PROJECT SUMMARY EPR oximetry has been established in preclinical models to be a very reliable method for the direct and repeated measurement of tissue oxygen levels. Ongoing clinical studies in cancer patients have confirmed that EPR oximetry can be extended to human subjects very effectively, thus addressing a critical, but unmet, need—the ability to make repeated measurements of tissue oxygen concentration in the clinic. These studies have also revealed the potential shortcomings of the technology and informed further developments that are needed to achieve the full clinical potential of EPR oximetry for cancer therapy. The current EPR scanners are based on the traditional CW (continuous wave, frequency-domain) mode of operation using magnetic-field seep and modulation, both of which are necessary for measuring paramagnetic probes with much shorter relaxation times (typically on the order of nanoseconds), for which a pulse (time-domain) mode of operation is prohibitively difficult. The CW EPR oximetry requires stringent magnetic field homogeneity, longer data acquisition time, motion-induced artifacts in the data, and, more importantly, a cumbersome operation. On the other hand, pulse EPR technology, which uses spin echo-based relaxation-time measurements for oximetry, is a simple and straightforward approach that is more efficient, faster, and less dependent on field homogeneity and object motions. This proposal will combine pulse technology with a suite of oxygen-sensing probes (OxyChip and related materials) developed as part of the PI’s current NIH funding program to develop an advanced pulse EPR scanner, as well as procedures for the scanner’s use in human subjects in a clinical setting. We propose the following specific aims to achieve the overall objective of developing an advanced pulse EPR scanner for the repeated measurement of oxygen concentration in human subjects: (1) Development of a multi-modal EPR scanner for clinical oximetry and imaging. (2) Validation of the multi-modal pulse EPR scanner for repeated measurements of oxygen concentration using tissue phantoms and animal models of tumor. (3) Clinical evaluation of the pulse EPR scanner, for the measurement of oxygen concentration in healthy and cancer patients. The availability of this capability would be a new addition to clinical medicine that would immediately and significantly enhance the treatment of various disease states and malignancies.