bluuebunny/crossref_metadata_2025 huggingface dataset:
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

Tissue 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

High 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

A 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

High 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

An 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.
Gradient moment smoothing: A new flow compensation technique for multi‐shot echo‐planar imaging
Authors: Slavin Glenn S. , Riederer Stephen J.
Published: March 2007

This work identifies an additional source of phase error across ky in multi‐shot echo‐planar imaging resulting from flow or motion along the phase‐encoding direction. A velocity‐independent flow compensation technique, gradient moment smoothing, is presented that corrects this error by forcing the phase to have smooth quadratic behavior. The correction is implemented, without compromising scan time, by changing the first moment of a bipolar prephaser pulse on a shot‐by‐shot basis. In phantom and in vivo experiments, gradient moment smoothing effectively eliminates ghosting and signal loss due to phase‐encoding flow. When used in conjunction with a “flyback” echo‐planar readout, which compensates for flow in the frequency‐encoding direction, gradient moment smoothing renders multi‐shot echo‐planar imaging relatively insensitive to in‐plane flow. This can make multi‐shot echo‐planar imaging a viable technique for accurately imaging in‐plane flow and may desensitize it to the otherwise serious problem of in‐plane motion.
Calculations and experimental studies of the lineshape of the lactate doublet in PRESS‐localized 1H MRS
Authors: Marshall Ian , Wild James M.
Published: March 2007

Accurate quantification of NMR metabolites by spectral modeling requires assumed lineshape functions. For singlet resonances, a combination of Lorentzian and Gaussian lineshapes is sufficient, but for weakly J‐coupled resonances such as lactate, more complex lineshapes are necessary. In this work, the lactate lineshape is calculated for the PRESS sequence using standard RF pulses, and compared with experimental measurements. A similar comparison is made for PRESS‐localized spectroscopic imaging, in which the lineshape varies from voxel to voxel across the field of view. These observations have important implications for the quantification of lactate in experimental and human studies.
In Vivo1H‐NMR microimaging with respiratory triggering for monitoring adoptive immunotherapy of metastatic mouse lymphoma
Authors: Fichtner Klaus‐Peter , Schirrmacher Volker , Griesbach Andreais , Hull William E.
Published: March 2007

The metastatic ESb‐MP murine lymphoma in DBN2 mice has been used as a model for investigating metastatic disease and its cure by adoptive immunotherapy (ADI) as monitored by in vivo multislice spin‐echo 1H NMR microimaging at 7 T. Isoflurane inhalation anesthesia facilitated long measurement sessions, and respiratory gating with a fiber‐optic sensor greatly reduced motional artifacts. With T2 weighting (TR = 2 s, TE = 30 ms) mean signal‐to‐noise ratios of 30 and 15 for kidney and liver, respectively, were achieved in 20 min (100‐μm pixels, 1‐mm slices, 25‐mm field of view). Without the use of contrast agents, metastases with diameters > 0.3 mm in the imaged plane could be detected as hyperintense lesions in kidney (contrast ratio ca. 1.4) and liver (contrast ratio ca. 2) with a confidence level of > 98%. For the first time the complete eradication of late‐stage macroscopic metastases by ADI could be demonstrated noninvasively by MRI.
Diffusion‐weighted imaging of the human optic nerve: A New approach to evaluate optic neuritis in multiple sclerosis
Authors: Iwasawa Tae , Matoba Hiroko , Ogi Akiko , Kurihara Hiroaki , Saito Kimihiko , Yoshida Tetsuo , Matsubara Sho , Nozaki Atsushi
Published: March 2007

The apparent diffusion coefficient (ADC) in the optic nerve was measured from diffusion‐weighted magnetic resonance imaging using an intravoxel incoherent motion (IVIM) sequence. The subjects were seven normal volunteers and eight patients with multiple sclerosis (MS) with a total of four optic nerves with acute neuritis and nine nerves with chronic neuritis. The mean ADC (4.18 ± 1.13 × 10−3 mm2/s, n = 9) in the optic nerves with chronic neuritis was significantly higher than that in normal volunteers (1.56 ± 0.675 × 10−3 mm2/s, n = 14) and that in the nerves with acute neuritis (0.94 ± 0.43 × 10−3 mm2/s n = 4) (P < 0.001). The ADC is useful in assessing MS foci in the optic nerves.
A Method to measure arbitrary k‐space trajectories for rapid MR imaging
Authors: Mason Graeme F. , Harshbarger Todd , Hetherington Hoby P. , Zhang Yantian , Pohost Gerald M. , Twieg Donald B.
Published: March 2007

A method to measure arbitrary k‐space trajectories was developed to compensate for nonideal gradient performance during rapid magnetic resonance (MR) imaging with actively or nonactively shielded gradients at a magnetic field strength of 4.1 T. Accurate MR image reconstruction requires knowledge of the k‐trajectory produced by the gradient waveforms during k‐space sampling. Even with shielded gradients, residual eddy currents and imperfections in gradient amplifier performance can cause the true k‐space trajectory to deviate from the ideal trajectory. The k‐space determination was used for spiral gradient‐echo imaging of the human brain. While individual calibrations are needed for new pulse sequences, the method of k‐space determination can be used for any sequence of preparation pulses and readout gradient waveforms and should prove useful for other trajectories, including the rastered lines of echo‐planar imaging.
Phase insensitive preparation of single‐shot RARE: Application to diffusion imaging in humans
Authors: Alsop David C.
Published: March 2007

Although RARE and GRASE can produce single‐shot images of excellent quality, their utility has been restricted because preparation of the magnetization with interesting contrast before imaging can cause severe artifacts. These artifacts relate to the strong sensitivity of multiple spin echo sequences to the phase of the prepared magnetization. Modifications of the RARE sequence to eliminate these artifacts are discussed, and an approach that eliminates the artifact producing signals from the very first echo is presented. The approach is applied to diffusion imaging of the human brain in normal volunteers and one patient.
Increase of creatine kinase activity in the visual cortex of human brain during visual stimulation: A 31p NMR magnetization transfer study
Authors: Chen Wei , Zhu Xiao‐Hong , Adriany Gregor , Uǧurbil Kǎmil
Published: March 2007

31P magnetic resonance spectroscopy was used to investigate changes in high energy phosphate levels and creatine kinase (CK) kinetics induced in the human visual cortex during photic stimulation. CK kinetics was evaluated by measuring the apparent unidirectional rate constant (kf) in the “forward” direction (i.e., in the direction of ATP synthesis from phosphocreatine). kf increased 34% in the visual cortex areas during stimulation without significant changes of steady‐state concentration of high energy phosphate compounds, indicating that CK turnover is elevated during increased neuronal activity.
Determination and characterization of nitric oxide generation in mice by in vivo l‐band EPR spectroscopy
Authors: Fujii Hirotada , Koscielniak Janusz , Berliner Lawrence J.
Published: March 2007

The authors have shown direct, real‐time, in vivo measurement of nitric oxide (NO) in mice by using the water soluble metal chelator complex, N‐methyl‐D‐glucamine dithiocarbamate (MGD), and Fe(II) as monitored by EPR at L‐band. The three‐line EPR spectrum from the product [(MGD)2‐Fe(II)‐NO] was observed noninvasively in lipopolysaccharide (LPS)‐treated mice. The spectrum was markedly suppressed by the administration, before LPS injection, of phenyl N‐tert‐butyl nitrone (PBN), an inhibitor of the expression of induced nitric oxide synthase (iNOS). When 15N‐arginine was administered to LPS‐treated mice, a diagnostic EPR spectrum was observed, consisting of both three‐ and two‐line EPR signals, due to (MGD)2‐Fe(II)‐14NO and (MGD)2‐Fe(II)‐16NO, respectively. The results strongly suggested that the NO detected in these experiments was synthesized by iNOS. In vivo EPR measurements of [(MGD)2‐Fe(II)‐NO] at several regions in the body (from the head to the tail) indicated that the NO was generated mostly in the upper abdomen near the liver. These observations were confirmed by ex vivo EPR measurements on isolated organs where higher NO levels were detected in vivo in the liver and kidney. The spectroscopic results, combined with the pharmacokinetic data, support the model that NO detected in LPS‐treated mice was produced mainly in the liver, and that it did not reflect NO‐adduct complex accumulated in the liver via the blood circulation.
BURST excitation pulses
Authors: Heid Oliver
Published: March 2007

A theory of optimum burst excitation is developed in the frame‐work of the Shinnar‐LeRoux spinor formalism. An N pulse RF train of constant or linear phase cannot improve on an average echo strength of M0/N, and a phase modulation of the pulse train is necessary to improve the signal yield to the theoretical maximum value M0√;N. Several methods are presented yielding pulse trains of nearly optimum average amplitude for arbitrary N. It is shown that RF phase spoiling can be analyzed with the same framework. The presented pulse trains may also be useful when ultrawide spectrum hard pulses are required, but only limited RF power is available, e.g., for NMR experiments in extremely inhomogeneous B0 fields.
Simultaneous acquisition of spatial harmonics (SMASH): Fast imaging with radiofrequency coil arrays
Authors: Sodickson Daniel K. , Manning Warren J.
Published: March 2007

SiMultaneous Acquisition of Spatial Harmonics (SMASH) is a new fast‐imaging technique that increases MR image acquisition speed by an integer factor over existing fast‐imaging methods, without significant sacrifices in spatial resolution or signal‐to‐noise ratio. Image acquisition time is reduced by exploiting spatial information inherent in the geometry of a surface coil array to substitute for some of the phase encoding usually produced by magnetic field gradients. This allows for partially parallel image acquisitions using many of the existing fast‐imaging sequences. Unlike the data combination algorithms of prior proposals for parallel imaging, SMASH reconstruction involves a small set of MR signal combinations prior to Fourier transformation, which can be advantageous for artifact handling and practical implementation. A twofold savings in image acquisition time is demonstrated here using commercial phased array coils on two different MR‐imaging systems. Larger time savings factors can be expected for appropriate coil designs.
Simulated phase evolution rewinding (SPHERE): A technique for reducing B0 inhomogeneity effects in MR images
Authors: Kadah Yasser M. , Hu Xiaoping
Published: March 2007

A novel method for reducing field inhomogeneity effects in magnetic resonance images is described in this paper. Observing that image degradation arises from B0 inhomogeneity‐induced phase accrual during data acquisition, the present method numerically rewinds the accumulated phase in the k‐space data based on an initial estimate of the image and a corresponding field map. The rewinding process generates a corrected k‐space data set that is subsequently Fourier transformed to produce the final image. In this paper, a theoretical analysis of the method and applications of the technique to magnetic resonance imaging data are presented. The theoretical analysis of the method indicates that it is a general approach applicable to a variety of sequences. Results obtained by applying the method to experimental data acquired with single‐shot echo‐planar imaging, segmented echo‐planar imaging with centric reordering, and spiral sequences demonstrate that it is robust in reducing image degradation induced by B0 inhomogeneity.
STEAM‐Burst: A single‐shot, multi‐slice imaging sequence without rapid gradient switching
Authors: Crémillieux Yannick , Wheeler‐Kingshott Claudia A. , Briguet André , Doran Simon J.
Published: March 2007

The stimulated‐echo acquisition mode‐Burst sequence is a single‐shot, multi‐slice imaging technique that does not involve rapid gradient switching. A Burst excitation pulse train is followed by a 90° hard pulse and, after a mixing time, by a 90° slice‐selective pulse. A read gradient refocuses a set of stimulated echoes, which can be phase encoded to form an image. By repeating the selective pulse N times, each time with the carrier frequency offset differently, it is possible to sample N slices in a single‐shot. A comparison is made of the sequence with other three‐dimensional single‐shot methods. Experiments implementing the technique on a 3 T whole‐body imaging system and a 2 T, 31‐cm bore animal imager are described. Both phantom and brain images are presented. The principal advantages of the new sequence are its speed, the absence of rapid gradient switching and corresponding freedom from artifacts, its insensitivity to static magnetic field inhomogeneities, and its low acoustic noise. The main disadvantages are the low signal‐to‐noise ratio of the images produced and the concomitant limitation in resolution.
Theoretical basis for sodium and potassium MRI of the human heart at 1.5 T
Authors: Parish Todd B. , Fieno David S. , Fitzgerald Steven W. , Judd Robert M.
Published: March 2007

Knowledge of the extent and location of viable tissue is important to clinical diagnosis. In principle, sodium (23Na) and potassium (39K) MRI could noninvasively provide information about tissue viability. In practice, imaging of these nuclei is difficult because, compared with water protons (1H), 23Na and 39K have lower MR sensitivities (9.2 and 0.051%, respectively), and lower in vivo concentrations (ca. 1000‐fold). On the other hand, the relatively short T1 relaxation times of 23Na and 39K (ca. 30 and 10 ms, respectively) suggest that optimized imaging pulse sequences may in part alleviate the weak signal of these nuclei. In this study, numerical simulations of high‐speed imaging sequences were developed and used to maximize 23Na and 39K image signal‐to‐noise ratio (SNR) per unit time within the constraints of existing gradient hardware. The simulation demonstrated that decreasing receiver bandwidth at the expense of echo time (TE) results in a substantial increase in 23Na and 39K image SNR/time despite the short T2 and T2* of these nuclei. Referenced to the available 1H signal on existing 1.5 T scanners, the simulation suggested that it should be possible to acquire three‐dimensional 23Na images of the human heart with 7 × 7 × 7 mm resolution and 39K images with 26 × 26 × 26 mm resolution in 30 min. Experimentally in humans at 1.5 T, three‐dimensional 23Na images of the heart were acquired in 15 min with 6 × 6 × 12 mm resolution and signal‐to‐noise ratios of 11 and 7 in the left ventricular cavity and myocardium, respectively, which is very similar to the predicted result. The results demonstrate that by choosing imaging pulse sequence parameters that fully exploit the short relaxation times of 23Na and 39K, potassium MRI is improved but remains impractical, whereas sodium MRI improves to the point where 23Na imaging of the human heart may be clinically feasible on existing 1.5 T scanners.
An orthogonal correlation algorithm for ghost reduction in MRI
Authors: Kruger David G. , Slavin Glenn S. , Muthupillai Raja , Grimm Roger C. , Riederer Stephen J.
Published: March 2007

Ghosting in MRI due to modulation of k‐space data can be caused by motion of the subject or characteristics of the sequence. A general solution for 2DFT MRI that reduces ghosting without causal modeling is presented. Separate image data sets are acquired in which the phase and frequency directions are swapped. In these two data sets, the image signal is correlated, whereas the ghost signals are not. By taking a correlation of these two data sets, an image with greatly reduced ghosting is obtained. The reduction is shown to depend both on the correct signal intensity of the image, as well as the ghost intensity in the ghosted region. The reduction approaches 100% in regions of low image signal, and is more moderate in regions of higher image signal. The process was applied to conventional spin‐echo, fast‐spin‐echo, and gradient echo imaging of volunteers and a phantom. Results of a reader study of the volunteer images reflected a significant overall reduction of ghosting artifacts in all volunteer experiments.
Multi‐turn split‐conductor transmission‐line resonators
Authors: Serfaty Stéphane , Haziza Nathalie , Darrasse Luc , Kan Siew
Published: March 2007

Multi‐turn split‐conductor transmission‐line resonators (MSTR) can be designed to operate over a wide range of NMR operating frequencies without lumped tuning capacitors. This architecture constitutes an alternative solution to recent designs proposed for high‐Q, thin‐film, high‐temperature superconducting NMR probes. An advantage is that the resonant frequency can be calculated in a relatively simple manner in terms of coil turns or total coil length, coil width, substrate thickness, and dielectric constant. Analytical calculation of the resonant frequency is provided. Also, a series of MSTRs was constructed on a double copper‐clad substrate, and their resonant frequencies are noted. The results obtained were in good agreement with the predicted values.
Lateral geniculate activations can be detected using intersubject averaging and fMRI
Authors: Büchel Christian , Turner Robert , Friston Karl
Published: March 2007

Applications of fMRI in functional brain imaging are mainly confined to single subject designs, prohibiting the assessment of subject or group by condition interactions (i.e., differential activations) or areas of conjoint activation. In this paper a framework for fMRI group designs, using statistical parametric mapping, is introduced. It is generally believed that intersubject averaging, which requires spatial normalization and smoothing, will decrease the effective spatial resolution of fMRI or its sensitivity. A subcortical activation of the lateral geniculate nucleus (LGN) was therefore chosen to demonstrate the feasibility and power of intersubject averaging in the context of fMRI. Seven volunteers were studied, while looking at a blank screen or radially moving dots. LGN activation was demonstrated in all single subject analyses and in the group analysis.
Brain MRI with laser‐polarized 129xe
Authors: Swanson Scott D. , Rosen Matthew S. , Agranoff Bernard W. , Coulter Kevin P. , Welsh Robert C. , Chupp Timothy E.
Published: March 2007

The feasibility of brain MRI with laser‐polarized 129Xe in a small animal model is demonstrated. Naturally abundant 129Xe is polarized and introduced into the lungs of Sprague‐Dawley rats. Polarized xenon gas dissolves in the blood and is transported to the brain where it accumulates in brain tissue. Spectroscopic studies reveal a single, dominant, tissue‐phase NMR resonance in the head at 194.5 ppm relative to the gas phase resonance. Images of 129Xe in the rat head were obtained with 98‐μl voxels by 2D chemical shift imaging and show that xenon is localized to the brain. This work establishes that nuclear polarization produced in the gas phases survives transport to the brain where it may be imaged. Increases in polarization and delivered volume of 129Xe will allow clinical measurements of regional cerebral blood flow.
Low visibility of lactate in excised rat muscle using double quantum proton spectroscopy
Authors: Jouvensal Laurence , Carlier Pierre G. , Bloch Gilles
Published: March 2007

Lactate NMR visibility was investigated in excised rat muscle at 3 T by comparing the concentration determined in situ by double quantum (DQ) proton spectroscopy (150 ms effective echo time) to the concentration measured in vitro from perchloric acid extracts of the same muscle samples. After 1‐2 h of ischemia, lactate NMR visibility was 32 ± 3% (±SE, n = 9), and was only 21 ± 1% (n = 6) after 10–12 h. Muscle lactate T2 was 140 ± 11 ms and 184 ± 6 ms, respectively. All potential mechanisms of DQ lactate signal attenuation (B0, and B1 inhomogeneity, DQ transverse relaxation, diffusion) were examined, and accounted for when necessary. A significant increase in lactate NMR visibility was demonstrated using a shorter effective echo time (79 ms) DQ editing sequence. These results are interpreted as reflecting muscle lactate compartmentation between a long T2 pool predominantly detected by DQ spectroscopy, and a short T2 pool virtually invisible with longer echo time NMR techniques.
Dual‐frequency, dual‐quadrature, birdcage RF coil design with identical b1pattern for sodium and proton imaging of the human brain at 1.5 T
Authors: Shen Gary X. , Boada Fernando E. , Thulborn Keith R.
Published: March 2007

Rapid quantification of tissue metabolitesin vivoby MRS or MRI can be achieved using dual‐frequency RF coils with identical B1field distributions at the observation frequencies of the metabolites and tissue water protons. Tissue sodium is used as an example for optimizing the dual‐frequency, dual‐quadrature RF coils for such measurements in humans. In the setting of sodium imaging, the challenge of dual‐quadrature birdcage configurations is to decouple the sodium and proton channels because the fourth harmonic of the sodium frequency is very close to the proton frequency. A generalizable method for effectively decoupling these two RF frequencies is presented in this paper. The method is demonstrated with the design of an EPI compatible, dual‐quadrature, double‐tuned,23Na/1H birdcage coil. The performance of the RF probe is reported at 1.5 Tesla in terms of signal‐to‐noise ratio, B1homogeneity and image quality.
Theory of the quadrature elliptic birdcage coil
Authors: Leifer Mark C.
Published: March 2007

This paper presents the theory of the quadrature birdcage coil wound on an elliptic cylindrical former. A conformal transformation of the ellipse to a circular geometry is used to derive the optimal sampling of the continuous surface current distribution to produce uniform magnetic fields within an elliptic cylinder. The analysis is rigorous for ellipses of any aspect ratio and shows how to produce quadrature operation of the elliptic birdcage with a conventional hybrid combiner. Insight gained from the transformation is also used to analyze field homogeneity, find the optimal RF shield shape, and specify component values to produce the correct current distribution in practice. Measurements and images from a 16‐leg elliptic birdcage coil at both low and high frequencies show good quadrature performance, homogeneity, and sensitivity.
Axonal stimulation under MRI magnetic field z gradients: A modeling study
Authors: Carbunaru Rafael , Durand Dominique M.
Published: March 2007

The stimulation of axons under MRI magnetic fields is analyzed solving the cable equation along the axons in the presence of magnetic field z gradients. Axons are represented using a one‐dimensional compartmental cable model including the kinetics of mammalian myelinated fibers. Computer simulations of the model were performed for sinusoidal and trapezoidal fields. Several axon and field parameters were tested to determine the threshold values for axonal stimulation of the magnetic field, induced electric field and time derivative of the magnetic field. The results indicate that 1) threshold for stimulation is lowest for the largest diameter axons terminating in the region where the magnetic field is maximum, 2) trapezoidal waveforms can be optimized to allow better sub‐threshold resolution than sinusoidal wave‐forms, and 3) the induced electric field is better than the magnetic field and time derivative of the magnetic field as indicators of stimulation threshold.
T2 Accuracy on a whole‐body imager
Authors: Foltz Warren D. , Stainsby Jeffery A. , Wright Graham A.
Published: March 2007

MR 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

The 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

An 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

Triple 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

A 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

Spin‐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

A 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

It 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

A 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

To 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

Small 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

The 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

The 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

Multivoxel 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

This 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

High‐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

The 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.