Calcium Tungstate Screens Research Articles

Imaging performance of a CaWO4/CMOS sensor

The aim of this study was to investigate the modulation transfer function (MTF) and the effective gain transfer function (eGTF) of a non-destruc­­tive testing (NDT)/industrial inspection complementary metal oxide semi­conductor (CMOS) sensor in conjunction with a thin calcium tungstate (CaWO4) screen. Thin screen samples, with dimensions of 2.7x3.6 cm2 and thick­ness of 118.9 μm, estimated from scanning electron microscopy-SEM im­ages, were extracted from an Agfa Curix universal screen and coupled to the active area of an active pixel (APS) CMOS sensor. MTF was assessed using the slanted-edge method, following the IEC 62220-1-1:2015 method. MTF values were found high across the examined spatial frequency range. eGTF was found maximum when CaWO4 was combined with charge-coupled devices (CCD) of broadband anti-reflection (AR) coating (17.52 at 0 cycles/mm). The com­bi­nation of the thin CaWO4 screen with the CMOS sensor provided very pro­mis­ing image resolution and adequate efficiency properties, thus could be also con­sidered for use in CMOS based X-ray imaging devices, for various applications.

SURVEY OF IMAGING TECHNOLOGY AND PATIENT DOSE RECORDING PRACTICE IN DEVELOPING COUNTRIES

A survey was conducted through the International Atomic Energy Agency (IAEA) to assess the use of older imaging technologies and availability of dose information in radiography, computed tomography (CT), mammography and interventional machines. Responses from 55 countries indicated 55.8% having non-digital radiography systems, only 9.1% having all systems that are digital (CR and DR), nearly one-third (36.4%) still use calcium tungstate screens in radiography cassettes and only 16.9% having all mammography systems as digital. On the dose features, not all CT machines provide CTDI, DLP values directly as only about half (50.6%) stated all machines having CTDI and DLP display, nearly half (46.8%) indicated the absence of KAP meters in radiography systems. Only 53.2% stated all angiography machines having DAP/KAP display. Only about a quarter (23.4%) having picture archiving and communication system in all hospitals in the country. Nearly one-third (36.4%) have no difficulty in maintaining patient identification number while sending dose data.

Towards the enhancement of medical imaging with non-destructive testing (NDT) CMOS sensors. Evaluation following IEC 62220-1-1:2015 international standard

The aim of the current work was to investigate the modulation transfer function (MTF) of a non-destructive testing (NDT)/ industrial inspection CMOS sensor in conjunction with a calcium tungstate (CaWO4) thin screen, following both the IEC 62220-1:2003 and IEC 62220-1-1:2015 methods. Thin screen samples, with dimensions of 2.7x3.6 cm2 and mean coating thickness of 36.26 mg/cm2 (actual thickness: 118.9 μm estimated from scanning electron microscopy-SEM images) were extracted from an Agfa Curix universal screen and were manually coupled to the active area of a high resolution, active pixel (APS), complementary metal oxide semiconductor (CMOS) sensor. Experiments were performed using the RQA-5 beam quality, as described in the IEC series. Modulation transfer function was assessed using the slanted-edge method. The final MTF, following IEC 62220-1-1:2015 was obtained through averaging the oversampled edge spread function (ESF), using a custom-made software developed in our laboratory. MTF values were found with close agreement in the low and medium spatial frequency ranges, for both methods. Thereafter, MTFs calculated following the 62220-1:2003 protocol, were found overestimated for spatial frequencies higher than 5.7 cycles/mm. The combination of the thin calcium tungstate screen and the CMOS sensor provided very promising image resolution properties and thus could be also considered for use in CMOS based X-ray imaging devices, for various applications.

Resolution Properties of a Calcium Tungstate (CaWO4) Screen Coupled to a CMOS Imaging Detector

The aim of the current work was to assess the resolution properties of a calcium tungstate (CaWO4) screen (screen coating thickness: 50.09 mg/cm2, actual thickness: 167.2 μm) coupled to a high resolution complementary metal oxide semiconductor (CMOS) digital imaging sensor. A 2.7x3.6 cm2 CaWO4 sample was extracted from an Agfa Curix universal screen and was coupled directly with the active area of the active pixel sensor (APS) CMOS sensor. Experiments were performed following the new IEC 62220-1-1:2015 International Standard, using an RQA-5 beam quality. Resolution was assessed in terms of the Modulation Transfer Function (MTF), using the slanted-edge method. The CaWO4/CMOS detector configuration was found with linear response, in the exposure range under investigation. The final MTF was obtained through averaging the oversampled edge spread function (ESF), using a custom-made software developed by our team, according to the IEC 62220-1-1:2015. Considering the renewed interest in calcium tungstate for various applications, along with the resolution results of this work, CaWO4 could be also considered for use in X-ray imaging devices such as charged-coupled devices (CCD) and CMOS.

Understanding the relative sensitivity of radiographic screens to scattered radiation.

This study compared the relative response of various screen-film and computed radiography (CR) systems to diagnostic radiation exposure. An analytic model was developed to calculate the total energy deposition within the depth of screen and the readout signal generated from this energy for the x-ray detection system. The model was used to predict the relative sensitivity of several screen-film and CR systems to scattered radiation as a function of various parameters, such as x-ray spectra, phantom thickness, phosphor composition, screen thickness, screen configuration (single front screen, single back screen, screen pair), and readout conditions. In addition, measurements of the scatter degradation factor (SDF) for different screen systems by using the beam stop technique with water phantoms were made to verify the model results. Theoretically calculated values of SDF were in good agreement with experimental data. These results are consistent with the common observation that rare-earth screens generally produce better image quality than calcium tungstate screens and the CR screen.

Image information transfer properties of x-ray intensifying screens in the energy range from 17 to 320 keV.

The image information transfer properties of a number of x-ray fluorescent screens have been measured for x-ray energies from 17 to 320 keV. The detective quantum efficiency of the screens at each x-ray energy has been determined by separate measurements of the x-ray absorption efficiency and the statistical factor associated with the emission of optical photons upon absorption of an incident x-ray. Data have been recorded for both rare-earth phosphor screens and calcium tungstate screens. The value of the statistical factor for optical photon emission tends toward a constant value as the incident energy increases. Comparisons of the image information transfer properties are presented for several screens, which have been measured over a ten year interval. The utility of the screens for high-energy radiography is discussed.

A physical evaluation of the imaging performance of a high-resolution fluorography system.

A physical evaluation of the imaging performance of a unique high-resolution fluorography system is presented. Measurements have been made of contrast performance, unsharpness (modulation transfer function) and noise (Wiener spectrum and threshold contrast detectability), and the results have been compared with those measured from medium-fast calcium tungstate and rare-earth screen--film combinations and a standard fluorography system. The results have proved sufficiently encouraging that a full-scale trial is now underway to evaluate the system's performance in routine clinical application.

Kodak T-Mat G film in rotational panoramic radiography

Panoramic radiographs were taken of a tissue-equivalent phantom to evaluate T-Mat G and Ortho G films in combination with rare earth screens. The radiographs were compared to radiographs made with high-speed calcium tungstate screens and Kodak XRP film. The reduction in the amount of radiation necessary for the use of rare earth screens (50% to 70%) was accomplished by lowering the mA and adding filtration. All evaluated films were diagnostically acceptable. There was a marked preference of the T-Mat radiographs over the Ortho G radiographs and a slight preference over radiographs made with the standard calcium-tungstate screen-film system.

Effect of kilovoltage on the relative speed of rare-earth screens.

Three screen-film combinations were evaluated with respect to relative speeds at various kilovoltages by comparing them to a reference medium speed calcium tungstate screen. Kodak Lanex Regular (rare-earth) screens did not exhibit a maximum relative speed at 80 kVp, although this has been reported previously. In the experiment the relative speeds of all screen-film combinations increased with an increase in kilovoltage from 50 to 100.

Rare earth screens for panoramic radiography

Six panoramic radiographs, each made with a different screen/film combination, were compared for “periapical visibility” by fourteen dentists. Two combinations (a par-speed and a high-speed) used calcium tungstate screens, and three used rare earth screen systems four times faster than par-speed. One other combination consisted of a single rare earth screen plus a single-coated detail film. The radiographs produced by the faster rare earth screen/film combinations were found to be similar in quality to those produced by calcium tungstate screens. Rare earth screens could be used for panoramic radiography while maintaining quality and reducing exposure.

Absolute measures of physical image quality: measurement and application to radiographic magnification.

Two absolute measures of physical image quality—noise equivalent quanta (NEQ) and detective quantum efficiency (DQE)—are reviewed and calculated as functions of spatial frequency from measurements of the imaging characteristics of radiographic screen–film systems. NEQ is the effective number of information bearing quanta used to form an image, and DQE is the ratio of NEQ to the actual number of quanta used. NEQ and DQE are derived from the sensitometric characteristic, modulation transfer function (MTF), and noise power spectrum (NPS) of a screen–film system. Spatial frequency ( f) distributions of NEQ and DQE were determined for three calcium tungstate screens—Du Pont Cronex Detail, Par Speed, and Hi‐Plus—used with Kodak XRP film. The relative ranking of NEQ( f) of these three systems followed the ranking of MTF and the exposure required to produce a given film density (Detail>Par Speed>Hi Plus). The ranking of DQE( f) followed that of system speed—Hi Plus>Par Speed>Detail—at frequencies <2.5 cycles/mm, but was the same as the ranking of NEQ( f) at frequencies >5 cycles/mm. Noise equivalent apertures defined in terms of MTF were larger than analogous apertures defined in terms of NEQ for these screen–film systems, suggesting that, for a specific imaging task, these systems introduce more degradation in image quality due to loss of contrast than to loss of signal‐to‐noise ratio (SNR). Finally, the application of NEQ( f) and DQE( f) distributions to imaging system analysis is demonstrated by applying them to an optimization of magnification radiography with respect to image SNR.

Utilization of Monoenergetic X-Ray Beams to Examine the Properties of Radiographic Intensifying Screens

Monoenergetic x-ray beams, produced by the direct electron excitation of characteristic K x rays in elemental targets, are important for measuring the response of different components of radiologic imaging systems. Previous studies have investigated the energy dependence of the scatter produced as an x-ray beam traverses the object to be radiographed and the efficacy of antiscatter grids in reducing this unwanted component of the image signal. Presently, these beams are being. utilized to examine the image information transfer properties of x-ray intensifying screens conventionally used in screen-film imaging systems. The technique of single photon counting is being employed to determine the relative probability. distribution of the number of optical photons emitted from the screen after absorption of an incident x ray. These data can be, used to determine the average number of light quanta emitted per absorbed x ray. In addition, the shape of these distributions can be used to determine the image information transfer properties of a particular screen. Results of these measurements for a typical calcium tungstate screen and one of the new rare-earth phosphor screens are presented.

An asymmetric screen pair for angiography.

Commercially available rare-earth screens are mechanically softer and less abrasion resistant than calcium tungstate screens. This leads to problems in radiography without the use of cassettes in areas where particulate matter appears. The benefits of an asymmetric screen pair in Schonander changers in which the movable front screen is calcium tungstate and the back screen is a Du Pont Quanta III are investigated. The image is sharper, the quantum mottle is less apparent, the speed remains high, and the physical durability of the system is improved in comparison to a pair of Quanta III screens.

Bildgüte bei der Mittelformat-Aufnahmetechnik (Fluorographie)

The paper deals with image quality of films using a medium format type film and fluorography. Contrast transmission function is achieved by special calcium tungstate screens. In practice, image quality equals that of full size films with universal screens. By increasing lens size it is possible to reduce the necessary x-ray dose without losing image quality.

X-Ray Absorption, Speed, and Luminescent Efficiency of Rare Earth and Other Intensifying Screens

Seven new and two conventional screens were investigated. Measurements were performed at different energies of the photon fraction interacting in the screens, and energy losses due to escape of K-fluorescent radiations were calculated. The speed of the screens was dependent on the energy; this was primarily due to the fact that x-ray absorption is dependent on energy. Rare earth and other new screens absorb a maximum of 1.5 times as much energy and emit twice as much light as comparable calcium tungstate screens. Reabsorbed K-fluorescent radiations might contribute to image unsharpness in the new screens.

Rare-earth and calcium tungstate intensifying screens, a comparative study of relative speed, radiation doses and resolving power.

Using calcium tungstate (CaWO4) and rare-earth intensitying screens, their relative speeds were compared in exposures of a pelvis phantom, and radiation doses were measured at several kilovoltages. Using a slit pattern, the resolving power was assessed by a modified contrast-frequency response and by direct viewing of the radiographs. In the comparison of the rare-earth and CaWO4 screens the following differences were found: (1) A great reduction in the radiation dose can be achieved by the use of the fast screen types, yet at the cost of some loss of resolving power; (2) the film contrast can be improved by a reduction in the kilovoltage without an increase in the radiation dose; (3) the short exposure times and the characteristic course of the dose curve necessitate an adjustment of the phototimer or a change of the control unit; and (4) with the slower screen type tested, the resolving power can be improved without increasing the radiation dose.

Sensitivity of Intensifying Screens to Scattered Radiation in Radiography: Physical Measurements

The x‐ray absorption of rare earth (gadolinium‐lanthanum) and medium speed calcium tungstate screens was measured in regions of high and low radiographic density of a chest phantom. The ratio of absorptions for the two regions was higher for the rare earth screens. This confirms by direct measurement that the higher observed radiographic contrast for the rare earth screens when compared to calcium tungstate screens is caused by the change in absorption of the screen phosphors with change in transmitted beam quality. The net effect in rare earth screens of this type is a reduction in the recording of scattered radiation with an increase in the percentage of information‐containing (primary) radiation. The degree of this effect is a function of the amount of scattered radiation in the transmitted beam incident on the cassette.

Technical notes. Test of rare earth oxysulphide intensifying screens.

The introduction of fast tungstate intensifying screens was a remarkable step toward dose reduction in diagnostic radiology (Soila and Suolanen, 1963). Intensifying screens made of terbium activated gadolinium and lanthanum oxysulphides have a higher X-ray absorption and exhibit a larger conversion efficiency from X-rays to visible light than calcium tungstate screens (Buchanan et al., 1972). The use of rare-earth screens thus provides a considerable reduction of radiation dose as compared with conventional screens (Ardran et al., 1975). Different types of rare-earth intensifying screens are available; the physical properties of the screens are well documented (Stevels, 1975). There are, however, few reports on the clinical use of rare-earth screens and possible differences between them and conventional screens in practical work (Evans et al., 1975; Wiljasalo et al., 1975). The resolution capacity of each film-screen combination was tested with an E-plate especially designed for mimicking clinical situations (Soila and Tähti, 1965; Laasonen, 1968). The test films were exposed through layers of 3.5, 7, 14 and 21 cm of water, using an X-ray equipment with a 12 pulse generator and electronic contactor. The film-focus distance was 90 cm and the diameter of the focal spot was 0.6 mm. In Table I the tested film-screen combinations are listed. The doses were measured by thermoluminescent dosimeters at the surface of the 3.5 cm phantom and the optical densities were measured with an electronic densitometer.

New intensifying screens in clinical radiology. Paper I-Experimental investigations (author's transl)

The degree of intensification and resolution was compared using rare-earth-screens and conventional calcium tungstate screens. -In order to approximate usual phantom (DIN 6, 815) was equipped with a lead grid at a depth of 6 cm. Nine screen-film combinations were exposed at 70 to 80 kv., using a 12-pulse generator and Bucky grid till blackening of S = 1 was obtained. -Resolution depends substantially on the inherent noise of the system and improves from 0.6 Lp/mm. for the most sensitive combinations (intensitification factor 8.0) to 1.15 Lp/mm (intensification factor 1.7). Compared with the standard combination with a resolution of about 0.9 Lp/mm., dose was reduced by more than 40% with an increase in resolution of 45% (combination 3M-Trimax A4/XD).

X-ray exposure reduction using rare-earth oxysulfide intensifying screens.

Abstract The characteristics of terbium-activated gadolinium oxysulfide and lanthanum oxysulfide are compared with those of calcium tungstate. The new rare-earth phosphors have higher x-ray absorption in the 38 to 70 keV region relative to calcium tungstate. They also exhibit a larger x-ray-to-visible light energy conversion efficiency. Rare-earth phosphors, however, emit primarily in the green spectral region and thus poorly match the spectral response of blue-sensitive x-ray film. Despite this disadvantage, gadolinium oxysulfide screens allow a significant x-ray exposure reduction relative to calcium tungstate screens providing equivalent resolution. An additional exposure reduction is anticipated when faster green-sensitive x-ray film becomes available.