Determination of oxygen relaxivity in oxygen nanobubbles at 3 and 7 Tesla

Objective Oxygen-loaded nanobubbles have shown potential for reducing tumour hypoxia and improving treatment outcomes, however, it remains difficult to noninvasively measure the changes in partial pressure of oxygen ( P O 2 ) in vivo. The linear relationship between P O 2 and longitudinal relaxation rate ( R 1 ) has been used to noninvasively infer P O 2 in vitreous and cerebrospinal fluid, and therefore, this experiment aimed to investigate whether R 1 is a suitable measurement to study oxygen delivery from such oxygen carriers. Methods T 1 mapping was used to measure R 1 in phantoms containing nanobubbles with varied P O 2 to measure the relaxivity of oxygen ( r 1Ox ) in the phantoms at 7 and 3 T. These measurements were used to estimate the limit of detection (LOD) in two experimental settings: preclinical 7 T and clinical 3 T MRI. Results The r 1Ox in the nanobubble solution was 0.00057 and 0.000235 s −1 /mmHg, corresponding to a LOD of 111 and 103 mmHg with 95% confidence at 7 and 3 T, respectively. Conclusion This suggests that T 1 mapping could provide a noninvasive method of measuring a > 100 mmHg oxygen delivery from therapeutic nanobubbles.