Download Document

DIGITAL PROJECTION
RADIOGRAPHY
COMPUTED RADIOGRAPHY
DIGITAL PROJECTION
RADIOGRAPHY DEPENDS ON
COMPUTER TECHNOLOGY TO
PRODUCE DIGITAL
RADIOGRAPHIC IMAGE
DIGITAL PROJECTION RADIOGRAPHY
COMPUTED RADIOGRAPHY (CR)
DIGITAL RADIOGRAPHY (DR)
DIGITAL RADIOGRAPHY
vs
COMPUTED RADIOGRAPHY
DR
X-RAYS
DETECTORS
ADC
11011101
CR
X-RAYS
CR PLATE
SCANNER
ADC
11011101
COMPUTED RADIOGRAPHY (CR):
USES PHOTOSTIMULABLE PLATE (IMAGE PLATE)
INSTEAD OF FILM, AS THE IMAGE RECEPTOR.
CR USES CASSETTE THAT LOOKS VERY SIMILAR
TO CONVENTIONAL RADIOGRAPHIC CASSETTE.
CR PLATE !!!!!
DOES NOT USE SCREENS OR
FILM ( CONVENTIONAL FILM)
CR PLATE
CASETTE CONSTRUCTION
► LIGHTWEIGHT
ALUMINUM
► PLASTIC
► STEEL
FRAME
FRONT PANEL MADE OF LOW ATTENUATION
CARBON FIBER
CR CASSETTES ARE NOT LIGHTTIGHT
IMAGE PLATE IS NOT SENSITIVE
TO LIGHT
CR CASSETTE IS PROTECTING
IMAGE PLATE FROM DAMAGE
+
IS THE STABLE VEHICLE FOR
TRANSPORT AND PLACEMENT
OF THE CASSETTE UNDER THE
PATIENT
BACK PANEL CONTAINS LEAD
FOIL TO PROTECT THE PLATE
FROM BACKSCATTER
BACK PANEL ALSO CONTAINS
CHIP TO RECORD PATIENT
DEMOGRAPHIC INFORMATION
REMOTE OPERATOR PANEL
IMAGE PLATE
APPROX. 1MM
CROSS-SECTION OF CR PLATE
PROTECTIVE LAYER
PHOSPHOR LAYER
ANTI-HALO & REFLECTIVE LAYER
BASE
BACKING LAYER
BASE
► PET-
POLYETHYLENE TERAPHTALATE
PHOSPHOR LAYER
► Ba
FX: Eu
+2
PROTECTIVE LAYER
► FLUORINATED
POLYMER MATERIAL
PROTECTS PHOSPHOR LAYER
ANTI-HALO LAYER + REFLECTIVE
LAYER
► PREVENTS
LASER FROM PASSING
THROUGH.
► REFLECTED LIGHT FROM PHOSPHOR IS
ALLOWED TO PASS.
BACKING LAYER
► PROTECTS
THE BASE FROM DAMAGE.
LEAD COATING
SCATTER
The simplest explanation for luminescence is
that impurities in the crystal lattice are
responsible for luminescence. As the
concentration of impurity ions increase the
greater the intensity of the luminescence. CR
screens use barium fluorohalides doped with
europium (europium is the impurity in the
crystal).
When phosphors are stimulated with x-ray
photon energy electron pair holes are created.
In effect, europium is raised to an excited
state and upon luminescence it is returned to
its ground Eu2+ state. This mechanism holds
for both spontaneous luminescence and
photostimulated luminescence
The shifting of europium from its excited state
back to its ground state for both spontaneous
and photostimulated luminescence is about
0.6 - 0.8 microseconds. With screen-film
imaging these crystals spontaneously
luminescence to expose a film, but with CR
imaging the luminescence occurs, then there
is also photoluminescence that occurs when
the screen is stimulated by a narrow beam of
infrared light.
The holes or vacancies in the lattice are
portions of the lattice normally occupied by
halogens (fluoride, bromide, or iodine). These
vacancies will trap free electrons when
irradiated and are called Farbzentren centers
or F-centers.
When the photostimulable plate is exposed to
high frequency light, usually from a helium
laser, the electrons in these F-centers are
liberated and cause luminescence at readout.
THE LATENT IMAGE WILL
REMAIN STORED FOR 24
HOURS.
IT WILL FADE THROUGH
PHOSPHORESCENCE
FADING
► 25%
OF STORED
ENERGY
► WITHIN
8 HOURS
CR AFTER EXPOSURE IS
INSERTED INTO CR SCANNER
CR PLATE
CR SCANNER/READER
THE SCANNER SCANS THE
IMAGE PLATE WITH LASER
LIGHT AND READS THE LATENT
IMAGE FROM THE PHOSPHOR
READOUT
LASER LIGHT
HELIUM-NEON (633 nm) LASER
BEAM SWEEPS THE PLATE IN
RASTER FASHION
RASTER
LASER
PM TUBE
LIGHT
GATE
AMPLIFIER
ADC
SWEEPING OF THE LASER ACROSS
THE PLATE
► FAST
SCAN DIRECTION
► SCAN DIRECTION
► LASER SCAN DIRECTION
PLATE MOVEMENT IN THE SCANNER
► SLOW
SCAN DIRECTION
► SUB-SCAN DIRECTION
► PLATE SCAN DIRECTION
THE SPEED OF THE SCANNING
LASER BEAM IS ADJUSTED TO
THE LUMINESCENT SIGNAL
LASER 633 nm
PHOTOSTIMULABLE LUMINESCENCE
390-400 nm
PLATE & F-CENTERS
LASER LASER SPOT SIZE - 100
MICROMETERS
PIXEL SIZE - 100 MICROMETERS
P.M. TUBE DETECTION
SENSITIVITY MATCHED TO
BLUE-PURPLE PSL (390-400 nm)
AFTER SCANNING THE CR
PLATE NEED TO BE CLEARED
FROM RESIDUAL SIGNAL
PLATE EXPOSED TO HIGH
INTENSITY SODIUM VAPOR OR
FLUORESCENT LIGHT
Typical image plate can be
reused thousands of times
IMAGE DISPLAY
IMAGE CHECK-INDEX???
IMAGE MANIPULATIONWINDOWING
PICTURE DESTINATION -PACS
PACS
CR CHARACTERISTIC CURVE
► VERY
WIDE LATITUDE
CR SYSTEMS
► LESS
RADIOGRAPH REPEATS
CHARACTERISTIC CURVE
LATITTUDE OF FILM
LATITTUDE OF CR
IMAGE CAN BE SALVAGED WITH CR
► 500%
► 80%
OVEREXPOSURE
UNDEREXPOSURE
USE ADEQUATE KVP
HARD COPY GENERATED IN LASER
PRINTER
CR SYSTEM EFFICIENCY
CHARACTERISTIC
► QDE-
QUANTUM DETECTION EFFICIENCY
NO NEED FOR SCREENS
QDE:
THE MEASURE OF THE EFFICIENCY OF CR
SYSTEM TO CONVERT REMNANT X-RAYS TO
USEFUL IMAGE SIGNAL
QDE
PATIENT DOSE
DIGITAL RADIOGRAPHY
ELECTRONIC DETECTORS
► INDIRECT
TECHNIQUE CLASS
► DIRECT TECHNIQUE CLASS
INDIRECT TECHNIQUE
► INDIRECT
CONVERSION FLAT PANEL
DETECTORS
► INDIRECT CONVERSION CHARGED COUPLE
DEVICE DETECTORS
INDIRECT CONVERSION FLAT
PANEL DETECTORS
► USE
THIN LAYERS OF SILICON
COMBINED WITH THE ARRAYS OF
PHOTODIODES. P.D. IS COATED WITH
CESIUM IODIDE OR RARE EARTH
MATERIAL- WHEN STRUCK BY
REMNANT X-RAYS THEY EMIT LIGHT.
ADVANTAGE: HIGH QDE
DISADVANTAGE: LIGHT DIVERGENCE
PHOTODIODE
INDIRECT CONVERSION CHARGE
COUPLED DEVICE DETECTORS
► USE
CCD ARRAYS TO RECORD THE VISIBLE
LIGHT EMITTED BY A SCINTILLATION
CRYSTAL THAT HAS ABSORBED THE
ENERGY. AN OPTICAL COUPLING SYSTEM
DEMAGNIFIES THIS LIGHT AND SENDS IT
TO THE CCD ARRAY.
OPTICAL COUPLING
CCD
DIRECT TECHNIQUE
► DIRECT
CONVERSION FLAT PANEL
DETECTORS
DIRECT CONVERSION FLAT
PANEL DETECTORS
►
USE AN AMORPHOUS SELENIUM COATED THIN FILM
TRANSISTOR ARRAY THAT DIRECTLY CONVERTS XRAY ENERGY INTO ELECTRICAL SIGNAL. REMNANT XRAYS ARE ABSORBED IN THE SELENIUM WHERE
THEY FORM ELECTRON-HOLES. CAPACITORS
COLLECT THE CHARGE.
ADVANTAGE: NO LIGHT DIVERGENCE
READY - The 17"x17"
amorphous selenium-based
sensor is electrically sensitized
and readied for exposure.
EXPOSE - Incident X-ray photons are
converted into electron-hole pairs. A
transverse electric field separates the
electrons and holes and creates a
charge image precisely corresponding
to the X-ray image.
READOUT - The microplasma line
scanner sweeps across the sensor in
less than 2 seconds causing readout
of the charge image and sensor reset.
DR vs CR COMPARISON
DR SYSTEM
ADVANTAGE
►
►
►
►
►
IMMEDIATE REDOUT
CASSETTE FREE
OPERATION
GOOD FOR HIGH
VOLUME RADIOGRAPHY
CAN BE USED FOR
MAMMOGRAPHY
DETECTORS CAN BE REEXPOSED IMMEDIATELY
DISADVANTAGE
►
►
►
MORE EXPENSIVE
CAN NOT BE USED IN
PORTABLE
RADIOGRAPHY??????
NOT COMPATIBLE WITH
EXISTING TABLES
PORTABLE????
PORTABLE CT???!!
MONITORS IN DR
► 1K
► 2K
► 4K
► 1,000
X 1,000
PIXELS
► 2,000
X 2,000
PIXELS
► 4,000
X 4,000
4K MONITORS USED IN MAMMOGRAPHY
PIXELS
AND CHEST RADIOGRAPHY
DR TERMS TO KNOW
PIXEL
► VOXEL
► SPATIAL RESOLUTION
► DYNAMIC RANGE
► CONTRAST RESOLUTION
► IMAGE NOISE
►
PIXEL
.
VOXEL
The formation