M. Alecci

Nitroxide free radical clearance in the live rat monitored by radio-frequency CW-EPR and PEDRI

The use of RF (100 to 300 MHz) PEDRI and CW-EPR techniques allows the in vivo study of large animals such as whole rats and rabbits. Recently a PEDRI instrument was modified to also allow CW-EPR spectroscopy with samples of similar size and under the same experimental conditions. In the present study, this CW-EPR and PEDRI apparatus was used to assess the feasibility of the detection of a pyrrolidine nitroxide free radical (2,2,5,5,-tetramethylpyrrolidine-1-oxyl-3-carboxylic acid, PCA) in the abdomen of rats. In particular, we have shown that after the PCA administration (4 mmol kg(-1) b.w.): (i) the PCA EPR linewidth does not show line broadening due to concentration effects; (ii) a similar PCA up-take phase is observed by EPR and PEDRI; and (iii) the PCA half-lives in the whole abdomen of rats measured with the CW-EPR (T1/2=26+/-4 min, mean+/-sd, n=10) and PEDRI (T1/2=29+/-4 min, mean+/-sd, n=4) techniques were not significantly different (p > 0.05). These results show, for the first time, that information about PCA pharmacokinetics obtained by CW-EPR is the same as that from PEDRI under the same experimental conditions.

Multipolar magnet for low-frequency ESR imaging (with computer controlled power supply)

Low-frequency ESR imaging requires field configurations which are difficult to obtain using traditional electromagnets. Multipolar magnets represent a convenient way to obtain both the main magnetic fields and two of the three gradients necessary for a 3D image reconstruction. The authors present the first experimental implementation of this design in the form of a 16-pole electromagnet of cylindrical symmetry, with an aperture of 27 cm and a length of 60 cm. To achieve a given field profile the currents at each pole must be individually controlled and the whole experiment must be under computer control. The power supply consists of 16 bipolar sections that deliver 10 A through a load of 3.7 Omega . The field is sensed by Hall probes at the 16 pole positions and this information is used to correct the field. Details are provided of the power supply and of the interface between power supply and computer.

Versatile coil design and positioning of transverse-field RF surface coils for clinical 1.5-T MRI applications

Clinical MRI/MRS applications require radio frequency (RF) surface coils positioned at an arbitrary angle α with respect to B0. In these experimental conditions the standard circular loop (CL) coil, producing an axial RF field, shows a large signal loss in the central region of interest (ROI). We demonstrate that transverse-field figure-of-eight (FO8) RF surface coils design are not subject to the same amount of signal loss in the central ROI as loop coils when their orientations are changed. The 1.5-T CL and FO8 prototypes (diameter = 10 cm) were built on Plexiglas using copper strips (width = 4 mm, thickness = 100 μm). The two linear elements of the FO8 coil were 1 cm apart. Axial spoiled gradient echo (SPGR) images of a phantom containing doped water were acquired with the coil plane at α=0°, 45°, and 90°. As α increases, the CL images show, in the central ROI, a signal that decreases from a maximum value to zero. Whereas the FO8 images show, in the same ROI, a signal that varies little from the maximum value (20%). Optimized FO8 coils can be oriented with the coil plane positioned along any direction with respect to B0 without significant signal loss. Transverse RF coil design should be useful for clinical MRS studies and also for parallel imaging techniques where versatile RF coils disposed along arbitrary directions are required.

Modification of a whole-body NMR imager into a radio frequency EPR spectrometer suitable for in vivo measurements

We report the modification of a low-field whole-body NMR imager to allow radio frequency EPR spectroscopy. The instrument is designed to give optimum sensitivity for in vivo detection of free radicals. The RF circuit is able to operate over a wide frequency range (240 - 320 MHz) and is designed to handle input power levels of up to 12.5 W. The EPR resonator is of the loop - gap type suitable for samples up to 400 ml. A remote resonator coupling method has been developed enabling convenient matching adjustment. An automatic frequency control circuit is able to adjust for frequency deviations caused by animal motion. Where possible, existing imager hardware and commercially available instruments have been used. The instrument is controlled from a central computer via an IEEE 488 instrumentation bus. Here we present sensitivity measurements obtained from a variety of large aqueous samples containing nitroxide free radicals. We show that the instrument is suitable for the detection of exogenous free radicals in 200 g rats.

Young investigator award presentation at the 13th annual meeting of the esmrmb, september 1996, prague

The detection of free radicalsin vivo is very important for the study of many physiologic and pathologic conditions. Free radicals have been implicated in a number of diseases such as ischemia, inflammation, kidney damage, and cancer. Proton-electron double-resonance imaging (PEDRI) allows the indirect detection of free radicals via the Overhauser effect. Nitroxide free radicals used forin vivo PEDRI studies present spectra with two or three lines, but most PEDRI experiments performed to date have used only single-line electron paramagnetic resonance (EPR) irradiation. There is theoretical evidence that simultaneous irradiation of multiple EPR transitions could increase the maximum achievable PEDRI enhancement. From the experimental point of view, this requires the combined use of a suitable multiple-frequency EPR source and a multiple-tuned EPR resonator. A novel radiofrequency (RF) triple-tuned loop-gap resonator for use in PEDRI has recently been developed, and dynamic nuclear polarization (DNP) data were reported. In the present study we describe a new PEDRI apparatus, equipped with a triple-tuned resonator, that is suitable for simultaneous double-or triple-EPR irradiation of nitroxide free radicals. In particular, the details of the EPR hardware used to generate the two or three EPR frequencies are given, and PEDRI images obtained with simultaneous multiple EPR irradiation are shown. Moreover, DNP experimental results showing the increase of the enhancement as a function of the EPR power for single and simultaneous double EPR irradiation are presented. The main goal of this apparatus is to improve the sensitivity and/or to reduce EPR irradiation power in a PEDRI experiment. This is likely to be particularly important in future biologic applications of PEDRI where the applied power must be optimized to reduce sample heating.

Improved 1.5 T Magnetic Resonance Spectroscopy in the Human Calf with a Spatially Selective Radio Frequency Surface Coil

We describe the use of a transverse field RF surface coil that improves 1.5 T proton MR spectroscopy in the human calf. A 2-element figure-of-eight (FO8) transverse field RF surface coil (diameter 2R=10 cm; separation between the two linear current elements 2s=1cm) and a circular loop (CL) coil of equal diameter where built and tested with proton PRESS spectra at 1.5 T. The 1 H PRESS spectra obtained in the resting calf muscle of healthy volunteers showed that the FO8 coil allows a higher PRESS SNR (up to a factor 4.5) within a region of about 20 mm centred at about 12 mm from the coil plane, as compared to a standard CL coil. We found also a faster PRESS SNR decrease in the muscle tissue for anterior/posterior distance >20 mm using the FO8 coil. The measured PRESS SNR in the fat tissues of the calf showed a signal mostly localised within 10 mm from the coils surface and with an improved SNR (up to 5.5 times) observed in the presence of the FO8 coil as compared to the CL coil of equal diameter. The FO8 coil design can be advantageous for MRS applications, since it allows higher SNR from a small VOI positioned centrally within a relatively narrow region at a given depth in the human calf. The reported spatial SNR features of the FO8 coil design should also be useful for 1 H and 31 P MRS metabolites quantification in the human brain.