A novel programmable pulse generator with nanosecond resolution for pulsed electron paramagnetic resonance applications

Hypoxia Imaging As a Guide for Hypoxia-Modulated and Hypoxia-Activated Therapy

Significance: Oxygen imaging techniques, which can probe the spatiotemporal heterogeneity of tumor oxygenation, could be of significant clinical utility in radiation treatment planning and in evaluating the effectiveness of hypoxia-activated prodrugs. To fulfill these goals, oxygen imaging techniques should be noninvasive, quantitative, and capable of serial imaging, as well as having sufficient temporal resolution to detect the dynamics of tumor oxygenation to distinguish regions of chronic and acute hypoxia. Recent Advances: No current technique meets all these requirements, although all have strengths in certain areas. The current status of positron emission tomography (PET)-based hypoxia imaging, oxygen-enhanced magnetic resonance imaging (MRI), 19F MRI, and electron paramagnetic resonance (EPR) oximetry are reviewed along with their strengths and weaknesses for planning hypoxia-guided, intensity-modulated radiation therapy and detecting treatment response for hypoxia-targeted prodrugs. Critical Issues: Spatial and temporal resolution emerges as a major concern for these areas along with specificity and quantitative response. Although multiple oxygen imaging techniques have reached the investigative stage, clinical trials to test the therapeutic effectiveness of hypoxia imaging have been limited. Future Directions: Imaging elements of the redox environment besides oxygen by EPR and hyperpolarized MRI may have a significant impact on our understanding of the basic biology of the reactive oxygen species response and may extend treatment possibilities.

Echo-based Single Point Imaging (ESPI): a novel pulsed EPR imaging modality for high spatial resolution and quantitative oximetry

A novel time-domain spectroscopic EPR imaging approach, that is a unique combination of already known techniques, is described. The first one is multi-gradient Single Point Imaging involving pure phase-encoding where the oximetry is based on T(2)(∗). Line width derived from T(2)(∗) is subject to susceptibility effects and therefore needs system-dependent line width calibrations. The second approach utilizes the conventional 90°-τ-180° Spin-Echo pulse sequence where the images are obtained by the filtered back-projection after FT of the echoes collected under frequency-encoding gradients. The spatially resolved oximetry information is derived from a set of T(2)-weighted images. The back-projection images suffer susceptibility artifacts with resolution determined by T(2)(∗), but the oximetry based on T(2) is quite reliable. The current approach combines Single Point Imaging and the Spin-Echo procedure to take advantage the enhanced spatial resolution associated with the former and the T(2) dependent contrast of the latter. Pairs of images are derived choosing two time points located at identical time intervals on either side of the 180° pulse. The refocusing pulse being exactly in the middle of the two points ensures that artifacts associated with susceptibility and field inhomogeneities are eliminated. In addition, the net phase accumulated by the two time points being identical results in identical field of views, thus avoiding the zoom-in effect as a function delay in regular SPI and the associated interpolation requirements employed in T(2)(∗)-weighted oximetry. The end result is superior image resolution and reliable oximetry. In spite of the fact that projection-reconstruction methods require less number of measurements compared to SPI, the enormous advantage in SNR of the SPI procedure makes the echo-based SPI equally efficient in terms of measurement time. The Fourier reconstruction, line width independent resolution and the true T(2)-weighting make this novel procedure very attractive for in vivo EPR imaging of tissue oxygen quantitatively.