bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic resonance imaging of transverse acoustic strain waves
Authors: Muthupillai Raja , Rossman Phillip J. , Lomas David J. , Greenleaf James F. , Riederer Stephen J. , Ehman Richard L.
Published: March 2007

We describe a phase contrast based MRI technique with high sensitivity to cyclic displacement that is capable of quantitatively imaging acoustic strain waves in tissue‐like materials. A formalism for considering gradient waveforms as basis functions to measure arbitrary cyclic motion waveforms is introduced. Experiments with tissue‐like agarose gel phantoms show that it is possible to measure small cyclic displacements at a submicron level by an appropriate choice of the applied gradient basis function and to use this capability to observe the spatial and temporal pattern of displacements caused by acoustic strain waves. The propagation characteristics of strain waves are determined by the mechanical properties of the media. It is therefore possible to use this technique to noninvasively estimate material properties such as elastic modulus.
Off‐resonance image artifacts in interleaved‐EPI and GRASE pulse sequences
Authors: Mugler John P. , Brookeman James R.
Published: March 2007

Echo‐time shifting (ETS) is used in GRASE and interleaved‐EPI sequences to improve the phase evolutions for off‐resonance signal sources. However, even with ETS the phase evolutions still exhibit discontinuities. In this work, we extend previous studies of ETS by quantitatively evaluating the magnitude and form of the image artifacts that result from these phase discontinuities. The functional form of the phase evolution is used to derive the general conditions under which artifacts are expected. The artifacts for two sequence structures are then evaluated as a function of off‐resonance frequency and data sampling period by calculating point spread functions and simulated images. It was found that even when ETS is used to improve the phase evolutions, periodic phase discontinuities may degrade image quality by producing ghosting artifacts of edges. These artifacts are similar to those that commonly occur with periodic motion. From our results recommendations are derived for limiting the ghosting artifacts.
OIL FLAIR: Optimized interleaved fluid‐attenuated inversion recovery in 2D fast spin echo
Authors: Listerud John , Mitchell Jim , Bagley Linda , Grossman Robert
Published: March 2007

Inversion recovery may be used to suppress signal from cerebrospinal fluid, a technique which has been named “fluid attenuated with inversion recovery” (FLAIR). This report describes interleaving a slice selective inversion pulse within a rapid spin‐echo sequence to obtain the desirable contrast characteristics of FLAIR in imaging times comparable to standard rapid spin echo. Additionally, the pulse repetition time is allowed to float above a defined minimum, which can further shorten scan times and dramatically ease the optimization process. The optimized interleaved sequence is referred to as OIL FLAIR.
Examples of the design of screened and shielded RF receiver coils
Authors: Burl Michael , Young Ian R.
Published: March 2007

Three methods of screening RF surface coil receivers are discussed. The aim is to design coils with very much greater sensitivity on one side than on the other so that signal and noise contributions from unwanted regions of the body can be minimized. These methods can usefully be applied to situations wherein data are only wanted from one leg, although the coil is most conveniently located between the two, or from an arm lying adjacent to the body.
Quantitation of automated single‐voxel proton MRS using cerebral water as an internal reference
Authors: Soher Brian J. , Hurd Ralph E. , Sailasuta Napapon , Barker Peter B.
Published: March 2007

Data from a previously published, multi‐site trial (P. G. Webb, N. Sailasuta, S. J. Kohler, T. Raidy, R. A. Moats, R. E. Hurd, Automated single‐voxel proton MRS: technical development and multisite verification. Magn. Reson. Med. 31, 365–373 (1994)) of a fully automatic, single‐voxel, proton spectroscopy package (PROBE/SV, GE Medical Systems) was re‐analyzed in terms of absolute metabolite concentrations using the cerebral water signal as an internal reference. In 100 spectra from parietal white matter in normal volunteers ranging in age from 22 to 34 years at eight sites, overall concentrations of choline (Cho), creatine (Cr), and N‐acetyl‐aspartate (NAA) resonances were found to be 2.00 ± 0.50, 8.43 ± 1.26, and 12.55 ± 1.76 μmol/g wet weight, respectively. These values are in good general agreement with previously published values from quantitative, single‐voxel studies. Metabolite concentrations for NAA, Cr, and Cho across all sites had standard deviations of 14.1%, 14.9%, and 25.1%, respectively. Quantitation of PROBE data sets is routinely possible by using the cerebral water signal as an internal reference.
Determinants of tissue delivery for 129Xe magnetic resonance in humans
Authors: Peled Sharon , Jolesz Ferenc A. , Tseng Ching‐Hua , Nascimben Luigino , Albert Mitchell S. , Walsworth Ronald L.
Published: March 2007

Magnetic resonance imaging using laser‐polarized 129Xe is a new technique first demonstrated by Albert et al. (Nature 370, 1994) who obtained a 129Xe image of an excised mouse lung. This paper describes the factors influencing the accumulation of inhaled, polarized 129Xe in human tissue. The resulting model predicts the 129Xe magnetization in different tissues as a function of the time from the start of inhalation, the tissue perfusion rate and partition coefficient for xenon, and the relevant T1 decay times. The relaxation times of 129Xe in biological tissues are not yet known precisely. Substitution of estimated values for these parameters results in an expected signal‐to‐noise ratio (SNR) from polarized 129Xe MR in the brain of approximately 2% of the equivalent SNR from proton MR.
Time‐resolved contrast‐enhanced 3D MR angiography
Authors: Korosec Frank R. , Frayne Richard , Grist Thomas M. , Mistretta Charles A.
Published: March 2007

An MR angiographic technique, referred to as 3D TRICKS (3D time‐resolved imaging of contrast kinetics) has been developed. This technique combines and extends to 3D imaging several previously published elements. These elements include an increased sampling rate for lower spatial frequencies, temporal interpolation of k‐space views, and zero‐filling in the slice‐encoding dimension. When appropriately combined, these elements permit reconstruction of a series of 3D image sets having an effective temporal frame rate of one volume every 2‐6 s. Acquiring a temporal series of images offers advantages over the current contrast‐enhanced 3D MRA techniques in that it i) increases the likelihood that an arterial‐only 3D image set will be obtained, ii) permits the passage of the contrast agent to be observed, and iii) allows temporal‐processing techniques to be applied to yield additional information, or improve image quality.
Quantification of citrate concentration in the prostate by proton magnetic resonance spectroscopy: Zonal and age‐related differences
Authors: Lowry Martin , Liney G. P. , Turnbull L. W. , Manton D. J. , Blackband S. J. , Horsman A.
Published: March 2007

A commercial phased‐array multicoil was used to acquire water‐suppressed localized proton spectra of the two major anatomical regions of the prostate. The signal‐to‐noise ratio and spectral resolution allowed identification of peaks from choline and creatine, as well as a major peak from citrate. Quantification of the citrate peak using experimentally determined relaxation parameters with tissue water as an internal concentration reference revealed a marked variability between different volunteers. Nevertheless, in each case, the citrate concentration was up to fourfold greater in the peripheral zone than in the central gland. Furthermore, the difference in citrate concentration between these two regions was positively correlated with the subjects age. The results indicate a consistent difference in cellular function between the major anatomical regions within the prostate and may have important consequences for the application of magnetic resonance spectroscopy to the diagnosis of prostatic pathology.
A computational strategy for the deconvolution of NMR spectra with multiplet structures and constraints: Analysis of overlapping 13C‐2H multiplets of 13C enriched metabolites from cell suspensions incubated in deuterated media
Authors: Laatikainen Reino , Niemitz Matthias , Malaisse Willy J. , Biesemans Monique , Willem Rudolph
Published: March 2007

A computational strategy for the deconvolution of complex spectra involving scalar multiplet patterns is presented. This approach fits spectra that can be composed of single resonances as well as scalar coupling multiplets for which resonance frequencies, intensities, and lineshape parameters can be optimized. For multiplets, the coupling constant also is optimized. Any external information about the optimizable parameters can be taken into account as external constraints. A lineshape described by absorptive and dispersive Lorentzian and Gaussian contributions and the baseline with up to 40 Fourier and polynomial terms can likewise be optimized. The effectiveness of the procedure is assessed on the basis of computer simulated deconvolutions of a composite of 1J(13C−2H) multiplets arising from a mixture of all possible 13C−2H isotopomers of deuterated L‐[3‐13C]lactate generated from cell preparations incubated with D‐[1‐13C]glucose in D2O, which was analyzed previously with a manual deconvolution procedure (R. Willem, M. Biesemans, F. Kayser, W. J. Malaisse, Magn. Reson. Med. 31, 259–267 (1994)). The use of constraints is shown to lead to an improvement in the results. The fitting strategies and the importance of the baseline as an origin of bias are discussed.
An evaluation of the effects of susceptibility changes on the water chemical shift method of temperature measurement in human peripheral muscle
Authors: Young Ian R. , Hajnal Joseph V. , Roberts Idris G. , Ling Junxiao X. , Hill‐Cottingham Roger J. , Oatridge Angela , Wilson Jason A.
Published: March 2007

Divergencies between chemical shift measurements of temperature and directly measured values using optical sensors have been studied in vivo in human peripheral muscle with the assistance of a variety of experimental and theoretical techniques. These include the modeling of both thermal and susceptibility changes using two‐ and three‐dimensional finite element methods, as well as the use of multi‐wavelength near infrared observations. The conclusion of these studies is that a simple temperature calibration is not accessible, with results affected by the complex response of the tissue itself.
A method to improve the BO homogeneity of the heart in vivo
Authors: Jaffer Farouc A. , Wen Han , Balaban Robert S. , Wolff Steven D.
Published: March 2007

A homogeneous static (Bo) magnetic field is required for many NMR experiments such as echo planar imaging, localized spectroscopy, and spiral scan imaging. Although semi‐automated techniques have been described to improve the Bo field homogeneity, none has been applied to the in vivo heart. The acquisition of cardiac field maps is complicated by motion, blood flow, and chemical shift artifact from epicardial fat. To overcome these problems, an ungated three‐dimensional (3D) chemical shift image (CSI) was collected to generate a time and motion‐averaged Bo field map. Bo heterogeneity in the heart was minimized by using a previous algorithm that solves for the optimal shim coil currents for an input field map, using up to third‐order current‐bounded shims (1). The method improved the Bo homogeneity of the heart in all 11 normal volunteers studied. After application of the algorithm to the unshimmed cardiac field maps, the standard deviation of proton frequency decreased by 43%, the magnitude 1H spectral linewidth decreased by 24%, and the peak‐peak gradient decreased by 35%. Simulations of the high‐order (second‐ and third‐order) shims in Bo field correction of the heart show that high order shims are important, resulting for nearly half of the improvement in homogeneity for several subjects. The T2* of the left ventricular anterior wall before and after field correction was determined at 4.0 Tesla. Finally, results show that cardiac shimming is of benefit in cardiac 31P NMR spectroscopy and cardiac echo planar imaging.
Improved centric phase encoding orders for three‐dimensional magnetization‐prepared mr angiography
Authors: Wilman Alan H. , Riederer Stephen J.
Published: March 2007

New centric phase encoding orders are introduced for transient state three‐dimensional MR imaging. Unlike previous centric orders, these variants are not fixed with respect to differences in the relative y and z fields of view, but are tailored to account for the differential degrees of gradient dephasing in the ky and kz phase encoding directions and thus maintain the highest weighting for the most central k‐space values. The new centric orders are compared with previous methods in both simulation and experimental magnetization‐prepared MR angiography of the carotid and renal arteries. In comparison with other tested centric orders, the new methods provided better retention of true vessel shape caused by isotropic blurring across the phase encoding plane. In addition, the new methods provided increased contrast, with the degree of improvement over previous methods depending on vessel orientation, size, and degree of inflow.
Apparent diffusion coefficients in benign and secondary progressive multiple sclerosis by nuclear magnetic resonance
Authors: Horsfield Mark A. , Lai Ming , Webb Stephanie L. , Barker Gareth J. , Tofts Paul S. , Turner Robert , Rudge Peter , Miller David H.
Published: March 2007

The diffusion characteristics of water in brain white matter were studied in patients with benign and secondary progressive multiple sclerosis (MS), and also in normal controls. In the MS patients, both lesions and normal‐appearing white matter (NAWM) were examined to assess whether pathological differences might be evident from the diffusion behavior. A volume‐selective technique was used to reduce data acquisition time and improve the reliability and precision of the measurements. This also allowed the time‐dependence of apparent diffusion coefficients to be assessed. While lesions from both patient groups showed an elevated diffusion coefficient, no differences between the two groups were found. In addition, NAWM was elevated for both patient groups compared with the control group, although this was only statistically significant for patients with a benign disease course. The degree of elevation of the diffusion coefficient within the individual lesions measured was not related to the disability of the patient. Pathological differences between lesions in patients with different disease courses, if they exist, have not been detected in this study of brain water diffusion.
Design of adiabatic selective pulses using optimal control theory
Authors: Rosenfeld Daniel , Zur Yuval
Published: March 2007

Optimal control theory has been applied in the past for the design of RF pulses for selective excitation. This was the outcome of having established the controllability of the MR spin system for the selective excitation problem. “Minimum distance” was the main formulation used for the solution. Because of their robust behavior in the presence of inhomogeneous RF fields, adiabatic pulses play an important role in spin inversion and excitation. In this study, we present a method for incorporating adiabaticity into the optimal control problem by enhancing the cost functional with an appropriate term. Two different types of adiabatic terms are proposed. Furthermore, two methods are used to solve the optimal control problem, namely the Hamiltonian approach and the solution by mathematical programming. Design examples include both a frequency selective pulse for performing fat suppression by inversion and a regular inversion pulse. It is shown that, in the course of optimization, the pulse designer can trade‐off slice resolution against pulse adiabaticity.
Recordings of eye movements for stimulus control during fMRI by means of electro‐oculographic methods
Authors: Felblinger Jacques , Müri René M. , Ozdoba Christoph , Schroth Gerhard , Hess Christian W. , Boesch Chris
Published: March 2007

A method for monitoring eye movements in humans during functional MRI is presented. It is based on the acquisition of electro‐oculographic (EOG) signals near one eye. EOG potentials were amplified and converted into an optical signal just outside the head coil. An optical fiber was used for signal transmission from inside the magnet bore to the control room. The EOG sensor was tested during EPI sequences at 1.5 Tesla without contamination of the MR signal. Some flow related artifacts on the EOG were observed inside the magnet, but no additional interactions from the MR sequence. An analysis of the latency, direction, and amplitude of the saccadic eye movements was possible.
Contribution of Sinerem® used as blood‐pool contrast agent: Detection of cerebral blood volume changes during apnea in the rabbit
Authors: Berry Isabelle , Benderbous Soraya , Ranjeva Jean‐Philippe , Gracia‐Meavilla Dominique , Manelfe Claude , Le Bihan D.
Published: March 2007

The authors suggest that ultra‐small paramagnetic iron oxide (USPIO) particles used as blood pool contrast agents may increase the sensitivity of midfield MRI (i.e., less than 1.5 Tesla) to physiological variations in cerebral blood volume. This hypothesis was tested on a rabbit model of apnea which increases pCO2 and cerebral blood volume. Using Sinerem® as the USPIO at a blood concentration of 60 μmol iron/kg body weight, an 8% T2*‐weighted signal decrease could be observed at 1.0 T with 25–33% increase in pCO2. Comparatively, in the absence of USPIO, T2*‐weighted signal dropped only 4% during apnea and after mild hyperoxygenation beforehand, due to increased deoxyhemoglobin content. These preliminary data suggest that USPIOs could play an important role in functional MRI at midfield strength, by sensitizing the signal to cerebral blood volume changes.
Influence of membrane lipid packing on T2‐weighted magnetic resonance images: Study of relaxation parameters in model membrane systems
Authors: Lee Jumi , Barry Judith A.
Published: March 2007

The detailed mechanisms leading to soft tissue contrast in MRI are not known. To explore the physical basis for this phenomena, the effect of acyl chain packing of phospholipid model membranes on water proton relaxation were investigated at 20 MHz at 40°C. Three variables affecting lipid packing were examined: chain composition, toluene concentration, and pH/ionic strength. It is demonstrated that the T2 of water protons is strongly influenced by changes in lipid packing, while T1 is generally insensitive to these variables. It is also shown that small changes in water content (7 wt%) altered both T1 and T2 in all systems studied. The results suggest that the physical properties of biological membranes in tissues are an important factor in determining T2.
A comparison of phase encoding ordering schemes in T2‐weighted GRASE imaging
Authors: Johnson Glyn , Feinberg David A. , Venkataraman Viswanathan
Published: March 2007

Gradient and spin echo (GRASE) imaging is an echo train imaging sequence that combines gradient and RF refocusing. This combination introduces phase modulations into the echo train. If the phase encoding order is linear with echo time, these modulations cause severe ghosting artifacts. Changing the order of phase encoding can greatly reduce these artifacts. Several phase encoding orders for T2‐weighted sequences are compared in this paper: linear, partially randomized, standard GRASE ordering, and k‐banded (kb) GRASE ordering. Different possible implementations of GRASE and kbGRASE are also considered. Computer simulation is used to compare resolution and artifact levels. Phantom and volunteer images are presented. The linear order is most sensitive to ghosting artifacts associated with chemical shift, susceptibility differences and static field inhomogeneities. The standard GRASE order is least sensitive to these but most vulnerable to artifacts associated with short T2 signals. kbGRASE is a good intermediate between linear and standard GRASE and generally shows the lowest artifact levels. The partially randomized order gives the most diffuse artifacts. Computer simulations show that spatial resolution and contrast with all phase encoding orders are similar.
Real‐time adaptive motion correction in functional MRI
Authors: Lee Christine C. , Jack Clifford R. , Grimm Roger C. , Rossman Phillip J. , Felmlee Joel P. , Ehman Richard L. , Riederer Stephen J.
Published: March 2007

Functional magnetic resonance imaging (fMRI) of the brain is often degraded by bulk head motion. Algorithms that address this by retrospective re‐registration of images in an fMRI time series are all fundamentally limited by any motion that occurs through‐plane. Here, a technique is described that can account for such motion by prospective. correction in real time. A navigator echo is used before every image acquisition to detect superior/inferior displacements of the head. The displacement information is then used to adjust the plane of excitation of the ensuing single‐shot echo‐planar fMRI axial image. These correction updates can be completed in 100 ms with motion sensitivity at least as small as 0.5 mm. The efficacy of this method is documented in phantom and human studies.
Oxidation of acetate in rabbit skeletal muscle: Detection by 13C NMR spectroscopy in vivo
Authors: Szczepaniak Lidia , Babcock Evelyn E. , Malloy Craig R. , Sherry A. Dean
Published: March 2007

The results of a proton‐decoupled and Overhauser‐enhanced 13C NMR study of acetate metabolism in skeletal muscle are reported. [2‐13C]Acetate was infused intravenously over 2 h into anesthetized rabbits, and skeletal muscle in the lateral thigh was monitored by 13C NMR spectroscopy at 4.7 T. Stable 13C enrichment in carbons 2, 3, and 4 of glutamate was observed at the end of the infusion, and the half‐time for enrichment was 17 min for glutamate C4 and 50 min for glutamate C2 and C3. The contribution of exogenous acetate to acetyl‐coenzyme A was nearly equal in skeletal muscle and heart in vivo (83–87%, measured in tissue extracts), comparable with earlier perfused heart studies in which acetate was the sole available substrate. Although relative flux through the combined anaplerotic pathways (relative to citric acid cycle flux) was higher in quiescent skeletal muscle (26%) compared with hearts (3%) from the same animals, actual anaplerotic flux was estimated to be substantially higher in heart than in skeletal muscle after correcting for differences in citric acid cycle flux in the two tissues.
Elimination of artifacts in short echo time 1H MR spectroscopy of the frontal lobe
Authors: Ernst Thomas , Chang Linda
Published: March 2007

Localized 1H spectra from the frontal part of the human brain are often distorted by “ghost” artifacts, especially with short echo time PRESS sequences. These artifacts are caused by insufficient dephasing of unwanted coherences involving magnetization in the sinuses and the mouth. Because the various unwanted coherences of a PRESS sequence are dephased differently, the order of the slice gradients has a pronounced effect on the spectral quality. For frontal voxels, the last slice gradient has to be applied in the axial direction. With this slice order, the major metabolites can be quantified with an interindividual variability of about 10%.
Noise in MRI
Authors: Macovski Albert
Published: March 2007

This study analyzes the signal‐to‐noise ratio (SNR) in magnetic resonance imaging. The factors that determine the SNR are derived starting from basic principles. The SNR, for a given object, is shown to be proportional to the voxel volume and the square root of the acquisition time. The noise generated by the body is derived using a cylindrical model and is shown to be proportional to the square of the radius and the square root of the length.
Pulmonary perfusion: Qualitative assessment with dynamic contrast‐enhanced MRI using ultra‐short TE and inversion recovery turbo FLASH
Authors: Hatabu Hiroto , Gaa Jochen , Kim Ducksoo , Li Wei , Prasad Pottumarthi V. , Edelman Robert R.
Published: March 2007

The accurate assessment of pulmonary perfusion is especially important in the evaluation of patients with suspected pulmonary embolism, a common and potentially lethal disorder that can be treated by aggressive anticoagulation. In this study, we demonstrate for the first time the use of MR to image pulmonary perfusion in humans by using dynamic imaging after contrast administration. The technique, which uses an inversion recovery turbo FLASH sequence with ultrashort TE (1.4 ms) and 1‐s temporal resolution, was tested in a series of eight healthy subjects and in a porcine model of pulmonary embolism. After the administration of gadopentetate dimeglumine in humans and animal models, there was serial enhancement of the systemic veins, right atrium, right ventricle, and pulmonary arteries. The pulmonary arterial tree was visualized beyond the segmental branches, followed by a gradual diffuse increase in signal intensity of the lung parenchyma over a period of 4.0–7.0 s. Pulmonary circulation times ranged from 3.0–3.4 s. Whereas a high dose (20 or 40 ml) of contrast agent tended to produce the most intense parenchymal enhancement, a low dose (5 ml) was best for showing recirculation. In the animal model, a perfusion defect due to a pulmonary embolus was clearly shown and confirmed by cine angiography. It is concluded that MRI of lung perfusion is feasible. With further development, perfusion MRI could eventually have a significant clinical role in the diagnostic evaluation of pulmonary embolism.
Line scan diffusion imaging
Authors: Gudbjartsson Hákon , Maier Stephan E. , Mulkern Robert V. , Mórocz István Á. , Patz Samuel , Jolesz Ferenc A.
Published: March 2007

A novel line scan diffusion imaging sequence (LSDI) is introduced. LSDI is inherently insensitive to motion artifacts and high quality diffusion maps of the brain can be obtained rapidly without the use of head restraints or cardiac gating. Results from a stroke study and abdominal diffusion images are presented. The results indicate that it is feasible to use the LSDI technique for clinical evaluation of acute ischemic stroke. In contrast to echo‐planar diffusion imaging, LSDI does not require modified gradient hardware and can be implemented on conventional scanners. Thus, LSDI should dramatically increase the general availability of robust clinical diffusion imaging.
Computation of flow pressure fields from magnetic resonance velocity mapping
Authors: Yang Guang‐Zhong , Kilner Philip J. , Wood Nigel B. , Underwood S. Richard , Firmin David N.
Published: March 2007

Magnetic resonance phase velocity mapping has unrivalled capacities for acquiringin vivomulti‐directional blood flow information. In this study, the authors set out to derive both spatial and temporal components of acceleration, and hence differences of pressure in a flow field using cine magnetic resonance velocity data. An efficient numerical algorithm based on the Navier‐Stokes equations for incompressible Newtonian fluid was used. The computational approach was validated within vitroflow phantoms. This work aims to contribute to a better understanding of cardiovascular dynamics and to serve as a basis for investigating pulsatile pressure/ flow relationships associated with normal and impaired cardiovascular function.
A readout magnet for prepolarized MRI
Authors: Morgan Patrick , Conolly Steven , Scott Greig , Macovski Albert
Published: March 2007

Conventional MRI systems rely on large magnets to generate a field that is both strong and extremely uniform. This field is usually produced by a heavy permanent magnet or a cryogenically cooled superconductor. An alternative approach, called prepolarized MRI (PMRI), employs two separate fields produced by two different magnets. A strong and inhomogeneous magnetic field is used to polarize the sample. After polarization, a weak magnetic field is used for readout. These fields can be produced by two separate resistive electromagnets that cost significantly less than a single permanent or superconducting magnet. At Stanford, the authors are constructing a PMRI prototype scanner suitable for imaging human extremities roughly 20 cm in diameter. With this system the authors hope to demonstrate comparable image quality to MRI with reduced system cost. The authors' initial work on low‐frequency reception indicates that it will be possible to obtain comparable image signal‐to‐noise ratio to an MRI scanner operating at the same polarizing field strength. To reduce the capital cost of the system, the authors use resistive electromagnets. Here the authors discuss the full development of the readout magnet including important design considerations, shimming, and field plots. These encouraging results are an important step toward evaluating the cost effectiveness of PMRI.
Three‐dimensional strain‐rate imaging
Authors: Robson Matthew D. , Constable R. Todd
Published: March 2007

Strain‐rate imaging uses large velocity encoding gradients to obtain measurements of velocity that are extremely insensitive to the effects of random noise. The spatial differential of velocity yields the velocity gradient from which the strain‐rate and twist‐rate tensors can be determined. These tensors represent the distortion of the material and are of interest in the analysis of the dynamic behavior of living tissue (e.g., that of the myocardium). This work presents a new technique that uses the magnitude of the signal in the velocity encoded data to measure through‐plane velocity variations at the resolution of the voxel size. The magnitude of the MR signal contains information about the range of phases present within a voxel. When the phase is dependent on the velocity (as in phase velocity imaging), the magnitude contains information about the range of velocities within a voxel. The method presented in this work uses unbalanced slice‐refocusing gradients to sample the magnitude variation introduced by the interaction of velocity encoding gradients with spatially dependent velocities. The previously developed in‐plane velocity gradient methods can be easily integrated with this new through‐plane measurement to characterize the deformation of the myocardium in three spatial dimensions with high accuracy. The applicability of these methods is demonstrated theoretically, in phantoms and in vivo.
Dynamic electron spin resonance (ESR) imaging of the distribution of spin labeled dextran in a mouse
Authors: Kazama Shunsuke , Takashige Gou , Yoshioka Hisashi , Tanizawa Hisayuki , Ogata Tateaki , Koscielniak Janusz , Berliner Lawrence J.
Published: March 2007

The nitroxide group 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) was covalently bound to a dextran via an N‐acetamido linkage to produce a novel spin probe, TEMPO‐dextran (TEMPO‐DX), which circulates for long time periods in an animal without metabolic degradation. TEMPO‐DX was stable in mice, while small TEMPO analogs quickly disappeared after administration. Since dextran was reduced somewhat in size during synthesis, the resultant spin‐labeled dextran could be excreted through the kidney. A strong L‐band electron spin resonance signal was obtained shortly after Intravenous administration of TEMPO‐DX into the tail vein of a mouse, from which three‐dimensional images of specific organs were calculated. The signal was found to persist for well over 1 h.
Dynamic MR digital subtraction angiography using contrast enhancement, fast data acquisition, and complex subtraction
Authors: Wang Yi , Johnston Donald L. , Breen Jerome F. , Iii John Huston , Jack Clifford R. , Julsrud Paul R. , Kiely Michael J. , King Bernard F. , Riederer Stephen L. , Ehman Richard L.
Published: March 2007

A dynamic MR angiography technique, MR digital subtraction angiography (MR DSA), is proposed using fast acquisition, contrast enhancement, and complex subtraction. When a bolus of contrast is injected into a patient, data acquisition begins, dynamically acquiring a thick slab using a fast gradient echo sequence for 10–100 s. Similar to x‐ray DSA, a mask is selected from the images without contrast enhancement, and later images are subtracted from the mask to generate angiograms. Complex subtraction is used to overcome the partial volume effects related to the phase difference between the flowing and stationary magnetization in a voxel. Vessel signal is the enhancement of flow magnetization resulting from the contrast bolus. MR DSA was performed in 28 patients, including vessels in the lungs, brains, legs, abdomen, and pelvis. All targeted vessels were well depicted with MR DSA. Corresponding dynamic information (contrast arrival time ta and duration of the arterial phase tav) was measured: ta/tav = 3.4/4.7 s for the lung, 10.3/4.9 s for the brain, 12.8/19.3 for the aorta, 15.2/12.6 s for the leg. MR DSA can provide dynamic angiographic images using a very short acquisition time.
Radial BURST imaging
Authors: Jakob Peter M. , Kober Frank , Haase Axel
Published: March 2007

A radial BURST approach combining BURST excitations with projection reconstruction (PR) gradients for excitation and acquisition is introduced. With the application of PR gradients during excitation, the applied RF energy is distributed more homogeneously across the sample than in the initial BURST approach. Computer simulations of the radial BURST excitation scheme are presented and experimentally verified. Actual radial BURST implementation acquiring a 64 × 64 radial BURST image in 45 ms is reported. Extensions of the radial BURST imaging technique placing a magnetization preparation experiment in front of the whole imaging sequence are also described. Furthermore, the feasibility of combining radial BURST with a multishot approach is demonstrated.
Frequency offset corrected inversion (FOCI) pulses for use in localized spectroscopy
Authors: Ordidge Roger J. , Wylezinska Marzena , Hugg James W. , Butterworth Edward , Franconi Florence
Published: March 2007

Gradient localized spectroscopy techniques suffer from a well documented spatial localization error caused by the difference in chemical shifts between resonances. This results in the acquisition of spectra from partially overlapping spatial regions of the sample, with each resonance representing a different region. The image‐selected in vivo spectroscopy technique uses hyperbolic secant inversion pulses, where the main limitation in reducing this error is in the RF power available for application of the selective RF pulse. This spatial localization error may be dramatically reduced by increasing, and temporally shaping, the gradient pulse during slice‐selective spin inversion. The performance of these RF pulses have been experimentally verified.
Effect of ovariectomy on magnetic resonance T2* in rat femur
Authors: Checkley David , Millest Andrew J. , Johnstone Donna , Waterton John C.
Published: March 2007

Measurements have been made in the rat femur in vivo and ex vivo by using an asymmetric spin echo technique of T2′, the susceptibility contribution to T2*. The trabecular spacing in this study in rat bone is considerably less than in previous studies in the human. A significant increase in T2′ was seen in vivo 3 mm proximal to the growth plate with ovariectomy (a model of osteopenia), from 8.1 ± 0.7 to 10.0 ± 0.6 ms. Parallel changes in trabecular bone mineral density measured by quantitative computed tomography were found. T2′ was higher in living bones than in the same bones measured post mortem.
Measurement of Gd‐DTPA dialysis clearance rates by using a look‐locker imaging technique
Authors: Niendorf Eric R. , Santyr Giles E. , Brazy Peter C. , Grist Thomas M.
Published: March 2007

The exponential clearance rate constant, (k), and filtration fraction (FF) have been measured for dialysis of Gadoliniumdiethylenetriaminepentaacetic acid (Gd‐DTPA) (Magnevist, Berlex, Wayne, NJ) solutions by using a Look‐Locker imaging technique under conditions of flow. The measured values of k for the Baxter CA‐50, CA‐110, and CA‐210 filters were 0.0037 ± 0.0003, 0.0057 ± 0.0017, and 0.0092 ± 0.0018 min−1, respectively, for dialysis of 4.0 liters of aqueous Gd‐DTPA solutions. The measured values of FF for the Baxter CA‐110 and CA‐210 filters were 0.060 ± 0.013 and 0.089 ± 0.015, respectively, for dialysis of aqueous Gd‐DTPA at 350 ml/min. The k and FF measurements agree with values that use inversion recovery (IR) on static samples obtained by drawing aliquots of solution during the course of dialysis. This in vitro experiment suggests that accurate in vivo measurements of filtration fraction and glomerular filtration rate (GFR) may be possible.
3D MR angiography of pulmonary arteries using realtime navigator gating and magnetization preparation
Authors: Wang Yi , Rossman Phillip J. , Grimm Roger C. , Wilman Alan H. , Riederer Stephen J. , Ehman Richard L.
Published: March 2007

An ECG‐triggered magnetization‐prepared segmented 3D fast gradient echo sequence was developed to perform pulmonary arterial MR angiography. A selective inversion recovery pulse was used in the magnetization preparation to suppress venous vasculature. A real‐time gating technique based on navigator echoes was implemented to reduce respiration effects. Pencil‐beam navigator echoes were acquired immediately before and after the readout train and processed in real‐time to dynamically measure the diaphragm position, which was used to control data acquisition with an accept‐or‐reject‐reacquire logic. In a study of 10 volunteers, a gated 3D acquisition with 28 slices required on average approximately 4 min of acquisition time, and six to seven segmental arteries related to the interlobar trunk of the pulmonary artery were depicted. The use of SIR pulse reduced venous signal by 99%. The gated acquisitions were superior to the ungated acquisitions (n = 10, P < 0.005). The real‐time navigator gating technique is effective for reduction of respiration effects and thereby makes high resolution 3D MRA of the pulmonary arteries feasible.
Cardiac magnetic resonance fluoroscopy
Authors: Debbins Josef P. , Riederer Stephen J. , Rossman Phillip J. , Grimm Roger C. , Felmlee Joel P. , Breen Jerome F. , Ehman Richard L.
Published: March 2007

A technique is described for high speed interactive imaging of the heart with either white or black blood contrast. Thirty‐two views of a segmented, magnetization‐prepared gradient echo sequence are acquired during diastole. Using three‐quarter partial Fourier sampling, data for a complete 128 × 128 image are acquired in three cardiac cycles. High speed reconstruction provides an image update of each cardiac cycle 159 ms after measurement. An independent graphical user interface facilitates interactive control of section localization and contrast by permitting pulse sequence parameter modification during scanning. The efficiency and image quality of the cardiac MR fluoroscopy technique were evaluated in 11 subjects. Compared with the conventional graphic prescription method, the cardiac fluoroscopy technique provides an approximate eightfold reduction in the time required to obtain subject‐specific double oblique sections. Image quality for these scout acquisitions performed during free breathing was sufficient to identify small cardiac structures.
High resolution intravascular MRI and MRS by using a catheter receiver coil
Authors: Atalar Ergin , Bottomley Paul A. , Ocali Ogan , Correia Luis C. L. , Kelemen Mark D. , Lima Joao A. C. , Zerhouni Elias A.
Published: March 2007

Potentially important diagnostic information about atherosclerosis can be obtained by using magnetic resonance imaging and spectroscopy techniques. Because critical vessels such as the aorta, coronary arteries, and renal arteries are not near the surface of the body, surface coils are not adequate to increase the data quality to desired levels. A few catheter MR receiver coil designs have been proposed for imaging the walls of large blood vessels such as the aorta. These coils have limited longitudinal coverage and they are too thick to be placed into small vessels. A flexible, long and narrow receiver coil that can be placed on the tip of a catheter and will enable multi‐slice high resolution imaging of small vessels has been developed. The authors describe the theory of the coil design technique, derive formulae for the signal‐to‐noise ratio characteristics of the coil, and show examples of high resolution cross‐sectional images from isolated human aortas acquired by using this catheter coil. In addition, high resolution in vivo rabbit aorta images were obtained as well as a set of spatially resolved chemical shift spectra from a dog circumflex coronary artery.
Sensitivity‐enhanced echo‐planar MRI at 1.5T using a 5 × 5 mesh dome resonator
Authors: Meyer Kristen L. , Kim Karl , Li Tao , Tulipano Paola K. , Lee Kyoung‐Min , Delapaz Robert , Hirsch Joy , Ballon Douglas
Published: March 2007

In this work a 5 × 5 mesh dome resonator that has been optimized for functional brain imaging is presented. The resonator was reduced in length and diameter compared with previous versions to reduce sample losses, thus enhancing the signal‐to‐noise ratio of the acquired data. In addition, a 5 × 5 mesh design was employed, which offered improved axial homogeneity over an earlier 3 × 3 mesh version. The new resonator exhibited high sensitivity and good homogeneity over the brain volume, permitting analysis of functional activation over large areas of the cerebral cortex. In a direct comparison with a standard clinical head‐imaging resonator, the high sensitivity of the 5 × 5 mesh dome resonator resulted in greater statistical confidence in functional activation.
Wavelet encoding for 3D gradient‐echo MR imaging
Authors: Gelman Neil , Wood Michael L.
Published: March 2007

An implementation of wavelet encoding to resolve one spatial direction of a three‐dimensional gradient‐echo magnetic resonance image is described. This is the first report of the use of wavelet encoding with a relatively short repetition time for rapid image acquisition. An important feature of this implementation is that it allows wavelet‐encoded images to be reconstructed without phase correction. The images show no discernible degradation compared with images acquired with phase encoding or scaling‐function encoding instead of wavelet encoding. In addition to describing the implementation, typical sources of phase errors are investigated both theoretically and experimentally.
Fast 3D functional magnetic resonance imaging at 1.5 T with spiral acquisition
Authors: Yang Yihong , Glover Gary H. , van Gelderen Peter , Mattay Venkata S. , Santha Attanagoda K. S. , Sexton Roy H. , Ramsey Nick F. , Moonen Chrit T. W. , Weinberger Daniel R. , Frank Joseph A. , Duyn Jeff H.
Published: March 2007

A new method to perform rapid 3D fMRI in human brain is introduced and evaluated in normal subjects, on a standard clinical scanner at 1.5 Tesla. The method combines a highly stable gradient echo technique with a spiral scan method, to detect brain activation related changes in blood oxygenation with high sensitivity. A motor activation paradigm with a duration of less than 5 min, performed on 10 subjects, consistently showed significant changes in signal intensity in the area of the motor cortex. In all subjects, these changes survived high statistical thresholds.
Changes in water diffusion due to Wallerian degeneration in peripheral nerve
Authors: Beaulieu Christian , Does Mark D. , Snyder Richard E. , Allen Peter S.
Published: March 2007

The authors report NMR measurements of the changes in water diffusion brought about by in vivo Wallerian degeneration due to either crush‐ or tie‐injuries in the sciatic nerve of the frog. Using a pulsed‐gradient spin‐echo sequence with a diffusion measurement time of 28 ms, the degree of diffusion coefficient anisotropy {D(longitudinal)/D(transverse)} 4 weeks after injury in both crush‐ and tie‐injured nerves (2.3 ± 0.4 and 1.7 ± 0.1, respectively) is significantly less than in normal frog sciatic nerve (3.9 ± 0.4). The decrease of anisotropy in the degenerated nerves is due to both a decrease in longitudinal diffusion and an increase in transverse diffusion. The changes in diffusion coefficients are compared with the degree of axonal and myelin breakdown observed in light and electron micrographs of the nerves.
Quantitative volumetric magnetization transfer analysis in multiple sclerosis: Estimation of macroscopic and microscopic disease burden
Authors: van Buchem Mark A. , McGowan Joseph C. , Kolson Dennis L. , Polansky Marcia , Grossman Robert I.
Published: March 2007

Magnetization transfer imaging (MTI) has been shown to be sensitive to both macroscopic and microscopic disease in multiple sclerosis (MS). In this study three‐dimensional MTI was used to estimate the global burden of disease in large volumes of brain tissue. MTI was performed in 15 MS patients and 11 normal controls. In seven MS patients MTI was performed on two different occasions. MTI data were displayed as magnetization transfer ratio (MTR) histograms and analyzed. The peak height of the histograms, presumably reflecting the residual amount of normal brain tissue, was lower in MS patients as compared with normal controls (P < 0.001), and was found to correlate with the duration of disease (P < 0.05). A decrease of the MTR histogram peak height was observed in the course of the disease (P < 0.01). These findings suggest that in MS, volumetric MTI provides quantitative information reflecting the global burden of disease.
In vivo rapid magnetic field measurement and shimming using single scan differential phase mapping
Authors: Kanayamay Shoichi , Kuhara Shigehide , Satoh Kozo
Published: March 2007

In vivo shimming is a powerful tool for alleviating subjectdependent magnetic field distortions in MRI, although the usually lengthy procedure has limited its use. A magnetic field measurement method called single scan differential phase mapping that can reduce the measurement time of conventional methods by more than half when used with multiple gradient‐recalled echo imaging was developed. An automatic high‐order shimming using the present method was implemented in a commercial MRI system. In vivo field inhomogeneity features and the shimming performances were investigated. The method was found to be useful for improving the quality of echo‐planar images.
Simple measurement of scanner stability for functional NMR imaging of activation in the brain
Authors: Weisskoff Robert M.
Published: March 2007

Functional MR imaging (fMRI) of activation in the brain is one of the more demanding applications required of an NMR imaging instrument. Since the signal changes in activation imaging are quite small, any instrumental variations can easily compromise the fMRI contrast. The authors describe a simple method for measuring these scanner instabilities, which is implementable on all scanners, to determine whether instability is degrading the fMRI contrast‐to‐noise. In this method, a long time series of images is acquired with identical imaging parameters to the chosen fMRI scans, the fluctuations are measured as a function of region‐of‐interest (ROI) size, and compared with the single image SNR. By plotting these fluctuations versus ROI size, and comparing them with the theoretically approachable value, the impact of these fluctuations can be assessed. We also present a simplified version of the test, which can be implemented with minimal calculations.