Investigation and optimization of PET-guided SPECT reconstructions for improved radionuclide therapy dosimetry estimates

Abstract

AimsTo investigate and optimize the SPECTRE (Single Photon Emission Computed Theranostic REconstruction) reconstruction approach, using the hybrid kernelized expectation maximization (HKEM) algorithm implemented in the software for tomographic image reconstruction (STIR) software library, and to demonstrate the feasibility of performing algorithm exploration and optimization in 2D. Optimal SPECTRE parameters were investigated for the purpose of improving SPECT-based radionuclide therapy (RNT) dosimetry estimates.MethodsUsing the NEMA IEC body phantom as the test object, SPECT data were simulated to model an early and late imaging time point following a typical therapeutic dose of 8 GBq of 177Lu. A theranostic 68Ga PET-prior was simulated for the SPECTRE reconstructions. The HKEM algorithm parameter space was investigated for SPECT-unique and PET-SPECT mutual features to characterize optimal SPECTRE parameters for the simulated data. Mean and maximum bias, coefficient of variation (COV %), recovery, SNR and root-mean-square error (RMSE) were used to facilitate comparisons between SPECTRE reconstructions and OSEM reconstructions with resolution modelling (OSEM_RM). 2D reconstructions were compared to those performed in 3D in order to evaluate the utility of accelerated algorithm optimization in 2D. Segmentation accuracy was evaluated using a 42% fixed threshold (FT) on the 3D reconstructed data.ResultsSPECTRE parameters that demonstrated improved image quality and quantitative accuracy were determined through investigation of the HKEM algorithm parameter space. OSEM_RM and SPECTRE reconstructions performed in 2D & 3D were qualitatively and quantitatively similar, with SPECTRE showing an average reduction in background COV % by a factor of 2.7 and 3.3 for the 2D case and 3D case respectively. The 42% FT analysis produced an average % volume difference from ground truth of 158% and 26%, for the OSEM_RM and SPECTRE reconstructions, respectively.ConclusionsThe SPECTRE reconstruction approach demonstrates significant potential for improved SPECT image quality, leading to more accurate RNT dosimetry estimates when conventional segmentation methods are used. Exploration and optimization of SPECTRE benefited from both fast reconstruction times afforded by first considering the 2D case. This is the first in-depth exploration of the SPECTRE reconstruction approach, and as such, it reveals several insights for reconstructing SPECT data using PET side-information.

Prediction of 177Lu-DOTATATE Therapy Outcomes in Neuroendocrine Tumor Patients Using Semi-Automatic Tumor Delineation on 68Ga-DOTATATE PET/CT

“…The present study is also subject to the limitations of a single-center retrospective design. Partial volume effect was reduced but not eliminated by our application of the minimum tumor size threshold, where a compromise between lesion detection and SUV measurement had to be made [18,41]. The semi-automatic tumor delineation method had drawbacks compared to manual contouring.…”

The lesion detection rate of Ga‐68 DOTATATE PET/MR in multiple endocrine neoplasia type 1

“…Nevertheless, all imaging modalities proved inadequate in visualizing lesions measuring 5 and 6 mm in size. This may be attributed to the partial volume effect caused by the limited spatial resolution of PET in small‐sized lesions, resulting in a typical underestimation of radioactive tracer uptake 17 . In addition, MRI has limited spatial resolution in cases of small lesions with small fractions of voxels, and cannot distinguish them from surrounding normal tissue.…”

An anthropomorphic body phantom for the determination of calibration factor in radionuclide treatment dosimetry

“…A standard volume is not used in the literature to determine the CF. PVE, whose effects depend on the volume of radioactivity, is an important problem in quantitative radioisotope imaging ( 15 , 27 ) . Wevrett et al.…”

“…However, there are several effects that cause the reconstructed SPECT image to differ from the real activity distribution in the body, such as (i) dead time occurred in the detector at high count rates with high activity ( 11 ) , (ii) low signal-to-noise ratio (SNR) due to sort imaging time ( 12 ) , (iii) effects of spatial resolution on SPECT ( 13 ) , (iv) depth-dependent spatial resolution (DDSR) due to varying distance from the imaging system ( 14 ) , (v) partial volume effect (PVE) due to the limited spatial resolution of SPECT/CT system ( 15 ) , (vi) reconstruction algorithm effects ( 16 ) , (vii) scattering ( 17 ) and (viii) attenuation effects caused by tissue ( 18 , 19 ) . The effect of the phantom type, which reflects the scatter and attenuation effects mentioned in the last above two, on the determination of CK is a subject that is still being studied and there is no consensus ( 20–26 ) .…”