226 research outputs found
A Mathematica Package for Computing N=2 Superfield Operator Product Expansions
We describe a general purpose Mathematica package for computing Superfield
Operator Product Expansions in meromorphic superconformal field theory.
Given the SOPEs for a set of ``basic" superfields, SOPEs of arbitrarily
complicated composites can be computed automatically. Normal ordered products
are always reduced to a standard form. It is possible to check the Jacobi
identities, and to compute Poisson brackets (``classical SOPEs''). We present
two explicit examples: a construction of the ``small'' superconformal
algebra in terms of superfields, and a realisation of the
superconformal algebra in terms of chiral and antichiral fermionic superfields.Comment: 15 pages, LaTeX. Minor corrections, particularly to Mathematica
output Out[6],Out[9] in section 4. Available through anonymous ftp from
ftp://euclid.tp.ph.ic.ac.uk/papers/ or on WWW at
http://euclid.tp.ph.ic.ac.uk/Papers
Higher-spin strings and W minimal models
We study the spectrum of physical states for higher-spin generalisations of
string theory, based on two-dimensional theories with local spin-2 and spin-
symmetries. We explore the relation of the resulting effective Virasoro string
theories to certain minimal models. In particular, we show how the
highest-weight states of the minimal models decompose into Virasoro
primaries.Comment: 13 pages, CTP TAMU-43/93, KUL-TF-93/9
Effect of positron range on PET quantification in diseased and normal lungs
The impact of positron range on PET image reconstruction has often been
investigated as a blurring effect that can be partly corrected by adding an element to
the PET system matrix in the reconstruction, usually based on a Gaussian kernel
constructed from the attenuation values. However, the physics involved in PET is
more complex. In regions where density does not vary, positron range indeed involves
mainly blurring. However, in more heterogeneous media it can cause other effects. This
work focuses on positron range in the lungs and its impact on quantification, especially
in the case of pathologies such as cancer or pulmonary fibrosis, for which the lungs have
localised varying density. Using Monte Carlo simulations, we evaluate the effects of
positron range for multiple radionuclides (18F, 15O, 68Ga, 89Zr, 82Rb, 64Cu and 124I) as,
for novel radiotracers, the choice of the labelling radionuclide is important. The results
demonstrate quantification biases in highly heterogeneous media, where the measured
uptake of high-density regions can be increased by the neighbouring radioactivity from
regions of lower density, with the effect more noticeable for radionuclides with highenergy positron emission. When the low-density regions are considered to have less
radioactive uptake (e.g. due to the presence of air), the effect is less severe
Immunogenicity of targeted lentivectors
To increase the safety and possibly efficacy of HIV-1 derived lentivectors (LVs) as an anti-cancer vaccine, we recently developed the Nanobody (Nb) display technology to target LVs to antigen presenting cells (APCs). In this study, we extend these data with exclusive targeting of LVs to conventional dendritic cells (DCs), which are believed to be the main cross-presenting APCs for the induction of a TH1-conducted antitumor immune response. The immunogenicity of these DC-subtype targeted LVs was compared to that of broad tropism, general APC-targeted and non-infectious LVs. Intranodal immunization with ovalbumin encoding LVs induced proliferation of antigen specific CD4(+) T cells, irrespective of the LVs' targeting ability. However, the cytokine secretion profile of the restimulated CD4(+) T cells demonstrated that general APC targeting induced a similar TH1-profile as the broad tropism LVs while transduction of conventional DCs alone induced a similar and less potent TH1 profile as the non-infectious LVs. This observation contradicts the hypothesis that conventional DCs are the most important APCs and suggests that the activation of other APCs is also meaningful. Despite these differences, all targeted LVs were able to stimulate cytotoxic T lymphocytes, be it to a lesser extent than broad tropism LVs. Furthermore this induction was shown to be dependent on type I interferon for the targeted and non-infectious LVs, but not for broad tropism LVs. Finally we demonstrated that the APC-targeted LVs were as potent in therapy as broad tropism LVs and as such deliver on their promise as safer and efficacious LV-based vaccines
Estimation of Timing Resolution for Very Fast Time-Of-Flight Detectors in Monte Carlo Simulations
In PET imaging the depth of absorption in the
crystal contributes to the detection time uncertainty, which
impacts the time resolution of the scatter. In addition, affects
the nature of the timing distribution. It was found that when
Photon Travel Spread (PTS) in the crystal is the only factor
affecting the timing uncertainty, in which case, a Laplace kernel
might describe the measured data, more accurately. It was shown
that for crystals as thin as 20 mm the RMSE of the Laplace was
smaller than that of a Normal. While when PTS is combined
with an addition coincidence detection resolution (CDR) then,
a Normal achieves better RMSE, but with dependency on the
crystal size. Results in terms of CRC, of a simulated NEMA
phantom, confirmed that reconstruction using a Laplace kernel
can model the data better for thicker crystals
The BV-algebra structure of W_3 cohomology
We summarize some recent results obtained in collaboration with J. McCarthy
on the spectrum of physical states in gravity coupled to matter. We
show that the space of physical states, defined as a semi-infinite (or BRST)
cohomology of the algebra, carries the structure of a BV-algebra. This
BV-algebra has a quotient which is isomorphic to the BV-algebra of polyvector
fields on the base affine space of . Details have appeared elsewhere.
[Published in the proceedings of "Gursey Memorial Conference I: Strings and
Symmetries," Istanbul, June 1994, eds. G. Aktas et al., Lect. Notes in Phys.
447, (Springer Verlag, Berlin, 1995)]Comment: 8 pages; uses macros tables.tex and amssym.def (version 2.1 or later
Estimation of optimal number of gates in dual gated ¹⁸F-FDG cardiac PET
Gating of positron emission tomography images has been shown to reduce the motion effects, especially when imaging small targets, such as coronary plaques. However, the selection of optimal number of gates for gating remains a challenge. Selecting too high number of gates results in a loss of signal-to-noise ratio, while too low number of gates does remove only part of the motion. Here, we introduce a respiratory-cardiac motion model to determine the optimal number of respiratory and cardiac gates. We evaluate the model using a realistic heart phantom and data from 12 cardiac patients (47–77 years, 64.5 on average). To demonstrate the benefits of our model, we compared it with an existing respiratory model. Based on our study, the optimal number of gates was determined to be five respiratory and four cardiac gates in the phantom and patient studies. In the phantom study, the diameter of the most active hot spot was reduced by 24% in the dual gated images compared to non-gated images. In the patient study, the thickness of myocardium wall was reduced on average by 21%. In conclusion, the motion model can be used for estimating the optimal number of respiratory and cardiac gates for dual gating
Motion-corrected reconstruction of parametric images from dynamic PET data with the Synergistic Image Reconstruction Framework (SIRF)
Motion correction has been added to a PET-MR reconstruction tool, SIRF, by incorporating a registration package, NiftyReg. New functionality has been demonstrated in the context of estimating kinetic parameters in the left temporal lobe, comparing direct and indirect reconstructions and evaluating the impact of using motion correction.Principal component analysis was used to detect motion and to determine time frames, while STIR's parametric-OSEM was used to perform the motion-corrected direct parametric reconstruction.It was found that the variance in the left temporal lobe decreased when motion correction was performed, and the same was true of direct reconstructions compared to indirect.With SIRF, the entirety of the demonstrated functionality can be performed from a single Matlab or Python script
Versatile regularisation toolkit for iterative image reconstruction with proximal splitting algorithms
Ill-posed image recovery requires regularisation to ensure stability. The presented open-source regularisation toolkit consists of state-of-the-art variational algorithms which can be embedded in a plug-and-play fashion
into the general framework of proximal splitting methods. The packaged regularisers aim to satisfy various prior expectations of the investigated objects, e.g., their structural characteristics, smooth or non-smooth surface morphology.
The flexibility of the toolkit helps with the design of more advanced model-based iterative reconstruction methods
for different imaging modalities while operating with simpler building blocks. The toolkit is written for CPU and
GPU architectures and wrapped for Python/MATLAB. We demonstrate the functionality of the toolkit in application
to Positron Emission Tomography (PET) and X-ray synchrotron computed tomography (CT)
Формування теоретичної моделі геополітичного дискурсу у вітчизняній політичній думці кінця ХХ – початку ХХІ століття
У статті висвітлюються питання щодо започаткування новітньої дослідницької традиції геополітичного дискурсу у проблематиці вітчизняної політичної думки ХХ – початку ХХІ століття. Зазначено позиції провідних вітчизняних вчених щодо формування емпіричного та ідейно-теоретичного підґрунтя для утвердження цієї традиції політичного дослідження.The article considers the questions of the becoming of a new research tradition of
geopolitical discourse in the topic of native political thought of the 20-th – the beginning of the 21-st century. The views of leading home scientists about the development of empirical, ideological and theoretical basis for the maintenance of this tradition of political research are pointed out
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