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B ASIC P HYSICS OF D IGITAL R ADIOGRAPHY By : Maisa Alhassoun Recommended Book: Walter Huda, REVIEW OF RADIOLOGIC PHYSICS &

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Presentation on theme: "B ASIC P HYSICS OF D IGITAL R ADIOGRAPHY By : Maisa Alhassoun Recommended Book: Walter Huda, REVIEW OF RADIOLOGIC PHYSICS &"— Presentation transcript:

1 B ASIC P HYSICS OF D IGITAL R ADIOGRAPHY By : Maisa Alhassoun maisa@inaya.edu.sa Recommended Book: Walter Huda, REVIEW OF RADIOLOGIC PHYSICS & http://en.wikibooks.org/wiki/Basic_Physics_of_Digital_Ra diography/The_Computer

2 D IRECT D IGITAL I MAGE R ECEPTOR With direct digital imaging, the phosphor is built into the Wall Bucky or Table Bucky. Almost as soon as the image is produced, it can be viewed. Great through-put but less flexible than CR.

3 D IRECT D IGITAL I MAGE R ECEPTOR The direct digital receptor is basically a digital x-ray camera. The pixel area is exposed by the x-rays exiting the patient. The photons are absorbed and the energy produces an electrical signal.

4 D IRECT D IGITAL I MAGE R ECEPTOR The electrical signal is a form of analog data that is converted into a digital number and stored as one pixel in the image.

5 C OMPUTED R ADIOGRAPHY R ECEPTOR CR uses a Stimualible Phosphor Receptor inside a cassette. It can be used with existing radiographic systems. The x-ray exposure produces an invisible latent image.

6 C OMPUTED R ADIOGRAPHY R ECEPTOR The difference between intensifying screens and computed radiography is that there is a delay between exposure and the production of the light. Here is how it works:

7 C OMPUTED R ADIOGRAPHY R ECEPTOR First a receptor containing the phosphor plate is exposed to record an image. At this point it is an invisible latent image. The next step is the cassette is placed into reader and processor unit. The plate is scanned with a very small laser beam.

8 C OMPUTED R ADIOGRAPHY R ECEPTOR The laser beam stimulates the plate causing light to be produced. The light that is produced is proportional to the x-ray exposure to that specific spot. The result is an image formed by the light produced on surface of the plate or screen.

9 R EADING P HASE A light detector measures the light and sends the data on to produce a digitized image. As the surface of the stimualible phosphor screen is scanned by the laser beam, the analog data representing the brightness of the light at each point is converted to digital values for each pixel.

10 R EADING P HASE The digital data is stored as a digital image. The process takes about 50 seconds compared to two minutes or more with film.

11 This is the digital radiographic processor used in the clinic.

12 D IGITAL R ECEPTOR D YNAMIC R ANGE A wide dynamic range of exposure is a characteristic of many digital radiography systems. This means that the receptor respond to x-ray exposure and produce digital data over a wide range of exposure.

13 F ILM L ATITUDE OR D YNAMIC R ANGE Most film systems have a very limited dynamic range of exposure. Latitude is the range of exposure that forms an image. Latitude is associated with the slope part of the H & D curve.

14 F ILM L ATITUDE OR D YNAMIC R ANGE The region of the toe of the curve has no significant contrast and it corresponds to the underexposed areas of the image. The shoulder region also has no significant contrast and corresponds to over exposure.

15 F ILM L ATITUDE OR D YNAMIC R ANGE The limited latitude is due to the way the image is formed with the silver halide crystals. Digital receptors do not have this limitation.

16 T HE E XPOSURE H ISTOGRAM Before we go deeper into characteristics of digital receptors, lets develop the concept of the exposure histogram. The x-ray image and contrast are formed as the beam passes through the body and experiences different levels of attenuation through the anatomical regions.

17 T HE E XPOSURE H ISTOGRAM In the chest, the low- density lung areas produce a relatively high exposure to the receptor and dark areas of the image. The more dense areas like the spine and below the diaphragm produce relatively low exposure and light areas of the image.

18 T HE E XPOSURE H ISTOGRAM The histogram as we see it here, shows the amount of image area that receives the different levels of exposure that forms the image. At this time our primary interest is in the range of exposures.

19 I MAGING WITH F ILM The greatest challenge of film radiography is to get the range of exposure to fit into the dynamic range of the film. If the exposure falls outside the latitude, there will be little or no image contrast formed.

20 I MAGING WITH F ILM There are two conditions that contribute to receptor exposure outside the film latitude: Error in setting the correct exposure factors. Some regions of the body have wide dynamic of exposure that exceeds the latitude of the film. Using a wider latitude film can correct this but results in lower image contrast.


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