Monte Carlo simulation of the acquisition conditions for ¹⁷⁷Lu molecular imaging of hepatic tumors

Objectives: ¹⁷⁷Lu is used for peptide receptor radionuclide therapy of neuroendocrine tumors. This study aimed to examine the influence of acquisition time on ¹⁷⁷Lu single-photon emission computed tomography (SPECT) images using Monte Carlo simulation.

Methods: Gamma camera simulation of SPECT was performed based on Monte Carlo electron and photon simulation equipped with a medium-energy, general-purpose collimator; 96 projections were generated across 360°¹. The phantom background was modeled on a NEMA IEC BODY phantom; spheres were installed in the phantom with diameters of 10, 13, 17, 22, 28, and 37 mm. The phantom background was assumed to be a healthy liver tissue, whereas the spheres were assumed to be tumors. Radioactivity concentrations of the background and hot sphere were set at 155 kBq/mL and 2.12 MBq/mL, respectively, 24 h after administration according to a previously reported time-activity curve². Concentrations after 6 and 72 h were also set, and ¹⁷⁷Lu SPECT images were simulated for each. Acquisition times were set as 12, 6, 3, 2, 1.5, and 1.2 min in each case. Projection images were pre-filtered using a Butterworth filter (cut-off frequency: 0.50 cycle/cm, order: 8), and filtered images were reconstructed using ordered subset expectation maximization (iteration: 8, subset: 6). Reconstructed images were then corrected using attenuation correction and collimator response correction. To evaluate image quality, contrast-to-noise ratios (CNRs) were calculated for each image. In this study, the threshold of CNR for detectability was assumed to be 5.0.

Results: In images obtained after 6 h, 17-37-mm spheres were detectable across all acquisition times. The 13-mm sphere required an acquisition time of at least 2.0 min, whereas the 10-mm sphere was not detectable under any condition. In images obtained following 24 and 72 h, the 13-37-mm spheres were detectable across all acquisition times; however, the 10-mm sphere required an acquisition time of at least 6.0 and 3.0 min, respectively. Conclusion: ¹⁷⁷Lu SPECT images were successfully generated using Monte Carlo simulation. In images obtained 24 h after administration, an acquisition time of 6 min was required to detect 10-mm spheres.