bluuebunny/crossref_metadata_2025_split huggingface dataset:
VET imaging: Magnetic resonance imaging with variable encoding time
Authors: Feinberg David A. 
Published: March 2007

AbstractA methodology of MRI data acquisition is introduced which involves lengthening the duration of signal readout period with complementary shortening of the phase encode pulses, and vice versa, in sequences where gradient encoding time is limited by RF pulse spacing. This variable encoding time (VET) methodology can be used to increase spatial resolution or reduce data acquisition time in 2D and 3D FT MRI based on CPMG and gradient echo sequences. Advantages of higher image SNR and different spatial frequency distributions in k‐space are discussed and evaluated in preliminary experiments.
Spectroscopic imaging of the water resonance with short repetition time to study tumor response to hyperoxia
Authors: Oikawa Hiroshi , Al‐Hallaq Hania A. , Lewis Marta Z. , River Jonathan N. , Kovar David A. , Karczmar Gregory S. 
Published: March 2007

AbstractA variety of treatments that modulate tumor oxygen tension are used clinically to improve the outcome of radiotherapy. High resolution, noninvasive measurements of the effects of these treatments would greatly facilitate the development of improved therapies and could guide treatment of cancer patients. Previous work demonstrated that magnetic resonance (MR) gradient echo imaging of the water proton resonance detects changes in T2* and T1 in tumors during hyperoxia that may reflect increased tumor oxygenation. This report describes the use of high resolution MR spectroscopic imaging with short repetition time (TR = 0.2 s) to improve the accuracy with which changes in T2* and T1 are measured. Mammary adenocarcinomas grown in the hind limbs of rats were studied. Carbogen inhalation was used to induce hyperoxia. A single 2‐mm slice through the center of tumors and underlying muscle was imaged at 4.7 Tesla with in‐plane resolution of approximately 1.2 mm and frequency resolution of 5.8 Hz. The peak integral increased by an average of 6% in tumors during carbogen inhalation suggesting a decrease in T1 (n = 8, P <0.001). Peak height increased by an average of 15% in tumors during carbogen inhalation (n = 8, P <0.001). The large difference between increases in peak height and peak integral demonstrates that the width of the water resonance decreased. Assuming a Lorentzian lineshape, an average increase of 12% in T2* was observed in tumors. In muscle, peak integral and peak height increased slightly (about 1.2% and 3%. respectively; P <0.02) during carbogen inhalation but no significant change in T2*was observed. Spectroscopic imaging detects changes in the water proton resonance in tumors during hyperoxia accurately and reproducibly with high signal‐to‐noise ratio and allows clear separation of T1 and T2* effects. Increases in T2* may be due to decreased deoxyhemoglobin in tumor blood vessels (i.e., the BOLD effect) and may provide a clinically useful index of increases in tumor oxygenation.
Dipolar resonance frequency shifts in 1H MR spectra of skeletal muscle: Confirmation in rats at 4.7 T in Vivo and observation of changes postmortem
Authors: Ntziachristos Vasilis , Kreis Roland , Boesch Chris , Quistorff Bjørn 
Published: March 2007

AbstractNon‐isotropic contributions to 1H MR spectra from human skeletal muscle in vivo have recently been observed in the 0‐to 5‐ppm region. One pair of peaks has been identified to be subject to dipolar couplings. The corresponding changes in resonance frequency are related to the orientation of muscle fibers with respect to the external magnetic field and are analogous to the behavior of small molecules dissolved in liquid crystals. Image‐guided localized spectroscopy based on the STEAM method has been applied to verify these phenomena in rat skeletal muscle in vivo and to investigate the effect postmortem. Residual dipolar couplings and anisotropic contributions to 1H MR spectra of skeletal muscle have been confirmed in animals and at a higher field strength—albeit with a slightly different spectral pattern compared to the human study. The most prominent dipolar doublet due to creatine and/or phosphocreatine vanishes postmortem with a rate similar to the disappearance of phosphocreatine, and is no longer observable 2 h postmortem.
Skeletal muscle perfusion measurements using adiabatic inversion of arterial water
Authors: Marro Kenneth I. , Kushmerick Martin J. 
Published: March 2007

AbstractQuantitative NMR measurements of perfusion using magnetic labeling of arterial water have been demonstrated previously in several different highly perfused organs. The success of these previous experiments suggested that arterial labeling may be of use in measuring perfusion in skeletal muscle, where resting perfusion is very low and where increased perfusion after exercise is transient. In the experiments described in this paper, adiabatic inversion of arterial water has been used to make single‐voxel measurements of perfusion in the lower hind limb of rats. At rest, the NMR results were quantified to yield a perfusion rate of about 13.8 ml/100g/min. After perturbation due to ischemic exercise, large relative changes in the NMR signal were observed. The peak change of about 2.5% of the NMR signal occurred shortly after perturbation and was followed by a return to resting levels over a period of about 4 min.
Diffusion‐weighted multiple shot echo planar imaging of humans without navigation
Authors: Robson Matthew D. , Anderson Adam W. , Gore John C. 
Published: March 2007

AbstractA new method is proposed for reducing the artifacts produced in diffusion‐weighted imaging. When data are acquired using multiple shot echo planar acquisitions, conventional reconstruction methods produce artifactual images as a consequence of diffusion weighting and small amounts of bulk motion of the subject. If the amount of motion can be determined, it is possible to correct the data before reconstruction, which removes the artifact. A method for estimating the motion from the acquired data has been developed and evaluated. This method assumes that ghost image effects will be minimized when motion has been correctly compensated. By considering the amount of signal in the background of the image, appropriate corrections to the data can be made, and the accuracy of the motion compensation may be estimated. This technique has been evaluated by computer simulation, and its performance has been demonstrated in a phantom and humans with both two‐ and four‐shot echo planar acquisitions and using both “mosaic” and “interleaved” sampling schemes.
Density compensation functions for spiral MRI
Authors: Hoge Richard D. , Kwan Remi K. S. , Bruce Pike G. 
Published: March 2007

AbstractIn interleaved spiral MRI, an object's Fourier transform is sampled along a set of curved trajectories in the spatial frequency domain (k‐space). An image of the object is then reconstructed, usually by interpolating the sampled Fourier data onto a Cartesian grid and applying the fast Fourier transform (FFT) algorithm. To obtain accurate results, it is necessary to account for the nonuniform density with which k‐space is sampled. An analytic density compensation function (DCF) for spiral MRI, based on the Jacobian determinant for the transformation between Cartesian coordinates and the spiral sampling parameters of time and interleaf rotation angle, is derived in this paper, and the reconstruction accuracy achieved using this function is compared with that obtained using several previously published expressions. Various non‐ideal conditions, including intersecting trajectories, are considered. The new DCF eliminated intensity cupping that was encountered in images reconstructed with other functions, and significantly reduced the level of artifact observed when unevenly spaced sampling trajectories, such as those achieved with trapezoidal gradient waveforms, were employed. Modified forms of this function were found to provide similar improvements when intersecting trajectories made the spiral‐Cartesian transformation noninvertible, and when the shape of the spiral trajectory varied between interleaves.
Efficient design of pulses with trapezoidal magnitude and linear phase response profiles
Authors: Pickup Stephen , Popescu Mihail 
Published: March 2007

AbstractA variation of the Shinnar‐Le Roux (SLR) method of pulse envelope design that allows for control of the phase of the frequency response profile has been developed. The method makes use of the fact that a knowledge of one of the SLR polynomials in combination with a root inversion pattern for the other polynomial is sufficient to fully define the second polynomial. Optimization of the first polynomial, when cast in this form, remains nonlinear. However, it was demonstrated that the relationship between the SLR polynomials and the frequency response profile may be used to generate an initial guess for the SLR polynomials that is sufficiently accurate to allow for the application of linear optimization techniques in most cases. In practice several pulse envelopes having different root inversion patterns are investigated for each target profile. The resulting collection of pulses allows the user to trade off pulse power for profile accuracy. The proposed technique was used to design a large number of amplitude modulated excitation pulses having trapezoidal magnitude and linear phase frequency response profiles. A few examples of the resulting pulses and their response profiles are presented.
A spectral approach to analyzing slice selection in planar imaging: Optimization for through‐plane interpolation
Authors: Noll Douglas C. , Boada Fernando E. , Eddy William F. 
Published: March 2007

AbstractInterpolation between slices is necessary whenever reslicing a volume of data into a different coordinate frame. This may be done to view the data from different perspectives, to align data from different sessions, or to remove the effects of head movement in functional imaging studies. In this paper, issues surrounding slice‐selection in two‐dimensional imaging are examined in the context of through‐plane interpolation and a spectral framework is introduced to describe the sources of error when interpolating between slices. This framework suggests that there is a trade‐off between precision in localization, which requires high spatial frequencies, and interpolation, which requires a narrow spectrum of spatial frequencies. An analysis of the sources of error has lead to several approaches to reducing interpolation error including elimination of interslice gaps or making slices overlap, use of a slice profile with a narrower spatial frequency bandwidth such as a Gaussian profile, and use of high‐order interpolation. The simulation and experimental data demonstrate significant reductions in interpolation error for these approaches.
The functional scout image: Immediate mapping of cortical function at 4 tesla using receiver phase cycling
Authors: Goodyear Bradley G. , Gati Joseph S. , Menon Ravi S. 
Published: March 2007

AbstractA rapid method of generating functional scout images to map cortical activity is presented. Maps were produced immediately “on‐line” on the scanner's console using fast low‐angle shot gradient‐recalled echo and echo‐planar imaging sequences without special hardware. To demonstrate the technique, raw data collected during photic stimulation and dark control periods were subtracted through phase alternation of the receiver with constant transmitted radio frequency phase. A blood oxygen level‐dependent signal difference map was produced by magnitude reconstruction of the resulting complex‐difference data. Maps improved in contrast with an increasing number of stimulation‐control cycles and showed similar areas of activation to traditional fMRI processing methods. Such a functional scout procedure allows rapid localization of brain function within the anatomical region of interest. From these functional scout images, slices or voxels may be planned by using the scanner manufacturers prescription tools for further specialized studies.
Quantitative studies of magnetization transfer by selective excitation and T1 recovery
Authors: Gochberg Daniel F. , Kennan Richard P. , Gore John C. 
Published: March 2007

AbstractWater proton longitudinal relaxation has been measured in agar and cross‐linked bovine serum albumin (BSA) using modified selective excitation (Goldman‐Shen and Edzes‐Samulski) pulse sequences. The resulting recovery curves are fit to biexponentials. The fast recovery rate gives magnetization transfer (MT) information, which is complementary to that given by steady‐state saturation methods. This rate provides an estimate of the strength of the coupling of the immobile proton pool to the mobile proton pool. Near their optimal pulse power values, the Goldman‐Shen and Edzes‐Samulski sequences give fast recovery rates that agree with each other. However, these measured fast recovery rates are dependent on the pulse power, an effect not predicted by the coupled two‐pool model. For 8% agar and 17% BSA, both methods (at optimal pulse powers) give rates in the neighborhoods of 210 and 64 Hz, respectively. The Goldman‐Shen and Edzes‐Samulski pulse sequences have several advantages over those techniques based on steady state saturation: no long saturating pulses, shorter measurement time, and reduced necessity for making lineshape or fitting technique assumptions. The principle disadvantages are smaller effects on the NMR signal, less complete characterization of the MT system, and, in the case of the Goldman‐Shen sequence, greater pulse power.
Evidence for the exchange of arterial spin‐labeled water with tissue water in rat brain from diffusion‐sensitized measurements of perfusion
Authors: Silva Afonso C. , Williams Donald S. , Koretsky Alan P. 
Published: March 2007

AbstractThe extraction fraction of vascular water in rat brain is investigated by means of diffusion measurements of arterial spin labeled water at varying cerebral blood flow (CBF) values. The apparent diffusion coefficient (ADC) of the difference of the proton magnetization signal in the brain acquired with and without continuous arterial spin labeling is modeled to provide a measure of the amount of arterial water in tissue and vasculature and thus of the extraction fraction. The tissue and vascular portion of the arterial spin labeled water are differentiated based on their diffusion characteristics in a manner analogous to the intravoxel incoherent motion (IVIM) method. The amount of labeled arterial water that exchanges with tissue water is determined by estimating the fraction of the total signal that is associated with the slow‐decaying component of a biexponential fit to the normalized difference signal between the magnetization of brain tissue acquired with and without arterial spin labeling. The results indicate that, at normal CBF (1.15 ± 0.21 ml g‐1 min‐1), about 90% of the arterial spin labeled water diffuses with an ADC of (1.21 ± 0.37) 10‐3mm2 s‐1), which is equal to tissue. At high CBF, an increasing fraction of the labeling water has a fast‐pseudo‐diffusion coefficient due to a decrease in water extraction fractions. The results also show that the contribution of vascular water to the measurement of perfusion by techniques that use endogenous water as a tracer can be efficiently eliminated by the use of diffusion sensitizing gradients with small effective b values (b ≈ 20 s/mm2), enabling these techniques to monitor true changes in tissue perfusion.
MRI using piecewise‐linear spiral trajectory
Authors: Liao Jan‐Ray , Pauly John M. , Pelc Norbert J. 
Published: March 2007

AbstractA new generation of high power gradient systems which allow much faster MR imaging as well as shorter echo times has recently become available. Some of these high‐speed gradient systems impose limits on the percentage of time during which the gradient can change in amplitude (slewing duty cycle). While this limitation may be immaterial to many 2DFT and echo planar imaging methods, a traditional circular spiral trajectory is difficult to use on these systems because its gradient waveforms change during the entire course of the trajectory so that the slewing duty cycle during the readout period is 100%. We describe a piecewise‐linear spiral trajectory which is composed of linear segments and rounded corners. This trajectory reduces the slewing duty cycle while maintaining the desirable imaging properties of circular spirals including interleaving by simple gradient rotation. For one representative example, the slewing duty cycle is reduced to 46%. A conventional gridding method was used for image reconstruction, but a new numerical algorithm to calculate the density compensation factor was required. Use of piecewise‐linear spiral trajectories reduces the impact imposed by limited gradient slewing duty cycle.
In vivo imaging of extraction fraction of low molecular weight mr contrast agents and perfusion rate in rodent tumors
Authors: Kovar David A. , Lewis Marta Z. , River Jonathan N. , Lipton Martin J. , Karczmar Gregory S. 
Published: March 2007

AbstractTissue uptake of a fully extractable MR detectable tracer, deuterated water (D2O), was compared with that of a less extractable contrast agent, Gadolinium‐DTPA‐dimeglumine (Gd‐DTPA), in rodent tumor and muscle tissue. This dual tracer method allowed calculation of relative (to muscle) tissue perfusion and extraction fraction of Gd‐DTPA in each image pixel in vivo. Solutions of Gd‐DTPA and D2O were injected intravenously into Fisher female rats (n = 9) with R3230 mammary adenocarcinomas implanted in the hind limb. Perfusion rate was approximately two times greater (P < 0.005 by paired t test) in tumor than in muscle. Gd‐DTPA extraction fraction at the interface between tumor and muscle was 2.0 times the extraction fraction in normal muscle (P <0.005 by paired t test). Extraction fraction at the tumor center was 1.6 times the extraction fraction in muscle (P <0.01 by paired t test). High extraction fraction of Gd‐DTPA correlated with high capillary permeability determined from Evans Blue staining. Low molecular weight Gd‐DTPA derivatives are widely used in clinical practice, and their extraction fractions are crucial determinants of image contrast during the first few passes of the contrast agent bolus. Therefore spatially resolved measurements of contrast agent extraction fractions obtained in vivo have significant clinical utility. The data demonstrate that extraction of low molecular weight tracers is sensitive to increased permeability in tumor vasculature and that this increased permeability can be imaged.
Experimental determination of the BOLD field strength dependence in vessels and tissue
Authors: Gati Joseph S. , Menon Ravi S. , Uǧurbil Kǎmil , Rutt Brian K. 
Published: March 2007

AbstractHigh resolution functional MRI (fMRI) experiments were performed in human visual cortex at 0.5, 1.5, and 4 T to determine the blood oxygenation level dependent (BOLD) field strength response within regions of obvious venous vessels and cortical gray matter (“tissue”). T2*‐weighted FLASH images were collected in single‐ and multi‐echo mode and used to determine the intrinsic BOLD parameters, namely, signal‐to‐noise ratio (Ψ), the apparent transverse relaxation rate (R2*) and the change in R2* (ΔR2*) between the activated and baseline states. The authors find the average percentage signal change (ΔS/S, measured at TE = T2*) to be large in vessels (13.3 ± 2.3%, 18.4 ± 4.0%, and 15.1 ± 1.2%) compared with that in tissue (1.4 ± 0.7%, 1.9 ± 0.7%, and 3.3 ± 0.2%) at 0.5, 1.5, and 4 T, respectively. The signal‐to‐noise ratio in optimized, fully relaxed proton density weighted gradient echo images was found to increase linearly with respect to the static magnetic field strength (B0). The predicted upper bound on BOLD contrast‐to‐noise ratio (ΔS/R)max as a function of field strength was calculated and found to behave less than linearly in voxels containing vessels larger than the voxel itself and greater than linearly in voxels containing a mixture of capillaries and veins/venules with a diameter less than that of the voxel.
Motion‐adapted gating based on k‐space weighting for reduction of respiratory motion artifacts
Authors: Weiger Markus , Börnert Peter , Proksa Roland , Schäffter Tobias , Haase Axel 
Published: March 2007

AbstractA new modified type of gating is presented that shows the ability to reduce the total scan time with almost conserved image quality compared with conventional gating. This new motion‐adapted gating approach is based on a k‐space‐dependent gating threshold function. MR data acquired are only accepted if the motion‐induced displacements measured from a reference position are below the chosen gating threshold function. During the MR measurement the scanner analyses respiratory motion decides in real‐time which data in k‐space could be measured according to the gating threshold function and performs data acquisition. In the present paper the approach will be described and discussed. Simulations based on in vivo data and initial in vivo experiments are presented to compare different variants of the new approach mutually and to the conventional technique. The analysis given is focused on spin warp type sequences, which are the best candidates for this approach.
Direct observation of resolved intracellular and extracellular water signals in intact human red blood cells using 1H MAS NMR spectroscopy
Authors: Humpfer Eberhard , Spraul Manfred , Nicholls Andrew W. , Nicholson Jeremy K. , Lindon John C. 
Published: March 2007

AbstractHigh resolution 400 MHz 1H NMR spectra of red blood cell suspensions when measured using magic angle spinning (MAS) show two water resonances separated by 15 Hz. Based on addition of a paramagnetic Mn‐EDTA complex, measurement of relaxation times and variation of extracellular H2O/D2O ratios, these have been assigned as intracellular (line‐width 17.5 Hz) and extracellular water (linewidth 4.6 Hz). This is the first direct observation of intracellular water using NMR spectroscopy and the 1H MAS NMR spectroscopic approach offers the possibility of studying directly the compartmentation of substances in cells and kinetics of molecular transport.
A dual approach to linogram imaging for MRI
Authors: Gai Neville , Axel Leon 
Published: March 2007

AbstractAn alternate scheme for linogram image reconstruction, which is more logical from the viewpoint of its applicability to MRI data, is presented here. As a result, an intermediate step for this method, the direct Fourier method (DFM) gives the same results as the earlier developed general reconstruction algorithm labeled as the linogram method (LM). However, the two differ in the pathways taken to the solution. As a result, an intermediate step for DFM corresponds directly with the geometry of linogram data collected for MRI, in contrast to the LM reconstruction. The two reconstruction methods are delineated within the context of MRI data reconstruction, and applied to reconstruct images from linogram spin‐echo data of a physical phantom, obtained on a clinical 1.5 T scanner.