Time-resolved Phase-contrast Imaging Research Articles

Factors affecting electron beam damage in calcite nanoparticles

We report electron fluence thresholds for the degradation of calcite nanoparticles under electron irradiation by both conventional and scanning TEM (CTEM and STEM), using time resolved phase contrast imaging and EDX spectroscopy at both 80 kV and 300 kV accelerating voltages. We show that the degradation pathway of calcite involves disruption of the crystal lattice with the evolution of pores and transformation to calcium oxide and carbon dioxide. Depending on irradiation conditions (CTEM or STEM), the calcium oxide formed can be either amorphous or crystalline, with the formation of the latter apparently being hindered by hydrocarbon contamination build up in STEM. For a given electron flux, irradiation at 300 kV prolongs the characteristic lifetime of the calcite lattice as compared to irradiation at 80 kV but with a corresponding reduction in both image contrast and energy dispersive X-ray (EDX) signal, consistent with the change in inelastic mean free path for electron scattering. STEM offers significant benefits over CTEM, however only in the presence of hydrocarbon contamination, increasing the fluence threshold for the detection of irradiation induced faults in the calcite lattice from 2.7 × 107 e− nm-2 for 300 kV CTEM to over 1.8 × 108 e− nm-2 for 300 kV STEM. This work forms a framework for reliable identification of discrete particle crystallinity in nominally amorphous, nanoscale calcium carbonate particles which is of importance for fundamental studies of crystallisation and also for the process control during the synthesis of such surfactant stabilised nanoparticles for application as over-based fuel detergents.

THz band-stop filter using metamaterials surfaced on LiNbO(3) sub-wavelength slab waveguide.

We designed and implemented periodic bar arrays metamaterials to select appropriate frequencies of terahertz (THz) waves propagating in a LiNbO(3) sub-wavelength waveguide. The spatial and temporal electric field profiles of the THz waves were recorded using a time-resolved phase-contrast imaging system. The metamaterials can operate as a band-stop filter to realize blocking back THz waves in a band range of 0.6-1.0 THz, while transparent transmission for the fundamental mode of the slab over a range of 0.3-0.6 THz.

USPIO enhanced 4D flow imaging of the mouse cardiovascular system at 7T with an ultrashort echo time sequence

Background Time resolved phase contrast imaging is hardly used in small animal studies due to a lack of signal-to-noise ratio. Some studies were performed in 2D and one publication (Janiczek et al, MRM 2011) reported flow measurement in mouse aortic arch in 3D due to the use of a thin excitation slab and high Time-Of-Flight (TOF) effect. However, this strategy is not applicable on the whole cardiopulmonary system. To overcome this problem we proposed to combined Ultra-short Echo Time sequence with an injection of Ultra Small Particles of Iron Oxide (USPIO) to generate a stable, positive and high blood signal. The method was first validated on phantom and then applied on the whole cardiopulmonary system of normal mice.

X-ray phase-contrast imaging: the quantum perspective

Time-resolved phase-contrast imaging using ultrafast x-ray sources is an emerging method to investigate ultrafast dynamical processes in matter. Schemes to generate attosecond x-ray pulses have been proposed, bringing electronic timescales into reach and emphasizing the demand for a quantum description. In this paper, we present a method to describe propagation-based x-ray phase-contrast imaging in nonrelativistic quantum electrodynamics. We explain why the standard scattering treatment via Fermi’s golden rule cannot be applied. Instead, the quantum electrodynamical treatment of phase-contrast imaging must be based on a different approach. It turns out that it is essential to select a suitable observable. Here, we choose the quantum-mechanical Poynting operator. We determine the expectation value of our observable and demonstrate that the leading order term describes phase-contrast imaging. It recovers the classical expression of phase-contrast imaging. Thus, it makes the instantaneous electron density of non-stationary electronic states accessible to time-resolved imaging. Interestingly, inelastic (Compton) scattering does automatically not contribute in leading order, explaining the success of the semiclassical description.

Photothermal time-resolved imaging of living cells.

Thermal effects of laser radiation at cell level play very important role in cell functioning and in many laser applications. The aim of this study was to evaluate a new method of photothermal imaging (PTI) for monitoring short-time nano-scale thermal effects in individual living cells. PTI is based on the irradiation of a cell with a short laser pump pulse (8 nanoseconds, 532 nm) and on registration of the laser-induced local thermal effects using time-resolved phase-contrast imaging with a pulsed probe laser. PT images of lymphocytes, lympholeukemia cells in vitro were obtained at different laser energies. PTI in time-resolved mode allowed visualizing the structures with size less than diffraction limit (90-nm liposomes). The photodamage process was visualized for a single human leukocyte in suspension. PTI in non-invasive mode offered better contrast of living cell image than conventional optical phase-contrast microscopy. The data obtained showed that PTI is in perspective for studies of live non-fluorescent cells.