1. Apparatus for combining two X-ray images of an anatomical region recorded simultaneously on film where one of the images corresponds to having exposed said region primarily to a high energy spectral band in a broad energy spectrum X-ray beam and the other image corresponds to having exposed said region primarily to a relatively lower energy spectral band in said broad spectrum and said images on the film are represented by a sequence of alternative lines of picture elements (pixels) composing the different images, said apparatus comprising: means for optically scanning said film transversely to said lines repeatedly over the width of the film, and detector means responsive to said scanning by producing alternating amplitude analog signals representative of the intensities of the pixels in the alternate low energy and higher energy image lines, clock pulse generating means coupled to said detector means and operative to produce clock pulses in synchronism with the alternate low and high energy analog signals, analog-to-digital converter (ADC) means having input means for said analog signals and having output means, said ADC means responding to occurrence of clock pulses by converting the corresponding analog pixel signal to alternate low and high energy digital pixel signals, first and second multiple stage shift registers having input means and means coupling said input means, respectively, to the output means of said ADC means, said shift registers responding to occurrence of alternate clock pulses corresponding to low and high energy analog signals, respectively, by entering a low and high energy digital pixel signal in the first stage of each register and by shifting from stage to stage in response to occurrence of successive clock pulses, spatial interpolation circuit means for obtaining substantial coincidence between low and high energy pixels in the adjacent lines on the film, said circuit means comprising sets of digital signal multipliers for each register, said multipliers in a set having inputs coupled to the stages of said registers and an input for a spatial weighting coefficient signal and each multiplier having an output, said multipliers being operative to multiply the signals presently in the low and high energy shift register stages by said weighting coefficient signals, and, circuit means for performing arithmetic and logic operations and having inputs coupled to the outputs of the multipliers and having output means and operative to combine the weighted pixel signals from the multiplier sets to produce composite interpolated low and high energy pixel signals and said circuit means is further operative to subtract related high and low energy pixel signals and to output a sequence of difference pixel signals for composing a single difference image.