COMPUTED RADIOGRAPHY

1. COMPUTED
RADIOGRAPHY

2. OUTLINE
 INTRODUCTION
 HISTORY
 LIMITATIONS OF CONVENTIONAL RADIOGRAPHY
 COMPONENTS OF COMPUTED RADIOGRAPHY
 PHOTOSTIMULABLE PHOSPHOR
 X-RAY EXPOSURE
 STEPS INVOLVED
 IMAGE SCANNING
 ERASING PROCESS
 ADVANTAGES AND DISADVANTAGES OF CR RADIOGRAPHY

3. INTRODUCTION:
COMPUTED RADIOGRAPHY ALSO CALLED AS “FILM REPLACEMENT
TECHNOLOGY” USES A FLEXIBLE PHOSPHOR IMAGING PLATE TO CAPTURE
DIGITAL IMAGES INSTEAD OF CONVENTIONAL PHOTOGRAPHIC FILM.

4. HISTORICAL DEVELOPMENT:
CT, MRI AND ULTRASOUND WERE ALL DIGITAL SINCE THEIR
INTRODUCTION.
BUT GENERAL RADIOGRAPHY REMAINED ANALOG.
IN 1981, AT INTERNATIONAL CONFERENCE OF RADIOLOGY AT
BRUSSELS , FUJI PRESENTED THE CONCEPT OF CR
EMPLOYING PHOTOSTIMULABLE PHOSPHOR.
IN 1983, CR FIRST CLINICALLY USED IN JAPAN.

6. LIMITATIONS OF CONVENTIONAL RADIOGRAPHY
 Image cannot be adjusted once taken
 Require space for dark room and film/chemicals
storage..
 Price of films and chemicals is constantly going up (4-
10% per year)
 Image developing time is more (manual about 60
minutes)

7. COMPONENTS OF COMPUTED RADIOGRAPHY
THE CR SYSTEM CONSISTS OF IMAGING PLATE,A
READER,COMPUTER AND A PRINTER IN ADDITION TO X RAY UNIT.
IN CR, IMAGE IS CAPTURED IN THIN SHEET OF PLASTIC KNOWN AS IMAGING PLATE.
HAS SEVERAL LAYERS:
PROTECTIVE LAYER
PHOSPHOR/ ACTIVE LAYER
REFLECTIVE LAYER
CONDUCTIVE LAYER
SUPPORT LAYER
BACKING LAYER

8. PROTECTIVE LAYER: THIN, TOUGH, CLEAR, PLASTIC THAT
PROTECTS THE PHOSPHOR LAYER.
PHOSPHOR LAYER: THIS IS A LAYER OF PHOTOSTIMULABLE
PHOSPHOR THAT “TRAPS” ELECTRONS DURING EXPOSURE.
IT IS USUALLY MADE OF PHOSPHOR FROM THE BARIUM
FLUOROHALIDE FAMILY (EG, BARIUM
FLUOROHALIDE,CHLOROHALIDE OR BROMOHALIDE CRYSTALS)

9. REFLECTIVE LAYER:THIS IS A LAYER THAT SENDS LIGHT IN A
FORWARD DIRECTION WHEN RELEASED IN THE CASSETTE READER,
THIS LAYER MAY BE BLACK TO REDUCE THE SPREAD OF
STIMULATING LIGHT AND THE ESCAPE OF EMITTED LIGHT.
CONDUCTIVE LAYER:THIS IS A LAYER OF MATERIAL THAT
ABSORBS AND REDUCE STATIC ELECTRICITY.

10. READER
 THE READER IS THE MOST CRITICAL PART OF CR IMAGING
SYSTEM.THE CR CONSISTS OF:
 ENTRY SYSTEM FOR IMAGING PLATE
 LASER LIGHT SOURCE
 PHOTOMULTIPLIER TUBE

12.  AFTER THE RADIATION EXPOSURE THE CR CASETTE IS
INSERTED INTO THE READER, WHERE THE IMAGING PLATE IS
REMOVED AND FITTED INTO DRIVE MECHANICS.
 THE DRIVE MECHANICS MOVES THE PLATE WITH CONSTANT
VELOCITY.THIS IS USUALLY DONE WITH SLOW MOTION AND IS
ALSO CALLED AS SLOW SCAN MODE.
 A ROTATING AND MULTIFACED MIRROR REFLECTS THE LIGHT
FROM LASER LIGHT SOURCE.
 THIS LIGHT IS REFLECTED BACK AND FORTH ACROSS THE
PHOSPHOR PLATE IN THE HORIZONTAL, X DIRECTION, WHICH
RELEASES VISIBLE, BLUE-GREEN LIGHT .
 THIS IS DONE BY FAST SCAN MODE.

14. PHOTOMULTIPLIER TUBE:
PHOTOMULTIPLIER TUBE IS USED TO COLLECT THE RELEASED BLUE-
GREEN OR UV LIGHT DURING SCANNING, AND GIVES AN ELECTRONIC
SIGNAL.
THE PMT AMPLIFIES THE SIGNAL AND GIVES THE OUTPUT ELECTRONIC
SIGNAL, WHICH IS FED INTO THE COMPUTER FOR FURTHER PROCESSING.
ELECTRONIC SIGNAL IS DIGITIZED AND STORED IN MEMORY.

15. PHOTOSTIMULABLE PHOSPHOR
 THE COMMONLY USED PHOSPHOR IS BARIUM FLUOROHALIDES;
BAFBR(85%) AND BAFI(15%):EU(EUROPIUM)
 THE EUROPIUM IS CALLED ACTIVATOR AND IT IS PRESENT IN SMALL
QUANTITY, WHICH IS RESPONSIBLE FOR THE PSL PROPERTY.
 THE ACTIVATOR CREATES DEFECTS IN THE F CENTRE WHICH CAN TRAP
ELECTRONS.

16. X FRAY EXPOSURE
 WHEN THE PHOSPHOR IS EXPOSED TO RADIATION, THE DIVALENT
EUROPIUM ATOMS(EU+2) GET OXIDIZED INTO TRIVALENT
EUROPIUM(EU+3) WITH RELEASE OF ELECTRONS IN THE VALENCE
BAND.
 THESE ELECTRONS MOVE FROM THE VALENCE BAND TO THE
CONDUCTION BAND, LATER THEY ARE TRAPPED AT THE F- CENTRE
IN THE FORBIDDEN ZONE.
 THE ELECTRONS CAN STAY IN THESE CENTRES FOR LONG PERIOD
OF TIME.
 THUS BILLIONS OF ELECTRONS ARE TRAPPED IN THE F CENTRES.
 THE NUMBER OF ELECTRONS PER UNIT AREA IS PROPORTIONAL TO
THE ABSORBED RADIATION ENERGY.

18.  OVER THE TIME THESE ELECTRONS MAY RETURN TO THE GROUND STATE
ON THEIR OWN.
 HOWEVER EXPOSING THE PHOSPHOR TO A RED LIGHT SOURCE MAY
ACCELERATE THE ELECTRON RETURN PROCESS.
 WHEN THE IMAGING PLATE IS SCANNED BY RED LASER LIGHT, THE F
CENTRE ABSORBS THE ENERGYAND TRANSFER THE SAME TO THE
ELECTRONS.
 THE ELECTRONS REACH TO THE CONDUCTION BAND, WHERE THEY
BECOME MOBILE AGAIN.
 THEY MOVE TO THE VALENCE BAND, WITH EMISSION OF BLUE-GREEN
LIGHT.
 THE ELECTRON JOINS WITH TRIVALENT EUROPIUM AND IS CONVERTED
INTO DIVALENT EUROPIUM
 THE BLUE GREEN LIGHT IS GREATER THAN THAT OF LAAER LIGHT
ENERGY.

19.  THE PHOSPHOR WILL NOT GIVE UP ALL ITS TRAPPED
ELECTRONS IN THE FIRST STAGE OF LASER LIGHT.IT WILL
RETAIN SOME AMOUNT OF TRAPPED ELECTRONS.
 HENCE IT IS TO BE EXPOSED TO VERY BRIGHT LIGHT
SOURCE,WHICH MOVES ALL THE TRAPPED ELECTRONS TO THE
VALENCE BAND, THUS EMPTYING THE F CENTERS.
 NOW THE PHOSPHOR CAN BE USED FOR ANOTHER RADIATION
EXPOSURE.

20. STEPS INVOVLVED
 CR CASSETTE IS EXPOSED TO X-RAY BEAM.
 CASSETTE IS INSERTED INTO THE READER.
 IMAGING PLATE IS REMOVED FROM THE CASSETTE.
 SCANNED BY THE HE-NE LASER BEAM,BLUE-GREEN OR UV LIGHT IS
RELEASED FROM THAT LOCATION.
 PMT IS USED TO COLLECT THIS LIGHT AND GIVES AN ELECTRONIC
SIGNAL.
 ELECTRONIC SIGNAL IS DIGITIZED AND STORED IN MEMORY.
 THE PLATE IS EXPOSED TO BRIGHT WHITE LIGHT, TO ERASE THE
RESIDUAL ENERGY FOR ANOTHER USE.

21. IMAGE SCANNING:
PLATE IS SCANNED BY THE LASER CAUSING THE EMISSION OF BLUE LIGHT
FROM THE PLATE WHICH IS THEN DETECTED BY THE PHOTOMULTIPLIER
PLATE, IS ERASED WITH LIGHT(NOT LASER)AND RETURNED TO THE
CASSETTE.

22. ERASING PROCESS
 Residual latent image information is erased by using an intense light
(consisting of wavelengths that remove electrons in traps without
stimulating further electron trapping), and the image plate is
reinserted into the cassette for reuse.
 Exposure to a bright fluorescent light removes the remaining
information in 10-15 sec.

23. ADVANTAGES
 CR technology can be considered as the digital replacement of
conventional X-ray film.
 Post-processing, manipulation & storage of images is easy.
 Repeat examinations are reduced due to wide exposure latitude.
 Image Plate is reusable.
 Ability to produce consistent high quality images
 Ability to deliver images quickly to those who need to make critical
decisions.
 Decreased time to acquire images thereby increasing patient throughput in
medical diagnosis.
 Increased Radiologist reading capacity and the ability to have instant
comparison images along with previous reports

24.  Ability to print on paper or make CD media instead of film, economically
profitable.
 Increased savings: no film, chemicals, dark room and storage room
required
 Computer processing of raw image brightness, contrast, sharpness
enhancement, zooming, measurements.
 Reduction in exposure and processing time
 Digital magnification allows better detail viewing.

25. DISADVANTAGES
 Additional cost (Service and maintenance)
 Plate is sensitive to fogging: use grid, need to be erased daily
 Dose-creep: since, exposure latitude is wide, high exposure
technique is used which increases the patient dose which is
called dose-creep.