Patent Number: 
Section: claims

1. An imaging system comprising: a radiation source comprising an x-ray generator for emitting a beam of penetrating radiation toward an object to be imaged;  a spectral radiation filter positioned between said radiation source and said object to be imaged, said spectral radiation filter being positioned in the path of said beam of penetrating radiation and being sized to intercept substantially all x-rays in said radiation beam;  said at least one spectral radiation filter comprising a material having atomic number (Z) of at least 58, said radiation filter further being disposed to attenuate x-rays having energies at a low end and at a high end of a spectrum of x-rays emitted by said x-ray generator from passing therethrough so as to provide preferential passage of a selected intermediate spectrum of x-rays for imaging of said object; and  a radiation detector positioned to receive the x-rays passing through said radiation filter and through said object;  said radiation filter further comprises a plurality of filter material layers, each of said filter material layers comprising a material having a respective Z value, said filter materials being disposed such that material having a Z value higher than the Z value of another filter material are disposed closer to said radiation source, each of said filter materials having a respective Z value that differs from an adjacent layer by a value of at least 6 so as to reduce x-ray fluorescence radiation reaching said object to be imaged. 2. An imaging system as recited in  claim 1 , wherein said radiation filter comprises a material having an atomic number (Z) in the range of about 58 to about 83. claim 1 3. An imaging system as recited in  claim 2 , wherein said radiation filter comprises a material having an atomic number (Z) of at least 65. claim 2 4. An imaging system as recited in  claim 2 , wherein said radiation filter material is selected from the group consisting of thulium (Tm), lead (Pb), and gold (Au). claim 2 5. An imaging system as recited in  claim 2 , wherein said low end spectrum of x-rays has an energy less than about 35 KeV and said high end spectrum of x-rays has an energy of greater than about 80 KeV. claim 2 6. An imaging system as recited in  claim 1  wherein said at least one radiation filter comprises a plurality of interchangeable radiation filters, wherein at least a first filter of said plurality of filters comprises a first selected material and having a first selected thickness for use with objects of a first size, and wherein at least a second filter of said plurality of filters comprises a second material that is different from said first selected material. claim 1 7. An imaging system as recited in  claim 1  wherein said at least one radiation filter comprises a plurality of interchangeable radiation filters, wherein at least a first filter of said plurality of filters comprises a first selected material having a first selected thickness for use with objects of a first size, and wherein at least a second filter of said plurality of filters comprises said first selected material having a second selected thickness for use with objects of a second size. claim 1