A novel embedding technique for multimodality imaging of xenotransplanted tumor models

Introduction
Multimodality imaging (magnetic resonance imaging and spectroscopy (MRI/MRS), positron emission tomography (PET), computer tomography (CT), and autoradiography) of xenotransplanted or syngene tumors on the legs of mice requires the solving of two main problems:
a) the magnetic field inhomogeneity in the tumor periphery caused by the transition of the magnetic field from tissue to the surrounding air, which makes MRS in the tumor periphery difficult or impossible and
b) reproducible positioning for subsequent histological sectioning of the separated tumor.

The aim of the present study was to find an embedding of tumors to solve these two problems simultaneously.

Materials and Methods
Phantom studies were performed using balloons filled with a water/ethanol mixture. Animal experiments were performed using 7-14-week-old NMRI mice. We used a 7 Tesla magnetic resonance tomograph. Chemical shift imaging (CSI) was performed to detect magnetic field inhomogeneities. For phantom embedding, three different materials were used: i) alginate, ii) gelatin, and iii) a mixture (“dough”) of wheat flour, sodium chloride, and potassium aluminum sulfate dodecahydrate. For tissue/tumor embedding alginate was used. The animals were placed in an animal bed including position markers which are visible in MR, PET, and CT images. Prior to histological sectioning the embedded phantoms and tumors were shock frozen in liquid nitrogen.

Results
The easy handling of alginate was superior to that of gelatin and dough. Gelatin disrupted during freezing and did not yield improvement of magnetic field homogeneity. Dough showed improvement of magnetic field homogeneity, but was not completely frozen and therefore not suitable. Alginate showed improvement of magnetic field homogeneity. Also histological sectioning after freezing succeeded very well. Therefore, we selected alginate for tumor embedding. The embedding with alginate showed a significant improvement of the full width at half maximum (FWHM) of the water peak in the peripheral rim of the tumor in comparison to the FWHM of the spectra without embedding (21+/-8 Hz vs. 54+/-29 Hz).

Discussion and conclusion
Our investigation showed that alginate exhibits the properties needed for multimodality investigations with MRS and histological sectioning simultaneously, namely easy handling, magnetic field homogeneity, and the consistency required for histological sectioning. Freezing immediately after imaging, especially after PET imaging is necessary, to avoid dislocation of radioactivity before and after sectioning.