Dual isotope (In-111/Tc-99 m) SPECT: noise reduction with an analytic attenuation correction method

Noise and artefacts may reduce lesion detectability in dual isotope (Tc-99 m/In-111) SPECT studies. The effects of attenuation can lead to artefacts near high activity structures, e.g., the liver, spleen, kidneys, or urinary bladder. Without effective attenuation correction, these artefacts may reduce diagnostic utility. This study compares three processing methods: (1) zero order Chang's attenuation correction (ZOCAC) with 3D Butterworth filtering, (2) quasi-optimal attenuation correction (QOAC) with 3D Butterworth filtering, and (3) QOAC with a new, adaptive filter. A physical phantom is used to determine effective attenuation coefficients to use with Tc-99 m and with In-111. Computer simulation studies are used to optimize the parameters of the filters and to compare the three processing methods using bias vs. variance plots. The results indicate that QOAC and the adaptive filter both improve the bias-variance tradeoff. Differences between the ZOCAC and the QOAC reconstructions are seen in clinical studies. QOAC noticeably reduces cold spot artefacts near high activity structures, e.g., near the urinary bladder and great vessels in Tc-99 m labeled red blood cell images. QOAC combined with the new, adaptive filter holds promise for improving Tc-99 m/In-111 SPECT reconstructions.