Effective atomic numbers for low-energy total photon interactions in human tissues

Area of Science:

• Medical Physics
• Biophysics
• Radiological Sciences

Background:

• Accurate effective atomic numbers (Zeff) are crucial for radiation transport calculations in human tissues.
• Previous Zeff definitions showed significant energy dependence, limiting their applicability in diagnostic and therapeutic radiology.
• A need exists for a more robust and energy-independent method to determine Zeff for various human tissues.

Purpose of the Study:

• To introduce a new method for defining effective atomic numbers (Zeff) for human tissues.
• To evaluate the energy dependence of the proposed Zeff definition across a range of photon energies (10-200 keV).
• To compare the new method with existing Zeff determination techniques.

Main Methods:

• Calculated total photon interaction cross sections per electron for various human tissues (blood, bone, brain, etc.) and water.
• Defined effective atomic numbers (Zeff) based on these per-electron cross sections.
• Analyzed the energy dependence of the new Zeff values from 10 to 200 keV.
• Compared the results with Zeff values derived from per-atom cross sections.

Main Results:

• The new Zeff definition showed less than 4% variation within soft tissues across the 10-200 keV energy range.
• For bone tissues, Zeff variations ranged from 2.86% to 5.03%, demonstrating improved stability.
• The proposed method significantly reduced energy dependence compared to previous per-atom methods, which showed up to 50% variation in bone.

Conclusions:

• The new method provides a more energy-independent and accurate determination of effective atomic numbers for human tissues.
• This approach enhances the reliability of radiation interaction calculations in medical physics and dosimetry.
• The findings support the use of this Zeff definition for improved accuracy in medical imaging and radiation therapy planning.