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MRI Characteristics of Cobalt Dichloride N-Acetyl Cysteine (C4) as a Contrast Agent Marker for Prostate Brachytherapy

T Lim

Tze Yee Lim1,5*, R Jason Stafford2,5, Madhuri Sankaranarayanapillai2, Karen Martirosyan3, Rajat Kudchadker1,5, Steven Frank4, (1) Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX 77030. (2) Department of Imaging Physics, Division of Diagnostic Imaging, The University of Texas MD Anderson Cancer Center, Houston, TX 77030. (3) Department of Physics and Astronomy, The University of Texas at Brownsville and Texas Southmost College, Brownsville, TX 78520. (4) Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX 77030. (5) The University of Texas Graduate School of Biomedical Sciences at Houston, Houston, TX 77030.

TU-G-134-5 Tuesday 4:30PM - 6:00PM Room: 134

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, T₁ and spin-spin relaxation time, T₂ 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, R₁ and R₂, versus C4 concentrations gave the relaxivities, r₁ and r₂.

Results: At 1.5T, r₁ = 0.151 ± 0.003 mM⁻¹s⁻¹ and r₂ = 0.208 ± 0.002 mM⁻¹s⁻¹; at 3.0T, r₁ = 0.148 ± 0.002 mM⁻¹s⁻¹ and r₂ = 0.328 ± 0.006 mM⁻¹s⁻¹. Accordingly, the relaxivity ratio, r₂/r₁ = 1.38 ± 0.030 at 1.5T and 2.22 ± 0.050 at 3.0T. r₁ showed a modest decrease with increasing field strength, consistent with the small C4 size. Conversely, r₂ increased with increasing field strength, thus the r₂/r₁ increased with increasing field strength. The C4 concentration of 1.0% was chosen, which balanced the desired effect of T₁ shortening and undesired domineering effect of T₂ 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.

Funding Support, Disclosures, and Conflict of Interest: Steven Frank and Karen Martirosyan are cofounders of C4 Imaging, LLC.

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