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Biomedical subjects

X L Meng

Publications and source records attributed to X L Meng.

9 recordsLinked to original sources
PubMed2006

Polarization-resolved study of diode-pumped passively Q-switched Nd:GdVO4 lasers.

By using the natural birefringence of an a-cut Nd:GdVO(4) crystal, we experimentally investigate laser operation under different light polarizations (pi- and sigma-polarized emissions) and compare their performance. The laser performance is further compared with a diode-pumped c-cut Nd:GdVO(4) laser of the same parameters. We show experimentally that the c-cut Nd:GdVO(4) laser could have good passively Q-switched performance in a low-pump-power region but is influenced by other intrinsic laser effects, such as the thermally induced cavity losses; its performance is severely degraded under high pump power. The best passively Q-switched performance obtained in our experiments is from the a-cut pi-polarized laser, which gives the narrowest pulse of 5.4 ns with the highest peak power of 20 kW. Our experimental results show that, to achieve the best performance of the passively Q-switched laser operation, an overall consideration of laser parameters is important.

Journal Article↗
PubMed2003

Deterministic chaos in a diode-pumped Nd:YAG laser passively Q switched by a Cr4+:YAG crystal.

We report the experimental observation and numerical simulation of deterministic chaos in a diode-pumped Nd:YAG solid-state laser that is passively Q switched by a Cr4+:YAG crystal saturable absorber. We show that apart from the stable Q-switched operation the laser can also exhibit complicated nonlinear dynamics, including the period-doubling route to chaos and periodic windows.

Journal Article↗
PubMed2001

Cytotoxicity of ascorbate, lipoic acid, and other antioxidants in hollow fibre in vitro tumours.

Vitamin C (ascorbate) is toxic to tumour cells, and has been suggested as an adjuvant cancer treatment. Our goal was to determine if ascorbate, in combination with other antioxidants, could kill cells in the SW620 hollow fibre in vitro solid tumour model at clinically achievable concentrations. Ascorbate anti-cancer efficacy, alone or in combination with lipoic acid, vitamin K3, phenyl ascorbate, or doxorubicin, was assessed using annexin V staining and standard survival assays. 2-day treatments with 10 mM ascorbate increased the percentage of apoptotic cells in SW620 hollow fibre tumours. Lipoic acid synergistically enhanced ascorbate cytotoxicity, reducing the 2-day LC(50)in hollow fibre tumours from 34 mM to 4 mM. Lipoic acid, unlike ascorbate, was equally effective against proliferating and non-proliferating cells. Ascorbate levels in human blood plasma were measured during and after intravenous ascorbate infusions. Infusions of 60 g produced peak plasma concentrations exceeding 20 mM with an area under the curve (24 h) of 76 mM h. Thus, tumoricidal concentrations may be achievable in vivo. Ascorbate efficacy was enhanced in an additive fashion by phenyl ascorbate or vitamin K3. The effect of ascorbate on doxorubicin efficacy was concentration dependent; low doses were protective while high doses increased cell killing.

Antioxidants↗
PubMed2000

[Expression of 2.1 kb enhancin gene fragment from Helicoverpa armigera granulosis virus in Escherichia coli].

The 2.1 kb fragment of enhancin gene from Helicoverpa armigera granulosis virus was inserted into vector pQE-30 and expressed successfully in E. coli M15(pREP4). The synergy of expression product(P78) on AcMNPV against the larvae of Plutella xylostella was also studied. The results indicated that the percentage of correct mortality of the larvae increased 27.88%-32.92% in 10 post-infection days.

Baculoviridae↗
PubMed1999

Applications of multiple imputation in medical studies: from AIDS to NHANES.

Rubin's multiple imputation is a three-step method for handling complex missing data, or more generally, incomplete-data problems, which arise frequently in medical studies. At the first step, m (> 1) completed-data sets are created by imputing the unobserved data m times using m independent draws from an imputation model, which is constructed to reasonably approximate the true distributional relationship between the unobserved data and the available information, and thus reduce potentially very serious nonresponse bias due to systematic difference between the observed data and the unobserved ones. At the second step, m complete-data analyses are performed by treating each completed-data set as a real complete-data set, and thus standard complete-data procedures and software can be utilized directly. At the third step, the results from the m complete-data analyses are combined in a simple, appropriate way to obtain the so-called repeated-imputation inference, which properly takes into account the uncertainty in the imputed values. This paper reviews three applications of Rubin's method that are directly relevant for medical studies. The first is about estimating the reporting delay in acquired immune deficiency syndrome (AIDS) surveillance systems for the purpose of estimating survival time after AIDS diagnosis. The second focuses on the issue of missing data and noncompliance in randomized experiments, where a school choice experiment is used as an illustration. The third looks at handling nonresponse in United States National Health and Nutrition Examination Surveys (NHANES). The emphasis of our review is on the building of imputation models (i.e. the first step), which is the most fundamental aspect of the method.

Acquired Immunodeficiency Syndrome↗
PubMed1997

The EM algorithm and medical studies: a historical link.

Anderson Gray McKendrick's 1926 paper, 'Applications of mathematics to medical problems', was the earliest reference cited in Dempster et al.'s 1977 paper that defined and popularized the EM algorithm. McKendrick's paper was prominently featured by Joseph Oscar Irwin in his 1962 inaugural address as the President of the Royal Statistical Society (in the UK), entitled 'The place of mathematics in medical and biological statistics'. The link of McKendrick's work to the EM algorithm is due to an improvement made by Irwin on a novel method McKendrick used for estimating an infection rate when the observed data do not distinguish between those individuals who are not susceptible to the infection and those who are susceptible, but do not develop symptoms. This article examines this link, along the way illustrating the central ideas underlying the EM algorithm as well as its properties; the examination also suggests a profiling strategy for speeding up EM, which may be worthy of general investigation. McKendrick's data on an epidemic of cholera are used for illustration and to compare EM with Irwin's method as well as the Newton-Raphson algorithm. Issues beyond computation are also discussed whenever appropriate.

Algorithms↗
PubMed1993

Seroepidemiology of hepatitis C virus in Beijing, China.

To investigate the seroprevalence of hepatitis C virus (HCV) in China we tested sera from healthy individuals without hepatitis and no history of parenteral blood exposure and from patients admitted to a hepatitis hospital in Beijing. Sera were tested for anti-HCV by first-generation enzyme immunoassay; selected positives were tested with two second-generation EIAs, one utilizing recombinant antigens and the other synthetic peptides. We found anti-HCV with the following frequencies: 10 of 164 (6%) individuals with no disease; 2 of 36 (5.5%) patients with acute non-A non-B hepatitis (NANBH); 26 of 39 (67%) patients with post-transfusion NANBH; 10 of 34 (29%) patients with chronic hepatitis negative for hepatitis B surface antigen (HBsAg); 3 of 30 (10%) patients with chronic HBsAg-positive hepatitis; 0 of 19 patients with acute HBsAg-positive hepatitis. Of 24 repeat-positive sera, 19 were positive by both and 4 by one second-generation tests. We conclude that hepatitis C infection is common in China, that it contributes substantially to the incidence of post-transfusion hepatitis, and that HCV plays a significant role in both acute and chronic hepatitis. Further studies are needed to extend these observations and to define the predominant routes of transmission of HCV in China.

Journal Article↗
PubMed1993

Survival differences and trends in patients with AIDS in the United States.

The AIDS surveillance system maintained by the Centers for Disease Control and Prevention (CDC) provides a unique data base for estimating survival after a diagnosis of AIDS for the general AIDS population in the United States. Because patients enrolled in most AIDS clinical trial studies receive unusual medical care that may not be available to the general public and typically have relatively longer survival time, estimates obtained from these studies may not be of direct use in assessing the national health-care needs. Furthermore, such studies are usually of short duration and may not be very informative for long-term health-policy planning. We present survival estimates obtained from the CDC surveillance data for the adult/adolescent AIDS population in the United States and compare their survival and trend in survival on gender, sexual behavior, and injection-drug use status. These estimates provide information for mortality risk after an AIDS diagnosis over a period of 8 years and for trend of survival during the period between 1983 and 1991.

Acquired Immunodeficiency Syndrome↗