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Fluoroscopy in Specials: Flat Detector Vs. Image Intensification

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Fluoroscopy in Specials: Flat Detector Vs. Image Intensification Fluoroscopy in Specials: Flat Detector vs. Image Intensification William R. Geisler M.S. DABR Department of Radiology/Medical Physics Fluoroscopy in Specials: Flat Detector vs. Image Intensification • Acknowledgements: – Perry Juel, SIEMENS – Dean Rindlisbach, PHILIPS – John Rowlands, Univ. Toronto – Phil Rauch, Henry Ford Health System Fluoroscopy in Specials: Digital Flat Detector vs. Image Intensification • Differences and Similarities with image intensification (II) and flat detector (FD) – Brief review II technology – Overview FD technology • Artifacts associated with II and FD Comparison of Image Intensifier and Digital Flat Panel Detector Review II Tube • Vacuum tube • Curvature at input window required to allow focusing of e- and minimize stress from vacuum • Thin (~1 mm) layer of Aluminum at input housing – Attenuates x-rays striking input phosphor • Output phosphor coupled to camera or CCD CsI Input Phosphor/ZnCdS Output Phosphor X-ray Ö Visible Photon Ö e- Ö Visible Photon • X-ray exits patient and interacts with CsI (200 - 400 μm thick for II) • X-ray converted to visible light • light travels along CsI and interacts with photocathode Bushberg 2nd ed fig. 9-3 • photocathode emits e- • 25 – 35 kV accelerates e-sto output phosphor (ZnCdS, < 10 μm) to increase light output Bushberg 2nd ed fig. 9-6 II Gain Factors • Flux Gain (FG) – Increase no. light photons emitted from output phosphor compared to input phosphor; 50 - 100 typical • Minification Gain (MG) – (dia. Input/dia. Output)2 – e.g., 23 cm to 2.5 cm => 85 • Brightness Gain – = FG × MG – Typical overall gain 5,000:1 – BIG advantage of II over FD is brightness gain factor – FD uses pixel binning and image frame averaging to Carlton, Adler 4th ed fig 40-2 reduce noise Flat Detector Design • FD also uses CsI to convert x-rays to visible light – Use of scintillator only similarity of FD to II – CsI used due to Si detector being far more (Cesium Iodide) sensitive to visible light than to x-rays – FD does not have e- acceleration or minification gain to reduce noise • However can combine adjacent pixels (called pixel binning) to reduce noise • Binning also permits fast frame rate imaging – Additionally FD does not have Al housing so less attenuation of x-rays vs II – Additionally, CsI thicker for FD compared to II (more x-rays absorbed for FD vs II) • Light sensor (photodiode) behind CsI adjusts technique Flat Detector Design FD Reading Image Info • Image “stored” in pixels on detector as electrical charge • Each line of data read sequentially • Detector cannot acquire another image until all data is read • Time needed to read data part of reason for need to bin pixels for fast frame rate • HOWEVER, not all of the information/charge is removed – Combine two rows of pixels into one and read as single during read process line of data – If not removed will see “ghost” – Binning pixels also results in image less noise – Bright light removes residual charge (every 30 msec for Philips) FD Data Collection For higher frame rate imaging require combining adjacent pixels; less information required to be read by electronics II Magnification • II magnification done by moving out focal point of e-s inside II – Now central region of e-s strike output phosphor and outer region e-s miss output phosphor – Results in larger image and increased resolution – Fewer e-s striking output also results in dimmer image – Increase technique required to maintain same amount of light output – That is, technique increased to keep magnification noise level same as unmagnified noise level – Technique increases with area (diameter2) FD magnification • FD magnification done one of two ways, depending on design of unit 1. Electronically zoom image; no actual increase in resolution – mag imaging will still be binned (if binning available) – No true increase in resolution, image displayed larger electronically, similar to enlarging digital picture 2. Unbin pixels and have true increased resolution Resolution Comparison II vs FD Allura 9 (23 cm II) Allura FD 10 (25 cm) Mode LP/mm Mode LP/mm 23 cm 2.2 25 cm 3.4 (diagonal) 17 cm 2.8 20 cm 3.4 13 cm 3.1 15 cm 3.4 ***Philips Allura FD 10 does not bin pixels*** Effect of Pixel Binning This is a portion of a Mars Observer Camera image of gullies cut into a crater wall. The portion shown is about 1 km across. The left hand image has not been binned. The right hand image has been binned 4 x 4. Note that binning reduces the No binning 4x4 binning spatial resolution and the finest details can no longer been seen. Specifications Philips FD10, FD20 and Siemens dBA Philips FD10/20 Siemens Axiom Artis dBA • FD10 • 2480 x 1920, 48 cm in – 1024 x 1024 diagonal direction – No binning of pixels • Bins pixels until max mag – Mag modes uses fewer pixels but no change in resolution (zoom 3 = 22 cm FOV) • FD20 – Overview, zoom 1, zoom 2 – 2480 x 1920, 48 cm in diagonal 0.5 – 7.5 f/sec 2K x 2K direction – All Zoom Modes: 0.5-7.5 f/sec – Bins pixels until max mag – 10/15/30 f/sec with High Speed – Max mag provides highest Acquisition in 1024 x 1024 resolution (= 22 cm FOV) matrix – 0.5 – 6 f/sec 2K x 2K – 15 and 30 fps 1Kx1K Magnification and Dose • For image intensifier dose increases with area (or diameter2) • For digital flat panel, how magnification affects dose slightly more complicated – If magnification unbins pixels then dose increases to maintain same S/N per pixel – If magnification is actually electronic mag with no actual increase in resolution, dose will increase—based on linear distance (diagonal), not area [not all vendors may follow this convention] – Why? Both unmagnified and magnified image will have same S/N but apparent noise (noise/mm2 as viewed on monitor) will increase Electronic Magnification and Dose Identical images, second image electronically magnified. Both images have same SNR (largest disc SNR = 10) but different apparent noise characteristics DQE • For radiographic applications the DQE of digital flat panels is typically greater than for screen-film • For fluoroscopic applications DQE is more difficult to compare – Pixels have varying sensitivity; radiographic FP will adjust for pixel response – With fluoroscopic imaging ability to correct for varying pixel sensitivity depends on frame rate • Low frame rate FD DQE > II DQE • High frame rate FD DQE ? II DQE – At high frame rates it is possible that for FD the DQE per pixel is lower—however since high frame rate required binning pixels, this would off-set decreased DQE • Additionally, DF rooms typically use greater filtration at x-ray tube as compared to II rooms, resulting in the possibility of greater radiation exposure reaching detector with concurrent reduction in patient dose Dynamic Range • Dynamic range of flat panel is greater than an image intensifier • Issues such as burnout (blooming) and blackout (saturation into black) regions in image is not as significant an issue with FD as it is with II Images taken from Jack Cusma AAPM 2006 Image Artifacts for II and FD • II image distortion (pincushion, vignetting and S distortion) caused by – curvature of II surface – e-s repel each other – stray magnetic fields affecting e- path • FD does not suffer from these types of artifacts • No. FD also has inherent • Does this mean FD images artifacts are artifact free? FD Artifacts 1 mR • All large area digital detectors have defects that results in artifacts – Raw image unsuitable for clinical display • Multiple causes – Different pixels have different sensitivities to radiation FD Artifacts • Can correct for different pixel sensitivities – Referred to as pixel gain – Expose FD to uniform 250 275 250 radiation and measure response of each pixel 250225 250 – Correct pixel sensitivity 300 200 275 during patient imaging FD Image Artifacts Correction • All large area digital detectors suffer from artifacts – Pixels can have different sensitivity to radiation (gain factors) – Pixels can be unresponsive – Data line drop off – Clusters of adjacent pixels – …. • These artifacts can be corrected by software • Illustration of artifacts and corrections based on 1st generation experimental FD FD Image Artifacts Correction FD Image Artifacts Correction FD Image Artifacts Correction .
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