bluuebunny/crossref_metadata_2025 huggingface dataset:
Design of adiabatic pulses for fat‐suppression using analytic solutions of the bloch equation
Authors: Rosenfeld Daniel , Panfil Shimon L. , Zur Yuval
Published: March 2007

Discrimination between signals produced by fat and by water is an important issue in MRI. One efficient approach is to perform fat‐suppression by selective inversion. This technique exploits the transition region of a selective RF pulse to invert the longitudinal lipid magnetization while leaving the magnetization of the water protons untouched. The damaging effects of RF field inhomogeneity may be overcome by using pulses based on the adiabatic fast passage principle (AFP). In particular, the well‐known sech/tanh adiabatic pulse is a robust and efficient pulse that is obtained as an analytic solution of the Bloch equation. In this paper, a wider class of analytic solutions of the Bloch equation is presented of which the sech/tanh driving function is merely a particular case. The new pulse exhibits an asymmetric distribution of magnetization with one transition sharper than the other. The sharper transition can be used to perform the required selective discrimination between signals. The resulting pulse features excellent adiabatic behavior. Moreover, the transition width of the new pulse can be reduced by a factor of about 2/3 with respect to an equal‐duration sech/tanh pulse. The performance of the new pulse is compared with a similar sech/tanh pulse with the aid of a practical design example.
Real time monitoring of laser‐induced thermal changes in cartilage in vitro by using snapshot FLASH
Authors: Jakob Peter M. , Hendrich Christian , Breitling Thomas , Schäfer Achim , Berden Astrid , Haase Axel
Published: March 2007

Snapshot FLASH imaging has been applied to study the spatial and temporal spreading of thermal changes caused by a Holmium:YAG laser in patella cartilage in vitro at 7 T. The temperature dependence of the proton resonance frequency was used to demonstrate the thermal energy deposition. A series of cartilage images, with a time resolution of 512 ms and a spatial resolution of 400 × 200 μm, showed dynamic changes of the temperature‐related image phase in the regions irradiated by the laser.
MR imaging and spectroscopy using hyperpolarized 129Xe gas: Preliminary human results
Authors: Mugler John P. , Driehuys Bastiaan , Brookeman James R. , Cates Gordon D. , Berr Stuart S. , Bryant Robert G. , Daniel Thomas M. , De Lange Eduard E. , Downs J. Hunter , Erickson Christopher J. , Happer William , Hinton Denise P. , Kassel Neal F. , Maier Therese , Phillips C. Douglas , Saam Brian T. , Sauer Karen L. , Wagshul Mark E.
Published: March 2007

Using a new method of xenon laser‐polarization that permits the generation of liter quantities of hyperpolarized 129Xe gas, the first 129Xe imaging results from the human chest and the first 129Xe spectroscopy results from the human chest and head have been obtained. With polarization levels of approximately 2%, cross‐sectional images of the lung gas‐spaces with a voxel volume of 0.9 cm3 (signal‐to‐noise ratio (SNR), 28) were acquired and three dissolved‐phase resonances in spectra from the chest were detected. In spectra from the head, one prominent dissolved‐phase resonance, presumably from brain parenchyma, was detected. With anticipated improvements in the 129Xe polarization system, pulse sequences, RF coils, and breathing maneuvers, these results suggest the possibility for 129Xe gas‐phase imaging of the lungs with a resolution approaching that of current conventional thoracic proton imaging. Moreover, the results suggest the feasibility of dissolved‐phase imaging of both the chest and brain with a resolution similar to that obtained with the gas‐phase images.
Evidence for 100% 13C NMR visibility of glucose in human skeletal muscle
Authors: Roussel Ronan , Carlier Pierre G. , Wary Claire , Velho Gilberto , Bloch Gilles
Published: March 2007

The accuracy of the measurement of total muscle glucose by in vivo 13C NMR spectroscopy was tested in five normal volunteers during a euglycemic [1‐13C]glucose infusion. The NMR visible concentration calibrated using an external reference was compared with that calculated from plasma glucose concentration, assuming that glucose remained extracellular. The NMR measurement always provided higher values than the calculation from plasma glucose: 0.51 ± 0.035 (mean ± SE) versus 0.38 ± 0.005 mmol/liter of muscle on average. This systematic difference was interpreted as reflecting the presence of muscle glucose‐6‐phospnate, co‐resonating with free glucose. Thus, glucose appeared to be virtually 100% NMR visible in human skeletal muscle.
Quantitative lactate‐specific MR imaging and 1H spectroscopy of skeletal muscle at macroscopic and microscopic resolutions using a zero‐quantum/double‐quantum coherence filter and SLIM/GSLIM localization
Authors: Kmiecik Joseph A. , Gregory Carl D. , Liang Zhi‐Pei , Hrad Daniel E. , Lauterbur Paul C. , Dawson M. Joan
Published: March 2007

Quantitative lactate imaging and spectroscopy were performed on phantoms and on electrically stimulated, excised frog skeletal muscle at macroscopic and microscopic resolutions. Lactate selectivity was achieved by use of a zero‐quantum/double‐quantum coherence (ZQC/DQC) lactate filter, which suppressed all signals besides lactate, including water and lipid, to below noise level. Three‐dimensional lactate data sets were acquired in 1–3 h; one of these spatial dimensions was frequency‐encoded and the other two were phase‐encoded. High‐resolution images were reconstructed using the spectral localization by imaging (SLIM) and generalized SLIM (GSLIM) techniques. Lactate quantitation was achieved by employing an external lactate concentration standard and was verified by comparison to quantitative STEAM‐localized and nonlocalized spectra that used total creatine as an internal concentration reference. Additionally, quantitatively accurate behavior of the SLIM and GSLIM techniques as applied to data sets of low signal‐to‐noise ratio and to macroscopically heterogeneous objects was verified using simulations and real muscle lactate data sets with known heterogeneity.
In vivo measurement of regional brain metabolic response to hyperventilation using magnetic resonance: Proton echo planar spectroscopic imaging (PEPSI)
Authors: Posse Stefan , Dager Stephen R. , Richards Todd L. , Yuan Chun , Ogg Robert , Artru Alan A. , Müller‐Gärtner Hans‐Wilhelm , Hayes Cecil
Published: March 2007

A new rapid spectroscopic imaging technique with improved sensitivity and lipid suppression, referred to as Proton Echo Planar Spectroscopic Imaging (PEPSI), has been developed to measure the 2‐dimensional distribution of brain lactate increases during hyperventilation on a conventional clincal scanner equipped with a head surface coil phased array. PEPSI images (nominal voxel size: 1.125 cm3) in five healthy subjects from an axial section approximately 20 mm inferior to the intercommissural line were obtained during an 8.5‐min baseline period of normocapnia and during the final 8.5 min of a 10‐min period of capnometry‐controlled hyperventilation (end‐tidal PCO2 of 20 mmHg). The lactate/N‐acetyl aspartate signal increased significantly from baseline during hyperventilation for the insular cortex, temporal cortex, and occipital regions of both the right and left hemispheres, but not in the basal ganglia. Regional or hemispheric right‐to‐left differences were not found. The study extends previous work using single‐voxel MR spectroscopy to dynamically study hyperventilation effects on brain metabolism.
A flexible magnetization transfer line shape derived from tissue experimental data
Authors: Li Jonathan G. , Graham Simon J. , Henkelman R. Mark
Published: March 2007

To summarize and compare magnetization transfer data from biological tissues, a method was developed to extract the average absorption lineshape of the semi‐solid pool directly from magnetization transfer experimental data along with the four other parameters that characterize the two‐pool model of exchange. Magnetization transfer data for several biological tissues were analyzed using this method and the resulting “flexible” lineshapes were compared with super‐Lorentzian and “Kubo‐Tomita” lineshapes. The use of flexible lineshapes noticeably improves the fit of the two‐pool model to the data. The derived flexible lineshapes of all the tissues analyzed are physically realistic and show remarkably consistent behavior.
An MRI method for measuring T2 in the presence of static and RF magnetic field Inhomogeneities
Authors: Yablonskiy Dmitriy A. , Haacke E. Mark
Published: March 2007

A magnetic resonance imaging method for measuring the T2 relaxation time constant is proposed. It is based on the assumption that, under very general conditions, the MR signal near a spin echo has a special symmetry arising from the refocusing nature of the 180° RF pulse. A gradient echo sampling of the spin echo (GESSE) sequence is implemented to evaluate T2 by collecting multiple gradient echoes before and after the spin echo. This approach is a modification of the GESFIDE sequence proposed by Ma and Wehrli. However, our approach compares images that are not separated by any RF pulses and, as a result, is insensitive to slice profile imperfections. In addition, the calculated T2 value does not rely on any special assumptions about the MRI signal behavior in the presence of an inhomogeneous static magnetic field and, hence, is insensitive to the presence of static magnetic field inhomogeneities.
Dynamic liver imaging with iron oxide agents: Effects of size and biodistribution on contrast
Authors: Mandeville Joseph B. , Moore John , Chesler David A. , Garrido Leoncio , Weissleder Ralph , Weisskoff Robert M.
Published: March 2007

In vivo effective relaxation rates in normal rat liver were evaluated for four dextran coated iron oxide agents: monocrystal‐line iron oxide nanocolloid (MION) with a mean particle diameter of 3.9 nm, a polycrystalline agent (PION) with a larger mean diameter of 12 nm, and these two agents labeled with the asialofetuin (ASF) protein for high hepatocytic receptor binding affinity (MION‐ASF and PION‐ASF). Using echo planar imaging at 2 Tesla, dose response was measured with high temporal resolution for 3 h after injection of agent, and by comparing with relaxivities in vitro and in brain, dominant in vivo contrast phenomena were elucidated. While transverse relaxivity for PION‐ASF exceeded that for MION‐ASF by almost a factor of 2 in solution, relaxation rates in vivo became equivalent. Liver relaxation using non‐ASF agents was consistent with rapid water exchange between vascular and extravascular compartments, which dominated relaxation as a result of agent accumulation in Kupffer cells.
Guidewire antennas for MR fluoroscopy
Authors: Ladd Mark E. , Erhart Peter , Debatin Jörg F. , Hofmann Eugen , Boesiger Peter , Von Schulthess Gustav K. , McKinnon Graeme C.
Published: March 2007

Before MRI can be used for guiding vascular interventions, the problem of dependably visualizing guidewires must be solved. Ideally, the position and curvature of the tip of the guidewire should be visible to facilitate steering through the vascular system. In this paper, various antennas are described that can be incorporated into a guidewire tip. These antennas allow the guidewire to be visualized with high contrast. Computer simulation and experimental evidence are presented showing the value of adding a passive MR signal source, with a shortT1, internal to the coil. The internal source increases the available signal, narrows the apparent width of the guidewire, and allows the use of fast imaging sequences.
Background suppression with multiple inversion recovery nulling: Applications to projective angiography
Authors: Mani Sanjay , Pauly John , Conolly Steven , Meyer Craig , Nishimura Dwight
Published: March 2007

We have developed a technique to accurately null the longitudinal magnetization (Mz) of background material. This suppression involves first saturating the longitudinal magnetization (Mz) of a region, and then applying several nonselective inversions. The inversions are timed relative to the saturation such that Mz is nulled across a broad range of T1 at a predetermined time after the initial saturation. B1 and B0 inhomogeneity, which could lead to inaccurate suppression, are dealt with by the combination of a multiple tip saturation sequence and four adiabatic inversion pulses. The suppression sequence can be used to form projective angiograms by selectively tagging the imaging region with the saturation pulse. After the inversions are played out, a projection taken through the tag region when Mz is nulled will only contain signal from blood that has flown into the region after the saturation. Since only two dimensions are acquired, the technique can acquire gated projection angiograms in reasonable scan times. Representative inflow MIR angiograms of the carotid arteries and renal arteries show excellent background suppression.
Excitation of arbitrary shapes by gradient optimized random walk in discrete k‐space
Authors: Serša Igor , Macura Slobodan
Published: March 2007

A new technique for the excitation of arbitrary shapes is proposed. It is based on a parallel sequence of small tip angle RF pulses and gradient pulses. The small tip angle rotations co‐add yielding a 90° excitation pulse within the selected excitation profile while outside the profile, the rotations cancel each other. A full theory of the completely arbitrary regional volume excitation (CARVE) method is presented and experimentally verified. In CARVE, k‐space is discrete because the RF is applied in pulses. The discrete character of k‐space permits an arbitrary trajectory for the k‐space walk. The optimal random trajectory is found by minimizing the gradient load using simulated annealing. It is shown, both theoretically and experimentally, that such a trajectory is much better than any other systematic or random trajectory in k‐space.
Dispersion in magnetization transfer contrast at a given specific absorption rate due to variations of RF pulse parameters in the magnetization transfer preparation
Authors: Wang Yi , Grist Thomas M. , Mistretta Charles A.
Published: March 2007

The effects of RF pulse parameters on magnetization transfer contrast (MTC) were investigated using a magnetization prepared segmented fast gradient echo sequence. MTC was found not to be uniquely determined by the specific absorption rate (SAR). RF pulse parameters (RF amplitude, number of RF pulses and RF duration) also affect MTC. There can be 40% variation in MTC due to differences in RF parameters at SAR = 1 W/kg for a 70‐kg subject. Increasing the number of RF pulses is a more efficient way to increase MTC than increasing RF amplitude. This phenomenon is likely caused by the fact that the time scale for magnetization transfer between the free and restricted proton pools is on the same order or longer than the duration of the MT pulse. Accordingly, increase in the MT pulse duration by increasing the number of RF pulses in the MT pulse allows more effective magnetization transfer. Such information can be used as a guide to select RF pulse parameters for a magnetization transfer (MT) pulse. An off‐resonance MT pulse designed under this guide for coronary MR angiography improved the depiction of distal vessels.
Functional magnetic resonance imaging of median nerve stimulation in rats at 2.0 T
Authors: Scanley B. Ellen , Kennan Richard P. , Cannan Susan , Skudlarski Pawel , Innis Robert B. , Gore John C.
Published: March 2007

An animal model of sensory activation using fMRI at 2.0 T has been developed, demonstrating that fMRI studies on animals need not be limited to high field magnets. These methods produced reliable image intensity changes of 2% using median nerve stimulation in rats at 3 Hz and propofol as the anesthetic agent. At 6 Hz the activation was slightly but not statistically significantly greater. The feasibility of fMRI studies in animals using propofol suggests that it may be a useful anesthetic for fMRI studies in agitated adult patients or in children.
Gloxy: An oxygen‐sensitive coal for accurate measurement of low oxygen tensions in biological systems
Authors: James Philip E. , Grinberg Oleg Y. , Goda Fuminori , Panz Tomasz , O'Hara Julia A. , Swartz Harold M.
Published: March 2007

This paper describes the characteristics of a new oxygen sensitive, paramagnetic material that has some significant advantages for measurements of tissue pO2 by in vivo EPR. This paramagnetic component of Welsh coal, termed “gloxy” was found to have valuable EPR features that allow accurate measurement of low oxygen tensions in vivo; these include large oxygen‐dependent changes in linewidth, a high number of paramagnetic spin centers (resulting in high signal amplitude), and stability in tissue allowing repeated pO2, measurements to be made in vivo with high precision. Renal pO2 was measured deep in the medulla region of isolated perfused kidneys and found to be lower than that in the cortex (1.7 ± 0.05 and 7.1 ± 0.3 mm Hg, respectively). The quality of the EPR signal obtained from the renal outer medulla and also from tumors in mice was such that the pO2 measurements were obtained with a precision of ±3% of the measured pO2 (Kidney: 1.7 ± 0.05 mmHg; Tumor: 1.37 ± 0.04 mmHg). In vitro tests on the viability of cells and in vivo studies using Gloxy demonstrate the stability and inertness of this oxygen‐sensitive material.
Comparison of blood oxygenattion and cerebral blood flow effect in fMRI: Estimation of relative oxygen consumption change
Authors: Kim Cmrr Seong‐gi , Uĝurbil Kaâmil
Published: March 2007

The most widely‐used functional magnetic resonance imaging (fMRI) technique is based on the blood oxygenation level dependent (BOLD) effect, which requires at least partial uncoupling between cerebral blood flow (CBF) and oxygen consumption changes during increased mental activity. To compare BOLD and CBF effects during tasking, BOLD and flowsensitive alternating inversion recovery (FAIR) images were acquired during visual stimulation with red goggles at a frequency of 8 Hz in an inteerleaved fashion. With the FAIR technique, absolute and relative CBF changes were determined. Relative oxygen consumption changes cdan be estimated using the BOLD and relative CBF changes. In gray matter areas in the visual cortex, absolute and relative CBF changes in humans during photic stimulation were 31 ± 11 SD ml/100 g tissue/min and 43 ± 16 SD % (n =12), respectively, while the relative oxygen consumption change was close to zero. These findings agree extremely well with previouse results using positron emission tomography. The BOLD signal change is not linearly correlated with the relative CBF increase across subjects and negatively correlates with the oxygen consumption change. Caution should be exercised when interpreting the BOLD percent change as a quantitative index of the CBF change, especially in inter‐subject comparisons.
Dynamics of magnetization in hyperpolarized gas MRI of the lung
Authors: Johnson G. Allan , Cates Gordon , Chen X. Josette , Cofer Gary P. , Driehuys Bastiaan , Happer Will , Hedlund Laurence W. , Saam Brian , Shattuck Mark D. , Swartz John
Published: March 2007

The magnetization in hyperpolarized gas (HP) MRI is generated by laser polarization that is independent of the magnet and imaging process. As a consequence, there is no equilibrium magnetization during the image acquisition. The competing processes of gas inflow and depolarization of the spins lead to large changes in signal as one samples k‐space. A model is developed of dynamic changes in polarization of hyperpolarized 3He during infusion and in vivo imaging of the lung and verified experimentally in a live guinea pig. Projection encoding is used to measure the view‐to‐view variation with temporal resolution <4 ms. Large excitation angles effectively sample the magnetization in the early stages of inflow, highlighting larger airways, while smaller excitation angles produce images of the more distal spaces. The work provides a basis for pulse sequences designed to effectively exploit HP MRI in the lung.
Short TE MR Microscopy: Accurate measurement and zonal differentiation of normal hyaline cartilage
Authors: Freeman Dominique M. , Bergman Gabrielle , Glover Gary
Published: March 2007

The purpose of this study was to use MR imaging to accurately measure the thickness of hyaline cartilage and determine the MR contrast parameters for differentiation of cartilage zones in normal human cartilage samples. Cartilage samples were examined using three dimensional spin‐echo MR microscopy at 9.4 T with a voxel size of 31 × 31 × 300 μm. Effects of T2 signal loss, susceptibility, and partial volume on measured thickness of cartilage were investigated. Thickness measurements were obtained on corresponding histological sections for comparison. Optimal contrast parameters for delineation of cartilage zones were evaluated using magnetization transfer, inversion recovery, T1, and T2 contrast. T2 relaxation losses were identified as the primary source of discrepancy between the measured thickness of cortical bone and hyaline cartilage. Good contrast for zonal differentiation was obtained using T1 weighting. We conclude that images obtained using short TE MR microscopy can be used to accurately measure cartilage and bone thickness in human specimens, and can demonstrate zones within normal cartilage.
Ghost artifact reduction for echo planar imaging using image phase correction
Authors: Buonocore Michael H. , Gao Lisheng
Published: March 2007

An algorithm is described for reducing ghost artifacts in echo planar imaging (EPI) using phase corrections derived from images reconstructed using only even or odd k‐space lines. The N/2 ghost, that arises principally from time‐reversal of alternate k‐space lines, was significantly reduced by this algorithm without the need for a calibration scan. In images obtained in eight subjects undergoing EPI for auditory functional MRI (fMRI) experiments, N/2 ghost intensity was reduced from 10.3% – 2.1% (range: 7.9–14.1%) to 4.5% ± 0.2% (range: 4.1–4.9%) of parent image intensity, corresponding to a percent reduction in ghost intensity of 54% ± 9% (range: 43–65%), and the algorithm restored this intensity to the parent image. It provided a significant improvement in image appearance, and increased the correlation coefficients related to neural activation in functional MRI studies. The algorithm provided reduction of artifacts from all polynomial orders of spatial phase errors in both spatial directions. The algorithm did not eliminate N/2 ghost intensity contributed by field inhomogeneities, susceptibility, or chemical shift.
Double line scan diffusion imaging
Authors: Gudbjartsson Hákon , Maier Stephan E. , Jolesz Ferenc A.
Published: March 2007

A new double line scan diffusion imaging sequence (DLSDI) is presented. In DLSDI, two lines from two separate slices are acquired in each shot. As its predecessor, LSDI, DLSDI is insensitive to motion artifacts and it can be used on conventional MR scanners. In addition, DLSDI is almost twice as fast as LSDI. Preliminary results from phantom and patient studies show excellent agreement between ADC trace maps obtained with DLSDI and LSDI. The technical and the theoretical aspects of DLSDI are studied, and it is shown how the conditional random walk model can be used as an analytical tool to derive the diffusion sensitivity in the DLSDI sequence.
Amplitude demodulation of the electrocardiogram signal (ECG) for respiration monitoring and compensation during MR examinations
Authors: Felblinger Jacques , Boesch Chris
Published: March 2007

A method for respiration monitoring during MR‐sequences using the electrocardiogram signal (ECG) is presented. The basic principle is known in conventional patient monitoring and is based on the fact that the amplitude of the ECG‐signal varies with respiratory motion. A demodulation of this “cardio‐respiratory signal” yields a respiration curve that can be used for patient monitoring, triggering, or respiration compensation of MRI sequences. Since no drift is introduced by digital demodulation, this method is superior to analog signal processing. The proposed method was applied during MR examinations using an optically coupled ECG sensor that has the advantage of almost negligible interference due to the MR sequences (<10% of the main ECG signal). The respiration signal obtained is strongly correlated (r= 0.79 to 0.98) with the position of the diaphragm in inferior‐superior direction measured during breath‐holding MRI.
Stability constants and 1H relaxation effects of ternary complexes formed between gd‐dtpa, gd‐dtpa‐bma, gd‐dota, and gd‐edta and citrate, phosphate, and carbonate ions
Authors: Burai László , Hietapelto Vesa , Király RóBert , Tóth éva , Brücher Ernő
Published: March 2007

Formation of ternary complexes between Gd‐DTPA, Gd‐DTPA‐BMA, and Gd‐DOTA, used as contrast enhancement agents in MRI and the endogenously available carbonate and phosphate ions, has been demonstrated. The extent of ternary complex formation and its effect on the proton relaxation, measured at 9 MHz, rates is negligible at around pH <8. The complex Gd‐EDTA forms more stable ternary complexes with carbonate and phosphate and it also strongly coordinates the terdentate citrate ligand. The formation of ternary complexes Gd‐EDTA(X) (X = CO32‐, Cit3‐) results in a significant decrease in the proton relaxation rates under physiological conditions.
A comparison of RIGR and SVD dynamic imaging methods
Authors: Hanson Jill M. , Liang Zhi‐Pei , Magin Richard L. , Duerk Jeff L. , Lauterbur Paul C.
Published: March 2007

Several constrained imaging methods have recently been proposed for dynamic imaging applications. This paper compares two of these methods: the Reduced‐encoding Imaging by Generalized‐series Reconstruction (RIGR) and Singular Value Decomposition (SVD) methods. RIGR utilizes a priori data for optimal image reconstruction whereas the SVD method seeks to optimize data acquisition. However, this study shows that the existing SVD encoding method tends to bias the data acquisition scheme toward reproducing the known features in the reference image. This characteristic of the SVD encoding method reduces its capability to capture new image features and makes it less suitable than RIGR for dynamic imaging applications.
Easy fabrication of a tunable high‐pass birdcage resonator
Authors: Xu Yan , Tang Pei
Published: March 2007

A practical design for a high‐pass birdcage resonator is presented. Precision seamless telescoping tubes were used for easy tuning of resonant frequency by adjusting the length of the coils. Three probes, of 4.4, 5.0, and 25.0 cm in diameter, respectively, were constructed and tested. An empirical formula is given that can be used to calculate the capacitance needed for a given frequency when the desired physical dimension and the number of elements of the coil are specified. A simple three‐step procedure is suggested for easy fabrication of resonators that are routinely tunable over tens of megahertz.
A new method for absolute quantitation MRS metabolites
Authors: Barantin Laurent , Pape Alain Le , Akoka Serge
Published: March 2007

A new method for absolute of MRS spectra is presented. This method is not based on a reference peak, derived from a real NMR signal, but rather on a synthesized NMR reference produced by an electronic device, transmitted by a broad‐band antenna to avoid quality factor variations. This signal is therefore received at the same time as the sample signal. The reference line produced is stable in time (maximum variation lower than 2%) and allows precise and accurate measurement of absolute concentrations (mean error lower than 3%) in vitro and in vivo.
The effect of magnetization transfer on functional MRI signals
Authors: Zhang Rongyan , Cox Robert W. , Hyde James S.
Published: March 2007

A magnetization transfer (MT)‐prepared echo‐planar imaging (EPI) pulse sequence was developed to study motor cortex activation, using a finger tapping paradigm. MT weighting resulted in a reduction of both the activated area and, in the majority of activated pixels, the functional MRI signal, regardless of the correlation coefficient threshold used in generating the activation map. The magnetization transfer ratio (MTR) was higher during task activation than during rest. Because the MT effect is strongly tissue‐dependent, these results support the hypothesis that incorporation of MT into functional MRI will help to understand the origin of the functional MRI signal.
Consistent fat suppression with compensated spectral‐spatial pulses
Authors: Block Walter , Pauly John , Kerr Adam , Nishimura Dwight
Published: March 2007

Reliable fat suppression is especially important with fast imaging techniques such as echo‐planar (EPI), spiral, and fast spin‐echo (FSE) T2‐weighted imaging. Spectral‐spatial excitation has a number of advantages over spectrally selective presaturation techniques, including better resilience to B0 and B1, inhomogeneity. In this paper, a FSE sequence using a spectral‐spatial excitation pulse for superior fat suppression is presented. Previous problems maintaining the CPMG condition are solved using simple methods to accurately program radio‐frequency (RF) phase. Next an analysis shows how B0 eddy currents can reduce fat suppression effectiveness for spectral‐spatial pulses designed for conventional gradient systems. Three methods to compensate for the degradation are provided. Both the causes of the degradation and the compensation techniques apply equally to gradient‐recalled applications using these pulses. These problems do not apply to pulses designed for high‐speed gradient systems. The spectral‐spatial FSE sequence delivers clinically lower fat signal with better uniformity than spectrally selective pre‐saturation techniques.
In Vivo sodium multiple quantum spectroscopy of human articular cartilage
Authors: Reddy Ravinder , Li Shuchun , Noyszewski Elizabeth A. , Kneeland J. Bruce , Leigh John S.
Published: March 2007

The authors report, for the first time, sodium properties of human articular cartilage in vivo using sodium multiple‐quantum‐filtered methods with a surface coil. A flip angle‐independent, phase‐cycled pulse sequence was used to obtain triple‐quantum‐filtered spectra as a function of preparation time. Biexponential relaxation rates were calculated by fitting the triple‐quantum‐filtered spectral amplitudes to a theoretical expression. Theoretical analysis of the flip angle dependence of even rank two‐quantum coherence (T22), odd rank two‐quantum coherence (T23), and triple‐quantum coherence are presented and verified against experimental results on a cartilage specimen. Sodium multiple‐quantum‐filtered spectral lineshapes obtained in vivo correlate well with those observed on in vivo specimens. Relaxation rates obtained from asymptomatic volunteers were found to be: T2rise= 1.0 ± 0.12 ms, T2decay= 12.0 ± 0.75 ms (mean ± SD). The diagnostic potential of this method in detecting early changes in articular cartilage is described.
In vivo validation of MR pulse pressure measurement in an aortic flow model: Preliminary results
Authors: Urchuk Steven N. , Fremes Stephen E. , Plewes Donald B.
Published: March 2007

MR imaging experiments were conducted to investigate the feasibility of estimating vascular pulse pressure waveforms from measurements of blood flow rates and vessel cross‐sectional area. Blood flow waveforms were measured in the aorta's of three 25–30‐kg pigs at multiple imaging sections using phase‐contrast velocity imaging. Estimates of pulse pressure were derived from these data by evaluating a model characterizing the relationship between pressure, flow, and the cross‐sectional area of a vessel segment. Comparisons between the MR‐derived estimates of pressure and those obtained from a micromanometer pressure catheter indicate that accurate measurements (mean error ± SD = 8.2 ± 3.4, n = 6) can be obtained using conventional velocity imaging techniques. Optimization of the method will require the application of rapid imaging techniques and the development of strategies for obtaining a more localized measurement. With these improvements, our results suggest that MR‐based measurement of pulse pressure and related elastic parameters is feasible.
Determination of laser‐induced temperature distributions using echo‐shifted turboflash
Authors: Harth Thorsten , Kahn Thomas , Rassek Margarethe , Schwabe Birgit , Schwarzmaier Hans‐Joachim , Lewin Jonathan S. , Mödder Ulrich
Published: March 2007

An echo‐shifted TurboFLASH sequence implemented on a clinical whole body MR scanner was used to determine thermal changes in tissue. With this snapshot‐like data acquisition, temperature‐related phase shifts were measured with a temporal resolution of 1.3 s. For different types of tissue (postmortem porcine brain, liver, and muscle) the temperature coefficients of the proton chemical shift were recorded during uniform heating of the specimen in a water bath. The specific temperature‐dependent frequency shifts appeared similar to the proton chemical shift of free water (‐0.01 ppm/°C). With this method, laser‐induced ablation in postmortem porcine brain was monitored by temperature mapping. Comparison of the induced temperature profiles measured with NiCrNi‐thermocouples with the MR calculated profiles demonstrated excellent temperature sensitivity and accuracy for this method of MR thermometry, with a maximum deviation of the determined temperatures of only 1.8°C. This investigation was designed as a feasibility study for this rapid version of the phase mapping method, and no in vivo studies were performed.
Hormonally induced modulation in the phosphate metabolites of breast cancer: Analysis of in vivo31P MRS signals with a modified prony method
Authors: Viti V. , Ragona R. , Guidoni L. , Barone P. , Furman E. , Degani H.
Published: March 2007

A modified Prony method (MPM) was applied to analyze the main signals present in spatially resolved 31P NMR spectra of MCF7 breast tumors implanted in nude mice. First, the method was tested on synthetic data to establish its limits of reliability. Its performance with respect to peak identification and quantification of signal intensities was then exploited on data from three implanted tumors during hormonal manipulation with estrogen and the antiestrogenic drug tamoxifen. The phosphomonoester peak was resolved into phosphocholine (PC) and phosphoethanolamine (PE). Treatment with tamoxifen led to a significant reduction in the PE to PE+PC peak amplitude ratio in the tumors under consideration. MPM analysis also revealed the presence of two different inorganic phosphate pools: a larger acidic pool and a smaller alkaline pool during estrogen‐induced growth and the reverse during tumor regression.
Quantitative regional brain water measurement with magnetic resonance imaging in a focal ischemia model
Authors: Lin Weili , Paczynski Richard P. , Venkatesan Ramesh , He Yong Y. , Powers William J. , Hsu Chung Y. , Haacke E. Mark
Published: March 2007

Therapeutic approaches to cerebral edema require an understanding of both the magnitude and location of changes in brain water content. It is desirable to have a sensitive, accurate means of measuring brain water noninvasively so that effective therapies for cerebral edema in stroke, head trauma, and other conditions can be investigated. In this work, a three‐dimensional magnetic resonance imaging technique that is able to provide both spin density and T1 simultaneously is described. This method was used to quantitate regional changes in brain water content in a rat model of focal cerebral ischemia. Brain water contents estimated from both relative spin density and relative T1 measurements made in vivo were compared with ex vivo measurements of relative tissue water content based on the wet‐dry technique. Correlation coefficients of 0.95 and 0.98 were obtained between the wet‐dry measurements and magnetic resonance measurements of T1 and spin density, respectively. Notably, the slope of the relationship between T1 and tissue water content changed dramatically after the injection of a paramagnetic contrast agent while precontrast and postcontrast spin density measurements remained essentially invariant. In addition, a plot of absolute spin density (obtained by normalizing spin density from agar gelatin phantoms of different water contents to the spin density of a sample of 100% water) was linearly related to wet‐dry measurements with a slope of 0.99 (R2 = 0.99).
Improved water and lipid suppression for 3D PRESS CSI using rf band selective inversion with gradient dephasing (basing)
Authors: Star‐Lack Josh , Nelson Sarah J. , Kurhanewicz John , Huang L. Royal , Vigneron Daniel B.
Published: March 2007

A T1 insensitive solvent suppression technique— band selective inversion with gradient dephasing (BASING)—was developed to suppress water and lipids for 1H magnetic resonance spectroscopy (MRS). BASING, which consists of a frequency selective RF inversion pulse surrounded by spoiler gradient pulses of opposite signs, was used to dephase stopband resonances and minimally impact passband metabolites. Passband phase linearity was achieved with a dual BASING scheme. Using the Shinnar‐Le Roux algorithm, a highpass filter was designed to suppress water and rephase the lactate methyl doublet independently of TE, and water/lipid bandstop filters were designed for the brain and prostate. Phantom and in vivo experimental 3D PRESS CSI data were acquired at 1.5 T to compare BASING with CHESS and STIR suppression. With BASING, the measured suppression factor was over 100 times higher than with CHESS or STIR causing baseline distortions to be removed. It was shown that BASING can be incorporated into a variety of sequences to offer improved suppression in the presence of B1 and T1 inhomogeneites.
Three‐dimensional flow‐independent peripheral angiography
Authors: Brittain Jean H. , Olcott Eric W. , Szuba Andrzej , Gold Garry E. , Wright Graham A. , Irarrazaval Pablo , Nishimura Dwight G.
Published: March 2007

A magnetization‐prepared sequence, T2‐Prep‐IR, exploits T1, T2, and chemical shift differences to suppress background tissues relative to arterial blood. The resulting flow‐independent angiograms depict vessels with any orientation and flow velocity. No extrinsic contrast agent is required. Muscle is the dciminant source of background signal in normal volunteers. However, long‐T2 deep venous blood and nonvascular fluids such as edema also contribute background signal in some patients. Three sets of imaging parameters are described to address patient‐specific contrast requirements. A rapid, spiral‐based, three‐dimensional readout is utilized to generate high‐resolution angiograms of the lower extremities. Comparisons with x‐ray angiography and two‐dimensional time‐of‐flight angiography indicate that this flow‐independent technique has unique capabilities to accurately depict stenoses and to visualize slow flow and in‐plane vessels.
Real‐time interactive MRI on a conventional scanner
Authors: Kerr Adam B. , Pauly John M. , Hu Bob S. , Li King C. , Hardy Christopher J. , Meyer Ciraig H. , Macovski Albert , Nishimura Dwight G.
Published: March 2007

A real‐time interactive MRI system capable of localizing coronary arteries and imaging arrhythmic hearts in real‐time is described. Non‐2DFT acquisition strategies such as spiral‐interleaf, spiral‐ring, and circular echo‐planar imaging provide short scan times on a conventional scanner. Real‐time grid‐ding reconstruction at 8–20 image is achieved by distributing the reconstruction on general‐purpose UNIX workstations. An X‐windows application provides interactive control. A six‐interleaf spiral sequence is used for cardiac imaging and can acquire six images/s. A sliding window reconstruction achieves display rates of 16–20 images/s. This allows cardiac images to be acquired in real‐time, with minimal motion and flow artifacts, and without breath holding or cardiac gating. Abdominal images are acquired at over 2.5 images/s with spiral‐ring or circular echo‐planar sequences. Reconstruction rates are 8–10 images/s. Rapid localization in the abdomen is demonstrated with the spiral‐ring acquisition, whereas peristaltic motion in the small bowel is well visualized using the circular echo‐planar sequence.
Manganese ion enhances T1‐weighted MRI during brain activation: An approach to direct imaging of brain function
Authors: Lin Yi‐Jen , Koretsky Alan P.
Published: March 2007

Present techniques for functional MRI rely on detecting changes in hemodynamics that result as a consequence of brain activation. It would be useful if MRI techniques could be developed that enable imaging of a parameter directly related to neuronal activity. Influx of calcium into neurons is necessary for release of neurotransmitters. Divalent manganese ions (Mn2+) can enter cells through voltage‐gated calcium channels and Mn2+ is paramagnetic. Mn2+ accumulation in brain due to activation should alter relaxation times offering an approach to sensitize MRI to calcium influx in the brain. To test this idea, T1‐weighted MRI was obtained from the rat brain in the presence of a continuous intravenous infusion of 3.6 μmol/min MnCl2. In the anesthetized rat brain, signal enhancement was detected in regions corresponding to ventricles. Activation of the brain with glutamate led to increase in MRI signal intensity in the brain to 238 ± 23% of the original. This increase in signal was dependent on the presence of MnCl2 and was not due to changes in blood flow. It was necessary to break the blood brain barrier with mannitol to make Mn2 accessible to the active sites for efficient detection. Enhancement of MRI signal in the brain was also detected with decreasing anesthesia and with somatosensory stimulation. Due to the slow clearance of Mn2+ from the stimulated region of the brain, MRI enhancement could also be detected after stimulation that occurred on awake, behaving rats outside the magnet. These data indicate that MnCl2 shows potential as a MRI contrast agent that is directly sensitive to brain activation.
Detection of dopaminergic neurotransmitter activity using pharmacologic MRI: Correlation with PET, microdialysis, and behavioral data
Authors: Chen Yin Ching I. , Galpern Wendy R. , Brownell Anna‐Liisa , Matthews Russell T. , Bogdanov Mikhail , Isacson Ole , Keltner John R. , Beal M. Flint , Rosen Bruce R. , Jenkins Bruce G.
Published: March 2007

The metabolic activation resulting from direct dopaminergic stimulation can be detected using auto‐radiography, positron emission tomography (PET) or, potentially, fMRI techniques. To establish the validity of the latter possibility, we have performed a number of experiments. We measured the regional selectivity of two different dopaminergic ligands: the dopamine release compound D‐amphetamine and the dopamine transporter antagonist 2β‐carbomethoxy‐3β‐(4‐fluoropheny) tropane (CFT). Both compounds led to increased signal intensity in gradient echo images in regions of the brain with high dopamine receptor density (frontal cortex, striatum, cingulate cortex ≫ parietal cortex). Lesioning the animals with unilaterally administered 6‐hydroxydopamine (6‐OHDA) led to ablation of the phMRI response on the ipsilateral side; control measurements of rCBV and rCBF using bolus injections of Gd,‐DTPA showed that the baseline rCBV and rCBF values weire intact on the lesioned side. The time course of the BOLD signal changes paralleled the changes observed by microdialyisis measurements of dopamine release in the striatum for both amphetamine and CFT; peaking at 20–40 min after injection and returning to baseline at about 70–90 min. Signal changes were not correlated with either heart rate, blood pressure or pCO2. Measurement of PET binding in the same animals showed an excellent correlation with the phMRI data when compared by either measurements of the number of pixels activated or percent signal change in a given region. The time course for the behavioral measurements of rotation in the 6‐OHDA lesioned animals correlated with the phMRI. These experiments demonstrate that phMRI will become a valuable, noninvasive tool for investigation of neurotransmitter activity in vivo. Key words: fMRI; dopamine; amphetamine; CFT.
Gradient, high‐resolution, magic‐angle spinning nuclear magnetic resonance spectroscopy of human adipocyte tissue
Authors: Millis Kevin K. , Maas Werner E. , Cory David G. , Singer Samuel
Published: March 2007

The recently developed technique of gradient, high‐resolution magic‐angle spinning NMR (g‐hr‐MAS‐NMR) spectroscopy was applied to the study of ex vivo human lipoma and liposarcoma tissue. Compared with conventional 1H‐NMR, the g‐hr‐MAS method yielded a large improvement in spectral resolution and permitted the detection of metabolite resonance's in a well‐differentiated liposarcoma that was not observed in spectra of similar samples obtained using nonspinning NMR methods. These findings suggest that g‐hr‐MAS‐NMR spectroscopy provides a key improvement in spectral quality for ex vivo lipoma and liposarcoma tissue thereby permitting a more precise determination of tissue metabolite composition than conventional nonspinning NMR methods. Key words: magic‐angle spinning; nuclear magnetic resonance; magnetic field gradients; sarcoma.
STAR‐HASTE: Perfusion imaging without magnetic susceptibility artifact
Authors: Chen Qun , Siewert Bettina , Bly Benjamin Martin , Warach Steven , Edelman Robert R.
Published: March 2007

A novel magnetic resonance imaging technique (STAR‐HASTE) based on pulsed arterial spin labeling using a single shot acquisition method is described for perfusion imaging. The method is similar to EPISTAR in using STAR (Signal Targeting with Alternating Radiofrequency) technique for pulsed radiofrequency labeling of inflowing blood, but uses a half‐Fourier single shot turbo spin‐echo (HASTE) sequence for data acquisition instead of echo‐planar imaging (EPI). Our preliminary studies show that STAR‐HASTE permits perfusion imaging to be performed without many of the artifacts encountered with other imaging methods based on EPI acquisition. The novel method not only provides similar perfusion information to that obtained by EPISTAR, as demonstrated in the functional brain imaging study, but also eliminates magnetic susceptibility artifacts and image distortion commonly observed in EPI images. Furthermore, this technique can be readily implemented in MR systems without EPI capability.
Quantitative measurements of regional cerebral blood volume using MRI in rats: Effects of arterial carbon dioxide tension and mannitol
Authors: Lin Weili , Paczynski Richard P. , Kuppusamy Karthikeyan , Hsu Chung Y. , Haacke E. Mark
Published: March 2007

A three‐dimensional (3D) T1‐weighted sequence was used to acquire high spatial resolution whole brain images in rats before and after the injection of an intravascular contrast agent. These T1‐weighted images were used to estimate regional cerebral blood volume (rCBV) as a percentage of blood volume in each voxel. Ventilation was manipulated to investigate the effects of altered arterial carbon dioxide tension (PaCO2) on rCBV. In addition, different doses of a hypertonic mannitol solution were used to investigate the sensitivity of the proposed method in a serial monitoring paradigm. An rCBV of 2.40% ± 0.34% was obtained before any physiological manipulation, in good agreement with literature values using alternative techniques. Using this method, it was found that there exists a linear relationship between PaCO2 and rCBV (R2 = 0.77) and that rCBV increased in a dose and time dependent fashion in mannitol‐treated rats. High signal‐to‐noise was available due to the substantial increase in blood signal from the intravascular contrast agent.