flat panel detectors/ DR

flat pannel detector

an array of pixel-sized detectors (DELs) thats very flat and thin

-contains capacitor and thin film translator (TFT) switch

DELs

• measures xray absorption

• array of pixels fastened to glass base/ substrate

• dexels - once exposure is recieved and signal is collected

absorption materials in detector

photo conductors- absorbs xrays and emits electric charge, also converts light to electronic signal

scintillators- phosphors that absorb xrays and emit light

absorption material determines whther IR is _______ or _________ capture

direct, indirect

two types of flat pannels

Direct and indirect

direct capture

-TFT converts xray to digital signal

-single step system, xrays strike photoconductor and converts to signal which is then temporarilly stored in DEL capacitor, TFT switching gate releases signal to be read out

-signal is rad out through edges of array of DELS (DEL contains signal), signal is read out line by line, signal is apmlified and sent to ADC for processing

-active layer: amorphous selenium (a-Se) photoconductor

array of hardware pixels or DELs (data elements)

-signal from attenuated beam is collected within DEL

• 3 components to label:

• semiconductor surface area (a-Se)

• capacitor - stores charge

• TFT switching gate - releases charge for readout

fill factor

the % of detectors elements area dedicated to xray absorption

-high fill factor equals: higher contrast resolution, higher spatial resolution

TFT readout

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

indirect capture

-scintillator and CCD (charge coupled device) or scintillator and TFT (thin film transistor)

-In CCD: light sensitive device that can respond to very low intensities, CsI is most common or GdOS, xray strikes scintilator and fiber optic cables collect light for read out, once recieved, applied charge is sent to ADC, uses A-Si

-TFT: xray strikes scintilator - CsI or GDOS and A-Si converts light to signal, TFT array captures charge in DEL and stores in capactor, TFT switch sends to ADC

-active layer - smorphous silicon (a-Si) photoconductor

scintillator: materila that absorbs xray and emits light

bucket brigade readout

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

-once bucket is full, can dump into amplifier at the end

DEL of indirect conversion system

-cesium iodide phosphor (top layer) absorbs xray and emits light, entire array of DELS covered with screen of cesium iodide

-amorphous silicon then absorbs light and emits electrons

unstructured vs structured scintillaotr construction

un:

-light scatters, reduced spatial resolution

Structured:

-needle like structure, increased interactions, less light speed, improved resolution

CMOS (complimentary metal oxide conductor)

indirect capture type

always compared to CCD array

scintillator works the same, each DEL contains its own transistor, amplifier, noise correction circuit and digitization circuit which results in lower fill factor and higher noise

signal readout is slightly different from CCD, DELs charge is sent across the chip and read in a corner of the array

signal sent to ADC

CCD vs CMOS

CCD

• better light sensitivity

• greater fill factor for better quality

• modular construction (easy for repairs)

• uses more power (110x)

CMOS

• more susceptible to noise

• lower light sensitivity

• lower fill factor

• less power

• inexpensive to manufacture

• lower quality and lower resolution

DR artifacts

IMG lag (ghosting)- detector doesnt clear signal from previous image

Back scatter- non-appropriate collimation

Gain calibration- used to compensate for sensitivity variation across the detector