A new Monte Carlo simulation model for the transport of optical photons over rough surfaces
ABSTRACT A new Monte Carlo program has been developed for the simulation of the transport of optical photons over a rough planar surface. The new simulation employs a full three-dimensional model of a rough surface and addresses some of the limitations of the UNIFIED surface of UNDETECT2000 that is frequently used for the simulation of optical transport in PET detector crystals. A comparative study with DETECT2000 shows significant differences between the DETECT2000 UNIFIED model and our new surface model.
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ABSTRACT: Monochorionic twins have placental anastomoses that to varying degrees create a common circulation. This presents unique challenges for the performance of selective fetal termination in cases of twin-twin transfusion syndrome, twin reversed arterial perfusion sequence, or discordant twin abnormalities. Multiple methods of interrupting the affected twin's circulation have been attempted with variable success. One of the most frequent complications of any approach is iatrogenic preterm premature rupture of membranes. Laser coagulation in the midtrimester appears to be safe and effective; however, it is only available at limited centers. Currently, bipolar coagulation is the method of choice in the second half of pregnancy. The recently reported technique of radiofrequency ablation appears to be successful with minimal complications. Exploration of further percutaneous and noninvasive techniques, as well as interventions to decrease the morbidity arising from preterm membrane rupture, may lead to increased survival of the remaining twin and reduced risk of maternal complications.Seminars in Perinatology 11/2005; 29(5):330-7. · 2.81 Impact Factor
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ABSTRACT: Data are analyzed for 54 women who made an appointment with a North American Center specializing in multifetal pregnancy reduction (MFPR) to be counseled and possibly have a reduction. The impact on decision difficulty of combinations of three frames through which patients may understand and consider their options and use to justify their decisions are examined: a conceptional frame marked by a belief that life begins at conception; a medical frame marked by a belief in the statistics regarding risk and risk prevention through selective reduction; and a lifestyle frame marked by a belief that a balance of children and career has normative value. All data were gathered through semi-structured interviews and observation during the visit to the center over an average 2.5h period. Decision difficulty was indicated by self-assessed decision difficulty and by residual emotional turmoil surrounding the decision. Qualitative comparative analysis was used to analyze the impact of combinations of frames on decision difficulty. Separate analyses were conducted for those reducing only to three fetuses (or deciding not to reduce) and women who chose to reduce below three fetuses. Results indicated that for those with a non-intense conceptional frame, the decision was comparatively easy no matter whether the patients had high or low values of medical and lifestyle frames. For those with an intense conceptional frame, the decision was almost uniformly difficult, with the exception of those who chose to reduce only to three fetuses. Simplifying the results to their most parsimonious scenarios oversimplifies the results and precludes an understanding of how women can feel pulled in different directions by the dictates of the frames they hold. Variations in the characterization of intense medical frames, for example, can both pull toward reduction to two fetuses and neutralize shame and guilt by seeming to remove personal responsibility for the decision. We conclude that the examination of frame combinations is an important tool for understanding the way women carrying multiple fetuses negotiate their way through multi-fetal pregnancies, and that it may have more general relevance for understanding pregnancy decisions in context.Social Science [?] Medicine 01/2008; 65(11):2342-56. · 2.73 Impact Factor
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ABSTRACT: To compare the outcome of trichorionic triplet pregnancies managed expectantly with those reduced to twins or singletons. This was a retrospective study of trichorionic triplet pregnancies with three live fetuses at 10-14 (median 12) weeks' gestation referred to our unit for consideration of embryo reduction. Women were counselled as to the available options of either expectant management or embryo reduction. In those choosing reduction, a needle was inserted into the uterus transabdominally and potassium chloride was injected into the fetal heart. Using data derived from this study and from a review of studies reporting on survival and handicap by gestational age in singletons, the effects of embryo reduction on survival and handicap rates were estimated. Main outcome measures were miscarriage before 24 weeks of gestation, preterm delivery before 32 weeks, perinatal death and handicap rates. In total, there were 280 trichorionic triplet pregnancies and 125 of these were managed expectantly, 133 were reduced to two fetuses and 22 were reduced to one fetus. The rates of miscarriage were 3.2% for those managed expectantly, 8.3% for those reduced to twins and 13.6% for those reduced to singletons. The rates of early preterm delivery in those pregnancies that did not miscarry were 23.1%, 9.8% and 5.3%, respectively. The percentages for pregnancies with at least one survivor were 95.2%, 91.0% and 81.8%, respectively, and the median gestation at delivery was 34 weeks for the non-reduced, 36 weeks for those reduced to twins and 38 weeks for those reduced to singletons. From the published series on early preterm delivery, it was estimated that survival increases from about 27% at 24 weeks to about 98% at 32 weeks, and handicap decreases from 28% at 24 weeks to less than 5% at 32 weeks. From these estimates and the data on triplet pregnancies, it was calculated that, in triplets reduced to twins, compared to those managed expectantly, the chance of survival is similar (90.3% compared to 93.3%), but the risk of handicap may be lower (0.6% compared to 1.5% per fetus). In trichorionic triplet pregnancies, embryo reduction to twins does not improve the chance of survival but may reduce the rate of handicap. Reduction from triplets to singletons may reduce both the survival rate and the handicap rate among survivors.Journal of Maternal-Fetal and Neonatal Medicine 06/2002; 11(5):307-12. · 1.52 Impact Factor