Ch.22 Digital Radiography Flat Panel Detector Systems Flashcards
Referred to as digital radiography (DR)
Flat- Panel Detector digital systems
When was DR introduced ?
1995
what are the two general classes of DR panels
Indirect and Direct
Steps of Indirect Conversion:
xrays to light, then to an electrical charge
Steps of Direct conversion
xrays directly to electrical charge
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. Indirect DR systems include either a charge-coupled device (CCD) or amorphous silicon with a thin film transistor (TFT) array.
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.
Two types of Indirect DR detectors
TFT and CCD
How do scintillators emit light
Isotropically
conversion of incoming xray photons to an electronic signal without scintillation
Direct Conversion
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
Out of direct and indirect, which has the better image
direct. because there is one less step, so less info lost
-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
Requires a scintillator because the atomic number is only 14
Amorphous silicon
what are two types of scintillators that amorphous silicon would require
-Cesium Iodide
-Gadolinium Oxysulfide
what are the three ways the signal is captured by :
-charge-coupled device (CCD)
-complimentary metal oxide semiconductor (CMOS)
-Thin Film Transistor (TFT)
Designed to convert light energy into electrical signal
Transmission along detector circuit pathways
Analog-to-digital convertor (ADC)
-Used in both indirect and direct detectors
-Electronic readout of signal
-arranged in an array of detector elements (DELs)
TFT
how is pixel size related to DEL size
directly related
Collects the electric charges produced by either the selenium or silicon as an array or matrix of pixel size detector elements (DEL)
TFT
where is the latent image stored
TFT
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 photodetector 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 spacial 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
-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
Image processing consists of steps to optimize image quality:
-Exposure field recognition
-Histogram creation and analysis
-Rescaling
-L U T application
-Latent image with DR is electrical signal values (voltages) created in each DEL
-Readout of signal completed in a logical sequence
-Only D E L s with signal values are read out and make up image
-Image processing similar to CR processing
DR panels capable of pre-processing and post-processing
Image Acquisition, Processing, and Display
-Image processing consists of steps to optimize image quality
-Brightness and contrast tables applied to image data set
-Processing techniques applied if selected
-Spatial location
-Frequency processing
-Final image data set presented for video display
Image Acquisition, Processing, and Display
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
Be aware of DR panel Precautions
-avoid panel dropping
-clean panels
-fluid invasion (encase protective bags)
-weight load restrictions
-Respect level of sophistication
Soft copy
what you see on the monitor
Hard Copy
have actual film
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
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 couples 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 swicth 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 and fiber optics
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
What part of the DEL detects radiation
Capture Area
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 spacial 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 spacial resolution
large matrix and low pitch
(more dels which means more spacial resolution)
A device capable of accumulating and storing an electrical charge
Capacitor
which is capable of converting light into an electronic signal, used in direct conversion
photoconductor
converts incoming xray photons to light
scintillator
used in conversion of light photons to an electric signal, used in indirect conversion
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 director
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
Selenium
Denotes that crystals in the phosphor are randomly distributed
Amorphous
term for directer element
Dexel
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
