Diego Pontoni

Low-dose phase contrast x-ray medical imaging

Phase contrast x-ray imaging is a powerful technique for the detection of low-contrast details in weakly absorbing objects. This method is of possible relevance in the field of diagnostic radiology. In fact, imaging low-contrast details within soft tissue does not give satisfactory results in conventional x-ray absorption radiology, mammography being a typical example. Nevertheless, up to now all applications of the phase contrast technique, carried out on thin samples, have required radiation doses substantially higher than those delivered in conventional radiological examinations. To demonstrate the applicability of the method to mammography we produced phase contrast images of objects a few centimetres thick while delivering radiation doses lower than or comparable to doses needed in standard mammographic examinations (typically approximately 1 mGy mean glandular dose (MGD)). We show images of a custom mammographic phantom and of two specimens of human breast tissue obtained at the SYRMEP bending magnet beamline at Elettra, the Trieste synchrotron radiation facility. The introduction of an intensifier screen enabled us to obtain phase contrast images of these thick samples with radiation doses comparable to those used in mammography. Low absorbing details such as 50 microm thick nylon wires or thin calcium deposits (approximately 50 microm) within breast tissue, invisible with conventional techniques, are detected by means of the proposed method. We also find that the use of a bending magnet radiation source relaxes the previously reported requirements on source size for phase contrast imaging. Finally, the consistency of the results has been checked by theoretical simulations carried out for the purposes of this experiment.

On the origin of the hydrophobic water gap: An X-ray reflectivity and MD simulation study.

The density deficit of water at hydrophobic interfaces, frequently called the hydrophobic gap, has been the subject of numerous experimental and theoretical studies in the past decade. Recent experiments give values for the interfacial depletion that consistently correspond to less than a monolayer of water. The main question which remained so far unanswered is its origin and the mechanisms affected by the chemistry and molecular geometry of a particular hydrophobic coating. In this work, we present a combined high-energy X-ray reflectivity and molecular dynamics simulation study of the water depletion at a perfluorinated hydrophobic interface with a spatial resolution on the molecular scale. A comparison of our experimental and computational results elucidates the underlying mechanisms that affect the extent of the interfacial depletion. The complex interplay between surface chemistry and topography precludes the existence of a direct and universal relation between the macroscopic contact angle and the nanoscopic water depletion.

Atomic-Scale Surface Demixing in a Eutectic Liquid BiSn Alloy

Resonant x-ray reflectivity of the surface of the liquid phase of the Bi(43)Sn(57) eutectic alloy reveals atomic-scale demixing extending over three near-surface atomic layers. Because of the absence of an underlying atomic lattice which typically defines adsorption in crystalline alloys, studies of adsorption in liquid alloys provide unique insight on interatomic interactions at the surface. The observed composition modulation could be accounted for quantitatively by the Defay-Prigogine and Strohl-King multilayer extensions of the single-layer Gibbs model, revealing a near-surface domination of the attractive Bi-Sn interaction over the entropy.

Design and evaluation of AC-coupled, FOXFET-biased, “edge-on” silicon strip detectors for X-ray imaging

Abstract A silicon strip detector for the SYRMEP (SYnchrotron Radiation for MEdical Physics) experiment has been designed and realised. The main features of this detector are AC-coupling through integrated coupling capacitors, DC bias of the strips by means of a gated punch-through structure, bulk contact on the junction side through a forward-biased p + implant, thinned entrance window for the incoming radiation (in an “edge-on” geometry) and integrated fan-in on active silicon. Results of laboratory tests of the detector parameters, allowing a thorough evaluation of the technological solutions employed, are presented.

Interactions and kinetic arrest in an adhesive hard-sphere colloidal system

The evolution of microstructure and dynamics of a colloidal suspension transforming from hard-sphere to sticky hard-sphere system is investigated by small-angle x-ray scattering techniques. The colloidal system comprised of sterically stabilized silica particles suspended in a marginal solvent. The repulsive to attractive transition was realized by varying the temperature. While the particle form factor showed few changes, the structure factor of interparticle interactions exhibited liquidlike features in the attractive phase. The measured structure factors up to a gelation transition can be adequately described by the square-well model of short-ranged attractive fluids. The particle dynamics showed a continuous change from single to stretched exponential decay as the system transformed from repulsive to attractive behavior. A complete jamming of the particle dynamics was observed when the depth of attractive well attained several kBT. Although, static and dynamic behavior are reversible with respect to t...

Modification of deeply buried hydrophobic interfaces by ionic surfactants

Hydrophobicity, the spontaneous segregation of oil and water, can be modified by surfactants. The way this modification occurs is studied at the oil–water interface for a range of alkanes and two ionic surfactants. A liquid interfacial monolayer, consisting of a mixture of alkane molecules and surfactant tails, is found. Upon cooling, it freezes at Ts, well above the alkane’s bulk freezing temperature, Tb. The monolayer’s phase diagram, derived by surface tensiometry, is accounted for by a mixtures-based theory. The monolayer’s structure is measured by high-energy X-ray reflectivity above and below Ts. A solid–solid transition in the frozen monolayer, occurring approximately 3 °C below Ts, is discovered and tentatively suggested to be a rotator-to-crystal transition.

Castor 1.0, a VLSI analog-digital circuit for pixel imaging applications

Abstract We present a VLSI CMOS-mixed analog-digital circuit for high-rate pixel X-ray imaging applications. It consists of 32 channels at 80 μm pitch. The total die size is 3.7×14 mm 2 . Each channel features: a low-noise charge preamplifier, a CR-RC shaper, a buffer, a threshold discriminator and a 16-bit binary counter. The readout is done serially on a tri-state buffer. The main parameters of the analog part are: shaping time of 850 ns at 5 pF input capacitance, gain of 180 mV/fC, ENC( e − rms) = 60 + 17 C d (pF) and a power consumption of 3.8 mW/channel. The counting rate is limited by the analog part to around 100 kHz/channel for 1 fC charge pulses. Due to the parallelism of the circuit, photon rate in the order of 1 GHz/cm 2 can be measured for a pixel size of the order of 200 × 200 μ m 2 . The parameters of the circuit were optimised for the Syrmep experiment, an R&D project in digital mammography. The circuit was produced in 1.2 μm CMOS technology by AMS (Austria). Characterisation of the circuit, as well as first-imaging results of the circuit connected to microstrips or pixel detectors are presented. They show the circuit works according to specification and can be used for imaging applications.

Analysis of the correlation of counterions to rod-like macroions by anomalous small-angle X-ray scattering

We present a study of a rod-like polyelectrolyte by anomalous small-angle X-ray scattering (ASAXS). The polyelectrolyte consists of a stiff poly(p-phenylene) backbone with attached positive charged groups that are balanced by bromine counterions. The scattering data are taken far below the absorption edge (13 473.7 eV) and in its immediate neighborhood. The decrease of the measured intensity predicted by theory is directly observed. A new analysis of these ASAXS-data leads to three partial intensities in a numerically self-consistent fashion. In particular, the scattering intensity that is solely due to the cloud of the counterions could be determined and compared to the prediction of the Poisson–Boltzmann cell model. Quantitative agreement is found. ASAXS is thus shown to be a new and highly effective tool for the analysis of polyelectrolytes.

Core-shell nanoparticle monolayers at planar liquid- liquid interfaces: effects of polymer architecture on the interface microstructure†

Self-assembly of core–shell nanoparticles (NPs) at liquid–liquid interfaces is rapidly emerging as a strategy for the production of novel nano-materials bearing vast potential for applications, including membrane fabrication, drug delivery and emulsion stabilization. The development of such nanoparticle-based materials is facilitated by structural characterization techniques that are able to monitor in situ the self-assembly process during its evolution. Here, we present an in situ high-energy X-ray reflectivity study of the evolution of the vertical position (contact angle) and inter-particle spacing of core–shell iron oxide–poly(ethylene glycol) (PEG) nanoparticles adsorbing at flat, horizontal buried water–n-decane interfaces. The results are compared with time-resolved interfacial tension data acquired with the conventional pendant drop method. We investigate in particular the effect of varying polymer molecular weights (2–5 kDa) and architectures (linear vs. dendritic) on the self-assembly process and the final structure of the interfacially adsorbed NP monolayers. Linear PEG particles adsorb more rapidly than dendritic PEG ones and reach full interface coverage and stable NP monolayer structure, while dendritic PEG particles undergo a slower adsorption process, which is not completed within the experimental time window of ∼6 hours. All NPs are highly hydrophilic with effective contact angles that depend weakly on PEG molecular weight and architecture. Conversely, the in-plane NP separation depends strongly on PEG molecular weight. The measured inter-particle separation at full interface coverage yields low iron oxide core content, indicating a strong deformation and flattening of the linear PEG shell at the interface. This finding is supported by modeling and has direct implications for materials fabrication, e.g. for the realization of core–shell NP membranes by in situ cross-linking of the polymer shells.

High-dynamic range SAXS data acquisition with an X-ray image intensifier

Data with a wide dynamic range of intensity can be collected with a pinhole high-brilliance small-angle X-ray scattering (SAXS) camera using an image-intensified charge-coupled device (CCD) detector. The point spread function (PSF) of this detector has a narrow peak with a broad low tail such that a high level of scattered intensity at small angles can cause a significant background in the detector elements at higher angles. A correction scheme for the long tail of the PSF of the detector is needed when this integrating area detector is used for measuring intensity that spans a dynamic range of four to five orders of magnitude. A procedure is described for measuring the PSF contribution by masking a small part of the detector from the scattered radiation with an absorbing material. In order to measure the PSF, it is necessary to use a high-intensity spot, which is readily achieved by using a sample that scatters strongly at small angles. Although this intensity is spread over many pixels, the sharp features in the scattering from the silica sample chosen for this study permit one to obtain simultaneously both the narrow and the broad parts of the PSF. The data are compared with the actual scattering function, which has been measured exactly with a point-geometry Bonse–Hart camera. The advantages of this procedure are discussed.