bluuebunny/crossref_metadata_2025_split huggingface dataset:
Multiple tuning of birdcage resonators
Authors: Amari Smain , Müfit Uluǧ Aziz , Bornemann Jens , Van Zijl Peter C. M. , Barker Peter B. 
Published: March 2007

AbstractA theoretical framework is presented for designing birdcage resonators for MRI and MR spectroscopy. The analogy between the birdcage problem and the phonon problem in solid‐state physics is used to achieve multiple tuning. Allowing that the capacitances in the columns of the cage assume unequal values, it is possible to achieve multiple tuning and simultaneously preserve the sinusoidal current distribution necessary to set a homogeneous magnetic field. Given the physical dimensions of the columns and branches of the cage as well as the desired resonant frequencies, the corresponding values of the capacitances can be calculated exactly. Closed‐form expressions for the capacitances are given in terms of the mutual inductances and the desired resonant frequencies. A detailed analysis for a symmetrical low‐pass birdcage is presented. The expressions for the resonant frequencies reduce to those given by other authors when only nearestneighbor mutual inductances are included.
Three‐dimensional magnetization‐prepared time‐of‐flight MR angiography of the carotid and vertebral arteries
Authors: Wilman Alan H. , Iii John Huston , Riederer Stephen J. 
Published: March 2007

AbstractThree‐dimensional magnetization‐prepared (MP) MR angiography (MRA) was applied to the carotid and vertebral arteries by using ECG triggering, a slab selective RF inversion pulse, centric phase encoding and acquisition during diastole. Both theoretically and experimentally, the MP MRA sequence was shown to perform well in cases where there was substantial blood replenishment during the inversion time Tl (>330 ms). In comparison with standard, ungated, steady‐state 3D time‐of‐flight MRA in 13 consecutive volunteers, the MP MRA images demonstrated better background suppression with less artifact and generally had more uniform vessel depiction. The MP MRA sequence was generally superior for portraying vessels exhibiting high pulsatility such as the carotid siphon. However, in distal vessels with slow flow and incomplete blood replenishment, the MP MRA technique was inferior due to increased loss of vessel signal. The reasons for this increased signal loss are identified and improvements to the MP MRA sequence are suggested.
Angiographic Imaging with 2D RF Pulses
Authors: Alley Marcus T. , Pauly John M. , Sommer F. Graham , Pelc Norbert J. 
Published: March 2007

AbstractMagnetic resonance angiography (MRA) was performed by using RF pulses designed to excite a limited spatial extent in two orthogonal directions. The restriction in the second spatial dimension can be used to increase inflow enhancement and to improve small field‐of‐view imaging. A rectangular excitation was produced with an “echo‐planar” k‐space trajectory and a sine‐modulated RF waveform. In vivo images have demonstrated that vessels are more clearly delineated with the two‐dimensional excitation. Aliasing artifacts in small field‐of‐view imaging are significantly reduced, although in some cases complete elimination is not possible due to the nature of the gradient trajectory.
Improvement of brain lesion detection at 0.1 T by simultaneous use of Gd‐DTPA and magnetization transfer imaging
Authors: Ramadan Usama Abo , Aronen Hannu J. , Tanttu Jukka I. , Karjalainen Pertti T. , Kairemo Kalevi J. , Jääskeläinen Juha , Sipilä Leena , Häkkinen Anna‐Maija 
Published: March 2007

AbstractImaging parameters were optimized at 0.1 T to improve contrast‐to‐noise ratios (CNR) when combining magnetization transfer (MT) imaging and the use of paramagnetic contrast medium. This was accomplished by imaging a phantom containing serial concentrations of Gd‐DTPA in cross‐linked bovine serum albumin. With the use of simulations, the dependence of CNR on imaging parameters was studied. Conventional and MT images were obtained from 10 brain tumor patients with single and triple doses of Gd‐DTPA. Simulations demonstrated the importance of TR in postcontrast sequences. The CNR in MT images is less sensitive to TR than in conventional images. A significant CNR improvement caused by MT remains at longer TR when there is no contrast enhancement without MT. The clinical results indicate that a single dose of Gd‐DTPA combined with MT cannot replace imaging with a triple dose. However, MT significantly improved the CNR after single and triple Gd‐DTPA‐doses on T1‐weighted and proton‐density images.
Characterization of and correction for artifacts in linogram MRI
Authors: Gai Neville , Axel Leon 
Published: March 2007

AbstractChemical shift differences, field inhomogeneity, and gradient nonlinearity result in artifacts in magnetic resonance imaging. Three artifacts are characterized for linogram imaging and it is shown that, based on computer simulations and theory, linogram MRI behaves similarly to 2DFT. A correction technique similar to a scheme for 2DFT imaging based on the Dixon technique and coordinate transform methods is proposed. The algorithm is applied to correct for field inhomogeneity and gradient nonlinearity‐induced artifacts in both simulations and images of a clinical phantom. The results show good correlation with the theory.It is concluded that linogram imaging offers certain attractive features of both 2DFT and PR imaging techniques, and is a potentially viable alternative to PR imaging in the presence of field inhomogeneity.
Improved 2D time‐of‐flight angiography using a radial‐line k‐space acquisition
Authors: Nielsen Hatsumi T. C. , Olcott Eric W. , Nishimura Dwight G. 
Published: March 2007

AbstractFor flow imaging applications, radial‐line k‐space acquisition methods offer advantages over conventional 2DFT methods. Specifically, radial‐line acquisition methods mitigate artifacts resulting from pulsatile flow while offering a potential reduction in scan times. In this paper, radial‐line and 2DFT acquisitions are compared in a two‐dimensional time‐of‐flight angiography sequence. The twisting radial‐line (TwiRL) trajectory, a variant of 2D projection reconstruction, is used to represent the family of radial‐line trajectories. In both phantom and in vivo studies, the TwiRL images demonstrate improved vessel depiction including a more uniform signal intensity and better delineation of the vasculature in comparison with images obtained via the 2DFT method.
The b matrix in diffusion tensor echo‐planar imaging
Authors: Mattiello James , Basser Peter J. , Le Bihan Denis 
Published: March 2007

AbstractIn diffusion tensor imaging (DTI) an effective diffusion tensor in each voxel is measured by using a set of diffusion‐weighted images (DWIs) in which diffusion gradients are applied in a multiplicity of oblique directions. However, to estimate the diffusion tensor accurately, one must account for the effects of all imaging and diffusion gradient pulses on each signal echo, which are embodied in the b matrix. For DTI to be practical clinically, one must also acquire DWIs rapidly and free of motion artifacts, which is now possible with diffusion‐weighted echo‐planar imaging (DW‐EPI). An analytical expression for the b matrix of a general DW‐EPI pulse sequence is presented and then validated experimentally by measuring the diffusion tensor in an isotropic phantom whose diffusivity is already known. The b matrix is written in a convenient tabular form as a sum of individual pair‐wise contributions arising from gradient pulses applied along parallel and perpendicular directions. While the contributions from readout and phase‐encode gradient pulse trains are predicted to have a negligible effect on the echo, the contributions from other imaging and diffusion gradient pulses applied in both parallel and orthogonal directions are shown to be significant in our sequence. In general, one must understand and account for the multiplicity of interactions between gradient pulses and the echo signal to ensure that diffusion tensor imaging is quantitative.
In vivo phosphorus polarization transfer and decoupling from protons in three‐dimensional localized nuclear magnetic resonance spectroscopy of human brain
Authors: Gonen Oded , Mohebbi Afsaneh , Stoyanova Radka , Brown Truman R. 
Published: March 2007

AbstractRefocused insensitive nucleus enhancement by polarization transfer (RINEPT) from protons (1H) to a J‐coupled phosphorus (31P) has been incorporated into three‐dimensional (3D) chemical‐shift‐imaging (CSI) sequence on a clinical imager. The technique is demonstrated on a phantom and in in vivo human brain. The polarization‐transfer efficiency (∼1.2) is lower than the theoretical maximum of γ1H/γ31P≈ 2.4 resulting from 1H‐1H homonuclear J couplings of similar magnitude competing with the 1H →31P transfer. Nevertheless, compared with direct 31P Ernst‐angle excitation, signal gains of up to × 1.8 were obtained mainly as a result of T1 differences between 31P and the 1H. Spectral interpretation is simplified by editing out all non‐proton‐coupled 31P signals. The duration, ∼50 min, and power deposition, ∼1 W · kg−1, make the application suitable for human studies.
Application of a 3D volume 19FMR imaging protocol for mapping oxygen tension (pO2) in perfluorocarbons at low field
Authors: Pratt Ronald G. , Zheng Jie , Stewart Brent K. , Shiferaw Yoseph , McGoron Anthony J. , Samaratunga Ranasinghage C. , Thomas Stephen R. 
Published: March 2007

AbstractA limited flip angle gradient‐echo 3D volume acquisition imaging protocol for mapping partial pressure of oxygen (pO2) in perfluorocarbon compounds (PFCs) at low field (0.14 T) is presented. The pO2 measurement method is based on the paramagnetic effect of dissolved molecular oxygen (O2) which reduces the PFC 19F T1− Specific objectives related to imaging of PFCs through use of the protocol include improved image signal‐to‐noise characteristics and elimination of 19F chemical shift artifacts. A parametric Wiener deconvolution filtering algorithm is used for suppression of 19F chemical shift artifacts. Application of the protocol is illustrated in a series of calculated pO2 maps of a gas equilibrated, multi‐chamber phantom containing perfluorotributylamine (FC‐43). The utility of the protocol is demonstrated in vivo through images of a commercially available perfluorocarbon based blood substitute emulsion containing FC‐43 sequestered in the liver and spleen of a rat.
Polyvinyl alcohol cryogel: An ideal phantom material for MR studies of arterial flow and elasticity
Authors: Chu Kenneth C. , Rutt Brian K. 
Published: March 2007

AbstractThe authors present a unique application of polyvinyl alcohol (PVA) cryogel as an anthropomorphic, elastic, vascular phantom material that can be used in MR imaging. The composition consists of two nontoxic ingredients: water and PVA. The biomechanical and MR properties can be adjusted to be similar to those of excised porcine aortas by varying the number of freeze‐thaw cycles to which the PVA solution is exposed. The authors present the T1, T2, shrinkage, and tensile properties of PVA cryogel tubes as a function of freeze‐thaw cycles. MR images of a dual elastic aortic phantom undergoing pulsatile motion are shown.
Proton‐decoupled 19F spectroscopy of 5‐FU catabolites in human liver
Authors: Murphy‐Boesch Joseph , Li Libo He Chun‐Wei , Padavic‐Shaller Kristin A. , Negendank William , Brown Truman R. 
Published: March 2007

AbstractAn RF network and a dual‐tuned surface coil are described for obtaining proton‐decoupled, NOE enhanced 19F spectra from a whole body clinical imager operating at 1.5 Tesla. The network removes 19F frequency noise from the decoupler transmitter and prevents preamplifier saturation from high‐level decoupling signals. Proton decoupling of 19F spectra was optimized using a sample of urine containing 5‐fluorouracil (5‐FU) and its catabolite fluoro‐β‐alanine (FBAL). Protondecoupled 19F spectroscopy in vivo is demonstrated by obtaining both nonlocalized spectra and spectra localized with three‐dimensional chemical shift imaging from the liver of patients undergoing 5‐FU chemotherapy.
In vivo detection of GABA in human brain using a localized double‐quantum filter technique
Authors: Keltner John R. , Wald Lawrence L. , Frederick Blaise De B. , Renshaw Perry F. 
Published: March 2007

AbstractA proton MR spectral editing technique employing a spatially localized, double‐quantum filter (DQF) was used to measure γ‐aminobutyric acid (GABA) in the human brain at 1.5 T. The double‐quantum method provided robust, single‐shot suppression of uncoupled resonances from choline, creatine, and NAA and allowed detection of the γCH2 GABA (3.0 ppm) resonance with 30% efficiency. Spatial localization of the GABA measurement was achieved by incorporating PRESS localization within the double‐quantum excitation and detection sequence. A calibration technique was developed to adjust the relative phases of the RF pulses to maximize the in vivo double‐quantum detection efficiency for an arbitrary voxel location. The sequence efficiency, degree of suppression of uncoupled resonances, and characterization of the in vivo DQF technique was examined in phantom experiments and in a study of the occipital lobe of 10 normal subjects. The ratio of the 3.0‐ppm GABA resonance to the 3.0‐ppm creatine resonance was found to be 0.20 ± 0.05 (SD).
Differentiation between cortical atrophy and hydrocephalus using 1H MRS
Authors: Bluml Stefan , McComb J. Gordon , Ross Brian D. 
Published: March 2007

AbstractQuantitative 1H MRS to determine cerebral metabolite patterns and MRI to determine CSF flow were applied to 12 patients with ventricular dilation—Group A, cortical atrophy (N = 5); or Group B, hydrocephalus (N = 7)—and in 9 normal controls. While mean brain water (Group A = 80% ± 6; Group B = 86% ± 5; normal = 85% ± 4) did not differ between the two groups of patients and controls, 1H MRS distinguished those patients with cortical atrophy (Group A) (N‐acetylaspartate/creatine (NAA/Cr) = 0.69 ± 0.17, versus normal = 1.06 ± 0.16; P < 0.002; [NAA] = 5.9 ± 1.3 mmoles/kg, versus normal 8.0 ± 1.4; P < 0.02) from those with hydrocephalus (Group B) (NAA/Cr = 1.16 ± 0.11; [NAA] = 9.2 ± 1.2; P > 0.13 and P > 0.07). Lactate levels were elevated in 3/5 patients with cortical atrophy, but in 0/7 of those with hydrocephalus. Mean absolute concentrations (mmoles/kg) of the five major cerebral osmolytes were 41 ± 4 (Group A), 43 ± 6 (Group B), and 42 ± 4 (normal), so that despite massive brain deformation, constant osmolality was maintained. 1H MRS may directly benefit surgical planning in hydrocephalus infants by clearly identifying those with cortical atrophy who do not require CSF diversion. Thinning of the cortical mantle in hydrocephalus may result from osmotically driven reduction in individual cell volumes, (shrinkage), rather than brain‐compression.
Electric current density imaging of mice tumors
Authors: Serša Igor , Beravs Katarina , Dodd Nick J. F. , Zhao Sha , Miklavčič Damijan , Demsar Franci 
Published: March 2007

AbstractThe use of electric current density imaging (CDI) to map spatial distribution of electric currents through tumors is presented. Specifically, a method previously tested on phantoms was implemented in vivo and in vitro for mapping electric current pulses of the same order of magnitude (j ∽ 2500 A/m2) as in electrochemotherapy through T50/80 mammary carcinomas, B‐16 melanomas and SA‐1 sarcomas. A technically simplified method of electric current density imaging is discussed as well. Three geometries of electrodes (flat‐flat, point‐point, point‐flat) indicate altered electric current distribution for the same tumor. This indicates that the method can be used for monitoring the effects of electrochemotherapy as a function of electrode geometry.
Automatic cardiac MR image segmentation using edge detection by tissue classification in pixel neighborhoods
Authors: Singleton H. Ross , Pohost Gerald M. 
Published: March 2007

AbstractA highly sensitive edge detector has been developed that uses tissue classification of pixels based on analysis of data in their local neighborhoods. In conjunction with recursive region growing, it has been used successfully to define regions of interest (ROI) when applied specifically to gradient echo MR images of the heart. The detector adapts to nonuniformity by carrying out an independent analysis at each location. If two tissues are present in a neighborhood and the pixel at that location cannot be classified with the seed pixel, a region edge has been crossed and recursion is stopped. No geometric assumptions relating to object shape such as definition of a region center and radial search are required. The detector was applied to multi‐slice, multi‐phase images of the heart from 26 subjects. A segmentation strategy specified slice processing order, graded ROIs, and used successfully detected ROIs to guide subsequent detection. Segmentation of all images resulted in a 90.3% median edge pixel detection efficiency.
Alterations in T1 of normal and reperfused infarcted myocardium after Gd‐BOPTA versus GD‐DTPA on inversion recovery EPI
Authors: Wendland Michael F. , Saeed Maythem , Lauerma Kirsi , Derugin Nikita , Mintorovitch Jan , Cavagna Friedrich M. , Higgins Charles B. 
Published: March 2007

AbstractThis study tested whether Gd‐BOPTA/Dimeg or Gd‐DTPA exerts greater relaxation enhancement for blood and reperfused infarcted myocardium. Relaxivity of Gd‐BOPTA is increased by weak binding to serum albumin. Thirty‐six rats were subjected to reperfused infarction before contrast (doses = 0.05, 0.1, and 0.2 mmol/kg). ΔR1 was repeatedly measured over 30 min. Gd‐BOPTA caused greater ΔR1 for blood and myocardium than did Gd‐DTPA clearance of both agents from normal and infarcted myocardium was similar to blood clearance; plots of ΔR1myocardium/ΔR1blood showed equilibrium phase contrast distribution. Fractional contrast agent distribution volumes were approximately 0.24 for both agents in normal myocardium, 0.98 and 1.6 for Gd‐DTPA and Gd‐BOPTA, respectively, in reperfused infarction. The high value for Gd‐BOPTA was ascribed to greater relaxivity in infarction versus blood. It was concluded that Gd‐BOPTA/Dimeg causes a greater ΔR1 than Gd‐DTPA in regions which contain serum albumin.
Observer performance methodologies for evaluating blood vessel visibility in MR angiograms using accurate geometric registration to high resolution x‐ray angiograms
Authors: Chapman Brian E. , Sanderson Allen R. , Goodrich K. Craig , Alexander Andrew L. , Blatter Duane D. , Parker Dennis L. 
Published: March 2007

AbstractReceiver operator characteristic (ROC) and two alternative forced choice (2AFC) methodologies for quantitatively assessing vessel visibility in MR angiography (MRA) were examined using x‐ray angiography images as truth. The methodologies required MRA projection images with the same orientation and magnification as the x‐ray images. Geometric distortions limited the quality of the registration. The observer performance measurements were compared to vessel contrast‐to‐noise ratio (CNR) measurements. Receiver bandwidth (RBW) and magnetization transfer (MT) effects in 3D time‐offlight MRA were evaluated. Overall, applying MT significantly increased all three measurements while decreasing the RBW significantly improved the 2AFC and CNR measurements. The relative importance of both effects on the 2AFC measure increased as vessel diameter decreased, although statistical significance was limited by small samples for diameters less than approximately 0.7 mm. These results demonstrate the usefulness of these observer performance methodologies for MRA technique assessment.
Two methods for peak RF power minimization of multiple inversion‐band pulses
Authors: Goelman Gadi 
Published: March 2007

AbstractTwo novel methods to minimize peak RF power for high order longitudinal Hadamard encoding are described and demonstrated experimentally. The first method uses the fact that the choice of a reference phase in an inversion process does not affect the final frequency response. In this method, the different single inversion‐band pulses are added together, each with a different reference phase. For a proper phase choice, minimization of the peak RF power is obtained. Scaling laws are defined allowing the use of a given phase‐set in multiple cases. In the second method, single inversion‐band pulses are added together, each partially shifted in time. This results in a significant reduction in peak power with only a moderate increase in pulse length. Theoretical conditions outlining the optimal addition order are defined. Experimental results verify the theoretical conditions and demonstrate that the frequency response is not affected by the peak power minimization process. With the new low peak RF power, longitudinal Hadamard encoding of 8TH (or 16TH) order can be performed in any clinical setting.
A new diffusion SSFP imaging technique
Authors: Zur Yuval , Bosak Elyakim , Kaplan Noam 
Published: March 2007

AbstractIn this paper a new diffusion sensitive steady‐state free precession (SSFP) pulse sequence with a reduced sensitivity to physiological brain motion is presented. The signal attenuation due to diffusion in this SSFP sequence is derived theoretically and confirmed experimentally with a phantom. It is shown that for brain tissue this signal attenuation is approximately independent of T1 and T2, but depends only on the pulse sequence used, i.e., the timing and the size of the RF and the gradient pulses. On this basis the diffusion constant can be calculated for any region in the image. Diffusion sensitive images of the brain obtained with our pulse sequence are presented and shown to be superior over an image obtained with a “conventional” diffusion sensitive SSFP sequence.
Quantitative magnetic resonance imaging of capillary water permeability and regional blood volume with an intravascular MR contrast agent
Authors: Schwarzbauer Christian , Morrissey Sean P. , Deichmann Ralf , Hillenbrand Claudia , Syha Jutta , Adolf Holger , Nöth Ulrike , Haase Axel 
Published: March 2007

AbstractA novel method is presented to simultaneously measure the permeability surface area product of water (PS), also known as capillary diffusion capacity, and the regional blood volume (RBV). It is based on magnetic resonance imaging of the longitudinal relaxation times of tissue and blood at different concentrations of an intravascular MR contrast agent. PS and RBV were measured in vivo in different regions of the brain and the skeletal muscle of the rat. The average PS values (n = 5) obtained in cerebral cortex, corpus callosum, hippocampus, thalamus, jaw muscle, and tongue muscle were 3.31 ± 0.20, 1.81 ± 0.25, 3.37 ± 0.36, 3.68 ± 0.44, 10.6 ± 1.1, and 14.1 ± 2.51 ml ± min−1 ± g−1, respectively. The corresponding average RBV values were 1.63 ± 0.18,1.22 ± 0.25,3.30 ± 0.37, 3.03 ± 0.36, 1.66 ± 0.30, and 1.38 ± 0.33 ml ± 100g−1. These results are in good agreement with previously reported literature values obtained by means of autoradiography.
Improved sensitivity to overlapping multiplet signals in in vivo proton spectroscopy using a multiecho volume selective (CPRESS) experiment
Authors: Hennig Jürgen , Thiel Thorsten , Speck Oliver 
Published: March 2007

AbstractA method for volume selective proton spectroscopy is presented based on a multiecho sequence with short refocusing interval tcp. It is demonstrated, that by appropriate choice of tcp on the order of 4–6 ms, signals from overlapping multiplets like the glutamine and glutamate (Glu/Gln) resonances in spectra of the human brain are considerably increased compared with a conventional PRESS volume selection scheme. Thus proton spectra from J‐coupled multiplet signals can be acquired with TE on the order of 20–30 ms avoiding the baseline problems arising at shorter echo times due to broad resonances. This allows to selectively acquire spectra from substances with longer T2 without the confounding effects from J‐coupling occurring in conventional volume selection techniques.