In this study we characterized in vivo a functional superparagmagnetic iron-oxide magnetic resonance contrast agent that effects the T2 relaxation time in MRI. The agent was developed by conjugating Molday Ion Carboxyl-6 (MIC6), with a de-immunized mouse monoclonal antibody (muJ591) targeting prostate-specific membrane antigen (PSMA). We propose this functional contrast agent as a non-invasive method to detect prostate cancer cells that are PSMA positive to provide increased differentiability from surrounding tissues for treatment. PSMA-positive prostate tumours were induced into 20 immunocompromised mice. The functional contrast agent was injected into 14 mice leaving 6 mice as controls. MR imaging was performed on a clinical 3T scanner using different parameters on a MESE sequence to obtain T2 relaxation time values. Tumour size, signal intensity, and T2 relaxation time were obtained pre and post injection and were found to have a lower value for treated mice compared to controls. ICP confirmed the increased level of elemental iron in treated mice tumours compared to controls. H&E staining showed healthy morphology of all tissues collected. The reduction in T2 relaxation time for the functional contrast agent, combined with its specificity against PSMA suggest its potential as a biologically-specific MR contrast agent.