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S Yiu

Publications and source records attributed to S Yiu.

6 recordsLinked to original sources
PubMed1997

A study of thyroid radioiodine monitoring by Monte Carlo simulations: implications for equipment design.

Monte Carlo simulations have been performed to evaluate the design of collimated detectors used to measure 125I or 131I in the thyroid gland. Two detector sizes were simulated for each radioisotope: (i) for 125I monitoring 2.54 cm diameter and 7.62 cm diameter and 0.2 cm thickness and (ii) for 131I monitoring 2.54 cm diameter, 3.2 cm thickness and 7.62 cm diameter, 6.4 cm thickness. The virtual thyroid gland was 20 g. Activity was placed in both the gland and the remainder of the body in varying amounts to assess the efficacy of collimation. The results show that the detector should be sufficiently large so that its solid angle of acceptance when placed 15 cm anterior to the skin surface will include the whole of a moderately enlarged thyroid gland. Heavy collimation to reduce the contribution of extrathyroidal radioiodine within the subject's body is not normally required. It may be of more value as a positioning device and spacer ensuring an appropriate and constant neck to detector distance than in cutting down counts from extrathyroidal activity. In specifying a sensitive detector system for monitoring intrathyroidal radioiodine, a wide angle of acceptance and sufficient detector crystal thickness take precedence over collimation and shielding.

Computer Simulation↗
PubMed1997

Examination of the effect of counting geometry on 125I monitoring using MCNP.

This paper theoretically investigates how the counting efficiencies of three selected detector (NaI) sizes are affected by neck-detector distance, detector misplacement, thickness of overlaying tissue, and size of the thyroid gland. The detector sizes were small, 2.54 cm diameter and 0.2 cm crystal thickness; medium, 7.62 cm diameter and 0.2 cm crystal thickness; large 30.48 cm diameter and 0.2 cm crystal thickness. The evaluations were determined using a Monte Carlo technique. The simulations show that the large detector minimizes the uncertainties for all problems except the thickness of overlaying tissue.

Humans↗
PubMed1996

Synthesis, biological evaluation, calcium channel antagonist activity, and anticonvulsant activity of felodipine coupled to a dihydropyridine-pyridinium salt redox chemical delivery system.

3-(2-Hydroxyethyl) 5-methyl 1,4-dihydro-2,6-dimethyl-4-(2,3-dichlorophenyl)-3,5-pyridinedi-carboxyla te (7) was prepared using a modified Hantzsch reaction, which was then elaborated to 3-[2-[[(1-methyl-1,4-dihydropyrid-3-yl)carbonyl]oxy]ethyl]5-methyl 1,4-dihydro-2,6-dimethyl-4-(2,3-dichlorophenyl)-3,5-pyridinedicarboxylat e [10, felodipine-chemical delivery system (CDS)]. The equipotent 3-(2-hydroxyethyl) 7 (IC50 = 3.04 x 10(-8) M) and felodipine-CDS (10, IC50 = 3.10 x 10(-8) M) were, respectively, 2- and 21-fold less potent calcium channel antagonists than the reference drugs nimodipine (IC50 = 1.49 x 10(-8) M) and felodipine (IC50 = 1.45 x 10(-9) M). Compounds 7, 10, nimodipine, and felodipine are highly lipophilic (Kp = 236, 366, 187, and 442, respectively). 3-(2-Hydroxyethyl) 7, felodipine-CDS (10), and felodipine provided protection against maximal electroshock-induced seizures in mice but were inactive in the subcutaneous metrazol anticonvulsant screen. In vitro incubation studies of felodipine with rat plasma and 20% brain homogenates showed felodipine was very stable in both biological media. Similar incubations of felodipine-CDS showed its rate of biotransformation followed psuedo-first-order kinetics with half-lives of 15.5 h in rat plasma and 1.3 h in 20% rat brain homgenates. In vivo biodistribution of felodipine and felodipine-CDS was studied. Uptake of felodipine in brain produced a peak brain concentration of 5 micrograms/g of brain tissue at 5 min, after which it rapidly egressed from brain resulting in undetectable levels at 60 min. Peak blood concentrations of 10 occurred at about 7 min followed by a rapid decline to a near undetectable concentration by 17 min. The pyridinium salt species 9, resulting from oxidation of 10, also reached peak concentrations at about 7 min but it slowly decreased to undetectable concentrations at 2 h. 3-(2-Hydroxyethyl) 7 remained at near undetectable concentrations throughout a 2 h time period. Localization of 10 in brain provided a peak concentration of 4.2 micrograms/g of brain tissue at 5 min and then decreased to negligible concentrations at 15 min. The concentration of oxidized pyridinium species 9 in brain remained high providing detectable concentrations up to 4 days. In contrast, the concentration of the 3-(2-hydroxyethyl) hydrolysis product 7 in brain remained at very low levels throughout the study. The slow hydrolysis rate of the pyridinium ester 9 to the 3-(2-hydroxyethyl) 7 and the rapid egression of felodipine-CDS from brain are believed to contribute to the moderate anticonvulsant activity exhibited hy the felodipine-CDS (10).

Animals↗
PubMed1990

Delta and conjugated bilirubin as complementary markers of early rejection in liver-transplant recipients.

For 51 liver allograft recipients, we evaluated whether serum profiles of delta (Bd) and conjugated bilirubins (Bc) could be used to diagnose rejection during the first 30-50 postoperative days, in comparison with histology as the "gold standard." Daily measurements of aspartate aminotransferase, alkaline phosphatase, total bilirubin, Bd, and Bc were made, the last two by liquid chromatography. In 34 patients without any biochemical or histological evidence of rejection, within seven to 10 postoperative days Bd increased to greater than 40-50% of total bilirubin, while Bc decreased to less than 10%. In patients with severe rejections resulting in death, Bc increased rapidly to greater than 50%, while Bd remained less than 30%. In 24 histologically proven episodes of rejection in 17 patients there was either a rapid decrease in Bd or its persistence at 30%, plus either a steep increase in Bc, or its remaining at greater than 50% total bilirubin. Treatment of rejection resulted in a prompt reversal of these trends. Individually promising as prognostic adjuncts to existing liver-function markers, Bd and Bc also complemented each other, rendering the diagnosis of liver rejection much more incisive.

Adult↗
PubMed1986

Reliability of fixation preference testing in diagnosing amblyopia.

We evaluated the sensitivity and specificity of standard fixation preference testing and the 10-prism diopter (PD) fixation test in 427 patients. Standard fixation preference testing showed good sensitivity and specificity for than 10 PD. Patients with small angle tropias, however, had a high false-positive rate, with 40% of nonamblyopic patients inappropriately responding with strong fixation preference. The 10-PD fixation test reduced this high false-positive rate and showed only 1.5% false-positive results. Unfortunately, this shift to equal fixation preference was also seen in some patients with amblyopia, as five of 19 patients with three or more lines difference alternated fixation with the 10-PD fixation test. Retesting disclosed that, although these children would briefly alternate to the amblyopic eye while viewing a stationary target, they would not hold fixation with the amblyopic eye through smooth pursuit. Our conclusion is that standard fixation preference testing can be used to diagnose moderate to severe amblyopia in patients with tropias greater than 10 PD. Patients with small angle deviations or straight eyes should be examined with the 10-PD fixation test, with the criterion for equal vision being the ability to hold fixation well with either eye through smooth pursuit.

Amblyopia↗