Flat-panel X-ray Imaging Research Articles

1/f Conductance noise in stabilized amorphous selenium

We report measurements of low-frequency conductance fluctuations in samples of amorphous selenium alloys that find use in direct-conversion, flat-panel X-ray image detectors. The signal-to-noise ratio of these detectors depends in part on the fluctuations in the so-called dark current that flows through the selenium layer; proper modeling of the detector performance requires measurements of this noise signal. The conductance noise was measured for various applied voltages ranging from 400 to 2000V that produced dark currents ranging from 10−9 to 10−6A. The noise power density spectrum varies approximately as 1/fα from 0.5Hz to 1kHz with α in the range from 0.77 to 1.5. The noise spectrum depends on resting time after application of the voltage and on the metal used for the electrodes which implies that the noise is controlled by the metal semiconductor interface. Integration of the noise power density over the measured frequency range yields an rms fluctuation of the dark current that can be as large as 1% of the average value.

Determination of point spread function for a flat-panel X-ray imager and its application in image restoration

We investigate the image blur estimation methods, namely modified the Richardson–Lucy (R–L) estimator and the Wiener estimator. Based on the empirical model of the PSF, an image restoration is applied to radiological images. The accuracy of the PSF estimation under the Poisson noise and readout electronic noise is significantly better for the R–L estimator than the Wiener estimator. In the image restoration using the 2-D PSF from the R–L estimator, the result shows a good improvement in the low and middle range of spatial frequency.

Recent advances in X-ray photoconductors for direct conversion X-ray image detectors

Recent research on flat panel X-ray image detectors has shown their potential for replacing existing X-ray film/screen cassettes and capturing X-ray images electronically, thus enabling the clinical transition to digital radiography. The present work examines the imaging properties of a number of potential X-ray photoconductors for these new X-ray image detectors. The X-ray sensitivity is discussed in terms of the absorption efficiency, electron–hole pair creation energy (ionization energy), and charge transport and trapping limited collection efficiency. X-ray photoconductor properties of various semiconductors are compared and critically discussed with special attention to stabilized a-Se and HgI2, currently the most promising materials.

X-ray-induced recombination effects in a-Se-based x-ray photoconductors used in direct conversion x-ray sensors

Stabilized amorphous selenium (a-Se) is currently used as an x-ray photoconductor in direct conversion flat-panel digital x-ray image detectors. Therefore, there is much interest in x-ray-induced effects in a-Se, especially changes in charge carrier lifetimes that result from x-ray exposure. We have observed that the exposure of an a-Se x-ray detector sample to x rays induces negative capture centers in the bulk and thereby reduces the hole lifetime. By using conventional and interrupted field time-of-flight (IFTOF) transient photoconductivity techniques in a TOF-IFTOF-TOF sequence, we were able to develop a technique that allows the measurement of the capture coefficient Cr between free holes and x-ray-induced negative centers, which we believe to be trapped electrons. We find that the capture process follows the Langevin recombination mechanism, the same recombination mechanism that has been observed in the case of recombination between free holes and free electrons in a-Se. We have shown that the concentration of x-ray-induced negative centers increases almost linearly with the x-ray exposure. As a corollary, in terms of fundamental physics of amorphous semiconductors, we can also conclude that the influence of potential fluctuations in the noncrystalline structure in shielding a charged center in a-Se is relatively small.

Recombination of drifting holes with trapped electrons in stabilized a-Se photoconductors: Langevin recombination

Stabilized amorphous selenium (a-Se) is one of the x-ray photoconductors that is currently used in recently developed direct conversion flat panel x-ray image detectors. We have studied the recombination of free holes with trapped electrons in stabilized a-Se. Electrons were deeply trapped in a-Se by carrying out repetitive electron time-of-flight (TOF) transient photoconductivity experiments. By using conventional and interrupted field hole time-of-flight (IFTOF) transient photoconductivity techniques in a TOF, IFTOF, TOF sequence, we were able to develop a technique that allows the measurement of the capture coefficient between free holes and trapped electrons. We find that the capture process of holes by trapped electrons closely follows the Langevin recombination mechanism.

New materials and processes for flat panel X-ray detectors

Flat panel X-ray imagers using amorphous silicon active matrix addressing have been introduced to the medical imaging market at sizes up to 40×40 cm and with up to 10 million pixels. Some new technology developments, which can further increase the performance of these devices, are described. High atomic number polycrystalline X-ray photoconductors can operate near the theoretical sensitivity and at reasonably low bias voltages. The higher sensitivity obtained in HgI2 allows single-photon detection, which opens up new imaging opportunities. Another approach to improve sensitivity is to integrate an amplifier at the pixel level, which requires laser-recrystallised polysilicon transistors. A pixel-level source follower amplifier is shown to have enough gain to overcome other noise sources. A three-dimensional device structure is needed to accommodate the pixel electronics, and so the sensor is deposited on top of the electronics, separated by a thick passivation layer. Future possible detector technologies based on printing and organic semiconductors are discussed.

Direct conversion X-ray sensors: sensitivity, DQE and MTF

Imaging characteristics of photoconductive flat panel X-ray image detectors, such as X-ray sensitivity, detective quantum efficiency (DQE), and resolution in terms of the modulation transfer function (MTF) are examined with applications to amorphous selenium (a-Se), polycrystalline HgI2 and CdZnTe detectors. A theoretical model has been developed for the calculation of X-ray sensitivity of a pixellated detector by using the Shockley–Ramo theorem, the weighting potential of the individual pixel and the final trapped charge distribution across the photoconductor. The X-ray sensitivity of pixellated X-ray detectors mostly depends on the mobility and lifetime product of charges that move towards the pixel electrodes, and the extent of dependence increases with decreasing pixel per unit detector thickness. A cascaded linear system model that includes incomplete charge collection and interaction depth dependent conversion gain and charge collection stages is considered for the calculation of the zero spatial frequency detective quantum efficiency DQE(0) of a direct conversion X-ray image detector. The DQE(0) performance of a-Se, HgI2 and CdZnTe detectors is examined for fluoroscopic applications. It is shown that, in addition to high quantum efficiency, both high conversion gain and high charge collection efficiency are required to improve the DQE performance of an X-ray image detector. The factors affecting the MTF of a-Se flat-panel detectors for digital mammography are investigated. Both theoretical and experimental methods are examined. A theoretical model has been developed based on cascaded linear system analysis with parallel processes to take into account the effect of K-fluorescence. This model has been used to understand the performance of a small-area prototype detector with 85 μm pixel size. The presampling MTF of the prototype has been measured and compared to a theoretical calculation based on the model. The calculation shows that K-fluorescence accounts for a 15% reduction in the MTF at the Nyquist frequency of the prototype detector. The measurement of presampling MTF of the prototype detector reveals an additional source of blurring, which is probably due to charge carrier trapping in the blocking layer at the interface between a-Se and the active matrix. This introduces a drop in presampling MTF at high spatial frequencies.

Direct-conversion flat-panel X-ray image sensors for digital radiography

Advances in active-matrix array flat panels for displays over the last decade have led to the development of flat-panel X-ray image detectors. Recent flat-panel detectors have shown image quality exceeding that of X-ray film/screen cassettes. They can also permit the instantaneous capture, readout, and display of digital X-ray images and, hence, enable the clinical transition to digital radiography. There are two general approaches to flat panel detector technology: 1) direct and 2) indirect conversion. The present paper outlines the operating principles for direct-conversion detectors based on the use of photoconductors. It formulates and reviews the required X-ray photoconductor properties for such applications and examines to what extent potential materials fulfill these requirements. The quantum efficiency, X-ray sensitivity, noise, and detective quantum efficiency factors are discussed with reference to current and potential large area X-ray photoconductors.

Direct-conversion flat-panel X-ray image detectors

Flat-panel X-ray image detectors have been shown to be suitable to replace the conventional X-ray film/screen cassettes for medical radiography (static or snapshot imaging). They are capable of capturing the X-ray image digitally immediately after the X-ray exposure which permits a convenient clinical transition to digital radiography. There are two general approaches to the flat-panel X-ray detector technology: direct and indirect conversions. The authors review the operating principles for direct conversion, and formulate and review the required X-ray photoconductor properties for enabling a successful direct conversion detector. Two important photoconductor requirements are discussed in detail, the X-ray sensitivity and dark current, both of which are topical current research areas in seeking the best photoconductor amongst a number of candidate semiconductors such a-Se, PbI/sub 2/, HgI/sub 2/ and others. The requirements of medical fluoroscopy (real-time imaging at very low exposure levels) is challenging this technology and demanding even higher X-ray sensitivity.

A performance comparison of direct- and indirect-detection flat-panel imagers

A comparison of the performance of a direct- and an indirect-detection amorphous silicon flat-panel X-ray imager is presented for a 6 MV beam. Experimental measurements of the noise characteristics, image lag, spectral response, spatial resolution and quantum efficiency are described, compared and discussed. The two systems are comprised of 512×512 pixel, 400 μm pitch, arrays of a-Si:H p–i–n photodiodes and thin-film transistors. In the direct-detection system, X-rays interact to produce electron/hole pairs directly in the silicon photodiodes. For the indirect-detection system, a phosphor screen converts energy from the incident X-rays into visible light, which is then detected by the photodiodes. Both systems are shown to be quantum noise limited, with the total electronic noise in the detector 10–15 times smaller than the Poisson noise level in detected signal. The measured lag for both systems is 1.0±0.1% or less in the first frame with subsequent signals decaying exponentially with frame read-out, with a half-life of between 3.3 and 3.8 frames. Both systems are demonstrated to have a pronounced sensitivity to low-energy multiply scattered photons, although this is shown to be effectively filtered out using a 2 mm copper build-up plate. The direct-detection system, with the 2 mm Cu build-up, shows greater sensitivity to scattered radiation than the indirect system. The spatial resolutions of both systems were effectively equal with an f 50 of 0.25 mm −1 when pixels are binned 2×2, although a slight contribution from optical scattering in the phosphor screen is seen for the indirect-detection system. The quantum efficiency of the direct-detection system is a factor of 0.45 lower than that of the indirect-detection system. The application of these detectors to megavoltage CT is discussed, with the conclusion that the indirect-detection system is to be preferred.

Optimization of the temporal response of II-VI direct type semiconductor detectors for flat-panel pulsed X-ray imaging

The rising and falling edges of detected signal pulses have been measured utilizing X-ray ionization of a planar Cd/sub 1-x/Zn/sub x/Te system under different irradiation geometries, at different detector thicknesses, and applied electric fields. The experimental results of this study indicate that the time response of the CdZnTe based X-ray system is suitable for digital pulsed radiographic applications.

X-ray sensitivity of photoconductors: application to stabilizeda-Se

The x-ray sensitivity of a high-resistivity photoconductor sandwichedbetween two parallel plate electrodes and operating under a constant field isanalysed by considering charge carrier generation that follows the x-ray photonabsorption profile and taking into account both electron and hole trappingphenomena but neglecting recombination, bulk space charge and diffusion effects.The amount of collected charge in the external circuit due to distributedgeneration of electrons and holes through the detector is calculated byintegrating the Hecht collection efficiency with Ramo's theorem across thesample thickness. The results of the model allow the x-ray sensitivity to becalculated as a function of the applied field, detector thickness and electronand hole ranges (µτ), given the field and energy dependence of theelectron and hole pair creation energy, W±, and the energy spectrum ofincident x-ray radiation. The sensitivity model was applied to stabilized a-Sethat is currently used as a successful x-ray photoconductor in the recentlydeveloped flat panel x-ray image detectors. Recent free electron-hole paircreation energy versus electric field data at room temperature and appropriateelectron and hole drift mobilities were used to calculate the sensitivity formonoenergetic x-rays at 20 and at 60 keV. For the 20 keV radiation, it was shownthat a typical detector thickness of 200 µm (4 × attenuation depth at20 keV) with currently attainable electron and hole trapping parameters in a-Sewas operating optimally, the sensitivity of which can only be increased byfurther increasing the applied field. With the receiving electrode positively biased,the sensitivity was much more dependent onthe hole lifetime than electron lifetime. The absence of hole transport resultsin a reduction in sensitivity by a factor of about 4.4, whereas the absence ofelectron transport results in a sensitivity degradation of only 22%. The ratioof hole trapping limited sensitivity to electron trapping limited sensitivity isabout 0.3. For a detector of thickness 200 µm operating at 10 V µm-1,the maximum sensitivity is about 220 pC cm-2 mR-1, and thissensitivity degrades by more than 10% when either the electron lifetime fallsbelow ~20 µs or the hole lifetime falls below ~5 µs. When thehole lifetime is very short so that the sensitivity is substantially reduced,the sensitivity versus thickness dependence at a given field exhibits a maximum(an optimal thickness) that is less than that for full absorption. In the caseof 60 keV x-ray photons, it is more useful to examine the sensitivity as afunction of detector thickness given the practical bias voltage limit. Thesensitivity versus thickness behaviour for a given bias voltage exhibits amaximum, that is an optimal thickness, that is less than that for nearly fullabsorption. Electron lifetimes longer than ~200 µs and hole lifetimeslonger than ~10 µs do not significantly affect the sensitivity.

Properties of a-Se for use in flat panel X-ray image detectors

The present paper examines why a-Se has become the key photoconductor material in flat panel direct conversion X-ray image detectors. We critically examine the X-ray sensitivity of stabilized a-Se layers as a function of electric field and mean photon energy. The radiation energy W ± absorbed per free electron–hole pair liberated for photoconduction in both a-Se and a-Si:H seem to follow the Que–Rowlands rule of W ±≈2.2 E g +E phonon for amorphous semiconductors.

Photoconductor selection for digital flat panel x-ray image detectors based on the dark current

We consider a flat panel direct conversion x-ray image detector in which a photoconductor is used to directly convert absorbed x-ray photons to collectable electron hole pairs. The storage capacitor at each pixel stores a quantity of charge proportional to the incident radiation. The charge stored at each pixel is read out every Δt seconds. Based on a condition for a minimal dynamic constraint, we have examined the corresponding range of constraints on the material properties to establish approximate x-ray photoconductor selection criteria. We consider requirements for radiology and fluoroscopy in which Δt=1 and 1/30 s, respectively. When the electrical contacts are totally blocking, the dark current is limited by bulk thermal generation. Minimum band gap Eg requirements depend on the concentration of defects Nd around the Fermi level and their recombination cross section Sr. For single crystal photoconductors with a neutral-defect concentration of the order of ∼1014 cm−3, we need Eg>1.5 eV for fluoroscopy and Eg>1.7 eV for radiology. For high purity single crystals, these band gap values can be lower, for example, Eg≈1.42 eV (GaAs) requires Nd<1012 cm−3. In the case of amorphous semiconductors, the dark current is due to the emission of carriers from a distribution of localized states in the energy gap. For a maximum integrated concentration of ∼1016 cm−3 of localized states around the Fermi level, calculations suggest Eg>1.75 eV for fluoroscopy and Eg>1.9 eV for radiology; amorphous selenium readily satisfies these conditions. When the dark current is controlled to thermal emission from the metal electrode into the semiconductor over a potential barrier φB (i.e., bulk thermal generation of carriers is negligible) then we find that the required Schottky barrier φB can be expressed in terms of the band gap Eg. For Schottky barriers φB∼0.6Eg, Eg>1.8 eV for fluoroscopy and >2.0 eV for radiology.

Review X-ray photoconductors and stabilized a-Se for direct conversion digital flat-panel X-ray image-detectors

Recently developed direct conversion flat-panel X-ray image detectors provide not only potentially superior images but also enable a simple and convenient means of achieving digital radiography. The present flat-panel sensors use stabilized a-Se as an X-ray photoconductor to convert absorbed X-ray photons to collectable charge carriers. The present paper outlines the basics of the direct conversion flat-panel X-ray detector, formulates and reviews the required X-ray photoconductor properties and examines to what extent stabilized amorphous selenium (a-Se) fulfills these requirements and how it compares with other potential X-ray photoconductors that have been proposed. Charge transport and the electron–hole pair creation energy in stabilized a-Se are reviewed within the context of our present knowledge. Current materials problems and future potential developments are also critically discussed.

Digital radiology using active matrix readout of amorphous selenium: radiation hardness of cadmium selenide thin film transistors.

A flat-panel x-ray imaging detector using active matrix readout of amorphous selenium (a-Se) is being investigated for digital radiography and fluoroscopy. The active matrix consists of a two-dimensional array of thin film transistors (TFTs). Radiation penetrating through the a-Se layer will interact with the TFTs and it is important to ensure that radiation induced changes will not affect the operation of the x-ray imaging detector. The methodology of the present work is to investigate the effects of radiation on the characteristic curves of the TFTs using individual TFT samples made with cadmium selenide (CdSe) semiconductor. Four characteristic parameters, i.e., threshold voltage, subthreshold swing, field effect mobility, and leakage current, were examined. This choice of parameters was based on the well established radiation damage mechanisms for crystalline silicon metal-oxide-semiconductor field-effect transistors (MOSFETs), which have a similar principle of operation as CdSe TFTs. It was found that radiation had no measurable effect on the leakage current and the field effect mobility. However, radiation shifted the threshold voltage and increased the subthreshold swing. But even the estimated lifetime dose (50 Gy) of a diagnostic radiation detector will not affect the normal operation of an active matrix x-ray detector made with CdSe TFTs. The mechanisms of the effects of radiation will be discussed and compared with those for MOSFETs and hydrogenated amorphous silicon (a-Si:H) TFTs.

Digital radiology using active matrix readout of amorphous selenium: detectors with high voltage protection.

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.

Digital radiology using active matrix readout of amorphous selenium: Theoretical analysis of detective quantum efficiency

A flat-panel x-ray imaging detector using a layer of amorphous selenium (a-Se) for direct conversion of x rays (to charge) and an active matrix for self-scanned readout is being investigated for digital radiology. A theoretical analysis of the spatial frequency dependent detective quantum efficiency (DQE(f)) of the self-scanned a-Se detector is performed based on a model of signal and noise propagation in a cascaded imaging system. Because of the high intrinsic resolution of a-Se and the pixelated active matrix readout method, such detectors are inherently undersampled and aliasing is present. The presampling modulation transfer function (MTF) and aliased noise power spectrum (NPS) of the detector were used in the analysis of DQE(f). It is proven that the aliased NPS for the self-scanned a-Se detectors is white. Since the shape of DQE(f) is determined by the ratio of MTF squared and the NPS, the shape of DQE(f) follows the square of the presampling MTF of the detector as a result of the white NPS. The analysis also shows that DQE(0) is proportional to the pixel fill factor, i.e., the fraction of each pixel area used for image charge collection. The DQE analysis is applied to detector parameters for three x-ray imaging applications: mammography, chest radiography, and fluoroscopy. The effects of pixel fill factor, imaging geometry (i.e., incident angle of x rays), and various sources of electronic noise on the detector DQE(f) are discussed. Strategies for maximizing detector DQE for each x-ray imaging application are proposed.

Amorphous Silicon Sensor Arrays for X-Ray and Document Imaging

AbstractLarge area amorphous silicon image sensor arrays are important for x-ray medical imaging and document scanning as well as a variety of other applications where large sensor size is required. The paper first summarizes the present state of the flat panel x-ray imager technology, and compares the two main approaches for x-ray detection. We then describe the performance of a new, large area, high resolution, radiographic imager based on a single amorphous silicon array with 2304×3200 pixels, and an active area of 30×40 cm (12×1 6”).