bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantification and reduction of ghosting artifacts in interleaved echo‐planar imaging
Authors: Reeder Scott B. , Atalar Ergin , Bolster Bradley D. , McVeigh Elliot R.
Published: March 2007

A mathematical analysis of ghosting artifacts often seen in interleaved echo‐planar images (EPI) is presented. These artifacts result from phase and amplitude discontinuities between lines of k‐space in the phase‐encoding direction, and timing misregistrations from system filter delays. Phase offsets and time delays are often measured using “reference” scans, to reduce ghosting through post‐processing. From the expressions describing ghosting artifacts, criteria were established for reducing ghosting to acceptable levels., Subsequently, the signal‐to‐noise ratio (SNR) requirements for estimation of time delays and phase offsets, determined from reference scans, was evaluated to establish the effect of estimation emor on artifact reduction for interleaved EPI. Artifacts resulting from these effects can be reduced to very low levels when appropriate reference scan estimation is used. This has important implications for functional MRI (fMRI) and applications involving small changes in signal intensity.
Noise suppression digital filter for functional magnetic resonance imaging based on image reference data
Authors: Buonocore Michael H. , Maddock Richard J.
Published: March 2007

The central decision in every functional magnetic resonance imaging (fMRI) experiment is whether pixels in brain tissues are showing activation in response to neural stimulus or as a result of noise. Images are degraded not only by random (e.g., thermal) noise, but also by structured noise due to MR system characteristics, cardiac and respiratory pulsations, and patient motion. A novel digital filter has been developed to suppress cardiac and respiratory structured noise in fMRI images, using estimates of structured and random noise power spectra obtained directly from the images. It is an adaptive filter based on stationary noise statistics, and is equivalent in form to a Wiener filter. A mathematical model of the filtering process was developed to understand how the strength and distribution of structured and random noise power influenced filter performance. The filter was tested using images from an auditory activation study in ten subjects. In subjects whose structured noise power was localized to a relatively narrow frequency range, a strong relationship was found, both experimentally (R = 0.975, P < 0.0004 for Ho: R = 0) and using the model, between filter performance and the level of structured noise power contaminating the experiment frequency. The filter significantly reduced the rate of false‐positive activations in the subset of subjects whose experiment frequency was relatively heavily contaminated by structured noise. Notch filters, that simply eliminate unwanted frequencies, performed poorly in all subjects. Unlike the proposed Wiener filter, these filters did not suppress structured noise power at the experiment frequency that contributes to false‐positive activations.
Dynamic shimming for multi‐slice magnetic resonance imaging
Authors: Morrell Glen , Spielman Daniel
Published: March 2007

Dynamic shimming has been implemented in three pulse sequences on a commercial GE Signa 1.5‐T imaging system. Multi‐slice field maps are acquired before the imaging sequence, and linear shim terms and center frequencies are calculated for each slice. During the imaging scan, the linear shim terms and center frequency are set before each pulse sequence repetition according to the current slice. Acquisition of multi‐slice field maps and calculation of shim terms and center frequency for each slice are accomplished in a matter of seconds. Pulse sequences require only minimal modification to add dynamic shimming capability. Results are shown for a fat saturation spin‐echo sequence, a single‐shot echo‐planar gradient‐recalled echo sequence, and a spiral acquisition gradient‐recalled echo sequence. In all cases, dynamic shimming with shim currents and center frequency optimized for each slice is shown to give better results than constant shim currents and a single center frequency optimized for the entire volume of interest.
Estimating test‐retest reliability in functional MR imaging I: Statistical methodology
Authors: Genovese Christopher R. , Noll Douglas C. , Eddy William F.
Published: March 2007

A common problem in the analysis of functional magnetic resonance imaging (fMRI) data is quantifying the statistical reliability of an estimated activation map. While visual comparison of the classified active regions across replications of an experiment can sometimes be informative, it is typically difficult to draw firm conclusions by inspection; noise and complex patterns in the estimated map make it easy to be misled. Here, several statistical models, of increasing complexity, are developed, under which “test‐retest” reliability can be meaningfully defined and quantified. The method yields global measures of reliability that apply uniformly to a specified set of brain voxels. The estimates of these reliability measures and their associated uncertainties under these models can be used to compare statistical methods, to set thresholds for detecting activation, and to optimize the number of images that need to be acquired during an experiment.
Estimating test‐retest reliability in functional MR imaging II: Application to motor and cognitive activation studies
Authors: Noll Douglas C. , Genovese Christopher R. , Nystrom Leigh E. , Vazquez Alberto L. , Forman Steven D. , Eddy William F. , Cohen Jonathan D.
Published: March 2007

Functional magnetic resonance imaging (fMRI) using blood oxygenation contrast has rapidly spread into many application areas. In this paper, a new statistical model is used to evaluate the reliability of fMRI activation in a finger opposition motor paradigm for both within‐session and between‐session data and in a working memory paradigm for between‐session data. A slice prescription procedure for between‐session reproducibility is introduced. Estimates are made for the probabilities of correctly and falsely classifying voxels as active or inactive and receiver operator characteristic curves are generated. In the motor paradigm, estimated between‐session reliability was found to be somewhat reduced relative to within‐session reliability; however, this includes additional sources of variation and may not reflect intrinsically lower reliability. After matching false‐positive classification probabilities, between‐session reliability was found to be nearly identical for both motor and cognitive activation paradigms.
Half‐fourier BURST imaging on a clinical scanner
Authors: Jakob Peter M. , Griswold Mark A. , Lovblad Karl O. , Chen Qun , Edelman Robert R.
Published: March 2007

A modified BURST imaging technique with a nearly twofold reduction in acquisition time, improved signal‐to‐noise ratio and clear improvement of the image quality is introduced. The method applies half‐Fourier reconstruction and can be implemented on a standard clinical scanner. This approach allows the collection of single‐shot magnetization prepared images of the human brain with a matrix size of 112 × 128 within 86‐250 ms. The signal‐to‐noise properties of the pulse sequence are analyzed and compared with a standard BURST experiment.
Regional measurement of the Gd‐DTPA tissue partition coefficient in canine myocardium
Authors: Harris Paul A. , Lorenz Christine H. , Holburn George E. , Overholser Knowles A.
Published: March 2007

Quantifiable MRI perfusion studies using the contrast agent Gd‐DTPA require measurement or estimation of the tissue partition coefficient (λ) for tracer kinetic modeling. Radiotracer techniques were used to obtain regional λ measurements from the left ventricles of five dogs. Measurements were analyzed to determine whether spatial heterogeneity was a major component of λ variability. No systematic variations were identified in terms of radial position, short‐axis slice location, or wall position. The high λ variability seen in this study and in cited data of others may be due in part to tissue heterogeneity in interstitial volume, plasma volume, and perfusate hematocrit.
Slice profile effects in adiabatic inversion: Application to multislice perfusion imaging
Authors: Frank Lawrence R. , Wong Eric C. , Buxton Richard B.
Published: March 2007

Imperfections in the slice profile of the adiabatic inversion induced by relaxation effects are shown to cause signal variations in pulsed arterial tagging schemes on the order of magnitude of perfusion changes, and result in gross errors in perfusion quantitation. Significant improvement can be made with minor modifications to the inversion pulse which facilitate the acquisition of quantitative, multislice perfusion images, as demonstrated in both a phantom and a normal human volunteer.
Classification of 1H MR spectra of biopsies from untreated and recurrent ovarian cancer using linear discriminant analysis
Authors: Wallace J. C. , Raaphorst G. P. , Somorjai R. L. , Ng C. E. , Fung Fung M. Kee , Senterman M. , Smith Ianc. P.
Published: March 2007

Proton (1H) magnetic resonance (MR) spectra of ex vivo biopsy samples of ovarian cancers provided biochemical information that was used to discriminate cancer from normal ovarian tissue. Possible differences present in intrinsically resistant tumors or changes in biochemistry after the induction of resistance were identified. Using multivariate techniques, in particular linear discriminant analysis (LDA), ovarian cancer was distinguished from normal ovarian tissue with a sensitivity of 100%, a specificity of 95% and an accuracy of 98%. Moreover, LDA was able to distinguish untreated ovarian cancer from recurrent ovarian cancer with a sensitivity of 92%, a specificity of 100%, and an accuracy of 97%; removal of the single “fuzzy” specimen increased the accuracy to 100%. Applications of this knowledge to in vivo measurements could lead to noninvasive diagnosis of ovarian cancer.
SPLICE: Sub‐second diffusion‐sensitive MR imaging using a modified fast spin‐echo acquisition mode
Authors: Schick Fritz
Published: March 2007

Recent human studies for measuring of the apparent diffusion coefficient in tissue by magnetic resonance imaging have been conducted by time‐consuming standard spin‐echo acquisition sequences and phase correction with navigator echoes. Diffusion‐weighted echo‐planar sequences have been shown to be rapid alternatives for brain imaging. Both methods show inherent disadvantages in applications on thoracic or abdominal sites. A new approach combining single‐shot diffusion‐weighted imaging with a modified fast spin‐echo acquisition mode is reported here. The modification is necessary, because normal fast spin‐echo acquisition requires a particular phase relation between the magnetization and the refocusing pulses. Unfortunately, this phase relation is not provided after diffusion sensitive preparation. Therefore, the split echo acquisition mode was developed and is shown to be insensitive to the phase of the magnetization. The advantages of both fast spin‐echo acquisition and diffusion weighting can be combined in the SPLICE sequence (split aqcuisition of fast spin‐echo signals for diffusion imaging). The applicability of the new technique is shown by series of sub‐second diffusion‐weighted images from different parts of the body.
Effects of diffusion anisotropy on lesion delineation in a rat model of cerebral ischemia
Authors: Lythgoe Mark F. , Busza Albert L. , Calamante Fernando , Sotak Christopher H. , King Martin D. , Bingham Anna C. , Williams Stephen R. , Gadian David G.
Published: March 2007

The effects of white and gray matter diffusion anisotropy on ischemic lesion delineation have been studied in the rat model of middle cerebral artery occlusion. Apparent diffusion coefficient (ADC) maps obtained by conventional pulsed gradient spin echo diffusion‐weighted imaging (PGSE‐DWI) were compared with maps of the trace of the diffusion tensor in both normal and occluded animals. Diffusion tensor trace maps were derived from the average of the ADC maps from three separate experiments with diffusion weighting along three orthogonal axes, and also from a single‐scan method. A marked degree of diffusion anisotropy was observed in both cortical gray matter and white matter from ADC maps of the control animals. In the occluded animals, the systematic effects of anisotropy on ADC and lesion area influenced the delineation of the ischemic territory in the PGSE‐DWI ADC maps. However, the two trace methods eliminated these effects and gave consistent ischemic lesion depiction, despite the use of differing diffusion times in the two measurements.
An in vivo model for functional MRI in cat visual cortex
Authors: Jezzard Peter , Rauschecker Josef P. , Malonek Dov
Published: March 2007

A protocol is described for obtaining functional magnetic resonance images in anesthetized cat brain based on the blood oxygenation level dependent (BOLD) contrast mechanism. A visual stimulus was used, which consisted of a high‐contrast drifting grating, whose speed and spatial frequency was optimized for cat area 18 (V2). Experiments were conducted at 4.7 Tesla using a gradient echo EPI sequence with a 29‐ms echo time, yielding signal changes of between 0.7% and 2% in area 18.
Measurement of the point spread function in MRI using constant time imaging
Authors: Robson Matthew D. , Gore John C. , Constable R. Todd
Published: March 2007

The point spread function is a fundamental property of magnetic resonance imaging methods that affects image quality and spatial resolution. The point spread function is difficult to measure precisely in magnetic resonance even with the use of carefully designed phantoms, and it is difficult to calculate this function for complex sequences such as echo‐planar imaging. This report describes a method that measures the point spread function with high spatial resolution at each pixel in samples of uniform intensity distribution. This method uses additional phase encoding gradients before the echo‐planar acquisition that are constant in length but vary in amplitude. The additional gradients are applied to image the contents within each individual voxel. This method has been used to measure the point spread function for echo‐planar imaging to demonstrate the effects of limited k‐space sampling and transverse relaxation, as well as the effects of object motion. By considering the displacement of the point spread function, local distortions due to susceptibility and chemical shift effects have been quantified and corrected. The method allows rapid assessment of the point spread function in echo‐planar imaging, in vivo, and may also be applied to other rapid imaging sequences that can be modified to include these additional phase encoding gradients.
Isotropic diffusion‐weighted and spiral‐navigated interleaved EPI for routine imaging of acute stroke
Authors: Butts Kim , Pauly John , De Crespigny Alex , Moseley Michael
Published: March 2007

An interleaved echo‐planar imaging (EPI) technique is presented for the rapid acquisition of isotropic diffusion‐weighted images of stroke patients. Sixteen isotropic diffusion‐weighted images at three b values are acquired in less than 3 min. A spiral navigator echo is used to measure the constant and linear phase shifts across the head in both the x and y directions which result from motion during the isotropic diffusion‐ sensitizing gradients. The measured k‐space errors are corrected during a gridding reconstruction. The gridding kernel has a constant width in kx and a variable width in ky which eliminates variable data‐density ghosts. The resulting isotropic diffusion‐weighted images have excellent lesion‐to‐normal brain contrast, very good spatial resolution, and little sensitivity to susceptibility effects in the base of the brain. Examples of diffusion‐weighted images and ADC maps from several stroke patients are shown.
Noninvasive comprehensive characterization of renal artery stenosis by combination of STAR angiography and EPISTAR perfusion imaging
Authors: Prasad Pottumarthi V. , Kim Ducksoo , Kaiser Andreas M. , Chavez David , Gladstone Steven , Li Wei , Buxton Richard B. , Edelman Robert R.
Published: March 2007

In a small fraction of patients with hypertension, the cause is stenosis of the renal artery. To date, there is no established noninvasive screening technique available to identify this population, for whom treatment with a surgical procedure or percutaneous transluminal angioplasty is often possible. In this study, the sensitivity and specificity of STAR angiography and EPISTAR perfusion imaging in characterizing renal artery stenosis were evaluated in an animal model. Qualitatively, STAR provided projection angiograms that were comparably sensitive to x‐ray angiograms but obtained noninvasively. The sensitivity for detecting the stenosis was 100%. The specificity varied according to the definition of the threshold for significance, which varied 78–94%, depending on whether >70% or >50% diameter reduction was considered. Improvements in specificity will depend upon use of shorter echo times and higher spatial resolution. Our preliminary data indicate that EPISTAR provides sensitivity and specificity of 100% for characterizing stenosis with diameter reductions of >70%.
A simplified sequence for observing deoxymyoglobin signals in vivo: Myoglobin excitation with dynamic unexcitation and saturation of water and fat (MEDUSA)
Authors: Noyszewski Elizabeth A. , Chen Enn‐Ling , Reddy Ravinder , Wang Zhiyue , Leigh John S.
Published: March 2007

This paper describes a new, simplified pulse sequence for observing NMR signals from deoxymyoglobin in vivo. Paramagnetically shifted resonances from deoxymyoglobin can be exploited to noninvasively calculate intracellular oxygen tension in striated muscle. However, special sequences are required to observe these weak signals against the larger water and fat signals encountered in vivo. The pulse sequence described here, which is based on inversion recovery sequences, efficiently suppresses both water and fat resonances and can be implemented with short repetition rates. Moreover, it is perfectly suited for studies with surface coils, where RF inhomogeneities render other popular suppression sequences ineffective.
Statistical methods in functional magnetic resonance imaging with respect to nonstationary time‐series: Auditory cortex activity
Authors: Gaschler‐Markefski Birgit , Baumgart Frank , Tempelmann Claus , Schindler Frank , Stiller Detlet , Heinze Hans‐Jachen , Scheich Henning
Published: March 2007

In awake animal and human auditory cortices, it is a common experience with electrophysiological and suitable imaging methods for responses to steady stimulation to be strongly state‐dependent and to exhibit nonstationarities, even over short periods of observation. If such nonstationary behavior is also reflected by hemodynamic responses in the human auditory cortex, conventional methods of analysis of fMRI data, although applicable for instance to largely stationary responses in visual and other cortices, may be misleading in attempts to parcellate auditory cortex into fields and to demonstrate functional maps. Time‐Windows, described in this article as a convenient tool for the detection and analysis of time‐variant brain activities, solves some of these problems. Time‐Windows demonstrates that activity is evoked reliably in three separate territories of human auditory cortex, parts of which may show nonstationary behavior, depending on the auditory stimuli and tasks.
Implementation and evaluation of frequency offset corrected inversion (FOCI) pulses on a clinical MR system
Authors: Payne Geoffrey S. , Leach Martin O.
Published: March 2007

FOCI pulses are a variant of hyperbolic secant pulses in which the RF amplitude, RF frequency, and gradient waveform are all modulated by the same function A(t). This increases the usable gradient amplitude without requiring a corresponding increase in RF amplitude. In this paper the implementation and inversion profiles of FOCI pulses on a clinical MR system are described, showing improved slice definition and chemical‐shift offset behavior. Their adiabatic behavior is confirmed, and their use with an ISIS sequence for localized MRS is illustrated. Finally the effects of waveform digitization are considered, and the implications for SAR and db/dt are discussed.
Improvements on an in Vivo automatic shimming method (FASTERMAP)
Authors: Shen Jun , Rycyna Robert E. , Rothman Douglas L.
Published: March 2007

Improvements on a localized, automatic shimming method described by Gruetter and Boesch (J. Magn. Reson. 96, 323–334 (1992)) and Gruetter (Magn. Reson. Med. 29, 804–811 (1993)) are presented. A spin‐echo sequence employing a double sech refocusing scheme is used to acquire a field map along linear projections that improves the signal‐to‐noise ratio by at least a factor of two over the stimulated echo sequence previously used. To further improve the reliability of shim adjustments, a variance‐weighted polynomial regression analysis is performed. This also extends the scope of application of this technique to global shimming. Localized 1H spectra of human brain shimmed by this method are presented.
Signal‐to‐noise measurements in magnitude images from NMR phased arrays
Authors: Constantinides Chris D. , Atalar Ergin , McVeigh Elliot R.
Published: March 2007

A method is proposed to estimate signal‐to‐noise ratio (SNR) values in phased array magnitude images, based on a region‐of‐interest (ROI) analysis. It is shown that the SNR can be found by correcting the measured signal intensity for the noise bias effects and by evaluating the noise variance as the mean square value of all the pixel intensities in a chosen background ROI, divided by twice the number of receivers used. Estimated SNR values are shown to vary spatially within a bound of 20% with respect to the true SNR values as a result of noise correlations between receivers.
Planar gradient coil design by scaling the spatial frequencies of minimum‐inductance current density
Authors: Lee Soo Y. , Park Bu S. , Yi Jeong H. , Yi Wan
Published: March 2007

Gradient coil inductance has been remarkably reduced by the minimum‐inductance design technique, which minimizes the magnetic energy stored by the gradient coil. The planar gradient coil designed by this technique, however, often has poor magnetic field linearity. Scaling the spatial frequencies of the current density function derived by this method, the magnetic field linearity of the planar gradient coil can be greatly improved with a small sacrifice of gradient coil inductance. A figure of merit of the planar gradient coil has been found to be improved by scaling the spatial frequencies.
Functional MRI bold signal coincides with electrical activity in the rat whisker barrels
Authors: Yang Xiaojin , Hyder Fahmeed , Shulman Robert G.
Published: March 2007

Functional MRI (fMRI) provides a noninvasive method for mapping brain functional activity based on blood oxygenation level dependent (BOLD) image contrast that is primarily due to localized increases in perfusion. Recently, Malonek and Grinvald (Science 272:551–554, 1996) suggested that during sustained functional activation, the increases in perfusion were spread over a much larger area than the localized electrical activity. In this study, it is demonstrated that the spatial distribution of the BOLD fMRI signal during sustained stimulation of rat whiskers has the same spatial pattern and dimension as that of neuronal electrical activity in the rat whisker barrels.
Energetics of 3.5 s neural activation in humans: A 31P MR spectroscopy study
Authors: Rango Mario , Castelli Alberto , Scarlato Guglielmo
Published: March 2007

No direct information on brain energetics and energy‐related compounds in the first seconds of physiological activation has been reported to date. In this study visual cortex high energy phosphate changes were monitored in 11 normal subjects during 3.5 s activation and the following 23.5 s by a simple 31P magnetic resonance spectroscopic method. An intraactivation decrease of phosphocreatine (PCr) was observed in all subjects, with changes in pH in three, one of them also presenting a change in adenosine triphosphate (ATP). In the subgroup of eight subjects without changes in pH, the mean rate of mean PCr decrease (DPCr) was 7.24 ± 0.78 %/s, and the postactivation mean rate of mean PCr recovery was <1/2 DPCr. Short phasic neural activity requires a large amount of energy, i.e., at least three times basal consumption, in agreement with theoretical calculations. Additional energy demands in the visual cortex are several times those measured by positron emission tomography during prolonged stimulation studies, implying that mean energy requirements decrease with increases in duration of stimulation. During short activation, the vascular responses as detected by brainmapping techniques (BMT) are preceded by an important reduction of the intracellular high‐energy phosphate content, which returns to resting values during an interval that corresponds to the poststimulation return of BMT signals to baseline.
Two‐point Dixon technique for water‐fat signal decomposition with B0 inhomogeneity correction
Authors: Coombs Bernard D. , Szumowski Jerzy , Coshow William
Published: March 2007

To separate water and lipid resonance signals by phase‐sensitive MRI, a two‐point Dixon (2PD) reconstruction is presented in which phase‐unwrapping is used to obtain an inhomogeneity map based on only in‐phase and out‐of‐phase image data. Two relaxation‐weighted images, a “water image” and a “fat image,” representing a two‐resonance peak model of proton density, are output. The method is designed for T1‐ or density‐weighted spin‐echo imaging; a double‐echo scheme is more appropriate for T2‐weighted spin‐echo imaging. The technique is more time‐efficient for clinical fat‐water imaging than 3PD schemes, while still correcting for field inhomogeneity.
Dynamic 13C NMR analysis of pyruvate and lactate oxidation in the in vivo canine myocardium: Evidence of reduced utilization with increased work
Authors: Rath Dipti P. , Zhu Hui , Tong Xiaoxin , Jiang Zongcheng , Hamlin Robert L. , Robitaille Pierre‐Marie L.
Published: March 2007

In this work, substrate selection was monitored in the left ventricle of the canine myocardium by following pyruvate and lactate oxidation under in vivo conditions at basal and elevated workloads. These studies were conducted in the open chest model using dynamic 13C NMR techniques in the presence and absence of dichloroacetic acid (DCA), a well‐known activator of pyruvate dehydrogenase (PDH). Following the infusion of (3‐13C) pyruvate or (3‐13C) lactate into the left anterior descending artery, highly variable 13C enrichments of glutamate, alanine, aspartate, and citrate were noted under low (RPP <14,500 mmHg/min), intermediate (RPP = 15,000–25,000 mmHg/min), and high (RPP > 25,500 mmHg/min) rate pressure products (RPP). At low workloads, the myocardium typically oxidized the infused (3‐13C) pyruvate or (3‐13C) lactate and incorporated the labeled carbon into the glutamate pool as expected. However, in a few notable instances (n = 3), 13C‐enriched pyruvate and lactate were unable to label the glutamate pool under in vivo conditions even at the lowest RPPs, indicating a lack of selection for these substrates by the tricarboxylic acid (TCA) cycle. Nonetheless, the levels of glutamate C4 enrichment observed at low workloads could usually be enhanced by infusion of DCA. Importantly, 13C NMR extract analysis revealed that (3‐13C) pyruvate or (3‐13C) lactate labeling of the glutamate pool was reduced (<20%) at high workloads in spite of increased DCA concentrations.
19F NMR monitoring of in vivo tumor metabolism after biochemical modulation of 5‐fluorouracil by the uridine phosphorylase inhibitor 5‐benzylacyclouridine
Authors: Holland Scott K. , Bergman Andries M. , Zhao Yiming , Adams Earle R. , Pizzorno Giuseppe
Published: March 2007

A uridine phosphorylase inhibitor, 5‐benzylacyclouridine (BAU), has been utilized as biochemical modulator of 5‐fluorouracil (5‐FU) anti‐tumor activity in a murine tumor model. The effect of BAU on 5‐FU metabolism has been evaluated using in vitro and in vivo 19F NMR spectroscopy. The analysis of the NMR data revealed an increased formation and retention of fluorouracil nucleotides and fluorouridine in colon 38 tumors treated with the regimen containing BAU and a reduction in 5‐FU catabolites (α‐fluoro‐β‐ureidopropionic acid and α‐fluoro‐β‐alanine). In the normal tissues evaluated, the presence of BAU did not significantly alter the metabolism and presence of fluoropyrimidine species, indicating a more selective effect on tumor tissues. Therapy experiments on C57/BL6 mice bearing colon 38 tumor showed that the administration of 120 mg/kg BAU 30 min before 5‐FU at 85 mg/kg, on a weekly basis, resulted in an increased antineoplastic effect compared to the same dose of 5‐FU alone. A smaller dose of 5‐FU (60 mg/kg) also administered 30 min after 120 mg/kg BAU caused a reduction in tumor growth similar to 5‐FU alone. The addition of BAU to 5‐FU (85 mg/kg) resulted in a slight increase, although statistically nonsignificant, in host toxicity without causing any toxic death during the chemotherapeutic treatment. 19F NMR spectroscopy is here shown to be a powerful technique to evaluate changes in the metabolism of fluoropyrimidines after the use of biochemical modulator and to allow a correlation between improved therapeutic response with the biochemical effects generated in tissues.
The effect of perfusion on T1 after slice‐selective spin inversion in the isolated cardioplegic rat heart: Measurement of a lower bound of intracapillary‐extravascular water proton exchange rate
Authors: Bauer Wolfgang R. , Roder Fridtjof , Hiller Karl‐Heinz , Han Hong , Fröhlich Susanne , Rommel Eberhard , Haase Axel , Ertl Georg
Published: March 2007

Many NMR measurements of cardiac microcirculation (perfusion, intramyocardial blood volume) depend on some kind of assumption of intracapillary‐extravascular water exchange rate, e.g., fast exchange. The magnitude of this water exchange rate, however, is still unknown. The intention of this study was to determine a lower limit for this exchange rate by investigating the effect of perfusion on relaxation time. Studies were performed in the isolated perfused cardioplegic rat heart. After slice‐selective inversion, the spin lattice relaxation rate of myocardium within the slice was studied as a function of perfusion and compared with a mathematical model which predicts relaxation rate as a function of perfusion and intracapillary‐extravascular exchange rate. A linear relationship was found between relaxation rate T−1 and perfusion P normalized by perfusate/tissue partition coefficient of water, λ: ΔT−1 = m · ΔP/λ with 0.82 ≤ m ≤ 1.06. Insertion of experimental data in the model revealed that a lower bound of the exchange rate from intra‐to extravascular space is 6.6 s−1 (4.5 s−1, P < 0.05), i.e., the intracapillary lifetime of a water molecule is less than 150 ms (222 ms, P < 0.05). Based on this finding, the T1 mapping after slice‐selective inversion could become a valuable noncontrast NMR method to measure variations of perfusion.
Homocarnosine and the measurement of neuronal pH in patients with epilepsy
Authors: Rothman Douglas L. , Behar Kevin L. , Prichard James W. , Petroff Ognen A. C.
Published: March 2007

Homocarnosine is a dipeptide of gamma‐aminobutyric acid (GABA) and histidine found uniquely in the brain, most likely in a subclass of GABAergic neurons. By comparison of spectra from the occipital lobe of patients receiving a homocarnosine elevation drug to normal subjects we have assigned two elevated resonances in the short TE 1H MRS spectrum to homocarnosine. These resonances are partially resolved at 7.05 and 8.02 ppm in a short TE spectrum at 2.1 T when macromolecule resonances are removed by subtraction of a spectrum in which the metabolite resonances are nulled by inversion recovery. The chemical shift of both of these resonances is sensitive to pHi. By comparison with a titration curve the pHi was calculated from the downfield resonance to be 7.06 in the patient group which is similar to values reported using the Pi resonance. Based on the in vivo results and theoretical considerations the potential sensitivity for using nonelevated homocarnosine to measure pH is similar to that of Pi under physiological conditions.
Localizedq‐space imaging of the mouse brain
Authors: King Martin D. , Houseman J. , Gadian D. G. , Connelly A.
Published: March 2007

Localizedq‐space imaging was used to obtain water displacement profiles from mouse brain. These profiles take the form of unidirectional diffusive displacement probability distributions. Two groups of mice were studied, a normal group and a group in which surgery had been performed to produce a unilateral reduction in the supply of blood to the forebrain.q‐Space measurements were made bothin vivoand postmortem. The displacement profiles were characterized using the summary parameter prob[d< 10], which is the proportion of water molecules that undergo a net diffusive displacement that is less than ±10 μm, during the diffusion period (50 ms). The range of prob[d< 10] values in the normal group was 0.71 to 0.77in vivocompared with 0.78 to 0.87 in the impaired hemisphere of the surgically treated group. An increase in prob[d< 10] occurred postmortem to yield values in the range 0.79 to 0.81 and 0.80 to 0.89 in the normal and surgically treated group, respectively. These observations are consistent with the diffusion‐weighted image intensity changes that occur after a period of ischemia.
On the SAR and field inhomogeneity of birdcage coils loaded with the human head
Authors: Jin Jianming , Chen Ji
Published: March 2007

Birdcage coils are widely used as a radiofrequency (RF) resonator in magnetic resonance imaging (MRI) because of their capability to Produce a highly homogeneous B1 field Over a large volume within the coil. When they are employed for high‐frequency MRI, the interaction between the electromagnetic field and the object to be imaged deteriorates the B1‐field homogeneity and increases the specific absorption rate (SAR) in the object. To investigate this problem, a finite‐element method (FEM) is developed to analyze the SAR and the B1 field in a two‐dimensional (2D) model of a birdcage coil loaded with a 2D model of a human head. The electric field, magnetic field, and SAR distributions are shown, and a comprehensive study is carried out for both linear and quadrature birdcage coils at 64, 128, 171, and 256 MHz. It is that to generate the same value of the B1 field, the SAR is increased significantly with the frequency, and for the same imaging method the SAR produced by a quadrature coil is significantly lower than that of a linear coil. It is also shown that the B1‐field inhomogeneity is increased significantly with the frequency.
Non‐fourier encoding with multiple spin echoes
Authors: Panych Lawrence P. , Mulkern Robert V. , Saiviroonporn Pairash , Zientara Gary P. , Jolesz Ferenc A.
Published: March 2007

The advantages and limitations of multiple spin‐echo sequences for non‐Fourier encoding are investigated. Complications caused by improper encoding of alternate magnetization pathways due to imperfect refocusing pulses are analyzed. It is shown that mirror image ghosts result if the encoding RF pulse matrix is real‐valued. These ghosts can be avoided as long as the rows of the RF pulse matrix are conjugate symmetric, which implies that spatial profiles are real valued. Non‐Fourier encoding using bases derived from wavelet, Hadamard, and other real‐valued orthogonal functions does not result in a mirror ghost artifact. A RARE sequence for non‐Fourier encoding has been implemented on a clinical imaging system and successfully applied for brain imaging.
Magnetization transfer imaging in vivo of the rat brain at 4.7 T: Interpretation using a binary spin‐bath model with a superlorentzian lineshape
Authors: Quesson Bruno , Thiaudière Eric , Delalande Christophe , Dousset Vincent , Chateil Jean‐François , Canioni Paul
Published: March 2007

Proton magnetization transfer contrast (MTC) imaging, using continuous wave off‐resonance irradiation, was performed on the rat brain in vivo at 4.7 Tesla. The observed MTC was studied in three different brain regions: the corpus callosum, the basal ganglia, and the temporal lobe. By systematically varying the offset frequency and the amplitude of the RF irradiation, the observed signal intensities for each region of interest were modeled using a system including free water and a pool of protons with restricted motions (R. M. Henkelman, X. Huang, Q. Xiang, G. J. Stanisz, SD Swanson, M. J. Bronskill, Magn. Res. Med. 29, 759 (1993)). Most of the relaxation parameters of both proton pools remained fairly constant for the three regions of interest, with a T2 value of about 9 μs for the immobilized protons, whereas the rate of exchange increased significantly from the temporal lobe to the corpus callosum. The optimal acquisition parameters for the improved MTC under steady‐state saturation were found to be 2–10 kHz offset frequency and 500–800 Hz RF irradiation amplitude. Conversely, an irradiation amplitude of 3 kHz at an offset frequency of 12 kHz is required to minimize the direct effect of off‐resonance irradiation. Such an approach could be extended to human brain imaging with the aim of characterizing tissue‐specific disease.
MRI quantitative myocardial perfusion with compartmental analysis: A rest and stress study
Authors: Vallée Jean‐Paul , Sostman H. Dirk , Macfall James R. , Wheeler Ted , Hedlund Larry W. , Spritzer Charles E. , Coleman R. Edward
Published: March 2007

K1 (first‐order transfer constant from arterial plasma to myocardium for Gd‐DTPA) and Vd (distribution volume of Gd‐DTPA in myocardium) were measured in vivo in a canine model (n = 5) using MRI‐derived myocardial perfusion curves and a compartmental model. Perfusion curves were obtained after a bolus injection of Gd‐DTPA (0.04 mM/kg) with an inversion‐prepared fast gradient echo sequence. Myocardium and blood signal intensity were converted to a concentration of Gd‐DTPA, according to a model appropriate for short (<1 s) interimage intervals characteristic of cardiac‐triggered acquisitions. Before dipyridamole‐induced stress, K1 and Vd, obtained from the fit of the MRI‐derived perfusion curves, were 6.2 ± 1.4 (mHz) and 17.5 ± 4.2%, respectively. After dipyridamole infusion, a K1 increase of a factor of 2.82 ± 0.72 was measured (P = 0.003). No change was observed in Vd (P = 0.98). These results suggest that the K1 increase after dipyridamole reflects a flow‐related effect that can be useful to quantity the MRI‐derived perfusion curves.
Spectrally weighted twisted projection imaging: Reducing T2 signal attenuation effects in fast three‐dimensional sodium imaging
Authors: Boada Fernando E. , Shen Gary X. , Chang Sam Y. , Thulborn Keith R.
Published: March 2007

A scheme for the reduction of T2 signal attenuation effects in three‐dimensional twisted projection imaging is presented. By purposely reducing the sample density at the high spatial frequencies, a considerable reduction in readout time is achieved. The reduction in readout time leads to decreased T2 signal attenuation which translates into improved signal‐to‐noise ratio (SNR). The SNR improvement is achieved without decreasing the image's resolution since the point spread function depends on the sample weighting as well as the T2 attenuation. The results indicate that SNR improvements of up to 40% can be achieved using the proposed scheme.
k‐space detection and correction of physiological artifacts in fMRI
Authors: Wowk Brian , McIntyre Michael C. , Saunders John K.
Published: March 2007

Signal phase variations caused by physiology are a major source of instability in fMRI images produced by multiple RF pulses. k‐Space phase variation maps show cyclic phase variations at the frequency of respiration combined with a cardiac variation of lower amplitude. The amplitude of the variation increases with gradient echo time and proximity to the chest, suggesting that the dominant cause of the phase variation is a B0 shift (∼0.01 ppm) produced by movement of organs in the chest. A simple k‐space phase correction method is proposed and demonstrated for FLASH fMRI. The retrospective method requires no pulse sequence modification, and is more effective than navigator echo correction. Physiological noise is dramatically reduced, especially at inferior slice locations.
Navigated single‐voxel proton spectroscopy of the human liver
Authors: Tyszka J. Michael , Silverman Jeffrey M.
Published: March 2007

The addition of a navigator echo acquisition to a 1H spectroscopy sequence allows retrospective motion correction using acceptance criteria based on measured displacement of the diaphragm. This technique has been applied to the acquisition of short echo‐time 1H spectroscopy of the liver in seven normal subjects. Navigation correction significantly improved overall spectral quality as measured by the phase variance, linewidth and suppression ratio of the water resonance. The hepatic choline resonance was consistently observed in all navigated spectra. Retrospective navigation makes close to optimal use of data acquisition time and, unlike external respiratory transducers, allows direct measurement of diaphragm displacement. The reconstruction of multiple voxel spectra from a single data set was also demonstrated.
A fast spin echo technique with circular sampling
Authors: Zhou Xiaohong , Liang Zhi‐Pei , Gewalt Sally L. , Gofer Gary P. , Lauterbur Paul C. , Johnson G. Allan
Published: March 2007

This paper presents a fast spin echo (FSE) imaging method that employs circular sampling of Jr‐space. The technique has been implemented on a 2 Tesla imaging system and validated on both phantoms and living animals. Experimental studies have shown that circular sampling can produce artifact‐free FSE images without the need of phase correction. Although not fully explored, preliminary results also show that circular sampling may have advantages over the conventional rectilinear FSE in signal‐to‐noise ratio and imaging efficiency. A major disadvantage is the increased sensitivity to off‐resonance effects. The authors expect that the FSE technique with circular sampling will find its applications in magnetic resonance microscopy, neuro‐functional imaging, and real‐time dynamic studies.
Regional metabolite concentrations in human brain as determined by quantitative localized proton MRS
Authors: Pouwels Petra J. W. , Frahm Jens
Published: March 2007

The regional distribution of brain metabolites was studied in several cortical white and gray matter areas, cerebellum, and thalamus of young adults with use of quantitative single‐voxel proton MRS at 2.0 T. Whereas the neuronal compound N‐acetylaspartate is distributed homogeneously throughout the brain, N‐acetylaspartylglutamate increases caudally and exhibits higher concentrations in white matter than in gray matter. Creatine, myo‐inositol, glutamate, and glutamine are less concentrated in cortical white matter than in gray matter. The highest creatine levels are found in cerebellum, parallel to the distribution of creatine kinase and energy‐requiring processes in the brain. Also myo‐inositol has highest concentrations in the cerebellum. Choline‐containing compounds exhibit a marked regional variability with again highest concentrations in cerebellum and lowest levels and a strong caudally decreasing gradient in gray matter. The present findings neither support a metabolic gender difference (except for a 1.3‐fold higher myo‐inositol level in parietal white matter of female subjects) nor a metabolic hemispheric asymmetry.
A comparison of fast MR scan techniques for cerebral activation studies at 1.5 Tesla
Authors: Yang Yihong , Glover Gary H. , van Gelderen Peter , Patel Anand C. , Mattay Venkata S. , Frank Joseph A. , Duyn Jeff H.
Published: March 2007

To evaluate the sensitivity of fast, gradient‐echo MR scan techniques in their ability to detect blood oxygenation level dependent (BOLD) signal changes in task activation studies, three dedicated fast scan techniques, each with whole‐brain coverage, were compared during a 3‐min finger tapping paradigm on nine normal volunteers on a clinical 1.5 T scanner. Multislice (2D) single‐shot spiral, 3D spiral, and multislice (2D) single‐shot EPI scan techniques were done with similar temporal and spatial resolutions on each of the volunteers in random order. After image registration and statistical analysis, the sensitivity to detect activation was evaluated for the techniques by calculatingtscores and number of activated voxels in predetermined regions of interest, including the contralateral primary sensorimotor cortex, the premotor region, the parietal region, the supplementary motor area, and the ipsilateral cerebellum. Baseline images acquired with the three techniques were qualitatively comparable and had a similar effective spatial resolution of around 5 × 5 × 5 mm3, as determined from autocorrelation analysis. The anatomical coverage was somewhat reduced (4 less slices per volume) with EPI at the identical temporal resolution of 1.76 s for all techniques. The use of multislice 2D spiral scan for motor cortex fMRI experiments provided for a superior overall temporal stability, and an increased sensitivity compared with multislice 2D EPI, and 3D spiral scan. The difference in sensitivity between multislice 2D spiral and EPI scans was small, in particular in the case of a ramp‐sampled version of EPI. The difference in performance is attributed mainly to the difference in scan‐to‐scan stability.
Three‐dimensional spiral fMRI technique: A comparison with 2D spiral acquisition
Authors: Lai Song , Glover Gary H.
Published: March 2007

A comparison study was performed of 3D and 2D spiral k‐space fMRI techniques using BOLD contrast with a 4.25‐min finger‐tapping task paradigm. The 3D sequence uses the conventional 2D spiral technique in conjunction with Fourier phase encoding in the slice select direction. Characteristics that were compared included image appearances, signal‐to‐noise ratio (SNR), fluctuation noise, functional contrast‐to‐noise ratio (fCNR), detected activation areas, inflow effect, and large vessel involvement. The results showed that, with constant total scan time, the 3D spiral acquisition has higher SNR and fCNR despite its slightly higher fluctuation noise. The capability for thin‐slice incoherent averaging is a further advantage of 3D over 2D. With these advantages as well as its intrinsic ability for contiguous slices and image reformatting, the 3D spiral fMRI technique may be superior to its 2D counterpart.
Imaging lungs using inert fluorinated gases
Authors: Kuethe Dean O. , Caprihan Arvind , Fukushima Eiichi , Waggoner R. Allen
Published: March 2007

Rat lungs were imaged by 19F projection MRI of hexafluoroethane, mixed with 20% oxygen to form the inhaled gas. The 3D image had 700 μm resolution, and the data took 4.3 h to acquire. Free induction decays were collected in the presence of steady magnetic field gradients in 686 different directions. To take advantage of fast relaxation (T1 = 5.9 ± 0.2 ms), the repetition time was 5 ms. To eliminate signal loss from magnetic field inhomogeneities, data were collected within 2 ms of spin excitation (from 80 μs to 2 ms after the 42‐μs π/2 pulses). The singular value decomposition of the transform from frequency to time domain was used to obtain projections despite the absence of data during and immediately after the RF pulses. Inert fluorinated gas imaging may be less expensive than polarized noble gas imaging and is appropriate for imaging steadystate rather than transient gas concentrations.