Christabel Lee

National Institutes of Health, 베서스다, Maryland, United States

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Publications (2)0.2 Total impact

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    ABSTRACT: Low noise, high resolution, fast and accurate T₁ maps from MRI images of the brain can be performed using a dual flip angle method. However, B₁ field inhomogeneity, which is particularly problematic at high field strengths (e.g., 3T), limits the ability of the scanner to deliver the prescribed flip angle, introducing errors into the T₁ maps that limit the accuracy of quantitative analyses based on those maps. A dual repetition time method was used for acquiring a B₁ map to correct that inhomogeneity. Additional inaccuracies due to misregistration of the acquired T₁-weighted images were corrected by rigid registration, and the effects of misalignment on the T₁ maps were compared to those of B₁ inhomogeneity in 19 normal subjects. However, since B₁ map acquisition takes up precious scanning time and most retrospective studies do not have B₁ map, we designed a template-based correction strategy. B₁ maps from different subjects were aligned using a twelve-parameter affine registration. Recomputed T₁ maps showed an important improvement with respect to the noncorrected maps: histograms of all corrected maps exhibited two peaks corresponding to white and gray matter tissues, while unimodal histograms were observed in all uncorrected maps because of the inhomogeneity. A method to detect the best nonsubject-specific B₁ correction based on a set of features was designed. The optimum set of weighting factors for those features was computed. The best available B₁ correction was detected in almost all subjects while corrections comparable to the T₁ map corrected using the B₁ map from the same subject were detected in the others.
    11/2010; 29(11):1927-41. DOI:10.1109/TMI.2010.2053552
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    ABSTRACT: Accurate estimation of relaxation time T1 from MRI images is increasingly important for some clinical applications. Low noise, high resolution, fast and accurate T1 maps from MRI images of the brain can be performed using a dual flip angle method. However, accuracy is limited by the scanners ability to deliver the prescribed flip angle due to the B1 inhomogeneity, particularly at high field strengths (e.g. 3T). One of the most accurate methods to correct that inhomogeneity is to acquire a subject-specific B1 map. However, since B1 map acquisition takes up precious scanning time and most retrospective studies do not have B1 map, it would be desirable to perform that correction from a template. For this work a dual repetition time method was used for B1 map acquisition in five normal subjects. Inaccuracies due to misregistration of acquired T1-weighted images were corrected by rigid registration, and the effects of misalignment were compared to those of B1 inhomogeneity. T1-intensity histograms were produced and three-Gaussian curves were fitted for every fully-, partially- and non-corrected histogram in order to estimate and compare the white and gray matter peaks. In addition, in order to reduce the scanning time we designed a template based correction strategy. Images from different subjects were aligned using a twelve-parameter affine registration, and B1 maps were aligned according to that transformation. Recomputed T1 maps showed a significant improvement with respect to non-corrected ones. These results are very promising and have the potential for clinical application.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2009; DOI:10.1117/12.811180 · 0.20 Impact Factor