gms | German Medical Science

Objective: Sodium (²³Na) plays a crucial role in cellular function. Recently, MRI-based methods became available to measure the ²³Na signal in vivo. Unfortunately, conventional sequences yield the total tissue content only. To distinguish between intra- and extracelluar ²³Na a shift reagent such as Tm[DOTP]5- is needed. Here, we present a method to transiently open the blood-brain barrier (BBB) followed by loading the brain with Tm[DOTP]5- which does not cross the intact BBB.

Method: Male Sprague Dawley rats were anesthetized with thiobutabarbital (15 mg/100 g body weight) and equipped with a tracheal cannula and catheters in the femoral artery (blood pressure monitoring, withdrawal of blood samples) and the left carotid artery for bolus injection of mannitol (25% solution, 1.5 ml over 45 sec) followed by infusion of Tm[DOTP]5- (80 mM solution, 6 ml over 60 min). At the end the blood was flushed and the concentration of Tm measured in brain tissue samples by total reflection X-ray fluorescence analysis (T-XRF) methodology. MRI measurements were performed on a 94/20 Biospec (Bruker) scanner using a Hanning-weighted CSI sequence (parameters: TR, 50 ms; acquisition delay, 0.38 ms; number of scans, 14,340; FID sampling time, 400 ms). Each scan took 10 min. The spatial resolution was 1.7x1.7x1.6 mm³. To allow coregistration with 1H images references were placed on the surface coil. Coregistration of the 1H/²³Na data is exemplified in Figure 1 [Fig. 1]. T2* values were calculated following a mono-exponential linear least square fit to the FID. A Fourier transform was applied to the FIDs and the resonance frequency determined. Chemical shift maps were created 30 and 60 min after start of Tm[DOTP]5- infusion and the chemical shift determined spectroscopically in two regions of interest (ROIs; see Figure 1 [Fig. 1]).

Results: After bolus infusion of normotonic mannitol hardly any Tm[DOTP]5- was detectable and the ²³Na signal did not change. Following opening of the BBB loading with Tm[DOTP]5- resulted in the ipsilateral hemisphere in an estimated extracellular concentration of 2.3+1.8 mM, in a decrease of the T2* signal and of the total ²³Na signal, and in a chemical shift in the spectroscopic analysis (arrows in Figure 2 [Fig. 2]).

Conclusions: Despite high repellent properties of the BBB for Tm[DOTP]5- we successfully accumulated Tm[DOTP]5- in brain tissue high enough to induce a chemical shift. This approach for the first time allows distinction of the intra- and extracellular compartment of the ²³Na signal in the living brain.