Quantitative Monte Carlo–Based ⁹⁰Y SPECT Reconstruction

Abstract

The evaluation of radiation absorbed doses in tumorous and healthy tissues is of increasing interest for 90 Y microsphere radioembolization of liver malignancies. The objectives of this work were to introduce and validate a new reconstruction method for quantitative 90 Y bremsstrahlung SPECT to improve posttreatment dosimetry. Methods: A fast Monte Carlo simulator was adapted for 90 Y and incorporated into a statistical reconstruction algorithm (SPECT-MC). Photon scatter and attenuation for all photons sampled from the full 90 Y energy spectrum were modeled during reconstruction by Monte Carlo simulations. The energy-and distance-dependent collimator-detector response was modeled with precalculated convolution kernels. The National Electrical Manufacturers Association 2007/International Electrotechnical Commission 2008 image quality phantom was used to quantitatively evaluate the performance of SPECT-MC in comparison with those of state-of-the-art clinical SPECT reconstruction and PET. The liver radiation absorbed doses estimated by SPECT, PET, and SPECT-MC were evaluated in 5 patients consecutively treated with radioembolization. Results: In comparison with state-of-the-art clinical 90 Y SPECT reconstruction, SPECT-MC substantially improved image contrast (e.g., from 25% to 88% for the 37-mm sphere) and decreased the mean residual count error in the lung insert (from 73% to 15%) at the cost of higher image noise. Image noise and the mean count error were lower for SPECT-MC than for PET. Image contrast was higher in the larger spheres (diameter of $28 mm) but lower in the smaller spheres (#22 mm) for SPECT-MC than for PET. In the clinical study, mean absorbed dose estimates in liver regions with high absorbed doses were consistently higher for SPECT-MC than for SPECT (P 5 0.0625) and consistently higher for SPECT-MC than for PET (P 5 0.0625). Assessment of the 90 Y microsphere distribution can be performed by imaging bremsstrahlung photons with a SPECT camera or by imaging annihilation photons with a PET camera. Posttreatment dosimetry with 90 Y PET has advantages over SPECT, mainly because of higher resolution and image contrast (6,7). However, the low positron branch (32 · 10 26 ) in 90 Y decay requires a stateof-the-art lutetium-(yttrium)-orthosilicate time-of-flight PET/CT scanner to obtain images with sufficiently high quantitative accuracy for dosimetry purposes (8,9). Posttreatment imaging with a standard SPECT/CT system may be a more widely available and cost-effective option for most centers, but the image quality (IQ) of state-of-the-art clinical 90 Y bremsstrahlung SPECT is still limited (7). The wide range (0-2.3 MeV) and continuous nature of the 90 Y bremsstrahlung photon spectrum prohibit the use of simple energy window-based scatter rejection and correction techniques, hinder attenuation correction based on single-photon energy, and require compensation for collimator-and detector-related imagedegrading effects, such as collimator scatter, lead x-rays, septal penetration, camera (back)scatter, an...

Hybrid Imaging: Instrumentation and Data Processing

“…The most common examples are the Dual Energy Window (DEW) approach that neglects upper scatter or the Triple Energy Window (TEW) [150][151][152]. More complex solutions, based on Monte Carlo estimations of the scattered events, have been also proposed [153,154] and are expected to be widely used in the near future.…”

Practical reconstruction protocol for quantitative 90Y bremsstrahlung SPECT/CT

“…Elschot et al directly incorporated into the reconstruction algorithm the energy dependent photon scatter and attenuation estimated from Monte Carlo simulations. 15 Their approach demonstrated higher tumor contrast and lower mean residual count in lung insert albeit at the cost of higher image noise. These advanced Monte Carlo based approached are not commercially available and cannot be easily implemented in the routine clinical practice.…”

A review of 3D image-based dosimetry, technical considerations and emerging perspectives in 90Y microsphere therapy

“…Similar work along with a model of the decay location and photon emission location incorporated into an OSEM reconstruction, showed increased quantitative accuracy validated on digital (XCAT) and physical phantoms with errors between -1.6% and 11.9% [67]. Other work provides "on-the-fly" MC simulations of scatter and attenuation effects for each patient, and is less dependent on patient geometry [68]. They show improved image contrast and decreased count error compared to non-MC SPECT reconstruction, and also a more accurate mean absorbed dose to a phantom.…”

“…Significant corrections are required in order to accurately image these isotopes especially for quantitative 3D PET where the potential for spurious coincidence contamination is much increased. Other efforts into using biodegradable Chitosan glycol MS labeled to 68 Ga have been developed in preclinical models [132], although no clinical results are yet available.…”