Design Optimization Studies for Active Matrix Flat Panel Imagers Based on Segmented Crystalline Scintillators for Radiotherapy Imaging.

In this dissertation, a series of theoretical studies were performed using Monte Carlo simulation to optimize the design of active matrix flat panel imagers (AMFPIs) based on segmented scintillators for radiotherapy imaging. The influence of imager design specifications (such as use of a focused geometry, as well as the physical size and optical properties of scintillator elements) on imaging performance at megavoltage (MV) energies has been systematically investigated. The first study, involving simulation of radiation transport only, examined focused segmented scintillators as a potential solution to counter the detrimental effect of beam divergence. A focused planar geometry was found to effectively eliminate degradation in spatial resolution and detective quantum efficiency due to beam divergence, and to achieve uniform imaging performance across the entire detection area for thick, large-area, segmented scintillators. The second study, which involved simulation of both radiation and optical transport using a novel hybrid modeling technique, was performed to examine the influence of optical effects on the imaging performance of segmented scintillators. Based on the theoretical examination of various scintillator designs, an optimization map, which takes into account contrast-to-noise ratio and spatial resolution performance, was generated to guide decision-making in scintillator design. The final study explored the possibility of extending the clinical application of thick, segmented scintillators to include kilovoltage (kV) imaging using an extended hybrid modeling technique. A methodology was presented for identifying the most favorable design of a dual energy imager based on segmented scintillators. Such a design maintains the desirably high level of imaging performance at MV energies made possible by thick, segmented scintillators, while helping to provide performance comparable to that of commercial imagers at kV energies. The studies presented in this dissertation, which build upon the results of earlier empirical and theoretical characterizations of engineering prototypes, provide valuable insight for the design of future prototypes. It is anticipated that, through careful design assisted by theoretical modeling and empirical measurements, AMFPIs based on segmented scintillators can provide significantly improved performance compared to that of existing imagers in the treatment room, thereby increasing the clinical utility of in-room kV and MV imaging.PhDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111394/1/llgc_1.pd

Low-dose megavoltage cone-beam CT imaging using thick, segmented scintillators

Megavoltage, cone-beam computed tomography (MV CBCT) employing an electronic portal imaging device (EPID) is a highly promising technique for providing soft-tissue visualization in image-guided radiotherapy. However, current EPIDs based on active matrix flat-panel imagers (AMFPIs), which are regarded as the gold standard for portal imaging and referred to as conventional MV AMFPIs, require high radiation doses to achieve this goal due to poor x-ray detection efficiency (~2% at 6 MV). To overcome this limitation, the incorporation of thick, segmented, crystalline scintillators, as a replacement for the phosphor screens used in these AMFPIs, has been shown to significantly improve the detective quantum efficiency (DQE) performance, leading to improved image quality for projection imaging at low dose. Toward the realization of practical AMFPIs capable of low dose, soft-tissue visualization using MV CBCT imaging, two prototype AMFPIs incorporating segmented scintillators with ~11 mm thick CsI:Tl and Bi 4 Ge 3 O 12 (BGO) crystals were evaluated. Each scintillator consists of 120 _ 60 crystalline elements separated by reflective septal walls, with an element-to-element pitch of 1.016 mm. The prototypes were evaluated using a bench-top CBCT system, allowing the acquisition of 180 projection, 360° tomographic scans with a 6 MV radiotherapy photon beam. Reconstructed images of a spatial resolution phantom, as well as of a water-equivalent phantom, embedded with tissue equivalent objects having electron densities (relative to water) varying from ~0.28 to ~1.70, were obtained down to one beam pulse per projection image, corresponding to a scan dose of ~4 cGy–-a dose similar to that required for a single portal image obtained from a conventional MV AMFPI. By virtue of their significantly improved DQE, the prototypes provided low contrast visualization, allowing clear delineation of an object with an electron density difference of ~2.76%. Results of contrast, noise and contrast-to-noise ratio are presented as a function of dose and compared to those from a conventional MV AMFPI.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90773/1/0031-9155_56_6_001.pd

Countering beam divergence effects with focused segmented scintillators for high DQE megavoltage active matrix imagers

The imaging performance of active matrix flat-panel imagers designed for megavoltage imaging (MV AMFPIs) is severely constrained by relatively low x-ray detection efficiency, which leads to a detective quantum efficiency (DQE) of only ∼1%. Previous theoretical and empirical studies by our group have demonstrated the potential for addressing this constraint through the utilization of thick, two-dimensional, segmented scintillators with optically isolated crystals. However, this strategy is constrained by the degradation of high-frequency DQE resulting from spatial resolution loss at locations away from the central beam axis due to oblique incidence of radiation. To address this challenge, segmented scintillators constructed so that the crystals are individually focused toward the radiation source are proposed and theoretically investigated. The study was performed using Monte Carlo simulations of radiation transport to examine the modulation transfer function and DQE of focused segmented scintillators with thicknesses ranging from 5 to 60 mm. The results demonstrate that, independent of scintillator thickness, the introduction of focusing largely restores spatial resolution and DQE performance otherwise lost in thick, unfocused segmented scintillators. For the case of a 60 mm thick BGO scintillator and at a location 20 cm off the central beam axis, use of focusing improves DQE by up to a factor of ∼130 at non-zero spatial frequencies. The results also indicate relatively robust tolerance of such scintillators to positional displacements, of up to 10 cm in the source-to-detector direction and 2 cm in the lateral direction, from their optimal focusing position, which could potentially enhance practical clinical use of focused segmented scintillators in MV AMFPIs.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98594/1/0031-9155_57_16_5343.pd

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

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134977/1/mp5120.pd

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

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135068/1/mp5724.pd