Direct Digital Radiographyor Direct Capture
Radiography
Bushong Ch. 27
Late 1990’s
A new approach to imaging appeared
DR or DDR or Direct Capture imaging
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 PSP – photostimulable phosphor plate REPLACES FILM IN THE CASSETTE
DR – NO CASSETTE – PHOTONS CAPTURED DIRECTLY ONTO A TRANSISTOR SENT DIRECTLY TO A MONITOR
DIRECT RADIOGRAPHY
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 DRCR imaging plate
processed in a Digital Reader
Signal sent to computer
Viewed on a monitor
DR transistor receiver
(like bucky)
directly into digital signal
seen immediately on monitor –
Digital
Radiography
DirectCapture
IndirectCapture
Direct-to-DigitalRadiography
(DDR)-Selenium
ComputedRadiography
(CR) - PSL
LaserScanningDigitizers
Direct-to-DigitalRadiographySilicon Scint.
DDR CR
Two types of DDR systems
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
Each TFT and detector represents a pixel
DEL = charge collecting detector element
DEL Digital Value
Digital Value depends on: 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.
One exposure = Detector Readout
DDR using cesium iodide scintillation phosphors 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
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)
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
CsI phosphors have high detective quantum efficiency (DQE) = lower patient dose 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*
525 vs 1000 line more pixels = more
memory needed to store
resolution dependent on pixel size
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 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
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 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
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
Important factors that affect patient dose
DQE: when using CsI systems Both systems “fill factor”
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
Post processing & PACS
Questions ?