Three-dimensional nuclear and optical fusion imaging for sentinel node mapping

Objectives Sentinel node (SN) mapping is now very important to less invasive therapy for various kinds of malignant tumors with early stage. To accurately identify SNs, the combined method using both optical and nuclear agents is useful. Recently, we found a mixture of Tc-99m phytate and indocyanine green (ICG) could be a promising multimodality tracer of optical and nuclear fusion imaging (Otolaryngol Head Neck Surg 2014). In this study, we experimentally investigated the usefulness of 3-dimentional fusion imaging for sentinel node mapping with this unique tracer.

Methods Tc-99m phytate (300 MBq/ml) and ICG (1 mg/ml) was mixed with the ratio of 9:1 and this mixture solution was used as a dual modality tracer. Five to ten microliter of this mixture was injected to footpads of forearms, tails and auricles of balb/c mice. The biodistribution of this mixture was images by a SPECT/CT scanner dedicated for small animal imaging and a near-infrared (NIR) fluorescence imaging system. Obtained NIR fluorescence image data were 3-dimensionally reconstructed using an algorithm incorporating lp (0<p<1) sparsity regularization (Biomed Opt Express 2014). SPECT and reconstructed NIR fluorescence images were superimposed using commercially available software. The visualization of SNs and preciseness of superimposition was evaluated.

Results SPECT could clearly visualize SNs as hot spots and these hot spots corresponded with LNs of CT images. On the contrary, reconstructed optical images successfully depicted SNs only when tracers were injected in footpads and tails. When tracers were injected in auricles, SNs located near the injection site were often obscure and distorted on reconstructed optical images.

Conclusions Our nuclear and optical fusion imaging would be useful for SN mapping when SNs are located away from the injection site. Since identification of SNs under such conditions is often difficult, this method would be promising for clinical application.