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

Abstract

Megavoltage cone-beam computed tomography (MY CBCT) is a highly promising technique for providing volumetric patient position information in the radiation treatment room. Such information has the potential to greatly assist in registering the patient to the planned treatment position, helping to ensure accurate delivery of the high energy therapy beam to the tumor volume while sparing the surrounding normal tissues. Presently, CBCT systems using conventional MV active matrix flat-panel imagers (AMFPIs), which are commonly used in portal imaging, require a relatively large amount of dose to create images that are clinically useful. This is due to the fact that the phosphor screen detector employed in conventional MV AMFPIs utilizes only ~2% of the incident radiation (for a 6 MV x-ray spectrum). Fortunately, thick, segmented scintillating detectors can overcome this limitation, and the first prototype imager has demonstrated highly promising performance for projection imaging at low doses. It is therefore of definite interest to examine the potential performance of such thick, segmented scintillating detectors for MV CBCT. In this study, Monte Carlo simulations of radiation energy deposition were used to examine reconstructed images of cylindrical CT contrast phantoms, embedded with tissue-equivalent objects. The phantoms were scanned at 6 MV using segmented detectors having various design parameters (i.e., detector thickness, as well as scintillator and septal wall materials). Due to constraints imposed by the nature of this study, the size of the phantoms was limited to ~6 cm. For such phantoms, the simulation results suggest that a 40 mm thick, segmented CsI detector with low density septal walls can delineate electron density differences of ~2.3% and 1.3% at doses of 1.54 and 3.08 cGy, respectively. In addition, it was found that segmented detectors with greater thickness, higher density scintillator material, or lower density septal walls exhibit higher contrastto-noise performance. Finally, the performance of various segmented detectors obtained at a relatively low dose (1.54 cGy) was compared to that of a phosphor screen similar to that employed in conventional MV AMFPIs. This comparison indicates that, for a phosphor screen to achieve the same contrast-to-noise performance as the segmented detectors, ~18 to 59 times more dose is required, depending on the configuration of the segmented detectors.

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

“…Such an increase in DQE͑0͒ compared to that of conventional 2 as well as an earlier experimental prototype 13 reflects the more efficient use of the incident x-ray quanta, which should lead to better contrast-to-noise ratio and thus better soft-tissue delineation in MV CBCT images. 39 Although the present prototypes are far from optimal, it is anticipated that, through further iteration of design, fabrication, and characterization, MV CBCT systems based on high-efficiency, segmented scintillators can provide high quality images for soft-tissue delineation at clinically practical doses.…”

“…[26][27][28][29][30][31][32][33][34] A series of theoretical and empirical studies has been previously reported on 2D segmented crystalline scintillators, which consists of matrices of scintillating crystals separated by septal walls to limit the lateral spread of optical photons. 25,[35][36][37][38][39][40] Monte Carlo simulations of radiation and optical transport have suggested that EPIDs employing segmented CsI:Tl and Bi 4 Ge 3 O 12 ͑BGO͒ detectors up to 40 mm thick can offer DQE values of up to 29% and 42%, respectively. 36,38 Such significant improvement in DQE should allow acquisition of MV CBCT images with softtissue visualization at doses as low as ϳ3 cGy.…”

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

“…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%.…”

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

“…As a result, the detective quantum efficiency ͑DQE͒ for conventional MV AMFPIs is only ϳ1%, 3 which is much lower than that for kilovoltage AMFPIs ͑ϳ40% to 80%͒. [4][5][6] In order to significantly improve portal imaging performance, 7 as well as reduce the dose requirement for MV CBCT imaging, [7][8][9] it is necessary to substantially increase the DQE of MV AMFPIs.…”