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At least 1,351 records · Page 75Linked to original sources
PubMed1994

Simulation of reduced band broadening during single-stranded DNA pulsed field electrophoresis in polyacrylamide gels.

Using a computer simulation algorithm based on the reptation model, we investigated the effects of pulsed field gel electrophoresis on the separation of single-stranded DNA molecules in denaturing polyacrylamide gels. Pulsed fields that combine two different field intensities were found to affect the orientation of the reptation tube as well as the electrophoretic velocity, the rate of band broadening, the plate height and the molecular length for which the minimum of mobility was found, in agreement with available experimental results. Due to "memory" effects, pulses alternating between the forward and backward directions can reduce the diffusion constant by many orders of magnitude. Pulses of identical polarity but different intensities reduce the diffusion because stretches of less-oriented DNA segments are conserved during the migration. We suggest that, for DNA sequencing, pulsed fields of fixed polarity should be used to reduce band broadening and not to overcome band inversion or suppress molecular orientation. We conclude that, using a low intensity, constant electric field will lead to smaller band widths than any pulsed field regime, in a similar length of experimental time, but with less complications.

Algorithms↗
PubMed1989

Single-scan volume-selective spectral editing by homonuclear polarization transfer.

An RF and field gradient pulse sequence is presented permitting the single-scan volume-selective spectral editing of the signals of coupled spins. The attribute "single scan" means that the editing of resonance lines is performed in each accumulation or phase cycling scan rather than by subtraction of independently acquired signals. As an example the methyl line of lactate has been investigated. The method is considered to be a useful tool in biomedical research using lactate as an ischemic parameter. The basic principles applied are the VOSY pulse scheme for volume selection and the INEPT sequence for homonuclear polarization transfer from the CH to the CH3 groups. As an intrinsically single-scan editing method ("homonuclear single-scan VOSING procedure"), the pulse train is expected to provide results that are more reliable than the two-scan subtraction methods previously reported.

Lactates↗
PubMed2001

Diffusion and relaxation effects in general stray field NMR experiments.

We analyze the evolution of magnetization following any series of radiofrequency pulses in strongly inhomogeneous fields, with particular attention to diffusion and relaxation effects. When the inhomogeneity of the static magnetic field approaches or exceeds the strength of the RF field, the magnetization has contributions from different coherence pathways. The diffusion or relaxation induced decay of the signal amplitude is in general nonexponential, even if the sample has single relaxation times T(1), T(2) and a single diffusion coefficient D. In addition, the shape of the echo depends on diffusion and relaxation. It is possible to separate contributions from different coherence pathways by phase cycling of the RF pulses. The general analysis is tested on stray field measurements using two different pulse sequences. We find excellent agreement between measurements and calculations. The inversion recovery sequence is used to study the relaxation effects. We demonstrate two different approaches of data analysis to extract the relaxation time T(1). Finite pulse width effects on the timing of the echo formation are also studied. Diffusion effects are analyzed using the Carr--Purcell--Meiboom--Gill sequence. In a stray field of a constant gradient g, we find that unrestricted diffusion leads to nonexponential signal decay versus echo number N, but within experimental error the diffusion attenuation is still only a function of g(2)Dt(3)(E)N, where t(E) is the echo spacing.

Journal Article↗
PubMedDate not supplied

Multiple spin-echo MR imaging of the body: image contrast and motion-induced artifact.

For a given TR and TE, image quality changes when the number of spin echoes obtained is varied. To investigate the importance of this in clinical imaging, a total of 4 patients and 9 volunteers had MRI examinations of the abdomen (n = 7) and/or pelvis (n = 8) which included at least 2 sequences with identical TR (2000 or 2500 ms), TE (80 ms) and other parameters, but with a different series of refocusing pulses. Sequences included single-echo (S), asymmetric and symmetric double-echo (AD and SD) and quadruple-echo (Q) techniques. Image contrast and severity of motion-induced artifact was measured via blind examination by 3 independent MRI radiologists and calculation of signal-difference, signal-difference-to-noise ratios and intensity of motion-induced "ghost artifact." The order of decreasing signal differences was S, SD, AD and Q, and all of three liver lesions were better seen with S than with SD techniques. These observations are consistent with signal loss from cumulative inaccuracies from multiple 180 degrees RF pulses. The order of increasing intensity of ghost artifact was Q, SD, AD and S, consistent with the beneficial motion artifact-reducing effects of even-echo rephasing. Knowledge of these effects of multi-echo imaging allows one to make informed decisions about imaging protocols rather than to simply obtain multiple echoes "because they are free."

Abdomen↗
PubMedDate not supplied

MR imaging evaluation of knee collateral ligaments and related injuries: comparison of T1-weighted, T2-weighted, and fat-saturated T2-weighted sequences--correlation with clinical findings.

The objectives of this study were to compare the ability of T1-weighted (T1W), proton density/T2-weighted (PD/T2W), and fat saturation (FS) PD/T2W magnetic resonance (MR) sequences for depiction of the knee collateral ligaments and related injuries, and to compare MR findings with clinical findings. Ten subjects with normal knee ligaments and 64 patients with suspected collateral ligament injuries underwent coronal T1W, PD/T2W, and FS PD/T2W imaging. Abnormalities ranged from edema surrounding the collateral ligaments (grade I) to complete disruption of ligamentous fibers (grade III). FS PD/T2W images improved definition of the medial collateral ligament (MCL) and lateral collateral ligament (LCL) compared with other sequences in 78% and 81% of patients, respectively. While the apparent grade of collateral ligament injury was similar with all pulse sequences in most patients, depiction of such injury was usually most conspicuous on FS PD/T2W images (MCL, 92% of patients; LCL, 38% of patients). In no patients were clinically diagnosed collateral ligament injuries undetected or understaged with MR imaging. MR findings indicated higher-grade MCL and LCL injuries than did clinical examination in 24 and 15 patients, respectively.

Adipose Tissue↗
PubMed2006

Continuously moving table 3D MRI with lateral frequency-encoding direction.

A method is presented for 3D MRI in an extended field of view (FOV) based on continuous motion of the patient table and an efficient acquisition scheme. A gradient-echo MR pulse sequence is applied with lateral (left-right (L/R)) frequency-encoding direction and slab selection along the direction of motion. Compensation for the table motion is achieved by a combination of slab tracking and data alignment in hybrid space. The method allows fast k-space coverage to be achieved, especially when a short sampling FOV is chosen along the direction of table motion, as is desirable for good image quality. The method can be incorporated into different acquisitions schemes, including segmented k-space scanning, which allows for contrast variation with the use of magnetization preparation. Head-to-toe images of volunteers were obtained with good quality using 3D spoiled gradient-echo sequences. As an example of magnetization-prepared imaging, fat/water separated images were acquired using chemical shift selective (CHESS) presaturation pulses.

Beds↗
PubMedDate not supplied

Multi-gradient pulse investigations of fluid transport in porous media.

NMR pulsed field gradient (PFG) experiments employing the application of n gradient pulses k(1) ellipsis k(n) are discussed in a general way as an n-fold encoding of position at successive times. The experiments are then represented by a sampling of n-dimensional k-space, K(n). Various parameters of motion can be derived from the evolution of correlations within the n-dimensional position (r-)space, R(n), which is the Fourier conjugate space to K(n). A wide class of NMR experiments may be described by this formalism, where the dimension of the experiment is often reduced by imposing conditions to the free variables. This is demonstrated for the case of displacement measurements where the condition summation operatork(i) = 0 is met. The two simplest pulse sequences which allow one to correlate displacements at two different times with each other are presented. While the three-pulse version of SERPENT encodes displacements in two interleaved time intervals Delta(1) and Delta(2), the four-pulse VEXSY experiment includes a mixing time tau(m) in between both encoding intervals Delta. The behaviour of fluid transport subject to external pressure through a model porous system is demonstrated by means of numerical simulations of SERPENT and VEXSY experiments for water flowing through a packed bed of monosized spherical particles. Displacements parallel (Z) and perpendicular (X) to the main flow direction are determined and the 2-D joint probability densities and the conditional probabilities are discussed along with the correlation coefficients related to the displacements at different encoding times. It is shown that all possible correlations between Z and X(2) in VEXSY decay with time constants comparable to the average time needed for a fluid molecule to cover one bead diameter, while a negative correlation is observed between transverse (X) displacements which is explained by molecules flowing along streamlines which follow the circumference of the spherical particles. Correlations for displacements during the different times in SERPENT generally decay much slower and provide complementary information about the evolution of displacements with time.

Computer Simulation↗
PubMed1996

Magnetic resonance guidance of percutaneous ethanol injection in liver.

RATIONALE AND OBJECTIVES: Percutaneous ethanol injection (PEI) is used as a form of treatment for cancer, particularly malignant hepatic tumors. Little is known about the intratumoral distributions of ethanol following PEI. We assessed, using magnetic resonance (MR) imaging, the distribution of ethanol in liver and the concentration of ethanol needed to kill tumor cells in vivo. METHODS: MR imaging studies were performed using phantoms of alcohol, ex vivo bovine liver, and healthy human volunteers. A variety of pulse sequences were tested for their ability to maximize the signal intensity from alcohol while minimizing the signal from liver tissues as well as the regions of necrosis following ethanol injection. A cell culture model of in vitro cytotoxicity was developed to predict the target concentration of alcohol necessary for killing tumor cells. RESULTS: At 1.5 T, we found that an inversion-recovery spin-echo sequence using an inversion time of 250 msec and an echo time of 150 msec in combination with water saturation pulses effectively suppressed the tissue water signal from human liver while obtaining a clear signal from the ethanol. The cytotoxicity experiments suggested that a concentration of 20% or more ethanol is sufficient to completely kill all the tumor cells. CONCLUSION: A critical concentration of ethanol (e.g., 10%) is necessary for full tumoricidal effect. MR imaging should be able to determine the volume of distribution and the intratumoral concentrations of ethanol, thus potentially allowing researchers to achieve the requisite concentrations for maximal tumoricidal effects.

Animals↗
PubMed2004

High-resolution contrast-enhanced magnetic resonance angiography of the carotid arteries using fluoroscopic monitoring of contrast arrival: diagnostic accuracy and interobserver variability.

PURPOSE: To evaluate the diagnostic accuracy of high-resolution contrast-enhanced magnetic resonance angiography (CE-MRA) of the supra-aortic arteries using the CareBolus technique. Digital subtraction angiography was the standard of reference. MATERIAL AND METHODS: Fifty consecutive patients with suspected internal carotid artery stenosis underwent CE-MRA and digital subtraction angiography. CE-MRA was performed on a 1.5-T superconducting scanner with the CareBolus technique. CareBolus combines a nearly real-time 2D-FLASH (fast low angle shot) sequence for fluoroscopic triggering and a high-resolution 3D-FLASH with elliptical centric view order for the angiographic pulse sequence (6.0/2.16 ms [TR/TE], 30 degrees flip angle, 30.98 s acquisition time, 0.88 mm effective (interpolated) partition thickness and a 160 x 512 matrix). Intra-arterial digital subtraction angiography and CE-MRA studies were evaluated independently by four blinded readers. Internal carotid artery stenoses were graded according to the NASCET criteria. RESULTS: CE-MRA had an accuracy of 92.53%, a sensitivity of 95.64%, and a specificity of 90.39% for the identification of carotid artery stenoses > or = 70% (grade 3). Image quality for suppression of stationary tissue and venous contrast was good, but was reduced in five cases due to patient motion. CONCLUSION: The CareBolus technique is a useful non-invasive method for high-resolution imaging of the supra-aortic vessels because of its easy application and high sensitivity and specificity. Limitations can occur in non-compliant patients due to motion artifacts during the measurement time.

Adult↗
PubMed1994

Magnetic resonance microscopy of the rat carotid artery at 300 megahertz.

RATIONALE AND OBJECTIVES: Magnetic resonance microscopy (MRM) has evolved from a technical curiosity to a tool with which researchers can study important disease models. But MRM is not simply an extension of clinical magnetic resonance imaging. In this article, the unique adaptations of MRM required in the study of carotid artery disease are outlined. The techniques of MRM are integrated into a specific model of carotid artery disease in the rat to allow in vivo studies of vascular wall thickening after removal of the vascular endothelium. METHODS: Imaging was performed at 300 MHz in 250-gm Sprague-Dawley rats using surgically implanted radiofrequency coils to localize the region of interest and to provide an increase in the signal-to-noise ratio over that of volume or surface coils. A three-dimensional Fourier encoding sequence was modified with flow-dephasing gradients to minimize signal and artifacts from moving blood. RESULTS: In vivo images were acquired with spatial resolution of 25 x 25 x 400 microns and signal-to-noise ratio more than sufficient to define the morphology of the vascular wall. Significant changes in the intensity and distribution of signal were visible in the area surrounding the vessel after angioplasty. CONCLUSIONS: Signal-to-noise improvements from surgically implanted coils coupled to a three-dimensional radiofrequency-refocused sequence with flow-dephasing gradients were sufficient to define the wall of the carotid artery. The diffusion-weighted pulse sequence detects signal changes in the area surrounding the vessel after angioplasty. The MRM techniques described and the contrast observed allow us, for the first time to follow in vivo the early stage of developing atherosclerosis in the vessel wall and closely surrounding tissue.

Angioplasty, Balloon↗
PubMed1994

Evaluation of tilted, optimized, non-saturating excitation pulses in 3D magnetic resonance angiography of the abdominal aorta and major branches in volunteers.

Ten normotensive volunteers were imaged with FISP-3D (Fast Imaging Steady Precession) pulse sequences (a steady state gradient recalled echo, SS-GRE, technique) without and with modification by tilted, optimized non-saturating excitation (TONE) customized radio frequency (RF) pulses, for evaluation of the abdominal aorta--in particular the renal arteries. Maximum intensity projection (MIP) angiograms were constructed for each sequence in each volunteer; 15 observers "blinded" to sequence type preferred the images generated from the TONE applied sequence. Analysis showed that this preference was significant.

Aorta, Abdominal↗
PubMed2005

Adiabatic pulses in 1H-15N direct and long-range heteronuclear correlations.

The application of adiabatic inversion pulses to the detection of (1)H-(15)N heteronuclear correlations is described. The pulse sequences studied were gHSQC, CRISIS-gHSQC, gHMBC and CRISIS-gHMBC. The poor inversion quality of rectangular 180 degrees X pulses can lead to a loss of signal at the peripheries of the spectrum. Replacing these pulses with adiabatic sweeps significantly improves sensitivity across the potentially large (15)N spectral window. Satellite spectrum profiles are shown to demonstrate the increase in sensitivity when employing adiabatic pulses on wide spectral widths. Additionally, the active pharmaceutical nizatidine was used as a model compound to demonstrate the improvements in the long-range correlation data.

Computer Simulation↗
PubMed1984

Driven-equilibrium radiofrequency pulses in NMR imaging.

Driven-equilibrium pulse techniques are applied to NMR imaging to extend the possibilities of manipulating image contrasts in pulse sequences with a high repetition rate. In many cases the data acquisition time can be much shorter than in more conventional pulse techniques. Both calculations and experiments reveal that the intensity of tissue with slowly relaxing nuclear magnetizations can significantly be enhanced, thus facilitating the detection of a number of pathologies.

Head↗
PubMed1991

Practical implementation and optimization of one-shot T1 imaging.

Longitudinal relaxation times (T1) can be measured rapidly in an imaging context using a "one-shot" method based on the pulse sequence originally proposed by D. C. Look and D. R. Locker (Rev. Sci. Instrum. 41, 250 1970). This sequence is significantly faster than either repeated inversion recovery or repeated saturation recovery methods. The method uses a 180 degrees inversion pulse followed by multiple small-angle alpha pulses that sample the longitudinal magnetization during its recovery. Choices of inversion pulse, tip angle, and time intervals are discussed for optimal clinical use. We can produce 29 images sampling the full T1 recovery curve with a 256 x 256 resolution in about 10 min. From this data, T1 images can be calculated with a precision of 10%.

Humans↗
PubMed2005

Optimizing brain tissue contrast with EPI: a simulated annealing approach.

A new magnetization preparation and image acquisition scheme was developed to obtain high-resolution brain images with optimal tissue contrast. The pulse sequence was derived from an optimization process using simulated annealing, without prior assumptions with regard to the number of radiofrequency (RF) pulses and flip angles. The resulting scheme combined two inversion pulses with the acquisition of three images with varying contrast. The combination of the three images allowed separation of gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF) based on T1, contrast. It also enabled the correction of small errors in the initial T1 estimates in postprocessing. The use of three-dimensional (3D) sensitivity-encoded (SENSE) echo-planar imaging (EPI) for image acquisition made it possible to achieve a 1.15(3) mm3 isotropic resolution within a scan time of 10 min 21 s. The cortical GM signal-to-noise ratio (SNR) in the calculated GM-only image varied between 30 and 100. The novel technique was evaluated in combination with blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) on human subjects, and provided for excellent coregistration of anatomical and functional data.

Brain Mapping↗
PubMed1987

Maximizing signal-to-noise and contrast-to-noise ratios in FLASH imaging.

This paper presents an analysis of signal-to-noise and contrast-to-noise ratios from small tip angle, gradient reversal (FLASH) imaging. Analytic and numerical techniques are used to determine the delay times and tip angles that maximize signal-to-noise per unit time from a single tissue. Similar procedures are used to determine the delay times and tip angles that maximize both T1-induced and T-2*-induced contrast-to-noise per unit time for a pair of tissues as a function of tissue characteristics and pulse sequence sampling times. The advantage of optimized FLASH imaging over optimized spin-echo imaging is quantitated by comparing signal-to-noise and contrast-to-noise ratios per unit time from the two sequences. Images are used to confirm these numerical results, to compare noise levels resulting from gradient reversals versus 180 degrees rephasing pulses and to assess the possible adverse effects of static magnetic field inhomogeneities on FLASH imaging.

Humans↗
PubMed2005

Determination of transverse relaxation rate for estimating iron deposits in central nervous system.

To determine the amount of iron deposits in the basal ganglia, we examined 13 healthy volunteers with a 1.5 T MRI system using three transverse relaxation rates measured with two sequences. The transverse relaxation rates comprise the reversible contribution (R2') and irreversible contribution (R2) to a phase-reversal 180 degrees -pulse sequence. The transverse relaxation rates with the estimated iron indicated that both R2 and R2' had a robust relationship with brain iron level as determined from published post mortem data. This was the case only when the analyses are limited to the subcortical gray matter regions, however. R2' was affected by macroscopic magnetic field inhomogeneity arising from the skull bases, so that it was less robust for estimating the amount of iron deposits in the basal ganglia.

Adult↗
PubMedDate not supplied

The earliest manifestation of focal encephalitis on diffusion-weighted MRI.

We report the case of an adult patient with a focus of meningioencephalitis in whom the earliest magnetic resonance imaging (MRI) manifestation was hyperintense signal on diffusion-weighted (DW) sequence only. To our knowledge, this is the first adult encephalitis case to be reported with the MRI diffusion-weighted pulse sequence being the only positive MRI finding. A brief review of the pertinent literature is included.

Aged↗