32,388 research outputs found
On the Adjoint Operator in Photoacoustic Tomography
Photoacoustic Tomography (PAT) is an emerging biomedical "imaging from
coupled physics" technique, in which the image contrast is due to optical
absorption, but the information is carried to the surface of the tissue as
ultrasound pulses. Many algorithms and formulae for PAT image reconstruction
have been proposed for the case when a complete data set is available. In many
practical imaging scenarios, however, it is not possible to obtain the full
data, or the data may be sub-sampled for faster data acquisition. In such
cases, image reconstruction algorithms that can incorporate prior knowledge to
ameliorate the loss of data are required. Hence, recently there has been an
increased interest in using variational image reconstruction. A crucial
ingredient for the application of these techniques is the adjoint of the PAT
forward operator, which is described in this article from physical, theoretical
and numerical perspectives. First, a simple mathematical derivation of the
adjoint of the PAT forward operator in the continuous framework is presented.
Then, an efficient numerical implementation of the adjoint using a k-space time
domain wave propagation model is described and illustrated in the context of
variational PAT image reconstruction, on both 2D and 3D examples including
inhomogeneous sound speed. The principal advantage of this analytical adjoint
over an algebraic adjoint (obtained by taking the direct adjoint of the
particular numerical forward scheme used) is that it can be implemented using
currently available fast wave propagation solvers.Comment: submitted to "Inverse Problems
Continuous-time perpetuities and time reversal of diffusions
We consider the problem of estimating the joint distribution of a
continuous-time perpetuity and the underlying factors which govern the cash
flow rate, in an ergodic Markov model. Two approaches are used to obtain the
distribution. The first identifies a partial differential equation for the
conditional cumulative distribution function of the perpetuity given the
initial factor value, which under certain conditions ensures the existence of a
density for the perpetuity. The second (and more general) approach, identifies
the joint law as the stationary distribution of an ergodic multi-dimensional
diffusion using techniques of time reversal. This later approach allows for
efficient use of Monte-Carlo simulation when estimating the distribution, as
the distribution is obtained by sampling a single path of the reversed process.Comment: 42 pages; added numerical exampl
Irving Fisher and Index Number Theory
There are four main approaches to bilateral index number theory: the fixed basket, stochastic, test and economic approaches. The paper reviews the contributions of Irving Fisher to these approaches to index number theory which are still in use today. The paper also reviews Fisherâs contributions to multilateral index number theory. The main themes of the paper are developed in the context of a review of the early history of index number theory: a history that conveys a wealth of information and insight into the making and use of index numbers today.Price indexes, quantity indexes, bilateral price indexes, multilateral price indexes, the Fisher ideal index, the test approach to index number theory
Near-Infrared Light Curves of the Brown Dwarf Eclipsing Binary 2MASS J05352184-0546085: Can Spots Explain the Temperature Reversal?
We present the JHKs light curves for the double-lined eclipsing binary 2MASS
J05352184-0546085, in which both components are brown dwarfs. We analyze these
light curves with the published Ic-band light curve and radial velocities to
provide refined measurements of the system's physical parameters. The component
masses and radii are here determined with an accuracy of ~6.5% and ~1.5%,
respectively. We confirm the previous surprising finding that the primary brown
dwarf has a cooler effective temperature than its companion. Next, we perform a
detailed study of the variations in the out-of-eclipse phases of the light
curves to ascertain the properties of any inhomogeneities on the surfaces of
the brown dwarfs. Our analysis reveals two low-amplitude periodic signals, one
attributable to the rotation of the primary (with a period of 3.293+/-0.001 d)
and the other to that of the secondary (14.05+/-0.05 d). Finally, we explore
the effects on the derived physical parameters of the system when spots are
included in the modeling. The observed low-amplitude rotational modulations are
well fit by cool spots covering a small fraction of their surfaces. To mimic
the observed ~200 K suppression of the primary's temperature, our model
requires that the primary possess a very large spot coverage fraction of ~65%.
Altogether, a spot configuration in which the primary is heavily spotted while
the secondary is lightly spotted can explain the apparent temperature reversal
and can bring the temperatures of the brown dwarfs into agreement with the
predictions of theoretical models.Comment: Accepted for publication in The Astrophysical Journal; 13 pages, 8
figures, 6 tables. Updated parameter uncertaintie
Motorway Tidal Flow Lane Control
A traffic control case of particular importance occurs when inbound and outbound traffic on a motorway stretch is unbalanced throughout the day. This scenario may benefit of a lane management strategy called tidal flow (or reversible) lane control, in which case the direction of a contraflow buffer lane is reversed according to the needs of each direction. This paper proposes a simple and practical real-time strategy for efficient motorway tidal flow lane control. A switching policy based on the fundamental diagram, that requires only aggregated measurements of density (or occupancy), is adopted. A kinematic wave theory-based traffic flow analysis shows that the proposed strategy provides a Pareto-optimal solution. Simulation studies of the A38(M) Aston Expressway (Birmingham, UK), are used to demonstrate its operation. The results confirm an increase of motorway throughput and a smooth operation of the strategy
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