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Successful employment of multimodal molecular imaging for cancer targeting entails the development of safe nanoparticle contrast agents (NPCAs), detects at least by two nonionizing imaging techniques. This paper presents a quantitative assessment of the effectiveness of both pure silica nanospheres (SiNSs) and composite silica/superparamagnetic NPCAs as scatterers for low-frequency diagnostic ultrasound (US) (3 MHz) in very low range of concentrations (1.5-5 mg/mL). Iron oxide (IO) and FePt-IO nanocrystals are employed for SiNS magnetic coating. Different samples of NPCA-containing agarose gel are US imaged through a commercially available system and acquired data are processed through a dedicated prototypal platform to extract image backscatter information and perform evaluation of the image gray level. The pure silica NPCAs confirms recent reports for higher concentrations at higher frequencies. The FePt-IO-coated NPCAs show similar behavior, although with lower values of image backscatter, with a marked effectiveness peak for 330-nm SiNSs, particularly useful for tumor targeting purposes. Finally, the IO-coated SiNSs presented a marked lowering of US enhancement potential and a peak efficiency for a particle diameter of 660 nm. The extent of US backscatter reduction is found to be a function of the number of magnetic nanoparticles per mL of NPCA-containing gel and decreased with increasing NPCA concentrations. These results broadened our knowledge of dual-mode molecular imaging of deep tumors, employing US, and magnetic resonance techniques for the accurate, safe and early detection of cancer cells located in internal organs.