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

Abstract

Purpose: In ⁹⁰ Y 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 ⁹⁰ Y imaging by either SPECT or PET is challenging. In ⁹⁰ Y 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 ⁹⁰ Y bremsstrahlung SPECT/CT imaging.

Methods: Since a fully Monte Carlo (MC) approach to ⁹⁰ Y 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 intrahepatic lesions and low-uptake extrahepatic objects was imaged to evaluate SPECT/CT reconstruction without and with scatter correction. Clinical application was demonstrated by applying the reconstruction approach to five 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 two 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 intrahepatic 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 extrahepatic 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 2.7 vs. 4.3 (P = 0.0402) for reconstruction without and with scatter correction respectively.

Conclusions: Quantitative accuracy of ⁹⁰ Y bremsstrahlung imaging can be substantially improved with MC scatter modeling without significant degradation in image noise or intensive computational requirements.

Keywords: 90Y; SPECT/CT; bremsstrahlung; radioembolization; reconstruction.

Figure 1

Flowchart illustrating combination of Monte Carlo scatter estimation with an analytical forward projector.

Figure 2

The liver/lung torso phantom (left) was positioned in‐between a skull and elliptical phantom for SPECT/CT imaging (right). [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

Profile (10‐pixel wide) across the measured and simulated line source. Profiles have been normalized to their maximum value. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

Measured and simulated projection images (at 0⁰) of the liver phantom and corresponding line profiles. Profiles were summed over 10 bins centered on the center sphere. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5

(a) Lesion CNR, (b) lesion activity recovery, (c) normal liver activity recovery and (d) lung activity recovery for the different reconstructions of the phantom data. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 6

An axial slice of the true activity map and the different reconstructions w/o and with SC and the corresponding line profiles for this slice. Profiles were summed over 30 bins centered on the center sphere. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 7

Coronal SPECT/CT slices without (top row) and with (bottom row) scatter correction, demonstrating visibility of the hepatic lesions (left) and the extra hepatic objects (right). [Color figure can be viewed at wileyonlinelibrary.com]

Figure 8

For patient #3 the SPECT reconstruction without and with scatter correction is compared with ⁹⁰Y PET. Profiles are across the center of the large necrotic lesion contoured on the catheter directed selective hepatic arterial contrast enhanced CT. [Color figure can be viewed at wileyonlinelibrary.com]