Magnetic Resonance Imaging (MRI) is a non-invasive imaging technique to visualize different tissues and anatomical structures in vivo. MRI is not dependent on ionizing irradiation, in contrast to Positron Emission Tomography (PET) or Computer Tomography (CT) imaging. Used are instead of ionizing irradiation, magnetic properties of atomic nuclei. Concentrations of metabolites can be obtained using Magnetic Resonance Spectroscopy (MRS). Comparing the relative metabolite concentrations, conclusions from the properties of pathological tissue can be drawn.
The 7 tesla small-animal-magnetic-resonance-tomograph BioSpec 70/30 (Bruker BioSpin MRI GmbH, Ettlingen, Germany) operates since 2005 at the PET-Center (Figure 1).
1. Cancer research
In this area the tomograph is applied to obtain morphological images with high special resolution of the xenotransplanted tumors. These images can be overlayed by means of image fusion with functional PET data, see Figure 2. Specific information about the metabolism of a tumor are obtained by MRS. As an example; figure 3 shows two different spectra of one tumor before and after irradiation. Interesting is the peak at 3.2 ppm; he shows the signal of the molecule choline, an indicator of strong cell proliferation. The peak is not visible after irradiation, this leads to the conclusion, that the applied radiation dose caused a strong reduction in cell proliferation.
2. Metabolism of adipose tissue
The differentiation of different tissues is possible via MRI through the appropriate selection of the imaging sequence. This difference in contrast is very useful to differentiate intra-abdominal and subcutaneous adipose tissue from other tissues (Figure 4). Accumulation of excess intra-abdominal adipose tissue has been shown to play a crucial role in the development of cardiovascular, metabolic, and renal disorders including insulin-resistance, diabetes mellitus, hyperlipidemia, atherosclerosis, hypertension, and chronic renal disease, many of which are interdependent. The high resolution spectra obtained at 7 T allow identification of at least 9 different proton resonances specific for lipids, and, thus, for calculation of mono- to polyunsaturated fatty acid ratio in vivo (Figure 5).
In collaboration with the OncoRay are experiments in progress to explore new strategies for cell labeling. Figure 6 shows for instance, that marked cells (marked with Gadolinium or Manganese) alter the contrast in their surrounding.
Figure 1: Small animal magnetic resonance tomograph BioSpec 70/30
Figure 2: PET image (upper row), MR image (middle row), and the fusion of the images.
Figure 3: Proton spectra of a tumor before (black curve) and after (red curve) irradiation.
Figure 4: Deposits of adipose tissue (orange) of a mouse.
Figure 5: MR spectra of intra abdominal adipose tissue of a mouse.
Figure 6: MR image of cells marked with gadolinium (upper row, right), manganese (lower row, left), and unmarked cells.