Large area x-ray detectors based on phosphors coupled to flat panel amorphous silicon diode technology offer significant
advances for cargo radiologic imaging. Flat panel area detectors provide large object coverage offering high throughput
inspections to meet the high flow rate of container commerce. These detectors provide excellent spatial resolution when
needed, and enhanced SNR through low noise electronics. If the resolution is reduced through pixel binning, further
advances in SNR are achievable. Extended exposure imaging and frame averaging enables improved x-ray penetration
of ultra-thick objects, or "select-your-own" contrast sensitivity at a rate many times faster than LDAs. The areal
coverage of flat panel technology provides inherent volumetric imaging with the appropriate scanning methods. Flat
panel area detectors have flexible designs in terms of electronic control, scintillator selection, pixel pitch, and frame
rates. Their cost is becoming more competitive as production ramps up for the healthcare, nondestructive testing (NDT),
and homeland protection industries. Typically used medical and industrial polycrystalline phosphor materials such as
Gd2O2S:Tb (GOS) can be applied to megavolt applications if the phosphor layer is sufficiently thick to enhance x-ray
absorption, and if a metal radiator is used to augment the quantum detection efficiency and reduce x-ray scatter.
Phosphor layers ranging from 0.2-mm to 1-mm can be "sandwiched" between amorphous silicon flat panel diode arrays
and metal radiators. Metal plates consisting of W, Pb or Cu, with thicknesses ranging from 0.25-mm to well over 1-mm
can be used by covering the entire area of the phosphor plate. In some combinations of high density metal and phosphor
layers, the metal plate provides an intensification of 25% in signal due to electron emission from the plate and
subsequent excitation within the phosphor material. This further improves the SNR of the system.
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 photomechanical picture of laser ablation of biological tissue asserts that ablation is initiated when the laser-induced tensile stress exceeds the ultimate tensile strength of the target. We have developed a three-dimensional theoretical model of the thermoelastic response of tissue to short-pulsed laser irradiation which allows the time-dependent stress distribution to be calculated given the optical, thermal and mechanical properties of the target. In addition, we have developed a complimentary interferometric technique which can measure the laser- induced thermoelastic expansion of a material with nanometer spatial resolution on a nanosecond time scale. The complex features of this expansion allow the needed optical, thermal, and mechanical properties of the target to be determined, which then allows the stress distribution to be calculated. This work has led to several significant results which support the photomechanical model of ablation of biological tissue. First, unlike the one-dimensional model predicts the development of significant tensile stressed on the surface of the target, precisely where ablation is observed to occur. Experimental results from bone are consistent with mechanical fracture caused by laser-induced stresses. Experimental results from human meniscus, a representative soft tissue, show additional behavior consistent with the growth and collapse of cavitation bubbles within the tissue caused by laser-induced stresses.
In the regime where the specific time for propagation of stress waves is longer than the laser pulse duration, but shorter than the heat dissipation time, stress can be one of the governing mechanisms of laser-induced ablation of biological tissue. In such inertially confined regimes, knowing the mechanical properties of biological tissue an the kinetics of cracking (in hard tissue represented by bone) and cavitation (in soft tissue represented by meniscus) are important to understand the ablation process. An experimental technique has been developed to study laser-induced stress generation and mechanical properties of tissue in such regimes. This technique is based on monitoring the tissue surface after laser irradiation, using an interferometer that can measure submicron surface displacements on a nanosecond time scale. The subablation threshold laser-induced surface displacements can be related to the stress within the tissue and mechanical properties of the tissue. The surface movement of aqueous solution and meniscus tissue irradiated by 7.5-ns pulses of 355 nm light was consistent with growth and collapse of cavitation bubble. Bone movement was qualitatively consistent with theoretical predictions obtained by solving the equation of motion both analytically and numerically. In the regime where laser beam radius and optical absorption depth are comparable, it is shown that a full 3D analysis is necessary to understand the observed results.
The dynamics of the ablation process was investigated in bone using two pulses separated by a variable time delay. In one experiment two sub- threshold 7.5 nsec duration pulses at (lambda) equals 355 nm with pulse separation ranging between 1 ns and 100 msec were used to ablate bone. Crater depths remained approximately constant for pulse separations up to approximately equals 100 nsec, then decreased monotonically in time to zero at 10 msec pulse separation. In another experiment the second pulse was replaced by a 7.5 nsec duration pulse at (lambda) equals 532 nm. The combination of sub-threshold pulses at two different wavelengths also ablated bone with a cutting quality matching that of the more strongly absorbed (lambda) equals 355 nm wavelength. Crater depths from dual wavelength ablation increased with increasing fluence in either contributing pulse. Practical consequences of these experiments are discussed.
Laser-induced surface breakdown in fused silica has been studied as a function of pulse width in the
nanosecond regime. The third harmonic of a Nd:YAG laser was used to produce 7.5 ns duration pulses
(FWHM) at 355 nm. A novel system using optical delay lines was used to extend these pulses to variable
widths between 7.5 and 400 ns. At each pulse width, the beam was focused onto the surface of a
commercially available fused silica flat and the breakdown fluence was determined. The breakdown fluence
threshold was found to scale as the pulse width to the 0.8 power, significantly higher than the 0.5 power
reported elsewhere for similar cases. Experiments were also performed on 200 jim core fused silica optical
fibers and the results Obtained were consistent with a 0.8 power scaling law. This strong scaling law led to
a dramatic increase in the amount of 355 nm light that could be transmitted through 200 p.m core fibers -
from 1-2 mJ at 7.5 ns up to over 30 mJ at 400 ns. An experiment was also performed to probe the
recovery time of fused silica (the time separation between pulses such that their effects are independent).
This time was determined to be less than 25 ns.
Based upon a consideration of the morphology of calcified tissue, a two component picture of ablation is postulated in
which the soft connective tissue matrix is vaporized and entrains and removes the hard calcium salts. The dependence of the
ablation process on laser irradiance, fluence and wavelength is discussed, including estimates of optimal ranges of those
parameters for practical applications.
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