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Zusammenfassung:
Calcium sensitive MRI contrast agents can only yield quantitative results if the agent concentration in the tissue is known. The agent concentration could be determined by diffusion modeling, if relevant parameters were available. We have established an in vivo MRI based method capable of determining diffusion properties of conventional and calcium sensitive agents. Simulations and experiment demonstrate that the method is applicable both for conventional contrast agents with a fixed relaxivity value, and for calcium sensitive contrast agents. Sprague-Dawley rats (N=19, 220-300g) were used as approved by the local authorities in compliance with guidelines EUVD 86/609/EEC. Physiological signs were monitored throughout the experiment and remained within physiological limits. Surgery was performed after anesthesia (2.0%isoflurane, Forene, urethane (1.5 g/kg, i.p.) and xylocain (locally)). A burr hole was made above the primary sensory cortex (S1) and a guiding cannula was implanted and fixed prior to positioning in a stereotaxic holder and insertion of a glass capillary (tip: OD: 21-30μm, ID: 6-10μm) in the center of S1. The glass capillary was connected to an injection pump via 6.5m long fused silica tubes and the rat positioned in the center of a 7T Bruker Biospec 70/30 scanner (BGA-9S, Helmholtz RF volume transmission coil, 2cm single loop receiving surface coil). Biodistribution and diffusion of the contrast agents were imaged continuously before, during and after a 1.1±0.3µl slow bolus infusion, with a rate of 32±9nl/min by a T1-weighted RARE sequence (TE/TR 9/290ms) and by quantitative mapping of T1 times by a Look-Locker inversion recovery sequence with a single-shot EPI read out (11.5/8000ms; TI1=35ms, TIdelay=250ms, total of 18 LL images). Data analysis consisted of estimation of the Gadolinium concentration from the images and non-linear fitting of the diffusion equation (Figure). The D* values observed for conventional and responsive contrast agents[1-3] in vivo by MRI were in agreement with model predictions for extra-cellular diffusion and previous findings using other measurement techniques[4], the only exception being Dextran10k (Table 1). This compound has been used both as an extra-cellular tracer for diffusion measurements, and as a neuroanatomical connectivity tracer. The MRI method described is based on long observation times (up to 20h) and assessed both extra-cellular diffusion and intra-cellular transport across extended brain regions of Dextran10k. The apparent diffusion coefficient determined for the calcium sensitive contrast agents may be used to determine local tissue concentrations and to design infusion protocols that maintain the agent concentration at a steady-state, hereby enabling quantitative sensing of the local calcium concentration.
