Diffusion q-space measurements in-vivo does sequence timing influence the result?

Sammanfattning: Magnetic resonance imaging (MRI) is still a fast growing area of research and is an important part of the clinical tools used for diagnosis of various diseases. The diagnostic values of diffusion imaging are well known especially for ischemic stroke in the acute phase. This work concerns whether the apparent diffusion coefficient (ADC) and the full width at half maximum (FWHM) changes when the imaging protocol changes e.g. when the diffusion time or the duration of the diffusion encoding gradient duration, d, changes. All measurements were performed in-vivo using either on a Siemens Allegra 3T head scanner or a Philips Achieva 3T whole body system. The result of varying the different diffusion time shows that FWHM in of the probability distribution in white brain matter increases linearly in the direction parallel to the fibre structure, as expected from Einstein’s equation. Whereas in the direction orthogonal to thefibres FWHM is increases only slightly and this is assumed to reflect the restriction of the diffusion in this direction. The FWHM in grey brain matter shows the same tendency to increase as for the parallel direction in white brain matter. When the duration of the diffusion encoding gradients, d, is increased, simulations indicate that both ADC and FWHM should decrease. This was verified in the in-vivo measurementsalthough the decrease in the orthogonal direction was smaller than expected. In most of the literature today it is assumed that the ADC and the FWHM values are independent of the duration of the pulsed gradients. The result presented in this work suggests that this assumption should be reconsidered.