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At least 973 records · Page 54Linked to original sources
PubMed2000

Separation of quadrupolar and magnetic contributions to spin-lattice relaxation in the case of a single isotope

We present a NMR pulse double-irradiation method which allows one to separate magnetic from quadrupolar contributions in the spin-lattice relaxation. The pulse sequence fully saturates one transition while another is observed. In the presence of a ||Deltam || = 2 quadrupolar contribution, the intensity of the observed line is altered compared to a standard spin-echo experiment. We calculated analytically this intensity change for spins I = 1, 32, 52, thus providing a quantitative analysis of the experimental results. Since the pulse sequence we used takes care of the absorbed radiofrequency power, no problems due to heating arise. The method is especially suited when only one NMR sensitive isotope is available. Different cross-checks were performed to prove the reliability of the results obtained. The applicability of this method is demonstrated by a study of the plane oxygen (17)O (I = 52) in the high-temperature superconductor YBa(2)Cu(4)O(8): the (17)O spin-lattice relaxation rate consists of magnetic as well as quadrupolar contributions. Copyright 2000 Academic Press.

Journal Article↗
PubMed1989

[Experimental determination of weighting functions in magnetic resonance tomography].

Weighting functions are defined and calculated for the pulse sequences spin-echo and inversion recovery. These weighting functions make possible a qualitative discussion of the term "weighting" in MR imaging. Furthermore, a new method has been developed to determine weighting functions experimentally. A comparison between the results obtained and the theoretical predictions shows a sufficiently good agreement. This method can also be applied with other pulse sequences.

Humans↗
PubMed2003

In vivo proton MR three-dimensional T1rho mapping of human articular cartilage: initial experience.

The purpose of this study was to demonstrate the feasibility of computing three-dimensional relaxation maps of spin-lattice relaxation time in the rotating frame (T1rho) from in vivo magnetic resonance (MR) images of the human patellofemoral joint. T1rho was measured by applying a three-dimensional gradient-echo pulse sequence in six healthy subjects and one symptomatic subject by using a 1.5-T MR imager and a 15-cm-diameter transmit-receive quadrature birdcage radiofrequency coil. Average T1rho measured in healthy patellar cartilage was 49.7 msec +/- 3.2 (mean +/- SD). Two-dimensional T1rho-weighted images were obtained with a fast spin-echo pulse sequence for comparison. There was good correlation between two-dimensional and three-dimensional T1rho values for the six healthy subjects (R2 = 0.88, slope = 1.16).

Adult↗
PubMed1991

Generation of color composites for enhanced tissue differentiation in magnetic resonance imaging of the brain.

Currently, the diagnostic interpretation of magnetic resonance (MR) images requires that radiologists integrate specific tissue contrast information from several different images obtained at the same anatomic slice position. Each of these images has its own unique tissue contrast patterns which are based on the image acquisition parameters (pulse sequence) selected. The complex contrast patterns observable in these images reflect the inherent biophysical characteristics of the tissues and fluids present in the imaged section. In an effort to increase the diagnostic accuracy and efficiency of MR image interpretation, we have generated color composite images from quantitatively analyzed achromatic MR images of the brain, obtained while utilizing different pulse sequences. By using a DEC MicroVAX II computer with Interactive Digital Language (IDL), this color display method has been applied to images obtained from General Electric Signa and Siemens Magnatom imagers. For this study, our image sets included T1-weighted, T2-weighted, and proton density spin echo sequences as well as both high and low flip angle gradient echo sequences. Advantages of our color composite methods, in contrast to many other image processing techniques that have been described, are that minimal information is lost, computer misclassification of tissues is avoided, and the conspicuity of specific tissues is enhanced. Furthermore, with this method it is possible to produce composite images whose color renditions approach a natural anatomic tissue appearance. Availability of these color composites to radiologists may improve the efficiency and accuracy of the diagnostic interpretation of MR images.

Brain↗
PubMed2005

Magnetic resonance imaging of in vivo kinematics after total knee arthroplasty.

PURPOSE: To improve the quality of magnetic resonance imaging (MRI) on knees after total knee arthroplasty (TKA) by minimizing image artifacts caused by metallic implants, and to establish a method determining in vivo kinematics of TKA knees using MRI. MATERIALS AND METHODS: Two knee implants made of cobalt-chrome and oxidized zirconium were tested with different pulse sequences and imaging parameters. Then, in vivo kinematic MRI was performed on five well-functioning TKAs under simulated weight-bearing conditions. Kinematic measurements were made and a linear correlation test was run between the tibio- and patellofemoral measurements. RESULTS: The best images with minimum metallic artifacts were observed using oxidized zirconium implants, a fast spin echo sequence (FSE), thin slice thickness, and high readout gradient. TKA kinematics exhibited a large deviation from the normal kinematics and considerable patient-to-patient variability. However, significant linear correlations between tibiofemoral and patellofemoral kinematics were observed (R = -0.96, 0.92, 0.88). CONCLUSION: Metallic artifacts due to orthopedic implants can be reduced in MR images for some materials, appropriate pulse sequence, and imaging parameters selection, enabling MR quantification of knee kinematics. Tibiofemoral kinematics appears to affect patellofemoral position after total knee arthroplasty.

Aged↗
PubMed2003

Restricted diffusion in grossly inhomogeneous fields.

We analyze the effects of geometrical restriction on the nuclear magnetization of spins diffusing in grossly inhomogeneous fields where radio-frequency (RF) pulses are weak relative to the total field inhomogeneity, making the rotation angle space-dependent and thus exciting multiple coherence pathways. We show how to separate the effects of restricted diffusion from the effects of the pulses in the case when the change in the field experienced by a diffusing spin in the course of the experiment is small compared to the RF magnitude. We then derive explicit formulas for the contribution of individual coherence pathways to the total magnetization in arbitrary pulse sequences. We find that, for long diffusion times, restriction can dramatically alter the spectrum and the shape of a particular echo, while for short times, the correction will be proportional to the pore space surface-to-volume ratio. We demonstrate these results on the example of the early echoes of the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence.

Journal Article↗
PubMedDate not supplied

[Intracranial hemorrhage studied with a high-field (1.5T) NMR apparatus].

Fifty-two patients were studied with high-field magnetic resonance imaging (1.5T), with T1- and T2-weighted spin-echo pulse sequences. The study was aimed at assessing the efficacy of MR imaging in the evaluation of intracranial hematomas. Characteristic intensity patterns were observed in the evolution of the hematomas, due to the physicochemical changes in hemoglobin. No acute hematomas were observed. In 35 sub-acute hematomas, peripheral hyperintensity could be observed on T1- and T2-weighted pulse sequences. This hyperintensity eventually fills in the hematoma in the chronic stage. In 17 chronic hematomas, a peripheral hypointense ring due to hemosiderin deposits was seen on T1- and T2-weighted scans. The authors conclude that high-field MR imaging is a very sensible diagnostic method in the evaluation of sub-acute and chronic hematomas.

Adult↗
PubMed1993

[Usefulness of superparamagnetic iron oxide particles in the detection of hepatocellular carcinoma by MR imaging].

Eight patients with histologically or clinically diagnosed hepatocellular carcinoma and a total of 23 nodules were studied. SE 2000/20 (PDW-SE), SE-2000/80 (T2W-SE), SE 500/20 (T1W-SE), GRE 167/13/70 degrees (T1W-GE), and GRE 167/13/15 degrees (T2*W-GE) were used. Using each pulse sequence the pre-and post-contrast detection rates of the 23 nodules visualized by CT arterial portography (CTAP) were compared. With plain MRI, the detection rates were PDW-SE: 48%, T2W-SE: 74%, T1W-SE: 52%, T1W-GE 48% and T2*W-GE: 48%. After the administration of contrast medium all pulses sequences showed enhanced detection rates: 100%, 83%, 87%, 91%, and 96% respectively. Superparamagnetic iron oxide particle enhanced MRI showed a high detection rate comparable to CTAP, suggesting that this agent can make a major contribution to the diagnosis of HCC, particularly the detection of small HCC.

Carcinoma, Hepatocellular↗
PubMed2002

MR detection of mechanical vibrations using a radiofrequency field gradient.

A new method for NMR characterization of mechanical waves, based upon radiofrequency field gradient for motion encoding, is proposed. A binomial B1 gradient excitation scheme was used to visualize the mobile spins undergoing a periodic transverse mechanical excitation. A simple model was designed to simulate the NMR signal as a function of the wave frequency excitation and the periodicity of the NMR pulse sequence. The preliminary results were obtained on a gel phantom at low vibration frequencies (0-200 Hz) by using a ladder-shaped coil generating a nearly constant RF field gradient along a specific known direction. For very small displacements and/or B1 gradients, the NMR signal measured on a gel phantom was proportional to the vibration amplitude and the pulse sequence was shown to be selective with respect to the vibration frequency. A good estimation of the direction of vibrations was obtained by varying the angle between the motion direction and the B1 gradient. The method and its use in parallel to more conventional MR elastography techniques are discussed. The presented approach might be of interest for noninvasive investigation of elastic properties of soft tissues and other materials.

Magnetic Resonance Spectroscopy↗
PubMed2005

Detection of intracellular lactate with localized diffusion {1H-13C}-spectroscopy in rat glioma in vivo.

The aim of this study was to compare the diffusion characteristic of lactate and alanine in a brain tumor model to that of normal brain metabolites known to be mainly intracellular such as N-acetylaspartate or creatine. The diffusion of (13)C-labeled metabolites was measured in vivo with localized NMR spectroscopy at 9.4 T (400 MHz) using a previously described localization and editing pulse sequence known as ACED-STEAM ('adiabatic carbon editing and decoupling'). (13)C-labeled glucose was administered and the apparent diffusion coefficients of the glycolytic products, {(1)H-(13)C}-lactate and {(1)H-(13)C}-alanine, were determined in rat intracerebral 9L glioma. To obtain insights into {(1)H-(13)C}-lactate compartmentation (intra- versus extracellular), the pulse sequence used very large diffusion weighting (50 ms/microm(2)). Multi-exponential diffusion attenuation of the lactate metabolite signals was observed. The persistence of a lactate signal at very large diffusion weighting provided direct experimental evidence of significant intracellular lactate concentration. To investigate the spatial distribution of lactate and other metabolites, (1)H spectroscopic images were also acquired. Lactate and choline-containing compounds were consistently elevated in tumor tissue, but not in necrotic regions and surrounding normal-appearing brain. Overall, these findings suggest that lactate is mainly associated with tumor tissue and that within the time-frame of these experiments at least some of the glycolytic product ([(13)C] lactate) originates from an intracellular compartment.

Alanine↗
PubMed2002

Numerical calculations of switched magnetic field gradients during magnetic resonance imaging.

During magnetic resonance imaging (MRI) pulse-sequences the human body is exposed to switched magnetic gradient fields. These gradients become stronger and are switched faster for fast imaging. Effects resulting from these fields with trapezoidal waveforms are on the one hand sensory perception of induced currents and on the other hand muscular and cardiac stimulation. All three components of the current density induced by gradient pulse sequence were analysed in a high-resolution model of the human torso. The evaluation of the calculated data was performed thoroughly in the region of the heart muscle of the torso model to find out how different waveforms of the switched gradient field influence strength and direction of induced currents.

Electrocardiography↗
PubMed1992

[Value of MR imaging in the staging of rectal carcinoma].

Correlative studies of MRI and pathologic specimens were done in 35 patients with rectal cancer. The MR manifestations of the primary tumor and its invasion into the surrounding structures were investigated with reference to the staging of rectal cancer. Prone positioning and the procedure of hypotonic air-distension of rectum was the method of choice to depict the primary tumor and tumor invasion. The spin-echo (SE)pulse sequence with TR/TE: 500/32 ms (T1-weighted image) was selected to show the anatomical structures in the pelvis and tumor spread in the surrounding fatty space and lymph node metastasis. Owing to the reduced contrast between tumor invasion and fatty tissue and decreasing signal intensity on multi-echo T2-weighted images the long repetiting time (TR) pulse sequence could not provide significant contribution in tumor staging. The MR appearance of rectal carcinoma was categorized as polypoidnodular, cauliflowermassive and protuberant-ring types. Ulceration was often seen in the latter two types. Peripheral invasion often manifested as spotty-nodular, sawtoothed-wavy and tumefied shape with medium signal intensity on T1-weighted images. The presence of a lump of small nodes, round or oval nodules within 2 cm from the rectal wall or nodular mass in the perirectal fatty space could be considered as possible lymph node metastasis. Following the modified Dukes Staging System of rectal cancer proposed by Astler-Coller all patients were staged preoperatively and correlated with surgical specimens. The accuracy of staging was 74.3%, compatible with the results of studies published.

Adenocarcinoma↗
PubMed2004

ODIN-object-oriented development interface for NMR.

A cross-platform development environment for nuclear magnetic resonance (NMR) experiments is presented. It allows rapid prototyping of new pulse sequences and provides a common programming interface for different system types. With this object-oriented interface implemented in C++, the programmer is capable of writing applications to control an experiment that can be executed on different measurement devices, even from different manufacturers, without the need to modify the source code. Due to the clear design of the software, new pulse sequences can be created, tested, and executed within a short time. To post-process the acquired data, an interface to well-known numerical libraries is part of the framework. This allows a transparent integration of the data processing instructions into the measurement module. The software focuses mainly on NMR imaging, but can also be used with limitations for spectroscopic experiments. To demonstrate the capabilities of the framework, results of the same experiment, carried out on two NMR imaging systems from different manufacturers are shown and compared with the results of a simulation.

Brain Mapping↗
PubMed2003

Contrast-enhanced magnetic resonance angiography: development and optimization of techniques for paramagnetic and hyperpolarized contrast media.

Contrast-enhanced magnetic resonance angiography (CE-MRA) is a diagnostic method for imaging of vascular structures based on nuclear magnetic resonance. Vascular enhancement is achieved by injection of a contrast medium (CM). Studies were performed using two different types of CM: conventional paramagnetic CM, and a new type of CM based on hyperpolarized (HP) nuclei. The effects of varying CM concentration with time during image acquisition were studied by means of computer simulations using two different models. It was shown that a rapid concentration variation during encoding of the central parts of k-space could result in signal loss and severe image artifacts. The results were confirmed qualitatively with phantom experiments. A postprocessing method was developed to address problems with simultaneous enhancement of arteries and veins in CE-MRA of the lower extremities. The method was based on the difference in flow-induced phase in the two vessel types. Evaluation of the method was performed with flow phantom measurements and with CE-MRA in two volunteers using standard pulse sequences. The flow-induced phase in the vessels of interest was sufficient to distinguish arteries from veins in the superior-inferior direction. Using this method, the venous enhancement could be extinguished. The possibility of using HP nuclei as CM for CE-MRA was evaluated. Signal expressions for a flow of HP CM imaged with a gradient echo sequence were derived. These signal expressions were confirmed in phantom experiments using HP 129Xe dissolved in ethanol. Studies were also performed with a new CM based on HP 13C. The CM had very long relaxation times (T1, in vivo/T2, in vivo approximately 38/1.3 s). The long relaxation times were utilized in imaging with a fully balanced steady-state free precession pulse sequence (trueFISP), where the optimal flip angle was found to be 180 degrees. CE-MRA with the 13C-based CM in rats resulted in images with high vascular SNR (approximately 500). CE-MRA is a useful clinical tool for diagnosing vascular disease. With the development of new contrast media, based on hyperpolarized nuclei for example, there is a potential for further improvement in the signal levels that can be achieved, enabling a standard of imaging of vessels that is not possible today.

Animals↗
PubMed2004

Second order average Hamiltonian theory of symmetry-based pulse schemes in the nuclear magnetic resonance of rotating solids: application to triple-quantum dipolar recoupling.

The average Hamiltonian theory (AHT) of several classes of symmetry-based radio-frequency pulse sequences is developed to second order, allowing quantitative analyses of a wide range of recoupling and decoupling applications in magic-angle-spinning solid state nuclear magnetic resonance. General closed analytical expressions are presented for a cross term between any two interactions recoupled to second order AHT. We classify them into different categories and show that some properties of the recoupling pulse sequence may be predicted directly from this classification. These results are applied to examine a novel homonuclear recoupling strategy, effecting a second order average dipolar Hamiltonian comprising trilinear triple quantum (3Q) spin operators. We discuss general features and design principles of such 3Q recoupling sequences and demonstrate by numerical simulations and experiments that they provide more efficient excitation of (13)C 3Q coherences compared to previous techniques. We passed up to 15% of the signal through a state of 3Q coherence in rotating powders of uniformly (13)C-labeled alanine and tyrosine. Second order recoupling-based (13)C homonuclear 3Q correlation spectroscopy is introduced and demonstrated on tyrosine.

Journal Article↗
PubMed2005

Spherical tensor analysis of nuclear magnetic resonance signals.

In a nuclear magnetic-resonance (NMR) experiment, the spin density operator may be regarded as a superposition of irreducible spherical tensor operators. Each of these spin operators evolves during the NMR experiment and may give rise to an NMR signal at a later time. The NMR signal at the end of a pulse sequence may, therefore, be regarded as a superposition of spherical components, each derived from a different spherical tensor operator. We describe an experimental method, called spherical tensor analysis (STA), which allows the complete resolution of the NMR signal into its individual spherical components. The method is demonstrated on a powder of a (13)C-labeled amino acid, exposed to a pulse sequence generating a double-quantum effective Hamiltonian. The propagation of spin order through the space of spherical tensor operators is revealed by the STA procedure, both in static and rotating solids. Possible applications of STA to the NMR of liquids, liquid crystals, and solids are discussed.

Journal Article↗
PubMed1984

MR imaging of orbital tumors with CT and ultrasound correlations.

Fourteen patients with various orbital space-occupying lesions were studied by magnetic resonance (MR) imaging with a 0.15 T imager using spin-echo and inversion-recovery pulse sequences. Computed tomographic and ultrasonographic (US) examinations were done in 13 of these patients. All 14 lesions could be demonstrated by MR. Magnetic resonance was found to be better than US but not as good as CT in providing morphological details. Regarding tissue characterization, US was the most useful among the three modalities. With the T2-weighted spin-echo pulse sequence, a possible trend in tissue characterization could be shown. From our preliminary experience, we conclude that MR imaging using a 0.15 T resistive magnet offers no distinct advantage over the combination of CT and US in the diagnosis of orbital tumors.

Adult↗
PubMedDate not supplied

MR velocity imaging by phase display.

The ability of the nuclear magnetic resonance signal to encode information about macroscopic motion has been recognized since the works of Hahn and Carr and Purcell. In the medical imaging setting this ability has led to a variety of ingenious magnetic resonance flow imaging schemes that ultimately may become competitive with X-ray angiography in sensitivity and specificity while remaining radically noninvasive. This work demonstrates that conventional spin-echo Fourier transform image acquisitions naturally encode a component of flow velocity that lies within the image plane. By displacing just the real part of the complex image data (phase display), the velocity distribution within the subject is revealed in image form. This method of flow imaging requires neither special pulse sequences nor image reconstruction and format software for its implementation. Further, images that intersect a flow channel longitudinally, demonstrating in-plane flow, yield an unusually large quantity of physiologic information per image. Phantom and in vivo flow images are presented. Also described is a phantom based on a rotating disk that enables calibration of the velocity/phase-shift constant for an untested pulse sequence.

Data Display↗