The document discusses the components and functioning of conventional fluoroscopy, digital fluoroscopy, and digital subtraction angiography (DSA). It describes the key components of an image intensifier tube used in conventional fluoroscopy including the glass envelope, input phosphor, photocathode, electrostatic focusing lens, and output phosphor. Digital fluoroscopy systems use a charge-coupled device (CCD) instead of a television camera and can acquire images faster with less radiation exposure to the patient. Flat panel detectors are now also used as alternatives to image intensifier tubes.

1. CONVENTIONAL,DIGITAL
FLUOROSCOPY AND Digital
SUBTRACTION ANGIOGRAPHY
Subodh Dhungana
Chitwan Medical College

2. INTRODUCTION
• Refers to the way of obtaining image in the fluorescent screen.
• Conventional fluoroscopy
• Modern fluoroscopy
Advantages
• Real-time imaging & visualization of anatomy & dynamic processes.
• Also used to position the patient for subsequent image recording or
devices for interventional procedures.
• Possible to examine the patient in different positions & to evaluate
profile views of abnormalities found.

3. • Fluoroscopy is actually a rather routine type of x-ray examination
except for its application in the visualization of vessels, called
angiography.
• The fluoroscope is used for examination of moving internal structures
and fluids.

4. HISTORY
• In 1896 Thomas Addison discovered fluoroscopy using barium
platinocyanide as screen phosphor
• – Modified II/TV system with more brightness gain(1948)
• – Temporal and energy subtraction (1960)
• – Clinical DSA systems(1977)
• – Qualitative and quantitative improvements
• – Image processing advances

5. • Early fluoroscopes: Cardboard funnels, open at narrow end for viewing,
the wide end closed with a thin cardboard piece that coated on the inside
with a layer of fluorescent material.
• Direct viewing of the fluoroscopic image is defective in Color, Sharpness &
contrast than the image produced on the film radiographs.
• Red adaptation goggles were developed by Wilhelm Trendelenburg in
1916. The resulting red light from it’s filtration correctly sensitized the
observer’s eyes prior to procedure.

7. Fluoroscopic Acquisition Components

8. Fluoroscopy Composition / Structure
• Fluoroscopic imaging system, the x-ray tube is usually hidden under
the patient table.
• The image intensifier are set over the patient table.
• With some fluoroscopes, the x-ray tube is over the patient table, and
the image receptor is under the patient table.
• Some fluoroscopes are operated remotely from outside the x-ray
room.
• During image-intensified fluoroscopy, the radiologic image is
displayed on a television monitor or flat panel monitor.
• Image intensifier that converts x-rays into visible light at high intensity

9. • During fluoroscopy, the x-ray tube is operated at less than 5 mA.
• Despite the lower mA, however, the patient dose is considerably
higher during fluoroscopy than during radiographic examinations
because the x- ray beam exposes the patient constantly for a
considerably longer time.
• The kilovolt peak (kVp) of operation depends entirely on the section
of the body that is being examined.

10. Image-Intensifier Tube
• The image-intensifier tube is approximately 50 cm long.
• The image-intensifier tube is a complex electronic device that
receives the image-forming x-ray beam and converts it into a visible-
light image of high intensity.

11. Image-Intensifier Tube
1. Glass envelope
2. Input phosphor
3. Photocathode
4. Electrostatic focusing lens
5. Output phosphor

12. Glass envelope
• Maintains tube vacuum to allow control of electrons flow.
• When installed, the tube is mounted inside a metal container to
protect it from rough handling and breakage.
• The tube components are contained within a glass or metal envelope
that provides structural support.

13. Input phosphor
• X-rays that exit the patient and are incident on the image intensifier
tube are transmitted through the glass envelope and interact with the
input phosphor, which is cesium iodine.
• The CsI crystals are grown as tiny needles and are tightly packed in a
layer of approximately 300 μm, Each crystal is approximately 5 μm in
diameter.
• When an x-ray interacts with the input phosphor, its energy is
converted into visible light.

14. 1st Generation Image Intensifiers
• Input phosphor – Zinc Cadmium Sulfide
• Output phosphor – Zinc Cadmium Sulfide
2nd Generation Image Intensifiers
• Input Phosphor – Cesium Iodide
(smaller crystals with greater packing density)
• Output phosphor – Zinc Cadmium Sulfide

15. Photocathode
• It is bonded directly to the input phosphor with a thin, transparent,
adhesive layer.
• The photocathode is a thin metal layer, usually composed of cesium
and antimony compounds, that respond to stimulation of light with
the emission of electron. This process is known as photoemission.
• The number of electrons emitted by the photocathode is directly
proportional to the intensity of light that reaches it.
• The photocathode emits electrons when illuminated by the input
phosphor.

16. Electrostatic Focusing Lens
• Located along length of the tube, responsible for focusing the
electrons across the tube from input to output phosphor.
• Image is reversed from input to output phosphor (right becomes left,
superior to inferior)
• The concave input screen reduces distortion by keeping the same
distance between all points on the input and output screens.

17. Anode
• Anode is usually charged with 25kV and used to accelerate electron
across the tube.
• Electrons produced by photoemission will be accelerated to the
anode.
• The anode is a circular plate with a hole in the middle through which
electrons pass to the output phosphor, which is just the other side of
the anode and is usually made of zinc cadmium sulfide.

18. Output Phosphor
• The output Phosphor is usually made up of zinc cadmium sulfide
crystals.
• Each photoelectron that arrives at the output Phosphor results in
approximately 50-70 times magnification.
• The output phosphor is the site where electrons interact and
produce light.

19. • For the image pattern to be accurate,
the electron path from the
photocathode to the output
phosphor must be precise.
• The engineering aspects of
maintaining proper electron travel
are called electron optics.
• The interaction of these high-energy
electrons with the output phosphor
produces a considerable amount of
light.
• Each photoelectron that arrives at
the output phosphor produces 50 to
75 times as many light photons as
were necessary to create it.

20. Summary
Glass envelope
• Surrounds all of the components and provides mechanical
support of internal components has a vacuum tube.
Input phosphor
• Receives incident x-rays from the x-ray tube and converts them
into light Composed of cesium iodide
Photocathode
• Attached to the input phosphor by an adhesive layer, Converts
light from input phosphor to electrons by photoemission
Negative portion of the tube
Anode
• Positive portion of the tube. A circular plate with a hole in it in
which electrons are focused to which goes to the output
phosphor
Electrostatic focusing lenses
• Focuses electron path form photocathode to anode by means of
repulsion
Output phosphor
• Converts electrons from anode to light

21. Camera
• The television camera consist of
cylindrical housing, approximately
15 mm in diameter by 25 cm in
length, that contains the heart of
the camera, TV camera tube.
• It also contains electromagnetic coils
that are used to properly steer the
electron beam inside the tube.
• A number of such television camera
tubes are available for television
fluoroscopy, but the vidicon and its
modified version, the plumbicon,
are used most often.

22. • The glass envelope serves the same
function that it does for the X-ray tube;
to maintain a vacuum and provide
mechanical support for the internal
elements.
• These internal elements include the
cathode, its electron gun, assorted
elelctrostatic grids, and a target
assembly that serves as an anode.
• The electron gun is a heated filament
that supplies a constant electron
current by thermionic emission.
• Theses electrons are formed into an
electron beam by the control grid,
which also helps to accelerate the
electrons to the anode.

23. • At the anode end of the tube,
the electron beam passes
through a wire mesh- like
structure and interacts with the
target assembly. The target
assembly consist of three layers
that are sandwiched together.
1. Window
2. Signal plate
3. Target

24. • The target of a television camera
tube conducts electrons, creating
a video signal only when
illuminated.
• Image intensifiers and television
camera tubes are manufactured
so that the output phosphor of
the image intensifier tube is the
same diameter as the window of
the television camera tube,
usually 2.5 or 5cm.

25. Optical coupling
• Television camera tubes and
charge coupled devices (CCDs)
are coupled to an image
intensifier tube in two ways.
A. Fiberoptics
B. Lens system

26. • The simplest method is to use a
bundle of fiber optics.
• One advantage of this type of
coupling is its compact assembly,
which makes it easy to move the
image intensifier tower. This
coupling is rugged and can
withstand relatively rough handling.
• The principal disadvantage is that it
cannot accommodate the
additional optics required for
devices such as cine or photospot
cameras.

27. Lens Coupling:
• To accept a cine or photoshot camera,
lens coupling is required. This type of
coupling results in a much larger
assembly that should be handled with
care.
Working:
• The objective lens accepts light from
the output Phosphor and converts it
into a parallel beam.
• When an image is recorded on film, this
beam is interrupted by a beam-splitting
mirror so that only a portion is
transmitted to the television camera;
the remainder is reflected to a film
camera. Such a system allows the
fluroscopist to view the image while it
being recorded.

28. THREE FORMS OF FLUOROSCOPY
• DIRECT FLUOROSCOPY
• TV FLUOROSCOPY
• DIGITAL FLUOROSCOPY

29. DIRECT FLUOROSCOPY

30. DIRECT FLUOROSCOPY

31. DIRECT FLUOROSCOPY
Photo: Suitcase portable x-ray unit & hand fluoroscope

32. TV FLUOROSCOPY

33. TV FLUOROSCOPY
• Image intensifier- TV system

34. TV FLUOROSCOPY

35. WHY DIGITAL FLUOROSCOPY?
• Image acquistion faster
• Post processing is done
• Linear response
• Low patient dose(last frame hold)
• Pulse progessive fluoroscopy
• Temporal frame averaging

36. DIGITAL FLUOROSCOPY IMAGING SYSTEM
• A digital fluoroscopy examination is conducted in the much same
manner as a conventional fluoroscopic study but a computer has
been added and have multiple monitors and a more complex
operating system

37. DIGITAL FLUOROSCOPY IMAGING SYSTEM
• Digital Fluoroscopy Imaging System contains:
• alphanumeric and special function keys for patient data and
communication with the computer,
• Additional special function keys for data acquisition and image
display,
• Computer interactive video control,
• A pad for cursor and Region of interest (ROI) manipulation or
trackball, joystick, mouse.
• Al least two monitors- one for edit patient and examination data and
to annotate final image and other for subtracted images.

39. DIGITAL FLUOROSCOPY IMAGING SYSTEM
• During DF, the X-ray tube actually operates in the radiographic mode.
• This is not a problem as images from Digital Fluoroscopy are obtained by
pulsing the X-ray beam in a manner called Pulse-Progressive Fluoroscopy.
• PPF contains 3 stages:
• INTERROGATION TIME
• EXTINCTION TIME
• DUTY TIME
• DF system must incorporate high frequency generators with very rapid
switching on and off with interrogation time and extinction times of less
than 1 ms.

40. CHARGE-COUPLED DEVICE
• A major change from conventional fluoroscopy to Digital fluoroscopy
is the use of a charge-coupled device (CCD) instead of TV camera
pickup tube.
• Developed in 1970s for military applications especially in night vision
scopes.
• The application of CCD in fluoroscopy is a recent development.
The Sensitive component is a layer of crystalline silicon.

41. CHARGE-COUPLED DEVICE

42. CHARGE-COUPLED DEVICE

43. CCD
• CCDs can be tiled to receive the light from
an area X-ray beam as it interacts with a
scintillation phosphor such as Cesium
Iodide (CsI).
• The scintillation light from a CsI phosphor is
efficiently transmitted through fiber optic
bundles to the CCD array.
• The result is the high X-ray capture
efficiency and good spatial resolution – up
to 5lp/mm.
• CsI/CCD is an indirect Digital Fluoroscopic
process by which X-rays are first converted
into light and then to electric signal.

44. ADVANTAGES OF CCD
• High spatial resolution
• High signal to noise ratio
• High quantam detective efficiency
• No warm up required
• No spatial distortion
• No maintenance
• Unlimited life
• Not affected by magnetic field
• Linear response
• Low patient dose

45. CCD
• The spatial resolution of a CCD is determined by its physical size and
pixel count.
• 1024 matrix can produce images with 10lp/min spatial resolution.

46. FLAT PANEL IMAGE RECEPTOR(FPIR)
• Next development in Direct Fluoroscopy.
• Composed of either cesium iodide(CsI) or amorphous silicon and
amorphous selenium(a-Se)
• Two types:
• Direct conversion FPIR
• Indirect conversion FPIR

47. INDIRECT CONVERSION
• Indirect conversion FPIR involves the use of CsI to capture the X-ray as
well as transmission of the resulting scintillation light to a collection
element.
• The collection element is silicon sandwiched as a Thin Film
Transistor(TFT).
• Silicon is a semiconductor that usually is grown as a crystal but when
identified as amorphous Silicon (a-Si), silicon is not crystalline but is
fluid that can be painted onto a supporting surface.
• CsI has a high photoelectric capture as Cs has atomic no. 55 and
Iodine has 53

48. INDIRECT CONVERSION

49. • CsI/a-Si is an indirect Direct Fluoroscopy process by which X-rays are
converted first to light by CsI and then to electrical signal by a-Si.
• The image receptor is fabricated into individual pixels which has light
sensitive face of a-Si with a capacitor and a TFT embedded in it.

50. • The geometry of each individual pixel is very important as a portion of
the pixel face is occupied by conductors, capacitors and TFT.
• It is not totally sensitive to the incident image forming X-ray beam.
• The percentage of the pixel face that is sensitive to X-rays is the fill
factor, which is approximately 80%; therefore 20% of the X-ray beam
does not contribute to the image.
• As the pixel size is reduced, spatial resolution improves but at the
expense of the patient radiation dose.
• With smaller pixels, the fill factor is reduced and the X-ray intensity
must be increased to maintain adequate signal strength.

51. DIRECT CONVERSION
• Amorphous Selenium is the direct DF
process by which X-rays are converted to
electric signal as no scintillation phosphor
is involved.
• The imaging forming X-ray beam interacts
directly with a-Se producing a charged
pair.
• Amorphous-Se is both the capture
element and the coupling element.
• Amorphous-Se is approx. 200 m thick and
is sand-wiched between charged
electrodes.
• X-rays incident on a-Se create electron
hole pairs through direct ionization of
Selenium.
The created charge is collected by a storage
capacitor and remains there until the signal
is read by the switching action of the TFT.

52. FPIR

53. FPIR

54. Field coverage / size advantage to flat panel Image distortion advantage to flat panel

55. FPIR
• FPIR is much small and lighter and is manipulated more easily than an
image intensifier.
• FPIR imaging suite provides easier patient manipulation and
radiologist/technologist movement.
• No radiographic cassettes
• No pincushion distortion

56. FPIR
• image guided catheter navigation.

57. FINAL COMPARISION

58. IMAGE DISPLAY-VIDEO SYSTEM
• A 525-line system of conventional fluoroscopy is inadequate for DF.
• Limitations of conventional video that restrict its application in digital
techniques are:
• Interlaced mode of reading the target of the television camera can
significantly degrade image.
• Conventional television camera tubes are relatively noisy which have SNR
about 200:1 where as 1000:1 is necessary of DF.
• Interlaced Versus Progressive Mode:
• Conventional television camera tube reads its target assembly by interlaced
mode, wherein two fields of 262½ lines are read individually in 1/60s
(17ms) to form a 525-line video frame in 1/30s (33ms).
• In DF, the TV camera tube reads in progressive mode in which the electron
beam of the TV camera sweeps the target assembly continuously from top
to bottom in 33 ms.

59. IMAGE DISPLAY-VIDEO SYSTEM
• The video image is formed similarly on the television monitor.
• No interlace of one field with another.
• Produce a sharper image with less flicker.
• Signal-to-noise Ratio:
• Background noise
• As conventional TV camera tubes have and SNR 200:1, the maximum
output signal will be 200 times greater than the background electronic
noise.
• SNR 200:1 is not sufficient for DF because the video signal is rarely at
maximum and lower signals become even more lost in the noise.
• This affect in subtraction techniques and image contrast resolution is
severely degraded by a system with a low SNR.

60. FLAT PANEL IMAGE DISPLAY
• Flat Panel Display technology is rapidly replacing the Cathode Ray
Tube (CRT) in all applications like Television, Computer.
• Radiography and fluoroscopy is the similar field in which CRTs are
rapidly being replaced by the Flat Panel Image Display.
• In fluoroscopic image viewing, Flat Panel Image Display is usually
Active Matrix Liquid Crystal Display (AMLCD)
• Liquid crystals materials -a natural molecular dipole.

61. Active Matrix Liquid Crystal Display (AMLCD)
• Display Characteristics:
• AMLCDs are fashioned pixel by pixel.
• A 1-megapixel display will have a 1000 x 1000 pixel arrangement
and a high resolution monitor of 5-megapixel display has 2000 x
2500 pixel arrangement.
• Image luminance:
• Better grayscale definition than CRTs.
• Not limited by Veiling glare or reflections in the glass faceplate
• AMLCDs have less intrinsic noise than that of a CRT.
• Ambient light:
• Designed to better reduce the influence of ambient light on image
contrast.
• Angular dependence

62. DIFFERENCES BETWEEN CRT AND AMLCD

63. TOTAL PROCESS
Acquisition Processing Display
FLUORO
UNIT ADC
COMPUTER
HARDWARE
AND
SOFTWARE
ALGORITHMS
DAC
FLAT PANEL
IMAGE
DISPLAY
STORAGE

64. Automatic Brightness Control
• Automatic Brightness control (ABC) is a mechanism, which can keep
the brightness of the image constant ate the monitor.
• Basically a feedback circuit, which measure the light intensity of the
output screen or video camera signal.
• A photomultiplier or a photodiode is used to monitor the light output
of the tube.

65. DIGITAL SUBTRACTION ANGIOGRAPHY
• Digital subtraction angiography (DSA) is a new radiographic
technology used in diagnosing vascular disease
• The digitalized image information makes it possible to “subtract” the
pre contrast images from those obtained after contrast injection so as
to visualize arterial structures without direct arterial puncture and
injection
• DSA is a gold standard investigation for renal artery stenosis, cerebral
aneurysms & arteriovenous malformations (AVM).

67. DSA
• Image contrast can be enhanced electronically and image contrast is
improved by subtraction techniques which provide instantaneous
viewing of the subtracted image during passage of a bolus of contrast
medium.
• So Digital fluoroscopy provides better contrast resolution through
post processing of image subtraction.
• Types of image subtraction:
• Temporal subtraction
• Energy subtraction
• Hybrid subtraction

68. TEMPORAL SUBTRACTION
• Image obtained at one time subtracted from image at later time
• Shows only blood vessels with contrast
• Two types:
1. Mask mode
2. Time interval display mode(TID)

69. MASK MODE
Amount of contrast
• 4-10 sec delayed image before bolus of CM reaches the anatomic site
is taken as mask image and stored in primary memory and is
displayed.
• The mask image if followed by a series of additional images with
bolus of CM and stored in adjacent memory locations.
• These each subsequent images are acquired with subtraction from
the mask image and stored in primary memory and displayed in
monitor as well

70. MASK MODE

71. MASK MODE
Image Integration
• Imaging sequence after acquisition of the mask can be controlled. e.g.
after acquiring mask image after 2 sec. of injection, after 2 sec
another delay, images are obtained at the rate of 2/s for 3s, 1/s for 5s
and one every other second for another 14 second.

72. MASK MODE
REMASKING
• On subsequent examination, if the initial mask image is inadequate
because of the patient motion or improper technique or any other reason,
later images may be used as the mask image.
• e.g. if the intended mask image is technically inadequate and maximum
contrast appears during the 5th image, a better subtraction image may be
obtained by using image number 5 as the mask image rather than image
number 1.
• Several images (e.g. image numbers four through eight) even can be
integrated using the composite image as the mask image.
• Unacceptable masking images can be caused by noise, motion and
technical factors.

73. Time-Interval Difference Mode
• Some examinations call for each subtracted image to be made form a
different mask and follow-up frame.
• TID mode produces subtracted images form progressive masks and
following frames.

74. Time-Interval Difference Mode
• In real time, the images observed convey the flow of CM dynamically.
• TID images are relatively free of motion artifacts but have less
contrast than mask-mode imaging.
• TID imaging is applied principally in cardiac evaluation.

75. TEMPORAL SUBTRACTION
MISREGISTRNATION
• If the patient motion occurs between the mask image and a
subsequent image, the subtracted image will contain misregistration
artifacts.
• The same anatomy is not registered in the same pixel of the image
matrix.
• This type of artifact can be eliminated by the registration of the mask
by shifting the mask by one or more pixels so that superimposition of
images is again obtained.

76. ENERGY SUBTRACTION
• The temporal subtraction techniques take advantage of changing contrast
media during the time of examination and require no special demands on
the high voltage generator.
• Energy subtraction uses two different X-ray beam alternately to provide a
subtraction image that results form differences in photoelectric interaction.
• Energy subtraction is based on the abrupt change in the photoelectric
absorption of the K edge of contrast media compared with that for soft
tissue and bone.
• The probability of photoelectric absorption in iodine of CM, bone and
muscle decreases with the increasing X-ray energy.
• At an energy of 33 keV, an abrupt increase in absorption is noted in iodine
and a modest decrease in soft tissue and bone.
• The energy corresponds to the binding energy of the two K-shell electrons
of iodine.

77. ENERGY SUBTRACTION
• If monoenergetic X-ray beams of 32 and 34 keV could be used
alternately, the difference in absorption of Iodine would be enormous
and the resultant subtraction image would have very high contrast.
• But such is not the case because every X-ray beam contains a wide
spectrum of energies.
• Energy subtraction has the decided disadvantage of requiring some
method of providing an alternating X-ray beam of two different
emission spectra.
• Two methods have been devised –
• alternating pulsing the X-ray beam at 70kVp and then 90 kVp
• Introducing dissimilar metal filters into the X-ray beam alternately on
a flywheel.

78. DIFFERENCES

79. HYBRID SUBTRACTION
• Some Digital Fluoroscopy systems are capable of combining Temporal
and Energy subtraction techniques – Hybrid Subtraction.
• In Hybrid subtraction, image acquisition follows the mask-mode
procedure.
• The mask and each subsequent images are formed by an energy
subtraction technique.
• If patient motion can be controlled, hybrid imaging theoretically can
produce the highest-quality DF images.

80. HYBRID SUBTRACTION

81. DSA
ROADMAPPING
• Roadmapping is a special application of DSA.
• A mask image is acquired and stored and CM is injected and
subtraction images are acquired as in DSA (A).
• As the catheter is fluoroscopically advanced, the image is formed by
subtraction form the second mask (B).
• Final DSA image shows completer vasculature tree with good contrast
(C).
• This image is inverted and used as the mask for additional DSA images.

83. DSA
PATIENT DOSE
• Potential advantage of DF is reduced patient dose.
• However, DF images appear to be continuous, in fact they are
discrete.
• DF X-ray beams are pulsed to fill one or more 33-ms video frames; so
fluoroscopic dose rate is lower than that for continuous analog
fluoroscopy.
• Static images with DF also are made with a lower dose per frame than
those attained with a 100-mm spot film camera.
• Both television camera tube and the CCD have greater sensitivity than
the spot film.
• Digital spot images are so easy to acquire that it is possible to mare
more exposures than are necessary.

84. DSA

85. SUMMARY
• Digital fluoroscopy has replaced the conventional type for its
advantages.
• In case of conventional fluoroscopy, what we get is what we see but
in case of digital fluoroscopy many post processing can be done.

87. Advances
• A future prospect is that DSA may be performed with energy
subtraction rather than temporal subtraction.
• Energy subtraction, an alternative being tested in several centers, is
based on subtraction of images of different kilovoltage, rather than in
different time.
• The advantage is that the different kilovoltages can be programmed
within milliseconds of each other, so that motion no longer
introduces an artifact.
• Current fluoroscopically based DSA units are incapable of energy
subtraction although one manufacturer is planning to provide such a
system in the future.

88. • The cesium-iodide image intensifier is the standard for production DSA
units, but state-of-the-art image intensifiers, such as the Thompson CSF 96
intensifier and the Philips 14-inch (36 cm) intensifier, are proposed for
future DSA units.
• Further modifications will include thicker image phosphors in order to
better control and use the light output.
• The size of the image intensifier is also an important factor, especially for
demonstrating large vascular areas, as in the extremities.
• Large intensifiers, such as the Philips 14-inch (36 cm), offer superior
contrast resolution capabilities, but have the disadvantages of decrease in
spatial resolution due to the fixed matrix size of the image processor and
the considerable increase in cost.

89. • Newly developed video tubes, such as the Amperex 45-XQ (‘‘frog’s
head’ ‘ plumbicon), as incorporated into a Sierra Scientific camera,
and the lead oxide Videcon tube are examples.
• Snapshot mode; slow scan video technique where the image is stored
on the target of the of the TV pickup tube & then read out digitized.

90. QUESTIONS???
• What are the forms of fluroscopy?
• Why digital fluroscopy has replaced other forms of fluroscopy?
• What are the advantages of FPIR?
• Why AMLCD has replaced CRT?
• What is meant by DSA and what are its type?
• What is meant by remasking?
• What is meant by misregistration and how can it be reduced?
• How are the temporal and energy substraction differ with each
other?

91. THANK YOU……