Understanding kVp & mAs (X-Ray Exposure Factors)

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kVp is actually an acronym that stands for:

Kilovoltage peak
This is the maximum (or Peak) electron energy as they flow through the x-ray tube.

Here’s what happening:

During an x-ray exposure, the cathode is negatively charged and the anode is positively charged. This difference in charge is called the kilovoltage of the “tube potential”.

Increasing the kVp creates a larger voltage difference across the tube. This causes the electrons to move more quickly, which gives them more energy.

Decreasing the kVp creates a smaller voltage difference across the tube. This causes the electrons to move more slowly, which gives them less energy.

Why does this matter?

Changes in kVp have very important effects on the x-ray beam. (Large arrows)

Increasing the kVp increases the beam intensity (like increasing mAs), but…
Increasing the kVp also increases the beam energy.

In other words, kVp affects both the beam quantity (intensity/number of photons) and the beam quality (energy).

KVp is unique for this reason: it is the only prime exposure variable that can change the energy of the x-ray beam. No other variable does this…

Not the mA
Not the exposure time
Not the distance

The only kVp controls the energy of the x-ray beam.
The mathematical relationship between kVp and beam energy is very simple.

The kVp is mathematically equal to the maximum beam energy.
For example: If the kVp is set to 100, the maximum energy photons in the beam will be 100 keV. Not all photons in the beam will be 100 keV, but some while.

KeV stands for kilo electron volt, which is how we measure the energy of photons.

The mathematical relationship between kVp and beam intensity (quantity) is slightly more complex. It looks like this:

I_1/I_2 = (〖kVp〗_1^2)/(〖kVp〗_2^2 )

Just know that changing the kVp does have a large effect on the beam intensity.

KVp affects other aspects of the exposure too.

Increasing the kVp increases the patient dose. This is because increasing the kVp creates more x-ray photons, which means more photons are striking the patient.

The reverse is also true:

Decreasing kVp decreases the patient dose.

KVp also affects receptor exposure in a similar way.

Increasing the kVp increases receptor exposure. This is an effect of the increased beam intensity and because of the increased energy results in more penetration through the patient.

The opposite is also true:

Decreasing the kVp decreases receptor exposure. Fewer photons are created and fewer penetrate through the patient.

The relationship between kVp and receptor exposure id defined by the 15% rule…

Increasing kVp by 15% will double receptor exposure.
Decreasing the kVp by 15% will reduce receptor exposure by half.

kVp also affects the image contrast. The reasons for this are discussed in other lessons, but know…

Increasing the kVp decreases the image contrast
Decreasing the kVp increases image contrast.

In summary:

Increasing the kVp increases the beam energy and the beam intensity.
Increasing kVp also increases the patient dose and receptor but decreases the image contrast.

Видео Understanding kVp & mAs (X-Ray Exposure Factors) канала Clover Learning