TU‐G‐134‐05: MRI Characteristics of Cobalt Dichloride N‐Acetyl Cysteine (C4) as a Contrast Agent Marker for Prostate Brachytherapy

Purpose: The use of MRI in post‐brachytherapy‐implant evaluation is limited due to challenges in localizing dark‐appearing seeds in the prostatic stroma. Previous studies showed cobalt dichloride N‐acetyl cysteine complex (C4) as a promising positive contrast agent to indicate seed location. In this study, we investigated the potential benefit of using C4, through relaxivity measurements at 1.5T and 3.0T. Methods: Eight vials containing cobalt dichloride at varying concentrations (0%, 0.10%, 0.20%, 0.50%, 1.0%, 1.5%, 2.0% and 5.0%) and a control vial containing water only were imaged in a head coil at two magnetic field strengths, 1.5T and 3.0T. At each concentration, spin‐lattice relaxation time, Ti and spin‐spin relaxation time, Tz were estimated using nonlinear least‐squares fitting of mean signal intensities versus time to standard exponential models. The slopes from linear fits of relaxation rates, Ri and Rz, versus C4 concentrations gave the relaxivities, ri and rz. Results: At 1.5T, ri = 0.151 ± 0.003 mMV1 and rz = 0.208 ± 0.002 mM_1s_1; at 3.0T, ri = 0.148 ± 0.002 mM“is”i and rz = 0.328 ± 0.006 mM'Vi. Accordingly, the relaxivity ratio, rz/ri = 1.38 ± 0.030 at 1.5T and 2.22 ± 0.050 at 3.0T. ri showed a modest decrease with increasing field strength, consistent with the small C4 size. Conversely, rz increased with increasing field strength, thus the rz /ri increased with increasing field strength. The C4 concentration of 1.0% was chosen, which balanced the desired effect of Ti shortening and undesired domineering effect of Tz damping at higher field strengths, to preserve C4 as a positive contrast agent. Conclusion: With these measured relaxation characteristics, we can optimize the pulse sequence parameters for use with C4 by minimizing susceptibility effects and enhancing C4 signal. Overall, this study confirmed the potential of C4 as a positive contrast marker for MRI‐based prostate brachytherapy. Steven Frank and Karen Martirosyan are cofounders of C4 Imaging, LLC.