Phantom study to determine optimal PET reconstruction parameters for PET/MR imaging of 90 Y microspheres following radioembolization

Y-90 PET/MR imaging optimization with a Bayesian penalized likelihood reconstruction algorithm

Positron Emission Tomography (PET) imaging after90 Y liver radioembolization is used for both lesion identification and dosimetry. Bayesian penalized likelihood (BPL) reconstruction algorithms are an alternative to ordered subset expectation maximization (OSEM) with improved image quality and lesion detectability. The investigation of optimal parameters for90 Y image reconstruction of Q.Clear, a commercial BPL algorithm developed by General Electric (GE), in PET/MR is a field of interest and the subject of this study. The NEMA phantom was filled at an 8:1 sphere-to-background ratio. Acquisitions were performed on a PET/MR scanner for clinically relevant activities between 0.7 and 3.3 MBq/ml. Reconstructions with Q.Clear were performed varying the β penalty parameter between 20 and 6000, the acquisition time between 5 and 20 min and pixel size between 1.56 and 4.69 mm. OSEM reconstructions of 28 subsets with 2 and 4 iterations with and without Time-of-Flight (TOF) were compared to Q.Clear with β = 4000. Recovery coefficients (RC), their coefficient of variation (COV), background variability (BV), contrast-to-noise ratio (CNR) and residual activity in the cold insert were evaluated. Increasing β parameter lowered RC, COV and BV, while CNR was maximized at β = 4000; further increase resulted in oversmoothing. For quantification purposes, β = 1000-2000 could be more appropriate. Longer acquisition times resulted in larger CNR due to reduced image noise. Q.Clear reconstructions led to higher CNR than OSEM. A β of 4000 was obtained for optimal image quality, although lower values could be considered for quantification purposes. An optimal acquisition time of 15 min was proposed considering its clinical use.

Tumour-to-normal tissue (T/N) dosimetry ratios role in assessment of 90Y selective internal radiation therapy (SIRT)

OBJECTIVE

The purpose of our work is to assess the role of tumour-to-normal tissue (T/N) dosimetry ratios for predicting response in patients undergoing locoregional therapy to the liver with ⁹⁰Y microspheres.

METHODS

A total of 39 patients (7 female:32 male, mean age 68.3 ± 7.6 years), underwent positron emission tomography (PET)/CT imaging after treatment with ⁹⁰Y microspheres. For attenuation correction and localization of the ⁹⁰Y microspheres, the low-dose, non-diagnostic CT images from PET/CT were used. The acquisition took 15 min and the reconstruction matrix size was 200 × 200 × 75 mm and voxel size of 4.07 × 4.07 × 3.00 mm. For dosimetry calculations, the local deposition method with known activity of ⁹⁰Y was used. For each patient, regions of interest for tumour(s) and whole liver were manually created; the normal tissue region of interest was created automatically. mRECIST criteria on MRI done at 1 month post-treatment and subsequently every 3 months after ⁹⁰Y treatment, were used to assess response.

RESULTS

For 39 patients, the mean liver, tumour and normal tissue doses (mean ± SD) were, 55.17 ± 26.04 Gy, 911.87 ± 866.54 Gy and 47.79 ± 20.47 Gy, respectively. Among these patients, 31 (79%) showed complete response (CR) and 8 (21%) showed progression of disease (PD). For patients with CR, the mean T/N dose ratio obtained was 24.91 (range 3.09-80.12) and for patients with PD, the mean T/N dose ratio was significantly lower, at 6.69 (range 0.36-14.75).

CONCLUSION

Our data show that patients with CR have a statistically higher T/N dose ratio than those with PD. Because, the number of PD cases was limited and partial volume effect was not considered, further investigation is warranted.

ADVANCES IN KNOWLEDGE

T/N dosimetry ratios can be used for assessing response in patients undergoing locoregional therapy to the liver with ⁹⁰Y microspheres.

An estimate of 90Y dosimetry for bremsstrahlung SPECT/CT imaging in liver therapy with 90Y microspheres

OBJECTIVE

Bremsstrahlung SPECT/CT (bSPECT/CT) is one of the most common methods for post-therapy imaging in ⁹⁰Y microspheres selective internal radiation therapy (SIRT) of liver cancers. Here, we are proposing a simple approach using bSPECT/CT to estimate mean absorbed dose to the liver in patients undergoing treatment with ⁹⁰Y microspheres.

MATERIALS AND METHODS

In our previous study comparing ⁹⁰Y dosimetry obtained using bSPECT/CT vs PET/CT, we found that there was a large difference between the mean absorbed dose values to the whole-liver. However, there was a high linear correlation between the doses, which presented an opportunity for quantitative assessment using bSPECT/CT ⁹⁰Y imaging. In this study, after treatment with ⁹⁰Y microspheres, 43 patients were immediately imaged on a dual-head Infinia SPECT/CT gamma camera and on a mCT PET/CT system. Images from 25 of the patients, randomly selected, were used to calculate the correlation of mean liver doses obtained from bSPECT/CT vs. PET/CT. For the remaining 18 patients, the calculated correlation was used to estimate doses obtained from bSPECT/CT, and these estimations were then compared to the doses obtained from PET/CT, considered the gold standard for quantitative analysis.

RESULTS

From the 25 selected patients, the calculated linear correlation between bSPECT/CT and PET/CT ⁹⁰Y mean absorbed doses in whole liver was high (r^2 = 0.97), with a slope of 2.80 and an intercept of -0.63. This linear fit was used to calculate the bSPECT/CT doses for the remaining 18 patients. For these patients, the mean whole-liver dose obtained from bSPECT/CT fitted data vs that obtained from PET/CT were 50.59 Gy and 50.81 Gy, respectively. The average dose difference was 0.2 ± 5.4 Gy (range -18.2%-13.0%). The repeatability coefficient was 10.5 (20.8 % of the mean).

CONCLUSION

Although quantitative bremsstrahlung imaging is difficult, it is possible to calculate adequate estimates of whole-liver dosimetry from bSPECT/CT imaging that is calibrated using its correlation with post-therapy PET/CT ⁹⁰Y images.

Validation of post-treatment PET-based dosimetry software for hepatic radioembolization of Yttrium-90 microspheres

PURPOSE

Yttrium-90 (⁹⁰ Y) microsphere radioembolization enables selective internal radiotherapy for hepatic malignancies. Currently, there is no standard postdelivery imaging and dosimetry of the microsphere distribution to verify treatment. Recent studies have reported utilizing the small positron yield of ⁹⁰ Y (32 ppm) with positron emission tomography (PET) to perform treatment verification and dosimetry analysis. In this study, we validated a commercial dosimetry software, MIM SurePlan™ LiverY90 (MIM Software Inc., Cleveland, OH), for clinical use.

METHODS

A MATLAB-based algorithm for ⁹⁰ Y PET-based dosimetry was developed in-house and validated for the purpose of commissioning the commercial software. The algorithm is based on voxel S values and dosimetry formalism reported in MIRD Pamphlet 17. We validated the in-house algorithm to establish it as the ground truth by comparing results from a digital point phantom and a digital uniform cylinder to manual calculations. Once we validated our in-house MATLAB-based algorithm, we used it to perform acceptance testing and commissioning of the commercial dosimetry software, MIM SurePlan, which uses the same dosimetry formalism. A 0.4 cm/5% gamma test was performed on PET-derived dose maps from each algorithm of uniform digital and nonuniform physical phantoms filled with ⁹⁰ Y chloride solution. Average dose (D_avg ) and minimum dose to 70% (D₇₀ ) of a given volume of interest (VOI) were compared for the digital phantom, the physical phantom, and five patient cases (27 tumor VOIs), representing different clinical scenarios.

RESULTS

The gamma-pass rates were 97.26% and 97.66% for the digital and physical phantoms, respectively. The differences between D_avg and D₇₀ were 0.076% and 0.10% for the digital phantom, respectively, and <5.2% for various VOIs in the physical phantom. In the clinical cases, 96.3% of the VOIs had a difference <5% for D_avg , and 88.9% of the VOIs had a difference <5% for D₇₀ .

CONCLUSIONS

Dose calculation results from MIM SurePlan were found to be in good agreement with our in-house algorithm. This indicates that MIM SurePlan performs as it should and, hence, can be deemed accepted and commissioned for clinical use for post-implant PET-based dosimetry of ⁹⁰ Y radioembolization.

Quantitative comparison of pre-therapy 99mTc-macroaggregated albumin SPECT/CT and post-therapy PET/MR studies of patients who have received intra-arterial radioembolization therapy with 90Y microspheres

OBJECTIVE

The aim of our study was to compare yttrium -90 (⁹⁰Y) dosimetry obtained from pre-therapy ^99mTc-macroaggregated albumin (MAA) SPECT/CT versus post-therapy PET/MRI imaging among patients with primary or metastatic hepatic tumors.

MATERIALS AND METHODS

Prior to ⁹⁰Y radioembolization (RE), 32 patients underwent a scan using MAA mimicking ⁹⁰Y distribution. After RE with ⁹⁰Y microspheres, the patients were imaged on a PET/MRI system. Reconstructed images were transferred to a common platform and used to calculate ⁹⁰Y dosimetry. The Passing-Bablok regression scatter diagram and the Bland and Altman method were used to analyze the difference between dosimetry values.

RESULTS

For MAA and PET/MRI modalities, the mean liver doses for all 32 subjects were 43.0 ± 20.9 Gy and 46.5 ± 22.7 Gy, respectively, with a mean difference of 3.4 ± 6.2 Gy. The repeatibility coefficient was 12.1 (27.0% of the mean). The Spearman rank correlation coefficient was high (ρ = 0.92). Although, there was a substantial difference in the maximum doses to the liver between the modalities, the mean liver doses were relatively close, with a difference of 24.0% or less.

CONCLUSIONS

The two main contributors to the difference between dosimetry calculations using MAA versus ⁹⁰Y PET/MRI can be attributed to the changes in catheter positioning as well as the liver ROIs used for the calculations. In spite of these differences, our results demonstrate that the dosimetry values obtained from pre-therapy MAA SPECT/CT scans and PET/MRI post-therapy ⁹⁰Y studies were not significantly different.

Comparison of PET/CT and PET/MR imaging and dosimetry of yttrium-90 (90Y) in patients with unresectable hepatic tumors who have received intra-arterial radioembolization therapy with 90Y microspheres

BACKGROUND

The aim of our study was to compare 90Y dosimetry obtained from PET/MRI versus PET/CT post-therapy imaging among patients with primary or metastatic hepatic tumors. First, a water-filled Jaszczak phantom containing fillable sphere with 90Y-chloride was acquired on both the PET/CT and PET/MRI systems, in order to check the cross-calibration of the modalities. Following selective internal radiation therapy (SIRT) with 90Y microspheres, 32 patients were imaged on a PET/CT system, immediately followed by a PET/MRI study. Reconstructed images were transferred to a common platform and used to calculate 90Y dosimetry. A Passing-Bablok regression scatter diagram and the Bland and Altman method were used to analyze the difference between the dosimetry values.

RESULTS

The phantom study showed that both modalities were calibrated with less than 1% error. The mean liver doses for the 32 subjects calculated from PET/CT and PET/MRI were 51.6 ± 24.7 Gy and 46.5 ± 22.7 Gy, respectively, with a mean difference of 5.1 ± 5.0 Gy. The repeatability coefficient was 9.0 (18.5% of the mean). The Spearman rank correlation coefficient was very high, ρ = 0.97. Although the maximum dose to the liver can be significantly different (up to 40%), mean liver doses from each modalities were relatively close, with a difference of 18.5% or less.

CONCLUSIONS

The two main contributors to the difference in 90Y dosimetry calculations using PET/CT versus PET/MRI can be attributed to the differences in regions of interest (ROIs) and differences attributed to attenuation correction. Due to the superior soft-tissue contrast of MRI, liver contours are usually better seen than in CT images. However, PET/CT provides better quantification of PET images, due to better attenuation correction. In spite of these differences, our results demonstrate that the dosimetry values obtained from PET/MRI and PET/CT in post-therapy 90Y studies were similar.

Multi institutional quantitative phantom study of yttrium-90 PET in PET/MRI: the MR-QUEST study

Background

Yttrium-90 (⁹⁰Y) radioembolization involves the intra-arterial delivery of radioactive microspheres to treat hepatic malignancies. Though this therapy involves careful pre-treatment planning and imaging, little is known about the precise location of the microspheres once they are administered. Recently, there has been growing interest post-radioembolization imaging using positron-emission tomography (PET) for quantitative dosimetry and identifying lesions that may benefit from additional salvage therapy. In this study, we aim to measure the inter-center variability of ⁹⁰Y PET measurements as measured on PET/MRI in preparation for a multi-institutional prospective phase I/II clinical trial.

Eight institutions participated in this study and followed a standardized phantom filling and imaging protocol. The NEMA NU2-2012 body phantom was filled with 3 GBq of ⁹⁰Y chloride solution. The phantom was imaged for 30 min in listmode on a Siemens Biograph mMR non-TOF PET/MRI scanner at five time points across 10 days (0.3–3.0 GBq). Raw PET data were sent to a central site for image reconstruction and data analysis. Images were reconstructed with optimal parameters determined from a previous study. Volumes of interest (VOIs) matching the known sphere diameters were drawn on the vendor-provided attenuation map and propagated to the PET images. Recovery coefficients (RCs) and coefficient of variation of the RCs (COV) were calculated from these VOIs for each sphere size and activity level.

Results

Mean RCs ranged from 14.5 to 75.4%, with the lowest mean RC coming from the smallest sphere (10 mm) on the last day of imaging (0.16 MBq/ml) and the highest mean RC coming from the largest sphere (37 mm) on the first day of imaging (2.16 MBq/ml). The smaller spheres tended to exhibit higher COVs. In contrast, the larger spheres tended to exhibit lower COVs. COVs from the 37 mm sphere were < 25.3% in all scans. For scans with ≥ 0.60 MBq/ml, COVs were ≤ 25% in spheres ≥ 22 mm. However, for all other spheres sizes and activity levels, COVs were usually > 25%.

Conclusions

Post-radioembolization dosimetry of lesions or other VOIs ≥ 22 mm in diameter can be consistently obtained (< 25% variability) at a multi-institutional level using PET/MRI for any clinically significant activity for ⁹⁰Y radioembolization.

The origin and reduction of spurious extrahepatic counts observed in 90Y non-TOF PET imaging post radioembolization

Our literature survey revealed a physical effect unknown to the nuclear medicine community, i.e. internal bremsstrahlung emission, and also the existence of long energy resolution tails in crystal scintillation. None of these effects has ever been modelled in PET Monte Carlo (MC) simulations. This study investigates whether these two effects could be at the origin of two unexplained observations in ⁹⁰Y imaging by PET: the increasing tails in the radial profile of true coincidences, and the presence of spurious extrahepatic counts post radioembolization in non-TOF PET and their absence in TOF PET. These spurious extrahepatic counts hamper the microsphere delivery check in liver radioembolization. An acquisition of a ³²P vial was performed on a GSO PET system. This is the ideal setup to study the impact of bremsstrahlung x-rays on the true coincidence rate when no positron emission and no crystal radioactivity are present. A MC simulation of the acquisition was performed using Gate-Geant4. MC simulations of non-TOF PET and TOF-PET imaging of a synthetic ⁹⁰Y human liver radioembolization phantom were also performed. Internal bremsstrahlung and long energy resolution tails inclusion in MC simulations quantitatively predict the increasing tails in the radial profile. In addition, internal bremsstrahlung explains the discrepancy previously observed in bremsstrahlung SPECT between the measure of the ⁹⁰Y bremsstrahlung spectrum and its simulation with Gate-Geant4. However the spurious extrahepatic counts in non-TOF PET mainly result from the failure of conventional random correction methods in such low count rate studies and poor robustness versus emission-transmission inconsistency. A novel proposed random correction method succeeds in cleaning the spurious extrahepatic counts in non-TOF PET. Two physical effects not considered up to now in nuclear medicine were identified to be at the origin of the unusual ⁹⁰Y true coincidences radial profile. TOF reconstruction removing of the spurious extrahepatic counts was theoretically explained by a better robustness against emission-transmission inconsistency. A novel random correction method was proposed to overcome the issue in non-TOF PET. Further studies are needed to assess the novel random correction method robustness.

PET/MRI of Hepatic 90Y Microsphere Deposition Determines Individual Tumor Response

Purpose

The purpose of our study is to determine if there is a relationship between dose deposition measured by PET/MRI and individual lesion response to yttrium-90 (⁹⁰Y) microsphere radioembolization.

Materials and Methods

26 patients undergoing lobar treatment with ⁹⁰Y microspheres underwent PET/MRI within 66 h of treatment and had follow-up imaging available. Adequate visualization of tumor was available in 24 patients, and contours were drawn on simultaneously acquired PET/MRI data. Dose volume histograms (DVHs) were extracted from dose maps, which were generated using a voxelized dose kernel. Similar contours to capture dimensional and volumetric change of tumors were drawn on follow-up imaging. Response was analyzed using both RECIST and volumetric RECIST (vRECIST) criteria.

Results

A total of 8 hepatocellular carcinoma (HCC), 4 neuroendocrine tumor (NET), 9 colorectal metastases (CRC) patients, and 3 patients with other metastatic disease met inclusion criteria. Average dose was useful in predicting response between responders and non-responders for all lesion types and for CRC lesions alone using both response criteria (p < 0.05). D70 (minimum dose to 70 % of volume) was also useful in predicting response when using vRECIST. No significant trend was seen in the other tumor types. For CRC lesions, an average dose of 29.8 Gy offered 76.9 % sensitivity and 75.9 % specificity for response.

Conclusions

PET/MRI of ⁹⁰Y microsphere distribution showed significantly higher DVH values for responders than non-responders in patients with CRC. DVH analysis of ⁹⁰Y microsphere distribution following treatment may be an important predictor of response and could be used to guide future adaptive therapy trials.