Benchmarking transverse spin relaxation based oxygenation measurements in the brain during hypercapnia and hypoxia

To simultaneously assess reproducibility of three MRI transverse relaxation parameters ( R2', R2*, and R₂ ) for brain tissue oxygenation mapping and to assess changes in these parameters with inhalation of gases that increase and decrease oxygenation, to identify the most sensitive parameter for imaging brain oxygenation.

Forty-eight healthy subjects (25 male, ages 35 ± 8 years) were scanned at 3.0 Tesla, each with one of four gases (mildly and strongly hypercapnic and hypoxic) administered in a challenge paradigm, using a gas delivery setup designed for patient use. Cerebral blood flow mapping with arterial spin labeling, and simultaneous R2', R2*, and R₂ mapping with gradient-echo sampling of free induction decay and echo (GESFIDE) were performed. Reproducibility in air and gas-induced changes were evaluated using nonparametric analysis with correction for multiple comparisons.

Our gas delivery setup achieved stable gas challenges as shown by physiological monitoring. Test-retest variability of R2', R2*, and R₂ were found to be 0.24 s^-1 (8.6% of mean), 0.24 s^-1 (1.3% of mean), and 0.15 s^-1 (1.0% of mean), respectively. Strong hypoxia produced the most conclusive oxygenation-driven relaxation change, inducing increases in R2' (25 ± 13%, P = 0.03), R2* (5 ± 2%, P = 0.02), and R₂ (2 ± 2%, NS).

We benchmarked the intra-scan test-retest variability in GESFIDE-based transverse relaxation rate mapping. Using a reliable framework for gas challenge paradigms, we recommend strong hypoxia for validating oxygenation mapping methods, and the use of tissue R2' change, instead of R2* or R₂ , as a metric for studying brain tissue oxygenation using transverse relaxation methods.

1 Technical Efficacy: Stage 1 J. MAGN. RESON. IMAGING 2017;46:704-714.