1. In a dual isotope subtraction method of imaging a tumor, lesion, organ or tissue, wherein an amount sufficient for imaging of a radiolabeled antibody or antibody fragment which specifically binds a marker produced by or associated with said tumor, lesion, organ or tissue is injected into a patient having such tumor, lesion, organ or tissue, an amount sufficient for background subtraction of a reference substance labeled with a different and separately detectable radioisotope is injected into the patient prior to scanning, the patient is scanned with a scintigraphic camera equipped with means to separately acquire emission data in energy windows corresponding to radioactive emissions from the two different radioisotope labels, as a function of the scanning position of the camera, and to store the data in digital form, said scanning being effected after a time sufficient to localize said radiolabeled antibody or antibody fragment, and the scans from the two energy windows are processed and subtracted such that the san data from the reference label are subtracted from the scan data from the specific antibody or antibody fragment label, to produce background-compensated image data for the tumor, lesion, organ or tissue, suitable for display in a form permitting visualization of the image, the improvement which comprises: (a) processing the raw digital data from the scans in the two energy windows, in the spatial frequency domain, using a band pass spatial frequency filter; wherein the raw scan data for each of the radioisotopes, after normalization to the same number of total counts per image and transformation into the spatial frequency domain, are multiplied by the same filter function, determined for the less-well-resolved radioisotope (LWRR); wherein the filter function is a two-dimensional circularly symmetric Gaussian filter function, the low frequency portion of which is substantially equal to the inverse of the modulation transfer function (MTF) for the scanning camera and collimator system, the MTF is obtained from the line spread function of a line source of the LWRR, the high frequency cutoff portion of the filter is a smoothly decreasing function going to zero over a short but finite range of frequencies, a twodimensional power spectrum of the spatial frequency domain-transformed scan data from the LWRR is formed, a one-dimensional power spectrum is formed from the two-dimensional power spectrum by averaging over annuli, the noise level is determined by averaging over the high frequency components, and the rolloff frequency of the filter is the point on the one-dimensional power spectrum which is about 1.5-3 times the noise level; and wherein the processed data are transformed back to the spatial domain; and (b) subtracting the processed data for the reference radioisotope from those of the specific antibody or antibody fragment label, to produce enhanced background-compensated image data suitable for display and visualization.