René Debuyst

The second international intercomparison on EPR tooth dosimetry

Eighteen international EPR laboratories participated in the second intercomparison programme. Each participant had to prepare enamel samples and evaluate the absorbed dose from molars that were irradiated in vitro in the range 0-1000 mGy. The objective of the programme was to bring together all methods which are currently applied by different laboratories for EPR dose reconstruction and to demonstrate the present state of dosimetry. An overview of the essential features of the different methods is presented. The current accuracy of EPR tooth enamel dosimetry under defined conditions of irradiation is evaluated

Spin labelled arabinogalactan as MRI contrast agent

In this study, we report the synthesis and the evaluation as MRI contrast agent of arabinogalactan/pyrrolidinoxyl radicals (PCA) covalent adduct (SLAG:Spin Labelled ArabinoGalactan). Arabinogalactan was used as targeting device, as it is recognized by the asialoglycoprotein receptor specific to the hepatocytes. The higher relaxivity R1 in water of SLAG, compared with small hydrophilic nitroxyl radicals, was explained by the molecular dynamics study using EPR spectroscopy that showed some immobilization of the radical into the polysaccharide. A binding study on isolated hepatocytes revealed that SLAG still recognizes the asialoglycoprotein receptor. MR imaging was performed using spin-echo T1 weighted images on mice to compare the contrast effect obtained with SLAG and PCA after IV injection (1 mmol/kg free radical). The percent signal enhancement observed in the liver 5 min after IV injection was 40 +/- 3% and 13 +/- 5% for SLAG and PCA, respectively. The signal was also dramatically increased in the renal cortex. This latter effect as well as the prolonged duration of the contrast (+/- 3 h), indicates at least a partial nonselective biodistribution; the high concentration needed to obtain a contrast effect could account for the saturation of the asialoglycoprotein receptor and hence for the apparent nonselective biodistribution.

Evaluation of nonionic nitroxyl lipids as potential organ-specific contrast agents for magnetic resonance imaging

Considering their intrinsic properties of accumulation in the hepatic tissue, we have synthesized nitroxyl-containing lipids as potential organ-specific contrast agents for magnetic resonance imaging (MRI). Their resistance to reduction by ascorbate and in liver homogenates, and their relaxivity in different media were investigated and compared to those of free carboxyl-Proxyl (3-carboxy-2,2,5,5-tetramethylpyrrolidine-1-oxyl) and Tempamaine (4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl). With respect to the reduction rates by ascorbate, the lipid derivatives show the same well-known order of reactivity as carboxy-Proxyl and Tempamine, the five-membered nitroxyls being more stable than the six-membered compounds. However the binding of the piperidinoxyl compounds to the fatty acids confers to those lipid derivatives a markedly increased stability. Similarly, in liver homogenates, the nitroxyl lipids remained unchanged more than 20 min, contrarily to carboxy-Proxyl and Tempamine. The measurements of spin-lattice relaxation time (T1) in biological media have demonstrated a higher relaxivity of nitroxyl lipids, which can be related to their interaction with proteins. Tested in vivo, one of the synthesized compounds (0.75 mmol/kg) produced an enhancement of 44 +/- 12% of the hepatic signal 5 min after intraportal injection in T1-weighted images. The potential applicability of the other nitroxyl lipids as contrast agents for MRI was limited in the in vivo studies by an unexpected toxicity. Work is currently in progress to improve the therapeutic index of the present class of nitroxyl lipids.

An EPR study of γ- and α-irradiated synthetic powdered calcite labelled with 13C

Abstract Samples of 13 C enriched (99%) calcite were exposed to γ ray and α particle irradiations. Due to their g and hyperfine coupling values, the EPR signals could be attributed to four CO - 2 radicals located in different environments, two CO - 3 radicals and two non-carbon centred species (at g = 2.0052 and 2.0036). The isotropic dating signal at g = 2.0007 is attributed to a rapidly tumbling CO - 2 radical. An axial CO - 2 radical is selectively produced by α particles.

An in vitro L-band EPR study with whole human teeth in a surface coil resonator

L-band EPR measurements were done in vitro on extracted human teeth with the objective to evaluate the feasibility of retrospective in vivo EPR dosimetry. In a recent study, the relative contributions of individual tooth components (enamel, crown dentin and root) to the total response of a whole tooth inside an L-band surface coil resonator was investigated. In the present work, the gamma-dose response curves were evaluated for different EPR signal evaluation methods, using 35 whole teeth with absorbed doses in the range 1-100 Gy. The paper reports on the first attempt to deconvolute the single composite L-band EPR line in components due to CO2- and native radicals. The L-band EPR spectrum of teeth could be approximated by combining powder-simulated spectra of orthorhombic and quasi-axial signals of the CO2- radical and an isotropic signal of the native radicals. Among the applied EPR signal evaluation methods, the evaluation by spectrum deconvolution. revealed the lowest detection limit for absorbed dose. A detection limit of about 0.5 Gy was estimated for the currently available L-band equipment

ESR dating: A study of 210Po-coated geological and synthetic samples

Abstract Several synthetic (calcite, calcite doped with humic acids, carbonate apatite) and natural (tooth, mollusc, speleothem) samples are irradiated with a pure alpha emitter, 210 Po, adsorbed on the sample grains surface. The ESR spectra of these “ external ” alpha irradiated samples are compared with those of gamma irradiated ones. Both radiations globally induce the same ESR signals. On the contrary, radiolytical effects in pure synthetic calcite of “ external ” and “ internal ” alpha irradiations appear to be different. “ External ” alpha efficiency factors (ESR k -values) are estimated for some ESR lines and range from 0.08 to 0.3.

Location and Motion of Isotropic Paramagnetic Centres in Synthetic Monohydrocalcite

The isotropic CO-2 and CO-3 centres in CaCO3H2O were examined with electron spin resonance (ESR), electron-nuclear double resonance (ENDOR) and thermogravimetric analysis (TGA). From the absence of anisotropy and the small value of the 1H ENDOR splitting ( ~0.3 MHz), the effect of crushing and/or thermal annealing on the ESR and TGA spectra, it is concluded that both isotropic centres are located in the occluded water, surrounding the constituent crystallites of the spherulites. ESR line width and 13C hyperfine coupling parameter measurements yield approximate values for the rotation activation energy (ΔE( CO3-<ΔE( CO2-) with ΔE~ 0.2 eV) and for the vibrational frequency ( ~2.1012 Hz for CO2-).

A critical discussion of the 2nd intercomparison on electron paramagnetic resonance dosimetry with tooth enamel

Recently, we have participated in The 2nd International Intercomparison on EPR Tooth Dosimetry wherein 18 laboratories had to evaluate low-radiation doses (100-1000 mGy) in intact teeth (Wieser et al., Radiat. Meas., 32 (2000a) 549). The results of this international intercomparison seem to indicate a promising picture of EPR tooth dosimetry. In this paper, the two Belgian EPR participants present a more detailed and critical study of their contribution to this intercomparison. The methods used were maximum likelihood common factor analysis (MLCFA) and spectrum subtraction. Special attention is paid to potential problems with sample preparation, intrinsic dose evaluation, linearity of the dose response, and determination of dose uncertainties

Retrospective Dosimetry after Criticality Accidents Using Low-Frequency EPR: A Study of Whole Human Teeth Irradiated in a Mixed Neutron and Gamma-Radiation Field

Abstract Zdravkova, M., Crokart, N., Trompier, F., Asselineau, B., Gallez, B., Gaillard-Lecanu, E. and Debuyst, R. Retrospective Dosimetry after Criticality Accidents Using Low-Frequency EPR: A Study of Whole Human Teeth Irradiated in a Mixed Neutron and Gamma-Radiation Field. Radiat. Res. 160, 168–173 (2003). In the context of accidental or intentional radiation exposures (nuclear terrorism), it is essential to separate rapidly those individuals with substantial exposures from those with exposures that do not constitute an immediate threat to health. Low-frequency electron paramagnetic resonance (EPR) spectroscopy provides the potential advantage of making accurate and sensitive measurements of absorbed radiation dose in teeth without removing the teeth from the potential victims. Up to now, most studies focused on the dose–response curves obtained for γ radiation. In radiation accidents, however, the contribution of neutrons to the total radiation dose should not be neglected. To determine how neutrons contribute to the apparent dose estimated by EPR dosimetry, extracted whole human teeth were irradiated at the SILENE reactor in a mixed neutron and γ-radiation field simulating criticality accidents. The teeth were irradiated in free air as well as in a paraffin head phantom. Lead screens were also used to eliminate to a large extent the contribution of the γ radiation to the dose received by the teeth. The EPR signals, obtained with a low-frequency (1.2 GHz) spectrometer, were compared to dosimetry measurements at the same location. The contribution of neutrons to the EPR dosimetric signal was negligible in the range of 0 to 10 Gy and was rather small (neutron/γ-ray sensitivity in the range 0–0.2) at higher doses. This indicates that the method essentially provides information on the dose received from the γ-ray component of the radiation.

ESR dating: A study of humic acids incorporated in synthetic calcite

Calcite doped with humic acids (HA) exhibits an ESR signal at g = 2.0043, with a line width (ΔHpp) of about 0.6 mT, that appears to be very radiosensitive. It seems that one may conclude that this line, due to humic acids, contributes at least partly to the line at 2.003 ± 0.001 observed in natural samples. Moreover, upon γ-irradiation, ESR lines at g=2.0052 (ΔHpp∽ 0.06 mT) and at g=2.0035 (ΔHpp ∽ 0.05 mT) which are also present in γ-irradiated pure calcite, are amplified by the presence of humic acids. The line at g = 2.0052 observed in γ-irradiated HA-doped calcite might well belong to the same paramagnetic center as the line at 2.0057 observed in natural samples. In that case, the last one should not be assigned to a humic acid-clay complex.