Showing papers in "Magnetic Resonance in Medicine in 1992"
TL;DR: Using gradient‐echo echo‐planar MRI, a local signal increase is observed in the human brain during task activation, suggesting a local decrease in blood deoxyhemoglobin concentration and an increase in blood oxygenation.
Abstract: Using gradient-echo echo-planar MRI, a local signal increase of 4.3 +/- 0.3% is observed in the human brain during task activation, suggesting a local decrease in blood deoxyhemoglobin concentration and an increase in blood oxygenation. Images highlighting areas of signal enhancement temporally correlated to the task are created.
1,877 citations
TL;DR: A flow‐independent method for imaging the coronary arteries within a breath‐hold on a standard whole‐body MR imager was developed and fat suppression by a spectral‐spatial pulse improves the visualization of the arteries.
Abstract: A flow-independent method for imaging the coronary arteries within a breath-hold on a standard whole-body MR imager was developed. The technique is based on interleaved spiral k-space scanning and forms a cardiac-gated image in 20 heartbeats. The spiral readouts have good flow properties and generate minimal flow artifacts. The oblique slices are positioned so that the arteries are in the plane and so that the chamber blood does not obscure the arteries. Fat suppression by a spectral-spatial pulse improves the visualization of the arteries.© 1992 Academic Press,Inc.
889 citations
TL;DR: Projection reconstruction techniques are shown to have intrinsic advantages over spin‐warp methods with respect to diminished artifacts from respiratory motion, and respiratory‐ordered view angle (ROVA) acquisition is found to diminish residual streaking significantly by reducing interview inconsistencies.
Abstract: Projection reconstruction (PR) techniques are shown to have intrinsic advantages over spin-warp (2DFT) methods with respect to diminished artifacts from respiratory motion. The benefits result from (1) portrayal of artifacts as radial streaks, with the amplitude smallest near the moving elements; (2) streak deployment perpendicular to the direction of motion of moving elements and often residing outside the anatomic boundaries of the subject; (3) inherent signal averaging of low spatial frequencies from oversampling of central k-space data. In addition, respiratory-ordered view angle (ROVA) acquisition is found to diminish residual streaking significantly by reducing interview inconsistencies. Comparisons of 2DFT and PR acquisitions are made with and without ROVA. Reconstructions from magnitude-only projections are found to have increased streaks from motion-induced phase shifts.
556 citations
TL;DR: This note shows that the knowledge of the capillary network organization may provide a link between these measurements and those obtained by conventional and MR perfusion techniques based on tracer uptake by tissues.
Abstract: MR measurements based on motion encoding gradients, such as intravoxel incoherent motion imaging, could provide, in principle, information on flowing blood volume and blood velocity. This note shows that, in addition, the knowledge of the capillary network organization may provide a link between these measurements and those obtained by conventional and MR perfusion techniques based on tracer uptake by tissues.
378 citations
TL;DR: A novel NMR imaging technique is presented that allows absolute determination of the magnetic susceptibility constant, χ, of a solution by comparing the phase difference of MR images produced with an instant (echo planar) “offset” spin‐echo sequence.
Abstract: We present a novel NMR imaging technique that allows absolute determination of the magnetic susceptibility constant, χ, of a solution. By comparing the phase difference of MR images produced with an instant (echo planar) “offset” spin-echo sequence, we obtain a direct measure of the magnetic field perturbations caused by the solution. We demonstrate this method with Gd(DTPA), Dy(DTPA), human red blood cells, and superparamagnetic iron oxide particles. © 1992 Academic Press, Inc.
376 citations
TL;DR: Filtering methods are described to minimize ghosting artifact that is typical in echo planar imaging and results from computer simulation and experiments will be presented.
Abstract: Echo planar imaging is characterized by scanning the 2D k-space after a single excitation. Different sampling patterns have been proposed. A technically feasible method uses a sinusoidal readout gradient resulting is measured data that does not sample k-space in an equidistant manner. In order to employ a conventional 2D-FFT image reconstruction, the data have to be converted to a cartesian grid. This can be done either by interpolation or alternatively by a generalized transformation. Filtering methods are described to minimize ghosting artifact that is typical in echo planar imaging. Results both from computer simulation and from experiments will be presented. Experimental images were obtained using a 2-T whole-body research system.
297 citations
TL;DR: It is shown that the PSF can produce significant blurring and loss of object contrast in fast spin‐echo images but that this blurring may be not be obvious in practice because the noise is unaffected by thePSF.
Abstract: The importance to MR image quality of the order of acquisition of different phaseencoded views with sequences that have variable TR and TE has been recently reported. It has been shown that the effective point spread function (PSF) may be manipulated by varying TE or TR, or both, with each phase-encoding step. This paper explores the behavior of the PSF in a variable TE sequence and its dependence on both imaging and tissue parameters. It is shown that the PSF is different for each tissue type and that its effect on tissue contrast is a function of both the shape and size of the structure. The important problem of signal loss from small objects that arises when the effective PSF is broad and the difficulty in detecting this phenomenon in practical MR images is illustrated. It is shown that the PSF can produce significant blurring and loss of object contrast in fast spin-echo images but that this blurring may be not be obvious in practice because the noise is unaffected by the PSF. It is also shown that the signal from small lesions with short T2 can easily be lost through this blurring mechanism. The importance of signal loss from small objects and its implication for the clinical use of such sequences as fast spinecho or rapid acquisition relaxation-enhanced and echo planar imaging is stressed.© 1992 Academic Press,Inc.
260 citations
TL;DR: This work introduces here a method that automatically removes blur introduced by magnetic field inhomogeneity and susceptibility without using a resonant frequency map, making these imaging methods more useful.
Abstract: For several non-2D Fourier transform imaging methods, off-resonant reconstruction does not just cause geometric distortion, but changes the shape of the point spread function and causes blurring. This effect is well known for projection reconstruction and spiral k-space scanning sequences. We introduce here a method that automatically removes blur introduced by magnetic field inhomogeneity and susceptibility without using a resonant frequency map, making these imaging methods more useful. In this method, the raw data are modulated to several different frequencies and reconstructed to create a series of base images. Determination of degree of blur is done by calculating a focusing measure for each point in each base image and a composite image is then constructed using only the unblurred regions from each base image. This method has been successfully applied to phantom and in vivo images using projection-reconstruction and spiral-scan sequences.
240 citations
TL;DR: High‐resolution 1H NMR spectroscopy has been used to measure the concentrations of metabolites in perchloric acid extracts of human epileptic cortex and brain tumors, finding differences in metabolite concentrations that may be of use in differential tumor diagnosis.
Abstract: High-resolution 1H NMR spectroscopy has been used to measure the concentrations of metabolites (alanine, N-acetylaspartate, γ-aminobutyric acid, glutamate, glutamine, aspartate, taurine, glycine, succinate, creatine, cholines, inositol, and glucose) in perchloric acid extracts of human epileptic cortex and brain tumors. All tissue was obtained by surgical biopsy, excised before thermal coagulation, and immediately frozen in liquid nitrogen. Lower levels of N-acetylaspartate and γ-aminobutyric acid and a shift in the glutamate/glutamine ratio toward glutamine in the tumors reflect neuronal loss. Abnormal glucose metabolism (aerobic glycolysis) in the tumors gives decreased levels of succinate, glutamate, aspartate, glutamine, and creatine and generally increased concentrations of glycine and alanine. Differences in metabolite concentrations that may be of use in differential tumor diagnosis include lower creatine and inositol in meningiomas than in astrocytomas. Lower taurine differentiates benign from malignant astrocytomas. Malignant astrocytomas and metastatic tumors are more regionally heterogeneous than meningiomas or benign astrocytomas. Mannitol, administered perioperatively to all patients from whom tissue was obtained, was observed only in the spectra of extracts of tissue from tumors which enhanced on computerized tomographic imaging. © 1992 Academic Press, Inc.
222 citations
TL;DR: A measure of the relative longitudinal and transverse displacements of the upper abdominal organs during breathing using an MR line scan technique is obtained.
Abstract: Despite the fact that respiratory motion is a major factor limiting the image quality of MR examinations in the upper abdomen, little quantitative information is available about the kinematics of visceral motion during respiration. The objective of this study was to obtain a measure of the relative longitudinal and transverse displacements of the upper abdominal organs during breathing using an MR line scan technique.
221 citations
TL;DR: A method is presented for measuring myocardial deformation in a breath‐hold with tagged cine MRI with the scan time reduced 16‐ to 64‐fold from previous techniques.
Abstract: A method is presented for measuring myocardial deformation in a breath-hold with tagged cine MRI. Tagged cine images of human hearts are obtained in arbitrary oblique planes on a standard imager with as few as four heartbeats. The scan time has been reduced 16- to 64-fold from previous techniques.
TL;DR: This study suggests that repeat, rapid measurements of myocardial T1 relaxation rates following the bolus injection of Gd‐DTPA are required to calculate MBF (i.e.,Myocardial tissue perfusion) and MECV.
Abstract: In order to clarify the relationship between coronary artery disease (including myocardial infarction) and image contrast in gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA-enhanced MRI it was decided to model the myocardial tissue distribution and clearance of Gd-DTPA using the modified Kety equation. Using a canine model, myocardial tissue Gd-DTPA concentrations ([Gd-DTPA]m) were measured 1 or 5 min after a bolus injection of Gd-DTPA or immediately after the end of a constant infusion of Gd-DTPA in a total of 35 dogs. It was found that within 5 min of a bolus injection [Gd-DTPA]m is determined primarily by myocardial blood flow (MBF) and after about 10 min primarily by myocardial extracellular volumes (MECV). This study suggests that repeat, rapid (every 2–4 s) measurements of myocardial T1 relaxation rates following the bolus injection of Gd-DTPA are required to calculate MBF (i.e., myocardial tissue perfusion) and MECV. © 1992 Academic Press, Inc.
TL;DR: The ultra‐fast application of the RARE experiment is described in detail, with special emphasis on its multifarious applications with preparation experiments that produce transverse magnetization.
Abstract: The ultra-fast application of the RARE experiment is described in detail, with special emphasis on its multifarious applications with preparation experiments that produce transverse magnetization. The factors affecting the temporal evolution of the magnetization during the experiment are described, and the implications for the slice profile when using a Gaussian refocusing pulse are experimentally examined. The choice of phase-encoding scheme for use with preparation experiments is discussed, as is the use of various phase-encoding schemes to reduce line broadening in the phase-encoding direction if a number of averages are acquired. An explanation for the decomposition of the echo are into two components if the read gradient is imbalanced is given, and the experimental conditions necessary for the coherent addition of these two echo groups are described. An alternative sequence that removes one of these groups from the acquisition window is proposed. The sensitivity of the sequence to flow and motion is investigated, and the drastic loss of signal in this situation explained. The in vivo and in vitro application of preparation experiments leading to the accurate measurement of T1, T2, diffusion constant, and magnetization transfer characteristics is presented. The implementation of zoom-imaging using spin- and stimulated-echo preparation is described, and 3D in vivo spin-echo zoom images are presented. Simple phantom experiments demonstrating the feasibility of chemical-shift selective and spectroscopic imaging are also given.
TL;DR: A new TOF sequence in which magnetization transfer contrast is used to supplement wash is presented, in which angiograms demonstrating the superior performance of this technique are presented.
Abstract: Time-of-flight (TOF) angiography based on inflow enhancement is limited by the steady-state signal differences between blood and the surrounding stationary tissues. We present a new TOF sequence in which magnetization transfer contrast is used to supplement wash. in effects. Angiograms demonstrating the superior performance of this technique are presented. © 1992 Academic Press, Inc.
TL;DR: This verifies the feasibility of generating three‐dimensional “neurographic” images, analogous to angiograms, but which demonstrate the peripheral nerve tracts in apparent isolation.
Abstract: We have made cross-sectional image “neurograms” in which peripheral nerve has a greater signal intensity than that of other tissue. Neurographic images of the rabbit forelimb were obtained using a spin-echo magnetic resonance imaging (MRI) technique that combines fat suppression and diffusion weighting. After fat suppression the nerve shows up in relative isolation and is brighter than the surrounding tissue due to its longer T2 relaxation time of ≈ 50 ms compared to ≈ 27 ms for muscle. The addition of pulsed gradients for diffusion weighting of the MR signal further enhances the intensity of the nerve signal relative to that of surrounding muscle tissue. The greater diffusional anisotropy of nerve tissue (D∥/D∥ = 3.1) compared to that of muscle (D∥/D∥ = 1.9) allows further enhancement of the nerve by a subtraction of two diffusion-weighted images, one with the gradients oriented parallel and one with the gradients oriented perpendicular to the nerve orientation. We show that by manipulation of the MRI parameters, either echo time or pulsed gradient strength, the nerves can be made to show up as the most intense feature. This verifies the feasibility of generating three-dimensional “neurographic” images, analogous to angiograms, but which demonstrate the peripheral nerve tracts in apparent isolation.© 1992 Academic Press,Inc.
TL;DR: A new technique with which susceptibility artifact in gradient‐echo imaging can be reduced substantially by use of a tailored RF pulse is described and experimental results obtained using a human volunteer with a 2.0‐T KAIS NMR system are presented.
Abstract: A new technique with which susceptibility artifact in gradient-echo imaging can be reduced substantially by use of a tailored RF pulse is described. The proposed technique can ideally be applied to the case where high local magnetic field inhomogeneity is dominated by the susceptibility. The signal loss and void phenomena due to susceptibility in a voxel are studied and a correction method is also proposed. The description of the tailored RF pulse and its proposed application are given and experimental results obtained using a human volunteer with a 2.0-T KAIS NMR system are presented.
TL;DR: Human lymphocytes were labeled with biotinylated anti‐lymphocyte‐directed monoclonal antibodies, to which streptavidin and subsequently dextran‐magnetite particles were coupled, resulting in a strong and selective negative contrast enhancement of lymphocyte suspensions at 2.0 T.
Abstract: Human lymphocytes were labeled with biotinylated anti-lymphocyte-directed monoclonal antibodies, to which streptavidin and subsequently biotinylated dextran-magnetite particles were coupled. This labeling resulted in a strong and selective negative contrast enhancement of lymphocyte suspensions at 2.0 T, caused predominantly by the specific increase of R2 with a small but significant specific increase of R2. The R1 was found to decrease with increasing field strength. The immunolabeling procedure described here may be used for the selective signal depletion of target cells in MR imaging.
TL;DR: This investigation explores the feasibility of a catheter‐based receiver probe for NMR study of arterial walls and demonstrates the spatial response of several “inside‐out” probe coil designs, including loop, “birdcage,” “multipole,’ “center return,“ and opposed solenoids.
Abstract: This investigation explores the feasibility of a catheter-based receiver probe for NMR study of arterial walls. Simulations and phantom experiments demonstrate the spatial response of several "inside-out" probe coil designs, including loop, "birdcage," "multipole," "center return," and opposed solenoids. For a target defined by an annulus in a plane perpendicular to B0, the opposed solenoid design provides substantially superior homogeneity to other designs considered. Canine iliofemoral artery images were acquired using a catheter probe in a whole-body, 1.5-T clinical imaging system. In situ (cadaver) images acquired with TE 70, TR 2400, 2-mm slice thickness, and 78 x 78-microns in-plane voxel size in 10-min acquisition times show vessel wall structures identified as intima, internal elastic lamina, media, and adventitia. In vivo images from similar acquistion conditions are much more poorly resolved, presumably due to motion, despite the use of cardiac gating and gradient moment nulling, so the feasibility of obtaining high-resolution in vivo MR images of the arterial wall remains in doubt.
TL;DR: A quantitative SP‐MRI methodology suited to conventional MRI is presented, and an “echo‐planar” methodology, able to produce qualitative functional images of myocardial kinematics at almost real‐time speeds is presented.
Abstract: A magnetic resonance imaging (MRI) method is presented to detect, localize, and quantify myocardial kinematics by measuring the material rate-of-strain tensor at each pixel in gated NMR images of the heart. The immediate, local effect of muscular activity is self-deformation, and the strain tensor is the basic mathematical device by which such deformation may be quantified. The present method, called "strain-phase" MRI (SP-MRI), entails four steps: (1) the velocity of the myocardium is encoded by means of a set of motion-sensitive NMR image acquisitions, one image per velocity component; (2) the spatial derivatives of the velocity are computed at each pixel; (3) the velocity-derivative data are combined to compute an approximation of the strain-rate tensor of the myocardium at each pixel; and (4) the strain-rate tensor data are simplified to produce a color-coded functional image which represents strain-rate components which are of particular biomedical interest in the myocardium. We present a quantitative SP-MRI methodology suited to conventional MRI, and in addition present an "echo-planar" methodology, able to produce qualitative functional images of myocardial kinematics at almost real-time speeds. Two-dimensional strain-phase MRI data acquired in normal human subjects are presented. These data demonstrate the practicability of SP-MRI in vivo, that SP-MRI resolves myocardial kinematics at the single-pixel scale, having resolution comparable to that of conventional MRI, and that SP-MRI data may have a signal-to-noise ratio up to 50% as great as that of the conventional MRI data from which they are produced. SP-MRI measurements of the local instantaneous strain rates in the human left ventricular myocardium are quantitatively consistent with known transmural average values of myocardial strain.
TL;DR: Computer simulation is used to assess the precision and accuracy of diffusion and perfusion parameters derived from a set of gradient‐sensitized images and shows that systematic errors in perfusion fraction estimation, as well as understimation of the uncertainties in perfusions parameters will be found at moderate signal‐to‐noise levels.
Abstract: Computer simulation is used to assess the precision and accuracy of diffusion and perfusion parameters derived from a set of gradient-sensitized images. Under ideal experimental conditions, a moderate signal-to-noise level (ca. 40) suffices to estimate diffusion coefficients to within 20% relative precision. However, estimation of a typical cerebral perfusion fraction of 5% to within 20% relative precision requires signal-to-noise levels of ca. 400. Simulations also show that systematic errors in perfusion fraction estimation, as well as understimation of the uncertainties in perfusion parameters (by X-squared analysis), will be found at moderate signal-to-noise levels. © 1992 Academic Press, Inc.
TL;DR: It is shown that short‐T2 spin pools can be selectively saturated with short intense RF pulses and cross‐relaxation time constants can be measured using the technique of saturation recovery.
Abstract: In vivo 1H conventional NMR image contrast generation usually relies on the macroscopic T1 and T2 relaxation parameters of the tissues of interest. Recently cross-relaxation related image contrast has been reported by Wolff and Balaban in animal models. Due primarily to the broad lineshape of the intended saturation spin pool and the use of off-resonance irradiation, high specific absorption rate and an auxiliary RF amplifier have been necessary to produce these images. The relatively long spin-lattice relaxation property of this spin pool, however, suggests the use of pulse methods to achieve saturation. In this paper, we show that short-T2 spin pools can be selectively saturated with short intense RF pulses. Cross-relaxation time constants can be measured using the technique of saturation recovery. In vivo magnetization-transfer-weighted images can be produced using pulses on commercial whole-body imagers without additional hardware.
TL;DR: A modified ISIS method, for image‐selected localized proton magnetic resonance spectroscopy (1H MRS), was used to determine the ratios and T2 relaxation times of proton metabolites in normal subjects and in patients with chronic infarction and MRI white matter signal hyperintensities (WMSH).
Abstract: A modified ISIS method, for image-selected localized proton magnetic resonance spectroscopy (1H MRS), was used to determine the ratios and T2 relaxation times of proton metabolites in normal subjects and in patients with chronic infarction and MRI white matter signal hyperintensities (WMSH). First, in patients with cerebral infarctions, increased concentrations of lactate were found in the majority of patients, and N-acetyl aspartate (NAA) was reduced to a significantly greater extent than choline (Cho) or creatine (Cre). For TE = 270 ms, the raw ratios of Cho/NAA, Cre/NAA, and Lac/NAA were significantly (P less than 0.05) increased from 0.23 +/- 0.02 (mean +/- SE), 0.20 +/- 0.01, and 0.05 +/- 0.01, respectively in the normal group to 0.39 +/- 0.08, 0.37 +/- 0.05, and 0.48 +/- 0.15 in the stroke group. Also, the T2 relaxation time of creatine was significantly (P = 0.007) increased from 136 ms in normal white matter to 171 ms in cerebral infarcts. Second, in patients with WMSH, no significant change of the proton metabolite concentrations could be detected with the exception of the choline which was significantly (P = 0.003) altered. The Cho/NAA ratio, after T2 and excitation profile correction, increased from 0.47 +/- 0.02 in the normal group to 0.64 +/- 0.05 in the WMSH group. Third, in normal white matter, the concentration of N-acetyl aspartate, choline, and lactate was estimated to 11.5, 2.0, and 0.6 mM, respectively, by assuming a total creatine concentration of 10 mM.
TL;DR: To model the uptake it is necessary to know the arterial concentration of Gd‐DTPA; in this study the signal in the sagittal sinus was measured to avoid the need to take repeated blood samples.
Abstract: Echo-planar imaging has been used to observe the dynamics of Gd-DTPA uptake in brain tumors. It has been possible to examine both vascular uptake and diffusion across the blood-brain barrier in a single experiment, by using the IR-MBEST echo-planar sequence which combines a high temporal resolution (approximately 3 s) with strong T1 weighting. To model the uptake it is necessary to know the arterial concentration of Gd-DTPA; in this study the signal in the sagittal sinus was measured to avoid the need to take repeated blood samples. The time constant for transfer across the blood-brain barrier was measured to be between 20 and 1050 s for different tumors. The results of the modeling correlated with the results of other assessments of tumor vascularity.
TL;DR: Of clinical significance to RARE practitioners is the increase of contrast‐to‐noise ratios between gray and white matter on proton density‐weighted images with increasing slice number.
Abstract: Magnetization transfer effects are demonstrated to be significant in determining the signal intensity from brain tissues on images acquired with multislice rapid acquisition relaxation enhanced (RARE) sequences. We report studies designed to determine how the signal intensities vary with slice number or, equivalently, off-resonance power deposition. The results obtained in fat, gray matter, and white matter are similar in form to those reported in kidney tissues during classic magnetization transfer experiments (J. Eng, T. L. Ceckler, and R. S. Balaban, Magn. Reson. Med. 17, 304 (1991)). Of clinical significance to RARE practitioners is the increase of contrast-to-noise ratios between gray and white matter on proton density-weighted images with increasing slice number.
TL;DR: The hypothesis of a rapid efflux of lactate from brain tissue under activated conditions underactivated conditions is supported.
Abstract: Localized proton NMR spectroscopy was used to study cerebral metabolism in the visual cortex of healthy adults during rest and photic stimulation. Basal lactate levels showed considerable interindividual differences ranging from below detectability (less than 0.3 mM) to about 1 mM without consistent alteration during photic stimulation. Local brain glucose levels were significantly reduced (approximately 50%) during the entire period of photic stimulation and recovered to resting levels (approximately 0.8 mM) within 10 min after the end of stimulation. This decrease reflects the establishment of a new equilibrium due to enhanced delivery (blood flow) and enhanced consumption. The absence of lactate accumulation supports the hypothesis of a rapid efflux of lactate from brain tissue under activated conditions.
TL;DR: A fast imaging method that is based on gradient‐recalled echoes of spins whose excitation and echo formation are separated by more than one TR period is presented, resulting in drastically increased sensitivity to dynamic susceptibility erects, while maintaining a short total imaging time.
Abstract: A fast imaging method that is based on gradient-recalled echoes of spins whose excitation and echo formation are separated by more than one TR period is presented. This method does not incorporate chemical-shift refocusing and thus results in drastically increased sensitivity to dynamic susceptibility effects, while maintaining a short total imaging time. The efficiency of the new technique is demonstrated in dynamic contrast-enhanced experiments (bolus tracking) in the cat brain using a duration of 600 ms for each image. Blood volume maps are derived with expected contrast between white and gray matter.
TL;DR: The theoretical model for perfusion measurement by NMR using arterial labeling of endogenous water is extended to include the effects of transit time and cross‐relaxation of tissue water with macromolecules, and it is demonstrated that the transit time in rat brain is quite short, and thus its effect on perfusions measurement is small.
Abstract: The theoretical model for perfusion measurement by NMR using arterial labeling of endogenous water is extended to include the effects of transit time and cross-relaxation of tissue water with macromolecules. Water magnetization in rat brain is monitored using the STEAM method to simultaneously determine the transit time, magnetization transfer rate constant, and perfusion. The results show that the transit time in rat brain is quite short, and thus its effect on perfusion measurement is small. It is also demonstrated both theoretically and experimentally that the steady-state effects of cross-relaxation with macromolecules on perfusion measurement are accounted for by a proper control experiment.
TL;DR: T1 was relatively constant, but 1/T2 increased linearly with field strength, in agreement with earlier MRI observations in the monkey brain, and supports the theory that ferritin is responsible for T2 shortening in brain nuclei containing iron.
Abstract: Nuclear magnetic relaxation times T1 and T2 were measured in ferritin solutions at field strengths from 0.04 to 1.5 T. T1 was relatively constant, but 1/T2 increased linearly with field strength, in agreement with earlier MRI observations in the monkey brain. This finding supports the theory that ferritin is responsible for T2 shortening in brain nuclei containing iron. The linear dependence of 1/T2 on magnetic field is unique and not explained by present theories of the magnetic properties of ferritin.
TL;DR: Techniques are described for the design of shielded gradient coils for superconducting MRI systems suited for constructing the most efficient gradient coil that meets a specified homogeneity requirement.
Abstract: Techniques are described for the design of shielded gradient coils for superconducting MRI systems. These design methods are suited for constructing the most efficient gradient coil that meets a specified homogeneity requirement. Tradeoffs in coil design of efficiency with coil size and gap size are discussed. Residual eddy currents from coils constructed with a finite number of wires are calculated and give guidelines for the construction of efficient, whole-body gradient coils.
TL;DR: Wavelet encoding has three advantages over phase encoding: there is no Gibblquot's ringing from partial volume effects, the effective repetition time can be 36 times the repetition time for a 256 × 256 image, and motion artifacts are local and dramatically reduced.
Abstract: Wavelet encoding is presented and compared to phase encoding. In wavelet encoding a distribution of spins is excited by a slice selective RF pulse; for each repetition time the distribution excited has the profile of a wavelet at different scale and translation. The spin density can be reconstructed with an inverse wavelet transform. Wavelet encoding has three advantages over phase encoding: (1) there is no Gibblquot's ringing from partial volume effects, (2) the effective repetition time can be 36 times the repetition time for a 256 × 256 image, and (3) motion artifacts are local and dramatically reduced. Using wavelet encoding, a 256 × 256 T2-weighted projection image can be acquired in 33s. © 1992 Academic Press, Inc.