We propose a new algorithm for registering 3D reconstructions of blood vessels from DSA with MR images of the brain. The registration transformation is determined by fitting the blood vessel tree reconstructed from DSA projections into the fissures of the brain derived from MR images, somewhat in the manner of fitting a key into a lock. The fit of the vessels into the fissures is guided by specific but simple anatomical knowledge. Preliminary evaluation of the algorithm has been carried out using data derived from a cadaver brain.