Localizing prostate brachytherapy seeds with SGM

Introduction Brachytherapy is a common radiation treatment option for prostate cancer, in which many small radioactive seeds are implanted into the prostate gland under image guidance [1]. Precise knowledge of the prostate volume and the distribution of the seeds are required for accurate radiotherapy dosimetry. Imaging modalities like 2D X-ray or computed tomography allow precise seed localization but do not offer the soft-tissue contrast for accurate depiction of the prostate. MRI offers superior soft tissue contrast of the prostate but radioactive seeds produce local susceptibility changes and thus hypo-intensities in gradient echo MR images. These signal voids may be confused with other structures, such as blood vessels, making distinction sometimes difficult. Therefore, the use of a combined X-ray and MR-system (XMR) has been proposed for the visualization of brachytherapy seeds (from X-ray) overlaid to the MR-image of the prostate through registration [2]. In this work we applied susceptibility gradient mapping (SGM) using the original resolution for accurate seed localization, which can be overlaid onto MR data of the prostate. Furthermore, we investigated the strength of the induced susceptibility gradient as a basis for dosimetry.