Unidirectional blood-to-brain and blood-to-tumor transport rate constants for rubidium 82 were determined using dynamic positron emission tomography in patients with primary or metastatic brain tumors. Regional influx rate constants (K1) and plasma water volume (Vp) were estimated from the time course of blood and brain radioactivity following a bolus injection of tracer. Eight patients were studied before and 24 to 72 hours after treatment using pharmacological doses of dexamethasone, and 6 additional patients with metastatic brain tumors were studied before and within 60 to 90 minutes after 200- to 600-rad whole-brain radiation therapy. Steroid treatment was associated with a 9 to 48% decrease in tumor K1 and a 21% mean decrease in tumor Vp. No consistent changes in K1 or Vp were observed in control brain regions. Tumor K1 and Vp did not increase in patients undergoing whole-brain radiation therapy, all of whom were taking dexamethasone at the time of study. These data suggest that corticosteroids decrease the permeability of tumor capillaries to small hydrophilic molecules (including those of some chemotherapeutic agents) and that steroid pretreatment prevents acute, and potentially dangerous, increases in tumor capillary permeability following cranial irradiation.

Unidirectional blood-to-brain and blood-to-tumour transport rate constants (K1) for 82Rb (half-life 76 s) and plasma water volume per unit mass of brain/tumour tissue (Vp) can be estimated in vivo using dynamic positron emission tomography (PET). The accuracy of these estimates depends upon the accuracy of PET measurements of regional brain/tumour radioactivity and scintillation well detector measurements of whole-blood radioactivity, which, in turn, depend upon the time course of arterial blood radioactivity. A two-compartmental model has been employed to derive estimates for K1, k2 (efflux rate constant) and Vp from 82Rb/PET data. Errors in these parameter estimates have been studied (1) qualitatively using sensitivity function analysis and (2) quantitatively using computer simulations. The effect of adding a third irreversible compartment and its unidirectional rate constant, k3, has also been investigated. The advantages and disadvantages of bolus injection vs continuous infusion protocols are discussed. Precision in estimated parameters from actual patient data is compared to that obtained from computer simulations in part II of this paper.

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Background: Brain-states, which correlate with specific cognitive states, may be monitored with non-invasive techniques such as EEG and MEG that indirectly measure cortical activity. These cortical signatures provide insight into the neuronal activity which has been used to identify abnormal cortical function in numerous neurological and psychiatric conditions. Further, the induction of entrained cortical rhythms via transcranial stimulation is effective in imbuing brain-states correlated with such cortical rhythms.Objective/Hypothesis: Mental states have distinct neural correlates. It may be possible to induce a desired mental state by replicating these neural correlates.Methods: EEG/MEG recording of brainwaves from a “donor” in a particular mental state provide a specific signature. This signature is then inverted, filtered, and applied via transcranial stimulation.Conclusion: We propose that mental states may be transferred between subjects by acquiring an associated cortical signature from a donor which, following processing, may be applied to a recipient via transcranial stimulation.