According to SC35 (2008), "The contrast detectability is the ability to resolve different objects from the background when the difference in attenuation between the objects and the background is small compared to noise" (pg. 54). In order to radiographically identify an abnomality that has a similar tissue density as the surrounding tissues, it is crucial for one to be able to perform the contrast detail detectability test and regularly assess the parameter. For both of the following tests, the TOR CDR was used with its electronic manual provided. The low contrast sensitivity was measured by counting the number of visible large details (11mm). On the other hand, the high contrast sensitivity was measured by counting the number of very small details (0.5mm). The numerical values were compared with the tables in the manual to find the corresponding threshold contrast. ("TOR CDR," n.d.). Threshold Contrast Detectability CR The purpose of this lab is to be able to monitor image quality by assessing the visibility of low contrast details. Usually, it is done 3-6 times per month. As SC35 (2008) states that test procedures recommended by the manufacturer should be followed, the manual was referred throughout the lab. The test was performed in Room 6. A TOR CDR test object was placed on the table with a focus-receptor distance of 1-1.5m. 1mm copper (Cu) filter was added and the beam was collimated to the CR cassette. It was then exposed at 70kVp and 10mAs. Two more exposures were taken under the same conditions. The exposed cassette was processed by a Kodak system - on 'Pattern' mode with no data and no edge enhancement. The exposure index (EI) value of each image was recorded. The window width and level of the resulted images were adjusted to better visualize the background noise. The numbers of visible details were counted and recorded. The EI values of the three images were 3020, 2040, and 2010, respectively. As the first image's recorded EI value exceeds the acceptable range between 1800 and 2200, indicating overexposure, the data resulted from the first exposure were considered irrelevant and dropped from the study. The recorded EI values of the other two images, however, falls under the range, and their data were analysed. For both images, 13 large details and 11 smaller details could be resolved. The result indicates that it has the low-contrast sensitivity of 0.009, and the high-contrast sensitivity of 0.128. Threshold Contrast Detectability DR The purpose of this lab is to monitor image quality by assessing the visibility of low contrast details. Usually, it is done 3-6 times per month. As SC35 (2008) states that test procedures recommended by the manufacturer should be followed, the manual was referred throughout the lab. The test was performed in Room 6. A TOR CDR test object was placed on the table with a focus-receptor distance of 1-1.5m. 1mm copper (Cu) filter was added and then the test tool was exposed at 70kVp and 6.3mAs. Two more exposures were taken under the same conditions. The dose area product (DAP) and the dose of each image were recorded. The numbers of visible details were counted and recorded. The DAP's of the three images were 0.271 dGycm^2, 0.268 dGycm^2, and 0.270 dGycm^2, respectively, and the doses were 0.101 mGy, 0.100 mGy, and 0.101 mGy. In the first two images, 12 large details and 9 small details could be resolved. 12 large details and 10 smalls details could be resolved in the last image. The result indicates that it has the low-contrast sensitivity of 0.011, and the high-contrast sensitivity of 0.203.
1 Comment
|