Presentation on theme: "1 Fluoroscopy Equipment Operation Rad T 290. 2 Topics for WEEK 2 Describe the components of an image intensifier. Describe the components of flat panel."— Presentation transcript:
2 Topics for WEEK 2 Describe the components of an image intensifier. Describe the components of flat panel digital fluoroscopy. TV & viewing system……..etc
II Fluoroscopy The II was developed to replace the conventional fluorescent screen. The II raised illumination into the cone vision region, where visual acuity is greatest. Technical factors is similar to radiographic image quality. Generally, high kVp and low mA are preferred. 3
4 Image Intensifier VACUUM TUBE ENCASED IN A LEAD HOUSING = 2MM PB (PRIMARY BARRIER)
6 Image Intensification Tube Components Input screen and photocathode Electrostatic lenses Anode and output screen
Steps to image intensification Object of the II is to convert remnant radiation into an amplified light image 5 basic parts Input phosphor Photocathode Electrostatic lenses Accelerating anode Output phosphor 7
8 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 converted into photoelectrons * Electrodes (lenses): focalization of electrons onto the output screen * electrodes provide the electronic magnification * Output screen: conversion of accelerated electrons into light photons
II Fluoroscopy During image-intensified fluoroscopy, the radiologic image is displayed on a television monitor or flat panel monitor. X-ray tube is operated at less than 5 mA. Radiographic exams the x-ray tube current is measured in hundreds of mA. Despite this fluoro dose tends to be much higher? 9
kVp KVp depends entirely on the anatomy being examined. Fluoroscopic equipment operates by selecting an image brightness. The automatic brightness control (ABC) The ABC maintaines image brighness automatically by varying the kVp, the mA, or sometimes both. Generally kVp is maintained by adjust the mA depending on part/patient thickness 10
Image-intensifier Remnant photons enter the image-intensifier tube transmitted through the glass envelope and interact with the input phosphor, which is cesium iodide (CsI). When an x-ray interacts with the input phosphor, its energy is converted into visible light. Where else does this occur in radiography? 12
Cesium Iodide microlight pipes CsI crystals are grown as tiny needles and are tightly packed in a layer of approximately 300 µm 14
Input phosphor Is a round tube that can A diameter of 6, 9, 12 or 16 inches 15
Photocathode The next active element of the image- intensifier tube is the photocathode. Bonded directly to the input phosphor with a thin, transparent adhesive layer. The photocathode is a thin metal layer composed of cesium and antimony compounds that respond to stimulation of input phosphor light by the emission of electrons. The photocathode emits e- when illuminated by the input phosphor 16
Photoemission This process is known as photoemission. Photoemission is electron emission that follows light stimulation. The number of electrons emitted by the photocathode is directly proportional to the intensity of light that reaches it. 17
Electrostatic Focusing Lenses 18 A series of metal rings which have varying positive voltage. They pull the e- from the input side toward the put out phosphor. This process is called minification.
19 The image intensifier (I.I.) + I.I. Input Screen I.I.Output Screen Photocathode Electrode E 1 Electrode E 3 Electrode E 2 Electrons Path
20 The anode of the II The anode is a circular plate about 20” away from the photocathode. It has a hole in the middle of it allowing electrons to pass through and hit the output phosphor made of zinc cadmium sulfide. Electrostatic lenses have a negative charge to repel the negative electrons and push them to the anode and focus them to a narrow beam. The electrons are carrying the latent image and when they hit the output phosphor they are turned into light again.
Accelerating Anode II tube is approximately 50 cm long Potential difference between photocathode and anode of 25,000 - 30, 000 V 21
22 Flux gain (flow) The ratio of the number of light photons striking the output screen to the ratio of the number of x-ray photons striking the input screen is called fluxgain
24 1000 light photons at the photocathode from 1 x-ray photon Output phosphor = 3000 light photons (3 X more than at the input phosphor!) This increase is called the flux gain FLUX GAIN
Output Phosphor a 1” circular plate with a hole in the middle through which electrons pass. Made of zinc cadmium sulfide that produces light by interacting with e-. Output phosphor is always 1”. Very concentrated bright light is direct to a TV camera tub or CCD. 25
26 Minification ( ↑ BRIGHTNESS OF LIGHT) Electrons had to be focused down to fit through the hole at the anode. Input phosphor is much bigger than the anode opening Input phosphors are 10-35 cm in diameter* (6, 9, 12 inches) Output phosphors are 2.5 to 5 cm (1 in) in diameter* Most fluoro tubes have the ability to operate in 2 sizes (just like small and large focal spot sizes) Bi focus or newer units - tri focus
27 Total brightness gain (BG) The II makes the image brighter because it is minified and amplified (more light photons). BG = MG X FG Multiply the minification gain times the flux gain.
28 Intensifier Brightness Gain (BG) BG = MG x FG Minification Gain x Flux Gain Minification gain (MG): The ratio of the squares of the input and output phosphor diameters. This corresponds to “concentrating” the light into a smaller area, thus increasing brightness MG = (Input Diameter ) 2 (Output Diameter) 2
29 Minification gain - again BG = MINIFICATION GAIN X FLUX GAIN MINIFICATION GAIN – same # e at input condensed to output phosphor – ratio of surface area on input screen over surface area of output screen IP SIZE 2 OP SIZE 2
30 BG = MG X FG FLUX GAIN – increase of light brightness due to the conversion efficiency of the output screen (estimation) 1 electron = 50 light photons is 50 FG Can decrease as II ages Flux gain is almost always 50
31 Intensifier Brightness Gain Example: Input Phosphor Diameter = 9” Output Phosphor Diameter = 1” Flux Gain = 50 BG = FG x MG = 50 x (9/1) 2 = 4,050 Typical values: a few thousand to >10,000 for modern image intensifiers
32 Intensifier Brightness Gain Flux Gain (FG): Produced by accelerating the photoelectrons across a high voltage (>20 keV), thus allowing each electron to produce many more light photons in the output phosphor than was required to eject them from the photcathode. Summary: Combining minification and flux gains:
33 Image Intensifier FORMULAS Flux Gain (usually stated rather than calculated) Brightness Gain Ability of II to increase illumination Minification Gain
Conversion Factor International Commission of Radiologic Units and Measurements (ICRU) recommends evaluating the brightness gain of the II based upon the conversion factor. 34
35 Image Intensifier Performance Conversion factor is the ratio of output phosphor image luminance (candelas/m 2 ) to x-ray exposure rate entering the image intensifier (mR/second). II has conversion factors between 50 - 300 Usually 5000 to 30,000 brightness gains
36 Image Intensifier Tube Vacuum diode tube 1.Input phosphor (CsI) X-rays light 2.Photocathode Photoemission Light electron beam 3.Electrostatic lenses Maintain & minify e - 4.Anode Attracts e - in beam 5.Output phosphor (ZnS-CdS) e - light 1 5 2 3 4
Multifield Image Intensification FOV selection gives you the active diameter of the input phosphor. 6, 9, 12 or 16” In 16” mode photoelectrons from the entire input phosphor are accelerated to the output phosphor. 12” mode, the voltage on the electrostatic focusing lenses increase causing the electron focal point to move farther from the output phosphor. Only 12’ of input phosphor are on the output phosphor. 37
38 Magnification Tubes Greater voltage to electrostatic lenses Increases acceleration of electrons Shifts focal point away from anode Dual focus 23/15 cm 9/6 inches Tri focus 12/9/6 inches
39 Intensifier Format and Modes Note focal point moves farther from output in mag mode
FOV This change in focal point will reduce the FOV and the image appears magnified. Using the smaller dimension of a multifield image-intensifier tube always results in a magnified image, with a magnification factor in direct proportion to the ratio of the diameters. 41
42 Magnification Factor FORMULA IP OLD SIZE IP NEW SIZE = %mag
What’s the catch? Image will be much dimmer, less light entering II = less light per output pixel. Minification gain is reduced. Reduced signal-to-noise ratio (SNR). Noise will become more visible in the image. ABC will compensate, how? 44
Image Quality in Mag Mode Improved spatial resolution 45
46 MAG MODE VS PT DOSE MAG USED TO ENLARGE SMALL STRUCTURE OR TO PENETRATE THROUGH LARGER PARTS FORMULA: PATIENT DOSE IS INCREASED IN THE MAG MODE – DEPENDANT ON SIZE OF INPUT PHOSPHOR
47 Basic Componets of “NEW DIGITAL” Fluoro“Imaging Chain” Fluoro TUBE Primary Radiation PATIENT EXIT Radiation Image Intensifier ABC CCD Analog to Digital Converter ADC TV
Flat-Panel Detectors (FPD) II tubes are being replaced by Flat- panel detectors.
Coating for DR AMORPHOUS SILICON (indirect) X-ray photon to light photon AMORPHOUS SELENIUM (direct = trapped e-) No light
Flat-Panel Detectors (FPD) Two types of dynamic FPDs Indirect using cesium iodide (CsI) phosphors coupled to an active matrix array of amorphous silicon (a-Si), which holds a charge on its surface that can then be read out by a TFT.
Active Matrix Array (AMA) Pixels are read sequentially, one at a time Each TFT or CCD detector represents a pixel DEL = charge collecting detector element
Flat-Panel Detectors (FPD) Direct capture detector using an AMA of Amorphous selenium (a-Se) TFTs Direct e- capture
Capture Element Where the remnant photons are captured. DR = Cesium iodide (CsI), Gadolium oxysulfide (GdOS), or Amorphous selenium (a-Se).
Collection element Collects converted x-ray signal. Types: Photodiode, A charge-coupled device (CCD), or A thin-film transistor (TFT). Photodiode & CCD collect light. TFT is charge sensitive and collects E-.
Charge-Coupled Device CCD, which is the light-sensing element. The CCD is a silicon-based semiconductor has three principal advantageous imaging characteristics: sensitivity, dynamic range, and size.
Sensitivity is the ability of the CCD to detect and respond to very low levels of visible light This sensitivity is important for low patient radiation dose in digital imaging.
Direct vs Indirect Conversion In direct conversion, x-ray photons are absorbed by the coating material and immediately converted into an electrical signal. The DR plate has a radiation-conversion material or scintillator, typically made of a-Se. This material absorbs x-rays and converts them to electrons, which are stored in the TFT detectors.
Indirect Conversion Indirect conversion is a two-step process: x-ray photons are converted to light, and then the light photons are converted to an electrical signal. A scintillator converts x-rays into visible light. The light is then converted into an electric charge by photodetectors such as amorphous silicon photodiode arrays or charge-coupled devices (CCDs).
TFT The thin-film transistor (TFT) is a photosensitive array made up of small (about 100 to 200μm) pixels. Each pixel contains a photodiode that absorbs the electrons and generates electrical charges.
DR A field-effect transistor (FET) or silicon TFT isolates each pixel element and reacts like a switch to send the electrical charges to the image processor.
Amorphous Selenium No scintillation phosphor is involved The image-forming x-ray beam interacts directly with amorphous selenium (a-Se), producing a charged pair.
Amorphous Selenium The a-Se is both the capture element and the converting element. a-Se is a direct DR process by which x- rays are converted to electric signal
DDR only using amorphous selenium (a-Se) The exit x-ray photon interact with the a-Si (detector element/DEL). Photon energy is trapped on detector (signal) The TFT stores the signal until readout, one pixel at a time
FPD vs. dynamic FPD Fluoroscopy FPD are larger and have larger matrix sizes. Pixel sizes?
Digital Fluoroscopy (DF) DF, the under-table x-ray tube operates in the radiographic mode. Tube current is measured in hundreds of mA instead of less than 5 mA, as in image-intensifying fluoroscopy. Pulse-progressive fluoroscopy
72 Image Display 2 Methods: Thermionic television camera tube Solid state charge-coupled device (CCD) Coupling I.I. to TV or CCD Fiber optics Lens system
73 Viewing The output phosphor of the II is connected by fiber optic cables directly to a TV camera tube when the viewing is done through a television monitor. The most commonly used camera tube - vidicon Inside the glass envelope that surrounds the TV camera tube is a cathode, an electron gun, grids and a target. Past the target is a signal plate that sends the signal from the camera tube to the external video device
74 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 motions) 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 lp/mm) for a 1000 line TV system
77 Bandpass/Horizantal Resolution Horizontal resolution is determined by the bandpass. Bandpass is expressed in frequency (Hz) and describes the number of times per second the electron beam can be modulated. The higher the bandpass, the better the resolution
78 TV RESOLUTION-Horizontal Along a TV line, resolution is limited by how fast the camera electronic signal and monitor’s electron beam intensity can change from minimum to maximum. This is bandwidth. For similar horiz and vertical resolution, need 525 changes (262 full cycles) per line. Example (at 30 frames/second): 262 cycles/line x 525 lines/frame x 30 frames/second = 4.2 million cycles/second or 4.2 Megahertz (MHz)
79 Video Camera Charged Coupled Devices (CCD) Operate at lower voltages than video tubes More durable than video tubes Semiconducting device Emits electrons in proportion to amount of light striking photoelectric cathode Fast discharge eliminates lag