Low-energy Imaging Research Articles

Health risks of low photon energy imaging

The major health risk associated with low photon energy imaging is thought to be the induction of cancer as a consequence of the radiation exposure and this is the focus of this paper. Low photon energy imaging typically involves exposure to a low dose (<50 mGy) of low linear energy transfer (LET) radiation delivered at high dose-rate. Since epidemiologic data cannot provide an accurate assessment of risk at the doses used in imaging, risk estimates are currently made by fitting a linear response to intermediate and high dose data for cancer induction in radiation-exposed human populations. This method assumes a linear no-threshold (LNT) response and implies that no dose of radiation is safe. This assumption is not borne out by many laboratory studies of cancer-related endpoints that would suggest that the risk at low doses is much less than would be estimated from linear extrapolation from intermediate to high doses. It is also well recognised that the dose-response from many epidemiologic studies could equally well be fit by threshold models. Through the study of radiation-induced neoplastic transformation in vitro J-shaped dose-response curves for a variety of low LET radiations, including those used in low photon energy imaging, have been demonstrated. The relative risks calculated from this data compare remarkably well with those for breast cancer and leukemia incidence in radiation-exposed populations. From this it is concluded that the LNT hypothesis is likely to overestimate the risk of cancer induction by low photon energy imaging, at least for certain tumors.

Characterisation of adhesively bonded laminates using radiography and infrared thermal imaging techniques

This paper discusses the studies undertaken to establish the limits of detectability of glue variations and defect detection in adhesively bonded canopy specimens used in fighter aircrafts, by low-energy radiography and thermal imaging techniques. The complementary nature of radiography and thermal imaging is also highlighted. From the results of the experiments, it is shown that low-energy radiography reveals volumetric defects such as porosities as small as 170 microns; thermal imaging is more sensitive to evaluate variations in glue content, having the capability of detection of variations in glue content of the order of 20%. Thermal imaging did not indicate the presence of any major defects such as delaminations and debonds. While reflection and through-transmission-based thermal imaging techniques can be utilised for the detection of glue variations, the rate of change of temperature by transmission technique being higher, this can serve as a more sensitive indicator of glue variations, if both sides access is feasible.

In situformation of a new Al-Pd-Mn-Si quasicrystalline phase on the pentagonal surface of the Al-Pd-Mn quasicrystal

Growth of thin Si films deposited on the 5-fold symmetry surface of the icosahedral Al-Pd-Mn quasicrystal is monitored by low-energy electron diffraction, secondary-electron imaging, and Auger electron spectroscopy. We observe that below a sample temperature of 370K, Si grows in an amorphous structure. Above 370K, a new Al-Pd-Mn-Si quasicrystalline phase, which exhibits the same icosahedral symmetry as the substrate, is formed at the surface by substitutionally replacing Al by absorbed Si.

High-resolution, back-side illuminated monolithic active pixel sensor for low-energy electron imaging

A new domain of application of monolithic active pixel sensors, with respect to particle tracking, has been triggered by the innovative idea of a nondestructive beam monitoring system in the extraction lines of a hadron-therapy center. The beam monitoring exploits secondary electrons emitted from a submicrometer thick Al foil, intersecting the beam. Electrons are accelerated in an electrostatic field. The detection of low-energy electrons, up to 30 keV, is required. The sensitivity to these energies is obtained by thinning the detector, originally fabricated in a standard VLSI technology, down to the thickness of the radiation sensitive layer. A thin entrance window, in the order of 100 nm, is provided. Monolithic active pixel sensors for low-energy electron imaging can be prospectively used in several domains: in bioscience for cell process study using radiotracers, e.g., /sup 3/H(18 keV/spl beta//sup -/) or fluorescence imaging exploiting the Hybrid Photodiodes principle, in safety or environmental studies for neutron imaging with converters directly deposited or in micro-beam facilities for position resolving in studies of living cell irradiations. The low-energy electron imaging capabilities for installation inside an HPD test facility and the results obtained with a /sup 3/H marked source are shown. The detector used is the 1/spl times/10/sup 6/ pixel MIMOSA V chip. The back-thinning up to the epitaxial layer was applied, resulting in a high resolution, back-side illuminated imager.

Direct imaging and elemental mapping of microgels in natural rubber particles

Elemental distribution maps of Hevea brasiliensis natural rubber gels have been obtained using electron energy-loss spectroscopy imaging in a low-energy (80 kV) electron spectroscopy imaging transmission electron microscope. Two types of gels have been investigated: a microgel contained within the natural rubber particle, and a macrogel prepared by equilibrating dry natural rubber in toluene. Both types of gels are found to contain a high amount of calcium. The intraparticle microgel is dense and rich in calcium but poor in nitrogen, indicating the predominant role of calcium in cross-link formation. The macroscopic gel is inhomogeneous, with dense calcium-rich microgels interspersed in a matrix of a less dense gel. The significant level of nitrogen associated with the matrix of the less dense gel supports the role of proteinaceous materials in the formation of the macroscopic gel.

A LEEM study of bamboo-like growth of Ag crystals on Si(0 0 1) surfaces

We report a low-energy electron microscopy study of novel bamboo-like (quasi-one-dimensional) growth during deposition of Ag on Si(0 0 1) surfaces at elevated temperatures. The bamboo crystals, with typical dimensions of 10 μm in length and varied height and width (tens to hundred of nanometers), align primarily along Si[1 1 0] or Si[1 −1 0] orientation. Low-energy electron microscopy imaging further demonstrates that the Ag bamboo crystals are initially stable against annealing, but break into segments upon prolonged annealing at 843 K. Possible growth mechanisms of the bamboo-like crystals are discussed.

Growth of thick CdTe epilayers on GaAs substrates and evaluation of CdTe/n+-GaAs heterojunction diodes for an X-ray imaging detector

The growth characteristics of thick (100) CdTe epitaxial layers of a thickness up to 200 µm on a (100) GaAs substrate in a metal-organic vapor-phase epitaxy (MOVPE) system and fabrication of CdTe/n+-GaAs heterojunction diodes for their possible applications in low-energy x-ray imaging detectors are reported. The grown epilayers were of high structural quality as revealed from the x-ray double-crystal rocking curve (DCRC) analysis, where the full-width at half-maximum (FWHM) values of the (400) diffraction peaks was between 50 arcsec and 70 arcsec. The 4.2-K photoluminescence (PL) showed high-intensity bound-excitonic emission and very small defect-related peaks. The heterojunction diode fabricated had a good rectification property with a low value of reverse-bias current. The x-ray detection capability of the diode was examined by the time-of-flight (TOF) measurement, where good bias-dependent photoresponse was observed, but no carrier transport property could be deduced. It was found that the CdTe layer has a large number of trapping states as attributed to the cadmium-related vacancy and Ga-impurity, diffused from the substrate, related defect complexes.

Real-time perfusion imaging: a new echocardiographic technique for simultaneous evaluation of myocardial perfusion and contraction.

Myocardial contrast echocardiography (MCE) with high acoustic energy and triggered harmonic imaging is the best established ultrasound technique to date for the assessment of myocardial perfusion. With this technique, however, the ultimate goal of MCE (noninvasive real-time simultaneous assessment of myocardial perfusion and function after an intravenous injection of microbubbles) is not met. Recently, technologic advances have enabled myocardial opacification to be visualized during low-energy real-time imaging. During real-time perfusion imaging, wall motion and myocardial perfusion may be assessed simultaneously, obviating the need of the presently time-consuming combination of different imaging modalities. When high-energy ultrasound bursts are periodically transmitted to produce bubble destruction during low-power imaging, the consecutive frames after destruction delineate the restoration of contrast intensity. Microbubble replenishment rate and peak intensity may be determined subsequently, and provide reliable quantitative parameters of regional microcirculatory flow. This review will introduce the modalities used for real-time perfusion imaging with focus on power pulse inversion imaging and quantitative analysis. Furthermore, we will describe the clinical role the technique may have in the identification of coronary artery disease, quantification of coronary stenosis severity, assessment of myocardial viability, determination of infarction size, and evaluation of reflow and no- or low-reflow after acute myocardial infarction.

Fabrication process validation for a double-sided silicon pixel detector

Previous results on the performance under irradiation of a pixel detector matrix (Nucl. Instr. and Meth. A 487 (2002) 170) have been extended in order to test the readiness for commercial detector wafer production, using I–V, I–V under low-energy irradiation (UV responsivity) and C–V characterisation methods. The data analysis includes a study of the C–V characteristic interpolation. The conclusion points to the fact that state-of-the-art double-sided silicon pixel detector fabrication allows reliable industrialisation of low-energy imaging devices.

RMDP: a dedicated package for 131I SPECT quantification, registration and patient-specific dosimetry.

The limitations of traditional targeted radionuclide therapy (TRT) dosimetry can be overcome by using voxel-based techniques. All dosimetry techniques are reliant on a sequence of quantitative emission and transmission data. The use of (131)I, for example, with NaI or mIBG, presents additional quantification challenges beyond those encountered in low-energy NM diagnostic imaging, including dead-time correction and additional photon scatter and penetration in the camera head. The Royal Marsden Dosimetry Package (RMDP) offers a complete package for the accurate processing and analysis of raw emission and transmission patient data. Quantitative SPECT reconstruction is possible using either FBP or OS-EM algorithms. Manual, marker- or voxel-based registration can be used to register images from different modalities and the sequence of SPECT studies required for 3-D dosimetry calculations. The 3-D patient-specific dosimetry routines, using either a beta-kernel or voxel S-factor, are included. Phase-fitting each voxel's activity series enables more robust maps to be generated in the presence of imaging noise, such as is encountered during late, low-count scans or when there is significant redistribution within the VOI between scans. Error analysis can be applied to each generated dose-map. Patients receiving (131)I-mIBG, (131)I-NaI, and (186)Re-HEDP therapies have been analyzed using RMDP. A Monte-Carlo package, developed specifically to address the problems of (131)I quantification by including full photon interactions in a hexagonal-hole collimator and the gamma camera crystal, has been included in the dosimetry package. It is hoped that the addition of this code will lead to improved (131)I image quantification and will contribute towards more accurate 3-D dosimetry.

The First two Years of Image

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) is the first satellite mission that is dedicated to imaging the Earth's magnetosphere. Using advanced multispectral imaging techniques along with omnidirectional radio sounding, IMAGE has provided the first glimpses into the global structure and behavior of plasmas in the inner magnetosphere. Scientific results from the two-year prime mission include the confirmation of the theory of plasmaspheric tails and the discovery of several new and unpredicted features of the plasmasphere. Neutral-atom imaging has shown how the ring current develops during magnetic storms and how ionospheric ions are injected into the ring current during substorms. The first global imaging of proton auroras has allowed the identification of the ionospheric footprint of the polar cusp and its response to changes in the interplanetary magnetic field. Detached subauroral proton arcs have been found to appear in the afternoon sector following south-north and east-west rotations of the IMF. Low-energy neutral atom imaging has shown global-scale ionospheric outflow to be an immediate response to solar-wind pressure pulses. Such imaging has also provided the first measurements of solar wind and interstellar neutral atoms from inside the magnetosphere. Radio sounding has revealed the internal structure of the plasmasphere and identified plasma cavities as the source of kilometric continuum radiation. These and numerous other scientific results now set the stage for the extended mission of IMAGE in which the imaging perspective will change markedly owing to orbital evolution while the magnetospheric environment undergoes a transition from solar maximum toward solar minimum.

Chromophore Mapping via Low-Energy Loss Imaging in a Modified EM902: Formation of the Immune Synapse

Chromophore Mapping via Low-Energy Loss Imaging in a Modified EM902: Formation of the Immune Synapse

Fuel-Coolant Interactions: Visualization and Mixing Measurements

Dynamic X-ray imaging of fuel-coolant interactions (FCI), including quantitative measurement of fuel-coolant volume fractions and length scales, has been accomplished with a novel imaging system at the Nuclear Safety Research Center at the University of Wisconsin, Madison. The imaging system consists of visible-light high-speed digital video, low-energy X-ray digital imaging, and high-energy X-ray digital imaging subsystems. The data provide information concerning the melt jet velocity, melt jet configuration, melt volume fractions, void fractions, and spatial and temporal quantification of premixing length scales for a model fuel-coolant system of molten lead poured into a water pool (fuel temperatures 500 to 1000 K; jet diameters 10 to 30 mm; coolant temperatures 20 to 90°C). Overall results indicate that the FCI has three general regions of behavior, with the high fuel-coolant temperature region similar to what might be expected under severe accident conditions. It was observed that the melt jet leading edge has the highest void fraction and readily fragments into discrete masses, which then subsequently subdivide into smaller masses of length scales <10 mm. The intact jet penetrates <3 to 5 jet length/jet diameter before this breakup occurs into discrete masses, which continue to subdivide. Hydrodynamic instabilities can be visually identified at the leading edge and along the jet column with an interfacial region that consists of melt, vapor, and water. This interface region was observed to grow in size as the water pool temperature was increased, indicating mixing enhancement by boiling processes.

ENHANCEMENT OF IMMUNOGOLD-LABELLED FOCAL ADHESION SITES IN FIBROBLASTS CULTURED ON METAL SUBSTRATES: PROBLEMS AND SOLUTIONS

Visualisation of cell adhesion patterns by scanning electron microscopy requires special preparation and labelling. The membranes and cytoplasm must be removed, without damaging the antigen, to facilitate antibody access to vinculin in the focal adhesions. Low beam energy imaging is used to visualise the cell undersurface (embedded in resin after staining with osmium tetroxide) and immunogold-labelled adhesion sites. The gold probe, must be large enough (>40nm) for detection, while viewing the whole cell, but large gold markers increase steric hindrance and decrease labelling efficiency. This problem can be overcome by using small gold probes (1–5nm) followed by enlargement with silver enhancement, but osmium tetroxide stain etches the silver. We demonstrated that metal substrates increased this etching. Reducing the concentration of osmium tetroxide and incubation time reduced the amount of etching. We have demonstrated that gold enhancement was not etched by osmium tetroxide, irrespective of the substrate. Therefore, comparative studies of cell adhesion to different biomaterial substrates can be performed using immunogold-labelling with gold enhancement.

The cosmic electron background in low energy imaging atmospheric Cherenkov telescopes: effect of the geomagnetic field

A new generation of low threshold imaging atmospheric Cherenkov telescopes (IACTs) may reach γ-ray energies about 10 GeV with high sensitivities and very large collection areas. At these low energies cosmic electrons significantly contribute to the telescope background and are in principle indistinguishable from γ-rays. In this paper we estimate the electron background expected for two configurations of the low energy IACT MAGIC. We discuss in particular the reduction of the background caused by the geomagnetic field at different locations on the Earth's surface.

A monolithic active pixel sensor for charged particle tracking and imaging using standard VLSI CMOS technology

A novel Monolithic Active Pixel Sensor (MAPS) for charged particle tracking made in a standard CMOS technology is proposed. The sensor is a photodiode, which is readily available in a CMOS technology. The diode has a special structure, which allows the high detection efficiency required for tracking applications. The partially depleted thin epitaxial silicon layer is used as a sensitive detector volume. Semiconductor device simulation, using either ToSCA based or 3-D ISE-TCAD software packages shows that the charge collection is efficient, reasonably fast (order of 100 ns), and the charge spreading limited to a few pixels only. A first prototype has been designed, fabricated and tested. It is made of four arrays each containing 64×64 pixels, with a readout pitch of 20 μm in both directions. The device is fabricated using standard submicron 0.6 μm CMOS process, which features twin-tub implanted in a p-type epitaxial layer, a characteristic common to many modern CMOS VLSI processes. Extensive tests made with soft X-ray source ( 55Fe) and minimum ionising particles (15 GeV/ c pions) fully demonstrate the predicted performances, with the individual pixel noise (ENC) below 20 electrons and the Signal-to-Noise ratio for both 5.9 keV X-rays and Minimum Ionising Particles (MIP) of the order of 30. This novel device opens new perspectives in high-precision vertex detectors in Particle Physics experiments, as well as in other application, like low-energy beta particle imaging, visible light single photon imaging (using the Hybrid Photon Detector approach) and high-precision slow neutron imaging.

Quantitative assessment of myocardial perfusion during graded coronary stenosis by real-time myocardial contrast echo refilling curves

OBJECTIVESThe present study examined the ability of real-time myocardial contrast echocardiography (MCE) to delineate abnormalities produced by graded coronary stenoses and to correlate signal intensity (SI) parameters derived from destruction/refilling curves with regional myocardial blood flow (MBF) and contractile function.BACKGROUNDRecent technological advances have enabled myocardial opacification by MCE to be achieved during real-time imaging.METHODSIn eight open-chest dogs, we created LAD occlusion and graded stenoses that were either flow-limiting at rest (FLS) or reduced adenosine hyperemia (non-flow-limiting at rest = NFLS). Myocardial contrast echo used Optison infusion and low-energy real-time power pulse inversion imaging. High-energy FLASH frames destroyed bubbles every 15 cardiac cycles. Myocardial SI-versus-time plots were fitted to a one-exponential function to obtain the rate of SI rise (b) and peak SI in the last frame.RESULTSDyssynergy was not observed during any NFLS, but perfusion abnormalities were. Visual detection of decreased opacification was possible with severe NFLS and FLS. bdemonstrated a significant reduction with severe NFLS and near significant with moderate NFLS; peak SI did not. All exponential parameters were significantly decreased with FL stenosis and occlusion. The MBF ratio in LAD/LCx beds (fluorescent microspheres) correlated with b(r = 0.79) and the product of the peak SI and b(r = 0.80).CONCLUSIONSIn an open-chest dog model, parameters derived from microbubble refilling of the imaging field by real-time MCE correlate well with myocardial blood flow and can identify coronary stenosis.

Threshold Energy Effects in Secondary Electron Emission

Abstract In large bandgap semiconductors and insulators, the threshold energies for e–h pair production and ionization damage can lie above the vacuum level. For low energy imaging, a window is then opened whose width is potentially sensitive to local changes in work function, doping level, or acidity. Recent progress and future opportunities for damage-free imaging of these properties using low energy electrons are discussed in the light of the underlying physics, as well as of recent instrumental developments in low energy electron microscopy (LEEM), environmental scanning electron microscopy (ESEM), photoelectron emission microscopy (PEEM), scanned probe microscopy (SPM), and projection electron microscopy.

Threshold Energy Effects in Secondary Electron Emission

AbstractIn large bandgap semiconductors and insulators, the threshold energies for e–h pair production and ionization damage can lie above the vacuum level. For low energy imaging, a window is then opened whose width is potentially sensitive to local changes in work function, doping level, or acidity. Recent progress and future opportunities for damage-free imaging of these properties using low energy electrons are discussed in the light of the underlying physics, as well as of recent instrumental developments in low energy electron microscopy (LEEM), environmental scanning electron microscopy (ESEM), photoelectron emission microscopy (PEEM), scanned probe microscopy (SPM), and projection electron microscopy.

Principle of Low-Energy Electron Beam-Induced Current Imaging for Ferroelectric Thin Films

Principle of Low-Energy Electron Beam-Induced Current Imaging for Ferroelectric Thin Films