Mapping and quantifying hyperpolarized 3He magnetic resonance imaging apparent diffusion coefficient gradients

We measured hyperpolarized He-3 magnetic resonance imaging (MRI) apparent diffusion coefficients (ADC) and quantified ADC gradients in each three-by-three voxel region of interest (ROI). Such local ADC gradients can be represented in vector maps showing the magnitude (vertical bar G(3x3)vertical bar) and direction of ADC gradients, providing a qualitative visualization tool and quantitative measurement of airway and air space heterogeneity. Twenty-four subjects (15 male, mean age = 67 +/- 7 yr) with global initiative for chronic obstructive lung disease (GOLD) stage II (n = 9, mean age 68 +/- 6 yr), GOLD stage III chronic obstructive pulmonary disease (COPD; n = 7, mean age 67 +/- 8 yr), and age- matched healthy volunteers (n = 8, mean age 67 +/- 6 yr) were enrolled based on their age and spirometry results. Hyperpolarized 3He MRI was performed on a whole body 3.0 Tesla system. Mean He-3 ADC and ADC standard deviation were calculated for the center coronal slice, and the mean magnitude and direction of the ADC gradient vectors were calculated for each three- by- three voxel matrix (vertical bar G(3x3)vertical bar). While the He-3 ADC standard deviation was not significantly different, mean vertical bar G(3x3)vertical bar was significantly different between subjects with stage II (0.14 +/- 0.03 cm/s) and stage III COPD (0.19 +/- 0.03 cm/s; P < 0.005) and between healthy subjects (0.12 +/- 0.03 cm/ s) and those with stage II COPD (P < 0.02). The second order statistic vertical bar G(3x3)vertical bar may provide a sensitive measure of ADC heterogeneity for ROI representing 9.4 x 9.4 x 30 mm or 2.6 cm(3) of lung tissue.