TOPICAL REVIEW: Electronic portal imaging devices: a review and historical perspective of contemporary technologies and research

A review of electronic portal imaging devices (EPIDs) used in external beam, megavoltage radiation therapy is presented. The review consists of a brief introduction to the definition, role and clinical significance of portal imaging, along with a discussion of radiotherapy film systems and the motivations for EPIDs. This is followed by a summary of the challenges and constraints inherent to portal imaging along with a concise, historical review of the technologies that have been explored and developed. The paper then examines, in greater depth, the two first-generation technologies that have found widespread clinical use starting from the late 1980s. This is followed by a broad overview of the physics, operation, properties and advantages of active matrix, flat-panel, megavoltage imagers, presently being commercially introduced to clinical environments or expected to be introduced in the future. Finally, a survey of contemporary research efforts focused on improving portal imaging performance by addressing various weaknesses in existing commercial systems is presented.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48974/2/m206r1.pd

Erratum: â Strategies to improve the signal and noise performance of active matrix, flatâ panel imagers for diagnostic xâ ray applicationsâ [Med. Phys. 27, 289â 306 (2000)] and â Determination of the detective quantum efficiency of a prototype, megavoltage indirect detection, active matrix flatâ panel imagerâ [Med. Phys. 28, 2538â 2550 (2001)]

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134876/1/mp5910.pd

Determination of the detective quantum efficiency of a prototype, megavoltage indirect detection, active matrix flatâ panel imager

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134773/1/mp3516.pd

An investigation of signal performance enhancements achieved through innovative pixel design across several generations of indirect detection, active matrix, flatâ panel arrays

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134990/1/mp9602.pd

Empirical noise performance of prototype active pixel arrays employing polycrystalline silicon thin- film transistors

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/162751/2/mp14321.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162751/1/mp14321_am.pd

Monte Carlo investigations of megavoltage coneâ beam CT using thick, segmented scintillating detectors for soft tissue visualization

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134940/1/mp8957.pd

Low-dose megavoltage cone-beam CT imaging using thick, segmented scintillators

Megavoltage, cone-beam computed tomography (MV CBCT) employing an electronic portal imaging device (EPID) is a highly promising technique for providing soft-tissue visualization in image-guided radiotherapy. However, current EPIDs based on active matrix flat-panel imagers (AMFPIs), which are regarded as the gold standard for portal imaging and referred to as conventional MV AMFPIs, require high radiation doses to achieve this goal due to poor x-ray detection efficiency (~2% at 6 MV). To overcome this limitation, the incorporation of thick, segmented, crystalline scintillators, as a replacement for the phosphor screens used in these AMFPIs, has been shown to significantly improve the detective quantum efficiency (DQE) performance, leading to improved image quality for projection imaging at low dose. Toward the realization of practical AMFPIs capable of low dose, soft-tissue visualization using MV CBCT imaging, two prototype AMFPIs incorporating segmented scintillators with ~11 mm thick CsI:Tl and Bi 4 Ge 3 O 12 (BGO) crystals were evaluated. Each scintillator consists of 120 _ 60 crystalline elements separated by reflective septal walls, with an element-to-element pitch of 1.016 mm. The prototypes were evaluated using a bench-top CBCT system, allowing the acquisition of 180 projection, 360° tomographic scans with a 6 MV radiotherapy photon beam. Reconstructed images of a spatial resolution phantom, as well as of a water-equivalent phantom, embedded with tissue equivalent objects having electron densities (relative to water) varying from ~0.28 to ~1.70, were obtained down to one beam pulse per projection image, corresponding to a scan dose of ~4 cGy–-a dose similar to that required for a single portal image obtained from a conventional MV AMFPI. By virtue of their significantly improved DQE, the prototypes provided low contrast visualization, allowing clear delineation of an object with an electron density difference of ~2.76%. Results of contrast, noise and contrast-to-noise ratio are presented as a function of dose and compared to those from a conventional MV AMFPI.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90773/1/0031-9155_56_6_001.pd

Effects of x-ray irradiation on polycrystalline silicon, thin-film transistors

The effects of x-ray irradiation on the transfer and noise characteristics of excimer-laser-annealed polycrystalline silicon (poly-Si) thin-film transistors (TFTs) have been examined at dose levels up to 1000 Gy1000Gy. Parameters including mobility, threshold voltage, subthreshold swing, and leakage current, as well as flicker and thermal noise coefficients, were determined as a function of dose. In addition, the physical mechanisms of the observed changes in these parameters are analyzed in terms of radiation-generated charge in the gate oxide, at the Si–SiO2Si–SiO2 interface, and at the grain boundaries. The results of the studies indicate that poly-Si TFTs exhibit sufficient radiation tolerance for the use in active-matrix flat-panel imagers for most medical x-ray applications.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87455/2/064501_1.pd

System performance of a prototype flatâ panel imager operated under mammographic conditions

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135010/1/mp5051.pd

Active pixel imagers incorporating pixelâ level amplifiers based on polycrystallineâ silicon thinâ film transistors

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135114/1/mp6364.pd