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The diffusion weighted images have high signal in the major airways (see Figure 4) biasing the ADC parameters to be calculated. The high signal intensity values vary between slices and subjects. Normally in DWI we obtain 18 or 20 images from which we calculate ADC maps. 9 or 10 ADC maps along with histograms are obtained. The whole lung ADC (including major airways) calculated shows substantially larger ADC values compared to the ADC values from the lung parenchyma. The trachea being larger in size than the alveoli contains more gas giving high signal in the MRI image.
In order to obtain the ADC mean from the lung parenchyma we need to segment out the major airways. This implies high signal from the airways skews the ADC values. We need to obtain ADC values from the lung parenchyma without the influence of the airways.