Indirect Flat-panel Detector Research Articles

Ｆｌａｔｐａｎｅｌｄｅｔｅｃｔｏｒ（ＦＰＤ）装置を使用した小児ディジタル胸部撮影における最適ヒストグラム解析の検討

Optical density of the lung area in pediatric digital chest radiography using an indirect flat panel detector (FPD) system were changed enormously by the influence of such factors as the size of lung area, gas of retroperitoneum, and so on. Our purpose in this study was to improve the stability of lung density on output images by means of optimizing histogram analysis. Chest images of a lung phantom were taken at various X-ray tube voltages and processed using a half-type region of interest (ROI) for the lung area. The shape of the histogram obtained by two different calculation methods including frequency distribution and cumulative relative frequency were compared, and digital chest images of 110 clinical cases in our hospital were classified into three age-groups (under 6 years old, over 6 years old and under 13 years old, and over 13 years old) and their histograms were analyzed. In conclusion, it was important that to analyze the histograms of age-groups, a cumulative relative frequency histogram should be used because the influence of X-ray tube voltage was lower than the frequency distribution histogram, and stabilized lung density under 6 years old could be obtained if the value of the imaging parameter for lung density from default was 90% to 85% or 80% because the distribution of lung area and its center value were significantly lower than in those over 13 years old.

X‐ray imaging using avalanche multiplication in amorphous selenium: Investigation of intrinsic avalanche noise

The flat-panel detector (FPD) is the state-of-the-art detector for digital radiography. The FPD can acquire images in real-time, has superior spatial resolution, and is free of the problems of x-ray image intensifiers-veiling glare, pin-cushion and magnetic distortion. However, FPDs suffer from poor signal to noise ratio performance at typical fluoroscopic exposure rates where the quantum noise is reduced to the point that it becomes comparable to the fixed electronic noise. It has been shown previously that avalanche multiplication gain in amorphous selenium (a-Se) can provide the necessary amplification to overcome the electronic noise of the FPD. Avalanche multiplication, however, comes with its own intrinsic contribution to the noise in the form of gain fluctuation noise. In this article a cascaded systems analysis is used to present a modified metric related to the detective quantum efficiency. The modified metric is used to study a diagnostic x-ray imaging system in the presence of intrinsic avalanche multiplication noise independently from other noise sources, such as electronic noise. An indirect conversion imaging system is considered to make the study independent of other avalanche multiplication related noise sources, such as the fluctuations arising from the depth of x-ray absorption. In this case all the avalanche events are initiated at the surface of the avalanche layer, and there are no fluctuations in the depth of absorption. Experiments on an indirect conversion x-ray imaging system using avalanche multiplication in a layer of a-Se are also presented. The cascaded systems analysis shows that intrinsic noise of avalanche multiplication will not have any deleterious influence on detector performance at zero spatial frequency in x-ray imaging provided the product of conversion gain, coupling efficiency, and optical quantum efficiency are much greater than a factor of 2. The experimental results show that avalanche multiplication in a-Se behaves as an intrinsic noise free avalanche multiplication, in accordance with our theory. Provided good coupling efficiency and high optical quantum efficiency are maintained, avalanche multiplication in a-Se has the potential to increase the gain and make negligible contribution to the noise, thereby improving the performance of indirect FPDs in fluoroscopy.

TH-C-L100F-01: Glare Characterization in Indirect Flat Panel Detectors

Purpose: To quantify veiling glare and determine the origins of the MTF low frequency drop (lfd) in indirect x-ray flat panel detectors (FPDs), by measuring the amplitude of lost scintillation light and its characteristic propagation length. Methods and Materials: A series of radio-opaque lead disks (1 to 35mm radius) were imaged on a GE structured CsI FPD, under different x-ray spectra (28 to 49 kVp with Rh/Rh anode/filter, with additional 0.3mm Cu filtration). The signal at the center of the disk as a function of disk radius, the disk transfer function (DTF), was used to determine the amplitude and decay length of scintillation light propagated horizontally in the detector. Results: It is found that there are two additive constituents of glare in this detector. They have similar amplitudes of ∼12%, but have respective propagation lengths of ∼2 and 10mm. The shorter-range contribution diminishes in amplitude and propagation length with increasing mean x-ray energy, while the other varies oppositely. By using the DTF it was possible to correct for the MTF lfd. Conclusions: The precise cause of the two additive contributions to glare is not yet known. Future investigations with monoenergetic x-ray beams will allow precise mapping of the energy dependence of the veiling glare. A better understanding of the physics of scintillation light propagation will allow the optimization of transmission properties in indirect detectors.

Imaging properties of digital magnification radiography

Flat panel detectors exhibit improved signal-to-noise ratio (SNR) and display capabilities compared to film. This improvement necessitates a new evaluation of optimal geometry for conventional projection imaging applications such as digital projection mammography as well as for advanced x-ray imaging applications including cone-beam computed tomography (CT), tomosynthesis, and mammotomography. Such an evaluation was undertaken in this study to examine the effects of x-ray source distribution, inherent detector resolution, magnification, scatter rejection, and noise characteristics including noise aliasing. A model for x-ray image acquisition was used to develop generic results applicable to flat panel detectors with similar x-ray absorption characteristics. The model assumed a Gaussian distribution for the focal spot and a rectangular distribution for a pixel. A generic model for the modulated transfer function (MTF) of indirect flat panel detectors was derived by a nonlinear fit of empirical receptor data to the Burgess model for phosphor MTFs. Noise characteristics were investigated using a generic noise power spectrum (NPS) model for indirect phosphor-based detectors. The detective quantum efficiency (DQE) was then calculated from the MTF and NPS models. The results were examined as a function of focal spot size (0.1, 0.3, and 0.6 mm) and pixel size (50, 100, 150, and 200 microm) for magnification ranges 1 to 3. Mammography, general radiography (also applicable to mammotomography), and chest radiography applications were explored using x-ray energies of 28, 74, and 120 kVp, respectively. Nodule detection was examined using the effective point source scatter model, effective DQE, and the Hotelling SNR2 efficiency. Results indicate that magnification can potentially improve the signal and noise performance of digital images. Results also show that a cross over point occurs in the spatial frequency above and below which the effects of magnification differ indicating that there are task dependent tradeoffs associated with magnification. The cross over point varies depending upon focal spot size, pixel size, x-ray energy, and source-to-image-distance (SID). For mammography, the cross over point occurs for a 0.3 mm focal spot while a 0.6 mm focal spot indicates that magnification does not improve image quality due to focal spot blurring. Thus, the benefit of magnification may be limited. For general radiography (as well as mammotomography), and chest radiography, the cross over point changes with SID. For a system with a 0.3 mm focal spot, 100 microm pixel size, a 2 m SID, and the applicable tissue thickness and scatter components, optimal magnification improved SNR2 by approximately 1.2 times for mammography and 1.5 times for general radiography (and mammotomography). These results indicate that the optimal geometry can improve image quality without changing patient dose or otherwise reduce dose without compromising image quality.

Portable Flat-Panel Detector for Low-Dose Imaging in a Pediatric Intensive Care Unit

We sought to evaluate the diagnostic performance of a portable indirect flat-panel detector for low-dose imaging as compared with an asymmetric film-screen system in a pediatric intensive care unit. A total of 120 neonates underwent chest radiographs using a portable flat-panel detector (digital speed 800) and an asymmetric film-screen system (400 speed). Four readers evaluated the detection of 11 anatomic and 5 pathologic landmarks and 4 support devices. Statistical analysis was performed using repeated analysis of variance. The level of statistical significance was P = 0.05. The detection of 4 anatomic/4 pathologic landmarks and 2 support devices was significantly better using the flat-panel detector as compared with the asymmetric film-screen system (P < 0.05). Another 8 anatomic and one pathologic landmarks were detected equally well or slightly better with the flat-panel detector (P > 0.05). The portable flat-panel detector offers the potential of a 50% dose reduction with equal or significantly better detection of clinically important structures.

Experimental Evaluation of a Portable Indirect Flat-Panel Detector for the Pediatric Chest: Comparison with Storage Phosphor Radiography at Different Exposures by Using a Chest Phantom

To compare the exposure dose requirements and performance of a portable indirect flat-panel detector for pediatric use in the depiction of catheters, simulated pulmonary nodules, and simulated interstitial lung disease with those of storage phosphor radiography. Catheters and simulated nodules and subtle interstitial lung disease (miliary, reticular, linear, and ground-glass patterns) were superimposed over an anthropomorphic chest phantom. Images were obtained with different exposures corresponding to simulated speeds of 400 and 800 with a portable flat-panel detector and printed on hard copies. These images were compared with those from storage phosphor radiography at a simulated speed of 400, which is typically used in pediatric radiology. Four independent readers recorded 7200 observations per pattern (for a total of 600 statistically independent observations), and these observations were subjected to receiver operating characteristic (ROC) analysis. Differences were considered significant at a P value of .05. Catheters over obscured chest areas, nodules 10 mm or smaller and larger than 10 mm over lucent lung, nodules 10 mm or smaller over obscured chest areas, and miliary and linear patterns over lucent lung showed higher areas under the ROC curve (A(z)) with the flat-panel detector at 400 and 800 digital speed compared with storage phosphor radiography. A(z) values for reticular and ground-glass patterns with the flat-panel detector were equal to or less than those with storage phosphor radiography. These differences, however, were not statistically significant. In the detection of catheters, nodules, and almost all interstitial lung disease, A(z) values were higher with the portable flat-panel detector than with storage phosphor radiography at equivalent and reduced speeds. These results suggest that the portable flat-panel detector could be used with reduced exposure dose in pediatric patients.

TH-C-I-611-08: A New Concept of Indirect Flat-Panel Detector with Avalanche Gain: SAPHIRE (Scitillator Avalanche Photoconductor with High Resolution Emitter Readout)

Purpose: To investigate the feasibility of a new concept of indirect flat-panel detector with avalanche gain and field emitter array (FEA) readout to improve the imaging performance at low dose with high resolution. Method and Materials: The proposed detector is made by optically coupling a structured scintillator (CsI) to a uniform layer of avalanche amorphous selenium (a-Se) photoconductor called HARP (High Avalanche Rushing amorphous Photoconductor). The HARP layer absorbs the visible photons emitted from the scintillator and generates electron-hole pairs. These carriers undergo avalanche multiplication under a sufficiently high electric field ESe and form an amplified charge image. A two-dimensional FEA, which is essentially a flat electron source, is proposed as a new method to read out the charge image. The proposed detector is called SAPHIRE (Scintillator Avalanche Photoconductor with HIgh Resolution Emitter readout). Experimental and theoretical methods were developed to study the factors affecting the x-ray response and spatial resolution of SAPHIRE. Results: The avalanche gain in the HARP layer depends on both its thickness and ESe. The avalanche gain for an 8-μm-thick HARP layer is 46 under an ESe of 110V/μm. The gain can be turned off in high dose applications (e.g. radiography) by decreasing the ESe. Our electron optics simulation of SAPHIRE shows the dominant source of blur is the lateral spread of the electron beams from the FEA, which depends on the geometry and bias potentials of different electrodes in the detector. With proper electron optics design a pixel size of 50 microns is achievable. Conclusion: The avalanche gain of SAPHIRE can provide x-ray quantum noise limited images down to a single x-ray photon. The gain is programmable by changing ESe, which ensures wide dynamic range for SAPHIRE. The FEA can potentially provide a smaller pixel size and higher readout speed than existing flat-panel detectors.

Comparison of technical and anatomical noise in digital thorax X-ray images

Former studies by Hoeschen and Buhr indicated a higher total noise in a thorax image than expected from technical noise, i.e. quantum and detector noise. This difference results from the overlay of many small anatomical structures along the X-ray beam, which leads to a noise-like appearance without distinguishable structures in the projected image. A method is proposed to quantitatively determine this 'anatomical noise' component, which is not to be confused with the anatomical background (e.g. ribs). This specific anatomical noise pattern in a radiograph changes completely when the imaging geometry changes because different small anatomical structures contribute to the projected image. Therefore, two images are taken using slightly different exposure geometry, and a correlation analysis based on wavelet transforms allows to determining the uncorrelated noise components. Since the technical noise also differs from image to image, which makes it difficult to separate the anatomical noise, images of a lung phantom were produced on a low-sensitive industrial X-ray film using high-exposure levels. From these results, the anatomical noise level in real clinical thorax radiographs using realistic exposure levels is predicted using the general dose dependence described in the paper text and compared with the quantum and detector noise level of an indirect flat-panel detector system. For consistency testing, the same lung phantom was imaged with the same digital flat-panel detector and the total image noise including anatomical noise is determined. The results show that the relative portion of anatomical noise may exceed the technical noise level. Anatomical noise is an important contributor to the total image noise and, therefore, impedes the recognition of anatomical structures.

Physical Imaging Properties and Low-contrast Performance of a Newly Developed Flat-panel Digital Radiographic System

We investigated the clinical usefulness of a newly developed flat-panel detector (FPD) system by comparing its physical imaging properties and low-contrast detectability with those of a current FPD system. The newly developed CsI-based indirect FPD (Canon, CXDI-40C) and current Gd(2)O(2)S-based FPD (Canon CXDI-11) systems were used. Characteristic curves, resolution properties, radiographic noise, detective quantum efficiencies (DQEs) and low-contrast detectability for both systems were measured. The new FPD system showed considerably lower noise levels than those of the current FPD system. DQE (0) s of the new and current FPD systems were 75% and 35%, respectively. Observer performance tests of the contrast-detail (C-D) phantom indicated that the new FPD system can significantly improve low-contrast performance over that obtainable with the current FPD system under the same conditions of exposure. The new FPD system provided approximately 50% reduction in exposure while providing comparable detectability. The newly developed FPD system provides radiographic images with excellent inherent physical image quality and low-contrast performance.