Photoacoustic staging of nodal metastases using SPIOs: Comparison between in vivo, in toto and ex vivo imaging in a rat model

Background and objectives: To determine prognosis and treatment, accurate nodal staging is essential in many tumor types. After injection of clinical grade superparamagnetic iron oxide (SPIO) nanoparticles, it has been shown that metastatic lymph nodes can be distinguished from benign specimens using MR imaging. However, MR does not benefit per-operative nodal staging which requires a non-ionizing, small volume, high resolution, fast imaging technique. In vivo non-invasive photoacoustic (PA) imaging of lymph nodes might facilitate nodal staging during surgery, thereby benefiting both surgeon and patient. Materials and methods: In order to investigate the feasibility of an in vivo nodal staging approach using photoacoustics, six Mat-lylu inoculated Copenhagen rats were photo-acoustically imaged after injection of a new Class IIa medical device SPIO magnetic tracer (Sienna+). Lymph nodes were imaged in vivo, in toto (after euthanization) and ex vivo using multiple wavelength illumination. Results were compared with MRI, immunohistochemistry and photographs of the sectioned nodes. Results: These experiments demonstrate that in an ex vivo setting, the PA contrast of Sienna+ is able to facilitate a distinction between metastatic and benign nodes. A non-invasive distinction between both groups is partially impeded by the low amount of PA contrast generated by the SPIO particles compared to that of endogenous absorbers such as hemoglobins. Conclusions: This comparison between in vivo, in toto and ex vivo PA imaging of lymph nodes after SPIO injection demonstrates that the clinical potential of combined PA/SPIO staging should initially be exploited in an ex vivo setting. Improved distinction between chromophores by for example multi-spectral unmixing might in the near future enable non-invasive assessment of nodal involvement.