Accelerated Monte Carlo based simultaneous ^99mTc/¹²³I SPECT reconstruction

In simultaneous dual isotope breast SPECT studies using 123I labelled Z-MIVE and 99mTc-sestamibi, 123I labelled Z-MIVE not only detects the presence of estrogen receptor (ER) but also thought to complement 99mTc-sestamibi in differentiating between benign and malignant breast lesions for patients with breast cancer. The major concern in simultaneous 99mTc/123I SPECT is the significant crosstalk contamination between the different isotopes used. The current study focuses on a method of crosstalk compensation between two isotopes in simultaneous 99mTc/123I SPECT in case of Thallium activated Sodium Iodide (NaI(Tl)) detector (Energy resolution 9.8% at 140 keV) and Cadmium Zinc Telluride (CZT) detector (Energy resolution 5% 140 keV) respectively. Monte Carlo (MC), which is thought to offer the most realistic crosstalk and scatter compensation modelling, in typical implementations, has inherent long calculation times (often several hours or days) associated with it. This makes MC unsuitable for clinical applications. Our group has previously incorporated convolution based forced detection (CFD) into SIMIND Monte Carlo program which have made MC feasible to use in clinical time frames. We have developed an iterative MC-based image reconstruction technique that simulates the photon downscatter from one isotope into the acquisition window of a second isotope. The MC based estimation of scatter contamination contained in projection views is then used to compensate for the photon contamination during iterative reconstruction. We use a modified ordered subset-expectation maximization (OS-EM), named as simultaneous ordered subset-expectation maximization (Sim-OSEM), to perform this step. We have undertaken a number of simulation studies using our modelled breast phantom to verify this approach. Reconstruction using Sim-OSEM showed very promising results in terms of crosstalk and scatter compensation and uniformity of background. In our case images obtained using Sim-OSEM were comparable or even better than the images reconstructed from separately acquired projection data using analytical attenuation based reconstruction. This may be due to better small angle scatter compensation in case of Sim-OSEM as MC based forward projector was used.