Current Electronic Portal Imaging Devices Research Articles

Beam's-Eye-View Imaging of Liver SBRT With a Novel Multi-Layer Imager

Beams-eye view (BEV) imaging during radiation therapy can be used for real-time localization of the treatment target. However, clinical implementation is limited by the poor performance of current electronic portal imaging devices (EPIDs). A novel multilayer imager (MLI), consisting of four stacked conventional flat-panel imagers, was recently shown to have 5.7 times the detective quantum efficiency and nearly 2 times the contrast-to-noise ratio of a conventional EPID. In the current study, we compare the MLI against a single layer imager for the application of fiducial tracking during liver SBRT procedures. The hypothesis is that the MLI will provide more accurate and efficient fiducial tracking.The prototype MLI was installed in the standard MV imager housing on a clinical LINAC, replacing the conventional EPID. The MV imager arm was extended during volumetric modulated arc therapy (VMAT) SBRT treatments for passive data acquisition. Six patients with liver metastases receiving SBRT in 3-5 fractions (900-1200 cGy/fx) were studied. Data was acquired for two fractions of each treatment course, one fraction with the MLI using all 4 layers and one with the MLI using the top layer only. Automatic fiducial localization was performed using a modification of a previously published algorithm. Truth data was derived using external surrogate respiratory traces combined with manual tracking. Results for 4- and 1-layer modes were compared against truth data to determine tracking accuracy and efficiency. Tracking and noise improvements were assessed for any correlation.Between 2771 to 6505 image frames were acquired for each patient, with roughly half of each corresponding to acquisition in 1-layer mode and the other half 4-layer mode. Tracking accuracy with the 1-layer imager returned a root mean square error (RMSE) of 2.1 mm compared to 4-layer RMSE of 1.5 mm, a statistically significant (P < 0.001) improvement of 0.6 mm. Successful tracking efficiency with 4 layers improved to 82.8% versus 58.4% for the 1-layer mode, a relative improvement of 41.8% (P < 0.001). There was a strong correlation (r = 0.913) between the increase in successfully tracked frames and reduction in noise when using the 4-layer mode.Noise reduction and increased MV photon detection efficiency achieved by utilizing a multi-layer MV imager results in improved fiducial tracking for liver SBRT treatments. Future BEV clinical applications may be improved by pursuing similar noise reduction and photon detection enhancement strategies.

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

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

TU‐D‐BRC‐00: The EPID Strikes Back

There has been increased deployment of gantry‐mounted linacs with onboard kilovoltage x‐ray imagers. Despite this trend, most clinics still employ beam's‐eye‐view (BEV) imaging using flat panel electronic portal imaging devices (EPIDs) as a highly useful means of verifying geometric localization pre‐ and post‐delivery. In recent years, there has been increased interest in the use of EPIDs for a variety of clinical and investigational applications ranging from megavoltage cone‐beam CT, in vivo dosimetry, patient‐specific QA to real‐time target localization for dynamic MLC‐based tumor tracking. In conjunction, there are several innovative EPID designs currently under investigation that hold great promise in bringing novel imaging and image‐guidance applications into clinical practice.This symposium is aimed at clinical and research‐oriented medical physicists who want to learn more about current and future megavoltage imaging technology and applications. In this symposium, we will examine the (i) interesting applications that make use of current EPID technology such as volumetric 4D image‐based models, real‐time position estimation for dynamic MLC tracking, etc., (ii) clinical applications such as in vivo EPID dosimetry and (iii) innovative high‐performance EPID designs and exciting future applications, including designs based on thick segmented scintillators, hybrid kV/MV imagers using segmented strip scintillators, and energy‐resolving EPIDs based on multilayer imagers and poly‐Silicon active matrix flat panel imagers.Learning Objectives – In this symposium, the attendees will learn Novel applications using currently available EPIDs Use of EPIDs to perform in vivo dosimetry Novel designs using high‐efficiency, energy resolving EPIDs and future applications of megavoltage imagingJ. Rottmann, Varian Medical SystemsP. Greer, Research Grant Funding from Radiation Oncology Institute (Grant ROI2013‐912), 2013–2014

TU‐D‐BRC‐03: Future EPID Technology and Applications

There has been increased deployment of gantry‐mounted linacs with onboard kilovoltage x‐ray imagers. Despite this trend, most clinics still employ beam's‐eye‐view (BEV) imaging using flat panel electronic portal imaging devices (EPIDs) as a highly useful means of verifying geometric localization pre‐ and post‐delivery. In recent years, there has been increased interest in the use of EPIDs for a variety of clinical and investigational applications ranging from megavoltage cone‐beam CT, in vivo dosimetry, patient‐specific QA to real‐time target localization for dynamic MLC‐based tumor tracking. In conjunction, there are several innovative EPID designs currently under investigation that hold great promise in bringing novel imaging and image‐guidance applications into clinical practice.This symposium is aimed at clinical and research‐oriented medical physicists who want to learn more about current and future megavoltage imaging technology and applications. In this symposium, we will examine the (i) interesting applications that make use of current EPID technology such as volumetric 4D image‐based models, real‐time position estimation for dynamic MLC tracking, etc., (ii) clinical applications such as in vivo EPID dosimetry and (iii) innovative high‐performance EPID designs and exciting future applications, including designs based on thick segmented scintillators, hybrid kV/MV imagers using segmented strip scintillators, and energy‐resolving EPIDs based on multilayer imagers and poly‐Silicon active matrix flat panel imagers.Learning Objectives – In this symposium, the attendees will learn Novel applications using currently available EPIDs Use of EPIDs to perform in vivo dosimetry Novel designs using high‐efficiency, energy resolving EPIDs and future applications of megavoltage imagingJ. Rottmann, Varian Medical SystemsP. Greer, Research Grant Funding from Radiation Oncology Institute (Grant ROI2013‐912), 2013–2014

TU‐D‐BRC‐02: Free Information ‐ EPID‐Based In Vivo Dosimetry

There has been increased deployment of gantry‐mounted linacs with onboard kilovoltage x‐ray imagers. Despite this trend, most clinics still employ beam's‐eye‐view (BEV) imaging using flat panel electronic portal imaging devices (EPIDs) as a highly useful means of verifying geometric localization pre‐ and post‐delivery. In recent years, there has been increased interest in the use of EPIDs for a variety of clinical and investigational applications ranging from megavoltage cone‐beam CT, in vivo dosimetry, patient‐specific QA to real‐time target localization for dynamic MLC‐based tumor tracking. In conjunction, there are several innovative EPID designs currently under investigation that hold great promise in bringing novel imaging and image‐guidance applications into clinical practice.This symposium is aimed at clinical and research‐oriented medical physicists who want to learn more about current and future megavoltage imaging technology and applications. In this symposium, we will examine the (i) interesting applications that make use of current EPID technology such as volumetric 4D image‐based models, real‐time position estimation for dynamic MLC tracking, etc., (ii) clinical applications such as in vivo EPID dosimetry and (iii) innovative high‐performance EPID designs and exciting future applications, including designs based on thick segmented scintillators, hybrid kV/MV imagers using segmented strip scintillators, and energy‐resolving EPIDs based on multilayer imagers and poly‐Silicon active matrix flat panel imagers.Learning Objectives – In this symposium, the attendees will learn Novel applications using currently available EPIDs Use of EPIDs to perform in vivo dosimetry Novel designs using high‐efficiency, energy resolving EPIDs and future applications of megavoltage imagingJ. Rottmann, Varian Medical SystemsP. Greer, Research Grant Funding from Radiation Oncology Institute (Grant ROI2013‐912), 2013–2014

TU‐D‐BRC‐01: Novel Applications for Current EPID Technology

There has been increased deployment of gantry‐mounted linacs with onboard kilovoltage x‐ray imagers. Despite this trend, most clinics still employ beam's‐eye‐view (BEV) imaging using flat panel electronic portal imaging devices (EPIDs) as a highly useful means of verifying geometric localization pre‐ and post‐delivery. In recent years, there has been increased interest in the use of EPIDs for a variety of clinical and investigational applications ranging from megavoltage cone‐beam CT, in vivo dosimetry, patient‐specific QA to real‐time target localization for dynamic MLC‐based tumor tracking. In conjunction, there are several innovative EPID designs currently under investigation that hold great promise in bringing novel imaging and image‐guidance applications into clinical practice.This symposium is aimed at clinical and research‐oriented medical physicists who want to learn more about current and future megavoltage imaging technology and applications. In this symposium, we will examine the (i) interesting applications that make use of current EPID technology such as volumetric 4D image‐based models, real‐time position estimation for dynamic MLC tracking, etc., (ii) clinical applications such as in vivo EPID dosimetry and (iii) innovative high‐performance EPID designs and exciting future applications, including designs based on thick segmented scintillators, hybrid kV/MV imagers using segmented strip scintillators, and energy‐resolving EPIDs based on multilayer imagers and poly‐Silicon active matrix flat panel imagers.Learning Objectives – In this symposium, the attendees will learn Novel applications using currently available EPIDs Use of EPIDs to perform in vivo dosimetry Novel designs using high‐efficiency, energy resolving EPIDs and future applications of megavoltage imagingJ. Rottmann, Varian Medical SystemsP. Greer, Research Grant Funding from Radiation Oncology Institute (Grant ROI2013‐912), 2013–2014

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.

Megavoltage X-Ray Imaging Based on Cerenkov Effect: A New Application of Optical Fibres to Radiation Therapy

A Monte Carlo simulation was used to study imaging and dosimetric characteristics of a novel design of megavoltage (MV) X-ray detectors for radiotherapy applications. The new design uses Cerenkov effect to convert X-ray energy absorbed in optical fibres into light for MV X-ray imaging. The proposed detector consists of a matrix of optical fibres aligned with the incident X rays and coupled to an active matrix flat-panel imager (AMFPI) for image readout. Properties, such as modulation transfer function, detection quantum efficiency (DQE), and energy response of the detector, were 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. The proposed detector is also less sensitive to scattered X rays from patients than current EPIDs.

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 Bi4Ge3O12 (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.

Image-Guided Radiotherapy in Near Real Time With Intensity-Modulated Radiotherapy Megavoltage Treatment Beam Imaging

To utilize image-guided radiotherapy (IGRT) in near real time by obtaining and evaluating the online positions of implanted fiducials from continuous electronic portal imaging device (EPID) imaging of prostate intensity-modulated radiotherapy (IMRT) delivery. Upon initial setup using two orthogonal images, the three-dimensional (3D) positions of all implanted fiducial markers are obtained, and their expected two-dimensional (2D) locations in the beam's-eye-view (BEV) projection are calculated for each treatment field. During IMRT beam delivery, EPID images of the megavoltage treatment beam are acquired in cine mode and subsequently analyzed to locate 2D locations of fiducials in the BEV. Simultaneously, 3D positions are estimated according to the current EPID image, information from the setup portal images, and images acquired at other gantry angles (the completed treatment fields). The measured 2D and 3D positions of each fiducial are compared with their expected 2D and 3D setup positions, respectively. Any displacements larger than a predefined tolerance may cause the treatment system to suspend the beam delivery and direct the therapists to reposition the patient. Phantom studies indicate that the accuracy of 2D BEV and 3D tracking are better than 1 mm and 1.4 mm, respectively. A total of 7330 images from prostate treatments were acquired and analyzed, showing a maximum 2D displacement of 6.7 mm and a maximum 3D displacement of 6.9 mm over 34 fractions. This EPID-based, real-time IGRT method can be implemented on any external beam machine with portal imaging capabilities without purchasing any additional equipment, and there is no extra dose delivered to the patient.

MO‐E‐L100J‐03: Development of a Novel Electronic Portal Imaging Device Based On Cherenkov Radiation

Purpose: Most electronic portal imaging devices (EPIDs) developed so far use a Cu plate/phosphor screen to absorb x rays. The main problem with this approach is that the Cu plate/phosphor screen must be thin (∼ 2 mm) in order to obtain a high spatial resolution, resulting in a low quantum efficiency (QE) for megavoltage (MV) x rays (typically 2–4%). In addition, the phosphor screen contains high atomic number (high‐Z) materials, resulting in an over‐response of the detector to low energy x rays in dosimetric verification. Our goal is to develop a new high QE MV x‐ray detector made of a low‐Z material for both geometrical and dosimetric verification in radiotherapy. Method and Materials: Our approach uses radiation‐induced light (Cherenkov radiation) in optical fibers that are made of low‐Z materials. With our approach, a thick (∼ 10–30 cm) fiber‐optic taper consisting of a matrix of optical fibers aligned with the incident x rays is used to replace the thin Cu plate/phosphor screen to dramatically increase the QE. The feasibility of this approach has been investigated using a single optical fiber embedded in a solid material. The spatial resolution expressed by the modulation transfer function (MTF) and the signal‐to‐noise ratio of the proposed detector at low doses (∼ one Linac pulse) have been measured. Results: It is predicted that, using this approach, a detective quantum efficiency (DQE) of an order of magnitude higher at zero frequency can be obtained while maintaining a reasonable MTF, as compared to current EPIDs. Conclusion: This work demonstrated the feasibility of using Cherenkov radiation for portal imaging applications [Work supported by the Individual Discovery Grant Program awarded by National Sciences and Engineering Research Council of Canada (NSERC)].

Electronic portal imaging based on Cerenkov radiation: A new approach and its feasibility

Most electronic portal imaging devices (EPIDs) developed so far use a Cu plate/phosphor screen to absorb x rays and convert their energies into light, and the light image is then read out. The main problem with this approach is that the Cu plate/phosphor screen must be thin (approximately 2 mm thick) in order to obtain a high spatial resolution, resulting in a low x-ray absorption or low quantum efficiency for megavoltage x rays (typically 2-4%). In addition, the phosphor screen contains high atomic number (high-Z) materials, resulting in an over-response of the detector to low-energy x rays in dosimetric verification. In this paper, we propose a new approach that uses Cerenkov radiation to convert x-ray energy absorbed by the detector into light for portal imaging applications. With our approach, a thick (approximately 10-30 cm) energy conversion layer made of a low-Z dielectric medium, such as a large-area, thick fiber-optic taper consisting of a matrix of optical fibers aligned with the incident x rays, is used to replace the thin Cu plate/phosphor screen. The feasibility of this approach has been investigated using a single optical fiber embedded in a solid material. The spatial resolution expressed by the modulation transfer function (MTF) and the sensitivity of the detector at low doses (approximately one Linac pulse) have been measured. It is predicted that, using this approach, a detective quantum efficiency of an order of magnitude higher at zero frequency can be obtained while maintaining a reasonable MTF, as compared to current EPIDs.

Study of a prototype high quantum efficiency thick scintillation crystal video-electronic portal imaging device

Image quality in portal imaging suffers significantly from the loss in contrast and spatial resolution that results from the excessive Compton scatter associated with megavoltage x rays. In addition, portal image quality is further reduced due to the poor quantum efficiency (QE) of current electronic portal imaging devices (EPIDs). Commercial video-camera-based EPIDs or VEPIDs that utilize a thin phosphor screen in conjunction with a metal buildup plate to convert the incident x rays to light suffer from reduced light production due to low QE (<2% for Eastman Kodak Lanex Fast-B). Flat-panel EPIDs that utilize the same luminescent screen along with an a-Si:H photodiode array provide improved image quality compared to VEPIDs, but they are expensive and can be susceptible to radiation damage to the peripheral electronics. In this article, we present a prototype VEPID system for high quality portal imaging at sub-monitor-unit (subMU) exposures based on a thick scintillation crystal (TSC) that acts as a high QE luminescent screen. The prototype TSC system utilizes a 12 mm thick transparent CsI(Tl) (thallium-activated cesium iodide) scintillator for QE=0.24, resulting in significantly higher light production compared to commercial phosphor screens. The 25 X 25 cm2 CsI(Tl) screen is coupled to a high spatial and contrast resolution Video-Optics plumbicon-tube camera system (1240 X 1024 pixels, 250 microm pixel width at isocenter, 12-bit ADC). As a proof-of-principle prototype, the TSC system with user-controlled camera target integration was adapted for use in an existing clinical gantry (Siemens BEAMVIEW(PLUS)) with the capability for online intratreatment fluoroscopy. Measurements of modulation transfer function (MTF) were conducted to characterize the TSC spatial resolution. The measured MTF along with measurements of the TSC noise power spectrum (NPS) were used to determine the system detective quantum efficiency (DQE). A theoretical expression of DQE(0) was developed to be used as a predictive model to propose improvements in the optics associated with the light detection. The prototype TSC provides DQE(0)=0.02 with its current imaging geometry, which is an order of magnitude greater than that for commercial VEPID systems and comparable to flat-panel imaging systems. Following optimization in the imaging geometry and the use of a high-end, cooled charge-coupled-device (CCD) camera system, the performance of the TSC is expected to improve even further. Based on our theoretical model, the expected DQE(0)=0.12 for the TSC system with the proposed improvements, which exceeds the performance of current flat-panel EPIDs. The prototype TSC provides high quality imaging even at subMU exposures (typical imaging dose is 0.2 MU per image), which offers the potential for daily patient localization imaging without increasing the weekly dose to the patient. Currently, the TSC is capable of limited frame-rate fluoroscopy for intratreatment visualization of patient motion at approximately 3 frames/second, since the achievable frame rate is significantly reduced by the limitations of the camera-control processor. With optimized processor control, the TSC is expected to be capable of intratreatment imaging exceeding 10 frames/second to monitor patient motion.

TH‐D‐ValB‐07: Development of a Novel High Quantum Efficiency Flat Panel Detector for Megavoltage Cone Beam CT/DT: Construction and Evaluation of a Prototype Single‐Row Detector

Purpose: Most electronic portal imaging devices (EPIDs) developed so far have low x‐ray absorption, i.e., low quantum efficiency (QE) of 2–4% for megavoltage x rays. A significant increase of QE is desirable for applications such as megavoltage cone‐beam CT (MV‐CBCT) and digital tomosynthesis (MV‐CBDT). Our overall goal is to develop a new generation of area detectors for MV‐CBCT/DT, with a QE an order of magnitude higher than that of current EPIDs and yet an equivalent spatial resolution. To this end, a novel direct‐conversion design of such a high QE detector was introduced recently [Pang and Rowlands, Med. Phys. 31, 3004 (2004)]. The purpose of this work is to construct and evaluate a prototype single‐row detector. Method and Materials: A prototype single‐row detector was constructed based on the novel design. It consists of a single custom‐made printed circuit board (with microsize cavities, charge collection electrodes and microstrip spacers) sandwiched between two identical tungsten plates. The detector array has 128 pixels each with dimensions of 0.45mm (width)×0.6mm (length)×22mm (height). The detector array was placed inside a sealed vessel filled with Xe gas (ionization medium) and then connected to a data acquisition board (XDAS, Electron Tubes Ltd.) for readout. Some fundamental imaging properties including QE, noise power spectrum (NPS) and detective quantum efficiency (DQE) were measured with a 6MV beam. Results: Phantom images were obtained using a dose as low as one Linac pulse. The QE of the prototype is ∼ 66% at 6MV. The DQE at zero frequency is more than an order of magnitude higher than that of current EPIDs. Conclusion: This work demonstrated the feasibility of our novel design for a high QE MV detector. Construction and evaluation of a prototype flat‐panel area detector is in progress.Conflict of Interest: Supported by the DOD Prostate Cancer Research Program (DAMD17‐04‐1‐0276).

Sci‐PM Sat ‐ 02: Development of a novel high quantum efficiency flat panel detector for megavoltage cone beam CT: An experimental study

Soft tissue imaging in the treatment room is one of the main challenges faced today in high precision radiotherapy. Megavoltage cone‐beam CT (MVCT) is a promising new imaging technique for image‐guided radiotherapy due to its simplicity and its potentially higher accuracy. However, currently the dose required to achieve sufficient soft tissue contrast to visualize and delineate, e.g., the prostate using MVCT is prohibitively large for daily verification. This is due to the low x‐ray absorption of the electronic portal imaging devices (EPIDs) used, i.e., low quantum efficiency (QE), which is typically on the order of 2–4%. Our overall goal is to develop a new generation of area detectors for MVCT, with a QE an order of magnitude higher than that of current EPIDs and yet an equivalent spatial resolution. With this new generation of detectors, the large dose currently required to visualize and delineate soft‐tissue targets with MVCT will be significantly reduced, and image‐guided radiotherapy using MVCT can be realized. In this work, we constructed a prototype single‐pixel detector based on the novel design introduced recently by Pang and Rowlands. Some fundamental imaging properties including the QE, spatial resolution, and sensitivity of the prototype detector were measured with a 6MV beam. It has been shown that the experimental results agree with our theoretical predictions and further development based on the novel design including the construction of a prototype area detector is warranted. This work was supported by the Department of Defense Prostate Cancer Research Program (DAMD17‐04‐1‐0276).

WE‐D‐I‐6B‐06: Development and Analysis of a High Quantum Efficiency Thick Scintillator Based Video EPID for Sub‐MU Verification Imaging

Purpose: The major limitation of current electronic portal imaging devices (EPIDs) is poor quantum efficiency (QE) with less than 5% of incident radiation being typically detected. We present a high QE prototype EPID based on a CsI(Tl) thick scintillation crystal (TSC) for high quality sub‐MU verification imaging. Method and Materials: The prototype TSC utilizes a 12 mm thick clear 17×17cm2 CsI(Tl) screen (QE=24%) coupled to a 12‐bit, 1024×1240 Plumbicon‐tube camera (250 μm pixel‐size at isocenter) installed on a clinical Siemens BEAMVIEWPLUS gantry. The initial performance of the TSC was studied with experimental measurements of spatial resolution and noise, along with a quantum accounting diagram (QAD) based on linear cascaded systems theory to predict system detective quantum efficiency (DQE). The theoretical model was used to study the effect of various components of the detector geometry on image quality, and to optimize the TSC for improved image quality. Results: The current TSC prototype with DQE(0)∼0.01 provides an order‐of‐magnitude improvement in image quality over traditional VEPIDs, while matching the performance of flat‐panel imagers at low frequencies. Since the optical geometry of the BEAMVIEW gantry was not optimized for the TSC, there was some loss in spatial resolution resulting in diminished performance at high frequency. Overall good image quality was observed even at low exposures. Conclusion: The enhanced luminescent output of the TSC results in high quality portal images even at sub‐MU dose, and significant potential for daily verification imaging without risk of additional patient dose and intra‐treatment imaging to monitor patient motion. Proposed improvements in the TSC include the optimization of the detector geometry for greater light collection efficiency and spatial resolution. Based on the QAD model, the performance of the TSC with proposed improvements is expected to improve significantly (DQE(0)∼0.1) and exceed that of current flat‐panels.

Segmented phosphors: MEMS-based high quantum efficiency detectors for megavoltage x-ray imaging

Current electronic portal imaging devices (EPIDs) based on active matrix flat panel imager (AMFPI) technology use a metal plate+phosphor screen combination for x-ray conversion. As a result, these devices face a severe trade-off between x-ray quantum efficiency (QE) and spatial resolution, thus, significantly limiting their imaging performance. In this work, we present a novel detector design for indirect detection-based AMFPI EPIDs that aims to circumvent this trade-off. The detectors were developed using micro-electro-mechanical system (MEMS)-based fabrication techniques and consist of a grid of up to approximately 2 mm tall, optically isolated cells of a photoresist material, SU-8. The cells are dimensionally matched to the pixels of the AMFPI array, and packed with a scintillating phosphor. In this paper, various design considerations for such detectors are examined. An empirical evaluation of three small-area (approximately 7 x 7 cm2) prototype detectors is performed in order to study the effects of two design parameters--cell height and phosphor packing density, both of which are important determinants of the imaging performance. Measurements of the x-ray sensitivity, modulation transfer function (MTF) and noise power spectrum (NPS) were performed under radiotherapy conditions (6 MV), and the detective quantum efficiency (DQE) was determined for each prototype SU-8 detector. In addition, theoretical calculations using Monte Carlo simulations were performed to determine the QE of each detector, as well as the inherent spatial resolution due to the spread of absorbed energy. The results of the present studies were compared with corresponding measurements published in an earlier study using a Lanex Fast-B phosphor screen coupled to an indirect detection array of the same design. The SU-8 detectors exhibit up to 3 times higher QE, while achieving spatial resolution comparable or superior to Lanex Fast-B. However, the DQE performance of these early prototypes is significantly lower than expected due to high levels of optical Swank noise. Consequently, the SU-8 detectors presently exhibit DQE values comparable to Lanex Fast-B at zero spatial frequency and significantly lower than Fast-B at higher frequencies. Finally, strategies for reducing Swank noise are discussed and theoretical calculations, based on the cascaded systems model, are presented in order to estimate the performance improvement that can be achieved through such noise reduction.

Development of high quantum efficiency flat panel detectors for portal imaging: intrinsic spatial resolution.

Recently developed flat panel detectors have been proven to have a much better image quality than conventional electronic portal imaging devices (EPIDs). They are, however, not yet the ideal systems for portal imaging application due to the low x-ray absorption, i.e., low quantum efficiency (QE), which is typically on the order of 2-4% as compared to the theoretical limit of 100%. The QE of current flat panel systems can be improved by significantly increasing the thickness of the energy conversion layer (i.e., amorphous selenium or phosphor screen). This, however, will be at the expense of a decrease in spatial resolution mainly due to x-ray scatter in the conversion layer (and also the spread of optical photons in the case of phosphor screen). In this paper, we investigate theoretically the intrinsic spatial resolution of a high QE flat panel detector with a new energy conversion layer that is much denser and thicker than that of current flat panel systems. The modulation transfer function (MTF) of the system is calculated based on a theoretical model using a novel approach, which uses an analytical expression for absorbed dose. It is found that if appropriate materials are used for the conversion layer, then the intrinsic MTF of the high QE flat panel is better than that of current EPIDs, and in addition they have a high QE (e.g., approximately 60%). Some general rules for the design of the conversion layer to achieve both high QE and high resolution as well as high DQE are also discussed.

Potential and role of a prototype amorphous silicon array electronic portal imaging device in breathing synchronized radiotherapy.

Current electronic portal imaging devices (EPID) are limited in their ability to provide direct and quick verification and monitoring of patients during both setup and treatment of breathing synchronized radiotherapy (BSRT, including breathing gated, voluntary and forced breath-hold radiotherapy treatment.) These limitations are largely due to their slow image capture rate and poor image quality. An amorphous silicon array flat panel electronic portal imaging device (si-EPID) is emerging to meet the challenge. The purpose of this study is threefold: (1) to characterize the performance of a prototype si-EPID; (2) to compare image quality against that of digitized films; and (3) to evaluate the device in terms of verification of patient setup and monitoring during BSRT. In this study a Varian prototype si-EPID detector array and Clinic accelerator at the University of California Davis Cancer Center were used for imaging. Three quality assurance phantoms: a Lutz PVC phantom, a modified "Las Vegas" phantom, and a RMI model 1151 phantom, were used to characterize the imaging system. A Rando head phantom was used for anthropomorphic imaging tests. Images were obtained with the si-EPID and a Fuji RX film in a Kodak X-Omatic cassette. To investigate the clinical application, two sets of si-EPID images were collected from a lung cancer patient during a 22 s breath-hold and normal breathing. The quality of images obtained with the fast mode was found to be comparable to that obtained with the digitized films. The images with the standard mode were found to be better than the digitized film images. With this prototype si-EPID, it is possible to collect the images at the beginning, middle, and end of each breath-hold for those patients who can hold their breath for longer than 15 s. The si-EPID images can provide a quick verification of the initial patient setup and subsequent treatment position throughout the daily fractionation.