Optimization of the performance of segmented scintillators for radiotherapy imaging through novel binning techniques

Abstract

Thick, segmented crystalline scintillators have shown increasing promise as replacement x-ray converters for the phosphor screens currently used in active matrix flat-panel imagers (AMFPIs) in radiotherapy, by virtue of providing over an order of magnitude improvement in the DQE. However, element-to-element misalignment in current segmented scintillator prototypes creates a challenge for optimal registration with underlying AMFPI arrays, resulting in degradation of spatial resolution. To overcome this challenge, a methodology involving the use of a relatively high resolution AMFPI array in combination with novel binning techniques is presented. The array, which has a pixel pitch of 0.127 mm, was coupled to prototype segmented scintillators based on BGO, LYSO and CsI:Tl materials, each having a nominal element-to-element pitch of 1.016 mm and thickness of ~1 cm. The AMFPI systems incorporating these prototypes were characterized at a radiotherapy energy of 6 MV in terms of MTF, NPS, DQE, and reconstructed images of a resolution phantom acquired using a cone-beam CT geometry. For each prototype, the application of 8×8 pixel binning to achieve a sampling pitch of 1.016 mm was optimized through use of an alignment metric which minimized misregistration and thereby improved spatial resolution. In addition, the application of alternative binning techniques that exclude the collection of signal near septal walls resulted in further significant improvement in spatial resolution for the BGO and LYSO prototypes, though not for the CsI:Tl prototype due to the large amount of optical cross-talk resulting from significant light spread between scintillator elements in that device. The efficacy of these techniques for improving spatial resolution appears to be enhanced for scintillator materials that exhibit mechanical hardness, high density and high refractive index, such as BGO. Moreover, materials that exhibit these properties as well as offer significantly higher light output than BGO, such as CdWO4, should provide the additional benefit of preserving DQE performance.

Theoretical investigation of the design and performance of a dual energy (kV and MV) radiotherapy imager

“…BGO was chosen due to the promising performance exhibited by previous prototypes based on this material, which offers desirable properties such as high electron density, high refractive index, and high optical transparency. 19,21,23 The lower limit for thickness corresponds to a point where the QE for MV imaging is ∼15%, representing a substantial improvement compared to that of conventional MV AMFPIs, whereas the 4 cm upper limit (for which the QE is ∼80%) corresponds to a point beyond which the rate of improvement in QE as a function of thickness diminishes rapidly. The lower limit for pitch corresponds to a point below which the scintillator fill factor drops sharply (due to the volume occupied by the fixed septal wall thickness), while the upper limit roughly corresponds to a point beyond which the advantageous spatial resolution offered by kV imaging would be severely compromised.…”

“…6,7 To address this challenge, various approaches for increasing the QE of MV imagers have been investigated or implemented, such as xenon gas ion chambers arranged in a fan-beam geometry, 8 and thick, segmented scintillating crystals arranged in the form of a linear array [9][10][11][12] or a 2D matrix. [13][14][15][16][17][18][19][20][21][22][23][24] In particular, the approach involving a 2D matrix of segmented scintillator elements (based on CsI:Tl, Bi 4 SiO 5 [LYSO]) has been explored both theoretically and empirically. [17][18][19][20][21][22][23][24] Based on this approach, one prototype employing ∼1.13 cm thick BGO crystals has demonstrated DQE values as high as ∼20%-representing an ∼20-fold increase compared with that of conventional MV AMFPIs.…”

“…[13][14][15][16][17][18][19][20][21][22][23][24] In particular, the approach involving a 2D matrix of segmented scintillator elements (based on CsI:Tl, Bi 4 SiO 5 [LYSO]) has been explored both theoretically and empirically. [17][18][19][20][21][22][23][24] Based on this approach, one prototype employing ∼1.13 cm thick BGO crystals has demonstrated DQE values as high as ∼20%-representing an ∼20-fold increase compared with that of conventional MV AMFPIs. 19 Such a large increase in DQE has been shown to enable the acquisition of high quality MV CBCT images at a total dose as low as ∼4 cGy (Ref.…”

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

“…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. This, plus the fact that the number of x-ray histories required for such studies is itself often very large (so as to achieve clinically realistic doses or a desired level of statistical precision), means that studies can become too computationally burdensome to be carried out on practical timescales.…”

“…BGO was chosen due to the generally promising performance of previous prototype segmented scintillators incorporating the material, resulting from properties such as high electron density and refractive index, as well as a high degree of optical transparency. 6,11,26 The present investigation involves simulation of reconstructed images of a contrast phantom using a CBCT geometry at clinically realistic radiation doses-accounting for both radiation and optical transport effects. Given the very large number of x-ray histories required for the study, a purely event-by-event Monte Carlo modeling approach would have been computationally prohibitive even had the scintillation yield been reduced to a level consistent with avoiding deleterious effect on accuracy.…”