Optimization of metal/phosphor screens for on‐line portal imaging

Wowk, B.; Radcliffe, T. J.; Leszczyński, Konrad; Shalev, Shlomo; Rajapakshe, Rasika

doi:10.1118/1.597299

Cited by 26 publications

(25 citation statements)

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“…However, its use is limited for pre-treatment or inter treatment imaging due to high imaging dose and/or poor image quality (when compared to kilovoltage imaging systems). Potential methods for improving megavoltage imaging have included improving detector 55 quantum efficiency 24,25,33,39,40 and modification of the megavoltage beam line, such that the linac produces lower energy x-rays more suitable for imaging. The first improvement has resulted in the ability to acquire cone beam computed tomography images using the therapy beam.…”

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confidence: 99%

Kilovoltage energy imaging with a radiotherapy linac with a continuously variable energy range

et al. 2012

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Purpose: 10In this article the effect on image quality of significantly reducing the primary electron energy of a radiotherapy accelerator is investigated using a novel waveguide test piece. The waveguide contains a novel variable coupling device (rotovane) allowing for a wide continuously variable energy range of between 1.4 and 9 MeV suitable for both imaging and therapy. Method: 15Imaging at linac accelerating potentials close to 1 MV was investigated experimentally and via Monte Carlo simulations. An imaging beam line was designed, and planar and cone beam computed tomography images were obtained to enable qualitative and quantitative comparisons with kilovoltage and megavoltage imaging systems. The imaging beam had an electron energy of 1.4MeV which was incident on a water cooled electron window consisting of stainless steel, a 5 mm 20 Carbon electron absorber and 2.5 mm aluminium filtration. Images were acquired with an amorphous silicon detector sensitive to diagnostic x-ray energies. Results:The x-ray beam had an average energy of 220 keV and half value layer of 5.9 mm of copper. Cone Page 2 of 28 beam CT images with the same contrast to noise ratio as a gantry mounted kV imaging system were 25 obtained with doses as low as 2 cGy. This dose is equivalent to a single 6 MV portal image. While 12 times higher than a 100 kVp CBCT system (Elekta XVI), this dose is 140 times lower than a 6MV cone beam imaging system, and 6 times lower than previously published LowZ imaging beams operating at higher (4-5 MeV) energies. Conclusion 30The novel coupling device provides for a wide range of electron energies that are suitable for kilovoltage quality imaging and therapy. The imaging system provides high contrast images from the therapy portal at low dose, approaching that of gantry mounted kilovoltage x-ray systems.Additionally the system provides low dose imaging directly from the therapy portal potentially allowing for target tracking during radiotherapy treatment. There is the scope with such a tuneable 35 system for further energy reduction and subsequent improvement in image quality.

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confidence: 99%

Kilovoltage energy imaging with a radiotherapy linac with a continuously variable energy range

et al. 2012

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“…It is usually accepted that the screen spatial response is degraded with the increasing thickness of the substrate layer [7], [9], [10]. Different behaviors of the screen Line Spread Function (LSF) and Modulation Transfer Function (MTF) are observed in the case of lead and PET screens for a 300 keV incident X-ray energy.…”

Section: Influence Of Substrate Parameters On Screen Spatial Resmentioning

confidence: 98%

“…The choice for an optimal system is not simple. Thicker metal plates increase the X-ray photon absorption inside the metal screen but do not necessarily reinforce the absorption inside the phosphor screen [6], [7]. Once the metal is sufficiently thick, the electrons from the upper layers cannot escape and reach the phosphor screen so that the metal has no further role to play.…”

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confidence: 97%

“…For example the electron range in lead, calculated in the continuous-slowing-down approximation, is 19 and 145 m for electron kinetic energies of 80 and 300 keV, respectively (see NIST ESTAR database for further details [8]). Thicker phosphor screens increase the screen brightness but degrade the spatial response [7]. Moreover, the optimum screen thickness depends on the object to be imaged [9]: thicker screens are required for detecting large, low-contrast structures (e.g., pelvic structures), whereas thinner ones are needed for detecting small structures or edges between different media.…”

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Simulation Study of the Role Played by Intensifying or Support Layers in Scintillation Screens

Pistrui-Maximean

Freud

Létang

et al. 2007

IEEE Trans. Nucl. Sci.

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The advantage of using reinforcing substrate layers with phosphor screens is that they increase the phosphor sensitivity and filter the scattered radiation, while providing a mechanical support for the phosphor layer. A simulation model based on the Monte Carlo code Geant4 was developed to study the influence of the substrate properties (material, thickness) on the phosphor screen spatial response. The optimal substrate/phosphor combinations have been determined at given sensitivity of the screen. Of all studied cases at medium energy (300 keV), a configuration with slight improvement in the screen spatial response has been found. Moreover, the results show that, without degrading the performances of the screen, the phosphor thickness may be significantly reduced (by 35% in the case of a 0.05 mm Pb substrate). Besides, PolyEthylene Terephthalate (PET) makes a good mechanical support (of up to minimum 4 mm in thickness) for the phosphor layer. The simulation methodology permitted to study the role played by the X-ray and electron processes inside the substrate layer.

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“…Increase in the thickness of the phosphor screen or the build up plate increases the detection efficiency and SNR, but causes a significant decrease in spatial resolution. 31,32 TV camera based EPIDs although light quantum limited, also face this problem of a compromise between QDE and spatial resolution. As opposed to this, a thick transparent CsI (Tl) scintillator provides a longer path for x-ray interactions and thus improves the QDE at the detector by an order of magnitude as compared to the existing EPID technologies (i.e.…”

Section: Qdementioning

confidence: 99%

Portal Imaging Using a CSI (TL) Scintillator Coupled to a Cooled CCD Camera

Sawant¹

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Optimization of metal/phosphor screens for on‐line portal imaging

Cited by 26 publications

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Kilovoltage energy imaging with a radiotherapy linac with a continuously variable energy range

Kilovoltage energy imaging with a radiotherapy linac with a continuously variable energy range

Simulation Study of the Role Played by Intensifying or Support Layers in Scintillation Screens

Portal Imaging Using a CSI (TL) Scintillator Coupled to a Cooled CCD Camera

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