Dual modality imaging combining positron emissions tomography (PET) and magnetic resonance (MR) imaging can improve diagnosis and disease tracking by providing concurrent, high-resolution anatomical and functional views. MR imaging provides good tissue contrast while avoiding radiation exposure of CT. PET provides functional imaging of targeted tissues, such as tumors, within the MR anatomical context, for non-invasive diagnosis and disease monitoring.
To facilitate concurrent PET and MR imaging in oncology, Abou et al. demonstrate a new technique to load liposomes with simultaneous paramagnetic and radiopharmaceutical probes, and target them to a specific tumor type.
Liposomes are well established as low-toxicity, biodegradable vehicles for pharmaceutical delivery. Covalent attachment of polyethylene glycol (PEG) - or pegylation - ensures a long circulation time. Abou et al. developed liposomes carrying both the common MR T1 contrast agent gadolinium (Gd) and the positron-emitting radionucleotide 89-zirconium (89Zr). The researchers conjugated the liposomes with the peptide octreotide to selectively target neuroendocrine tumors via human somatostatin receptor subtype 2 (SSTr2).
After confirming successful labeling of the liposomes in vitro, the authors performed PET and MR imaging of mice carrying both wild-type and SSTr2-positive tumors. The researchers acquired MR images on a Bruker 4.7 T 40 cm bore scanner equipped with 400 mT/m shielded gradients, using a 36-mm ID quadrature resonator for excitation and detection. The team localized tumors in T2 scout images before taking five, 1-mm-thick T1-weighted axial gradient-echo images across the tumor. Using a deionized water phantom as a signal intensity reference, they quantified tumor contrast using ImageJ software.