Speaker
Description
Ac-225 is a very promising radionuclide for targeted alpha therapy, as demonstrated in a number of pre-clinical experiments and clinical trials. However, due to the extremely low injected activities and the number of emitted gamma-rays, imaging via single-photon emission computed tomography (SPECT) is very challenging. Moreover, SPECT works best at energies in the ~100keV range, so its performance is largely sub-optimal for imaging of Ac-225. We propose Compton imaging to provide accurate and high-sensitivity imaging of the two most imporant daughters of Ac-225: Fr-221 and Bi-213. We also perform collimated SPECT imaging of lower energy gamma-rays produced directly by Ac-225 in order to obtain a multi-energy imaging solution. In order to do this, we employ a 3D-positioning CZT camera (M400 by H3D, Michigan, USA) with an energy resolution below 1% at 662keV, submillimeter depth-of-interaction resolution enabled by dual readout, and subpixel resolution via a neighbouring pixel interpolation.
We have performed a caracterization campaign of the camera with and without a custom-made tungsten collimator. We measured the sensitivity and angular resolution with several sealed sources and unsealed sources of Ac-225 to demonstrate the capabilities of our system. Compared to standard SPECT with a collimator, collimatorless Compton imaging results in a sensitivity increase of a factor of ~100 for the 218keV gamma-rays from Fr-221 and of ~10 for the 440keV of Bi-213. We also show in a mouse phantom study that an amount of 15µCi of Ac-225 can be imaged in only 9 seconds for Bi-213 and 35 seconds for Fr-221 with a single detector head in a single bed position. This means that imaging of relevant activities of Ac-225 injected in vivo (less than 0.5µCi) can be achieved with a multi-head detector in minutes.
At the time of the conference we will show characterization results of the CZT camera and imaging studies including mice phantoms and in-vivo mouse imaging of Ac-225 radiopharmaceuticals. Additionally, we will benchmark our simulation framework with data from our prototype to demonstrate that a full system is capable of imaging low activities of Ac-225 in imaging sessions of a few minutes. This would provide a practical tool to accurately and efficiently evaluate Ac-225 radiopharmaceuticals in-vivo for the first time.
