Modelling SPECT auto-contouring acquisitions for ¹⁷⁷Lu & ¹³¹I molecular radiotherapy using new developments in...

Monte Carlo modelling of SPECT imaging in Molecular Radiotherapy can improve activity quantification. Until now, SPECT modelling with GATE only considered circular orbit (CO) acquisitions. This cannot reproduce auto-contour acquisitions, where the detector head moves close to the patient to improve image resolution. The aim of this work is to develop and validate an auto-contouring step-and-shoot acquisition mode for GATE SPECT modelling.

¹⁷⁷Lu and ¹³¹I SPECT experimental acquisitions performed on a Siemens Symbia T2 and GE Discovery 670 gamma camera, respectively, were modelled. SPECT projections were obtained for a cylindrical Jaszczak phantom and a lung and spine phantom. Detector head parameters (radial positions and acquisition angles) were extracted from the experimental projections to model the non-circular orbit (NCO) detector motion. The gamma camera model was validated against the experimental projections obtained with the cylindrical Jaszczak (¹⁷⁷Lu) and lung and spine phantom (¹³¹I). Then, ¹⁷⁷Lu and ¹³¹I CO and NCO SPECT projections were simulated to validate the impact of explicit NCO modelling on simulated projections.

Experimental and simulated SPECT images were compared using the gamma index, and were in good agreement with gamma index passing rate (GIPR) and gamma_avg of 96.27%, 0.242 (¹⁷⁷Lu) and 92.89%, 0.36 (¹³¹I). Then, simulated ¹⁷⁷Lu and ¹³¹I CO and NCO SPECT projections were compared. The GIPR, gamma_avg between the two gamma camera motions was 99.85%, 0.108 for ¹⁷⁷Lu and 75.58%, 0.6 for ¹³¹I.

This work thereby justifies the need for auto-contouring modelling for isotopes with high septal penetration.