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PubMed2001

Double-quantum-filtered NMR signals in inhomogeneous magnetic fields.

The possibility of exciting and detecting proton NMR double-quantum coherences in inhomogeneous static and radiofrequency magnetic fields was investigated. For this purpose specialized pulse sequences which partially refocus the strongly inhomogeneous evolution of the spin system and generate double-quantum buildup and decay curves were implemented on the NMR MOUSE (mobile universal surface explorer). The theoretical justification of the method was developed for the simple two-spin-1/2 system. The performances of the same pulse sequences were also tested on a solid-state high-field NMR spectrometer. It was shown that DQ decay curves have a better signal-to-noise ratio in the initial time regime than DQ buildup curves. The double-quantum buildup and decay curves were recorded for a series of cross-linked natural rubber samples. These curves give access to quantitative values of the ratio of proton total residual dipolar couplings which are in good agreement with those measured in homogeneous fields. A linear dependence of these ratios on the sulfur-accelerator content was found.

Journal Article↗

PubMed1998

Subcutaneous granuloma annulare: radiologic appearance.

OBJECTIVE: Granuloma annulare is an uncommon benign inflammatory dermatosis characterized by the formation of dermal papules with a tendency to form rings. There are several clinically distinct forms. The subcutaneous form is the most frequently encountered by radiologists, with the lesion presenting as a superficial mass. There are only a few scattered reports of the imaging appearance of this entity in the literature. We report the radiologic appearance of five cases of subcutaneous granuloma annulare. DESIGN AND PATIENTS: The radiologic images of five patients (three male, two female) with subcutaneous granuloma annulare were retrospectively studied. Mean patient age was 6.4 years (range, 2-13 years). The lesions occurred in the lower leg (two), foot, forearm, and hand. MR images were available for all lesions, gadolinium-enhanced imaging in three cases, radiographs in four, and bone scintigraphy in one. RESULTS: Radiographs showed unmineralized nodular masses localized to the subcutaneous adipose tissue. The size range, in greatest dimension on imaging studies, was 1-4 cm. MR images show a mass with relatively decreased signal intensity on all pulse sequences, with variable but generally relatively well defined margins. There was extensive diffuse enhancement following gadolinium administration. CONCLUSION: The radiologic appearance of subcutaneous granuloma annulare is characteristic, typically demonstrating a nodular soft-tissue mass involving the subcutaneous adipose tissue. MR images show a mass with relatively decreased signal intensity on all pulse sequences and variable but generally well defined margins. There is extensive diffuse enhancement following gadolinium administration. Radiographs show a soft-tissue mass or soft-tissue swelling without evidence of bone involvement or mineralization. This radiologic appearance in a young individual is highly suggestive of subcutaneous granuloma annulare.

Adipose Tissue↗

PubMed2004

ECG-gated 23Na-MRI of the human heart using a 3D-radial projection technique with ultra-short echo times.

Pathological changes in tissue often manifest themselves in an altered sodium gradient between intra- and extracellular space due to a malfunctioning Na+-K+ pump, resulting in an increase in total sodium concentration in ischaemic regions. Therefore, 23Na-MRI has the potential to non-invasively differentiate viable from non-viable tissue by detecting concentration changes of intra- and extracellular sodium. As the in vivo sodium signal shows a bi-exponential T2 decay, with a short component of less than 1 ms, the accurate quantification of the total sodium content requires imaging techniques with ultra-short echo times (TE) below 0.5 ms. A 3D-radial projection technique has been developed which allows the acquisition of ECG-triggered sodium images of the human heart with a TE of 0.4 ms. With this pulse sequence 23Na-MRI volunteer measurements of the head or the heart were performed in less than 18 min on a 1.5-T clinical scanner with an isotropic resolution of 10 mm3. The signal to noise ratio of the radial projection technique is twofold higher than that of a Cartesian gradient echo pulse sequence (TE = 3.2 ms). Radial 23Na-MRI provides a tool for clinical studies, aiming at the differentiation of viable and non-viable tissue.

Brain↗

PubMed2005

Rapid high-resolution four-dimensional NMR spectroscopy using the filter diagonalization method and its advantages for detailed structural elucidation of oligosaccharides.

Four-dimensional nuclear magnetic resonance spectroscopy with high resolution of signals in the indirect dimensions is reported as an implementation of the filter diagonalization method (FDM). Using an oligosaccharide derivatized with 13C-labeled acetyl isotags, a four-dimensional constant-time pulse sequence was tailored for conjoint use with the FDM. Results demonstrate that high resolution in all dimensions can be achieved using a relatively short experimental time period (19 h), even though the spectrum is highly congested in the direct and all three indirect dimensions. The combined use of isotags, constant-time pulse sequences, and FDM permits rapid isolation of sugar ring proton spin systems in multiple dimensions and enables all endocyclic J-couplings to be simply measured, the key goal to assigning sugar stereochemistry and anomeric configuration. A general method for rapid, unambiguous elucidation of spin systems in oligosaccharides has been a long-sought goal of carbohydrate NMR, and isotags combined with the FDM now enable this to be easily performed. Additional general advantages of the FDM program for generating high-resolution 2D slices in any dimension from a 4D spectrum are emphasized.

Carbon Isotopes↗

PubMed1995

MR contrast due to intravascular magnetic susceptibility perturbations.

A particularly powerful paradigm for functional MR imaging of microvascular hemodynamics incorporates paramagnetic materials that create significant image contrast. These include exogenous (lanthanide chelates) and endogenous (deoxygenated hemoglobin) agents for mapping cerebral blood volume and neuronal activity, respectively. Accurate interpretation of these maps requires an understanding of the biophysics of susceptibility-based image contrast. The authors developed a novel Monte Carlo model with which the authors quantified the relationship between microscopic tissue parameters, NMR imaging parameters, and susceptibility contrast in vivo. The authors found vascular permeability to water and the flow of erythrocytes to be relatively unimportant contributors to susceptibility-induced delta R2. However, pulse sequence, echo time, and concentration of contrast agent have profound effects on the vessel size dependence of delta R2. For a model vasculature containing both capillaries and venules, the authors predicted a linear volume fraction dependence for physiological volume changes based on recruitment and dilation, and a concentration dependence that is nonlinear and pulse sequence dependent. Using the model, the authors demonstrated that spin echo functional images have greater microvascular sensitivity than gradient echo images, and that the specifies of the volume fraction and concentration dependence of transverse relaxivity change should allow for robust mapping of relative blood volume. The authors also demonstrated excellent agreement between the predictions of their model and experimental data obtained from the serial injection of superparamagnetic contrast agent in a rat model.

Animals↗

PubMed1983

Nuclear magnetic resonance imaging of the kidney.

The role of nuclear magnetic resonance (NMR) imaging of the kidney was analyzed in 18 persons (6 normal volunteers, 3 patients with pelvocaliectasis, 2 with peripelvic cysts, 1 with renal sinus lipomatosis, 3 with renal failure, 1 with glycogen storage disease, and 2 with polycystic kidney disease). Ultrasound and/or computed tomography (CT) studies were available for comparison in every case. In the normal kidney distinct anatomical structures were clearly differentiated by NMR. The best anatomical detail was obtained with spin echo (SE) imaging, using a pulse sequence interval of 1,000 msec and an echo delay time of 28 msec. However, in the evaluation of normal and pathological conditions, all four intensity images (SE 500/28, SE 500/56, SE 1,000/28, and SE 1,000/56) have to be analyzed. No definite advantage was found in using SE imaging with a pulse sequence interval of 1,500 msec. Inversion recovery imaging enhanced the differences between the cortex and medulla, but it had a low signal-to-noise level and, therefore, a suboptimal overall resolution. The advantages of NMR compared with CT and ultrasound are discussed, and it is concluded that NMR imaging will prove to be a useful modality in the evaluation of renal disease.

Glomerulonephritis↗

PubMed1999

Prospective MR signal-based cardiac triggering.

A cardiac motion compensation method using magnetic resonance signal-based triggering is presented. The method interlaces a triggering pulse sequence with an imaging sequence. The triggering sequence is designed to measure aortic blood velocity, from which cardiac phase can be inferred. The triggering sequence is executed repeatedly and the acquired data processed after each sequence iteration. When the desired phase of the cardiac cycle is detected, data are acquired using the imaging sequence. A signal-processing unit of a conventional scanner is used to process the triggering data in real time and issue triggering commands. Alternatively, a workstation, with a bus adaptor, can access data as they are acquired, process and display the data, and issue triggering commands. With a graphical user interface, the triggering pulse sequence and data-processing techniques can be modified instantaneously to optimize triggering. The technique is demonstrated with coronary artery imaging using both conventional two-dimensional Fourier transform scans and spiral trajectories.

Aorta, Thoracic↗

PubMed2001

Magnetic resonance imaging of injected silicone: findings in seven male-to-female transsexuals.

Our goal was to describe the MR findings of subcutaneously injected silicone in male-to-female transsexuals and to determine the most optimal combination of pulse sequences for discrimination of injected silicone from the surrounding structures. Seven male-to-female transsexuals were referred for MRI in order to detect the injected silicone in the subcutaneous tissues and to visualize distant migration and possible complications. Conventional spin-echo (SE) T1-weighted and turbo SE proton-density/T2-weighted, plain short tau inversion recovery (STIR), and two STIR sequences with, respectively, water and silicone suppression were applied at 1.5 T. Three observers trained in MRI analysed the images separately for the abnormalities and rated the images using a four-level scale. The STIR with water suppression provided the highest contrast between silicone and the surrounding adipose tissue. Silicone-specific STIR pulse sequences provided clear images for the evaluation of the amount and patterns of local and distant spread of injected fluid silicone. Magnetic resonance imaging readily identifies the subcutaneously injected silicone, its distant migration and the amount of tissue disorganization around the silicone deposits. Two silicone-specific STIR sequences are usually sufficient for imaging the spread of silicone.

Adult↗

PubMed2006

Effective dipolar couplings determined by dipolar dephasing of double-quantum coherences.

It is shown how homonuclear distances and homonuclear dipolar lattice sums between spin-1/2 nuclei can be measured by a pulsed solid-state NMR experiment under magic-angle spinning conditions. The presented technique is based on double-quantum coherence filtering. Instead of measuring a build-up of double-quantum coherence the pulse sequence is designed to dephase double-quantum coherence. This is achieved by exciting double-quantum coherence either with the help of the through-space dipolar coupling or the through-bond dipolar coupling while the dephasing relies on the through-space dipolar coupling as selected by a gamma-encoded pulse sequence from the C/R symmetry class. Since dephasing curves can be normalized on zero dephasing, it is possible to analyze the initial dephasing regime and hence determine dipolar lattice sums (effective dipolar couplings) in multiple-spin systems. A formula for the effective dipolar coupling is derived theoretically and validated by numerical calculations and experiments on crystalline model compounds for (13)C and (31)P spin systems. The double-quantum dephasing experiment can be combined with constant-time data sampling to compensate for relaxation effects, consequently only two experimental data points are necessary for a single distance measurement. The phase cycling overhead for the constant-time experiment is minimal because a short cogwheel phase cycle exists. A 2D implementation is demonstrated on [(13)C(3)]alanine.

Alanine↗

PubMed1984

A strategy for magnetic resonance imaging of the head: results of a semi-empirical model. Part II.

A semi-empirical model was used to identify specific pulse sequences that cause most lesions to appear distinctly brighter than normal tissues in magnetic resonance (MR) images of the head. Clinical trials confirm the utility of these sequences for patient screening. As a result, a strategy for effective and efficient MR imaging of the head is proposed. The previously described gray-scale model has been modified to account for the effect of image noise. By means of computer simulation, 13,800 different hypothetical cerebral lesions were imaged with a variety of pulse sequences. A number of conclusions resulted. First, two sequences are expected to be sufficient to visualize most intracranial lesions, a "diagonal" SE sequence (e.g., SE 2500/80) and an IR sequence with a short inversion time (e.g., IR 1800/200). These sequences are orthogonal, i.e., lesions missed by one are likely to be detected by the other. Second, signal averaging the screening sequences is expected to be more effective than optimized sequences when lesion tissue parameters differ little from brain. Finally, the effectiveness of unaveraged screening sequences suggests that improved signal-to-noise ratio (SNR) is not necessary for the detection of most large lesions. Therefore, the increased SNR achievable through signal averaging or increased field strength might best be utilized to improve spatial resolution so that smaller lesions can be detected.

Brain Diseases↗

PubMed2001

An improved MR imaging technique for the visualization of myocardial infarction.

PURPOSE: To design a segmented inversion-recovery turbo fast low-angle shot (turboFLASH) magnetic resonance (MR) imaging pulse sequence for the visualization of myocardial infarction, compare this technique with other MR imaging approaches in a canine model of ischemic injury, and evaluate its utility in patients with coronary artery disease. MATERIALS AND METHODS: Six dogs and 18 patients were examined. In dogs, infarction was produced and images were acquired by using 10 different pulse sequences. In patients, the segmented turboFLASH technique was used to acquire contrast material-enhanced images 19 days +/- 7 (SD) after myocardial infarction. RESULTS: Myocardial regions of increased signal intensity were observed in all animals and patients at imaging. With the postcontrast segmented turboFLASH sequence, the signal intensity of the infarcted myocardium was 1,080% +/- 214 higher than that of the normal myocardium in dogs-nearly twice that of the next best sequence tested and approximately 10-fold greater than that in previous reports. All 18 patients with myocardial infarction demonstrated high signal intensity at imaging. On average, the signal intensity of the high-signal-intensity regions in patients was 485% +/- 43 higher than that of the normal myocardium. CONCLUSION: The segmented inversion-recovery turboFLASH sequence produced the greatest differences in regional myocardial signal intensity in animals. Application of this technique in patients with infarction substantially improved differentiation between injured and normal regions.

Adult↗

PubMed2003

Subchondral bone and cartilage thickness from MRI: effects of chemical-shift artifact.

Magnetic resonance imaging (MRI) is the modality of choice for visualizing and quantifying articular cartilage thickness. However, difficulties persist in MRI of subchondral bone using spoiled gradient-echo (SPGR) and other gradient-echo sequences, primarily due to the effects of chemical-shift artifact. Fat suppression techniques are often used to reduce these artifacts, but they prevent measurement of bone thickness. In this report, we assess the magnitude of chemical-shift effects (phase-cancellation and misregistration artifacts) on subchondral bone and cartilage thickness measurements in human femoral heads using a variety of pulse sequence parameters. Phase-cancellation effects were quantified by comparing measurements from in-phase images (TE=13.5 ms) to out-of-phase images (TE=15.8 ms). We also tested the assumption of the optimal in-phase TE by comparing thickness measures at small variations on TE (13.0, 13.5 and 14.0 ms). Misregistration effects were quantified by comparing measurements from water+fat images (water-only+fat-only images) to the measurements from in-phase (TE=13.5) images. A correction algorithm was developed and applied to the in-phase measurements and then compared to measurements from water+fat images. We also compared thickness measurements at different image resolutions. Results showed that both phase-cancellation artifact and misregistration artifact were significant for bone thickness measurement, but not for cartilage thickness measurement. Using an in-phase TE and correction algorithm for misregistration artifact, the errors in bone thickness relative to water+fat images were non-significant. This information may be useful for developing pulse sequences for optimal imaging of both cartilage and subchondral bone.

Adipose Tissue↗

PubMed1997

ROESY with water flip back for high-field NMR of biomolecules.

We report a version of the ROESY experiment in which saturation of the water magnetization is avoided without compromising suppression of the water signal during acquisition. Field gradient and selective RF pulses are used to maintain precise control of the water magnetization throughout the experiment and avoid signal losses due to radiation damping and molecular diffusion effects. The pulse sequence includes a delay for intentional radiation damping prior to mixing period. The optimal length of this delay is field and sample dependent, but easily determined from the apparent linewidth of the water signal. NOESY and TOCSY variants of the same experiment are presented which make use of identical manipulations of the water magnetization. The three pulse sequences constitute a suite for which little parameter adjustment is required once one of the experiments has been configured.

Electromagnetic Fields↗

PubMed2006

Improving whole brain structural MRI at 4.7 Tesla using 4 irregularly shaped receiver coils.

Both higher magnetic field strengths (> or =3 T) and multiple receiver "array coils" can provide increased signal-to-noise ratio (SNR) for MRI. This increase in SNR can be used to obtain images with higher resolution, enabling better visualisation of structures within the human brain. However, high field strength systems also suffer from increased B(1) non-uniformity and increased power deposition, reaching specific absorption rate (SAR) limits more quickly. For these problems to be mitigated, a careful choice of both the pulse sequence design and transmit RF coil is required. This paper describes the use of a prototype array coil consisting of 4 irregularly shaped coils within a standard configuration for neuroimaging at 4.7 T (a head transmit/receive volume coil to minimise SAR and a head gradient insert for maximum gradient performance). With a fast spin echo (FSE) pulse sequence optimised for 4.7 T, this provides dramatically increased quality and resolution over a large brain volume. Using the array coil, a SNR improvement relative to the volume coil of 1-1.5 times in central brain areas and 2-3 times in cortical regions was obtained. Array coil images with a resolution of 352 x 352 x 2000 mum had a SNR of 16.0 to 26.2 in central regions and 19.9 to 34.8 in cortical areas. Such images easily demonstrate cortical myeloarchitecture, while still covering most of the brain in a approximately 12 min scan.

Algorithms↗

PubMed2000

Transverse relaxation optimized 3D and 4D 15n/15N separated NOESY experiments of 15N labeled proteins.

NMR studies of protein structures require knowledge of spectral assignments through correlation spectroscopy and the measurement of dipolar interactions by NOESY-type experiments. In order to obtain NOEs for protons with degenerate chemical shifts, which is particularly common for large proteins with significant helical content, 3D and 4D 15N/15N separated NOESY experiments (HSQC-NOESY-HSQC) are essential for NMR studies of these proteins. TROSY sections could replace the latter or both HSQC parts of the 3D and 4D 15N/15N separated HSQC-NOESY-HSQC pulse sequences to enhance signal sensitivity and improve resolution. For a 1.0 mM, 100% 15N and 70% 2H-labeled Trichosanthin sample (approximately 27 kDa) at 5 degrees C it is found that sensitivity enhancements could only be obtained when TROSY sections replace the latter HSQC parts of 3D and 4D 15N/15N separated HSQC-NOESY-HSQC pulse sequences. The sensitivities of 3D and 4D HSQC-NOESY-TROSY experiments are enhanced by 62% and 8% at 5 degrees C, respectively, compared to their corresponding 3D and 4D HSQC-NOESY-HSQC experiments. Furthermore, the corresponding linewidths are, on average, decreased by 20% and 18% Hz in the HN and N2 dimensions, respectively. This enhancement of sensitivity depends on the molecular mass of the sample used and the lengths of the evolution times in the indirectly and directly detected dimensions.

Deuterium↗

PubMed2001

[Perfusion MR imaging of the heart with TrueFISP].

OBJECTIVE: Development and test of a saturation-recovery TrueFISP (SR-Trufi) pulse sequence for myocardial perfusion MR imaging (MRI) using improved gradient hardware. MATERIAL AND METHODS: Measurements were performed on a 1.5 T scanner with prototype gradients (50 mT/m, minimum rise time 300 microseconds). T1-weighted first-pass MRI of Gd-DTPA (0.025 mumol/kg) kinetics in the myocardium was performed using an SR-Trufi pulse sequence (TR/TE/alpha = 2.6 ms/1.4 ms/55 degrees) with a saturation preparation of TD = 30 ms before the TrueFISP readout. Measurements were performed in volunteers (n = 4) and in a pig model of chronic ischemia (n = 1). RESULTS: In phantoms, the signal intensity was linear with contrast concentration up to 0.9 mmol/kg Gd-DTPA. MR images obtained with SR-Trufi had a good image quality and high spatial resolution of 2.1 mm x 2.1 mm. Differences of the contrast agent's kinetics between a subendocardial perfusion deficit and neighboring myocardium were well visible on both MR images and signal-time curves derived from the region-of-interest analysis. CONCLUSION: SR-Trufi appears to be an interesting new technique for the assessment of myocardial microcirculation using dedicated cardiovascular MR systems.

Adult↗

PubMed1987

Evaluation of pleural and pericardial effusions by magnetic resonance imaging.

MR examinations of 36 patients with pleural and/or pericardial effusions were retrospectively evaluated. The purpose of this study was to determine of MR imaging is capable of differentiating between pleural and pericardial effusions of different compositions using standard electrocardiogram (ECG)-gated and non-gated spin echo pulse sequences. Additional data was obtained from experimental pleural effusions in 10 dogs. The results of this study indicate that old hemorrhages into the pleural or pericardial space can be differentiated from other pleural or pericardial effusions. However, further differentiation between transudates, exudates and sanguinous effusions is not possible on MR images acquired with standard spin echo pulse sequences. Respiratory and cardiac motion are responsible for signal loss, particularly on first echo images. This was documented in experiments in dogs with induced effusions of known composition; "negative" T2 values consistent with fluid motion during imaging sequences were observed in 80% of cases. However, postmortem studies of the dogs with experimental effusions showed differences between effusions with low protein concentrations and higher protein concentrations. We conclude from our study that characterization of pleural and pericardial effusions on standard ECG-gated and non-gated MR examinations is limited to the positive identification of hemorrhage. Motion of the fluid due to cardiac and respiratory activity causes artifactual and unpredictable changes in intensity values negating the more subtle differences in intensity associated with increasing protein content.

Animals↗

PubMed1996

Diffusion weighted fMRI at 1.5 T.

Functional magnetic resonance imaging (fMRI) is capable of detecting task-induced blood oxygenation changes using susceptibility sensitive pulse sequences such as gradient-recalled echo-planar imaging (EPI). The local signal increases seen in the time course are believed to be due to an increase in oxygen delivery that is incommensurate with oxygen demands. To help isolate the sources of functional signal changes, the authors have incorporated various forms of diffusion weighting into EPI pulse sequences to characterize the apparent mobility of the functionally modulated protons. Results suggest that the majority of the functional signal at 1.5 T arises from protons that have apparent diffusion coefficients that are approximately four or five times higher than that of brain tissue. This implies that significant functional signal sources are either protons within the vascular space or protons from the perivascular space that is occupied by cerebrospinal fluid.

Adult↗