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PubMedDate not supplied

MR sialography: initial experience using a T2-weighted fast SE sequence.

PURPOSE: The aim of this study was to evaluate an MR technique optimized for imaging of the parotid gland ductal system. METHOD: The pulse sequence was optimized in 10 volunteers to depict static or nearly static fluid in the parotid ductal system. A heavily T2-weighted fast SE sequence (TR 3,600 ms/TE 800 ms) with a slice thickness of 30-40 mm using an 8 cm surface coil allowed depiction of the fluid-filled parotid duct. Thirteen patients with benign as well as malignant parotid gland pathologies were examined: sialadenitis (n = 2), sialadenosis (n = 3), Heerfordt syndrome (n = 1), pleomorphic adenoma (n = 2), parotid carcinoma (n = 1), lymphoepithelial carcinoma (n = 1), cystadenolymphoma (n = 2), and non-Hodgkin lymphoma (n = 1). RESULTS: The heavily T2-weighted projection image yielded good quality sialographic images. The main duct and primary branching ducts were clearly depicted in all normal cases. The main duct was visualized in all patients. Intra- and extraglandular duct widening and ductal strictures were well depicted. Sialolithiasis with a calculus in the main duct was correctly demonstrated in one case. CONCLUSION: MR sialography is noninvasive and does not depend on duct cannulation or contrast agent injection. Initial experience with a thick slice projection technique indicates that MR sialography can be successfully applied to image the parotid gland ductal system.

Adenolymphoma↗
PubMed2006

3D MR sialography protocol for postradiotherapy follow-up of the salivary duct system.

PURPOSE: To develop and evaluate an MR sialography protocol that reproducibly images the parotid and submandibular ducts over time, in 3D. Such a protocol is needed in order to investigate the possible radiation-induced changes to the salivary ducts in patients receiving radiotherapy to the head-and-neck. MATERIALS AND METHODS: MR sialography was performed on a 1.5-T MR scanner. Sequence parameters were optimized on 11 healthy volunteers. A 3D water-selective turbo spin echo (TSE) pulse sequence (TR/TE = 6000 msec/190 msec), using a two-element circular surface coil was applied twice in one MR session. In order to assess the reproducibility, the same procedure was repeated four to six months later. The quality of the MR sialograms was measured subjectively by developing a visibility scoring system and objectively by the means of contrast-to-noise ratio (CNR) of the ducts vs. fat (CNR(duct-fat)). RESULTS: High-quality, 3D MR sialographic images were obtained. The quality of the MR sialograms and the subjective visibility score of the salivary ducts were constant over time. The CNR(duct-fat) varied between volunteers (standard deviation, SD 26%) but it was relatively constant per volunteer (SD 5%). CONCLUSION: The MR sialography protocol presented in this study provides good quality 3D imaging of the major salivary ducts, submandibular duct, and the parotid duct and it can be used for the comparison of the salivary duct system of an individual over time.

Head and Neck Neoplasms↗
PubMed1998

Artifacts induced by concomitant magnetic field in fast spin-echo imaging.

It has been observed that fast spin-echo (FSE) images with a large field of view (>40 cm) in certain directions exhibit unusual ghosting artifacts that cannot be eliminated with existing ghost removal methods. These artifacts have been related to a higher-order magnetic field perturbation (known as the concomitant field, or Maxwell field) concomitant to the linear imaging gradient, in accordance with the Maxwell equations V z B = 0 and V x B approximately 0. Several methods have been developed to eliminate or minimize the effects of the concomitant magnetic field by redesigning the FSE pulse sequences. In the slice-selection direction, the gradient waveforms are made symmetrical about the refocusing RF pulses wherever possible. Surrounding the first refocusing pulse, such symmetry cannot be achieved due to the slice-refocusing gradient, which is often combined with the left crusher. In this case, it is shown how crusher gradients can be reshaped to nullify the phase due to the concomitant field. In the phase-encoding direction, the gradient amplitude is reduced and its duration is prolonged. Artifacts due to the readout gradient are eliminated by reshaping the prephasing lobe, while keeping its area fixed. In all the three directions, the gradient waveforms are adjusted so that they have minimal overlap. Selected methods have been implemented on a clinical scanner, and typically reduce the ghost intensities in phantom and human images by a factor of 3.

Artifacts↗
PubMed1998

Five-quantum coherence of I = 52 nuclei: 27Al in an Al2O3 single crystal

Optimal conditions were calculated for excitation and detection of the five-quantum coherence of quadrupolar nuclei with I = 52 in single crystals, observed by the two-pulse sequence (theta1)x - tau1 - (theta2)alpha - tau2, where alpha is the phase cycling angle. Variations in the pulse lengths, the relative values of the nutation frequency omega1 = gammaB1, and the quadrupolar frequency omegaQ as well as in the resonance offset were taken into account. In addition, the effect of the pulse length on the intensity of spectral lines was considered. Theoretical results were compared with experiments on 27Al nuclei in an Al2O3 single crystal. Copyright 1998 Academic Press.

Journal Article↗
PubMed2002

Electron dipole-dipole ESEEM in field-step ELDOR of nitroxide biradicals.

The use of a rapid stepping of the magnetic field for investigation of electron dipole-dipole ESEEM in pulsed X-band ELDOR is described. The magnetic field jump, synchronized with a microwave pumping pulse, is positioned between the second and the third pulses of the stimulated echo pulse sequence. This echo is measured as a function of the delay between the first and the second pulses. The data are analyzed for a Fourier transform resulting in a Pake resonance pattern. To remove the electron-nuclear contributions to ESEEM, time traces with pumping were divided by those without. This resulted in complete elimination of electron-nuclear contributions, which is seen from the absence of peaks at nuclear frequencies and the similarity of results for protonated and deuterated solvents. For increasing the electron-electron modulation depth, a scanning of the magnetic field during the microwave pumping is proposed. The interspin distances and their distribution are determined for two long-chained (ca. 2 nm) nitroxide biradicals in glassy toluene and in frozen nematic liquid crystal 4-cyano-4'-pentyl-biphenyl. For the latter solvent, the alignment of the axis connecting two nitroxides in biradicals is quantitatively analyzed.

Journal Article↗
PubMed2000

Bridging the gap: A set of selective 1H-15N-correlations to link sequential neighbors of prolines.

Triple-resonance experiments are standard in the assignment of protein spectra. Conventional assignment strategies use 1H-15N-correlations as a starting point and therefore have problems when proline appears in the amino acid sequence, which lacks a signal in these correlations. Here we present a set of amino acid selective pulse sequences which provide the information to link the amino acid on either side of proline residues and thus complete the sequential assignment. The experiments yield amino acid type selective 1H-15N-correlations which contain signals from the amino protons of the residues either preceding or following proline in the amino acid sequence. These protons are correlated with their own nitrogen or with that of the proline. The new experiments are recorded as two-dimensional experiments and their performance is demonstrated by application to a 115-residue protein domain.

Amino Acids↗
PubMed2001

A TROSY relayed HCCH-COSY experiment for correlating adenine H2/H8 resonances in uniformly 13C-labeled RNA molecules.

A new TROSY relayed HCCH-COSY pulse sequence is introduced for correlating adenine H2 and H8 resonances in 13C-labeled RNA molecules. The pulse scheme provides substantial improvements in signal-to-noise compared to previously suggested experiments, and therefore will be suitable for NMR studies of larger RNA molecules. The experiment provides 13C chemical shifts for all carbon nuclei in the adenine base. This is advantageous for resolving spectral overlap in larger RNA molecules and provides a starting point for measuring additional parameters for these carbons in the adenine spin system.

Adenine↗
PubMed2006

Proton line narrowing in solid-state nuclear magnetic resonance: new insights from windowed phase-modulated Lee-Goldburg sequence.

We present here a bimodal Floquet analysis of the windowed phase-modulated Lee-Goldburg (wPMLG) sequence for homonuclear dipolar decoupling. One of the main criteria for an efficient homonuclear dipolar decoupling scheme is an effective z-rotation condition. This is brought about by the presence of radio-frequency imperfections in the pulse sequence together with a systematic manipulation of the wPMLG pulses. Additional improvement in the (1)H spectral resolution was obtained by a proper understanding of the off-resonance dependence of the wPMLG irradiation scheme based on bimodal Floquet theory. Numerical investigations further corroborate both theoretical and experimental findings. Theoretical analysis points to accidental degeneracies between the cycle time of the wPMLG sequence and the rotor period leading to the experimentally observed off-resonance dependence of the resolution. Two-dimensional (1)H-(1)H homonuclear single-quantum correlation spectra of model amino acids are also presented, highlighting the improved spectral resolution of wPMLG sequences.

Journal Article↗
PubMed2006

Generalized self-navigated motion detection technique: Preliminary investigation in abdominal imaging.

Patient motion remains a primary obstacle to diagnostic image quality, especially in the abdomen, despite the existence of various motion artifact reduction techniques. This work presents a self-navigated motion detection method that can be generalized for most pulse sequences and k-space trajectories. Motion information is extracted directly from raw MR data, requiring no additional gradient or RF pulses, no physiologic monitoring equipment, and minimal--if any--impact on scan time. Initial feasibility results with a two-dimensional gradient echo sequence are shown in phantom studies and in vivo volunteer abdominal studies, demonstrating the sensitivity of the method to both respiratory motion and cardiovascular pulsatility. Prospectively gated images were acquired using the self-navigated data to synchronize image acquisition with motion. These preliminary results suggest that the self-navigated method is a promising technique for reducing motion artifacts in clinical abdominal and cardiac applications.

Abdomen↗
PubMed2004

Evaluation of balanced steady-state free precession (TrueFISP) and K-space segmented gradient echo sequences for 3D coronary MR angiography with navigator gating at 3 Tesla.

PURPOSE: To test the feasibility of k-space segmented gradient-echo pulse sequences for free-breathing coronary magnetic resonance angiography (cMRA) on a clinical 3T system. MATERIALS AND METHODS: T2-prepared, fat-suppressed turbo field echo (TFE, turboFLASH, SFPGR) as well as balanced TFE (b-TFE, trueFISP, FIESTA, segmented SSFP) sequences with navigator gating for prospective motion correction were applied on a 3T system equipped with a six-element phased-array cardiac coil. In 15 healthy volunteers, the right coronary artery (RCA) was examined with TFE and b-TFE sequences. Due to examination time limitations, the left coronary artery (LM/LAD) was examined exclusively with the TFE sequence in ten volunteers. Image quality was graded on a five point scale (0 = not visualized to 4 = excellent). The length, diameter and sharpness of the vessels and the contrast-to-noise ratios (CNR) were measured. RESULTS: 98 % of all major segments (proximal/middle/distal) of the RCA could be seen with the TFE sequence and 82 % with the b-TFE sequence. The image quality for the three segments was graded higher for the TFE sequence (2.7/2.7/1.5) than for the b-TFE sequence (1.9/1.6/0.9) with P: (< or = 0.001/< or = 0.004/< or = 0.056). The kappa of the interobserver variability was 0.75 for the TFE sequence and 0.8 for the b-TFE sequence. The measured vessel lengths were longer for the TFE sequence (95 +/- 22 mm) than for the b-TFE sequence (80 +/- 40 mm; P < or = 0.115). No significant changes (P < or = 0.074, P < or = 0.145) in diameter and vessel sharpness of the RCAs were observed between the TFE (2.4 +/- 0.3 mm, 60 % +/- 5) and b-TFE sequences (2.4 +/- 0.3 mm, 62 % +/- 6). The CNR was higher for the TFE sequence (10.1 +/- 3.4) than for the b-TFE sequence (6.6 +/- 2.1; P < or = 0.014). All ten main and proximal segments of the LM/LAD, which were examined exclusively with the TFE sequence, were visible with grade 2.5 and 2.1. The middle segment was visible in seven cases with grade 1.3. In three cases, the distal segment was visible with grade 0.5. The vessel length was 78 +/- 27 mm and the CNR 11.9 +/- 2.4. CONCLUSION: The conventional TFE technique has demonstrated good feasibility for cMRA at 3T. In its operational availability at 3T, the b-TFE sequence is inferior to the TFE sequence.

Adult↗
PubMed1997

The b matrix in diffusion tensor echo-planar imaging.

In diffusion tensor imaging (DTI) an effective diffusion tensor in each voxel is measured by using a set of diffusion-weighted images (DWIs) in which diffusion gradients are applied in a multiplicity of oblique directions. However, to estimate the diffusion tensor accurately, one must account for the effects of all imaging and diffusion gradient pulses on each signal echo, which are embodied in the b matrix. For DTI to be practical clinically, one must also acquire DWIs rapidly and free of motion artifacts, which is now possible with diffusion-weighted echo-planar imaging (DW-EPI). An analytical expression for the b matrix of a general DW-EPI pulse sequence is presented and then validated experimentally by measuring the diffusion tensor in an isotropic phantom whose diffusivity is already known. The b matrix is written in a convenient tabular form as a sum of individual pair-wise contributions arising from gradient pulses applied along parallel and perpendicular directions. While the contributions from readout and phase-encode gradient pulse trains are predicted to have a negligible effect on the echo, the contributions from other imaging and diffusion gradient pulses applied in both parallel and orthogonal directions are shown to be significant in our sequence. In general, one must understand and account for the multiplicity of interactions between gradient pulses and the echo signal to ensure that diffusion tensor imaging is quantitative.

Algorithms↗
PubMedDate not supplied

[Natural polymers according to NMR data: cross-relaxation in hydrated collagen macromolecules from two connective tissues].

Spin-lattice relaxation and cross-relaxation in oriented and randomly oriented collagen fibers from two connective tissues (15-month-old calf and 8-year-old steer) at a water content of 0.6 g H2O/g dry matter were studied. Collagens were chosen according to different numbers of covalent nonreducible cross-links, which increase during the life of the animal. The spin-lattice relaxation curves for all the collagens after a 180 degree-tau-90 degree pulse sequence were described by two exponential components. The dependences of two components of spin lattice relaxation time and their populations on the length of the 180 degree-pulse were obtained. On the basis of data of Goldman-Shen sequence and the two-phase model, the populations of proton fractions (p(w) and p(c)) as well as the rates of transfer of magnetization between water protons and collagen protons (k(w) and k(c)) were calculated. No significant difference between k(w) (k(c)) in oriented and randomly oriented fibers as well as in fibers with different cross-linking was found. The estimates of the cross-relaxation times for low cross-link collagen and high cross-link one were done. The correlation times of dipole-dipole interactions for both connective tissues were calculated using the cross-relaxation theory.

Animals↗
PubMed2007

Fast 3D 1H spectroscopic imaging at 3 Tesla using spectroscopic missing-pulse SSFP with 3D spatial preselection.

Three-dimensional (3D) (1)H MR spectroscopic imaging (SI) allows metabolic changes in human tissue to be identified. In clinical practice, fast acquisition techniques are required to achieve an adequate spatial resolution within acceptable total measurement times. In this study a novel fast pulse sequence for 3D (1)H SI based on the condition of steady-state free precession (SSFP), termed "spectroscopic missing-pulse SSFP" (spMP-SSFP), is proposed. It combines 3D spatial preselection with the acquisition of full spin echoes (SEs), and thus makes subsequent phase correction of spectra redundant. The sequence was applied to a phantom and healthy human brains in vivo at 3 Tesla. Metabolic images are acquired with a spatial resolution of 1.8 cm(3) within a total measurement time of about 6 min. With a lower signal-to-noise ratio (SNR) per unit measurement time compared to previous spectroscopic SSFP implementations, 3D spatial preselection can now be realized with spMP-SSFP. Since the method does not require separate techniques for water and lipid suppression, and employs a simple data-processing approach, spMP-SSFP is a robust, fast SI method that requires only minimal user interaction.

Brain↗
PubMed1995

The effect of inhomogeneous sample susceptibility on measured diffusion anisotropy using NMR imaging.

Water diffusion measurements in white matter of freshly excised pig spinal cord and in parenchyma of fresh celery (excluding the fibers along the edge of the stalk) were performed using NMR at 200 MHz. In white matter of pig spinal cord, the measured diffusion coefficient is anisotropic and independent of sample orientation with respect to the magnetic field. In celery parenchyma, diffusion is isotropic and independent of orientation in the magnetic field when using a diffusion sequence that gives results independent of self-induced magnetic-field gradients. However, when the standard diffusion pulse sequence that gives results dependent upon self-induced magnetic-field gradients is used, diffusion in celery appears isotropic when the stalk is oriented parallel to the magnetic field but anisotropic when oriented perpendicular. Susceptibility variations leading to anisotropic self-induced magnetic-field gradients approximately 3 kHz/cm in magnitude when the celery is oriented perpendicular to the magnetic field can explain this apparent anisotropic diffusion. A study of the apparent diffusion coefficient (ADC) in celery as a function of diffusion times ranging from 8 to 22 ms indicates that the motion is at most only slightly restricted. Therefore, although the effect is not seen in all types of samples, one must be aware that self-induced gradients may affect the ADC and may cause isotropic diffusion to appear anisotropic. In addition, NMR experiments that change diffusion-sensitizing gradient timings to study restricted diffusion change the effects of the self-induced gradients as well as the effect of barriers on the ADC, complicating interpretation.

Algorithms↗
PubMed2006

Theory and applications of supercycled symmetry-based recoupling sequences in solid-state nuclear magnetic resonance.

We present the theoretical principles of supercycled symmetry-based recoupling sequences in solid-state magic-angle-spinning NMR. We discuss the construction procedure of the SR26 pulse sequence, which is a particularly robust sequence for double-quantum homonuclear dipole-dipole recoupling. The supercycle removes destructive higher-order average Hamiltonian terms and renders the sequence robust over long time intervals. We demonstrate applications of the SR26 sequence to double-quantum spectroscopy, homonuclear spin counting, and determination of the relative orientations of chemical shift anisotropy tensors.

Journal Article↗
PubMed1990

Effect of trabecular bone on the appearance of marrow in gradient-echo imaging of the appendicular skeleton.

This prospective study evaluated the effect of trabecular bone on the appearance of marrow in gradient-echo (GRE) images of the appendicular skeleton in vivo at high magnetic field strength. Magnetic resonance (MR) imaging of 10 normal extremities in five patients was performed with spin-echo (SE) and GRE sequences. The latter were obtained with gradient recalled acquisition in a steady state. SE and GRE sequences had identical spacing and planes of imaging. Cortical bone appeared as a signal void regardless of the pulse sequences and parameters. Marrow in contact with trabecular bone exhibited a shortened effective transverse relaxation time (T2*) and resultant signal loss because of local field inhomogeneities where mineralized matrix interfaced with it. This T2* effect was increased in regions with more trabecular bone (epiphysis) than regions with little trabecular bone (diaphysis). A low signal intensity on GRE images may represent fatty marrow with a high content of trabecular bone and should not be interpreted only as hematopoietic marrow.

Bone Marrow↗
PubMed1991

[Localized nuclear magnetic resonance spectroscopy in vivo. Physical principles and quantitative problems].

Recent technological developments extended the use of nuclear magnetic resonance to clinical imaging (MRI), as well as to in vivo metabolic studies on tissues and organs by means of localized spectroscopy (MRS). The quantitative determination of absolute metabolic concentrations by in vivo MRS still represents a challenge for technological efforts and biochemical investigations. In fact, the different methodologies today available for spectral acquisition from localized regions within a body organ or pathological lesion should ideally lead to a complete suppression of signals from surrounding tissues, without any appreciable signal loss from the volume of interest. Accurate assessment of deviations of the real from the ideal case represents an essential requirement for either relative or absolute quantification of in vivo localized MR spectra. A review will be presented of the most utilized MRS methodologies, together with a discussion on their potentialities and limitations. Techniques have been classified according to the use of either r.f. (B1) and/or static magnetic field (B0) gradients. The last session will be devoted to the presentation of some results obtained in our laboratory on the use of a particular test-object (constructed at the Deft Institute of Technology) for assessing signal localization efficiency provided by a surface coil in a small scale MRS/MRI equipment (for in vivo biochemical studies on small animals). The surface coil was used in conjunction with a number of sequence (t1-90 degrees-acq; 1-D "chemical shift imaging"; "depth") of either square or adiabatic r.f. pulses. The results allow the selection, on a quantitative basis, of the most appropriate pulse sequence(s) to be used for metabolic studies on superficial experimental tumours (implanted s.c. in small animals), according to the mean area and thickness of the neoplastic lesion. The latter parameters can be non-invasively assessed by previous MRI analysis of the tumour. This study was carried out in the frame of the EC COMAC-BME Concerted Research Project on Tissue Characterization by MRS and MRI.

Animals↗
PubMed1999

Comparison of four magnetic resonance methods for mapping small temperature changes.

Non-invasive detection of small temperature changes (< 1 degree C) is pivotal to the further advance of regional hyperthermia as a treatment modality for deep-seated tumours. Magnetic resonance (MR) thermography methods are considered to be a promising approach. Four methods exploiting temperature-dependent parameters were evaluated in phantom experiments. The investigated temperature indicators were spin-lattice relaxation time T1, diffusion coefficient D, shift of water proton resonance frequency (water PRF) and resonance frequency shift of the methoxy group of the praseodymium complex (Pr probe). The respective pulse sequences employed to detect temperature-dependent signal changes were the multiple readout single inversion recovery (T One by Multiple Read Out Pulses; TOMROP), the pulsed gradient spin echo (PGSE), the fast low-angle shot (FLASH) with phase difference reconstruction, and the classical chemical shift imaging (CSI). Applying these sequences, experiments were performed in two separate and consecutive steps. In the first step, calibration curves were recorded for all four methods. In the second step, applying these calibration data, maps of temperature changes were generated and verified. With the equal total acquisition time of approximately 4 min for all four methods, the uncertainties of temperature changes derived from the calibration curves were less than 1 degree C (Pr probe 0.11 degrees C, water PRF 0.22 degrees C, D 0.48 degrees C and T1 0.93 degrees C). The corresponding maps of temperature changes exhibited slightly higher errors but still in the range or less than 1 degree C (0.97 degrees C, 0.41 degrees C, 0.70 degrees C, 1.06 degrees C respectively). The calibration results indicate the Pr probe method to be most sensitive and accurate. However, this advantage could only be partially transferred to the thermographic maps because of the coarse 16 x 16 matrix of the classical CSI sequence. Therefore, at present the water PRF method appears to be most suitable for MR monitoring of small temperature changes during hyperthermia treatment.

Body Temperature↗