Physics Question related to X-ray Tube.

NocturnalMedic · Joined: 14 Aug 2009 Posts: 2

Hey all, im just studying for an upcoming medical imaging exam however im having some trouble understanding a concept:

With regards to X-ray beam production, the effective photon energy of a beam is roughly 60-70% of the maximum photon energy (Emax) at a particular Tube voltage in the range of 40-150KV. For example a 100KV tube voltage produces an effective photon energy of approx. 60keV.

my questions are:

1) why does this occur what is the physics behind the value being roughly 60% of Emax
2) how is the effective photon energy different to the Average photon energy (the most frequent beam) or is this the same thing.

any help is greatly appreciated!

wvaio · Joined: 22 Sep 2007 Posts: 82

I'll see if I can help you with this.

These are the factors that contribute:

1. Less than 1% of the kinetic energy of the electrons coming from cathode to anode will produce X-rays. 99% will be transferred to heat.

2. Xrays are polyenergetic. Ranging from 0 to Emax(kVp).

3. Filtration is added to reduce the number of low energy photons emitted from the tube; therefore, raising the average effective photon energy and reducing patient dose.

4. Not all projectile electrons from the cathode will have the peak potential energy, due to voltage ripple this is inherent in any generator.

5. Many low energy X-rays will be absorbed by the target. i.e. the anode heel effect.

6. The target is thick, and many of the Brems interactions happen after the projectile electrons have interacted previously. Every interaction causes a further loss of potential energy.

As for the difference between effective and average photon, the terms are NEARLY interchangeable. Effective means leaving the tube toward the patient. Average could be used to describe the energy at any given point. There must be another word attached to it to describe the point of quantification, because there is filtration between production and exit.. They are usually used together as average effective energy.

X-ray production is extremely inefficient, and these are the main reasons why.One more final thing to remember is that MOST of the X-rays produced will possess an energy 1/3 of the kVp. Hope this helps.

Wvaio

NocturnalMedic · Joined: 14 Aug 2009 Posts: 2

Excellent response! thank you.

matt_giftfromgod · Joined: 22 Aug 2009 Posts: 7

dont think i couldve put it better myself...actually i couldnt have put it AS well. long time since radiation physics. Just remember that whenever a KvP is selected for a tube voltage that it is just that -- a PEAK voltage or electron energy-- so if it is 100 kvp then that is as high as the energy can be and that beam will contain electron energies from every voltage from there to zero. maybe that was just repetitive, but i thought it might help expound more on the nice peice by wvaio Smile