Dmitry S .Novikov1, Valerij G. Kiselev2
1Radiology, NYU School of Medicine, New York, NY, United States; 2Diagnostic Radiology, Uniklinikum Freiburg, Freiburg, Germany
NMR diffusion measurements are confounded by microscopic tissue heterogeneity. A known phenomenon is the underestimation of apparent diffusion coefficient (ADC) in the presence of susceptibility-induced microscopic magnetic field when the diffusion length is shorter than the field correlation length. This study demonstrates for the first time that the effect is opposite in the case of long diffusion length: The ADC overestimates the genuine diffusion coefficient. This effect is anomalously strong for effectively two-dimensional disordered objects, such as blood capillaries, which agrees with experimental data on hypercapnia and diffusion fMRI. Conversely, heterogeneous transverse relaxation rate results in a decrease of ADC.