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CR Image Acquisition By Professor Stelmark
Unlike a film radiograph that is made up of minute deposits of black metallic silver, a digital image is recorded as a matrix or combination of rows and columns (array) of small, usually square, “picture elements” called pixels. Each pixel is recorded as a single numerical value, which is represented as a single brightness level on a display monitor. The location of the pixel within the image matrix corresponds to an area within the patient or volume of tissue.
Assume that pixel values from 0 to 2048 are used to represent the full range of digital image densities or brightness levels. A high pixel value could represent a volume of tissue that attenuated fewer x-ray photons and is displayed as a decreased brightness level or increased density. Therefore a low pixel value represents a volume of tissue that attenuates more x-ray photons and is displayed as increased brightness or decreased density.
Acquisition During image acquisition the computer creates a histogram A histogram is a graphic representation of a data set. This graph represents the number of digital pixel values versus the relative prevalence of those values in the image. The x-axis represents the amount of exposure and the y-axis the incidence of pixels for each exposure level. The computer then analyzes the histogram using processing algorithms and compares it to a preestablished histogram specific to the anatomic part being imaged. This process is called histogram analysis.
The computer software has histogram models for all menu choices. These stored histogram models have values of interest (VOI) and determine what section of the histogram data set should be included in the displayed image. During this process of “recognition” the computer identifies the exposure field and the edges of the image, and all exposure data outside this field are excluded from the histogram. Ideally, all four edges of a collimated field are recognized. If at least three edges are not identified, then all data, including raw exposure or scatter outside the field, may be included in the histogram, resulting in a histogram analysis error.
Histogram analysis is also employed to maintain consistent image brightness despite overexposure or underexposure of the IR. This procedure is known as automatic rescaling. The computer rescales the image based on the comparison of the histograms, which is actually a process of mapping the grayscale to the value of interest VOI to present a specific display of brightness.
Part Selection Once the patient has been positioned and the plate has been exposed, you must select the examination or body part from the menu choices on your workstation. For example, if you are performing a skull examination, select “skull” from the workstation menu. Selecting the proper body part and position is important for the proper conversion to take place. Image recognition is accomplished through complex mathematical computer algorithms, and if the improper part and/or position is selected, the computer will misinterpret the image. For example, if a knee examination is to be performed and the examination selected is for skull, the computer will interpret the exposure for the skull, resulting in improper density and contrast and inconsistent image graininess .
It is not acceptable to select a body part or position different from that being performed simply because it looks better. If the proper examination/part selection results in a suboptimal image, then service personnel should be notified of the problem to correct it as soon as possible. Improper menu selections may lead to overexposure of the patient and/or repeats.
Kilovoltage Peak Selection Kilovoltage peak (kVp), milliamperage seconds (mAs), and distance are chosen in exactly the same manner as for conventional film/screen radiography. kVp must be chosen for penetration and the type and amount of contrast desired. In the early days of CR, kVp minimum values were set at about 70kVp. This is no longer necessary. kVp values now range from around 45 to 120. It is not recommended that kVp values less than 45 or greater than 120 be used because those values may be inconsistent and produce too little or too much excitation of the phosphors. However, exposures outside that range are widely used and will 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 kVp 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 and image contrast be achieved.
Milliamperage Seconds Selection The mAs is selected according to the number of photons needed for a particular part. If there are too few photons, no matter what level of kVp is chosen, the result will be a lack of sufficient phosphor stimulation. When insufficient light is produced, the image is grainy, a condition known as quantum mottle or quantum noise.
Imaging Plate Selection Two important factors should be considered when selecting the CR imaging cassette: type and size. Most manufacturers produce two types of imaging plates: standard and high resolution. Cassettes should be marked on the outside to indicate high resolution imaging plates. Typically, high resolution imaging plates are limited to size range and are most often used for extremities, mammography, and other examinations requiring increased detail.
In conventional film/screen radiography, we are taught to select a cassette appropriate to the size of the body part being imaged. CR cassette selection is the same but even more critical. CR digital images are displayed in a matrix of pixels, and 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. Using the smallest imaging plate possible for each examination 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. A 2000 × 2000 matrix on an 8′′ × 10′′ cassette results in much smaller pixel size, thereby increasing resolution. If, for example, a hand was imaged on a 14′′ × 17′′ cassette , the entire cassette is read according to a 14′′ × 17′′ matrix size with much larger pixels so that the resultant image is very large.
Grid Selection Digital images are displayed in tiny rows of picture elements or pixels. Grid lines that are projected onto the imaging plate when using a stationary grid can interfere with the image. This results in a wavy artifact known as a moiré pattern that occurs because the grid lines and the scanning laser are parallel The oscillating motion of a moving grid, or Bucky, blurs the grid lines and 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, and size.
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 and the less they interfere with the image. Typical grid frequency is between 80 and 152lines/in. Some manufacturers recommend no fewer than 103lines/in and strongly suggest grid frequencies greater than 150. The higher the frequency, the less positioning latitude is available, increasing the risk for grid cutoff errors, especially in mobile radiography. In addition, the closer the grid frequency is to the laser scanning frequency, the greater likelihood of frequency harmonics or matching and the more likely the risk of moiré effects.
Ratio The relationship between the height of the lead strips and the space between the lead strips is known as grid ratio. The higher the ratio, the more scatter radiation is absorbed. However, the higher the ratio, the more critical the positioning is, so high grid ratio is not a good choice for mobile radiography. A grid ratio of 6:1 would be proper for mobile radiography, whereas a 12:1 grid ratio would be appropriate for departmental grids that are more stable and less likely to be mispositioned, causing grid cutoff errors.
Size The physical size of the grid matters in CR examinations. The smaller the cassette being used, the higher the sampling rate. When using cassettes that are 10′′ × 12′′ or smaller, it is important to select a high frequency grid to eliminate scatter that will interfere with quality image interpretation by the computer algorithm. Remember that the CR imaging plate is able to record a wider range of exposure, including scatter.
Collimation When exposing a patient, the larger the volume of tissue being irradiated and the greater the kVp used, the more likely it is that Compton interactions, or scatter, will be produced. Whereas the use of a grid absorbs the scatter that exits the patient and affects latent image formation, properly used collimation reduces the area of irradiation and the volume of tissue in which scatter can be created. Collimation is the reduction of the area of beam that reaches the patient through the use of two pairs of lead shutters encased in a housing attached to the x-ray tube. Collimation results in increased contrast as a result of the reduction of scatter as fog and reduces the amount of grid cleanup necessary for increased resolution.
Use of Lead Masks Use of lead masks/blocker for multiple images on a single IR is recommended when CR is used .This recommendation is due to the hypersensitivity of image plate phosphors to lower-energy scatter radiation; even small amounts may affect the image. (Note: Some manufacturers recommend that only one image be centered and placed per IP. Check with your department to find out whether multiple images can be placed on a single IP.)
Through postexposure image manipulation known as shuttering, a black background can be added around the original collimation edges, virtually eliminating the distracting white or clear areas. However, this technique is not a replacement for proper preexposure collimation. It is an image aesthetic only and does not change the amount or angles of scatter. There is no substitute for appropriate collimation because collimation reduces patient dose.
Side/Position Markers If you have used CR image processing equipment, you already know that it is very easy to mark images with left and right side markers or other position or text markers after the exposure has been made. However, we strongly advise that conventional lead markers be used the same way they are used in film/screen systems. Marking the patient examination at the time of exposure not only identifies the patient’s side but also identifies the technologist performing the examination. This is also an issue of legality. If the examination is used in a court case, the images that include the technologist’s markers allow the possibility of technologist testimony and lend credibility to his or her expertise.
Exposure Indicators The amount of light given off by the imaging plate is a result of the radiation exposure the plate has received. The light is converted into a signal that is used to calculate the exposure indicator number. This number varies from one vendor to another (The total signal is not a measure of the dose to the patient but indicates how much radiation was absorbed by the plate, which gives only an idea of what the patient received.) The base exposure indicator number for all systems designates the middle of the detector operating range.
For the Fuji (Tokyo, Japan), Philips (Eindhoven, The Netherlands), and Konica Minolta (Tokyo, Japan) systems, the exposure indicator is known as the S or sensitivity number. It is the amount of luminescence emitted at 1mR at 80kVp and has a value of 200. The higher the S number with these systems, the lower the exposure. For example, an S number of 400 is half the exposure of an S number of 200, and an S number of 100 is twice the exposure of an S number of 200. The numbers have an inverse relationship to the amount of exposure so that each change of 200 results in a change in exposure by a factor of 2.
Kodak (Rochester, NY) uses exposure index (EI) as the exposure indicator. A 1-mR exposure at 80kVp combined with aluminum/copper filtration yields an EI number of 2000. An EI number plus 300 (EI + 300) is equal to a doubling of exposure, and an EI number of –300 is equal to halving the exposure. The numbers for the Kodak system have a direct relationship to the amount of exposure, so that each change of 300 results in change in exposure by a factor of 2.
The term for exposure indicator in an Agfa (Mortsel, Belgium) system is the logarithm of the median exposure (lgM). An exposure of 20μGy at 75kVp with copper filtration yields a lgM number of 2.6. Each step of 0.3 above or below 2.6 equals an exposure factor of 2
Low exposure index (high “S” number) indicates underexposure with “noisy” undesirable image.
