Improved imaging performance of a 14"x17" direct radiography system using a Se/TFT detector

Lee, Denny L. Y.; Cheung, Lawrence; Rodricks, Brian G.; Powell, Gregory F.

doi:10.1117/12.317017

Cited by 67 publications

(45 citation statements)

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“…1 Their work assumes that the detector is operated in drift mode, so that the point spread function (PSF) for normal incidence is a delta function and hence the MTF for normal incidence is unity at all frequencies. 2 Oblique incidence is more readily observed in digital mammography (DM) than many other imaging studies. A DM detector is placed closer to the focal spot than most modalities 3 to counteract the loss in x-ray penetration resulting from the use of relatively low energies ($20 keV).…”

Section: Introductionmentioning

confidence: 99%

Optimization of phosphor‐based detector design for oblique x‐ray incidence in digital breast tomosynthesis

Acciavatti

Maidment

2011

Medical Physics

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Purpose: In digital breast tomosynthesis (DBT), a volumetric reconstruction of the breast is generated from a limited range of x-ray projections. One trade-off of DBT is resolution loss in the projections due to non-normal (i.e., oblique) x-ray incidence. Although degradation in image quality due to oblique incidence has been studied using empirical data and Monte Carlo simulations, a theoretical treatment has been lacking. The purpose of this work is to extend Swank's calculations of the transfer functions of turbid granular phosphors to oblique incidence. The model is ultimately used as a tool for optimizing the design of DBT detectors. Methods: A quantum-limited system and 20 keV x-rays are considered. Under these assumptions, the modulation transfer function (MTF) and noise power spectra (NPS) are derived using the diffusion approximation to the Boltzmann equation to model optical scatter within the phosphor. This approach is applicable to a nonstructured scintillator such as gadolinium oxysulfide doped with terbium (Gd 2 O 2 S:Tb), which is commonly used in breast imaging and which can reasonably approximate other detector materials. The detective quantum efficiency (DQE) is then determined from the Nishikawa formulation, where it is written as the product of the x-ray quantum detection efficiency, the Swank factor, and the Lubberts fraction. Transfer functions are calculated for both front-and back-screen configurations, which differ by positioning the photocathode at the exit or entrance point of the x-ray beam, respectively. Results: In the front-screen configuration, MTF and DQE are found to have considerable angular dependence, while NPS is shown to vary minimally with projection angle. As expected, the high frequency MTF and DQE are degraded substantially at large angles. By contrast, all transfer functions for the back-screen configuration have the advantage of significantly less angular dependence. Using these models, we investigated the possibility for optimizing the design of DBT detectors. As an example optimization strategy, the phosphor thickness which maximizes the DQE at a fixed frequency is analyzed. This work demonstrates that the optimal phosphor thickness for the frontscreen is angularly dependent, shifting to lower thickness at higher angles. Conversely, the backscreen is not optimized by a single thickness but instead attains reasonably high DQE values over a large range of thicknesses. Although the back-screen configuration is not suited for current detectors using a glass substrate, it may prove to be preferred in future detectors using newly proposed plastic thin-film transistor (TFT) substrates. Conclusions: Using the diffusion approximation to the Boltzmann equation to model the spread of light in a scintillator, this paper develops an analytical model of MTF, NPS, and DQE for a phosphor irradiated obliquely. The model is set apart from other studies on oblique incidence in being derived from first principles. This work has applications in the optimization of DBT detector design.

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Section: Introductionmentioning

confidence: 99%

Optimization of phosphor‐based detector design for oblique x‐ray incidence in digital breast tomosynthesis

Acciavatti

Maidment

2011

Medical Physics

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“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] While many improvements have been made on the detector technology, image acquisition methods, and image display system, less attention has been paid to the long recognized problem of scattered x-rays in projection imaging.…”

Section: Introductionmentioning

confidence: 99%

Scatter rejection and low‐contrast performance of a slot‐scan digital chest radiography system with electronic aft‐collimation: A chest phantom study

et al. 2008

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Anti-scatter grids have been widely used to reject scatter and increase the perceptibility of lowcontrast object in chest radiography; however they also attenuate the primary x-rays, resulting in a substantial degradation of primary information. Compensation for this degradation requires the use of higher exposure technique hence higher dose to the patient. A more efficient approach to reject scatter is the slot-scan imaging technique which employs a narrow scanning x-ray fan beam in conjunction with a slit or slot shaped solid state detector or an area detector used with an aftcollimator. With this approach, scatter can be rejected effectively without the need to attenuate primary x-rays. This paper demonstrates an electronic aft-collimation method, referred to as the alternate line erasure and readout ͑ALER͒ technique, for implementing the slot-scan digital radiography with a modern flat-panel detector. With this technique, instead of first exposing the detector and then reading the image line by line, the image line on the leading edge of the scanning fan beam is reset to erase the scatter accumulated prior to the arrival of the fan beam x-rays, while the image line on the trailing edge of the scanning fan beam is read out to acquire the image signals following the fan-beam exposure. These reset and readout processes are alternated and repeated as the x-ray fan beam scans across the detector. An anthropomorphic chest phantom was imaged to evaluate the scatter rejection ability and the low-contrast performance for the ALER technique and compare them with those for the anti-scatter grid method in full-field chest imaging. With a projected beam width of 16 mm, the slot-scan/ALER technique resulted in an average reduction of the scatter-toprimary ratios by 81%, 84%, 82%, and 86% versus 65%, 73%, 74%, and 73% with the anti-scatter grid method in the lungs, mediastinum, retrocardium, and subdiaphragm, respectively. The average CNR for the slot-scan/ALER technique was found to improve by 135%, 133%, 176%, and 87% versus 15%, 15%, 38%, and −11% with the anti-scatter grid method in the mediastinum, retrocardium, subdiaphragm, and lungs, respectively. These results demonstrated that the slot-scan/ALER technique can be used to achieve equally effective scatter rejection but substantially higher lowcontrast performance than the anti-scatter grid method.

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“…These results were consistent with those reported by other groups. [10][11][12] In the measurement of profile curves on the edges of static and moving tungsten material plates, the effect of image lag was seen as blurred edges of the plate. In the MTF measurement, the blurred edges of a moving target were indicated as reduction of MTF.…”

Section: Discussionmentioning

confidence: 99%

“…[6][7][8][9] Modifications in the technology of the a-Se detectors appear to have resulted in marked decreases in both lag and ghosting effects in more recent systems. [10][11][12] Recently, several methods for measuring temporal modulation transfer function (MTF) and detective quantum efficiency (DQE) have been proposed and the properties of FPDs used in dynamic imaging have been reported. [13][14][15] However, there have been no studies regarding how image lag and ghosting affect motion tracking by dynamic imaging with FPDs.…”

Section: Introductionmentioning

confidence: 99%

Effects of image lag on real-time target tracking in radiotherapy

Tanaka

Ichikawa

Mori³

et al. 2010

SPIE Proceedings

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There is a concern that image lag may reduce accuracy of real-time target tracking in radiotherapy. This study was performed to investigate influence of image lag on the accuracy of target tracking in radiotherapy. Fluoroscopic images were obtained using a direct type of dynamic flat-panel detector (FPD) system under conditions of target tracking during radiotherapy. The images continued to be read out after X-irradiations and cutoff, and image lag properties in the system were then determined. Subsequently, a tungsten materials plate with a precision edge was mounted on to a motor control device, which provided a constant velocity. The plate was moved into the center of the detector at movement rate of 10 and 20 mm/s, covering lung tumor movement of normal breathing, and MTF and profile curves were measured on the edges covering and uncovering the detector. A lung tumor with blurred edge due to image lag was simulated using the results and then superimposed on breathing chest radiographs of a patient. The moving target with and without image lag was traced using a template-matching technique. In the results, the target could be traced within a margin for error in external radiotherapy. The results indicated that there was no effect of image lag on target tracking in usual breathing speed in a radiotherapy situation. Further studies are required to investigate influence by the other factors, such as exposure dose, target size and shape, imaging rate, and thickness of a patient's body.

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Improved imaging performance of a 14"x17" direct radiography system using a Se/TFT detector

Cited by 67 publications

References 0 publications

Optimization of phosphor‐based detector design for oblique x‐ray incidence in digital breast tomosynthesis

Optimization of phosphor‐based detector design for oblique x‐ray incidence in digital breast tomosynthesis

Scatter rejection and low‐contrast performance of a slot‐scan digital chest radiography system with electronic aft‐collimation: A chest phantom study

Effects of image lag on real-time target tracking in radiotherapy

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