Thick, segmented CdWO4-photodiode detector for cone beam megavoltage CT: a Monte Carlo study of system design parameters

Full modulation transfer functions of thick parallel- and focused-element scintillator arrays obtained by a Monte Carlo optical transport model

BACKGROUND

Arrays of thick segmented crystalline scintillators are useful x-ray converters for image-guided radiation therapy using electronic portal imaging (EPI) and megavoltage cone-beam computed tomography (MV-CBCT). Ionizing-radiation-only simulations previously showed relatively low modulation transfer function (MTF) in parallel-element arrays because of beam divergence. Hence, a focused-element geometry (matching the beam divergence) has been proposed. The "full" (ionizing and optical) MTF performance of such a focused geometry compared to its radiation-only MTF has, however, not been fully investigated.

PURPOSE

To study the full MTF performance of such arrays in a more realistic situation in which optical characteristics are also included using an in-house detector model that supports light transport, and quantify the errors in MTF estimation when the optical stage is ignored.

METHODS

First, radiation (x-ray and electron) transport was simulated. Then, transport of the generated optical photons was modeled using ScintSim2, an optical Monte Carlo (MC) code developed in MATLAB for simulation of two-dimensional (2D) parallel- and focused-element scintillator arrays. The full-MTF responses of focused- and parallel-element geometries, for a large array of 3 × 3 mm² CsI:Tl detector elements of 10, 40, and 60 mm thicknesses, were examined. For each configuration, a composite line spread function (LSF) was calculated to obtain the MTF.

RESULTS

At the Nyquist frequency, for 10 mm-thick central elements and 60 mm-thick peripheral parallel elements, full-MTF exhibited a drop of up to 15 and 79 times, respectively, compared with radiation-only MTF. This was found to be partly attributable to the angular distribution of the light emerging from the detector-element exit face and the dependence on its aspect ratio, since the light exiting thicker scintillators exhibited a more forward-directed distribution. Focused elements provided an increase of up to nine times in peripheral-area full MTF values.

CONCLUSIONS

Full MTF was up to 79 times lower than radiation-only MTF. Focused arrays preserved full MTF by up to nine times compared to parallel elements. The differences in the results obtained with and without inclusion of optical photons emphasize the need to include light transport when optimizing thick segmented scintillation detectors. Besides their application in detector optimization for radiotherapy megavoltage photon imaging, these findings can also be useful for other segmented-scintillator-based imaging systems, for example, in nuclear medicine, or in 2D detection systems for quality assurance of MR-linacs.

Development of a novel high quantum efficiency MV x-ray detector for image-guided radiotherapy: A feasibility study

Purpose

To develop a new scintillating fiber‐based electronic portal imaging device (EPID) with a high quantum efficiency (QE) while preserving an adequate spatial resolution.

Methods

Two prototypes were built: one with a single pixel readout and the other with an active matrix flat‐panel imager (AMFPI) for readout. The energy conversion layer of both prototypes was made of scintillating fiber layers interleaved with corrugated lead sheets to form a honeycomb pattern. The scintillating fibers have a diameter of 1 mm and the distance between the centers of neighboring fibers on the same layer is 1.35 mm. The layers have 1.22 mm spacing between them. The energy conversion layer has a thickness of 2 cm. The modulation transfer function (MTF), antiscatter properties and sensitivity of the detector with a single pixel readout were measured using a 6‐MV beam on a LINAC machine. In addition, a Monte Carlo simulation was conducted to calculate the zero‐frequency detective quantum efficiency (DQE(0)) of the proposed detector with an active matrix flat‐panel imager for readout.

Results

The DQE(0) of the proposed detector can be 11.5%, which is about an order of magnitude higher than that of current EPIDs. The frequency of 50% modulation (f50) of the measured MTF is 0.2 mm-1 at 6 MV, which is comparable to that of video‐based EPIDs. The scatter to primary ratio (SPR) measured with the detector at 10 cm air gap and 20 × 20 cm2 field size is approximately 30% lower than that of ionization chamber–based detectors with a comparable QE. The detector noise which includes the x‐ray quantum noise and absorption noise is much larger than the electronic noise per pixel of the flat‐panel imager at a dose of less than two LINAC pulses. Thus, the proposed detector is quantum noise limited down to very low doses (∼a couple of radiation pulses of the LINAC). A proof‐of‐concept image has been obtained using a 6‐MV beam.

Conclusions

This work indicates that by using scintillating fibers and lead layers it is possible to increase the thickness of the detecting materials, and therefore the QE or the DQE(0) of the detector, while maintaining an adequate spatial resolution for MV x‐ray imaging. Due to the use of lead as the spacing material, the new detector also has antiscatter property, which will help improve the signal‐to‐noise ratio of the images. Further investigation to optimize the design of the detector and achieve a better combination of DQE and spatial resolution is warranted.

Modelling the transport of optical photons in scintillation detectors for diagnostic and radiotherapy imaging

Computational modelling of radiation transport can enhance the understanding of the relative importance of individual processes involved in imaging systems. Modelling is a powerful tool for improving detector designs in ways that are impractical or impossible to achieve through experimental measurements. Modelling of light transport in scintillation detectors used in radiology and radiotherapy imaging that rely on the detection of visible light plays an increasingly important role in detector design. Historically, researchers have invested heavily in modelling the transport of ionizing radiation while light transport is often ignored or coarsely modelled. Due to the complexity of existing light transport simulation tools and the breadth of custom codes developed by users, light transport studies are seldom fully exploited and have not reached their full potential. This topical review aims at providing an overview of the methods employed in freely available and other described optical Monte Carlo packages and analytical models and discussing their respective advantages and limitations. In particular, applications of optical transport modelling in nuclear medicine, diagnostic and radiotherapy imaging are described. A discussion on the evolution of these modelling tools into future developments and applications is presented.

A piecewise-focused high DQE detector for MV imaging

PURPOSE

Electronic portal imagers (EPIDs) with high detective quantum efficiencies (DQEs) are sought to facilitate the use of the megavoltage (MV) radiotherapy treatment beam for image guidance. Potential advantages include high quality (treatment) beam's eye view imaging, and improved cone-beam computed tomography (CBCT) generating images with more accurate electron density maps with immunity to metal artifacts. One approach to increasing detector sensitivity is to couple a thick pixelated scintillator array to an active matrix flat panel imager (AMFPI) incorporating amorphous silicon thin film electronics. Cadmium tungstate (CWO) has many desirable scintillation properties including good light output, a high index of refraction, high optical transparency, and reasonable cost. However, due to the 0 1 0 cleave plane inherent in its crystalline structure, the difficulty of cutting and polishing CWO has, in part, limited its study relative to other scintillators such as cesium iodide and bismuth germanate (BGO). The goal of this work was to build and test a focused large-area pixelated "strip" CWO detector.

METHODS

A 361 × 52 mm scintillator assembly that contained a total of 28 072 pixels was constructed. The assembly comprised seven subarrays, each 15 mm thick. Six of the subarrays were fabricated from CWO with a pixel pitch of 0.784 mm, while one array was constructed from BGO for comparison. Focusing was achieved by coupling the arrays to the Varian AS1000 AMFPI through a piecewise linear arc-shaped fiber optic plate. Simulation and experimental studies of modulation transfer function (MTF) and DQE were undertaken using a 6 MV beam, and comparisons were made between the performance of the pixelated strip assembly and the most common EPID configuration comprising a 1 mm-thick copper build-up plate attached to a 133 mg/cm(2) gadolinium oxysulfide scintillator screen (Cu-GOS). Projection radiographs and CBCT images of phantoms were acquired. The work also introduces the use of a lightweight edge phantom to generate MTF measurements at MV energies and shows its functional equivalence to the more cumbersome slit-based method.

RESULTS

Measured and simulated DQE(0)'s of the pixelated CWO detector were 22% and 26%, respectively. The average measured and simulated ratios of CWO DQE(f) to Cu-GOS DQE(f) across the frequency range of 0.0-0.62 mm(-1) were 23 and 29, respectively. 2D and 3D imaging studies confirmed the large dose efficiency improvement and that focus was maintained across the field of view. In the CWO CBCT images, the measured spatial resolution was 7 lp/cm. The contrast-to-noise ratio was dramatically improved reflecting a 22 × sensitivity increase relative to Cu-GOS. The CWO scintillator material showed significantly higher stability and light yield than the BGO material.

CONCLUSIONS

An efficient piecewise-focused pixelated strip scintillator for MV imaging is described that offers more than a 20-fold dose efficiency improvement over Cu-GOS.

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

PURPOSE

Active matrix flat-panel imagers (AMFPIs) incorporating thick, segmented scintillators have demonstrated order-of-magnitude improvements in detective quantum efficiency (DQE) at radiotherapy energies compared to systems based on conventional phosphor screens. Such improved DQE values facilitate megavoltage cone-beam CT (MV CBCT) imaging at clinically practical doses. However, the MV CBCT performance of such AMFPIs is highly dependent on the design parameters of the scintillators. In this paper, optimization of the design of segmented scintillators was explored using a hybrid modeling technique which encompasses both radiation and optical effects.

METHODS

Imaging performance in terms of the contrast-to-noise ratio (CNR) and spatial resolution of various hypothetical scintillator designs was examined through a hybrid technique involving Monte Carlo simulation of radiation transport in combination with simulation of optical gain distributions and optical point spread functions. The optical simulations employed optical parameters extracted from a best fit to measurement results reported in a previous investigation of a 1.13 cm thick, 1016 μm pitch prototype BGO segmented scintillator. All hypothetical designs employed BGO material with a thickness and element-to-element pitch ranging from 0.5 to 6 cm and from 0.508 to 1.524 mm, respectively. In the CNR study, for each design, full tomographic scans of a contrast phantom incorporating various soft-tissue inserts were simulated at a total dose of 4 cGy.

RESULTS

Theoretical values for contrast, noise, and CNR were found to be in close agreement with empirical results from the BGO prototype, strongly supporting the validity of the modeling technique. CNR and spatial resolution for the various scintillator designs demonstrate complex behavior as scintillator thickness and element pitch are varied--with a clear trade-off between these two imaging metrics up to a thickness of ~3 cm. Based on these results, an optimization map indicating the regions of design that provide a balance between these metrics was obtained. The map shows that, for a given set of optical parameters, scintillator thickness and pixel pitch can be judiciously chosen to maximize performance without resorting to thicker, more costly scintillators.

CONCLUSIONS

Modeling radiation and optical effects in thick, segmented scintillators through use of a hybrid technique can provide a practical way to gain insight as to how to optimize the performance of such devices in radiotherapy imaging. Assisted by such modeling, the development of practical designs should greatly facilitate low-dose, soft tissue visualization employing MV CBCT imaging in external beam radiotherapy.

Monte Carlo simulation of a quantum noise limited Čerenkov detector based on air-spaced light guiding taper for megavoltage x-ray imaging

PURPOSE

Electronic Portal Imaging Devices (EPIDs) have been widely used in radiation therapy and are still needed on linear accelerators (Linacs) equipped with kilovoltage cone beam CT (kV-CBCT) or MRI systems. Our aim is to develop a new high quantum efficiency (QE) Čerenkov Portal Imaging Device (CPID) that is quantum noise limited at dose levels corresponding to a single Linac pulse.

METHODS

Recently a new concept of CPID for MV x-ray imaging in radiation therapy was introduced. It relies on Čerenkov effect for x-ray detection. The proposed design consisted of a matrix of optical fibers aligned with the incident x-rays and coupled to an active matrix flat panel imager (AMFPI) for image readout. A weakness of such design is that too few Čerenkov light photons reach the AMFPI for each incident x-ray and an AMFPI with an avalanche gain is required in order to overcome the readout noise for portal imaging application. In this work the authors propose to replace the optical fibers in the CPID with light guides without a cladding layer that are suspended in air. The air between the light guides takes on the role of the cladding layer found in a regular optical fiber. Since air has a significantly lower refractive index (∼ 1 versus 1.38 in a typical cladding layer), a much superior light collection efficiency is achieved.

RESULTS

A Monte Carlo simulation of the new design has been conducted to investigate its feasibility. Detector quantities such as quantum efficiency (QE), spatial resolution (MTF), and frequency dependent detective quantum efficiency (DQE) have been evaluated. The detector signal and the quantum noise have been compared to the readout noise.

CONCLUSIONS

Our studies show that the modified new CPID has a QE and DQE more than an order of magnitude greater than that of current clinical systems and yet a spatial resolution similar to that of current low-QE flat-panel based EPIDs. Furthermore it was demonstrated that the new CPID does not require an avalanche gain in the AMFPI and is quantum noise limited at dose levels corresponding to a single Linac pulse.

Rapid Monte Carlo simulation of detector DQE(f)

PURPOSE

Performance optimization of indirect x-ray detectors requires proper characterization of both ionizing (gamma) and optical photon transport in a heterogeneous medium. As the tool of choice for modeling detector physics, Monte Carlo methods have failed to gain traction as a design utility, due mostly to excessive simulation times and a lack of convenient simulation packages. The most important figure-of-merit in assessing detector performance is the detective quantum efficiency (DQE), for which most of the computational burden has traditionally been associated with the determination of the noise power spectrum (NPS) from an ensemble of flood images, each conventionally having 10(7) - 10(9) detected gamma photons. In this work, the authors show that the idealized conditions inherent in a numerical simulation allow for a dramatic reduction in the number of gamma and optical photons required to accurately predict the NPS.

METHODS

The authors derived an expression for the mean squared error (MSE) of a simulated NPS when computed using the International Electrotechnical Commission-recommended technique based on taking the 2D Fourier transform of flood images. It is shown that the MSE is inversely proportional to the number of flood images, and is independent of the input fluence provided that the input fluence is above a minimal value that avoids biasing the estimate. The authors then propose to further lower the input fluence so that each event creates a point-spread function rather than a flood field. The authors use this finding as the foundation for a novel algorithm in which the characteristic MTF(f), NPS(f), and DQE(f) curves are simultaneously generated from the results of a single run. The authors also investigate lowering the number of optical photons used in a scintillator simulation to further increase efficiency. Simulation results are compared with measurements performed on a Varian AS1000 portal imager, and with a previously published simulation performed using clinical fluence levels.

RESULTS

On the order of only 10-100 gamma photons per flood image were required to be detected to avoid biasing the NPS estimate. This allowed for a factor of 10(7) reduction in fluence compared to clinical levels with no loss of accuracy. An optimal signal-to-noise ratio (SNR) was achieved by increasing the number of flood images from a typical value of 100 up to 500, thereby illustrating the importance of flood image quantity over the number of gammas per flood. For the point-spread ensemble technique, an additional 2× reduction in the number of incident gammas was realized. As a result, when modeling gamma transport in a thick pixelated array, the simulation time was reduced from 2.5 × 10(6) CPU min if using clinical fluence levels to 3.1 CPU min if using optimized fluence levels while also producing a higher SNR. The AS1000 DQE(f) simulation entailing both optical and radiative transport matched experimental results to within 11%, and required 14.5 min to complete on a single CPU.

CONCLUSIONS

The authors demonstrate the feasibility of accurately modeling x-ray detector DQE(f) with completion times on the order of several minutes using a single CPU. Convenience of simulation can be achieved using GEANT4 which offers both gamma and optical photon transport capabilities.

ScintSim1: A new Monte Carlo simulation code for transport of optical photons in 2D arrays of scintillation detectors

Two-dimensional (2D) arrays of thick segmented scintillators are of interest as X-ray detectors for both 2D and 3D image-guided radiotherapy (IGRT). Their detection process involves ionizing radiation energy deposition followed by production and transport of optical photons. Only a very limited number of optical Monte Carlo simulation models exist, which has limited the number of modeling studies that have considered both stages of the detection process. We present ScintSim1, an in-house optical Monte Carlo simulation code for 2D arrays of scintillation crystals, developed in the MATLAB programming environment. The code was rewritten and revised based on an existing program for single-element detectors, with the additional capability to model 2D arrays of elements with configurable dimensions, material, etc., The code generates and follows each optical photon history through the detector element (and, in case of cross-talk, the surrounding ones) until it reaches a configurable receptor, or is attenuated. The new model was verified by testing against relevant theoretically known behaviors or quantities and the results of a validated single-element model. For both sets of comparisons, the discrepancies in the calculated quantities were all <1%. The results validate the accuracy of the new code, which is a useful tool in scintillation detector optimization.

Does kV-MV dual-energy computed tomography have an advantage in determining proton stopping power ratios in patients?

Conventional kilovoltage (kV) x-ray-based dual-energy CT (DECT) imaging using two different x-ray energy spectra is sensitive to image noise and beam hardening effects. The purpose of this study was to evaluate the theoretical advantage of the DECT method for determining proton stopping power ratios (SPRs) using a combination of kV and megavoltage (MV) x-ray energies. We investigated three representative x-ray energy pairs: 100 and 140 kVp comprised the kV-kV pair, 100 kVp and 1 MV comprised the kV-MV pair, and two 1 MV x-ray beams-one with and one without external filtration-comprised the MV-MV pair. The SPRs of 34 human tissues were determined using the DECT method with these three x-ray energy pairs. Small perturbations were introduced into the CT numbers and x-ray spectra used for the DECT calculation to simulate the effects of random noise and beam hardening. An error propagation analysis was performed on the DECT calculation algorithm to investigate the propagation of CT number uncertainty to final SPR estimation and to suggest the best x-ray energy combination. We found that the DECT method using each of the three beam pairs achieved similar accuracy in determining the SPRs of human tissues in ideal conditions. However, when CT number uncertainties and artifacts such as imaging noise and beam hardening effects were considered, the kV-MV DECT improved the accuracy of SPR estimation substantially over the kV-kV or MV-MV DECT methods. Furthermore, our error propagation analysis showed that the combination of 100 kVp and 1 MV beams was close to the optimal selection when using the DECT method to determine SPRs. Overall, the kV-MV combination makes the DECT method more robust in resolving the effective atomic numbers for biological tissues than the traditional kV-kV DECT method.

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.

Monte Carlo investigations of the effect of beam divergence on thick, segmented crystalline scintillators for radiotherapy imaging

The use of thick, segmented scintillators in electronic portal imagers offers the potential for significant improvement in x-ray detection efficiency compared to conventional phosphor screens. Such improvement substantially increases the detective quantum efficiency (DQE), leading to the possibility of achieving soft-tissue visualization at clinically practical (i.e. low) doses using megavoltage (MV) cone-beam computed tomography. While these DQE increases are greatest at zero spatial frequency, they are diminished at higher frequencies as a result of degradation of spatial resolution due to lateral spreading of secondary radiation within the scintillator--an effect that is more pronounced for thicker scintillators. The extent of this spreading is even more accentuated for radiation impinging the scintillator at oblique angles of incidence due to beam divergence. In this paper, Monte Carlo simulations of radiation transport, performed to investigate and quantify the effects of beam divergence on the imaging performance of MV imagers based on two promising scintillators (BGO and CsI:Tl), are reported. In these studies, 10-40 mm thick scintillators, incorporating low-density polymer, or high-density tungsten septal walls, were examined for incident angles corresponding to that encountered at locations up to approximately 15 cm from the central beam axis (for an imager located 130 cm from a radiotherapy x-ray source). The simulations demonstrate progressively more severe spatial resolution degradation (quantified in terms of the effect on the modulation transfer function) as a function of increasing angle of incidence (as well as of the scintillator thickness). Since the noise power behavior was found to be largely independent of the incident angle, the dependence of the DQE on the incident angle is therefore primarily determined by the spatial resolution. The observed DQE degradation suggests that 10 mm thick scintillators are not strongly affected by beam divergence for detector areas up to approximately 30x30 cm2. For thicker scintillators, the area that is relatively unaffected is significantly reduced, requiring a focused scintillator geometry in order to preserve spatial resolution, and thus DQE.

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

PURPOSE

Electronic portal imaging devices (EPIDs) based on active matrix, flat-panel imagers (AMFPIs) have become the gold standard for portal imaging and are currently being investigated for megavoltage cone-beam computed tomography (CBCT) and cone-beam digital tomosynthesis (CBDT). However, the practical realization of such volumetric imaging techniques is constrained by the relatively low detective quantum efficiency (DQE) of AMFPI-based EPIDs at radiotherapy energies, approximately 1% 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 thick) and three segmented CsI:Tl scintillators (11.4, 25.6, and 40.0 mm thick) 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 approximately 12 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:T1 scintillators provide significantly improved portal imaging performance at extremely low dose (i.e., down to 1 beam pulse corresponding to approximately 0.022 cGy), creating the possibility of soft-tissue visualization using MV CBCT and CBDT at clinically practical dose.

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

Thick, segmented scintillating detectors, consisting of 2D matrices of scintillator crystals separated by optically opaque septal walls, hold considerable potential for significantly improving the performance of megavoltage (MV) active matrix, flat-panel imagers (AMFPIs). Initial simulation studies of the radiation transport properties of segmented detectors have indicated the possibility of significant improvement in DQE compared to conventional MV AMFPIs based on phosphor screen detectors. It is therefore interesting to investigate how the generation and transport of secondary optical photons affect the DQE performance of such segmented detectors. One effect that can degrade DQE performance is optical Swank noise (quantified by the optical Swank factor I(opt)), which is induced by depth-dependent variations in optical gain. In this study, Monte Carlo simulations of radiation and optical transport have been used to examine I(opt) and zero-frequency DQE for segmented CsI:Tl and BGO detectors at different thicknesses and element-to-element pitches. For these detectors, I(opt) and DQE were studied as a function of various optical parameters, including absorption and scattering in the scintillator, absorption at the top reflector and septal walls, as well as scattering at the side surfaces of the scintillator crystals. The results indicate that I(opt) and DQE are only weakly affected by absorption and scattering in the scintillator, as well as by absorption at the top reflector. However, in some cases, these metrics were found to be significantly degraded by absorption at the septal walls and scattering at the scintillator side surfaces. Moreover, such degradations are more significant for detectors with greater thickness or smaller element pitch. At 1.016 mm pitch and with optimized optical properties, 40 mm thick segmented CsI:Tl and BGO detectors are predicted to provide DQE values of approximately 29% and 42%, corresponding to improvement by factors of approximately 29 and 42, respectively, compared to that of conventional MV AMFPIs.

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

Megavoltage cone-beam computed tomography (MV 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 approximately 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 approximately 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 approximately 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 contrast-to-noise performance. Finally, the performance of various segmented detectors obtained at a relatively low dose (1.54 cGy) was compared with 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 approximately 18 to 59 times more dose is required, depending on the configuration of the segmented detectors.