bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical model for calculation of apparent diffusion coefficients (ADC) in permeable cylinders—comparison with measured ADC in spinal cord white matter
Authors: Ford J. Chetley , Hackney David B.
Published: March 2007

We have implemented a numerical method for calculation of the apparent diffusion coefficient (ADC) in spinal cord injury, which takes into account the distribution of axon diameters and permeability found in spinal cord white matter, as well as relative axonal volume. We propose a procedure for determining the status of axonal integrity from measured ADC values. These methods have been applied to a well characterized rat spinal cord injury model, affording a prediction of the increase in axonal permeability which is presumed to be closely related to functional deficit. ADC values are compared to those calculated from analytical formulas in the literature, and possible factors underlying the ADC behavior are explored. Calculated results indicate both axonal swelling and cell membrane permeability to be important factors contributing to ADC in traumatic spinal cord injury.
Rapid cardiac imaging with turbo BRISK
Authors: Doyle Mark , Walsh Edward G. , Foster Robert E. , Pohost Gerald M.
Published: March 2007

A variety of variable and constant rate, sparse sampling strategies have previously been proposed to rapidly image dynamically changing objects. The majority of these strategies compile a k‐space data set for any given time point by substituting k‐space data from the most recently sampled time positions (extracted from the sparsely sampled set). The BRISK technique, is a variable rate, sparse sampling technique which additionally incorporates an interpolation scheme to more accurately represent k‐space data at positions which were not directly sampled. Here, strategies are introduced that allow turbo concepts to be incorporated with BRISK. Simulations are conducted to compare the efficacy of the turbo BRISK acquisition and processing strategy against a constant rate, sparse sampling strategy with direct substitution of the most recently acquired k‐space lines. It is shown that turbo BRISK generates images of similar quality in approximately half the time as the uniform sampling rate, sparse sampling strategy. Data from turbo BRISK acquisitions of multicardiac phase image sets, obtained on a normal volunteer and cardiac patients are presented.
Three‐dimensional projection imaging with half the number of projections
Authors: Boada Fernando E. , Christensen James D. , Gillen Joseph S. , Thulborn Keith R.
Published: March 2007

This study demonstrates a sampling scheme for three‐dimensional projection imaging with half the number of projections. The angular distribution of projections is designed so that the oversampling of the low spatial frequencies, in tandem with a partial Fourier algorithm, can be used to recreate purposely missing projections. The performance of the sampling scheme and its associated reconstruction algorithm are illustrated with computer‐simulated as well as experimental data sets. We find that this technique produces images of comparable quality to the conventional projection imaging scheme. Although a 30% loss of signal‐to‐noise ratio (SNR) results from its use, the algorithm should prove useful for applications where robustness against motion artifacts and reduced T2* signal loss are desired.
High resolution electron paramagnetic resonance imaging of biological samples with a single line paramagnetic label
Authors: Kuppusamy Periannan , Wang Penghai , Chzhan Michael , Zweier Jay L.
Published: March 2007

The application of electron paramagnetic resonance imaging (EPRI) to obtain information from biological samples has been limited by the lack of ideal single line radical labels. The commonly used nitroxides exhibit multiple lines causing either hyperfine‐based limitations in the maximum obtainable image resolution or hyperfine‐based artifacts in the reconstructed image. The use of a novel single‐line triarylmethyl paramagnetic label that enables marked enhancement in image quality and resolution is reported. This label exhibits a single line EPR spectrum that is sharp (linewidth ∼60 mG in the absence of oxygen) and relatively stable in tissues. The potential of this label in enabling high resolution EPR imaging of biological samples was demonstrated in a series of phantoms and isolated biological organs such as the rat kidney. The images demonstrate that resolutions better than 100 μm could be obtained at L‐band on samples of up to 20 mm in size.
In vivo determination of intra‐myocellular lipids in human muscle by means of localized 1H‐MR‐spectroscopy
Authors: Boesch Chris , Slotboom Johannes , Hoppeler Hans , Kreis Roland
Published: March 2007

Intra‐myocellular lipids (IMCL) are stored in droplets in the cytoplasm of muscle cells and are an energy storage form readily accessed during long‐term exercise. 1H‐MR spectroscopy methods are presented for noninvasive determination of IMCL in human muscle. This is based on (a) the separation of two resonances in the lipid‐CH2‐region, with the one assigned to IMCL being independent of muscle orientation relative to the magnetic field and (b) the fact that IMCL resonances scale along with signal amplitudes of metabolites in the muscle cell (e.g., creatine) when voxel size is increased, while lipid signals of bulk fat show a disproportionate growth. Inter‐individual and intra‐individual reproducibility studies indicate that the error of the method is about 6% and that IMCL levels differ significantly between identical muscles in different subjects, as well as intra‐individually when measured at 1 week intervals. IMCL determinations in a single subject before and after strenuous exercise indicate that lipid stores recover with a t1/2 of about 1 day.
Magnetic susceptibility measurement of insoluble solids by NMR: Magnetic susceptibility of bone.
Authors: Hopkins Jeffrey A. , Wehrli Felix W.
Published: March 2007

Measurements of the volume magnetic susceptibility of solids using the classical Gouy balance approach are hampered by variations in apparent density of the packed powder. In this paper a quantitative NMR measurement of the volume magnetic susceptibility of powdered solids is described and the volume susceptibility of bone is reported. The technique is based on the measurement of changes in incremental linewidth (1/πT2′) induced in a marker fluid whose susceptibility can be predictably modified by changing the composition, such as by addition of a soluble diamagnetic compound. The spectroscopic linewidth of the marker fluid is determined by the susceptibility difference between the fluid and the suspended solid. Changes in the linewidth are accompanied by bulk magnetic susceptibility induced frequency shifts in the fluid resonance. Correlating the two dependencies allows measurement of the absolute volume susceptibility of the solid. The susceptibility of bovine rib bone was found to be −0.90 ± 0.02 × − 10−6 (CGS) confirming previous estimates which suggested bone to be more diamagnetic than the marrow constituents. Knowledge of the susceptibility of bone is relevant in view of the growing interest in MRI osteodensitometric techniques.
Coronary flow and coronary flow reserve measurements in humans with breath‐held magnetic resonance phase contrast velocity mapping
Authors: Davis Clarence P. , Liu Pei‐Fang , Hauser Markus , Göhde Susanne C. , Von Schulthess Gustav K. , Debatin Jörg F.
Published: March 2007

Objective evidence for coronary lesion significance can be obtained with ischemic stress testing. Since flow‐limiting stenoses have already undergone compensatory vasodilatation to maintain flow, the response to vasoactive stimulation is dampened. The degree of response limitation is reflected by the coronary flow reserve (CFR). Absolute volume flow rates can be accurately and noninvasively measured with MRI techniques. The purpose was to assess the ability to measure coronary volume flow rate noninvasively, and characterize the effect of pharmacologic stress on coronary flow quantitatively by using ultrafast, breath‐held segmented k‐space phasecontrast‐MR imaging (PC‐MRI). Ten healthy volunteers were examined by using ultrafast breath‐held PC‐MRI. Coronary volume flow rates were measured in the anterior descending coronary artery (LAD) at rest and following intravenous administration of dipyridamole. CFR was determined based on these data. Mean LAD volume flow rates increased from 38 ± 11 ml/min before application of dipyridamole to 169 ± 42 ml/min. The mean CFR amounted to 5.0 ± 2.6 (median = 4.15). This study demonstrates the feasibility of breath‐held PC‐MRI to noninvasively quantify coronary volume flow rates over the cardiac cycle. Pharmacologically induced changes in volume flow rate and thus CFR can be quantitated.
Slice location dependence of aortic regurgitation measurements with MR phase velocity mapping
Authors: Chatzimavroudis George P. , Walker Peter G. , Oshinski John N. , Franch Robert H. , Pettigrew Roderic I. , Yoganathan Ajit P.
Published: March 2007

Although several methods have been used clinically to assess aortic regurgitation (AR), there is no “gold standard” for regurgitant volume measurement. Magnetic resonance phase velocity mapping (PVM) can be used for noninvasive blood flow measurements. To evaluate the accuracy of PVM in quantifying AR with a single imaging slice in the ascending aorta, in vitro experiments were performed by using a compliant aortic model. Attention was focused on determining the slice location that provided the best results. The most accurate measurements were taken between the aortic valve annulus and the coronary ostia where the measured (Y) and actual (X) flow rate had close agreement (Y = 0.954 × + 0.126, r2 = 0.995, standard deviation of error = 0.139 L/min). Beyond the coronary ostia, coronary flow and aortic compliance negatively affected the accuracy of the measurements. In vivo measurements taken on patients with AR showed the same tendency with the in vitro results. In making decisions regarding patient treatment, diagnostic accuracy is very important. The results from this study suggest that higher accuracy is achieved by placing the slice between the aortic valve and the coronary ostia and that this is the region where attention should be focused for further clinical investigation.
Steady state effects in fast gradient echo magnetic resonance imaging
Authors: Duyn Jeff H.
Published: March 2007

Fourier methodology is applied to analyze steady state effects in fast gradient echo imaging. Simulations and MR imaging experiments on phantoms demonstrate the effectiveness of existing schemes under different experimental conditions, and modifications are introduced which result in reduced sensitivity to slow object motion as compared to conventional phase modulation schemes. In addition, phase modulation schemes are introduced which more than double temporal signal stability in human brain scans, at flip angles well above the Ernst angle and TR >> T1, T2. Application of phase modulation to the measurement of higher order signals shows increased sensitivity, particularly for the highest order signals.
RARE spiral T2‐weighted imaging
Authors: Block Walter , Pauly John , Nishimura Dwight
Published: March 2007

Spiral imaging has a number of advantages for fast imaging, including an efficient use of gradient hardware. However, inhomogeneity‐induced blurring is proportional to the data acquisition duration. In this paper, we combine spiral data acquisition with a RARE echo train. This allows a long data acquisition interval per excitation, while limiting the effects of inhomogeneity. Long spiral k‐space trajectories are partitioned into smaller, annular ring trajectories. Each of these annular rings is acquired during echoes of a RARE echo train. The RARE refocusing RF pulses periodically refocus off‐resonant spins while building a long data acquisition. We describe both T2‐weighted single excitation and interleaved RARE spiral sequences. A typical sequence acquires a complete data set in three excitations (32 cm FOV, 192 × 192 matrix). At a TR = 2000 ms, we can average two acquisitions in an easy breath‐hold interval. A multifrequency reconstruction algorithm minimizes the effects of any off‐resonant spins. Though this algorithm needs a field map, we demonstrate how signal averaging can provide the necessary phase data while increasing SNR. The field map creation causes no scan time penalty and essentially no loss in SNR efficiency. Multiple slice, 14‐s breath‐hold scans acquired on a conventional gradient system demonstrate the performance.
Detection of glutamate/glutamine resonances by 1H magnetic resonance spectroscopy at 0.5 tesla
Authors: Prost Robert W. , Mark Leighton , Mewissen Mark , Li Shi‐Jiang
Published: March 2007

Midfield proton magnetic resonance spectroscopy (MRS) provides a noninvasive method to monitor glutamate and glutamine (Glx) levels in vivo. Experiments to detect the γ and β resonances of Glx have been performed by using commercial 0.5 T and 1.5 T MR scanners on seven patients with elevated blood ammonia and eight normal volunteers. Compared with the spectral sensitivity obtained on an otherwise identical system operating at 1.5 T, the singlet resonance of N‐acetyl aspartate (NAA) was decreased by a factor of 1.48, which is significantly less than expected using the ratio of Boltzman populations at the two field strengths. However, the resonances of Glx at 0.5 T increased in signal‐to‐noise ratio (SNR) by a factor of 2. The increased SNR of Glx is principally due to improved B 0 main‐field homogeneity and collapse of the strongly J ‐coupled Glx resonances. Our preliminary results suggest that midfield proton MRS will provide significant clinical utility in the detection of Glx levels in human brain.
Use of voigt lineshape for quantification of in vivo1H spectra
Authors: Marshall Ian , Higinbotham John , Bruce Stephen , Freise Andreas
Published: March 2007

Quantification of NMR visible metabolites by spectral modeling usually assumes a Lorentzian or Gaussian lineshape, despite the fact that experimental lineshapes are neither. To minimize systematic fitting errors, a mixed Lorentzian‐Gaussian (Voigt) lineshape model was developed. When tested with synthetic FIDs, the Voigt lineshape model gave more accurate results (maximum error 2%) than either Lorentzian (maximum error 20%) or Gaussian models (maximum error 12%). The three lineshape models gave substantially different peak areas in an in vitro experiment, with the Voigt model having a much lower X2 (2.1 compared with 5.2 for the Lorentzian model and 6.2 for the Gaussian model). In a group of 10 healthy volunteers, fitting of 1H spectra from cerebral white matter gave significantly different peak areas between the methods. Even when area ratios were taken, the Lorentzian model gave higher values (+5% for NAA/choline and +2% for NAA/creatine) than the Voigt lineshape model, whereas the Gaussian model gave lower values (−2% and −1%, respectively).
Theory of heterogeneous relaxation in compartmentalized tissues
Authors: Barsky Daniel , Pütz Benno , Schulten Klaus
Published: March 2007

A new model of compartmentalized relaxation—that which occurs for spins (protons) exchanging between compartments of different relaxation rates—is presented. This model generalizes previous ones by allowing spatially dependent relaxation within compartments. Solutions for the diffusion‐Bloch equations are found via an efficient numerical technique known as the generalized moment expansion, and they agree well with the solutions to the standard two‐site exchange equations (TSEE) for many typical situations. Specific models are developed for liposomes, red blood cells, capillaries, and arteries with respect to applied contrast agents. A parameter derived from tissue characteristics is introduced to predict the nature of the solutions. A new method is proposed for using contrast agents to detect capillaries, which exploits their high surface‐to‐volume ratio relative to the other elements of the vasculature.
Different effects of exercise and edema on T2 relaxation in skeletal muscle
Authors: Ploutz‐Snyder Lori L. , Nyren Sven , Cooper Thomas G. , Potchen E. James , Meyer Ronald A.
Published: March 2007

The hypothesis that increased muscle T2 after exercise is caused by increased extracellular fluid volume was tested by comparing the effects of exercise versus external leg negative pressure on muscle T2 relaxation in normal human subjects. T2 in lower leg muscles was measured by echo‐planar imaging at 63 echo times from 24 to 272 ms, and the relaxation spectrum was calculated by using a non‐negative least squares algorithm. T2 relaxation in anterior leg muscle before exercise was characterized by a single component with mean T2 = 29.3 ± 0.7 (SE, n = 5). After ankle dorsiflexion exercise, this single component broadened, and mean T2 increased to 38.3 ± 0.7 ms. In contrast, after leg negative pressure, which increased the total leg muscle cross‐sectional area by 21% (range 12–32% n = 6), there was a variable appearance of much slower‐relaxing components (60–500 ms). The results suggest that increased extracellular fluid can account for only a minor portion of the increase in muscle T2 observed during exercise.
Global cardiac function using fast breath‐hold MRI: Validation of new acquisition and analysis techniques
Authors: Bloomgarden Daniel C. , Fayad Zahi A. , Ferrari Victor A. , Chin Bennett , St. John Sutton Martin G. , Axel Leon
Published: March 2007

Calculation of global cardiac function parameters has been validated using fast, segmentedk‐space, breath‐hold, gradient‐echo, magnetic resonance images. Images of phantoms, experimental animals, normal volunteers, and patients were acquired with a 1.5 T clinical scanner. Humans were imaged using two phased‐array surface coils in multicoil mode. Myo‐cardial contours were extracted using a new interactive, semi‐automated method based on the active contour model method. Images were acquired in the short‐axis orientation, and, using a new imaging and analysis strategy, in rotating plane long‐axis orientations, to provide better definition of the valve planes and the apex, and also to reduce the number of slices (compared with the short‐axis method) required to sample the whole heart. Validation was accomplished through calculation of the volumes of phantoms and left and right ventricular masses of animal hearts. Functional parameters from MRI were compared with those from echocardiograms and radionuclide angiograms in normal volunteers and patients, respectively.
Fast three dimensional sodium imaging
Authors: Boada Fernando E. , Gillen Joseph S. , Shen Gary X. , Chang Sam Y , Thulborn Keith R.
Published: March 2007

An efficient scheme for fast three dimensional acquisition of sodium MR images is described. This scheme relies on the use of three dimensional k‐space trajectories with constant sample density to achieve significant (60–70%) reductions in total data acquisition time over conventional projection imaging schemes. The performance of this data acquisition scheme is demonstrated with acquisition of sodium data sets on phantoms and normal human volunteers at 1.5 and 3.0 Tesla. The experimental results demonstrate that high quality three dimensional sodium images (0.2 cc voxel size, 10:1 signal‐to‐noise ratio) can be acquired at clinical field strengths (1.5 Tesla) in under 10 min.
Relaxation of solvent protons by solute Gd3+‐chelates, revisited
Authors: Kellar Kenneth E. , Henrichs P. Mark , Spiller Marga , Koenig Seymour H.
Published: March 2007

The magnetic field dependence (NMRD profile) of 1/T1, of solvent protons in an aqueous solution of Gd(DTPA)2− was remeasured at 5,15,25,30, and 35°C. The data were reanalyzed with the usual low‐field theory, using recently published values for TM, the residence lifetime of the single inner‐coordinated waters of solute Gd(DTPA)2−. (These TM values are significantly longer than earlier estimates). Values were obtained for three dynamic parameters: TR, the rotational relaxation time of solute ions, and TSO and TV, the low‐field relaxation time of the Gd3+ magnetic moment and the related correlation time. These Gd(DTPA)2+ values, together with recent results for TM for Gd(DTPA‐BMA) — a nonionic structural analog of Gd(DTPA)2− with an unusually long TM — were used to calculate NMRD profiles at 5 and 35°C. These profiles agree very well with new data given here for a solution of Gd(DTPA‐BMA). This reaffirms the importance of knowing the temperaturedependent values of TM a priori in order to obtain unambiguous quantitative theoretical analyses of NMRD profiles of chelates of known structure. Additionally, the theory of inner sphere relaxation is extended to high fields, at which the magnetic energy of a solute moment is greater than its thermal energy.
31P NMR saturation transfer study of the creatine kinase reaction in human skeletal muscle at rest and during exercise
Authors: Goudemant Jean‐François , Francaux Marc , Mottet Isabelle , Demeure Roger , Sibomana Merence , Sturbois Xavier
Published: March 2007

The creatine kinase reaction has been studied by 31P NMR in exercising human calf muscle. Quantitative analysis of high energy phosphates and saturation transfer study of the creatine kinase flux in the direction of ATP synthesis (Vfor) were performed at rest and during exercise. As expected, exercise induced a [PCr] decrease (from 28.5 ± 0.9 to 21.9 ± 1.5 mM, P < 0.01) matched by a P1, increase (from 4.5 ± 0.2 to 8.9 ± 1.8 mM,P = 0.06). pHi and [ATP] remained unchanged. Vfor did not change from rest (12.4 ± 0.9 mM s−1) to moderate exercise and decreased at the highest exercise level (8.4 ± 1.4 mM s−1, P = 0.006). This observation differs from the prediction of the creatine kinase rate equation, showing an increase in the flux with exercise intensity. Computations suggest that this discrepancy arises from metabolite compartmentalization and/or from the reaction kinetics of a dead end complex stabilized by planar anions.
Multicoil array for high resolution imaging of the breast
Authors: Insko Erik K. , Connick Thomas J. , Schnall Mitchell D. , Orel Susan G.
Published: March 2007

An RF multicoil array for high resolution imaging of the breast is presented. The RF multicoil is incorporated into a dedicated breast imaging apparatus that holds either breast in gentle compression during imaging. Simulations and imaging experiments demonstrate the high sensitivity and RF homogeneity of the multicoil array. Direct comparisons are made between the multicoil and a surface coil. In vivo images are presented to demonstrate the homogeneity of the RF field of the multicoil over the entire breast. An in vivo image of both high temporal and spatial resolution is shown, which demonstrates the sensitivity of the RF multicoil array that may be achieved in a practical imaging experiment.
Phosphorus J‐coupling constants of ATP in human brain
Authors: Jung Wulf‐Ingo , Staubert Andreas , Widmaier Stefan , Hoess Thomas , Bunse Michael , van Erckelens Franz , Dietze Günther , Lutz Otto
Published: March 2007

Proton decoupled 31P NMR spectroscopy of the occipital brain of healthy volunteers was performed with a 1.5 T whole‐body imager. By use of two‐dimensional chemical‐shift imaging in combination with slice‐selective excitation well resolved localized spectra (38 ml) were obtained within 34 min from which the homonuclear 31P‐31P J‐coupling constants of ATP could be determined: Jαβ = 16.1 Hz ± 0.2 Hz and Jαβ = 16.3 Hz ± 0.1 Hz (mean ± SEM, n = 14). Both, the J‐coupling constants and the chemical‐shift difference between α‐ and β‐ATP (δαβ = 8.61 ppm ± 0.01 ppm) were used to calculate the concentration of intracellular free magnesium. The concentrations are 0.39 mM ± 0.09 mM by using the average of both coupling constants of each spectrum, which is in fair agreement with 0.32 mM ± 0.01 mM obtained from the chemical shift of α and β phosphate resonances, which is the more accurate result.
Intracellular volume and apparent diffusion constants of perfused cancer cell cultures, as measured by NMR
Authors: Pilatus Ulrich , Shim Hyunsuk , Artemov Dmitri , Davis Delphine , Van Zijl Peter C. M. , Glickson Jerry D.
Published: March 2007

Diffusion NMR spectroscopy was used to study intracellular volume and apparent water diffusion constants in different cell lines (DU145, human prostate cancer; AT3, rat prostate cancer; MCF‐7, human breast cancer; RIF‐1, mouse fibrosacroma). The cells were grown on various matrices (collagen sponge, collagen beads, polystyrene beads) which enabled continuous growth in perfused high density cell culture suitable for NMR studies. In perfused cell systems, the attenuation of the water signal versus the squared gradient strength was fitted by the sum of two decaying exponentials. For the slowly decaying component the apparent water diffusion constant at 37°C was 0.22 (±0.02) × 10−9 s/m2 for all cell lines at diffusion times > 100 ms. It continuously increased up to 0.47(±0.05) × 10−9 s/m2 when the diffusion time was decreased to 8 ms, indicating restricted diffusion. No significant effect of the matrices was observed. The fractional volume of the slow component as determined from the biexponential diffusion curve correlated with the relative intracellular volume, as obtained from the cell density in the sample and the cell size as measured by light microsocopy. Therefore, this simple NMR approach can be used to determine intracellular volume in perfused cell cultures suitable for NMR studies. Using this information in combination with spectroscopic data, changes in intracellular metabolite concentration can be detected even when the cellular volume is changing during the experiment. The apparent diffusion constant for the fast diffusing component varied with growth matrix, cell density and cell type and also showed the typical characteristics of restricted diffusion (increase of apparent diffusion constant with time).
Effect of brain, body, and magnet bore temperatures on energy metabolism during global cerebral ischemia and reperfusion monitored by magnetic resonance spectroscopy in rats
Authors: Shimizu Hiroaki , Chang Lee‐Hong , Litt Lawrence , Zarow Gregory , Weinstein Philip R.
Published: March 2007

To record brain temperature for comparison with rectal and temporalis muscle temperatures in preliminary studies before MR spectroscopy experiments, a thermistor was inserted into the basal ganglia in eight anesthetized, ventilated, and physiologically monitored rats. The rats were placed in an MR spectrometer and subjected to 60 min of global cerebral ischemia and 2 h of reperfusion without radiofrequency (RF) pulsing. Body temperature was maintained at 37.5–38.0°C (normothermia) or 36.5–37.0°C (mild hypothermia). Brain temperature during ischemia, which dropped to 31.9 ± 0.3 (hypothermia) and 33.6 ± 0.5±C (normothermia), correlated with temporalis muscle temperature (r2 = 0.92) but not with body or magnet bore temperature measurements. Ischemia reduced brain temperature approximately 1.7°C in rats subjected to mild hypothermia (1° reduction of body temperature). Parallel MR spectroscopy experiments showed no significant difference in energy metabolites between normothermic and hypothermic rats during ischemia. However, the metabolic recovery was more extensive 20–60 min after the onset of reperfusion in hypothermia rats, although not thereafter (P < 0.05). Mild hypothermia speeds metabolic recovery temporarily during reperfusion but does not retard energy failure during global ischemia in rats.
Evaluation of the early response in fMRI in individual subjects using short stimulus duration
Authors: Hu Xiaoping , Le Tuong Huu , Uǧurbil Kǎmil
Published: March 2007

Optical imaging studies have provided evidence of an initial increase in deoxyhemoglobin following the onset of neuronal stimulation/activation and demonstrated that this initial increase could be spatially more specific to the site of neuronal activity. These studies also raised the possibility of improving the specificity of fMRI by selective mapping of this early response. Previous MR studies reported the observation of this early response but were limited in scope and not in full agreement. This paper presents a more extensive study that (a) demonstrates the initial signal decrease in individual subjects and (b) examines its dependence on stimulus duration and subject. Binocular visual stimulation experiments were performed on 14 subjects using echo‐planar imaging (EPI) with high temporal resolution. An initial signal decrease was consistently observed in regions that were more localized than those displaying the delayed positive response. In agreement with previous fMRI and optical imaging findings, the maximum signal decrease was 1–2% and occurred at approximately 2 s after the onset of the stimulus, depending on the subject. For stimulus longer than 3.0 s, the temporal dynamics and the amount of signal change of the early response was essentially independent of the stimulus duration, while the delayed response and the post‐stimulus undershoot increased both in terms of magnitude and rise time as the duration of the stimulus increased; this observation is concordant with the recent optical imaging study.
Physiologic basis for BOLD MR signal changes due to hypoxia/hyperoxia: Separation of blood volume and magnetic susceptibility effects
Authors: Kennan Richard P. , Scanley B. Ellen , Gore John C.
Published: March 2007

An NMR method is presented for separating blood volume and magnetic susceptibility effects in response to respiratory challenges such as hypoxia and hyperoxia. The technique employs high susceptibility contrast agents to enhance blood volume induced signal changes. The results show that for a rat model the dominant source of signal variation upon changing breathing gas from 100% oxygen to 10% oxygen/90% nitrogen is the change in blood magnetic susceptibility associated with the BOLD effect. The results imply that signal changes associated with respiratory challenges can be regarded as indicators of local blood oxygenation in vivo.
Mechanism and echo time dependence of the fast response in FMR
Authors: Dymond Rosemary C. , Norris David G.
Published: March 2007

The fast response (FR) is an early reduction in the amplitude of the fMR signal occurring shortly after the onset of stimulation. Owing to its potential advantages in terms of temporal and spatial resolution, it may be of considerable significance in functional experiments. A model for the mechanism of the FR was developed which accounts for the fMRS finding that the amplitude of the dip decreases with increasing echo time. Two computer simulations that confirmed the predictions of the model and showed that it is possible to select optimal echo times for the detection of the FR were performed.
VET imaging: Magnetic resonance imaging with variable encoding time
Authors: Feinberg David A.
Published: March 2007

A methodology of MRI data acquisition is introduced which involves lengthening the duration of signal readout period with complementary shortening of the phase encode pulses, and vice versa, in sequences where gradient encoding time is limited by RF pulse spacing. This variable encoding time (VET) methodology can be used to increase spatial resolution or reduce data acquisition time in 2D and 3D FT MRI based on CPMG and gradient echo sequences. Advantages of higher image SNR and different spatial frequency distributions in k‐space are discussed and evaluated in preliminary experiments.
Spectroscopic imaging of the water resonance with short repetition time to study tumor response to hyperoxia
Authors: Oikawa Hiroshi , Al‐Hallaq Hania A. , Lewis Marta Z. , River Jonathan N. , Kovar David A. , Karczmar Gregory S.
Published: March 2007

A variety of treatments that modulate tumor oxygen tension are used clinically to improve the outcome of radiotherapy. High resolution, noninvasive measurements of the effects of these treatments would greatly facilitate the development of improved therapies and could guide treatment of cancer patients. Previous work demonstrated that magnetic resonance (MR) gradient echo imaging of the water proton resonance detects changes in T2* and T1 in tumors during hyperoxia that may reflect increased tumor oxygenation. This report describes the use of high resolution MR spectroscopic imaging with short repetition time (TR = 0.2 s) to improve the accuracy with which changes in T2* and T1 are measured. Mammary adenocarcinomas grown in the hind limbs of rats were studied. Carbogen inhalation was used to induce hyperoxia. A single 2‐mm slice through the center of tumors and underlying muscle was imaged at 4.7 Tesla with in‐plane resolution of approximately 1.2 mm and frequency resolution of 5.8 Hz. The peak integral increased by an average of 6% in tumors during carbogen inhalation suggesting a decrease in T1 (n = 8, P <0.001). Peak height increased by an average of 15% in tumors during carbogen inhalation (n = 8, P <0.001). The large difference between increases in peak height and peak integral demonstrates that the width of the water resonance decreased. Assuming a Lorentzian lineshape, an average increase of 12% in T2* was observed in tumors. In muscle, peak integral and peak height increased slightly (about 1.2% and 3%. respectively; P <0.02) during carbogen inhalation but no significant change in T2*was observed. Spectroscopic imaging detects changes in the water proton resonance in tumors during hyperoxia accurately and reproducibly with high signal‐to‐noise ratio and allows clear separation of T1 and T2* effects. Increases in T2* may be due to decreased deoxyhemoglobin in tumor blood vessels (i.e., the BOLD effect) and may provide a clinically useful index of increases in tumor oxygenation.
Dipolar resonance frequency shifts in 1H MR spectra of skeletal muscle: Confirmation in rats at 4.7 T in Vivo and observation of changes postmortem
Authors: Ntziachristos Vasilis , Kreis Roland , Boesch Chris , Quistorff Bjørn
Published: March 2007

Non‐isotropic contributions to 1H MR spectra from human skeletal muscle in vivo have recently been observed in the 0‐to 5‐ppm region. One pair of peaks has been identified to be subject to dipolar couplings. The corresponding changes in resonance frequency are related to the orientation of muscle fibers with respect to the external magnetic field and are analogous to the behavior of small molecules dissolved in liquid crystals. Image‐guided localized spectroscopy based on the STEAM method has been applied to verify these phenomena in rat skeletal muscle in vivo and to investigate the effect postmortem. Residual dipolar couplings and anisotropic contributions to 1H MR spectra of skeletal muscle have been confirmed in animals and at a higher field strength—albeit with a slightly different spectral pattern compared to the human study. The most prominent dipolar doublet due to creatine and/or phosphocreatine vanishes postmortem with a rate similar to the disappearance of phosphocreatine, and is no longer observable 2 h postmortem.
Skeletal muscle perfusion measurements using adiabatic inversion of arterial water
Authors: Marro Kenneth I. , Kushmerick Martin J.
Published: March 2007

Quantitative NMR measurements of perfusion using magnetic labeling of arterial water have been demonstrated previously in several different highly perfused organs. The success of these previous experiments suggested that arterial labeling may be of use in measuring perfusion in skeletal muscle, where resting perfusion is very low and where increased perfusion after exercise is transient. In the experiments described in this paper, adiabatic inversion of arterial water has been used to make single‐voxel measurements of perfusion in the lower hind limb of rats. At rest, the NMR results were quantified to yield a perfusion rate of about 13.8 ml/100g/min. After perturbation due to ischemic exercise, large relative changes in the NMR signal were observed. The peak change of about 2.5% of the NMR signal occurred shortly after perturbation and was followed by a return to resting levels over a period of about 4 min.
Diffusion‐weighted multiple shot echo planar imaging of humans without navigation
Authors: Robson Matthew D. , Anderson Adam W. , Gore John C.
Published: March 2007

A new method is proposed for reducing the artifacts produced in diffusion‐weighted imaging. When data are acquired using multiple shot echo planar acquisitions, conventional reconstruction methods produce artifactual images as a consequence of diffusion weighting and small amounts of bulk motion of the subject. If the amount of motion can be determined, it is possible to correct the data before reconstruction, which removes the artifact. A method for estimating the motion from the acquired data has been developed and evaluated. This method assumes that ghost image effects will be minimized when motion has been correctly compensated. By considering the amount of signal in the background of the image, appropriate corrections to the data can be made, and the accuracy of the motion compensation may be estimated. This technique has been evaluated by computer simulation, and its performance has been demonstrated in a phantom and humans with both two‐ and four‐shot echo planar acquisitions and using both “mosaic” and “interleaved” sampling schemes.
Density compensation functions for spiral MRI
Authors: Hoge Richard D. , Kwan Remi K. S. , Bruce Pike G.
Published: March 2007

In interleaved spiral MRI, an object's Fourier transform is sampled along a set of curved trajectories in the spatial frequency domain (k‐space). An image of the object is then reconstructed, usually by interpolating the sampled Fourier data onto a Cartesian grid and applying the fast Fourier transform (FFT) algorithm. To obtain accurate results, it is necessary to account for the nonuniform density with which k‐space is sampled. An analytic density compensation function (DCF) for spiral MRI, based on the Jacobian determinant for the transformation between Cartesian coordinates and the spiral sampling parameters of time and interleaf rotation angle, is derived in this paper, and the reconstruction accuracy achieved using this function is compared with that obtained using several previously published expressions. Various non‐ideal conditions, including intersecting trajectories, are considered. The new DCF eliminated intensity cupping that was encountered in images reconstructed with other functions, and significantly reduced the level of artifact observed when unevenly spaced sampling trajectories, such as those achieved with trapezoidal gradient waveforms, were employed. Modified forms of this function were found to provide similar improvements when intersecting trajectories made the spiral‐Cartesian transformation noninvertible, and when the shape of the spiral trajectory varied between interleaves.
Efficient design of pulses with trapezoidal magnitude and linear phase response profiles
Authors: Pickup Stephen , Popescu Mihail
Published: March 2007

A variation of the Shinnar‐Le Roux (SLR) method of pulse envelope design that allows for control of the phase of the frequency response profile has been developed. The method makes use of the fact that a knowledge of one of the SLR polynomials in combination with a root inversion pattern for the other polynomial is sufficient to fully define the second polynomial. Optimization of the first polynomial, when cast in this form, remains nonlinear. However, it was demonstrated that the relationship between the SLR polynomials and the frequency response profile may be used to generate an initial guess for the SLR polynomials that is sufficiently accurate to allow for the application of linear optimization techniques in most cases. In practice several pulse envelopes having different root inversion patterns are investigated for each target profile. The resulting collection of pulses allows the user to trade off pulse power for profile accuracy. The proposed technique was used to design a large number of amplitude modulated excitation pulses having trapezoidal magnitude and linear phase frequency response profiles. A few examples of the resulting pulses and their response profiles are presented.
A spectral approach to analyzing slice selection in planar imaging: Optimization for through‐plane interpolation
Authors: Noll Douglas C. , Boada Fernando E. , Eddy William F.
Published: March 2007

Interpolation between slices is necessary whenever reslicing a volume of data into a different coordinate frame. This may be done to view the data from different perspectives, to align data from different sessions, or to remove the effects of head movement in functional imaging studies. In this paper, issues surrounding slice‐selection in two‐dimensional imaging are examined in the context of through‐plane interpolation and a spectral framework is introduced to describe the sources of error when interpolating between slices. This framework suggests that there is a trade‐off between precision in localization, which requires high spatial frequencies, and interpolation, which requires a narrow spectrum of spatial frequencies. An analysis of the sources of error has lead to several approaches to reducing interpolation error including elimination of interslice gaps or making slices overlap, use of a slice profile with a narrower spatial frequency bandwidth such as a Gaussian profile, and use of high‐order interpolation. The simulation and experimental data demonstrate significant reductions in interpolation error for these approaches.
The functional scout image: Immediate mapping of cortical function at 4 tesla using receiver phase cycling
Authors: Goodyear Bradley G. , Gati Joseph S. , Menon Ravi S.
Published: March 2007

A rapid method of generating functional scout images to map cortical activity is presented. Maps were produced immediately “on‐line” on the scanner's console using fast low‐angle shot gradient‐recalled echo and echo‐planar imaging sequences without special hardware. To demonstrate the technique, raw data collected during photic stimulation and dark control periods were subtracted through phase alternation of the receiver with constant transmitted radio frequency phase. A blood oxygen level‐dependent signal difference map was produced by magnitude reconstruction of the resulting complex‐difference data. Maps improved in contrast with an increasing number of stimulation‐control cycles and showed similar areas of activation to traditional fMRI processing methods. Such a functional scout procedure allows rapid localization of brain function within the anatomical region of interest. From these functional scout images, slices or voxels may be planned by using the scanner manufacturers prescription tools for further specialized studies.
Quantitative studies of magnetization transfer by selective excitation and T1 recovery
Authors: Gochberg Daniel F. , Kennan Richard P. , Gore John C.
Published: March 2007

Water proton longitudinal relaxation has been measured in agar and cross‐linked bovine serum albumin (BSA) using modified selective excitation (Goldman‐Shen and Edzes‐Samulski) pulse sequences. The resulting recovery curves are fit to biexponentials. The fast recovery rate gives magnetization transfer (MT) information, which is complementary to that given by steady‐state saturation methods. This rate provides an estimate of the strength of the coupling of the immobile proton pool to the mobile proton pool. Near their optimal pulse power values, the Goldman‐Shen and Edzes‐Samulski sequences give fast recovery rates that agree with each other. However, these measured fast recovery rates are dependent on the pulse power, an effect not predicted by the coupled two‐pool model. For 8% agar and 17% BSA, both methods (at optimal pulse powers) give rates in the neighborhoods of 210 and 64 Hz, respectively. The Goldman‐Shen and Edzes‐Samulski pulse sequences have several advantages over those techniques based on steady state saturation: no long saturating pulses, shorter measurement time, and reduced necessity for making lineshape or fitting technique assumptions. The principle disadvantages are smaller effects on the NMR signal, less complete characterization of the MT system, and, in the case of the Goldman‐Shen sequence, greater pulse power.
Evidence for the exchange of arterial spin‐labeled water with tissue water in rat brain from diffusion‐sensitized measurements of perfusion
Authors: Silva Afonso C. , Williams Donald S. , Koretsky Alan P.
Published: March 2007

The extraction fraction of vascular water in rat brain is investigated by means of diffusion measurements of arterial spin labeled water at varying cerebral blood flow (CBF) values. The apparent diffusion coefficient (ADC) of the difference of the proton magnetization signal in the brain acquired with and without continuous arterial spin labeling is modeled to provide a measure of the amount of arterial water in tissue and vasculature and thus of the extraction fraction. The tissue and vascular portion of the arterial spin labeled water are differentiated based on their diffusion characteristics in a manner analogous to the intravoxel incoherent motion (IVIM) method. The amount of labeled arterial water that exchanges with tissue water is determined by estimating the fraction of the total signal that is associated with the slow‐decaying component of a biexponential fit to the normalized difference signal between the magnetization of brain tissue acquired with and without arterial spin labeling. The results indicate that, at normal CBF (1.15 ± 0.21 ml g‐1 min‐1), about 90% of the arterial spin labeled water diffuses with an ADC of (1.21 ± 0.37) 10‐3mm2 s‐1), which is equal to tissue. At high CBF, an increasing fraction of the labeling water has a fast‐pseudo‐diffusion coefficient due to a decrease in water extraction fractions. The results also show that the contribution of vascular water to the measurement of perfusion by techniques that use endogenous water as a tracer can be efficiently eliminated by the use of diffusion sensitizing gradients with small effective b values (b ≈ 20 s/mm2), enabling these techniques to monitor true changes in tissue perfusion.
MRI using piecewise‐linear spiral trajectory
Authors: Liao Jan‐Ray , Pauly John M. , Pelc Norbert J.
Published: March 2007

A new generation of high power gradient systems which allow much faster MR imaging as well as shorter echo times has recently become available. Some of these high‐speed gradient systems impose limits on the percentage of time during which the gradient can change in amplitude (slewing duty cycle). While this limitation may be immaterial to many 2DFT and echo planar imaging methods, a traditional circular spiral trajectory is difficult to use on these systems because its gradient waveforms change during the entire course of the trajectory so that the slewing duty cycle during the readout period is 100%. We describe a piecewise‐linear spiral trajectory which is composed of linear segments and rounded corners. This trajectory reduces the slewing duty cycle while maintaining the desirable imaging properties of circular spirals including interleaving by simple gradient rotation. For one representative example, the slewing duty cycle is reduced to 46%. A conventional gridding method was used for image reconstruction, but a new numerical algorithm to calculate the density compensation factor was required. Use of piecewise‐linear spiral trajectories reduces the impact imposed by limited gradient slewing duty cycle.