bluuebunny/crossref_metadata_2025 huggingface dataset:
Reversible, reproducible reduction of brain water apparent diffusion coefficient by cortical electroshocks
Authors: Zhong Jianhui , Petroff Ognen A. C. , Pleban Lisa A. , Gore John C. , Prichard James W.
Published: March 2007

Rat brains were imaged after cortical electroshock pulse trains (1 ms pulses at 100 Hz) of varying durations (0.1–10 s), with diffusion‐weighted echo planar imaging sequences at 2.0 T. The apparent water diffusion coefficient (ADC) decreased after either single or repeat electroshock trains. ADC reductions were observed within 6 s after the first shock. The size of the affected area of the brain increased in subsequent images during the 1st min after a 10‐pulse (0.1 s) train, and also increased with the duration of electroshock trains. ADC reduction was reproducible in extent and time course after single 10‐shock trains and was reversible. In the affected pixels the mean ADC reduction was 4% for a single shock train (0.1 s), and 7–8% for trains repeated once a minute, independent of electroshock train duration. The results indicate that neuronal activity associated with electrostimulation may be monitored with water diffusion measurements, and they may be useful for measuring the severity of seizure activity in patients with medically intractable epilepsy.
Characterization and prediction of gradient acoustic noise in MR imagers
Authors: Hedeen Robert A. , Edelstein William A
Published: March 2007

Gradient acoustic noise has been measured and characterized for an epoxy‐potted, shielded gradient assembly in a 1.5 T MRI system. Noise levels vary by 10 dB or more as a function of longitudinal position in the scanner and reflect the pattern of forces applied to the gradient assembly. The noise level increases slightly (1–3 dB) with a patient in the scanner. The spectrum of the noise is similar (but not identical) to the spectrum of the input signal. A gradient‐pulse‐to‐acoustic‐noise transfer function was obtained by using a white noise voltage input to the gradient system. The transfer function enabled us to accurately predict acoustic noise output for a pulse sequence consisting of a series of trapezoidal pulses on a single axis and for a clinical fast spin echo sequence with gradients present on all three axes.
Multinuclear solid‐state nmr spectroscopy of envelopes from virgin and explanted silicone breast prostheses: An exploratory study
Authors: Picard Frédéric , Alikacem Nadir , Guidoin Robert , Auger Michèle
Published: March 2007

The structure and dynamics of silicone envelopes from virgin and explanted breast prostheses have been studied by 1H, 13C, and 29Si high‐resolution solid‐state NMR spectroscopy. The results indicate that this combination of techniques can be used to unambiguously identify the presence of methyltri‐fluoropropylsiloxane units and diphenylsiloxane units at concentrations of about 2.0% and 3.5%, respectively, in some of the envelopes. The presence of about 10% silica also was detected and a trace of lipids was found in the explanted silicone envelopes. We have also measured the proton T2 and T1 relaxation times of the envelopes to characterize the dynamics of the silicone in the envelopes.
Temporal lobe epilepsy: Correlation of proton magnetic resonance spectroscopy and 18F‐fluorodeoxyglucose positron emission tomography
Authors: Lu Dongfeng , Margouleff Claude , Rubin Eric , Labar Douglas , Schaul Neil , Ishikawa Tatsuya , Kazumata Ken , Antonini Angelo , Dhawan Vijay , Hyman Roger A. , Eidelberg David
Published: March 2007

Proton magnetic resonance spectroscopy (MRS) has demonstrated reduction of N‐acetylaspartate (NAA) in the epileptogenic temporal lobe. However, the correlation of NAA reduction with cerebral metabolic abnormalities is unknown in temporal lobe epilepsy (TLE). Proton MRS and 18F‐fluorodeoxyglucose positron emission tomography (FDG/PET) were used to study 12 unilateral TLE patients with medically intractable seizures and 26 age‐matched healthy volunteers. The epileptogenic temporal lobe of each patient was determined by both electroencephalography and FDG/PET. The NAA/choline‐plus‐creatine (NAA/(Cho+Cr)) ratio correlated significantly with the interictal glucose metabolism (r = 0.54, P 0.01) in 12 TLE patients. The mean NAA/(Cho + Cr) ratio in the epileptogenic temporal lobe was significantly less than that in the contralateral side (P < 0.01), and less than that in normal control temporal lobes (P < 0.0001). These results suggest that quantitative MRS abnormalities reflect underlying metabolic pathology in TLE.
Histological and 1H magnetic resonance spectroscopic imaging analysis of quinolinic acid‐induced damage to the rat striatum
Authors: Strauss Ian , Fernandez Erik J. , Williamson John M. , Bertram Edward H. , Lothmant Eric W.
Published: March 2007

NAA has been described as a neuron‐specific compound. NAA levels as determined by magnetic resonance spectroscopic imaging (MRSI) have been used to determine degree of neuronal loss in several neurological diseases, but there has been limited work to document the accuracy and reliability of this technique. This study addresses this question quantitatively with histological analysis of cell viability and tissue shrinkage in quinolinic acid (QA)‐induced damage of the rat striatum compared with 1H MRSI measurement of N‐acetyl aspartate (NAA) as a noninvasive measure of neuronal loss. Both 'H MRSI and histology detect damage to the lesioned striatum; however, there are differences in the degree of damage as assessed by the two methods. Although partial‐volume effects and tissue shrinkage may decrease the sensitivity of MR to such damage, the sparing of axons by QA may be another important factor in the differences in assessment. These results indicate that further studies of NAA metabolism and its distribution within neurons are warranted.
In vivo measurement of T2 distributions and water contents in normal human brain
Authors: Whittall Kenneth P , Mackay Alex L. , Graeb Douglas A. , Nugent Robert A. , Li David K. B. , Paty Donald W.
Published: March 2007

Using a 32‐echo imaging pulse sequence, T2 relaxation decay curves were acquired from five white‐ and six gray‐matter brain structures outlined in 12 normal volunteers. The water contents of white and gray matter were 0.71 (0.01) and 0.83 (0.03) g/ml, respectively. All white‐matter structures had significantly higher myelin water percentages (signal percentage with T2 between 10 and 50 ms) than all gray‐matter structures. The range in geometric mean T2 of the main peak for both white and gray matter was from 70 to 86 ms. T2 distributions from the posterior internal capsules and splenium of the corpus callosum were significantly wider (width is related to water environment inhomogeneity) than those from any other white‐ or gray‐matter structures. Thus, quantitative measurement and analysis of T2 relaxation reveals differences in brain tissue water environments not discernible on conventional MR images. These differences may make short T2 components reliable markers for normal myelin.
Diffusion of solutes in agarose and alginate gels: 1H and 23Na PFGSE and 23Na TQF NMR studies
Authors: Lundberg Peter , Kuchel Philip W
Published: March 2007

Cells immobilized in gels experience potential metabolic restrictions in the form of reduced diffusion rates of metabolites and ions and their possible selective adsorption on the gel matrix. Diffusion and relaxation characteristics of common solutes in agarose and barium alginate gels were investigated at 37°C by using 1H PFGSE and 23Na TQF NMR spectroscopy. Glucose, glycine, alanine, lactate, sodium ions, and HDO were studied. There were no selective interactions between any of the metabolites and the gel materials but the diffusion coefficients were uniformly reduced. The effects of metabolite diffusion and utilization, in gel beads and threads containing cells, were simulated by using a reaction diffusion model incorporating the measured diffusion coefficients. Metabolism is expected to be very significantly limited by diffusion of solutes to and from the cells that are centrally located within gel threads or spheres of radius ∼2.0 mm, which is a cornmonly used size.
T1ρ of protein solutions at very low fields: Dependence on molecular weight, concentration, and structure
Authors: Virta Anette , Komu Markku , Kormano Martti
Published: March 2007

The effect of molecular weight, concentration, and structure on 1/T1ρ, the rotating frame relaxation rate, was investigated for several proteins using the on‐resonance spin‐lock technique, for locking fields B1 < 200 μT. The measured values of 1/T1ρ, were fitted to a simple theoretical model to obtain the dispersion curves 1/T1ρ(ω1) and the relaxation rate at zero B1 field, 1/T1ρ,(O). 1/T1ρ, was highly sensitive to the molecular weight, concentration, and structure of the protein. The amount of intra‐ and intermolecular hydrogen and disulfide bonds especially contributed to 1/T1ρ. In all samples, 1/T1ρ(O) was equal to 1/T2ρ measured at the main magnetic field Bo = 0.1 T, but at higher locking fields the dispersion curves mono‐tonically decreased. The results of this work indicate that a model considering the effective correlation time of molecular motions as the main determinant for 1/T1ρ relaxation in protein solutions is not valid at very low B1 fields. The underlying mechanism for the relaxation rate 1/T1ρ at B1 fields below 200 μT is discussed.
Estimation of water extraction fractions in rat brain using magnetic resonance measurement of perfusion with arterial spin labeling
Authors: Silva Afonso C. , Zhang Weiguo , Williams Donald S. , Koretsky Alan P , Koretsky Alan P
Published: March 2007

The model used for calculating perfusion by MRI techniques that use endogenous water as a tracer assumes that arterial water is a freely diffusible tracer. Evidence shows that this assumption is not valid in the brain at high blood flow rates, at which movement of water into and out of the microvasculature becomes limited by diffusion across the blood‐brain barrier. In this work, the arterial spin‐labeling technique is used to show that fraction of arterial water that is dependent on blood flow rate remains in the vasculature and does not exchange with brain tissue water. By using perfusion measurements without and with magnetization transfer (MT) effects, one can distinguish arterial label that exchanges into tissue because blood has much smaller MT than brain tissue. Using this technique, the extraction fraction for water is measured in the rat brain at various cerebral blood flow rates. At high flow rates (‐5 ml/g/min), the extraction fraction for water is found to be about 45% in rat brain. Disruption of the blood‐brain barrier with D‐mannitol caused an increase in the extraction fraction for water. It was possible to form an image related to the extraction fraction for water. The ability to estimate the amount of vascular water exchanging with tissue water by MRI may represent a noninvasive approach to detect the integrity of the blood‐brain barrier.
Measurement of strain exerted on blood vessel walls by double‐quantum‐filtered 2H NMR
Authors: Sharf Yehuda , Akselrod Solange , Navon Gil
Published: March 2007

In this study 2H double quantum‐filtered (DQF) NMR spectros‐copy of water molecules inside the blood vessel wall was used to monitor structural changes after the application of strain. This method was applied to a variety of bovine large blood vessels, including aorta, coronary artery, carotid artery, and vena cava. Inducing strain in coronary arteries by inflating them by air or water revealed dramatic changes in the 2H DQF spectral lineshapes. Uniaxial extensions of longitudinal and circumferential strips led to distinct spectral responses for the different blood vessels. While the spectral lineshapes for the coronary and the carotid arteries were very sensitive to longitudinal strain, they were not affected by circumferential stretching. For the aorta there were no changes in the line‐shape upon strain. For the vena cava a large effect of strain was found that was independent of the choice of elongation axis. The effect of elongation on the spectra was assigned to the outer layer, the adventitia, whereas spectral lineshapes that originated from inner layers, media and intima, were almost insensitive to strain. Angular dependence experiments proved the presence of macroscopic order in the strained state superimposed on a broad distribution of the local directors. Our results demonstrate the sensitivity of 2H DQF NMR as a probe for dynamic processes occurring on the microscopic scale in a molecular network. Key words: blood vessels; strain; double‐quantum; 2H NMR.
An fem approach for the characterization of the RF field homogeneity at high field
Authors: Guclu Cecil , Kashmar George , Nalcioglu Orhan , Hacinliyan Avadis
Published: March 2007

High field magnetic resonance offers new opportunities because of its high SNR and better spectral resolution for MRI and MRS. However, new problems also emerge at high field. As the field strength increases, the wavelength in the tissue becomes shorter and comparable with the body dimensions. This perturbs the field and also causes standing waves within the patient as a result of the impedance mismatching at the tissue interfaces. Due to the Complexity of the boundary conditions and the solution of Maxwell's equations, an exact analytical calculation for a loaded RF resonator has not been possible. In this paper, we present a birdcage coil simulation study based on a 3D finite element method (FEM) model for the characterization of the field within the tissue. First, the accuracy of the FEM solutions is validated by the 2D analytical solutions at 64 and 223 MHz. In these solutions, the frequency dependence of the conductivity and permittivity is also taken into account. Then, a more realistic 3D model is studied. The results are compared with the experimental measurements. It is shown that the 3D model makes it possible to explore the effects of the end rings in the presence of a tissue sample inside the coil.
Unsupervised, automated segmentation of the normal brain using a multispectral relaxometric magnetic resonance approach
Authors: Alfano Bruno , Brunetti Arturo , Covelli Eugenio M. , Quarantelli Mario , Panico Maria Rosaria , Ciarmiello Andrea , Salvatore Marco
Published: March 2007

The purpose of this study was the development and testing of a method for unsupervised, automated brain segmentation. Two spin‐echo sequences were used to obtain relaxation rates and proton‐density maps from 1.5 T MR studies, with two axial data sets including the entire brain. Fifty normal subjects (age range, 16 to 76 years) were studied. A Three‐dimensional (3D) spectrum of the tissue voxels was used for automatic segmentation of gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF) and for calculation of their volumes. Accuracy and reproducibility were tested with a three‐compartment phantom simulating GM, WM, and CSF. In the normal subjects, a significant decrease of GM fractional volume and increased CSF volume with age were observed (P < 0.0001), with no significant changes in WM. This multi‐spectral segmentation method permits reproducible, operator‐independent volumetric measurements.
A dual approach for minimizing false lesion classifications on magnetic resonance images
Authors: Bedell Barry J. , Narayana Ponnada A , Wolinsky Jerry S.
Published: March 2007

Segmentation methods based on dual‐echo MR images are generally prone to significant false lesion classifications. We have minimized these false classifications by (1) improving the lesion‐to‐tissue contrast on MR images by developing a fast spin‐echo sequence that incorporates both cerebrospinal fluid signal attenuation and magnetization transfer contrast and (2) including information from MR flow images. Studies on patients with multiple sclerosis indicate that this dual approach to tissue segmentation reduces the volume of false lesion classifications by an average of 87%.
An analytical model of restricted diffusion in bovine optic nerve
Authors: Stanisz Greg J. , Wright Graham A. , Henkelman R. Mark , Szafer Aaron
Published: March 2007

An analytical model of restricted diffusion in bovine optic nerve is presented. The nerve tissue model is composed of two different objects: prolate ellipsoids (axons) and spheres (glial cells) surrounded by partially permeable membranes. The free diffusion coefficients of intracellular and extra cellular water may differ. Analytical formulas for signal loss due to diffusion in the pulsed gradient spin echo (PGSE) experiment for this tissue model are derived. The model is fitted to experimental data for bovine optic nerve. The obtained model parameters are shown to be reasonable. The model describes all of the characteristics of the PGSE data: anisotropy, upward curvature of decay curves, and diffusion time dependence. The validity and sensitivity of the model are also discussed.
Intravascular magnetic resonance imaging using a loopless catheter antenna
Authors: Ocali Ogan , Atalar Ergin
Published: March 2007

Recently, intravascular catheter probes have been developed to increase signal‐to‐noise ratio (SNR) for MR imaging of blood vessels. Miniaturization of these catheter probes without degrading their performances is very critical in imaging small vessels such as coronary arteries. Catheter coils have a loop incorporated in their structure and have limitations in physical dimensions and electromagnetic properties. The use of a loopless intravascular catheter antenna is proposed to overcome these problems. The catheter antenna is essentially a dipole, which makes a very thin diameter possible, and its electronic circuitry can be placed outside the blood vessels without performance degradation. The theoretical foundation for the design and operation of the catheter antenna is presented. Several catheter antennae, as small as 1.5 French, were constructed and tested on phantoms and rabbits with great success. The catheter antenna has a simple structure and is easy to design, implement, and operate.
Cerebrospinal fluid‐suppressed high‐resolution diffusion imaging of human brain
Authors: Falconer James C. , Narayana Ponnada A
Published: March 2007

A cerebrospinal fluid (CSF)‐suppressed flow‐attenuated inversion recovery (FLAIR) double‐shot diffusion echo‐planar imaging (EPI) sequence was developed and used, along with a non‐CSF‐suppressed version of the sequence, to determine the extent of the contribution of CSF partial‐volume averaging to the apparent diffusion coefficients (ADCs) of normal human brain in vivo. Regional analysis indicates that cortical gray matter and parenchymal tissues bordering the ventricles are most affected by CSF contamination, leading to elevated ADC values. Only slight differences in gray‐ and white‐matter average ADCs were detected after CSF suppression. The human brain average ADCs calculated from high‐resolution CSF‐sup‐pressed diffusion‐weighted images in these studies are similar to those reported in animals. FLAIR diffusion sequences remove CSF as a source of error in ADC determination and ischemic lesion discrimination in diffusion‐weighted images (DWI) and ADC maps.
Techniques for high‐speed cardiac magnetic resonance imaging in rats and rabbits
Authors: Rehwald Wolfgang G. , Reeder Scott B. , McVeigh Elliot R. , Judd Robert M
Published: March 2007

Progress in research on hypertension, heart failure, aging, post‐infarct remodeling, and the molecular basis of cardiovascular diseases in general has been greatly facilitated in recent years by the development of specialized small‐mam‐mal models by selective breeding and/or genetic alteration. Routine noninvasive evaluation of cardiac function and perfusion in these animals models, however, is difficult using existing methods. In principle, MRI can be used for this purpose, but in practice this is difficult because of problems related to RF coils, cardiac gating, and imaging pulse sequences. In this article, solutions to these problems are described that have allowed us to use MRI to routinely image the hearts of rats and rabbits. Specifically described are four RF coils, cardiac gating schemes, and an imaging pulse sequence specially designed for cardiac imaging in these animals on a 4.7 T Omega chemical‐shift imaging (CSI) spectrometer. These techniques can be used to obtain, within 2 min, eight double‐oblique short‐axis images of the rat at different cardiac phases with 200 × 400μm in‐plane resolution and a slice thickness of 2 mm. Moreover, myocardial tissue tagging can be performed with tag thicknesses and separations comparable to those used routinely in humans. The technical information is presented in sufficient detail to allow researchers at other sites to reproduce the results. This information should facilitate the use of MRI for the noninvasive examination of cardiac function and perfusion, which can be combined with other established techniques for the study of cardiovascular disease in specialized animal models.
Decoupled automated rotational and translational registration for functional MRI time series data: The dart registration algorithm
Authors: Maas Luis C. , deB. Frederick Blaise , Renshaw Perry F.
Published: March 2007

A rapid, in‐plane image registration algorithm that accurately estimates and corrects for rotational and translational motion is described. This automated, one‐pass method achieves its computational efficiency by decoupling the estimation of rotation and translation, allowing the application of rapid cross‐correlation and cross‐spectrum techniques for the determination of displacement parameters. k‐space regridding and modulation techniques are used for image correction as alternatives to linear interpolation. The performance of this method was analyzed with simulations and echo‐planar image data from both phantoms and human subjects. The processing time for image registration on a Hewlett‐Packard 735/125 is 7.5 s for a 128 × 128 pixel image and 1.7 s for a 64 × 64 pixel image. Imaging phantom data demonstrate the accuracy of the method (mean rotational error, −0.09°; standard deviation = 0.17°; range, −0.44° to + 0.31°; mean translational error = −0.035 pixels; standard deviation = 0.054 pixels; range, −0.16 to + 0.06 pixels). Registered human functional imaging data demonstrate a significant reduction in motion artifacts such as linear trends in pixel time series and activation artifacts due to stimulus‐correlated motion. The advantages of this technique are its noniterative one‐pass nature, the reduction in image degradation as compared to previous methods, and the speed of computation.
Normal myocardial perfwsion assessed with multishot echo‐planar imaging
Authors: Schwitter Jürg , Debatin Jorg F. , von Schulthess Gustav K. , McKinnon Graeme C.
Published: March 2007

A new magnetic resonance imaging strategy is presented for accessing myocardial perfusion. Most previous work has relied on using T1‐weighted fast gradient‐echo imaging to monitor dynamically the signal changes during the passage of a contrast media bolus. However, the gradient‐echo approach is limited by an inability to image the entire heart with adequate temporal resolution.

This paper focuses on a electrocardiogram‐gated multishot echo‐planar imaging sequence, using the simple strategy of using the intrinsic T1 weighting produced by a repetition time equal to the heart period. To quantitate the sequence's performance with respect to normal myocardial perfusion, seven volunteers were imaged, each with three different doses of the contrast medium gadolinium diethylenetriamine penta‐acetic acid (Gd‐DTPA). The first‐pass dynamics of the contrast were quantified in 13 regions per heart for each examination.

In all volunteers, the complete heart could be covered, with five to seven slices, every two heartbeats. Enhancement was homogeneous throughout the left ventricular myocardium, with an enhancement of approximately 50% for the optimum contrast dose of 0.05 mmol/kg Gd‐DTPA.
Prospective adaptive navigator correction for breath‐hold MR coronary angiography
Authors: McConnell Michael V. , Khasgiwala Vaibhav C. , Savord Bernard J. , Chen Ming Hui , Chuang Michael L. , Manning Warren J , Edelman Robert R.
Published: March 2007

Current MR coronary angiography (MRCA) methods use breath‐holding to minimize respiratory motion. A major limitation to this technique is misregistration between imaging slices due to breath‐hold variability. Prospective adaptive correction of image location using real‐time navigator measurement of diaphragm position is a potential method for improving slice registration in breath‐hold MRCA. Ten subjects underwent MRCA using an ECG‐gated, fat‐suppressed, segmented k‐space, gradient‐echo sequence. Transverse and coronal images were acquired using standard breath‐holding with and without prospective navigator correction. Breath‐hold MRCA with prospective navigator correction resulted in a 47% reduction in craniocaudal slice registration error compared to standard breath‐holding (0.9 ± 0.2 mm versus 1.7 2 0.4 mm, P = 0.04). Prospective adaptive navigator correction of image location significantly improves slice registration for breath‐hold MRCA and is a promising motion correction technique for cardiac MR.
Magnetization and diffusion effects in NMR imaging of hyperpolarized substances
Authors: Gao Jia‐Hong , Lemen Lisa , Xiong Jinhu , Patyal Baldev , Fox Peter T.
Published: March 2007

The special magnetization characteristics of hyperpolarized noble gases have led to an interest in using these agents for new MRI applications. In this note, the magnetization effects and NMR signal dependence of two noble gases, 3He and 129Xe, are modeled across a range of gradient‐echo imaging parameters. Pulse‐sequence analysis shows a wide variation in optimum flip angles between imaging of gas (e.g., 3He or 129Xe) in air spaces (e.g., trachea and lung) and in blood vessels. To optimize imaging of the air spaces, it is also necessary to reduce the otherwise substantial signal losses from diffusion effects by increasing voxel size. The possibility of using hyperpolarized 129Xe for functional MRI (fMRI) is discussed in view of the results from the blood flow analysis. The short‐lived nature of the hyperpolarization opens up new possibilities, as well as new technical challenges, in its potential application as a blood‐flow tracer.
Peak assignment in localized 1H MR spectra of human muscle based on oral creatine supplementation
Authors: Kreis Roland , Koster Markus , Kamber Matthias , Hoppeler Hans , Boesch Chris
Published: March 2007

1H‐MR spectra of human skeletal muscle feature peak splittings due to dipolar coupling. Quantitative difference spectroscopy in a double‐blind cross‐over trial testing oral creatine supplementation revealed that most of the resonances affected by dipolar coupling can be assigned to creatine and/or phosphocreatine. The assignment was performed in two different skeletal muscles and confirmed by measurements at the magic angle where dipolar splittings vanish. Numerical spectral simulations revealed that the observed spectra are consistent with partly‐averaged dipolar coupling among methylene and methyl protons of (phospho)creatine. The possible nature of the molecular dynamics leading to incomplete dipolar averaging is discussed.
Simultaneous 3D NMR spectroscopy of proton‐decoupled fluorine and phosphorus in human liver during 5‐fluorouracil chemotherapy
Authors: Gonen Oded , Murphy‐Boesch Joseph , Li Chun‐Wei , Padavic‐Shaller Kristin , Negendank William G. , Brown Truman R.
Published: March 2007

Simultaneous acquisition of 1H‐decoupled 31P and 19F 3D CSI is demonstrated in the liver of a patient undergoing 5‐fluorouracil chemotherapy. Both 31P and 19F shared the same voxel size (64 or 27 ml), bi‐level 1H‐decoupling and 0.35 s TR. The measurements were done in a 1.5 Tesla clinical imager with three radio‐frequency (RF) channels and a triple‐tuned surface‐coil. The overall MRI and MRS examination time was under 90 min. Simultaneous acquisition of 31P and 19F permits localized study of the influence of hepatic metabolism on the uptake and catabolism of fluoropyrimidine drugs without extra measurement time or higher SAR.
Neuronal nitric oxide synthase mutant mice show smaller infarcts and attenuated apparent diffusion coefficient changes in the peri‐infarct zone during focal cerebral ischemia
Authors: Zaharchuk Greg , Hara Hideaki , Huang Paul L. , Fishman Mark C. , Moskowitz Michael A. , Jenkins Bruce G. , Rosen Bruce R.
Published: March 2007

Diffusion‐weighted MRI at 2 T was used to monitor and assess tissue damage after permanent middle cerebral artery occlusion (MCAO) in wild‐type (WT) and mice deficient in nitric oxide synthase gene expression (nNOS−). The ischemic lesion was evaluated 3 h after occlusion and subdivided into the lesion core and peri‐infarct zone based on the magnitude of the apparent diffusion coefficient (ADC) change. Infarct volume, measured by using histochemical staining 24 h after MCA occlusion, correlated best with MRI infarct volume as assessed by an ADC threshold of 25% decrease from baseline at 3 h. For ADC thresholds of greater than 25% decrease, lesion size was not significantly different in nNOS− and WT mice. However, brain tissue showing ADC decreases of 10–25% was significantly smaller in the ipsilateral hemisphere of mutants (27 · 2% and 21 · 2% in WT and nNOS−, respectively; P < 0.05). These findings occurred independently of infarct volume and are consistent with a smaller peri‐infarct zone in nNOS− mice. We postulate that the smaller peri‐infarct zone is a reflection of less severe metabolic disturbance after ischemia in nNOS− mice, possibly related to diminished production of nitric oxide (NO) or a related product. We conclude that magnetic resonance techniques previously used to assess ischemic damage in larger animals can be extended to the mouse, raising the possibility that the molecular mechanisms leading to ischemic damage can be examined by using genetically engineered mice.
The sensitivity of low flip angle RARE imaging
Authors: Alsop David C.
Published: March 2007

It is demonstrated that the stability of the Carr‐Purcell‐Mei‐boom‐Gill (CPMG) sequence reflects the existence of a steady state solution to the Bloch equations in the absence of T2 and T1 decay. The steady state theory is then used to evaluate the performance of low flip angle RARE imaging sequences with both constant and optimally varied refocusing flip angles. The theory is experimentally verified in phantoms and then optimized, single shot, low flip angle RARE is used to obtain artifact‐free images from the brain of a normal volunteer.
In vivo MR microscopy of the human skin
Authors: Song Hee Kwon , Wehrli Felix W. , Ma Jingfei
Published: March 2007

The requirements for imaging the skin are dictated by the organ's layered structure, which extends only a few millimeters from the surface and thus demands extremely high resolution in this direction. While less critical, resolution in the remaining two dimensions determines whether the skin's accessory structures can be resolved. The problem is compounded by short transverse relaxation times, in particular of the dermis, the structure of most clinical interest. In this work images of the normal human skin were obtained in vivo at voxel sizes as small as 19 × 78 × 800 μm3, by means of customized 3D gradient and partial flip‐angle spin‐echo pulse sequences and very small transmit/receive coils on a 1.5T clinical imager equipped with high‐power whole‐body gradients. Structures resolved include hair follicles and the sublayers of the dermis. The very short time constant for the major component (91 %) for transverse relaxation in the dermis (T2* ∼10 ms) suggests the potential of substantial gains in achievable signal‐to‐noise ratio by shortening the echo time.
In vivo23Na NMR studies of myotonic dystrophy
Authors: Kushnir Tammar , Knubovets Tatyana , Itzchak Yacov , Eliav Uzi , Sadeh Menachem , Rapoport Lubov , Kott Edna , Navon Gil
Published: March 2007

Myotonic dystrophy is an inherited multi‐system disease. Its pathophysiology leading to muscle malfunction and damage is not well understood. 23Na NMR spectroscopy was applied here for an in vivo comparative study of the calf muscles of 7 myotonic dystrophy patients at various stages of the disease and 11 healthy volunteers. Both the total sodium content, expressed as the ratio of the 23Na and 1H water signals, and the fast transverse relaxation time, T21, determined from the triple quantum‐filtered spectra, increased in correlation with the severity of the disease. The results demonstrate that 23Na NMR enables the quantitation of myotonic dystrophy progression.
In vivo observation of anisotropic motion of brain water using 2H double quantum filtered NMR spectroscopy
Authors: Assaf Yaniv , Navon Gil , Cohen Yoram
Published: March 2007

The 2H DQF NMR spectra of deuterated water molecules were measured for the first time in in vivo rat brain. The observation of the DQF signal indicates that there is a water population that exhibits anisotropic motion. The characteristics of the DQF spectra premortem and postmortem are very similar (lineshape and relaxation times). In the 1st h there is a 10–15% decrease in the signal intensity of the DQF spectra followed by a gradual but a much slower decrease in the DQF signal intensity that reaches 65–70% of its initial value after only 12 h. When the brains were kept at 4°C, a 40% decrease in the DQ signal intensity was observed only after 7 days. Mechanical chopping of the brain tissues causes an immediate loss of more that 97% of the DQ signals. The slow, temperature‐sensitive decay of the signal, and its sensitivity to mechanical treatment point out that these signals originate from water molecules that interact with structural components in the brain. The characteristics of the DQF spectra depend on the amount of bulk water as exemplified by increased residual quadrupolar interaction and relaxation rates obtained when dehydrating the brain tissue.
Proton MR spectroscopy of the normal human prostate with an endorectal coil and a double spin‐echo pulse sequence
Authors: Heerschap Arend , Jager Gerrit J. , Van Der Graaf Marinette , Barentsz Jelle O. , Ruijs Sjef H. J.
Published: March 2007

This report describes the use of an endorectal coil and a double spin‐echo pulse sequence for localized 1H MR spectroscopy of the normal prostate in volunteers. The spectra showed well‐resolved signals for citrate, (phospho)choline, and creatine protons. Additional signals were assigned to taurine and myoinositol protons. J modulation of the main and outer peaks of citrate could be monitored in vivo. Apparent relaxation times T1 and T2 have been estimated for the methyl protons of cholines and creatine. An effective T1 relaxation time was estimated for the main peaks of the citrate multiplet. Ratios of the integrals of these resonances have been evaluated, and tissue contents of choline and creatine were estimated using the H2O signal as an internal reference. Spectro‐scopic imaging experiments revealed a lower relative citrate signal in central parts of the prostate than in peripheral parts.
Quantitation of T2′ anisotropic effects on magnetic resonance bone mineral density measurement
Authors: Yablonskiy Dmitriy A. , Reinus William R. , Stark Helmut , Haacke E. Mark
Published: March 2007

In this paper, the authors quantitate the anisotropy of susceptibility effects in an uniaxial trabecular bone model and show its relevance to clinical MR bone mineral density measurements. A physical model is described that quantitates the anisotropic MR behavior of uniaxial trabecular bone. To test the model, a phantom of parallel polyethylene filaments was scanned every 15° between 0° and 90° with respect to the system's main magnetic field (B0). The distal radial metaphysis of a healthy female volunteer was scanned in orthogonal projections. The signal from each phantom image and each radial image was separated in a pixel‐wise fashion into R2 and R2′ maps. As predicted, R2′ relaxation showed anisotropic behavior and changed according to sin2 (ϕ), confirming that columnar structures parallel with B0 will cause no MR susceptibility effects. Scans of the distal radius showed that R2′ relaxation was twice as great with the forearm perpendicular to B0 as when it was parallel to it, demonstrating different contributions from struts and columns. For both phantom and radial bone scans, R2 relaxation was isotropic and did not change with object orientation.
1H chemical shift imaging reveals loss of brain tumor choline signal after administration of Gd‐contrast
Authors: Sijens Paul E. , Van Den Bent Martin J. , Nowak Peter J. C. M. , van Dijk Pieter , Oudkerk Matthijs
Published: March 2007

1H MR spectra obtained by chemical shift imaging (CSI) of contrast‐enhancing brain tumors before and after the administration of Gd‐contrast agent were quantitated and compared with the results in normal brain tissue included in the volume of interest. Twenty‐seven combined magnetic resonance imaging and spectroscopy (MRI, MRS) examinations of brain tumor lesions included T1‐weighted MRI and CSI (TR/TE 1500/135 ms double‐spin echo) repeated 5–10 min after the administration of Gd‐contrast agent (0.1–0.2 mM). In 1H MR spectra of contrast‐enhancing tumor Gd‐contrast induced a mean loss of 15% of the peak area of choline‐containing compounds (Cho, P < 0.001) that was correlated with precontrast Cho linewidth (r = −0.72, P < 0.00001). This phenomenon limits the diagnostic use of brain tumor MRS examinations performed immediately after contrast‐enhanced MRI.
Correction for vascular artifacts in cerebral blood flow values measured by using arterial spin tagging techniques
Authors: Ye Frank Q. , Mattay Venkata S. , Jezzard Peter , Frank Joseph A. , Weinberger Daniel R. , McLaughlin Alan C.
Published: March 2007

„Vascular”︁ artifacts can have substantial effects on human cerebral blood flow values calculated by using arterial spin tagging approaches. One vascular artifact arises from the contribution of „tagged”︁ arterial water spins to the observed change in brain water MR signal. This artifact can be reduced if large bipolar gradients are used to „crush”︁ the MR signal from moving arterial water spins. A second vascular artifact arises from relaxation of „tagged”︁ arterial blood during transit from the tagging plane to the capillary exchange site in the imaging slice. This artifact can be corrected if the arterial transit times are measured by using „dynamic”︁ spin tagging approaches. The mean transit time from the tagging plane to capillary exchange sites in a gray matter region of interest was calculated to be ∼0.94 s. Cerebral blood flow values calculated for seven normal volunteers agree reasonably well with values calculated by using radioactive tracer approaches.
A MRI spatial mapping technique for microvascular permeability and tissue blood volume based on macromolecular contrast agent distribution
Authors: Demsar Franci , Roberts Timothy P. L. , Schwickert Heidi C. , Shames David M. , Van Dijke Cornelis F. , Mann Jeffry S. , Saeed Maythem , Brasch Robert C.
Published: March 2007

A rapid and automated method for two‐dimensional spatial depiction (mapping) of quantitative physiological tissue characteristics derived from contrast enhanced MR imaging was developed and tested in disease models of cancer, inflammation, and myocardial reperfusion injury. Specifically, an established two‐compartment kinetic model of unidirectional mass transport was implemented on a pixel‐by‐pixel basis to generate maps of tissue permeability surface area product (PS) and fractional blood volume (BV) based on dynamic MRI intensity data after administration of albumin‐(Gd‐DTPA)30, a prototype macromolecular contrast medium (MMCM) designed for blood pool enhancement. Maps of PS and BV in disease models of adenocarcinoma, intramuscular abscess inflammation, and myocardial reperfusion injury clearly depicted zones of increased permeability (up to approximately 500.μl/cc/h‐compared to <25 μl/cc/h in normal tissues). As revealed on PS maps, the rank ordering of studied permeability abnormalities was reperfusion injury > inflammation > tumors. A rapid, automated mapping technique derived from dynamic contrast‐enhanced MRI data can be used to facilitate the identification and characterization of pathophysiologic abnormalities, specifically relative increases in blood volume and/or microvascular permeability.
Multiple tuning of birdcage resonators
Authors: Amari Smain , Müfit Uluǧ Aziz , Bornemann Jens , Van Zijl Peter C. M. , Barker Peter B.
Published: March 2007

A theoretical framework is presented for designing birdcage resonators for MRI and MR spectroscopy. The analogy between the birdcage problem and the phonon problem in solid‐state physics is used to achieve multiple tuning. Allowing that the capacitances in the columns of the cage assume unequal values, it is possible to achieve multiple tuning and simultaneously preserve the sinusoidal current distribution necessary to set a homogeneous magnetic field. Given the physical dimensions of the columns and branches of the cage as well as the desired resonant frequencies, the corresponding values of the capacitances can be calculated exactly. Closed‐form expressions for the capacitances are given in terms of the mutual inductances and the desired resonant frequencies. A detailed analysis for a symmetrical low‐pass birdcage is presented. The expressions for the resonant frequencies reduce to those given by other authors when only nearestneighbor mutual inductances are included.
Three‐dimensional magnetization‐prepared time‐of‐flight MR angiography of the carotid and vertebral arteries
Authors: Wilman Alan H. , Iii John Huston , Riederer Stephen J.
Published: March 2007

Three‐dimensional magnetization‐prepared (MP) MR angiography (MRA) was applied to the carotid and vertebral arteries by using ECG triggering, a slab selective RF inversion pulse, centric phase encoding and acquisition during diastole. Both theoretically and experimentally, the MP MRA sequence was shown to perform well in cases where there was substantial blood replenishment during the inversion time Tl (>330 ms). In comparison with standard, ungated, steady‐state 3D time‐of‐flight MRA in 13 consecutive volunteers, the MP MRA images demonstrated better background suppression with less artifact and generally had more uniform vessel depiction. The MP MRA sequence was generally superior for portraying vessels exhibiting high pulsatility such as the carotid siphon. However, in distal vessels with slow flow and incomplete blood replenishment, the MP MRA technique was inferior due to increased loss of vessel signal. The reasons for this increased signal loss are identified and improvements to the MP MRA sequence are suggested.
Angiographic Imaging with 2D RF Pulses
Authors: Alley Marcus T. , Pauly John M. , Sommer F. Graham , Pelc Norbert J.
Published: March 2007

Magnetic resonance angiography (MRA) was performed by using RF pulses designed to excite a limited spatial extent in two orthogonal directions. The restriction in the second spatial dimension can be used to increase inflow enhancement and to improve small field‐of‐view imaging. A rectangular excitation was produced with an “echo‐planar” k‐space trajectory and a sine‐modulated RF waveform. In vivo images have demonstrated that vessels are more clearly delineated with the two‐dimensional excitation. Aliasing artifacts in small field‐of‐view imaging are significantly reduced, although in some cases complete elimination is not possible due to the nature of the gradient trajectory.
Improvement of brain lesion detection at 0.1 T by simultaneous use of Gd‐DTPA and magnetization transfer imaging
Authors: Ramadan Usama Abo , Aronen Hannu J. , Tanttu Jukka I. , Karjalainen Pertti T. , Kairemo Kalevi J. , Jääskeläinen Juha , Sipilä Leena , Häkkinen Anna‐Maija
Published: March 2007

Imaging parameters were optimized at 0.1 T to improve contrast‐to‐noise ratios (CNR) when combining magnetization transfer (MT) imaging and the use of paramagnetic contrast medium. This was accomplished by imaging a phantom containing serial concentrations of Gd‐DTPA in cross‐linked bovine serum albumin. With the use of simulations, the dependence of CNR on imaging parameters was studied. Conventional and MT images were obtained from 10 brain tumor patients with single and triple doses of Gd‐DTPA. Simulations demonstrated the importance of TR in postcontrast sequences. The CNR in MT images is less sensitive to TR than in conventional images. A significant CNR improvement caused by MT remains at longer TR when there is no contrast enhancement without MT. The clinical results indicate that a single dose of Gd‐DTPA combined with MT cannot replace imaging with a triple dose. However, MT significantly improved the CNR after single and triple Gd‐DTPA‐doses on T1‐weighted and proton‐density images.
Characterization of and correction for artifacts in linogram MRI
Authors: Gai Neville , Axel Leon
Published: March 2007

Chemical shift differences, field inhomogeneity, and gradient nonlinearity result in artifacts in magnetic resonance imaging. Three artifacts are characterized for linogram imaging and it is shown that, based on computer simulations and theory, linogram MRI behaves similarly to 2DFT. A correction technique similar to a scheme for 2DFT imaging based on the Dixon technique and coordinate transform methods is proposed. The algorithm is applied to correct for field inhomogeneity and gradient nonlinearity‐induced artifacts in both simulations and images of a clinical phantom. The results show good correlation with the theory.

It is concluded that linogram imaging offers certain attractive features of both 2DFT and PR imaging techniques, and is a potentially viable alternative to PR imaging in the presence of field inhomogeneity.
Improved 2D time‐of‐flight angiography using a radial‐line k‐space acquisition
Authors: Nielsen Hatsumi T. C. , Olcott Eric W. , Nishimura Dwight G.
Published: March 2007

For flow imaging applications, radial‐line k‐space acquisition methods offer advantages over conventional 2DFT methods. Specifically, radial‐line acquisition methods mitigate artifacts resulting from pulsatile flow while offering a potential reduction in scan times. In this paper, radial‐line and 2DFT acquisitions are compared in a two‐dimensional time‐of‐flight angiography sequence. The twisting radial‐line (TwiRL) trajectory, a variant of 2D projection reconstruction, is used to represent the family of radial‐line trajectories. In both phantom and in vivo studies, the TwiRL images demonstrate improved vessel depiction including a more uniform signal intensity and better delineation of the vasculature in comparison with images obtained via the 2DFT method.
The b matrix in diffusion tensor echo‐planar imaging
Authors: Mattiello James , Basser Peter J. , Le Bihan Denis
Published: March 2007

In diffusion tensor imaging (DTI) an effective diffusion tensor in each voxel is measured by using a set of diffusion‐weighted images (DWIs) in which diffusion gradients are applied in a multiplicity of oblique directions. However, to estimate the diffusion tensor accurately, one must account for the effects of all imaging and diffusion gradient pulses on each signal echo, which are embodied in the b matrix. For DTI to be practical clinically, one must also acquire DWIs rapidly and free of motion artifacts, which is now possible with diffusion‐weighted echo‐planar imaging (DW‐EPI). An analytical expression for the b matrix of a general DW‐EPI pulse sequence is presented and then validated experimentally by measuring the diffusion tensor in an isotropic phantom whose diffusivity is already known. The b matrix is written in a convenient tabular form as a sum of individual pair‐wise contributions arising from gradient pulses applied along parallel and perpendicular directions. While the contributions from readout and phase‐encode gradient pulse trains are predicted to have a negligible effect on the echo, the contributions from other imaging and diffusion gradient pulses applied in both parallel and orthogonal directions are shown to be significant in our sequence. In general, one must understand and account for the multiplicity of interactions between gradient pulses and the echo signal to ensure that diffusion tensor imaging is quantitative.