The vast majority of patients diagnosed with prostate cancer in the modern era of PSA screening have localized and potentially curable disease. Patients have a number of excellent treatment options to consider, and quality of life following treatment is often a consideration in treatment choice. One critical quality of life concern for patients diagnosed with prostate cancer is sexual function. Studies of sexual function post-radiation therapy fall into three categories: incidence studies; correlative (dose and toxicity) studies; mechanism(functional) studies. All these studies point to a vascular mechanism of impotence post-radiation therapy, as opposed to a nerve-based mechanism following surgery. In this study, the critical vascular structures (internal pudendal artery and corpus cavernosum) are defined by MRI-based imaging and included in treatment planning for radiation treatments. Investigators propose that this technique will improve quality of life, sexual function, and relapse-free survival.