Integration of a model-independent interface for RBE predictions in a treatment planning system for active particle beam scanning.

Especially for heavier ions such as carbon ions, treatment planning systems (TPSs) for ion radiotherapy depend on models predicting the relative biological effectiveness (RBE) of the particles involved. Such models are subject to intensive research and the choice of the optimal RBE model is a matter of debate. On the other hand TPSs are often strongly coupled to particular RBE models and transition even to extended models of the same family can be difficult. We present here a model-independent interface which allows the unbiased use of any RBE model capable of providing dose-effect curves (even sampled curves) for a TPS. The full decoupling between the RBE model and TPS is based on the beam-mixing model proposed by Lam which is, in contrast to the often-used Zaider-Rossi model, independent of the explicit form of the underlying dose-effect curves. This approach not only supports the refinement of RBE models without adaptations of the TPS--which we demonstrate by means of the local effect model (LEM)--but also allows the comparison of very different model approaches on a common basis. We exemplify this by a comparison between the LEM and a model from the literature for proton RBE prediction.

Scalp position and efficacy of transcranial magnetic stimulation.

OBJECTIVE: To assess the impact of the scalp site on the biological effects of TMS. METHODS: We performed high-resolution, three-dimensional whole head magnetic resonance imaging (MRI) in a healthy subject, systematically measured the scalp-to-cortex distance across the head and calculated the resulting electric field in the superficial cortex. RESULTS: The variability in scalp-to-cortex distance led to differences in calculated cortical electric field strengths of a factor of two. A major portion of this variability was explained by a lateral to medial gradient with scalp-to-cortex distances being greatest close to the midline and smallest towards the temporal coordinates. CONCLUSIONS: Because of the medio-lateral gradient in scalp-to-cortex distance interventions tailored on the basis of effects of TMS in the motor system will systematically induce stronger than expected electric currents when performed laterally to the motor spot. SIGNIFICANCE: The biological effects of TMS outside the motor spot may be markedly different from those observed in the motor system and this should be taken into account to optimize TMS for the evaluation or treatment of neuropsychiatric disorders.

Dehydration confounds the assessment of brain atrophy.

Computerized brain volumetry has potential value for diagnosis and the follow-up evaluation of degenerative disorders. A potential pitfall of this method is the extent of physiologic variations in brain volume. The authors show that dehydration and rehydration can significantly change brain volume: lack of fluid intake for 16 hours decreased brain volume by 0.55% (SD, +/-0.69), and after rehydration total cerebral volume increased by 0.72% (SD, +/-0.21).

Localization of primary auditory cortex in humans by magnetoencephalography.

Brief auditory stimuli activate the primary auditory cortex (PAC) earlier than any other cortical area so, within a certain latency range, the PAC is the only cortical source contributing to the auditory evoked field (AEF). Nevertheless, there is no AEF component specific to PAC that can be reliably detected in all individuals. The present study suggests that a peak in the first temporal derivative of the magnetic field at about 20 ms (dP20m) is a genuine correlate of PAC activity. AEFs in response to clicks presented to the right ear were recorded with a 37-channel axial gradiometer system positioned over the left hemisphere in nine normal-hearing subjects. More than 8500 stimuli were presented in each of two independent sessions at a rate of approximately 3/s. The dipole coordinates for the dP20m derived from the two sessions typically differed by only a few millimeters. Coregistration of the dipoles with structural magnetic resonance images suggests that dP20m arises from an area close to the retroinsular origin of Heschl's gyrus. Although the dP20m is simply the point of steepest slope on the well-known middle-latency peak, P30m or Pam, it would appear that dP20m and P30m do not have the same cortical origin. Evidence is provided that P30m receives major contributions from at least two distinct cortical areas, only one of which is PAC.

Three-dimensional reconstruction of the auditory cortical areas from magnetic resonance images.

The future of neuromagnetic research will be highly dependent on the development of analysis procedures utilizing morphological information derived from magnetic resonance (MR) images. However, constraining the biomagnetic inverse problem by using such information may lead to serious misinterpretations if the reconstruction algorithm for the cortical surface overlooks boundaries between grey matter and cerebrospinal fluid (CSF) or artificially generates them. The purpose of this study was to check as to what extent an advanced automatic three-dimensional reconstruction procedure is able to segment the cortical structures located hidden in the Sylvian fissure (especially Heschl's gyrus and planum temporale). The procedure consisted of four processes: a coarse segmentation, a refined segmentation of the white matter, a skeletonization of the sulci and a segmentation of the cortical surface by concurrent region growing for brain and CSF. The reconstruction result for single slices basically agrees with the impression obtained upon visual inspection of the original MR data. Photorealistic visualizations, showing a good qualitative agreement with anatomical images, suggest that the reconstructed surfaces are realistic and detailed enough to be applicable in source analyses of auditory evoked fields.

High-precision neuromagnetic study of the functional organization of the human auditory cortex.

Previous studies have proven that a dipole source analysis of the auditory evoked field is capable of providing evidence of the tonotopic organization of the human auditory cortex. To explore the nature of the estimated dipoles in greater detail, a single subject was extensively studied, and the estimated sources were registered in a three-dimensional reconstruction of the cortical surface derived from magnetic resonance images. The stimuli were 500-ms tone bursts with frequencies of 250, 500, 1,000, and 2,000 Hz (mean intensity of 60 dB SL). The total number of stimuli presented per condition was about 3,600 (36 independent experiments spread over 4 days). Using special postprocessing techniques, the relative localization accuracy could be enhanced to such an extent that differences in the dipole locations of 1 mm could be clearly distinguished. The results suggest that peak N1m (latency around 100 ms) arises from the planum temporale, whereas peak P2m (latency around 170 ms) appears to correspond to a center of activity in (or close to) Heschl's gyrus. The tonotopic organization found for the generator of N1m was consistent with earlier studies ("the higher the frequency the deeper the source"). However, additional findings (time dependence of the estimated sources; slightly different tonotopy obtained for field change; dependence of the estimated sources on the estimation technique) indicate that multiple areas are involved in the generation of N1m. Evidence of a frequency-dependent source location was found also for P2m.

Neuromagnetic source analysis using magnetic resonance images for the construction of source and volume conductor model.

Sources of the somatosensory evoked fields (SEF) for one subject were estimated using constraints from the magnetic resonance images (MRI) of the same subject. A realistic volume conductor model was shaped corresponding to the inside of the skull. Sources were restricted to a dipole patch riding on the surface of the cortex, reconstructed from the individual MRI. Such a patch can be considered as a uniformly activated cortical area giving rise to distributed currents which flow perpendicular to the cortical surface. Source locations obtained for the SEF in response to separate stimulations of lower lip, first and fifth digit, and collarbone followed the course of the contralateral central sulcus. The order of the estimated source locations was in agreement with the somatosensory homunculus of Penfield and Rasmussen. Similar results were obtained with the simple model of a current dipole in a homogeneous sphere. In contrast, combining a current dipole model with a realistic volume conductor model was rather problematic as it overestimates the radial dipole component by an order of magnitude.