To evaluate the hemodynamic-independent biomechanical-information of the brain, we determined changes in the regional apparent diffusion coefficient(ADC)and total cerebral blood flow(tCBF)during the cardiac cycle using magnetic resonance imaging(MRI). The water fluctuation index(WFI), which was defined as a ratio of the regional ADC and the tCBF changes during the cardiac cycle(delta-ADC and delta-tCBF), was obtained from ECG-triggered single-shot diffusion echo planar imaging and phase-contrast cine MRI, respectively. The WFI was assessed in patients with idiopathic normal pressure hydrocephalus(I-NPH, n=2), brain atrophy or asymptomatic ventricular dilation(VD, n=2)and in healthy volunteers(control group, n=9). WFIs in I-NPH were clearly higher than those in VD and control because the water molecules in cerebral white matter in I-NPH easily fluctuate owing to the arterial blood volume loading of the cranium, due to the decreasing the lower compliance in I-NPH, compared with that in control. On the other hand, delta-ADC in I-NPH was higher than that in control while delta-tCBF in I-NPH was lower. However, the differences between them were lower than the WFI. WFI analysis makes it possible to obtain biomechanical information as the fluctuation of the water molecules hemodynamic-independently in the brain, and may assist in the diagnosis of I-NPH.