Electron paramagnetic resonance imaging of tumor hypoxia: Enhanced spatial and temporal resolution for in vivo pO₂ determination

Abstract

The time-domain (TD) mode of electron paramagnetic resonance (EPR) data collection offers a means of estimating the concentration of a paramagnetic probe and the oxygen-dependent linewidth (LW) to generate pO 2 maps with minimal errors. A methodology for noninvasive pO 2 imaging based on the application of TD-EPR using oxygen-induced LW broadening of a triarylmethyl (TAM)-based radical is presented. The decay of pixel intensities in an image is used to estimate T* 2 , which is inversely proportional to pO 2 . Factors affecting T* 2 in each pixel are critically analyzed to extract the contribution of dissolved oxygen to EPR line-broadening. Suitable experimental and image-processing parameters were obtained to produce pO 2 maps with minimal artifacts. Image artifacts were also minimized with the use of a novel data collection strategy using multiple gradients. Results from a phantom and in vivo imaging of tumor-bearing mice validated this novel method of noninvasive oximetry. The current imaging protocols achieve a spatial resolution of ϳ1.0 mm and a temporal resolution of ϳ9 s for 2D pO 2 mapping, with a reliable oxygen resolution of ϳ1 mmHg (0.12% oxygen in gas phase). This work demonstrates that in vivo oximetry can be performed with good sensitivity, accuracy, and high spatial and temporal resolution.

Low-field paramagnetic resonance imaging of tumor oxygenation and glycolytic activity in mice

“…Figure 2C shows the anatomic image from MRI of a squamous cell carcinoma (SCC) tumor-bearing mouse (implanted on the right hind leg), and Figure 2D shows the corresponding pO 2 map from EPRI. Previous studies showed that the SCC tumor at a size of 1 cm 3 exhibits hypoxia in a significant fraction (25,30). There was no significant difference in pO 2 between normal muscle tissues with or without tumor as opposed to significant lower pO 2 level (10.5 ± 3.6 mmHg; n = 6; P < 0.01) in tumor region ( Figure 2E).…”

“…The actual line widths (FWHM) at 300 MHz are 180 mG and 133 mG, respectively, for oxo63 and oxo31, due to the presence of unresolved hyperfine coupling from 13 Pulsed (time domain) EPRI and oximetry. Technical details of the pulsed EPR scanner operating at 300 MHz, data acquisition based on the singlepoint imaging (SPI) modality, image reconstruction, and the oximetry calculation procedure were described in the earlier reports (25,29,46). Briefly, the SPI is a pure phase-encoding imaging technique.…”

“…Since FIDs last for a couple of microseconds, 1 could generate a sequence of T2*-weighted images, which allowed pixel-wise estimation of in vivo pO2. In order to get reproducible values of T2* and to retain a more or less uniform image resolution, a set of 3 gradients were used, and data for corresponding images were collected in an interleaved fashion (25). The spatial resolution of SPI modality, in which all 3 directions were phase encoded, employed here does not, in principle, depend on the line width (FWHM) of the TAM probe.…”

“…EPR detects paramagnetic species that have unpaired electrons such as transition metal complexes and free radicals. With the recent availability of triarylmethyl radical (TAM) probes as in vivo compatible paramagnetic tracers (23), EPR imaging (EPRI) is being explored for mapping tissue oxygen in live animals (24)(25)(26). The collisional interaction between TAM and dissolved paramagnetic oxygen leads to a broadening of the spectral line width of TAM.…”

“…Furthermore, using magnetic field gradients, as in MRI, the spatial distribution of the TAM tracer can be obtained in a living subject (28). By extracting the pO 2 -dependent EPR line widths, a pO 2 map within the body can be generated (25).…”