The goal of physics is to formulate theories, or “laws,”
which summarize our knowledge of the natural world. We don’t yet know
all the basic laws of nature,or how many laws there are.
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Science Poster Session
15 March 2010
Coherent and coherently scattered x-ray in bio-medical imaging
Time: 12:30pm - 1:30pm
Location: 2L17 (map)
Congwu Cui, CancerCare, MB, Department of Medical Physics
X-ray is a form of electromagnetic wave with a wavelength in the range of 10~0.01 nanometer. Because of its ability of penetrating solid objects, x-ray is used to "see" the inside of objects as small as a cell to as big as a truck. In medical imaging, x-ray is used to take images of human body for diagnosing or treatment planning.
Being a wave, x-ray has both amplitude and phase. Although in the x-ray range the phase may be a thousand times more sensitive than the amplitude and has a great potential in medical and biological imaging applications, only the amplitude information is used in the majority applications because the current detector technique cannot directly record the phase information. To obtain the phase information, coherent source is needed to form interference patterns of the x-rays after interacting with an object. A new technique taking advantage of the phase of x-rays, coherent x-ray diffraction imaging (CXDI), has been developed to image objects with nanometer spatial resolution. The CXDI technique can be used in biomedicine, material science, and other fields. This presentation will briefly talk about how the CXDI technique works, its applications in measuring the three dimensional local structures of several materials, e.g. a fabricated 3D object, Ta2O5 aerogel, pumice, etc., and application in massively parallel x-ray holography.
When interacting with matter, the coherently scattered x-rays by neighboring atoms contain the information of molecular structures of the matter. In the bio-medical applications, because the normal and pathologically changed tissues have different molecular structures but very similar attenuation to x-rays, the coherently scattered x-ray may be used to identify and map a specific component, such as a tumor, which may be difficult to be distinguished from tissues around it in the traditional attenuation contrast images. A new computed tomography method, direct 3D coherently scattered x-ray tomography for directly generating 3D images of small animals, biopsies, or other small objects with low attenuation contrast, will also be talked about in this presentation.
19 March 2010
Theoretical Limits to Fluorescence Microscopy With Nanometer Resolution
Time: 12:30pm - 1:30pm
Location: 2L17 (map)
Alex Small, Calpoly Pomona
Conventional optical microscopes cannot resolve cellular features smaller than approximately half the wavelength of light: The lens diffracts light, causing it to spread out and form a small blur rather than focus to a single point. This has limited the ability of biologists to detect fine details in live cells. Recent experimental work has shown that it is possible to get past this limitation and form images of fluorescently labeled cells with resolution as fine as 10 nanometers or better. These techniques work by switching fluorescent molecules on and off: At any given time only a small fraction of the molecules are emitting light, and they form non-overlapping blurs on the detector. The centers of these blurs are determined, leading to a map of molecular positions. There is a trade-off between speed and image quality in these approaches, however, as activating more molecules at once speeds up the imaging process but also runs the risk of forming overlapping blurs. Biophysically relevant details revealed by these techniques include cytoskeletal proteins, membranes, and the co-location of mitochondria and microtubules.
My group has been working on understanding the theoretical limits to what is achievable in these new microscopy techniques. Statistical calculations show that there is a maximum acquisition rate, but also a maximum error rate. We also show that the achievable speed and image quality depend on the algorithms used to analyze images and form molecular maps, and not just on the hardware used to acquire the image. Our models predict the properties required for optimal image analysis algorithms, and we are working on translating these predictions into useful computational tools. Interestingly, our work leads to a way of benchmarking different image analysis algorithms, leading to the interesting finding that algorithms of widely different speed and complexity can yield similar performance on the most relevant metrics. In addition, our model predicts the effects of photobleaching on image quality, and provides a framework for analyzing the effects of "blinking" by molecules.
22 March 2010
Asymmetric Stem Cell Division in Drosophila Testis
Time: 12:30pm - 1:30pm
Location: 2L17 (map)
Jun Cheng, University of Michigan
Stem cells have received much attention due to their scientific importance and potential medical applications. The balance of stem cell's unique abilities, self-renewal and differentiation, must be tightly regulated to maintain tissue homeostasis, avoiding carcinogenesis or tissue aging. However, the regulatory mechanisms maintaining this balance remain to be elucidated. The Drosophila male gonad is amongst the best characterized model systems to study the regulation of stem cells. Previous results showed that centrosomes in germline stem cells are oriented and this ensures asymmetric stem cell division, generating one self-renewing stem cell and one differentiating cell. Here, to overcome the limitation associated with fixed tissue, the tissue-culture protocol is developed to perform time-lapse live-imaging microscopy of stem cells in Drosophila testis, revolutionizing the study of stem cell biology. Results of the time-lapse live-imaging microscopy reveal that there exists a checkpoint in germline stem cell monitoring centrosome orientation and the activation of the checkpoint contributes to the disruption of tissue homeostasis during aging. Furthermore, results show that the somatic cyst stem cells in Drosophila testis divide asymmetrically through the mechanism of anaphase spindle repositioning. Also suggested is a tight division coordination between the germline stem cells and somatic cyst stem cells.
26 March 2010
Research Talk
Time: 12:30pm - 1:30pm
Location: 2L17 (map)
Esmat Elhami, University of Manitoba
