Chapter 22: Digital Radiographer Flat Panel Detector Systems Flashcards by Jennifer Gonzalez

Digital is classified as what two types?

CR + DR

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Referred to as digital radiography (DR)

Flat- Panel Detector Evolution

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When was DR introduced ?

1995

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What are two advantages of digital DR?

post processing options
contrast resolution

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what are the two general types of DR panels?

Indirect and Direct

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Correct order for Indirect Conversion:

x-ray photons to light, then to an electrical signal then finally the image

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Correct order of Direct conversion

x-rays photons directly to electrical signal then finally the image

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Steps of Indirect Conversion: (with more detail)

Indirect conversion detectors are used in a two-step process. The first step involves converting incom-ing x-ray photons to light first, which is accomplished through the use of a scintillator. The second step involves a conversion of light photons to an electronic signal using a photodetector. Which is your amorphous silicon. Indirect DR systems include either a charge-coupled device (CCD) or amorphous silicon with a thin film transistor (TFT) array.

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Direct Conversion (more detail)

Direct DR systems are capable of directly converting incoming x-ray photons to an electronic signal. These systems use amorphous sele-nium and a TFT.

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Two types of Indirect DR detectors

TFT and CCD

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Indirect detectors use what kind of material

amorphous silicon layer

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How do scintillators emit light

Isotropically

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conversion of incoming xray photons to an electronic signal without scintillation

Direct Conversion

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What layer deals with indirect

amorphous silicon

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A two part process involving a scintillator (which converts incoming xray photons to light) and a photodetector (which converts light into an electronic signal)

Indirect Conversion

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Out of direct and indirecting, which has the better image

direct. because there is one less step, so less info lost

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-uses a scintillator to convert xray energy to light
-light energy converted to eletrical signal in amorphous silicon layer
-2 possible method : Charge couple device (CCD) or Thin Film Transistor (TFT)

Indirect detectors

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Requires a scintillator because the atomic number is only 14

Amorphous silicon

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what are two types of scintillators that amorphous silicon would require

-Cesium Iodide
-Gadolinium Oxysulfide

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what are the three ways the signal is captured by :

-charge-coupled device (CCD)
-complimentary metal oxide semiconductor (CMOS)
-Thin Film Transistor (TFT)

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-Used in both indirect and direct detectors
-Electronic readout of signal
-arranged in an array of detector elements (DELs)

TFT

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how is pixel size related to DEL size

directly related

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Collects the electric charges produced by either the selenium or silicon as an array or matrix of pixel size detector elements (DEL)

TFT

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where is the latent image stored

TFT

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which two use thin film transistors for electronic read out

-Amorphous silicon (indirect conversion ) -amorphous selenium (direct conversion)

out of CCD and CMOS which has the high quality, low noise

CCD

a photoconductor that converts light to electric charge and stores it. The stored charge releases line by line to the ADC. The electrical signal produced is then sent to the computer for image processing

CCD

-more susceptible to noise -light sensitivity tends to be lower

CMOS

-uses scintillator -closely associated with CCD chips -image sensors, which convert light to electrons

CMOS

-storing electric charge -latent image stored here

TFT

Explain steps of TFT:

when taking an exposure, flap on TFT closes, capturing signal, when done TFT opens (releases signal), then reading line by line

Each DEL has :

-Sensor Surface (picking up that radiation) -Storage Capactor -TFT

how is spatial resolution related to DEL dimension?

indirectly

-determined by percentage of DEL consisting of sensing material -how efficient is it picking up the dose of radiation

TFT Fill Factor

fill factors relationship with detector dose efficiency

Higher fill factor increases x-ray dose efficiency

How does fill factor affect image quality

Higher fill factor yields higher spatial and contrast resolution

Increase sensing area

increase fill factor and increase image quality

What is in each del?

TFT

What is the DEL size expressed in?

Micrometers

-Use amorphous selenium (a-S e) as photoconductor -No scintillating layer -Direct action of x-ray photons converted to electrons in a-Se layer -Charges collected by T F T for readout

Direct Detectors

Term for detector element

DEL

Dexel and del are what?

The same thing

The more efficient your fill factor is

the better your image

-Exposure field recognition -Histogram creation and analysis -Rescaling -L U T application

Image processing to optimize image quality

Only DELs with signal values

are read out and make up image

How much percent should there be for light field

30 to 35 percent

Exposure field recognition is also known as

your field of view

Histogram creation is read

left to right white to black values of interest

If the image is white and has high atomic number

Is more absorption less transmission

two general classes of DR panel

Portable (wireless of hard wired with tether cable) Integral to equipment deign (typically hard wired to radiographic equipment)

-DR panels typically require communication with xray generator -interface is vendor specific -newer DR panels eliminate the traditional generator interface -Panels are wireless -No true generator interface TFT is charged by on-board capacitor prior to exposure -Presence of x-radiation hitting panel “triggers” TFT to collect signal -Exposure terminated by manual timing or automatic exposure control (A E C) -Data read-out completed and panel recharges for next exposure

Auto-Detection Technology

Enough radiation has reached and back-up timer it will shut off

AEC

What are the three kinds of generator?

- single phase - three phase - high frequency (the best)

Be aware of DR panel Precautions

-avoid panel dropping -clean panels -fluid invasion (encase protective bags) -weight load restrictions -Respect level of sophistication

What you see on the monitor is your manifest image

Soft copy

Have actual film

Hard copy

Which three components are included in indirect capture flat panel digital image detector systems?

- photodiode - scintillation layer - TFT array

Which component of an indirect capture flat panel detector converts incident xray photons into visible light photons?

Scintillation layer

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

Photodiode

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 - charge-coupled device (CCD)

which element is used in the construction of a flat panel detector is able to convert xray 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 detector element?

Thin film transistor (TFT)

Cesium Iodide is the preferred material for the scintillation layer because it can be formed into

Columns with focus light photons

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

Lenses or fiber options

the digital image receptor is comprised of small elements called

Detector Elements (DELs)

Which of the following makes up a single detetctor element

- capture area - storage capacitor - thin film transistor (TFT) gate

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

Top electrode detection layer bottom electrode

The storage capacitor of the detetcor element (DEL) stores which of the following

A positive charge

What is the percentage of the detector element 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 detetcor elements into rows and columns

Matrix

Which of the following would lead to an increase in patient dose

decreased fill factor (smaller dels have reduced fill factor which reduced efficiency )

which of the following will produce the highest spatial resolution

large matrix and low pitch (more dels which means more spatial resolution)

A device capable of accumulating and storing an electrical charge

Capacitor

which is capable of converting light into an electronic signal

photoconductor

converts incoming xray photons to light

scintillator

used in conversion of light photons to an electric signal

photodetector

refers to the sensing area compared to the non-sensing area, and can be expressed as a percentage.

fill factor

being used to describe both the indirect amorphous silicon and the direct amorphous selenium plates that are being used in some digital systems.

flat panel detector

are positioned in a matrix that allows the charge pattern to be read out on a pixel-by-pixel and column-by-column basis. Includes sensing area, capacitor , and tft layer

Dexel

is a photodetector, a device that is capable of converting visible light into an electric charge and storing it in a sequential pattern.

CCD

designed as image sensors, which convert light to electrons. closely associated with CCD chips

CMOS

are built into the design of a radiographic table or upright holder.

integral detectors

An electronic switch on flat-panel detectors commonly made of amorphous silicon. allows the charge collected at each pixel to be independently transferred to external electronics, where it is amplified and quantized.

TFT

An electronic element which converts light into charge. With indirect TFT detectors this is accomplished by a light-sensitive amorphous silicon photodiode on top of the TFT array

photodiode

The scintillator crystal commonly used in indirect capture DR

cesium iodide

The photoconductor used in direct radiography

amorphous selenium

Denotes that crystals in the phosphor are randomly distributed

Amorphous

What are the advantage of DR compared to film?

- post processing options - higher contrast resolution - ability to take multiple pictures - more faster - produces higher quality images - tech can manipulate image - image is displayed on monitor - pre processing

What does film deal with?

- You have to process the film on your own - the hard copy - you get what you get - better spatial resolution

What are the disadvantages of DR?

- really expensive - advance technology can make it difficult for techs to learn - technology doesn't always work in your favor

Look over the worksheets she gave us

Look over the worksheets she gave us!!!

What is being replaced when we change to digital?

- eliminated the need of cassettes - reduce the number of steps required to perform exams

Direct detectors use what type of material?

Photo conductor

What does a photoconductor do for direct detectors?

capable of converting light into an electronic signal

What does a scintillator do?

Convert incoming x-ray photons to light

Why is photo detector photo idoide used for indirect detectors?

conversion of light photons to an electronic signal

What can't amorphous silicon do?

They cannot directly convert x-rays into an electric charge

What does a amorphous silicon do instead?

It works as a light detector ( photodiode) to capture fluorescent light

Scintillators emit light isotropically which can cause what?

Reduce spatial resolution

Is manufactured as structured crystals in the form of needles or columns 10-20 micrometers in diameter

Cesium iodide (CsI)

Indirec detectors once the light reaches the amorphous silicon what happens?

is converted into electrical charges that are then deposited on a TFT

semiconductors

Amorphous silicon, amorphous selenium, CCD

when is a photoconductor used

In direct conversion

is CCD or TFT a photodetector

CCD

what is the readout scheme

read line by line

when is a photodetector used

indirect