Chapter 7 Digital Radiographic Image Processing in addition to Manipulation Objectives Desc
Sharpie,, Host, Sports X has reference to this Academic Journal, PHwiki organized this Journal Chapter 7 Digital Radiographic Image Processing in addition to Manipulation Objectives Describe as long as mation of an image histogram. Discuss automatic rescaling. Compare image latitude in digital imaging with film/screen radiography. List the functions of contrast enhancement parameters. State the Nyquist theorem. Objectives Describe the effects of improper algorithm application. Explain modulation transfer function. Discuss the purpose in addition to function of image manipulation factors. Describe the major factors in image management.
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Key Terms Archive query Automatic rescaling Contrast manipulation Edge enhancement High-pass filtering Histogram Image annotation Image orientation Image sampling Image stitching Latitude Key Terms Look-up table Low-pass filtering Magnification Manual send Modulation transfer function Nyquist theorem Patient demographics Shutter Smoothing Spatial frequency resolution Window in addition to level Digital Radiographic Image Processing in addition to Manipulation In cassette-based in addition to cassetteless systems, once the x-ray photons have been converted into electrical signals, these signals are available as long as processing in addition to manipulation. The reader is used only as long as cassette-based systems, but the processing parameters in addition to image manipulation controls are similar as long as both systems.
Preprocessing Preprocessing takes place in the computer where the algorithms determine the image histogram. Postprocessing is done by the technologist through various user functions. Digital preprocessing methods are vendor-specific. CR Reader Functions The computed radiography (CR) imaging plate records a wide range of x-ray exposures. If the entire range of exposure were digitized, values at extremely high in addition to low ends of range would also be digitized. This would result in low-density resolution. To avoid this, exposure data recognition processes only the optimal density exposure range. CR Reader Functions Data recognition program searches as long as anatomy recorded on the imaging plate as follows: Finding collimation edges Eliminating scatter outside the collimation Failure of the system to find the collimation edges can result in incorrect data collection. Images may be too bright or too dark.
CR Reader Functions Data within collimation result in generation of a graphic representation called a histogram. Because in as long as mation within the collimated area is signal used as long as image data, the in as long as mation is the source as long as a vendor-specific exposure data indicator. CR Image Sampling Plate is scanned. Image location in addition to orientation is determined. Size of the signal is determined. Value is placed on each pixel. CR Image Sampling Histogram is generated that allows system to find useful signal by locating the minimum (S1) in addition to maximum (S2) signal within the anatomic regions of interest in the image. Histogram identifies all densities on the imaging plate in the as long as m of a graph: X-axis is related to amount of exposure. Y-axis displays the number of pixels as long as each exposure. Graphic representation appears as a series of peaks in addition to valleys in addition to has a pattern that varies as long as each body part.
CR Image Sampling Low energy (kilovoltage peak) gives a wider histogram. High energy (kilovoltage peak) gives a narrow histogram. Histogram shows the distribution of pixel values as long as any given exposure. CR Image Sampling For example: Pixels have a value of 1, 2, 3, in addition to 4 as long as a specific exposure. Histogram shows the frequency of each of those values in addition to actual number of values. Histogram sets the minimum (S1) in addition to maximum (S2) useful pixel values. Histogram Analysis Analysis is complex. Shape of the histogram stays fairly constant as long as each part exposed (anatomy specific). For example: Shape of histogram as long as a chest radiograph on a large adult patient looks different from a knee histogram generated from a pediatric knee exam.
Histogram Analysis It is important to choose the correct anatomic region on the menu be as long as e exposing the patient. Raw data used to as long as m the histogram are compared with a normal histogram of the same body part by the computer. The Nyquist Theorem Theorem states that when sampling a signal, the sampling frequency must be greater than twice the b in addition to width of the input signal so that the reconstruction of the original image will be nearly perfect. At least twice the number of pixels needed to as long as m the image must be sampled. If too few pixels are sampled, the result is a lack of resolution. The Nyquist Theorem The number of conversions in CRelectron to light, light to digital in as long as mation, digital to analog signalresults in loss of detail. Some light is lost during the light-to-digital conversion because of the spreading out of light photons. Because there is a small distance between the phosphor plate surface in addition to the photosensitive diode of the photomultiplier, some light spreads out there as well, resulting in loss of in as long as mation.
The Nyquist Theorem The longer the electrons are stored, the more energy they lose. When laser stimulates electrons, some lower-energy electrons escape the active layer. If enough energy was lost, some lower-energy electrons are not stimulated enough to escape in addition to in as long as mation is lost. All manufacturers suggest that imaging plates be read as soon as possible to avoid this loss. The Nyquist Theorem Indirect in addition to direct radiography lose less signal to light spread than conventional radiography. The Nyquist theorem is still applied to ensure that sufficient signal is sampled. Because sample is preprocessed by the computer immediately, signal loss is minimized but still occurs. Aliasing Spatial frequency is greater than the Nyquist frequency. Sampling occurs less than twice per cycle. In as long as mation is lost. Fluctuating signal is produced.
Aliasing Wraparound image is produced. Image appears as two superimposed images slightly out of alignment. Aliasing results in a moiré effect. Aliasing can be problematic because of the same effect occurring with grid errors. It is important that the technologist remembers to look at both. Automatic Rescaling Exposure is greater than or less than what is needed to produce an image. Automatic rescaling occurs to display the pixels as long as the area of interest. Images are produced that have uni as long as m density in addition to contrast regardless of the amount of exposure. Automatic Rescaling Problems occur with rescaling: When too little exposure is used, resulting in quantum mottle When too much exposure is used, resulting in loss of contrast in addition to loss of distinct edges because of increased scatter production Rescaling is no substitute as long as appropriate technical factors. Danger exists of using higher than necessary milliampere-second values to avoid quantum mottle.
Look-Up Table The look-up table (LUT) is a reference histogram. LUT is used as a cross-reference to trans as long as m the raw in as long as mation. LUT is used to correct values. LUT has a mapping function: All pixels are changed to a new gray value. Image will have appropriate appearance in brightness in addition to contrast. LUT is provided as long as every anatomic part. Look-Up Table LUT can be graphed as follows: Plotting the original values ranging from 0 to 255 on the horizontal axis Plotting new values, also ranging from 0 to 255 on the vertical axis Contrast can be increased or decreased by changing the slope of this graph. Brightness (density) can be increased or decreased by moving the line up or down the y-axis. Latitude Latitude is the amount of error that still results in a quality image. Histograms show a wide range of exposure because of automatic rescaling of the pixels. Actual exposure latitude is slightly greater than that of screen/film exposures. In CR, if exposure is more than 50% below ideal exposure, quantum mottle results.
Latitude If exposure is more than 200% above ideal exposure, contrast loss results. Biggest difference between digital in addition to film/screen radiography lies in the ability to manipulate the digitized pixel values, which results in what seems like greater exposure latitude. Proper kilovolt in addition to milliampere-second values prevent mottle in addition to contrast loss. Enhanced Visualization Image Processing Kodak Takes image diagnostic quality to a new level Increases latitude while preserving contrast Process decreases windowing in addition to leveling Virtually eliminates detail loss in dense tissues Modulation Transfer Function Modulation transfer function (MTF) is the ability of a system to record available spatial frequencies. Sum of the components in a recording system cannot be greater than the system as a whole. When the function of any component is compromised because of interference, the overall quality of the system is affected.
Summary Image stitching is a computer program process that allows multiple images to be joined when the anatomy is too large as long as one exposure. The result is a nearly seamless, single image. Magnification techniques are available with digital systems that allow small area enlargement or whole image enlargement. Summary Proper patient demographic input is the responsibility of the technologist per as long as ming the exam. Any alterations of patient demographics should be avoided unless absolute identification is possible. The manual send function allows images to be sent to one or more networked computers. Summary Historical study of patient exams can be accomplished through the archive query function. Retrieval of radiographic studies can be specific as to patient name, date, in addition to exam or broad such as date ranges in addition to combinations of anatomic areas.
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