Bone metastases in prostate cancer patients are a clinical and diagnostic challenge to image. Sometimes very small metastatic bone lesions may only become detectable by imaging in response to therapy due to increased bone deposition during the first three months after therapy. Commonly used imaging tests (such as regular CT or bone scan) are unable to reliably tell the difference between increased bone deposition (therapy response) and growth of the lesion (progressive disease). This diagnostic challenge may have profound negative effects on patient management since it may require additional imaging before an accurate determination of tumor response can be made. An appropriate determination of tumor response is needed for appropriate management of prostate cancer. The investigators anticipate that the new imaging tested in this study (called dual energy CT) may provide additional critical information in this clinical and diagnostic challenge.