Results: The DIR performs well visually and the sum of squares for the difference between the deformed CTs and the real CTs ranged between 4x10 9 and 5x10 9 . For comparison, the difference between four volumes in which only noise generates differences between the images, gave a comparable SQD of around 3.7 x 10 9 . For all the image pairs, the SQD of the deformed CTs is approximately half of that of the non-registered images (ranging from 0.45 to 0.59). Conclusions: A limitation of an US based

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... deformation field is that the area of the CT on which one can perform the deformation field is limited to the area of which US data is available. However, this phantom experiment does show that the application of such an US based CT DIR in principle could work. Whether the US based CT DIR is also valid for patient cases, has to be studied further with patient examples.