8/3/2016. Outline. The EPID Strikes Back: Future EPID Technology and Applications. Active Matrix Flat-Panel Imagers (AMFPIs)

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1 8//6 The EPID Strikes Back: Future EPID Technology and Applications Larry E. Antonuk Department of Radiation Oncology University of Michigan, Ann Arbor Acknowledgements: Youcef El-Mohri, Qihua Zhao (U. Michigan) Some illustrations courtesy of: Gino Fallone Cross Cancer Center Josh Star-Lack, Daniel Morf Varian Medical Systems Larry Partain TeleSecurity Sciences Joerg Rottmann, Ross Berbeco Brigham and Women s Hospital Outline Challenges in improving EPID performance Pathways for improvement: Fluence Imaging - Improve DQE of MV imagers - Introduce dual-energy BEV imagers Photon Counting - Prospects for large-area photon counting array (PCA) imagers Observations Active Matrix Flat-Panel Imagers (AMFPIs) Based on D matrix of thin-film photodiodes and transistors + mm Cu ~6, photons/mev; Optical efficiency of ~6% However DQE small, ~% low compared to modern diagnostic systems: ~ to 7% Limits image quality at any dose Volumetric (CBCT) MV imaging requires doses >> cgy X-ray Cu plate Phosphor screen Pixelated array Backplane MV AMFPI Imager Antonuk et al., Med. Phys. 9(6), 99

2 8//6 X-rays Segmented Crystalline Converters X-ray source DQE QE MTF NPS Strategy: Significantly enhance QE while limiting MTF loss through use of matrices of optically isolated crystalline scintillator elements Thicknesses up to mm are practical (increasing QE up to ~8% at 6 MV) Very good element-to-element uniformity & good optical transport Design septal walls to minimize optical cross-talk and absorption Large area detectors are manufacturable Desirable properties: high density + good light yield + high refractive index + rad-tolerant Wang et al., Med. Phys. 6(), 9; Liu et al., Phys. Med. Biol. 7, Linear, Piecewise-Focused Strip Detector Maximum remaining beam divergence = degree (SDD = cm) Pixelated Submatrices 6⁰ Wa 6⁰ ⁰ ⁰ ⁰ ⁰ W Wb Wb Wb Wa Wa Piecewise-Focused Fiber Optic Faceplate (FOP) Amorphous Silicon Flat Panel Imager Fiber optics Six CdWO (k photons/mev) subarrays with.78 mm pitch, BGO subarray (8k photons/mev),.7 mm pitch Total: 8,7 elements. DQE() ~% Slide courtesy of Josh Star-Lack & Daniel Morf; Med. Phys. (9), Performance Improvement with a D Segmented Scint. BGO (8k photons/mev) prototype: mm thick;.6 mm pitch Optical efficiency ~% DQE() ~ % Quantum limited down to ~. cgy Wang et al. Med. Phys. 6, 9

3 CNR 8//6 Empirical Demonstration of CBCT Improvement BGO ~ cgy - - BGO, cgy Conv., FastB, cgy Relative Electron Density Conv. ~ cgy Conv. ~6 cgy El-Mohri et al Phys. Med. Biol. 6, 9 () AS Multi-Layer Imager control board System AS MLI Layers Thickness 9 mm mm DQE () ~.% ~6.7% layer of readout modules layer of gate driver modules Courtesy of Joerg Rottmann, Ross Berbeco, Josh Star-Lack AS Multi-Layer Imager control board System AS MLI Layers Thickness 9 mm mm DQE () ~.% ~6.7% layer of readout modules layer of gate driver modules Scintillator Density Ref. Index Light Yield Manuf. Cost (g/cm ) Per MeV LKH-.8.6 k $ BGO 7.. 8k $$$ CdWO 7.9. k $$$.7. 6k $ Courtesy of Joerg Rottmann, Ross Berbeco, Josh Star-Lack

4 Relative Fluence 8//6 kv Imagers in the Treatment Room X-ray attenuation dominated by photoelectric High-contrast projection & volumetric images Constraints: - kv beam direction not parallel to BEV - photon starvation from implants leads to artifacts MV MV kv kv X-ray Sources Integrated into Treatment Head 9 source TumoTrak TM kv x-ray design Partain et al., SPIE 978, Improve Contrast Dual-Energy Imagers Design segmented scintillator for multiple energies For example: Provide high-dqe MV performance Preserve performance of kv imaging systems 6 MV kvp.. (Liu et al. Med. Phys., )

5 Relative Fluence Relative Fluence 8//6 Improve Contrast Dual-Energy Imagers Design segmented scintillator for multiple energies For example: Provide high-dqe MV performance Preserve performance of kv imaging systems Favorable design: cm BGO;.8 mm element pitch + rear illumination + black reflector + high-res & low-res operation for kv and MV 6 MV kvp.. (Liu et al. Med. Phys., ) Improve Contrast Dual-Energy Imagers Design segmented scintillator for multiple energies For example: Provide high-dqe MV performance Preserve performance of kv imaging systems Favorable design: cm BGO;.8 mm element pitch + rear illumination + black reflector + high-res & low-res operation for kv and MV cm BGO MV ~ cgy sim. cm BGO kv.9 cgy sim 6µm CsI:Tl kv.9 cgy sim 6 MV kvp.. 6µm CsI:Tl kv.9 cgy meas. (Liu et al. Med. Phys., ) Improve Contrast Photon Counting Array (PCA) Conventional Imager Simple AMFPI backplane TFT + storage capacitor Integrates deposited energy No energy resolution PD TFT AMFPI Pixel Circuit 8 µm

6 Relative Fluence Relative Fluence Relative Fluence 8//6 Improve Contrast Photon Counting Array (PCA) Conventional Imager Simple AMFPI backplane TFT + storage capacitor Integrates deposited energy No energy resolution PCA Imager components Counts individual x-rays kev -level energy resolution PD TFT AMFPI Pixel Circuit 8 µm PCA Pixel Circuit mm PCA: Energy Windowing Entails measuring & recording information from individual x-rays Increases signal-to-noise Potential for improvements in contrast & tumor identif. & tracking But requires complex detector backplane Threshold Th Th.. Conventional Threshold Scatter rejection. Thresholds Diagnostic component -Threshold / -Window PCA Pixel Circuit Comparator (Threshold #) Clock-generator Input Amplifier Counter Output Comparator (Threshold #) µm minimum feature size 8 metal layers µm pixel pitch µm 6

7 Relative Fluence 8//6 Count Rate Considerations Treatment machines provide radiation in form of trains of brief, high intensity pulses (e.g., ~6, ~ µs pulses/mu) Results in high instantaneous fluence rates ~6 9 x-rays / second /. mm pixel at cm SDD much higher than for other common x-ray procedures and cmpd to the count rate capabilities of st gen. poly-si PCA ~6 counts / second for a. mm pixel Mammography Radiography Therapy- 6 Co Therapy-Linac Fluoroscopy Instantaneous Incident X-ray Count Rate (counts/sec/pixel) Summary Excellent prospects for significantly improved real-time tumor tracking Near-term: Practical implementation of segmented scintillators gradually increasing MV EPID DQE eventually up to ~% Mid-term: Introduction of dual-energy imagers operated with with BEV kv, low-mv or flattening-filter-free beams Long-term: Photon counting detectors with multi-energy-windowing offering new contrast capabilties with a MV or kv beam.. 6 MV 6 MV FFF. MV kvp 7

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9 8//6 The EPID Strikes Back: Future EPID Technology and Applications Larry E. Antonuk Department of Radiation Oncology University of Michigan, Ann Arbor Acknowledgements: Youcef El-Mohri, Qihua Zhao (U. Michigan) Some illustrations courtesy of: Gino Fallone Cross Cancer Center Josh Star-Lack, Daniel Morf Varian Medical Systems Larry Partain TeleSecurity Sciences Joerg Rottmann, Ross Berbeco Brigham and Women s Hospital Linear Segmented Scintillator Array Kirvan & Fallone Cross Cancer Centre Prototype benchtop MVCT system: mm pitch mm CdWO crystals (k/mev) coupled to a photodiode array, ~9% DQE 6 6 element modules detection area of 6 mm mm focused in an arc with radius of 9. cm Picture courtesy of Gino Fallone; Med. Phys. 7(), 9