bluuebunny/crossref_metadata_2025 huggingface dataset:
Microscopic susceptibility variation and transverse relaxation: Theory and experiment
Authors: Weisskoff Robert , Zuo Chun S. , Boxerman Jerrold L. , Rosen Bruce R.
Published: March 2007

Microscopic susceptibility variations invariably increase apparent transverse relaxation rates. In this paper, we present comparisons between Monte Carlo simulations and experiments with polystyrene microspheres to demonstrate that this enhanced relaxation can be explained quantitatively for both spin echo and gradient echo imaging experiments. The spheres used (1 to 30 μ), and degree of susceptibility variation (caused by 0‐12 mM Dy‐DTPA) covered a wide range of biologically relevant compartment sizes and contrast agent concentrations. These results show that several regimes of behavior exist, and that contrast dependence is quite different in these regimes. For a given susceptibility, δχ, a small range of particle sizes show peak transverse relaxation. For the range of susceptibilities found in the first pass of a clinical IV contrast agent bolus, this size range is 5 to 10 μ, or roughly capillary sized compartments. In both our simulations and experiments, smaller spheres showed quadratic relaxation versus concentration curves, and larger particles showed sublinear behavior. For particles corresponding to the peak relaxivity, the relaxation‐concentration curves were linear. In addition, we demonstrated that increasing the diffusion coefficient can increase, decrease, or, paradoxically, leave unaffected the apparent relaxation rate. The regime for which the diffusion coefficient is relatively unimportant corresponds to the region of peak relaxivity. By using the Bloch‐Torrey equation to produce scaling rules, the specific Monte Carlo simulations were extended to more general cases. We use these scaling rules to demonstrate why we often find that susceptibility‐induced relaxation rates vary approximately linearly with concentration of injected agent.
Analysis of discrete T2 components of NMR relaxation for aqueous solutions in hollow fiber capillaries
Authors: Bronskill Michael J. , Santyr Giles E. , Walters Blake , Henkelman R. Mark
Published: March 2007

An analysis is presented of proton NMR T2 relaxation times measured for aqueous solutions In simple bundles of hollow fibers. The relaxation times are calculated with a two‐compartment diffusive exchange model using the known relaxation times of the aqueous solutions and the fiber geometry. When the relaxation time outside the fibers is short (∼1 ms), three or more relaxation components are observed from this two compartment system, in agreement with the calculation. The amplitude and relaxation times of the third component are consistent with those of a diffusion‐mediated mode, as suggested theoretically by Brownstein and Tarr (Phys. Rev. A 19, 2446 (1979)). The possible contribution of such modes to the multicomponent relaxation observed in tissues is discussed.
Self‐diffusion in CNS tissue by volume‐selective proton NMR
Authors: Horsfield Mark A. , Barker Gareth J. , McDonald W. Ian
Published: March 2007

We have used a volume‐selective NMR pulse sequence to measure the diffusion coefficient (D) inside a selected cuboid volume. The sequence is a variant of the stimulated echo method for volume‐selective spectroscopy, and allows sequential measurement of D in any of three orthogonal directions and for a range of diffusion times. The echo attenuation from which D is calculated is free from the cross terms that can arise because of interactions between volume selection and diffusion‐sensitizing gradients. Anisotropic and diffusion‐time‐dependent behavior is observed in brain structures known to have axon fibers aligned primarily in one direction.
Reduced phase encoding in spectroscopic imaging
Authors: Maudsley Andrew A. , Matson G. B. , Hugg J. W. , Weiner M. W.
Published: March 2007

The effect of different spatial‐encoding (k‐space) sampling distributions are evaluated for magnetic resonance spectroscopic imaging (MRSI) using Fourier reconstruction. Previously, most MRSI studies have used square or cubic k‐space functions, symmetrically distributed. These studies examine the conventional k‐space distribution with spherical distribution, and 1/2 k‐space acquisition, using computer simulation studies of the MRSI acquisition for three spatial dimensions and experimental results. Results compare the spatial response function, Gibbs ringing effects, and signal contamination for different spatial‐encoding distribution functions. Results indicate that spherical encoding, in comparison with cubic encoding, results in a modest improvement of the re sponse function with approximately equivalent spatial resolution for the same acquisition time. For spin‐echo acquired data, reduced acquisition times can readily be obtained using 1/2 k‐space methods, with a concomitant reduction in signal to noise ratio.
Interleaved echo planar phase contrast angiography
Authors: McKinnon Graeme
Published: March 2007

Technical aspects of a new rapid 2D multiplanar phase contrast angiographic sequence are presented. The sequence uses multiple excitation echo planar imaging concepts. The advantage of using multiple excitations is that the resolution can be increased over that obtainable with single shot echo planar imaging. By combining phase contrast measurements with echo planar imaging, the measurement time is considerably reduced compared with conventional phase contrast magnetic resonance angiography. The present implementation still requires some improvements before being suitable for clinical applications. Future embodiments could, however, permit angiograms of vessels subject to respiratory motion.
An evaluation of the sensitivity of the intravoxel incoherent motion (IVIM) method of blood flow measurement to changes in cerebral blood flow
Authors: Neil Jeffrey J. , Bosch Coleen S. , Ackerman Joseph J. H.
Published: March 2007

To evaluate the sensitivity of the intravoxel incoherent motion (IVIM) technique to changes in cerebral blood flow, we made measurements of IVIM parameters in rat brain under conditions of altered arterial pCO2. The arterial pCO2, was varied over a range which would be expected to change cerebral blood flow from roughly 50 to 500 ml/(100 g·min). The IVIM measurements were made with suppression of extravascular water signal. The parameters f′ (the apparent fraction of spins which have “fast” pseudodiffusion), D (the “fast‐pseudodiffusion” coefficient), and D(the “slow‐pseudodiffusion” coefficient) all showed statistically significant positive linear correlations with arterial pCO2. These results suggest that the IVIM method, when used with suppression of extravascular water signal, is sensitive to changes in blood flow.
MRI signal void due to in‐plane motion is all‐or‐none
Authors: Wedeen Van J. , Weisskoff Robert M. , Poncelet Brigitte P.
Published: March 2007

The process of MRI signal attenuation due to in‐plane intra‐voxel velocity inhomogeneity is described. Given rigid rotation or linear shear, velocity phase‐sensitivity will induce a phase distribution that varies linearly with position, which is exactly equivalent to the effect of a spatial phase encoding gradient pulse. It follows that the effect of such motion on the raw MRI signal is to displace it a fixed distance in k‐space. Attenuation becomes marked when the center of the spinecho reaches an edge of k‐space, which happens when intra‐voxlel phase shifts reach π radianlvoxel. Because spin echoes are typically peaked sharply at center, this attenuation usually is abrupt. Analytic and numerical simulations of linear and nonlinear velocity fields confirm abrupt MRI attenuation where phase dispersion exceeds π radianlvoxel. Examples of this phenomenon include the abrupt loss of blood signal adjacent the vessel wall in laminar flow, abrupt loss of subendocardial signal in early diastole, and sudden disappearance due to rotation of a kidney during a measurement of diffusion.
Optimum flip‐angles for exciting NMR with uncertain T1 values
Authors: Bottomley Paul A. , Ouwerkerk Ronald
Published: March 2007

T1 is often ill‐determined. This means that an Ernst angle excitation often cannot be precisely defined for the simple pulse and acquire experiment. Here, published 31P T1 values of metabolites in human muscle, liver, heart, and brain are archived, some new data on heart and brain added, and overall confidence intervals determined. Strategies for setting the flip‐angle based on the confidence intervals are examined, and an optimum flip‐angle derived which minimizes the signal loss relative to what could have been realized if T1 were precisely known. With such optimized pulses, signal loss can be limited to ≤14% for up to a 10‐fold variation in T1, with TR ≤ T1 The effect that an uncertainty in T1 by a factor of two has on the saturation corrected signal is limited to ≤20% in the optimum flip‐angle experiment. Adiabatic B1‐independent rotation phase‐cycled (BIRP) excitation pulses are ideal as optimum flip‐angle pulses as they can be prescribed without calibration.
Observation of a fast response in functional MR
Authors: Ernst Thomas , Hennig Jürgen
Published: March 2007

A spectroscopic MR technique was used to investigate the time course of the MR signal following a single visual stimulus. Gated experiments demonstrate there is an early response (500 ms after stimulus) leading to a reduction of the MR signal by −0.25% (P = 0.02), whereas a slower response (>1500 ms after stimulus) results in a signal increase of +0.59% (P = 0.01). The fast negative response may be attributed to increased oxygen consumption, followed by a slower vascular response with overcompensation in blood oxygenatioln. This explanation would be in agreement with the blood oxygenation level dependent (BOLD) contrast mechanism considered the basis of functional MR. However, other physiological events might also be responsible for the signal drop observed.
3‐dimensional functional imaging of human brain using echo‐shifted FLASH MRI
Authors: Duyn Jeff H. , Mattay Venhatha S. , Sexton Roy H. , Sobering Geoffrey S. , Barrios Fernando A. , Liu Guoying , Frank Joseph A. , Weinberger Daniel R. , Moonen Chrit T. W.
Published: March 2007

A 3‐dimensional MRI method has been developed for functional mapping of the human brain, based on blood oxygenation level dependent (BOLD) contrast mechanisms. The method uses recently introduced principles of echo‐shifted FLASH to acquire a single 3D data set in 20 s. The technique was tested on a conventional 1.5 Tesla clinical scanner with a standard head coil using visual stimulation with a 8 Hz flashing white light, or a varying checkerboard pattern. Areas of increased signal intensity were identified in the visual cortex, consistent with the known functional organization.
In Vivo detection of a boron‐neutron‐capture agent in melanoma by proton observed 1H ‐ 10B double resonance
Authors: Bendel Peter , Zilberstein Judith , Salomon Yoram
Published: March 2007

The Cesium salt of BSSB (Cs4B24H22S2), a common boron‐neutron‐capture‐therapy (BNCT) agent, was injected into M2R mouse melanoma xenografts, and detected in vivo by 1H‐observed, 10B‐edited NMR spectroscopy. The technique of spin‐echo difference spectroscopy, in which a proton spin‐echo is detected following the alternating presence and absence of a 10B 180° pulse was used. This method provides much higher sensitivity than direct 10B NMR detection, and should thus be suitable for in vivo detection in patients about to undergo BNCT treatment, where the infused agents are 95% isotopically enriched in 10B.
Intracellular and extracellular spaces and the direct quantification of molar intracellular concentrations of phosphorus metabolites in the isolated rat heart using 31P NMR spectroscopy and phosphonate markers
Authors: Clarke Kieran , Anderson Russell E. , Nédélec Jean‐François , Foster David O. , Ally Ariff
Published: March 2007

To quantify metabolite and cation concentrations using NMR spectroscopy, the volumes of intracellular and extracellular spaces must be known. We describe a simple 31P NMR spectroscopic method that employs dimethyl methylphosphonate (DMMP) as a marker of total water space and phenylphosphoriate (PPA) as a marker of extracellular space to determine intracellular and extracellular space volumes in the isolated, perfused rat heart. In vivo and in vitro radiolabel studies were used to verify this method. The difference between the total and extracellular water spaces, determined as milliliters/heart, gave the intracellular volume and allowed direct calculation of myocardial creatine phosphate, ATP, and inorganic phosphate concentrations, which were 13.4 mM, 10.1 mM, and 3.4 mM, respectively, for the glucose‐perfused rat heart. The extracellular volume decreased by 84% in hearts subjected to 28 min total, global ischemia and increased by 15% during reperfusion. The method described allows the determination of intracellular energy metabolite concentrations in perfused rat heart directly from a single, fully relaxed 31P NMR spectrum.
Avoiding peripheral nerve stimulation: Gradient waveform criteria for optimum resolution in echo‐planar imaging
Authors: Harvey Paul R. , Mansfield Peter
Published: March 2007

Adaptation of Mansfield and Harvey's theory of magnetically induced neural stimulation indicates that, for trapezoidal gradient waveforms and at the relatively low modulation frequencies commonly used in echo‐planar imaging, an increase in rise time even to the point of creating a triangular waveform can prove advantageous over the use of simple sinusoidal modulation. For example, a higher image resolution is in principle possible before the neural stimulation limit is reached. For higher modulation frequencies and fast ramp trapezoidal waveforms, the peak gradient can be lower than that for a sinusoidal waveform by a factor approaching 2/π for equal resolution. This means that by using a multimode resonant gradient drive, the amplifier power requirements are greatly relaxed. The signal sampling bandwidth required is also lower, because, due to the trapezoidal nonlinear sample pattern, sample points are spaced linearly during the waveform plateaus.
Localized two‐dimensional shift correlated spectroscopy in humans at 2 Tesla
Authors: Brereton Ian M. , Galloway Graham J. , Rose Stephen E. , Doddrell David M.
Published: March 2007

A method for the acquisition of localized 2D shift‐correlated spectra, based on the combination of the stimulated‐echo volume‐selection and gradient‐enhanced COSY experiments, is described. The sequence can be modified to perform a number of localized experiments including HOHAHA and DQF‐COSY. The method is demonstrated in vivo by presentation of localized COSY and HOHAHA spectra of human tibia marrow, and a localized COSY spectrum of human brain acquired at a field strength of 2 Tesla. Cross peaks corresponding to correlations between coupled groups along the acyl chains of triglycerides are observed in the spectra of marrow. The major cerebral metabolites are represented in the in vivo COSY brain spectrum, including N‐acetylaspartate, glutamate/glutamine, total creatine, aspartate, and myo‐inositol. Difficulties in the implementation of localized shift‐correlation spectroscopy, including water suppression and T2 relaxation, are discussed.
On the field inhomogeneity of a birdcage coil
Authors: Jin Jianming , Shen Gary , Perkins Thomas
Published: March 2007

A mathematical model is developed for the numerical analysis of birdcage coils constructed using capacitors having different capacitance. With the aid of this model, it is shown that the deviation of some capacitance introduced by tune, balance, and drive mechanisms can cause inhomogeneity in the B1, field produced by the coil. Useful curves and formulae are developed to provide a guideline for the design of birdcage coils.
Fast volume scanning with frequency‐shifted burst MRI
Authors: Duyn Jeff H. , van Gelderen Peter , Liu Guoying , Moonen Chrit T. W.
Published: March 2007

We introduce a modified BURST imaging technique with reduced saturation effects and improved signal‐to‐noise ratio. The method applies a frequency shift to the RF excitation pulse on successive repetitions. It allows collection of 3D datasets of human brain within a few seconds, on a standard clinical scanner at 1.5 Tesla.
Abstract scoring for the annual SMR program: Significance of reviewer score normalization
Authors: Glover Gary H. , Henkelman R. Mark
Published: March 2007

Presently, the scores of three to four reviewers of each abstract are averaged and form a major component of the input for decisions made in choosing papers for oral and poster presentations, and in rejecting others. No normalization is made for differences in either mean or standard deviation between the reviewers of the same abstracts. In this paper, several techniques for normalizing the scores of reviewers are examined, and the consequences of applying such normalization is to several categories of the 1994 abstract submissions are examined. It was found that some alterations in the acceptance and assignments of papers as oral and posters resulted for one of the categories, and the normalized scores were used during the program assembly. It is recommended that future review procedures utilize reviewer normalization.
Assessment of absolute metabolite concentrations in human tissue by31P MRS invivo. Part I: Cerebrum, cerebellum, cerebral gray and white matter
Authors: Buchli Reto , Duc Corinne O. , Martin Ernst , Boesiger Peter
Published: March 2007

Absolute metabolite concentrations were determined in four different brain regions using phosphorus magnetic resonance spectroscopy (31P MRS) on 10 healthy adult volunteers. Localized spectra were collected simultaneously from the cerebellum and the cerebrum and, later, from deep white matter and cortical gray matter by means of a two‐volume lSlS pulse sequence and a Helmholtz‐type RF‐coil. Each brain spectrum was quantified with a calibration spectrum from a head‐shaped simulation phantom. A time‐domain fitting routine was used to process the fully relaxed data. Several metabolite concentrations (mmolAiter) differed significantly between the cerebrum and the cerebellum (PME = 3.2 f 0.3 and 4.0 & 0.6, PC:r = 2.9 & 0.3 and 3.9 f 0.4, NTP = 2.9 f 0.2 and 2.6 & 0.2, respectively) and between cortical gray matter and deep white matter (PME = 3.1 f 0.4 and 4.3 ± 0.8, PDE = 10.1 f 2.5 and 14.2 & 2.6, respectively). The concentration of free magnesium ion was found to be similar in all four brain regions (0.53 ± 0.21 mmol/liter) but the intracellular pH was significantly higher in the cerebellum (7.04 ± 0.03) than in the cerebrum (6.99 ± 0.02).
Transition from lorentzian to gaussian line shape of magnetization transfer spectrum in bovine serum albumin solutions
Authors: Iino Maisaaki
Published: March 2007

Magnetization transfer experiments using an off‐resonance irradiation technique were performed on bovine serum albumin solutions by varying the irradiation frequency and the concentration. A transition of the magnetization transfer spectrunr from Lorentzian to Gaussian line shape was observed around the critical concentration of 6.2 mmoles of protein to kg ad solution. Observed magnetizations were well expressed by the rate equations of populations for spins below and above the transition, which yielded the magnetization transfer rates, the intrinsic relaxation rates of both protein and water protons, and the effective tumbling time and the rigid line width of the protein. The result showed that the estimates of the values for magnetization transfer rate do not change once the critical concentration is reached.
The design and test of a new volume coil for high field imaging
Authors: Wen Han , Chesnick Andrew S. , Balaban Robert S.
Published: March 2007

A major problem in the development of high field (>100 MHz) large volume (>6000 cm3) MR coils is the interaction of the coil with the subject as well as the radiation loss to the environment. To reduce subject perturbation of the coil resonance modes, a volume coil that uses an array of freely rotating resonant elements radially mounted between two concentric cylinders was designed for operation at 170 MHz. Substantial electromagnetic energy is stored in the resonant elements outside the sample region without compromising the efficiency of the overall coil. This stored energy reduces the effect of the subject on the circuit and maintains a high Q, facilitating the tuning and matching of the coil. The unloaded Q of the coil is 680; when loaded with a head, it was 129. The ratio of 5.3 of the unloaded to loaded Q supports the notion that the efficiency of the coil was maintained in comparison with previous designs. The power requirement and signal‐to‐noise performance are significantly improved. The coil is tuned by a mechanism that imparts the same degree of rotation on all of the elements simultaneously, varying their degree of mutual coupling and preserving the overall coil symmetry. A thin radiofrequency shield is an integral part of the coil to reduce the radiation effect, which is a significant loss mechanism at high fields. MR images were collected at 4T using this coil design with high sensitivity and B1, homogeneity.
Selective saturation in magnetization transfer experiments
Authors: McGowan Joseph C. , Leigh John S.
Published: March 2007

The concept of magnetic saturation in single and binary spin systems is essential to the understanding of magnetization transfer experiments. This paper outlines the requirements for selective magnetic saturation of either of two spin pools contained in a mixture, both having identical chemical shifts. Following a brief discussion of saturation in a homogeneous sample, simple relationships are derived that predict the maximum degree of selective saturation in a two‐spin nonexchanging system. Optimal saturation conditions are described in terms of the intrinsic relaxation parameters as well as the experimental conditions of saturation offset frequency and amplitude. In addition, a novel method is proposed that may be applied to saturate the “free” water as opposed to the “bound” water spins in a magnetization transfer experiment. Saturation in an exchanging system is well approximated by these relationships under some physiologically relevant conditions, examples of which are provided. Application of the relationships presented may be useful in the design of experiments to produce maximum magnetization transfer.
MR gradient echo imaging of intravascular blood oxygenation: T2* determination in the presence of flow
Authors: Chien Daisy , Levin David L. , Anderson Charles M.
Published: March 2007

The T2* relaxation time of blood varies with its oxygen saturation. To evaluate the feasibility of imaging intravascular blood oxygenation in humans using a conventional 1.5T MR system, we have implemented a method to measure T2* of blood despite the presence of pulsatile flow. The method was tested in a) stationary and flow phantoms, b) blood samples at different levels of oxygen saturation, and c) a human hypoxia model. Our results demonstrate the ability of cardiac‐triggered, flow compensated gradient echo imaging to obtain reproducible T2* measurements of flowing blood in vivo.
Water diffusion in the giant axon of the squid: Implications for diffusion‐weighted MRI of the nervous system
Authors: Beaulieu Christian , Allen Peter S.
Published: March 2007

To clarify the result that marked diffusional anisotropy had been found in nonmyelinated nerve, and in completion of an evaluation of the role of all longitudinal axonal structures, we report NMR measurements of water diffusion in the giant axon of the squid, where diffusional anisotropy is determined by the neurofilamentary structure. The diffusion coefficients of water parallel and perpendicular to the long axis of the squid giant axon at 20°C are (1.61 ± 0.06) × 10−5 cm2 s−1 and (1.33 ± 0.09) × 10−5 cm2 s−1, respectively, which yield an anisotropic diffusion ratio of 1.2 2 0.1. Water diffusion in the squid giant axon is therefore quite rapid and nearly isotropic, thus eliminating the possibility of a significant role for the longitudinally oriented neurofilaments in producing diffusional anisotropy within the axoplasm. In conjunction with our work on garfish nerves therefore, only membranes, either as numerous axonal membranes or as myelin (if present), remain to fulfill the role of the primary determinant of anisotropic water diffusion in nerve and in white matter.
Anisotropy of NMR properties of tissues
Authors: Henkelman R. Mark , Stanisz Greg J. , Kim Jae K. , Bronskill Michael J.
Published: March 2007

Orientational anisotropy of T2 and T1 relaxation times, diffusion, and magnetization transfer has been investigated for six different tissues: tendon, cartilage, kidney, muscle, white matter, and optic nerve. Relaxation anisotropy was observed for tendon and cartilage, and diffusional anisotropy was measured in kidney, muscle, white matter, and optic nerve. All other NMR measurements of these tissues showed no orientational dependence. This pattern of NMR anisotropies can be interpreted from the underlying geometrical structures of the tissues.
Real‐time motion detection in spiral MRI using navigators
Authors: Sachs Todd S. , Meyer Craig H. , Hu Bob S. , Kohli Jim , Nishimura Dwight G. , Macovski Albert
Published: March 2007

A technique has been developed whereby motion can be detected in real time during the acquisition of data. This enables the implementation of an algorithm to accept or reject and reacquire data during a scan. Frames of data with motion are rejected and reacquired on the fly so that by the end of the scan, a complete motion‐free data set has been acquired. The algorithm has been implemented on several different types of sequences. Preliminary in vivo studies indicate that motion artifacts are dramatically reduced.
Quantitative analysis of multi‐slice Gd‐DTPA enhanced dynamic MR images using an automated simplex minimization procedure
Authors: Buckley David L. , Kerslake Robert W. , Blackband Stephen J. , Horsman Anthony
Published: March 2007

Quantitative analysis of Gd‐DTPA enhanced dynamic MR images has potential for discriminating lesions, especially because the introduction of clinical fast imaging techniques has enabled good sampling of the relatively rapid Gd‐DTPA wash‐in curves. Analysis of such data requires curve fitting to a nonlinear model, which to date has been performed using a nonlinear least squares (NLLS) fitting procedure. However, this method often fails to converge to the appropriate minima without good initial parameter estimates when multi‐exponential models are involved, making automated analysis of complete multislice or volume data sets problematic. In this report we demonstrate the robust performance of a simplex minimization procedure compared with NLLS, by the method of Marquardt, using a Monte Carlo simulation. Further, we illustrate the applicability of such a technique to the analysis of dynamic contrast enhanced images on a pixel‐by‐pixel basis. As a preliminary example, the technique is applied to a breast lesion but is expected to be suitable for examination of many lesion types.
Detection of apparent restricted diffusion in healthy rat brain at short diffusion times
Authors: Niendorf Thoralf , Norris David G. , Leibfritz Dieter
Published: March 2007

The application of bipolar diffusion sensitizing gradient pulses to significantly reduce the diffusion time is described. This approach is combined with the rapid U‐FLARE imaging sequence. Three diffusion‐sensitized types of experiments are compared and their suitability for detecting restricted diffusion is discussed. Experiments using a modification of the diffusion weighting by varying the diffusion time between 1.6 and 6.0 ms obtained nonmonoexponential signal attenuation curves from both healthy brains and postmortem. This behavior is indicative of restricted diffusion, but as it is detectable only at short diffusion times, in contrast to a restriction due to impermeable barriers, we have termed this “apparent restriction”.
A gradient scheme suitable for localized shimming and in vivo1H/31P STEAM and ISIS NMR spectroscopy
Authors: Yongbi Ngong M. , Payne Geoffrey S. , Leach Martin O.
Published: March 2007

A gradient scheme is presented which may be used for STEAM or ISIS localization. One application of the scheme is to perform single‐shot STEAM shimming prior to data acquisition with STEAM and ISIS, using identical gradient amplitudes and durations. Using conventional STEAM to shim for ISIS can produce line‐shape distortions induced by different eddy currents in the two sequences; with this gradient scheme the problem is minimized. Line‐shape improvements of STEAM and ISIS localized data obtained after volume localized shimming with the proposed STEAM sequence are demonstrated. The localization performance of the STEAM and ISIS sequences are demonstrated on phantoms and in vivo for 1H and 31P metabolites.
A strategy for sampling on a sphere applied to 3D selective RF pulse design
Authors: Wong Sam T. S. , Roos Mark S.
Published: March 2007

Conventional constant angular velocity sampling of the surface of a sphere results in a higher sampling density near the two poles relative to the equatorial region. More samples, and hence longer sampling time, are required to achieve a given sampling density in the equatorial region when compared with uniform sampling. This paper presents a simple expression for a continuous sample path through a nearly uniform distribution of points on the surface of a sphere. Sampling of concentric spherical shells in k‐space with the new strategy is used to design 3D selective inversion and spin‐echo pulses. These new 3D selective pulses have been implemented and verified experimentally.
Breath‐Hold Projection Magnetic Resonance‐Cholangio‐Pancreaticography (MRCP): a New Method for the Examination of the Bile and Pancreatic Ducts
Authors: Laubenberger JÜRg , Büchert Martin , Schneider Britta , Blum Ulrich , Hennig JÜRgen , Langer Mathias
Published: March 2007

Breath‐hold MR cholangio‐pancreaticography using the RARE method was evaluated as a noninvasive alternative to direct cholangio‐pancreaticography in 30 healthy volunteers and in 30 patients with various diseases of the respective duct systems. In contrast to 3D MR cholangiography, RARE MRCP generates projection images requiring a 4‐s acquisition time and no postprocessing. Diagnostic information gained from the cholangio‐pancreaticograms was fully equivalent to that of conventional x‐ray imaging by invasive procedures. Additional information was gained whenever endoscopic access was impossible and/or cystic malformations were present.
Echo‐Time Reduction for Submillimeter Resolution Imaging with a 3D Phase Encode Time Reduced Acquisition Method
Authors: Ying K. , Schmalbrock P. , Clymer B.
Published: March 2007

To achieve optimal image quality and highest spatial resolution for inner ear imaging with a 3D gradient echo sequence, it is necessary to minimize susceptibility dephasing effects by using very short TE. Fractional RF pulses and echoes can yield short TE for moderate spatial resolution; however, for voxel size of less than 1 mm, TE is limited by the phase encode gradients. We present a method to obtain very short effective TE by using short triangular shaped phase encode gradients to sample the central portions of k‐space and progressively longer trapezoidal gradients for the outer portions of k‐space. A 3D pulse sequence employing the modified phase encoding scheme for both in‐plane and slice phase encoding directions was implemented and tested on phantoms and in vivo. The effective TE equals the minimal TE used for the central k‐space portions. Submillimeter resolution (0.35 x 0.35 x 0.7 mm3) images of the inner ear were obtained with effective TE of 3.2 ms and were compared with standard 3D images with TE of 8 ms. With this pronounced TE reduction, the susceptibility artifacts at air/fluid interfaces are significantly reduced.
Measurements of Magnetic Field Variations in the Human Brain Using a 3D‐FT Multiple Gradient Echo Technique
Authors: Ericsson Anders , Weis Jan , Hemmingsson Anders , Wikström Mats , Sperber Göran O.
Published: March 2007

A magnetic resonance 3DFT multiple gradient‐echo technique was used for measurements of the proton spectrum for each voxel in the measured slice. Water, fat, magnetic field and T2 distributions in the head of a normal volunteer and a patient with intracerebral hematoma were computed. Magnetic field variations caused by the head were calculated after correction for the static magnetic field inhomogeneity. Large local magnetic field variations up to 3 ppm were found in the human brain near interfaces between air or bone and brain tissues and 0.5 ppm between hematoma and brain tissue. Information about magnetic field variations could be useful for shimming procedures in vivo and for correcting artifacts in imaging and spectroscopy.
Magnetic Resonance Imaging and Magnetization Transfer in Experimental Myonecrosis in the Rat
Authors: Mattila Kimmo T. , Lukka Riitta , Hurme Timo , Komu Markku , Alanen Anu , Kalimo Hannu
Published: March 2007

Experimental myonecrosi–induced by injection of notexin into rat tibialis anterior muscle–and subsequent regeneration were studied from 1 h to 20 days postinjury with magnetic resonance imaging using conventional and magnetization transfer sequences, and these findings were correlated with histopathology. MR images revealed necrosis within 1 h postinjection. Histopathologically, necrotized fibers enlarged and intercellular spaces widened, indicating intracellular and extracellular edema, which began to decrease after 48 h, whereafter the formation of new myofibers predominated. T2 increased progressively until 7.5 h, while T1 increased until 24 h. Magnetization transfer contrast (MTC) and magnetization transfer rate (Rwm) decreased rapidly postinjection; the decrease in Rwm lasted longer than in MTC (96 h versus 48 h, respectively). Spin echo, inversion recovery and magnetization transfer sequences revealed the lesions equally effectively. MR images and relaxation parameters reflect well the extent of histopathological injury and edema in the acute phase, whereas specific tissue changes in the regenerative phase were not detectable by MRI. MT imaging and especially magnetization transfer rate are as sensitive as conventional T2 contrast to alterations in water imbalance.
Turbulent Fluctuation Velocity: The Most Significant Determinant of Signal Loss in Stenotic Vessels
Authors: Oshinski John N. , Ku David N. , Pettigrew Roderic I.
Published: March 2007

Studies of flow in a 90%‐stenosis phantom were conducted to elucidate the parameters and mechanisms responsible for signal loss in MR angiographic images. The studies independently evaluated the effect of velocity, Reynolds number, turbulent fluctuation velocity, and turbulence intensity on the amount of post‐stenotic signal loss. Results suggested that the magnitude of the turbulent fluctuation velocity, not merely the presence of turbulence or the intensity of turbulence, was the parameter that determined the extent of the signal loss. The study suggests that future flow phantom studies should be conducted with fluids having physiologic velocities and viscosities to obtain accurate levels of turbulent fluctuation velocities and hence reproduce results of in‐vivo signal‐loss patterns. The mechanism for signal loss is that the temporal and spatial variations of the turbulent fluctuation velocity cause a range of phases to be present within a voxel. Examination of the theoretical aspects of fluid turbulence suggest that shortening gradient durations and imaging during diastole may help reduce signal loss.
Dual‐Echo Interleaved Echo‐Planar Imaging of the Brain
Authors: Slavin Glenn S. , Butts Kim , Rydberg John N. , Jack Clifford R. , Riederer Stephen J.
Published: March 2007

An interleaved echo‐planar imaging (EPI) technique is described that provides images from 20 sections of the brain at two echo times (27 and 84 ms) in 1:05. Six echoes per image per repetition are collected in 24 repetitions of the pulse sequence. MR images of the brain obtained from five volunteers using the dual‐echo EPI sequence, fast spin‐echo (FSE), and conventional dual‐echo spin‐echo were evaluated qualitatively for diagnostic use and quantitatively for relative signal‐to‐noise ratio (SNR), contrast, and contrast‐to‐noise ratios (CNR).
Active Acoustic Screening: Reduction of Noise in Gradient Coils by Lorentz Force Balancing
Authors: Mansfield Peter , Chapman Barry L. W. , Bowtell Richard , Glover Paul , Coxon Ron , Harvey Paul R.
Published: March 2007

We have designed and constructed a quiet gradient set with restricted access for the combined purposes of evaluating the principles of active acoustic screening, recently introduced by Mansfield, Glover, and Bowtell, and for EPI studies of the head at 3.0 T. The design utilizes the return paths of the conductors in a closed arc loop arrangement to eliminate net Lorentz forces thereby attenuating acoustic noise especially at low frequency. This design should significantly reduce the dangers to patients of high noise levels, especially in high field magnetic resonance imaging systems.
Hybrid Three Dimensional (1D‐Hadamard, 2D‐Chemical Shift Imaging) Phosphorus Localized Spectroscopy of Phantom and Human Brain
Authors: Gonen Oded , Hu Jiani , Stoyanova Radka , Leigh John S. , Goelman Gadi , Brown Truman R.
Published: March 2007

A hybrid of two localized spectroscopy techniques, chemical shift imaging (CSI) and Hadamard spectroscopic imaging (HSI), is used to obtain an array of 16 x 16 x 4 (3 x3 x 3 cm3 voxels) proton‐decoupled phosphorus (31P) spectra of human brain. For equal spatial resolution, this organ's oblate shape requires fewer axial than coronal or sagittal slices. These different spatial requirements are well suited to 1D, 4th order, transverse HSI in the axial direction, combined with 2D 16 x 16 CSI in the other two orientations. The reduced localization matrix (16 x 16 x 4 over just the brain versus a cubic‐16 x 16 x 16 matrix of equal resolution, over the entire head) may proportionally shorten data acquisition if the voxel size is not signal‐to‐noise limited. In addition, the use of Hadamard encoding can improve the intervoxel spectral isolation.
A Single‐Step Method for Estimation of Local Cerebral Blood Volume from Susceptibility Contrast MRI Images
Authors: Bahn Mark M.
Published: March 2007

A new single‐step method is described for estimation of local cerebral blood volume (CBV) from multiple rapidly acquired T2‐ or T2*‐weighted MRI images after bolus administration of a susceptibility contrast agent. This method improves on existing methods in three ways. First, the method includes the baseline scan intensity value as a fitted parameter. Second, the model is fitted directly to the original scan intensity values instead of to transformed concentration curves, avoiding the propagation of errors that occurs in the transformation. Third, the equations are reparameterized with CBV fitted directly as a model parameter. Parameter estimation methods are compared by Monte Carlo simulation. The direct method described here yields more precise parameter estimates, with smaller mean absolute deviations from the true parameter values, than the existing method when compared using simulated data. Implementation is discussed and numerical evaluation of the Digamma or Psi function is described.
Continuous Update with Random Encoding (CURE): A New Strategy for Dynamic Imaging
Authors: Parrish Todd , Hu Xiaoping
Published: March 2007

Although dynamic imaging is presently used for various applications, it is still limited by the temporal resolution. In this paper, we present a new technique that uses a random phase‐encoding strategy to facilitate faster and smoother update of images and to improve the temporal resolution in dynamic studies. The technique was implemented on a conventional clinical scanner and demonstrated with various in vivo studies. Technical details, simulations, and experimental results are described. Images from experimental studies indicate that the new technique is robust in generating dynamic images and can be potentially utilized for clinical applications.
Time Resolved Flow Quantification with MRI Using Phase Methods: A Linear Systems Approach
Authors: Peeters Frank , Luypaert Robert , Eisendrath Henri , Osteaux Michel
Published: March 2007

Phase‐related unsteady (pulsatile) flow effects in MRI have been studied by means of linear response theory. These flow effects can be described in the frequency domain: the influence of the gradients on the phase shift is described by a transfer function, the spectrum of the gradient being the determining factor. An analysis of this transfer function is shown to provide information about the process of flow encoding: instant of encoding, induced distortions and how they are related to the gradient waveform. The connection with the traditional description in terms of the gradient moment expansion has also been investigated and clarified. This approach was applied to study the response of two time‐resolved flow quantification techniques (Fourier flow method and phase mapping) by analyzing their amplitude and phase transfer functions. By simulation it is shown that a better interpretation of the measured velocity waveform is obtained and that Fourier analysis in combination with a correction by the inverse transfer function results in an accurate reconstruction of the velocity waveform studied.
Increase of GPC Levels in Cultured Mammalian Cells during Acidosis. A 31P MR Spectroscopy Study Using a Continuous Bioreactor System
Authors: Galons Jean‐Phillipe , Job Constantin , Gillies Robert J.
Published: March 2007

The purpose of this study was to study the metabolic events during a slow acidosis in three different cell lines by combining 31P magnetic resonance spectroscopy and hollow fiber bioreactor technology. The rate of change in intracellular pH, glycerophosphorylcholine (GPC), phophorylcholine (PCho), and nucleoside‐triphosphate (NTP) levels were measured during 8 h of acidosis and 16 h of recovery in EPO, EAT, and RN1a cells, three cultured mammalians cell lines. Our results show a significant increase in GPC levels to 330 ± 21, 540 ± 25, and 220 ± 21% of their initial value correlated to a decrease of PCho levels to 57 ± 14, 58 ± 17 and 45 ± 15% of their initial value in EAT, RN1a, and EPO cells, respectively. These changes are discussed in terms of perturbation of energetic metabolism in cells undergoing a slow acidosis.
Three‐Dimensional Spectroscopic Imaging with Time‐Varying Gradients
Authors: Adalsteinsson Elfar , Irarrazabal Pablo , Spielman Daniel M. , Macovski Albert
Published: March 2007

A spectroscopic imaging sequence with a time‐varying readout gradient in the slice selection direction is used to image multiple contiguous slices. For a given voxel size, the imaging time and signal‐to‐noise ratio of the three‐dimensional spectroscopic sequence are the same as for a single slice acquisition without the oscillating readout gradient. The data reconstruction employs a gridding algorithm in two dimensions to interpolate the nonuniformly sampled data onto a Cartesian grid, and a fast Fourier transform in four dimensions: three spatial dimensions and the spectral dimension. The method is demonstrated by in vivo imaging of NAA in human brain at 1.5 T with 10 slices of 16 x 16 pixels spectroscopic images acquired in a total scan time of 17 min.
Interpretation of 31P MRS Spectra in Determining Intracellular Free Magnesium and Potassium Ion Concentrations
Authors: Golding Elke M. , Golding Raymund M.
Published: March 2007

The current paper develops a simple mathematical model for the determination of the concentration of ionic species using the chemical shifts of ATP from phosphorus magnetic resonance spectroscopy (31P MRS) spectra. An iterative method is used to evaluate the equilibrium constants for a range of Mg/ATP equilibria. Initially, an expression for free magnesium concentration is determined by consideration of four equilibria. We then demonstrate how any number of equilibria may be included using this simplified analysis, by incorporation of pseudo equilibrium constants. This approach may be applied to the interpretation of both ex vivo and, more importantly, in vivo MRS spectra. Furthermore, the authors suggest that the commonly used equation for determining free intracellular magnesium concentration as presented by Gupta and colleagues is an oversimplification, particularly under conditions of fluctuating pH.
Action of Compression and Cations on the Proton and Deuterium Relaxation in Cartilage
Authors: Lüsse Steffen , Knauss Robert , Werner Annett , Gründer Wilfried , Arnold Klaus
Published: March 2007

In this paper, investigations are described on the influence of osmotic pressures and of varying cation concentrations on water relaxation times in cartilage (pig articular cartilage and bovine nasal cartilage). Both water content and relaxation times decrease strongly with increasing osmotic pressure. This relaxation behavior can be explained in terms of a fast chemical exchange between unbound and bound water. Na+ does not influence water content or relaxation times, whereas Ca2+ causes a small reduction in these parameters.
Magnetization Transfer and T2 Relaxation Components in Tissue
Authors: Harrison Rodney , Bronskill Michael J. , Mark Henkelman R.
Published: March 2007

T2 relaxation makes an important contribution to tissue contrast in magnetic resonance (MR) imaging. Many tissues are known to exhibit multicomponent T2 relaxation that suggests some compartmental segregation of mobile protons on a T2 timescale. Magnetization transfer (MT) is another relaxation mechanism that can be used to produce tissue contrast in MR imaging. The MT process depends strongly on water‐macromolecular interactions. To investigate the relationship between multicomponent T2 relaxation and the MT process, multiecho T2 measurements have been combined with MT measurements for freshly excised samples of cardiac muscle, striated muscle, and white matter. For muscle, short T2 components show greater MT than long T2 components, consistent with the belief that they represent distinct water environments. For white matter, quantitative MT measurements were identical for the two major T2 components, apparently because of exchange between the T2 compartments on a timescale characteristic of the MT experiment. Implications for accurate modeling of MT in tissue and the use of MT for MR image contrast are discussed.
An Algorithm for MR Angiography Image Enhancement
Authors: Chen Haiguang , Hale James
Published: March 2007

This study describes a three‐dimensional (3D) filtering algorithm for magnetic resonance angiography (MRA) image enhancement. Based on the properties of 3D MRA data, the algorithm uses the dispersion range of the 13 outputs of a 3D directional low‐pass filter bank to identify the vessel structure and control the processing of MRA data. Vascular structures in the data are preserved while the components of static tissues and isolated noise impulses are reduced by the processing. As a result, the small vessels have been enhanced and visibility of vessels in the projected image has been improved.