bluuebunny/crossref_metadata_2025 huggingface dataset:
Time and temperature dependence of MR parameters during thermal coagulation of ex vivo rabbit muscle
Authors: Graham Simon J. , Bronskill Michael J. , Henkelman R. Mark
Published: March 2007

Detailed measurements of the T1‐weighted, T2‐weighted, and MT‐weighted signal were performed for ex vivo muscle samples heated to various temperatures for different times. Consistent, monotonic increases in signal intensity were observed with progressive thermal coagulation, corresponding to an increase in T2 relaxation time and an increase in MT‐weighted signal for temperatures above 60°C. The relationship for T1 relaxation was more complex, showing a decrease in T1 relaxation from 40 to 60°C and an increase above 60°C. These techniques provide a more direct measure of tissue thermal coagulation than that provided by MR thermometry and suggest MR imaging strategies for the optimization and monitoring of thermal coagulation therapy protocols that create thermal damage in target tissues.
Applicability and efficiency of near‐optimal spatial encoding for dynamically adaptive MRI
Authors: Zientara Gary P. , Panych Lawrence P. , Jolesz Ferenc A.
Published: March 2007

Adaptive near‐optimal MRI spatial encoding entails, for the acquisition of each image update in a dynamic series, the computation of encodes in the form of a linear algebra‐derived orthogonal basis set determined from an image estimate. The origins of adaptive encoding relevant to MRI are reviewed. Sources of error of this approach are identified from the linear algebraic perspective where MRI data acquisition is viewed as the projection of information from the field‐of‐view onto the encoding basis set. The definitions of ideal and non‐ideal encoding follow, with nonideal encoding characterized by the principal angles between two vector spaces. An analysis of the distribution of principal angles is introduced and applied in several example cases to quantitatively describe the suitability of a basis set derived from a specific image estimate for the spatial encoding of a given field‐of‐view. The robustness of adaptive near‐optimal spatial encoding for dynamic MRI is favorably shown by results computed using singular value decomposition encoding that simulates specific instances of worst case data acquisition when all objects have changed or new objects have appeared in the field‐of‐view. The mathematical analysis and simulations presented clarify the applicability and efficiency of adaptively determined near‐optimal spatial encoding throughout a range of circumstances as may typically occur during use of dynamic MRI.
Water dynamics in human blood via combined measurements of T2relaxation and diffusion in the presence of gadolinium
Authors: Stanisz Greg J. , Li Jonathan G. , Wright Graham A. , Henkelman R. Mark
Published: March 2007

An analytical model of tissue relaxation and restricted diffusion in human blood is presented. The blood tissue model is composed of three different compartments: red blood cells, plasma, and macromolecular protons. The relaxation rate constants and free diffusion coefficients of intracellular and extracellular water may differ. Analytical formulas for signal loss due to relaxation and diffusion in the Carr‐Purcell Meiboom‐Gill and pulsed‐field‐gradient multispin echo experiments for this tissue model are derived. The model is fitted to the experimental data for human blood with various concentrations of Gadolinium contrast agent. The obtained model parameters are realistic. The validity and sensitivity of the model are also discussed.
Tumor characterization with dynamic contrast–enhanced MRI using mr contrast agents of various molecular weights
Authors: Su Min‐Ying , Mühler Andreas , Lao Xiaoyan , Nalcioglu Orhan
Published: March 2007

Dynamic contrast‐enhanced MR imaging was used to measure the kinetics of enhancement in three different animal tumor models (Walker 256, R3230 AC, MCF7) using three different Gd complexes (Gd‐DTPA, Gd‐DTPA‐24‐cascade‐poly‐mer 30 kD, and polylysine‐Gd‐DTPA 50 kD). The three tumor models varied in growth rate, with the most rapid growth demonstrated by Walker 256 cells and the slowest growth occurring in the MCF7 cells. For each tumor, the kinetics of enhancement using polylysine‐Gd‐DTPA was analyzed using a pharmacokinetic model to estimate the vascular volume of the tumor. The rate of entry of the contrast agent into the interstitial space served as the measure of vascular permeability. The smallest molecular‐weight agent, Gd‐DTPA, could not provide information about vascular permeability. The intermediate and the largest agents both demonstrated that the faster‐growing Walker 256 tumor had greater vascular permeability than did the slower‐growing R3230 AC tumor. The degree of vascular permeability in the MCF7 tumor could not be assessed fairly due to insufficient statistics. The current study provides evidence supporting the hypothesis that more rapidly growing tumors have higher vascular permeability than do tumors that grow more slowly.
Constrained length minimum inductance gradient coil design
Authors: Chronik Elaine A. , Rutt Brian K.
Published: March 2007

A gradient coil design algorithm capable of controlling the position of the homogeneous region of interest (ROI) with respect to the current‐carrying wires is required for many advanced imaging and spectroscopy applications. A modified minimum inductance target field method that allows the placement of a set of constraints on the final current density is presented. This constrained current minimum inductance method is derived in the context of previous target field methods. Complete details are shown and all equations required for implementation of the algorithm are given. The method has been implemented on computer and applied to the design of both a 1:1 aspect ratio (length:diameter) central ROI and a 2:1 aspect ratio edge ROI gradient coil. The 1:1 design demonstrates that a general analytic method can be used to easily obtain very short gradient coil designs for use with specialized magnet systems. The edge gradient design demonstrates that designs that allow imaging of the neck region with a head‐sized gradient coil can be obtained, as well as other applications requiring edge‐of‐cylinder regions of uniformity.
Comparison of four magnetization preparation schemes to improve blood‐wall contrast in cine short‐axis cardiac imaging
Authors: Semple Scott I. K. , Redpath Thomas W. , McKiddie Fergus I. , Waiter Gordon D.
Published: March 2007

Improvements in short‐axis blood‐myocardium contrast in the heart with the use of four magnetization preparation schemes applied before the imaging sequence are demonstrated. Gradient‐echo cine cardiac images are acquired and compared at 0.95 T incorporating T2, T1p, magnetization transfer, and double inversion (black blood) preparations in a series of volunteer studies over the first 550 ms of the cardiac cycle. T2 and T1p preparations exhibit improvements of 100% and above in image contrast. Magnetization transfer preparation exhibits improvements of 50% in image contrast, whereas an initial improvement (50%) followed by a large loss in contrast is observed using the black blood preparation. Improvements in contrast are dependent on tissue relaxation parameters and therefore are suitable for studies involving patients exhibiting poor in‐flow enhancement of blood caused by poor heart function.
Concomitant gradient terms in phase contrast MR: Analysis and correction
Authors: Bernstein Mat A. , Zhou Xiaohong Joe , Polzin Jason A. , King Kevin F. , Ganin Alexander , Pelc Norbert J. , Glover Gary H.
Published: March 2007

Whenever a linear gradient is activated, concomitant magnetic fields with non‐linear spatial dependence result. This is a consequence of Maxwell's equations, i.e., within the imaging volume the magnetic field must have zero divergence, and has negligible curl. The concomitant, or Maxwell field has been described in the MRI literature for over 10 years. In this paper, we theoretically and experimentally show the existence of two additional lowest‐order terms in the concomitant field, which we call cross‐terms. The concomitant gradient cross‐terms only arise when the longitudinal gradient Gz is simultaneously active with a transverse gradient (Gx or Gy). The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail. Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described. The feasibility of a joint pulse sequence‐reconstruction method, which requires no increase in minimum TE, is demonstrated. Since the lowest‐order terms of the concomitant field are proportional to G2/B0, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields.
Localized proton NMR spectroscopy in the striatum of patients with idiopathic spasmodic torticollis
Authors: Möller Harald E. , Vollmer‐Haase Juliane , Wiedermann Dirk , Schuierer Gerhard
Published: March 2007

Single voxel proton spectroscopy was used to study brain metabolism in idiopathic spasmodic torticollis. Peak metabolite ratios in long echo time (135 ms) spectra were evaluated in the basal ganglia of 16 patients (29‐65 years). Eight of them were untreated; the other eight were examined 4‐6 weeks after administration of botulinum toxin type A. As compared with 60 control spectra, patients showed a significant, therapy‐resistant decrease in N‐acetyl‐L‐aspartate (NAA)/choline (Cho). NAA/cre‐atine (Cr) and Cho/Cr were close to normal in the post‐treatment group but significantly reduced in untreated patients. This result is consistent with the hypothesis that striatal Cr is reversibly elevated in chronic muscle stimulation. The right and left stria‐tum were affected to the same extent.
Evaluation of 31P metabolite differences in human cerebral gray and white matter
Authors: Mason Graeme F. , Chu Wen‐Jang , Vaughan J. Thomas , Ponder Steven L. , Twieg Donald B. , Adams Dorothy , Hetherington Hoby P.
Published: March 2007

31P NMR is commonly used to study brain energetics in health and disease. Due to sensitivity constraints, the NMR measurements are typically made in volumes that do not contain pure gray or white matter. For accurate evaluation of abnormalities in brain metabolite levels, it is necessary to consider the differences in normal levels of 31P metabolites in gray and white matter. In this study, voxels from a three‐dimensional spectroscopic image acquisition were analyzed for their dependence on tissue type to assess differences in metabolite levels between gray and white matter. Specifically, gray matter was found to have significantly higher ratios of phosphocreatine (PCr) to γ‐ATP and PCr to the total 31P metabolite signal, whereas pH and the ratio of PCr to inorganic phosphate (P1) were found to differ insignificantly between gray and white matter. Thus, tissue type can be an important factor to consider for alterations in bioenergetics by 31P NMR spectroscopic studies of the brain.
MR‐guided interstitial cryotherapy of the liver with a novel, nitrogen‐cooled cryoprobe
Authors: Tacke Josef , Adam Gerhard , Speetzen Ralf , Brucksch Kerstin , Sticker Arno , Heschel Ingo , Prescher Andreas , van Vaals Joop J. , Hunter David W. , Rau Günter , Günther Rolf W.
Published: March 2007

The purpose of the study was to test a newly developed, MR‐compatible, liquid nitrogen‐cooled cryoprobe. The probe has an outer diameter of 3.5 mm and was specifically developed for percutaneous, MR‐guided, interstitial cryotherapy. The probe was inserted percutaneously into the livers of 10 rabbits. The cryotherapy procedure was monitored with a surface coil in a 1.5 Tesla magnet using a gradient echo sequence. Follow‐up examinations were performed 3 and 7 days after the freezing procedure using T1‐ and T2‐weighted spin echo sequences. At 7 days the animals were sacrificed and the cryolesions were examined histologically.

The cryoprobe enabled artifact‐free MR imaging of the “ice‐ball” formation during freezing of the rabbit liver. After 1 min of freezing, the iceball at the tip of the probe showed an average maximum diameter of 10.8 mm. No bleeding complications were observed during or after the freezing procedure. Histologic examination 7 days after cryotherapy confirmed that the liver lesions were the same size as had been predicted by the images of the acute iceball.

This new, percutaneously inserted, MR‐compatible, liquid‐nitrogen cooled cryoprobe allows accurate, artifact‐free MR imaging of interstitial cryotherapy.
Wavelet encoding for improved SNR and retrospective slice thickness adjustment
Authors: Gelman Neil , Wood Michael L.
Published: March 2007

A method to encode the slice‐select direction in magnetic resonance imaging through the use of a discrete wavelet transform is presented. The paper reports the first implementation of wavelet encoding using the Haar basis. Theoretically, the signal‐to‐noise ratio (SNR) for two levels of wavelet encoding is expected to be approximately 1.6 times higher than for conventional multislice imaging in which each slice is excited separately. A slightly lower SNR is encountered experimentally because of slice interference. The design of radiofrequency (RF) pulses to excite slices approximating Haar wavelet basis functions is discussed. Wavelet encoding also provides an efficient method for balancing slice thickness and SNR after an image has been acquired. The experiments have been conducted at 1.5 Tesla. Wavelet encoding of slices should be especially suitable for low‐field MRI, in which the SNR is lower and the characteristically lower T, values favor the use of shorter TR, which sometimes forces the slices to be acquired in multiple passes.
Practical design of a high‐strength breast gradient coil
Authors: Maier Cynthia F. , Nikolov Hristo N. , Chu Kenneth C. , Chronik Elaine A. , Rutt Brian K.
Published: March 2007

A high‐strength three‐axis local gradient coil set was constructed for MRI of the breast. Gradient fields with good uniformity (<10% deviation from the desired gradient) over most of the volume required for breast imaging were generated with efficiencies of up to 3.3 mT/m/A. The coils will allow diffusion breast imaging in clinically acceptable examination times. The electrical design, water cooling system, and fabrication techniques are described. Preliminary tests of the coil included images of a grid phantom and diffusion measurements in a short‐T2 agarose gel phantom.
Functional MRI of brain activation induced by scanner acoustic noise
Authors: Bandettini Peter A. , Jesmanowicz A. , van Kylen Joel , Birn Rasmus M. , Hyde James S.
Published: March 2007

A method is introduced by which brain activation caused by the acoustic noise associated with echo planar imaging (EPI) is mapped. Two types of time series were compared. The first time series, considered the “task,” involved applying only EPI gradients for 20 s without the application of RF pulses, then, without pause, starting image collection. The second, considered the “control,” involved typical sequential image acquisition without the prior gradient pulses. Subtraction of the first 5 s of the two time series revealed signal enhancement mainly in the primary auditory cortex. The technique was validated using a motor cortex task that mimicked the hypothesized scanner noise induced activation.
Quantitative 1H MRS of the human brain in vivo based on the simulation phantom calibration strategy
Authors: Due Corinne O. , Weber Oliver M. , Trabesinger Andreas H. , Meier Dieter , Boesiger Peter
Published: March 2007

Normal metabolite concentrations were determined in five different brain regions of healthy adult volunteers using proton magnetic resonance spectroscopy (1H MRS) in vivo. The absolute in vivo concentrations of N‐acetylaspartate (NAA), creatine and phosphocreatine (Cre), and choline containing compounds (Cho) were quantified from measurements obtained with a head‐shaped simulation phantom. Scanner performance and calibration accuracy were assessed by phantom experiments. Localized spectra were acquired on clinical 1.5 T systems using the PRESS localization sequence with frequency selective water suppression. Comparison of the results obtained from phantom experiments and human brain in vivo strongly suggests that reproducibility in vivo mainly depends on the topologic metabolite heterogeneity of brain tissue in combination with relative volume dislocalization.
A critique of the use of the Kolmogorov‐Smirnov (KS) statistic for the analysis of BOLD fMRI data
Authors: Aguirre Geoffrey K. , Zarahn Eric , D′Esposito Mark
Published: March 2007

The use of the Kolmogorov‐Smirnov (KS) statistic for testing hypotheses regarding activation in blood oxygenation level‐dependent functional MRI data is critiqued on both theoretical and empirical grounds. Theoretically, it is argued that the KS test is formally unable to support inferences of interest to most neuro‐imaging studies and has reduced sensitivity compared with parametric alternatives. Empirically, false‐positive rates yielded by the KS test in human data collected under the null hypothesis were significantly in excess of tabular values. These excessive false‐positive rates could be explained by the presence of temporal autocorrelation. We also present evidence that the distribution of blood oxygenation level‐dependent functional MRI data is only slightly nonnormal, questioning the initial impetus for the use of the KS test in this context. Finally, it is noted that parametric alternatives exist that do provide adequate control of the false‐positive rate and can support inferences of interest.
Improved coverage in dynamic contrast‐enhanced cardiac MRI using interleaved gradient‐echo EPI
Authors: Ding Shujun , Wolff Steven D. , Epstein Frederick H.
Published: March 2007

An interleaved gradient‐echo echo‐planar imaging (IGEPI) sequence was modified for and applied to dynamic contrast‐enhanced imaging of the heart. Using IGEPI, images with 3.0 × 3.9 mm nominal in‐plane resolution are acquired in 100 ms, enabling eight slices per heartbeat for a heart rate of 60 beats/min. The acquisition speed and use of saturation pre‐pulses allows acquisition of short‐ and long‐axis images during the same contrast bolus. IGEPI maintains the acquisition characteristics required for performing a quantitative first‐pass perfusion analysis as well as providing improved coverage compared with conventional fast gradient echo.
Spatial and temporal differentiation of fMRI BOLD response in primary visual cortex of human brain during sustained visual simulation
Authors: Chen Wei , Zhu Xiao‐Hong , Kato Toshinori , Andersen Peter , Uǧurbil Kâmil
Published: March 2007

The blood oxygenation level dependent (BOLD) response during sustained visual stimulation has been studied by several groups using fMRI with controversial conclusions. This issue was investigated for the human brain at high (4 Tesla) magnetic field strength using a flashing goggle at 8 Hz. The results demonstrate that the overall BOLD response in the primary visual cortex has an initial overshoot after the onset of visual stimulation and an undershoot after the termination of visual stimulation. A significant and positive BOLD response, however, remains constant between the initial and terminal transient responses. The temporal BOLD responses in the primary visual cortex were spatially dependent. The regions identified as draining veins in images displayed proportionately larger initial and terminal transient responses, whereas regions devoid of such vessels and associated mainly with parenchyma exhibited a more time‐independent BOLD response. These results reveal that the BOLD effect and, presumably, the uncoupling between cerebral blood flow and cerebral metabolic rate of oxygen consumption, are maintained in the primary visual cortex during sustained visual stimulation, and the temporal characteristics of the BOLD effect are spatially dependent.
Multiple‐echo proton spectroscopic imaging using time domain parametric spectral analysis
Authors: Kiefer Andreas P. , Govindaraju Varanavasi , Matson Gerald B. , Weiner Michael W. , Maudsley Andrew A.
Published: March 2007

A multiple‐echo MR spectroscopic imaging (MRSI) method is presented that enables improved metabolite imaging in the presence of local field inhomogeneities and measurement of transverse relaxation parameters. Short echo spacing is used to maximize signal energy from inhomogeneously line‐broadened resonances, and time domain parametric spectral analysis of the entire echo train is used to obtain sufficient spectral resolution from the shortened sampling periods. Optimal sequence parameters for 1H MRSI are determined by computer simulation, and performance is compared with conventional single‐echo acquisition using phantom studies at a field strength of 4.7 T. A preliminary example for use at 1.5 T is also presented using phantom and human brain MRSI studies. This technique is shown to offer improved performance relative to single‐echo MRSI for imaging of metabolites with shortened T2* values due to the presence of local field inhomogeneities. Additional advantages are the intrinsic measurement of metabolite T2 values and determination of metabolite integrals without T2 weighting, thereby facilitating quantitative metabolite imaging.
Simultaneous intracellular and extracelular pH measurement in the heart by 19F NMR of 6‐fluoropyridoxol
Authors: Hunjan Sandeep , Mason Ralph P. , Mehta Vimal D. , Kulkarni Padmakar V. , Aravind Subramanian , Arora Veeva , Antich Peter P.
Published: March 2007

6‐Fluoropyridoxol (6‐FPOL) was evaluated as a simultaneous indicator of intracellular and extracellular pH and, hence, pH gradient in perfused rat hearts. After infusion, 19F NMR spectra rapidly showed two well‐resolved peaks assigned to the intracellular and extracellular compartments, and pH was calculated on the basis of chemical shift with respect to a sodium trifluoroacetate standard. To demonstrate use of this molecule, dynamic changes in myocardial pH were assessed with a time resolution of 2 min during respiratory and metabolic alkalosis or acidosis and ischemia. For a typical heart, intracellular pH (pHi) = 7.14 ± 0.01 and extracellular pH (pHe) = 7.52 ± 0.02. In response to metabolic alkalosis, pHi remained relatively constant and the pH gradient increased. In contrast, respiratory challenge caused a significant increase in pHi. Independent measurements using pH electrodes and 31P NMR confirmed validity of the 19F NMR results.
Quantitative measurements of cerebral blood flow in rats using the FAIR technique: Correlation with previous lodoantipyrine autoradiographic studies
Authors: Tsekos Nikolaos V. , Zhang Fangyi , Merkle Hellmut , Nagayama Masao , Ladecola Costantino , Kim Seong‐Gi
Published: March 2007

Flow‐sensitive alternating inversion recovery (FAIR) is a recently introduced MRI technique for assessment of perfusion that uses blood water as an endogenous contrast agent. To characterize the FAIR signal dependency on spin tagging time (inversion time (Tl)) and to validate FAIR for cerebral blood flow (CBF) quantification, studies were conducted on the rat brain at 9.4 T using a conventional gradient‐recalled echo sequence. The 71, of cerebral cortex and blood was found to be 1.9 and 2.2 s, respectively, and was used for CBF calculations. At short Tls (<0.8 s), the FAIR signal originates largely from vascular components with fast flows, resulting in an overestimation of CBF. For Tl > 1.5 s, the CBF calculated from FAIR is independent of the spin tagging time, suggesting that the observed FAIR signal originates predominantly from tissue/capillary components. CBF values measured by FAIR with Tl of 2.0 s were found to be in good agreement with those measured by the iodoantipyrine technique with autoradiogra‐phy in rats under the same conditions of anesthesia and arterial pCO2. The measured pCO2 index on the parietal cortex using the FAIR technique was 6.07 ml/100 g/min per mmHg, which compares well with the pCO2 index measured by other techniques. The FAIR technique was also able to detect the regional reduction in CBF produced by middle cerebral artery occlusion in rats.
Off‐resonance proton T1p dispersion imaging of 17O‐enriched tissue phantoms
Authors: Charagundla Sridhar R. , Stolpen Alan H. , Leigh John S. , Reddy Ravinder
Published: March 2007

Proton T1p dispersion imaging is a recently described method for indirect detection of 17O. However, clinical implementation of this technique is hindered by the requirement for a high‐amplitude spin‐locking field (γB1 > 1 kHz) that exceeds current limitations in specific absorption rate (SAR). Here, a strategy is offered for circumventing high SAR in T1p dispersion imaging of 17O through the use of low‐amplitude off‐resonance spin‐locking pulses (γB1 < 300 Hz). Proton spin‐lattice relaxation times in the off‐resonance rotating frame were measured in H217O‐enriched tissue phantoms. On‐ and off‐resonance T1p dispersion imaging was implemented at 2 T using a spin‐locking preparatory pulse cluster appended to a standard spin‐echo sequence. On‐ and off‐resonance dispersion images exhibited similar 17O‐based image contrast. Magnetization transfer effects did not depend on 17O concentration and had no effect on image contrast. In conclusion, off‐resonance proton T1p dispersion imaging shows promise as a safe, sensitive technique for generating 17O‐based T1p contrast without exceeding SAR limitations.
Concomitant magnetic‐field‐induced artifacts in axial echo planar imaging
Authors: Zhou Xiaohong Joe , Du Yiping P. , Bernstein Matt A. , Reynolds H. Glenn , Maier Joseph K. , Polzin Jason A.
Published: March 2007

When a linear magnetic field gradient is used, spatially higher‐order magnetic fields are produced to satisfy the Maxwell equations. It has been observed that the higher‐order magnetic field produced by the readout gradient causes axial echo planar images acquired with a horizontal solenoid magnet to shift along the phase‐encoding direction and lose image intensities. Both the shift and intensity reduction become increasingly severe as the slice offset from the isocenter increases. These phenomena are quantitatively analyzed, and good correlation between experiments and theory has been established. The analysis also predicts a previously unre‐ported Nyquist ghost on images with very large slice offsets. This ghost has been verified with computer simulations. Based on the analysis, several methods have been developed to eliminate the image shift, the intensity reduction, and the ghost. Selected methods have been implemented on a commercial scanner and proved effective in removing these image artifacts.
Proton T2 relaxation of cerebral metabolites during transient global ischemia in rat brain
Authors: Fujimori Hiroyuki , Michaelis Thomas , Wick Markus , Frahm Jens
Published: March 2007

Putative changes of metabolite T2 relaxation times were investigated before and after a 20‐min period of global ischemia in rat brain in vivo (n= 10) using localized proton MRS at different echo times (2.35 T). Neither absolute T2 relaxation times (TE = 20‐270 ms) nor time courses of T2‐weighted metabolite signals (TE = 135 ms) revealed statistically significant changes during the occlusion or early reperfusion relative to pre‐ischemic baseline. These findings are in line with reports of relaxation changes at much later stages and further demonstrate that altered T2 relaxation is not a confounding factor in diffusion‐weighted long‐TE proton MRS during early ischemic events.
Improved sampling of myocardial motion with variable separation tagging
Authors: McVeigh Elliot R. , Bolster Bradley D.
Published: March 2007

A novel tagging method is introduced that increases the spatial resolution of estimates of myocardial radial thickening. The separation of adjacent parallel tag planes is customized to match the expected motion of specific regions of the heart wall. In regions in which the tags increase their separation over systole (radial thickening), the tag planes are placed close together at end diastole. In regions in which the tags decrease their separation over systole (circumferential shortening), the tags are placed farther apart so they remain detectable at end systole. With variable separation tagging (VTAG), parallel plane tagging can be used to obtain higher‐resolution estimates of radial thickening and circumferential shortening simultaneously.
Simultaneous multislice acquisition with arterial‐flow tagging (SMART) using echo planar imaging (EPI)
Authors: Kao Yi‐Hsuan , Wan Xin , MacFall James R.
Published: March 2007

Arterial spin tagging techniques have been used to image tissue perfusion in MR without contrast injection or ionizing radiation. Currently, spin tagging studies are performed primarily using single‐slice imaging sequences, which are time consuming. This note reports a multislice echo‐planar arterial spin tagging technique (Simultaneous Multislice Acquisition with aRterial‐flow Tagging, or “SMART”). Multiband RF encoding (Hadamard) is used to provide simultaneous multislice acquisition capability for spin tagging techniques (such as echo planar imaging signal targeting with alternating radio frequency and flow‐sensitive alternative inversion recovery). The method is illustrated with a two‐slice pulse sequence that was implemented using the FAIR technique to generate two perfusion weighted images simultaneously. Compared with single‐slice sequences, this two‐slice sequence provided similar image quality, signal‐to‐noise ratio, and twice the spatial coverage compared with the single‐slice technique within the same scan time.
FID‐based lung MRI at 0.5 T: Theoretical considerations and practical implications
Authors: Schmidt Maria A. , Yang Guang‐Zhong , Gatehouse Peter D. , Firmin David N.
Published: March 2007

Pulse sequences based on FID signals and projection reconstruction (PR) were investigated for lung MRI at 0.5 T and evaluated for artifacts caused by: (1) k‐space mismapping due to either delay or distortion of the readout gradient waveform, (2) cardiac motion and pulsatile flow, and (3) respiratory motion. Nonstructured artifacts were described, simulated, and experimentally confirmed for the first time. Nonstructured artifacts did not impair the demonstration of structures of high signal‐to‐noise ratio (SNR) but generated quantitative errors in the image intensity analysis over the lung parenchyma. The use of FID‐based PR techniques for lung MRI is not justified at 0.5 T.
Time‐course imaging of rat embryos in utero with magnetic resonance microscopy
Authors: Smith Bradley R. , Shattuck Mark D. , Hedlund Laurence W. , Johnson G. Allan
Published: March 2007

Magnetic resonance (MR) microscopy was used to noninvasively investigate the development of live rat embryos in utero. The difficulty in making sequential observations of a developing mammalian embryo has frustrated developmental biologists for many years. Most current technologies analyze normal and abnormal development by observing end point phenotypes (in fixed specimens) rather than investigating the live embryo. MR microscopy was adapted to allow rat litters to be scanned three times each (at 1‐ to 3‐day intervals) and has produced images of live developing embryos. It was demonstrated that repeated anesthesia and imaging protocols produced no gross malformations in the rat pups that were subsequently delivered and observed. Three‐dimensional projection encoding with phase re‐winders produced isotropic (2563) image data sets in about 30 minutes with excellent tissue contrast arising from steady‐state effects in the amniotic fluid.
Encapsulation and perfusion of mitochondria in agarose beads for functional studies with 31P‐NMR spectroscopy
Authors: Doliba Nicolai M. , Wehrli Suzanne L. , Babsky Andriy M. , Doliba Nataliya M. , Osbakken Mary D.
Published: March 2007

An NMR method to study on‐line mitochondrial function was developed. Mitochondria were maintained in a stable physiologic state in agarose beads that were continuously superfused with oxygenated buffer at 28°C. Oxidative function of both heart and liver mitochondria was evaluated with 31P NMR at 9.4 T using pyruvate plus malate as substrate. This method allows clear resolution of adenosine triphosphate‐γ (ATPγ) and adenosine diphosphate‐β (ADPβ) phosphate signals, whereas a signals of ATP and ADP overlap. ATP production by mitochondria was documented to be very sensitive to different interventions (hypoxia, ischemia, carbonyl cyanide p‐(tri‐fluoromethoxy)phenylhydrazone (FCCP)) and depended on the ADP concentration in supervision medium. These data demonstrate that the new application of NMR to study mitochondrial function can discriminate, on‐line, between several physiologic and biochemical processes in intact physiologically stable mitochondria.
Blood oxygen saturation assessment in vivo using T2 * estimation
Authors: Li Debiao , Wang Yi , Waight David J.
Published: March 2007

The feasibility of noninvasively assessing hemoglobin oxygen saturation of deep blood vessels in vivo by measuring blood T2* is investigated. Techniques for blood T2* measurements in major arteries and veins in the presence of pulsatile blood flow are presented and validated using a flow phantom. Images of multiple TEs were collected in a paired fashion. Cardiac triggering was used to eliminate image artifacts caused by pulsatile arterial blood flow. Using these techniques, it was found that the T2* of arterial blood (199 ± 8 ms) is significantly greater than that of venous blood (108 ± 6 ms) in 10 volunteers, consistent with the fact that the oxygen saturation level of arterial blood is much larger than that of venous blood. Various oxygen saturation levels were created in vivo in a pig and the blood T2* was shown to increase with oxygen saturation levels over a wide range. Preliminary results of this study indicate that it is feasible to assess local oxygen saturation by measuring blood T2* using the proposed techniques.
Timing algorithm for bolus chase MR digital subtraction angiography
Authors: Wang Yi , Lee Howard M. , Avakian Rosalynn , Winchester Priscilla A. , Khilnani Neil M. , Trost David
Published: March 2007

Acquire multiple longitudinal locations in the lower extremity after a single contrast injection, appropriate table translation and contrast injection are required. An approximate model based on constant bolus velocity was developed to describe the space‐time course of a contrast bolus in the lower extremity. This model was verified in dynamic MR angiograms acquired in a group of patients using time‐resolved 2D MR digital subtraction angiography (MRDSA). From this contrast bolus passage model, a timing algorithm for table translation and contrast injection was developed for bolus chase MRDSA, subsequently validated in bolus chase 2D MRDSA experiments. All targeted major peripheral arteries were well depicted in bolus chase 2D MRDSA using this timing algorithm and a single 15‐ml contrast dose.
Quantitative imaging of perfusion using a single subtraction (QUIPSS and QUIPSS II)
Authors: Wong Eric C. , Buxton Richard B. , Frank Lawrence R.
Published: March 2007

In the pulsed arterial spin labeling (ASL) techniques EPISTAR, PICORE, and FAIR, subtraction of two images in which inflowing blood is first tagged and then not tagged yields a qualitative map of perfusion. An important reason this map is not quantitative is that there is a spatially varying delay in the transit of blood from the tagging region to the imaging slice that cannot be measured from a single subtraction. We introduce here two modifications of pulsed ASL (QUIPSS and QUIPSS II) that avoid this problem by applying additional saturation pulses to control the time duration of the tagged bolus, rendering the technique relatively insensitive to transit delays and improving the quantitation of perfusion.
Simultaneous multislice acquisition using rosette trajectories (SMART): A new imaging method for functional MRI
Authors: Noll Douglas C. , Peltier Scott J. , Boada Fernando E.
Published: March 2007

A new acquisition technique for rapid, whole‐brain functional MRI is presented. In this technique, several slices are simultaneously acquired using rosettek‐spacetrajectories and a gradient‐induced frequency modulation. This modulation together with the spectral properties of the rosette acquisition allow all slices to be reconstructed individually. In functional MRI studies, acquisition rates of 16.7 to 25 images/s were achieved, a threefold improvement over single‐slice acquisitions. The raw images showed some increase in noise. However, because this increase is mostly stationary, the functional activation maps showed only a slight increase in noise (8%).
Heuristic optimization algorithms applied to the quantification of spectroscopic data
Authors: Weber Oliver M. , Duc Corinne O. , Meier Dieter , Boesiger Peter
Published: March 2007

The quantification of in vivo MR spectra imposes severe problems because of low spectral resolution and poor signal‐to‐noise ratio. Maximum likelihood methods are often applied. However, with conventional spectrum analysis procedures, the search for a global minimum in a multidimensional space often terminates in only a local minimum. Heuristic optimization procedures are able to circumvent this difficulty. Two approaches, the genetic algorithm and the simulated annealing, have been adapted to the quantification of MR spectra. For evaluation purposes, the procedures have been applied to synthetic and in vivo spectra with different noise levels. They both allowed a reliable spectrum quantification. The areas of most peaks were quantified reproducibly, although in some cases, the discrimination between spectroscopically almost identical metabolites (e.g., glutamate and glutamine) was not completely satisfactory. The two algorithms are found to be valuable alternative methods in the quantification of in vivo MR spectra.
Assessment of glucosylifosfamide mustard biodistribution in rats with prostate adenocarcinomas by means of in vivo31P NMR and in vitro uptake experiments
Authors: Haberkorn Uwe , Krems Boris , Gerlach Ludwig , Bachert Peter , Morr Iris , Wiessler Manfred , van Kaick Gerhard
Published: March 2007

A combined in vitro/in vivo study was performed to evaluate the possible application of phosphorus (31P) NMR spectros‐copy for therapy monitoring and to investigate glucosylifos‐famide mustard (Glc‐IPM) transport and biodistribution by radiotracer techniques. Dynamic in vivo 31P NMR measurements were performed in rats with prostate adenocarcinoma after iv injection of 1 mmol/kg body weight (bw) of ifosfamide (IFO) (n = 4) and 1 mmol/kg bw (n = 4) or 2.15 mmol/kg bw (n = 9) of Glc‐IPM. In a biodistribution study with 14C‐labeled Glc‐IPM and a final dose of 0.8 mmol Glc‐IPM/kg bw, the animals were killed 5, 30, 60, and 120 min after drug administration, an ethanol extraction was performed from several tissues, and the dose per g tissue was calculated. The same tumor cell line was used in saturation and competition experiments to further elucidate the transport mechanism. The 31P NMR signals of IFO and Glc‐IPM showed no overlap with the endogenous phosphorus peaks. A rapid washout with a half‐life between 25.9 ± 5.6 min for the lower dose and 34.3 ± 4.2 min for the higher dose of Glc‐IPM was observed in the tumor. No statistically significant change of the pH value was observed during the examination period. The β‐nucleoside 5′‐triphosphate (NTP)/inorganic phosphate (Pi) signal intensity ratio showed a tendency to decrease but without statistical significance. A rapid elimination was demonstrated by both the noninvasive NMR technique and the biodistribution study. No saturation was found in vitro for the Glc‐IPM uptake, even at the concentration of 5 m/W. Furthermore, the Glc‐IPM uptake was not inhibited by the presence of 2‐deoxyglucose and vice versa. The data show that the pharmacokinetics of Glc‐IPM in the tumor can be followed in vivo by 31P NMR. The results presented are evidence for diffusion as the transport mechanism for Glc‐IPM in this tumor model. However, the better visualization of Glc‐IPM as compared to ifosfamide may be due to metabolic trapping of a negatively charged metabolite after deglycosylation.
Characterization of and correction for eddy current artifacts in echo planar diffusion imaging
Authors: Jezzard Peter , Barnett Alan S. , Pierpaoli Carlo
Published: March 2007

Magnetic resonance diffusion imaging is potentially an important tool for the noninvasive characterization of normal and pathological tissue. The technique, however, is prone to a number of artifacts that can severely affect its ability to provide clinically useful information. In this study, the problem of eddy current‐induced geometric distortions that occur in diffusion images acquired with echo planar sequences was addressed. These geometric distortions produce artifacts in computed maps of diffusion parameters and are caused by misalignments in the individual diffusion‐weighted images that comprise the diffusion data set. A new approach is presented to characterize and calibrate the eddy current effects, enabling the eddy current distortions to be corrected in sets of Interleaved (or snapshot) echo planar diffusion images. Correction is achieved by acquiring one‐dimensional field maps in the read and phase encode direction for each slice and each diffusion step. The method is then demonstrated through the correction of distortions in diffusion images of the human brain. It is shown that by using the eddy current correction scheme outlined, the eddy current‐induced artifacts in the diffusion‐weighted images are almost completely eliminated. In addition, there is a significant improvement in the quality of the resulting diffusion tensor maps.
Versatile frequency domain fitting using time domain models and prior knowledge
Authors: Slotboom Johannes , Boesch Chris , Kreis Roland
Published: March 2007

An iterative nonlinear least‐squares fitting algorithm in the frequency domain using time domain models for quantification of complex frequency domain MR spectra is presented. The algorithm allows incorporation of prior knowledge and has both the advantage of time‐domain fitting with respect to handling the problem of missing data points and truncated data sets and of frequency‐domain fitting with respect to multiple frequency‐selective fitting. The described algorithm can handle, in addition to Lorentzian and Gaussian line‐shapes, Voigt and nonanalytic lineshapes. The program allows the user the design of his own fitting strategy to optimize the probability of reaching the global least‐squares minimum. The application of the fitting program is illustrated with examples from in vivo 1H– 31P‐, and 13C‐MR spectroscopy.
Temporal characteristics of oxygenation‐sensitive MRI responses to visual activation in humans
Authors: Fransson Peter , Krüger Gunnar , Merboldt Klaus‐Dietmar , Frahm Jens
Published: March 2007

Series of single‐shot blipped echo‐planar images with spin‐density weighting and T2* sensitivity (2.0 T, TR = 400 ms, TE = 54 ms, flip angle = 30°) were used to study the temporal response profiles to repetitive visual activation (5 Hz, reversing black and white checkerboard versus darkness) for protocols comprising multiple cycles of a 1.6‐s stimulus in conjunction with a 8.4‐s or 90‐s recovery phase and a 10‐s stimulus with a 20‐s or 90‐s recovery phase. Analysis of the real‐time data from all activated pixels resulted in a strong positive MRI response (mean values 3‐6%) as well as a marked poststimulus undershoot (mean values 1‐2%, duration 60‐90 s) for all paradigms. Repetitive protocols with insufficient recovery periods of 8.4 s or 20 s gave rise to a wraparound effect when analyzing time‐locked averages from multiple activation cycles. This problem may lead to an early signal decrease that originates from the ongoing undershoot of preceding activations folded back into the initial latency phase of a subsequent activation. When ensuring complete decoupling of responses to successive stimuli by using a 90‐s recovery period, the wraparound effect vanished and an initial dip was observed in one of seven subjects for a 10‐S/90‐S protocol.
Automatic segmentation of gadolinium‐enhanced multiple sclerosis lesions
Authors: Bedell Barry J. , Narayana Ponnada A.
Published: March 2007

Automatic detection and quantitation of contrast‐enhanced lesions on MRI is expected to be useful in characterizing the disease state in multiple sclerosis (MS). The enhancing structures such as cerebral vasculature and regions with no blood‐brain barrier complicate automated analysis of lesion enhancement. A pulse sequence that incorporates both stationary and marching saturation bands and gradient dephasing is described for suppressing the enhancements within the cerebral vasculature. A postprocessing technique based on automatic image segmentation is implemented for eliminating enhancing structures such as choroid plexus. All MS lesions larger than 5 mm3 have been successfully identified. The automated analysis produced no false‐positives or false‐negative lesions above this volume in 13 patients with MS who were evaluated using the acquisition and evaluation techniques described.
Relaxation anisotropy in cartilage by NMR microscopy (μMRI) at 14‐μm resolution
Authors: Xia Yang
Published: March 2007

To study the structural anisotropy and the magic‐angle effect in articular cartilage,T1, andT2images were constructed at a series of orientations of cartilage specimens in the magnetic field by using NMR microscopy (μMRI). An isotropic T1, and a strong anisotropic T2were observed across the cartilage tissue thickness. Three distinct regions in the microscopic MR images corresponded approximately to the superficial, transitional, and radial histological zones in the cartilage. The percentage decrease ofT2follows the pattern of the curve of (3cos2θ − 1)2at the radial zone, where the collagen fibrils are perpendicular to the articular surface. In contrast, little orientational dependence of T2was observed at the transitional zone, where the collagen fibrils are more randomly oriented. The result suggests that the interactions between water molecules and proteoglycans have a directional nature, which is somehow influenced by collagen fibril orientation. Hence, T2anisotropy could serve as a sensitive and noninvasive marker for molecular‐level orientations in articular cartilage.
GRASE imaging at 3 Tesla with template interactive phase–encoding
Authors: Jovicich Jorge , Norris David G.
Published: March 2007

A new method for ordering the phase‐encoding gradient is proposed, and an application for short effective TE gradient‐and spin‐echo (GRASE) imaging is demonstrated. The proposed method calculates the phase‐encoding order from the signal decay of a template scan (hence “template interactive phase‐encoding” or TIPE). Computer simulations are used to compare the point spread functions of different phase‐encoding orders giving short effective echo times (kb centric GRASE, centric GRASE, centric TIPE). The conventional centric phase‐encoding order is also considered for GRASE. The conventional centric method is sensitive to both amplitude and phase modulation of the signal in K‐space. The centric TIPE method gives the least amplitude modulation artifacts but is vulnerable to phase artifacts. The TIPE experiment was implemented on a 3 Tesla system. To the best of our knowledge, we present the first in vivo GRASE images at this field strength.
Measurement of chemical shifts and coupling constants for glutamate and glutamine
Authors: Govindaraju Varanavasi , Basus Vladimir J. , Matson Gerald B. , Maudsley Andrew A.
Published: March 2007

Proton chemical shifts and coupling constants were obtained for glutamate and glutamine in water (D2O) at pH = 6.6. Initial chemical shift and coupling constant values obtained from experimental spectra were refined using a spectral simulation and optimization program to get a complete set of values that could not otherwise be measured directly from the experimental spectra due to strong spin‐spin couplings. These values are essential for automated spectral fitting procedures that require a priori information.