High‐DQE EPIDs based on thick, segmented BGO and CsI:Tl scintillators: Performance evaluation at extremely low dose

Abstract

Purpose: Electronic portal imaging devices ͑EPIDs͒ based on active matrix, flat-panel imagers ͑AMFPIs͒ have become the gold standard for portal imaging and are currently being investigated for megavoltage cone-beam computed tomography ͑CBCT͒ and cone-beam digital tomosynthesis ͑CBDT͒. However, the practical realization of such volumetric imaging techniques is constrained by the relatively low detective quantum efficiency ͑DQE͒ of AMFPI-based EPIDs at radiotherapy energies, ϳ1% at 6 MV. In order to significantly improve DQE, the authors are investigating thick, segmented scintillators, consisting of 2D matrices of scintillating crystals separated by septal walls. Methods: A newly constructed segmented BGO scintillator ͑11.3 mm thick͒ and three segmented CsI:Tl scintillators ͑11.4, 25.6, and 40.0 mm thick͒ were evaluated using a 6 MV photon beam. X-ray sensitivity, modulation transfer function, noise power spectrum, DQE, and phantom images were obtained using prototype EPIDs based on the four scintillators. Results: The BGO and CsI:Tl prototypes were found to exhibit improvement in DQE ranging from ϳ12 to 25 times that of a conventional AMFPI-based EPID at zero spatial frequency. All four prototype EPIDs provide significantly improved contrast resolution at extremely low doses, extending down to a single beam pulse. In particular, the BGO prototype provides contrast resolution comparable to that of the conventional EPID, but at 20 times less dose, with spatial resolution sufficient for identifying the boundaries of low-contrast objects. For this prototype, however, the BGO scintillator exhibited an undesirable radiation-induced variation in x-ray sensitivity. Conclusions: Prototype EPIDs based on thick, segmented BGO and CsI:Tl scintillators provide significantly improved portal imaging performance at extremely low dose ͑i.e., down to 1 beam pulse corresponding to ϳ0.022 cGy͒, creating the possibility of soft-tissue visualization using MV CBCT and CBDT at clinically practical dose.

Optimization of the design of thick, segmented scintillators for megavoltage cone-beam CT using a novel, hybrid modeling technique

“…The use of segmented scintillators in early prototypes has, thus far, been shown to significantly increase DQE values up to levels of ∼25%. 6,9,10 The primary importance of such order of magnitude increases in DQE is that the dose necessary to capture a single projection image is greatly reduced. This, in turn, makes it possible to acquire the large number of images required to perform megavoltage cone-beam computed tomography (MV CBCT) at a clinically practical dose-comparable to that required to capture a single projection image with a conventional MV AMFPI.…”

“…The crystals are arranged in the form of a twodimensional matrix of optically isolated elements-with each element consisting of a scintillating crystal surrounded by optically opaque septal wall material. Such segmented scintillators have been examined in numerous empirical 6,[8][9][10][11] and theoretical [12][13][14][15][16] studies of various designs with crystal thicknesses ranging up to 4 and 6 cm, respectively. The use of segmented scintillators in early prototypes has, thus far, been shown to significantly increase DQE values up to levels of ∼25%.…”

“…6,9,[12][13][14][15][23][24][25] Such modeling should account for both radiation and optical effects and an obvious approach would involve Monte Carlo based, event-by-event simulation of both radiation and optical transport so as to account for the most important physical effects. 13,23 However, since scintillation yields range from ∼8000 to 54 000 optical photons per MeV of deposited x-ray energy for the most promising scintillators for this application (Bi 4 4 and CsI:Tl), 26, 27 the computational demands related to optical transport can greatly exceed those related to radiation transport.…”

“…1,[4][5][6][7] These approaches employ both indirect and direct detection of the incident radiationcorresponding to use of scintillating and nonscintillating converters, respectively.…”

Performance evaluation of polycrystalline photoconductors for radiation therapy imaging

“…35 Furthermore, 2D scintillators integrated into MV AMFPIs include a 0.8 mm thick CsI:Tl needle scintillator, 36 a 2 mm thick segmented phosphor scintillator, 22 and several segmented CsI:Tl and BGO scintillators with thicknesses ranging from 8 to 40 mm. 18,19,37,38 An alternative approach for improving DQE performance involves the development of direct detection MV AMFPIs incorporating thick films of polycrystalline mercuric iodide ͑HgI 2 ͒ photoconductor. In direct detection AMFPIs, the charge created by x-ray interactions within the photoconductor is collected by means of an externally applied electric field, resulting in minimal lateral spread of the imaging signal.…”