Analysis of signal propagation in optically coupled detectors for digital mammography: II. Lens and fibre optics

An x-ray detector for digital x-ray mammography is under investigation, which consists of a phosphor screen coupled by a demagnifying fibre-optic taper to a time-delay integration mode, charge-coupled device (CCD) image array. The signal propagation through such a detector depends on the intensity and angular emission of light from the phosphor screen, the angular acceptance and transmission of light through the optics, and the spectral sensitivity of the CCD to the fluorescent light. The production of light by the phosphor screen was considered in a previous paper. Here, the issues related to the optics are examined. For phosphor screens coupled by lenses with limiting acceptance angles of less than 30 degrees, it was calculated that the coupling efficiency would be 10% greater than would be estimated under the assumption of a Lambertian source. These increases occur because a phosphor screen typically produces light which is more forward directed than a Lambertian source. Similar increases in efficiency are observed when a phosphor screen is coupled to a fibre-optic faceplate or taper. For fibre optics, exact estimation of the optical coupling efficiency requires knowledge of the angular-dependent transmission efficiency of the fibres.     

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.     

Initial performance evaluation of an indirect-detection, active matrix flat-panel imager (AMFPI) prototype for megavoltage imaging

PURPOSE: The development of the first prototype active matrix flat-panel imager (AMFPI) capable of radiographic and fluoroscopic megavoltage operation is reported. The signal and noise performance of individual pixels is empirically quantified. Results of an observer-dependent study of imaging performance, using a contrast-detail phantom, are detailed and radiographic patient images are shown. Finally, a theoretical investigation of the zero-frequency detective quantum efficiency (DQE) performance of such imagers, using a cascaded systems formalism, is presented. METHODS AND MATERIALS: The imager is based on a 508-microm pitch, 26 x 26 cm2 array which detects radiation indirectly via an overlying copper plate + phosphor screen converter. RESULTS: Due to its excellent optical coupling, the imager exhibits sensitivity superior to that of video-based systems. With an approximately 133 mg/cm2 Gd2O2S:Tb screen the system is x-ray quantum-noise-limited down to approximately 0.3 cGy, conservatively, and extensions of this behavior to even lower doses by means of reduced additive electronic noise is predicted. The observer-dependent study indicates performance superior to that of conventional radiotherapy film while the patient images demonstrate good image quality at 1 to 4 MU. The theoretical studies suggest that, with a 133 mg/cm2 Gd2O2S:Tb screen, the system would provide DQE performance equivalent to that of video-based systems and that almost a factor of two improvement in DQE is achievable through the incorporation of a 400 mg/cm2 screen. CONCLUSION: The reported prototype imager is the first megavoltage AMFPI having performance characteristics consistent with practical clinical operation. The superior contrast-detail sensitivity of the imager allows the capture of high-quality 6- and 15-MV images at minimal dose. Moreover, significant performance improvements, including extension of the operational range up to full portal doses, appear feasible. Such capabilities could be of considerable practical benefit in patient localization and verification.     

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.     

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.     

Methods to calculate the lens efficiency in optically coupled CCD x-ray imaging systems

In lens-coupled CCD x-ray imaging, the lens efficiency is of critical importance for attaining an x-ray quantum-noise-limited system. However, the equations and the associated parameters of this process have not always been used properly in some of the literature. This can result in the incorrect estimation of the signal-to-noise ratio (SNR). In practice, it is found more convenient to use lens-coupling efficiency equations which assume that the scintillating screen is an extended Lambertian source, since the number of light photons emerging from various scintillating screens has been measured and reported by several investigators. This communication examines several of the most common types of lens-coupling efficiency equations and explains how to use them correctly with other parameters to calculate the SNR of a lens-coupled CCD x-ray imaging system.     

Performance of various X-ray film/screen systems in demonstrating small simulated low-density lung nodules

A high-quality, conventional chest radiograph should be obtained in lung cancer screening programs to efficiently detect the faint, small nodular shadows due to early lung cancer in the lung fields, although, it is difficult to image the entire lung field within the linear part of the characteristic curve of the screen/film system owing to the wide variation in tissue density in the thorax, which ranges from the well aerated lung superimposed on the intercostal spaces to the lung area superimposed on the heart or diaphragm. In the detection of early cancer in the lung fields, it is important to consider the very low density (CT values of nearly -600 HU to -300 HU) commonly exhibited by early adenocarcinomas in the lung. The detectability of such nodules would be greatly influenced by the contrast and noise characteristics of the photographic system, the complexity of anatomical structures around the nodule, the characteristics of the observation system for x-ray films, the interpreter, and so on. The choice of x-ray film/screen system basically determines the efficacy of screening; specifically, the contrast characteristics of x-ray films together with noise level, which varies at different optical densities, affect the detectability of nodular shadows. The present study was carried out to determine the most suitable sensitometric characteristics and to assess the effects of the noise level of the x-ray film/screen system on the detectability of nodular shadows within the lung fields, particularly in low-density areas.     

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.     

On the dynamic range of different X-ray photon detectors in intra-oral radiography. A comparison of image quality in film, charge-coupled device and storage phosphor systems

OBJECTIVE: To compare film for intra-oral radiography with two charge-coupled device (CCD) and one storage phosphor system for digital imaging in respect of subjective image quality, detectability of small mass differences and appearance of burn-out effects and blooming phenomena at various exposure times. METHODS: Dried mandibles with teeth from different areas were radiographed at exposures covering a relative range from 1 to 100. Image quality was subjectively evaluated after image processing, when applicable, using a visual grading scale from 0 to 10. The number of visible holes in an aluminium block was used to measure the detectability of small mass differences. Burn-out effects and blooming were evaluated by measuring widths of roots and of aluminium and plastic cylinders. RESULTS: Radiographs with the storage phosphor system achieved image quality scores similar to those of film but over a larger exposure range, while CCD images were rated lower and over a smaller range. All holes in the aluminium block were only detected with the storage phosphor system. While the widths of roots were strongly affected by sensor saturation in CCD images and by burn-out in film images, smaller effects were seen with the storage phosphor system. Similar results were obtained with aluminium and plastic cylinders. CONCLUSIONS: Higher image quality was achieved over a much wider exposure range with the storage phosphor system than with either film or the CCD systems.     

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.     

Optimization of thin-foil based phosphor screens for CCD imaging in TEM in the voltage range of 80-400 kV

The voltage dependence characteristics of thin-foil based phosphor screens in the thickness range of approximately 10-60 microns are examined for CCD imaging in transmission electron microscopy (TEM) in the voltage range of 80-400 kV. The brightest screen is obtained with a P20 layer of about 12 microns at 80 kV, and a thicker screen lowers both the screen brightness and resolution. The thickness of the brightest screen is higher at higher voltage, but other considerations for a practical CCD imaging system suggest that the P20 layer should not be greater than approximately 18 microns for the voltage range stated above.     

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.     

Impact of extinction coefficient of phosphor on thermal load of color conversion elements of phosphor converted LEDs

Besides their direct impact on the respective correlated color temperature, the extinction coefficient and the quantum effi- ciency of the phosphor also have tremendous impact on the thermal load of the color conversion elements of phosphor converted LEDs under operation. Because of the low thermal conductivity of the silicone matrix in which the phosphor particles are typically embedded, the by far highest temperatures within the LED assembly are reached within the color conversion element. Based on a combined optical and thermal simulation procedure we show that in particular a larger value for the extinction coefficient might have a beneficial impact on the resulting thermal load.     

A room-based diagnostic imaging system for measurement of patient setup

A room-based diagnostic x-ray imaging system for routine measurement of radiotherapy patient orientation has been developed. The system consists of a pair of room-mounted x-ray tubes and a portable imager consisting of an orthogonal pair of phosphor screens, a mirror/lens system, a CCD camera, and computer software for comparing images of the patient to reference images. Orthogonal pairs of images can be acquired quickly and with relatively little exposure, allowing correction of patient setup on a daily basis. This could limit patient setup error to the uncertainty in the measurement and repositioning processes, a potentially significant improvement over the present standard.     

Observer performance comparison of digital radiograph systems for stereotactic breast needle biopsy

RATIONALE AND OBJECTIVES: Two digital radiograph systems for stereotactic mammography, one using a lens to couple a Lanex Regular screen to a back-illuminated charge-coupled device (CCD) and one using a fiber-optic taper to couple a Min-R Regular-type screen to a front-illuminated CCD, were evaluated with respect to observer performance. METHODS: A contrast-detail phantom was imaged in a variety of equivalent exposure conditions on both systems. Six observers viewed images on a video monitor and recorded which objects were detected. RESULTS: Performance (percent correct detections) with the lens-coupled system using the Lanex Regular screen was significantly higher than with the fiber-optic-coupled system using the Min-R Regular-type screen. CONCLUSION: Differences in absorption efficiencies of phosphors used, as well as differences in design of the two cameras, can explain differences in observer detection performance.     

Adaptive streak artifact reduction in computed tomography resulting from excessive x-ray photon noise

The quality of a computed tomography (CT) image is often degraded by streaking artifacts resulting from excessive x-ray quantum noise. Often, a patient has to be rescanned at a higher technique or at a larger slice thickness in order to obtain an acceptable image for diagnosis. This results in a higher dose to the patient, a degraded cross plane resolution, or a reduced patient throughput. In this paper, we propose an adaptive filtering approach in Radon space based on the local statistical properties of the CT projections. We first model the noise characteristics of a projection sample undergoing important preprocessing steps. A filter is then designed such that its parameters are dynamically adjusted to adapt to the local noise characteristics. Because of the adaptive nature of the filter, a proper balance between streak artifact suppression and spatial resolution preservation is achieved. Phantom and clinical studies have been conducted to evaluate the robustness of our approach. Results demonstrate that the adaptive filtering approach is effective in reducing or eliminating quantum noise induced artifacts in CT. At the same time, the impact on the spatial resolution is kept at a low level.     

Lens coupling efficiency: derivation and application under differing geometrical assumptions

The calculation of lens coupling efficiency is often performed in the design of lens coupled digital radiographic system, now currently under development. With these systems, an electronic camera is focused onto a planar scintillator such as a conventional intensifying screen. Historically, this calculation has relied on certain assumptions concerning the emission properties of the scintillator, and primarily this assumption is that the scintillator is either a Lambertain emitter (light is emitted over all angles equally) or a point radiator. Because there now exists new classes of scintillators such as scintillating fiber optic screens and pixelated intensifying screens, it is sometimes necessary to perform lens coupling calculations in the absence of the Lambertian and point source assumptions. In this paper we describe the necessary equations to accurately calculate lens coupling efficiency in the most general of cases. Graphical examples demonstrate the lens coupling efficiency for hypothetical Lambertain scintillating sources, for a rare earth intensifying screen, and for a scintillating fiber optical screen.     

The effects of depression and chronic pain on psychosocial and physical functioning

Ray tracing with a personal computer allows realistic simulation of optical properties of the human eye. Patterns of point sources are used as objects. The path of light rays is calculated between the point source and the retina for a Gullstrand eye model with improved parameters; the normal eye model has a resolution limit close to the natural resolution limit of the human eye. The image formed on the retina is projected back to a screen at the distance of the object so as to simulate image interpretation by the brain. Refractive errors are modeled by a change in eye parameters and corrected by eyeglasses or/and contact lenses or by an artificial intraocular lens. For optic correction the parameters of seeing aids can be fitted automatically by a least-squares routine. The effect of faulty eye correction on image quality is visualized by using a photograph of a realistic scene as an object.     

Amplification and entailed resolution degradation in high-pressure gas ionography

This work deals with the introduction of a gas electronic gain factor in high-pressure ionography (or "electron radiography"). The purpose is to check the possibility of a further reduction of radiation dose per radiograph. It is shown tha xenon or krypton can be mixed with special molecular gases in order to achieve charge-carrier multiplication at comparatively weak electrostatic fields and in an easily controllable manner. Some radiographs are reproduced which have been obtained in the amplifying working modes of the system. Relevant image characteristics and their limits are discussed. The obtainable resolution is limited by electron diffusion and by quantum noise (lowering of the effective quantum-detection efficiency).     

Image quality in dynamic CT: a clinical discussion

Modern state-of-the-art computed tomographic (CT) scanners emphasize three capabilities: image quality, dynamic scan capability, and a high-resolution thin-section technique. Image quality is fundamental and dependent on optimum performance and the interrelationship of all system components. Variables that affect the performance of the scanner include x-ray tube output and rate of heat dissipation; quantum detection efficiency; electronic noise in the acquisition system; speed, accuracy, and integration of mechanical motion in the gantry and table; and the algorithm used for image reconstruction. System design must allow for dynamic scan operation, either in the single-scan or cluster mode, with short interscan or intergroup delays or, as more recently developed, with helical acquisition. Dynamic scanning is frequently used for nonneurologic applications, including diagnosis of vascular and perivascular diseases and multifocal organ disease, particularly hepatic disease. Efficient operation depends on rapid reconstruction and display capability. Modern systems have been engineered to provide flexible modes of operation, particularly in dynamic scanning, and rapid on-line review and analysis, all of which serve to improve the quality of images produced with dynamic CT scanning.