Variable Flip Angle MR Imaging of 3He Spin Lattice Relaxation Times for Measurement of Alveolar Oxygen Partial Pressure

INTRODUCTION: Measurement of the spin-lattice relaxation time of helium due to oxygen in the lung (T1,O2) has been proposed as a method for estimation of the regional alveolar oxygen partial pressure, pAO2 (1). In diseased animals, where ventilation and perfusion can be strongly impaired, regional variations in pAO2 are expected and can be used to compute ventilation/perfusion mismatch (2). However, measurement of T1,O2 using hyperpolarized gases is complicated by the cumulative effects of RF pulses (‘history’), leading to image blurring. Variable flip angle (VFA) pulse sequences are less sensitive to RF pulse history and therefore may provide a more robust method for multislice mapping of pAO2. In this work, a VFA approach is described theoretically and tested experimentally in rats and compared to results obtained using the traditional constant flip angle (CFA) RF pulse method. The feasibility of the VFA technique is also demonstrated in normal human subjects. The results confirm that the VFA technique can reduce image blurring and provide more accurate pAO2 estimates. THEORY: Including the effects of T1 (4), the MR signal strength of the VFA method (5) can be written as: