Non-invasive imaging methods and minimally invasive sensing methods are used for assessing the viability of cells implanted in the central nervous system for therapeutic purposes and for detecting the transformation of such cells, including embryonic stem cells, into brain tumors. In particular, the present invention provides an imaging means for differentiating normal cell proliferation and angiogenesis following a cell implant from abnormal tumor growth and neovascularization associated with teratoma-inducing implanted embryonic stem cells.