Determining the minimum number of detectable cardiac-transplanted 111In-tropolone-labelled bone-marrow-derived mesenchymal stem cells by SPECT

In this work, we determined the minimum number of detectable(111) In-tropolone-labelled bone-marrow-derived stem cells from the maximum activity per cell which did not affect viability, proliferation and differentiation, and the minimum detectable activity (MDA) of In-111 by SPECT. Canine bone marrow mesenchymal cells were isolated, cultured and expanded. A number of samples, each containing 5 x 10(6) cells, were labelled with In-111-tropolone from 0.1 to 18 MBq, and cell viability was measured afterwards for each sample for 2 weeks. To determine the MDA, the anthropomorphic torso phantom (DataSpectrum Corporation, Hillsborough, NC) was used. A point source of 202 kBq In-111 was placed on the surface of the heart compartment, and the phantom and all compartments were then filled with water. Three In-111 SPECT scans (duration: 16, 32 and 64 min; parameters: 128 x 128 matrix with 128 projections over 360 degrees) were acquired every three days until the In-111 radioactivity decayed to undetectable quantities. In-111 SPECT images were reconstructed using OSEM with and without background, scatter or attenuation corrections. Contrast-to-noise ratio (CNR) in the reconstructed image was calculated, and MDA was set equal to the In-111 activity corresponding to a CNR of 4. The cells had 100% viability when incubated with no more than 0.9 MBq of In-111 (80% labelling efficiency), which corresponded to 0.14 Bq per cell. Background correction improved the detection limits for In-111-tropolone-labelled cells. The MDAs for 16, 32 and 64 min scans with background correction were observed to be 1.4 kBq, 700 Bq and 400 Bq, which implies that, in the case where the location of the transplantation is known and fixed, as few as 10 000, 5000 and 2900 cells respectively can be detected.