2Topics for WEEK 2 Describe the components of an image intensifier. Describe the components of flat panel digital fluoroscopy.TV & viewing system……..etcExplanation or/and additional informationInstructions for the lecturer/trainer
3II FluoroscopyThe 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.
4Image Intensifier VACUUM TUBE ENCASED IN A LEAD HOUSING = 2MM PB (PRIMARY BARRIER)
6Image Intensification Tube Components Input screen and photocathodeElectrostatic lensesAnode and output screen
7Steps to image intensification Object of the II is to convert remnant radiation into an amplified light image5 basic partsInput phosphorPhotocathodeElectrostatic lensesAccelerating anodeOutput phosphor
8Image intensifier component Input screen: conversion of incident X Rays into light photons (CsI)1 X Ray photon creates 3,000 light photonsPhotocathode: conversion of light photons into electronsonly 10 to 20% of light photons are converted into photoelectronsElectrodes (lenses): focalization of electrons onto the output screenelectrodes provide the electronic magnificationOutput screen: conversion of accelerated electrons into light photons
9II FluoroscopyDuring image-intensified fluoroscopy, the radiologic image is displayed on a television monitor or flat panel monitor.X-ray tube is operated at lessthan 5 mA. Radiographic exams the x-raytube current is measured in hundreds of mA.Despite this fluoro dose tends tobe much higher?
10kVpKVp 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
12Image-intensifierRemnant 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?
14Cesium Iodide microlight pipes CsI crystals are grown as tiny needles andare tightly packed in alayer of approximately300 µm
15Input phosphor Is a round tube that can A diameter of 6, 9, 12 or 16 inches
16PhotocathodeThe 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
17Photoemission 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.
18Electrostatic Focusing Lenses 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 calledminification.
20The anode of the IIThe 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.20
21Accelerating Anode II tube is approximately 50 cm long Potential differencebetween photocathodeand anode of 25,000- 30, 000 V
22Flux 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
241000 light photons at the photocathode from 1 x-ray photon FLUX GAIN1000 light photons at the photocathodefrom 1 x-ray photonOutput phosphor =3000 light photons (3 X more than at the input phosphor!)This increase is called the flux gain
25Output Phosphora 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.
26Minification (↑ 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 openingInput phosphors are 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 focus26
27Total brightness gain (BG) The II makes the image brighter because it is minified and amplified (more light photons).BG = MG X FGMultiply the minification gain times the flux gain.27
28Intensifier Brightness Gain (BG) BG = MG x FGMinification Gain x Flux GainMinification 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 brightnessMG = (Input Diameter )2(Output Diameter)2
29Minification gain - again BG = MINIFICATION GAIN X FLUX GAINMINIFICATION GAIN – same # e at input condensed to output phosphor – ratio of surface area on input screen over surface area of output screenIP SIZE 2OP SIZE 2
30BG = MG X FGFLUX GAIN – increase of light brightness due to the conversion efficiency of the output screen (estimation)1 electron = 50 light photons is 50 FGCan decrease as II agesFlux gain is almost always 50
31Intensifier Brightness Gain Example:Input Phosphor Diameter = 9”Output Phosphor Diameter = 1”Flux Gain = 50BG = FG x MG = 50 x (9/1)2 = 4,050Typical values: a few thousand to >10,000 for modern image intensifiers
32Intensifier 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:
33Image Intensifier FORMULAS Brightness GainAbility of II to increase illuminationMinification GainFlux Gain (usually stated rather than calculated)
34Conversion FactorInternational Commission of Radiologic Units and Measurements (ICRU) recommends evaluating the brightness gain of the II based upon the conversion factor.
35Image Intensifier Performance Conversion factor is the ratio of output phosphor image luminance (candelas/m2) to x-ray exposure rate entering the image intensifier (mR/second).II has conversion factors betweenUsually 5000 to 30,000 brightness gains
37Multifield 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.
38Magnification Tubes Greater voltage to electrostatic lenses Dual focus Increases acceleration of electronsShifts focal point away from anodeDual focus23/15 cm /6 inchesTri focus12/9/6 inches
39Intensifier Format and Modes Note focal point moves farther from output in mag mode
41FOVThis 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.
42Magnification Factor FORMULA IP OLD SIZEIP NEW SIZE = %mag
44What’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?
45Image Quality in Mag Mode Improved spatial resolution
46MAG MODE VS PT DOSEMAG USED TO ENLARGE SMALL STRUCTURE OR TO PENETRATE THROUGH LARGER PARTSFORMULA:PATIENT DOSE IS INCREASED IN THE MAG MODE –DEPENDANT ON SIZE OF INPUT PHOSPHOR
49Flat-Panel Detectors (FPD) II tubes are being replaced by Flat-panel detectors.
50Coating for DR AMORPHOUS SILICON (indirect) X-ray photon to light photonAMORPHOUS SELENIUM (direct = trapped e-)No light
51Flat-Panel Detectors (FPD) Two types of dynamic FPDsIndirect 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.
52Active Matrix Array (AMA) Pixels are read sequentially, one at a time Each TFT or CCD detector represents a pixelDEL = charge collecting detector element
53Flat-Panel Detectors (FPD) Direct capture detector using an AMA of Amorphous selenium (a-Se) TFTsDirect e- capture
54Capture Element Where the remnant photons are captured. DR = Cesium iodide (CsI), Gadolium oxysulfide (GdOS), or Amorphous selenium (a-Se).
55Collection 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-.
56Charge-Coupled Device CCD, which is the light-sensing element.The CCD is a silicon-based semiconductorhas three principal advantageous imaging characteristics: sensitivity, dynamic range, and size.
57Sensitivityis the ability of the CCD to detect and respond to very low levels of visible lightThis sensitivity is important for low patient radiation dose in digital imaging.
58Direct 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.
59Indirect ConversionIndirect 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).
62TFTThe 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.
63DRA 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.
64Amorphous Selenium No scintillation phosphor is involved The image-forming x-ray beam interacts directly with amorphous selenium (a-Se),producing acharged pair.
65Amorphous SeleniumThe a-Se is both the capture element and the converting element.a-Se is a direct DR process by which x-rays are convertedto electric signal
66DDR 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
68FPD vs. dynamic FPDFluoroscopy FPD are larger and have larger matrix sizes. Pixel sizes?
69Digital 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
72Image Display 2 Methods: Coupling I.I. to TV or CCD Thermionic television camera tubeSolid state charge-coupled device (CCD)Coupling I.I. to TV or CCDFiber opticsLens system
73ViewingThe 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 - vidiconInside 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 device73
74Type of TV camera VIDICON TV camera improvement of contrastimprovement of signal to noise ratiohigh image lagPLUMBICON TV camera (suitable for cardiology)lower image lag (follow up of organ motions)higher quantum noise levelCCD 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
77Bandpass/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
78TV 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)
79Video Camera Charged Coupled Devices (CCD) Operate at lower voltages than video tubesMore durable than video tubesSemiconducting deviceEmits electrons in proportion to amount of light striking photoelectric cathodeFast discharge eliminates lag