Introduction
Three-dimensional Magnetic Resonance Imaging (MRI) can achieve microscopic resolution to monitor cellular dynamical processes. In order to discriminate implanted cells from host tissue, they must be labelled. Iron oxide nanoparticles (USPIO) function as such label and generate pronounced contrast in T2*-weighted images. This approach is now well established1, 2. However, intrinsic T2 and T2* effects caused by paramagnetic deoxyhemoglobin in red blood cells of blood vessels may complicate the data analysis. Changes in the deoxyhemoglobin level were the basis of MRI studies that have addressed the effect of blood oxygenation level-dependent (BOLD) contrast 3 . We have utilized the BOLD effect for the suppression of intrinsic T2 and T2* effects and improved stem cell detectability by modification of the inhalation gas in an animal model.
Methods
Experiments were performed in accordance with NIH animal protection guidelines and approved by governmental authorities. Male Wistar rats (n=10, 250–550 g) were anesthetized with 1% halothane. Three animals were part of a study were USPIO labeled embryonic stem cells were implanted in a stroke model (see 2 ). The gas mixture for anesthesia was varied between: (A) 30–35% oxygen (O2) and 65–70% N2O and (B) 95% O2 and 5% CO2. The gas mixture was changed from mixture (A) to (B) after completion of the MRI experiments with mixture (A), without repositioning the animal. MRI experiments were performed on an experimental animal scanner at 4.7 Tesla or 7 Tesla (Bruker BioSpec) equipped with actively shielded gradient sets 200 mT m−1. For rf irradiation and signal detection custom-built coils were used. A 12-cm-diameter Helmholtz coil arrangement served for rf excitation, whereas signal detection was achieved with a 2.3 cm (4.7 T)/ 3.0 cm (7 T) surface coil.
Results
Rats tolerated the 4 hours procedure very well. Two animals were imaged multiple times (up to four times). Images with gas mixture B (95% O2, 5% CO2) resulted in substantial reduction of intrinsic T2 and T2* effects by blood vessels compared to the usually used gas mixture A (30–35% O2, 65–70% N2O). Small blood vessels were not visible and major blood vessels were reduced in size by up to 80%. The hypointense signal of USPIO labelled embryonic stem cells was not affected.
Conclusion
The increased amount of oxygen in the anesthesia gas and the resulting BOLD effect enables a clear distinction between hypointensity effects caused by labelled cells and intrinsic hypointensity caused by paramagnetic oxyhemoglobin in blood vessels. This improves the unambiguous monitoring of cell migration substantially. (See Figure 1).
Suppression of intrinsic T2* effects by modification of the anesthesia gas.