Search

We report the design and experimental validation of a compact positron emission tomography (PET) detector module (DM) intended for building a preclinical PET and electron-paramagnetic-resonance-imaging hybrid system that supports submillimeter image resolution and high-sensitivity, whole-body animal imaging. The DM is eight detector units (DUs) in a row. Each DU contains $12\times 12$ lutetium–yttrium oxyorthosilicate (LYSO) crystals having a 1.05-mm pitch read by $4\times 4$ silicon photomultipliers (SiPMs) having a 3.2-mm pitch. A small-footprint, highly multiplexing readout employing only passive electronics is devised to produce six outputs for the DM, including two outputs derived from SiPM cathodes for determining event time and active DU and four outputs derived from SiPM anodes for determining energy and active crystal. Presently, we have developed two DMs that are $1.28\times 10.4$ cm2 in extent and approximately 1.8 cm in thickness, with their outputs sampled at 0.7GS/s and analyzed offline. For both DMs, our results show successfully discriminated DUs and crystals. With no correction for SiPM nonlinearity, the average energy resolution for crystals in a DU ranges from 14% to 16%. While not needed for preclinical imaging, the DM may support 300–400-ps time-of-flight resolution.

Oxygen is the single most important molecule for sustaining life and, therefore, an important variable in tissue engineering and regenerative medicine. It has been shown that the change in oxygen concentration in an artificial or tissue-engineered graft affects cell survival, differentiation, and tissue growth in profound ways. However, at present, there are no reliable methods to map partial oxygen pressure (pO2) in growing artificial tissues. Here, we adapt and test the suitability of electron paramagnetic resonance oxygen imaging (EPROI) in assessing tissue graft oxygenation in vitro. EPROI is an established method to assess absolute pO2and has been widely applied to study tumor hypoxia in small animals. In this study, we demonstrate the feasibility of EPROI in evaluating oxygen dynamics in tissue grafts. We measured oxygen concentration in mesenchymal stem cell (MSC)-seeded polylactic-co-glycolic acid (PLGA) scaffolds with variable porosity. The pO2maps of these scaffolds showed that the mean pO2inside the scaffolds was smaller than the ambient air pO2(21% oxygen, 160 torr) and was gradually increased with increasing pore size. We assessed the local oxygen dynamics of the MSC-seeded osteogenic scaffold made from collagen–chitosan hydrogels in a partially sealed Eppendorf tube. The change in pO2values as a function of time inside the graft showed that the cells had used available oxygen within first 2 h of the experiment and then went to a dormant low oxygen consumption state until the oxygen supply was reestablished. Collectively, these data suggest that EPROI could be successfully used for mapping pO2in tissue-engineered grafts. The knowledge of tissue graft oxygenation may be used to improve scaffold design and to assess the tissue viability and growth.

In vivo oximetry by pulsed electron paramagnetic resonance is based on measurements of changes in electron spin relaxation rates of probe molecules, such as the triarylmethyl radicals. A series of experiments was performed at frequencies between 250 MHz and 1.5 GHz to assist in the selection of an optimum frequency for oximetry. Electron spin relaxation rates for the triarylmethyl radical OX063 as a function of radical concentration, salt concentration, and resonance frequency were measured by electron spin echo 2-pulse decay and 3-pulse inversion recovery in the frequency range of 250 MHz–1.5 GHz. At constant OX063 concentration, 1/T1decreases with increasing frequency because the tumbling dependent processes that dominate relaxation at 250 MHz are less effective at higher frequency. 1/T2also decreases with increasing frequency because 1/T1is a significant contribution to 1/T2for trityl radicals in fluid solution. 1/T2–1/T1, the incomplete motional averaging contribution to 1/T2, increases with increasing frequency. At constant frequency, relaxation rates increase with increasing radical concentration due to contributions from collisions that are more effective for 1/T2than 1/T1. The collisional contribution to relaxation increases as the concentration of counter-ions in solution increases, which is attributed to interactions of cations with the negatively charged radicals that decrease repulsion between trityl radicals. The Signal-to-Noise ratio (S/N) of field-swept echo-detected spectra of OX063 were measured in the frequency range of 400 MHz–1 GHz. S/N values, normalized by √Q, increase as frequency increases. Adding salt to the radical solution decreased S/N because salt lowers the resonator Q. Changing the temperature from 19 to 37°C caused little change in S/N at 700 MHz. Both slower relaxation rates and higher S/N at higher frequencies are advantageous for oximetry. The potential disadvantage of higher frequencies is the decreased depth of penetration into tissue.

The purpose of this study was to assess the natural pO2 of subcutaneous (SC) and intraperitoneal (IP) sites in mice to determine their relative suitability as sites for placement of implants. The pO2 measurements were performed using oxygen imaging of solid probes using lithium phthalocyanine (LiPc) as the oxygen sensitive material. LiPc is a water-insoluble crystalline probe whose spin-lattice and spin-spin relaxation rates (R1 and R2) are sensitive to the local oxygen concentration. To facilitate direct in vivo oxygen imaging, we prepared a solid probe containing encapsulated LiPc crystals in polydimethylsiloxane (PDMS), an oxygen-permeable and bioinert polymer. While LiPc-PDMS or similar probes have been used in repeated spectroscopic or average oxygen measurements using continuous wave (CW) electron paramagnetic resonance (EPR) since the late 1990s and now have advanced to clinical applications, they have not been used for pulse EPR oxygen imaging. One LiPc-PDMS probe of 2 mm diameter and 10 mm length was implanted in subcutaneous or intraperitoneal sites (left or right side) in each animal. The pO2 imaging of implanted LiPc-PDMS probes was performed weekly for six weeks using O2M’s preclinical 25 mT oxygen imager, JIVA-25, using the pulse inversion recovery electron spin echo (IRESE) method. At week 6, the probes were recovered, and histological examinations were performed. We report here, first-ever solid probe oxygen imaging of implanted devices and pO2 assessment of subcutaneous and intraperitoneal sites.

To protect the lownoise amplifier LNA in the receive arm of a pulsed 250 MHz EPR bridge, it is necessary to install as much isolation as possible between the power exciting the spin system and the LNA when high power is present in the receive arm of the bridge, while allowing the voltage induced by the magnetization in the spin sample to be passed undistorted and undiminished to the LNA once power is reduced below the level that can cause LNA damage. We discuss a combination of techniques to accomplish this involving the powerrouting circulator in the bridge, a second circulator acting as an isolator with passive shunt PIN diodes immediately following the second circulator. The low resistance of the forward biased PIN diode passively generates an impedance mismatch at the second circulator output port during the highpower excitation pulse and resonator ring down. The mismatch reflects the high power to the remaining port of the second circulator, dumping it into a system impedance matched load. Only when the power diminishes below the diode conduction threshold, the resistance of the PIN diode will rise to a value much higher than the system impedance. This brings the device into conduction mode. We find that the present design passively limits the output power to 14 dBm independent of the input power. For highinput power levels, the isolation may exceed 60 dB. This level of isolation is sufficient to fully protect the LNA of a pulse EPR bridge. © 2009 Wiley Periodicals, Inc. Concepts Magn Reson Part B Magn Reson Engineering 35B: 133–138, 2009

A versatile 250MHz pulse electron paramagnetic resonance EPR instrument for imaging of small animals is presented. Flexible design of the imager hardware and software makes it possible to use virtually any pulse EPR imaging modality. A fast pulse generation and data acquisition system based on general purpose PCI boards performs measurements with minimal additional delays. Careful design of receiver protection circuitry allowed us to achieve very high sensitivity of the instrument. In this article, we demonstrate the ability of the instrument to obtain threedimensional 3D images using the electron spin echo ESE and singlepoint imaging SPI methods. In a phantom that contains a 1 mM solution of narrow line 16 μT, peaktopeak paramagnetic spin probe, we achieved an acquisition time of 32 s per image with a fast 3D ESE imaging protocol. Using an 18min 3D phase relaxation T2e ESE imaging protocol in a homogeneous sample, a spatial resolution of 1.4 mm and a standard deviation of T2eof 8.5 were achieved. When applied to in vivo imaging this precision of T2edetermination would be equivalent to 2 Torr resolution of oxygen partial pressure in animal tissues. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part B Magn Reson Engineering 33B: 163–176, 2008

A versatile control software for pulse EPR spectrometers is introduced. Common and task‐specific problems are discussed and their solutions are described. The software provides the full spectrum of possibilities needed to perform arbitrary multidimensional pulse experiments. It allows for an easy interfacing of commonly used hardware components and enables straightforward modifications of the spectrometer. Good performance, configurability, and a number of unique features turn this software into an excellent tool for the operation of modern EPR spectrometers and upgrading old ones. © 2005 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 26B: 36–45, 2005

Abstract Pulse EPR, 55Mn-ENDOR and ELDOR-detected NMR experiments were performed on the S2-state of the oxygen-evolving complex from spinach Photosystem II. The novel technique of random acquisition in ENDOR was used to suppress heating artefacts. Our data unambiguously shows that four Mn ions have significant hyperfine coupling constants. Numerical simulation of the 55Mn-ENDOR spectrum allowed the determination of the principal values of the hyperfine interaction tensors for all four Mn ions of the oxygen-evolving complex. The results of our 55Mn-ENDOR experiments are in good agreement with previously published data [Peloquin JM et al. (2000) J Am Chem Soc 122: 10926–10942]. For the first time ELDOR-detected NMR was applied to the S2-state and revealed a broad peak that can be simulated numerically with the same parameters that were used for the simulation of the 55Mn-ENDOR spectrum. This provides strong independent support for the assigned hyperfine parameters.