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 (PO(2)) in vivo. The linear relationship between PO(2) and longitudinal relaxation rate (R(1)) has been used to noninvasively infer PO(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 PO(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. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10334-022-01009-3.