Table 1.

Notation summary.

Variable	Description	Units	Size
Nj	Number of voxels in the image	—	—
Ni	Number of measurements	—	—
Nm	Number of materials	—	—
NE	Number of energies	—	—
A	System matrix	mm	Ni × Nj
p(m)	Vector of densities for material m	g mm−3	Nj × 1
μ(m) (E)	Mass attenuation coefficient of material m at energy E	mm2g−1
S(i) (E)	System spectral response of measurement i at energy E	photon kV−1
x	Concatenated density vectors of all materials	g mm−3	(Nm · Nj) × 1
M		mm3 g−1	(NE · Ni) × (Nm · Nj)
B		photon	Ni × (NE · Ni)