Transcript Comparison of Film v. Digital Image Display

Comparison of Film v. Digital
Image Display
Process of data capture
• All image recording systems rely on
differential absorption within the patient to
produce a radiographic image
• Each system analog (film/screen) versus
digital (CR and DDR) records photon
intensity and displays the data.
– Differences in photon density become
radiographic contrast
Radiographic Contrast
• Radiographic contrast is dependent on subject
contrast and technical factors
– Subject contrast is basically fixed for each patient.
Within certain body/organ systems we can alter
subject contrast by introducing contrast media.
• Some technical factors include;
–
–
–
–
kVp
Grid ratio
Screen speed
Processor temperature
• Altering technical factors can improve or
degrade radiographic contrast.
Film
•
Films produce an analog image
that allows edges to gradually
blend into one another. This is
how we see images normally.
Conversely, digital images are
broken up into pixels (tiny
squares). Because the image
consists of pixels there is an
inherent loss of spatial resolution
when compared to analog film.
– 2.5 – 5 lp/mm versus 10
lp/mm
Computed Radiography
• Photostimulable plate (PSP)
– Barium-fluorohalide doped with europium
• When the PSP is struck by a photon
electrons are released in the crystal lattice
and stored in F centers within the lattice
• In the CR reader, a helium-neon laser (red
light) stimulates the crystals of the PSP.
Once stimulated the crystals emit blueviolet light.
• The amount of light emitted is proportional
to the amount of radiation absorbed by the
PSP.
• The emitted light is read by a
photomultiplier tube within the CR reader
producing an electronic signal. The
electronic signal is then fed through an
analog-to-digital converter (ADC usually
12 bit) producing a digital signal.
• Essentially, the PSP is a OSL radiation
monitor
ADC
• While converting the analog signal to a
digital the ADC also breaks the pieces or
squares that represent pixels in the final
image.
Digital Radiography (DR)
• Direct DR
– X-ray photons are absorbed by the DR plate.
The active ingredient is usually amorphous
selenium. The a-Se is ionized by the photons
and the released electrons are stored in the
TFT (thin film transistor) array. The TFT
consists of multitude of individual elements
that represent the pixels of the final image
Direct TFT
• Indirect DR
– Similar to direct DR but x-ray photons are
converted to light photons (cesium iodide) and
then pass through a photo-multiplier tube
coupled with amorphous silicon.
• Cesium iodide crystals resemble needles
in appearance resulting in very little light
spread and high spatial resolution.
Indirect DR w/CCD
Indirect DR w/CMOS
Indirect DR
Deterioration of image quality due to light diffusion
Device
Active Ingredient
Grayscale/
dynamic
range
Processor
Film/
Screen
Phosphorescent screens
and silver halide film
emulsion
1000
Wet or dry
chemical action
CR
Barium fluorobromide
doped with europium
14 bit
16000
Helium-neon red
laser stimulates
the PSP to emit
light
Direct DR
Amorphous selenium
14 to 16 bit
16000 - 65000
shades of gray
Indirect DR
Cesium iodide scintillator
coupled with amorphous
silicon
14 to 16 bit
16000 - 65000
shades of gray
Film/screen
• Optical density ranges from 0 to 3
– This represents a range of 3 orders of
magnitude or 1000.
– View boxes can only display 30 shades of
gray
• Film/screen grayscale is 1000
CR
• Four orders of magnitude are possible
– 10000 shades of gray
• CT and MR
– 12 bit or 4096
• CR and DR
– 14 bit or 16384
• Digital Mammography
– 16 bit or 65536 shades of gray
Contrast Resolution
• Principal descriptor
– Grayscale
– Dynamic range
Window level
• Window and leveling of the digital image allows
us to see the entire grayscale of the digital
image. Even though the eye is limited to 30
shades of gray by manipulating the window
width and level we can determine where the 30
shades occur.
• We determine the center point of the 30 shades,
level, and the width is how many grays will be
displayed.
Window Width and Level
Processing the Image
• Image pre-processing
– Find the pertinent image (histogram)
– Scale data to appropriate range
• Contrast enhancement
– Anatomy specific gray scale manipulation
• Spatial frequency enhancement
Raw Image
• Inherent subject
contrast displayed
• Contrast inverted
• PSL signal amplitude
log amplified
Preprocessed raw image
Scaled and inverted:
Unprocessed image
Contrast Enhancement
• Optimize image contrast via non-linear
transformation curves
• Unprocessed images have linear ‘subject
contrast’
– Gradiation processing – Fuji
– Tone scaling – Kodak
– MUSICA - Agfa
Contrast Enhancement
Scaled and inverted:
Unprocessed image
Contrast enhanced
• CsI crystals
Spatial Resolution and Monitor
Performance
• In the analog environment, spatial
resolution is a affected by SID, OID, RSV,
FFS, and type of film to name a few.
• In the digital domain, the viewing monitor
also affects spatial resolution as well as
contrast resolution or dynamic range.
Monitor Factors Affecting Image
Quality
• Monitor brightness
– The brighter the monitor the better the image
quality
• Color v/ B & W
– For CR and DR color monitors do not offer
enough dynamic range
• Matrix size
– Most PC monitors are 1024 x 1280 or 1200 x
1600. These will allow you to view CT and
MR at resolution.
Diagnostic Monitors
• Matrix size
– 2048 x 2560 at minimum
• Black and white not color
• Brightness
– 600 cd/m2 v 300 cd/m2
What are radiation protection and
safety issues?
• Unique characteristics of screen/film
imaging systems
– “self limitation” of patient exposure
– concept of "speed" defined and understood
• New considerations for digital radiography
– no “self limitation” as in screen/film systems
– no consensus on “speed”
– "inefficient" systems possible?
TERPSSC 2001
Robert M. Gagne
Film/Screen “Self Limitation”
• Imaging task with large dynamic range
• Be careful not to under or over expose film
• “Self limitation” of patient exposure
TERPSSC 2001
Robert M. Gagne
Film/Screen “Speed”
Difference in
speed of
about 2
• Film/screen “speed”
– speed = 100/E where E is exposure in mR to produce
an optical density of 1.0
– position on exposure axis dependent on “speed”
– higher “speed” number translates to lower patient
exposure
TERPSSC 2001
Robert M. Gagne
DR “Speed”
4500
4000
3500
Pixel Value
3000
Gain 1
Gain 2
2500
Gain 3
2000
1500
1000
500
0
0
20
40
60
80
100
120
Relative Exposure
• DR operating point
– equivalence to film/screen “speed” set at installation?
– no “self limitation” except at extreme ends of the grayscale transfer curve
– patient exposure increase / decrease / equivalence
compared to film/screen?
TERPSSC 2001
Robert M. Gagne
Elements of DR Imaging
Systems
• Capture element
– The element/material used to capture the x-ray
photon
• Coupling element
– Transfers the x-ray generated signal to the
collection element
• Fiber optics, lens, contact layer, or a-Se
• Collection element
– Photodiode, CCD, TFT, CMOS
• Photodiodes, CCDs, and CMOS are light sensitive
devices that collect light photons
• TFT collect electrons
Digital Radiography
Type
CR
Indirect DR
Indirect DR
Indirect DR
Direct DR
Capture
Element
Barium
fluorohalide
CsI
CsI – cesium
iodide/GdOD
gadolinium
oxysulfide
CsI – cesium
iodide
a-Se
amorphous
selenium
Coupling
element
Lens or fiber a-Si
optics
Amorphous
silicon
Fiber optics
or lens
a-Se
Collection
element
CMOS
CCD
TFT
TFT
Indirect w/ CCD and fiber optics
Multiple detectors
Single detector
CCD
Fiber
optic
coupling
CsI capture element
Indirect w/CMOS and lens
Indirect TFT
a-Si
Coupling
element
Direct TFT
Review of Indirect DR
• Basically there are three types of indirect
systems
– CCD
• A multitude of small CCD (charged coupled
devices) receive light from the cesium iodide
scintillator.
– TFT
• The light from scintillator is recorded by a TFT (thin
film transistor) that is also called a flat panel
detector.
– CMOS (complementary metal oxide
semiconductor)
Flat panel technology (TFT)
indirect and direct DR applications