An efficient noninvasive method for in vivo imaging of tumor oxygenation by using a low-field magnetic resonance scanner and a paramagnetic contrast agent is described. The methodology is based on Overhauser enhanced magnetic resonance imaging (OMRI), a functional imaging technique. OMRI experiments were performed on tumor-bearing mice (squamous cell carcinoma) by i.v. administration of the contrast agent Oxo63 (a highly derivatized triarylmethyl radical) at nontoxic doses in the range of 2-7 mmol/kg either as a bolus or as a continuous infusion. Spatially resolved pO 2 (oxygen concentration) images from OMRI experiments of tumor-bearing mice exhibited heterogeneous oxygenation profiles and revealed regions of hypoxia in tumors (<10 mmHg; 1 mmHg ‫؍‬ 133 Pa). Oxygenation of tumors was enhanced on carbogen (95% O 2͞5% CO2) inhalation. The pO2 measurements from OMRI were found to be in agreement with those obtained by independent polarographic measurements using a pO 2 Eppendorf electrode. This work illustrates that anatomically coregistered pO 2 maps of tumors can be readily obtained by combining the good anatomical resolution of water proton-based MRI, and the superior pO 2 sensitivity of EPR. OMRI affords the opportunity to perform noninvasive and repeated pO 2 measurements of the same animal with useful spatial (Ϸ1 mm) and temporal (2 min) resolution, making this method a powerful imaging modality for small animal research to understand tumor physiology and potentially for human applications.A bnormal values of pO 2 (the partial pressure of O 2 ) are linked to many pathophysiological conditions (e.g., ischemic diseases, reperfusion injury, and oxygen toxicity). Approximately one-third of human tumors evaluated for oxygen status have shown significant oxygen deficiency, and oxygen deficiency increases the tumor's resistance toward cancer treatment modalities, including radiation and chemotherapy (1, 2). Additionally, hypoxic microenvironments in tumors are known to promote processes driving malignant progression, such as angiogenesis, elimination of p53 tumor suppressor activity, genetic instability, and metastasis (3-5). Understanding of tumor hypoxia could lead to the discovery of diagnostic and prognostic markers for malignant progression, discovery of novel therapeutic targets, and the development of new constructs for gene therapy applications in human cancer. Hence, a noninvasive technique that could accurately and repetitively measure tissue oxygenation would find broad application in clinical and basic research. Unfortunately, the currently used electrochemical method (6) for in vivo oxygen measurement is an invasive technique applicable only to accessible tumors. Further, the technique is hampered by measurements of only a small part of the total tumor, which cannot be re-evaluated. Several magnetic resonance techniques (7, 8) have been developed for in vivo oximetry, including spin label oximetry (9), MRI (10), and electron paramagnetic resonance imaging (EPRI) (11,12). The blood oxygen level-dependent...

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.

A priori knowledge of spatial and temporal changes in partial pressure of oxygen (oxygenation; pO 2 ) in solid tumors, a key prognostic factor in cancer treatment outcome, could greatly improve treatment planning in radiotherapy and chemotherapy. Pulsed electron paramagnetic resonance imaging (EPRI) provides quantitative 3D maps of tissue pO 2 in living objects. In this study, we implemented an EPRI set-up that could acquire pO 2 maps in almost real time for 2D and in minutes for 3D. We also designed a combined EPRI and MRI system that enabled generation of pO 2 maps with anatomic guidance. Using EPRI and an air/carbogen (95% O 2 plus 5% CO 2 ) breathing cycle, we visualized perfusion-limited hypoxia in murine tumors. The relationship between tumor blood perfusion and pO 2 status was examined, and it was found that significant hypoxia existed even in regions that exhibited blood flow. In addition, high levels of lactate were identified even in normoxic tumor regions, suggesting the predominance of aerobic glycolysis in murine tumors. This report presents a rapid, noninvasive method to obtain quantitative maps of pO 2 in tumors, reported with anatomy, with precision. In addition, this method may also be useful for studying the relationship between pO 2 status and tumor-specific phenotypes such as aerobic glycolysis.