it was a seminar presentation, on the basic principles of CR. I presented at aminu Kano Teaching Hospital.

1. COMPUTED RADIOGRAPHY (CR)
BY
Rad. Aminu Abubakar Abubakar
Intern Radiographer
Aminu Kano Teaching Hospital
30th June,2015

2. SYNOPSIS
 Introduction
 Classification of DR
 CR imaging cycle
Image Acquisition
Readout
Erasure
 Reference

3. INTRODUCTION
 During the past two decades, digital radiography has
supplanted screen-film radiography in many radiology
departments
 Today, manufacturers provide a variety of digital
imaging solutions based on various detectors and
readout technologies
 Hence DR can broadly be divided into two
Indirect
Direct

4. INTRODUCTION
Digital
Radiography
Computed
Radiography CR
Direct Radiography
DR
Indirect Conversion
Photostorage
phosphor
Indirect Conversion
Scintillator- TFT
Scintillator-CCD
Direct conversion
photocondutor
Selenium-

5. COMPUTED RADIOGRAPHY
 (CR) was the first available digital technology
for projection radiography
 An indirect radiography
 Uses storage phosphor screen (SPS) or
image plate (IP)
(Lanca & Silver, 2013

6. Fig.1 IP & cassette

7. LAYERS OF IMAGE PLATE
 It has five layers
 Protective layer
Made of Fluorinated polymer material
The phosphor is coated with it
It protects the phosphor
(Shepard,2004 p.322)
 Phosphor layer (active layer)
Based on phosphor crystals
Median particle size of 1 to 15µ
Individual particles may be sub-µ or as large as 30 to 40µ
(Paul et. al.,2011)

8. LAYERS OF IMAGE PLATE CONT…
 Anti-halo & Reflective layer
Blue tinted material couple with reflective layer
It prevents laser light from penetrating
Allows reflected light emitted by the phosphor to pass through
(Shepard,2004 p.322)
 Base
Made of PET
It supports the active layer
 Backing layer
It protects the base from damage
(Shepard,2004 p.321)

9. LAYERS OF UNSTRUCTURED IMAGE PLATE
Protective layer
Phosphor layer
Anti-halo&
Reflective layer
Base
Backing layer
Fig.2 Cross-sectional diagram of a typical PIP

10.  Protective layer
 Phosphor layer
 Anti-halo &
Reflective layer
Base
Reflective layer
Fig.3 Cross-sectional diagram of a typical NIP
LAYERS OF STRUCTURED IMAGE PLATE

11. TYPES OF PHOTOSTIMULABLE PHOSPHORS
 Unstructured
 PIP
 BaFX:EU2+
 The spectrum of light emitted by an efficient
phosphor is controlled by an impurity called
activator (Shepard, 2004)
 X= Cl,I,Br
 Structured
 NIP
 CsBr:Eu2+
 RbBr:Tl+

12. Fig.4 image of Europioum activator

13. TYPES OF PHOTOSTIMULABLE PHOSPHORS
 The photostimulable
phosphor first used for CR
was BaFBr:Eu2+.
 Its crystal structure is non-
cubic,
 i.e. a layered structure that
gives rise to phosphor
grains with a plate-like
rather than the more
desirable cubic
morphology
(Blasse and Grabmaier 1994)
 The NIP is the latest
technology
 Its crystal structure is in
cubic form
 i.e. a layered structure that
gives rise to phosphor
grains with a needle shape
 Act as light guide
 ↓ the light diverges
 Produces better image

14. LAYERS OF UNSTRUCTURED IMAGE PLATE
Protective layer
Phosphor layer
Anti-halo&
Reflective layer
Base
Backing layer
Fig.5 Cross-sectional diagram of a typical PIP

15.  Protective layer
 Phosphor layer
 Anti-halo &
Reflective layer
Base
Reflective layer
Fig.6 Cross-sectional diagram of a typical NIP
LAYERS OF STRUCTURED IMAGE PLATE

16. Fig.7 cross-sectional diagram of a typical NIP

17. REQUIREMENTS FOR A GOOD STORAGE PHOSPHOR
 High X-ray absorption
for energies ranging from 20 keV to 140 keV.
 High conversion efficiency
This implies that a large fraction of absorbed X-ray quanta is
converted into trapped electrons and holes leading to PSL
 Should have slow fading
electron- and hole traps should be stable
dark-decay of the stored image in a CR plate is between 10
and 25% in the first hour after X-ray exposure
(Paul et. al.,2011)

18. REQUIREMENTS FOR A GOOD STORAGE PHOSPHOR
 The emission should match the sensitivity spectrum
of the light detector
below 500 nm is preparable
 Should be stable under normal room conditions
its performance should not degrade when it is exposed
to humidity and daylight
(Paul et. al.,2011)

19. PHYSICAL PROPERTIES OF PHOTOSTIMULABLE PHOSPHORS
Phosphor EK(k
e)
G(PH
OTON
S/50
(keV)
Decay
time
(µs)
Light
emission
peak (nm)
SE
µJ/mm2
CE
(pJ/mm2/
mR)
HYGROS
COPICITY
BaFBr:Eu2
+
37.4 140 0.7 390 16 25 Normal
BaFBrI:Eu
2+
37.4 140 0.7 390 16 25 Normal
BaFI:Eu2+ 37.4 140 0.6 390 16 25 Much
higher
CsBr:Eu2+ 36 200 0.7 440 4 35 Normal
RbBr:Tl+ 15.2 *** 0.35 433 4 *** Higher
*** not established (Paul et. al.,2011 & Roland,2002)

20. UNSTRUCTURED
 In the course of the storage phosphor
development it was discovered that partial
replacement of Br by I (BaFBxIy:EU2 ) almost
doubled the storage phosphor efficiency.
 In the commercial phosphor of Agfa, Fuji and
Kodak 15 to 20% of Br is replaced by I.
(Roland,2002)

21. UNSTRUCTURED
 A logical further modification is the complete replacement
of Br by I
 i.e., the transition to BaFI:Eu2+
 it leads to a higher X-ray absorption, especially for
general radiography exposures.
 A disadvantage is the much higher hygroscopicity of BaFI.
Much stronger efforts must be made to shield the
phosphor from moisture.
(Nakano et al.,2002)

22. STRUCTURED
 Most important structured phosphors for medical CR
are of the CsBr:Eu2+ family
 This phosphor has an excellent intrinsic X-ray
absorption, being made up of Cs with a K-edge of 36
 CsBr:Eu2+ is an efficient X-ray storage phosphor,
having adequate spectroscopic properties with a blue
emission
(Kato et al.2002)

23. STRUCTURED
 CsBr:Eu2+ has a light output per absorbed dose that
is higher than that of BaFX:Eu2+
 The CE is about 35 pJ/mm2/mR vs. about 25
pJ/mm2/mR for the best BaFX:Eu2+materials
 Less light is needed to stimulate the phosphor,
allowing the use of a less powerful laser and erasure
source in the scanner
 The stimulation energy is only 4 µJ/mm2
(Kato et al.2002)

24. STRUCTURED
 Konica discovered that RbBr:Tl+ is an
efficient storage phosphor with excellent PSL
properties
 RbBr:Tl+ plate has all the described NIP
benefits.
 However, Rb with its K-edge of 15.2 keV has
a relatively low intrinsic X-ray absorption.
(Kengyelics et al 1998 & Matsuda et al 1993)

25. STRUCTURED
 Therefore, it has to be two times thicker for adequate X-ray absorption
and more expensive NIP.
 It is even slightly below that of the BaFBr:Eu2+ PIP.
 The 0.3 µs decay-time of the PSL is sufficiently short for fast read-out
 In addition, RbBr:Tl+ is much more hygroscopic than BaFBr:Eu2+
 This made the use of RbBr:Tl + plates in a cassette system, in which the
atmospheric conditions cannot be controlled and where the protective
coating can be damaged.
(Leblans et al 2001).

26. STRUCTURED
 Needle imaging plates (NIP’s) have a number of advantages over
PIP’s.
 They have lower self-absorption of emitted light leading to higher
sensitivity.
 Lower self-absorption also allows the use of thicker layers,
having higher X-ray absorption.
 Higher X-ray absorption also results from a higher packing
density.
 In addition, suppression of lateral light diffusion in a vapor
deposited layer leads to improved resolution.

27. CR CYCLES
 It involves the following cycles:
Image acquisition
Readout
Erasure

28. IMAGE ACQUISITION
 When exposure is made ; two things happen
 Conversion gain
 Latent Image formation
(Yaffe & Rowlands, 1996)

29. LATENT IMAGE FORMATION
 A key concept in latent image formation is exciton and F-
centers
 exciton Is a hydrogen-like pseudo-atom consisting of a
bound electron and hole.
 The exciton is a neutral entity that can form in crystalline
phosphor after radiation has created ionization.
 The exciton can move freely within the crystal
(Kato et al.2002)

30. LATENT IMAGE FORMATION
 An F center is an electron trapped in an anion vacancy
generated by X-rays
 Taking BaFBr:Eu2+ crystal as example
 Ba2+ layers are alternately interspaced by Br− layers and
F− layers
 Hence, F(Br−) and F(F−) centers are created as electron
traps
(Kato et al.2002)

31. LATENT IMAGE FORMATION
Fig.8 Matlockite F center model for BaFBr

32. LATENT IMAGE FORMATION
 The holes are trapped by Eu2+ ions, which
are thus oxidized to Eu3+
Fig.9 Energy diagram showing electron and hole trapping in a storage
phosphor

33. READOUT PROCESS
 The Laser Scanner are of three types:
 Flying Point-scan Laser Readout
 Dual-side Laser Readout
 Line-scan Laser Readout

34. READOUT PROCESS
 The readout process entails three steps:
Stimulation with laser light
Detection and conversion of PSL to electrical signal
Conversion of electrical signals to digital signal
 Laser source
 Beam splitter
 Reference detector
 Beam deflector Stimulate the F centers
 F-θ lens
 Cylindrical mirror
 Light channeling guide (Shepard,2004)

35. STIMULATION WITH LASER LIGHT
 latent image imprinted on the exposed phosphor IP
corresponds to the activated F-centers, whose local
population of electrons is directly proportional to the
incident x-ray fluence
 Stimulation of the F-center and release of the stored
electrons requires a minimum energy of ~2 eV
 Most easily deposited by a highly focused laser light
source of a specific wavelength
(Bogucki,1995)

36. STIMULATION WITH LASER LIGHT
 A HeNe (helium-neon, λ633 nm) and “diode”
(λ≅680 nm) laser sources are most often used,
with the latter becoming much more prominent.
 The incident laser energy excites electrons in the
local F-center sites of the phosphor
 the electrons recombine with the hole at the Eu3+
complex

37. STIMULATION WITH LASER LIGHT
 The recombination energy is transferred to an
electron of the activator (EU3+)
 A light photon of 3eV (λ≅390-440 nm)
immediately follows as the electron drops
through the energy level of the (EU3+) complex
to the more stable Eu2+ energy level
 The above phenomenon is called PSL
(Shepard,2004&
kato,2002)

38. FLYING POINT-SCAN LASER READOUT
Fig.10 Schematic diagram of Flying point –scan laser
scanner

39. DUAL-SIDE LASER READOUT
Figure 10 Schematic diagram of dual side scanner.

40. LINE-SCAN LASER READOUT
Fig.11Schematic diagram of a “line-scan” PSP system

41. DETECTION AND CONVERSION OF PSL TO ELECTRICAL SIGNAL
 Detection and conversion of PSL to
electrical signal is done by
PMT: in Flying Point-scan dual-side Laser
Readout
CCD photodiode in Line-scan Laser Readout
Fig.12 image of CCD

42. PMT
 The photomultiplier tube is a vacuum tube with a
photocathode on the end
 A photocathode is a clear photosensitive glass surface
 The light striking the photocathode causes it to emit
electrons, referred to as photoelectrons
 The number of electrons produced at the photocathode is
greatly increased by the multiplying action within the tube
(Cherry et al.,2003)

43. PMT
 As soon as they are produced, the electrons cascade
along the multiplier portion of the tube successively
striking each of the tube’s dynodes
 As an electron strikes a dynode, it knocks out two to four
new electrons, each of which joins the progressively
larger pulse of electrons cascading toward the anode at
the end of the tube
 The electrical signal from the PMT must be further
amplified before it can be processed and counted
(Cherry et al.,2003)

44. Fig.13 image of PMT showing dynodes

45. CONVERSION OF ELECTRICAL SIGNALS TO DIGITAL SIGNAL
 Conversion of electrical signals to digital
signal by
ADC
Fig.14 image of ADC

46. ERASURE
 Residual latent image signals are retained on
the phosphor plate after readout.
 Residual signals are erased using a high
intensity light source of white or
polychromatic content that flushes the traps
without reintroducing electrons from the
ground energy level
(Shepard,2004)

47. Fig. 15 image of sodium vapor light

48. SUMMARY
 CR system is separated into three steps.
 First, the image plate (IP) is exposed to x-ray energy, part
of which is stored within the detective layer of the plate.
 Second, the image plate is scanned with a laser beam, so
that the stored energy is set free and light is emitted. An
array of photomultipliers collects the light, which is
converted into electrical charges by an ADC.
 Third, the residual energy is erase by sodium vapor light .

49. Fig.16 Drawing illustrates a CR system based on storage-phosphor image plates.

50. THANK YOU FOR LISTENING
sadiqbabahabu@gmail.com

51. REFERENCE
 Blasse G and Grabmaier B C 1994 Luminescent Materials (Berlin:
Springer)
 Kato H 2002 Private communication
 Cherry, SR, Sorenson, JA, and Phelps, ME, (2003) Physics in Nuclear
Medicine, 3rd edition, Saunders, Philadelphia,

 Kengyelics S M, Davies A G and Cowen A R (1998) A comparison of the
physical imaging properties of Fuji ST-V, ST-VA, and ST-VN computed
radiography image plates Med. Phys. 25 2163–9
 Lanc L¸ Silva A (2013), Digital Radiography Detectors: A Technical
Overview, Springer Science+Business Media New York, accessed 7th
June 2013, http://www.10.1007/978-1-4614-5067-2_2
 Leblans P, Struye L and Willems P (2001) New needle-crystalline CR
detector Proc. SPIE 4320 59–67

52. REFERENCE
 Nakano Y, Gido T, Honda S, Maezawa A, Wakamatsu H and Yanagita T
2002 Improved computed radiography image quality from a BaFI:Eu
photostimulable phosphor plate Med. Phys. 29 592–7

 Lo J Y, Floyd C E Jr, Baker J A and Ravin C E (199)4 Scatter
compensation in digital chest radiography using the posterior beam stop
technique Med. Phys. 21 435–43
 Sephert CT (2000), Radiographic Image Production and Manupulation,
MC Graw Hill Companies, UK
 T. Bogucki, D. Trauernicht, and T. Kocher. Company
(1995)Characteristics of a Storage Phosphor System for Medical
Imaging. Kodak Health Sciences Division. Rochester, NY: Eastman
Kodak.