In VivoImaging of Nitroxide-Free-Radical Clearance in the Rat, Using Radiofrequency Longitudinally Detected ESR Imaging
✍ Scribed by I. Nicholson; M.A. Foster; F.J.L. Robb; J.M.S. Hutchison; D.J. Lurie
- Publisher
- Elsevier Science
- Year
- 1996
- Tongue
- English
- Weight
- 928 KB
- Volume
- 113
- Category
- Article
- ISSN
- 1064-1866
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✦ Synopsis
Naturally occurring free radicals have been implicated in to detect nitroxide free radicals in aqueous samples ( volume 15 ml ) with concentrations down to the order of many disease processes including inflammatory disease, toxic injury, carcinogenesis, and reperfusion injury. The free micromolar. More recently, we have extended this approach to obtain two-dimensional in vivo LODESR im-radicals involved are highly reactive with very short lifetimes, on the order of microseconds. As a result, the concen-ages of the rat ( 16) . In this work, we describe further improvements to our LODESR imager which allow us to trations are relatively low, being on the order of micromolar at most. Any interpretation of the direct involvement in a collect two-dimensional images with an effective slice selection in the third direction. Sprague -Dawley rats, up particular disease process is rarely conclusive due to the difficulty in detecting and identifying the free radicals. A to 250 g weight, were used to image the distribution and clearance of the nitroxide free radical PCA ( 2,2,5,5-tet-method of imaging the distribution of these free radicals in vivo would be of much use in helping to characterize the ramethyl pyrrolidine-1-oxyl-3-carboxylic acid ) following intravenous administration. time course of such processes. Recent developments in radiofrequency ESR imaging (1-5) and proton-electron dou-
The components of the LODESR imaging system are shown in Fig. 1. The main magnet is a resistive, horizontal-ble-resonance imaging (PEDRI) (6-8) have resulted in the detection and imaging of free radicals injected intravenously field, four-coil electromagnet, with an internal bore of 34 cm. This can be swept over a 6 mT range centered on 10 in live animals. Current research is centered on improving the concentration sensitivity of these techniques to the stage mT. The magnet power supply (Farnell AP100-90A, Wetherby, UK) was operated in constant current mode, pro-where naturally occurring free radicals might eventually be studied.
grammed with an external voltage source derived from the output of a dual multiplying DAC circuit. The field sweep Over the past few years, we have investigated an alternative detection scheme for radiofrequency ESR imaging amplitude and sweep rate were computer controlled (Acorn Risc PC600, Cambridge, UK) through an eight-line digital of biological samples using longitudinally detected ESR ( LODESR ) ( 9 -14 ) . Conventional continuous-wave ra-output user port.
The RF coil assembly used to excite the ESR resonance diofrequency ESR requires excitation of the ESR resonance at frequencies on the order of 300 MHz with detec-and detect oscillations in the spin magnetization was placed inside the main magnet bore. This consisted of tion at the same frequency. The main magnetic field is swept to cover the resonance. With LODESR, the ESR a birdcage resonator ( length 3.0 cm, diameter 7.5 cm) constructed from 12 parallel wires ( copper, 1 mm diame-irradiation at 300 MHz is applied continuously and amplitude modulated at a frequency much lower than the ESR ter ) . Inductive coupling was used to drive the coil which was tuned to 300 MHz. With the coil matched to 50 V, an frequency. This modulation causes the component of the spin magnetization parallel to the main field to oscillate unloaded Q of 180 was measured. The signal was detected using a continuously wound solenoid ( length 4.5 cm, di-at the modulation frequency. The oscillation is detected with a coil tuned to the modulation frequency, placed ameter 6.4 cm) placed inside the birdcage. This was tuned to 200 kHz and, when matched to 50 V, had an unloaded parallel to the main field.
We have previously described this alternative approach Q of 44. The orientation of the coil system is shown schematically in Fig. 2. A rat ( 188 g ) was placed inside the to ESR imaging and have discussed the potential advantages for biological samples ( 15 ) . These initial studies at assembled coil to determine the effect of sample loading on coil performance. The Q of the solenoid was not sig-radiofrequencies ( 300 MHz ) have shown that it is possible 256