thin film transistor

DEL abbreviation

digital element

what is DEL

an individual hardware cell in the detector capable of producing a single electronic read out from incoming photon energy

TFT abbreviation

thin film transistor

what is TFT

a photosensitive array made up of small (about 100-200 μm) detector element (del) in TFT arrays

DQE abbreviation

detective quantum effieciency

what is DQE

a measure of how efficiently a system converts an x-ray or light (analog) signal into a useful output image (digital)

AMFPI abbreviation

active matrix flat panel imagers

what is AMFPI

a large 2d array of dels that measure the response of the material to x-ray absorption

what are the two main types of x-ray absorption materials used and how each one works (indirect or direct)

1. photoconductors: materials that absorb x-rays resulting directly to an electrical charge (direct)

2. scintillators: phosphors that absorb x-rays —> produce light —> then must be converted to an electrical charge (indirect)

what does the AMFPI detector measure

measures the response of the material to x-ray absorption

how is the absorption material attached to the surface of the AMFPI

either electrically (photoconductors) or physically (phosphor aka scintillator material)

what material (photoconductor) is used to immediately convert x-ray photons into an electrical signal

amorphous selenium

what is the primary function of the TFT in each pixel

acts as a switch to transfer charge for readout

what is the 2 step process used in indirect conversion

1. X-ray photons are converted to light by a phosphor layer (aka scintillation layer)

2. the light is then converted into an electrical charge by a photodetector such as a hydrogenated amorphous silicon photodiode array

what 2 things are used for the phosphor layer (aka scintillation layer) in indirect conversion

1. gadolinium oxysulfide

2. thallium-doped cesium iodide

the scintillation layer can be ___ or ___

structured or unstructured

which structure of the scintillation layer is less efficient

unstructured layers produce more scattered light than structured layers = decreases the efficiency of the detector

in an indirect conversion, what converts the light into an electric charge and what material is it made of?

a photodetector such as a hydrogenated amorphous silicon photodiode array

how long does it take the system to read and convert 1 million pixels into a digital image

less than 1 second

what is unstructured also known as

turbid (powdered granules)

which indirect material is unstructured

gadolinium oxysulfide (Gd2O2S)

what is the most popular type of cesium iodide detector (what material)

amorphus silicon

what indirect matieral is structured

cesium iodide

why is cesium iodide the best

almost no light spread=higher spatial resolution

greater detection of x-rays= higher DQE than a Gd2O2S scintillator

what does each pixel contain regarding flat panel array design

sensing component and switching component

what does the sensing component depend on

depends on the type of absorbing material and the vendors choice of construction

what sensing element is needed for the direct (a-Se)

a capacitor that measures charge (aka storage capacitor)

what is required if a phosphor/scintillator is used

a photodioder phototransistor is required to detect the light

how is image data that is from an active-matrix flat panel detector read?

row by row using control line voltage changes

what are the 2 ways to measure detector performance

1. numerical measurement of spatial resolution and DQE

2. observation of low contrast objects in a contrast detail phantom

what is a dead pixel?

a possibility of an imaging system of losing or misrepresenting image information because of defects in the operational components of the device

what causes a dead pixel?

dust, scratches, static discharge, chemical corrosion, and interactions between materials

how can dead pixels be corrected?

build system programs into systems to identify and isolate dead pixels, this software uses an interpolation method to “fill in” the dead pixels with information using surrounding pixels as a guide

what is the manufacturing standards for dead pixels

less than 0.1%-0.2% defective pixels

what is incorrect gain calibration

a mask of defect is created, and the software will use the mask information

what causes incorrect gain calibration

there was a defect in the detector, so masking is used and automatically created

what is image lag and incorrect offset correction

an image is taken prior to the detector releasing all of the previous image exposure may be visible

what causes image lag and incorrect offset correction

some flat panel systems can take images faster than the detector can accommodate

how can image lag and incorrect offset correction be corrected or avoided?

increase the amount of time between exposures
reduce the amount of attenuated beam by collimating close
set appropriate technical factors
use of offset correction - determines the amount of signal inherent in the detector

how do FPD systems make digital tomosynthesis today?

the detector is able to acquire images at a rate of 30 frames per second