Five energy window scatter correction for 111In microSPECT

Objectives In preclinical molecular imaging, large interest exists for absolute quantification, since this would allow for an accurate evaluation of disease progression. Photon scatter is one of the limiting factors for quantitative imaging. This study investigates a scatter correction (SC) method using five energy windows (FEW) for 111In µ-SPECT imaging. 111In can be used for in vivo evaluation of tumor selectivity, and hence the potential therapeutic performance, of a tracer. The monoclonal antibody 14C5, labeled to 111In-DOTA, has shown to be a selective tracer for pancreatic cancer.

Methods Our SC method uses 5 energy windows (EW): a 20% main EW around each photopeak, an 8% scatter EW at both sides of the 171keV photopeak and an 8% scatter EW at the left side of the 245keV photopeak. To evaluate the SC method, measurements were carried out on a U-SPECT-II camera (Milabs). A comparison between reconstructed uncorrected and corrected images was performed for the following parameters: (1) point spread function (PSF) of 1mm Ø cylinders in a resolution phantom; (2) contrast recovery for two 8mm Ø cold spots (air & water) in the small animal NEMA image quality phantom; (3) tumor-to-background ratio of a tumor bearing mouse injected with 111In-DOTA-14C5 .

Results The FEW SC gives rise to a mean improvement of 3.8% of the FWHM of the PSF for the cylinders of the resolution phantom. After SC, contrast improved with 4.3% and 2.8% for respectively the air and water filled cold spot of the NEMA phantom. In the preclinical mouse study, SC leads to a significant enhancement of the tumor-to-background ratio from 6.7 to 9.0.

Conclusions The FEW SC method will offer a significant improvement in the absolute quantification of 111In µ-SPECT images.