Flat Panel Detectors - DR

flat panel detector

an array of pixel-size detectors (DELs)

• array of pixels fastened to a glass base/substrate

contains a capacitor and TFT switch

photoconductors 

part of the detector that:

• absorbs x-rays and emits an electrical charge

• converts light to electronic signal 

scintillators

part of the detector that:

• absorbs x-rays and emits light

• made of phosphorus

difference between DEL and DEXEL

DEL - before exposure

DEXEL - after being exposed

what determines whether an IR is direct or indirect?

the absorption material

• indirect has a cesium iodide scintillator

• indirect uses amorphous silicon (a-Si) photoconductor

• direct uses amorphous selenium (a-Se) photoconductor

direct capture IR system 

single step system 

x-rays strike photoconductor 

• made of amorphous selenium (a-Se)

converts x-rays to electronic signal

signal is stored in DEL capacitor temporarily 

TFT switching gate releases signal to be read out

read out for direct capture system 

DELS are read out line by line 

amplifier amplifies the signal

ADC converts signal 

signal is sent to computer for processing 

3 components of a DEL

semiconductor surface area

capacitor

TFT switching gate 

fill factor of the DEL

the % of a detector’s elements area dedicated to x-ray absorption

80% is high

higher fill factor equals

higher contrast resolution

higher spatial resolution

TFT readout 

DEL chargers are read out on a pixel by pixel and column by column basis 

indirect capture types

scintillator used with charge coupled device (CDC)

scintillator used with thin film transistor (TFT) detector 

charged coupled device (CDC)

light sensitive device that can respond to very low intensities

CDC in indirect capture systems

x-ray strikes scintillator (CsI or GDOS) which emits light

signal is transmitted by fiber optic cables to CCD

photoconductor made of amorphous silicon (a-Si) converts light to electronic

charge is released and sent to ADC

ADC sends electronic signal to computer

bucket brigade readout 

each DEL contains 3 electrodes that hold electrons in a potential “well”

voltage gates open to allow electrons to flow down row to “read out” row

data sent sequentially to amplifier, which then digitizes signal

system of read out known as a bucket brigade scheme

TFT in indirect capture systems

x-ray strikes scintillator (CsI or GDOS) which emits light

photoconductor made of amorphous silicon (a-Si) converts light to electronic signal 

TFT array captures electrical charge in DEL and stores in capacitor 

TFT switch sends signal to ADC

ADC sends electronic signal to computer

DEL of indirect conversion system

cesium iodide phosphor absorbs x-ray and emits light 

amorphous silicon then absorbs and emits electrons

structured scintillator construction 

needle like structure

increased x-ray interactions

less light spread 

improved resolution

unstructured scintillator construction 

light scatters

reduced spatial resolution

CMOS stands for 

complementary metal oxide conductor

CMOS in indirect capture system

x-ray strikes scintillator (CsI or GDOS) which emits light

each DEL contains its own transmitter, amplifier, noise correction circuit, and digitization circuit 

• decreases fill factors 

• increases noise

signal read out is different from CCD

• DELs charge is sent across the chip and read in a corner of the array

signal is dent to ADC

CCD vs CMOS

CCD

• better light sensitivity 

• greater fill factor for better image quality 

• modular construction

  • easy to repair and upgrade

• uses more power 

  • 110 times more than CMOS

CMOS

• more susceptible to noise

• lower light sensitivity

• lower fill factor

• uses less power

• inexpensive to manufacture

• lower quality

  • lower resolution

image lag (ghosting)

detector doesn’t clear signal from previous image

gain calibration 

used to compensate for sensitivity variation across the detector