ISSN:1053-1807    

DOI:10.1002/jmri.1880010402

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

年代:1991

数据来源: WILEY

摘要:

AbstractA technique for the simultaneous acquisition of three‐dimensional phase‐contrast angiograms and stationary‐tissue images is described. Hadamard multiplexed encoding of flow information permits image acquisition times that are a third shorter than those of previous phase‐contrast methods. The encoding scheme described also enables differentiation of flow‐induced phase shifts from phase shifts due to resonance offset conditions such as field inhomogeneities and chemical shift. Display strategies that combine this phase information with the flow image are

ISSN:1053-1807    

DOI:10.1002/jmri.1880010403

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

年代:1991

数据来源: WILEY

摘要:

AbstractThree encoding strategies for the measurement of flow velocities in arbitrary directions with phase‐contrast magnetic resonance imaging are presented; their noise and dynamic range performance are compared by means of theoretical analysis and computer simulation. A six‐point measurement strategy is shown to be quite inefficient in terms of velocity variance per unit time. A simple four‐point method exhibits equal dynamic range; its noise depends on flow direction but on average is equal to that of the six‐point method. An alternate, balanced four‐point method has noise that is direction independent and has, depending on implementation, possibly lower noise levels. Either four‐point method is more efficient and is preferred over the six‐p

ISSN:1053-1807    

DOI:10.1002/jmri.1880010404

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

年代:1991

数据来源: WILEY

摘要:

AbstractA new acquisition scheme for three‐dimensional (3D) phase‐contrast MR angiography reduces by 33% the measurement time for a data set sensitive to flow in all three orthogonal directions. Background suppression is achieved by acquiring a flow‐compensated data set and three data sets flow encoded in the three orthogonal directions, with subsequent complex subtraction. The data are acquired in an interleaved fashion, eliminating misregistration artifacts due to patient motion between measurements sensitive to different flow directions. A standard maximum‐intensity‐projection algorithm is applied to the combined 3D data set to obtain angiographic projections sensitive to all three orthogonal flow directions. The theory and implementation of the method are described and examples of its application to the intracranial and abdominal circulation are

ISSN:1053-1807    

DOI:10.1002/jmri.1880010405

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

年代:1991

数据来源: WILEY

摘要:

AbstractA connected‐voxel algorithm (CVA) that improves the contrast and conspicuity of blood vessels in maximum‐intensity‐projection (MIP) magnetic resonance (MR) angiography is described. Images from a variety of anatomic regions in healthy volunteers were calculated with either an MIP procedure alone or with data that had first been processed with the CVA. A low‐signal‐intensity threshold is first applied to separate groups of voxels associated with different vessels from one another and to eliminate the contribution from low‐intensity stationary material. The remaining voxels are grouped by a connectivity criterion into discrete “objects.” Vessels are represented by extended objects, and small objects are discarded. The CVA, therefore, reduces the full three‐dimensional data set into a small number of discrete objects. It is a powerful technique that can be used to remove signal from vessels overlying the vessel of interest, to separate objects representing arterial flow from those representing venous flow, to eliminate flow artifact from projection images, and to more completely retain signal within the vascular lumen. This technique has been successfully demonstrated with MR angiography in h

ISSN:1053-1807    

DOI:10.1002/jmri.1880010406

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

年代:1991

数据来源: WILEY

摘要:

AbstractMagnetic resonance (MR) images made with the IVIM (intravoxel incoherent motion) technique for demonstrating tissue microcirculation are limited in sensitivity because of the small volume of blood involved. This limitation may be overcome by incorporating magnetic particles into the flow. The magnetic perturbation caused by the particles extends beyond the walls of the capillary and affects a much larger volume than that of the flowing material. Imaging experiments conducted with an artificial capillary system for renal dialysis, containing large magnetic particles, showed that signal intensity decreased with increasing flow rate through the dialysis bundle and with increasing particle concentration. Predictions of the effect based on a theoretical model of spin dephasing in the field of a magnetic dipole agreed with the experimental data. The results hold promise for development of the technique in vivo.

ISSN:1053-1807    

DOI:10.1002/jmri.1880010407

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

年代:1991

数据来源: WILEY

摘要:

AbstractTo determine the frequency and clinical significance of tumor‐associated muscle edema, magnetic resonance (MR) imaging findings in 46 consecutive patients with benign or malignant musculoskeletal lesions were reviewed. Increased muscle signal intensity on T2‐weighted, STIR (short‐inversion‐time inversion‐recovery), and gadopentetate dimeglumine‐enhanced TI‐weighted images was present in 41 cases, with the clearest delineation of tumor margins seen on T2‐weighted images. Typical peri/paratu‐moral edema (PTE) was present in equal proportions of malignant (25 of 37) and benign lesions (six of nine). Massive edema involving the entirety of at least one contiguous muscle‐to the authors' knowledge, a previously undescribed finding‐was identified on MR images of eight malignant and two benign lesions (22% of both groups). All cases of massive edema had a substantial soft‐tissue component and involved muscles disrupted by tumor at the point of attachment to bone. Malignant tumors associated with massive edema were larger than those with typical or no PTE, showed a poorer response to initial chemotherapy, and had a higher frequency of metastases at diagnosis. Thus, the presence of massive muscle edema appears to be an ominous clinical finding in patients with malignant

ISSN:1053-1807    

DOI:10.1002/jmri.1880010408

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

年代:1991

数据来源: WILEY

摘要:

AbstractThe T1 of soft tissues increases with magnetic field strength. Some tissue contrast may be diminished on high‐field‐strength magnetic resonance (MR) images when conventional TRs are used, because of altered T1 effects on the MR signals. This necessitates longer TRs in techniques that use long TRs, which prolongs the examination excessively. Behavior of macroscopic magnetization is governed by the Bloch equations. Therefore, T1 contributions to the MR signal can be modulated by means of both timing intervals and radio‐frequency pulses. The analytic solution to the Bloch equations allowed calculation of white matter/gray matter and gray matter/cerebrospinal fluid contrast in both spin‐echo and inversion‐recovery (IR) imaging. Rabbit brains (normal and tumor‐containing) were then imaged in vivo at 1.5 and 4.7 T. In addition, MR images of a human head were obtained at 4.O T. Experimental results supported the theoretical predictions that brain contrast on long TR spinecho or IR images increases with field strength. However, varying the excitation flip angle allowed optimization of the T1 contribution to the MR signals, improving image contrast and/or reducing examination time. Thus, the dependence of T1 on field strength determines the optimum choice of imaging techniques and parameters in a predicta

ISSN:1053-1807    

DOI:10.1002/jmri.1880010409

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

年代:1991

数据来源: WILEY

摘要:

AbstractPhase‐offset multiplanar (POMP) imaging is a technique that excites several sections simultaneously for improved imaging efficiency. The centers of the reconstructed images from each of the POMP sections are offset from each other in the phase‐encoding direction by means of view‐dependent phase modulation of the radio‐frequency (RF) excitation pulses and are placed adjacent to each other in the reconstruction. With a suitable reconstruction matrix size, the images can be made nonoverlapping and stored separately. At constant imaging time, signal‐to‐noise ratio (S/N), and resolution, POMP imaging produces a factor In: Book of abstracts: Society of Magnetic Resonance in Medicine In: Book of abstracts: Society of Magnetic Resonance in MedicineNpmore sections than a conventinal sequence but with a reduced field of view. Alternatively, imaging time may be invreased by the factorNpto retain the same field of view but with the expected S/N advantage. The average RF power deposited by the 90° composite RF pulse is greater by the factorNp, but the power for the 180° pulse is unchanged. The POMP method is discussed and compared with three‐dimensional and Hada

ISSN:1053-1807    

DOI:10.1002/jmri.1880010410

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

年代:1991

数据来源: WILEY

摘要:

AbstractCombining either paramagnetic (gadolinium chelatea) or superparamagnetic (ferrite) contrast agents with polymers such as polyethylene glycol or cellulose, or with simple sugars such as dextrose, results in mixtures that exhibit improved T1 and/or T2 relaxivity compared with that of the contrast agent alone. It is suggested that the addition of such inexpensive and nontoxic polymers or saccharides may improve the effectiveness and decrease the cost of enteric contrast agents.

ISSN:1053-1807    

DOI:10.1002/jmri.1880010411

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

年代:1991

数据来源: WILEY