Relating diffusion-weighted magnetic resonance imaging of brain white matter to cognitive processing-speed deficits in schizophrenia

Diffusion tensor imaging (DTI) and diffusion kurtosis imaging (DKI) analyses of diffusion-weighted magnetic resonance imaging (MRI) show that diffusional fractional anisotropy (FA) and kurtosis anisotropy (KA) of water inside brain white matter decrease for schizophrenic patients from that for healthy persons. DTI and DKI are statistical approaches and do not directly point to the underlying neurobiological reasons. In schizophrenia, it is believed that the demyelination of axons-microstructures that constitute the brain white matter-increases lateral diffusion of water and causes defective neural communications, resulting cognitive processing-speed deficits. Here, we use a simple but realistic neurobiological model for brain white matter and solve the Bloch-Torrey equation using numerical finite-element method to find out the underlying reasons of cognitive deficits in schizophrenia. FA and KA are calculated from computationally obtained diffusion-weighted MRI data after a Stejskal-Tanner gradient pulse sequence is applied to a periodic array of tubular axons with circular cross-sections. The calculated FA and KA decrease when the axon walls are more permeable to water, agree with the experimental findings, and correlate with the cognitive processing speeds of healthy persons and schizophrenic patients, and thus, help to understand the underlying reasons of cognitive processing-speed deficits in schizophrenia.