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DIGITAL RADIOGRAPHY

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Digital Radiography

• A “filmless” imaging system introduced in 1987

• Digital radiography uses an electronic sensor, instead of “film”

• Requires a computerized imaging system to produce an image.

• No film is used, and no processing chemicals are required.

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Analog vs. Digital

• “film based”• Produces a “radiograph”• Produced when x-ray

photons strike the film• Shows on film as a

continuous spectrum of gray shades between the extremes of white and black

• The shades “flow into” one another like a painting

• “sensor” is recording medium

• Produces a computerized “image”

• Uses an array of “pixel” elements with exact gray and discrete gray values for each pixel.

• More like a mosaic pattern instead of the shades “flowing together”

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Digital vs. Analog

• X-ray photons• Strike sensor creating a

surface electrical charge• Which is converted to

digital form (digitized)• Sensor transmits

digitized image to computer

• Image is produced almost instantly!

• Computer stores the information

• X-ray photons• Interact with silver

halide crystals• Produces a latent

image• Chemical processing

takes time• Visible image• Stored on mounts

within the patient’s paper record

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Analog to Digital Converter

• The data acquired by the sensor is communicated to the computer in analog form, then converted by the use of the analog –to-digital-converter (ADC)

• The image is displayed within seconds and may be readily manipulated to enhance diagnosis.

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Types of Digital Sensors

Charge-Coupled Device (CCD)Complimentary Metal Oxide Semiconductor/Active Pixel

Sensor (CMOS/APS)

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CCD

• One of the most common digital sensors used in dental digital imaging

• The CCD is a sensor that contains a silicon chip with an electric circuit built into it.

• The silicon chip is sensitive to x-radiation• The electrons that compose the silicon CCD

are arranged in “blocks” or picture elements known as pixels.

• A pixel is the digital equivalent of the silver halide crystal on conventional film.

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CCD

• Unlike the silver halide crystals contained in the film emulsion, which is randomly distributed, the pixel arrangement is ordered.

• The sensor will contain 307, 200 pixels! Each pixel is linked to a specific area on the computer screen.

• As the x-ray photons come in to contact with the sensor, this produces an electronic charge that is connected to a specific area on the computer screen

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CMOS Sensor

• The CMOS differs from the CCD in the way that the pixels are read by the computer.

• They claim a 25% greater resolution than CCD technology.

• The CMOS sensor is also less costly to produce and the sensor has greater durability

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So what does the computer do in digital

radiography??

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The Computer

• It stores the incoming electronic signal.• It also converts the the signal into a shade

of grey that is viewed on the computer monitor.

• Each pixel is represented in the computer by location and color level of the gray.

• The pixel can create 256 shades of gray, but the human eye can only perceive 32 shades of gray!

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Types of Digital Imaging

DirectIndirect

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Two Ways to Obtain a Digital Image

Direct• Sensor placed in

mouth and exposed to x-rays.

• Sensor captures image and transmits to computer monitor

• Image appears within seconds

Indirect• Scanning in traditional

radiographs• This method is inferior

because the resulting image is a “copy” vs. the original

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Analog vs. Digital

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Analog vs. Digital

Analog• Silver halide crystals• Random arrangement

on the film

Digital• Pixel – the digital

equivalent of a silver halide crystal, but in an ordered arrangement

• 307,200 pixels on a sensor!

• Produce a sort of “electronic” latent image.

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Digital Imaging

Advantages• Superior gray scale resolution• Decreased patient exposure

to radiation• Increased speed of image

viewing• Some cost reductions (no film,

processing or darkroom needed)

• Increased efficiency• Enhanced diagnostic image

known as digital subtraction• Effective patient education

tool

Disadvantages• Initial set-up costs• Image quality (on-

going debate)• Sensor size – thicker

and less flexible for patient

• Legal Issues – because the image can be enhanced

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Other Features of Digital Imaging

“Subtraction”MagnificationMeasurement

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Digital Subtraction

• With digital subtraction, the gray scale is reversed so that radiolucent images (normally black), appear white and radiopaque images (normally white), appear black.

• Digital subtraction helps to eliminate distracting background information.

• This feature permits the operator to remove all anatomic structures that have not changed between radiographic examinations to facilitate identification of changes in diagnostic information.

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Digital Subtraction

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Measurement

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Magnification