mod XERORADIOGRAPHY & STEREOSCOPIC IMAGING.pdf

Health care science

1. XERORADIOGRAPHY
&
STEREOSCOPIC IMAGING

2. XERORADIOGRAPHY

3. Xeroradiography was invented by Chester F . Carlsonin 1937

4. Introduction
Production of a visible image using the charged surface of
a photoconductor-detecting media.
Dissipates charge by exposure to x-rays
Latent image formed
Xerographic processing
Visible image

5. General Principles
A complex electrostatic process based on a Photoconductor.
Photoconductor used is Amorphous Selenium
SHEET OF ALUMINIUM
THIN LAYER OF SELENIUM
A material which conducts electric current when exposed to
radiation
CASSETTE

6. Uniform charge –Surface of the plate
Plate –Placed in a light tight cassette
Expose –Photoconductor layer loses charge α
intensity of X –ray
Remaining charge pattern –Latent image
Developing process by Toner
Toner image transferred to a receiver sheet
The plate is cleaned for reuse

7. Xeroradiographicplate

8. ALUMINIUM SUBSTRATE :
•Cleaned Aluminium
•Exceedingly Smooth Surface
•High sensitivity
INTERFACE LAYER :
•Thin layer of Aluminium Oxide –an insulator
•Prevents negative charges induced in the
aluminiumfrom migrating into the selenium &
dissipating the positive charge induced on the
selenium surface.
SELENIUM COATING :
•Highly purified selenium in the Amorphous / Vitrous
form
•Deposited on the aluminium substrate by condensation
of vaporized liquid selenium in a high vacuum.
•Thickness = 150 microns ( for powder toner
development plates)
320 microns (for liquid toner development
plates)
PROTECTIVE OVERCOATING:
•Cellulose acetate
•Applied by dip-coating from solution
•Thickness = 0.1 microns
•Extends the life of the plate by a factor of about 10

9. Photoconductive layer
3 properties:
•Electrical conductivity in the dark must of a
good insulator
•Must become electrically conducting during
exposure to x-rays
•Posses mechanical properties –durability &
ease of fabrication (Amorphous Selenium)

10. Photoconductive layer
•Pure amorphous selenium is mandatory as the presence
of any impurity increases the DARK DECAY RATE
•DARK DECAY
It is the reduction of plate voltage while the plate
remains in darkness
•SENSITIVITY
It depends on selenium thickness (150 µ, 320 µ) &
the energy (KVp) of the x-ray beam

12. •First step in the process
•Sensitize the photoconductor by applying a uniform
electrostatic charge to its surface in the dark.
•Xeroradiographicplate considered like a Parallel Plate
Capacitor.
•Charging the plate is done on the principle of
Corona Discharge in a gas….
ALUMINIUM SUBSTRATE
SELENIUM

14. •Corona threshold voltage is applied between a fine
wire & ground causing air near the wire to ionize.
•If voltage applied is positive, free electrons in the air
move toward wire
•Electrons will interact with many molecules in air
and create additional ions
•Positive ions move outward from the wire called
Corona Current
Corona discharge

15. contd…
Corona threshold voltage -4400 V
Plate range : 1000 –1600 V

16. •Plate range -2200V
Two problems
1) Overcharging of plate must be prevented
2) Small variationin wire will affect the threshold
voltage & causing non uniformity of corona current
Variations Minimized byCorona threshold voltage
: 7500 V

17. PLATE CHARGING –CHARGING DEVICES
PLATE CHARGING DEVICES
SCOROTRON COROTRON
Consists of :
•A control screen/grid
•Corona emitting wire / wires
•Grounded backing plate
•Grid of parallel wires –3/32 inch apart
placed between the corona wire and the
surface of the Xeroradiographic plate.

18. PLATE CHARGING DEVICES
SCOROTRON COROTRON
Consists of :
•A long ‘U’ shaped channel called
Shield maintained at ground
potential.
•Corona wire

19. After Corona charging
Exposure of charged plate
PLATE
Cassette

20. Plate exposed to X-rays
electron hole pair
electron , surface of plate ( dischargethe +veexisting)
+vecharge , Al substrate (neutralize induced –vecharge)
Discharge αintensity of X-ray
Remaining Charge pattern -Electrostatic Latent Image

21. PHOTOCONDUCTIVITY
OF THE PLATE
Photoconductivity
X-ray photons can penetrate
further into selenium and their
absorption may be almost
uniform through out selenium
layer
The energy of an absorbed /
scattered X-ray photon is
transferred to a photoelectron

22. Electric field distribution above latent image
•Development of the electrostatic image depends on
the attraction of charged particles to the surface of
the plate
Electric field
Both vertical & horizontal
component
Field strength declines rapidly as
one moves away from the charged
surface

23. Discharged portion Discharged portion

24. POWDER DEVELOPMENT
•Selective deposition of imaging material onto a surface in
response to electrostatic forces.
•Development consists of attracting small charged dust
particles, called toner.
•Commonly used toner is Charcoalwith a particle size of
1microns.
•Requires the use of pigmented thermoplastic material.
•Average diameter of particle is 4 microns

25. Powder cloud development chamber
DIFFUSION
AIR
DEVELOPMENT
ELECTRODE
+500 V
INSULATED
SEAL
+1750 V BACK BIAS
PLATE
SELENIUM
NOZZLE
TONER
GAS
TONER
TONER
CLOUD

26. •Agglomeration of toner into large particles is
prevented because of the turbulent flow produced
when the aerosol passes through the small bore
nozzle
•Electric charge on the toner particles is produced
by friction b/w the toner and the wall of the nozzle
-Triboelectrification/ Contact Electrification

27. Development process
•Consists of attracting charged toner particles to the configuration
of residual positive electrostatic charges on the selenium surface.
•The plate containing latent image is clamped onto the top of a
powder cloud chamber and toner introduced as aerosol
•Diffusion air is also introduced into the chamber to create air
turbulence
•+veimage -+vevoltage is applied to Al backing of plate
( Back Bias voltage )

28. Two electrostatic forces influencing motion of
particles:
•The uniform electric field of the back-bias voltage
•Non-uniform field of the latent image
For Positivedevelopment = 2000 V DC
For Negativedevelopment = -3350 V DC

29. Size of the deletion depends on:
•Voltage contrast ( voltage difference) at an edge
•Magnitude of the back-bias voltage
CONTRAST DELETION
WIDTH
High back-bias Low Small
High plate
voltage
High Large
High contrast
image
High Large
Low contrast
image
low small

30. Development electrode
•It is an electrode placed in front of the plate during powder
cloud development, and is given a voltage of appropriate
sign to superimpose a uniform helping field in the direction
of the field for the toner particles of desired charge.
“Causes a visible image to be developed in uniformly
exposed regions of the electrostatic latent image.”

31. Development electrode
•It is a grid (series of wires) placed close (1.5-2.5mm) to the
surface of the plate.
•Potential applied = +500V DC
“ The development electrode allows satisfactory detail to be
developed in broad exposure areas without sacrificing
information in areas of fine detail.. “

32. IMAGE TRANSFER & FIXING
•An electrostatic transfer process
•PAPER coated with a deformable layer of plastic, a
polyethylene material
•Toner particles embedded in the plastic when the paper is
pushed against the powder image under relatively high
pressure

33. •After development, plate is passed over a pretransfer
Corotronthat has a negative charge.
•Image is transferred to the paper
•Loosely held powder is made into a permanent bond by
heating the paper to about 475
o
F

34. •Plate exposed to electroluminescent strip that reduces the
bond holding residual toner to the plate.
•Precleancorotronexposes the plate to an alternating
current that serves to neutralize the electrostatic forces
holding toner to plate.
•A cleaning brush mechanically brushes the residual toner
from the plate.
Plate cleaning

35. Relaxation ( Plate Fatigue)
•It is done to prevent faint “ghost” images from appearing.
•Residual photoconductivity of the selenium would cause
some discharge in the dark and create a “ghost” image of
the previous exposure.
•Rest period can be reduced to 2 or 3 minutes if the plate is
relaxed by heating it to 140
o
F for 150 sec.
•Plates are stored in the storage compartment at 89
o
F until
needed for another exposure.

36. LIFE OF A XERORADIOGRAPHIC PLATE:
•It is in the order of several thousand exposures.
•Main cause of plate failure is physical damage caused
by handling and the cleaning process.

37. Automatic Xeroradiographic System
1.
2.
3.
4.
5.
CONDITIONER
1 -Plate storage Box 2-Relaxation Oven 3 –Plate Storage Compartment 4 –Plate Charging 5 –Cassette

38. 5.
1.
7.
6.
14.13. 8.
12.
11.
10.
PROCESSOR
Automatic Xeroradiographic System
6 –Plate Transport Mechanism 7 –Development Chamber 8 –Pre –Transfer station 9 –Paper Storage and Feeder 10 –Transfer Station11 -Fusing Oven12 –Paper Print Tray 13 –Pre-Cleaning Station 14 –Plate Cleaning Station

39. Liquid Toner Xeroradiography
•Efforts to reduce patient dose resulted in the introduction
of a liquid development system-Xerox 175 System, in 1985.
•Three factors contributing to dose reduction:
–Different photoreceptor manufacturing process
–Thicker photoreceptor
–More sensitive developer

40. Photoreceptor made of Amorphous Selenium
2 factors contributing to the increased sensitivity of the
receptor :
•A thicker photoreceptor layer increases x-ray absorption
•A higher conversion of absorbed x-ray energy to charge
signals.
Liquid Toner Xeroradiography

41. STEREOSCOPIC IMAGING

42. Stereoscopy(stereoscopicsor3D imaging) is a
technique for creating or enhancing theillusion
of depth in an image by means of stereopsis
forbinocular vision

43. Stereoscopic radiologyis the use of stereoscopic
imaging principles on radiographs and volumetric
data.

44. Physiology of Depth Perception
•Depth Perception is extremely complex &
depends on two independent mechanism
–Monocular / Photographic Depth ( 1 Eye )
–Stereopsis ( Binocular vision )

45. Monocular Depth Perception
Artist depicts depth the same way , see depth with one eye
•Size -Near objects larger than distant object
•Overlapping contours –Near objects overlap
distant object
•Perspective -See objects from a particular
vantage point
•Shading -A sphere has the contour of a circle
but with proper shading , perceive its true
shape

46. •Air haze –Water vapour , dust and smoke in
the atmosphere absorb and reflect light casting
a veil over a distant objects and causing them
to appear farther away and less sharp

47. STEREOPSIS
•Dependent on the brain’s ability to receive
slightly different images from each eye ,
Discrepant images and then fuse to them into a
single image that has depth.
•If the images are too different the brain can’t
fuse them
•If the difference is too small, the fused images
are appear flat.

48. Accommodation & Convergence
•Accommodation –Ability to change the
curvature of the lens with a change in visual
disturbance
•Convergence -A turning point in of the optical
axes so that both eyes can see the same subject

49. Depth Perception
Relative Absolute
•Ability to distinguish which
of 2 objects is closer to the
observer
•Qualitative
•Ability to judge how far
away an object is or the
distance b/w 2 objects
•Quantitative

50. Parallax
•Apparent displacement of an object when
viewed from two different vantage points
Convergent Angle α, is larger from A than from B (β) &
difference is called Angle of Instantaneous Parallax

51. StereoscopicFilming
2 films are exposed one for each eye

52. 4 in.
40 in.
StereoscopicFilming
2 films are exposed one for each eye

53. Magnitude of Tube Shift
•A 10 % shift produces discrepant images
∅
4 in.
40 in.
Tan θ= 4 / 40
= 0.10
Θ˜ 6 degree

54. Direction of Tube Shift
•Important for 2 reasons
–Examinations are done with grid technique
–Undesirable to shift across the long axis of a grid
because of grid cut off from lateral decentring

55. STEREOSCOPIC VIEWING
•3 preliminary steps
–Identify tube side of the film
–Determine direction of the x-ray tube shift
–Determine which film is to be viewed by the left
eye , and which by the right eye

56. •Edges of film not superimpose due to
–Patient movement
–Second film was put in a different position than the
first film
–Tube moved in two direction during the stereo
shift

57. ViewingSystems
•Stereoscopic radiographs are incapable of
judging the absolute distance between objects
•Stereopsis are discrepant images , and the
degree of discrepancy must be sufficiently
great to permit an appreciation of depth , and
yet not so great than the brain can’t fuse the
images

58. Cross-eyed stereoscopy
•Two films are interpreted at the same time

59. •Accommodation & convergence must be
dissociated
•Eyes cross so the right eyes see film on the
left side & both eyes converge to point C
•If eyes also accommodated for point C ,the
films would be out of focus .

60. •Advantages
–No cost
–Always available
–Most convenient system to use
•Disadvantages
–Requires a lot of practice
–Eye strain
–Reduced in size or out of focus

61. Wheatstone stereoscope
•Used to illustrate how they function

62. •Convergence is assisted by a pair of mirrors that
are located halfway films
•The mirrors are adjusted to superimpose the RT
& LT eye films with out changing our convergent
effort from what it would normally be for the
viewing distance
•Can see a stereoscopic image hanging in space
behind the mirrors

63. Binocular prism stereoscope
•Consists of a pair of prisms mounted into
binoculars

64. Single –mirror stereoscope
•The small pocket mirror is held against the
side of the nose and both eyes focus on the LT
eye film

65. •The mirror is carefully adjusted until the LT &
RT eye images superimpose and are fused into
a combined image having depth
•The mirror should be silvered on its front
surface because of the high angle of reflection
•Disadvantage :-
–Difficult to hold the mirror steady

66. Advantages
•Education of normal anatomy
•Foreign body localization
•Localization of intracranial calcifications
•Unimposing confusion shadows

67. DisAdvantages
•Expense
•Patient exposure
•Need of patient co-operation
•Need of practice