424 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
Adaptive adjustment of the number of subsets during iterative image reconstruction
A common strategy to speed-up image reconstruction in tomography is to use subsets, i.e. only part of the data is used to compute the update, as for instance in the OSEM algorithm. However, most subset algorithms do not convergence or have a limit cycle. Different strategies to solve this problem exist, for instance using relaxation. The conceptually easiest mechanism is to reduce the number of subsets gradually during iterations. However, the optimal point to reduce the number of subsets is usually depends on many factors such as initialisation, the object itself, amount of noise etc. In this paper, we propose a simple scheme to automatically compute if the number of subsets is too large (or too small) and adjust the size of the data to consider in the next update automatically. The scheme is based on idea of computing two image updates corresponding to different parts of the data. A comparison of these updates then allows to see if the updates were sufficiently consistent or not. We illustrate this idea using 2 different subset algorithms: OSEM and OSSPS
Advances in Clinical Molecular Imaging Instrumentation
In this article, we describe recent developments in the design of both single-photon emission computed tomography (SPECT) and positron emission tomography (PET) instrumentation that have led to the current range of superior performance instruments. The adoption of solid-state technology for either complete detectors [e.g., cadmium zinc telluride (CZT)] or read-out systems that replace photomultiplier tubes [avalanche photodiodes (APD) or silicon photomultipliers (SiPM)] provide the advantage of compact technology, enabling flexible system design. In SPECT, CZT is well suited to multi-radionuclide and kinetic studies. For PET, SiPM technology provides MR compatibility and superior time-of-flight resolution, resulting in improved signal-to-noise ratio. Similar SiPM technology has also been used in the construction of the first SPECT insert for clinical brain SPECT/MRI
Quantising Higher-spin String Theories
In this paper, we examine the conditions under which a higher-spin string
theory can be quantised. The quantisability is crucially dependent on the way
in which the matter currents are realised at the classical level. In
particular, we construct classical realisations for the algebra,
which is generated by a primary spin- current in addition to the
energy-momentum tensor, and discuss the quantisation for . From these
examples we see that quantum BRST operators can exist even when there is no
quantum generalisation of the classical algebra. Moreover, we find
that there can be several inequivalent ways of quantising a given classical
theory, leading to different BRST operators with inequivalent cohomologies. We
discuss their relation to certain minimal models. We also consider the
hierarchical embeddings of string theories proposed recently by Berkovits and
Vafa, and show how the already-known strings provide examples of this
phenomenon. Attempts to find higher-spin fermionic generalisations lead us to
examine the whether classical BRST operators for ( odd)
algebras can exist. We find that even though such fermionic algebras close up
to null fields, one cannot build nilpotent BRST operators, at least of the
standard form.Comment: CTP TAMU-24/94, KUL-TF-94/11, SISSA-135/94/E
Magnetic coupling with 3D knitted helical coils
Continuous power supply for wearable electronics can be facilitated using wireless power transfer (WPT). We use a 3D knitted helical coil as the receiver coil in the wrist or the waist of a garment. This 3D knitted helical coil is a novel approach to integrate coils in garments that maintains full flexibility of the garment. Measurements and simulations of coil-coil coupling give compelling evidence of the feasibility of this approach for wearable WTP. The coupling factor between a closely wound and knitted coil is found to be ~0.25 and ~0.55 for adjacent coils for a knit in the border of a cuff and waist, respectively. Using a simple circuit, we demonstrate a 30% efficiency between a closely wound transmitter coil worn on the wrist and a 3D knitted helical receiver coil integrated in the cuff of a garment at 6 mm distance
Improved correction for the tissue fraction effect in lung PET/CT imaging
Recently, there has been an increased interest in imaging different pulmonary disorders using PET techniques. Previous work has shown, for static PET/CT, that air content in the lung influences reconstructed image values and that it is vital to correct for this 'tissue fraction effect' (TFE). In this paper, we extend this work to include the blood component and also investigate the TFE in dynamic imaging. CT imaging and PET kinetic modelling are used to determine fractional air and blood voxel volumes in six patients with idiopathic pulmonary fibrosis. These values are used to illustrate best and worst case scenarios when interpreting images without correcting for the TFE. In addition, the fractional volumes were used to determine correction factors for the SUV and the kinetic parameters. These were then applied to the patient images. The kinetic parameters K1 and Ki along with the static parameter SUV were all found to be affected by the TFE with both air and blood providing a significant contribution to the errors. Without corrections, errors range from 34-80% in the best case and 29-96% in the worst case. In the patient data, without correcting for the TFE, regions of high density (fibrosis) appeared to have a higher uptake than lower density (normal appearing tissue), however this was reversed after air and blood correction. The proposed correction methods are vital for quantitative and relative accuracy. Without these corrections, images may be misinterpreted
W-algebras with set of primary fields of dimensions (3, 4, 5) and (3,4,5,6)
We show that that the Jacobi-identities for a W-algebra with primary fields
of dimensions 3, 4 and 5 allow two different solutions. The first solution can
be identified with WA_4. The second is special in the sense that, even though
associative for general value of the central charge, null-fields appear that
violate some of the Jacobi-identities, a fact that is usually linked to
exceptional W-algebras. In contrast we find for the algebra that has an
additional spin 6 field only the solution WA_5.Comment: 17 pages, LaTeX, KCL-TH-92-9, DFFT-70/9
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
- …