Unit 3 (Flat-Panel Digital Radiography)
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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
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
When was the first flat panel digital detector introduced?
Name two types of substances used in flat panel digital radiography:
- amorphous silicon
- amorphous selenium
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
with a selenium drum, a ___________ charge is first placed on the drum, which is then ______________________.
with a selenium drum, xrays change the charge in the selenium to produce a ___________ ________ stored on the drum.
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
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.
What electronics are contained in a flat-panel digital detector?
- flat-panel x-ray detection array
- switching control
- central logic circuits
- Internal memory
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
In flat-panel DR, what applies the appropriate image processing "to perform image correction and optimization for display?"
the host computer
In DR, the _________ provides a means to distribute images to anywhere in the world.
Two categories of flat-panel digital radiography detectors:
- indirect detectors
- direct detectors
Indirect detectors use ________________, while direct detectors use ________________.
- phosphors (scintillators)
What are not classified as a flat-panel digital detector?
CCD Digital detectors
what are the main components of a CCD digital detector?
- x-ray absorber
- light optics
- CCD-sensor (used to capture light and electrical signals)
_______________ are based on an indirect conversion process and use a chip to convert light to electrical charge.
CCD Digital Detectors
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)
Name three kinds of CCD digital detectors:
- Lens-Coupled CCD detector
- Fiber optic-coupled scanning array
- CCD linear array
What uses several physical components to convert x-rays into light that is subsequently converted into an electrical charge?
TFT Digital Detector
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
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
What two substances usually make up the x-ray scintillator layer of a TFT Digital Detector (indirect)?
- cesium iodide (CsI)
- gadolinium oxysulfide (Gd2O2S)
Name some components of a Direct plat-panel TFT:
- the source of high voltage
- Top electrode
- Dielectric layer
- Collection electrode
- Storage capacitor
- Glass substrate
Name some substances used as photoconductors for direct digital detectors:
- amorphous selenium (common)
- lead oxide
- lead iodide
- thallium bromide
- gadolinium compounds
Why is amorphous selenium usually used as a photoconductor?
because of its excellent x-ray detection and high spatial resolution
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
In the configuration of the flat-panel, each pixel contains:
- a TFT (switch)
- a storage capacitor
- a sensing area (sensing/storage element)
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
what activates each row of pixels and electronic amplifiers and associated electronic devices (multiplexer) for signal readout from each column of pixels?
What are different sizes of detectors available for various body parts?
- 43x43 (cm)
- 30x40 (cm)
- 18x18 (cm)
what are different matrix sizes?
What is the distance from the midpoint of one pixel to the midpoint of another pixel?
How can the number of pixels be obtained?
by multiplying the dimensions of the matrix size
What is defined as the ratio of the sensing area of the pixel to the area of the pixel itself?
the fill factor
If a pixel has a fill factor of 80%, then how much is occupied by electronics?
A(n) __________ in fill factor provides better spatial and contrast resolution.
What is a CCD silicon chip made of?
millions of pixels
In CCD, light from the scintillator falls upon each pixel to produce _________________.
- electron hole pairs (charges)
- (they have a direct relationship)
In CCD, readout is provided by a collection of changes on each chip called a _______________________.
In CCD, limitations are due to the light optics:
For direct flat panel TFT detectors, the x-ray photoconductor is:
Name three important characteristics of scintillators:
- atomic number (Z)
- thickness of attenuating material
Flat-panel TFT digital detectors are commonly used in these three areas:
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)
What is a term to describe a continuous x-ray?
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
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.
What measures the response of the image receptor (SF or DR) to the radiation falling upon it?
CR/DR have a ________ response while FS has a __________.
- non linear response curve
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:
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.
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?
- 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:
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:
"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:
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.
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
the digital detector responds to a ________ range of exposure compared with film-screen image receptors:
_________ __________ _____________ 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:
_______ ________ refers to the persistence of the image charge still being produced after the radiation beam from the x-ray tube has been turned off.
name several sources of artifacts in flat-panel digital detectors:
- chemical reactions
- defective pixels
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