009 Digital Radiography
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Transcript of 009 Digital Radiography
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Digital Radiography
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Analog imaging modalities
Screen-film radiography
Image intensifier fluoroscopy
Screen-film mammography
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Digital imaging modalities
Computed radiography (CR)
Digital radiography (DR)
Flat panel detector fluoroscopy
Computed tomography (CT)
Magnetic resonance imaging (MRI)
Ultrasound
Single photon emission computedtomography (SPECT)
Positron emission tomography (PET)
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Digitization
In a nutshell, to digitize = to quantize
Analog information
The data is overlapping and continuous. Digitization of the analog data
The data is broken into discrete blocks of
data, which is given a specific valuegrayscale
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Digital concepts
Pixel
Image matrix
Display resolution Image / Spatial resolution
Bit depth
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Pixel
Picture element
Smallest unit of a digital image
A digital image is made up of thousands(and thousands) of pixels.
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Matrix size
The physical size of pixels cannot bechanged, may be different from display todisplay.
Matrix size 256 x 256, 512 x 512
Can be manipulated by the video card of thecomputer controlling the display
Matrix size is limited by storageconsiderations and processing power/time.
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Display resolution
This display has anative resolution of1280 x 1024 pixels.
Medical gradedisplays employ aportrait style
display.
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Image resolution
Determined primarily by the physical sizeof pixels, the smaller the better.
For a fixed FOV, the larger the displaymatrix, the better the spatial resolution.
Theoretically, there is no limit to the spatialresolution of analog film, but digital iscatching up.
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Matrix size and image resolution
512*512 256*256 128*128 64*64
32*32 16*16 8*8 4*4
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Grayscale
A black and white image isbasically that an image whichhas pixels filled with the colour of
either white, or black.
A grayscale image is an imagewhich pixels have various
intensities ranging from full black,to full white and everything inbetween.
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Bit depth
Bit-depth describes how many shades ofgray that each pixel can take.
1-bit (21) display 2 shades
4-bit (24) display 16 shades
8-bit (28) display 256 shades
10-bit (210
) display 1024 shades
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Bit depth256 graylevels: 128 graylevels : 64 graylevels: 32 graylevels:
16 graylevels: 8 graylevels : 4 graylevels: 2 graylevels:
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Digital systems
Computed Radiography (CR)
Digital Radiography (DR)
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Computed radiography (CR)
Marketing term to differentiate from full
digital radiography.
Cassette based system.
Similar to screen-film, the imaging processis non-integrated.
Separate process for image capture, andimage processing
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Computed radiography (CR)
Consider a simplified workflow of aconventional film screen system
And for a typical CR system workflow
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Computed radiography (CR)
The basic workflow remains largely thesame:
The film cassette is replaced with the imaging
plate (IP) cassette.
The film processor is replaced with the CRreader.
The final output may be hardcopy (printer) orsoftcopy (digital storage and viewing viaworkstation monitors).
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Computed radiography (CR)
Advantages of CR systems:
Relatively low cost of setup
Able to be used in mobile x-ray units
No difference in x-ray source, smoothtransition from analog to digital for imagingdepartments
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CR imaging plate (IP)
Also known as photostimulable phosphordetector (PSP) screens.
Phosphors used in screen-film
radiography emit all their lightinstantaneously. In contrast, PSPs absorba large fraction of the x-ray energy andtrap it.
CR imaging plates are usually made ofBaFBr and BaFl, commonly called bariumfluorohalide.
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CR reader
The laser lightstimulates the trappedenergy in the IP, andvisible light is emitted.
The light from the IP iscollected by a fiberoptic light guide andfunneled to aphotomultiplier tube(PMT).
The plate is exposed tobright white light whicherases all trapped
energy.
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CR image characteristics
CR has much larger dynamic range compared toconventional screen-film systems.
Digital image manipulation allows bad
exposures to be corrected, largely eliminatingretakes for over- and under-exposures. This isparticularly useful for mobile units.
Underexposed images can be corrected to aproper grayscale level, but will still suffer fromhigh levels of x-ray quantum noise.
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Digital radiography (DR)
Charged-Coupled Devices (CCD)
Flat Panel Detectors (FPD)
Direct Flat Panel Detectors
Indirect Flat Panel Detectors
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Digital radiography (DR)
In DR, the main mechanism is theconversion of one signal (light for CCD,light or x-ray for FPD) to electrical
charges, which can then be digitized.
Both CCDs and FPDs are designed as agrid of individual detector elements, each
one representing a single pixel.
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Charged-coupled devices (CCD)
A CCD detector chip is an integratedcircuit that can convert incident light intoan electric charge proportional to the light
intensity. Common in modern digital cameras
Chip sizes vary from 4x3mm 4x3cm
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Charged-coupled devices (CCD)
The surface of a CCD is photosensitive whena photon strikes an atom on the surface, it freeselectrons which then accumulate. This forms an
electric charge proportional to the light intensity. CCDs are commonly used where the desired
field-of-view is small, such as dental radiographyor mammography biopsy systems.
Efficiency depends onratio of photosensitivearea to size of detector
element.
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Indirect flat panel detectors
Similar to CCDs, indirect FPDs consist of a gridarray of photosensitive detector elements.
FPD is the same size as the desired FOV
The term indirect comes from the x-ray light electric charge digitization sequence. In a
sense this is similar to conventional screen-filmsystems, but with the film capture and develop
stages integrated.
a FPD is the same size as the desired field-of-view
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Indirect flat panel detectors
An x-ray intensifying screen (typicallyGadolinium Oxide Sulfide Gd2O2S, or CesiumIodide CsI) is installed in front of the panel. The
screen converts incident x-rays to light, which isthen detected by the FPD.
Light spreadingtends to occur, acommon problemwith all scintillators.
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Indirect flat panel detectors
The size of the individual detector elementdetermines the resolution of the FPD.
Decreasing the pixel size (which increasesthe resolution) results in smallerphotosensitive area (lower contrastresolution).
A tradeoff must be made between spatialand contrast resolution.
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Direct flat panel detectors
Similar to the indirect FPD, but theintensifying screen is replaced by a speciallayer of photoconductive material (usually
selenium), which interacts with x-rays toemit electrons.
Selenium layer canbe made thicker forbetter detectionefficiency.
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CR and DR implementation
CR systems are more flexible (ability to doportable examinations) and can integratewith existing conventional radiography
equipment.
On average, a single CR reader canhandle three radiographic rooms.
DR systems excel in high throughputrooms due to the lack of cassettehandling.
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Dose considerations
Due to automatic digital image correction, wrongexposures can be easily swept under the
carpet. This is particularly dangerous for
overexposure. CR systems are generally rated to be
comparable in image quality to 200-speed film.
Flat panel detectors have better quantumabsorption and conversion efficiency, whichallows for a dose reduction of around two- tothreefold (varies).