Optical coherence tomography (OCT) is a high resolution imaging technology that is rapidly being adopted as the
standard of care for medical applications such as ocular and intravascular imaging. However, clinical translation has
been hampered by the lack of standardized test methods for performance evaluation as well as consensus standards
analogous to those that have been developed for established medical imaging modalities (e.g., ultrasound). In this study,
we address low contrast detectability, specifically, the ability of systems to differentiate between regions exhibiting small
differences in scattering coefficient. Based on standard test methods for established medical imaging modalities, we
have developed layered phantoms with well-characterized scattering properties in a biologically relevant range. The
phantoms consisted of polydimethylsiloxane (PDMS) doped with varying concentrations of BaSO<sub>4</sub> microparticles.
Microfabrication processes were used to create layered and channel schemes. Two spectral domain OCT systems - a
Fourier domain system at 855 nm and a swept-source device at 1310 nm - were then used to image the phantoms. The
detectability of regions with different scattering levels was evaluated for each system by measuring pixel intensity
differences. Confounding factors such as the inherent attenuation of the phantoms, signal intensity decay due to focusing
and system roll-off were also encountered and addressed. Significant differences between systems were noted. The
minimum differences in scattering coefficient that the Fourier domain and swept source systems could differentiate was
1.50 and 0.46 mm<sup>-1</sup> respectively. Overall, this approach to evaluating low contrast detectability represents a key step
towards the development of standard test methods to facilitate clinical translation of novel OCT systems.
In biophotonic imaging, turbid phantoms that are low-cost, biologically-relevant, and durable are desired for
standardized performance assessment. Such phantoms often contain inclusions of varying depths and sizes in order to
quantify key image quality characteristics such as penetration depth, sensitivity and contrast detectability. The emerging
technique of rapid prototyping with three-dimensional (3D) printers provides a potentially revolutionary way to fabricate
these structures. Towards this goal, we have characterized the optical properties and morphology of phantoms fabricated
by two 3D printing approaches: thermosoftening and photopolymerization. Material optical properties were measured by
spectrophotometry while the morphology of phantoms incorporating 0.2-1.0 mm diameter channels was studied by μCT,
optical coherence tomography (OCT) and optical microscopy. A near-infrared absorbing dye and nanorods at several
concentrations were injected into channels to evaluate detectability with a near-infrared hyperspectral reflectance
imaging (HRI) system (650-1100 nm). Phantoms exhibited biologically-relevant scattering and low absorption across
visible and near-infrared wavelengths. Although limitations in resolution were noted, channels with diameters of 0.4
mm or more could be reliably fabricated. The most significant problem noted was the porosity of phantoms generated
with the thermosoftening-based printer. The aforementioned three imaging methods provided a valuable mix of insights
into phantom morphology and may also be useful for detailed structural inspection of medical devices fabricated by
rapid prototyping, such as customized implants. Overall, our findings indicate that 3D printing has significant potential
as a method for fabricating well-characterized, standard phantoms for medical imaging modalities such as HRI.
The emerging technique of three-dimensional (3D) printing provides a simple, fast, and flexible way to fabricate structures with arbitrary spatial features and may prove useful in the development of standardized, phantom-based performance test methods for biophotonic imaging. Acrylonitrile Butadiene Styrene (ABS) is commonly used in the printing process, given its low cost and strength. In this study, we evaluate 3D printing as an approach for fabricating biologically-relevant optical phantoms for hyperspectral reflectance imaging (HRI). The initial phase of this work involved characterization of absorption and scattering coefficients using spectrophotometry. The morphology of phantoms incorporating vessel-like channels with diameters on the order of hundreds of microns was examined by microscopy and OCT. A near-infrared absorbing dye was injected into channels located at a range of depths within the phantom and imaged with a near-infrared HRI system (650-1100 nm). ABS was found to have scattering coefficients comparable to biological tissue and low absorption throughout much of the visible and infrared range. Channels with dimensions on the order of the resolution limit of the 3D printer (~0.2 mm) exhibited pixelation effects as well as a degree of distortion along their edges. Furthermore, phantom porosity sometimes resulted in leakage from channel regions. Contrast-enhanced channel visualization with HRI was possible to a depth of nearly 1 mm – a level similar to that seen previously in biological tissue. Overall, our ABS phantoms demonstrated a high level of optical similarity to biological tissue. While limitations in printer resolution, matrix homogeneity and optical property tunability remain challenging, 3D printed phantoms have significant promise as samples for objective, quantitative evaluation of performance for biophotonic imaging modalities such as HRI.
A detailed knowledge of the physical phenomena underlying the generation and the transport of fast electrons
generated in high-intensity laser-matter interactions is of fundamental importance for the fast ignition scheme
for inertial confinement fusion.
Here we report on an experiment carried out with the VULCAN Petawatt beam and aimed at investigating
the role of collisional return currents in the dynamics of the fast electron beam. To that scope, in the experiment
counter-propagating electron beams were generated by double-sided irradiation of layered target foils containing
a Ti layer. The experimental results were obtained for different time delays between the two laser beams as
well as for single-sided irradiation of the target foils. The main diagnostics consisted of two bent mica crystal
spectrometers placed at either side of the target foil. High-resolution X-ray spectra of the Ti emission lines in
the range from the Lyα to the Kα line were recorded. In addition, 2D X-ray images with spectral resolution were
obtained by means of a novel diagnostic technique, the energy-encoded pin-hole camera, based on the use of a
pin-hole array equipped with a CCD detector working in single-photon regime. The spectroscopic measurements
suggest a higher target temperature for well-aligned laser beams and a precise timing between the two beams.
The experimental results are presented and compared to simulation results.
The VULCAN laser at the Central Laser Facility has been used for laboratory simulations of collision-less astrophysical shocks. By ensuring that the experiment exhibit similar values of key dimension-less parameters to those in supernova remnants the hydrodynamics and magnetic field of the experiment are scaled to those of SNR. This enables some of the most challenging aspects of shock behavior to be tested directly against experiment. The experiments provide dat against which to test current theory. Collision-less shock formation, and plasma interaction of two counter- propagating colliding foils permeated by a strong magnetic field are discussed.
X-ray diffraction from dynamically compressed solids has been an area of active research for more than half a century. As early as 1950, Schall obtained submicrosecond, single-shot x-ray diffraction patterns of single crystals under dynamic deformation. Almost two decades later Q. Johnson and coworkers succeeded in obtaining diffraction patterns with an exposure time of tens of nanoseconds from an explosively shocked crystal, and were the first to demonstrate diffraction evidence for a shock induced phase transition. Over the past few years we have shown that even shorter exposure times can be achieved by using a laser-plasma as the source of x-rays, synchronous to a laser driven shock. In this paper we will review the progress made in this field, emphasising the potential applications fo time-resolved x-ray diffraction for addressing some of the fundamental problems of shock wave physics.
In situ x-ray diffraction from laser-shocked crystals provides one means of diagnosing high strain-rate (10<SUP>8</SUP> - 10<SUP>9</SUP> s<SUP>-1</SUP>) compression/tension waves in solids. Typically the radiation diffracted from the shocked crystals is relatively broadband, consisting of the resonance and intercombination lines of the helium-like ions of medium Z atoms, and their associated lithium-like dielectronic satellites. Deconvolution of this time-dependent x-ray spectrum will yield more detailed information on the strain profiles within the crystal. Preliminary results of a maximum entropy routine are presented.