spinal cords. To reduce the time required to determine the tensor, the sequence utilized a new single‐shot approach with multiple spin echoes. An assumption of cylindrical symmetry in the sample was made, thus requiring the measurement of only four of the six unique elements of the tensor. This assumption was found experimentally to be valid, and the results obtained using the new sequence were found to be quantitatively the same as results obtained using a standard spin‐echo sequence.
Simultaneous T2* and diffusion measurements with 3He
Authors: Bock Michael 
Published: March 2007

AbstractIt has recently been demonstrated that magnetic resonance (MR) imaging of human lungs and airways is possible with hyperpolarized gases such as 3He. Because the influence of the apparent transversal relaxation (T2* decay) and diffusion in 3He imaging have not been quantified, an imaging pulse sequence was developed to measure these two parameters simultaneously. The imaging pulse sequence generates two series of multiply recalled gradient echo images with both different echo‐spacings and diffusion‐sensitizing gradients. From differences in exponential signal decay between the two series, T2* and diffusion coefficients, D, of both hyperpolarized and unpolarized 3He samples could be measured on a standard clinical imager using a home‐built Helmholtz coil. In a hyperpolarized sample of pure 3He values of D = (1.8 ± 0.2) ± 10−4 m2/s and T2* = 36 ± 13 ms were measured, while D = (0.3 ± 0.1) · 10−4 m2/s and T2* = 136 ± 66 ms were found in a Boltzmann‐polarized 3He/O2 mixture.
Contrast‐agent phase effects: An experimental system for analysis of susceptibility, concentration, and bolus input function kinetics
Authors: Akbudak Erbil , Norberg Richard E. , Conturo Thomas E. 
Published: March 2007

AbstractA system is presented for experimental arterial input function (AIF) simulation and for accurate measurement of the concentration, susceptibility effects, and magnetic moment of paramagnetic MR contrast agents. Signal effects of contrast agents are evaluated with a stable, well‐characterized, and precise experimental setup. A cylindrical phantom and a closed‐loop circulating flow system were designed for AIF simulation, assessment of the physical determinants of contrast‐agent phase effects, and measurement of contrast agent properties under controlled conditions. A mathematical model of the AIF dynamics is proposed. From the experimental phase shift (Δϕ), either the concentration or molar susceptibility, xM, is determined. The linear dependence of Δϕ on concentration and echo time (TE), the orientation dependence, and the lack of dependence on T1, T2, and diffusion time are proven precisely for water solutions under a wide variety of conditions. The measured effective magnetic moment of Gd+3, μeff, was 7.924 ± 0.015 Bohr magnetons in agreement with the theoretical value of 7.937.
MR microscopy of perfused brain slices
Authors: Blackband Stephen J. , Bui Jonathan D. , Buckley David L. , Zelles Tibor , Plant H. Daniel , Inglis Benjamin A. , Phillips M. Ian 
Published: March 2007

AbstractTo study the origins of signal changes in clinical MRI we have previously studied isolated single neuronal cells by MR microscopy. To account for the extracellular environment of the cells, we have developed a prototype perfusion chamber for MR microimaging of perfused rat hippocampal brain slices. To demonstrate the utility of this model, brain slices were initially perfused in isotonic solutions and then subjected to osmotic perturbations via perfusate exchange with 20% hypertonic and 20% hypotonic solutions. In diffusion weighted images, signal intensity changes of +16(σn‐1= 11)%(hypotonic) and ‐26(σn‐1= 10)% (hypertonic) were observed. No significant variation in response was observed across the slice when several subregions were examined. These observations are consistent with the view that contrast changes are driven primarily by changes in the intra‐ and extracellular compartmentation of water. This is the first report of MR microimaging of the isolated brain slice. The technique will enable the correlation of MR microimaging measurements with microscopic changes using other modalities and techniques to provide a better understanding of signals in clinical MRI.
Elimination of eddy current artifacts in diffusion‐weighted echo‐planar images: The use of bipolar gradients
Authors: Alexander Andrew L. , Tsuruda Jay S. , Parker Dennis L. 
Published: March 2007

AbstractSmall gradient fields resulting from incompletely canceled eddy currents can cause geometric distortion in echo‐planar images. Although this distortion is negligible in most echo‐planar applications, the large gradient pulses used in diffusion‐weighted echo‐planar imaging can result in significant image distortion. In this report, it is shown that this distortion can be significantly reduced by the application of bipolar gradient waveforms. Both bipolar diffusion‐sensitizing gradients and an inverted gradient preparatory pulse were examined for minimizing the eddy currents responsible for these distortions.
Observation of a 1H double quantum filtered signal of water in biological tissues
Authors: Tsoref Liat , Shinar Hadassah , Navon Gil 
Published: March 2007

AbstractThe observation of a 1H double quantum filtered (DQF) NMR signal of water in bovine sciatic nerve, bovine articular cartilage, rat tail tendon, and rat brain is reported. The origin of this signal in rat tail tendon was found to be a result of residual dipolar interaction between water protons and macromolecular protons. The dependence of the width of the 1H DQF spectra on the orientation indicated that in rat tail tendon the effective director of the residual dipolar interaction is parallel to the collagen fibers. 1H DQF NMR may be applied in imaging where the contrast obtained is related to the degree of order in the tissue.
Recording of electrical brain activity in a magnetic resonance environment: Distorting effects of the static magnetic field
Authors: Müri René M. , Felblinger Jacques , Rösier Kai M. , Jung Bruno , Hess Christian W. , Boesch Chris 
Published: March 2007

AbstractThe technical limitations of electroencephalography (EEG) and flashed visually‐evoked potentials (VEP) recordings in the static magnetic field of the MR system were systematically studied. A main artifact occurring in the magnetic field was found to be correlated with the heart cycle and had amplitudes in the range of EEG and VEP signals. For VEP recordings, a substantial reduction of this effect was achieved by subtraction of the averaged artifact from the averaged composed signal (VEP and artifact) resulting in the VEP signal alone. However, for continuous EEG recordings, there is no such solution, since the observed effect is not sufficiently constant in amplitude, and the standard deviation of the amplitude of the effect is often larger than the EEG amplitude.
3D multivoxel proton spectroscopy of human brain using a hybrid of 8th‐order hadamard encoding with 2D chemical shift imaging
Authors: Gonen Oded , Murdoch James B. , Stoyanova Radka , Goelman Gadi 
Published: March 2007

AbstractMultivoxel 3D localized proton spectroscopy using a hybrid of 1D 8th‐order transverse Hadamard spectroscopic imaging (HSI) with 2D chemical shift imaging (CSI) is demonstrated in human brain. The spatially selective HSI pulse incorporates naturally into the PRESS sequence (TE = 135 ms), which then both excites an 8 × 8 × 6 cm parallelepiped volume of interest (VOI) and subdivides it into eight slices. The planes of these slices are further partitioned into 16 × 16 voxel arrays using 2D CSI to yield 8 × 8 × 8 voxels within the VOI. Simultaneous 3D coverage yields good voxel signal‐to‐noise (8, 12, and 22 for choline, creatine, and N‐acetylaspartate, respectively) from these 0.75‐ml voxels, in ˜45 min. The high spatial isolation allows localization to within less than 1 cm from the skull without fat contamination.
Nonlinear event‐related responses in fMRI
Authors: Friston Karl J. , Josephs Oliver , Rees Geraint , Turner Robert 
Published: March 2007

AbstractThis paper presents an approach to characterizing evoked hemodynamic responses in fMRI based on nonlinear system identification, in particular the use of Volterra series. The approach employed enables one to estimate Volterra kernels that describe the relationship between stimulus presentation and the hemodynamic responses that ensue. Volterra series are essentially high‐order extensions of linear convolution or “smoothing.” These kernels, therefore, represent a nonlinear characterization of the hemodynamic response function that can model the responses to stimuli in different contexts (in this work, different rates of word presentation) and interactions among stimuli. The nonlinear components of the responses were shown to be statistically significant, and the kernel estimates were validated using an independent event‐related fMRI experiment. One important manifestation of these nonlinear effects is a modulation of stimulus‐specific responses by preceding stimuli that are proximate in time. This means that responses at high‐stimulus presentation rates saturate and, in some instances, show an inverted U behavior. This behavior appears to be specific to BOLD effects (as distinct from evoked changes in cerebral blood flow) and may represent a hemodynamic “refractoriness.” The aim of this paper is to describe the theory and techniques upon which these conclusions were based and to discuss the implications for experimental design and analysis.
In vivo measurement of changes in venous blood‐oxygenation with high resolution functional MRI at 0.95 Tesla by measuring changes in susceptibility and velocity
Authors: Hoogenraad Frank G. C. , Reichenbach Jürgen R. , Haacke E. Mark , Lai Song , Kuppusamy Karthikeyan , Sprenger Michiel 
Published: March 2007

AbstractHigh‐resolution functional imaging experiments at 0.95 Tesla have been performed to determine the changes in oxygen saturation in pial veins during motor activation by measuring both flow and susceptibility changes in the blood. Averaging across subjects, mean values for the change of the oxygenation level, δY = 0.16 ± 0.08 (n = 7) and δY = 0.13 ± 0.09 (n = 4), were obtained from the susceptibility sensitive and the flow sensitive acquisitions, respectively. The results suggest that the increase in blood flow is largely uncoupled from the oxygen consumption. The quoted errors reflect mainly the inter‐subject variability. In addition, low‐resolution echo planar imaging (EPI) measurements were performed on the same volunteers to quantify signal intensity changes. Using the measured change in oxygenation, the observed signal changes in the EPI experiments can be attributed to a 5% venous blood volume.
A novel multi‐segment surface coil for neuro‐functional magnetic resonance imaging
Authors: Lin Chin‐Shoou , Rajan Sundar S. , Gold Jonathan 
Published: March 2007

AbstractThe construction of two novel mult‐segment (MS) resonators are described. The signal‐to‐noise ratio (SNR) and B1 homogeneity of the coils are compared with that of a surface coil and a standard quadrature head coil. The images obtained with the MS designs revealed a surface coil‐like fall‐off in signal with depth. The SNR offered by MS coils was found to be better than the head coil at depths less than approximately 6 cm. Bilateral motor cortex activation on normal subjects performing finger tapping tasks is demonstrated.
Time and temperature dependence of MR parameters during thermal coagulation of ex vivo rabbit muscle
Authors: Graham Simon J. , Bronskill Michael J. , Henkelman R. Mark 
Published: March 2007

AbstractDetailed measurements of the T1‐weighted, T2‐weighted, and MT‐weighted signal were performed for ex vivo muscle samples heated to various temperatures for different times. Consistent, monotonic increases in signal intensity were observed with progressive thermal coagulation, corresponding to an increase in T2 relaxation time and an increase in MT‐weighted signal for temperatures above 60°C. The relationship for T1 relaxation was more complex, showing a decrease in T1 relaxation from 40 to 60°C and an increase above 60°C. These techniques provide a more direct measure of tissue thermal coagulation than that provided by MR thermometry and suggest MR imaging strategies for the optimization and monitoring of thermal coagulation therapy protocols that create thermal damage in target tissues.
Applicability and efficiency of near‐optimal spatial encoding for dynamically adaptive MRI
Authors: Zientara Gary P. , Panych Lawrence P. , Jolesz Ferenc A. 
Published: March 2007

AbstractAdaptive near‐optimal MRI spatial encoding entails, for the acquisition of each image update in a dynamic series, the computation of encodes in the form of a linear algebra‐derived orthogonal basis set determined from an image estimate. The origins of adaptive encoding relevant to MRI are reviewed. Sources of error of this approach are identified from the linear algebraic perspective where MRI data acquisition is viewed as the projection of information from the field‐of‐view onto the encoding basis set. The definitions of ideal and non‐ideal encoding follow, with nonideal encoding characterized by the principal angles between two vector spaces. An analysis of the distribution of principal angles is introduced and applied in several example cases to quantitatively describe the suitability of a basis set derived from a specific image estimate for the spatial encoding of a given field‐of‐view. The robustness of adaptive near‐optimal spatial encoding for dynamic MRI is favorably shown by results computed using singular value decomposition encoding that simulates specific instances of worst case data acquisition when all objects have changed or new objects have appeared in the field‐of‐view. The mathematical analysis and simulations presented clarify the applicability and efficiency of adaptively determined near‐optimal spatial encoding throughout a range of circumstances as may typically occur during use of dynamic MRI.
Water dynamics in human blood via combined measurements of T2relaxation and diffusion in the presence of gadolinium
Authors: Stanisz Greg J. , Li Jonathan G. , Wright Graham A. , Henkelman R. Mark 
Published: March 2007

AbstractAn analytical model of tissue relaxation and restricted diffusion in human blood is presented. The blood tissue model is composed of three different compartments: red blood cells, plasma, and macromolecular protons. The relaxation rate constants and free diffusion coefficients of intracellular and extracellular water may differ. Analytical formulas for signal loss due to relaxation and diffusion in the Carr‐Purcell Meiboom‐Gill and pulsed‐field‐gradient multispin echo experiments for this tissue model are derived. The model is fitted to the experimental data for human blood with various concentrations of Gadolinium contrast agent. The obtained model parameters are realistic. The validity and sensitivity of the model are also discussed.
Tumor characterization with dynamic contrast–enhanced MRI using mr contrast agents of various molecular weights
Authors: Su Min‐Ying , Mühler Andreas , Lao Xiaoyan , Nalcioglu Orhan 
Published: March 2007

AbstractDynamic contrast‐enhanced MR imaging was used to measure the kinetics of enhancement in three different animal tumor models (Walker 256, R3230 AC, MCF7) using three different Gd complexes (Gd‐DTPA, Gd‐DTPA‐24‐cascade‐poly‐mer 30 kD, and polylysine‐Gd‐DTPA 50 kD). The three tumor models varied in growth rate, with the most rapid growth demonstrated by Walker 256 cells and the slowest growth occurring in the MCF7 cells. For each tumor, the kinetics of enhancement using polylysine‐Gd‐DTPA was analyzed using a pharmacokinetic model to estimate the vascular volume of the tumor. The rate of entry of the contrast agent into the interstitial space served as the measure of vascular permeability. The smallest molecular‐weight agent, Gd‐DTPA, could not provide information about vascular permeability. The intermediate and the largest agents both demonstrated that the faster‐growing Walker 256 tumor had greater vascular permeability than did the slower‐growing R3230 AC tumor. The degree of vascular permeability in the MCF7 tumor could not be assessed fairly due to insufficient statistics. The current study provides evidence supporting the hypothesis that more rapidly growing tumors have higher vascular permeability than do tumors that grow more slowly.