Transcript Direct Digital Radiography or Direct Capture Radiography Bushong Ch. 27

Direct Digital Radiography
or Direct Capture Radiography
Bushong Ch. 27
Late 1990’s
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A new approach to imaging appeared
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DR or DDR or Direct Capture imaging
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Too early to tell which system will
prevail
Directed Digital Radiography
(DDR)
Directed digital radiography, a
term used to describe total
electronic imaging capturing.
Eliminates the need for an
image plate altogether.
DDR Systems
IMAGE CAPTURE
CR
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PSP – photostimulable phosphor plate
REPLACES FILM IN THE CASSETTE
DR – NO CASSETTE – PHOTONS
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CAPTURED DIRECTLY
ONTO A TRANSISTOR
SENT DIRECTLY TO A MONITOR
DIRECT RADIOGRAPHY
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uses a transistor receiver (like bucky)
that captures and converts x-ray energy
directly into digital signal
seen immediately on monitor
then sent to PACS/ printer/ other
workstations FOR VIEWING
CR vs DR
CR
 imaging plate
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processed in a
Digital Reader
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Signal sent to
computer
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Viewed on a monitor
DR
 transistor receiver
(like bucky)
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directly into digital
signal
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seen immediately on
monitor –
DDR
Digital
CR
Radiography
Direct
Capture
Indirect
Capture
Direct-to-Digital
Radiography
(DDR)-Selenium
Computed
Radiography
(CR) - PSL
Direct-to-Digital
Radiography
Silicon Scint.
Laser
Scanning
Digitizers
Two types of DDR systems
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Both are based on the thin-film
transistor as an active matrix array
(AMA)
Built the size of a conventional S/F
receptor
Active Matrix Array (AMA)
Pixels are read sequentially, one at a time
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Each TFT and
detector represents
a pixel
DEL = charge
collecting detector
element
DEL Digital Value
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Digital Value depends on:
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Charge collected by DEL.
Bit depth
10 bit = 1 – 1024
12 bit =1 - 4096
DEL collects e-
Unlike CR plates, only the
exposed pixels contribute to the
image data base.
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One exposure = Detector Readout
DDR using cesium iodide
scintillation phosphors
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CsI is coated over an active matrix array
(AMA) of amorphus silicon (a-Si)
photodiodes
Amorphus means without shape
Photodiodes are used to detect light or
measure its intensity also called a
charge coupled device (CCD)
DDR steps using cesium iodide
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Exit x-rays interact with CsI scintillation
phosphor to produce light
The light interact with the a-Si to
produce a signal
The TFT stores the signal until readout,
one pixel at a time
CsI phosphor light detected by
the AMA of silicon photodiodes
DDR only using amorphous
selenium (a-Se)
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The exit x-ray photon interact with the
a-Si (detector element/DEL). Photon
energy is trapped on detector (signal)
The TFT stores the signal until readout,
one pixel at a time
Active matrix array of silicon
photodiodes
Advantages/Disadvantages
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CsI phosphors have high detective
quantum efficiency (DQE) = lower
patient dose
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DQE = % of x-rays absorbed by the
phosphors
a-Se only: there is no spreading of light
in the phosphor = better spatial
resolution
F/S & DDR imaging systems
F/S & DDR imaging systems
Image Resolution –
(how sharply is the image seen)
CR & DR
 4000 x 4000
 image only as good a
monitor*
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525 vs 1000 line
more pixels = more
memory needed to
store
resolution dependent on
pixel size
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CR 2 - 5 lp/mm
RAD 3-6 lp/mm
DR 3 - 5 lp/mm
IMAGE APPEARS
SHARPER BECAUSE
CONTRAST CAN BE
ADJUSTED BY THE
COMPUTER –
(DIFFERENCES IN
DENSITY)
Image Resolution
Pixel Pitch
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Spatial resolution
determined by
pixel pitch.
Detector element
(DEL) size
140 μm = ~3.7
lp/mm
100 μm = ~ 5.0
lp/mm
Signal Sampling Frequency
Good sampling
under sampling
DR
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Initial expense high
very low dose to pt –
image quality of 100s using a 400s
technique
Therfore ¼ the dose needed to make
the image
Flat Panel TFT Detectors
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Have to be very careful with terminology
One vendor claims: “Detector has sharpness of
100 speed screen”
May be true: TFT detectors can have very
sharp edges due to DEL alignment
But !
Spatial resolution is not as good as 100 speed
screen.
TFT detector = 3.4 lp/mm
100 speed screen = 8 – 10 lp/mm
TFT Array Detectors
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Detector is refreshed after
exposure
If no exposures are produced. . .
detector refreshed every 30 – 45
sec
Built in AEC, An ion chamber
between grid and detector
Patient Dose
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Important factors that affect patient
dose
DQE: when using CsI systems
Both systems “fill factor”
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The percentage of the pixel face that
contains the x-ray detector.
Fill factor is approximately 80%
Fill Factor
DDR has all the advantages of
CR imaging techniques
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Post processing & PACS
Questions ?