This allows evaluating the tissue’s response to therapy and monitoring progression of disease. (2) The patient can also be measured again at any time, over a period of up to many months if appropriate. These EPR measurements can be made repetitively in two clinically important ways: (1) Because scans can be taken virtually continuously for several minutes while patients rest comfortably, minute by minute changes can be assessed, including evaluating whether the tissue is responsive to the patient’s breathing enriched oxygen. Several unique features of EPR oximetry have the potential to address these needs, i.e., after a paramagnetic sensor is initially injected or implanted into a tissue of interest, EPR oximetry can make repeated and noninvasive measurements that directly assess tissue oxygen.
Developing treatment plans taking this into account, especially if the oxygen in the patient’s tissues can be successfully manipulated to improve the therapeutic ratio, is a key motive for identifying oximetry methods than can provide this information to clinicians while being readily and noninvasively integrated into clinical care for these patient groups. For example, in radiation treatment of cancers, the level of oxygen in the tumor is the single most important variable that affects the treatment outcomes. Diseases whose treatment strategies would benefit from taking tissue oxygen into account include tumors, peripheral vascular disease, and wound healing. If measured at all, oxygen level is assessed in the vascular system, while the greater need for many clinical applications is to know the oxygen level in tissues. Despite this importance, oxygen in tissues is seldom measured directly. The clinical value of repeated measurements of oxygen in tissues rests fundamentally on the central roles that oxygen levels play in a large array of physiological and pathophysiological processes. While India ink has a long history of safe use in tattoos, a systematic research search regarding its safety for marking tissues for medical uses and an examination of the evidence that differentiates between ink based on charcoal or carbon black has not been conducted. The most immediately available oxygen sensor is India ink, where two classes of carbon (carbon black and charcoal) have been identified as having acceptable paramagnetic properties for oximetry.
EPR oximetry has many potential advantages over other ways to measure oxygen in tissues, including directly measuring oxygen in tissues and being particularly sensitive to low oxygen, repeatable, and non-invasive after an initial injection of the EPR-sensing material is placed in the tumor.
Oximetry can provide important information useful for prognosis and to improve patient outcomes. Clinical EPR spectroscopy is emerging as an important modality, with the potential to be used in standard clinical practice to determine the extent of hypoxia in tissues and whether hypoxic tissues respond to breathing enriched oxygen during therapy.
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