bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of bloch model MT spin system parameters from Z‐spectral data
Authors: Holt R. W. , Duerk J. L. , Hua J. , Hurst Gregoly C.
Published: March 2007

Previous studies have described magnetization transfer (MT) Z‐spectra in terms of a two‐pool Bloch model, with six spin‐system parameters KA, F, T1A, T1B, and T2B. By simulation, we show that a process including nonlinear constrained optimization can be used to accurately and uniquely estimate spin‐system parameters from MT 2‐spectra prepared by continuous wave (CW) RF irradiation. Experiments producing Z‐spectra by pulsed RF irradiation give substantially different magnetization values, relative to MT acquisitions obtained by CW RF irradiation, at small offset frequencies, with a consequence that only T2B can be uniquely determined. However, several equalities and bounds involving four of the other parameters (KA, F, T1A, T1B) are derived, which are applicable to pulsed data. These relationships allow calculation of “free pool” magnetization corresponding to complete saturation of the restricted pool, without requiring that this complete saturation be experimentally achieved. MT experimental data from pulsed RF irradiation on boiled egg albumin, obtained using a clinical whole‐body MRI system, are analyzed using an optimization algorithm and the derived expressions.
In Vivo and in vitro EPR oximetry with fusinite: A new coal‐derived, particulate EPR probe
Authors: Vahidi N. , Clarkson R. B. , Liu K. J. , Norby S. W. , Wu M. , Swartz H. M.
Published: March 2007

The peak‐to‐peak line width (LW) of the first derivative electron spin resonance (EPR) spectrum of the coal maceral fusinite is reversibly broadened by O2. The extent of broadening per unit of partial pressure of oxygen (pO2) is unusually large, exceeding that of nitroxides by almost two orders of magnitude. This paramagnetic properly of fusinite, combined with its very stable physicochemical properties and low toxicity, is shown to be of utility in the measurement of pO2 in vitro and in vivo. Fusinite particles are endocytosed by Chinese hamster ovary (CHO) cells in vitro; this is useful for intracellular O2 measurements with commercially available EPR spectrometers operating at 9.1‐9.3 GHz. For measurement of oxygen in vivo using low frequency EPR (1.l‐1.3 GHr), fusinite provides a sensitive and persistent means to measure pop in tissues. Particles implanted into the gastrocnemius muscle of NJ mice remained interstitially in the same position for months with undiminished sensitivity to pO2 and no specific toxic effects.
MRA studies of arterial stenosis: Improvements by diastolic acquisition
Authors: Saloner David , Selby Kathy , Anderson Charles M.
Published: March 2007

Cardiac‐phase‐specific data acquisition is used to reduce signal loss in MR Angiography resulting from disturbed flow. RF pulses are delivered continuously throughout the cardiac cycle, but incrementation of phase‐encoding gradients and data storage are enabled only during the chosen part of the cycle. Studies in a stenotic pulsatile flow phantom demonstrate that poststenotic signal loss is primarily determined by the mean flow velocity, and is not appreciably affected by acceleration or deceleration of the mean flow rate. The signal loss is least in diastole. In vivo studies in patients with carotid artery disease show that data acquisition in diastole reduces the apparent degree and extent of carotid bifurcation stenosis and provides a crisper definition of the vascular lumen. The additional time required for cardiac‐phase‐specific acquisition can be reduced by gating only the lower‐order phase‐encoding lines while retaining acceptable image quality.
The presence of cytoplasmic lipid droplets is not sufficient to account for neutral lipid signals in the 1H spectra of neutrophils
Authors: May G. L. , Sztelma K. , Sorrell T. C.
Published: March 2007

Stimulation of human peripheral blood neutrophils with lipopolysaccharide (LPS), arachidonic acid (AA) and oleic acid (OA) resulted in significant increases in cytoplasmic lipid droplets. This phenomenon was also observed in enucleated and degranulated cytoplasts prepared from neutrophils stimulated with LPS. In contrast, only LPS and high concentrations of OA (10 μM) produced an increase in the lipid intensities of the MR spectra of neutrophils as determined by COSY cross peak volume measurements. Lipid intensities in cells stimulated with OA (2.5 μM) and AA (2.5 μM) and phorbol myristate acetate (20 nM) were not elevated. LPS stimulation of resting cytoplasts resulted in increased lipid droplets but not MR lipid intensities. These data suggest that while cytoplasmic lipid droplets may correlate with MR lipid intensity under some circumstances, their presence is not sufficient to account for increased neutral lipid signals.
Comparisons of ATP turnover in human muscle during ischemic and aerobic exercise using 31P magnetic resonance spectroscopy
Authors: Kemp G. J. , Thompson C. H. , Barnes P. R. J. , Radda G. K.
Published: March 2007

To investigate human muscle bioenergetics quantitatively in vivo, we used 31P magnetic resonance spectroscopy to study the flexor digitorum superficialis of four adult males during dynamic ischemic and aerobic exercise at 0.50–1.00 W and during recovery from aerobic exercise. During exercise, changes in pH and [PCr] were larger at higher power, but in aerobic exercise neither end‐exercise [ADP] nor the initial postexercise PCr resynthesis rate altered with power. In ischemic exercise we estimated total ATP synthesis from the rates of PCr depletion and glycogenolysis (inferred using an analysis of proton buffering); this was linear with power output. In aerobic exercise, again we estimated ATP synthesis rates due to phosphocreatine hydrolysis and glycogenolysis (incorporating a correction for proton efflux) and also estimated oxidative ATP synthesis by difference, using the total ATP turnover rate established during ischemic exercise. We conclude that in early exercise oxidative ATP synthesis was small, increasing by the end of exercise to a value close (as predicted) to the initial postexercise rate of PCr resynthesis. Furthermore, a plausible estimate of proton efflux during aerobic exercise can be inferred from the pH‐dependence of proton efflux in recovery.
Phantom studies simulating the impact of trabecular structure on marrow relaxation time, T2′
Authors: Engelke Klaus , Majumdar S. , Genant H. K.
Published: March 2007

Phantom studies were conducted to investigate the impact of trabecular structure on the T2′ signal measured by MRI. For a separation of density from structural effects, several phantoms were built. They consisted of parallel polyethylene strings arranged in a variety of different patterns to simulate (a) a constant uniform trabecular distribution with increasing trabecular thickness and (b) different structures with identical overall trabecular density. An asymmetric spin echo sequence was used to determine the apparent relaxation time T2′. Changes in T2′ are induced by susceptibility differences between the polyethylene strings simulating trabeculae and Gd‐TPA doped saline simulating bone marrow. The results showed an increasing T2′ decay rate with (a) decreasing spacing while the string density was constant and (b) with increasing string density while the string arrangement was constant. The results demonstrate that the T2′ signal is affected not only by density but also by spatial distribution. However, the results also indicate that a separation of the two effects is not possible from a T2′ measurement alone, but that e.g., a matching CT slice that would provide purely density information is additionally needed. Theoretical simulations confirmed these results.
Gd‐DOTA as a gastrointestinal contrast agent for gastric emptying measurements with MRI
Authors: Schwizer W. , Fraser R. , Maecke H. , Siebold K. , Funck R. , Fried M.
Published: March 2007

Current MR meal markers may interfere with gastric motility and secretion restricting the use of MRI in the measurement of gastric physiology. We therefore evaluated Gd‐DOTA as a liquid phase marker, in vitro by determining dissociation, and adherence to the solids, and in vivo by simultaneous MRI (0.35 T scanner, multiple T1‐weighted sections of the upper abdomen) and double indicator (perfusion marker PEG 4000, meal marker 99mTc‐DTPA) measurements of emptying and secretion, following ingestion of 500 ml 10% glucose. In vitro Gd‐DOTA was stable at a pH > 2 with < 2% dissociation at 24 h during incubation with HCI. Dissociation during incubation with HCI was linearly dependent on H+ concentration (0.77 < pH < 2.02). Less Gd‐DOTA was absorbed onto the solid phase than 99mTc‐DTPA (25% cf 36%). in vivo Gd‐DOTA marked gastric contents provided strong positive contrast. Similar emptying curves were observed with both MRI and double‐indicator techniques (r = 0.987, P < 0.001). Gd‐DOTA has the potential to be a useful liquid phase contrast agent in MR studies on gastric function.
Localized proton MR spectroscopy of the human kidney in vivo by means of short echo time STEAM sequences
Authors: Dixon Ruth M. , Frahm Jens
Published: March 2007

In order to obtain proton magnetic resonance spectra from the normal human kidney in vivo, we employed a STEAM sequence with delay times TE= 10 ms and TM = 30 ms. Signals are attenuated during STEAM sequences by J‐coupling effects and by macroscopic movement of the sample. The combination of short echo times and respiratory triggering ensured that the kidney was stationary during the pulse sequence, and allowed us to detect strongly coupled resonances between 3 and 4.2 ppm. Analysis of spectra of extracts of bovine kidneys suggested that the renal MR‐visible metabolites could include the osmolytes betaine, myo‐inositol, and glycerophosphocholine. Four volunteers were subjected to overnight dehydration followed by rehydration, and we found that these signals increased significantly after dehydration, and decreased significantly 4 h after rehydration, thus supporting the assignment of the resonances as osmotically active metabolites.
Reduction of signal fluctuation in functional MRI using navigator echoes
Authors: Hu Xiaoping , Kim Seong‐Gi
Published: March 2007

Functional magnetic resonance imaging is sensitive to signal fluctuations due to physiological motion and system instability. In this paper, motion‐related signal fluctuations are studied, and a method that uses navigator echoes to monitor and compensate for signal fluctuations in a gradient‐echo sequence is described. The technique acquires a “navigator” signal before the application of the phase‐encoding and readout gradients and corrects the phase of the subsequently acquired imaging data. This technique was implemented on a 4 Tesla whole body system and validated on normal volunteers. With this technique, temporal fluctuations in image intensity were substantially reduced and improved functional activation maps were obtained.
Real time blood flow imaging by spiral scan phase velocity mapping
Authors: Gatehouse P. D. , Firmin D. N. , Collins S. , Longmore D. B.
Published: March 2007

The work describes the development of a novel sequence that uses rapid spiral k‐space sampling, combined with phase velocity mapping, for real time flow velocity imaging. The performance of the technique is assessed on phantoms for both through‐plane and in‐plane flows. The flow measurements compared well with those measured using a bucket and stopwatch. One advantage of the technique is that flow related signal loss is minimal due to the early acquisition of the center of k‐space data. Flow artifacts were observed for in‐plane flow and these were understood with the aid of computer simulations. In vivo studies involved cine velocity mapping in normal volunteers; aortic blood flow waveforms acquired by spiral scanning in two cardiac cycles compared well with data from a conventional gradient‐echo sequence. Potential applications of the method are demonstrated by studying the response of aortic flow to physical exercise and the real time monitoring of aortic flow during a valsalver maneuver.
Double‐echo multislice proton spectroscopic imaging using Hadamard slice encoding
Authors: Dreher Wolfgang , Leibfritz Dieter
Published: March 2007

Simultaneous multislice proton spectroscopic imaging (SI) is presented using a pulse sequence with multifrequency selective RF excitation and Hadamard encoding in the slice direction, and conventional Fourier phase encoding in the in‐plane directions. Double‐echo data acquisition is used to increase the spectral information of the experiment. Tests on a phantom demonstrate the quality of the slice selection. Results of in vivo measurements on the healthy rat brain show that spectra with a high signal‐to‐noise ratio can be acquired from four slices within 32 min. The measurements were performed at 4.7 T using a field of view of 32 × 32 mm2, a slice thickness of 3 mm, and a voxel size of 12 μl. The proposed method is a useful alternative to sequential multislice SI and 3D SI. Furthermore, it is possible to combine sequential and simultaneous multi‐slice SI.
Studies on bromobenzene‐induced hepatotoxicity using in vivo MR microscopy with surgically implanted RF coils
Authors: Zhou Xiaohong , Maronpot Robert R. , Gofer Gary P. , Hedlund Laurence W. , Johnson G. Allan
Published: March 2007

Using surgically implanted RF coils at 300 MHz, three‐dimensional microscopic MR images of rat liver were obtained in vivo to follow the development of pathology induced by bromobenzene exposure. Formalin fixed specimens of liver from these animals were also imaged using in vitro MR microscopy, followed by conventional optical microscopy. All MR images were acquired using a spin‐warp pulse sequence with TR = 950 ms and TE= 23 ms. The in vivo images were reconstructed as 2562 × 32 arrays with a voxel size of (50 μm)2 × 219 pm, while the in vitro images were reconstructed as 2562 × 128 arrays, giving an isotropic resolution at (39 μm)2. Based on results from six animals, we have found in all animals exposed to bromobenzene, image intensity decreased in specific hepatic tissue regions. These regions were well correlated to low signal intensity areas observed in in vitro MR images at higher resolution. Conventional optical microscopy indicated that the low signal intensity regions corresponded to areas of necrosis. The decrease in signal intensity is consistent with increased local diffusion coefficients as a result of necrosis. This study demonstrates that MR microscopy with implanted RF coils can be successfully used to follow tissue pathological changes in living tissues.
Measurement of the intracellular concentration of oxygen in a cell perfusion system
Authors: Chen Kai , Ng Cheng E. , Zweier Jay L. , Kuppusamy Periannan , Glickson Jerry D. , Swartz Harold M.
Published: March 2007

[O2] was measured in the embedding material (alginate) in a typical apparatus for conducting studies of viable cells with NMR, using low frequency EPR. In suspension cultures respiration was independent of [O2] in the perfusing media down to about 1 μM while in alginate beads, the comparable value was 70 μM, indicating that the alginate was a very substantial barrier to the free diffusion of oxygen. With knowledge of [O2] in the various compartments, [O2] in the perfusing medium can be increased and the full power of NMR can be used to provide information on metabolism under various conditions. These results also provide evidence supporting the feasibility and usefulness of EPR techniques using nitroxides to measure [O2] in macroscopic samples such as NMR perfusion tubes. This technique is rapid, apparently nonperturbing, and enables one to differentiate between the concentrations of oxygen in different compartments.
Relaxivity enhancement of low molecular weight nitroxide stable free radicals: Importance of structure and medium
Authors: Vallet Patrick , van Haverbeke Yves , Bonnet Pierre Antoine , Subra Guy , Chapat Jean‐Pierre , Muller Robert N.
Published: March 2007

The longitudinal relaxivities of seven water‐soluble nitroxide derivatives ot low‐molecular weight have been measured at 5°C and 37°C in water and in serum between 0.01 and 200 MHz. The nuclear magnetic relaxation dispersion (NMRD) profiles show a clear relationship between the relaxivity observed in serum and the relative balance of the hydrophobichydrophilic character of the paramagnetic molecules. From the data analysis, contributions arising from a population of nitroxides characterized by reduced mobility can be extracted. The values of the correlation times are consistent with a system involving nitroxides adsorbed at the surface of albumin and magnetically interacting with the protons of hydrogen bonded water molecules.
T1 discrimination contributions to proton magnetization transfer in heterogeneous biological systems
Authors: Yang Hua , Schleich Thomas
Published: March 2007

Water proton spin‐lattice relaxation times are commonly used as a guide in establishing the off‐resonance irradiation time as well as the repetition time of the magnetization transfer experiment. T1 discrimination effects occur if the motionally restricted spin bath longitudinal magnetization does not reach thermal equilibrium. In this study we developed the formalism necessary for the evaluation of T1 discrimination contributions to proton magnetization transfer arising from the use of a short repetition time relative to the spin‐lattice relaxation time of the motionally restricted spin bath. The results of computer simulation indicate that T1, discrimination contributions occur when the repetition time is small relative to the spin‐lattice relaxation time of the motionally restricted spin bath, and when the off‐resonance irradiation is weak and far off‐resonance. For somewhat longer repetition times, T1 discrimination contributions become important only when the cross relaxation rate is small, and the fractional amount of motionally restricted component large. The occurrence of T1 discrimination effects results in distortion of water proton intensity ratio dispersion curves thereby resulting in the estimation of erroneous magnetization transfer parameters, whereas in magnetization transfer contrast enhanced imaging, such contributions are manifested by a decrease in image contrast.
Projection‐reconstruction methods: Fast imaging sequences and data processing
Authors: Crémillieux Yannick , Briguet André , Deguin André
Published: March 2007

New, rapid two‐ and three‐dimensional imaging sequences based on steady‐state gradient echoes and projection‐reconstruction (PR) techniques are proposed. Quantitative studies show that fast PR sequences and classical, fast gradient‐echo Fourier transform sequences lead to identical contrasts. In order to minimize inhomogeneity effects, a particular focus has been placed on echo‐time reduction. The use of a weighting window permits one to acquire severely truncated echoes; partial k‐space scanning may be considered.
Studies of Gd‐DTPA relaxivity and proton exchange rates in tissue
Authors: Donahue Kathleen M. , Burstein Deborah , Manning Warren J. , Gray Martha L.
Published: March 2007

The image intensity in many contrast agent perfusion studies is designed to be a function of bulk tissue T1, which is, in turn, a function of the compartmental (vascular, interstitial, and cellular) T1s, and the rate of proton exchange between the compartments. The goal of this study was to characterize the compartmental tissue Gd‐DTPA relaxivities and to determine the proton exchange rate between the compartments. Expressing [Gd‐DTPA] as mmol/liter tissue water, the relaxivities at 8.45 T and room temperature were: saline, 3.87 ± 0.06 (mM. s)−1 (mean ± SE; n = 29); plasma, 3.98 ± 0.05 (mM·s)−1 (n = 6); and control cartilage (primarily an interstitium), 4.08 f 0.08 (mM·s)−1 (n = 17), none of which are significantly different. The relexivity of cartilage did not change with compression, trypsinization, or equilibration in plasma, suggesting relaxivity is not influenced by interstitial solid matrix density, charge, or the presence of plasma proteins. T1 relaxation studies on isolated perfused hearts demonstrated that the cellular‐interstitial water exchange rate is between 8 and 27 Hz, while the interstitial‐vascular water exchange rate is less than 7 Hz. Thus, for Gd‐DTPA concentrations, which would be used clinicallly, the T1 relaxation rate behavior of intact hearts can be modeled as being in the fast exchange regime for cellular‐interstitial exchange but slow exchange for interstitial‐vascular exchange. A measured relaxivity of 3.82 ± 0.05 (mM·s)−1 (n = 8) for whole blood (red blood cells and plasma) and 4.16 ± 0.02 (mM·s)−1 (n = 3) for frog heart tissue (cells and interstitium) (with T1 and Gd‐DTPA concentration defined from the total tissue water volume) supports the conclusion of fast cellular‐extracellular exchange. Knowledge of the Gd‐DTPA relaxivity and maintaining Gd‐DTPA concentration in the range so as to maintain fast cellular‐interstitial exchange allows for calculation of bulk Gd‐DTPA concentration from bulk tissue T1 within a calculable error due to slow vascular exchange.
Dual interleaved 1H and proton‐decoupled‐31P in Vivo chemical shift imaging of human brain
Authors: Gonen Oded , Hu Jiani , Murphy‐Boesch Joseph , Stoyanova Radka , Brown Truman R.
Published: March 2007

A technique is demonstrated to obtain interleaved proton (1H) and 1H‐decoupled phosphorus (31P) spectra of human brain using 2D CSI. A modified commercial full‐body imager and a dual‐tuned birdcage head‐coil were employed. Because proton relaxation times are shorter than those of phosphorus, TR(1H) can be chosen to be shorter than TR(31P), thus permitting a 1H acquisition to be inserted in each 31P cycle. The scheme results in significant time savings as both CSI data sets are obtained concurrently with patient loading, coil tuningi, shimming, and imaging needed to be done only once.
Abnormal cerebral metabolite concentrations in patients with probable alzheimer disease
Authors: Moats Rex A. , Ernst Thomas , Shonk Truda K. , Ross Brian D.
Published: March 2007

To establish whether recently described abnormalities of peak ratios are the result of changes in metabolite concentrations, quaintitative 1H magnetic resonance spectroscopy was performed in 10 patients with Alzheimer disease (AD) and seven normal elderly. CSF volumes, metabolite T1 and T2relaxation rates, ratios and concentrations of Kacetyl residues, creatine, choline residues, myeinositol, glutamine plus glutamate (Glx), and glucose were obtained. Difference spectroscopy and quantitative assays showed a 50% increase in myo‐inositol (6.4–9.8 mM; P < 0.005) and a decrease in N‐acetyl in occipital gray matter. A reduction in β,γ‐Glx and a significant increase in intracerebral [glucose], greater than attributable to CSF, were defined. Choline concentration increased with age, but was not elevated above normal in AD patients. These findings indicate the need for quantitative 1H MRS to substantiate metabolite ratios. The increased myo‐inositol concentration in AD is demonstrated by these studies.
Quantitative proton spectroscopy of canine brain: in Vivo and in Vitro correlations
Authors: Barker Peter B. , Breiter Steven N. , Soher Brian J. , Chatham John C. , Forder John R. , Samphilipo Michael A. , Magee Carolyn A. , Anderson James H.
Published: March 2007

Quantitative, single‐voxel proton NMR spectroscopy of normal brain was performed in five adult beagle dogs using the cerebral water signal as an internal intensity reference. The same brain regions were then rapidly isolated and frozen using a pneumatic biopsy drill, perchloric acid extracted, and analyzed by biochemical assay and high‐resolution NMR spectroscopy. The concentrations of the major resonances in the in vivo and in vitro spectra were compared, and good agreement was found between the different measurements. The in vivo spectra contained three peaks at 3.21, 3.04, and 2.02 ppm, which are usually assigned to trimethylamines (TMA), creatines, and N‐acetyl derivatives (NAc), which corresponded to be the following metabolite concentration values: 1.7 ± 0.6, 7.7 ± 2.1, and 10.9 ± 2.7 μmol/g wet weight respectively. In vitro, the following metabolite concentrations were measured: glycerophosphocholine (GPC) 1.3 ± 0.2, phosphocholine (PC) 0.5 ± 0.1, phosphocreatine (PCr) 2.6 ± 0.4, creatine (Cr) 5.9 ± 1.4, and N‐Acetyl aspartate (NAA) 8.9 ± 1.8 μmol/g wet weight. Therefore, the 3.21 ppm resonance observed in the in vivo spectrum is predominantly GPC and PC in a ratio of 2.6:1, the 3.04 ppm resonance is Cr and PCr in a ratio of 2.3:1, and the 2.02 ppm resonance is predominantly (≈︁80%) NAA with small contributions from N‐acetylaspartyl‐glutamate (NAAG) and glutamate. The data presented here validate the technique of water referencing as a simple and convenient means of quantitating single‐voxel in vivo proton NMR spectra of the brain.
Three dimensional magnetic resonance microangiography of rat neurovasculature
Authors: Mellin Andrew F. , Cofer Gary P. , Smith Bradley R. , Suddarth Steven A. , Hedlund Laurence W. , Johnson G. Allan
Published: March 2007

Techniques are described to perform three dimensional (3D) MR microangiography. We have combined the use of a blood pool agent (Gd‐DTPA‐complexed with bovine serum albumin), three dimensional Fourier encoding, careful animal stabilization, and volume rendering to permit imaging with voxels of 60 × 60 × 60 pm. 3DFT encoding has been performed at 7.1 T with very large arrays (256 × 512 × 512). Interactive volume rendering allows a number of unique display opportunities that effectively exploit these isotropic 3D arrays.
High frequency volume coils for clinical NMR imaging and spectroscopy
Authors: Vaughan J. Thomas , Hetherington Hoby P. , Otu Joe O. , Pan Jullie W. , Pohost Gerald M.
Published: March 2007

A tuned transmission line resonator has been developed in theory and in practical design for the clinical NMR volume coil application at 4.1 tesla. The distributed circuit transmission line resonator was designed for high frequency, large conductive volume applications where conventional lumped element coil designs perform less efficiently. The resonator design has made use of a resonant coaxial cavity, which could be variably tuned to the Larmor frequency of interest by tunable transmission line elements. Large head‐ and body‐sized volumes, high efficiencies, and broad tuning ranges have been shown to be characteristic of the transmission line resonator to frequencies of 500 MHz. The B1 homogeneity of the resonator has been demonstrated to be a function of the electromagnetic properties of the load itself. By numerically solving Maxwell's: equations for the fully time‐dependent B1 field, coil homogeneity was predicted with finite‐element models of anatomic structure, and inhomogeneities corrected for. A how‐to exposition of coil design and construction has been included. Simple methods of quadrature driving and double tuning the transmission line resonator have also been presented. Human head images obtained with a tuned transmission line resonator at 175 MHz have clearly demonstrated uncompromised high field advantages of signal‐to‐noise and spatial resolution.
A modified quadrupole gradient set for use in high resolution MRI tagging
Authors: O'Dell Walter G. , Schoeniger Joseph S. , Blackband Stephen J. , McVeigh Elliot R.
Published: March 2007

A special purpose gradient set with a Cos(20) current distribution has been constructed for high resolution MR imaging of small samples in a clinical Signa 1.5 T scanner using the existing gradient amplifiers. The X, Y, and Z gradient coils can attain gradient field strengths of 11.3, 4.7, and 15.2 G/cm at 100 amps current, respectively, with a slew rate of 20 G/cm/ms and usable ramp time of 150 μs. Field distortions are less than 2% over the central 8 cm of the bore, suitable for high resolution tagging of isolated canine hearts.
Interleaved echo planar flow quantitation
Authors: McKinnon Graeme C. , Debatin Jörg F. , Wetter Denis R. , von Schulthess Gustav K.
Published: March 2007

A method is presented for rapid blood flow quantitation. It combines multiple shot echo planar imaging principles with phase contrast velocity encoding gradients. Preliminary validation experiments performed on a flow phantom indicate a good correlation with actual flow values. Measurements of pulsatile flow in the ascending and descending aorta, acquired in just 4 s1 with a temporal resolution of 50 ms, are demonstrated.
Correlation of spectroscopy and magnetization transfer imaging in the evaluation of demyelinating lesions and normal appearing white matter in multiple sclerosis
Authors: Hiehle John F. , Lenkinski Robert E. , Grossman Robert I. , Dousset Vincent , Ramer Karen N. , Schnall Mitchell D. , Cohen Jeffrey A. , Gonzalez‐Scarano Francisco
Published: March 2007

Magnetization transfer imaging (MT) and localized proton spectroscopy (1H‐MRS) were utilized in the evaluation of lesioins (high signal abnormalities on T2‐weighted images) and normal‐appearing white matter (NAWM) in multiple sclerosis (MI). Eleven patients with a clinical diagnosis of MS were independently evaluated with both 1H‐MRS and MT. The magnetization transfer ratio (MTR) of lesions was compared with the relative concentration of Kacetyl‐aspartate (NAA) and a composite peak at 2.1 to 2.6 ppm termed “marker peaks”. The MTR of white matter lesions in the MS patients was markedly decreased (6–34%; normal ≈︁42%), and correlated well with increase in the marker peaks region (0.94–3.89). There was no correlation between the relative concentration of NAA and MTR. Increased resonance peaks in the 2.1 to 2.6 ppm range and marked decreases in MTR may be a relatively specific indicators of demyelination.
Reconstructions of phase contrast, phased array multicoil data
Authors: Bernstein Matt A. , Grgic Mladen , Brosnan Thomas J. , Pelc Norbert J.
Published: March 2007

We present a reconstruction method for phased array multicoil data that is compatible with phase contrast MR angiography. The proposed algorithm can produce either complex difference or phase difference angiograms. Directional flow and quantitative information are preserved with the phase difference reconstruction. The proposed method is computationally efficient and avoids intercoil cancellation errors near the velocity aliasing boundary. Feasibility of the method is demonstrated on human scans.
Assessment of relative brain iron concentrations using T2‐weighted and T2*‐weighted MRI at 3 Tesla
Authors: Ordidge Roger J. , Gorell Jay M. , Deniau Jean C. , Knight Robert A. , Helpern Joseph A.
Published: March 2007

In this paper a new method is presented for the relative assessment of brain iron concentrations based on the evaluation of T2 and T2* ‐weighted images. A multiecho sequence is employed for rapid measurement of T2 and T2*, enabling calculation of the line broadening effect ( T2′). Several groups have failed to show a correlation between T2 and brain iron content. However, quantification of T2′, and the associated relaxation rate R2′, may provide a more specific relative measure of brain iron concentration. This may find application in the study of brain diseases, which cause associated changes in brain iron levels. A new method of field inhomogeneity correction is presented that allows the separation of global and local field inhomogeneities, leading to more accurate T2* measurements and hence, T2′ values. The combination of T2*, and T2‐weighted MRI methods enables the differentiation of Parlkinson's disease patients from normal age‐matched controls based on differences in iron content within the substantia nigra.
Dual‐echo MRI segmentation using vector decomposition and probability techniques: A two‐tissue model
Authors: Kao Yi‐Hsuan , Sorenson James A. , Bahn Mark M. , Winkler Stefan S.
Published: March 2007

We combined a vector decomposition technique with Gaussian probability thresholding in feature space to segment normal brain tissues, tumors, or other abnormalities on dual‐echo MR images. The vector decomposition technique assigns to each voxel a fractional volume for each of two tissues. A probability threshold, based on an assumed Gaussian probability density function describing random noise, isolates a region in feature space for fractional volume calculation that minimizes contamination from other tissues. The calculated fractional volumes are unbiased estimates of the true fractional volumes. The contrast‐to‐noise ratio (CNR) between tissues on the segmented images is the same as the Euclidean norm of CNRs in the original images. The method is capable of segmenting more than two tissues from a set ot dual‐echo images by sequentially analyzing different pairs of tissues. The model is analyzed mathematically and in experiments with a phantom. Two clinical examples are presented.
Modeling and observation of temperature changes in Vivo using MRI
Authors: Young Iain R. , Hand Jeffrey W. , Oatridge Angela , Prior Michael V.
Published: March 2007

Models have been developed for the signal changes occurring in human peripheral muscle after thermal stress for some methods of measuring temperature in vivo. These include the dependency of T1, on temperature, observed by both direct, and magnetization transfer, experiments, and the temperature sensitivity of the diffusion coefficient and the chemical shift of the water proton line. Results show the relative complexity of the T1‐dependent models when there are significant variations in, possibly amongst other things, tissue perfusion, and how the predictions made using them match the behavior of the practical values quite well. Using changes in the diffusion coefficient appears less problematic, although there can still be significant errors in temperature calibrations. The problems with this approach are likely to be due to the consequences of anisotropy in tissue.
Analysis and correction of motion artifacts in diffusion weighted imaging
Authors: Anderson Adam W. , Gore John C.
Published: March 2007

For diffusion‐weighted magnetic resonance imaging and under circumstances where patient movement can be modeled as rigid body motion, it is shown both theoretically and experimentally that translations and rotations produce phase errors which are zero‐ and first‐order, respectively, in position. Whlile a navigator echo can be used to correct the imaging data for arbitrary translations, only when the diffusion gradient is applied in the phase encode direction is there sufficient information to correct for rotations around all axes, and therefore for general rigid body motion. Experiments in test objects and human brain imaging confirm theoretical predictions and demonstrate that appropriate corrections dramatically improve image quality in vivo.
In vivo 31P NMR studies of orientation effects upon rat brain metabolism during mild hypoxia
Authors: Chen Meng , Stolk Jacob A. , Olsen Jay I. , Schweizer Martin P.
Published: March 2007

Energy metabolites in rat brain under the same level of hypoxia were monitored by 31P NMR in both horizontal and vertical magnets. The changes in PCr, P|, and pHI in the vertical setting toward the end of hypoxia were significantly larger and the recovery in the horizontally held animals was more complete. The results demonstrated quantitatively that the stress of the alignment is superimposed on the stress of hypoxia in the vertical magnet.
Assessment of absolute metabolite concentrations in human tissue by 31P MRS in vivo. Part II: Muscle, liver, kidney
Authors: Buchli Reto , Meier Dieter , Martin Ernst , Boesiger Peter
Published: March 2007

Absolute metabolite concentrations were assessed in the muscle, the liver, and the kidney of healthy human volunteers by 31P MRS. Fully relaxed in vivo spectra were acquired with a surface coil and were localized with an adiabatic lSlS pulse sequence. The spectra were quantified with a subsequent measurement of a calibration phantom and were processed iteratively in the time domain. The following mean metabolite concentrations (mmollliter) were measured in the resting male calf muscle (n = 9), in the fasting liver (n = 12), and in the orthiotopic kidney (n= 5): [PME] = 2.0 ± 0.6, 3.8 ± 0.7, and 2.6 ± 0.9, [Pi] = 2.9 ± 0.3, 1.8 ± 0.3, and 1.6 ± 0.4, [PDE] = 3.8 ± 0.8, 9.7 ± 1.5, and 4.9 ± 1.1, [PCr] = 22.0 ± 1.2, 0, and 0, [NTP] = 5.7 ± 0.4, 2.9 ± 0.4, and 2.0 ± 0.3, respectively. Several interesting findings are to be emphasized: The concentrations of Pi, PCr, and NTP were 20% lower in the muscle of women than of men. In addition, the pH, was significantly lower in female muscle (6.99 ± 0.03) than in male muscle (7.05 ± 0.03). The pH, in the liver (7.12 ± 0.09) and in the kidney (7.09 ± 0.08) were higher than in the muscle of both genders. The free magnesium concentration (mmollliter) was higher in the lliver (1.40 ± 0.64) than in the kidney (0.79 ± 0.39) and in the muscle (0.52 ± 0.10).
A four‐channel time domain multiplexer: A cost‐effective alternative to multiple receivers
Authors: Porter Jay R. , Wright Steven M. , Famili Nader
Published: March 2007

The concept of simultaneous image acquisition as originally conceived by Hyde has been implemented by Roemer et al. using a multiple receiver system. This article describes an alternative technique that uses time domain multiplexing (TDM) to acquire simultaneous images using a single receiver channel. This method requires few modifications to the existing receiver and has been shown to be equivalent to a true four channel receiver in most applications. The multiplexing technique was implemented and tested on a standard commercial scanner that has also been equipped with a four channel receiver. Signal‐to‐noise results demonstrate that four independent images can be multiplexed through a single receiver channel with no degradation in image quality. Additionally, spine images that were obtained from a normal volunteer with both a multiplexed single channel receiver and a true four channel receiver system are presented.
2D 1H spectroscopic imaging of the human brain at 4.1 T
Authors: Hetherington Hoby P. , Pan Jullie W. , Mason Graeme F. , Ponder Steven L. , Twieg Donald B. , Deutsch Georg , Mountz James , Pohost Gerald M.
Published: March 2007

A two‐dimensional spectroscopic imaging sequence consisting of an inversion recovery pulse, a plane selective prefocused pulse, and a semiselective water suppression pulse has been used to create 1H spectroscopic images of the human brain with nominal voxels of 0.5 cc. Due to the excellent lipid suppression provided by the inversion recovery pulse and subsequent delay, only planar volume selection is required enabling the entire brain within the slice to be imaged without contamination from extracerebral lipids in the brain voxels. The use of a semiselective refocusing pulse for water suppression permits any echo evolution time to be used, minimizing J‐modulation and T2 losses, while retaining full sensitivity in the lactate resonance. Using this sequence we have visualized the lactate elevation in the peri‐infarct region about a 6‐week‐old stroke.
Phase errors in multi‐shot echo planar imaging
Authors: Feinberg David A. , Oshio Koichi
Published: March 2007

Field inhomogeneity related phase errors in multi‐shot echo planar imaging (EPI) are directly visualized and analyzed in the spatial frequency domain data or ‘k‐space’. The echo time shift (ETS) technique incrementally moves the position of the echo train and improves the phase error function by redistributing phase discontinuities away from the center of k‐space.
31P NMR and triple quantum filtered 23Na NMR studies of the effects of inhibition of Na+/H+ exchange on intracellular sodium and pH in working and ischemic hearts
Authors: Navon Gil , Werrmann Jeffrey G. , Maron Ron , Cohen Sheila M.
Published: March 2007

The triple quantum filtered 23Na NMR method is applied here to measure the effects of EIPA, a specific inhibitor of the Na+/H+antiporter, on relative intracellular sodium concentrations in isolated working hearts at baseline, during ischemia, and at subsequent reperfusion. In analogy to the spectrophotometric isosbestic point, an approach is developed that defines a value of τ at which the effect of the relaxation times on the TQF signal intensities is minimized, and the signals are proportional to the sodium concentration for both ischemic and working hearts. EIPA at 1.5 μ significantly inhibited (P < 0.01) the influx of intracellular Na+ during 20 min of ischemia at 36.2°C in this rat heart model. In parallel31P NMR studies, EIPA had no effect on either the development of acidosis during ischemia or on the recovery of pH, during reperfusion despite its profound effect on intracellular Na+ influx. Thus, under our conditions the Na+/H+ antiporter did not play a critical role in the maintenance of intracellular pH. EIPA treatment resulted in improved recovery (P < 0.005) of mechanical function after 20 min of ischemia. [ATP] was higher in treated hearts during ischemia and reperfusion.
Dynamic contrast‐enhanced MR imaging of the upper abdomen: Enhancement properties of gadobutrol, gadolinium‐DTPA‐polylysine, and gadolinium‐DTPA‐cascade‐polymer
Authors: Adam Gerhard , Neuerburg Jörg , Spüntrup Elmar , Mühler Andreas , Scherer Kira , Günther Rolf W.
Published: March 2007

The enhancement properties of gadobutrol (40 and 80 μmol/kg body weight, 550 daltons), gadolinium‐DTPA‐polylysine (20 μmol/kg body weight, 53,000 daltons) and gadolinium‐DTPA‐cascade‐polymer (20 μmol/kg body weight, < 30,000 Daltons) were investigated in abdominal MR imaging using a pig model (n = 24). Signal intensities before and after contrast media application were assessed using a fast single slice FLASH sequence. Measurements were made every 4 s within the first 116 s, every minute between 4 and 10 min and after 15,20,30,40, 50,60,90, and 120 min after contrast media injection. Injection of gadobutrol resulted in typical signal intensity curves characterizing it as an extracellular agent similar to gadopentetate dimeglumine. Significant enhancement was found in all tissues except the trunk muscles when the lower dose was administered. Gadolinium‐DTPA‐polylysine injection resulted also in significant enhancement of the liver, the pancreas, and the renal cortex, but not of the trunk muscle, reflecting its blood pool properties known also from other macromolecular contrast agents. The signal intensity curves obtained after gadolinium‐DTPA‐cascade‐polymer injection were similar to those obtained after polylysine injection, stressing the blood pool character of this new type of blood pool agent.
A new T2 preparation technique for ultrafast gradient‐echo sequence
Authors: Parrish Todd , Hu Xiaoping
Published: March 2007

The T2, contrast in images obtained with driven equilibrium (90°x−180°x−90°x) prepared ultrafast gradient‐echo sequences is compromised by the longitudinal magnetization build‐up after the second 90°x pulse, which does not carry T2 information. This paper describes a new T2 contrast preparation technique for ultrafast gradient‐echo sequence that suppresses the signal arising from the build‐up. By dephasing in the preparation and rephasing in the acquisition of the gradient echoes, the new technique eliminates signals that are not dictated by the T2 contrast in a driven‐equilibrium approach. Consequently, it generates an image that is essentially T2‐weighted. Phantom and in vivo experiments were conducted to validate the technique and to demonstrate its clinical utility. These studies indicate that the technique works properly and can be used for in vivo studies.
Reduced power selective excitation radio frequency pulses
Authors: Shinnar Meir
Published: March 2007

A pulse synthesis algorithm is described that allows for the synthesis of selective radio frequency pulses requiring less peak power. The pulses thus synthesized have the same duration, total energy, and frequency response as those synthesized by the standard algorithm. Their phase function is, however, different. The power reduction is typically on the order of 60–70%. This modification will increase the utility of optimized selective pulses.
Experimental encephalomyelitis modulates inositol and taurine in the spinal cord of biozzi mice
Authors: Preece Nicholas E. , Amor Sandra , Baker David , Gadian David G. , O'Neill Janet K. , Urenjak Jutta
Published: March 2007

In this high resolution magnetic resonance spectroscopic study of experimental allergic encephalomyelitis (EAE) and Semliki Forest virus (SFV) infection of the Bioui AB/H mouse, marked increases in the initially low levels of N‐trimethyl compounds in the spinal cord were observed during probable demyelinating episodes. There was also a pronounced and reproducible modulation of the levels of taurine and myo‐inositol during acute and again during chronic relapsing EAE. The ratio of N‐acetyl‐aspartate to creatine in the spinal cord of mice infected with the mutant M9 strain of SFV decreased to approximately 70% of that seen in normal mice.
q‐Space imaging of the brain
Authors: King Martin D. , Houseman John , Roussel Simon A. , Van Bruggen Nicholas , Williams Stephen R. , Gadian David G.
Published: March 2007

q‐Space imaging (Callaghan, J. Magn. Reson. 88,493 (1990)) has been used to obtain mouse brain water displacement profiles. These profiles take the form of a unidirectional incoherent‐displacement probability density distribution. Two groups of mice were studied, a normal group and one in which surgery had been performed to reduce the supply of blood to the forebrain. In the normal group the incoherent displacement of water was reducedpostmortem. Four of the surgically treated mice yielded displacement profiles that resembled those obtained postmortem; the remaining two were near normal. This study demonstrates the feasibility of in vivo q‐space imaging. The displacement profile changes that occur subsequent to an interruption of the forebrain blood supply are consistent with the hyperintensity changes seen in diffusion‐weighted imaging.
Measurement of vascular volume fraction and blood‐tissue permeability constants with a pharmacokinetic model: Studies in rat muscle tumors with dynamic Gd‐DTPA enhanced MRI
Authors: Su Min‐Ying , Jao Jo‐Chi , Nalcioglu Orhan
Published: March 2007

We propose a compartmental model to explain the signal enhancement curves following the bolus injection of Gd‐DTPA. The model incorporates vascular volume fraction contribution, and the possibility of having different transport constants between the plasma and extravascular components. A Walker 256 carcinoma grown in rat muscle was used to demonstrate the capability of this model. Several different types of tissues were included in the measurements: normal, quickly enhanced, slowly enhanced, and necrotic tissues. Blood volume and blood‐tissue permeability information can be derived from the dynamic contrast‐enhanced MRI study employing the proposed model. In the tissue contrast enhancement curve, the initial rising slope after injection is related to the blood volume (or, vascular volume fraction), the maximum enhancement ratio is related to the uptake of tissue, and the decay rate is related to the clearance of tracer from tissue. The measured permeability constant is not the conventional permeability; instead they are contrast agents uptake and clearance rates, which are limited by the blood perfusion. These parameters can be used to characterize different enhancement patterns.