Quantitative Simultaneous In-111/Tc-99m Planar Imaging of the Foot

Tc-99m and In-111-white blood cell (WBC) imaging are useful for discriminating osteomyelitis from cellulitis of the foot. Accurate diagnosis from sequential studies is challenging because the images are not registered and the Tc-99m/In-111 ratio is assessed subjectively. We developed a simultaneous dual-radionuclide imaging method in which Tc-99m scans are acquired in perfect spatial registration with In-111 WBC images. A foot phantom was made using five tubes containing Tc/In activity ratios from 0.13 to 3.80. The tubes were positioned at a shallow angle in a 5-cm-deep water layer. A single tube was first imaged in list-mode over 18 hours, providing a wide range of Tc/In activity ratios for optimizing triple-energy-window (TEW) parameters for scatter and crosstalk correction of both nuclides. Twelve Poisson noise realizations were generated from very low-noise opposed images of the foot phantom. We measured the bias of Tc/In ratios in regions-of-interest (ROIs) on each tube, along with the uniformity and precision of ROI counts for both nuclides, with (cor) and without (uncor) TEW and geometric-mean (GM) attenuation compensation. The average uniformity along the tubes improved for Tc from 13% (uncor) to 5.6% (cor), and for In from 12.8% (uncor) to 4.4% (cor). The average absolute bias of the Tc/In ratio over all tubes was 4.5% (cor) vs. 48.4% (uncor), with comparable precision. Simultaneous Tc/In acquisition with TEW+GM corrections provides a useful approach for foot imaging.