We report on a set of tests that measure the performance of a-Si flat panel TFT arrays used in digital x-ray detectors. During production of high performance TFT panels for applications such as mammography it is important to verify the integrity and quality of the TFT array at progressive stages of production. Early identification of failing TFT arrays as well as continuous monitoring of the production process can result in early termination of poor quality panels, quick identification of the root cause of failures, and correction of process drift to prevent failures from occurring. We present results of a system designed to test the performance of a-Si TFT arrays during the production process. Metrics which are important to x-ray image quality were tested, including FET performance, pixel capacitance, storage capacitor lag and diode leakage. Functional tests were performed entirely on pixels in the imaging array using timing and biasing conditions that mimic x-ray illumination.
The purpose of this paper is to provide a performance characterization of a new large field-of-view (LFOV) flat panel detector with a novel pixel design that has been optimized for both screening mammography and low dose advanced applications such as tomosynthesis. The measurements reported here were performed on prototype x-ray imagers for GE's upcoming LFOV mammography system. In addition to a light sensitive photodiode and a field effect transistor (FET), a storage capacitor has been added to each pixel in order to increase the dynamic range. In order to characterize the performance of the detector, measurements of the MTF, noise power spectrum, DQE, electronic noise, conversion factor, and lag were made. The results show that the new detector can deliver a DQE at 0 and 5 lp/mm of 72% and 28% while maintaining an MTF at 5 lp/mm of 30%. The addition of a storage capacitor at each pixel allows the conversion factor to be increased to reduce the noise floor - leading to a 400% extension of the dynamic range. Finally, a re-design of the FET and photodiode to reduce the time constants allows a 10X reduction in the lag that enables up to 4 frame per second imaging with less than 1% lag. This work represents the first results from a next generation large field of view a Si/CsI based x-ray imager for mammography and shows that a single detector can achieve high performance standards for both high dose screening and low dose, fast acquisition tomosynthesis simultaneously.
The GE Senographe 2000D, the first full field digital mammography system based on amorphous silicon (a-Si) flat panel arrays and a cesium iodide (CsI) scintillator, has been in clinical use for over five years. One of the major advantages of this technology platform over competing platforms is the inherent flexibility of the design. Specifically, it is possible to optimize the x-ray conversion layer (scintillator) independently of the light conversion layer (panel) and vice versa. This is illustrated by a new detector utilizing the same amorphous silicon (a-Si) flat panel design, but an optimized scintillator layer, which provides up to 15% higher DQE than the existing detector. By utilizing the existing flat panel with an optimized scintillator layer, it is possible to significantly boost performance without changes to the panel design. Future enhancements to both the panel and scintillator will raise the DQE at zero frequency to more than 80%. The a-Si/CsI platform is especially well suited to advanced applications utilizing very low doses.
The GE Senographe 2000D, the first full field digital mammography system based on amorphous Silicon (a-Si) flat panel arrays and a Cesium-Iodide (CsI) scintillator, has been in clinical use for several years. The purpose of this paper is to demonstrate and quantify improvements in the detective quantum efficiency (DQE) for both typical screening and ultra-low exposure levels for this technology platform. A new figure of merit, the electronic noise factor, is introduced to explicitly quantify the influence of the electronic noise, conversion factor, modulation transfer function (MTF), and pixel pitch towards the reduction of DQE at low exposure levels. Methods to improve the DQE through an optimization of both the flat panel design and the scintillator deposition process are discussed. The results show a substantial improvement in the DQE(f) at all frequencies and demonstrate the potential for DQE(0) to exceed 80%. The combination of high DQE at ultra low exposures and the inherent fast read-out capability makes this technology platform ideal for both current clinical procedures and advanced applications that may use multiple projections (tomosynthesis) or contrast media to enhance digital mammography.
The modulation transfer function and detective quantum efficiency are modeled for a Full Field Digital Mammography detector constructed with a CsI scintillator deposited on an amorphous silicon active matrix array. The model is evaluated against experimental measurements using different exposure levels, x-ray tube voltages, target composition and beam filtrations as well as varying thicknesses and compositions of filtration materials placed in the path between the tube and detector. Available x-ray tube emission spectrum models were evaluated by comparison against the measured transmission through aluminum. The observed variation of DQE at zero spatial frequency among different target/filter conditions, acrylic filtration thicknesses and kVp is well characterized by a x-ray model. This variation is largely accounted for by just two effects -- the attenuation of x-rays through the detector enclosure and the stopping power of x-rays in the CsI layer. Additional considerations such as the Lubberts effect were included in the analysis in order to match the measured DQE(k) as a function of spatial frequency, k. The pixel aperture and light channeling through the scintillator shape the MTF which acts favorably to avoid aliasing due to digital sampling.
We report the results of performance measurements for an amorphous silicon flat panel detector used in a cardiovascular imaging system. The detector contains 1024 x 1024 elements on a 0.2 mm pitch for an active image area of about 20.5 x 20.5 cm2. The system allows imaging at fluoroscopic and dynamic cardiac record exposure levels at rates of up to 30 Hz. We measured MTF, NPS, DQE, contrast ratio, response uniformity, resolution uniformity, and lag. Measurements were made on 28 production detectors. The MTF was greater than 0.2 at 2.5 cycles/mm. Contrast ratio was several hundred, indicating negligible long range scatter (veiling glare) within the detector. The DQE of the detector was measured at exposures typical of fluoroscopic imaging, dynamic cardiac record imaging, and digital subtraction angiography (DSA). The DQE was at least 0.65, 0.54, and 0.34 at 0, 1, and 2 cycles/mm, respectively, for all of these exposure levels. The response of the detector varied by less than 12% across its surface. The MTF, measured at nine positions over the surface of the detector, was found to have a maximum difference among positions of less than 0.05 at both 1 and 2 cycles/mm. First frame lag was less than 5%.
The noise power spectrum, NPS, is a key imaging property of a detector and one of the principle quantities needed to compute the detective quantum efficiency. NPS is measured by computing the Fourier transform of flat field images. Different measurement methods are investigated and evaluated with images obtained from an amorphous silicon flat panel x-ray imaging detector. First, the influence of fixed pattern structures is minimized by appropriate background corrections. For a given data set the effect of using different types of windowing functions is studied. Also different window sizes and amounts of overlap between windows are evaluated and compared to theoretical predictions. Results indicate that measurement error is minimized when applying overlapping Hanning windows on the raw data. Finally it is shown that radial averaging is a useful method of reducing the two-dimensional noise power spectrum to one dimension.
We discuss how the frequency dependent detective quantum efficiency [DQE(f)] in a well-designed amorphous silicon flat panel detector is affected by several phenomena that reduce the DQE in other types of medical imaging detectors. The detector examined employs a CsI(Tl) scintillator and is designed for general diagnostic x-ray imaging applications. We consider DQE degradation due to incomplete x-ray absorption, secondary quantum noise, Swank factor, Lubberts effect, spatial variation in gain, noise aliasing, and additive system noise. The influences of detector design parameters on the frequency- and exposure-dependent DQE are also examined. We find that the DQE does not depend directly on MTF and that DQE is independent of exposure within the detector's operating range, except at the lowest exposures. Likewise the signal per absorbed x-ray, which contains the fill factor as one of several multiplicative components, does not affect DQE except at the lowest exposures. A methodology for determining DQE(f) from measurements of MTF(f), noise power spectrum (NPS), average signal, and x-ray exposure is presented. We find that it is important to incorporate several corrections in the NPS measurement procedure in order to obtain accurate results. These include corrections for lag, non-linearity, response variation from pixel to pixel, and use of a finite number of flat-field images. MTF, NPS, and DQE results are presented for a 41 X 41-cm2 flat panel detector designed for radiographic applications.
An x-ray imaging detector designed for both radiographic and fluoroscopic medical applications has been developed. The requirement that the detector provide superior imaging in both fluoroscopic and radiographic operation put severe constraints on its design. User requirements and a translation of those requirements to detector performance parameters guided detector design. This paper reports on the performance of a 20.5 X 20.5 cm prototype detector which was a product of this design effort. The detector was tested using both physical measurements and clinical imaging trials. The frequency dependent DQE is used as a measure of contrast to noise performance. Measurements of DQE were made at both fluoroscopic and radiographic signal levels. In fluoroscopic operation, measurements of lag were also made. The detector performance is compared to that of existing and emerging technologies. Results of clinical studies in both radiographic mode (chest imaging) and fluoroscopic mode (cardiac imaging) are reported.
Because of traps in the photodiodes, amorphous silicon detectors retain charge and release it slowly after exposure. One effect of this property is that after an exposure, a ghost image may appear in subsequent images. This is a particular problem when fluoroscopic imaging follows radiographic exposure. In this paper we propose a method for predicting the magnitude of the ghost image so that it can be eliminated. The method uses linear systems theory to model the time behavior of the signal. Then the model is used to predict the offset as a function of time so that offset correction can be performed.