Simultaneous ⁹⁹m̳Tc-MDP/¹²³I-MIBG imaging of neuroblastoma using SPECT-CT

Abstract

Simultaneous ⁹⁹m̳̳Tc-MDP/ ¹²³I-MIBG SPECT has the potential to replace current clinical sequential acquisitions of ⁹⁹m̳Tc-MDP and ¹²³-MIBG SPECT studies, and therefore has great potential to reduce imaging time, sedation time, and injection dose on patients with neuroblastoma. Simultaneous ⁹⁹m̳Tc/¹²³ imaging is challenging because of the crosstalk between the ⁹⁹m̳Tc and ¹²³I photo-peak windows due to down-scatter of ¹²³I photons (159keV) to the ⁹⁹m̳Tc (140keV) photo-peak window and limited energy resolution of the scanner. Additionally, the counts of detected photons are limited because the injection dose as well as scan time are limited for neuroblastoma patients and scan acquisition cannot be performed for at least 24 hours after ¹²³I-MIBG injection. These factors hinder the separation of images of these two radionuclides. An enhanced fast Monte Carlo based joint ordered-subset expectation maximization (MC-JOSEM) reconstruction algorithm has been been developed for simultaneous ⁹⁹m̳Tc/¹²³ imaging by Ouyang, El Fakhri, and Moore (2007). MC-JOSEM incorporates attenuation into a full system matrix to greatly improve image accuracy and include both primary and scattered photons in the reconstruction process to significantly reduce image noise. Separate ⁹⁹m̳Tc-MDP and ¹²³I-MIBG Monte Carlo simulations were performed. For each isotope, noise-free projection data sets were generated first. Lesions were then added to the ⁹⁹m̳Tc and ¹²³I data separately. Mimicked dual-isotope data were then generated by combining the ⁹⁹m̳Tc and ¹²³I data. Images for single-isotope and dual-isotope were reconstructed by using standard clinical single-isotope OSEM and MC-JOSEM, respectively. Channel Hotelling observer (CHO) was used to calculate lesion detectability. On average, the CHO SNR obtained from dual-isotope studies is close to that of single-isotope studies for each radionuclide. (SNR: 3.521 for dual-isotope versus 3.828 for single-isotope). Hence, simultaneous ⁹⁹m̳Tc-MDP/ ¹²³-MIBG has the potential to replace sequential ⁹⁹m̳Tc-MDP and ¹²³I-MIBG for neuroblastoma imaging.