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

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Abstract
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/mm2) 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 mm2/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.