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 ?