Segmented crystalline scintillators: Empirical and theoretical investigation of a high quantum efficiency EPID based on an initial engineering prototype CsI(Tl) detector

Abstract

Modern-day radiotherapy relies on highly sophisticated forms of image guidance in order to implement increasingly conformal treatment plans and achieve precise dose delivery. One of the most important goals of such image guidance is to delineate the clinical target volume from surrounding normal tissue during patient setup and dose delivery, thereby avoiding dependence on surrogates such as bony landmarks. In order to achieve this goal, it is necessary to integrate highly efficient imaging technology, capable of resolving soft-tissue contrast at very low doses, within the treatment setup. In this paper we report on the development of one such modality, which comprises a nonoptimized, prototype electronic portal imaging device (EPID) based on a 40 mm thick, segmented crystalline CsI(Tl) detector incorporated into an indirect-detection active matrix flat panel imager (AMFPI). The segmented detector consists of a matrix of 160 x 160 optically isolated, crystalline CsI(Tl) elements spaced at 1016 microm pitch. The detector was coupled to an indirect detection-based active matrix array having a pixel pitch of 508 microm, with each detector element registered to 2 x 2 array pixels. The performance of the prototype imager was evaluated under very low-dose radiotherapy conditions and compared to that of a conventional megavoltage AMFPI based on a Lanex Fast-B phosphor screen. Detailed quantitative measurements were performed in order to determine the x-ray sensitivity, modulation transfer function, noise power spectrum, and detective quantum efficiency (DQE). In addition, images of a contrast-detail phantom and an anthropomorphic head phantom were also acquired. The prototype imager exhibited approximately 22 times higher zero-frequency DQE (approximately 22%) compared to that of the conventional AMFPI (approximately 1%). The measured zero-frequency DQE was found to be lower than theoretical upper limits (approximately 27%) calculated from Monte Carlo simulations, which were based solely on the x-ray energy absorbed in the detector-indicating the presence of optical Swank noise. Moreover, due to the nonoptimized nature of this prototype, the spatial resolution was observed to be significantly lower than theoretical expectations. Nevertheless, due to its high quantum efficiency (approximately 55%), the prototype imager exhibited significantly higher DQE than that of the conventional AMFPI across all spatial frequencies. In addition, the frequency-dependent DQE was observed to be relatively invariant with respect to the amount of incident radiation, indicating x-ray quantum limited behavior. Images of the contrast-detail phantom and the head phantom obtained using the prototype system exhibit good visualization of relatively large, low-contrast features, and appear significantly less noisy compared to similar images from a conventional AMFPI. Finally, Monte Carlo-based theoretical calculations indicate that, with proper optimization, further, significant improvements in the DQE performance of such imagers could be achieved...

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

“…In this study, 10 and 40 mm thick, segmented CsI and BGO detectors, separated with low density (polystyrene), equivalent density (scintillator material) and high density (tungsten) septal walls were examined, for a total of 12 detector configurations. The detector element-toelement pitch was chosen to be 0.508 mm, equal to the pixel pitch of an indirect-detection MV AMFPI array 54 that has been used in empirical evaluation of a recent prototype 51 . The width of the septal walls was chosen to be 0.05 mm, corresponding to the minimum value that can be achieved with present fabrication techniques.…”

“…50,51 A prototype AMFPI incorporating such a detector design has been constructed and demonstrated highly promising performance at low doses. 51 These promising early results motivate the present investigation of the potential performance of such detector designs for MV CBCT.…”

A Monte Carlo investigation of Swank noise for thick, segmented, crystalline scintillators for radiotherapy imaging

“…Recently, a 40 mm thick segmented CsI:Tl detector, with an element-toelement pitch of 1.016 mm and an area of 16.25 ϫ 16.25 cm 2 , has been developed and evaluated by our group. 16 A prototype MV AMFPI incorporating this detector exhibited a DQE of ϳ22% for a 6 MV x-ray beam. 16 However, the construction and performance of this initial prototype were far from optimal.…”

“…16 A prototype MV AMFPI incorporating this detector exhibited a DQE of ϳ22% for a 6 MV x-ray beam. 16 However, the construction and performance of this initial prototype were far from optimal.…”

“…These detectors are based on concepts involving high pressure gas ͑e.g., xenon͒ chambers employing tungsten walls, [10][11][12] thick optical fibers detecting Cerenkov radiation, 13 thick HgI 2 photoconductors, 14 segmented phosphors based on photopolymer matrices, 15 and segmented crystalline scintillators which consist of 2D matrices ͑or 1D arrays͒ of scintillator crystals separated by optically opaque septal walls. 7,[16][17][18][19] Among these approaches, segmented crystalline scintillators offer significantly improved QE, with only limited loss in spatial resolution, and possibly no substantial increase in noise compared to conventional AMFPIs. In order to examine this strategy, 1D segmented arrays incorporating zinc tungstate ͑ZnWO 4 ͒ and cadmium tungstate ͑CdWO 4 ͒ scintillators, [17][18][19] as well as 2D segmented matrices employing bismuth germanate ͑BGO͒, thallium-doped cesium iodide ͑CsI:Tl͒ and CdWO 4 scintillators, 7,[20][21][22] have been investigated by different research groups.…”