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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology. RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY. L16.1: Optimization of protection in fluoroscopy. Introduction. Subject matter : fluoroscopy equipment and accessories

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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

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  1. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology RADIATION PROTECTION INDIAGNOSTIC ANDINTERVENTIONAL RADIOLOGY L16.1: Optimization of protection in fluoroscopy

  2. Introduction • Subject matter : fluoroscopy equipment and accessories • Different electronic component contribute to the image formation in fluoroscopy. • Good knowledge of their respective role and consistent Quality Control policy are the essential tools for an appropriate use of such equipment. 16.1: Optimization of protection in fluoroscopy

  3. Topics • Example of fluoroscopy systems • Image intensifier component and parameters • Image intensifier and TV system 16.1: Optimization of protection in fluoroscopy

  4. Overview • To become familiar with the components of the fluoroscopy system (design, technical parameters that affect the fluoroscopic image quality and Quality Control). 16.1: Optimization of protection in fluoroscopy

  5. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 16.1: Optimization of protection in fluoroscopy Topic 1: Example of fluoroscopy systems

  6. Fluoroscopy: a “see-through” operation with motion • Used to visualize motion of internal fluid, structures • Operator controls activation of tube and position over patient • Early fluoroscopy gave dim image on fluorescent screen • Modern systems include image intensifier with television screen display and choice of recording devices 16.1: Optimization of protection in fluoroscopy

  7. Fluoroscopy • X-ray transmitted trough patient • The photographic plate replaced by fluorescent screen • Screen fluoresces under irradiation and gives a live image • Older systems— direct viewing of screen • Screen part of an Image Intensifier system • Coupled to a television camera • Radiologist can watch the images “live” on TV-monitor; images can be recorded • Fluoroscopy often used to observe digestive tract • Upper GI series, Barium Swallow • Lower GI series Barium Enema 16.1: Optimization of protection in fluoroscopy

  8. Direct Fluoroscopy: obsolete In older fluoroscopic examinations radiologist stands behind screen and view the picture Radiologist receives high exposure; despite protective glass, lead shielding in stand, apron and perhaps goggles Main source of staff exposure is NOT the patient but direct beam 16.1: Optimization of protection in fluoroscopy

  9. Older Fluoroscopic Equipment(still in use in some countries) Staff in DIRECT beam 16.1: Optimization of protection in fluoroscopy

  10. Direct fluoroscopy • AVOID USE OF DIRECT FLUOROSCOPY • Directive 97/43Euratom Art 8.4. • In the case of fluoroscopy, examinations without an image intensification or equivalent techniques are not justified and shall therefore be prohibited. • Direct fluoroscopy will not comply with BSS • “… performance of diagnostic radiography and fluoroscopy equipment and of nuclear medicine equipment should be assessed on the basis of comparison with the diagnostic reference levels 16.1: Optimization of protection in fluoroscopy

  11. Modern Image Intensifier based fluoroscopy system 16.1: Optimization of protection in fluoroscopy

  12. Display control Automatic control display brightness radiation dose film exposure Timer Modern fluoroscopic system components 16.1: Optimization of protection in fluoroscopy

  13. Different fluoroscopy systems • Remote control systems • Not requiring the presence of medical specialists inside the X Ray room • Mobile C-arms • Mostly used in surgical theatres. 16.1: Optimization of protection in fluoroscopy

  14. Different fluoroscopy systems • Interventional radiology systems • Requires specific safety considerations. In interventional radiology the physician can be near the patient during the procedure. • Multipurpose fluoroscopy systems • Can be used as a remote control system or as a system to perform simple interventional procedures 16.1: Optimization of protection in fluoroscopy

  15. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 16.1: Optimization of protection in fluoroscopy Topic 2: Image Intensifier component and parameters

  16. The image intensifier (I.I.) I.I. Input Screen Electrode E1 Electrode E2 Electrode E3 Electrons Path I.I.Output Screen Photocathode + 16.1: Optimization of protection in fluoroscopy

  17. Image intensifier systems 16.1: Optimization of protection in fluoroscopy

  18. Image intensifier component • Input screen: conversion of incident X Rays into light photons (CsI) • 1 X Ray photon creates  3,000 light photons • Photocathode: conversion of light photons into electrons • only 10 to 20% of light photons are convertedinto photoelectrons • Electrodes : focusing of electrons onto the output screen • electrodes provide the electronic de-magnification • Output screen: conversion of accelerated electrons into light photons 16.1: Optimization of protection in fluoroscopy

  19. Image intensifier parameters (I) • Conversion coefficient (Gx): the ratio of the output screen brightness to the input screen dose rate— Gx = cd.m-2Gys-1 • Gx depends on : • the applied tube potential • the diameter ()of the input screen • I.I. input screen ()of 22 cm  Gx = 200 • I.I. input screen ()of 16 cm  Gx = 200 x (16/22)2 = 105 • I.I. input screen ()of 11 cm  Gx = 200 x (11/22)2 = 50 16.1: Optimization of protection in fluoroscopy

  20. Image intensifier parameters (II) • Brightness Uniformity: the input screen brightness may vary from the center of the image intensifier to the periphery Uniformity = (Brightness(c) - Brightness(p)) x 100 / Brightness(c) • Geometrical distortion: all X Ray image intensifiers exhibit some degree of pincushion distortion. This is usually caused by a local magnetic field. 16.1: Optimization of protection in fluoroscopy

  21. Image distortion 16.1: Optimization of protection in fluoroscopy

  22. Image intensifier parameters (III) • Spatial resolution limit: the value of the highest spatial frequency that can be visually detected • it provides a sensitive measure of the state of focusing of a system • it is quoted by manufacturer and usually measured optically and under fully optimized conditions. This value correlates well with the high frequency limit of the Modulation Transfer Function (MTF) • it can be assessed with a resolution pattern which contains several sets of patterns at various frequency 16.1: Optimization of protection in fluoroscopy

  23. Line pair gauges 16.1: Optimization of protection in fluoroscopy

  24. Line pair gauges GOOD RESOLUTION POOR RESOLUTION 16.1: Optimization of protection in fluoroscopy

  25. Image intensifier parameters (IV) • Overall image quality - threshold contrast-detail detection • X Ray, electrons and light scatter process in an I.I. can result in a significant loss of contrast of radiological detail. The degree of contrast exhibited by an I.I. is defined by the design of the image tube and coupling optics. • Spurious sources of contrast loss are: • accumulation of dust and dirt on the various optical surfaces • reduction in the quality of the vacuum • aging process (deterioration of phosphor screen) • Sources of noise are: • X Rayquantum mottle • photon-conversion processes, film granularity, film processing 16.1: Optimization of protection in fluoroscopy

  26. Image intensifier parameters (V) • Overall image quality can be assessed using a suitable threshold contrast-detail detectability test object which comprises an array of disc-shaped metal details and gives a range of diameters and X Ray transmission • Sources of image degradation such as contrast loss, noise and unsharpness limit the number of details that are visible. • If performance is regularly monitored using this test, any sudden or gradual deterioration in image quality can be detected as a reduction in the number of low contrast or small details. 16.1: Optimization of protection in fluoroscopy

  27. Overall image quality 16.1: Optimization of protection in fluoroscopy

  28. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 16.1: Optimization of protection in fluoroscopy Topic 3: Image Intensifier and TV system

  29. Image intensifier - TV system • Output screen image can be transferred to different optical displaying systems: • conventional TV • 262,5 odd lines and 262,5 even lines generating a full frame of 525 lines (in USA) • 625 lines and 25 full frames/s up to 1000 lines (in Europe) • interlaced mode is used to prevent flickering • Cine film • 35 mm film format: from 25 to 150 images/s • photography • roll film,105 mm: max 6 images/s • film of 100 mm x 100 mm 16.1: Optimization of protection in fluoroscopy

  30. kV X Ray TUBE PM REFERENCE kV FILM CONTROLLER VIDICON GENERAL SCHEME OF FLUOROSCOPY 16.1: Optimization of protection in fluoroscopy

  31. kV X Ray TUBE CINE MODE I3 I2 CONTROLLER I1 C1 C2 Ref. PM FILM VIDICON 16.1: Optimization of protection in fluoroscopy

  32. Type of TV camera • VIDICON TV camera • improvement of contrast • improvement of signal to noise ratio • high image lag • PLUMBICON TV camera (suitable for cardiology) • lower image lag (follow up of organ motion) • higher quantum noise level • CCD TV camera (digital fluoroscopy) • digital fluoroscopy spot films are limited in resolution, since they depend on the TV camera (no better than about 2 c/mm) for a 1000 line TV system 16.1: Optimization of protection in fluoroscopy

  33. TV camera and video signal (I) • The output phosphor of the image intensifier is optically coupled to a television camera system. A pair of lenses focuses the output image onto the input surface of the television camera. • Often a beam splitting mirror is interposed between the two lenses. The purpose of this mirror is to reflect part of the light produced by the image intensifier onto a 100 mm camera or cine camera. • Typically, the mirror will reflect 90% of the incident light and transmit 10% onto the television camera. 16.1: Optimization of protection in fluoroscopy

  34. TV camera and video signal (II) • Older fluoroscopy equipment will have a television system using a camera tube. • The camera tube has a glass envelope containing a thin conductive layer coated onto the inside surface of the glass envelope. • In a PLUMBICON tube, this material is made out of lead oxide, whereas antimony trisulphide is used in a VIDICON tube. 16.1: Optimization of protection in fluoroscopy

  35. Deviation coil Steering coils Alignement coil Photoconductive layer Accelarator grids Focussing optical lens Input plate Control grid Electron beam Iris Video Signal Electron gun Signal electrode Field grid Electrode Photoconductive camera tube 16.1: Optimization of protection in fluoroscopy

  36. TV camera and video signal (III) • The surface of the photoconductor is scanned with an electron beam and the amount of current flowing is related to the amount of light falling on the television camera input surface. • The scanning electron beam is produced by a heated photocathode. Electrons are emitted into the vacuum and accelerated across the television camera tube by applying a voltage. The electron beam is focussed by a set of focussing coils. 16.1: Optimization of protection in fluoroscopy

  37. TV camera and video signal (IV) • This scanning electron beam moves across the surface of the TV camera tube in a series of lines. • This is achieved by a series of external coils, which are placed on the outside of the camera tube. In a typical television system, the image is formed from a set of 625 lines. On the first pass the set of odd numbered lines are scanned followed by the even numbers. This type of image is called interlaced. • The purpose of interlacing is to prevent flickering of the television image on the monitor, by increasing the apparent frequency of frames (50 half frames/second). • In Europe, 25 frames are updated every second. 16.1: Optimization of protection in fluoroscopy

  38. Different types of scanning 11 1 INTERLACED SCANNING 13 12 3 2 15 14 625 lines in 40 ms i.e. : 25 frames/s 5 4 17 16 7 6 19 18 9 8 21 20 10 1 2 3 4 5 6 7 PROGRESSIVE SCANNING 8 9 10 11 12 13 14 15 16 17 18 16.1: Optimization of protection in fluoroscopy

  39. TV camera and video signal (V) • On most fluoroscopy units, the resolution of the system is governed by the number of lines of the television system. • Thus, it is possible to improve the high contrast resolution by increasing the number of television lines. • Some systems have 1,000 or 2,000 16.1: Optimization of protection in fluoroscopy

  40. TV camera and video signal (VI) • Many modern fluoroscopy systems used CCD (charge coupled devices) TV cameras. • The front surface is a mosaic of detectors from which a signal is derived. • The video signal comprises a set of repetitive synchronizing pulses. In between there is a signal that is produced by the light falling on the camera surface. The synchronizing voltage is used to trigger the TV system to begin sweeping across a raster line. • Another voltage pulse is used to trigger the system to start rescanning the television field. 16.1: Optimization of protection in fluoroscopy

  41. Schematic structure of a charged couple device (CCD) 16.1: Optimization of protection in fluoroscopy

  42. TV camera and video signal (VII) • A series of electronic circuits move the scanning beams of the TV camera and monitor in synchronism. This is achieved by the synchronizing voltage pulses. The current, which flows down the scanning beam in the TV monitor, is related to that in the TV camera. • Consequently, the brightness of the image on the video display is proportional to the amount of light falling on the corresponding position on the TV camera. 16.1: Optimization of protection in fluoroscopy

  43. TV image sampling IMAGE 512 x 512 PIXELS HEIGHT 512 WIDTH 512 ONE LINE VIDEO SIGNAL (1 LINE) 64 µs IMAGE LINE 52 µs SYNCHRO 12 µs DIGITIZED SIGNAL LIGHT INTENSITY SAMPLING SINGLE LINE TIME 16.1: Optimization of protection in fluoroscopy

  44. Digital radiography principle ANALOGUE SIGNAL I t ADC Memory DIGITAL SIGNAL Iris Clock t See more in Lecture L20 16.1: Optimization of protection in fluoroscopy

  45. Digital Image recording • In newer fluoroscopic systems film recording is replaced with digital image recording. • Digital photospots are acquired by recording a digitized video signal and storing it in computer memory. • Operation— fast, convenient. • Image quality can be enhanced by application of various image processing techniques, including window-level, frame averaging, and edge enhancement. • But the spatial resolution of digital photospots is less than that of film images. 16.1: Optimization of protection in fluoroscopy

  46. TV camera and video signal (VIII) • It is possible to adjust the brightness and contrast settings of the TV monitor to improve the quality of the displayed image. 16.1: Optimization of protection in fluoroscopy

  47. Summary • The main components of the fluoroscopy imaging chain and their role are explained: • Image Intensifier • Associated image TV system 16.1: Optimization of protection in fluoroscopy

  48. Where to Get More Information • The Essential Physics of Medical Imaging. JT Bushberg, JA Seibert, EM Leidholdt, JM Boone. Lippincott Williams & Wilkins, Philadelphia, 2011 • The physics of diagnostic imaging, Dowsett et al, Hodder Arnold, 2006 • Interventional Fluoroscopy: Physics, Technology, Safety, S. Balter, Wiley-Liss, 2001 16.1: Optimization of protection in fluoroscopy

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