Regional correlation of emphysematous changes in lung function and structure: a comparison between pulmonary function testing and hyperpolarized MRI metrics

Abstract

Regional and global relationships of lung function and structure were studied using hyperpolarized3He MRI in a rat elastase-induced model of emphysema ( n = 4) and healthy controls ( n = 5). Fractional ventilation ( r) and apparent diffusion coefficient (ADC) of3He were measured at a submillimeter planar resolution in ventral, middle, and dorsal slices 6 mo after model induction. Pulmonary function testing (PFT) was performed before MRI to yield forced expiratory volume in 50 ms (FEV50), airway resistance (RI), and dynamic compliance (Cdyn). Cutoff threshold values of ventilation and diffusion, r* and ADC*, were computed corresponding to 80% population of pixels falling above or below each threshold value, respectively. For correlation analysis, r* was compared with FEV50/functional residual capacity (FRC), RIand Cdyn, whereas ADC* was compared with FEV50/FRC, total lung capacity (TLC), and Cdyn. Regional correlation of r and ADC was evaluated by dividing each of the three lung slices into four quadrants. Cdynwas significantly larger in elastase rats (0.92 ± 0.16 vs. 0.61 ± 0.12 ml/cmH2O). The difference of RIand FEV50was insignificant between the two groups. The r* of healthy rats was significantly larger than the elastase group (0.42 ± 0.03 vs. 0.28 ± 0.06), whereas ADC* was significantly smaller in healthy animals (0.27 ± 0.04 vs. 0.36 ± 0.01 cm2/s). No systematic difference in these quantities was observed between the three lung slices. A significant 33% increase in ADC* and a significant 31% decline in r* for elastase rats was observed compared with a significant 51% increase in Cdynand a nonsignificant 26% decline in FEV50/FRC. Correlation of imaging and PFT metrics revealed that r and ADC divide the rats into two separate clusters in the sample space.