COMPUTED RADIOGRAPHY CHAPTER 4 EFFECTIVE USE OF CR

Transcription

1 This presentation is a professional collaboration of development time prepared by: Rex Christensen Terri Jurkiewicz and Diane Kawamura New Technology 7U COMPUTED RADIOGRAPHY CHAPTER 4 EFFECTIVE USE OF CR Something doesn t belong Cassettes CR imaging plates are similar in appearance to film screen cassettes. 1

2 Dynamic Range Imaging Plate Response It is the ability of the detector to adjust to a wide range of exposures and still obtain an optimum image. That is why exposure creep is so prevalent with digital imaging. The exposure can be times the correct exposure and still be a good image. Digital imaging systems have wide latitude is due to the fact that the imaging plate is sensitive to more energy levels of x-ray, including scatter Higher Kvp. Why? Imaging Plate Response Exposure Consequences The wide dynamic range is helpful in areas such as the chest where there is a wide variety of structures from extremely dense (ribs) to very light (air filled lungs). It allows the structures all to be seen without blacking out or having an area too light to be visualized. Underexposure noise (quantum mottle) Less photons hitting the detector Overexposure low contrast More photons hitting the detector 2

3 Histogram Analysis Automatic Rescaling The histogram is formed when the laser reads the entire CR plate. It identifies the collimation fields, so if the part is incorrectly centered, or there are other body parts on the image, the histogram will be incorrect, and the image will be either light or dark. (rescaling errors) Automatic rescaling is a compensatory tool. It can compensate for slight variances in part thickness and moderate technique errors. Automatic rescaling should not be relied upon, but is a nice feature. Histogram Analysis Stored histogram is the predetermined histogram for the study (ie forearm predetermined average histogram. Measured histogram is the actual histogram of the image taken Histogram Analysis All vendors use the histogram to determine if the exposure is adequate. So the reader and exposure field recognition must be accurate and calibrated to maintain accuracy of the histogram and exposure indicator. The Exposure indicator only displays an appropriate brightness level of the image. The image can be over or under exposed and still have the same brightness, but the contrast can be degraded. 3

4 Speed Class-Not related to Cars Speed Class = Balance The receptor determines the speed class at which the system is operating. (auto rescaling) Low mas -> High speed class (relative to mas) High mas -> Low speed class (relative to mas) Correct mas -> Correct speed class (relative to the mas) Speed class compensates for brightness not contrast. Low mas quantum mottle or noise High mas increased scatter, reduction in contrast Optimal Images 1. Technical exposure factors 2.Correct positioning of the part 3. Correct beam, part and receptor alignment Exposure Index = Optical Density With automatic rescaling and imaging processing it is difficult to differentiate between patient exposure and image appearance. 4

5 Exposure Indicators Exposure Indicators Each manufacture has an exposure indicator to show the technologist if the exposure is within acceptable limits. That is enough exposure to make a good image, but not too much to over-radiate the patient. At this time there is no standardization of these factors but several institutions are working on getting them standardized. Fuji Medical- Sensitivity number (S number) Agfa Medical- log of Median (lgm) Kodak-Exposure Index (EI) They each use a different name for the same thing Suboptimal Images What causes suboptimal images? Excessive noise mottle 50% below required exposure rate Low contrast 200% or greater exposure rate Background scatter Grid cutoff Nongrid images (of structures normally requiring a grid) and insufficent contrast improvement by grid. Incorrect menu selected for anatomy imaged Quantum Mottle 5

6 Technical Factors SID A technique chart or AEC system should be used. Centering is critical with CR systems as is appropriate collimation. The source to image distance is the same for CR as for film screen imaging. Although the exposure to the patient can be reduced by as much as 15% when the distance is changed to 48 inches SID rather than the usual 40 inches. Scatter It is thought by some that CR systems do not collimate well. Lack of collimation reduces contrast the same way it does with film screen systems. Use of grids also help with the contrast by removing scatter radiation. Remember the wide dynamic range? More sensitive to scatter. Are grids different in CR from film? Frequency = # of grid lp/in or lp/cm CR plate 8 feet from x-ray table with hand phantom in front of it. A lateral L- spine was exposed at 75 kvp, 200 mas at a 40" SID, and this is the image that was produced 8 feet from the x-ray table! Image contrast can be reduced by as much as 75% 6

7 How do you reduce scatter? Collimation - Scatter Erasing plates KVP Most vendors recommend that plates not used and erased within 48 hours should be erased prior to image acquisition. KVP does not have the impact that it did with film screen systems. (because of processing algorithms) Higher KVP lowers patient dose, and normally 10 kvp more than what is normally used in film screen gives a better image and lower dose to the patient. Because of scatter radiation, no more than 80 KVP should be used non grid. 7

8 mas Use of an accurate technique chart or appropriately adjusted AEC is best used for technique. Too much technique loss of contrast Too little by 50% noise or quantum mottle. Part, Beam, and Receptor Alignment On CR systems the exposure field must be identified (usually this is with the collimation ). On some exposures collimation is not possible or at best difficult causing a histogram error. Collimation must be equidistant from the edge of the imaging plate. Densities not normal Multiple exposure fields (CR only) 8

9 Symmetrical and Uniform Your chances of a histogram analysis errors goes up when you use split-field patterns. Most vendors recommend one image per imaging plate (IP). Over 1/3 of the imaging plate should be exposed. Collimation Horrible Much Better! Black Border Added What about size of the cassette? How does this affect resolution? Low Contrast Images Causes: High KVP No grid or inefficient grid Insufficient collimation Or incomplete plate erasure Overexposure A Large dynamic range does not mean there is no degradation of the image. If the image is overexposed by 4-5 times saturation occurs. Saturation is when a pixel that should be grey becomes black, and no post processing can change the color of the black pixels back to a shade of gray. 9

10 Saturation If you overexpose a CR image it degrades the contrast and detail, because of scattered radiation. If more than 2X the proper amount is used, the degradation is dependent on the amount of scatter. All of this diminishes the image quality, but also, it over exposes the patient hence ALARA is not maintained. Image MUST be repeated as a saturated image cannot be recovered Recorded Detail & Blur The more blur the less detail and conversely the less blur the more detail. Factors in recorded detail: Spatial Resolution Motion spatial resolution of lp/mm Longer exposure times are more likely to result in blur. Geometric factors: Short SIDs Longer OID larger focal spots distortion 10

11 CR and Spatial Resolution Spatial resolution is determined by sampling pitch (pixel pitch) and sampling frequency (pixel density) Sampling pitch = distance between pixels Sampling frequency higher frequency = smaller pixel pitch Lower frequency = larger pixel pitch Sampling is done in pixels/mm The End 11