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Name three basic steps included in CR:
- 1. exposure of imaging plate
- 2. transport IP to an IRD for image to be rendered
- 3. computer processes the signal and displays on a monitor
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Name four basic limitations of CR:
- insufficient detection of x-ray (affects the image quality and dose)
- the spatial resolution is less (than F.S.)
- IPs can be easily damaged
- physical transportation of IPs to an IRD
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When was the first flat panel digital detector introduced?
1995
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Name two types of substances used in flat panel digital radiography:
- amorphous silicon
- amorphous selenium
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Name the two main types of digital technology and when they were introduced:
- slot-scan digital detector (CCD chip), 1990
- selenium drum digital radiography, 1994
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with a selenium drum, a ___________ charge is first placed on the drum, which is then ______________________.
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with a selenium drum, xrays change the charge in the selenium to produce a ___________ ________ stored on the drum.
electrostatic image
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with a selenium drum, what occurs after the electrostatic image is produced and stored on the drum?
a probe scans the image to produce a voltage that is sent to an ADC
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with a selenium drum, what occurs after the probe scans the image to produce a voltage that is sent to an ADC?
the digital data stream is sent to the computer for digital processing that would in turn allow the image to be seen on a monitor.
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What electronics are contained in a flat-panel digital detector?
- flat-panel x-ray detection array
- pre-amplifier
- switching control
- central logic circuits
- ADCs
- Internal memory
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In flat-panel DR, what acts as an interface between the flat-panel detector and the x-ray machine and other system components?
the host computer
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In flat-panel DR, what applies the appropriate image processing "to perform image correction and optimization for display?"
the host computer
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In DR, the _________ provides a means to distribute images to anywhere in the world.
Display-Storage-Network
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Two categories of flat-panel digital radiography detectors:
- indirect detectors
- direct detectors
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Indirect detectors use ________________, while direct detectors use ________________.
- phosphors (scintillators)
- photoconductors
-
What are not classified as a flat-panel digital detector?
CCD Digital detectors
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what are the main components of a CCD digital detector?
- x-ray absorber
- light optics
- CCD-sensor (used to capture light and electrical signals)
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_______________ are based on an indirect conversion process and use a chip to convert light to electrical charge.
CCD Digital Detectors
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What are key technical components of CCD digital detectors?
- scintillation screen (detects x-ray and converts to light)
- light collection optics
- array of CCDs (CCD camera)
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Name three kinds of CCD digital detectors:
- Lens-Coupled CCD detector
- Fiber optic-coupled scanning array
- CCD linear array
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What uses several physical components to convert x-rays into light that is subsequently converted into an electrical charge?
TFT Digital Detector
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Main components for TFT (indirect) include:
- x-ray scintillator (x-ray conversion layer)
- amorphous silicon (a-Sl) photodiode flat-panel layer
- TFT array for readout of the electrical charges by the photodiode
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What does the a-Si photodiode layer of a TFT Digital Detector do?
converts the light from the x-ray detection scintillator into electrical charges
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What two substances usually make up the x-ray scintillator layer of a TFT Digital Detector (indirect)?
- cesium iodide (CsI)
- gadolinium oxysulfide (Gd2O2S)
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Name some components of a Direct plat-panel TFT:
- the source of high voltage
- Top electrode
- Dielectric layer
- photoconductor
- Collection electrode
- TFT
- Storage capacitor
- Glass substrate
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Name some substances used as photoconductors for direct digital detectors:
- amorphous selenium (common)
- lead oxide
- lead iodide
- thallium bromide
- gadolinium compounds
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Why is amorphous selenium usually used as a photoconductor?
because of its excellent x-ray detection and high spatial resolution
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the configuration of the flat-panel is designed as a matrix of detector elements (each regarded as a pixel) and this design is called a:
large integrated circuit
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In the configuration of the flat-panel, each pixel contains:
- a TFT (switch)
- a storage capacitor
- a sensing area (sensing/storage element)
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What a difference in the configuration of the direct and indirect TFT?
the sensing/storage element with the indirect TFT is the photodiode while the direct flat-panel TFT uses the a-Se photoconductor
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what activates each row of pixels and electronic amplifiers and associated electronic devices (multiplexer) for signal readout from each column of pixels?
switching electronics
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What are different sizes of detectors available for various body parts?
- 43x43 (cm)
- 30x40 (cm)
- 18x18 (cm)
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what are different matrix sizes?
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What is the distance from the midpoint of one pixel to the midpoint of another pixel?
pixel pitch
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How can the number of pixels be obtained?
by multiplying the dimensions of the matrix size
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What is defined as the ratio of the sensing area of the pixel to the area of the pixel itself?
the fill factor
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If a pixel has a fill factor of 80%, then how much is occupied by electronics?
20%
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A(n) __________ in fill factor provides better spatial and contrast resolution.
increase
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What is a CCD silicon chip made of?
millions of pixels
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In CCD, light from the scintillator falls upon each pixel to produce _________________.
- electron hole pairs (charges)
- (they have a direct relationship)
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In CCD, readout is provided by a collection of changes on each chip called a _______________________.
bucket brigade
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In CCD, limitations are due to the light optics:
- lenses
- mirrors
- fiber-optics
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For direct flat panel TFT detectors, the x-ray photoconductor is:
a-Se
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Name three important characteristics of scintillators:
- atomic number (Z)
- density
- thickness of attenuating material
-
Flat-panel TFT digital detectors are commonly used in these three areas:
- radiography
- fluoroscopy
- angiography
-
CsI atomic number and thickness:
- Z# 54
- thickness of about 600 micrometers
-
Is more or less thickness required of the amorphous selenium in mammography?
less (about 250 micrometers)
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What is a term to describe a continuous x-ray?
brems spectrum
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in an indirect flat-panel TFT detector, what occurs after a scintillator (CsI or Gd2O2S) converts x-ray to light?
the light then strikes an amorphous-silicon photodiode layer, which converts light into electrical charges
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in a direct flat panel TFT detector, what occurs after the x-ray photons fall upon the amorphous selenium photoconductor layered on top of a matrix of a-Se TFT array?
- an electric field is created between the top electrode and the TFT elements
- as the x-rays strike the a-Se, electrical charges are created and the electric field causes them to move towards the TFT elements, where they are collected and stored.
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What measures the response of the image receptor (SF or DR) to the radiation falling upon it?
exposure latitude
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CR/DR have a ________ response while FS has a __________.
- linear
- non linear response curve
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With DR/CR, what is the major advantage of wide exposure latitude?
the detector can respond to various energy levels of exposure and still provide an acceptable image for viewer
-
What is a name for the practice of using higher exposure in general to improve image quality?
exposure creep or dose creep
-
A numerical value usually displayed on the digital image to indicate the exposure to the digital detector:
exposure indicator
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The purpose of image processing in systems using flat-panel digital detectors are:
- to prevent artifacts by correcting for raw digital data from the detector
- to optimize the image by improving contrast and sharpness while reducing noise
-
Artifacts can be corrected by _____________ which is referred to as a system calibration.
flat-fielding
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name the steps that must be met when changing from a "for processing" image to a "for presentation" image:
- use processing algorithms and histograms to match anatomy being x-rayed
- scaling histogram based on exposure falling on the detector
- contrast enhancement and smoothing operations performed to enhance image quality
-
What offers details concerning image display optimization:
AAPM Task Group 18 (in 2006)
-
What contributes to image display optimization?
- post-processing
- technologist workstation
- communication of image from the technologist workstation to the PACS workstation for the radiologist to access
-
Processing optimization rules:
- avoid having techs manipulate processing on a case-by-case basis (inconsistency)
- ensure modified processing parameters work on a variety of cases and patient sizes
- evaluate processing parameter modifications on whatever the radiologist reads from, not including the aquisition station
-
Characteristics affecting image quality include:
- spatial resolution (sharpness)
- modulation transfer function (MTF)
- dynamic range
- detective quantom efficiency (DQE)
-
the ability of an imiging system to resolve the finde details in the object being imaged:
spatial resolution
-
What three things is spatial resolution dependent on?
-
measuring spatial resolution involves at least three methods:
- bar test pattern
- a sharp-edged object
- a narrow slit
-
a complex mathematical function that measures the ability of the detector to transfer its spatial resolution characteristics to the image:
modulation transfer function (MTF)
-
What represents perfect transfer of spatial and contrast information?
MTF of 1
-
the response of the detector to different levels of radiation exposure:
dynamic range
-
"the efficiency of a detector to convert the x-radiation signal at its entrance window into useful image signal" is dealt with by
DQE (detective quantom efficiency)
-
A ____________ is a "needle like" design that reduces the lateral spread of light which serves to improce the spatial resolution of the image.
- CsI crystal
- (a structured phosphor)
-
The image initially obtained by the detector is often referred to as a:
flat field
-
examples of pre-processing artifacts produced from bad detector elements:
- dead/bad pixels
- bad columns or pixels
- the seam where sub-panels are filled together
-
contrast enhancement takes place with ______ processing.
post
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A _________ MTF value at a higher spatial frequency means that the detector provides better ________ _________ than lower MTF values at low frequencies.
-
a higher MTF value at ___________ spatial frequencies means that the detector provides better contrast resolution
lower
-
the digital detector responds to a ________ range of exposure compared with film-screen image receptors:
wider
-
_________ __________ _____________ deals with the efficiency of a detector to convert the x-radiation signal at its entrance window into useful image signal
detective quantom efficiency
-
DQE provides information about the __________ to ________ ratio.
Signal to Noise ratio
-
When too few photons strike the detector, the noise is referred to as:
quantom noise
-
_______ ________ refers to the persistence of the image charge still being produced after the radiation beam from the x-ray tube has been turned off.
image lag
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name several sources of artifacts in flat-panel digital detectors:
- dust
- scratches
- chemical reactions
- defective pixels
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