Application of workplace correction factors to dosemeter results for the assessment of personal doses at nuclear facilities.

Ratios of H(p)(10) and H*(10) were determined with reference instruments in a number of workplace fields within the nuclear industry and used to derive workplace-specific correction factors. When commercial survey meter results together with these factors were applied to the results of the locally used personal dosemeters their results improved and became within 0.7 and 1.7 of the reference values or better depending on the response of the survey meter. A similar result was obtained when a correction was determined with a prototype reference instrument for H(p)(10) after adjustment of its response. Commercially available survey instruments both for photon and neutron H*(10) measurements agreed with the reference instruments in most cases to within 0.5-1.5. Those conclusions are derived from results reported within the EC supported EVIDOS contract.

Summary of personal neutron dosemeter results obtained within the EVIDOS project.

Within the EC project EVIDOS ('Evaluation of Individual Dosimetry in Mixed Neutron and Photon Radiation Fields'), different types of active neutron personal dosemeters (and some passive ones) were tested in workplace fields at nuclear installations in Europe. The results of the measurements which have been performed up to now are summarised and compared to our currently best estimates of the personal dose equivalent Hp(10). Under- and over-readings by more than a factor of two for the same dosemeter in different workplace fields indicate that in most cases the use of field-specific correction factors is required.

Distributed current analyses of bi-hemispheric magnetic N1m responses to ipsi/contralateral monaural stimuli from a single subject.

Magnetoencephalographic (MEG) responses of both auditory cortices to simple auditory stimuli presented monaurally to either ear were recorded from a single subject. A distributed current model and a current dipole model were used to analyse the responses at the latency of the dominant N1m complex. At the N1m the current density was localised to a single area and was consequently well modelled by a single current dipole close to the peak current density. In the left hemisphere, the contralateral response (as identified by the peak current density) preceded the ipsilateral response by 3 msec. This value was 7 msec for the right hemisphere. Evidence was found in the right hemisphere of a posterior-anterior movement along the sylvian fissure. Also, the left hemisphere N1m sources were all represented more posterior than the right hemisphere N1m sources.

The dipole density plot (DDP), a technique to show concentrations of dipoles.

To analyse spontaneous multichannel slow-wave MEG activity a tool was developed to extract focal abnormal activity with a higher spatial dipole density across time. The first version works on discrete volume units, the second version avoids a possible location error, works continuously and can be adjusted for individual slices three-dimensionally. The first version is used for screening and the second version is used to go into more detail. The dipole density plot (DDP) is not limited to single foci, nor to the single-dipole model; neither is it limited to the MEG. The DDP seems to be a valuable tool for detecting and locating lesions three-dimensionally within the brain.

Estimates of brain activity using magnetic field tomography in a GO/NOGO avoidance paradigm.

This paper presents the first estimates of three dimensional evolution of activity in the brain associated with a GO/NOGO avoidance (CNV) paradigm. These estimates are continuous probabilistic solutions (Ioannides et al. 1990) to the biomagnetic inverse problem, obtained from averaged multichannel magnetoencephalographic (MEG) recordings (Vieth et al. 1991). The emphasis here is placed on the comparison of the activity associated with the GO and NOGO conditions; estimates of activity are shown for the onset of warning stimulus (S1), the early response half a second after S1, the late response lasting for over one second before S2 (the time between S1 and S2 is 3.5 seconds) and the onset of the imperative stimulus (S2). We find responses in regions of the brain implicated with hearing the stimulus, task engagement and motor output. Differences in the images corresponding to GO and NOGO conditions are significant because they reflect differences in brain function when a motor response is required or must be inhibited.

Functional 3D localization of cerebrovascular accidents by magnetoencephalography (MEG).

Spontaneous magnetic slow wave brain activity can be used to locate the underlying sources with sufficient accuracy by using the single current dipole model. To locate focal sources from spontaneous activity a tool had to be developed to extract focal densities of dipoles across time-the dipole density plot. The first version works on discrete volume units and is used for screening. The second version avoids a possible localization error and works continuously and this even is done on individual slices. The DDP seems to be a valuable tool for extracting and separating different focal sources from the background activity. Not only brain infarctions and haemorrhages (and cysts and angiomas) could be located, but also functional sources associated with TIAs even one week after the symptoms. First results let us assume that clinically silent TIAs also (in analogy to clinically silent brain infarctions) could be detected and located.