bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetization transfer, cross‐relaxation, and chemical exchange in rotationally immobilized protein gels
Authors: Zhou Dawei , Bryant Robert G.
Published: March 2007

Water proton spin‐lattice relaxation rates are reported as a function of the magnetic field strength for cross‐linked bovine Serum albumin samples. The relaxation dispersion profile is analyzed using a relaxation model where the solid components have the magnetic field dependence proportional to v−0.5 which may result from a defect diffusion model with two degrees of freedom. If the cross‐linking agent concentration is not sufficiently high, the relaxation dispersion curve may have significant contributions from freely rotating protein. The magnetic field dependence of the relaxation rates studied as a function of the proton mole fraction in the sample show that approximately 30% of the magnetization transfer rate is directly proportional to the proton mole fraction. This contribution is identified with the magnetization transfer from exchange of whole water molecules with buried binding sites on the protein. The second order magnetization transfer rate constant is 388 s−1 assuming unit water spin concentration. The solid component relaxation obeys an Arrhenius activation law, but the overall temperature dependence of the crossrelaxation is complicated by chemical exchange processes which enter with opposite sign.
Initial feasibility studies using single‐shot EPI for the detection of focal liver lesions
Authors: Reimer Peter , Ladebeck Ralph , Rummeny Ernst J. , Repp Heinz , Schmitt Franz , Bongartz Georg , Schuierer Gerhard , Lange Thomas , Peters Peter E.
Published: March 2007

Echo planar MR imaging (EPI) has been developed to completely eliminate motion artifacts and is currently being prepared for implementation into clinical MR systems. Thus, the purpose of this study was to evaluate the clinical utility of EPI in the detection of focal liver lesions and to compare EPI with contrast‐enhanced CT. EPI studies were performed on an experimental 1.0 Tesla whole body system using fat‐suppressed single‐shot spin echo (SE) and inversion recovery (IR) pulse sequences. A total of 26 liver tumors in 12 patients scheduled for liver resection were prospectively examined and correlated with intraoperative ultrasound, surgery, and pathology as the gold standard. Quantitative analysis of EPI was performed by means of liver signal‐to‐noise and tumorliver contrast‐to‐noise calculations. Diagnostic performance compared with contrast‐enhanced CT was assessed by means of ROC analysis. Lesion‐liver contrast was highest with EPI SE at a TE‐time of 70 ms and this technique showed best lesion detectability as measured by area under curve (AUC) values. Among EPI techniques, the IR sequence with an inversion time of 300 ms to null the liver signal showed high lesion‐liver contrast but all four reviewers reported problems assessing liver anatomy. Improved EPI techniques may prove useful for screening of focal liver lesions.
Theory of NMR signal behavior in magnetically inhomogeneous tissues: The static dephasing regime
Authors: Yablonskiy Dmitriy A. , Haacke E. Mark
Published: March 2007

This paper is devoted to a theory of the NMR signal behavior in biological tissues in the presence of static magnetic field inhomogeneities. We have developed an approach that analytically describes the NMR signal in the static dephasing regime where diffusion phenomena may be ignored. This approach has been applied to evaluate the NMR signal in the presence of a blood vessel network (with an application to functional imaging), bone marrow (for two specific trabecular structures, asymmetrical and columnar) and a ferrite contrast agent. All investigated systems have some common behavior. If the echo timeTEis less than a known characteristic timetcfor a given system, then the signal decays exponentially with an argument which depends quadratically onTE. This is equivalent to an R2* relaxation rate which is a linear function ofTE. In the opposite case, whenTEis greater thantc, the NMR signal follows a simple exponential decay and the relaxation rate does not depend on the echo time. For this time interval, R2* is a linear function of a) volume fraction ζ occupied by the field‐creating objects, b) magnetic field B0or just the objects' magnetic moment for ferrite particles, and c) susceptibility difference Δχ between the objects and the medium.
Detection of propan‐1,2‐diol in neonatal brain by in vivo proton magnetic resonance spectroscopy
Authors: Cady Ernest B. , Lorek Ann , Penrice Juliet , Reynolds E. Osmund R. , Iles Richard A. , Burns Shamus P. , Coutts Glyn A. , Cowan Frances M.
Published: March 2007

Cerebral in vivo proton magnetic resonance spectroscopy of 13 newborn infants displaying seizures and receiving phenobarbitone, in one case supplemented by phenytoin, showed signals from propan‐1,2‐diol (the injection vehicle for both these anticonvulsants). Subsequent in vitro spectroscopy of cerebro spinal fluid (CSF) from one of these infants also showed signals from this substance. The estimated in vivo propan‐1,2‐diol concentration (∼3 mM) was less than that measured in the CSF sample (14.4 mM). These observations suggest that propan‐1,2‐diol may accumulate in cerebral tissue and misidentification of its signals in both in vivo and in vitro proton spectra may confuse diagnoses of metabolic or other disorders.
Proton Magnetic Resonance Spectroscopy and Gadolinium‐DTPA Perfusion Imaging of Asymptomatic MRI White Matter Lesions
Authors: Oppenheimer Stephen M. , Nick Bryan R. , Conturo Thomas E. , Soher Brian J. , Preziosi Thomas J. , Barker Peter B.
Published: March 2007

In the elderly, asymptomatic white matter hyperintensities are common on T2‐weighted magnetic resonance imaging (MRI). In symptomatic patients, such MRI appearances correlate with varied postmortem findings including demyelination or stroke. What structural correlates underlie the T2 hyperintensities in patients whose lesions are asymptomatic is controversial. Therefore, in order to investigate the underlying metabolism and perfusion in white matter lesions (exhibiting T2 hyperintensity), 13 patients underwent proton magnetic resonance spectroscopy and dynamic gadolinium‐DTPA perfusion‐weighted MR imaging. N‐acetyl aspartate (NA) levels were reduced in the lesions compared with age‐matched controls (P = 0.031), implying neuronal/axonal loss. Creatine levels were also reduced (P = 0.001). Choline levels were unchanged in the lesions. Lactate was identified in the lesions of 5 of the 13 patients. Although not statistically significant, perfusion studies exhibited a trend toward lower cerebral blood volumes in patients with high grade extracranial carotid stenosis and lactate‐containing lesions. These findings suggest that neuronal/axonal loss underlies the majority of T2‐weighted asymptomatic lesions in the older population, and in many cases these changes may be due to chronic ischemia.
Calculation of Sensitivity Correction Factors for Surface Coil MRS
Authors: Doyle Victoria L. , Buil Michiel , Payne Geoffrey S. , Leach Martin O.
Published: March 2007

Quantification of MRS signals obtained with surface coils is difficult due to the inhomogeneous response of these coils. This inhomogeneity results in the measured signal from a defined volume of interest (VOI) being spatially dependent. To account for the sensitivity variation with position from the surface coil, we have developed a method of calculating correction factors for defined VOIs based on an experimentally obtained 3D sensitivity coil map. These factors may then be applied to spectra obtained from these VOIs to accurately take into consideration the varying coil sensitivity resulting in a reduction of measured signal. This method is demonstrated here to be able to correct for the inhomogeneity of surface coils over a range of two coil radii to within 4% accuracy.
Cerebral White Matter in the Centrum Semiovale Exhibits a Larger N‐acetyl Signal than Does Gray Matter in Long Echo Time 1H‐Magnetic Resonance Spectroscopic Imaging
Authors: Tedeschi Gioacchino , Righini Andrea , Bizzi Alberto , Barnett Alan S. , Alger Jeffry R.
Published: March 2007

Long echo time (272 ms) 1H magnetic resonance spectro‐scopic imaging was used to measure the relative magnitudes of the N‐acetylaspartate (NAA) signal in a variety of anatomically defined brain structures (centrum semiovale, thalamus, medial frontal cortex, and genu of the corpus callosum) composed primarily of gray matter or white matter. Six normal young adult humans aged 30–40 were studied. With a 95% level of statistical confidence, the white matter in the centrum semiovale (CSO) produced a more intense NAA signal than did the gray matter in the thalamus and the frontal cortex. Differences between the white matter regions were also noted. The CSO white matter's NAA signal yielded a larger NAA signal than did the white matter of the genu of the corpus callosum. Possible reasons for the anatomical variation in the cerebral NAA signal intensity are discussed.
Detection of Residual Quadrupolar Interaction in Human Skeletal Muscle and Brain in vivo via Multiple Quantum Filtered Sodium NMR Spectra
Authors: Reddy Ravinder , Bolinger Lizann , Shinnar Meir , Noyszewski Elizabeth , Leigh John S.
Published: March 2007

Using sodium multiple quantum filtered methods, we have, for the first time, demonstrated the presence of residual quadrupolar interaction in human skeletal muscle and brain in vivo. Surface coils were used in both skeletal muscle and brain studies on healthy human volunteers. Theoretical analysis shows that even with arbitrary flip angles the double quantum filter retains filtering capability; it passes even rank two quantum coherence more efficiently than odd rank two quantum coherence. Multiple quantum filtered spectra were obtained from the gastrocnemius muscle in the leg and from the parietal lobe region of the brain. In double quantum filtered spectra, we observed different proportions of narrow and broad components with preparation time, while the line shape of triple quantum filtered spectra remained similar to the one observed in isotropic environment. These results suggest that, in human skeletal muscle and brain, sodium ions are bound to some ordered structures.
Ultra‐Rapid Gradient Echo Imaging
Authors: Heid Oliver , Deimling M. , Huk W. J.
Published: March 2007

A novel ultra‐rapid gradient echo (URGE) NMR imaging technique is introduced, which is capable of continuous high resolution 3D scanning while neither subject to fast gradient switching nor excessive RF power deposition. Sampling free induction decays instead of creating spin echoes enables maintaining a workable steady state magnetization. Due to segmented k‐space acquisition, chemical shift, diffusion, and field inhomogeneity effects do not present major problems. We report on implementations acquiring from 32 x 64 x 64 partial‐Fourier image sets in 0.72 s, allowing for single‐shot magnetization‐prepared 3D imaging, to 128 x 128 x 128 image sets in 13.3 s and 21.5 s on a standard MRI scanner.
Block Regional Interpolation Scheme for k‐Space (BRISK): A Rapid Cardiac Imaging Technique
Authors: Doyle Mark , Walsh Edward G. , Blackwell Gerald G. , Pohost Gerald M.
Published: March 2007

We introduce an acquisition method, “block regional interpolation scheme for k‐space” (BRISK), to reduce the acquisition time for cardiac imaging. The method exploits the high degree of correlation that exists between time‐resolved cardiac images. For representative k‐space data sets, Fourier analysis was applied along the cardiac phase dimension to reveal that different regions of k‐space can be effectively sampled at different rates. A reduced sampling strategy was implemented, and unsampled points were generated by Fourier interpolation. Time savings of up to 75% are quite feasible and 25% BRISK scans compare well with 100% scans. Simulations and acquisitions using a normal volunteer and patients are presented.
Multi‐Slice MRI of Rat Brain Perfusion During Amphetamine Stimulation Using Arterial Spin Labeling
Authors: Silva Afonso C. , Zhang Weiguo , Williams Donald S. , Koretsky Alan P.
Published: March 2007

When a single coil is used to measure perfusion by arterial spin labeling, saturation of macromolecular protons occurs during the labeling period. Induced magnetization transfer contrast (MTC) effects decrease tissue water signal intensity, reducing the sensitivity of the technique. In addition, MTC effects must be properly accounted for in acquiring a control image. This forces the image to a single slice centered between the labeling plane and the control plane. In this work, a two‐coil system is presented as a way to avoid saturation of macromolecular spins during arterial spin labeling. The system consists of one small surface coil for labeling the arterial water spins, and a head coil for MRI, actively decoupled from the labeling coil by using PIN diodes. It is shown that no signal loss occurs due to MTC effects when the two‐coil system is used for MRI of rat brain perfusion, enabling three‐dimensional perfusion imaging. Using the two‐coil system, a multi‐slice MRI sequence was used to study the regional effects of amphetamine on brain perfusion. Amphetamine causes significant increases in perfusion in many areas of the brain including the cortex, cingulate, and caudate putamen, in agreement with previous results using deoxyglucose uptake to monitor brain activation.
Phased Array Coils for Upper Extremity MRA
Authors: Monroe James W. , Schmalbrock Petra , Spigos Dimitrios G.
Published: March 2007

A phased array coil was constructed for imaging the upper extremity vasculature for patients undergoing dialysis treatment. The phased array coil exhibits improved signal‐to‐noise ratio (SNR) over the body coil and allows imaging of the entire upper extremity. SNR as a function of depth was measured on a homogeneous phantom with the arm coil and compared with the body coil. Near the coil there is an improvement of a factor of 7 and at a depth of approximately 6–7 cm, there is an improvement of factor 2. In vivo SNR measurements resulted in similar improvements. Images of the upper extremity vasculature of healthy volunteers were generated using 2D‐time‐of‐flight (2DTOF) angiography. Blood flow velocity was assessed in a CINE‐phase contrast study.
Barbiturate‐Reversible Reduction of Water Diffusion Coefficient in Flurothyl‐Induced Status Epilepticus in Rats
Authors: Zhong Jianhui , Petroff Ogen A. C. , Prichard James W. , Gore John C.
Published: March 2007

Rat brains (n = 17) with flurothyl‐induced status epilepticus (SE) have been imaged with a gradient‐echo diffusion‐weighted imaging sequence at 2.0 T. The apparent water diffusion coefficient (ADC) decreased during seizure discharges. The magnitude of the ADC reduction correlated well with the duration of flurothyl exposure. A 17% reduction in the water ADC compared with preseizure condition was observed in rats with the longest flurothyl exposure time. In 13 rats, pentobarbital was used to arrest the electrographic seizure activity. ADC values began to return to normal a few minutes after the injection. In four rats with no pentobarbital administration, ADC values remained depressed up to 1 h after seizure onset. The results suggest that diffusion‐weighted MR imaging may be useful for mapping recent intense seizure activity in human patients with medically intractable epilepsy.
3D 31P Spectroscopic Imaging of the Human Heart at 4.1 T
Authors: Hetherington Hoby P. , Luney Derek J. E. , Thomas Vaughan J. , Pan Jullie W. , Ponder Steven L. , Tschendel Otmar , Twieg Donald B. , Pohost Gerald M.
Published: March 2007

High field (4 Tesla) spectroscopic imaging offers the advantages of increased signal‐to‐noise ratio and the possibility of acquiring high resolution metabolite images. We have applied a three dimensional spectroscopic imaging sequence using a sparse Gaussian sampling method to acquire phosphocreatine (PCr) images of the human heart with 8‐cc voxels. PCr images enabled observation of the septum, left ventricular free wall, apex, and skeletal muscle. Quantitative evaluation of the 50 myocardial voxels acquired from 10 studies of healthy adults revealed a PCr/adenosine triphosphate (ATP) ratio of 1.80 ± 0.32 after correction for saturation effects. Due to the small size of the voxels and the ability to choose the location of the volumes to minimize inclusion of blood, no correction for blood pool ATP was required. The calculated PCr/ATP ratio is in agreement with other studies at 1.5 and 4.0 T.
A Model for Magnetization Transfer in Tissues
Authors: Morrison Clare , Mark Henkelman R.
Published: March 2007

Magnetization transfer in several tissues is measured and successfully modeled using a two‐pool model of exchange. The line shape for the semi‐solid pool is characterized by a superLorentzian and the liquid pool by a Lorentzian. The tissues investigated were white and gray matter, optic nerve, muscle, and liver. All tissues the authors studied are characterized by the same model but differ in the parameter values of the model. Blood and cerebral spinal fluid (CSF) were also investigated. The two‐pool model with a Lorentzian line shape for both the semi‐solid and liquid pools modeled the magnetization transfer in blood. In CSF, as expected, there is no measurable exchange of magnetization. The T2B associated with the semi‐solid pool was short (˜10 μ) for all tissues indicating a fairly rigid semi‐solid pool. In addition characterization of the line shape as superLorentzian indicates molecules such as integral membrane proteins or lipids in membranes are likely molecules participating in the exchange. Conversely, in blood large globular proteins are indicated due to the Lorentzian nature of the semi‐solid pool and a T2B ≈ 300 μs.
Quantitative Combined Phosphorus and Proton PRESS of the Brains of Newborn Human Infants
Authors: Cady Ernest B.
Published: March 2007

Techniques for quantitative, combined phosphorus and proton, point‐resolved spectroscopy (PRESS) studies of newborn‐infant brain have been developed. Phosphorus PRESS advantages include: voxel‐shimming; rapid transmitter‐pulse setting; novel use of brain‐water as a localized quantitation reference; and reduced broad components. Proton spectra from 1‐ml voxels and phosphorus spectra can both be acquired quantitatively within acceptable time. Cerebral lactate was consistently detected by proton PRESS and the normal concentration (˜ 3 mmol/kg wet weight) may be higher than in adult brain. Phosphorus PRESS provided metabolite peak‐area ratios and concentrations comparable with those obtained using ISIS.
The effects of isoflurane and halothane on blood flow and 31P NMR spectra in murine RIF‐1 tumors
Authors: Zhao Ming , Fortan Ludwig G. , Evelhoch Jeffrey L.
Published: March 2007

The principal aim of these studies was to evaluate the utility of isoflurane and halothane for NMR investigations of tumor physiology. In vivo 31P and 2H NMR were used to examine RIF‐1 tumors before, during, and (for 31P) after anesthesia. In tumors, halothane decreases blood flow, [PCR]:[NTP], and pH indicated by the P1 chemical shift (pHnmr), while it increases [P1:[NTP]; effects consistent with well‐established cardiovascular effects of halothane. Isoflurane does not affect tumor blood flow or [PCr]:[NTP], but increases tumor [P1:[NTP] and decreases tumor pHnmr. In vivo 31P NMR measurements of normal mouse liver (upper abdomen) indicate that isoflurane has a similar effect in the liver. Although the mechanism for these effects is unknown, observation of a split P1 peak during isoflurane anesthesia suggests that a pool of P1 in a lower pH environment may become evident under isoflurane anesthesia. Regardless of the cause for increased [P1:[NTP] and decreased pHnmr the utility of isoflurane anesthesia for 31P NMR studies of energy metabolism is limited.
Pulses with fixed magnitude and variable phase response profiles
Authors: Pickup Stephen , Ding Xiaoning
Published: March 2007

The Shinnar‐Le Roux (SLR) method of pulse envelope design provides for the generation of nearly arbitrary magnitude response profiles with such great efficiency that the pulse envelopes may often be calculated at the time of sequence initiation. A significant limitation of the method is that it provides only limited control of the phase of the response profile. In the current manuscript it is demonstrated that the phase of the response profile can be modulated without affecting the magnitude profile by replacement of some of the roots of the SLR polynomials with one over their complex conjugate. This method allows interactive tailoring of the phase profile to the user's needs. Although the method does not allow for the optimization of arbitrary phase profiles, a variety of pulses, which are of general utility, have been generated. Some of these pulses and their response profiles are presented.
Spatial‐Frequency‐Tuned Markers and Adaptive Correction for Rotational Motion
Authors: Korin Hope W. , Felmlee Joel P. , Riederer Stephen J. , Ehman Richard L.
Published: March 2007

A common type of motion present in clinical magnetic resonance imaging examinations is rotational motion, such as that due to voluntary motion during head examinations. The correction scheme presented in this work offers a method for eliminating the effects of rotations within the imaging plane. Integral to the implementation of this technique is the concept and design of spatial‐frequency‐tuned markers, which are used to track the rotational motion. These studies showed that it is possible to accurately track the motion, measuring both axis and angle of rotation, and use this information to retrospectively correct the acquired images. These markers can also provide information about any translational motion present. The resulting images show a marked decrease in artifacts and improved clarity.
Respiratory Motion of the Heart: Kinematics and the Implications for the Spatial Resolution in Coronary Imaging
Authors: Wang Yi , Riederer Stephen J. , Ehman Richard L.
Published: March 2007

Respiratory motion is a major limiting factor in improving image resolution and signal‐to‐noise ratio in MR coronary imaging. In this work the effects of respiration on the cardiac position were studied quantitively by imaging the heart during diastole at various positions of tidal respiration with a breath‐hold segmented fast gradient echo technique. It was found that during tidal breathing the movement of the heart due to respiration is dominated by superior‐inferior (SI) motion, which is linearly related to the SI motion of the diaphragm. The motion of the heart due to respiration is approximately a global translation. These results provide motivation for employing adaptive motion correction techniques to reduce image blurring in nonbreath‐hold coronary MR imaging.
Measurement of Water Diffusion in Hormone‐Treated Rat Uteri by Diffusion‐Weighted Magnetic Resonance Imaging
Authors: Yang Yihong , Xu Su , Joan Dawson M.
Published: March 2007

In this study, apparent water diffusion coefficients in hormone‐treated rat uteri were measured by diffusion‐weighted NMR imaging. Four groups of ovariectomized, mature young rats were treated with estrogen, progesterone, estrogen plus progesterone, or saline (as control). It was found that water diffusion is anisotropic in the myometrium, but not in the endometrium, and that it depends on hormonal conditions in the endometrium, but not in the myometrium. The anisotropy of water diffusion can be exploited to improve image contrast among the various uterine tissue types. The hormonal response suggests that diffusion imaging of human uterus may be useful in detecting endometrial pathology.
Discrimination of Large Venous Vessels in Time‐Course Spiral Blood‐Oxygen‐Level‐Dependent Magnetic‐Resonance Functional Neuroimaging
Authors: Lee Adrian T. , Glover Gary H. , Meyer Craig H.
Published: March 2007

A technique is described for discriminating blood‐oxygen‐level‐dependent (BOLD) signal changes originating from large venous vessels and those that arise from the cortical parenchyma based on examining the temporal delay of each pixel's response. Photic stimulation experiments were performed with a conventional 1.5 T scanner and correlated each pixel's time‐course with sine and cosine functions at the frequency of the stimulus. It was found that the signal in pixels anatomically associated with gray matter was delayed between 4 and 8 s compared with the stimulus, whereas the signal in pixels correlated with visible vessels and sulci was generally delayed from 8 to 14 s. This larger delay is consistent with the longer time required for blood to reach the larger vessels. Finally, stimulus‐induced NMR phase changes were observed for the largest vessels, which are attributed to bulk susceptibility shifts.
Motion Induced Phase Shifts in MR: Acceleration Effects in Quantitative Flow Measurements—A Reconsideration
Authors: Kouwenhoven Marc , Hofman Mark B. M. , Sprenger Michiel
Published: March 2007

Magnetic resonance phase difference techniques are commonly used to study flow velocities in the human body. Acceleration is often present, either in the form of pulsatile flow, or in the form of convective acceleration. Questions have arisen about the exact time point at which the velocity is encoded, and also about the sensitivity to (convective) acceleration and higher order motion derivatives. It has become common practice to interpret the net phase shifts measured with a phase difference velocity technique as being the velocity at a certain (Taylor) expansion time point, chosen somewhere between the RF excitation and the echo readout. However, phase shifts are developed over the duration of the encoding magnetic field gradient wave form, and should therefore be interpreted as a more or less time‐averaged velocity. It will be shown that the phase shift as measured with a phase difference velocity technique represents the velocity at the “gravity” center of the encoding bipolar gradient (difference) function, without acceleration contribution. Any attempt to interpret the measured phase shift in terms of velocity on any other time point than the gradient gravity point will automatically introduce acceleration sensitivity.
In Vivo Validation of Magnetic Resonance Blood Volume Flow Measurements with Limited Spatial Resolution in Small Vessels
Authors: Hofman Mark B. M. , Visser Frans C. , Van Rossum Albert C. , Vink Ger Q. M. , Sprenger Michiel , Westerhof Nico
Published: March 2007

The accuracy of magnetic resonance phase contrast volume flow measurements in small blood vessels is expected to be smaller than in large vessels, because of partial volume effects at the vessel boundary. Accuracy was validated in the dog femoral artery, diameter 3.5 ± 0.7 mm, using an ultrasonic transit‐time flowmeter (TT). The number of pixels per vessel diameter (ND) ranged from 1.6 to 4.8. The vessel cross‐section was determined using a threshold in the magnitude image. Between the two methods the correlation coefficient was 0.95 (range 10–200 ml/min). The proportional difference (PD), (QTT – QMR/1/2(QTT + QMR), was 0.8%, showing no systematic difference between the methods. The PDs standard deviation was 27%, and 19% for flow rates above 30 ml/min. Only a significant decrease of the PDs variance was found at the highest ND values, suggesting other sources of error than partial volume effects. It is concluded that with an ND value of about 3, accurate blood volume flow rates can be determined.
NMR Microscopy of Single Neurons Using Spin Echo and Line Narrowed 2DFT Imaging
Authors: Bowtell Richard W. , Peters Andrew , Sharp Jonathan C. , Mansfield Peter , Hsu Edward W. , Aiken Nanci , Horsman Anthony , Blackband Stephen J.
Published: March 2007

NMR microimages of single neural cells were acquired at 500 MHz using a conventional spin echo pulse sequence and a line‐narrowing sequence that eliminates susceptibility effects. The data show that any contribution to the measuredT2relaxation rate arising from diffusion in local field inhomogeneities using spin echo sequences at high fields and high spatial resolution is relatively small. We conclude that the measuredT2difference between the nucleus and cytoplasm in these cells represents primarily a trueT2relaxation effect arising from the interactions of water with macromolecules in the two compartments and does not result from microsusceptibility differences. These observations have implications regarding water compartmentation in single cells and the interpretation of the MR characteristics of tissuesin vivo.
Accuracy of Oxygen Measurements in T2 (Line Width) EPR Oximetry
Authors: Smirnova Tatyana I. , Smirnov Alex I. , Clarkson Robert B. , Linn Belford R.
Published: March 2007

EPR oximetry is used for in vivo and in vitro measurements of oxygen in biological systems, including experimental animals. The accuracy of oxygen measurements in T2 (line width) EPR oximetry is significantly improved if least‐squares simulation is used to extract the line width parameters. The oxygen effect on the EPR spectra of nitroxide solutions and aqueous suspensions of fusinite can be described as an additional homogeneous broadening that modifies the EPR spectrum of the oxygen‐free probe. This allows one to use a one‐parameter line width model in most cases. The simulations were carried out with the use of a fast‐convolution algorithm followed by Levenberg‐Marquardt optimization. The validity of error estimates provided by this method was tested on sets of experimental spectra taken under common conditions. It is shown that the accuracy of oxygen measurements in line width (T2) oximetry is determined not only by the probe sensitivity (rate of line width change versus oxygen concentration), but also by the signal‐to‐noise ratio, inhomogeneous contribution to the line shape (e.g., unresolved proton superhyperfine structure), and the spectral window. The accuracy of oxygen measurements is compared for aqueous solutions of two nitroxide radicals with different superhyperfine structure and for aqueous suspensions of fusinite.
A Nyquist Modulated Echo‐to‐View Mapping Scheme for Fast Spin‐Echo Imaging
Authors: Keller Paul J. , Heiserman Joseph E. , Fram Evan K. , Rand Scott D. , Drayer Burton P.
Published: March 2007

A new scheme for assignment of echoes to views in fast spin‐echo imaging was developed. The scheme places early and late echoes in alternating lines of the periphery of k‐space; continued alternation of echoes arising closer to the middle of the echo train encodes the central portion of k‐space. The scheme has two effects: a) The echo‐to‐echo signal decay that usually gives rise to multiple faint ghosts, now yields a single Nyquist ghost that is removed by phase over‐sampling. b) Mapping earlier echoes (and thus more energy) to the periphery of k‐space yields sharper edges.
Design of Matching Networks for Low Noise Preamplifiers
Authors: Reykowski Arne , Wright Steven M. , Porter Jay R.
Published: March 2007

This paper discusses matching networks that minimize inductive coupling between the antennas within an array while simultaneously insuring minimum noise contributions from preamplifiers. Typical low noise preamplifier designs require a strong mismatch between the source impedance and the amplifier input impedance (reflection coefficient close to one) to achieve optimal noise performance. This is in contrast to the familiar impedance match known from communication theory where input and source impedances have complex conjugate values for maximizing the power transfer from source to amplifier. The high input reflection coefficient of low noise amplifiers can be exploited to reduce antenna currents by using lossless impedance transformations to create a high impedance at the coil terminals while simultaneously maintaining a low noise figure for the amplifier. The networks presented here constitute an improvement over previous work because they give additional freedoms regarding the values of the network components and the amplifier input impedance. The technique has been formalized and coded in MathCadTM, making the design of realizable networks a simple process.
Bsh distributions in the canine head and a human patient using 11b mri
Authors: Bradshaw Kenneth M. , Schweizer Martin P. , Glover Gary H. , Rock Hadley J. , Tippets Richard , Tang Peng‐Peng , Davis Wayne L. , Peter Heilbrun M. , Johnson Suzanne , Ghanem Tamer
Published: March 2007

A 3D projection reconstruction (3DPR) method was used to obtain in vivo 11B images in a large canine brain tumor model and in a human infused with borocaptate sodium (BSH). Studies were performed in dogs with and without gliosarcomas implanted and grown to a size of 2–3 cm. The 3DPR method demonstrates a signal‐to‐noise ratio (SNR) that allows qualitative kinetic studies of the boron compound in normal and tumor tissue of the head. The measurements indicate initial uptake of the BSH compound in tumor to be less than that in muscle with no uptake in normal brain tissue. Moreover, uptake of BSH in tissue was found to lag the boron concentration in blood with delays that depend on tissue type. In addition, the first human boron images were obtained on a patient who underwent surgical resection and volumetric debulking of a large (7 cm) glioblastoma multiforme. BSH was readily taken up in residual tumor tissue, while diffusion into the resection volume was slower.
Correction for geometric distortion in echo planar images from B0 field variations
Authors: Jezzard Peter , Balaban Robert S.
Published: March 2007

A method is described for the correction of geometric distortions occurring in echo planar images. The geometric distortions are caused in large part by static magnetic field inho‐mogeneities, leading to pixel shifts, particularly in the phase encode direction. By characterizing the field inhomogeneities from a field map, the image can be unwarped so that accurate alignment to conventionally collected images can be made. The algorithm to perform the unwarping is described, and results from echo planar images collected at 1.5 and 4 Tesla are shown.
Increased flexibility in GRASE imaging by k space‐banded phase encoding
Authors: Feinberg David A. , Johnson Glyn , Kiefer Berthold
Published: March 2007

GRASE (GRadient and Spin Echo) is an echo train imaging technique that combines gradient and RF refocusing. Although overall signal decay is with T2 and field inhomogeneity phase errors do not accumulate, the small residual phase errors are periodic with echo number. The echo order described previously eliminates the phase error periodicity in k space but instead creates periodicity in the T2 modulation function that can also cause artifacts. In addition, with this order, the effective TE must be half the echo train time, and asymmetric Fourier sampling is difficult to implement. A new method is described that greatly reduces artifacts due to T2 decay, permits greater control of T2 contrast, and lends itself to asymmetric Fourier sampling. Different time segments of the echo train are encoded with different bands of spatial frequency in k space (hence “k banding”). Both computer simulations and experimental results demonstrate improvements in GRASE images acquired by this method.
Modulation of water diffusion during gonadotropin‐induced ovulation: nmr microscopy of the ovarian follicle
Authors: Tempel Catherine , Schiffenbauer Yael S. , Meir Gila , Neeman Michal
Published: March 2007

The preovulatory rat follicle reaches a diameter of 1 mm with no internal blood vessels. Nutrient supply to the enclosed oocyte depends solely on passive diffusion across the follicular wall and the follicular fluid. Spin‐echo and stimulatedecho NMR microscopy experiments were applied here for studying modulations in water diffusion during gonadotropin‐induced maturation of perfused rat ovarian follicles (32°C). Two diffusion compartments were observed for the follicular wall. The intracellular water diffusion coefficient, measured at a short diffusion time (9 ms) was 0.28 x 10−5 cm2/s. Diffusion at long diffusion times was restricted to 16 μm, the size of cells in the follicular wall, and did not change during maturation. In the follicular fluid a transient 26% decrease in the diffusion coefficient was observed 4–7 h after gonadotropin stimulation, a change that is bound to affect the metabolic balance of the oocyte before ovulation.
Homonuclear J‐refocused spectral editing technique for quantification of glutamine and glutamate by 1H NMR spectroscopy
Authors: Lee Heung K. , Yaman A. , Nalcioglu Orhan
Published: March 2007

Quantification of the excitatory amino acid of glutamine and glutamate is extremely important partly because, when in excessive amounts, they can produce neuronal damage. However, quantification of such metabolites seems to be not straightforward owing to a large number of metabolic signals, particularly between the 2.0– and 3.0−ppm chemical shift range. The authors investigated a quantification method by using a homonuclear J‐refocused spectral editing technique, namely the J‐refocused PRESS (JPRESS), using both 1.5− and 4‐T whole body NMR systems. At first, the J‐coupling modulation of each metabolite at different TE values using the PRESS and JPRESS pulse sequences was compared. The suppression of J modulations with the JPRESS is well demonstrated on condition that TE values are less than 1/4 J. Then, the accuracy of glutamate quantification after subtracting the two spectra obtained at two different echo times and integrating the difference spectral area at different concentrations was evaluated. The results exhibit an excellent linear relationship between the actual concentration versus the difference spectral peak area at both 1.5 and 4 T. The achievable minimum voxel size was also considered and a 1‐ml voxel size seems to be possible at the typical glutamate concentration in vivo at a field strength of 4 T.
Development of a phased‐array coil for the lower extremities
Authors: Alley Marcus T. , Grist Thomas M. , Swan J. Shannon
Published: March 2007

A phased‐array coil was developed to facilitate imaging the vasculature of the lower extremities. The array consists of four surface coils placed in a Plexiglas “I‐beam” frame that are configured to allow bilateral studies with up to a 40‐cm field of view (FOV). Data from phantoms indicate an increase in signal‐to‐noise ratio (SNR) in the regions of interest by an average factor of 2.8 ± 0.9 over that of the body coil. Preliminary in vivo data have also been obtained from n = 8 subjects and demonstrate significant improvements in image quality. The coil design described here should lead to reduced scan times through the ability to image both legs simultaneously with less need for patient repositioning.
A computer simulation of the static magnetic field distribution in the human head
Authors: Li Shizhe , Williams Gerald D. , Frisk Timothy A. , Arnold Blake W. , Smith Michael B.
Published: March 2007

Distortion of the static magnetic field inside the human head is dependent on regional tissue susceptibility variations and geometrical shape. These effects result in resonance line broadening and frequency shifts and consequently, intensity and spatial errors in both magnetic resonance imaging (MRI) and magnetic resonance (MR) spectroscopy. To calculate the field distortion due to the susceptibility's geometry, two dimensional (2D) finite element analysis was applied to simulate the field distribution in a 2D model of the human head, placed in a uniform magnetic field. The model contains air‐filled cavities and sinuses, and the remainder is treated as water. The magnetic field deviation was evaluated using gray scale plots and histograms of the magnetic field. The shifts in parts/ million and broadening of the histograms correspond to the NMR of the sampled region. The field distribution of the human head was also experimentally mapped using the DANTE tagging sequence. The calculated and experimental field maps are in good agreement. Thus, geometric considerations with uniform susceptibilities are sufficient to explain most of the static magnetic field distribution in the human head.
A magnetization‐driven gradient echo pulse sequence for the study of myocardial perfusion
Authors: Judd Robert M. , Reeder Scott B. , Atalar Ergin , Mcveigh Elliot R. , Zerhouni Elias A.
Published: March 2007

A T1‐weighted imaging pulse sequence for contrast‐based studies of myocardial perfusion is presented and evaluated in phantoms and in vivo. The sequence is similar to spoiled gradient‐recalled echo equences except that nonselective preparatory RF pulses drive magnetization to steady state prior to image acquisition. Steady state is thus obtained in both tissue and blood resulting in a stable, homogeneous, and dark pre‐contrast baseline. Tip angles and timings are chosen so that pixel intensity approximates a linear relation to 1/T1. The dynamic range of signal response to contrast agent concentration is greater than that of an inversion‐recovery fast low angle shot sequence. The sequence proposed should be useful for myocardial perfusion studies.
Quantification of relative cerebral blood flow change by flow‐sensitive alternating inversion recovery (FAIR) technique: Application to functional mapping
Authors: Kim Seong‐Gi
Published: March 2007

Relative cerebral blood flow changes can be measured by a novel simple blood flow measurement technique with endogenous water protons as a tracer based on flow‐sensitive alternating inversion recovery (FAIR). Two inversion recovery (IR) images are acquired by interleaving slice‐selective inversion and nonselective inversion. During the inversion delay time after slice‐selective inversion, fully magnetized blood spins move into the imaging slice and exchange with tissue water. The signal enhancement (FAIR image) measured by the signal difference between two images is directly related to blood flow. For functional MR imaging studies, two IR images are alternatively and repeatedly acquired during control and task periods. Relative signal changes in the FAIR images during the task periods represent the relative regional cerebral blood flow changes. The FAIR technique has been successfully applied to functional brain mapping studies in humans during finger opposition movements. The technique is capable of generating microvascular‐based functional maps.
In Vivo localized proton NMR spectroscopy of thiamine‐deficient rat brain
Authors: Lee Haakil , Tarter James , Holburn George E. , Price Ronald R. , Weinstein David D. , Martin Peter R.
Published: March 2007

Thiamine deficiency (TD) in rats produces lesions similar to those found in humans suffering from Wernicke's encephalopathy, an organic mental disorder associated with alcoholism. Male Sprague‐Dawley rats (n = 29) were deprived of thiamine via a regimen of thiamine‐deficient chow and daily intraperitoneal injections of the thiamine antagonist pyrithiamine hydrobromide. Spectra were obtained by using the STEAM sequence. No significant change occurred in the ratio of Cr/NAA, while the ratio of Cho/NAA declined significantly (60 ± 11%) on Day 14. Eleven rats received intraperitoneal injections of thiamine hydrochloride at the end of 12 days, and dose‐dependent recovery in Cho/NAA was observed.
Evaluation of protective effects of prostaglandin E1 on ischemic liver damage with in vivo31P‐MR spectroscopy
Authors: Nabeshima Motoshige , Moriyasu Fuminori , Nishikawa Koji , Hamato Noriyuki , Fujimoto Masazumi , Nada Takayuki , Okuma Minoru , Morikawa Shigehiro , Inubushi Toshiro
Published: March 2007

The cytoprotective effect of prostaglandins (PG) was evaluated by in vivo 31P MR spectroscopy. Twenty rabbits were divided into two groups; the control group given physiological saline, and the PG group given prostaglandin E1 (0.5 μg/kg/min). Each solution was infused for 8 min, after which complete hepatic ischemia was induced for 20 min, followed by reperfusion for 40 min. During ischemia, β‐ATP decreased to 23.6% and 42.3%, phosphomonoester increased to 260% and 200% of their initial values in the control and the PG groups, respectively. Inorganic phosphate also increased. After reperfusion, β‐ATP recovered to 65.2% and 96.5%, phosphomonoester to 130% and 110%, inorganic phosphate to 140% and 120% in the control and PG groups, respectively. The changes during ischemia were significantly smaller and the recoveries during reperfusion were more complete in the PG group. These results may confirm the cytoprotective effect of prostaglandins by in vivo 31P‐MR spectroscopy.
Spiral imaging on a small‐bore system at 4.7t
Authors: Spielman Daniel M. , Pauly John M.
Published: March 2007

Spiral imaging has a number of advantages for ultrafast data acquisition. However, implementation on high‐field small‐bore systems requires carefully addressing the issues of in‐homogeneity‐induced blurring and gradient hardware constraints. In this paper, spiral imaging on a 40‐cm‐bore 4.7T CSI Omega System (Bruker Instruments) is discussed. A constant‐voltage gradient waveform design algorithm is developed to reduce readout times as well as minimize waveform distortions due to gradient amplifier nonlinearities. Residual errors are then measured and taken into account in the image reconstruction procedure. Multiple spiral interleaves as well as a multifrequency reconstruction algorithm are used to decrease blurring of off‐resonance spins. Both phantom and in vivo images demonstrate the performance of the resulting pulse sequences.
A computer simulation program for mr imaging: application to rf and static magnetic field imperfections
Authors: Olsson Magnus B. E. , Wirestam Ronnie , Persson Bertil R. R.
Published: March 2007

A computer simulation program capable of demonstrating various artifacts, such as image distortion caused by metallic implants in MR imaging, is presented. The structure of the program allows for the implementation of various imaging situations as long as spins only experience weak interaction, i.e., the Bloch equations are obeyed. The raw data are obtained by repeatedly applying the Bloch equations to the magnetization vector of each point of the simulated object, throughout the pulse sequence. With only a limited number of spins in each voxel, the effects of intravoxel dephasing and rephasing require special attention, and algorithms for this have been implemented.
A time encoding method for single‐shot imaging
Authors: Elizabeth Meyerand M. , Wong Eric C.
Published: March 2007

A new 2D single‐shot imaging technique is introduced that uses only one dimension of Fourier encoding. The second dimension is encoded in time, rather than using phase encoding. The data is acquired in the form of a closely spaced echo train with each echo produced from a different physical line in the object. A 1D Fourier transform is applied to each echo for image reconstruction. Because only the desired lines are excited, there can be no aliasing in the time encoding direction even when the object is much larger than the field of view. This technique is also very insensitive to motion, as motion‐related artifacts do not propagate in the time encoding direction.
Validation of 19f‐magnetic resonance determination of myocardial blood volume
Authors: Rottman Gerald A. , Judd Robert M. , Yin Frank C. P.
Published: March 2007

The use of perfluorochemical (PFC) emulsions to study the myocardial circulation by means of 19F NMR requires that the biodistribution of the PFC be known. The authors tested the hypothesis that PFC particles remain within the myocardial vascular space by infusing rats with both a PFC emulsion and 125I‐albumin. Measurement of myocardial vascular volume by 19F NMR and by standard radiotracer analysis of the same tissue yielded concordant values by the two methods, establishing the PFC emulsion as an intravascular agent in this tissue. Perfluorochemical emulsions should be useful for the noninvasive study of myocardial vascular physiology by magnetic resonance.