An MRI perfusion model incorporating nonequilibrium exchange between vascular and extravascular compartments

A model of MRI signal intensity which is a function of perfusion is developed based upon the assumption that biological tissue can be represented by a blood and tissue compartment. The longitudinal magnetization is derived from the Bloch equations which are modified to model the magnetization in both the blood and tissue as a function of the following physiological parameters: blood flow velocity; perfusion fraction, which in the model is parameterized in terms of the ratio of the cross-sectional areas of the tissue and blood compartments; diffusion; rate of exchange between the blood and extravascular tissue compartments. Simulations of slice profiles excited by a repetitive sequence of 90 degrees slice-selective pulses show that the signal intensity in the blood and tissue compartments are modulated by the physiological parameters. A key factor in the modulation of the MRI signal is a time-of-flight effect whereby unexcited spins perfuse the excited region and exchange with blood and tissue compartments, thus immediately increasing the slice signal intensity but also delaying the spin exits from the slice, thereby decreasing their contribution to slice signal intensity in future repetitive pulse measurements.     

Acoustic resonances and blade vibration in axial flow compressors

A study is made of the conditions in which vibration of the blades of an axial flow compressor can be excited by acoustic resonances of the annulus. Such resonances can be generated by stalled blade rows and therefore occur during part speed operation.     

Sum and difference frequency generation in acoustoelectric domains in GaAs

Very distinct sum and difference frequency generation has been observed by Brillouin scattering in acoustoelectric domains in GaAs. Comparison of the two processes show that sum frequency generation is the much stronger process in the earliest stages of flux growth.     

The effects of arsenic source contamination on doped GaAs grown by MBE

The presence of parts per million (ppm) levels of impurities in the low-temperature arsenic charge used in molecular beam epitaxy (MBE) is shown to have a dramatic effect on the quality of GaAs grown. Source charge impurities play a vital role in determining the apparent variation in reported doping behaviour in MBE:GaAs.     

Study of transverse momenta of charged hadrons produced in νp and\bar vp charged current interactions

Data from a neutrino and antineutrino hydrogen experiment with BEBC are used to investigate transverse properties of the produced charged hadrons. Measurements are presented on average transverse momenta of charged pions as functions of Feynman-x and the hadronic mass, on the transverse momentum flow within an event and on jet-related quantities. The main features of the data are well described by the LUND model. The data favour a version of the model in which soft gluon effects are included and the primordial transverse momentum of the quarks in the proton is small. Effects from 1^st order QCD (hard gluon emission) are negligible.     

A latin square with orthogonal mate and no automorphism

A latin square of order ten is displayed having an orthogonal mate, but no nontrivial automorphism obtainable by permutation of rows, columns and digits.     

Application of an americium-241 source for the determination of barium,lanthanum and cerium in lateritic material by X-ray fluorescence

Barium, lanthanum and cerium were determined by excitation using a 100 mCi²⁴¹Am radioisotope source with the utilization of the K peaks. This work forms part of the UNESCO Project IGGP-129. This work includes data from the MS Thesis of D. ADAMES.     

Monitoring the effect of ultrafast deactivation of the electronic excited states of DNA bases and polynucleotides following 267 nm laser excitation using picosecond time-resolved infrared spectroscopy

In this paper we demonstrate the use of picosecond time-resolved infrared spectroscopy (ps-TRIR) to monitor the early structural dynamics of DNA bases and polydeoxynucleotides following UV excitation in solution.