Noise correction for the exact determination of apparent diffusion coefficients at low SNR

Noise in MR image data increases the mean signal intensity of image regions due to the usually performed magnitude reconstruction. Diffusion-weighted imaging (DWI) is especially affected by high noise levels for several reaso ns, and a decreasing SNR at increasing diffusion weighting causes systemati c errors when calculating apparent diffusion coefficients (ADCs). Two diffe rent methods are presented to correct biased signal intensities due to the presence of complex noise: 1) with Gaussian intensity distribution, and 2) with arbitrary intensity distribution. The performance of the correction sc hemes is demonstrated by numerical simulations and DWI measurements on two different MR systems with different noise characteristics. These experiment s show that noise significantly influences the determination of ADCs. Apply ing the proposed correction schemes reduced the bias of the determined ADC to less than 10% of the bias without correction. (C) 2001 Wiley-Liss, Inc.