Method of imaging the atomic number of a sample

A method as recited in claim 6, where said step of using said two determined energy coefficients and attenuation coefficients for said unknown material at said first and second energies to determine an atomic number image of said unknown material comprises determining said atomic number image according to the following equation for each of the plurality of points in the atomic number image cross section ##EQU4## where Z.sub.x *:

• effective atomic number for said unknown material
• m:
• constant equal to 3.0
• a.sub.1 :
• determined energy coefficient at said first energy
• a.sub.2 :
• determined energy coefficient at said second energy
• b.sub.1 :
• determined energy coefficient at said first energy
• b.sub.2 :
• determined energy coefficient at said second energy
• .mu..sub.1x :
• attenuation coefficient for said unknown material at said first energy
• .mu..sub.2x :
• attenuation coefficient for said unknown material at said second energy.

A method of obtaining a density image of an unknown material that is corrected for the effects of atomic composition of said unknown material, said method comprising the steps of:

• scanning a plurality of calibration materials with a computerized axial tomographic scanner (CAT) at a first energy, said plurality of calibration materials having a plurality of different known atomic numbers, Z.sub.i, and densities, .rho..sub.i ;
• scanning said plurality of calibration materials with a CAT at a second energy;
• determining from attenuation coefficients from reconstructed images of said plurality of calibration materials at said first and second energies two energy-dependent coefficients, a and b, at said first and second energies according to the following equation
• .mu..sub.i =aZ.sup.m.sub.i .rho..sub.i +b.rho..sub.i
• where
• .mu..sub.i :
• attenuation coefficient for the i.sup.th calibration material
• a:
• energy-dependent coefficient
• Z.sub.i :
• atomic number for the i.sup.th calibration material
• m:
• constant in the range of 3.0-4.0
• .rho..sub.i :
• electron density of the i.sup.th calibration material
• b:
• energy-dependent coefficient;
• and scanning said unknown material with a CAT at said first energy;
• scanning said unknown material with a CAT at said second energy;
• and using said two determined energy coefficients and attenuation coefficients from reconstructed images for said unknown material at said first and second energies to determine a density image of said unknown material.

A method of obtaining an atomic number image of an unknown material, comprising:

• scanning a plurality of calibration materials having known different atomic numbers, Z.sub.i, and densities, .rho..sub.i, and said unknown material at a first energy with a computerized axial tomograph (CAT) scanner,
• scanning said plurality of calibration materials and said unknown material at a second energy with said CAT scanner,
• calculating two energy-dependent coefficients from attenuation coefficients of said calibration materials obtained from reconstructed images of said calibration materials at said first and second energies, and
• calculating an effective atomic number image for said unknown material from attenuation coefficients for said unknown material obtained from reconstructed images of said unknown material and said energy-dependent coefficients at said first and second energies.

A method of obtaining a density image of an unknown material, comprising;

• scanning a plurality of calibration materials having known different atomic numbers, Z.sub.i, and densities, .rho..sub.i, and said unknown material at a first energy with a computerized axial tomograph (CAT) scanner,
• scanning said plurality of calibration materials and said unknown material at a second energy with said CAT scanner,
• calculating two energy-dependent coefficients from attenuation coefficients of said calibration materials obtained from reconstructed images of said calibration materials at said first and second energies, and
• calculating a density image for said unknown material from attenuation coefficients for said unknown material obtained from reconstructed images of said unknown material and said energy-dependent coefficients at said first and second energies.