This study will evaluate the use of magnetic resonance imaging (MRI) for guiding placement of hollow needles into the prostate gland for delivering internal radiation therapy to patients with prostate cancer. Prostate cancer is often treated with a combination of external beam radiation therapy and brachytherapy (internal radiation delivered close to the tumor). This study will determine whether MRI is more accurate in guiding needle placement than ultrasound, which is currently used for this purpose. Patients will have one brachytherapy treatment followed by 5 weeks of external beam treatments and a second brachytherapy.