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At least 811 records · Page 45Linked to original sources
PubMed1994

Accurate T1 determination from inversion recovery images: application to human brain at 4 Tesla.

It is well known that the signal polarity in inversion-recovery (IR) images changes with inversion time, complicating the determination of T1. To avoid this problem, a simple subtraction method is implemented. In this method, k-space data of the longest inversion time are subtracted from the corresponding data of each inversion time. This subtraction yields IR images of same polarity, making it straightforward to derive T1 using a standard fitting routine. Phantom T1 studies with IR Turbo-FLASH images demonstrate that this technique is robust and accurate. Four Tesla T1 values of the human brain were also determined by this method to demonstrate its in vivo utility.

Brain↗
PubMed1997

Background suppression with multiple inversion recovery nulling: applications to projective angiography.

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.

Carotid Arteries↗
PubMed2005

An adjustable adiabatic pulse for selective population inversion.

A new adiabatic inversion pulse and its design principles are presented. An analytical expression in the pulse length, inversion bandwidth, inversion efficiency, peak RF amplitude, and width of the transition region is derived and validated. Accordingly, the pulse shape can be adapted to achieve a specified inversion performance. Adjusted for broadband application, population can be inverted band selectively, rather independently of spatial RF field inhomogeneities, and with significantly reduced peak RF amplitude in comparison with the well-known hyperbolic secant adiabatic pulse.

Head↗
PubMed2005

The effect of B1 field inhomogeneity and the nonselective inversion profile on the kinetics of FAIR-based perfusion MRI.

Perfusion imaging with pulsed arterial spin labeling techniques, like flow-sensitive alternating inversion recovery (FAIR), may suffer from inflow of fresh, i.e., unlabeled, spins. Inflow of fresh spins is caused by the arrival of unlabeled spins in the image slice and can lead to underestimation of the perfusion if not taken into account. In this study it was shown that a decrease in B(1) field strength toward the edge of the transmit coil and the consequent reduction in the inversion efficiency leads to a narrowing of the arterial delivery function and a reduction in FAIR signal. Increasing the B(1) amplitude of the adiabatic inversion pulse from 2.3 to 5.7 times its minimum amplitude requirement resulted in an observed increase of 40 to 80% in the rat brain FAIR signal at inflow times longer than 0.65 s. For coils with limited dimensions and significant B(1) inhomogeneity over the perfusion labeling slab, the application of an excessively large B(1) amplitude in combination with adiabatic inversion is recommended to optimize the FAIR perfusion contrast.

Animals↗
PubMed2005

Full-brain T1 mapping through inversion recovery fast spin echo imaging with time-efficient slice ordering.

Brain T1 mapping has important clinical applications in detecting brain disorders. Conventional T1 mapping techniques are usually based on inversion recovery spin echo (IRSE) imaging or its more time-efficient counterpart inversion recovery fast spin echo (IRFSE) imaging because they can deliver good image quality. Multiple inversion times are required to accurately estimate T1 over a wide range of values. Without acquisition optimization, both the IRSE and the IRFSE T1 mapping techniques require long scan times to image the whole brain. To reduce the scan time and maintain the quality of the T1 maps, we propose a new full-brain T1 mapping pulse sequence based on a multislice inversion recovery fast spin echo imaging using a time-efficient slice ordering technique.

Adult↗
PubMed2006

In vivo estimation of the flow-driven adiabatic inversion efficiency for continuous arterial spin labeling: a method using phase contrast magnetic resonance angiography.

The accurate quantification of perfusion with arterial spin labeling (ASL) requires consideration of a number of factors, including the efficiency of the inversion and control pulses used for spin labeling. In this study the effects of spin velocity on continuous ASL efficiency when using the amplitude modulated control strategy were investigated using simulations of the Bloch equations. The inversion efficiency was determined in vivo by combining the simulations with phase-contrast velocity mapping data acquired at the level of the tagging plane. Using this novel method, an average inversion efficiency of 69% was calculated for a group of 28 subjects, in good agreement with experimental data reported previously. There was, however, a large range in inversion efficiency measured across the subject group (50-76%), indicating that the velocity dependence of the amplitude modulated control efficiency may introduce additional variability into the perfusion calculations if not properly taken into account.

Algorithms↗
PubMed2006

Simultaneous outer volume and blood suppression by quadruple inversion-recovery.

A new method has been developed for reduced field-of-view (FOV) imaging with simultaneous blood suppression. This method combines suppression of signals from the outer volume and inflowing blood by using a small-FOV quadruple inversion-recovery (SFQIR) preparative pulse sequence consisting of two double-inversion pulse pairs separated by appropriate delays. Within each pair, inversion pulses are successively applied to the imaged slice and the slab orthogonal to the imaging plane with the thickness equal to the FOV size in the phase-encoding direction. Each double inversion results in the reinversion of the magnetization in the central part of the FOV, while the outer areas of the FOV and inflowing blood remain inverted. The SFQIR module was implemented for single- and multislice acquisition with a fast spin-echo readout sequence. Based on a theoretical model of the signal, the timing parameters of the sequence corresponding to the maximal suppression efficiency can be found by minimizing the variation of the normalized signal over the entire range of T1's that occur in tissues. The method was tested for black-blood imaging of the aorta and carotid arteries, and the results demonstrated its ability to eliminate motion and flow artifacts, reduce scan time, and improve spatial resolution.

Algorithms↗
PubMed2006

Decomposed direct matrix inversion for fast non-cartesian SENSE reconstructions.

A new k-space direct matrix inversion (DMI) method is proposed here to accelerate non-Cartesian SENSE reconstructions. In this method a global k-space matrix equation is established on basic MRI principles, and the inverse of the global encoding matrix is found from a set of local matrix equations by taking advantage of the small extension of k-space coil maps. The DMI algorithm's efficiency is achieved by reloading the precalculated global inverse when the coil maps and trajectories remain unchanged, such as in dynamic studies. Phantom and human subject experiments were performed on a 1.5T scanner with a standard four-channel phased-array cardiac coil. Interleaved spiral trajectories were used to collect fully sampled and undersampled 3D raw data. The equivalence of the global k-space matrix equation to its image-space version, was verified via conjugate gradient (CG) iterative algorithms on a 2x undersampled phantom and numerical-model data sets. When applied to the 2x undersampled phantom and human-subject raw data, the decomposed DMI method produced images with small errors (< or = 3.9%) relative to the reference images obtained from the fully-sampled data, at a rate of 2 s per slice (excluding 4 min for precalculating the global inverse at an image size of 256 x 256). The DMI method may be useful for noise evaluations in parallel coil designs, dynamic MRI, and 3D sodium MRI with fixed coils and trajectories.

Algorithms↗
PubMed2005

Optical properties of nanoparticle-based metallodielectric inverse opals.

Metallodielectric inverse opals were prepared by co-crystallizing silica-coated gold nanoparticles and polymer spheres, followed by removal of the crystal template. The inverse opals exhibit a distinct reflectance peak, which results from Bragg diffraction due to the highly ordered 3D macroporous structure. Photonic band-structure calculations indicate that the characteristic reflectance peaks observed are signatures of the directional gap at the L point. It is demonstrated that the optical properties (the position and magnitude of the electromagnetic bandgaps) of the gold-silica nanocomposite inverse opals can be engineered by varying the nanoparticle morphology (core size and shell thickness) and/or the nanoparticle volume-filling ratio of the composite. The use of metallodielectric nanoparticles to form inverse opals offers a versatile approach to prepare photonic materials that may exhibit absolute bandgaps.

Crystallization↗
PubMed2005

A novel method to improve prenatal diagnosis of abnormal systemic venous connections using three- and four-dimensional ultrasonography and 'inversion mode'.

OBJECTIVE: The precise prenatal diagnosis of abnormal venous connections of the fetal heart is challenging. Anatomical accuracy may be important in determining the best route for postnatal angiography, as well as the prognosis and treatment. This study was designed to determine the value of 'inversion mode', a new three- and four-dimensional (4D) rendering algorithm, in the visualization of the spatial relationships of an interrupted inferior vena cava (IVC) with azygos or hemiazygos vein continuation associated with and without heterotaxic syndromes. METHODS: Heart volumes were acquired using 4D ultrasonography and spatiotemporal image correlation in cases of interrupted IVC with azygos/hemiazygos continuation (n = 3). Volume datasets were rendered using the 'inversion mode' algorithm and abnormal images were compared to those generated from a library of normal fetuses. RESULTS: The 'inversion mode' rendering algorithm allowed the visualization of dilated azygos or hemiazygos veins and their spatial relationships with the descending aorta, the aortic arch, the superior vena cava, and the atria in cases of interrupted IVC with and without heterotaxic syndromes. CONCLUSIONS: The 'inversion mode' algorithm improves prenatal visualization of both dilated azygos and hemiazygos veins, as well as their spatial relationships with the surrounding vascular structures. This has implications for the accurate prenatal diagnosis and management of neonates with abnormal systemic venous connections.

Azygos Vein↗
PubMed2000

Accuracy of two dipolar inverse algorithms applying reciprocity for forward calculation.

Two inverse algorithms were applied for solving the EEG inverse problem assuming a single dipole as a source model. For increasing the efficiency of the forward computations the lead field approach based on the reciprocity theorem was applied. This method provides a procedure to calculate the computationally heavy forward problem by a single solution for each EEG lead. A realistically shaped volume conductor model with five major tissue compartments was employed to obtain the lead fields of the standard 10-20 EEG electrode system and the scalp potentials generated by simulated dipole sources. A least-squares method and a probability-based method were compared in their performance to reproduce the dipole source based on the reciprocal forward solution. The dipole localization errors were 0 to 9 mm and 2 to 22 mm without and with added noise in the simulated data, respectively. The two different inverse algorithms operated mainly very similarly. The lead field method appeared applicable for the solution of the inverse problem and especially useful when a number of sources, e.g., multiple EEG time instances, must be solved.

Algorithms↗
PubMed2000

RF pulse concatenation for spatially selective inversion

It is shown that spatially selective inversion and saturation can be achieved by concatenation of RF pulses with lower flip angles. A concatenation rule which enables global doubling of the flip angle of any given excitation pulse applied to initial z magnetization is proposed. In this fashion, the selectivity of the single pulse is preserved, making the high selectivity achievable in the low flip-angle regime available for inversion and large flip-angle saturation purposes. The profile quality achievable with exemplary concatenated pulses is investigated in comparison with adiabatic inversion. It is verified that by using concatenated inversion in the transfer insensitive labeling technique (TILT), the MT artifact is suppressed. Copyright 2000 Academic Press.

Journal Article↗
PubMed1996

Microwave and Infrared Spectra, ab Initio Calculation, and Two-Dimensional Model of Amino Group Inversion and Ring Puckering in 2,5-Dihydropyrrole

The microwave spectra of 2,5-dihydropyrrole and 2,5-dihydropyrrole-1-d1 have been measured with Stark and Fourier transform spectrometers in the range 10-39 GHz. Rotational constants, centrifugal distortion constants, and 14N quadrupole coupling constants have been determined from the observed transition frequencies for the ground vibrational state. In addition, two satellites of the normal species and one satellite of the deuterated species have been identified and measured. Splittings of the rotational transitions due to amino group inversion tunneling have been observed and analyzed. Infrared transitions of the amino group inversion mode have been measured in the range 490-720 cm-1. The effect of ring puckering on the inversion motion of the amino group in 2,5-dihydropyrrole and 2,5-dihydropyrrole-N-d1 has been investigated by ab initio calculations and two-dimensional flexible model calculations from the results of microwave and infrared spectroscopy. The observed molecular properties have been reproduced by a model which involved adjustable parameters for the potential energy surface and the structural relaxation of the CCC valence angles. Additional parameters have been transferred from the ab initio calculations. The adjustment of the model to the experimental data has yielded an equatorial equilibrium conformation with slightly larger CCC valence angle than in the most stable axial conformation. Excitation of the first ring puckering state has been found to enhance the inversion tunnel splittings.

Journal Article↗
PubMed2005

Stochastic inverse consistency in medical image registration.

An essential goal in medical image registration is, the forward and reverse mapping matrices should be inverse to each other, i.e., inverse consistency. Conventional approaches enforce consistency in deterministic fashions, through incorporation of sub-objective cost function to impose source-destination symmetric property during the registration process. Assuming that the initial forward and reverse matching matrices have been computed and used as the inputs to our system, this paper presents a stochastic framework which yields perfect inverse consistency with the simultaneous considerations of the errors underneath the registration matrices and the imperfectness of the consistent constraint. An iterative generalized total least square (GTLS) strategy has been developed such that the inverse consistency is optimally imposed.

Algorithms↗
PubMed1977

Risk for recombinants in pericentric inversions of the (p11 leads to q21) region of chromosome 18.

A child with female hypospadia complicated by bilateral hydronephrosis, hydroureter, and hydrocolpos was heterozygous for a pericentric inversion of chromosome 18, 46,XX,inv(18)(p11q21). The normal mother and her father had the same inversion. The abnormal phenotype of the girl could be due to undetectable recombination or to a position effect. She had a low level of the enzyme peptidase-A whose locus is on 18q, while her mother and grandfather had normal levels. The two other cases of familial inversions for chromosomes 18 in the literature both involve the same (p11 leads to q21) region. These three families give a tentative figure of at least 10% as the risk for a normal carrier of this pericentric inversion to have an affected offspring due to recombination.

Chromosome Aberrations↗
PubMed1985

Characterization and properties of very large inversions of the E. coli chromosome along the origin-to-terminus axis.

Suppression of a dnaA46 mutation by integration of plasmid R100.1 derivatives in the termination region of chromosome replication in E. coli results in medium dependence, the suppressed bacteria being sensitive to rich medium at 42 degrees C. Derivatives of such bacteria have been selected for growth at 42 degrees C in rich medium and we have analyzed representatives of the most frequently observed type: bacteria displaying, once cured of the suppressor plasmid, both rich-medium sensitivity and temperature sensitivity. We found, in all cases, that the chromosome had undergone a major inversion event between two inverted IS5's. One is located at 29.2 min on the chromosome map and the other at either one of two positions between 69 and 80 min. The consequences of such inversions for cell growth are discussed. Some of them result from the fact that the replication terminator T2 is located, in inverted chromosomes, close to oriC in the orientation which allows its functioning as a terminus (de Massy et al. in press). Our observations allow an estimation of the frequency of inversions arising from recombination between pairs of inverted chromosomal IS, which could be as high as 10(-2) per cell per generation. We also found that inversion reversal occurs frequently after Hfr conjugational transfer of one of the IS5's, in its wild-type location. This led us to propose a new mechanism of recombination, in which the incoming DNA strands serve as guides to favor recombination between the resident sequences.

Chromosome Mapping↗
PubMed1982

Recombination between two TnA transposon sequences oriented as inverse repeats is found less frequently than between direct repeats.

Inverse repeats of the transposon Tn2660 in either a ColE1 or an R6K replicon, with or without inversions of the parental DNA sequences between the repeats, show no detectable (less than 2%) evidence of recombination between the repeats after 60 generations of growth in either recA or RecA+ hosts. In contrast, attempts made to construct plasmids which carry two direct repeats by in vitro cleavage and ligation in the recA host were unsuccessful, although homologous plasmids with inverse repeats could be constructed, and other plasmids were found consistent with products of recombination between the direct repeats of the transient intermediate structure. It is concluded that in recA or recA+ hosts recombination between direct repeats of a transposon is frequent, whereas recombination between inverse repeats of a homologous structure has not been observed. A model to explain this difference depends upon a mechanisms that produces a nick in only one of the pair of strands at the internal resolution site (IRS) sequence of the transposon.

Cloning, Molecular↗
PubMed1993

Magnetic source imaging based on the minimum-norm least-squares inverse.

The flow of ionic currents within the neurons of cerebral cortex produces a magnetic field that can be detected outside the human scalp. The dominant contribution is attributed to pyramidal cells, which are preferentially oriented perpendicular to the cortical surface. In general, it is not possible to deduce a unique representation of the spatial configuration of these cortical sources from a measurement of their field pattern alone. However, accurate a priori knowledge of the geometry of the underlying cerebral cortex makes it possible to infer the spatial configuration of these transcortical current sources, moment by moment, without imposing a simplified model such as a small set of current dipoles. To achieve such a realistic magnetic source image, we have introduced what we call the "Minimum-Norm Least-Squares Inverse" (MNLS inverse) for the magnetic problem. The MNLS inverse provides the least residual error in accounting for the measured field pattern, with a source current distribution having minimum power. An extension of this procedure provides an inverse solution for average field power, as opposed to field per se. This makes it possible to define spatial configurations of spontaneous cortical activity not phase-locked to a sensory stimulus. Rhythmic activity such as the occipital alpha rhythm is one example. Thus, it is possible to determine spatial patterns of enhanced or suppressed cortical rhythms that accompany cognitive processes and some pathological conditions. This paper provides the necessary background for understanding these recent developments, as well as examples of how they might be used.

Brain↗