Antonello Sotgiu

Whole mouse nitroxide free radical pharmacokinetics by low frequency electron paramagnetic resonance

The in vivo uptake distribution and reduction of the oxygen-sensitive nitroxide spin label PCA in the mouse monitored by low frequency electron paramagnetic resonance (EPR) spectroscopy are reported. Spectra were obtained from the head and liver regions of pentobarbital anesthetized mice during different circulatory and ventilatory conditions. Identical clearances were found in these regions during normoxia. Moderate hypoxia (10% O2-90% N2) did not significantly affect the spin label reduction rate.

Three-dimensional in vivo ESR imaging in rats

The first experiment of tridimensional in vivo ESR imaging at 1.2 GHz is described in this paper. The tails of rats weighing 300-350 grams were visualized using 1 cc of a 50 * 10(-3) M solution of nitroxide free radical injected in the caudal vein. In an even distribution of spin label is assumed this would correspond to a final concentration of about 10(-4) M. A reconstruction from projections was used to obtain the images. The apparatus utilizes stationary field gradients. Projections were obtained by sweeping the main field. For 3D reconstructions, the projections were collected along 32 * 8 field gradient orientations. The whole procedure takes approximately 18 minutes.

Electron paramagnetic resonance spectrometer for three‐dimensional in vivo imaging at very low frequency

The paper describes an electron paramagnetic resonance apparatus for spectroscopy and imaging at very low frequency (283 MHz). The bridge operates in a reflection cavity homodyne configuration and can be used on a very broad frequency range. The sample cavity is a one loop‐two gap resonator and accepts samples up to 5 cm in diameter and 10 cm long. These sample dimensions make the apparatus suitable for observing living samples such as rats. For the main field and gradients the apparatus uses a newly designed multipolar magnet that can provide the main field and gradients for 2D image reconstruction. The third gradient is obtained by coils mounted in the bore of the magnet. Tests on peroxilamine disulfonate samples have shown micromolar sensitivity. The minimum spatial resolution is limited by the sample linewidth and by the signal/noise. In living samples the toxicity of nitroxide radicals restricts the maximum label concentrations to values of the order of 10−4 M, and the resolution seems to be limited by sensitivity to fractions of cm3.

Whole rat electron paramagnetic resonance imaging of a nitroxide free radical by a radio frequency (280 MHz) spectrometer

Low frequency (280 MHz) electron paramagnetic resonance spectroscopy has been used to follow uptake, distribution and reduction of the nitroxyl spin label PCA in the rat. No difference of half life was found in seven rats submitted to three administrations of PCA (11.3 +/- 0.4; 11.0 +/- 0.6 and 11.5 +/- 0.7 min). Transversal two-dimensional images of PCA distribution in the rat body were obtained over 6 min by means of field gradients. PCA was observed in three regions by projections along the longitudinal axis of the rat. PCA accumulation was found in the lower abdomen 12 min after the start of the PCA injection.

pH-sensitive imaging by low-frequency EPR : a model study for biological applications

The use of pH-sensitive nitroxides, in conjunction with low-frequency EPR, offers a unique opportunity for non-invasive assessment of pH values (in the range 0 to 14) in living animals. In the present study, we have investigated the potential use of pH-sensitive nitroxide free radicals in conjunction with EPR imaging techniques at low and very low frequencies (280 MHz-2.1 GHz). In particular, we have measured the hyperfine splitting (hfs) of a pH-sensitive probe at three different EPR frequencies: 280 MHz, 1.1 GHz and 2.1 GHz. We have also developed EPR imaging experiments with phantoms simulating in vivo conditions, using pH-sensitive probes at 280 MHz (spatial-spatial) and 1.1 GHz (spectral-spatial). Finally, we discuss the actual sensitivity/resolution limits of the EPR imaging techniques at low frequencies. Practical applications of this method in the biomedical field are suggested for the continuous and non-invasive localization of pH in vivo.

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.

Low-frequency three dimensional ESR imaging of large samples

Presents an apparatus for performing a true 3D electron spin resonance image reconstruction at low microwave frequency (1.3 GHz). The gradients Gzz, Gzx, Gzy of the field Bz along the three perpendicular axes are directly controlled by a personal computer which is also used to store the converted signals and to compute the spin-density function from the projections. The mathematical technique used is a filtered projection obtained by means of the Fourier transforms, called 3D Fourier reconstruction. Examples of signal reconstruction on phantoms are given.

Post-processing noise removal algorithm for magnetic resonance imaging based on edge detection and wavelet analysis

A post-processing noise suppression technique for biomedical MRI images is presented. The described procedure recovers both sharp edges and smooth surfaces from a given noisy MRI image; it does not blur the edges and does not introduce spikes or other artefacts. The fine details of the image are also preserved. The proposed algorithm first extracts the edges from the original image and then performs noise reduction by using a wavelet de-noise method. After the application of the wavelet method, the edges are restored to the filtered image. The result is the original image with less noise, fine detail and sharp edges. Edge extraction is performed by using an algorithm based on Sobel operators. The wavelet de-noise method is based on the calculation of the correlation factor between wavelet coefficients belonging to different scales. The algorithm was tested on several MRI images and, as an example of its application, we report the results obtained from a spin echo (multi echo) MRI image of a human wrist collected with a low field experimental scanner (the signal-to-noise ratio, SNR, of the experimental image was 12). Other filtering operations have been performed after the addition of white noise on both channels of the experimental image, before the magnitude calculation. The results at SNR = 7, SNR = 5 and SNR = 3 are also reported. For SNR values between 5 and 12, the improvement in SNR was substantial and the fine details were preserved, the edges were not blurred and no spikes or other artefacts were evident, demonstrating the good performances of our method. At very low SNR (SNR = 3) our result is worse than that obtained by a simpler filtering procedure.

Limits of deconvolution in enhancing the resolution in EPR imaging experiments

The limits in resolution enhancement, obtained by deconvoluting experimental spectra with the zero gradient line in the frame of electron paramagnetic resonance imaging (EPRI), are discussed. The enhancement is evaluated for different values of the signal-to-noise ratio and different line widths of the spectra. Deconvolution requires the use of a low-pass filter in the Fourier domain. The cut-off frequency of this filter can be taken at the intersection between the signal and noise power spectra. This choice gives an automatic criterion to select the filter bandwidth if deconvolution is performed on a large data sequence. The effect of deconvolution is described by a parameter r that gives an indication of the gain achieved in performing deconvolution. It is defined as the ratio between the widths of the natural line shape and the filter response. Its value depends on the signal-to-noise ratio and on the line width of the original spectrum.

New experimental apparatus for multimodal resonance imaging: initial EPRI and NMRI experimental results

Electron paramagnetic resonance imaging (EPRI) is a recently developed imaging technique employed in the study of free radicals in living systems. A full understanding of many physiological and pathological processes involving free radicals has not yet been attempted. The reason for this is that whilst nuclear magnetic resonance imaging (NMRI) is able to generate very accurate images of soft tissues and organs, EPRI does not have this capability because of its sensitivity limitations and the large linewidths of paramagnetic probes. This work describes the development and optimization of a multimodal apparatus capable of performing both pulsed EPRI and NMRI experiments on the same sample. The instrument combines the possibilities offered by both techniques: the functional and biochemical information achieved with EPRI, and the high-resolution anatomical images generated by NMRI. At present, these experiments are performed by moving the sample from an EPRI spectrometer to an NMRI apparatus. Consequently, the acquisition times are very long and several problems arise in image reconstruction. On the other hand, a unique apparatus operating in the two modalities greatly reduces the acquisition times and makes it possible to relate accurately the observed distribution of electron spin density with the anatomical description of individual organs. The experiments are performed at 357 Gauss, corresponding to a resonance frequency of 1.52 MHz for NMR and 1 GHz for EPR. In the present work, a detailed description of the apparatus is reported, including the main magnet, the gradient assembly, the multimodal cavity and the transmitter and receiver systems. The preliminary experimental results obtained by this apparatus are presented.