Improved image quality with new ultrasound imaging techniques

AbstractThis work addresses three key subjects to the image quality with phased arrays: timing accuracy, beamforming strategy and post-processing for increased resolution and suppression of grating and side lobes.Timing accuracy is achieved by defining a modular and scalable architecture which guarantees timing errors of a few tens of picoseconds, whatever is the system size. The proposed beamforming methodology follows the progressive focusing correction technique, which keeps low focusing errors, provides a high information density and has a simple implementation for real-time imaging. Then, phase coherence imaging is defined to suppress grating and sidelobe indications and increasing the lateral resolution

Numerical study of the development of frontal motions, A

January 1971.Includes bibliographical references.Prepared for Air Force Cambridge Research Laboratories, Air Force Systems Command, United States Air Force F19628-68-C-104 8628 862804 86280401.AFCRL-71-0320

Objective analysis of a two dimensional data field by the cubic spline technique

August, 1969.Includes bibliographical references.Sponsored by by the Air Force Cambridge Research Laboratories F19628-68-C-0104

Optical absorption of non-interacting tight-binding electrons in a Peierls-distorted chain at half band-filling

In this first of three articles on the optical absorption of electrons in half-filled Peierls-distorted chains we present analytical results for non-interacting tight-binding electrons. We carefully derive explicit expressions for the current operator, the dipole transition matrix elements, and the optical absorption for electrons with a cosine dispersion relation of band width $W$ and dimerization parameter $\delta$. New correction (``$\eta$''-)terms to the current operator are identified. A broad band-to-band transition is found in the frequency range $W\delta < \omega < W$ whose shape is determined by the joint density of states for the upper and lower Peierls subbands and the strong momentum dependence of the transition matrix elements.Comment: 17 pages REVTEX 3.0, 2 postscript figures; hardcopy versions before May 96 are obsolete; accepted for publication in The Philosophical Magazine

Neutron spectroscopic study of crystal field excitations in Tb2Ti2O7 and Tb2Sn2O7

We present time-of-flight inelastic neutron scattering measurements at low temperature on powder samples of the magnetic pyrochlore oxides Tb2Ti2O7 and Tb2Sn2O7. These two materials possess related, but different ground states, with Tb2Sn2O7 displaying "soft" spin ice order below Tn~0.87 K, while Tb2Ti2O7 enters a hybrid, glassy spin ice state below Tg~0.2 K. Our neutron measurements, performed at T=1.5 K and 30 K, probe the crystal field states associated with the J=6 states of Tb3+ within the appropriate Fd\bar{3}m pyrochlore environment. These crystal field states determine the size and anisotropy of the Tb3+ magnetic moment in each material's ground state, information that is an essential starting point for any description of the low-temperature phase behavior and spin dynamics in Tb2Ti2O7 and Tb2Sn2O7. While these two materials have much in common, the cubic stanate lattice is expanded compared to the cubic titanate lattice. As our measurements show, this translates into a factor of ~2 increase in the crystal field bandwidth of the 2J+1=13 states in Tb2Ti2O7 compared with Tb2Sn2O7. Our results are consistent with previous measurements on crystal field states in Tb2Sn2O7, wherein the ground state doublet corresponds primarily to m_J=|\pm 5> and the first excited state doublet to mJ=|\pm 4>. In contrast, our results on Tb2Ti2O7 differ markedly from earlier studies, showing that the ground state doublet corresponds to a significant mixture of mJ=|\pm 5>, |\mp 4>, and |\pm 2>, while the first excited state doublet corresponds to a mixture of mJ=|\pm 4>, |\mp 5>, and |\pm 1>. We discuss these results in the context of proposed mechanisms for the failure of Tb2Ti2O7 to develop conventional long-range order down to 50 mK.Comment: 12 pages, 6 figures. Version is the same as the published one, except for figure placement on page

Regional Trajectories of Entrepreneurship and Growth

The development of regions is considerably shaped by their history. We review research that finds significant persistence of regional levels of entrepreneurship over longer periods of time. It is argued that the long term persistence of regional entrepreneurship indicates the presence and effect of a culture of entrepreneurship that is conducive to new business formation and regional growth. Hence, regional development is characterized by long term trajectories of entrepreneurship. We derive a number of policy implications and propose avenues for further research.</p