1. A method of using an apparatus comprising of at least a first radiation source, a first set of detector elements and a computing means to compute a 3D profile of a parameter of interest of an object comprising the steps of: using said first radiation source to generate a first radiation; positioning said first set of detector elements so as to detect said first radiation; interposing said object in between said first radiation source and said first set of detector elements; defining a region of interest of said object as a first set of voxels having a size that is large relative to the size of elements of said first set of detector elements, further the number of voxels in said first set of voxels is less than or equal to the number of elements of said first set of detector elements; denoting the value of said parameter of interest within each said voxel by a variable and thereby generating a first set of variables associated with said first set of voxels; representing the signal detected by each element of said first set of detector elements as a ray sum that has contributions from all the voxels of said first set of voxels that intersect with the ray path connecting said first radiation source and said each element of said first set of detector elements; and using said computing means for computing the 3D profile over said region of interest of said object by performing computations on the ray sums which are defined equal to the detected signals from said first set of detector elements and solving for the values of said first set of variables associated with said first set of voxels.