Adrian D. Parasca

Quantitative tumor oxymetric images from 4D electron paramagnetic resonance imaging (EPRI): methodology and comparison with blood oxygen level-dependent (BOLD) MRI.

This work presents a methodology for obtaining quantitative oxygen concentration images in the tumor‐bearing legs of living C3H mice. The method uses high‐resolution electron paramagnetic resonance imaging (EPRI). Enabling aspects of the methodology include the use of injectable, narrow, single‐line triaryl methyl spin probes and an accurate model of overmodulated spectra. Both of these increase the signal‐to‐noise ratio (SNR), resulting in high resolution in space (1 mm)3 and oxygen concentrations (∼3 torr). Thresholding at 15% the maximum spectral amplitude gives leg/tumor shapes that reproduce those in photographs. The EPRI appears to give reasonable oxygen partial pressures, showing hypoxia (∼0–6 torr, 0–103 Pa) in many of the tumor voxels. EPRI was able to detect statistically significant changes in oxygen concentrations in the tumor with administration of carbogen, although the changes were not increased uniformly. As a demonstration of the method, EPRI was compared with nearly concurrent (same anesthesia) T  2* /blood oxygen level‐dependent (BOLD) MRI. There was a good spatial correlation between EPRI and MRI. Homogeneous and heterogeneous T  2* /BOLD MRI correlated well with the quantitative EPRI. This work demonstrates the potential for EPRI to display, at high spatial resolution, quantitative oxygen tension changes in the physiologic response to environmental changes. Magn Reson Med 49:682–691, 2003.

Electron paramagnetic resonance oxygen images correlate spatially and quantitatively with Oxylite oxygen measurements.

Tumor oxygenation predicts cancer therapy response and malignant phenotype. This has spawned a number of oxymetries. Comparison of different oxymetries is crucial for the validation and understanding of these techniques. Electron paramagnetic resonance (EPR) imaging is a novel technique for providing quantitative high-resolution images of tumor and tissue oxygenation. This work compares sequences of tumor pO2 values from EPR oxygen images with sequences of oxygen measurements made along a track with an Oxylite oxygen probe. Four-dimensional (three spatial and one spectral) EPR oxygen images used spectroscopic imaging techniques to measure the width of a spectral line in each image voxel from a trityl spin probe (OX063, Amersham Health R&D) in the tissues and tumor of mice after spin probe injection. A simple calibration allows direct, quantitative translation of each line width to an oxygen concentration. These four-dimensional EPR images, obtained in 45 minutes from FSa fibrosarcomas grown in the legs of C3H mice, have a spatial resolution of ∼1 mm and oxygen resolution of ∼3 Torr. The position of the Oxylite track was measured within a 2-mm accuracy using a custom stereotactic positioning device. A total of nine images that involve 17 tracks were obtained. Of these, most showed good correlation between the Oxylite measured pO2 and a track located in the tumor within the uncertainties of the Oxylite localizability. The correlation was good both in terms of spatial distribution pattern and pO2 magnitude. The strong correlation of the two modalities corroborates EPR imaging as a useful tool for the study of tumor oxygenation.

Characterization of response to radiation mediated gene therapy by means of multimodality imaging.

Imaging techniques are under development to facilitate early analysis of spatial patterns of tumor response to combined radiation and antivascular gene therapy. A genetically modified, replication defective adenoviral vector (Ad.EGR‐TNFα), injected intratumorally, mediates infected cells to express tumor necrosis factor alpha (TNFα), which is increased after exposure to radiation. The goal of this study was to characterize an image based “signature” for response to this combined radiation and gene therapy in mice with human prostate xenografts. This study is part of an imaged guided therapy project where such a signature would be useful in guiding subsequent treatments. Changes in the tumor micro‐environment were assessed using MRI registered with electron paramagnetic resonance imaging which provides images of tissue oxygenation. Dynamic contrast‐enhanced MRI was used to assess tissue perfusion. When compared with null vector (control) treatment, the ratio of contrast agent (Gd‐DTPA‐BMA) washout rate to uptake rate was lower (P = 0.001) after treatment, suggesting a more balanced perfusion. Concomitantly, oxygenation significantly increased in the treated animals and decreased or did not change in the control animals (P < 0.025). This is the first report of minimally invasive, quantitative, absolute oxygen measurements correlated with tissue perfusion in vivo. Magn Reson Med, 2009.

Reduction of image artifacts in mice by bladder flushing with a novel double-lumen urethral catheter.

In electron paramagnetic resonance imaging (EPRI), the accumulation of contrast agent in the bladder can create a very large source of signal, often far greater than that of the organ of interest. Mouse model images have become increasingly important in preclinical testing. To minimize bladder accumulation on mouse images, we developed a novel, minimally invasive, MRI/EPRI-friendly procedure for flushing a female mouse bladder. It is also applicable to other imaging techniques, for example, PET, SPECT, etc., where contrast agent accumulation in the bladder is also undesirable. A double-lumen urethral catheter was developed, using a standard IV catheter with a silicone tube extension, having a polyethylene tube threaded into the IV catheter. Flushing of the bladder provides a substantial reduction in artifacts, as shown in images of tumors in mice.

Oral administration is as effective as intraperitoneal administration of amifostine in decreasing nitroxide EPR signal decay in vivo

A rapid method to determine the systemic incorporation of amifostine has been sought in order to determine the effectiveness of different administration routes without the delay inherent in awaiting therapeutic results. Consistent changes in animal measurements of nitroxide signal decay were monitored using in vivo EPR at frequencies low enough to ensure uniform sensitivity to organs deep in 20-g C3H mice. Conditions included both co-administration of the amifostine with the carbamoyl-proxyl spin probe (CP) via i.p. injection (n=6) and oral administration (n=8) of the amifostine. These decreased the first order rate of decay of the CP EPR signal after a dose of 13.5 Gy radiation, by 23% and 18%, respectively. These changes were significantly different from the rate of decay of the CP EPR signal without amifostine, but were statistically indistinguishable from each other. These data demonstrate: (1) condition-dependent exponential decay of CP EPR signal allowing its use to determine systemic availability of a drug, and (2) that oral administration and i.p. injection of amifostine are both effective in affecting the CP EPR signal decay rate in a mouse model. This is a strong indicator of similar bioavailability in mice from both routes of administration.

5-Carboxy-5-methyl-1-pyrroline N-oxide: a spin trap for the hydroxyl radical

The in vivo in situ detection of hydroxyl radical (HO˙) in real time has been one of the great challenges of free radicals in biology. While we have been able to identify this free radical as a secondary biomarker of HO˙, the discovery that 5-carboxy-5-methyl-1-pyrroline N-oxide 2 can specifically spin trap HO˙ at the expense of superoxide (O2˙−) opens new avenues of research. In particular, nitrone 2 will allow us to detect HO˙ from low doses of radiation in animal tumors in real time.

WE‐D‐I‐609‐08: Multimodality Small Animal Imaging: Registration of Functional EPR Images with MRI Anatomy

Purpose:Electron paramagnetic resonanceimaging (EPRI) is a spectroscopicimaging modality being developed for functional and molecular imaging in small animals, and has potential for ultimate translation to clinical imaging. By utilizing contrast agents (“spin probes”) whose EPR spectra are modified by the local environment, it is possible to produce images which directly measure important physiologic quantities. In studies of tumor response to radiation‐mediated gene therapy we have utilized EPR oxygen images, which provide 3D oxygen maps with resolution of ∼1mm spatially and ∼3 Torr in oxygen partial pressure. Like functional nuclear medicineimages,EPR oxygen images do not depict anatomy directly. Analysis is facilitated by the ability to use an anatomic image as a “roadmap” for interpretation of the functional images. We have developed methods for registration of EPRI with MRI to allow anatomically‐based analysis of these functional images.Method and Materials: EPRI is performed using locally developed spectroscopicimaging systems. MRI is performed on a 4.7T dedicated small animal system. Several registration techniques have been newly developed or adapted from clinical multimodality imaging. Fiducial markers visible in both EPRI and MRI can be attached to the animal or to the immobilization device. Simple point‐to‐point registration is possible using this method. Surface‐based and intensity‐based registration methods have also been applied. Customized immobilization devices are fabricated using a polymer dental impression material, analogous to foam cradles used in radiotherapy. Results: Both anatomically directed analysis of functional EPRimages and analysis of serial changes in defined anatomical regions during a course of therapy are enabled by the use of customized immobilization and anatomic/functional image registration.Conclusion:Image registration is critical for accurate interpretation of multimodality anatomic/functional small animal imaging. Techniques analogous to those in clinical use can be used with success in this setting.