Tumors exhibit fluctuations in blood flow that influence
oxygen concentrations and therapeutic resistance. To assist therapeutic planning and improve prognosis, noninvasive dynamic imaging of spatial and temporal variations in
oxygen partial pressure (pO(2)) would be useful. Here, we illustrate the use of pulsed electron paramagnetic resonance imaging (EPRI) as a novel imaging method to directly monitor fluctuations in
oxygen concentrations in mouse models. A common resonator platform for both EPRI and magnetic resonance imaging (MRI) provided pO(2) maps with anatomic guidance and microvessel density.
Oxygen images acquired every 3 minutes for a total of 30 minutes in two different
tumor types revealed that fluctuation patterns in pO(2) are dependent on
tumor size and
tumor type. The magnitude of fluctuations in pO(2) in SCCVII
tumors ranged between 2- to 18-fold, whereas the fluctuations in HT29 xenografts were of lower magnitude. Alternating breathing cycles with air or
carbogen (95% O(2) plus 5% CO(2)) distinguished higher and lower sensitivity regions, which responded to
carbogen, corresponding to cycling
hypoxia and chronic
hypoxia, respectively. Immunohistochemical analysis suggests that the fluctuation in pO(2) correlated with pericyte density rather than vascular density in the
tumor. This EPRI technique, combined with MRI, may offer a powerful clinical tool to noninvasively detect variable oxygenation in
tumors.