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Moving from analog to digital 2.0
- 1. Moving From Analog to Digital Not if, but when! The X-Ray Academy of Texas
- 8. 19-Dec-09 Stacy L. Palmer R.T. (R) Cost of Film Over 7 Years: {$15.82 Per Film Per Exam Per Year} 100,000 exams/year @ $1,600,000 Dollars ( Purchase, sort, utilization, storage, handling, staffing, disposing) It Is Estimated That A Single Sheet Cost $50 Over Its Life Source: Mayo Clinic Film Expense Study, 1997
- 11. DR / Direct Radiography 19-Dec-09 Stacy L. Palmer R.T. (R) In a system of direct radiography, also called direct capture radiography, the image receptor is composed of an array of electronic sensors that respond to the radiation exiting the patient. These sensors send that information in digital format to a computer.
- 13. DR / Charge Coupled Device 19-Dec-09 Stacy L. Palmer R.T. (R)
- 14. DR / Charge Coupled Device Low Radiation / Low Noise Zone High Radiation / High Noise Zone CCD Lens Reflector Scintillator X-ray tube
- 15. CR / Computed Radiography 19-Dec-09 Stacy L. Palmer R.T. (R)
- 23. Image Matrix 19-Dec-09 Stacy L. Palmer R.T. (R)
- 25. Histogram/Brightness Level 19-Dec-09 Stacy L. Palmer R.T. (R)
- 26. 19-Dec-09 Stacy L. Palmer R.T. (R) Soft Tissue Lung
- 27. Window Level and Width 19-Dec-09 Stacy L. Palmer R.T. (R)
- 28. Window Level and Density 19-Dec-09 Stacy L. Palmer R.T. (R)
- 29. Window Width and Contrast 19-Dec-09 Stacy L. Palmer R.T. (R)
- 30. Window Width and Contrast 19-Dec-09 Stacy L. Palmer R.T. (R)
- 32. “Dose Creep”
- 33. Dose Response Curve 19-Dec-09 Stacy L. Palmer R.T. (R)
- 34. Noise 19-Dec-09 Stacy L. Palmer R.T. (R)
- 38. 19-Dec-09 Stacy L. Palmer R.T. (R) Digital Subtraction
- 39. Edge Enhancement 19-Dec-09 Stacy L. Palmer R.T. (R)
- 40. Contrast Enhancement 19-Dec-09 Stacy L. Palmer R.T. (R)
- 41. Black and White Reversal 19-Dec-09 Stacy L. Palmer R.T. (R)
- 56. Thank-You 19-Dec-09 Stacy L. Palmer R.T. (R) The End
Hinweis der Redaktion
- A digital image from either the imaging plate or direct capture radiography is composed of a matrix of small bits of numerical data. In this respect a digital image is very different from an analog image on film that is composed of shades of gray (densities). A digital image is composed of many thousands of tiny cells in a matrix of columns and rows, as in the simple matrix shown here. Each cell in this matrix is one tiny piece of the image. These cells are called pixels (picture elements). Pixels are the smallest elements in a digital image. Each pixel has a numerical value representing a gray-scale value somewhere between white (low density) and black (high density).
- The illustration shows an image broken up into a simplified matrix of pixels indicating how the image is composed of small units. Again, the pixels are the individual picture elements in the matrix. They correspond to areas within the patient called voxels (volume elements), for the volume of tissue being imaged. The term "dynamic range" refers to the range of the gray scale values that can be assigned to each pixel. Although the human eye is capable of distinguishing only about 32 shades of gray, digital imaging systems have a dynamic range from 256 to 4096 different shades of gray. This is much more information than the human eye can distinguish.
- After the digital image is acquired using an imaging plate, direct radiography, or analog converter, a specialized computer processes the digital information to create a visible image. Each pixel has a brightness level, a numeric value, within the dynamic range of the equipment. Its brightness represents the degree to which the tissue at that site attenuated the x-ray beam. Digital processing begins with the construction of a histogram, which is an analysis of the pixel values in the digital image. An example is shown in the illustration. The horizontal axis is the pixel values within the dynamic range; the vertical axis is the number of pixels with each value. This is done to determine the level of the exposure intensity to the image receptor. If the histogram indicates that the exposure intensity was too high or too low, the equipment makes automatic adjustments for the best possible image. Unless the exposure error is more than 50% too high or too low, the automatic correction will generally produce a diagnostic image. With some equipment the radiologic technologist also determines what algorithm should be used to process the digital information to construct the image. Algorithms are different computer formulas, or preprogrammed processing routines, used in processing images from different kinds of examinations. For example, a chest image will be processed differently from an abdominal image in order to obtain the most diagnostic image from the digital information.
- Again, the window level is the midpoint in the densities desired. In addition, the window width can be adjusted. Window width is the range of visible densities on both sides of that midpoint. In this illustration, for example, the window level is 70 and the window width is 100. As you can see, the densities range from white at 120 to black at 20, the two extremes for this window width. Everything more dense than 120 will be white in the image, and everything less dense than 20 will be black
- In digital radiography, the radiologic technologist can adjust the density level to emphasize different tissues and structures. This variable density level is determined by setting the window level of the image. Recall that digital imaging systems can distinguish over 1000 shades of gray in the dynamic range between black and white, even if the human eye can distinguish only about 32 shades. The window level determines the level of brightness throughout the image's range of densities by setting the midpoint of the densities desired to be visible in the image viewed. As shown in the scale, the density of water is assigned a value of zero. The least dense substance, air, has a value of -1000 (a negative number relative to water), and the most dense, bone, has a value of 1000.
- Window width is a control of contrast in the image because changing the window width determines the range of densities that will be visible. A wider window width increases the range of densities and, therefore, lowers contrast. A narrower window width reduces the range of densities and therefore increases contrast in the visible image. In the example shown in the illustration, the window width is made narrow to image only the tissue density range from fat to muscle. Since everything outside that range will turn white or black in the visible image, the image will have high contrast.
- Shown in the illustration is an example of how changing the window width changes the contrast in the visible image. The image on the left shows a lower contrast, or more shades of gray, due to a wide window width. When a narrow window width is displayed, the image will have higher contrast, or fewer shades of gray, as seen in the image on the right. Remember that changing window width is a postprocessing technique. This changes how the previously exposed image will appear. Contrast in digital images is also affected by the many factors that affect conventional film images. The kVp selection affects contrast most significantly. Because scatter radiation reduces contrast, grids are still often used. In order to further reduce scatter, collimation is used to limit the beam size to only that needed for the examination.
- Density is another image characteristic that differs somewhat in digital radiography from conventional radiography. Density is the amount of darkening that occurs in the image as a result of radiation striking the image receptor. Recall that the sensitivity of radiographic film has an S-shaped curve, such as shown in this graph with the red curve. Note that the detector response (vertical axis) of film does not begin at zero or continue upward beyond its peak. That means that film responds (with increasing density) only within a certain range of exposure. There is no response below that minimum radiation exposure, and after the maximum exposure level the density is maximum and the film does not respond further. The sensitometric curve for digital radiography, however, shows a linear response, as shown by the blue line in the illustration. This means that the response is gradual and continuous from the minimum exposure level to the maximum, giving digital images a wider exposure latitude
- A final image characteristic in digital radiography is called noise. At very low exposure levels, such as sometimes used with very high-speed film-screen combinations, a type of image noise called quantum mottle may occur in either digital or conventional radiography. This occurs as a result of a statistical fluctuation in the number of x-ray photons per unit of area in the image. Recall that water is given the density value of zero. In a radiographic image of water, all pixels should, therefore, register the exact same value. All pixels will average to zero, but because of variations in the numbers of photons striking different pixel-sized areas, some pixels will be darker and some lighter. This noise may cause the image to appear splotchy or to have a salt-and-pepper graininess, such as you can see in this enlargement of a section of an image affected by noise. As mentioned earlier, a low-exposure image can be adjusted to enhance density, but this increase in density also increases the visibility of noise. Although image noise can occur with both conventional and digital radiography, additional system noise can occur with digital radiography during the processes of image acquisition. This noise may occur with the digital conversion from the imaging plate phosphors or laser scanning and digitizing of an analog image on film. Usually, however, noise is not a significant factor when correct exposures are used.