Preclinical imaging characteristics and quantification of Platinum-195m SPECT

In vivo biodistribution imaging of platinum-based compounds may allow better patient selection for treatment with chemo(radio)therapy. Radiolabeling with Platinum-195m (^195mPt) allows SPECT imaging, without altering the chemical structure or biological activity of the compound. We have assessed the feasibility of ^195mPt SPECT imaging in mice, with the aim to determine the image quality and accuracy of quantification for current preclinical imaging equipment.

Enriched (>96%) ¹⁹⁴Pt was irradiated in the High Flux Reactor (HFR) in Petten, The Netherlands (NRG). A 0.05 M HCl ^195mPt-solution with a specific activity of 33 MBq/mg was obtained. Image quality was assessed for the NanoSPECT/CT (Bioscan Inc., Washington DC, USA) and U-SPECT⁺/CT (MILabs BV, Utrecht, the Netherlands) scanners. A radioactivity-filled rod phantom (rod diameter 0.85-1.7 mm) filled with 1 MBq ^195mPt was scanned with different acquisition durations (10-120 min). Four healthy mice were injected intravenously with 3-4 MBq ^195mPt. Mouse images were acquired with the NanoSPECT for 120 min at 0, 2, 4, or 24 h after injection. Organs were delineated to quantify ^195mPt concentrations. Immediately after scanning, the mice were sacrificed, and the platinum concentration was determined in organs using a gamma counter and graphite furnace – atomic absorption spectroscopy (GF-AAS) as reference standards.

A 30-min acquisition of the phantom provided visually adequate image quality for both scanners. The smallest visible rods were 0.95 mm in diameter on the NanoSPECT and 0.85 mm in diameter on the U-SPECT⁺. The image quality in mice was visually adequate. Uptake was seen in the kidneys with excretion to the bladder, and in the liver, blood, and intestine. No uptake was seen in the brain. The Spearman correlation between SPECT and gamma counter was 0.92, between SPECT and GF-AAS it was 0.84, and between GF-AAS and gamma counter it was0.97 (all p < 0.0001).

Preclinical ^195mPt SPECT is feasible with acceptable tracer doses and acquisition times, and provides good image quality and accurate signal quantification.