Chapter 5 Cassette-Based Image Acquisition Objectives Discuss the importance of
Labbe, Bob, Host, Reelin in the Years has reference to this Academic Journal, PHwiki organized this Journal Chapter 5 Cassette-Based Image Acquisition Objectives Discuss the importance of matching the body part being examined to the exam menu. Discuss the selection of technical factors as long as density, contrast, in addition to penetration. Relate imaging plate size selection to radiographic exams. Describe the grid selection process. Objectives Relate the importance of preprocessing collimation. Discuss the importance of patient side markers. Compare exposure indicators as long as the major computed radiography (CR) manufacturers in addition to vendors.
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Key Terms Artifacts Automatic data recognition Collimation EI Exposure index Exposure indicators Fixed mode Grid focus Grid frequency Grid ratio Histogram kVp Logarithm of the median mAs Matrix Key Terms Multiple manual selection mode Operator errors Quantum mottle Quantum noise Reader errors S Semiautomatic mode Sensitivity Shuttering Computed Radiography Image Acquisition Part selection Technical factors Equipment selection Collimation Side/position markers Exposure indicators Image data recognition in addition to preprocessing
Part Selection Once the patient has been positioned in addition to the plate has been exposed, you must select the exam or body part from the menu choices on your workstation. If you are per as long as ming a skull exam, you then select skull from the workstation menu. Part Selection Selecting the proper body part in addition to position is important as long as the proper conversion to take place. Image recognition is accomplished through complex mathematical computer algorithms, in addition to if the improper part in addition to /or position is selected, the computer will misinterpret the image. For example, if a knee exam is to be per as long as med in addition to the exam selected is as long as skull, the computer will interpret the exposure as long as the skull, resulting in improper density in addition to contrast in addition to inconsistent image graininess. Part Selection It is not acceptable to select a body part or position different from that actually being per as long as med simply because it looks better. If the proper exam/part selection results in a suboptimal image, then service personnel should be notified of the problem in addition to the problem should be corrected as soon as possible. Improper menu selections may lead to overexposure of the patient in addition to to repeats.
Technical Factors Kilovoltage peak selection Milliampere-second (mAs) selection Kilovoltage Peak Selection Kilovoltage peak, milliampere-second, in addition to distance are chosen in exactly the same manner as as long as conventional film/screen radiography. Kilovoltage peak must be chosen as long as penetration in addition to the type in addition to amount of contrast desired. In the early days of CR, kilovoltage peak minimum values were set at about 70. This is no longer true or necessary. Kilovoltage Peak Selection Kilovoltage peak values now range from around 45 to 120. It is not recommended that kilovoltage peak values less than 45 or greater than 120 be used because those values may be inconsistent in addition to may produce too little or too much excitation of the phosphors. The k-edge of phosphor imaging plates ranges from 30 to 50 keV so that exposure ranges of 60 to 110 kVp are optimum.
Kilovoltage Peak Selection However, exposures outside that range are widely used in addition to depend on the quality desired. Remember, the process of attenuation of the x-ray beam is exactly the same as in conventional film/screen radiography. It takes the same kilovoltage peak to penetrate the abdomen with CR systems as it did with a film/screen system. It is vital that the proper balance between patient dose in addition to image contrast be achieved. Milliampere-Second Selection Milliampere-second is selected according to the number of electrons needed as long as a particular part. Too few electrons in addition to no matter what level of kilovoltage peak is chosen, the result will be a lack of sufficient phosphor stimulation. When insufficient light is produced, the image will be grainy, a condition known as quantum mottle or quantum noise. Milliampere-Second Selection CR systems typically use automatic exposure controls just as many film/screen systems do. Backscatter from the cassette/detector influences the number of milliampere-seconds necessary to create the image. When converting from film/screen systems to a CR system, it is critical that the automatic exposure control be recalibrated.
Equipment Selection Imaging plate selection Grid selection Imaging Plate Selection Two important factors should be considered when selecting the CR imaging cassette: type in addition to size. Most manufacturers produce two types of imaging plates: st in addition to ard in addition to high resolution. Cassettes should be marked on the outside to indicate high-resolution imaging plates. Imaging Plate Selection Typically, high-resolution imaging plates are limited to size range in addition to are most often used as long as extremities, mammography, in addition to other exams requiring increased detail. In conventional film/screen radiography, one is taught to select a cassette appropriate to the size of the body part being imaged.
Imaging Plate Selection CR cassette selection is the same, but even more critical. CR digital images are displayed in a matrix of pixels, in addition to the pixel size is an important factor in determining the resolution of the displayed image. The CR reader scans the imaging plate at a relatively constant frequency, about 2000 × 2000 pixels. Imaging Plate Selection Use of the smallest imaging plate possible as long as each exam results in the highest sampling rate. When the smallest possible imaging plate is selected, a corresponding matrix is used by the computer algorithm to process the image. Imaging Plate Selection A 2000 × 2000 matrix on an 8 × 10-inch cassette results in much smaller pixel size, thereby increasing resolution. If, as long as example, a h in addition to were imaged on a 14 × 17-inch cassette, the entire cassette would be read according to a 14 × 17-inch matrix size with much larger pixels, in addition to the resultant image would be very large.
Imaging Plate Selection Postexposure manipulation of the image to a smaller size reduces the resolution. Appropriate image plate selection as long as the exam also eliminates scatter outside the initial collimation in addition to increases image resolution. In addition, the image size on hard copy in addition to soft copy is affected by cassette selection. Imaging Plate Selection Some units use newer CR imaging plate technology but are cassetteless. These units are typically used as long as chest imaging, in addition to the imaging plate is enclosed within the unit. Imaging Plate Selection The storage phosphors have a needle-like structure that allows light to be guided with little light spread. Combined with line-scan readouts in addition to charge-coupled device detectors, these units have a complex reader within the fixed system.
Grid Selection Digital images are displayed in tiny rows of picture elements or pixels. Grid lines that are projected on the imaging plate when using a stationary grid can interfere with the image, resulting in a wavy artifact known as a moiré pattern. This pattern occurs because the grid lines in addition to the scanning laser are not parallel. Grid Selection The oscillating motion of a moving grid or Bucky blurs the grid lines in addition to eliminates the interference. Because of the ability of CR imaging plates to record a very high number of x-ray photons, the use of a grid is much more critical than in film/screen radiography. Appropriate selection of stationary grids reduces this interference as well. Grid selection factors are frequency, ratio, focus, in addition to size. Grid Frequency Grid frequency refers to the number of grid lines per centimeter or lines per inch. The higher the frequency or the more lines per inch, the finer the grid lines in the image in addition to the less they interfere with the image. Typical grid frequency is between 80 in addition to 152 lines per inch.
Grid Frequency Some manufacturers recommend no less than 103 lines per inch in addition to strongly suggest grid frequencies greater than 150 lines per inch. The higher the frequency, the less positioning latitude is available, increasing the risk as long as grid cutoff errors, especially in mobile radiography. The closer the grid frequency is to the laser scanning frequency, the greater likelihood of frequency harmonics or matching in addition to the more likely the risk of moiré effects. Ratio The relationship between the height of the lead strips in addition to the space between the lead strips is known as grid ratio. The higher the ratio, the more scatter radiation is absorbed. The higher the ratio, the more critical the positioning is, such that high grid ratio is not the appropriate choice as long as mobile radiography. Ratio A grid ratio of 6:1 would be proper as long as mobile radiography, whereas a 12:1 grid ratio would be appropriate as long as departmental grids that are more stable in addition to less likely to be mispositioned, causing grid cutoff errors.
Summary Image recognition takes place through computer algorithms that determine collimation borders in addition to edges in addition to histogram as long as mation. Typical recognition programs are automatic, semiautomatic, multiple manual selection, in addition to fixed modes as long as Fuji systems. Common CR acquisition errors include imaging plate, plate reader, image processing, in addition to printer artifacts, as well as operator errors.
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