PurposeThe purpose of this study was to investigate an ultrashort echo time (UTE) imaging approach for improving the detection of receptor targeted magnetic nanoparticles in cancer xenograft models using positive contrast.Materials and MethodsIron oxide nanoparticle (IONP) conjugated with tumor targeting ligands were prepared. A 3D UTE gradient echo sequence with the shortest TE of 0.07 msec was evaluated on a 3T magnetic resonance imaging (MRI) scanner using IONP solution, cancer cells bound with targeted IONPs and orthotopic human pancreatic, and breast cancer mouse models administered tumor targeting IONPs. A simulation was performed to analyze contrast‐to‐noise ratios (CNR) of UTE images and subtraction of the images obtained UTE and longer TE (SubUTE). T2‐weighted imaging and T2 relaxometry mapping were applied for comparison and validation.ResultsUTE and SubUTE images showed positive contrast in pancreatic tumors accumulated with EGFR targeted ScFvEGFR‐IONPs and mammary tumors accumulated with uPAR targeted ATF‐IONPs. The positive contrast observed in UTE images was consistent with the negative contrast observed in the T2‐weighted images. A flip angle of 10° and a maximal possible TE for the second echo are suitable for SubUTE imaging.ConclusionUTE imaging is capable of detecting tumor targeted IONPs in vivo with positive contrast in molecular MRI applications. J. Magn. Reson. Imaging 2014;40:1071–1081. © 2013 Wiley Periodicals, Inc.