3. Computed Radiography: Physics and Technology Flashcards
TERMS SYNONYMOUS WITH CR
- Photostimulable Luminescence Radiography (PSL)
- Storage Phosphor Radiography (SPR)
- Digital Luminescence Radiography (DLR)
- Photostimulable Storage Phosphor Radiography
- Digital Storage Phosphor Radiography (DSP)
PSL
Photostimulable Luminescence Radiography
SPR
Storage Phosphor Radiography
DLR
Digital Luminescence Radiography
DSP
Digital Storage Phosphor Radiography
was discovered at the base of a volcano near Bologna
Bolognese stone
Bolognese stone was discovered by ____ in ____
Vincenzo Cascariolo, 1603
when treated with heat and exposed to sunlight it would glow for hours or days
Bolognese stone
CR imaging system was commercialized for use in digital radiography in ____ by ____
1983,
Fuji
introduced their FCR-101 unit (Fuji Computed Radiography-101 unit)
Fuji
FCR-101 unit
Fuji Computed Radiography-101 unit
was the pioneer of CR
Fuji
4 MANUFACTURERS ENGAGED IN CR RESEARCH, TECHNOLOGY DEVELOPMENT AND MARKETING
Fuji
Kodak
Agfa Gavert (Belgium)
Konica
Bolognese stone discovered in Italy
1603
Notion of de-excitation described by Becquerel
1867
Hirsch proposed way to retain fluorescent image by using storage phosphor-heat to restimualte and red light to erase
1926
KODAK patented an apparatus that used infrared-stimulable phosphor to store image
1975
FUJI patented process in which photostimulable phosphor was used to record image by absorbing radiation and then releasing stored image when stimulated by helium-neon laser
1980
FUJI’s first Commercialized Computed Radiography (FCR 101)
1983
FCR 201
1985
FCR 7000
1989
KODAK’s first commercial storage phosphor unit
1992
Agfa-Gavaert introduces CR Imaging Technology
1994
Others in CR Research include GE Medical Systems, Philips, Konica, etc.
2001
Fuji, Kodak, and Agfa actively engaged in research and development of state-of-the-art CR systems
Present
4 STEPS OF CR IMAGING
- Image Acquisition
- Imaging Plate Scanning and Erasure
- Image Processing
- Image Display, Storage and Communications
there are secondary steps that must be considered, the “implementation issues”
AAPM (American Association of Physicists in Medicine)
AAPM
American Association of Physicists in Medicine
AAPM (American Association of Physicists in Medicine) –there are secondary steps that must be considered, the “implementation issues”
- Practical use of CR
2. Technical concerns such as: use of grids, radiation exposure, and technique selection
x-ray exposure of the phosphor plate storage, or imaging plate (IP)
Image Acquisition
• Refers to the mechanism of X-ray interaction with the phosphor to produce a latent image and subsequent scanning of the IP by a laser beam to produce photostimulable luminescence (PSL)
Image Acquisition
the scanning of the IP takes place in the imaging plate reader/processor
Imaging Plate Scanning and Erasure
This readout process essentially consists of laser scanning, detection, conversion of the PSL and digitization of the signal by the analog-to-digital converter (ADC)
Imaging Plate Scanning and Erasure
use of several digital operations for pre-processing and post-processing of the CR image data
Image Processing
shading corrections, pattern recognition and exposure field recognition
Pre-processing
contrast enhance (grayscale), edge enhancement (spatial frequency)
Post-processing
uses special algorithm to create separate images of bone and soft tissue
Energy subtraction
the RT determines and assess the overall image quality of the image and subsequently send the image to the PACS
Image Display, Storage and Communications
once in the _____ images are retrieved for interpretation by a radiologist
PACS
- A digital radiographic imaging modality in which a digital detector is used to capture x-rays transmitted through the patient
Computed Radiography
- CR is based on the phenomenon of ______ that is exhibited by photostimulable phosphors, which when struck by x-rays, creates latent image
photostimulable luminescence
- The ______ is rendered visible when the detector is scanned by a laser beam to produce light (photostimulable luminescence) that is subsequently converted in electrical signals
latent image
- These signals are digitized and processed by a computer that produces the CR image using
special digital image processing algorithms
Commonly used phosphor material is
barium fluorohalide
is positioned within the cassette or imaging plate (IP)
Photostimulable phosphor (PSP) screen
The phosphor particulates are bonded with a cohesive material forming a ____ structure and deposited on a base for mechanical support
turbid
CR is based on certain halide-based phosphors having energy storage and excitation characteristics known as photostimulable luminance (PSL) which:
- Enable phosphors to store x-ray energy
2. Release that energy upon excitation by a laser beam
Standard resolution IP characteristics
- Thick phosphors layers
- Absorbs more radiation
- Faster speed
High resolution IP characteristics
- Thinner phosphor layers
- Sharper images
- Extremity imaging (small parts)
CR IMAGE PROCESSING
- When the photostimulable phosphor (PSP) screen is exposed to x-rays, a fraction of that energy is stored
- After exposure the IP is inserted in to a CR reader
•The IP is processed by a scanning system or reader which:
- Extracts the (PSP) screen from the cassette
- Moves the screen across a high intensity scanning laser beam
- Blue-violet light is emitted via PSL
- Light energy is read by a PMT, which converts the light into an electrical signal
a high intensity white light dumps all the remaining energy traps allowing the plate to be re-used thousands of times (Approximately up to 10,000 times before a new phosphor is needed)
Erasing the screen
could erase by placing against view box for several minutes
Erasing the screen
replaces Darkroom and Processor and Chemicals
QC – Reader
replaces film, storage and view boxes
Diagnostic Viewer
are housed in cassettes similar to conventional film-screen cassettes, with the IP replacing the film, and there are no intensifying screens
Imaging plates
LAYERS OF THE IMAGING PLATE
- Protective layer
- Phosphor layer
- Reflective layer
- Conductive layer
- Color layer
- Support layer
- Backing layer
this is a very thin, tough, clear plastic that protects the phosphor layer
Protective layer
or active layer, this a layer of photostimulable phosphor that “traps” electrons during exposure
Phosphor layer
contains light absorbing dye to prevent light spread
Phosphor layer
this is a layer that sends light in forwards direction when released in the cassette reader
Reflective layer
some detail is lost in this process
Reflective layer
this a layer of material that absorbs and reduces static electricity
Conductive layer
newer plates may contain this layer, located between the active layer and the support, that absorbs the stimulating light but reflects emitted light
Color layer
this is a semi-rigid material that gives the imaging sheet some strength
Support layer
this is a soft polymer that protects the back of the cassette
Backing layer
absorbs backscatter
Backing layer
the barcode label is found here
Backing layer
allows the technologists to match the image information with the patient identifying barcode on the examination request
Barcode label or barcode sticker
indicate the appropriate orientation of the cassette in relation to the patient
Marker / Orientation label
the tube side in conventional radiography
Marker / Orientation label
a device that creates and amplifies a narrow, intense beam of coherent light
Laser (Light Amplification of Stimulated Emission of Radiation)
Laser
Light Amplification of Stimulated Emission of Radiation
Bone
small signal (white) [radiopaque]
Soft tissue
medium signal (grey)
Air
large signal (black) [radiolucent]
meets the requirements
Barium Fluro Halide: Europium (BaFX:EU2+)
Phosphors should have good X-ray absorption efficiency and must be capable of being stimulated by a
helium neon (HeNe) laser
The luminescence light must be compatible with the ________ (for proper detection and capture)
PMT (photomultiplier tube) phosphor
Time for luminescence must be
< (lesser) 1 microsecond
Phosphors should be able to store the latent image for a number of _____ without comprising the signal from the IP
hours
Instead of halide, these could be used:
•chlorine (Cl)
•bromine (Br)
•iodine (I),
or a mixture of them
The phosphor is usually doped with ______, which acts as an activator to improve the efficiency of PSL
EU2+ (Europium)
Other phosphors used in CR is
BaFBr/I :Eu2+ (Barium Fluro Bromide or Iodide doped with Euporium)
and
CsBr:Eu2 (Cesium Bromide doped with Euporium)
Farbe
color (German word)
German word for F-center
Farbe
CR TECHNOLOGY IN THREE STEPS
- Instead of film, an imaging plate is exposed to X-ray
- The imaging plate is digitized by the scanner and then erased for immediate reuse
- The digital image is then displayed on a computer monitor for evaluation with specialized software
wavelength of bluish-purple light
415 nm λ
wavelength of lasers today
680 nm
wavelength of HeNe lasers
633 nm
BASIC PHYSICS OF CR IMAGE FORMATION
- Photostimulable phosphors have the property of creating and storing a latent image when exposed to x-rays
- To render the latent image visible, the PSP must be scanned by a laser beam of a specific wavelength
- Laser scanning produces a luminescence (light) that is proportional to the stored latent image
- This luminescence is referred to as photostimulated luminescence (PSL)
- After laser scanning, the PSP IP is erased by exposing it to a high intensity light beam to get rid of any residual latent image
- This step is important so that the IP can be used again and again for several x-ray exposures
NATURES of PSP’s
The phosphors used in radiology must have certain physical characteristics to be useful in CR imaging
- Phosphors should have good X-ray absorption efficiency and must be capable of being stimulated by a helium neon (HeNe) laser
- The luminescence light must be compatible with the PMT (photomultiplier tube) phosphor (for proper detection and capture)
- Time for luminescence must be < 1 microsecond
- Phosphors should be able to store the latent image for a number of hours without comprising the signal from the IP
HOW PHOSPHORS WHEN STIMULATED BY X-RAYS (SCANNING AND PROCESSING THE IMAGING PLATE)
- When the x-rays fall upon the PSP IP
- The europium atoms are ionized by the radiation
- The electrons move from the valence band (ground state) to conduction band (higher energy)
- Electrons in the conduction band are free to travel to a so-called “F-center” (Farbe [German word] =color)
- The number of trapped electrons is proportional to the absorbed radiation
- At this point, the electrons are spatially distributed to create the latent image
- X-ray exposure of the IP causes it to fluorescence (emit light when it is exposed to X-rays) for a very brief duration
- To render the latent image visible, the PSP IP is taken to the CR reader/processor to be scanned by a laser beam (photostimulable excitation)
- While in the CR reader, the PSP IP is scanned systematically
- The laser light used must be capable of being absorbed by the F-centers (Helium-Neon)
- This absorption causes the trapped electrons to move up to the conduction band, where they are free to return to the valence band, thus causing the Eu3+ to the Eu2+ state
- This transition of the electrons from a higher energy state to a lower energy state (ground state) results in an emission of bluish-purple light (415 nm λ)
- Referred to as photostimulable luminescence (PSL) in the IP
