Monte Carlo simulation of a quantum noise limited Cerenkov 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) Cerenkov 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 Cerenkov 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 Cerenkov 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.

Finding an improved amorphous-silicon x-ray flat-panel detector configuration for the in-line geometry.

We have previously investigated the use of a conventional amorphous-silicon flat-panel detector (FPD) for intrafractional image guidance in the in-line geometry. In this configuration, the FPD is mounted between the patient and the treatment head, with the front of the FPD facing towards the patient. By geometrically separating signals from the diagnostic (kV) and treatment (MV) beams, it is possible to monitor the patient and treatment beam at the same time. In this study, we propose an FPD design based on existing technology with a 70% reduced up-stream areal density that is more suited to this new application. We have investigated our FPD model by means of a validated Monte Carlo simulation. Experimentally, simple rectangular fields were used to irradiate through the detector and observe the impact of removing detector components such as the support structure or the phosphor screen on the measured signal. The proposed FPD performs better than the conventional FPD: (i) attenuation of the MV beam is decreased by 60%; (ii) the MV signal is reduced by 20% for the primary MV field region which can avoid saturation of the FPD; and (iii) long range scatter from the MV into the kV region of the detector is greatly reduced.

An inherent anti-scatter detector for megavoltage x-ray imaging.

Scattered x-rays are detrimental to the image quality of x-ray transmission radiography. Anti-scatter grids have been developed for kilovoltage (kV) x-ray imaging but are impractical to use for megavoltage (MV) x-ray imaging in radiation therapy. Our goal is to develop a new approach that uses an inherent anti-scatter detector for scatter reduction in MV x-ray imaging. A Monte Carlo simulation has been conducted to evaluate the response of a recently proposed Cerenkov electronic portal imaging device (CPID) to scattered x-rays. The proposed detector consists of a matrix of optical fibers aligned with the incident x-rays and coupled to an active matrix flat panel imager for image readout. The effects of scatter on the signal and noise of the CPID in comparison with those of conventional electronic portal imaging devices (EPIDs) have been investigated. It has been found that the CPID is ∼50% less sensitive to scattered x-rays than conventional EPIDs at 6 MV. The differential signal to noise ratio is also improved by up to 30% in the CPID. The results of our simulations have demonstrated that the recently proposed CPID system is an inherent anti-scatter detector, the first of this kind, for MV x-ray imaging.

Properties of noise in positron emission tomography images reconstructed with filtered-backprojection and row-action maximum likelihood algorithm.

Noise levels observed in positron emission tomography (PET) images complicate their geometric interpretation. Post-processing techniques aimed at noise reduction may be employed to overcome this problem. The detailed characteristics of the noise affecting PET images are, however, often not well known. Typically, it is assumed that overall the noise may be characterized as Gaussian. Other PET-imaging-related studies have been specifically aimed at the reduction of noise represented by a Poisson or mixed Poisson + Gaussian model. The effectiveness of any approach to noise reduction greatly depends on a proper quantification of the characteristics of the noise present. This work examines the statistical properties of noise in PET images acquired with a GEMINI PET/CT scanner. Noise measurements have been performed with a cylindrical phantom injected with (11)C and well mixed to provide a uniform activity distribution. Images were acquired using standard clinical protocols and reconstructed with filtered-backprojection (FBP) and row-action maximum likelihood algorithm (RAMLA). Statistical properties of the acquired data were evaluated and compared to five noise models (Poisson, normal, negative binomial, log-normal, and gamma). Histograms of the experimental data were used to calculate cumulative distribution functions and produce maximum likelihood estimates for the parameters of the model distributions. Results obtained confirm the poor representation of both RAMLA- and FBP-reconstructed PET data by the Poisson distribution. We demonstrate that the noise in RAMLA-reconstructed PET images is very well characterized by gamma distribution followed closely by normal distribution, while FBP produces comparable conformity with both normal and gamma statistics.

Monte Carlo simulation of a novel water-equivalent electronic portal imaging device using plastic scintillating fibers.

PURPOSE: Most electronic portal imaging devices (EPIDs) developed so far use a thin Cu plate/phosphor screen to convert x-ray energies into light photons, while maintaining a high spatial resolution. This results in a low x-ray absorption and thus a low quantum efficiency (QE) of approximately 2-4% for megavoltage (MV) x-rays. A significant increase of QE is desirable for applications such as MV cone-beam computed tomography (MV-CBCT). Furthermore, the Cu plate/phosphor screen contains high atomic number (high-Z) materials, resulting in an undesirable over-response to low energy x-rays (due to photoelectric effect) as well as high energy x-rays (due to pair production) when used for dosimetric verification. Our goal is to develop a new MV x-ray detector that has a high QE and uses low-Z materials to overcome the obstacles faced by current MV x-ray imaging technologies. METHODS: A new high QE and low-Z EPID is proposed. It consists of a matrix of plastic scintillating fibers embedded in a water-equivalent medium and coupled to an optically sensitive 2D active matrix flat panel imager (AMFPI) for image readout. It differs from the previous approach that uses segmented crystalline scintillators made of higher density and higher atomic number materials to detect MV x-rays. The plastic scintillating fibers are focused toward the x-ray source to avoid image blurring due to oblique incidence of off-axis x-rays. When MV x-rays interact with the scintillating fibers in the detector, scintillation light will be produced. The light photons produced in a fiber core and emitted within the acceptance angle of the fiber will be guided toward the AMFPI by total internal reflection. A Monte Carlo simulation has been used to investigate imaging and dosimetric characteristics of the proposed detector under irradiation of MV x-rays. RESULTS: Properties, such as detection efficiency, modulation transfer function, detective quantum efficiency (DQE), energy dependence of detector response, and water-equivalence of dose response have been investigated. It has been found that the zero frequency DQE of the proposed detector can be up to 37% at 6 MV. The detector, also, is water-equivalent with a relatively uniform response to different energy x-rays as compared to current EPIDs. CONCLUSIONS: The results of our simulations show that, using plastic scintillating fibers, it is possible to construct a water-equivalent EPID that has a better energy response and a higher detection efficiency than current flat panel based EPIDs.

Sci-Fri PM: Delivery - 10: Megavoltage x-ray imaging detector based on cerenkov effect.

A Monte Carlo simulation was used to study imaging and dosimetric characteristics of a novel design of a megavoltage (MV) x-ray imaging detector. The proposed detector consists 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. The new design relies on Cerenkov effect for MV x-ray imaging and is named CPID (for Cerenkov Portal Imaging Device). When MV x-rays are incident on CPID, they interact within the volume of the detector primarily via Compton effect and pair-production, resulting in electrons and positrons. From these charged particles, those with sufficient energy, trigger production of optical light via Cerenkov effect. The light that is generated in the optical fibre cores within the acceptance angle of the fibers is guided towards the AMFPI. Properties, such as detection efficiency, modulation transfer function, zero frequency detective quantum efficiency (DQE), and energy response of the detector, have been investigated. It has been shown that the proposed detector can have a zero-frequency DQE more than an order of magnitude higher than that of current electronic portal imaging device (EPID) systems and yet a spatial resolution comparable to that of video-based EPIDs. In additional the proposed detector is less sensitive to scattered x-rays than current EPIDs.

New Measurement of the π0 radiative decay width.

High precision measurements of the differential cross sections for π0 photoproduction at forward angles for two nuclei, 12C and 208Pb, have been performed for incident photon energies of 4.9-5.5 GeV to extract the π0→γγ decay width. The experiment was done at Jefferson Lab using the Hall B photon tagger and a high-resolution multichannel calorimeter. The π0→γγ decay width was extracted by fitting the measured cross sections using recently updated theoretical models for the process. The resulting value for the decay width is Γ(π0→γγ)=7.82±0.14(stat)±0.17(syst) eV. With the 2.8% total uncertainty, this result is a factor of 2.5 more precise than the current Particle Data Group average of this fundamental quantity, and it is consistent with current theoretical predictions.