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Kilovoltage peak (kVp) is the peak potential applied to the x-ray tube, which accelerates electrons from the cathode to the anode in radiography or computed tomography. Tube voltage, in turn, determines the quantity and quality of the photons generated. An increase in kVp extends and intensifies the x-ray emission spectrum, such that the maximal and average/effective energies are higher and the photon number/intensity is higher.
Along with the mAs (tube current and exposure time product) and filtration, kVp (tube voltage) is one of the primary settings that can be adjusted on x-ray machines to control the image quality and patient dose.
The first consideration when selecting the kVp is ensuring adequate penetration and exposure 5,7, which depends on photon number, photon energy, and tissue attenuation (which depends on attenuation coefficient and thickness). There must be an adequate number of sufficiently energetic photons that penetrate the patient and reach the image receptor. Exposure at the image receptor increases approximately by the fifth power of the change in kVp (due to a combination of increased photon number and penetrability), such that a 15% increase in kVp doubles the intensity at the detector 7,8. Particularly in larger body parts, such as obese adult torsos, lower energy photons are absorbed completely without contributing to image formation. In such situations, higher kVp is employed to improve the x-ray intensity reaching the receptor, thus increasing the signal to noise ratio on the images.
Once sufficient exposure is ensured, the next consideration for adjusting kVp is image contrast. At higher photon energies, photoelectric effect interactions are less frequent, so a higher proportion of photon interactions are Compton scatter, which varies less between different tissues than the photoelectric effect does. Moreover, higher energy photons are more likely to travel through the patient without any tissue interaction. Consequently, higher energy x-ray beams generate images with poorer contrast 6.
When given satisfactory penetration and contrast, the radiation dose to the patient must also be considered when setting the kVp. Photon quantity output is approximately proportional to a power function, kVpn, where n is approximately 2 in radiography and 2.6 in CT (with the variation related to differences in filtration and beam shape) 3,4. For example, an increase from 100 to 120 kVp would increase CT dose index by more than 50% if other parameters were held constant.
In reality, so as to not increase dose unnecessarily, the mAs setting is typically adjusted down to compensate for the increased photon quantity caused by increasing kVp. Modern digital imaging systems achieve this through automated methods. As the fifth-power rule of thumb goes 7, to maintain the same image receptor exposure, every 15% change in kVp should be accompanied by an inverse adjustment in mAs by a factor of 2.
The ability to reduce mAs by a fifth-power relation is more than adequate to balance the second to third power direct effect of increasing kVp on photon quantity. The difference is related to how the probability of energy absorption in various biologic tissues declines dramatically over the range of photon energies used for diagnostic imaging 9. Thus, absorbed dose can be reduced while keeping the image receptor exposure constant by choosing a higher kVp and lower mAs. As this reduction is achieved with higher energy photons, however, the tradeoff is lower image contrast. Lower kVp techniques may be employed at lower dose when contrast is more important (e.g. CT angiography of the abdomen 10) or when noise related to attenuation is less of a problem (e.g. children 11).
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