Improved quantitative ⁹⁰Y bremsstrahlung SPECT/CT reconstruction with Monte Carlo scatter modeling

Abstract

Purpose In 90Y microsphere radioembolization (RE), accurate post-therapy imaging-based dosimetry is important for establishing absorbed dose versus outcome relationships for developing future treatment planning strategies. Additionally, accurately assessing microsphere distributions is important because of concerns for unexpected activity deposition outside the liver. Quantitative 90Y imaging by either SPECT or PET is challenging. In 90Y SPECT model based methods are necessary for scatter correction because energy window-based methods are not feasible with the continuous bremsstrahlung energy spectrum. The objective of this work was to implement and evaluate a scatter estimation method for accurate 90Y bremsstrahlung SPECT/CT imaging. Methods Since a fully Monte Carlo (MC) approach to 90Y SPECT reconstruction is computationally very demanding, in the present study the scatter estimate generated by a MC simulator was combined with an analytical projector in the 3D OS-EM reconstruction model. A single window (105 to 195-keV) was used for both the acquisition and the projector modeling. A liver/lung torso phantom with intra-hepatic lesions and low-uptake extra-hepatic objects was imaged to evaluate SPECT/CT reconstruction without and with scatter correction. Clinical application was demonstrated by applying the reconstruction approach to 5 patients treated with RE to determine lesion and normal liver activity concentrations using a (liver) relative calibration. Results There was convergence of the scatter estimate after just 2 updates, greatly reducing computational requirements. In the phantom study, compared with reconstruction without scatter correction, with MC scatter modeling there was substantial improvement in activity recovery in intra-hepatic lesions (from > 55% to > 86%), normal liver (from 113% to 104%) and lungs (from 227% to 104%) with only a small degradation in noise (13% vs. 17%). Similarly, with scatter modeling contrast improved substantially both visually and in terms of a detectability index, which was especially relevant for the low uptake extra-hepatic objects. The trends observed for the phantom were also seen in the patient studies where lesion activity concentrations and lesion-to-liver concentration ratios were lower for SPECT without scatter correction compared with reconstruction with just two MC scatter updates: in eleven lesions the mean uptake was 4.9 vs. 7.1 MBq/mL (P=0.0547), the mean normal liver uptake was 1.6 vs. 1.5 MBq/mL (P = 0.056) and the mean lesion-to-liver uptake ratio was 2.9 vs. 4.2 (P=0.0402) for reconstruction without and with scatter correction, respectively. Conclusions Quantitative accuracy of 90Y bremsstrahlung imaging can be substantially improved with MC scatter modeling without significant degradation in image noise or intensive computational requirements.

Accelerated SPECT image reconstruction with FBP and an image enhancement convolutional neural network

“…Monte Carlo-based iterative reconstruction to correct for scatter and collimator effects has been proven to be superior over more analytical correction schemes, albeit at the cost of reconstruction times of potentially several hours [1–3]. This has up to now limited introduction of such techniques into daily clinical practice because the requirement of reconstruction job queues causing interrupted workflow is considered bothersome.…”

The value of 99mTc-MAA SPECT/CT for lung shunt estimation in 90Y radioembolization: a phantom and patient study

“…Additionally, post-therapy 90 Y imaging based on either bremsstrahlung SPECT/CT or 90 Y PET/CT is challenging, especially in low uptake regions such as the lung. The use of time-of-flight information in the case of PET [ 8 ] and model/Monte Carlo-based scatter correction in the case SPECT [ 9 – 11 ] has led to improved Y-90 imaging, but these methods are not always available with clinic systems. Apart from the imaging methodology, another factor that can impact 99m Tc-MAA-based lung shunt estimation is the apparent in vivo breakdown of 99m Tc-MAA into smaller aggregates and to pertechnetate over time [ 12 , 13 ].…”

Y-90 SPECT ML image reconstruction with a new model for tissue-dependent bremsstrahlung production using CT information: a proof-of-concept study

“…Although specialized reconstruction methods have been developed for Y-90 SPECT imaging (Rong et al (2012), Elschot et al (2013) and Dewaraja et al (2017)), these methods have not accounted for the tissue-dependent bremsstrahlung yield. This paper reports simulations and experimental measurements to demonstrate the effect and implement and test a reconstruction formulation that accounts for the tissue dependency of bremsstrahlung generation in the SPECT system model.…”