Figure 6a: Transverse MR/CT fusion imaging for radiation therapy treatment planning. Brain tumor is barely visible on (a) CT scan but is clearly evident on (b) MR image (repetition time msec/echo time msec, 10 000/138) and (c) fused MR/CT image. At present, nearly all treatment-planning systems used to generate isodose maps for radiation therapy can work with CT or with fused MR/CT images (as in d) but not with MR images alone. This situation is likely to improve soon. (Image courtesy of Allen Li, PhD, Medical College of Wisconsin, Milwaukee, Wis.)