Hybrid kernelised expectation maximisation for Bremsstrahlung SPECT reconstruction in SIRT with 90Y micro-spheres

Abstract

Background Selective internal radiation therapy with Yttrium-90 microspheres is an effective therapy for liver cancer and liver metastases. Yttrium-90 is mainly a high-energy beta particle emitter. These beta particles emit Bremsstrahlung radiation during their interaction with tissue making post-therapy imaging of the radioactivity distribution feasible. Nevertheless, image quality and quantification is difficult due to the continuous energy spectrum which makes resolution modelling, attenuation and scatter estimation challenging and therefore the dosimetry quantification is inaccurate. As a consequence a reconstruction algorithm able to improve resolution could be beneficial. Methods In this study, the hybrid kernelised expectation maximisation (HKEM) is used to improve resolution and contrast and reduce noise, in addition a modified HKEM called frozen HKEM (FHKEM) is investigated to further reduce noise. The iterative part of the FHKEM kernel was frozen at the 72nd sub-iteration. When using ordered subsets algorithms the data is divided in smaller subsets and the smallest algorithm iterative step is called sub-iteration. A NEMA phantom with spherical inserts was used for the optimisation and validation of the algorithm, and data from 5 patients treated with Selective internal radiation therapy were used as proof of clinical relevance of the method. Results The results suggest a maximum improvement of 56% for region of interest mean recovery coefficient at fixed coefficient of variation and better identification of the hot volumes in the NEMA phantom. Similar improvements were achieved with patient data, showing 47% mean value improvement over the gold standard used in hospitals. Conclusions Such quantitative improvements could facilitate improved dosimetry calculations with SPECT when treating patients with Selective internal radiation therapy, as well as provide a more visible position of the cancerous lesions in the liver.

Triple modality image reconstruction of PET data using SPECT, PET, CT information increases lesion uptake in images of patients treated with radioembolization with $$^{90}Y$$ micro-spheres

“…3 . The bremsstrahlung data (SPECT) were first reconstructed with HKEM using the SPECT image-update and CT information as in [ 7 ]. The PET data are reconstructed using MHKEM which uses SPECT and CT images as well as the PET image estimate.…”

“…The KEM options were not used in this case as the results from the NEMA phantom showed poor performance. The images used as side information are: the CT image (acquired with PET) and the SPECT image (reconstructed using HKEM with SPECT and CT as in [ 7 ]). These CT (acquired together with PET) and SPECT images are reported for one patient in Fig.…”

“…The study was carried out with a clinical example using Cu/ Cu. In our previous work [ 7 ], the HKEM algorithm was used with CT information to improve the SPECT images of the bremsstrahlung data when PET data are not available. This method was also used for the reconstruction of the SPECT images in this study.…”