Sensitivity of amorphous selenium to x rays from 40 kVp to 18 MV: measurements and implications for portal imaging

Recently, the clinical application of electronic portal imaging devices has enabled more frequent verification of patient setup for radiation treatment. However, the image quality has sometimes proven to be inadequate, motivating the investigation of alternative sensors with better image quality. Amorphous selenium (a-Se) is potentially one such sensor since the electrostatic image formation process has high resolution. To fully evaluate the potential of a-Se for portal imaging, it is necessary to investigate all the imaging properties at high x-ray energies. Here, measurements of the sensitivity of a-Se to incident x-ray spectra ranging in energy from 40 kVp to 18 MV and for a-Se thicknesses ranging from approximately 10 to 300 microns under full buildup conditions are described. When x rays or energetic electrons deposit energy in a photoconductor with an applied electric field, F, electrons and holes are released. The x-ray conversion sensitivity may be defined as 1/W +/-, where W +/- is the energy required to release an electron-hole pair. Consistent with the results of previous investigators, W +/- is found to vary approximately with F-2/3. Unexpectedly, over the energy range of 40 kVp to 18 MV, W +/- was found to decrease by a factor of nearly 3. These dependencies are compared to the predictions of two competing charge recombination models, geminate and columnar. The results are explained by a microdosimetric model in which the sensitivity at megavoltage energies is governed by geminate recombination, but at lower energies, both mechanisms are involved. Thus, the sensitivity of a-Se to x rays spanning the diagnostic and radiotherapy range has been measured and the physical basis for this behavior established.     

Overexpression of deltaEmBP, a truncated dominant negative version of the wheat G-box binding protein EmBP-1, alters vegetative development in transgenic tobacco

A flat-panel x-ray imaging detector is being investigated for digital radiography and fluoroscopy. The detector uses a layer of amorphous selenium (a-Se) to convert x rays to a charge image, which is then electronically read out with a two-dimensional array of thin film transistors (TFTs). In order to sensitize the a-Se layer to x rays, a high voltage (of the order of several thousand volts) is applied to its top surface. The TFTs, which are at the bottom surface of the a-Se layer, are not subjected to any high voltage under normal radiological operational conditions since the pixel potential is < 10 V. However under a fault condition where these two events occur simultaneously: (1) suspended detector scan; and (2) an x-ray exposure more than ten times higher than normal, the voltage on the TFTs could rise to a damaging value. This paper describes a method for protecting the TFTs from high voltage damage under this fault condition. It employs a dual-gate TFT structure, one gate is for scanning control and the other is connected to the pixel electrode for high voltage protection. Before the pixel potential reaches a damaging value, the protection gate turns on the TFT automatically and drains excess charge away from the pixel thus providing a safe pixel saturation potential. In this paper, the characteristic curves of dual-gate TFTs are studied both theoretically and experimentally. The pixel x-ray response for imaging detectors with high voltage protection are predicted, and it is shown that with practical TFT designs the detector can provide a safe pixel saturation potential as well as satisfy the dynamic range required for diagnostic x-ray imaging applications.     

X-ray quantum limited portal imaging using amorphous silicon flat-panel arrays

We have measured the linearity, spatial resolution (MTF), noise (NPS), and signal-to-noise characteristics (DQE) of an electronic portal imaging device (EPID) based on an amorphous silicon flat-panel array. The array has a 128 x 128-pixel matrix and each pixel is 0.75 x 0.75 mm2 in dimension so the array covers an area of 96 x 96 mm2. The array acts like a large area light sensor and records the optical signals generated in a metal plate/phosphor screen x-ray detector when the detector is irradiated by a megavoltage x-ray beam. In addition, approximately 0.5% of the total signal is generated by nonoptical processes. The noise measurements show that the device is quantum noise limited with the noise power generated by the x-ray quanta being up to 100 times greater than the noise added by the external readout electronics and flat-panel light sensor itself. However, the flat-panel light sensor does reduce the spatial resolution (compared to a perfect optical sensor with infinitesimal pixel size) because of its moderate pixel size and because optical spread can occur in the transparent glues used to attach the phosphor screen to the flat-panel light sensor. The response of the sensor is very linear and does not suffer from the glare phenomenon associated with TV camera-based EPIDs--characteristics which suggest that the amorphous silicon EPID will be well suited to transit dosimetry. Nevertheless, some limitations need to be overcome before these devices can be used clinically. These include developing larger flat-panel light sensors, the elimination of "noisy" pixels with high dark signal, and improvements in the uniform sensitivity of the sensors. This last requirement is only needed for transit dosimetry applications where it would greatly simplify calibration of the device. In addition, an image acquisition scheme must be developed to eliminate artifacts created by the pulsed x-ray beam generated by linear accelerators. Despite these limitations, our studies suggest that the amorphous silicon EPIDs are very well suited to portal imaging.     

A method for measuring the presampled MTF of digital radiographic systems using an edge test device

The modulation transfer function (MTF) of radiographic systems is frequently evaluated by measuring the system's line spread function (LSF) using narrow slits. The slit method requires precise fabrication and alignment of a slit and high radiation exposure. An alternative method for determining the MTF uses a sharp, attenuating edge device. We have constructed an edge device from a 250-microm-thick lead foil laminated between two thin slabs of acrylic. The device is placed near the detector and aligned with the aid of a laser beam and a holder such that a polished edge is parallel to the x-ray beam. A digital image of the edge is processed to obtain the presampled MTF. The image processing includes automated determination of the edge angle, reprojection, sub-binning, smoothing of the edge spread function (ESF), and spectral estimation. This edge method has been compared to the slit method using measurements on standard and high-resolution imaging plates of a digital storage phosphor (DSP) radiography system. The experimental results for both methods agree with a mean MTF difference of 0.008. The edge method provides a convenient measurement of the presampled MTF for digital radiographic systems with good response at low frequencies.     

X-ray imaging using amorphous selenium: feasibility of a flat panel self-scanned detector for digital radiology

We investigate a concept for making a large area, flat-panel detector for digital radiology. It employs an x-ray sensitive photoconductor to convert incident x-radiation to a charge image which is then electronically read out with a large area integrated circuit. The large area integrated circuit, also called an active matrix, consists of a two-dimensional array of thin film transistors (TFTs). The potential advantages of the flat-panel detector for digital radiography include: instantaneous digital radiographs without operator intervention; compact size approaching that of a screen-film cassette and thus compatibility with existing x-ray equipment; high quantum efficiency combined with high resolution. Its potential advantages over the x-ray image intensifier (XRII)/video systems for fluoroscopy include: compactness; geometric accuracy; high resolution, and absence of veiling glare. The feasibility of the detector for digital radiology was investigated using the properties of a particular photoconductor (amorphous selenium) and active matrix array (with cadmium selenide TFTs). The results showed that it can potentially satisfy the detector design requirements for radiography (e.g., chest radiography and mammography). For fluoroscopy, the images can be obtained in real-time but the detector is not quantum noise limited below the mean exposure rate typically used in fluoroscopy. Possible improvements in x-ray sensitivity and noise performance for the application in fluoroscopy are discussed.     

X-ray imaging using amorphous selenium: determination of Swank factor by pulse height spectroscopy

The use of photoconductors, especially amorphous selenium (a-Se), in x-ray imaging is currently of interest. A critical performance parameter of an imaging detector is the Swank factor for degradation of the signal to noise ratio, or DQE(0), due to variations in the detector response. The Swank factor is evaluated from measured pulse height spectra generated by the absorption of monoenergetic x-ray photons. The spectra show an additional width over previous theoretical expectations, but the Swank factor is still close to the high values previously predicted theoretically.     

The effect of helical pitch and beam collimation on the lesion contrast and slice profile in helical CT imaging

A model is presented in this paper to describe how the contrast of a reconstructed object and slice sensitivity profile are affected by (1) the table speed or helical pitch, (2) the x-ray collimations, (3) the size of the object, (4) the alignment between the reconstructed slice and the object, (5) the distance of the object from the axis of rotation, and (6) the helical CT reconstruction algorithm employed. This contrast model is validated by both computer simulations and experiments. With this model, the contrast of a reconstructed object, slice sensitivity profile, and the longitudinal MTF can be accurately predicted. The optimal scan strategy and the point of diminishing returns can be determined prior to scanning. Several conclusions can be drawn from this model. First, overlapping reconstruction significantly improves overall scan contrast sensitivity of helical CT. Second, with a given x-ray collimation, low pitch helical scans provide better longitudinal resolutions. Third, with a given volume coverage rate (i.e., a given table speed), narrow collimation high pitch helical scans provide better longitudinal resolutions than wide collimation low pitch ones and therefore are recommended for high-contrast thin-slice applications. A lesion conspicuity model is also established.     

Monocyte B7 and Sialyl Lewis X modulates the efficacy of IL-10 down-regulation of LPS-induced monocyte tissue factor in whole blood

Recent studies have investigated the use of anti-inflammatory cytokine, interleukin 10 (IL-10) to control the development of disseminated intravascular coagulation (DIC) in sepsis by down-regulation of monocyte tissue factor (MTF) induced by lipopolysaccharide (LPS) in the initial phase of the disease. In vitro and in vivo human studies have shown that a minimal (<1 h) delay in IL-10 treatment significantly reduces the cytokines ability to inhibit LPS-induced MTF expression and the end products of coagulation. In this whole blood in vitro study we investigated the role of lymphocyte and platelet interactions with monocytes to up-regulate MTF expression in the presence of IL-10 in the initial phase of exposure to LPS. Individual blockade of monocyte B7 or platelet P-selectin significantly (35%) reduced MTF expression (P<0.05). IL-10 showed a dose-dependent inhibition of LPS (0.1 microg/ml) induced MTF expression, with 56% inhibition at 1 ng/ml, maximizing at 5 ng/ml IL-10 (75%; P<0.05). Simultaneous exposure to LPS and IL-10 (1 ng/ml) or addition of IL-10 1 h after LPS, with individual B7 and P-selectin blockade significantly enhanced the inhibition of MTF expression by IL-10 (P<0.05). We conclude that the efficacy of IL-10 to control DIC could be enhanced by a simultaneous B7 and P-selectin blockade.     

Effects of peer presence on preschool children's television-viewing behavior.

Observed 299 3- and 5-yr-olds watching TV in the presence of an audiovisual slide distractor in groups of 1, 2, or 3 viewers. Peers viewing the TV together influenced each other's behavior in a synchronized fashion: When one looked at the TV, looked at the distractor, or demonstrated overt involvement with the TV, the other tended to do the same thing. This peer influence occurred above and beyond the dynamic common organizing influence of the TV program itself. There was little evidence that a given S consistently "led" another, nor were there effects of same-sex vs cross-sex pairings of viewers. The watching of TV by young children is interpreted in the results as an active transactional process among the viewer, the TV, and the TV-viewing environment. (24 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Assessment of target recovery after TV camera saturation in X-ray fluoroscopy using a personal computer with frame-grabber

X-ray fluoroscopy images were digitized with a personal computer and frame-grabber system. This system was used to measure the recovery from saturation of the target of a TV camera by calculating the time at which contrast falls to 50% (C 1/2). This measurement was performed on images of: (a) a rotating lead disk with a slit at its side and (b) a square hole in a thick lead sheet. These images represented the moving (MOTR) and stationary (SOTR) object target recovery. The SOTR method produced much higher C 1/2 values in two mobile fluoroscopy systems used for cardiac catheterizations where waveforms permitted the application of both methods. The SOTR method was also applied to three additional fluoroscopy systems. It was found that, contrary to expectations, C 1/2 increased with exposure rate in all five machines. Target recovery characteristics of the machines tested, represented by the gradient of C 1/2 against exposure rate, were found to increase with inherent noise level measured in the vidicon camera target. The set-up used here for measuring target recovery proved to work quickly and reproducibly for both SOTR and MOTR. If initial values are calculated then the condition of any fluoroscopy system may easily be followed irrespective of the method used.     

Physiological effects of ultraviolet light on Dictyostelium discoideum spore germination

The spore germination in Dictyostelium discoideum consists of four stages: activation, postactivation lag, swelling and emergence. Ultraviolet irradiation (total fluence of 250 J/m(2)) of spores at any time prior to late spore swelling allows full swelling, but inhibits the emergence of myxamoebae. In the case of freshly activated spores, a UV exposure time of 30 s (total fluence of 50 J/m(2)) is sufficient to reduce emergence to about 6% when measured after 24 h of incubation. This same fluence results in about 10% viability as measured by plaque forming ability. Experiments utilizing "fractionated exposures' result in the same percentage inhibition of emergence as that found for "single exposures' provided the total fluence is equivalent. The higher fluences (250 J/m(2)) which completely prevent emergence, do not affect the endogenous oxygen uptake of spores during swelling. Ultraviolet light irradiated spores respond to the same activation and deactivation treatments as control unirradiated spores. Ultraviolet irradiation after late spore swelling allows emergence to occur in only a small fraction of the population. This fraction of cells which can emerge after UV treatment is said to have passed a "competence point', which is believed to be the time when all the events necessary for emergence have been completed. Though the sites of UV inactivation in spores can only be postulated at present, it is apparent that the initial stages of germination (activation, postactivation lag and spore swelling) occur independently of the UV sensitive sites. The final stage of germination (emergence), however, is dependent on UV sensitive functions.     

The significance of isolating low numbers of mycobacterium tuberculosis in culture of sputum specimens

Because fewer than five Mycobacterium tuberculosis colonies were isolated from single sputum specimens from several patients with little clinical evidence of pulmonary tuberculosis, we reviewed all of our low colony isolates (LCIs) from an 18-month period. Thirty-one patients had 35 LCIs and were separated easily into groups that either appeared to have tuberculosis by clinical criteria or appeared to be uninfected. Patients in the "tuberculosis likely" group reasons such as concurrent antituberculosis therapy or an interstitial location of their pulmonary lesion to account for their LCIs. In contrast, patients in the "tuberculosis unlikely" group were all culture-negative when reevaluated two to four months after their first culture, and they had no evidence of tuberculosis on chest x-ray film despite having received no antituberculosis therapy. Half had negative tuberculin reactions, without generalized anergy. Epidemiologic study showed that most had been cultured the same day as patients with smear-positive tuberculosis, and the possibility of cross-contamination was investigated.     

Pepstatin, an ascites retardant of L1210 tumor-bearing mice

The effect of pepstatin on the kinetics of ascitic fluid accumulation in L1210 tumor-bearing mice (DBA/2) was observed. Following inoculation of 1.5x10(6) tumor cells, untreated mice reached a peak of fluid accumulation on day 6 and remained at this level until death on day 9. A "lag" phase of 4 days occurred before fluid accumulation was seen. Pepstatin administered SC in a single dose of 80 mg/kg during the lag phase, significantly retarded fluid accumulation as compared to untreated animals. Pepstatin administered following fluid accumulation was much less effective. We concluded that pepstatin prevents fluid accumulation rather than acts as a diuretic agent. The term "ascites retardant" is suggested for the pharmacologic actions of pepstatin, since it prevents fluid accumulation without diminishing the cell count.     

Evaluation of a high-density scintillating glass for portal imaging

One of the main factors that limits the performance of T.V. camera-based portal imaging systems is the poor light-collection efficiency of the lens and T.V. camera. An x-ray detector that produces more light per incident x ray would help overcome this limitation. We have been evaluating a high-density (3.8 g/cm3), thick (12 mm) glass scintillator for its suitability as an x-ray detector for T.V. camera-based portal imaging systems. The light output and spatial resolution of the glass scintillator has been compared to that of a copper plate/phosphor screen detector using radiographic film and the T.V. camera of our portal imaging system. The film measurements show that the light output of the glass scintillator is 82% of that of the copper plate/phosphor screen, while the T.V. camera measurements show that this value is 48%. A theoretical model of light transport described in this paper suggests that this discrepancy is due to refraction at the glass-air interface. Our measurements of the modulation transfer function (MTF) show that the spatial resolution obtained with the glass scintillator is similar to that obtained with the copper plate phosphor screen. However, the spatial resolution obtained with the glass scintillator decreases as the angle of x-ray incidence increase; this decrease, which is not observed for the copper plate/phosphor screen detector, is due to the large thickness of the glass scintillator. Due to the limited light output and the variable spatial resolution, the transparent glass scintillator, in its current form, is not suitable for portal imaging.     

A neutron television camera detector

The system under development has a large counting rate capability; this is extremely important where the total background count exceeds the total counts in the signals of interest. Its spatial resolution is of the order of one mm, which is perfectly adequate for neutron work, while the screen size of 400 mm is reasonable. The main limitation of the system is its limited counting efficiency, and this is directly attributable to the optical self-absorption of the neutron phosphor. Any newly developed transparent phosphor with the same light output would immediately change the situation. The success of the electronics hardware in reducing random noise is demonstrated in Figure 3, which shows in the bottom trace the live video output when the input to the system is a grey-scale test chart. The top trace is the output after the image has been digitally integrated. Figures 4 and 5 show the monitor outputs of the see articles x-ray system with a "still" diffraction pattern of a crystal of GPD (glyceraldehyde-3-phosphate dehydrogenase). Figure 4 is a photograph of the "live" video display, and Figure 5 is the digitally summed image. All coherent noise in the system, i.e., all noise synchronized with the TV scans has to be kept lower than the first bit threshold. However, this requirement can be relaxed when dealing with diffraction patterns, such as those from single crystals, for which a local background is subtracted from the pattern.     

The effect of intrauterinely injected lymphocytes prepared from sensitized or non-sensitized guinea-pigs on the ultrastructure of rat fetal membranes (visceral yolk sac)

The lag in phenotype expression of methylnitrosourea(MNU)-induced mutation to 6-thioguanine (6TG) resistance has been studied in a diploid human lymphoblastoid cell line. We find that a considerable period (8-12 days) elapses before new mutants appear in treated cultures; after 2 weeks, however, a stable maximum fraction is attained, as would be expected for a genetic mutation. We present preliminary data linking this phenotypic lag to the slow degradation rate of hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and to an apparent requirement for very low (less than 0.2% normal) cellular HGPRT content in order for cells to be resistant to 10 mug 6TG/ml. A series of reconstruction experiments are presented, the results of which support the conclusion that selective pressures in the assay procedure do not bias the quantitative estimates of induced mutant fraction.     

Peers as mediators of television food advertisements aimed at children.

Past research on the effects of TV has focused primarily on it as a unitary force rather than as 1 source of influence in a network of cross-pressures. The present study attempted to determine how peer-modeled food preferences combined with TV food commercials to influence the food preferences of school-aged children. 40 Black and 40 White 4th graders were assigned randomly to 1 of 4 conditions: TV control, TV advertisements with similar peer-modeled food preferences, and TV advertisements with dissimilar peer-modeled food preferences. An additional 40 children served as peer models. Ss in the TV advertisement condition preferred foods in the same class as the advertised foods more frequently than did Ss in the TV control condition. Ss in the TV advertisement/peer-similar condition preferred foods like the advertised food more often than Ss in the TV advertisement condition, whereas Ss in the TV/peer-dissimilar condition preferred foods like the advertised food less frequently than did Ss in the TV advertisement condition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effect of phosphor persistence on image quality in digital x-ray scanning systems

A digital x-ray scanning system offers several advantages over conventional film-screen systems. However, there are sources of image degradation resulting from the scanning motion, such as motion blur due to the temporal response of the phosphor. This mechanism produces an asymmetrical blur, requiring the use of the complex optical transfer function (OTF) rather than the normal modulation transfer function (MTF) for correct characterization of image resolution. The luminescence response of eight phosphors was measured under pulsed x-ray excitation. A weighted exponential model was used to represent the primary luminescence. The dominant luminescence life-times ranged from 2.7 microseconds for Gd2O2S:Pr to 558 microseconds for Gd2O2S:Tb. The long term response was also measured, monitoring significant increases in a slow form of luminescence known as afterglow. Afterglow was modeled by an inverse power law equation. Afterglow was found to be strong in two of the phosphors studied (ZnCdS:Ag and YTaO4). In selecting a phosphor for a scanning system, it must satisfy several criteria, including a fast temporal response. Thus, a phosphor like Gd2O2S:Tb, which has a slow luminescence, but otherwise excellent imaging properties, may not be as useful as a more rapid phosphor like CsI:Tl.     

Induced fit of a peptide loop of methionyl-tRNA formyltransferase triggered by the initiator tRNA substrate

A 16-aa insertion loop present in eubacterial methionyl-tRNA formyltransferases (MTF) is critical for specific recognition of the initiator tRNA in Escherichia coli. We have studied the interactions between this region of the E. coli enzyme and initiator methionyl-tRNA (Met-tRNA) by using two complementary protection experiments: protection of MTF against proteolytic cleavage by tRNA and protection of tRNA against nucleolytic cleavage by MTF. The insertion loop in MTF is uniquely sensitive to cleavage by trypsin. We show that the substrate initiator Met-tRNA protects MTF against trypsin cleavage, whereas a formylation-defective mutant initiator Met-tRNA, which binds to MTF with approximately the same affinity, does not. Also, mutants of MTF within the insertion loop (which are defective in formylation) are not protected by the initiator Met-tRNA. Thus, a functional enzyme-substrate complex is necessary for protection of MTF against trypsin cleavage. Along with other data, these results strongly suggest that a segment of the insertion loop, which is exposed and unstructured in MTF, undergoes an induced fit in the functional MTF.Met-tRNA complex but not in the nonfunctional one. Footprinting experiments show that MTF specifically protects the acceptor stem and the 3'-end region of the initiator Met-tRNA against cleavage by double and single strand-specific nucleases. This protection also depends on formation of a functional MTF.Met-tRNA complex. Thus, the insertion loop interacts mostly with the acceptor stem of the initiator Met-tRNA, which contains the critical determinants for formylation.     

Interaction between radiation and drug damage in mammalian cells. II. The effect of actinomycin-D on the repair of the sublethal radiation damage in plateau phase cells

The effect of actinomycin-D (AMD) on radiation damage repair was studied in plateau phase V79 Chinese hamster cells. Sublethal radiation damage repair, as demonstrated by survival fluctuations following two x-ray exposures separted by time, was observed in our plateau phase cells. Plateau phase cells exposed to 0.01-0.04 mug/ml AMD (a nontoxic regimen to 8 hours) between x-ray exposures were less able to repair sublethal damage. If plateau phase cells were plated at low dilutions into fresh medium (conditions for resuming exponential growth) immediately after the first x-ray dose, and exposed to 0.01--0.04 mug/ml AMD until the second dose, inhibition of sublethal damage repair and additional cell killing were observed particularly at 0.04 mug/ml AMD. It is suggested that radiation-drug damage interactions should be studied in plateau phase cells and in cells resuming exponential growth after plateau phase (possibly analogous to "recruitment"), as well as in exponential phase cultures.