We discuss computationally e cient techniques for conÿdential storage and transmission of medical image data. Two types of partial encryption techniques based on AES are proposed. The ÿrst encrypts a subset of bitplanes of plain image data whereas the second encrypts parts of the JPEG2000 bitstream. We ÿnd that encrypting between 20% and 50% of the visual data is su cient to provide high conÿdentiality. ?