Partial volume effect (PVE) in positron emission tomography (PET) leads to inaccurate estimation of regional metabolic activities among neighbouring tissues with different tracer concentration. This may be one of the main limiting factors in the utilization of PET in clinical practice. Partial volume correction (PVC) methods have been widely studied to address this issue. MRI based PVC methods are well-established.(1) Their performance depend on the quality of the co-registration of the MR and PET dataset, on the correctness of the estimated point-spread function (PSF) of the PET scanner and largely on the performance of the segmentation method that divide the brain into brain tissue compartments.(1,2) In the present study a method for PVC is suggested, that utilizes cortical surfaces, to obtain detailed anatomical information. The objectives are to improve the performance of PVC, facilitate a study of the relationship between metabolic activity in the cerebral cortex and cortical thicknesses, and to obtain an improved visualization of PET data. The gray matter metabolic activity after performing PVC was recovered by 99.7 - 99.8 %, in relation to the true activity when testing on simple simulated data with different PSFs and by 97.9 - 100 % when testing on simulated brain PET data at different cortical thicknesses. When studying the relationship between metabolic activities and anatomical structures it was shown on simulated brain PET data, that it is important to correct for PVE in order to get the true relationship.

The perennial species Senecio erucifolius L. is rare in the northern parts of The Netherlands. Its very local occurrence can hardly be explained by special environmental conditions. Ripe achenes are not formed until the middle of September. The achenes show no dormancy, but the initial temperature requirements for germination are relatively high. According to germination experiments, germination may take place during the whole winter half of the year, but the seedlings are very susceptible to frost, which reduces establishment from seeds. The apparent absence of seedlings in the field confirms these conclusions.

Within the section Jacobaei (Thunb.) Dumort. of the genus Senecio L. the species S. aquaticus L. is a monocarp while S erucifolius L. is a perennial. The life form of S. jacobaea L. is somewhat intermediate, tending more to monocarpism than to perenniality. As a result of flowering, the carbohydrate reserves in all structures of S. aquaticus gradually fall to very low values when the seed is maturing. Following a slight decline in summer, the carbohydrate reserves of S. erucifolius increase from the onset of flowering until the aerial parts die off in autumn. At this time the highest concentrations are found in the rhizomes and the roots. Though decreasing significantly during the generative phase of development, and even when this stage has passed, the carbohydrate reserves of S. jacobaea remain at a relatively high level. Neither S. aquaticus nor S. jacobaea develop specific structures for the storage of carbohydrates, like the rhizomes of S. erucifolius, which facilitate vegetative reproduction. Still, (adventitious) shoots at the roots and stembases may be formed. The carbohydrate reserves of S. aquaticus are insufficient for these shoots to develop into new individuals. Differences between the three species in the changing patterns of soluble carbohydrate reserves reflect the interspecific diversities of life form.

Seasonal fluctuations of dry matter, water soluble carbohydrates and total nitrogen content were traced in underground storage organs of Cirsium arvense and Tussilago farfara. Samples from experimentally planted monocultures of the two species were analysed. Both species showed identical fluctuations in dry matter and carbohydrate contents. A gradual decrease in autumn and winter, succeeded by a rapid decline in spring, after a distinct minimum in April-May was followed by a steep rise towards a maximum in late summer. Fluctuations seem bo be affected by weather conditions during the growing season. The nitrogen content decreased gradually from early spring until the middle of the summer and then increased similarly until the start of the cycle.