This paper describes techniques for producing FPGA-based designs that support free-form deformation in medical image processing. The free-form deformation method is based on a B-spline algorithm for modelling three-dimensional deformable objects. Our design includes four optimisations. First, we store the values of a third-order B-spline model in lookup tables. Second, we adopt a customised number representation format in our implementation. Third, we transform a nested loop so that conditionals are moved outside the loop. Fourth, we pipeline the design to increase its throughput, and we also deploy multiple pipelines such that each covers a different image. Our design description, captured in the Handel-C language, is parameterisable at compile time to support a range of image resolutions and computational precisions. An implementation on a Xilinx XC2V6000 device would be capable of processing images of resolution up to 256 by 256 pixels in real time.