009 Digital Radiography

8/3/2019 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



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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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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Consider a simplified workflow of aconventional film screen system

And for a typical CR system workflow


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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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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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Charged-Coupled Devices (CCD)

Flat Panel Detectors (FPD)

Direct Flat Panel Detectors

Indirect Flat Panel Detectors


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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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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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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).

009 Digital Radiography

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