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INNOVATIVE METHODOLOGY

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

¹Imaging Research Laboratories, Robarts Research Institute; ²Department of Medical Biophysics, ³Division of Respirology, Department of Medicine; ⁴Lawson Health Research Institute; and ⁵Department of Radiology and Nuclear Medicine University of Western Ontario, London, Canada

Submitted 14 February 2008 ; accepted in final form 30 May 2008

We measured hyperpolarized ³He 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 (|G_3x3|) 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 ³He 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 (|G_3x3|). While the ³He ADC standard deviation was not significantly different, mean |G_3x3| 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 |G_3x3| may provide a sensitive measure of ADC heterogeneity for ROI representing 9.4 x 9.4 x 30 mm or 2.6 cm³ of lung tissue.

emphysema; chronic obstructive pulmonary disease; thoracic imaging; noble gas lung imaging