摘要:

AbstractThe accurate assessment of pulmonary perfusion is especially important in the evaluation of patients with suspected pulmonary embolism, a common and potentially lethal disorder that can be treated by aggressive anticoagulation. In this study, we demonstrate for the first time the use of MR to image pulmonary perfusion in humans by using dynamic imaging after contrast administration. The technique, which uses an inversion recovery turbo FLASH sequence with ultrashortTE(1.4 ms) and 1‐s temporal resolution, was tested in a series of eight healthy subjects and in a porcine model of pulmonary embolism. After the administration of gadopentetate dimeglumine in humans and animal models, there was serial enhancement of the systemic veins, right atrium, right ventricle, and pulmonary arteries. The pulmonary arterial tree was visualized beyond the segmental branches, followed by a gradual diffuse increase in signal intensity of the lung parenchyma over a period of 4.0–7.0 s. Pulmonary circulation times ranged from 3.0–3.4 s. Whereas a high dose (20 or 40 ml) of contrast agent tended to produce the most intense parenchymal enhancement, a low dose (5 ml) was best for showing recirculation. In the animal model, a perfusion defect due to a pulmonary embolus was clearly shown and confirmed by cine angiography. It is concluded that MRI of lung perfusion is feasible. With further development, perfusion MRI could eventually have a significant clinical role in the diagnostic evaluation of pulmonary emb

ISSN:0740-3194    

DOI:10.1002/mrm.1910360402

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractA novel line scan diffusion imaging sequence (LSDI) is introduced. LSDI is inherently insensitive to motion artifacts and high quality diffusion maps of the brain can be obtained rapidly without the use of head restraints or cardiac gating. Results from a stroke study and abdominal diffusion images are presented. The results indicate that it is feasible to use the LSDI technique for clinical evaluation of acute ischemic stroke. In contrast to echo‐planar diffusion imaging, LSDI does not require modified gradient hardware and can be implemented on conventional scanners. Thus, LSDI should dramatically increase the general availability of robust clinical diffusion imagin

ISSN:0740-3194    

DOI:10.1002/mrm.1910360403

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractMagnetic resonance phase velocity mapping has unrivalled capacities for acquiringin vivomulti‐directional blood flow information. In this study, the authors set out to derive both spatial and temporal components of acceleration, and hence differences of pressure in a flow field using cine magnetic resonance velocity data. An efficient numerical algorithm based on the Navier‐Stokes equations for incompressible Newtonian fluid was used. The computational approach was validated within vitroflow phantoms. This work aims to contribute to a better understanding of cardiovascular dynamics and to serve as a basis for investigating pulsatile pressure/ flow relationships associated with normal and impaired cardiovascular funct

ISSN:0740-3194    

DOI:10.1002/mrm.1910360404

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractConventional MRI systems rely on large magnets to generate a field that is both strong and extremely uniform. This field is usually produced by a heavy permanent magnet or a cryogenically cooled superconductor. An alternative approach, called prepolarized MRI (PMRI), employs two separate fields produced by two different magnets. A strong and inhomogeneous magnetic field is used to polarize the sample. After polarization, a weak magnetic field is used for readout. These fields can be produced by two separate resistive electromagnets that cost significantly less than a single permanent or superconducting magnet. At Stanford, the authors are constructing a PMRI prototype scanner suitable for imaging human extremities roughly 20 cm in diameter. With this system the authors hope to demonstrate comparable image quality to MRI with reduced system cost. The authors' initial work on low‐frequency reception indicates that it will be possible to obtain comparable image signal‐to‐noise ratio to an MRI scanner operating at the same polarizing field strength. To reduce the capital cost of the system, the authors use resistive electromagnets. Here the authors discuss the full development of the readout magnet including important design considerations, shimming, and field plots. These encouraging results are an important step toward evaluating the cost effectiveness of

ISSN:0740-3194    

DOI:10.1002/mrm.1910360405

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractStrain‐rate imaging uses large velocity encoding gradients to obtain measurements of velocity that are extremely insensitive to the effects of random noise. The spatial differential of velocity yields the velocity gradient from which the strain‐rate and twist‐rate tensors can be determined. These tensors represent the distortion of the material and are of interest in the analysis of the dynamic behavior of living tissue (e.g., that of the myocardium). This work presents a new technique that uses the magnitude of the signal in the velocity encoded data to measure through‐plane velocity variations at the resolution of the voxel size. The magnitude of the MR signal contains information about the range of phases present within a voxel. When the phase is dependent on the velocity (as in phase velocity imaging), the magnitude contains information about the range of velocities within a voxel. The method presented in this work uses unbalanced slice‐refocusing gradients to sample the magnitude variation introduced by the interaction of velocity encoding gradients with spatially dependent velocities. The previously developed in‐plane velocity gradient methods can be easily integrated with this new through‐plane measurement to characterize the deformation of the myocardium in three spatial dimensions with high accuracy. The applicability of these methods is demonstrated theoretically, in phanto

ISSN:0740-3194    

DOI:10.1002/mrm.1910360406

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractThe nitroxide group 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) was covalently bound to a dextran via an N‐acetamido linkage to produce a novel spin probe, TEMPO‐dextran (TEMPO‐DX), which circulates for long time periods in an animal without metabolic degradation. TEMPO‐DX was stable in mice, while small TEMPO analogs quickly disappeared after administration. Since dextran was reduced somewhat in size during synthesis, the resultant spin‐labeled dextran could be excreted through the kidney. A strong L‐band electron spin resonance signal was obtained shortly after Intravenous administration of TEMPO‐DX into the tail vein of a mouse, from which three‐dimensional images of specific organs were calculated. The signal was found to pers

ISSN:0740-3194    

DOI:10.1002/mrm.1910360407

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractA dynamic MR angiography technique, MR digital subtraction angiography (MR DSA), is proposed using fast acquisition, contrast enhancement, and complex subtraction. When a bolus of contrast is injected into a patient, data acquisition begins, dynamically acquiring a thick slab using a fast gradient echo sequence for 10–100 s. Similar to x‐ray DSA, a mask is selected from the images without contrast enhancement, and later images are subtracted from the mask to generate angiograms. Complex subtraction is used to overcome the partial volume effects related to the phase difference between the flowing and stationary magnetization in a voxel. Vessel signal is the enhancement of flow magnetization resulting from the contrast bolus. MR DSA was performed in 28 patients, including vessels in the lungs, brains, legs, abdomen, and pelvis. All targeted vessels were well depicted with MR DSA. Corresponding dynamic information (contrast arrival time taand duration of the arterial phase tav) was measured: ta/tav= 3.4/4.7 s for the lung, 10.3/4.9 s for the brain, 12.8/19.3 for the aorta, 15.2/12.6 s for the leg. MR DSA can provide dynamic angiographic images using a very short acquisition t

ISSN:0740-3194    

DOI:10.1002/mrm.1910360408

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractA radial BURST approach combining BURST excitations with projection reconstruction (PR) gradients for excitation and acquisition is introduced. With the application of PR gradients during excitation, the applied RF energy is distributed more homogeneously across the sample than in the initial BURST approach. Computer simulations of the radial BURST excitation scheme are presented and experimentally verified. Actual radial BURST implementation acquiring a 64 × 64 radial BURST image in 45 ms is reported. Extensions of the radial BURST imaging technique placing a magnetization preparation experiment in front of the whole imaging sequence are also described. Furthermore, the feasibility of combining radial BURST with a multishot approach is demonstrated

ISSN:0740-3194    

DOI:10.1002/mrm.1910360409

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractGradient localized spectroscopy techniques suffer from a well documented spatial localization error caused by the difference in chemical shifts between resonances. This results in the acquisition of spectra from partially overlapping spatial regions of the sample, with each resonance representing a different region. The image‐selectedin vivospectroscopy technique uses hyperbolic secant inversion pulses, where the main limitation in reducing this error is in the RF power available for application of the selective RF pulse. This spatial localization error may be dramatically reduced by increasing, and temporally shaping, the gradient pulse during slice‐selective spin inversion. The performance of these RF pulses have been experimentally verif

ISSN:0740-3194    

DOI:10.1002/mrm.1910360410

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractMeasurements have been made in the rat femurin vivoandex vivoby using an asymmetric spin echo technique ofT2′, the susceptibility contribution toT2*. The trabecular spacing in this study in rat bone is considerably less than in previous studies in the human. A significant increase inT2′was seenin vivo3 mm proximal to the growth plate with ovariectomy (a model of osteopenia), from 8.1 ± 0.7 to 10.0 ± 0.6 ms. Parallel changes in trabecular bone mineral density measured by quantitative computed tomography were found.T2′was higher in living bones than in the same bones measuredpost

ISSN:0740-3194    

DOI:10.1002/mrm.1910360411

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY