This CR system only responds to certain kVp ranges: FALSE
We know that digital imaging systems no longer utilize the traditional H&D curve, but instead have a wider “Dynamic Range” of exposures that can be considered useful for the final image.
For this reason, processing algorithm (See post #2 in this series) has greater impact on image contrast than kVp selection
For this reason, processing algorithm (See post #2 in this series) has greater impact on image contrast than kVp selection
With CR systems, kVp does not have the same impact as with film/screen imaging. We know that the fundamentals of x-ray interaction with matter remain constant, but as we have always known, even with film/screen imaging, is that the interaction of the phosphor material will vary based on composition and kVp range. Consider the variation in k-edge absorption values. The k-shell electron binding energies of the phosphors that are used in traditional film/screen imaging, as well as the BaSrSO4 in the CR phosphor plate are compared below:
Y2O2S:Tb = 17 keV
BaFBr:Eu or BaFI:Eu = 37 keV
LaOBr:Tm = 38 keV
Gd2O2S:Tb = 50 keV
CaWO4 = 69 keV
In order to produce the most efficient beam, the average energy of the beam should be enough to free the k-shell electron from its orbit using one of these phosphor compounds. You could say that yttrium has an “edge” over the rest because it has the lowest binding energy of all the compounds… but is it going to be the most efficient for the diagnostic range that we need for plain radiography. The PSP compound is extremely low, meaning that you can have just as much screen efficiency at lower kVp values compared to most of the other screens. Anything above that range should not affect efficiency when comparing one screen to another, so the CR image plates should not be considered to require different kVp ranges, and certainly not one specific kVp range for that particular screen.
This doesn’t mean that we CAN’T use certain kVp ranges, it just means that there is an optimum efficiency that we should be taking advantage of. It also means that we have the capability with CR imaging to use higher kVp ranges. If the processing algorithm is primarily responsible for image contrast, there is a great bit of latitude with the range of kVp that we can use... as long as we maintain the appropriate exposure to the image receptor. The added benefits of that include more uniform part penetration with better visualization of anatomy (see Reducing Radiation Dose in Diagnostic Radiography), and that we can reduce the amount of mAs required at higher kVp ranges, thus extremely lowering patient dose. These can be accomplished without affecting image contrast to the degree in which it would be affected with film/screen systems.
The main take away with all of this should be that what the CR system needs in order to perform its job as designed is exposure to the plate. We still control that as the Technologist, and we still have to be educated properly to determine what the BEST method of exposure should be taking all things into account; patient dose, image quality, and variations with each patient and all of the different types of equipment we are using. It is far more important to have the proper beam/part/film alignment, along with exposure factors that produce the least exposure necessary for the highest image quality when using CR imaging systems.
I hope you have enjoyed this series on Myths about CR Imaging... here are the other posts in the series:
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