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Radiography is widely understood to provide information complementary to passive gamma-ray detection systems: while not directly sensitive to radiological materials, radiography can reveal highly shielded regions that may mask a passive radiological signal. We present a method for combining radiographic and passive gamma-ray measurements that uses the radiograph to provide an estimate of scatter and attenuation for possible sources. This approach allows quantitative use of radiographic images without relying on image interpretation, and results in a probabilistic description of likely source locations and strengths. We present first results for this method for a simple modeled test case of a cargo container driving through a plastic scintillator radiation portal monitor. With this probabilistic "inversion" approach, we address criteria for an integrated passive and radiographic screening system and how detection of special nuclear material might be improved in such a system.