chapter 22 Digital Radiography Flat Panel Detector systems

Amorphous silicon

Light energy converted to electrical signal in _______ ____

Requires a scintillator

Isotopic light emission

The same intensity in all directions

Electronic signal capture

Charge-coupled device (CCD)

Complimentary metal oxide semiconductor (CMOS) ( not used it’s insufficient)

Thin film transistor(TFT)

Designed to convert light energy into electrical signal

Transmission along detector circuit pathways

Analog-to-digital convertor (ADC)

Complimentary metal oxide semiconductor (CMOS)

Consists of a pair of semiconductors connected to a common secondary voltage such that they operate in opposite ( complementary) fashion . Thus when one transistor is turned on, the other is turned off, and vice versa

Why is CMOS not used

Not stable as CCD chips

More susceptible to noise

Light sensitivity is lower

Tin film transistor (TFT)

Used in both indirect and direct detectors

Electronic readout of signal

Arranged in an array of detector elements( DEL s)

Pixel size directly related to DEL size

Read out is pixel-by- pixel

Spatial resolution indirectly related to

DEL dimension

DEL design miniaturized

Sensing area

Signal amplifier and switching mechanism

Capacitive storage

95% fill factor

Indicates 5% of non-sensing electronics

Higher fill factor

Increase dose efficiency

Higher spatial and contrast resolution

Lower pt dose

Less radiation

Direct detectors

X-ray photons are absorbed by the coating material and immediately converted into an electrical signal

The radiation-conversion material or photo conductor is made of amorphous selenium

Silicon TFT

Isolates each pixel element

Reacts like a switch to send the electrical charges to the image processor

More than 1 million pixels can be read and converted to a composite digital image in less than 1 second

TFT switches

Allows electrical charge information to discharge when switches are closed

The information is discharged onto the data columns and is read out with dedicated electronics

Integrated circuits

One one side they control the line scanning sequence

On the other side, low-noise, high-sensitivity amplifiers perform the readout, amplification, and analog -to- digital conversion

Latent image

With DR is electrical signal values( voltages ) created in each DEL

Only DELs with signal values are read out and make up image

DR panel have a

Pre processing and post processing

pre-processing

Permits DEL calibratIons

Manufactures continue to improve features

Weight, durability, patient load capacity, etc

Auto-detection

Trigger panels

Wireless panels

No true generator interface