Assessment of the lung microstructure in patients with asthma using hyperpolarized ³He diffusion MRI at two time scales: Comparison with healthy subjects and patients with COPD

Purpose: To investigate short- and long-time-scale ³He diffusion in asthma. Materials and Methods: A hybrid MRI sequence was developed to obtain coregistered short- and long-time-scale apparent diffusion coefficient (ADC) maps during a single breath-hold. The study groups were: asthma (n = 14); healthy (n = 14); chronic obstructive pulmonary disease (COPD) (n = 9). Correlations were made between mean-ADC and %ADC-abn (abnormal) (%pixels with ADC > mean +2 SD of healthy) at both time scales and spirometry. Sensitivities were determined using receiver operating characteristic (ROC) analysis. Results: For asthmatics, the short- and long-time-scale group-mean ADCs were 0.254 ± 0.032 cm²/s and 0.0237 ± 0.0055 cm²/s, respectively, representing a 9% and 27% (P = 0.038 and P = 0.005) increase compared to the healthy group. The group-mean %ADC-abn were 6.4% ± 3.7% and 17.5% ± 14.2%, representing a 107% and 272% (P = 0.004 and P = 0.006) increase. For COPD much greater elevations were observed. %ADC-abn provided better discrimination than mean-ADC between asthmatic and healthy subjects. In asthmatics ADC did not correlate with spirometry. Conclusion: With long-time scale ³He diffusion magnetic resonance imaging (MRI) changes in lung microstructure were detected in asthma that more conspicuous regionally than at the short time scale. The hybrid diffusion method is a novel means of identifying small airway disease.