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1 Fluoroscopy Equipment Operation Rad T 290

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Fluoroscopy Equipment Operation. Rad T 290. 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. - PowerPoint PPT Presentation

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Page 1: Fluoroscopy  Equipment Operation

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Fluoroscopy Equipment Operation

Rad T 290

Page 2: Fluoroscopy  Equipment Operation

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Topics for WEEK 2

Describe the components of an image intensifier.

Describe the components of flat panel digital fluoroscopy.

TV & viewing system……..etc

Page 3: Fluoroscopy  Equipment Operation

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.

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Page 4: Fluoroscopy  Equipment Operation

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Image Intensifier

VACUUM TUBE ENCASED IN A LEAD

HOUSING = 2MM PB (PRIMARY BARRIER)

Page 5: Fluoroscopy  Equipment Operation

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Image intensifier systems

Page 6: Fluoroscopy  Equipment Operation

6Image Intensification Tube Components

Input screen and photocathode

Electrostatic lenses Anode and output

screen

Page 7: Fluoroscopy  Equipment Operation

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

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Page 8: Fluoroscopy  Equipment Operation

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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

Page 9: Fluoroscopy  Equipment Operation

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?

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Page 10: Fluoroscopy  Equipment Operation

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

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Page 11: Fluoroscopy  Equipment Operation

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IMAGE INTENSIFIER

Page 12: Fluoroscopy  Equipment Operation

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?

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Page 13: Fluoroscopy  Equipment Operation

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INPUT PHOSPHOR

Page 14: Fluoroscopy  Equipment Operation

Cesium Iodide microlight pipes

CsI crystals are grown as tiny needles and

are tightly packed in a

layer of approximately

300 µm

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Page 15: Fluoroscopy  Equipment Operation

Input phosphor

Is a round tube that can

A diameter of 6, 9,

12 or 16 inches

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Page 16: Fluoroscopy  Equipment Operation

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

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Page 17: Fluoroscopy  Equipment Operation

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.

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Page 18: Fluoroscopy  Equipment Operation

Electrostatic Focusing Lenses

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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.

Page 19: Fluoroscopy  Equipment Operation

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The image intensifier (I.I.)

+

I.I. Input Screen

I.I.Output Screen

Photocathode

Electrode E1

Electrode E3

Electrode E2

Electrons Path

Page 20: Fluoroscopy  Equipment Operation

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The 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.

Page 21: Fluoroscopy  Equipment Operation

Accelerating Anode

II tube is

approximately

50 cm long Potential difference

between photocathode

and anode of 25,000

- 30, 000 V

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Page 22: Fluoroscopy  Equipment Operation

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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

Page 23: Fluoroscopy  Equipment Operation

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Intensifier Flux Gain

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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

Page 25: Fluoroscopy  Equipment Operation

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.

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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

Page 27: Fluoroscopy  Equipment Operation

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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.

Page 28: Fluoroscopy  Equipment Operation

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

Page 29: Fluoroscopy  Equipment Operation

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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

Page 30: Fluoroscopy  Equipment Operation

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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

Page 31: Fluoroscopy  Equipment Operation

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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

Page 32: Fluoroscopy  Equipment Operation

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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:

Page 33: Fluoroscopy  Equipment Operation

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Image Intensifier FORMULAS

Flux Gain (usually stated rather than calculated)

input

output

photonsraysx#

photonslight#FG

Brightness Gain Ability of II to increase illumination

gainfluxgainonminificatiBG Minification Gain

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phosphoroutput

phosphorinputMG

Page 34: Fluoroscopy  Equipment Operation

Conversion Factor

International Commission of Radiologic Units and Measurements (ICRU) recommends evaluating the brightness gain of the II based upon the conversion factor.

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Image 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 between 50 - 300 Usually 5000 to 30,000 brightness gains

Page 36: Fluoroscopy  Equipment Operation

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Image Intensifier Tube

Vacuum diode tube1. Input phosphor (CsI)

X-rays light2. 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

Page 37: Fluoroscopy  Equipment Operation

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.

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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

Page 39: Fluoroscopy  Equipment Operation

39 Intensifier Format and Modes

Note focal point moves farther from output in mag mode

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Page 41: Fluoroscopy  Equipment Operation

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.

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Page 42: Fluoroscopy  Equipment Operation

42Magnification Factor FORMULA

IP OLD SIZE

IP NEW SIZE = %mag

Page 43: Fluoroscopy  Equipment Operation

43 Intensifier Format and Mag Modes

Page 44: Fluoroscopy  Equipment Operation

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?

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Page 45: Fluoroscopy  Equipment Operation

Image Quality in Mag Mode

Improved spatial resolution

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Page 46: Fluoroscopy  Equipment Operation

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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

Page 47: Fluoroscopy  Equipment Operation

47Basic Componets of “NEW DIGITAL” Fluoro“Imaging Chain”

Fluoro TUBE

Primary

Radiation PATIENT

EXIT Radiation

Image Intensifier

ABC CCD

Analog to

Digital

Converter

ADC

TV

Page 48: Fluoroscopy  Equipment Operation

48Dynamic Flat-Panel Digital Fluoroscopy

Page 49: Fluoroscopy  Equipment Operation

Flat-Panel Detectors (FPD)

II tubes are being replaced by Flat-panel detectors.

Page 50: Fluoroscopy  Equipment Operation

Coating for DR

AMORPHOUS SILICON (indirect) X-ray photon to light photon

AMORPHOUS SELENIUM (direct = trapped e-) No light

Page 51: Fluoroscopy  Equipment Operation

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.

Page 52: Fluoroscopy  Equipment Operation

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

Page 53: Fluoroscopy  Equipment Operation

Flat-Panel Detectors (FPD)

Direct capture detector using an AMA of Amorphous selenium (a-Se) TFTs

Direct e- capture

Page 54: Fluoroscopy  Equipment Operation

Capture Element Where the remnant photons are

captured. DR = Cesium iodide (CsI),

Gadolium oxysulfide (GdOS), or Amorphous selenium (a-Se).

Page 55: Fluoroscopy  Equipment Operation

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-.

Page 56: Fluoroscopy  Equipment Operation

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.

Page 57: Fluoroscopy  Equipment Operation

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.

Page 58: Fluoroscopy  Equipment Operation

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.

Page 59: Fluoroscopy  Equipment Operation

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).

Page 60: Fluoroscopy  Equipment Operation

Scintillation DR

Page 61: Fluoroscopy  Equipment Operation

CCD Array with a scintillation phosphor

Page 62: Fluoroscopy  Equipment Operation

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.

Page 63: Fluoroscopy  Equipment Operation

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.

Page 64: Fluoroscopy  Equipment Operation

Amorphous Selenium

No scintillation phosphor is involved

The image-forming x-ray beam interacts directly with amorphous selenium (a-Se),

producing a charged pair.

Page 65: Fluoroscopy  Equipment Operation

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

Page 66: Fluoroscopy  Equipment Operation

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

Page 67: Fluoroscopy  Equipment Operation

Direct vs Indirect DR

Page 68: Fluoroscopy  Equipment Operation

FPD vs. dynamic FPD

Fluoroscopy FPD are larger and have larger matrix sizes. Pixel sizes?

Page 69: Fluoroscopy  Equipment Operation

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

Page 70: Fluoroscopy  Equipment Operation

Pulsed Fluoroscopy

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Fluoroscopic Image Display

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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

Page 73: Fluoroscopy  Equipment Operation

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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 - 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 device

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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

Page 75: Fluoroscopy  Equipment Operation

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Vidicon (tube) TV Camera

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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

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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)

Page 79: Fluoroscopy  Equipment Operation

79Video 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

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CCD’s

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Digital Uses Progressive Scan

1024 x 1024 Higher spatial resolution As compared to 525

8 images/sec (compared to 30 in 525 system)

Page 82: Fluoroscopy  Equipment Operation

Monitors

Cathode ray tube (CRT)

Liquid crystal display (LCD)

Plasma screen

Page 83: Fluoroscopy  Equipment Operation

Soft copy viewingdigital cathode ray tube (CRT)

Page 84: Fluoroscopy  Equipment Operation

active matrix liquid crystal display (AMLCD)

Page 85: Fluoroscopy  Equipment Operation

Active matrix liquid crystal displays are superior to cathode ray tube displays.

LCD design – gives out more light,

reduces ambient light

Better contrast resolution

Less noise Less maintenance

Page 86: Fluoroscopy  Equipment Operation

Crystals can be aligned by an external electric field

Page 87: Fluoroscopy  Equipment Operation

Plasma Display

The plasma displays are made up of many small fluorescent lights that are illuminated to form the color of the image.

Page 88: Fluoroscopy  Equipment Operation

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Questions?