10/15/2012 SECTION III - CHAPTER 6 DIGITAL FLUOROSCOPY RADT 3463 COMPUTERIZED IMAGING

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1 RADT COMPUTERIZED IMAGING Section III: Chapter 6 RADT 3463 Computerized Imaging 1 SECTION III - CHAPTER 6 DIGITAL FLUOROSCOPY RADT 3463 COMPUTERIZED IMAGING Section III: Chapter 6 RADT 3463 Computerized Imaging 3 1

2 ACKNOWLEDGEMENTS This presentation is a professional collaboration of development time prepared by: Rex Christensen Terri Jurkiewicz and Diane Kawamura References and images are gathered from many sources including those copyrighted by Elsevier / Mosby Publishing as they appear in: Bushong, S. C. (2008). Radiologic science for technologists: Physics, biology, and protection, 9 th ed. Chapter 27, St. Louis, MO: Elsevier Mosby. Seeram, E. Digital Radiography: An Introduction, Delmar Cengage Learning. Section III: Chapter 6 RADT 3463 Computerized Imaging 4 DEFINITION - FLUOROSCOPY An imaging modality that produces dynamic or moving images, displayed in real time. Study of anatomical structures and the motion of organs and contrast media in organs and blood vessels. Identifies the function of organs and blood vessels. Section III: Chapter 6 RADT 3463 Computerized Imaging 5 CONVENTIONAL FLUOROSCOPY PRINCIPLES X-ray source -> Image intensifier -> Video camera -> Television monitor (analog signal) 30 frames per second (fps) Section III: Chapter 6 RADT 3463 Computerized Imaging 6 2

3 Section III: Chapter 6 RADT 3463 Computerized Imaging 7 X-RAY TUBE AND GENERATOR Continuous ms Pulsed lower patient dose (3-10 ms/image), less blurring High frequency generators Low ma (1-3 ma) and high kv ( kv) Switch from fluoroscopic mode to radiographic mode (spot films, radiographic cassettes) Section III: Chapter 6 RADT 3463 Computerized Imaging 8 IMAGE INTENSIFICATION The brightening of the fluoroscopic image using an image intensifier. Section III: Chapter 6 RADT 3463 Computerized Imaging 9 3

4 IMAGE INTENSIFIER TUBE Section III: Chapter 6 RADT 3463 Computerized Imaging 10 IMAGE INTENSIFICATION Parts of the image intensifier (enclosed in a vacuum tube) include: Input screen (x-ray to light) Phospher Cesium Iodide CsI Photocathode (photoelectrons) Phospher - Antimony Cesium SbCs Electrostatic lens (focus) kv Output screen (light - Increase) Phospher Zinc Cadmium Sulfide ZnCdS Section III: Chapter 6 RADT 3463 Computerized Imaging 11 IMAGE INTENSIFICATION Section III: Chapter 6 RADT 3463 Computerized Imaging 12 4

5 BRIGHTNESS GAIN (BG) Increase in brightness from the input phospher to the output phospher (5,000 30,000) BG = Minification Gain (MG) x Flux Gain (FG) MG = Diameter of the input screen 2 Diameter of the output screen 2 FG= Number of light photons at the output screen Number of light photons at the input screen BUT. Section III: Chapter 6 RADT 3463 Computerized Imaging 13 CONVERSION FACTOR (CF) The Brightness Gain (BG) method has been replaced by the Conversion Factor (CF) This is the light gain at the output phospher CF = Luminance of the output screen Exposure rate at the input screen Luminance is measured in candela/square meter (Cd/m2) Exposure rate is measured in milliroentgens/second (mr/sec) Conversion Factor (CF) ranges between The higher being more efficient. Section III: Chapter 6 RADT 3463 Computerized Imaging 14 FLUX GAIN 1000 light photons at the photocathode from 1 x-ray photon photocathode decreased the # of ë s so that they could fit through the anode Output phosphor = 3000 light photons (3 X more than at the input phosphor!) This increase is called the flux gain Section III: Chapter 6 RADT 3463 Computerized Imaging 15 5

6 Magnification Magnification enhances the image to help improve diagnostic interpretation. Improves spatial resolution Controlled by the input screen diameter Section III: Chapter 6 RADT 3463 Computerized Imaging 16 Multi-field Units Allows selection of different input phosphor sizes Types of multi-field units: Dual focus - 9/6 inches Tri focus - 12/9/6 inches Smaller input magnifies output by moving focal point away from output Greater voltage to electrostatic lenses Increases acceleration of electrons Shifts focal point away from anode Requires more x-rays to maintain brightness Section III: Chapter 6 RADT 3463 Computerized Imaging 17 Intensifier Format and Modes Note focal point moves farther from output in mag mode Section III: Chapter 6 RADT 3463 Computerized Imaging 18 6

7 Magnification and Patient Dose Magnification is used to enlarge small structures or to penetrate through larger parts Patient dose is INCREASED in the magnification mode Dose is dependent on the size of the Input Phosphor (IP) FORMULA: Section III: Chapter 6 RADT 3463 Computerized Imaging 19 MAGNIFICATION MODE FORMULA IP OLD SIZE IP NEW SIZE = % mag Section III: Chapter 6 RADT 3463 Computerized Imaging 20 PT dose in MAG MODE IP OLD SIZE 2 IP NEW SIZE 2 = (x) pt dose Section III: Chapter 6 RADT 3463 Computerized Imaging 21 7

8 Image Quality Characteristics Spatial Resolution Contrast Ratio Noise Artifacts Section III: Chapter 6 RADT 3463 Computerized Imaging 22 Spatial Resolution The ability to resolve fine details of the object being viewed (patient) Input screen is convex better resolution in the center Resolution gets better as the Input diameter gets smaller Measured in line pairs per mm (lp/mm) - how close lines can be to each other and still be visibly resolved. The more line pairs the better the resolution (spatial frequency). Section III: Chapter 6 RADT 3463 Computerized Imaging 23 Spatial Frequency One line pair = the line and an interspace the same width as the line Section III: Chapter 6 RADT 3463 Computerized Imaging 24 8

9 Spatial Frequency An imaging system with high spatial frequency has better spatial resolution APPROXIMATE SPATIAL RESOLUTION - MEDICAL IMAGING SYSTEMS Gamma camera 0.1 lp/mm Magnetic resonance imaging 1.5 lp/mm Computed tomography 1.5 lp/mm Diagnostic sonography 2 lp/mm Fluoroscopy 3 lp/mm Digital radiography 4 lp/mm Computed radiography 6 lp/mm Radiography 8 lp/mm Mammography 15 lp/mm Section III: Chapter 6 RADT 3463 Computerized Imaging 25 Line pair gauges GOOD RESOLUTION POOR RESOLUTION Section III: Chapter 6 RADT 3463 Computerized Imaging 26 Pixel Size = Image intensifier Size / Matrix Spatial Resolution A 1024 x 1024 image matrix is sometimes described as a 1000-line system Spatial resolution (how much information is stored within the space given) is determined by both the image matrix and by the size of the image intensifier. Section III: Chapter 6 RADT 3463 Computerized Imaging 27 9

10 Noise Low ma produces high amount of noise If you increase the ma to minimize the noise you increase patient dose. Section III: Chapter 6 RADT 3463 Computerized Imaging 28 How Noise Effects Contrast Section III: Chapter 6 RADT 3463 Computerized Imaging 29 Artifacts Image lag continuous emission of light from the screen after the radiation beam has been turned off. Vignetting - reduction of an image's brightness or saturation at the periphery compared to the image center. Veiling glare light is scattered in the intensifier tube Distortion artifacts: Pincushion S distortion Barrel Distortion Section III: Chapter 6 RADT 3463 Computerized Imaging 30 10

11 Artifacts Vignetting FALL-OFF OF BRIGHTNESS AT PERIPHERY (EDGES) OF THE IMAGE Section III: Chapter 6 RADT 3463 Computerized Imaging 31 Artifacts Veiling glare Scatter in the form of x-rays, light & electrons can reduce contrast of an image intensifier tube. Section III: Chapter 6 RADT 3463 Computerized Imaging 32 Artifacts Distortion Geometric problems in shape of input screen Pincushion rectangular grid used with a round input screen S distortion electromagnetic field is close to the intensifier Section III: Chapter 6 RADT 3463 Computerized Imaging 33 11

12 Fluoroscopic Television Chain Section III: Chapter 6 RADT 3463 Computerized Imaging 34 Fluoroscopic Television Chain Video Camera Television pick-up camera Charged Couple Device (CCD) more common More compact No image lag No spatial distortion High dynamic range 3000:1 Connected to the image intensifier by an image distributor Converts light to an electrical signal Display Monitor Cathode Ray Tube (CRT) Liquid Crystal Display (LCD) Scanning: Interlaced - odd/even Progressive sequentially (important in digital fluoro) Section III: Chapter 6 RADT 3463 Computerized Imaging 35 VIDEO CAMERA - CHARGE-COUPLED DEVICE Charge-coupled device is mounted to the output phosphor of the image-intensifier tube and is coupled through fiber optics or a lens system Section III: Chapter 6 RADT 3463 Computerized Imaging 36 12

13 DISPLAY MONITOR Conventional Fluoroscopy System Interlaced Mode Digital Fluoroscopy System Progressive Mode Signal-to-noise ratio 200:1 Signal-to-noise ratio 1000:1 Conventional Fluoroscopy System Usually a 525-line system Limitations restrict application in digital techniques 1. Interlaced mode of reading the target of the TV camera can significantly degrade a digital image 2. Conventional TV camera tubes are relatively noisy (compare signal-to-noise ratios on table) Section III: Chapter 6 RADT 3463 Computerized Imaging 37 DISPLAY MONITOR Interlaced Mode 2 fields 525-line system/2 = 262½ lines 262½ lines are read individually in 1/60 s (17 ms) to form a 525-line video frame in 1/30 s (33 ms) Section III: Chapter 6 RADT 3463 Computerized Imaging 38 DISPLAY MONITOR Progressive Mode The video signal is read and the electron beam of the TV camera tube sweeps the target assembly continuously from top to bottom in 33 ms The video image is formed similarly on the TV monitor with no interlace of one field with another occurs Interlaced Mode Progressive Mode Section III: Chapter 6 RADT 3463 Computerized Imaging 39 13

14 DISPLAY MONITOR Compared to cathode ray tubes (CRT), flat panel monitors are: 1. Easier to view 2. Easier to manipulate 3. Provide better images 4. Light in weight 5. Easy to See 6. Easy to mount or suspend in an angiographic room Section III: Chapter 6 RADT 3463 Computerized Imaging 40 Digital Fluoroscopy with Image Intensifiers Projecting a radiographic image on an imageintensifying fluorescent screen coupled to a digital video image processor. Section III: Chapter 6 RADT 3463 Computerized Imaging 41 DIGITAL FLUOROSCOPY Advantages Low dose fluoroscopic imaging (digital average, last frame hold) Pulsed fluoroscopy and variable frame rate Speed of image acquisition Postprocessing to enhance image artifacts Uses hundreds of ma settings compared to 5Ma or less in conventional Digital Subtraction Angiography (DSA) and non subtraction acquisition and display Image distribution and archiving, PACS Section III: Chapter 6 RADT 3463 Computerized Imaging 42 14

15 DIGITAL FLUOROSCOPY IMAGING SYSTEM If x-ray tube were energized continuously thermal overloading would cause tube failure patient dose would be high or exceeded quickly Pulse-Progressive Fluoroscopy Images obtained by pulsing the x-ray beam Section III: Chapter 6 RADT 3463 Computerized Imaging 43 DIGITAL FLUOROSCOPY IMAGING SYSTEM PULSE-PROGRESSIVE FLUOROSCOPY INTERROGATION TIME Time required for the x-ray tube to be switched on and reach selected levels of kvp and ma EXTINCTION TIME Time required for the x-ray tube to be switched off High frequency generators must be fast enough to have interrogation and extinction times of less than 1 ms Section III: Chapter 6 RADT 3463 Computerized Imaging 44 DIGITAL FLUOROSCOPY IMAGING SYSTEM PULSE-PROGRESSIVE FLUOROSCOPY DUTY CYCLE The fraction of time the x-ray tube is energized The illustration shows the x-ray tube is energized for 100 ms every second which equals a duty cycle of 10% (100/1000 = 0.1 = 10%) Can result in significant radiation dose reduction Section III: Chapter 6 RADT 3463 Computerized Imaging 45 15

16 ADC Analog Digital Convertor Receives the output video signal (Analog) from the video camera and converts it into Binary code (0 s and 1 s) digital language. Each 0 or 1 is called a BIT (BInary DigiT) Section III: Chapter 6 RADT 3463 Computerized Imaging 46 1 bit (2 1) = 2 tones 2 bits (2 2 ) = 4 tones 3 bits (2 3) = 8 tones 4 bits (2 4) = 16 tones BIT Depth 8 bits (2 8 ) = 256 tones 16 bits (2 16) = 65,536 tones 24 bits (2 24) = 16.7 million tones Section III: Chapter 6 RADT 3463 Computerized Imaging 47 BIT Depth and Dynamic Range (Shades of Gray) N = 2 n N = Number of values n = number of bits DYNAMIC DYNAMIC BIT DEPTH POWER BIT DEPTH POWER RANGE RANGE , , ,048,576 Section III: Chapter 6 RADT 3463 Computerized Imaging 48 16

17 BIT Depth and Imaging Modalities With digital imaging systems, dynamic range is identified by the bit capacity of each pixel DIGITAL MEDICAL IMAGING SYSTEMS DYNAMIC RANGE Dynamic Range Imaging System Bit Depth Shades of Gray Diagnostic Sonography Nuclear Medicine Computed Tomography Magnetic Resonance Imaging Digital Radiography ,384 Digital Mammography ,536 Section III: Chapter 6 RADT 3463 Computerized Imaging 49 BIT Depth Section III: Chapter 6 RADT 3463 Computerized Imaging 50 DIGITAL FLUOROSCOPY IMAGING SYSTEM Operating Console: The right side module contains: Computer-interactive video controls A pad for cursor and region-of interest (ROI) manipulation May use trackball, joystick or a mouse instead Section III: Chapter 6 RADT 3463 Computerized Imaging 51 17

18 DIGITAL FLUOROSCOPY IMAGING SYSTEM Monitors Two or more monitors are used One is used to edit Patient data Examination data Annotate final image One is used for subtraction images Section III: Chapter 6 RADT 3463 Computerized Imaging 52 Computers in Digital Fluoroscopy Digital fluoroscopy employs the use of minicomputers and microprocessors Computer capacity is an important factor in determining: 1. Image quality 2. The manner and speed of image acquisition 3. The means of image processing and manipulation Section III: Chapter 6 RADT 3463 Computerized Imaging 53 DIGITAL SUBTRACTION ANGIOGRAPHY Usually image storage occurs in primary memory where data acquisition and transfer can be as rapid as 30 images per second A system might be capable of acquiring 30 images per second in the 512 x 512 matrix mode Section III: Chapter 6 RADT 3463 Computerized Imaging 54 18

19 DIGITAL SUBTRACTION ANGIOGRAPHY Data Transfer Limitation If a higher spatial resolution image is required and the 1024 x 1024 mode is requested, then only 8 images per second can be acquired Limitation on data transfer is imposed by the time required to transport enormous quantities of data from one segment on memory to another Section III: Chapter 6 RADT 3463 Computerized Imaging 55 Digital Fluoroscopy with Flat-Panel Detectors (FPD) FPDs are used in regular radiographic imaging. When used in Fluoroscopy it is referred to as Dynamic FPD Section III: Chapter 6 RADT 3463 Computerized Imaging 56 IMAGE CAPTURE - FLAT PANEL IMAGE RECEPTOR Flat Panel Image Receptors (FPIRs) Composed of cesium iodide (CsI) / amorphous silicon (a-si) pixel detectors Much smaller and lighter and is manipulated more easily than an image intensifier Provides easier patient manipulation and radiologist / technologist movement There are no radiographic cassettes Section III: Chapter 6 RADT 3463 Computerized Imaging 57 19

20 IMAGE CAPTURE - FLAT PANEL IMAGE RECEPTOR In contrast to an image-intensifier tube, a flat panel image receptor is insensitive to external magnetic fields May allows advanced application in: Cardiology Radiology Neurovascular Radiology Interventional - Vascular Radiology Image-guided catheter - magnetic tip in vessels is manipulated remotely by two large steering magnets located on either side of the patient Section III: Chapter 6 RADT 3463 Computerized Imaging 58 IMAGE CAPTURE - FLAT PANEL IMAGE RECEPTOR DF equipped with a flat panel image receptor Section III: Chapter 6 RADT 3463 Computerized Imaging 59 POSTPROCESSING LAST IMAGE HOLD Displays the last image continuously when the x-ray beam is turned off Section III: Chapter 6 RADT 3463 Computerized Imaging 60 20

21 POSTPROCESSING TEMPORAL FRAME AVERAGING Averages the current frame with previous frames to reduce the noise in the image. Reduces noise by 44% This is sometimes called over-sampling an image Section III: Chapter 6 RADT 3463 Computerized Imaging 61 Postprocessing - Edge Enhancement A.Original image B.Blurred image C.A and B digitally subtracted D.C is added to the original (A) image to produce the edge-enhanced image Section III: Chapter 6 RADT 3463 Computerized Imaging 62 Postprocessing Images Image contrast can be enhanced electronically using subtraction techniques Subtraction techniques provide instantaneous viewing of the subtracted image during passage of a bolus of contrast medium Digital fluoroscopy provides better contrast resolution through postprocessing of image subtraction. Section III: Chapter 6 RADT 3463 Computerized Imaging 63 21

22 DIGITAL SUBTRACTION ANGIOGRAPHY DSA Techniques 1. Temporal Subtraction (used most often) 2. Energy Subtraction 3. Hybrid Subtraction (combines temporal and energy subtraction) Section III: Chapter 6 RADT 3463 Computerized Imaging 64 DIGITAL SUBTRACTION ANGIOGRAPHY COMPARISON OF TEMPORAL AND ENERGY SUBTRACTION TEMPORAL SUBTRACTION A single kvp setting is used Normal x-ray beam filtration is adequate Contrast resolution of 1 mm at 1% is achieved Simple arithmetic image subtraction is necessary Motion artifacts are a problem ENERGY SUBTRACTION Rapid voltage switching is required X-ray beam filter switching is preferred High x-ray intensity is required for comparable contrast resolution Complex image subtraction is necessary Motion artifacts are greatly reduced Total subtraction of common structures is achieved Subtraction possibilities are limited by the number of images Some residual bone may survive subtraction Many more types of subtraction images are possible Section III: Chapter 6 RADT 3463 Computerized Imaging 65 DIGITAL SUBTRACTION ANGIOGRAPHY Temporal Subtraction A number of computer-assisted techniques where an image obtained at one time is subtracted form an image obtained at a later time (temporal = time) In the interval period, if contrast material is introduced into the vasculature, the subtracted image will contain only the vessels filled with contrast material Section III: Chapter 6 RADT 3463 Computerized Imaging 66 22

23 DIGITAL SUBTRACTION ANGIOGRAPHY Temporal Subtraction Two methods are commonly used to obtain the temporal subtracted image are: 1. The mask mode 2. The time interval difference mode (TID) Section III: Chapter 6 RADT 3463 Computerized Imaging 67 DIGITAL SUBTRACTION ANGIOGRAPHY Mask Mode Temporal Subtraction Mask mode results in successive subtraction images of contrast vessels. Section III: Chapter 6 RADT 3463 Computerized Imaging 68 DIGITAL SUBTRACTION ANGIOGRAPHY Mask Mode Temporal Subtraction A, The preinjection mask. B, A postinjection image. C, Image produced when the preinjection mask is subtracted from the postinjection image. Section III: Chapter 6 RADT 3463 Computerized Imaging 69 23

24 DIGITAL SUBTRACTION ANGIOGRAPHY Time-Interval Difference Mode Temporal Subtraction Time-Interval Difference mode produces subtracted images form progressive masks and following frames Section III: Chapter 6 RADT 3463 Computerized Imaging 70 DIGITAL SUBTRACTION ANGIOGRAPHY Time-Interval Difference Mode Temporal Subtraction Section III: Chapter 6 RADT 3463 Computerized Imaging 71 DIGITAL SUBTRACTION ANGIOGRAPHY Time-Interval Difference Mode Temporal Subtraction Misregistration Artifacts - due to patient motion occurring between the mask image and a subsequent image. Section III: Chapter 6 RADT 3463 Computerized Imaging 72 24

25 DIGITAL SUBTRACTION ANGIOGRAPHY Energy Subtraction Based on abrupt change in photoelectric absorption at the K edge of contrast media compared with that for soft tissue Illustration shows the probability of x-ray interaction with iodine, bone, and muscle as a function of x-ray energy Section III: Chapter 6 RADT 3463 Computerized Imaging 73 DIGITAL SUBTRACTION ANGIOGRAPHY Energy Subtraction The probability of photoelectric absorption in all three decreases with increasing x-ray energy. When the incident x-ray energy is sufficient to overcome the K-shell electrons binding energy of iodine, an abrupt and large increase in absorption occurs Graphically, this increase is known as the K absorption edge Section III: Chapter 6 RADT 3463 Computerized Imaging 74 DIGITAL SUBTRACTION ANGIOGRAPHY Hybrid Subtraction Combines temporal and energy subtraction techniques. Produces highest quality digital fluoroscopy images if patient motion can be controlled Section III: Chapter 6 RADT 3463 Computerized Imaging 75 25

26 PATIENT DOSE DIGITAL FLUOROSCOPY Should result in reduced patient dose Uses pulsed beams to fill one or more 33-ms video frames ma settings are higher with digital fluoroscopy but the fluoroscopic dose rate is lower than continuous analog fluoroscopy Section III: Chapter 6 RADT 3463 Computerized Imaging 76 PATIENT DOSE DIGITAL FLUOROSCOPY With digital fluoroscopy, static images are made with lower dose per frame than those attained with a 100 mm spot film camera Section III: Chapter 6 RADT 3463 Computerized Imaging 77 PATIENT DOSE DIGITAL FLUOROSCOPY Digital spot images are easy to acquire making it possible to make more than the needed exposures Even with more exposures, the patient dose is lower with digital fluoroscopy Section III: Chapter 6 RADT 3463 Computerized Imaging 78 26

27 DIGITAL SUBTRACTION ANGIOGRAPHY PATIENT DOSE Approximate Patient Dose in Representative Fluoroscopic Examinations Patient Dose Imaging Mode Conventional Digital 5 minutes fluoroscopy 3 spot filmsnormal mode 3 spot filmsmag 1 mode Total dose 20 rad (200 mgy) 0.6 rad (6 mgy) 1.0 rad (10 mgy) 21.6 rad (216 mgy) 10 rad (100 mgy) 0.2 rad (2 mgy) 0.3 rad (3 mgy) 10.5 rad (105 mgy) Section III: Chapter 6 RADT 3463 Computerized Imaging 79 QUESTIONS?? 27