A faster PISTOL for ¹ H MR-based quantitative tissue oximetry

Quantitative mapping of oxygen tension (pO ₂ ), noninvasively, could potentially be beneficial in cancer and stroke therapy for monitoring therapy and predicting response to certain therapies. Intracellular pO ₂ measurements may also prove useful in tracking the health of labeled cells and understanding the dynamics of cell therapy in vivo. Proton Imaging of Siloxanes to map Tissue Oxygenation Levels (PISTOL) is a relatively new oximetry technique that measures the T ₁ of administered siloxanes such as hexamethyldisiloxane (HMDSO), to map the tissue pO ₂ at various locations with a temporal resolution of 3.5 minutes. We have now developed a siloxane-selective Look-Locker imaging sequence equipped with an echo planar imaging (EPI) readout to accelerate PISTOL acquisitions. The new tissue oximetry sequence, referred to as PISTOL-LL, enables the mapping of HMDSO T ₁ , and hence tissue pO ₂ in under one minute. PISTOL-LL was tested and compared with PISTOL in vitro and in vivo. Both sequences were used to record dynamic changes in pO ₂ of the rat thigh muscle (healthy Fischer rats, n = 6), and showed similar results (P > 0.05) as the other, with each sequence reporting a significant increase in pO ₂ (P < 0.05) under hyperoxia compared with steady state normoxia. This study demonstrates the ability of the new sequence in rapidly and accurately mapping the pO ₂ changes and accelerating quantitative ¹ H MR tissue oximetry by approximately 4-fold. The faster PISTOL-LL technique could enable dynamic ¹ H oximetry with higher temporal resolution for assesing tissue oxygentation and tracking the health of transplanted cells labeled with siloxane-based probes. With minor modifications, this sequence can be useful for ¹⁹ F applications as well.