MRI characterization of cobalt dichloride-N-acetyl cysteine (C4) contrast agent marker for prostate brachytherapy

Brachytherapy, a radiotherapy technique for treating prostate cancer, involves the implantation of numerous radioactive seeds into the prostate. While the implanted seeds can be easily identified on a computed tomography image, distinguishing the prostate and surrounding soft tissues is not as straightforward. Magnetic resonance imaging (MRI) offers superior anatomical delineation, but the seeds appear as dark voids and are difficult to identify, thus creating a conundrum. Cobalt dichloride-N-acetyl-cysteine (C4) has previously been shown to be promising as an encapsulated contrast agent marker. We performed spin–lattice relaxation time (T₁) and spin–spin relaxation time (T₂) measurements of C4 solutions with varying cobalt dichloride concentrations to determine the corresponding relaxivities, r₁ and r₂. These relaxation parameters were investigated at different field strengths, temperatures and orientations. T₁ measurements obtained at 1.5 and 3.0 T, as well as at room and body temperature, showed that r₁ is field-independent and temperature-independent. Conversely, the T₂ values at 3.0 T were shorter than at 1.5 T, while the T₂ values at body temperature were slightly higher than at room temperature. By examining the relaxivities with the C4 vials aligned in three different planes, we found no orientation-dependence. With these relaxation characteristics, we aim to develop pulse sequences that will enhance the C4 signal against prostatic stroma. Ultimately, the use of C4 as a positive contrast agent marker will encourage the use of MRI to obtain an accurate representation of the radiation dose delivered to the prostate and surrounding normal anatomical structures. (paper)