Digital Radiography: Flat Panel Detectors, Materials, and Image Artifacts

What is the term for a pixel that is not functioning at all?

Dead pixel

Detector performance meaesures

The numerical measurement of spatial resolution and DQE

In what year was the first TFT introduced?

1995

The needle type of light-directing tubes created by cesium iodide in an amorphous silicon detector are very similar (much like) to?

Fiber optics

What is image lag?

An artifact that occurs if an image is taken prior to the detector releasing all of the previous image, which would, in turn, showcase a faint image of the previous exposure. It is very similar to a double exposure.

What are the 2 main types of x-ray absorption materials being used in flat panel detectors?

Photoconductors and scintillators

How does the gadolinium oxysulphide (Gd2O2S) detecting material differ from the cesium iodide (CsI) detecting material?

Gadolinium oxysulphide is made from small crystals bound together in an unstructured/turbid layer. This allows for more trauma but also induces 30% more light loss, making it less efficient than CsI. Cesium iodide is made from very thin crystalline needles perpendicular to the detector. These needles work like light-directing tubes, which increases efficiency as well as spatial resolution.

Manufacturers make an effort to maintain a standard of less than ________ defective pixels and build software programs into their systems to identify and isolate dead pixels.

0.1 to 0.2%

What is the most popular type of amorphous silicon detector used?

Cesium iodide scintillator

What is used to correct flaws in the detector?

Gain calibration

What does AMFPI stand for?

Active-matrix flat panel imagers

What is the advantage that cesium iodide detectors have over gadolinium oxysulphide?

It has greater detection of X-rays and, due to minimal light loss, has higher spatial resolution.

Do indirect conversion detectors utilize a scintillator or a photoconductor?

Scintillator (light step)

In the past, why were gadolinium oxysulphide detectors better for portable use?

They can handle more ruggedness.

What are the 2 components found in each pixel of an a-Si:H array?

Sensing and switching component

What is the typical radiation-conversion material used in the photoconductor of a direct conversion detector?

a-Se that is about 500 μm thick for radiography and 200-250 μm thick for mammography.

What is the abbreviation for gadolinium oxysulfide

Gd2O2S

What is the abbreviation for cesium iodide

CsI

What is the abbreviation for amorphous selenium

a-Se

What is the abbreviation for amorphous silicon

a-Si

What is a scintillator

any substance that emits light photons

What is used in PSP phosphor

barium fluorohalide

Which material is similar to fiber optics/needles

cesium iodide

What is the photodiode made out of

amorphous silicon

Which indirect capturing material is considered unstructured

gadolinium oxysulfide

Which capture material is used in direct digital imaging

amorphous selenium

Which capture material is thallium-doped

cesium iodide

Which capture material can be described as powdered granules

gadolinium oxysulfide

Which capture material is considered a photoconductor

amorphous selenium

Which capture material absorbs x-rays and converts then straight into electrons

amorphous selenium

Which indirect capturing material is considered structured

cesium iodide

Is Cesium Iodide direct, indirect, both, or none

indirect

Is a-Si direct, indirect, both, or none

both

Is barium fluorohalide direct, indirect, both, or none

none

Is a capture element direct, indirect, both, or none

both

Is a-Se direct, indirect, both, or none

direct

Is an electrode direct, indirect, both, or none

both

Is Gd2O2S direct, indirect, both, or none

indirect

Is a photoconductor direct, indirect, both, or none

direct

Is a scintillator direct, indirect, both, or none

indirect

Is TFT direct, indirect, both, or none

both

Which of the following is responsible for storing electrons

a capacitor

A photoconductor converts what

x-rays into electrons

Which of the following would be considered a photodiode: gad engine, solar panel, a-Se, or a radiographic grid

solar panel

Which would be found in the pixel element of a flat-panel imaging detecting plate: a-Si, a-Se, Gd2O2S, or CsI

a-Si

Which of the following would be found in the pixel element of a flat-panel imaging detecting plate

TFT, capacitor, or photodiode (sensing area/DEL)

What would NOT be found in the pixel element of a Flat-panel imaging detector

PMT

What describes the difference between a-Si and a-Se

a-Si creates a photodiode and a-Se is classified as a photoconductor

Which of the following is responsible for reducing the sensing area on each pixel in an FPD plate

TFT

The capture element of a digital plate could be made what

a-Se, CsI, or Gd2O2S

In what direction on the FPD plate does the switch/control/gate lines run

horizontal

Which three of the following components are included in indirect capture flat-panel digital image detector systems

photodiode, scintillation layer, and thin-film transistor (TFT) array

Which component of an indirect capture flat panel detector converts incident x-ray photons into visible light photons

scintillation layer

Which component of an indirect capture flat-panel detector converts the visible light into electrons

photodiode

What are other names for photodiode

DEL/sensing area/pixel/electrode

Flat-panel detector systems typically use a thin-film transistor (TFT). This device functions to

collect electrical charge

Which two of the following flat-panel detector components can collect and convert light into an electrical signal

photodiode and charge-coupled device (CCD)

Which element used in the construction of a flat-panel detector is able to convert x-ray photons directly into an electrical signal

amorphous selenium

Which of the following devices acts as a switch within a flat-panel detector to release the electric charge collected by the detector element

thin-film transistor (TFT)

CsI is the preferred material for the scintillation layer because it can be formed into columns/squares, which focus x-ray photons/light photons

columns; light photons

In a CCD system, the scintillation layer is connected to the CCD sensor chips by which of the following

lenses or fiber optics

When compared to computed radiography (CR), digital radiography (DR) is

faster, and produces higher quality images

The digital image receptor is comprised of small elements called

detector elements (DELs)

Which of the following makes up a single detector element (DEL)

capture area, storage capacitor, and thin film transistor gate

Which of the following makes up the capture area of a DEL

top electrode, detection layer, and bottom electrode

What part of the DEL detects radiation

capture area

The storage capacitor of the DEL stores which of the following

a positive charge

What is the percentage of the DEL devoted to the detection layer called

fill factor

What is the relationship between fill factor and spatial resolution

directly proportional

Which of the following is created on an image receptor by the arrangement of detector elements (DELs) into rows and columns

matrix

A _______ fill factor would lead to an increase in patient dose

decreased

What combination of matrix and pitch would produce the highest spatial resolution

large matrix and low pitch

How many pixels may be read in less than a second

one million

What similarities do PSP and indirect FPD image capture have

convert x-rays into light and both create an electronic/digital signal from XRs

What are the differences between PSP and FPD image capture

PSP has a PMT; FPDs do not. PSP has an ADC inside; FPDs replace the ADC with a photodiode on the plate (made of a-Si)

Where is the ADC in a flat panel detector

on the outside

What are the similarities between Indirect and Direct image capture?

Both have a photodiode made of a-Si to convert energy into a digital signal and contain pixels underneath the scintillation/photoconductor layer

What are the differences between Indirect and Direct image capture?

Indirect uses a scintillator (CsI or Gd2O2S) to convert XRs into light, while direct uses a photoconductor (a-Se) to convert XRs straight into electrons

Why is cesium iodide preferred over gadolinium oxysulfide?

Efficiency; Less loss of light, fewer X-rays required, less dose (30% less)

Why is gadolinium oxysulfide preferred over cesium iodide?

Turbid, making it more apt for trauma

What does the photoconductor replace?

A scintillator

All digital imaging capture plates have _____ but only Direct Capture has _____

a-Si; a-Se

List the 3 parts of the pixel

photodiode, TFT, capacitor

Which part of the pixel stores electrons

capacitor

Which part of the pixel allows electrons to move out of storage?

TFT Switch

Which part of the pixel senses light photons and converts them to electrons?

Photodiode

Which line in the flat panel pixel array is responsible for moving the electrons out of the storage capacitor?

switch/gate/control lines

Which line in the flat panel pixel array is responsible for moving the electrons out of the plate to the computer?

Data Lines

The vertical wires are called

Data Lines

The horizontal wires are called

Control/Switch/ gate

The parts of the patient's body that allow x-rays to pass through are called

radiolucent

The parts of the patient's body that stop x-rays from passing through are called

radiopaque

The radiolucent parts of the body will result in more or fewer electrons captured in the pixel?

more

What causes an atom of calcium to result in fewer electrons being produced in the pixel beneath it?

Photoelectric Absorption

Which results in the most dose to the patient: large DEL or small DEL?

Small DEL with more capacitor and TFT area; less fill factor/a:si

Which results in the most resolution: large DEL or small DEL?

Small DEL with a high fill factor/a:si concentration; small pixel pitch will also increase resolution

What is pixel pitch?

Distance from the midpoint of one pixel to the midpoint of the next

What causes there to be a difference in the pixel pitch of some imaging plates?

Trauma to IR

Increasing pixel pitch will _______ resolution

decrease

What is the determining factor in a flat panel that distinguishes it as an indirect or direct detector

light step vs no light step