Agitation-free multiphoton microscopy of oblique planes

The scanning two-photon fluorescence microscope produces optically sectioned images from the focal plane. It is sometimes desirable to acquire images from other planes of the specimen that are inclined with respect to the focal plane. In this Letter, we discuss the issues concerned with acquiring such images together with the effects of the inclination angle on image resolution and sectioning strength. To obtain images from oblique planes at high speed, a two-photon system was built wherein a novel optical system is used to provide aberration-free scanning.     

Many-neighbour interaction and non-locality in traffic models

The optimal-velocity model, as proposed by Bando et al. [1], shows unrealistic values of the acceleration for various optimal-velocity functions [2,3]. We discuss different approaches of how to correct this problem. Multiple look-ahead (many-neighbour interaction) models are the most promising candidates in reducing accelerations and decelerations to realistic values. We focus on two such models and, in particular, their linear stability and how these affect the vehicle dynamics and wave solutions. As found earlier [4], multiple look-ahead models reproduce many real flow features, and our results further support the necessity of this ansatz. However, the problem of non-locality arises when they are transformed into the corresponding continuum model. We discuss three methods of how to interpret many-neighbour interaction in macroscopic models.Received: 27 March 2004, Published online: 12 July 2004PACS: 45.70.Vn Granular models of complex systems; traffic flow - 89.90. + n Other topics in areas of applied and interdisciplinary physics - 47.50. + d Non-Newtonian fluid flows     

Superparamagnetic iron oxide (SPIO) enhancement in the cirrhotic liver: a comparison of two doses of ferumoxides in patients with advanced disease

The aim of this study was to establish whether enhancement of the liver by the MRI contrast agent ferumoxides could be effectively achieved at a reduced dose of 7.5 micromol/kg in patients with advanced liver cirrhosis. Forty-two liver transplant candidates with end-stage cirrhosis underwent SPIO-enhanced MRI at 1.5T, using either 15 micromol/kg or 7.5 micromol/kg ferumoxides. The lower dose of ferumoxides was also used in 21 non-cirrhotic patients with colorectal liver metastases who acted as a control group. The percentage signal intensity loss (PSIL) after SPIO was measured in all patients, and in those patients with tumors the post-SPIO contrast-to-noise ratio (CNR) was measured. The median PSIL after SPIO in the high dose cirrhotic (HDLC), low dose non-cirrhotic (LDNC) and low dose cirrhotic (LDLC) patients was 86.3%, 74.6%, and 64.2% respectively. These differences were significant using the Mann-Whitney U test. Tumors were found in 8 patients in the high dose cirrhotic group, 9 in the low dose cirrhotic group, and all 21 of the control group. No significant differences were found between the CNR values after SPIO in the 3 groups (median values HDLC 15.1, LDNC 23.7, LDLC 19.5). In patients with late-stage cirrhosis the PSIL after SPIO was significantly less at 7.5 micromol/kg than at 15 micromol/kg, but both doses produced a substantial loss of signal. Lesion to liver CNR was not adversely affected by using the lower dose, so when imaging at 1.5T the authors would recommend using 7.5 micromol/kg in patients with liver cirrhosis.     

Micromachined array tip for multifocus fiber-based optical coherence tomography

High-resolution optical coherence tomography demands a large detector bandwidth and a high numerical aperture for real-time imaging, which is difficult to achieve over a large imaging depth. To resolve these conflicting requirements we propose a novel multifocus fiber-based optical coherence tomography system with a micromachined array tip. We demonstrate the fabrication of a prototype four-channel tip that maintains a 9-14-microm spot diameter with more than 500 microm of imaging depth. Images of a resolution target and a human tooth were obtained with this tip by use of a four-channel cascaded Michelson fiber-optic interferometer, scanned simultaneously at 8 kHz with geometric power distribution across the four channels.     

Forces between a single atom and its distant mirror image

An excited-state atom whose emitted light is backreflected by a distant mirror can experience trapping forces, because the presence of the mirror modifies both the electromagnetic vacuum field and the atom's own radiation reaction field. We demonstrate this mechanical action using a single trapped barium ion. We observe the trapping conditions to be notably altered when the distant mirror is translated across an optical wavelength. The well-localized barium ion enables the spatial dependence of the forces to be measured explicitly. The experiment has implications for quantum information processing and may be regarded as the most elementary optical tweezers.     

Quasi-wavelet calculations of sound scattering behind barriers

Quasi-wavelets (QWs) are a representation of turbulence consisting of self-similar, eddy-like structures with random orientations and positions in space. They are used in this paper to calculate the scattering, due to turbulent velocity fluctuations, of sound behind noise barriers as a function of the size and spatial location of the eddies. The sound scattering cross-section for QWs of an individual size class (eddy size) is derived and shown to reproduce results for the von Kármán spectrum when the scattered energies from a continuous distribution of QW sizes are combined. A Bragg resonance condition is derived for the eddy size that scatters most strongly for a given acoustic wavenumber and scattering angle. Results for scattering over barriers show that, for typical barrier conditions, most of the scattered energy originates from eddies in the size range of approximately one-half to twice the size of the eddies responsible for maximum scattering. The results also suggest that scattering over the barrier due to eddies with a line of sight to both the source and receiver is generally significant only for frequencies above several kilohertz, for sources and receivers no more than a few meters below the top of the barrier, and for very turbulent atmospheric conditions.