Regions Of High Attenuation Research Articles

Degradation monitoring of multilayer ceramic capacitors during high accelerated lifetime testing using ultrasound

Multilayer ceramic capacitors (MLCCs) are vital circuitry components where long-term stability at high temperature and fields is critical to device operation. High accelerated lifetime testing (HALT) statistically investigates reliability and lifetime of MLCCs through exposure to temperatures and voltages exceeding normal operating conditions. The long-term reliability of the dielectric layers in MLCCs strongly depends on potential variability in the dielectric microstructure, such as cracking, which can be challenging to detect. Nondestructive ultrasonic evaluation is sensitive to microstructural changes through measurements of wave scattering. This work uses high frequency (100 MHz) focused ultrasonic scattering methods to nondestructively monitor structural and microstructural changes in MLCCs from the pristine to the failed state during HALT to understand structural origins of electrical failure. Printed circuit board mounted MLCCs were electrically and ultrasonically characterized in a pristine state and during various degraded states before and after electrical failure under HALT. Evidence of damage pre-HALT was ultrasonically indicated by high attenuation regions on the ultrasonic maps suggesting that the mounting process may induce damage. The high attenuation regions in the pristine samples evolved throughout HALT indicating that the damage in the pristine state might be the spatial origin of part failure, which underscores the impact and importance of detection of structural defects on the reliability of MLCCs.

Assessment of flaws in cold‐sintered ZnO via acoustic wave speed and attenuation measurements

AbstractThe cold sintering process (CSP) is a low temperature processing technique that utilizes a transient phase to synthesize dense ceramics. However, some CSP parts contain microflaws that arise due to inhomogeneities in pressure, temperature, and transient phase. This work uses 20 MHz ultrasound to verify the presence of defects in CSP ZnO samples of varying densities (84%–97%). Acoustic metrics used in this work include wave speed, which is affected by differences in the effective elastic properties of the medium, and attenuation, which quantifies wave energy loss due to scattering from defects. Wave speed maps were inhomogeneous, suggesting density gradients which were verified with scanning electron microscopy. In addition, it was demonstrated that the pores produced by cold sintering are anisometric, which increases the anisotropy in the elastic properties. High attenuation regions (>300 Np/m) are present in all samples independent of relative density and correspond to defects identified in X‐ray computed tomography (XCT) which were as small as 38 µm in effective diameter. However, some high attenuation spots do not correspond to visible defects in XCT, which suggests the presence of features undetectable with XCT such as residual secondary phases at the grain boundaries.

榎茸による食餌性腸閉塞の1例(Dietary small bowel obstruction by enokitake mashrooms)

要旨症例は生来健康な67歳の男性。主訴は腹痛，嘔気。患者は榎茸をほとんど毎日摂取し，1日で最大500g（通常量の50倍）の榎茸を食べることもあった。受診10日前から間欠的な心窩部痛，腹部膨満感，嘔気がみられ，症状が持続するため救急外来を受診した。来院時バイタルはGCS E4V5M6，血圧96/55mmHg，心拍数95/分，SpO2 98%，体温36.2℃。腹部は膨満しており，圧痛は明らかではなかった。受診時の腹部エコーでは腸管拡張がみられ，腸閉塞の可能性を疑い，CT撮影した。CTでは小腸閉塞がみられ，閉塞部位に内部が不均一な高吸収の腫瘤様陰影がみられた。摂食歴と併せ，榎茸による食餌性腸閉塞と診断し入院加療とした。入院後イレウス管を留置したところ，症状改善がみられ，再度撮影したCTでは腫瘤様陰影は消失していた。榎茸の過剰摂取で，上記のような画像所見がみられる小腸閉塞では，榎茸による食餌性腸閉塞を鑑別にあげる必要がある。また治療後は，食餌性腸閉塞の再発予防のために食事指導を行うことが重要である。

Seismic source characterization of the Arabian Peninsula and Zagros Mountains from regional moment tensor and coda envelopes

Reliable estimates of moment magnitude and source mechanism for seismic events in the Middle East can be challenging due to a small number of openly available stations, the complex tectonic setting, and regions of high attenuation. Access to high-quality waveform data from well-calibrated regional seismic stations is fundamental in producing robust and stable estimates of earthquake source parameters, particularly when measurements of absolute ground motion amplitudes are required. Earthquake source mechanism and moment magnitude are invaluable information in the assessment of seismic hazard, plate motions, and the characterization of faults and regional stress field. The expansion of the Kingdom of Saudi Arabia’s national seismic network in the past several decades provides an opportunity to develop the capabilities of routine focal mechanism and moment magnitude estimations. In this study, we use time-domain full waveform moment tensor inversion and coda envelope–derived amplitude measurements to solve for earthquake source mechanism, moment magnitudes, and their source type. We compared the moment magnitudes calculated from the two methods and publicly available earthquake catalogs and discuss the implications of the obtained source parameters.

Crustal attenuation from USArray ML amplitude tomography

SUMMARYSeismic attenuation across the US is estimated using station ML magnitude data from the USArray. Station magnitudes are recalibrated back to amplitude and back projected in a 2-D tomography. Data represent the amplitudes of the horizontal components of the Lg phase. The western US shows regions of very high attenuation and contrasts with the lesser attenuation of the eastern US. Individual attenuation anomalies can be clearly tied to regional geology. Station gains show broad regional variations that match geographic regions. Most of the high-attenuation areas are regions of high geothermal activity suggesting that intrinsic attenuation dominates over scattering attenuation. An exception is the central San Andreas Fault zone because it lacks any localized heat-flow anomaly. The US east of the Rocky Mountains is bland and contains none of the high-attenuation regions of the western US. Instead, the central US has low-attenuation patches that do not obviously correspond to geologic province. Sediments of the Gulf Coast Plain, Willison Basin and Michigan Basin do show up as intermediate attenuation while the Illinois Basin, Appalachian Basin and other basins are not apparent. In Alaska, attenuation is generally less than the western US, but still much greater than the eastern US. In southeast Alaska, the Wrangell Volcanic Field causes a sizeable high-attenuation zone. The volcanic Aleutian Mountains also have high attenuation. However, moderate to high attenuation also correlates with the tertiary sedimentary basins in Alaska. The North Slope Basin does not seem to attenuate. Thicker crust and mountain roots tend to show less attenuation, if anything, but this correspondence is most likely due to differences in temperature and seismic velocity. Heat, scattering and young sedimentary basins create seismic attenuation in the continental crust.

A unilateral thalamic lesion referred for biopsy: Answer

•Venous thrombosis of the great cerebral vein can lead to thalamic infarction.•Venous infarction can have similar MR imaging appearance to a space occupying lesion.•CT venography or gadolinium-enhanced MR venography is the most frequently used technique to show venous thrombosis. Answer–Venous infarct involving the left great cerebral vein Computerised tomography of the body did not show any primary malignant disease. The patient was referred to neurosurgery for consideration of a brain biopsy in view of the presumed neoplastic aetiology. However, further neuroradiological review raised the possibility of a deep venous infarct. This was confirmed with subsequent MR venography which showed absence of the left internal cerebral veins due to thrombosis related filling defect (Fig. 1B). The patient was treated with systemic anticoagulation and recovered without sequalae. Screening for prothrombotic tendencies and underlying precipitants was negative. This image report illustrates the challenging clinic-radiological diagnosis of a deep venous infarct, and the potential risk of misdiagnosis as a space occupying lesion. It is essential to consider venous infarction in the differential diagnosis of space occupying lesions of the brain [1Bakshi R. Lindsay B.D. Bates V.E. Kinkel P.R. Mechtler L.L. Kinkel W.R. Cerebral venous infarctions presenting as enhancing space-occupying lesions: MRI findings.J Neuroimaging. 1998; 8: 210-215https://doi.org/10.1111/jon199884210Crossref PubMed Scopus (30) Google Scholar, 2Dmytriw A.A. Song J.S.A. Yu E. Poon C.S. Cerebral venous thrombosis: state of the art diagnosis and management.Neuroradiology. 2018; 60: 669-685https://doi.org/10.1007/s00234-018-2032-2Crossref PubMed Scopus (51) Google Scholar]. This ensures patients receive prompt treatment with anticoagulation or endovascular therapy, and avoids unnecessary and potentially harmful invasive investigations such as brain biopsy [[3]Renard D. Castelnovo G. Campello C. Bouly S. Le Floch A. Thouvenot E. et al.Behav Neurol. 2014; 2014: 1-17Crossref Scopus (26) Google Scholar]. The following are the Supplementary data to this article: Download .xml (.0 MB) Help with xml files Supplementary data 1 A unilateral thalamic lesion referred for biopsyJournal of Clinical NeuroscienceVol. 76PreviewA 44-year-old man presented to the emergency department suffering with a three-week history of unremitting headaches associated with evolving paraesthesia and numbness involving the whole right side of the body. There was no significant past medical or pharmacological history. Routine blood tests were normal, and a lumbar puncture for cytology and microscopy was unremarkable. FLAIR magnetic resonance (MR) brain imaging demonstrated a region of high-attenuation and oedema in the left thalamus (Fig. Full-Text PDF

3-D seismic attenuation structure of Long Valley caldera: looking for melt bodies in the shallow crust

SUMMARY Unrest at Long Valley caldera (California) during the past few decades has been attributed to the ascent of hydrothermal fluids or magma recharge. The difference is critical for assessing volcanic hazard. To better constrain subsurface structures in the upper crust and to help distinguish between these two competing hypotheses for the origin of unrest, we model the 3-D seismic attenuation structure because attenuation is particularly sensitive to the presence of melt. We analyse more than 47 000 vertical component waveforms recorded from January 2000 through November 2016 obtained from the Northern California Earthquake Data Center. We then inverted the S-to-coda energy ratios using the coda normalization method and obtained an average Q of 250. Low attenuation anomalies are imaged in the fluid-rich western and eastern areas of the caldera, one of which corresponds to the location of an earthquake swarm that occurred in 2014. From a comparison with other geophysical images (magnetotellurics, seismic tomography) we attribute the high attenuation anomalies to hydrothermal systems. Average to high attenuation values are also observed at Mammoth Mountain (southwest of the caldera), and may also have a hydrothermal origin. A large high attenuation anomaly within the caldera extends from the surface to the depths we can resolve at 9 km. Shallow rocks here are cold and this is where earthquakes occur. Together, these observations imply that the high attenuation region is not imaging a large magma body at shallow depths nor do we image any isolated high attenuation bodies in the upper ≈8 km that would be clear-cut evidence for partially molten bodies such as sills or other magma bodies.

First 2-D intrinsic and scattering attenuation images of Mt Etna volcano and surrounding region from active seismic data

SUMMARYWe present 2-D attenuation images of the Mt Etna volcanic region on the basis of separation of intrinsic and scattering effects. The analysis presented here exploits a large active seismic database that fully covers the area under study. We observe that scattering effects dominate over intrinsic attenuation, suggesting that the region is very heterogeneous. Comparison with analyses conducted at other volcanoes reveals that the Mt Etna region is characterized by high intrinsic attenuation, resulting from the presence of large volcanoclastic deposits at shallow depth. The 2-D distributions of intrinsic and scattering anomalies show the presence of regions characterized by high and low attenuation effects, corresponding to several tectonic and volcanic features. In particular, we identify a high attenuation region in the SW sector of the Mt Etna volcanic complex, which is correlated with high seismicity rates and volcanism. This work supports the hypothesis of a link between the dynamics of the SW flank and the recharge of the volcano in the last decades, occurring under the summit crater and, secondarily, the upper South rift zone.

Enhancing radiographic imaging of cementitious materials in composite structures with photon attenuating inclusions

ABSTRACTThis article investigates the use of photon attenuating inclusions (PAI) for enhancing radiographic imaging of cementitious materials in composite structures. By targeting characteristic high attenuation regions in the photon energy spectrum, PAI can improve void detection in the parent material by increasing contrast in radiographic images. Radiographic imaging of cementitious grout specimens with 5% and 15% weight fractions of candidate PAI materials (Fe, BaCO3, BaSO4) was performed to characterize X-ray attenuation. Virtual radiography simulations were then conducted to evaluate the application of PAI to grout void detection in post-tensioned concrete construction. The simulations demonstrate that grout void detection is most effective when the high energy region of the emission spectrum is near the K-edge of the PAI material.

Anelasticity beneath the Aegean inferred from Rayleigh wave attenuation

Anelasticity of the Earth crucially affects the propagation of seismic waves especially, in the long period range. However, even though the elastic properties of the Aegean deep lithosphère and upper mantle have been thoroughly investigated, their quantitative anelastic properties that influence the long period wavefield are still largely unknown. This work is towards contributing to the better knowledge of the deep structure of the Aegean by introducing experimental anelastic parameters via the study of long period Rayleigh waves attenuation. For this scope, fundamental mode attenuation coefficients (γ%) have been obtained for different two-station great-circle paths across the Aegean. The data used were provided by a broadband array installed in the area for 6 months in 1997. More than 1100 seismograms were analyzed in the 10-100 s range to obtain 17 sets of path average γR(T) functions. The attenuation coefficients are in the range 2.5*10~3 — 0.15 x 10' km' and correlate sufficiently with both experimental measurements in active tectonic regions elsewhere and synthetics generated with the use of an attenuation reference model inferred from other sources. By applying a stochastic uncoupled causal inversion method an average joint Qß'1 and shear velocity model representative of the under study area was obtained. Furthermore, path average JR(T) functions were combined in a continuous regionalization tomographic scheme to obtain local yR(T) and tomograms were constructed in the range 10-60 s. The most prominent feature in the tomograms is a high attenuation region in the central and north Aegean. This region is located south of the North Anatolian Trough and correlates well with a low shear velocity zone inferred from surface wave phase velocities. Moreover, it is associated with observed intense extensional deformation rates, mantle olivine anisotropy, recent volcanism and high heat flow.

Seismic High Attenuation Region Observed Beneath Southern New England From Teleseismic Body Wave Spectra: Evidence for High Asthenospheric Temperature Without Melt

AbstractSeismic attenuation exhibits strong geographic variability in northeastern North America, with the highest values associated with the previously recognized Northern Appalachian Anomaly (NAA) in southern New England. The shear wave quality factor at 100 km depth is 14 < QS < 25, the ratio of P wave and S wave quality factors is QP/QS = 1.2 ± 0.03 (95%), and the frequency dependence parameter is α = 0.39 ± 0.025 (95%). The high values of QP/QS and α are compatible with laboratory measurements of unmelted rock and, in the case of α, incompatible with widespread melting. The low QS implies high mantle temperatures (~1,550–1,650°C) at 100 km depth (assuming no melt). Small‐scale variations in attenuation suggest structural heterogeneity within the NAA, possibly due to lithospheric delamination caused by asthenospheric flow.

Moment Magnitudes of Local/Regional Events from 1D Coda Calibrations in the Broader Middle East Region

Abstract Reliable moment magnitude estimates for seismic events in the Middle East region can be difficult to obtain due to the uneven distribution of stations, the complex tectonic structure, and regions of high attenuation. In this study, we take advantage of the many new broadband seismic stations that have become available through improved national networks and numerous temporary deployments. We make coda envelope‐amplitude measurements for 2247 events recorded by 68 stations over 13 narrow frequency bands ranging between 0.03 and 8 Hz. The absolute scaling of these spectra was calculated based on independent waveform modeling solutions of the moment magnitudes for a subset of these events to avoid circularity. Using our 1D path calibrations, we determined coda‐based magnitudes for a majority of the events. We obtain fairly good agreement with waveform‐modeled seismic moments for the larger events ( M w >4.5) at low frequencies ( 0.7 Hz) because of unaccounted 2D path effects, as well as mixing of both Sn coda and Lg coda, which have different attenuation behavior. This scatter leads to increased variance in the magnitudes estimated for smaller events in which low‐frequency amplitudes are below the noise levels and the higher frequencies are the only signals available. We quantify the expected variance in coda envelope amplitudes as a function of frequency using interstation scatter as our metric. The net results of this study provide thousands of new 1D coda magnitude estimates for events in the broad region, as well as the necessary initial starting model for use in a new related 2D coda study (Pasyanos et al. , 2016). Online Material: Table of site terms and moment magnitudes.

Crustal structure of the Alps as seen by attenuation tomography

We develop a simple tomographic approach exploiting the decay rate of coda waves to map the absorption properties of the crust in a region delimited approximately by the Rhine Graben to the North, the Apennines to the South, the Massif Central to the West and the Dinarides to the East. Our dataset comprises 40 000 coda records of about 2000 weak to moderate crustal earthquakes, with magnitude ranging from 2.8 to 6 and recorded by broad-band, accelerometric and short-period stations. After proper choice of a coda window minimizing the effects of variable epicentral distances, we measure the coda quality factor Qc in five non-overlapping frequency windows covering the 1–32 Hz band for all available source station pairs. These measurements are subsequently converted into maps of absorption quality factor (Qi) using a linearized, approximate relation between Qc and Qi. In practice the following procedure is applied in each frequency band: (1) we divide the target region into 40×40 km cells; (2) for each source-station pair, we assign the measured Qc value to each pixel intercepted by the direct ray path; (3) the results are averaged over all paths and subsequently smoothed with a 3×3 pixels moving window. Our approach is consistent with the high sensitivity of Qc to the value of Qi between source and station.Our tomographic approach reveals strong lateral variations of absorption with length scales ranging from 100 km to 1000 km. At low frequency (∼1 Hz), the correlation with the surface geology is clear, Cenozoic and Mesozoic sedimentary basins (resp. crystalline massifs) being recognized as high (resp. low)-absorption regions. Furthermore the Qi map delineates finer geological features such as the Ivrea Body, the Rhône Valley, or felsic intrusions in the central Alps. At high-frequency (>16 Hz), only the thickest Cenozoic sedimentary deposits show up as high-attenuation regions and a north/south dichotomy is apparent in the absorption structure. The limit between low-attenuation regions to the North and high-attenuation region to the South correlates geographically with the location of the Periadriatic Lineament (PL), a major late-alpine crustal- to lithospheric-scale structure. Furthermore, the attenuation structure seems to prolong the PL to the West along a line marked by large historical earthquakes. The Apennines orogenic belts exhibit a distinct frequency behavior, with high attenuation at low-frequency and low-attenuation at high-frequency. Low-frequency absorption may likely be explained by the relatively thick cover of Cenozoic sedimentary materials, as well as by shallow geothermal activity. We hypothesize that the frequency dependence of the attenuation structure, in particular in the Apennines, is caused by a change of the wavefield composition which accentuates the sensitivity of the coda to the deeper parts of the medium as the frequency increases.

Effect of heat input and accumulated fatigue damage on mechanical properties of dissimilar AL-6XN/316L welded joints

In this study, plates of AL-6XN and 316L stainless steels were gas metal arc welded with an ER-NiCrMo3 filler wire and the effect of two different heat inputs and different accumulated fatigue damage is evaluated in terms of microstructure and mechanical behavior. Microstructural characterization of the as-welded samples revealed differences in dendrite size and interdendritic spacing in the weld metal with respect to the side of the dissimilar weld and to the heat input that results in weld metal regions with strong texture. Mo–Nb rich particles were found in the weld metal for both welding conditions and high heat input yielded very wide unmixed zones. Failure of the as-welded samples, under tensile stress, consistently occurred in the 316L base metal irrespective of the heat input. However, it was found that for a high heat input (1.6kJ/mm) accumulated fatigue damage changed the mechanical response of the welded dissimilar joints so that fracture took place in the weld metal. Ultrasonic attenuation measurements were used to characterize the welded joint and results showed regions of high attenuation that are related to dendritic structures.

Dual-exposure technique for extending the dynamic range of x-ray flat panel detectors

This work presents an approach to extend the dynamic range of x-ray flat panel detectors by combining two acquisitions of the same sample taken with two different x-ray photon flux levels and the same beam spectral configuration. In order to combine both datasets, the response of detector pixels was modelled in terms of mean and variance using a linear model. The model was extended to take into account the effect of pixel saturation. We estimated a joint probability density function (j-pdf) of the pixel values by assuming that each dataset follows an independent Gaussian distribution. This j-pdf was used for estimating the final pixel value of the high-dynamic-range dataset using a maximum likelihood method. The suitability of the pixel model for the representation of the detector signal was assessed using experimental data from a small-animal cone-beam micro-CT scanner equipped with a flat panel detector. The potential extension in dynamic range offered by our method was investigated for generic flat panel detectors using analytical expressions and simulations. The performance of the proposed dual-exposure approach in realistic imaging environments was compared with that of a regular single-exposure technique using experimental data from two different phantoms. Image quality was assessed in terms of signal-to-noise ratio, contrast, and analysis of profiles drawn on the images. The dynamic range, measured as the ratio between the exposure for saturation and the exposure equivalent to instrumentation noise, was increased from 76.9 to 166.7 when using our method. Dual-exposure results showed higher contrast-to-noise ratio and contrast resolution than the single-exposure acquisitions for the same x-ray dose. In addition, image artifacts were reduced in the combined dataset. This technique to extend the dynamic range of the detector without increasing the dose is particularly suited to image samples that contain both low and high attenuation regions.

Uppermost inner core seismic structure – new insights from body waveform inversion

Differential travel times and waveforms of PKIKP and PKiKP phases in the 129°–141° distance range, deconvolved for the effects of source time functions and average mantle attenuation operators, are inverted for velocity and attenuation in the uppermost 80 km of the inner core, and the velocity gradient and attenuation in the lowermost 200 km of the outer core (F region). Results confirm degree-one velocity structure in the inner core whose fast and slow regions appear to be separated by transitions with a velocity contrast of about 1%. Although attenuation in the inner core also exhibits degree-one pattern, a high attenuation region (Qp−1=[0.002,0.0067]) extends to the central Pacific, which has been previously considered a part of the western hemisphere characterized by slower velocity and lower attenuation. The rest of the inner core exhibits a positive correlation between velocity and attenuation as observed previously. An observed mantle-like relationship between velocity and attenuation in the central Pacific can be interpreted as originating from higher than average homologous temperature, consistent with a model of inner core solidification driven by outer core convection coupled to core–mantle boundary thermal heterogeneity. Alternative mechanisms, including differences in grain size and impurity concentration, and lateral differences in solidification rate might be required to explain the structure of the inner core. The F region velocity and attenuation are best explained by the AK135-F velocity model and Qp−1=[0,0.001] respectively.

Electronic Noise in CT Detectors: Impact on Image Noise and Artifacts

The objective of our study was to evaluate in phantoms the differences in CT image noise and artifact level between two types of commercial CT detectors: one with distributed electronics (conventional) and one with integrated electronics intended to decrease system electronic noise. Cylindric water phantoms of 20, 30, and 40 cm in diameter were scanned using two CT scanners, one equipped with integrated detector electronics and one with distributed detector electronics. All other scanning parameters were identical. Scans were acquired at four tube potentials and 10 tube currents. Semianthropomorphic phantoms were scanned to mimic the shoulder and abdominal regions. Images of two patients were also selected to show the clinical values of the integrated detector. Reduction of image noise with the integrated detector depended on phantom size, tube potential, and tube current. Scans that had low detected signal had the greatest reductions in noise, up to 40% for a 30-cm phantom scanned using 80 kV. This noise reduction translated into up to 50% in dose reduction to achieve equivalent image noise. Streak artifacts through regions of high attenuation were reduced by up to 45% on scans obtained using the integrated detector. Patient images also showed superior image quality for the integrated detector. For the same applied radiation level, the use of integrated electronics in a CT detector showed a substantially reduced level of electronic noise, resulting in reductions in image noise and artifacts, compared with detectors having distributed electronics.

Spatial distribution of seismicity parameters in the Persian Plateau

In this paper, seismicity parameters in the Persian Plateau are estimated and represented as colored maps. We depict the b-value variations as an indication of either changing material properties, or the heterogeneity of the stress field in this region. The lower b-values in some regions indicate that the seismic energy release in these areas is mostly provided by large magnitude earthquakes. Yet the b-value around northern CentralIran and Eastern Alborz seems to be near unity, which implies that the seismic energy release is compatible for larger and smaller earthquakes. We also compared our results with some existing seismological information on Iran. Regarding attenuation: low-Q (high attenuation) regions approximately correspond to relatively lower b-value regions, and areas with nearly medium/high b-values correspond to high-velocity anomaly zones inferred from tomographic inversions. Meanwhile, regions associated with thrust and strike-slip faulting was associated with low/medium b-values. The higher b-values in NW Iran agree with the regional extensional tectonics and low/medium b-values were in agreement with regional compressional/strike-slip tectonics. In terms of geodynamics, this may be interpreted as an indication of a weak crust in NW Iran versus stronger crust (zones under stress) in other parts.

Dose Reduction in Fluoroscopic Interventions Using a Combination of a Region of Interest (ROI) X-Ray Attenuator and Spatially-Different, Temporally-Variable Temporal Filtering.

A novel dose reduction technique for fluoroscopic interventions involving a combination of a material x-ray region of interest (ROI) attenuator and spatially different, temporally variable ROI temporal recursive filter, was used to guide the catheter to the ROI in three live animal studies, two involving rabbits and one involving a sheep. In the two rabbit studies presented, a catheter was guided to the entrance of the carotid artery. With the added ROI attenuator the image under the high attenuation region is very noisy. By using temporal filtering with a filter weight of 0.6 on previous frames, the noise is reduced. In the sheep study the catheter was guided to the descending aorta of the animal. The sheep offered a relatively higher attenuation to the incident x-rays and thus a higher temporal filter weight of 0.8 on previous frames was used during the procedure to reduce the noise to levels acceptable by the interventionalist. The image sequences from both studies show that significant dose reduction of 5-6 times can be achieved with acceptable image quality outside the ROI by using the above mentioned technique. Even though the temporal filter weighting outside the ROI is higher, the consequent lag does not prevent perception of catheter movement.

The "push-pull" dosimeter: When two pigments are better than one

A new kind of gel dosimeter (the "push-pull" dosimeter) is proposed which would contain two spectrally complementary pigments, one which darkens with increasing dose and another which bleaches. The bleaching pigment would be optimised for high sensitivity and the darkening pigment for low sensitivity. By employing dual pigments optimised independently, the usual requisite compromises between sensitivity at low dose and accuracy at high dose would be relaxed. Such a gel, after exposure would be read using two successive optical CT scans employing two different scan wavelengths. The use of dual pigments could also reduce the occurrence of regions of high optical attenuation which generate artefacts in optical CT. This paper also presents results of simulations of the behaviour of such a gel when scanned using optical cone beam CT.