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排序方式: 共有504条查询结果,搜索用时 15 毫秒
61.
Rohan Dharmakumar Juimiin Hong Jean H Brittain Donald B Plewes Graham A Wright 《Magnetic resonance in medicine》2005,53(3):574-583
Steady-state free precession (SSFP) methods have gained widespread recognition for their ability to provide fast scans at high signal-to-noise ratio. This paper demonstrates that such methods are also capable of reflecting functional information, particularly blood oxygenation state. It is well known that SSFP signals show substantial sensitivity to small off-resonance frequency variations. However, that mechanism cannot explain the oxygen-sensitive contrast in blood that was observed with steady-state methods using phase-cycled radiofrequency pulses. From theoretical and experimental models it is demonstrated that the mechanism responsible for such contrast originates from the motion of spins through local field inhomogeneities in and around deoxygenated red blood cells. In addition, this work shows that it is critical to choose the scan parameters carefully for robust oxygen-sensitive contrast. Finally, it is demonstrated that it is possible to build a quantitative model that incorporates the Luz-Meiboom model, which had been used in the past to estimate quantitative measures of vascular blood oxygen levels. It is envisioned that this method could be instrumental in real-time imaging focused on detecting diseases where the oxygen state of blood is impaired. 相似文献
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In spin echo imaging, magnetization is assigned to a location defined by its frequency of rotation. In the presence of a static magnetic field inhomogeneity, however, this location does not correspond to the true location of the magnetization. This paper describes a magnetic resonance imaging technique called phase constrained encoding (PACE) that assigns magnetization to its true location through the use of a spin echotrain and alternating readout gradients. Small artifactual sidebands occur in the point spread function but can be minimized or eliminated using higher gradient strengths, more echoes, and/or additional acquisitions. Implementation of a simple version of this technique confirms simulations. 相似文献
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Why fat is bright in RARE and fast spin-echo imaging. 总被引:2,自引:0,他引:2
R M Henkelman P A Hardy J E Bishop C S Poon D B Plewes 《Journal of magnetic resonance imaging : JMRI》1992,2(5):533-540
Fast spin-echo (FSE) sequences are becoming popular for T2-weighted clinical imaging because they result in a severalfold reduction in imaging time and because they provide conventional spin-echo contrast for most tissues. Fat, however, has been observed to have anomalously high signal intensity on FSE images. The present study shows that the brighter fat results from the multiple 180 degrees refocusing pulses, which eliminate diffusion-mediated susceptibility dephasing and suppress J-coupling modulation of the echo train. 相似文献
69.
Does preoperative needle localization lead to an increase in local breast cancer recurrence? 总被引:1,自引:0,他引:1
Kopans DB; Gallagher WJ; Swann CA; McCarthy KA; White G; Hall DA; Wood WC 《Radiology》1988,167(3):667-668
Between 1978 and 1981, 74 women with nonpalpable breast cancer underwent surgery after localization guides were placed. In 72 patients, guides were introduced parallel to the chest wall; in two the needle was positioned anteroposteriorly under computed tomographic guidance. Fifty-six cases (76%) were infiltrating cancer; 13 (17%), intraductal cancers; two (3%), inflammatory; and three (4%), lobular carcinoma in situ. Surgery was not used to treat the latter five patients. In the remaining 69 women, 42 (61%) were treated by means of modified radical mastectomy; six (9%), total mastectomy; 12 (17%), local excision and radiation therapy; and seven (10%), local excision alone; exact therapy for two women (3%) was unknown. At a minimum follow-up of 5 years, none of the 67 women in whom the parallel approach was used had a local recurrence. The authors conclude that preoperative placement of guides parallel to the chest wall does not appear to increase the risk of local breast cancer recurrence. 相似文献
70.
Matson GB; Twieg DB; Karczmar GS; Lawry TJ; Gober JR; Valenza M; Boska MD; Weiner MW 《Radiology》1988,169(2):541-547
Localized phosphorus-31 magnetic resonance (MR) spectroscopy in humans has previously been accomplished with surface coils by means of depth-resolved surface coil spectroscopy or rotating frame experiments, in which the extent of tissue sampled critically depends on surface coil placement. The authors' goal was to modify the surface coil image-selected in vivo spectroscopy (ISIS) experiment to accomplish three-dimensional volume selection through application of selective pulses in the presence of B0 gradients. Advantages of ISIS include the ability to use proton images to define the volume of interest (VOI) and reduced dependence on exact positioning of the surface coil. However, rapid replication of the surface coil ISIS experiment can cause spectral contamination from signals originating outside the VOI. A modified version of the ISIS experiment was developed to alleviate contamination under conditions of rapid replication. Applications of localized P-31 MR spectroscopy for observation of high-energy phosphorus metabolites are presented in human liver, heart, and transplanted and normal kidney. 相似文献