DIFFUSION MRI - PRECISION, ACCURACY AND FLOW EFFECTS

After a decade of evolution and application of diffusion imaging, a la rge body of Literature has been accumulated. It is in this context tha t the accuracy and precision of diffusion-weighted and quantitative di ffusion MRI are reviewed. The emphasis of the review is on practical m ethods for clinical human imaging, particularly in the brain. The requ irements for accuracy and precision are reviewed for various clinical and basic science applications. The methods of measuring and calculati ng diffusion effects with MRI are reviewed. The pulse gradient spin ec ho (PGSE) methods are emphasized as these methods are used most common ly in the clinical setting. Processing of PGSE data is reviewed. Vario us PGSE encoding schemes are also reviewed in terms of the accuracy an d precision of isotropic and anisotropic diffusion measurements. The b road range of factors impacting the accuracy of the PGSE methods and o ther encoding schemes is then considered. Firstly, system inaccuracies such as background imaging gradients, gradient linearity, refocusing RF pulses, eddy currents, image misregistration, noise and dynamic ran ge are considered. A second class of inaccuracies is contributed by th e bulk effects of the imaged object, and include sample background gra dients, subject motion of cerebrospinal fluid and organs, and aperiodi c organ motion. A final category of potential inaccuracies is classifi ed as being contributed by microscopic, biophysical tissue properties and include partial volume effects, anisotropy, restriction, diffusion distance, compartmentation, exchange, multiexponential diffusion deca y, T-2 weighting and microvascular perfusion. Finally, the application of diffusion methods to studies of blood Bow in the microvasculature (i.e. the arterioles, capillaries and venules) are reviewed in detail, particularly in terms of feasibility and the stringent accuracy and p recision requirements. Recent provocative studies examining the use of PGSE approaches to suppress microvascular signals in brain functional MRI (MRI) are also reviewed.