Browsing by Author "Lee, Y. -H."

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Evaluating Instantaneous Perfusion Responses of Parotid Glands to Gustatory Stimulation Using High-Temporal-Resolution Echo-Planar Diffusion-Weighted Imaging
(2016-10)
Chiu, T. -W.
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Liu, Y. -J.
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Chang, H. -C.
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Lee, Y. -H.
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Lee, J. -C.
BACKGROUND AND PURPOSE: Parotid glands secrete and empty saliva into the oral cavity rapidly after gustatory stimulation. However, the role of the temporal resolution of DWI in investigating parotid gland function remains uncertain. Our aim was to design a high-temporal-resolution echo-planar DWI pulse sequence and to evaluate the instantaneous MR perfusion responses of the parotid glands to gustatory stimulation. MATERIALS AND METHODS: This prospective study enrolled 21 healthy volunteers (M/F = 2:1; mean age, 45.2 12.9 years). All participants underwent echo-planar DWI (total scan time, 304 seconds; temporal resolution, 4 s/scan) on a 1.5T MR imaging scanner. T2WI (b = 0 s/mm(2)) and DWI (b = 200 s/mm(2)) were qualitatively assessed. Signal intensity of the parotid glands on T2WI, DWI, and ADC was quantitatively analyzed. One-way ANOVA with post hoc group comparisons with Bonferroni correction was used for statistical analysis. P < .05 was statistically significant. RESULTS: Almost perfect interobserver agreement was achieved ( 0.656). The parotid glands had magnetic susceptibility artifacts in 14.3% (3 of 21) of volunteers during swallowing on DWI but were free from perceptible artifacts at the baseline and at the end of scans on all images. Increased ADC and reduced signal intensity of the parotid glands on T2WI and DWI occurred immediately after oral administration of lemon juice. Maximal signal change of ADC (24.8% +/- 10.8%) was significantly higher than that of T2WI (-10.1% +/- 5.2%, P < .001). The recovery ratio of ADC (100.71% +/- 42.34%) was also significantly higher than that of T2WI (22.36% +/- 15.54%, P < .001). CONCLUSIONS: Instantaneous parotid perfusion responses to gustatory stimulation can be quantified by ADC by using high-temporal-resolution echo-planar DWI.
Thermodynamic Characterization of Polymorphs in Bulk-Crystallized Syndiotactic Polystyrene via Small/Wide-Angle X-ray Scattering and Differential Scanning Calorimetry
(2009-06-23)
Su, C. H.
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Jeng, U.
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Chen, S. H.
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Cheng, C. -Y.
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Lee, J. -J.
By means of in situ small/wide-angle X-ray scattering (SAXS/WAXS) and differential scanning calorimetry (DSC), we examined evolutions of lamellar crystal thickness for alpha and beta crystals, respectively, in bulk-crystallized syndiotactic polystyrene (sPS) during the partial melting-reorganization process upon progressive heating up to 290 degrees C. For the SAXS data analysis, the Kratky-Porod approximation proves to be particularly helpful in extracting the crystal thickness when approaching final melting where crystalline lamellae (near equilibration with the melt) exist in low concentrations as dispersed entities instead of in arrays. On the basis of the crystal thicknesses at elevated temperatures under solid-melt equilibration, we constructed melting lines of the two separate forms in the Gibbs-Thomson phase plane. The extrapolated (to infinite lamellar thickness) equilibrium melting temperature T(m,alpha)* approximate to 294 degrees C of the a form is moderately lower than T(m,beta)* approximate to 306 degrees C of the beta form. The two melting lines intercept at a crossover temperature T(Q) approximate to 284 degrees C and crystal thickness l(Q) approximate to 9.6 nm, where the relative thermal stability of the two phases inverses. For crystals thicker than l(Q) (practically hard to reach for bulk crystallization under ambient pressure), the beta form is the stable phase; for crystals thinner than I(Q) (the commonly accessible case), the a form is circumstantially more stable. With crystallinity-corrected values of the heat of fusion Delta H(f,alpha) approximate to 82 MJ m(-3) and Delta H(f,beta) approximate to 146 MJ m(-3) obtained from a combination of DSC and WAXS results, we determined from the slope (= 2 sigma(e)/Delta H(f)) of the melting line that basal surface energy sigma(e,alpha) approximate to 8.2 mJ m(-2) and sigma(e,beta) approximate to 26.8 mJ m(-2) which are considerably lower than those expected for tight folds, indicative of nonadjacently re-entered or loosely looped folds. The combination of lower T(m)*, Delta H(f), Delta S(f), and sigma(e) values renders the a phase highly competitive in the rate of nucleation at low temperatures but much less so at high temperatures as compared to the beta phase. The higher sigma(e,beta) value is also consistent with the observation that the beta phase is more responsive to externally added heterogeneous nucleation agents.