Digital Radiography and PACS Chapter 5 TFT Flat-Panel Array

Active-matrix flat-panel images (AMFPI) consist of a flat-panel array with an x-ray _________________ material

absorption

The two main types of x-ray absorption materials currently being used are

photoconductors and scintillators

materials that absorb x-rays, resulting in an electrical charge

photoconductors

phosphors that produce light when absorbing xrays

scintillators

True or False: An AMFPI detector measures the response of photoconductors and scintillators to xray absorption and is a large area 2D array of pixels fastened to a thin glass backing, or substrate

true

the absorption material (photoconductor) is attacted to the AMFPI

electrically

the absorption material (scintillator/phosphor) is attached to the AMFPI

physically

True or False: the choice of xray absorption material determines whether the detector is direct or indirect conversion

true

xray photons are absorbed by the coating material and immediately converted into an electrical signal

direct conversion

direct conversion uses photoconductors or scintillators

photoconductors

The photoconductor in direct conversion is usually made of

amorphous selenium (a-Se)

Amorphous selenium (photoconductor) in direct conversion absorbs the xrays and converts them to electrons which are stored in the

TFT detectors

TFT is a photosensitive array made up of small pixels, also called a

detector element (del)

True or false: with direct conversion, each pixel contains a photodiode that absorbs the electrons and generates electrical charges

true

isolates each pixel element and reacts like a switch to send the electrical charge to the image processor

field effect transistor (FET) or silicon TFT

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

1 million

high speed or low speed digital electronics are used to achieve fast image acquisition and processing

high speed

a two step process where xrays are converted into light, and then the light photons are converted to an electrical signal

indirect conversion

phosphors used in indirect conversion, also known as the scintillator/phosphor layer

gadolinium oxysulphide (Gd2O2S) and thallium doped cesium iodide (CsI[TT])

gadolinium oxysulphide and cesium iodide

rapidly absorb xrays and produce light

True or false: the sintillation layer can either be structured or unstructured

true

unstructured layer produce more

scatted light

hygrogenated amorphous silicon (a-Si:H) is a

photodetector

gadolinium oxysulphide detecting material is made from small crystals bound together in an ____________ or ___________ layer along with a polyurethane material

unstructured, turbid

the most popular type of amorphous silicon detector uses a

CsI scintillator

the scintillator in a CsI, is made by growing very thin crystalline needles __________ to the detector surface

perpendicular

cesium iodide are considered a _______________ scintillator

structured

each pixel contains a

sensing component and switching component

what are the 2 types of switching components

2 contact switching diode and 3 contact TFT

what are the 2 ways to measure detector performance

numerical measurement of spatial resolution and DQE and observation of low-contrast objects in a contrast phantom

how efficently a detector recieves exposure (input signal) and converts to a useful image (output signal)

detective quantum efficiency

a mathematical theorem, the ability to record spatial frequencies, ability to record detail

modulation transfer function

True or false: there is a possibility in any imaging system of losing or misrepresenting image information because of defects in the operation components of the device

true

image artifacts can be caused by _______________ in the detector

malfunctions

true or false: the amount of dead pixels increase with the age of the of the detector

true

______________ programs may "fill in" dead pixels

software

true or false: Image artifacts include malfunctioning pixels, dead pixels, dust scratches, static discharge, and chemical corrosion

true

to remove the potential of having that artifact interfere with diagnosis

gain calibration or flat fielding

gain calibration ____________ the pixels included in the artifact

masks

if an image is taken prior to the detector releasing all of the signal from the previous image, a faint image of the previous exposure may be visible

image lag

Image lag is almost like

double exposure

techniques to reduce image lag

increase time in between exposures, close collimation, and acquire the image with the highest technical factors last

determines the amount of signal inherent in the detector

offset correction

true or false: tomograms were acquired by attaching a fulcrum to the bucky and tubehead and they move in concert with each over and the tube would arc across the patient

true

a photoconductor

absorbs xrays and produces electrical charges

which of the following is not considered scintillator: Gd2O2S, CsI, a-Se.

a-Se

which of the following are not found in AMFPI:a-Si TFT with CsI, a-Se with TFT, PSP, Scintillator

PSP

which detector converts xray energy directly into electrons? a-Si TFT with CsI, a-Se with TFT, a-Si TFT with Gd2O2S

a-Se with TFT

True or False: Gain calibration will remove inherent detector artifacts from an image

true

true or false: offset correction is used to remove inherent signal that remains in a detector

true

which type of absorbing materials requires a capacitor as the storage element: a-Se, photoconductor, or scintillator

a-Se and photoconductor