Radiographic Image Contrast is Controlled only by kVp: FALSE
While it is true that kVp still controls the attenuation of the x-ray beam through the anatomy, there are some additional things that have a greater effect on image contrast:
As you can see, the kVp range differs drastically between exposures ranging from 70 kVp to 110 kVp. For the most part, when viewing the image from a distance (at the QC station for example), the images do not appear drastically different compared to what you might expect on a film/screen system. Once magnified, you can see subtle differences between the 70 kVp range exposure and the 110 kVp range exposure confirming that kVp range does affect image contrast, but not to the degree that it once did with conventional film/screen systems:
Just because there is little difference in appearance does not mean you can use any old technique and expect the software to adjust your image for you (more in tomorrow's post). It is even more important for us to utilize our training to harness all of the advantages that CR and DR imaging systems offer while maintaining the standards of ALARA.
Back to my point about the processing algorithm... we just compared a lateral knee radiograph at 70 kVp with one at 110 kVp and saw some subtle changes, but they were processed under the same algorithm of "lateral knee". Here is an optimum exposure of a lateral knee phantom taken at 70 kVp and 7.3 mAs (like image 1 above) table-top and small focal spot, processed as "lateral knee"... and to the right of it, a knee with identical exposure factors but processed as "PA chest":
As you can see, the image on the left appears as you would expect a knee radiograph to appear. The image on the right used a processing algorithm designed for a body part with more subject contrast. The software normally recognizes data collected on the image plate from a chest x-ray as having very light pixel values and very dark pixel values, and the processing algorithm is designed to reduce that black and white appearance for maximum visualization of both mediastinum and fine lung detail. If improperly applied to a body part with a lower subject contrast, the overall image contrast will decrease on the final image.
Additional posts in this series:
Radiology Myth-Busters: CR Myths #1 (An increase in exposure creates an increase in density)
While it is true that kVp still controls the attenuation of the x-ray beam through the anatomy, there are some additional things that have a greater effect on image contrast:
- The subject contrast - how the anatomy varies to influence differential absorption. An anatomic part with a high subject contrast (chest) will have greater image contrast than a part with low subject contrast (abdomen) if all factors remain the same. Additionally, we can influence subject contrast with positive or negative contrast media.
- Presence or absence of a grid - I see this often when I notice a Radiographer performing portable chest x-rays without a grid. There are often complaints from the Radiologists that the image quality is poor, and they have trouble seeing lines through the mediastinum. If a grid is added, the image contrast improves, and as a result of the higher kVp range applied when using a grid, the chest is more uniformly penetrated by the beam, revealing information through the mediastinum that could not be seen at a lower kVp range (click here to learn more about grid use)
- Processing algorithm - application of the appropriate processing algorithm is crucial during the initial image acquisition (click here to learn more about processing algorithms).
As you can see, the kVp range differs drastically between exposures ranging from 70 kVp to 110 kVp. For the most part, when viewing the image from a distance (at the QC station for example), the images do not appear drastically different compared to what you might expect on a film/screen system. Once magnified, you can see subtle differences between the 70 kVp range exposure and the 110 kVp range exposure confirming that kVp range does affect image contrast, but not to the degree that it once did with conventional film/screen systems:
Just because there is little difference in appearance does not mean you can use any old technique and expect the software to adjust your image for you (more in tomorrow's post). It is even more important for us to utilize our training to harness all of the advantages that CR and DR imaging systems offer while maintaining the standards of ALARA.
Back to my point about the processing algorithm... we just compared a lateral knee radiograph at 70 kVp with one at 110 kVp and saw some subtle changes, but they were processed under the same algorithm of "lateral knee". Here is an optimum exposure of a lateral knee phantom taken at 70 kVp and 7.3 mAs (like image 1 above) table-top and small focal spot, processed as "lateral knee"... and to the right of it, a knee with identical exposure factors but processed as "PA chest":
As you can see, the image on the left appears as you would expect a knee radiograph to appear. The image on the right used a processing algorithm designed for a body part with more subject contrast. The software normally recognizes data collected on the image plate from a chest x-ray as having very light pixel values and very dark pixel values, and the processing algorithm is designed to reduce that black and white appearance for maximum visualization of both mediastinum and fine lung detail. If improperly applied to a body part with a lower subject contrast, the overall image contrast will decrease on the final image.
Additional posts in this series:
Radiology Myth-Busters: CR Myths #1 (An increase in exposure creates an increase in density)
No comments:
Post a Comment