7.2. Optimizarea_ Fluoroscopie.ppt

IAEA Standard syllabus course on Radiation Protection in diagnostic and interventional radiology

PROTECTIA RADIOLOGICA IN DIAGNOSTICUL RADIOLOGIC

Optimizarea protectiei in fluoroscopie

Introducere: Echipamentul de fluoroscopie si accesorii  Diverse componente electronice care contribuie la formarea imaginei in fluoroscopie.  Rolul lor si politica de Controlul Calitatii. 

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Continut:  Exemple

de sisteme fluoroscopice

 Componenta

and parametrii Intensificatorul de Imagine

 Intensificatorul

de Imagine si sistemul

TV

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Scop: Familiarizarea cursantilor privind componentele sistemului fluoroscopic ( parametrii care afecteaza calitatea imaginei si Controlul Calitatii).



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IAEA Standard syllabus course on Radiation Protection in diagnostic and interventional radiology

Optimizarea protectiei in fluoroscopie

Exemple de sisteme fluoroscopice

Fluoroscopia

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Diverse sisteme fluoroscopice: 

Sisteme de control de la distanta 



Nu necesita prezenta medicului in interiorul camerei de radiatii X

Arc mobil, in forma de C 

Cel mai folosit in interventiile chirurgicale.

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continuare: 

Sisteme de radiologie interventionala 



Necesita consideratii specifice de siguranta,interventionistii fiind aproape de pacient.

Sisteme multifunctionale 

fluoroscopice

Folosite fie ca sisteme clasice de fluoroscopie , fie pentru proceduri interventionale simple

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IAEA Standard syllabus course on Radiation Protection in diagnostic and interventional radiology

Optimizarea protectiei in fluoroscopie

Componenta si parametrii-Intensificator de Imagine

Intensificatorul de Imagine (I.I.) Ecran Intrare II Electrod E1

Electrod E2 Electrod E3

Ecran Esire II

Fotocatoda +

Intensificatorul de Imagine

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Componentele Intensificatorului de Imagine  Ecranul

de Intrare: realizeaza conversia radiatiei X incidente in fotoni de lumina (CsI) 1

foton X creaza  3.000 fotoni de lumina

 Fotocatoda:realizeaza

conversia fotonilor de lumina

in electroni  numai

10 pana la 20% din fotonii de lumina sunt convertiti in fotoelectroni  Electrozii : focalizeaza electronii spre ecranul de Esire  Electrozii realizeaza multiplicarea electronica  Ecranul de Esire:realizeaza conversia electronilor accelerati in fotoni de lumina

Parametrii Intensificatorului de Imagine (I)  Coeficientul

de conversie(Gx): raportul intre luminozitatea ecranului de Esire si debitul dozei la ecranul de Intrare [cd.m Gys ] -2

-1



Gx depinde de calitatea fasciculului incident (Publicatia IEC nr. 573 recomanda un HVL de 7  0.2 mm Al)



Gx este direct proportional cu: Potentialul

aplicat pe tub

Diametrul

() ecranul de Intrare

 I.I.

ecran () de 22 cm  Gx = 200

 I.I.

ecran () de 16 cm  Gx = 200 x (16/22)2 = 105

 I.I.

ecran () de 11 cm  Gx = 200 x (11/22)2 = 50

continuare parametrii I.I.: (II) 

Uniformitatea luminozitatii: luminozitatea ecranului de Intrare poate varia de la centrul I.I. la periferie Uniformitatea = (Luminozitatea(c) - Luminozitatea(p)) x 100 / Luminozitatea(c)

Diformarea geometrica:toate I.I.prezinta intr-un

anumit grad diformarea pernutei de ace.Fenomenul este cauzat fie de contaminarea magnetica a tubului de imagine sau de instalarea intensificatorului intr-un

puternic camp magnetic

Diformarea imaginei

continuare parametrii I.I.: (III) 

Limita de rezolutie spatiala: valoarea celei mai mari frecvente spatiale ce poate fi detectata vizual 

Ea reprezinta o masura sensibila a starii de focalizare a unui sistem



Este evaluata de producatorsi uzual este masurata optic si in conditii complet optimizate.Aceasta valuare se coreleaza foarte bine cu limita de frecventa inalta a Functiei de Transfer a Modulatiei (MTF)



Ea poate fi estimata cu sistemul Hüttner pentru determinarea rezolutiei,care trebuie sa contina mai multe cercuri la fiecare frecventa, pentru a simula periodicitatea

Line pair gauges

Line pair gauges GOOD RESOLUTION

POOR RESOLUTION

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  reduction  aging



of dust and dirt on the various optical surfaces

in the quality of the vacuum

process (destruction of phosphor screen)

Sources of noise are:  X-ray

quantum mottle

 photo-conversion

processes, film granularity, film processing

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 and/or small details.

Overall image quality

IAEA Standard syllabus course on Radiation Protection in diagnostic and interventional radiology

Part 16.1 : Optimization of protection in fluoroscopy

Topic 3 : Image Intensifier and TV system

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

 cinema 

35 mm film format: from 25 to 150 images/s

 photography  

rolled film of 105 mm: max 6 images/s film of 100 mm x 100 mm

kV

X-RAY TUBE

FILM

PM

REFERENCE CONTROLLER kV

VIDICON

GENERAL SCHEME OF FLUOROSCOPY Add module code number and lesson title

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kV

X-RAY TUBE

CINE MODE

I2

CONTROLLER

I3

I1

FILM

PM

C1

C2

Ref.

VIDICON

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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

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.

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.

Photoconductive camera tube Steering coils Focussing optical lens Photoconductive layer

Deviation coil Alignement coil

Input plate

Accelarator grids Control grid

Electron beam

Iris Video Signal

Signal electrode

Electron gun Field grid

Electrode

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.

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.

Different types of scanning 1

11

13 3 15

12 2

5 17 7 19

4 16 6 18 8 20

INTERLACED SCANNING

14

9 21

625 lines in 40 ms i.e. : 25 frames/s

10 1 3 5 7 9 11 13 15 17

2 4 6 8 10 12 14 16 18

PROGRESSIVE SCANNING

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 lines and prototype systems with 2,000 lines are being developed.

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.

Schematic structure of a charged couple device (CCD)

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 TV monitor is proportional to the amount of light falling on the corresponding position on the TV camera.

TV image sampling IMAGE 512 x 512 PIXELS

HIGHT 512

WIDTH 512

ONE LINE

VIDEO SIGNAL (1 LINE)

64 µs IMAGE LINE 52 µs DIGITIZED SIGNAL

LIGHT INTENSITY

SYNCHRO

12 µs SAMPLING SINGLE LINE TIME

Digital radiography principle ANALOGUE SIGNAL

I

ADC

t

Memory

DIGITAL SIGNAL

Iris Clock

t

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.  This can be performed using a suitable test object or electronic pattern generator. 

Summary 

The main components of the fluoroscopy imaging chain and their role are explained: 

Image Intensifier



Associated image TV system

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Where to Get More Information Physics of diagnostic radiology, Curry et al, Lea & Febiger, 1990  Imaging systems in medical diagnostics, Krestel ed., Siemens, 1990  The physics of diagnostic imaging, Dowsett et al, Chapman&Hall, 1998 

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