bluuebunny/crossref_metadata_2025 huggingface dataset:
MR imaging of anisotropic and restricted diffusion by simultaneous use of spin and stimulated echoes
Authors: Sotak Christopher H. , Li Limin
Published: March 2007

A new magnetic resonance imaging technique for evaluating anisotropic and restricted diffusion effects in a single experiment is described. The method is based on a pulse sequence that simultaneously excites a spin echo (SE) and a stimulated echo (STE) and contains two pairs of diffusion‐sensitive gradient pulses. The diffusion attenuation of the SE and STE can be controlled independently by separate adjustment of the parameters associated with each pair of the diffusion‐sensitive gradient pulses. This method can be more time efficient than acquiring separate SE and STE images for the same study. Furthermore, the pulse sequence minimizes the effect of motion and instrumental and physiological variations that occur between separate image acquisitions which can give rise to artifactual results when difference imaging is employed. In addition to applications to anisotropic and restricted diffusion, this technique can be used for: (1) simultaneous acquisition of T2‐weighted and diffusion‐weighted images during cerebral ischemia studies; (2) calculation of diffusion coefficients from a pair of diffusion‐weighted images; (3) separating fast and slow diffusion components in a single imaging experiment; and (4) two‐dimensional velocity‐encoded imaging.
Partial RF echo planar imaging with the FAISE method. I. Experimental and theoretical assessment of artifact
Authors: Melki Philippe S. , Jolesz Ferenc A. , Mulkern Robert V.
Published: March 2007

The fast acquisition interleaved spin‐echo (FAISE) method is a partial RF echo‐planar technique which utilizes a specific phase‐encode reordering algorithm to manipulate image contrast (Melki et al., J. Magn. Reson. imaging 1:319, 1991). The technique can generate “spin‐echo” like images up to 16 times faster than conventional spin‐echo methods. However, the presence of T2 decay throughout the variable k‐space trajectories used to manipulate T2 contrast ensures the presence of image artifacts, especially along the phase‐encode direction. In this work, we experimentally and theoretically examine the type and extent of artifacts associated with the FAISE technique. We demonstrate the existence of well‐defined minima of phase‐encode ghost noise for selected k‐space trajectories, examine the extent of blurring and edge enhancement artifacts, demonstrate the influence of matrix size and number of echoes per train on phase‐encode artifact, and show how proper choice of FAISE sequence parameters can lead to proton density brain images which are practically indistinguishable from conventional spin‐echo proton density images. A comparison of contrast between FAISE and standard spin‐echo methods is presented in a companion article referred to as II.
Single‐Shot GRASE Imaging without Fast Gradients
Authors: Oshio Koichi , Feinberg David A.
Published: March 2007

Based on the CPMG sequence, gradient‐ and spin‐echo (GRASE) echo train length is limited by T2 decay rather than the T2* decay and phase error in echo‐planar techniques, permitting a longer image acquisition period. An ultrafast GRASE sequence, utilizing a single excitation, generates a 128 × 56 true T2‐weighted image in 200 ms on an unmodified commercial scanner without fast gradient switching, extreme field homogeneity, or fat signal suppression.
The capillary network: a link between ivim and classical perfusion
Authors: Bihan Denis Le , Turner Robert
Published: March 2007

MR measurements based on motion encoding gradients, such as intravoxel incoherent motion imaging, could provide, in principle, information on flowing blood volume and blood velocity. This note shows that, in addition, the knowledge of the capillary network organization may provide a link between these measurements and those obtained by conventional and MR perfusion techniques based on tracer uptake by tissues.
Nuclear magnetic resonance velocity spectra of pulsatile flow in a rigid tube
Authors: Wendt Richard E. , Wong Wai‐Fan
Published: March 2007

Velocity spectra can be derived from velocity‐encoded nuclear magnetic resonance (NMR) images. Velocity spectra are histograms showing the amounts of fluid flowing at different velocities in the sensitive volume of the measurement. Velocity spectra may prove to be useful in characterizing the flow of blood in small vessels, for example, in detecting the presence of stenoses and in evaluating their severity. NMR velocity spectra acquired in vivo are sufficiently complicated that a model system was esigned and tested to investigate the velocity spectra of pulsatile flow. This study measured the NMR velocity spectra of pulsatile flow in a rigid tube and compared them to velocity spectra derived from Doppler ultrasound measurements and to velocity spectra inferred from a theoretical model driven by the measured pressure difference function. The experimental results from each technique agree.
Direct Observation of the Magnetization Exchange Dynamics Responsible for Magnetization Transfer Contrast in Human Cartilage in Vitro
Authors: Morris G. A. , Freemont A. J.
Published: March 2007

Saturating irradiation far off‐resonance can lead to diminution in the water signal seen in MRI, giving rise to magnetization transfer contrast. This results from transfer of magnetization between “solid” protons with restricted motion, which give rise to a band some tens of kilohertz wide, and the narrow signal from mobile protons. In the work reported here a high‐power pulse spectrometer, which can detect signals from both mobile and immobile protons, was used to investigate the dynamics of magnetization transfer in cartilage in vitro. Magnetization transfer in modified Hoffman‐Forsén inversion transfer experiments was well‐described by a single rate constant model; full analytical solutions are offered for the resultant biexponential magnetization recovery curves. The use of pulsed methods to generate magnetization contrast may in some circumstances offer advantages over the steady‐state saturation methods used hitherto.© 1992 Academic Press,Inc.
A New Frontier of Blood Imaging Using Susceptibility Effect and Tailored RF Pulses
Authors: Ro Y. M. , Cho Z. H.
Published: March 2007

In MRI, image contrast can be controlled by use of the susceptibility effect if an object contains paramagnetic substances. The localized linear gradient dephases spins in the voxel, leading to phase cancellation and thus reduced signal. This signal void phenomenon, can be exploited if the intrinsic linear gradient is either enhanced or compensated by externally applied RF generated phase distributions. In this paper, a new concept which utilizes the susceptibility effect through the use of tailored RF pulses is proposed. As potential applications of the method, two different types of tailored RF pulses are introduced: one for the enhancement of the susceptibility effect and the other for the correction of the susceptibility artifact, respectively. The former, for example, can be applied to angiography utilizing the paramagnetic property of deoxygenated blood, suggesting a new avenue for the angiography which, for the first time, is not based on flow, although the method is currently limited to imaging of venous blood or venography. Both a theoretical study of the method and experimental results are reported.© 1992 Academic Press,Inc.
Selective MR imaging of labeled human peripheral blood mononuclear cells by liposome mediated incorporation of dextran‐magnetite particles
Authors: Bulte Jeff W. M. , Ma Loralie D. , Magin Richard L. , Kamman Richard L. , Hulstaert Caesar E. , Go K. Gwan , The T. Hauw , De Leij Lou
Published: March 2007

Human peripheral blood mononuclear cells (PBMCs) were incubated with large unilamellar vesicles (LUV) containing encapsulated dextran‐magnetite particles (DMP). This resulted in an efficient incorporation of DMP. Electron microscopy revealed the presence of DMP in cells mainly in phagosomes and secondary lysosomes. DMP‐labeled PBMCs showed a strong increase of the transverse relaxation rate (up to 16.6 s−1 for 5 x 107 cells/ml) and, accordingly, a great loss of signal intensity in MR imaging. The fraction of DMP containing PBMCs could be enriched by magnetic cell separation. The major population of the DMP containing cells proved to be monocytes. When PBMCs depleted of monocytes were used for labeling, DMP uptake was observed also in the peripheral blood lymphocytes. The labeling of PBMCs presented here may be used in future studies of selective MR imaging of in vivo cell migration in a variety of immunologically compromised tissue states, e.g., tumors, transplantations, and abscesses.
Functional MRI of human brain activation at high spatial resolution
Authors: Frahm Jens , Merboldt Klaus‐Dietmar , Hänicke Wolfgang
Published: March 2007

Functional activation maps of the human visual cortex were obtained at a spatial resolution almost two orders of magnitude better than achievable by positron emission tomography and within measuring times of a few seconds. Transient alterations in the concentration of paramagnetic deoxyhemoglobin were conveniently detected at 2.0‐T with use of RF‐spoiled FLASH MRI sequences employing gradient echo times of 6 to 60 ms and voxel sizes of 2.5 to 39 μl.
Quantitative velocity images from thick slab 2D phase contrast
Authors: Weber David M. , Wang Yi , Korosec Frank R. , Mistretta Charles A.
Published: March 2007

Thick slab two‐dimensional phase contrast (2D PC) angiography provides a large amount of anatomical information in a short acquisition time. Quantitative velocity information is, however, destroyed by the necessary projection dephasing gradient. We present a 2D PC acquisition scheme which retains quantitative velocity information in a thick slab acquisition. A thick slab acquisition produces an image which is a projection through an entire vessel. Accurate velocity measurements must take into account the intravoxel phase cancellation caused by a distribution of velocities within the vessel. In addition to the description of the data acquisition scheme, we outline a method for determining the mean velocity within a vessel having a laminar flow distribution. It is shown that without the consideration for intravoxel phase cancellation, the mean velocity measurement from thick slab images overestimates the true mean velocity by 10%.
Specific in vitro labeling of cells with a fluorine‐19 probe encapsulated in antibody ‐ targeted liposomes: A F‐19 NMR spectroscopy study
Authors: Vion‐Dury Jean , Machy Patrick , Confort‐Gouny Sylviane , Leserman Lee , Cozzone Patrick J.
Published: March 2007

Liposomes containing dexamethasone phosphate (DMp) were covalently coupled to protein A and then incubated with mu‐rine L929 fibroblast and RDM4 thymoma cells in the presence of monoclonal antibodies specific for the major histocompatibility complex. The detection of the specific F‐19 labeling of cells is rapid (minutes). Such a strategy might be useful to study the kinetics of internalization processes of bound lipo‐somes on cultured living cells.
The visualization of RF probe electric fields
Authors: Chen C‐N. , Hoult D. I.
Published: March 2007

An assembly of resistive paper and liquid crystal sheet, conveniently and cheaply constructed for visual detection of the electric fields associated with an rf probe, is presented. Electrical asymmetries, and “hot‐spots” usually associated with conservative electric fields, are easily visualized by the color patterns displayed. The device is of considerable assistance in probe design and the minimization of dielectric loss.
K‐space description for MR imaging of dynamic objects
Authors: Xiang Qing‐San , Henkelman R. Mark
Published: March 2007

The spatial frequency (k) space concept is extended to describe the imaging of time‐dependent objects. This work builds on the existing k‐space description of MRI and is useful for simplifying the analysis of explanations of motion artifacts, algorithms for the correction of motion, and efficient imaging schemes for dynamic objects. Specific examples of the use of this concept for different imaging techniques are presented.
Diffusion imaging of experimental allergic encephalomyelitis
Authors: Heide A. C. , Richards T. L. , Alvord E. C. , Peterson J. , Rose L. M.
Published: March 2007

Diffusion‐weighted magnetic resonance imaging (MRI) was compared with T2‐weighted MRI in longitudinal studies of experimental allergic encephalomyelitis (EAE), an animal model of multiple sclerosis, in five monkeys (Macaca fascicularis). In a region of the brain that had highly directional myelinated fibers (internal capsule) sequential changes were identified on diffusion‐weighted images on and before the day these changes were detected on conventional T2‐weighted images. Changes were also identified on diffusion‐weighted images in brain areas that did not develop T2‐weighted abnormalities. This result suggests that diffusion‐weighted image intensities are sensitive to pathologic conditions of the brain that can not be seen on T2‐weighted images.
High‐resolution variable flip angle 3D MR imaging of trabecular microstructure in vivo
Authors: Jara H. , Wehrli F. W. , Chung H. , Ford J. C.
Published: March 2007

Two conceptually related variable‐flip‐angle 3D spin‐echo pulse sequences were designed for imaging at voxel sizes of 2–5 × 10−3 mm3 corresponding to pixel areas of less than 100 × 100 μm2 and section thicknesses on the order of 300–400 μm on a conventional 1.5 T MR imaging system equipped with 1 G/cm imaging field gradients, providing 12 sections in 10 min imaging time. The pulse sequences make use of the concept of restoring longitudinal magnetization inverted by the 180° phase reversal pulse and are derivatives of pulse sequences previously dubbed “FATE” and “RASEE.” It is shown that even in the small‐voxel regime (< 10−2 mm3 voxel size) and at echo times on the order of 10 ms, gradient echo images are sensitive to intrinsic fields causing artifactual boundary effects, including signal loss from intravoxel phase scrambling and spatial mismapping. At this resolution the variable flipangle spin‐echo pulse sequences are demonstrated to be better suited for imaging magnetically heterogeneous systems such as trabecular bone microstructure in vivo. These pulse sequences are found to be substantially less sensitive to distortions from magnetic dipole fields occurring at the boundaries of two phases of different magnetic permeability.
Double inversion recovery improves water suppression in vivo
Authors: Shen Jie F. , Saunders John K.
Published: March 2007

It is shown that double inversion recovery used for water suppression can be made much less sensitive to pulse imperfections and to the variations in the relaxation times than a single inversion recovery. This insensitivity results in up to 10‐fold improvement in the water suppression in vivo. The excellent water suppression by double inversion recovery is demonstrated experimentally by the in vivo proton spectra obtained from a rat brain.
Resting and end‐diastolic [Ca2+]i measurements in the langendorff‐perfused ferret heart loaded with a 19F NMR indicator
Authors: Harding Deborah P. , Smith Gerry A. , Metcalfe James C. , Morris Peter G. , Kirschenlohr Heide L.
Published: March 2007

Intracellular free calcium concentration ([Ca2+]i) was measured in Langendorff‐perfused ferret hearts (30°C, pH 7,4) by loading paced hearts with the 19F NMR calcium indicator, the 5,5′‐difluoro derivative of 1,2‐bis(o‐aminophenoxy) ethane‐N, N,N′,N′‐tetraacetic acid (5FBAPTA), to an initial cytosolic concentration of approximately 120 uM. Increasing the pacing frequency raised the end‐diastolic [Ca2+]i, from 299 ± 44 nM (mean ± SEM) at 0.2 Hz to 522 ± 54 nM at 1.0 Hz and 691 ± 166 nM at 2.0 Hz. Raising [Ca]o from 1.8 to 7.0 mM at a pacing frequency of 1.0 Hz increased end‐diastolic [Ca2+]i to 625 ± 39 nM. In unpaced hearts perfused with diltiazem (100 uM), [Ca2+]i fell rapidly to a steady‐state value of <100 nM after 60 min. Raising [Ca]o from 1.8 to 7.0 mM had no detectable effect on resting [Ca2+]i. The time course of the [Ca2+]i transient was measured in hearts paced at 1.1 Hz and perfused with 1.8 mM [Ca]o. The peak [Ca2+]i was ∼ 2 uM at approximately 150 msec after the pacing pulse, and peak developed LVP occurred at 550 msec compared with 280 msec in control hearts not loaded with 5FBAPTA. Comparisons with data obtained by other techniques, including fluorescent [Ca2+]i indicators, imply that although the end‐diastolic [Ca2+]i values obtained with 5FBAPTA in beating hearts are elevated by the concentrations of intracellular 5FBAPTA required for signal detection, the changes in [Ca2+]i observed in response to experimental interventions are qualitatively consistent with previous data.
Continuous inversion angiography
Authors: Roberts D. A. , Bolinger L. , Detre J. A. , Insko E. K. , Berge P. , Leigh J. S.
Published: March 2007

A subtractive time‐of‐flight technique for magnetic resonance anglography is described. In this approach, the arterial supply to an organ is inverted in a steady‐state fashion by applying off‐resonance irradiation in the presence of a linear magnetic field gradient. An angiogram is formed by subtracting an image acquired with arterial inversion from a control image acquired with no arterial inversion. A single coil is used to apply both the inversion and observation pulses. Intracranial angiograms obtained from normal volunteers using a two‐dimensional projective implementation of this technique at 1.5 T illustrate excellent small vessel detail and background suppression.
Synchronized inversion recovery‐spin echo sequences for precise in vivo T1 measurement of human myocardium: A pilot study on 22 healthy subjects
Authors: Walker P. M. , Marie P. Y. , Mezeray C. , Bessieres M. , Escanyé J. M. , Karcher G. , Danchin N. , Mattei S. , Villemot J. P. , Bertrand A.
Published: March 2007

An ECG‐triggered, two‐sequence MRI technique is proposed for the precise measurement of proton T1 relaxation times of the human myocardium at a field strength of 0.5 T. The combination of an inversion recovery (IR) sequence and a spin echo (SE) sequence is not new. It is, however, rarely used in quantitative in vivo cardiac studies. Our approach employs a synchronization of the 90° read pulse to the systolic period. In a study of 22 healthy volunteers, the globally measured T1 value was estimated to be 714 ± 23 ms. Four of the volunteers also underwent additional imaging scans for the purposes of reproducibility assessment. The T1 precision was found to be 3.9 ± 1.1% for the IR/SE combination and 16.9 ± 5.3% for a combination of SE sequences. Total imaging time for the IR and SE sequences was 19.2 ± 3.0 mins. The relative rapidity of this classic technique and the T1 precision obtained give this technique an obvious application in the discrimination of normal and diseased myocardium. In the same study, valuable supplementary tissue characterization is provided by T2, calculated from the SE sequence. T2 was evaluated to be 50 ± 3 ms.
N‐acetyl‐L‐aspartate and acetate 1H NMR signal overlapping under mild acidic pH conditions
Authors: Marti Magali , Labouesse Julie , Canioni Paul , Merle Michel
Published: March 2007

The pH dependence of methyl proton chemical shifts of acetate, acetoacetate, N‐acetyl‐L‐aspartate (NAA), and N‐acetyl‐L‐aspartyl‐L‐glutamate (NAAG) were studied from pH 3 to pH 9. Only slight shifts of acetoacetate, NAA, and NAAG methyl signals were observed, whereas the acetate signal was largely shifted as a result of the titration of its acidic function. At pH 4.7, acetate and NAA methyl signals overlapped, whereas at more acidic pH, the acetate signal appeared downfield when compared to that of NAA. Results are discussed in terms of spectra misinterpretation risks linked to uncontrolled sample pH, on the one hand, and in terms of pH control and contamination by exogenous acetate during perchloric acid cell extract preparation, on the other.
Metabolic changes in the rat brain after acute and chronic ethanol intoxication: A 31P NMR spectroscopy study
Authors: Denays R. , Chao S. L. , Mathur‐Devré R. , Jeghers O. , Frühling J. , Noël P. , Ham H. R.
Published: March 2007

In this work, 31P phosphorus NMR (31P NMR) studies of the brain have been conducted in rats acutely and chronically intoxicated with ethanol. In both groups, changes in levels of high‐energy phosphates were observed: increase of phosphocreatinine (PCr)/β AaTP and PCr/inorganic phosphate (Pi) in acute and long‐term ethanol exposure, and decrease of Piβ ATP after acute ethanol administration. These changes in high‐energy phosphates, indicative of a reduction of adenosine triphosphate (ATP) and PCr consumption (PCr+ ADP + H+ ATP + Cr; ATP ADP + Pi), suggest a reduction of cerebral metabolism both in acute and chronic ethanol exposure. In addition, in the group of rats chronically intoxicated with ethanol, there were variations in phosphodiester peak intensities (decrease of phosphomonoester (PME)/phosphodiester (PDE), increase of PDE/β ATP), suggesting increased breakdown of membrane phospholipids. These changes could provide a metabolic explanation for the development of cerebral atrophy in chronic alcoholism.
Use of a projection reconstruction method to decrease motion sensitivity in diffusion‐weighted MRI
Authors: Gmitro Arthur F. , Alexander Andrew L.
Published: March 2007

Diffusion‐weighted MRI is a clinically useful technique, but its utility is compromised by high sensitivity to patient motion. Use of radial‐scan data acquisition and projection reconstruction, rather than the conventional Fourier imaging method, can substantially reduce the sensitivity to global translational motion of the object. The basis of this concept and a demonstration of the technique in an animal imaging experiment are presented.
Changes in water diffusion and relaxation properties of rat cerebrum during status epilepticus
Authors: Zhong J. , Petroff O. A. C. , Prichard J. W. , Gore J. C.
Published: March 2007

Diffusion‐weighted (DW) imaging has been used to record changes associated with status epilepticus (SE) in rat brain. It was found that the apparent diffusion coefficient (ADC) of water in brain decreased14–18% during SE, and it fell a further 20–22% when the animals were sacrificed. The transverse decay time constant T2 showed corresponding reductions, but no significant changes were seen in relaxation times T1 or T2 values. Changes in ADC in status epilepticus are similar to those seen in stroke and ischemia but occur under very differ ret conditions of blood flow and metabolism.
Correction of phase effects produced by eddy currents in solvent suppressed 1H‐CSI
Authors: Roebuck Joseph R. , Hearshen David O. , O'Donnell Matthew , Raidy Tom
Published: March 2007

Accurate phasing of MRS spectra is often difficult unless time varying phase effects produced by gradient‐induced eddy currents that persist during data acquisition are eliminated. This effect is particularly problematic in 1H‐CSI spectra where frequency shifts produced by static field inhomogeneity and phase shifts produced by eddy currents combine. In this paper we present a method that corrects both shifts and eliminates manual phasing of individual CSI spectra typically required to recover a pure absorption line shape. The method uses a time domain phase correction derived from the ambient water signal acquired under identical conditions (i.e., acquisition parameters, gradient sequence) as the solvent‐sup pressed CSI data. Results from CSI experiments on phantoms and in vivo solvent suppressed 1H‐CSI spectra from normal human brain are presented demonstrating the capabilities of the technique.
Snapshot MRI with T2*‐weighted magnetization preparation
Authors: Bendel Peter
Published: March 2007

A method for encoding T2*‐dependent contrasts into the preparation period of magnetization‐prepared uitrafast MRI sequences is demonstrated and validated. The preparation block consists of a 90°—T—90°p sequence, where p is shifted by 90° for successive images. By adding two such successive images in quadrature, a combined image is obtained, in which the pixel intensities depend on the incoherent intravoxel dephasing that occurred during T in a way that is similar to the dependence on TE in a gradient‐echo sequence. The method could therefore be useful for applications that require very short repetition times for good temporal resolution, together with detection of T2*‐weighted contrasts, such as functional MRI in the brain.
Comparison of methods for the determination of absolute metabolite concentrations in human muscles by 31P MRS
Authors: Buchli Reto , Boesiger Peter
Published: March 2007

In order to determine metabolite concentrations in human skeletal muscles by in vivo 31P MRS, different quantification methods were analyzed with regard to the accuracy and reproducibility of results and the simplicity of handling. Each quantification method comprised a calibration strategy and a localization technique. Extensive in vivo and in vitro tests showed that homonuclear phantom‐based calibration strategies yielded significantly more accurate (lower systematic errors) and more reproducible (lower statistical errors) concentration estimates than heteronuclear strategies using internal water as a concentration standard. Additionally, the former strategies are easier to handle than the latter. Localization with the volume‐selective sequence lSlS yielded slightly more reproducible results than localization by surface coil. We conclude that phosphorus metabolite concentrations are determined most accurately with phantom‐based calibration strategies in combination with lSlS localization (measurement errors ≈ 5–7%).
Quantifying MRI geometric distortion in tissue
Authors: Sumanaweera Thilaka , Glover Gary , Song Samuel , Adler John , Napel Sandy
Published: March 2007

We present a method to quantify the MR field inhomogeneity geometric distortion to subplxel accuracy without using objects of known dimensions and without using an external standard such as CT. Our method may be used to quantify the geometric accuracy of MR images of anatomical structures of unknown geometry and also to test any geometry correction scheme. We have quantified the distortion in a tissue phantom and found the largest error to be approximately 2.8 pixels (1.8 mm) for B0 = 1.5 T, G = 3.13 mT/m and FOV = 160 × 160 × 70.7 mm3. We also found that our previously published correction technique reduced the largest error to 0.3 pixels (μ = 0.02 and σ = 0.07 pixels).
Simultaneous 31P NMR spectroscopy and EMG in exercising and recovering human skeletal muscle: Technical aspects
Authors: Vestergaard‐Poulsen P. , Thomsen C. , Sinkjær T. , Henriksen O.
Published: March 2007

The bioenergetics of human skeletal muscle can be studied by 31P NMR spectroscopy (31P‐MRS) and by surface electromyography (SEMG). Simultaneous 31P‐MRS and SEMG permit accurate and noninvasive studies of the correlation between metabolic and electrical changes in exercising and recovering human skeletal muscle, a relationship that is still poorly understood. This study describes the optimization of skeletal muscle 31P‐MRS in a whole‐body magnet, involving surface coil design, utilization of adiabatic radio frequency pulses and advanced time‐domain fitting, to the technical design of SEMG. A nonmagnetic ergometer was used for ankle dorsi‐flexions that activated only the anterior tibia1 muscle as verified by post exercise imaging. The coil design and the adiabatic sechltanh pulse improved sensitivity by 45% and 56% respectively, compared with standard techniques. Simultaneous electromyographic recordings did not deteriorate the NMR spectra. The VARPRO time domain fitting routine was very suitable for estimating 31P muscle spectra. With these methods it was possible to accurately estimate parameters describing metabolic and electrical changes during rest, exercise and the entire recovery period with a 20‐s time resolution on a standard 1.5 T whole‐body NMR scanner.
31P NMR spectroscopic studies of the effects of cyclophosphamide on perfused RIF‐1 tumor cells
Authors: Aiken Nanci R. , McGovern Kathy A. , Ng Cheng E. , Wehrle Janna P. , Glickson Jerry D.
Published: March 2007

To determine whether direct cellular effects of chemotherapy are responsible for 31P NMR spectral changes observed in treated tumors in vivo, RIF‐1 fibrosarcoma cells were examined in vitro before, during, and after treatment with 4‐hy‐droperoxycyclophosphamide (4‐HC), an activated form of cyclophosphamide. When RIF‐1 cells were treated with 4‐HC in a metabolically stable but nonproliferating state, the 31P NMR spectra were identical with those of untreated cells for up to 70 h. When actively proliferating RIF‐1 cells were treated with 4‐W, the intensities of the nucleotide triphosphate resonances, which increased linearly during control cell growth, remained constant for 50 h or longer. These studies demonstrate that the bioenergetic improvement observed following treatment of RIF‐1 tumors in vivo [S.‐J. Li, J.P. Wehrle, S.S. Rajan, R.G. Steen, J.D. Glickson, and J. Hilton, Cancer Res. 48, 4736 (1988)] does not result from direct effects of cyclophosphamide metabolites on RIF‐1 cell metabolism, but rather from indirect effects of treatment on tumor or host physiology. Key words: 31P NMR spectroscopy; RIF‐1 tumor cells; cyclophosphamide; cancer.
A Bayesian approach to subvoxel tissue classification in NMR microscopic images of trabecular bone
Authors: Wu Zhenyu , Chung Hsiao‐Wen , Wehrli Felix W.
Published: March 2007

NMR microscopy is currently being used as an investigational tool for the evaluation of micromorphometric parameters of trabecular bone as a possible means to assess its strength. Since, typically, the image voxel size is not significantly smaller than individual trabecular elements, partial volume blurring can be a major complication for accurate tissue classification. In this paper, a Bayesian segmentation technique is reported that achieves improved subvoxel tissue classification. Each voxel is subdivided either into eight subvoxels twice the original resolution, or up to four subvoxels along the transaxial direction and the subvoxels optimally classified as either bone or marrow. Based on a statistical model for partial volume blurring, the likelihood for the number of marrow subvoxels in each voxel can be computed on the basis of its measured signal. To resolve the ambiguity of the location of the marrow subvoxels, a Gibbs distribution is introduced to model the interaction between the subvoxels. Neighboring subvoxel pairs with the same tissue label are encouraged, and pairs with distinct labels are penalized. The segmentation is achieved by maximizing thea posterioriprobability of the label image using the block ICM (iterative conditional mode) algorithm. The potential of the proposed technique is demonstrated in real and synthetic NMR microscopic images.
Simulation of spatial and contrast distortions in keyhole imaging
Authors: Spraggins Thomas A.
Published: March 2007

Keyhole imaging is a scheme introduced to improve temporal resolution in dynamic contrast‐enhanced MRI by a factor of four or more. A “full” acquisition before contrast administration is followed by truncated acquisitions sensitive primarily to changes in image contrast. Simulations of the point‐spread functions that obtain, and their effect on contrast and spatial resolution, reveal significant degradation only for the smallest objects. Our simulations also address the feasibility of three‐dimensional keyhole imaging, and demonstrate a potential 16‐fold increase in temporal resolution. This suggests roles for keyhole imaging in conventional (nondynamic) precon‐trast and postcontrast studies and other applications.
True myocardial motion tracking
Authors: Fischer Stefan E. , McKinnon Graeme C. , Scheidegger Markus B. , Prins Wim , Meier Dieter , Boesiger P.
Published: March 2007

Myocardial tagging is a powerful tool for the assessment of in‐plane cardiac motion. However, for previous myocardial tagging techniques, the imaged slice is fixed with respect to the magnet coordinate system. Thus, images acquired at different heart phases do not always represent the same slice of the myocardium. A new myocardial tagging technique is presented, which takes the through‐plane motion into consideration. It involves tagging of the desired myocardial slice and applying a subtraction imaging technique to image just that part of the myocardium. The examination time can be reduced considerably by the acquisition of two one‐dimensionally tagged images. To increase the signal‐to‐noise ratio especially at later heart phases, variable imaging RF excitation flip angles are applied. To reduce motion artifacts a repetitive breathhold scheme was applied. in vivo results demonstrate that the tags can be accurately tracked within the entire heart period with a temporal resolution of 35 ms, even at a top basal level of the heart and right ventricle.
Three‐dimensional time‐of‐flight MR angiography using selective inversion recovery RAGE with fat saturation and ECG‐triggering: Application to renal arteries
Authors: Li Debiao , Haacke E. Mark , Mugler John P. , Berr Stuart , Brookeman James R. , Hutton Max C.
Published: March 2007

A three‐dimensional (3D), ECG‐triggered, selective inversion recovery (SIR) rapid gradient‐echo (RAGE) technique is proposed to obtain MR angiograms of the main renal arteries. By using the selective inversion recovery and fat saturation, the background is significantly suppressed while blood maintains a high signal intensity as compared with conventional 3D time‐of‐flight (TOF) MR angiography. The sequence is ECG‐triggered so that blood in‐flow is maximized during systole, and intravoxel dephasing and pulsatile flow artifacts are minimized by collecting data during diastole. As a result, vessel boundary blurring and ghosting artifacts due to background motion are dramatically reduced, and the conspicuity and lumen definition of the arteries are significantly improved. High‐quality MR angiograms of the main renal arteries with excellent blood/tissue contrast and suppression of motion artifacts have been consistently obtained for normal volunteers, with the length of visualization being 51 ± 07 mm for the left, and 57 ± 06 mm for the right renal arteries, significantly greater than using conventional 3D TOF pulse sequences. Statistical analysis was performed by using a one‐sided Student's t test. Key words: renal artery; MR angiography; three‐dimensional MR imaging.
A microscope slide probe for high resolution imaging at 11.7 tesla
Authors: Glover P. M. , Bowtell R. W. , Brown G. D. , Mansfield Peter
Published: March 2007

We present details of a novel RF coil for use in NMR microscopy. This coil is an inductively coupled surface coil which is built into a standard microscope slide. The coil is highly sensitive and is also designed so that the sample to be imaged can be viewed under an optical microscope. This facility allows comparison of optical and NMR micrographs, as well as accurate sample positioning. Using the slide coil, images of onion epidermal cells with an in‐plane resolution of 4.5 μm have been produced.
Localized proton spectroscopy of focal brain pathology in humans: Significant effects of edema on spin–spin relaxation time
Authors: Kamada Kyousuke , Houkin Kiyohiro , Hida Kazutoshi , Matsuzawa Hitoshi , Iwasaki Yoshinobu , Abe Hiroshi , Nakada Tsutomu
Published: March 2007

Localized proton nuclear magnetic resonance (NMR) spectroscopy of human brain in two common focal pathologies producing brain edema (peritumor edema and acute edema–tous ischemic stroke) was performed utilizing point resolved spectroscopy (PRESS). The spectra obtained from the pathological tissues were characterized by a reduced N‐acetyl‐as–partate (NAA) to total creatine (Cr) ratio (NAA/Cr) and high level of lactate. While the spin lattice relaxation time (T1) of the main metabolite resonances, namely, those of NAA, Cr, and choline containing compounds (Cho), showed values similar to those of normal brain, the spin‐spin relaxation time (T2) of these metabolites exhibited a dramatic shortening in pathological tissues. Serial postoperative measurements of T2 in two patients with peritumor edema showed a gradual recovery of the T2 shortening corresponding to improvement of the edema. The majority of localized spectroscopy studies in humans is performed using a sequence which utilizes spin echo signals with a fixed single echo time. Hence, the signal intensities of the metabolite resonances obtained are inherently T2 dependent. The current study underscores that cautious interpretation of clinical data with respect to metabolite quantification is warranted.
A novel eddy current compensation scheme for pulsed gradient systems
Authors: Wysong R. E. , Madio D. P. , Lowe Irving J.
Published: March 2007

We describe a modified pre‐emphasis network that uses a number of fixed time constants, only whose amplitudes are adjustable. In comparison with variable time constant pre‐emphasis networks we have used, the fixed time constant network is more effective and easier to set on the fly. We have further developed a method to set the fixed time constant network mathematically using a linear least squares technique.
Off‐resonance spin locking for MR imaging
Authors: Santyr Giles E. , Fairbanks E. Jefferson , Kelcz Frederick , Sorenson James A.
Published: March 2007

Off‐resonance spin locking is investigated as a low power method for achieving low field spin‐lattice relaxation contrast using high field clinical MR imaging systems (e.g., 1.5 tesla). Spin‐lattice relaxation times and equilibrium magnetizations in the off‐resonance rotating frame (T β) were measured for tissue‐mimicking phantom materials as a function of the ratio of the amplitude to the resonance offset of the spin‐locking pulse (f1/Δ). The phantom materials consisted of vegetable oil to simulate fat and two different gels containing 2% and 4% agar to simulate nonfatty tissues with different macromolecular compositions. These measurements were used to verify a signal strength equation for a multislice off‐resonance spin‐locking technique implemented on a clinical MR imaging system operating at 1.5 tesla. Although the oil showed little change in image contrast with increasing f1/Δ, the two gels demonstrated a strong variation which provided improved discrimination compared to T1‐weighted imaging. Off‐resonance spin locking is suggested as a method for improving delineation of breast lesions and a preliminary clinical example from a patient volunteer is presented.
Intravascular MR imaging in a porcine animal model
Authors: Martin Alastair J. , Henkelman R. Mark
Published: March 2007

An opposed solenoid intravascular receiver coil is used in a series of five pigs to generate images of the aorta. Severe ghosting artifacts are shown to occur if the motion of the intravascular coil is not restricted with respect to the vessel wall. Restricting coil motion and saturating the blood signal superior to the imaging plane are shown to produce high quality MR images. Layers within the artery wall could be discerned and the results of the in vivo analysis compared favorably with high resolution MR images of the corresponding tissue after excision.
Conditions for achieving artifact‐free T preparation in MRI
Authors: Zhang Weiguo
Published: March 2007

A theoretical analysis of the stimulated gradient‐echo method for achieving T‐preparation in magnetic resonance imaging is performed. The results show that T‐weighting without the “shading effect” due to magnetic field variation can only be achieved if the dephasing requirement of the stimulated echo is satisfied. It is also shown that the effective echo‐time for T‐weighting depends on the polarities of the dephasing and the rephasing gradient pulses that generate the stimulated echo. The results from the theoretical analysis are experimentally validated.
A new 3D localization technique using quadratic field gradients
Authors: Wu Ed X. , Johnson Glyn , Hilal Sadek K. , Cho Zang‐Hee
Published: March 2007

A new method of 3D spatial selection using pulsed quadratic field gradients is presented. Like previous pulsed quadratic gradient methods the new method requires only two RF pulses to achieve selection in three dimensions. Unlike previous methods, however, precise alignment of linear and quadratic gradient coils is not required. This characteristic allows the method to be used with small or surface quadratic gradient coils. The method is demonstrated on phantoms and rats.
Glucose metabolism in RIF‐1 tumors after reduction in blood flow: An in Vivo13C and 31P NMR study
Authors: Bhujwalla Zaver M. , Shungu Dikoma C. , Chatham John C. , Wehrle Janna P. , Glickson Jerry D.
Published: March 2007

Low pH appears to enhance the effectiveness of therapeutic hyperthermia. 13C and 31P NMR spectroscopy have been employed to examine the possibility that elevating glucose in a solid tumor while simultaneously reducing tumor blood flow would induce a more profound acidosis than either treatment alone. When blood flow in RIF‐1 tumors was acutely reduced by administration of hydralazine and additional glucose was delivered locally by intratumoral injection, tumor acidosis (as determined by 31P NMR spectroscopy) during the period of reduced blood flow was not enhanced, relative to administration of hydralarine alone. Tumor NTP/P1 ratios decreased significantly within 20 min of hydralazine administration, whether or not glucose was injected, although NTP/P1 ratios were slightly higher in tumors that received extra glucose. Tumor lactate concentrations were not significantly different in glucose‐supplemented tumors, despite glucose concentratlons that were 4 to 5 times higher. When the added glucose was labeled with 13C, no correlation was detected between the pH in an individual tumor and the intensity of the 3‐[13C]‐lactate resonance in the same tumor.
Quantitation of lipid in biological tissue by chemical shift magnetic resonance imaging
Authors: Wong Wai‐Fan , Northrup S. Robert , Herrick Richard C. , Glornbicki Alan P. , Wood R. Patrick , Morrisett Joel D.
Published: March 2007

A method combining several previously used approaches is described for the rapid, accurate quantitation of the fat content of biological tissue based on chemical shift images (CSI) corrected for magnetic field inhomogeneity, and compensated for T1, and T2, effects. The gravimetrically determined lipid content of fatty tissues (pork fat, rabbit and human liver) that had been differentially depleted of lipid by chloroform extraction correlated well (r = 0.99) with the lipid image intensities of the respective tissues. This multi‐point CSI method was used to quantitate lipid in fresh fatty human liver tissue (wet and dry) containing varying amounts of lipid. Plots of integrated lipid intensity versus tissue lipid content gave straight parallel lines for hydrated (r = 0.94) and dehydrated (r = 0.98) tissues, permitting determination of a proportionality constant for measuring absolute amounts of lipid present in a specific biological tissue. These results suggest the feasibility of using the method in vivo for absolute quantitation of lipid in tissues of agricultural (e.g. pork, beef) and medical (e.g. human liver) interest.
A quantitative interpretation of IVIM measurements of vascular perfusion in the rat brain
Authors: Henkelman R. Mark , Neil Jeffrey J. , Xiang Qing‐San
Published: March 2007

Pulsed gradient spin echo (PGSE) sequences have been used to measure the signal loss of 19F in perfluorinated hydrocarbon blood substitutes moving within the vasculature of the rat brain in the experimental conditions of the study. The signal loss is not characterized by a single apparent pseudodiffusion coefficient. A simple vascular network model based on self‐similarity hss been used to calculate the shape of the signal loss. Excellent agreement with the experiment has been obtained showing that the IVlM measurements are sensitive to flow over a wide range of vessel diameters and flow rates. This model of vascular structure may serve well for other MR measurements that are sensitive to perfusion.
A new method for proton detection of H217O with potential applications for functional MRI
Authors: Ronen Itamar , Navon Gil
Published: March 2007

A method is presented for the sensitive detection of minute amounts of H217O. The method is based on the increase of the T2 of the water protons following an irradiation of the 17O resonance frequency, due to the partial or full decoupling of the 1H‐17O spin‐spin interaction. It is demonstrated that when 17O concentrations are low, full decoupling is achieved, and at short echo times the increase in the amplitude of the proton echo signal is proportional to the 17O content of the sample. The potential of the method for indirect 17O imaging is substantiated by a simple one dimensional projection of cylindrical phantoms containing various 17O concentrations. Using interleaved sequences with and without 17O decoupling, errors due to time dependent effects are minimized.
The Effect of Respiration on the Contrast and Sharpness of Liver Lesions in MRI
Authors: Butts Kim , Riederer Stephen J. , Ehman Richard L.
Published: March 2007

This work demonstrates the effects of through‐plane motion due to respiration on contrast and sharpness of liver lesions in MRI. The effects of slice coverage with and without such respiratory motion is also reported. This work is comprised of two parts: a theoretical prediction of liver‐lesion contrast and blur with and without respiration and an experimental validation using gel phantoms of the predicted results. Both theory and experiment show a loss of contrast, increasing with amplitude of the peak‐to‐peak motion. The loss of contrast for a 5‐mm lesion at normal respiration of 15 mm peak‐to‐peak superior‐inferior motion is ≈ 10% with a low order sorted respiratory ordered phase encoding acquisition and ≈ 50% for an unsorted acquisition. Lesion blur is greatest for the low order sorted acquisition while the unsorted and high sort acquisitions maintain edge definition. Breath‐hold imaging is potentially superior to nonbreath‐hold imaging in liver lesion contrast and edge definition, but is more sensitive to inadequate slice coverage.
Diffusion Weighting by the Trace of the Diffusion Tensor within a Single Scan
Authors: Mori Susumu , Van Zijl Peter C. M.
Published: March 2007

The anisotropy of the water diffusion tensor inside brain causes contrast in diffusion images, which depends on the relative orientation of the diffusion gradients and the subject. Because the trace of a tensor is invariant upon rotation, measurement of this trace can reduce the orientation effect. A family of imaging pulse sequences is presented in which the signal intensity is weighted by the trace of the diffusion tensor in a single scan. The methods are demonstrated for chicken gizzard in several orientations with respect to the gradient frame of reference, and for ischemic injury in cat brain after middle cerebral artery occlusion. The sensitivity of the techniques to the presence of background gradients is measured and discussed in detail. As a result, pulse sequences are suggested that provide reliable diffusion constants in both homogeneous and inhomogeneous magnetic fields. The efficiency of the techniques for clinical application is also evaluated.