kVp and X-Ray intensity

• X-ray intensity varies rapidly with changes in kilovolt peak (kVp).
• The change in x-ray intensity is proportional to the square of the of the kVp; in other words, if kVp were doubled, the x-ray intensity would increase by a factor of 4.
• X-ray intensity is proportional to kVp².
• Radiographic technique factors must be selected from a relatively narrow range of values, from approximately 50 to 120 kVp.
• Theoretically, doubling the x-ray intensity by kVp manipulation alone requires an increase of 40% in kVp.
• In practice, a slightly different situation prevails.
• As kVp is increased, the penetrability of the x-ray beam is increased, and relatively fewer x-rays are absorbed in the patient.
• More x-rays are transmitted through the patient and interact with the image receptor.
• To maintain a constant exposure of the image receptor, an increase of 15% in kVp should be accompanied by a reduction of one half in mAs.
• Such an increase in kVp results in doubling x-ray intensity.
• Increasing kVp and reducing mAs so that image receptor exposure remains constant reduces patient radiation dose significantly.
• The disadvantage of such a technique adjustment is reduced image contrast when screen-film is the image receptor.
• There is little change in image contrast when using digital image receptors.
• X-ray intensity is inversely proportional to the square of the distance from the source.
• When SID is increased, mAs must be increased by SID² to maintain constant exposure to the image receptor.
• Compensating for a change in SID by changing mAs by the factor SID² is known as the square law, a corollary to the inverse square law.