Development of a lead X-ray compensation method in simultaneous Tl-201 SPECT and Tc-99m TCT using a flood source

The authors have developed an uncollimated sheet line source with a fluoroplastic tube for gamma ray transmission CT (TCT). This source filled with Tc-99m was attached to the collimator face on one head of a dual head single photon emission computed tomography (SPECT) system. In simultaneous SPECT/TCT scans using Tl-201 as the SPECT source and Tc-99m as the TCT source, scatter and spilldown contamination in the Tc-99m (141 keV) data and the Tl-201 (75 keV) data can be removed with the triple energy window (TEW) method. However, lead X-ray contamination in the Tl-201 data cannot be removed with the TEW method. The authors have developed an estimation method of the lead X-ray contamination in the Tl-201 data. This method depends on the hypothesis that the lead X-ray is proportional to the amount of attenuating media and primary photons along each projection path. The amount of attenuating media is given by a transmission image and that of primary photons is estimated by the TEW method. The coefficients needed for this estimation are experimentally determined by only Tc-99m TCT scan data The feasibility of the proposed method was demonstrated by a chest phantom study and a myocardial clinical study.