Gamma camera characterization at high holmium-166 activity in liver radioembolization

Abstract

Background High activities of holmium-166 (166Ho)–labeled microspheres are used for therapeutic radioembolization, ideally directly followed by SPECT imaging for dosimetry purposes. The resulting high-count rate potentially impacts dead time, affecting the image quality and dosimetric accuracy. This study assesses gamma camera performance and SPECT image quality at high 166Ho activities of several GBq. To this purpose, the liver compartment, including two tumors, of an anthropomorphic phantom was filled with 166Ho-chloride, with a tumor to non-tumorous liver activity concentration ratio of 10:1. Multiple SPECT/CT scans were acquired over a range of activities up to 2.7 GBq. Images were reconstructed using a commercially available protocol incorporating attenuation and scatter correction. Dead time effects were assessed from the observed count rate in the photopeak (81 keV, 15% width) and upper scatter (118 keV, 12% width) window. Post reconstruction, each image was scaled with an individual conversion factor to match the known total activity in the phantom at scanning time. The resulting activity concentration was measured in the tumors and non-tumorous liver. The image quality as a function of activity was assessed by a visual check of the absence of artifacts by a nuclear medicine physician. The apparent lung shunt fraction (nonzero due to scatter) was estimated on planar and SPECT images. Results A 20% count loss due to dead time was observed around 0.7 GBq in the photopeak window. Independent of the count losses, the measured activity concentration was up to 100% of the real value for non-tumorous liver, when reconstructions were normalized to the known activity at scanning time. However, for tumor spheres, activity concentration recovery was ~80% at the lowest activity, decreasing with increasing activity in the phantom. Measured lung shunt fractions were relatively constant over the considered activity range. Conclusions At high 166Ho count rate, all images, visually assessed, presented no artifacts, even at considerable dead time losses. A quantitative evaluation revealed the possibility of reliable dosimetry within the healthy liver, as long as a post-reconstruction scaling to scanning activity is applied. Reliable tumor dosimetry, instead, remained hampered by the dead time.

Improving MRI‐based dosimetry for holmium‐166 transarterial radioembolization using a nonrigid image registration for voxelwise ΔR2∗$\Delta R_2^*$ calculation

“…In a recent publication, it was described that especially in high-activity regions such as tumors, reliable dosimetry is significantly hampered by dead time, and underestimations of the activity concentration map of up to 20-40% can occur. 23 Other aspects of 166 Ho-SPECT such as lack of respiratory gating during image acquisition and an increased partial volume effect due to a decreased imaging resolution leading to dose spill-out and projection outside of the actual source. In addition, we used a rigid, manual registration to copy the MRI-based tumor VOIs to the 166 Ho SPECT/CT images that were acquired 2 days after treatment.…”

Advances in Radionuclides and Radiolabelled Peptides for Cancer Therapeutics

“…Holmium-166 ( 166 Ho)-labelled microspheres with high activity are utilised in interventional radioembolization, along with SPECT (single-photon emission computed tomography) imaging, for dosimetry purposes. The ensuing high count rate may have an impact on dead time, lowering the dosimetric precision and image quality [ 49 ]. High purity 166 Ho radioisotopes can be synthesised by one neutron activation of the 165 Ho isotope.…”

Holmium-166 Radioembolization: Current Status and Future Prospective

“…On the other hand, immediately after administration, there is an abundance of gamma photons that significantly increases detector dead time (time duration during which the gamma camera is unable to detect a new scintillation after a previous event). In particular, using an acquisition and reconstruction protocol commonly applied in clinical practice, a 20% count loss due to dead time was observed around 0.7 GBq injected activity [ 8 ]. Thus, dependent on the amount of administered activity, it is advised to perform post-treatment 166 Ho SPECT/CT between 2 and 5 days after treatment, aiming at an activity < 0.7 GBq at the time of imaging.…”