Diffusion‐weighted imaging of the parotid gland: Influence of the choice of b ‐values on the apparent diffusion coefficient value

Purpose To determine how the ADC value of parotid glands is influenced by the choice of b ‐values. Materials and Methods In eight healthy volunteers, diffusion‐weighted echo‐planar imaging (DW‐EPI) was performed on a 1.5 T system, with b ‐values (in seconds/mm 2 ) of 0, 50, 100, 150, 200, 250, 300, 500, 750, and 1000. ADC values were calculated by two alternative methods (exponential vs. logarithmic fit) from five different sets of b ‐values: (A) all b ‐values; (B) b = 0, 50, and 100; (C) b = 0 and 750; (D) b = 0, 500, and 1000; and (E) b = 500, 750, and 1000. Results The mean ADC values for the different settings were (in 10 −3 mm 2 /second, exponential fit): (A) 0.732 ± 0.019, (B) 2.074 ± 0.084, (C) 0.947 ± 0.020, (D) 0.890 ± 0.023, and (E) 0.581 ± 0.021. ADC values were significantly ( P < 0.001) different for all pairwise comparisons of settings (A–E) of b ‐values, except for A vs. D ( P = 0.172) and C vs. D ( P = 0.380). The ADC(B) was significantly higher than ADC(C) or ADC(D), which was significantly higher than ADC(E). ADC values from exponential vs. logarithmic fit ( P = 0.542), as well as left vs. right parotid gland ( P = 0.962), were indistinguishable. Conclusion The ADC values calculated from low b ‐value settings were significantly higher than those calculated from high b ‐value settings. These results suggest that not only true diffusion but also perfusion and saliva flow may contribute to the ADC. J. Magn. Reson. Imaging 2004;20:786–790. © 2004 Wiley‐Liss, Inc.