Tissue parameter determination with MRI in the presence of imperfect radiofrequency pulses

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Title: Tissue parameter determination with MRI in the presence of imperfect radiofrequency pulses
Author: Li, Xing
Abstract: Magnetic resonance imaging has the major advantage over other medical imaging modalities in that it can provide superb soft tissue contrast. The contrast in a magnetic resonance image is determined by intrinsic tissue parameters such as spin-lattice relaxation time T1, spin-spin relaxation time T2, and spin density p. These tissue parameters can be extracted from a series of images obtained with appropriately designed pulse sequences, and be used in tissue characterization, classification, etc.. The accuracy of tissue parameter determination, however, is affected by a number of factors, one of which is the inhomogeneity of the radio-frequency (RF) electromagnetic field. In many cases the effect of RF field inhomogeneity is not just a modulation of the spatially varying field with the artifact-free image. It affects the estimation of tissue parameter in a still less straightforward way. In this study the effect of RF field imperfection in a spin-echo sequence was investigated. The signal intensity expression was derived first as a function of the perturbing pulse, the rephasing pulse, and other tissue and instrumental parameters. A correction scheme for RF field imperfections in tissue parameter determination was then devised. Two RF field mapping methods, the ratio method and the least-squares method were compared by imaging phantoms in a birdcage head coil and a single turn breast coil. It was found that the two methods had a small discrepancy of about two degrees for the head coil but agreed very well for the breast coil. The ratio method was used in the in vivo RF field mapping. The correction of RF field inhomogeneity resulted in reductions of spatial variation of about 16% and 40%, in terms of normalized standard deviation, for the spin density and T1 values of a chosen brain tissue. It resulted in a reduction of about 45% for both spin density and T1 values of a chosen breast tissue. Also, as two multi-echo spin echo images were used in calculating the T2 image in breast imaging, RF field correction resulted in a reduction of T2 spatial variation of about 50% for the same tissue. As the RF field inhomogeneity was large in the breast coil, its correction also noticeably reduced the overall tissue parameter variation of the same tissue type.
Record URI: http://hdl.handle.net/1850/12182
Date: 1994-11

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