ALTERNATIVES TO THE DISCRETE COSINE TRANSFORM FOR IRREVERSIBLE TOMOGRAPHIC IMAGE COMPRESSION

Full-frame irreversible compression of medical images is currently bei ng performed using the discrete cosine transform (DCT). Although the D CT is the optimum fast transform for video compression applications, w e show here that it is outperformed by the discrete Fourier transform (DFT) and discrete Hartley transform (DHT) for images obtained using p ositron emission tomography (PET) and magnetic resonance imaging (MRI) , and possibly for certain types of digitized radiographs. This differ ence occurs because PET and MRI images are characterized by a roughly circular region D of non-zero intensity bounded by a region R in which the image intensity is essentially zero. Clipping R to its minimum ex tent can reduce the number of low-intensity pixels but the practical r equirement that images be stored on a rectangular grid means that a si gnificant region of zero intensity must remain an integral part of the image to be compressed. With this constraint imposed, the DCT loses i ts advantage over the DFT because neither transform introduces signifi cant artificial discontinuities. The DFT and DHT have the further impo rtant advantage of requiring less computation time than the DCT.