164 research outputs found
Coherence effects in the transition radiation spectrum and practical consequences
In the framework of the pseudophotons method and under the hypothesis of the
far field approximation the formal evaluation of the transition radiation
spectrum produced by a three-dimensional charged beam interacting with a
metallic target is carried out. When spatial incoherent conditions are
occurring, the transverse size of a three-dimensional beam affects the spectrum
via a suitable dependence on the Fourier transform of the transverse
distribution, while the temporal coherent enhance of the spectrum depends only
on the longitudinal beam size via the longitudinal form factor in the same
manner as in the case of a one-dimensional beam. Spatial incoherent effects,
arising at short wave-length in the spectrum, on the one hand allow a better
comprehension of some aspects of the transition radiation phenomelogy - e.g. an
analytical formula for the total radiated energy - and on the other hand offer
new perspectives in the practical application of the transition radiation as
diagnostics method of the transverse beam size in a particle accelerator.Comment: 18 pages, 1 figures, Submitted to Nuclear Instruments and Methods in
Physics Research Section
Covariant Formulation of the Transition Radiation Energy Spectrum of an Electron Beam at a Normal Angle of Incidence onto a Round Metallic Screen
In the transition radiation emission from a N electron bunch hitting at a
normal angle of incidence a metallic screen, the transverse and the
longitudinal spatial coordinates of the electron bunch play different roles in
determining the N single electron radiation field amplitudes and their relative
phases in relation to the different physical constraints which an
electromagnetic radiative mechanism by a charged beam must meet: i.e., temporal
causality and covariance. The distribution of the N electron longitudinal
coordinates determines indeed the sequence of the N electron collisions onto
the metallic screen and, on the basis of the temporal causality principle, it
also determines the distribution function of the relative emission phases of
the N single electron field amplitudes from the metallic surface. The
distribution of the transverse coordinates of the N electrons contributes as
well to determine the relative phase distribution of the N electron field
amplitudes at the observation point - located on the longitudinal axis of the
reference frame - providing a further phase information that accounts for the
transverse displacement of the N electrons with respect to the beam axis. The
distribution of the transverse coordinates of the N electrons is a relativistic
invariant under a Lorentz transformation with respect to the direction of
motion of the beam and, consequently, it is expected to leave a covariant mark
on the N single electron amplitudes composing the radiation field. The
covariant imprinting of the N electron transverse density on the radiation
field affects both the temporal coherent and incoherent parts of the transition
radiation energy spectrum. Such a dependence of the N single electron radiation
field amplitudes on the electron density in the transverse plane manifests
itself as....(abstract partially missed because of lack of space
Thomson Scattering of Coherent Diffraction Radiation by an Electron Bunch
The paper considers the process of Thomson scattering of coherent diffraction
radiation (CDR) produced by the preceding bunch of the accelerator on one of
the following bunches. It is shown that the yield of scattered hard photons is
proportional to N, where N is the number of electrons per bunch. A
geometry is chosen for the CDR generation and an expression is obtained for the
scattered photon spectrum with regard to the geometry used, that depends in an
explicit form on the bunch size. A technique is proposed for measuring the
bunch length using scattered radiation characteristics.Comment: 14 pages, LATEX, 6 ps.gz figures, submitted to Phys.Rev.
Combined FDG-PET/CT for the detection of unknown primary tumors: systematic review and meta-analysis
The aim of this study was to systematically review and meta-analyze published data on the diagnostic performance of combined 18F-fluoro-2-deoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) in the detection of primary tumors in patients with cancer of unknown primary (CUP). A systematic search for relevant studies was performed of the PubMed/MEDLINE and Embase databases. Methodological quality of the included studies was assessed. Reported detection rates, sensitivities and specificities were meta-analyzed. Subgroup analyses were performed if results of individual studies were heterogeneous. The 11 included studies, comprising a total sample size of 433 patients with CUP, had moderate methodological quality. Overall primary tumor detection rate, pooled sensitivity and specificity of FDG-PET/CT were 37%, 84% (95% CI 78–88%) and 84% (95% CI 78–89%), respectively. Sensitivity was heterogeneous across studies (P = 0.0001), whereas specificity was homogeneous across studies (P = 0.2114). Completeness of diagnostic workup before FDG-PET/CT, location of metastases of unknown primary, administration of CT contrast agents, type of FDG-PET/CT images evaluated and way of FDG-PET/CT review did not significantly influence diagnostic performance. In conclusion, FDG-PET/CT can be a useful method for unknown primary tumor detection. Future studies are required to prove the assumed advantage of FDG-PET/CT over FDG-PET alone and to further explore causes of heterogeneity
FDG PET/CT in carcinoma of unknown primary
Carcinoma of unknown primary (CUP) is a heterogeneous group of metastatic malignancies in which a primary tumor could not be detected despite thorough diagnostic evaluation. Because of its high sensitivity for the detection of lesions, combined 18F-fluoro-2-deoxyglucose positron emission tomography (FDG PET)/computed tomography (CT) may be an excellent alternative to CT alone and conventional magnetic resonance imaging in detecting the unknown primary tumor. This article will review the use, diagnostic performance, and utility of FDG PET/CT in CUP and will discuss challenges and future considerations in the diagnostic management of CUP
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