Chapter 10 Flashcards
What are the two groups of digital image receptors
Direct Radiography
Computer Radiography
Which systems use a capture phosphor to temporarily store the energy that represents image signal?
CR
Which systems have detectors that are able to directly capture and read out an electric image signal?
DR
Which system does not use a removable imaging plate?
DR
Why were CR systems attractive upon introduction?
They could be easily integrated with exisiting x-ray setups that were designed for use with film-screen. Minor adjustments to AEC could be done quickly
What are the 4 parts to a CR system?
Cassette
Photostimuable phosphor plate
Plate reader
Computer workstation
What are CR cassettes made of and what is their function?
Lightweight plastic
To hold the PSP
CR cassettes are lined with felt to help reduce ______
Static
What type of backing material is in CR cassettes and what is the function?
Aluminum backing that helps reduce back-scatter
What are the layers of PSP?
Protective
Phosphor
Reflective
Conductive
Support (color and soft backing)
What is the main purpose of the protective layer?
Protect the phosphor layer
What is the phosphor layer made of in CR?
Material from the barium fluorohalide family coupled with a europium activator
What are the two types of phosophor layers?
Turbid or Structured
What is the difference between turbid and structured phosphor layer?
Turbid - phosphor crystals are randomly distributed
Structured - contains columnar phosphor crystals that resemble needles standing on end, packed tightly together
What is the purpose of the reflective layer?
Reflects the light released during reading towards the photodetector
What is the purpose of the conductive layer?
Helps to reduce and conduct away static electricity
What is the purpose of the color layer?
Absorb stimulating light and reflect emitted light
What is the purpose of the soft backing layer?
Protect the back of the PSP
When a PSP is exposed to radiation, some ______________ are removed from the atoms of the phosphor?
Electrons
What happens to the electrons that are removed from the phosphor atoms?
Half become trapped in the conduction band
The other half return immediately to the valence and release excess energy in the form of light
What is the conduction band?
Area of an atom just outside it’s outermost valence electron shell
When an electron is trapped in the conduction band, it is still ________ the atom, but is not part of the ____________
Orbiting
Structured valence
What allows the phosphor to store xray energy as the latent image?
Europium
How long can electrons stay in the conduction band?
For hours, but the longer you wait to process the PSP, the more image deterioration there will be
What is used to release the electrons from the conduction band?
A laser
When trapped electrons return to the valence, they release an energy known as:
Photostimulable luminescence
What is the purpose of the CR reader?
Houses the components needed to process the radiated PSP
What happens when the cassette is placed into the CR reader?
A drive mechanism moves the plate through the laser-scanning process (slow scan)
How does the fast scanning process work?
The optical system protects and guides a controlled laser beam back and forth across the plate (fast scan)
What makes up the optical system?
Laser, beam-shaping optics, light-collecting optics, and optical filters
What detects the light that is released from the PSP during scanning?
Photodetector
In CR, what does the photodetector do after detecting the light?
Amplifies the light in the form of an analog signal
The amplified analog signal is converted to a digital signal by an:
ADC (Analog-to-digital converter)
What are the two categories of direct radiography?
Direct and indirect
What are the two forms of Indirect capture?
CCD/CMOS
TFT
What are CCDs?
Charge-Coupled Devices; light sensitive devices that are capable of responding to very low levels of light
What type of scintillator is used for CCD?
Cesium iodide phosphor plate
What is a scintillator?
Absorbs x-rays and emits visible light in response
How is the cesium iodide coupled to the CCD?
Fiber-optics or an optical lens system
In CCD, how is an image created after a scintillator (cesium iodide) absorbs radiation?
The scintillator converts the energy into light
The light energy is captured with the fiber-optics/optical lenses and ends up in the CCD
The energy is convered into electric signal, which is then sent to the computer
What is the scintillator in a CMOS system?
The CMOS itself
What does a CMOS detector consist of?
Amplifier, photodiode, and storage capacitor and is surrounded by transistors
Is CCD or CMOS more light sensitive?
CCD
Which provides better resolution - CCD or CMOS?
CCD
Which is more efficient - CCD or CMOS?
CMOS
CMOS is currently available in what type of areas?
Mammography and dental
What is TFT?
A form of indirect capture - thin-film transistor
What is TFT coupled with?
Amorphous silicon
What is the function of amorphous silicon in TFT?
Acts as the photodetector - allows the capture element to absorb light emitted by the phosphor layer
How is an image created using TFT?
Thin-film transistors are laid out on a glass substrate
The capture element is painted with amorphous silcon, which allows it to absorb light emitted by the phosphor layer
The emitted light is converted to electric signal
In indirect systems, what is a scintillator used for?
A scintillator is used to convert radiation into light
In indirect capture systems, what is light coverted into?
Electric signal
What is the difference between direct capture and indirect capture?
Direct - turns radiation into electronic signal
Indirect - needs visible light for electronic signal
What is CMOS?
Complementary Metal Oxide Semiconductors
Direct capture systems use a photoconductor made of:
Amorphous selenium
What is the function of amorphous selenium?
Absorb radiation as input and create an electric charge as output
During image acquisition, the computer creates a:
Histogram
What is the x-axis and y-axis of a histogram?
X-axis: Amount of exposure
Y-axis: How many pixels were radiated for each exposure level
What is histogram analysis?
The computer analyzes the created histogram and compares it with a default histogram
During the recognition proess of histogram analysis, how many edges must be identified?
Minimum of 3, but ideally 4
When does sampling occur?
During transmission from the photodetector to the ADC
What is sampling?
Taking a peice of the original analog signal and using that sample piece to create the digital signal
The closer individual sample sections are to each other, the greater the:
Sampling frequency
Sampling frequency must be at least ______ times the original bandwidth of the analog signal in order to accurately digitize the signal (Nyquist Theorem)
2 times
What is the key difference between CR and DR when it comes to processing the histogram?
DR - only the detector pixels that were radiated contribute to image formation; you can center the part over any part of the cassette
CR - region of interest, must be in the center of the cassette
What is defined as the range of exposure values that a receptor can receive and still produce a diagnostic image
Exposure latitude
Numerical values that represent how much radiation exposure a receptor received
Exposure indicators
S numbers are ___________ proportional to the amount of radiation received
Inversely
EI numbers are _________ proportional to the amount of radiation received
Directly
What is DAP?
Dose Area Product - a measure of exposure in air
What are the three values when measuring DAP as an exposure indicator?
EI - Exposure index
EI-T - Exposure index target
DI - Deviation index
What is exposure index?
Exposure detected at the time of exposure
What is exposure index target?
The predetermined ideal exposure index
What is the deviation index?
The measure of difference between EI and EI-T
What is DQE?
Detective Quantum Efficiency - a meausrement of the efficiency of the receptor converting x-ray exposure into a diagnostic quality image
Spatial frequency is:
The amount of line pairs per millimeter within the matrix
Spatial frequency is ______ of the Nyquist frequency
1/2
Nyquist frequency is the ___________ spatial frequency that a digital detector can record
Highest
Monthly and quarterly quality checks should be done to evaluate:
Distortion
Ambient light reflection
Resolution
How often should imaging plates be cleaned?
Every 3 months or as needed
CR imaging plates need to be erased before use after ___ hours
48
What is PACS?
Picture Archiving and Communication Systems - integral part of digital imaging
What are the 3 parts of PACS?
Acquisition
Network
Storage
What is DICOM?
Digital Imaging and Communication in Medicine - file format that allows for exams from all modalities to be able to be stored and used at any PACS terminal across the network
One major limitation of PACS is:
Storage
3 main classifications of PACS storage
Online - data is stored on hard drives that provides near instant access
Nearline - data is stored on removable drives that are able to be loaded by a robatic arm on demand
Offline - data is stored on removable drives that must be manually retrieved
