Restoration of signal polarity in a set of inversion recovery NMR images

A method is described which enables unambiguous retrieval of sign information in a set of magnetic resonance magnitude images of the inversion recovery type. The proposed method starts from the observation that the inversion recovery curve S is a monotonically increasing function of the inversion time TI, and comes down to finding the zero-crossing time TI0 of this curve for each pixel within the image. Absolute Value(S)and S are then related by S(TI) = -Absolute Value(S(TI)) for TI =/< TI(0) and S(TI) = + Absolute Value(S(TI)) for TI =/> TI0. The method, which does not require additional knowledge with respect to any of the NMR parameters involved, is shown to be effective when at least four inversion recovery images with different inversion times of a particular object slice are available. The efficacy of sign retrieval is demonstrated by imaging experiments on phantoms and human subjects. The validity of the polarity restoration method is established by viewing its results against the results of conventional methods, i.e., NMR spectroscopy.     

Short-pulse effects in a free-electron laser

The Free-Electron Laser for Infrared eXperiments (FELIX) offers a unique combination of short electron bunches and long wavelengths, i.e. a slippage parameter μ_c ranging up to 10. As a consequence, pronounced short-pulse effects can be observed. In this paper the experimental observation of two of these effects is discussed, namely the occurrence of limit-cycle oscillations and the feasibility of tuning of the micropulse duration. The stable limit-cycle oscillation of the macropulse power is due to a modulation of the optical micropulse shape. This is a consequence of a combination of high optical power and short pulses. The former causes synchrotron oscillations of the electrons and the effect is, therefore, closely related to spiking phenomena. The short-pulse nature of FELIX ensures that the oscillations do not evolve into the chaotic behavior normally associated with spiking and the sideband instability. Experimental results are compared with numerical simulations     

An interactive computer system for guiding the surgical treatment of life-threatening ventricular tachycardias

Ventricular tachycardias that are medically refractory can be treated surgically by resection of the area that generates the arrhythmia. The origin of a tachycardia can be determined during operation by recording endocardial electrograms. Tachycardias occurring spontaneously, or induced during the operative procedure, often last for only a few cycles and require the simultaneous recording of endocardial signals from numerous sites (mapping). The conventional technique of sequential mapping by using a roving probe cannot be applied in such cases. We, therefore, developed a flexible computerized system which enables us to record simultaneously from a multiple of 16 electrode terminals. The basic concept can accommodate 64 channels. A multielectrode array consisting of an inflatable balloon, uniformly covered with electrode terminals, is used to map the whole endocardial surface simultaneously. A rectangular grid covered with electrode terminals is used for refined resolution. Optically isolated amplifiers are used, providing ohmic isolation between the patient and the support electronics. After amplification and filtering, the endocardial signals are collected in groups of 16 and transferred to data acquisition units. Each unit contains a 16-channel multiplexing A/D converter and a 16K circular buffer. An LSI-11 based minicomputer controls the acquisition units and accounts for the transition of data to disk. Preselection of data is carried out during acquisition in order to reduce operation time. After acquisition, hard copies of the signals are made on a 16-channel chart recorder.     

Intramolecular n-alkenylnitrone-additions: Regio- and stereochemistry

Efficient intramolecular cycloadditions of N-3-alkenyl- and N-4-alkenylnitrones proceed with opposite regioselection which is modified by dipolarophile-substituent effects. Polycyclic isoxazolidines are obtained in a highly stereocontrolled fashion, consistent with an endo-addition of the Z-nitrones.     

Metric semantics for concurrency

An overview is given of work we have done in recent years on the semantics of concurrency, concentrating on semantic models built on metric structures. Three contrasting themes are discussed, viz. (i) uniform or schematic versus nonuniform or interpreted languages; (ii) operational versus denotational semantics, and (iii) linear time versus branching time models. The operational models are based on Plotkin's transition systems. Language constructs which receive particular attention are recursion and merge, synchronization and global nondeterminacy, process creation, and communication with value passing. Various semantic equivalence results are established. Both in the definitions and in the derivation of these equivalences, essential use is made of Banach's theorem for contracting functions.Dedicated to Peter Naur on the occasion of his 60th birthday     

Precision in calculated rho, T1 and T2 images as a function of data analysis method

In NMR imaging rho, T1 and T2 images are usually calculated from a set of partial saturation, saturation recovery or inversion recovery experiments with multiple echoes and multiple repetition times. Several methods can be envisaged to extract parameter images from such a set of source images. These methods to a greater or lesser extent take advantage of the fact that a multiple echo/multiple repetition time experiment provides a set of largely independent T1 and T2 measurements. In this study several data analysis methods, including weighted and non-weighted averaging of results of independent T1 and T2 measurements, weighted and non-weighted averaging of source images prior to data reduction and simultaneous three-parameter fitting, were compared against another in terms of precision, computational efficiency and robustness. The predicted performance of the examined methods was verified by stochastic simulation experiments.     

Special purpose computers in physics

This talk describes a new approach for large-scale computational problems which is particularly effective when a relatively simple algorithm is used. We demonstrate that it is possible to design and construct, at modest cost, special purpose computers for various classes of problems. By exploiting the principles of pipelining and parallel processing, and by adapting the hardware design to the specific structure of a particular algorithm, one can obtain a device which is as fast as or faster than general-purpose commercial supercomputers. The user of a such a processor has the double advantage of its speed and of its continuous availability for the particular problem for which it was constructed. In statistical mechanics special purpose computers have been built recently (i) for Monte Carlo simulation of the Ising model, and (ii) for the molecular dynamics of classical many-particle systems with short-range interactions. The design and performance of these machines are discussed and compared to those of commercial computers.     

On the methodology of the analysis of Mössbauer spectra

A review is presented of the direct fitting procedures which are used in the analysis of Mössbauer spectra. Direct lineshape fitting with alternative profiles as well as shape-dependent, shape-independent and quasi shape-independent distribution fitting methods all can easily be incorporated in one computer program scheme yielding a large versatility for modification and/or extension of the programs according to specific spectra.Research director at the National Fund for Scientific Research, Belgium.