(68)Ga-labeled superparamagnetic iron oxide nanoparticles (SPIONs) for multi-modality PET/MR/Cherenkov luminescence imaging of sentinel lymph nodes

Abstract

The aim of this study was to develop (68)Ga-SPIONs for use as a single contrast agent for dynamic, quantitative and high resolution PET/MR imaging of Sentinel Lymph Node (SLN). In addition (68)Ga enables Cherenkov light emission which can be used for optical guidance during resection of SLN. SPIONs were labeled with (68)Ga in ammonium acetate buffer, pH 5.5. The labeling yield and stability in human serum were determined using instant thin layer chromatography. An amount of 0.07-0.1 mL (~5-10 MBq, 0.13 mg Fe) of (68)Ga-SPIONs was subcutaneously injected in the hind paw of rats. The animals were imaged at 0-3 h and 25 h post injection with PET/CT, 9.4 T MR and CCDbased Cherenkov optical systems. A biodistribution study was performed by dissecting and measuring the radioactivity in lymph nodes, kidneys, spleen, liver and the injection site. The labeling yield was 97.3 ± 0.05% after 15 min and the (68)Ga-SPIONs were stable in human serum. PET, MR and Cherenkov luminescence imaging clearly visualized the SLN. Biodistribution confirmed a high uptake of the (68)Ga-SPIONs within the SLN. We conclude that generator produced (68)Ga can be labeled to SPIONs. Subcutaneously injected (68)Ga-SPIONs can enhance the identification of the SLNs by combining sensitive PET and high resolution MR imaging. Clinically, hybrid PET/MR cameras are already in use and (68)Ga-SPIONs have a great potential as a single-dose, tri-modality agent for diagnostic imaging and potential Cherenkov luminescent guided resection of SLN.

Keywords: 68Ga; Cherenkov imaging; PET/MR imaging; lymphatics; sentinel lymph node (SLN); superparamagnetic iron oxide nanoparticle (SPION).

Figure 1

Radiolabeling of the ⁶⁸Ga-SPIONs. A: The Radiolabeling yield in ammonium acetate buffer (pH 5.5) exceeded 95% (n=3) after 10 minutes of incubation time. B: The radiolabeling yield is shown to be optimal when the labeling of ⁶⁸Ga-SPIONs is performed at pH 3-7 (n=3).

Figure 2

PET/CT image of one representative animal. A: Fused whole-body image of the rat 3 h after subcutaneous injection with ⁶⁸Ga-SPIONs. The arrows show accumulation of ⁶⁸Ga-SPIONs in the SLN (popliteal) and iliac node. B: The axial image visualizing the SLN in comparison with the contralateral reference node.

Figure 3

MR imaging of rats visualizing the SLN. A: Axial MR image of the same animal as shown in Figure 2. The uptake of the ⁶⁸Ga-SPIONs in SLN after 3 h is evidently seen in the image. B: The SLN are clearly visualized indicating hypointensity due to accumulation of the ⁶⁸Ga-SPIONs, 25 h after injection.

Figure 4

Optical images of ⁶⁸Ga-SPIONs visualizing the SLN 2.5-3 h p.i. A: Cherenkov emission image, 10 min exposure, where the skin was shaved but intact. B: Cherenkov emission image, 2 min exposure, after the skin was removed. C: Representative Cherenkov emission image, 2 min exposure, of a well plate filled with ⁶⁸Ga mixture with 20% Intralipid. The activity levels in the range of 0.08-1.8 MBq for each well are as indicated in the figure. D: The Cherenkov radiance plotted for the different levels of activity as extracted from each well. The linear relation is confirmed with an R2-value of 0.99. White bar in Figure 4A, 4B indicates the length scale 10 mm.

Figure 5

Biodistribution data of ⁶⁸Ga-SPIONs in 4 white Wistar rats 3 h after subcutaneous injection in the right hind paw. Uptake is shown as mean ± SD% IA/g.