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At least 1,387 records · Page 77Linked to original sources
PubMed2004

Magnetic resonance angiography is equivalent to X-ray coronary angiography for the evaluation of coronary arteries in Kawasaki disease.

OBJECTIVES: The purpose of this study was to compare the results of magnetic resonance angiography (MRA) with X-ray coronary angiography (XCA) in a pediatric population. BACKGROUND: Coronary artery abnormalities in Kawasaki disease (KD) develop in about 15% to 25% of young patients, mostly in the form of aneurysms. METHODS: Thirteen patients (12 male), age three to eight years, were studied. The maximal diameter and length of the aneurysm were recorded. Coronary MRA was performed using a 1.5 T Philips Intera CV magnetic resonance scanner with an electrocardiographically triggered pulse sequence. It was a three-dimensional segmented k-space gradient-echo sequence (TE = 2.1 ms, TR = 7.5 ms, flip angle = 30 degrees, slice thickness = 1.5 mm) employing a T2-weighted preparation pre-pulse and a frequency selective fat-saturation pre-pulse. Data acquisition was performed in mid-diastole. All scans were carried out with the patient free breathing using a two-dimensional real-time navigator beam. All patients underwent XCA within a week. RESULTS: In six patients, aneurysms of the coronary arteries were identified, while coronary ectasia alone was present in the remaining seven patients. Magnetic resonance angiography and XCA diagnosis of coronary artery aneurysm agreed completely. Maximal aneurysm diameter and length and ectasia diameter by MRA and XCA were similar. No stenotic lesion was identified by either technique. CONCLUSIONS: In conclusion, MRA is a reliable diagnostic tool, equivalent to XCA for coronary artery aneurysm identification in patients with KD. Magnetic resonance angiography may prove to be of great value for the serial non-invasive evaluation of these patients.

Child↗
PubMed1997

Measurement of magnetic resonance T2 for physiological experiments.

The proton transverse relaxation time (T2) of human skeletal muscles has been increasingly used in magnetic resonance imaging experiments to examine muscle physiology and neuromuscular control. However, little attention has been paid to the experimental factors affecting the accuracy or sensitivity of the T2 measurement. We have explored theoretically and experimentally the structure of several magnetic resonance pulse sequences for measuring T2 of the first dorsal interosseous muscle and found that a multiecho imaging technique using non-slice-selective refocusing pulses (MENSS) produces more accurate T2 estimates than multiecho slice-selective (MESS) imaging methods that are commonly used. Using either technique we acquired four 5-mm-thick transverse images of the first dorsal interosseous muscle with a spatial resolution of 0.6 mm within 5 min. The T2 measured by the MENSS method was closer to the true T2 than was the T2 estimated by the MESS method. After a given amount of exercise, the MENSS technique revealed an average 28 +/- 10% increase in T2 compared with a 13 +/- 3% increase measured with an equivalent MESS technique. We conclude that the MENSS method is a more accurate and sensitive procedure for studying neuromuscular physiology compared with the more commonly used MESS method.

Adult↗
PubMed1998

MR angiography of the thoracic aorta.

MR angiography is a powerful tool in evaluating the complete spectrum of congenital and acquired diseases of the thoracic aorta. The use of gadolinium-chelates in conjunction with rapid three-dimensional sequences performed with breath holding provides robust image quality with multiple display options. As a result of technological advances in hardware, software, and pulse sequence implementation combined with innovative use of contrast agents, MR angiography is poised to replace conventional catheter angiography of the thoracic aorta for many indications.

Aorta, Thoracic↗
PubMed2001

Steady-state preparation for spoiled gradient echo imaging.

While spoiled gradient echo sequences provide a rapid means of acquiring T(1)-weighted images, it is often desirable that the magnetization be in the steady state to avoid artifacts. For some applications, this requires many "dummy" repetitions of the pulse sequence prior to data collection, delaying image acquisition. A method is presented in which a saturation pulse, followed by a prescribed recovery period, places longitudinal magnetization levels of all materials near steady state, ready for data acquisition much sooner than when employing only dummy repetitions to achieve steady state. Effects of transverse coherences are studied using configuration theory. The method is shown to be effective in both phantom studies and in vivo applications, including real-time imaging, multiphase cardiac imaging, and triggered contrast-enhanced angiography. Magn Reson Med 45:653-661, 2001.

Humans↗
PubMed2003

Steady-state sequence synthesis and its application to efficient fat-suppressed imaging.

A new synthesis algorithm, based on the Shinnar-Le Roux (SLR) transform, can be used to generate fully refocused steady-state pulse sequences with arbitrary magnetization profiles as a function of off-resonant precession. This is accomplished by appropriate periodic oscillation of the RF excitation magnitude and phase from echo to echo. The technique is applied to the design of refocused steady-state free precession (SSFP) sequences with flat profiles, providing the opportunity for banding-artifact-free imaging with steady-state contrast. The algorithm is also used to generate refocused-SSFP sequences with an arbitrarily broad region of attenuated signal. These sequences are implemented and applied to the problem of steady-state fat suppression. Preliminary results show signal levels that agree well with theory, and a broad region of suppressed signal at each echo. Total imaging time is kept identical to that of a standard refocused-SSFP experiment through echo equalization and interleaving. 3D images from the leg of a normal volunteer acquired in 44 s demonstrate the applicability of the technique to fat-suppressed imaging.

Adipose Tissue↗
PubMedDate not supplied

Visualization of magnetic resonance-compatible needles at 1.5 and 0.2 Tesla.

PURPOSE: For two types of passively visualizable magnetic resonance (MR)-compatible needles, the size of susceptibility artifacts was investigated at 0.2 and 1.5 Tesla (T) and assessed regarding their suitability for needle visualization. METHODS: Phantom trials were performed using T1-weighted spin echo (SE), turbospin echo (TSE) and gradient echo (GE) sequences and different angles beta between the needles and the main magnetic field (B0). RESULTS: Depending on the needle angle beta and the applied pulse sequence, we found artifact diameters of 0-9.7 mm employing SE, of 1.7-9.4 mm employing TSE, and of 1.4-20.6 mm employing GE at 1.5 T. At 0.2 T, we found artifact diameters of 0-5.7 mm employing SE, of 0-6.3 mm employing TSE, and of 0-11.3 mm employing GE. CONCLUSION: Comparing artifact sizes at 1. 5 T and 0.2 T, low field strength is superior for passive visualization of the needles tested-especially if GE imaging is performed.

Artifacts↗
PubMed1993

A method for detection of spectral spin diffusion from minor peaks, and its application to 19F MAS NMR of antimony(III)-doped fluorapatite.

A new technique for detecting spectral spin diffusion in solids under MAS NMR conditions that is particularly well suited for accurately measuring cross-relaxation from minor spectral components is presented. The pulse sequence, SINK (Saturation Inter-Nuclear Kinetics), selectively saturates the magnetization of a minor spectral component with a series of rotor-synchronized DANTE pulse trains and monitors spin diffusion to other peaks with a non-selective 90 degrees pulse. We have used SINK with 19F MAS NMR on samples of calcium fluorapatite doped with Sb3+ to measure spin diffusion between a weak peak at 68.6 ppm due to fluoride ions associated with Sb3+ and other peaks in the spectrum. The SINK experiment clearly demonstrates that spin diffusion from the former peak to the main resonance of fluorapatite at 64.0 ppm is faster than spin diffusion to a second antimony-related peak at 65.6 ppm. These results strengthen our previous conclusion that antimony(III) occupies a phosphate site in the apatite lattice, with an SbO3(3-) group replacing a PO4(3-) group. The SINK experiment also enables the detection of a "hidden" peak at approximately 62.9 ppm that is otherwise obscured by the intense main peak at 64.0 ppm.

Antimony↗
PubMed2006

Cogwheel phase cycling in common triple resonance NMR experiments for the liquid phase.

We demonstrate that cogwheel phase cycling, which was previously only used in solid state NMR, can be applied to optimize the efficiency of commonly used pulse sequences in multiple resonance liquid-state biomolecular NMR. In favorable cases the required minimum number of scans can be reduced by more than 80% as compared to a corresponding sequence with nested phase cycles. Since cogwheel phase cycling procedures can be designed for a range of scan numbers, and can be combined with pulsed field gradients, the total experiment time can be adjusted closely to the required signal-to-noise ratio with minimal overhead. Examples are shown for 3D-TROSY-HNCO, 3D-TROSY-HNCACO, and 3D-HACACO experiments on diamagnetic and paramagnetic proteins.

Algorithms↗
PubMed2003

Frequency-domain Hadamard spectroscopy.

A new technique is proposed for multichannel excitation and detection of NMR signals in the frequency domain, an alternative to the widely used pulse-excited Fourier transform method. An extensive array of N radiofrequency irradiation channels covers the spectrum of interest. A selective radiofrequency pulse sequence is applied to each channel, generating a steady-state NMR response acquired one-point-at-a-time in the intervals between pulses. The excitation pattern is repeated N times, phase-encoded according to a Hadamard matrix, and the corresponding N composite responses are decoded by reference to the same matrix. This multiplex technique offers the same sensitivity advantage as conventional Fourier transform spectroscopy. The irradiation pattern may be tailored to concentrate on interesting spectral regions, to facilitate homonuclear double resonance, or to avoid exciting strong solvent peaks. As no free induction decay is involved, the new method avoids problems of pulse breakthrough or lineshape distortion by premature termination of the time-domain signal.

Fourier Analysis↗
PubMed1997

Hemodialysis-related arthropathy. A prospective MR study with SE and GRE sequences.

PURPOSE: A prospective examination of hemodialysis-related arthropathy by MR imaging with SE and GRE sequences was undertaken to verify whether the low signal intensity on T2-weighted SE images is caused by a paramagnetic susceptibility effect derived from amyloid or from hemosiderin. MATERIAL AND METHODS: Twenty-seven joints (19 hip joints, 6 discovertebral joints of 5 lumbosacral spines and 1 cervical spine, 2 shoulder joints) in 14 patients undergoing long-term hemodialysis (duration 10-22 years, mean 17) were examined with SE T1-weighted, T2-weighted and GRE T2*-weighted sequences. RESULTS: The signal intensity of the intraarticular and periarticular masses of the involved joints was basically isointense to the muscle in all 3 pulse sequences. No significantly low signal area was found on GRE T2*-weighted images. CONCLUSION: The low signal areas in the hemodialysis-related arthropathy on SE T2-weighted images are not caused by a paramagnetic susceptibility effect, but probably by the hypocellular and fibrous nature of amyloid-containing tissues. Therefore the GRE sequence can be helpful for differentiating hemodialysis-related amyloid arthropathy from chronic hemarthrosis or juxta-articular brown tumor.

Adult↗
PubMed1992

Central nodal necrosis and extracapsular neoplastic spread in cervical lymph nodes: MR imaging versus CT.

Computed tomographic (CT) scans and magnetic resonance (MR) images obtained in 24 patients with cervical lymphadenopathy were retrospectively and blindly evaluated by two readers for the presence of central nodal necrosis (CNN) and extracapsular nodal spread (ENS). The CT studies were all enhanced, and the MR images were obtained with short repetition time (TR)/echo time (TE), long TR/double echo, and enhanced short TR/TE fat-suppressed sequences. Each MR imaging sequence was interpreted separately and then collectively. Sixty lymph nodes were identified with CT. Sensitivity for CNN was 16%-67% with the unenhanced MR pulse sequences, 50% with enhanced sequences, and 83%-100% with CT. The most accurate reading of MR images for CNN was with the unenhanced T1-weighted and T2-weighted images (86%-87%); the accuracy of CT was 91%-96%. The accuracy of MR imaging for detecting ENS was maximal with T1-weighted images (78%-90%). Gadolinium-enhanced, fat-suppressed images did not improve accuracy in evaluating CNN or ENS. CT is currently more accurate than unenhanced or enhanced MR imaging in detecting CNN or ENS.

Carcinoma, Squamous Cell↗
PubMedDate not supplied

MR imaging of cardiac tumors.

Magnetic resonance (MR) imaging is an important tool in the evaluation of cardiac neoplasms. T1-weighted, T2-weighted, and gadolinium-enhanced sequences are used for anatomic definition and tissue characterization, whereas cine gradient-echo imaging is used to assess functional effects. Recent improvements in pulse sequences for cardiac MR imaging have led to superior image quality, with reduced motion artifact and improved signal-to-noise ratio and tissue contrast. Although there is some overlap in the MR imaging appearances of cardiac tumors, particularly of primary malignancies, differences in characteristic locations and features should allow confident differentiation between benign and malignant tumors. Indicators of malignancy at MR imaging are invasive behavior, involvement of the right side of the heart or the pericardium, tissue inhomogeneity, diameter greater than 5 cm, and enhancement after administration of gadolinium contrast material (as a result of higher tissue vascularity). Concomitant pericardial or pleural effusions are rare in benign processes but occur in about 50% of cases of malignant tumors. MR imaging offers improved resolution, a larger field of view, and superior soft-tissue contrast compared with those of echocardiography, suggesting that knowledge of the MR imaging features of cardiac neoplasms is important for accurate diagnosis and management.

Adult↗
PubMed1993

Fat suppressed magnetic resonance imaging at 0.5 T using binomial radiofrequency pulses.

Fat suppressed MR imaging can be achieved by selectively saturating lipid protons, just before applying a conventional spin-echo rf pulse sequence. The difference in the Larmor frequency between fat and water protons is only 3.5 ppm, so that the frequency response of the suppressing pulse, or pulses, has to be carefully designed. The choice of suitable binomial and modified trains of hard rf pulses was investigated by computer simulation, phantom testing and T1 weighted spin-echo imaging of a normal volunteer's optic nerve at 0.5 T.

Computer Simulation↗
PubMedDate not supplied

Spinal cord vascular disease: characterization with fast three-dimensional contrast-enhanced MR angiography.

BACKGROUND AND PURPOSE: Noninvasive characterization of spinal vascular lesions is essential for guiding clinical management, and several MR angiographic techniques have been applied in the past with variable results. The purpose of our study was to assess the potential of a dynamic 3D contrast-enhanced MR angiographic sequence to characterize spinal vascular lesions and to identify their arterial feeders and venous drainage. METHODS: A contrast-enhanced gradient-echo 3D pulse sequence providing angiographic information within 24 seconds was applied prospectively in 12 consecutive patients with a presumed spinal vascular lesion. The images were evaluated for visibility of the arterial feeder, and the results were compared with those of conventional angiography performed the next day. RESULTS: The MR angiographic findings proved that the lesions were correctly characterized as spinal arteriovenous malformations (AVMs) (n = 6), spinal dural arteriovenous fistulas (AVFs) (n = 3), a hemangioblastoma (n = 1), a teratoma (n = 1), and a vertebral hemangioma (n = 1). The arterial feeder was visible in all six AVMs and in the hemangioblastoma, corresponding to conventional angiographic findings. In two of three spinal dural AVFs, an enlarged draining medullary vein was seen within the neural foramen, providing correct localization. The third fistula could not be seen owing to reduced image quality from motion artifacts. CONCLUSION: Fast 3D contrast-enhanced MR angiography is a noninvasive technique with high accuracy in the characterization of spinal vascular disease. Visibility of the arterial pedicles corresponds well with that of digital subtraction angiography, facilitating the management of these patients.

Adult↗
PubMed1995

MR evaluation of vertebral metastases: T1-weighted, short-inversion-time inversion recovery, fast spin-echo, and inversion-recovery fast spin-echo sequences.

PURPOSE: To compare the detectability of vertebral metastatic disease on T1-weighted, short-inversion-time inversion recovery (STIR), fast spin-echo (FSE), fat-saturated FSE, and inversion recovery FSE (IRFSE) MR sequences using percent contrast and contrast-to-noise ratios. METHODS: Patients with proved metastatic disease underwent imaging on a 1.5-T MR system with sagittal T1-weighted (800/20/2 [repetition time/echo time/excitations]) (91 patients), STIR (1400/43/2; inversion time, 140) (91 patients), FSE (4000/180/2) (46 patients), fat-saturated FSE (4000/180/2) (16 patients), and IRFSE (29 patients) sequences. Percent contrast and contrast-to-noise ratio were calculated for the lesions. The number of metastatic lesions detected with each of the pulse sequences was also calculated. RESULTS: Mean percent contrast was, for T1-weighted sequence, -42.2 +/- 1%; STIR, 262 +/- 34%; FSE, 121 +/- 21%; fat-saturated FSE, 182 +/- 6%; and IRFSE, 272 +/- 47%. The mean contrast-to-noise ratio for T1-weighted was -4.63 +/- 1.7; STIR, 10.8 +/- .98; FSE, 4.16 +/- .76; fat-saturated FSE, 4.87 +/- .19; and IRFSE, 5.2 +/- .87. STIR and IRFSE showed the highest number of lesions, followed by T1-weighted, fat-saturated FSE, and FSE sequences. T1-weighted sequences showed 94%, FSE 55%, and fat-saturated FSE 78% of the lesions detected. Epidural metastatic lesions were better depicted on T1-weighted, FSE, and fat-saturated FSE sequences. CONCLUSION: STIR was superior to both T1-weighted and FSE (with and without fat saturation) for detection of metastatic lesions, in terms of both percent contrast and contrast-to-noise ratio and visibility. IRFSE was equal to STIR for the detection of metastasis by both subjective and objective criteria. T1-weighted, FSE, and fat-saturated FSE sequences were superior to STIR and IRFSE in the detection of epidural metastatic disease. IRFSE provided faster scanning time, which could be translated into greater resolution.

Adolescent↗
PubMed1999

A TROSY CPMG sequence for characterizing chemical exchange in large proteins.

A new NMR spin relaxation experiment is described for measuring chemical exchange time constants from approximately 0.5 ms to 5 ms in 15N-labeled macromolecules. The pulse sequence is based on the Carr-Purcell-Meiboom-Gill technique [Carr and Purcell (1954) Phys. Rev., 94, 630-638; Meiboom and Gill (1958) Rev. Sci. Instrum., 29, 688-691; Loria et al. (1999) J. Am. Chem. Soc., 121, 2331-2332], but implements TROSY selection [Pervushin et al. (1997) Proc. Natl. Acad. Sci. USA, 94, 12366-12371] to permit measurement of exchange linebroadening contributions to the narrower component of the 1H-15N scalar-coupled doublet. This modification extends the size limitation imposed on relaxation measurements due to the fast decay of transverse magnetization in larger macromolecules. The new TROSY-CPMG experiment is demonstrated on a [U-98% 15N] labeled sample of basic pancreatic trypsin inhibitor and a [U-83% 2H, U-98% 15N] labeled sample of triosephosphate isomerase, a 54 kDa homodimeric protein.

Binding Sites↗
PubMed1989

[MR imaging of femoral head avascular necrosis with STIR sequence].

The uncomplicated bone necrosis is usually demarcated by reactive interface (RI). To analyze signal characteristics of RI, MR imaging of the hip was done using short TI IR (STIR) as well as short/long TR pulse sequences at 0.22 tesla. On STIR sequence the RI was observed as a high-signal line or ring with the shape identical to that of short TR image. The double line sign was not seen in early stage. The best explanation of high signal intensity of RI on STIR is that it has prolonged T1 and T2 secondary to high water content. It is concluded that STIR is an unique way to demonstrate RI in early osteonecrosis.

Adult↗
PubMed1986

Errors in the measurements of T2 using multiple-echo MRI techniques. II. Effects of static field inhomogeneity.

The accurate estimation of the spin-spin relaxation time T2 is an important goal in magnetic resonance imaging particularly because it can be used for quantitative tissue characterization. The spin-spin relaxation time T2 may be estimated using multiecho pulse sequences, but the accuracy of the estimate is dependent on the fidelity of the spin-echo amplitudes, which may be severely compromised by rf pulse and static field imperfections. In this paper, the effects of static field inhomogeneities are investigated. The propagation of the errors introduced by off-resonance effects are analyzed through computer simulations and analytical solutions of the Bloch equations. A series of experiments performed on a simple tissue phantom using a whole-body imaging system operating at 6.35 MHz corroborates the simulation and analytical results. For accurate measurements of T2 using a whole-body imaging system it is necessary to correct for these inhomogeneities. A correction scheme which would enable a more accurate estimate of T2 is currently under investigation.

Biometry↗