Quantitative and qualitative assessment of Yttrium-90 PET/CT imaging

PET/CT and SPECT/CT imaging of 90Y hepatic radioembolization at therapeutic and diagnostic activity levels: Anthropomorphic phantom study

PURPOSE

Prior to 90Y radioembolization procedure, a pretherapy simulation using 99mTc-MAA is performed. Alternatively, a small dosage of 90Y microspheres could be used. We aimed to assess the accuracy of lung shunt fraction (LSF) estimation in both high activity 90Y posttreatment and pretreatment scans with isotope activity of ~100 MBq, using different imaging techniques. Additionally, we assessed the feasibility of visualising hot and cold hepatic tumours in PET/CT and Bremsstrahlung SPECT/CT images.

MATERIALS AND METHODS

Anthropomorphic phantom including liver (with two spherical tumours) and lung inserts was filled with 90Y chloride to simulate an LSF of 9.8%. The total initial activity in the liver was 1451 MBq, including 19.4 MBq in the hot sphere. Nine measurement sessions including PET/CT, SPECT/CT, and planar images were acquired at activities in the whole phantom ranging from 1618 MBq down to 43 MBq. The visibility of the tumours was appraised based on independent observers' scores. Quantitatively, contrast-to-noise ratio (CNR) was calculated for both spheres in all images.

RESULTS

LSF estimation. For high activity in the phantom, PET reconstructions slightly underestimated the LSF; absolute difference was <1.5pp (percent point). For activity <100 MBq, the LSF was overestimated. Both SPECT and planar scintigraphy overestimated the LSF for all activities. Lesion visibility. For SPECT/CT, the cold tumour proved too small to be discernible (CNR <0.5) regardless of the 90Y activity in the liver, while hot sphere was visible for activity >200 MBq (CNR>4). For PET/CT, the cold tumour was only visible with the highest 90Y activity (CNR>4), whereas the hot one was seen for activity >100 MBq (CNR>5).

CONCLUSIONS

PET/CT may accurately estimate the LSF in a 90Y posttreatment procedure. However, at low activities of about 100 MBq it seems to provide unreliable estimations. PET imaging provided better visualisation of both hot and cold tumours.

AAPM Medical Physics Practice Guideline 14.a: Yttrium-90 microsphere radioembolization

Radioembolization using Yttrium-90 (90 Y) microspheres is widely used to treat primary and metastatic liver tumors. The present work provides minimum practice guidelines for establishing and supporting such a program. Medical physicists play a key role in patient and staff safety during these procedures. Products currently available are identified and their properties and suppliers summarized. Appropriateness for use is the domain of the treating physician. Patient work up starts with pre-treatment imaging. First, a mapping study using Technetium-99m (Tc-99m ) is carried out to quantify the lung shunt fraction (LSF) and to characterize the vascular supply of the liver. An MRI, CT, or a PET-CT scan is used to obtain information on the tumor burden. The tumor volume, LSF, tumor histology, and other pertinent patient characteristics are used to decide the type and quantity of 90 Y to be ordered. On the day of treatment, the appropriate dose is assayed using a dose calibrator with a calibration traceable to a national standard. In the treatment suite, the care team led by an interventional radiologist delivers the dose using real-time image guidance. The treatment suite is posted as a radioactive area during the procedure and staff wear radiation dosimeters. The treatment room, patient, and staff are surveyed post-procedure. The dose delivered to the patient is determined from the ratio of pre-treatment and residual waste exposure rate measurements. Establishing such a treatment modality is a major undertaking requiring an institutional radioactive materials license amendment complying with appropriate federal and state radiation regulations and appropriate staff training commensurate with their respective role and function in the planning and delivery of the procedure. Training, documentation, and areas for potential failure modes are identified and guidance is provided to ameliorate them.

Image Quality and Quantitative PET Parameters of Low-Dose [18F]FDG PET in a Long Axial Field-of-View PET/CT Scanner

PET/CT scanners with a long axial field-of-view (LAFOV) provide increased sensitivity, enabling the adjustment of imaging parameters by reducing the injected activity or shortening the acquisition time. This study aimed to evaluate the limitations of reduced [18F]FDG activity doses on image quality, lesion detectability, and the quantification of lesion uptake in the Biograph Vision Quadra, as well as to assess the benefits of the recently introduced ultra-high sensitivity mode in a clinical setting. A number of 26 patients who underwent [18F]FDG-PET/CT (3.0 MBq/kg, 5 min scan time) were included in this analysis. The PET raw data was rebinned for shorter frame durations to simulate 5 min scans with lower activities in the high sensitivity (HS) and ultra-high sensitivity (UHS) modes. Image quality, noise, and lesion detectability (n = 82) were assessed using a 5-point Likert scale. The coefficient of variation (CoV), signal-to-noise ratio (SNR), tumor-to-background ratio (TBR), and standardized uptake values (SUV) including SUVmean, SUVmax, and SUVpeak were evaluated. Subjective image ratings were generally superior in UHS compared to the HS mode. At 0.5 MBq/kg, lesion detectability decreased to 95% (HS) and to 98% (UHS). SNR was comparable at 1.0 MBq/kg in HS (5.7 ± 0.6) and 0.5 MBq/kg in UHS (5.5 ± 0.5). With lower doses, there were negligible reductions in SUVmean and SUVpeak, whereas SUVmax increased steadily. Reducing the [18F]FDG activity to 1.0 MBq/kg (HS/UHS) in a LAFOV PET/CT provides diagnostic image quality without statistically significant changes in the uptake parameters. The UHS mode improves image quality, noise, and lesion detectability compared to the HS mode.

Comparison of PMT-based TF64 and SiPM-based Vereos PET/CT systems for 90 Y imaging and dosimetry optimization: A quantitative study

BACKGROUND

Selective internal radiotherapy based on transarterial radioembolization (TARE) with yttrium-90 (⁹⁰ Y) microspheres is an established treatment for primary or metastatic liver disease.

PURPOSE

The objective of this work is to optimize the dosimetry of patients treated with ⁹⁰ Y TARE, using positron emission tomography (PET) images.

METHODS

The NEMA 2012 PET phantom was filled with nearly 3.9 GBq of ⁹⁰ Y activity and acquired at days 0, 3, 5, 7, and 9 on a classic time-of-flight PET/computed tomography (CT) scanner (Philips TF64) and on a silicon photomultiplier (SiPM)-based PET/CT scanner (Philips Vereos). Acquisitions were carried on following the guidelines proposed in a previously published multicentric trial and images were reconstructed by varying and combining the available parameters. Comparisons were performed to identify the best set(s) of parameters leading to the most accurate ⁹⁰ Y-PET image(s), in terms of activity distribution. Then, for both scanners, the best images were analyzed with Simplicit⁹⁰ Y, a personalized dosimetry software using multicompartmental Medical Internal Radiation Dose model. The comparison between measured and true doses allowed to identify the image granting the most consistent dose estimations and, therefore, to designate the set of parameters to be applied on patients' data for the reconstruction of optimized clinical images. Posttreatment dosimetry of four patients was then realized with Simplicit⁹⁰ Y using optimized imaging datasets.

RESULTS

Based on activity distribution comparisons and dose estimations over phantom and patients data, the SiPM-based PET/CT system appeared more suitable than the photomultiplier tube-based TF64 for ⁹⁰ Y-PET imaging. With the SiPM-based PET/CT system, reconstructed images with a 2-mm voxel size combined with the application of the point spread function correction led to the most accurate results for quantitative ⁹⁰ Y measures.

CONCLUSIONS

For the SiPM-based PET/CT scanner, an optimized set of reconstruction parameters has been identified and applied on patients' data in order to generate the most accurate image to be used for an improved personalized ⁹⁰ Y-PET dosimetry, ensuring a reliable evaluation of the delivered doses.

Yttrium-90 TOF-PET-Based EUD Predicts Response Post Liver Radioembolizations Using Recommended Manufacturer FDG Reconstruction Parameters

Purpose

Explaining why ⁹⁰Y TOF-PET based equivalent uniform dose (EUD) using recommended manufacturer FDG reconstruction parameters has been shown to predict response.

Methods

The hot rods insert of a Jaszczak deluxe phantom was partially filled with a 2.65 GBq ⁹⁰Y - 300ml DTPA water solution resulting in a 100 Gy mean absorbed dose in the 6 sectors. A two bed 20min/position acquisition was performed on a 550ps- and on a 320ps- TOF-PET/CT and reconstructed with recommended manufacturer FDG reconstruction parameters, without and with additional filtering. The whole procedure was repeated on both PET after adding 300ml of water (50Gy setup). The phantom was acquired again after decay by a factor of 10 (5Gy setup), but with 200min per bed position. For comparison, the phantom was also acquired with ¹⁸F activity corresponding to a clinical FDG whole body acquisition.

Results

The 100Gy-setup provided a hot rod sectors image almost as good as the ¹⁸F phantom. However, despite acquisition time compensation, the 5Gy-setup provides much lower quality imaging. TOF-PET based sectors EUDs for the three large rod sectors agreed with the actual EUDs computed with a radiosensitivity of 0.021Gy^-1 well in the range observed in external beam radiotherapy (EBRT), i.e. 0.01-0.04Gy^-1. This agreement explains the reunification of the dose-response relationships of the glass and resin spheres in HCC using the TOF-PET based EUD. Additional filtering reduced the EUDs agreement quality.

Conclusions

Recommended manufacturer FDG reconstruction parameters are suitable in TOF-PET post ⁹⁰Y liver radioembolization for accurate tumour EUD computation. The present results rule out the use of low specific activity phantom studies to optimize reconstruction parameters.

Yttrium-90 quantitative phantom study using digital photon counting PET

BACKGROUND

PET imaging of 90Y-microsphere distribution following radioembolisation is challenging due to the count-starved statistics from the low branching ratio of e+/e- pair production during 90Y decay. PET systems using silicon photo-multipliers have shown better 90Y image quality compared to conventional photo-multiplier tubes. The main goal of the present study was to evaluate reconstruction parameters for different phantom configurations and varying listmode acquisition lengths to improve quantitative accuracy in 90Y dosimetry, using digital photon counting PET/CT.

METHODS

Quantitative PET and dosimetry accuracy were evaluated using two uniform cylindrical phantoms specific for PET calibration validation. A third body phantom with a 9:1 hot sphere-to-background ratio was scanned at different activity concentrations of 90Y. Reconstructions were performed using OSEM algorithm with varying parameters. Time-of-flight and point-spread function modellings were included in all reconstructions. Absorbed dose calculations were carried out using voxel S-values convolution and were compared to reference Monte Carlo simulations. Dose-volume histograms and root-mean-square deviations were used to evaluate reconstruction parameter sets. Using listmode data, phantom and patient datasets were rebinned into various lengths of time to assess the influence of count statistics on the calculation of absorbed dose. Comparisons between the local energy deposition method and the absorbed dose calculations were performed.

RESULTS

Using a 2-mm full width at half maximum post-reconstruction Gaussian filter, the dosimetric accuracy was found to be similar to that found with no filter applied but also reduced noise. Larger filter sizes should not be used. An acquisition length of more than 10 min/bed reduces image noise but has no significant impact in the quantification of phantom or patient data for the digital photon counting PET. 3 iterations with 10 subsets were found suitable for large spheres whereas 1 iteration with 30 subsets could improve dosimetry for smaller spheres.

CONCLUSION

The best choice of the combination of iterations and subsets depends on the size of the spheres. However, one should be careful on this choice, depending on the imaging conditions and setup. This study can be useful in this choice for future studies for more accurate 90Y post-dosimetry using a digital photon counting PET/CT.

Impact of image reconstruction method on dose distributions derived from 90Y PET images: phantom and liver radioembolization patient studies

PET images acquired after liver ⁹⁰Y radioembolization therapies are typically very noisy, which significantly challenges both visualization and quantification of activity distributions. To improve their noise characteristics, regularized iterative reconstruction algorithms such as block sequential regularized expectation maximization (Q.Clear for GE Healthcare, USA) have been proposed. In this study, we aimed to investigate the effects which different reconstruction algorithms may have on patient images, with reconstruction parameters initially narrowed down using phantom studies. Moreover, we evaluated the impact of these reconstruction methods on voxel-based dose distribution in phantom and patient studies (lesions and healthy livers). The International Electrotechnical Commission (IEC)/NEMA phantom, containing six spheres, was filled with ⁹⁰Y and imaged using a GE Discovery 690 PET/CT scanner with time-of-flight enabled. The images were reconstructed using Q.Clear (with β parameter ranging from 0 to 8000) and ordered subsets expectation maximization. The image quality and quantification accuracy were evaluated by computing the hot ([Formula: see text]) and cold ([Formula: see text]) contrast recovery coefficients, background variability (BV) and activity bias. Next, dose distributions and dose volume histograms were generated using MIM® software's SurePlan LiverY90 toolbox. Subsequently, parameters optimized in these phantom studies were applied to five patient datasets. Dose parameters, such as D_max, D_mean, D₇₀, and V_100Gy, were estimated, and their variability for different reconstruction methods was investigated. Based on phantom studies, the β parameter values optimized for image quality and quantification accuracy were 2500 and 300, respectively. When all investigated reconstructions were applied to patient studies, D_mean, D₅₀, D₇₀, and V_100Gy showed coefficients of variation below 8%; whereas the variability of D_max was up to 30% for both phantom and patient images. Although β = 300-1000 would provide accurate activity quantification for a region of interest, when considering activity/dose voxelized distribution, higher β value (e.g. 4000-5000) would provide the greatest accuracy for dose distributions. In this ⁹⁰Y radioembolization PET/CT study, the β parameter in regularized iterative (Q.Clear) reconstruction was investigated for image quality, accurate quantification and dose distributions based on phantom experiments and then applied to patient studies. Our results indicate that more accurate dose distribution can be achieved from smoother PET images, reconstructed with larger β values than those yielding the best activity quantifications but noisy images. Most importantly, these results suggest that quantitative measures, which are commonly used in clinics, such as SUV_max or SUV_peak( equivalent of D_max), should not be employed for ⁹⁰Y PET images, since their values would highly depend on the image reconstruction.

Cerenkov luminescence and PET imaging of 90Y: capabilities and limitations in small animal applications

The in vivo sensitivity limits and quantification performance of Cerenkov luminescence imaging have been studied using a tissue-like mouse phantom and ⁹⁰Y. For a small, 9 mm deep target in the phantom, with no background activity present, the Cerenkov luminescence ⁹⁰Y detection limit determined from contrast-to-noise ratios is 10 nCi for a 2 min exposure with a sensitive CCD camera and no filters. For quantitative performance, the values extracted from regions of interest on the images are linear within 5% of a straight line fit versus target activity for target activity of 70 nCi and above. The small branching ratio to decay with positron emission for ⁹⁰Y also permits low-statistics PET imaging of the radionuclide. For PET imaging of the same phantom, with a small animal LSO detector-based scanner, the ⁹⁰Y detection limit is approximately 3 orders of magnitude higher at 10 µCi.

The use of yttrium in medical imaging and therapy: historical background and future perspectives

Yttrium presents a wide palette of isotopes with interesting coordination and radiochemical properties. We review its most prominent isotopes and their diverse medical uses in therapy and imaging.

Measurement of the branching ratio related to the internal pair production of Y-90

The aim of the work is to determine the number of positron-electron pair creation in the E0 transition between the 1760.7 keV level and the ground level of the Zr-90 nucleus at the Y-90 decay. The number of conversions is determined from the number of 511 keV photons originating from positron annihilation corrected for the annihilation of positrons in flight. Emission of annihilation photons was determined from the measurements of 511 keV full-energy peak on two calibrated high-purity germanium detectors. The measurements were performed with two sources of a different construction. The first source was a 1 ml ampoule filled with Sr-90 solution (in equilibrium with Y-90) with an activity of 38.041 MBq inserted into an aluminum absorber. The other source was the evaporation residue of the Sr-90 solution (in equilibrium with Y-90) in a polyethylene absorber. In both cases, the annihilation of positrons occurred in the source materials. The efficiency of 511 keV photon detection was determined by Monte Carlo calculation, where the source was defined as a theoretical continuous positron spectrum with a maximum energy of 738 keV. The branching ratio related to the internal pair production during Y-90 decay was determined to be (3.26 ± 0.04) × 10^-5 pairs/decay.

Modeling sphere dynamics in blood vessels for SIRT pre-planning - To fathom the potential and limitations

For selective internal radiation therapy (SIRT) the calculation of the 3D distribution of spheres based on individual blood flow properties is still an open and relevant research question. The purpose of this work is to develop and analyze a new treatment planning method for SIRT to calculate the absorbed dose distribution. For this intention, flow dynamics of the SIRT-spheres inside the blood vessels was simulated. The challenge is treatment planning solely using high-resolution imaging data available before treatment. The resolution required to reliably predict the sphere distribution and hence the dose was investigated. For this purpose, arteries of the liver were segmented from a contrast-enhanced angiographic CT. Due to the limited resolution of the given CT, smaller vessels were generated via a vessel model. A combined 1D/3D-flow simulation model was implemented to simulate the final 3D distribution of spheres and dose. Results were evaluated against experimental data from Y90-PET. Analysis showed that the resolution of the vessels within the angiographic CT of about 0.5mm should be improved to a limit of about 150μm to reach a reliable prediction.

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.

99mTc-Labeled RGD-Polyethylenimine Conjugates with Entrapped Gold Nanoparticles in the Cavities for Dual-Mode SPECT/CT Imaging of Hepatic Carcinoma

We report the construction and characterization of ^99mTc-labeled arginine-glycine-aspartic acid (RGD)-polyethylenimine (PEI) conjugates with entrapped gold nanoparticles in the cavities (RGD-^99mTc-Au PENPs) for dual-mode single-photon emission computed tomography (SPECT)/computed tomography (CT) imaging of an orthotopic hepatic carcinoma model. In this study, PEI was successively decorated with diethylenetriaminepentaacetic acid, poly(ethylene glycol) (PEG), and PEGylated RGD segments, and was utilized as an effective nanoplatform to entrap Au NPs and to be labeled with ^99mTc. We showed that the designed RGD-^99mTc-Au PENPs displayed desirable colloidal stability and radiostability, and cytocompatibility in the investigated concentration range, and could be specifically uptaken by α_vβ₃ integrin-overexpressing liver cancer cells in vitro. In vivo CT and SPECT imaging results indicated that the particles were able to be accumulated within an orthotopic hepatic carcinoma and displayed both CT and SPECT contrast enhancement in the tumor tissue. With the proven biocompatibility in vivo via histological examinations, the designed RGD-^99mTc-Au PENPs may be potentially employed as an effective nanoprobe for a highly efficient dual-mode SPECT/CT imaging of various α_vβ₃ integrin-overexpressing tumors.

Phantom validation of quantitative Y-90 PET/CT-based dosimetry in liver radioembolization

Background

PET/CT has recently been shown to be a viable alternative to traditional post-infusion imaging methods providing good quality images of ⁹⁰Y-laden microspheres after selective internal radiation therapy (SIRT). In the present paper, first we assessed the quantitative accuracy of ⁹⁰Y-PET using an anthropomorphic phantom provided with lungs, liver, spine, and a cylindrical homemade lesion located into the hepatic compartment. Then, we explored the accuracy of different computational approaches on dose calculation, including (I) direct Monte Carlo radiation transport using Raydose, (II) Kernel convolution using Philips Stratos, (III) local deposition algorithm, (IV) Monte Carlo technique (MCNP) considering a uniform activity distribution, and (V) MIRD (Medical Internal Radiation Dose) analytical approach. Finally, calculated absorbed doses were compared with those obtained performing measurements with LiF:Mg,Cu,P TLD chips in a liquid environment.

Results

Our results indicate that despite ⁹⁰Y-PET being likely to provide high-resolution images, the ⁹⁰Y low branch ratio, along with other image-degrading factors, may produce non-uniform activity maps, even in the presence of uniform activity. A systematic underestimation of the recovered activity, both for the tumor insert and for the liver background, was found. This is particularly true if no partial volume correction is applied through recovery coefficients. All dose algorithms performed well, the worst case scenario providing an agreement between absorbed dose evaluations within 20%. Average absorbed doses determined with the local deposition method are in excellent agreement with those obtained using the MIRD and the kernel-convolution dose calculation approach.

Finally, absorbed dose assessed with MC codes are in good agreement with those obtained using TLD in liquid solution, thus confirming the soundness of both calculation approaches. This is especially true for Raydose, which provided an absorbed dose value within 3% of the measured dose, well within the stated uncertainties.

Conclusions

Patient-specific dosimetry is possible even in a scenario with low true coincidences and high random fraction, as in ⁹⁰Y–PET imaging, granted that accurate absolute PET calibration is performed and acquisition times are sufficiently long. Despite Monte Carlo calculations seeming to outperform all dose estimation algorithms, our data provide a strong argument for encouraging the use of the local deposition algorithm for routine ⁹⁰Y dosimetry based on PET/CT imaging, due to its simplicity of implementation.

Quantitative imaging, dosimetry and metrology; Where do National Metrology Institutes fit in?

In External Beam Radiotherapy, National Metrology Institutes (NMIs) play a critical role in the delivery of accurate absorbed doses to patients undergoing treatment. In contrast for nuclear medicine the role of the NMI is less clear and although significant work has been done in order to establish links for activity measurement, the calculation of administered absorbed doses is not traceable in the same manner as EBRT. Over recent decades the use of novel radiolabelled pharmaceuticals has increased dramatically. The limitation of secondary complications due to radiation damage to non-target tissue has historically been achieved by the use of activity escalation studies during clinical trials and this in turn has led to a chronic under dosing of the majority of patients. This paper looks to address the difficulties in combining clinical everyday practice with the grand challenges laid out by national metrology institutes to improve measurement capability in all walks of life. In the life sciences it can often be difficult to find the correct balance between pure research and practical solutions to measurement problems, and this paper is a discussion regarding these difficulties and how some NMIs have chosen to tackle these issues. The necessity of establishing strong links to underlying standards in the field of quantitative nuclear medicine imaging is highlighted. The difficulties and successes of current methods for providing traceability in nuclear medicine are discussed.

Investigation of the imaging characteristics of the ALBIRA II small animal PET system for 18F, 68Ga and 64Cu

AIM

In this study the performance characteristics of the Albira II PET sub-system and the response of the system for the following radionuclides ¹⁸F, ⁶⁸Ga and ⁶⁴Cu was analyzed.

MATERIALS AND METHODS

The Albira II tri-modal system (Bruker BioSpin MRI GmbH, Ettlingen, Germany) is a pre-clinical device for PET, SPECT and CT. The PET sub-system uses single continuous crystal detectors of lutetium yttrium orthosilicate (LYSO). The detector assembly consists of three rings of 8 detector modules. The transaxial field of view (FOV) has a diameter of 80mm and the axial FOV is 148mm. A NEMA NU-4 image quality phantom (Data Spectrum Corporation, Durham, USA) having five rods with diameters of 1, 2, 3, 4 and 5mm and a uniform central region was used. Measurements with ¹⁸F, ⁶⁸Ga and ⁶⁴Cu were performed in list mode acquisition over 10h. Data were reconstructed using a maximum-likelihood expectation-maximization (MLEM) algorithm with iteration numbers between 5 and 50. System sensitivity, count rate linearity, convergence and recovery coefficients were analyzed.

RESULTS

The sensitivities for the entire FOV (non-NEMA method) for ¹⁸F, ⁶⁸Ga and ⁶⁴Cu were (3.78±0.05)%, (3.97±0.18)% and (3.79±0.37)%, respectively. The sensitivity based on the NEMA protocol using the ²²Na point source yielded (5.53±0.06)%. Dead-time corrected true counts were linear for activities ≤7MBq (¹⁸F and ⁶⁸Ga) and ≤17MBq (⁶⁴Cu) in the phantom. The radial, tangential and axial full widths at half maximum (FWHMs) were 1.52, 1.47 and 1.48mm. Recovery coefficients for the uniform region with a total activity of 8MBq in the phantom were (0.97±0.05), (0.98±0.06), (0.98±0.06) for ¹⁸F, ⁶⁸Ga and ⁶⁴Cu, respectively.

CONCLUSION

The Albira II pre-clinical PET system has an adequate sensitivity range and the system linearity is suitable for the range of activities used for pre-clinical imaging. Overall, the system showed a favorable image quality for pre-clinical applications.

A gate evaluation of the sources of error in quantitative90 Y PET

PURPOSE

Accurate reconstruction of the dose delivered by ⁹⁰ Y microspheres using a postembolization PET scan would permit the establishment of more accurate dose-response relationships for treatment of hepatocellular carcinoma with ⁹⁰ Y. However, the quality of the PET data obtained is compromised by several factors, including poor count statistics and a very high random fraction. This work uses Monte Carlo simulations to investigate what impact factors other than low count statistics have on the quantification of⁹⁰ Y PET.

METHODS

PET acquisitions of two phantoms-a NEMA PET phantom and the NEMA IEC PET body phantom-containing either ⁹⁰ Y or ¹⁸ F were simulated using gate. Simulated projections were created with subsets of the simulation data allowing the contributions of random, scatter, and LSO background to be independently evaluated. The simulated projections were reconstructed using the commercial software for the simulated scanner, and the quantitative accuracy of the reconstruction and the contrast recovery of the reconstructed images were evaluated.

RESULTS

The quantitative accuracy of the ⁹⁰ Y reconstructions were not strongly influenced by the high random fraction present in the projection data, and the activity concentration was recovered to within 5% of the known value. The contrast recovery measured for simulated ⁹⁰ Y data was slightly poorer than that for simulated ¹⁸ F data with similar count statistics. However, the degradation was not strongly linked to any particular factor. Using a more restricted energy range to reduce the random fraction in the projections had no significant effect.

CONCLUSIONS

Simulations of ⁹⁰ Y PET confirm that quantitative ⁹⁰ Y is achievable with the same approach as that used for ¹⁸ F, and that there is likely very little margin for improvement by attempting to model aspects unique to ⁹⁰ Y, such as the much higher random fraction or the presence of bremsstrahlung in the singles data.

Evaluating the Application of Tissue-Specific Dose Kernels Instead of Water Dose Kernels in Internal Dosimetry: A Monte Carlo Study

PURPOSE

The aim of this work is to evaluate the application of tissue-specific dose kernels instead of water dose kernels to improve the accuracy of patient-specific dosimetry by taking tissue heterogeneities into consideration.

MATERIALS AND METHODS

Tissue-specific dose point kernels (DPKs) and dose voxel kernels (DVKs) for yttrium-90 (⁹⁰Y), lutetium-177 (¹⁷⁷Lu), and phosphorus-32 (³²P) are calculated using the Monte Carlo (MC) simulation code GATE (version 7). The calculated DPKs for bone, lung, adipose, breast, heart, intestine, kidney, liver, and spleen are compared with those of water. The dose distribution in normal and tumorous tissues in lung, liver, and bone of a Zubal phantom is calculated using tissue-specific DVKs instead of those of water in conventional methods. For a tumor defined in a heterogeneous region in the Zubal phantom, the absorbed dose is calculated using a proposed algorithm, taking tissue heterogeneity into account. The algorithm is validated against full MC simulations.

RESULTS

The simulation results indicate that the highest differences between water and other tissue DPKs occur in bone for ⁹⁰Y (12.2% ± 0.6%), ³²P (18.8% ± 1.3%), and ¹⁷⁷Lu (16.9% ± 1.3%). The second highest discrepancy corresponds to the lung for ⁹⁰Y (6.3% ± 0.2%), ³²P (8.9% ± 0.4%), and ¹⁷⁷Lu (7.7% ± 0.3%). For ⁹⁰Y, the mean absorbed dose in tumorous and normal tissues is calculated using tissue-specific DVKs in lung, liver, and bone. The results are compared with doses calculated considering the Zubal phantom water equivalent and the relative differences are 4.50%, 0.73%, and 12.23%, respectively. For the tumor in the heterogeneous region of the Zubal phantom that includes lung, liver, and bone, the relative difference between mean calculated dose in tumorous and normal tissues based on the proposed algorithm and the values obtained from full MC dosimetry is 5.18%.

CONCLUSIONS

A novel technique is proposed considering tissue-specific dose kernels in the dose calculation algorithm. This algorithm potentially enables patient-specific dosimetry and improves estimation of the average absorbed dose of ⁹⁰Y in a tumor located in lung, bone, and soft tissue interface by 6.98% compared with the conventional methods.

Computational simulation of the predicted dosimetric impact of adjuvant yttrium-90 PET/CT-guided percutaneous ablation following radioembolization

BACKGROUND

90Y PET/CT post-radioembolization imaging has demonstrated that the distribution of 90Y in a tumor can be non-uniform. Using computational modeling, we predicted the dosimetric impact of post-treatment 90Y PET/CT-guided percutaneous ablation of the portions of a tumor receiving the lowest absorbed dose. A cohort of fourteen patients with non-resectable liver cancer previously treated using 90Y radioembolization were included in this retrospective study. Each patient exhibited potentially under-treated areas of tumor following treatment based on quantitative 90Y PET/CT. 90Y PET/CT was used to guide electrode placement for simulated adjuvant radiofrequency ablation in areas of tumor receiving the lowest dose. The finite element method was used to solve Penne's bioheat transport equation, coupled with the Arrhenius thermal cell-death model to determine 3D thermal ablation zones. Tumor and unablated tumor absorbed-dose metrics (average dose, D50, D70, D90, V100) following ablation were compared, where D70 is the minimum dose to 70% of tumor and V100 is the fractional tumor volume receiving more than 100 Gy.

RESULTS

Compared to radioembolization alone, 90Y radioembolization with adjuvant ablation was associated with predicted increases in all tumor dose metrics evaluated. The mean average absorbed dose increased by 11.2 ± 6.9 Gy. Increases in D50, D70, and D90 were 11.0 ± 6.9 Gy, 13.3 ± 10.9 Gy, and 11.8 ± 10.8 Gy, respectively. The mean increase in V100 was 7.2 ± 4.2%. All changes were statistically significant (P < 0.01). A negative correlation between pre-ablation tumor volume and D50, average dose, and V100 was identified (ρ < - 0.5, P < 0.05) suggesting that adjuvant radiofrequency ablation may be less beneficial to patients with large tumor burdens.

CONCLUSIONS

This study has demonstrated that adjuvant 90Y PET/CT-guided radiofrequency ablation may improve tumor absorbed-dose metrics. These data may justify a prospective clinical trial to further evaluate this hybrid approach.

Optimization of Image Reconstruction for 90Y Selective Internal Radiotherapy on a Lutetium Yttrium Orthosilicate PET/CT System Using a Bayesian Penalized Likelihood Reconstruction Algorithm

Imaging on a γ-camera with ⁹⁰Y after selective internal radiotherapy (SIRT) may allow for verification of treatment delivery but suffers relatively poor spatial resolution and imprecise dosimetry calculation. ⁹⁰Y PET/CT imaging is possible on 3-dimensional, time-of-flight machines; however, images are usually poor because of low count statistics and noise. A new PET reconstruction software using a Bayesian penalized likelihood (BPL) reconstruction algorithm (termed Q.Clear) was investigated using phantom and patient scans to optimize the reconstruction for post-SIRT imaging and clarify whether BPL leads to an improvement in clinical image quality using ⁹⁰Y. Methods: Phantom studies over an activity range of 0.5-4.2 GBq were performed to assess the contrast recovery, background variability, and contrast-to-noise ratio for a range of BPL and ordered-subset expectation maximization (OSEM) reconstructions on a PET/CT scanner. Patient images after SIRT were reconstructed using the same parameters and were scored and ranked on the basis of image quality, as assessed by visual evaluation, with the corresponding SPECT/CT Bremsstrahlung images by 2 experienced radiologists. Results: Contrast-to-noise ratio was significantly better in BPL reconstructions when compared with OSEM in phantom studies. The patient-derived BPL and matching Bremsstrahlung images scored higher than OSEM reconstructions when scored by radiologists. BPL with a β value of 4,000 was ranked the highest of all images. Deadtime was apparent in the system above a total phantom activity of 3.3 GBq. Conclusion: BPL with a β value of 4,000 is the optimal image reconstruction in PET/CT for confident radiologic reading when compared with other reconstruction parameters for ⁹⁰Y imaging after SIRT imaging. Activity in the field of view should be below 3.3 GBq at the time of PET imaging to avoid deadtime losses for this scanner.

Physics of pure and non-pure positron emitters for PET: a review and a discussion

With the increased interest in new PET tracers, gene-targeted therapy, immunoPET, and theranostics, other radioisotopes will be increasingly used in clinical PET scanners, in addition to ¹⁸F. Some of the most interesting radioisotopes with prospective use in the new fields are not pure short-range β⁺ emitters but can be associated with gamma emissions in coincidence with the annihilation radiation (prompt gamma), gamma-gamma cascades, intense Bremsstrahlung radiation, high-energy positrons that may escape out of the patient skin, and high-energy gamma rays that result in some e⁺/e⁻ pair production. The high level of sophistication in data correction and excellent quantitative accuracy that has been reached for ¹⁸F in recent years can be questioned by these effects. In this work, we review the physics and the scientific literature and evaluate the effect of these additional phenomena on the PET data for each of a series of radioisotopes: ¹¹C, ¹³N, ¹⁵O, ¹⁸F, ⁶⁴Cu, ⁶⁸Ga, ⁷⁶Br, ⁸²Rb, ⁸⁶Y, ⁸⁹Zr, ⁹⁰Y, and ¹²⁴I. In particular, we discuss the present complications arising from the prompt gammas, and we review the scientific literature on prompt gamma correction. For some of the radioisotopes considered in this work, prompt gamma correction is definitely needed to assure acceptable image quality, and several approaches have been proposed in recent years. Bremsstrahlung photons and ¹⁷⁶Lu background were also evaluated.

Transarterial Radioembolization with Yttrium-90 for the Treatment of Hepatocellular Carcinoma

Background

Hepatocellular carcinoma (HCC) is a common cause of worldwide mortality. Transarterial radioembolization (TARE) with yttrium-90 (Y90), a transcatheter intra-arterial procedure performed by interventional radiology, has become widely utilized in managing HCC.

Methods

The following is a focused review of TARE covering its commercially available products, clinical considerations of treatment, salient clinical trial data establishing its utility, and the current and future roles of TARE in the management of HCC.

Results

TARE is indicated for patients with unresectable, intermediate stage HCC. The two available products are glass and resin microspheres. All patients undergoing TARE must be assessed with a history, physical examination, clinical laboratory tests, imaging, and arteriography with macroaggregated albumin. TARE is safe and effective in the treatment of unresectable HCC, as it has a safer toxicity profile than chemoembolization, longer time-to-progression, greater ability to downsize and/or bridge patients to liver transplant, and utility in tumor complicated by portal vein thrombosis. TARE can also serve as an alternative to ablation and chemotherapy.

Conclusion

TARE assumes an integral role in the management of unresectable HCC and has been validated by numerous studies.

Combination Radioimmunotherapy Approaches and Quantification of Immuno-PET

Monoclonal antibodies (mAbs), which play a prominent role in cancer therapy, can interact with specific antigens on cancer cells, thereby enhancing the patient's immune response via various mechanisms, or mAbs can act against cell growth factors and, thereby, arrest the proliferation of tumor cells. Radionuclide-labeled mAbs, which are used in radioimmunotherapy (RIT), are effective for cancer treatment because tumor associated-mAbs linked to cytotoxic radionuclides can selectively bind to tumor antigens and release targeted cytotoxic radiation. Immunological positron emission tomography (immuno-PET), which is the combination of PET with mAb, is an attractive option for improving tumor detection and mAb quantification. However, RIT remains a challenge because of the limited delivery of mAb into tumors. The transport and uptake of mAb into tumors is slow and heterogeneous. The tumor microenvironment contributed to the limited delivery of the mAb. During the delivery process of mAb to tumor, mechanical drug resistance such as collagen distribution or physiological drug resistance such as high intestinal pressure or absence of lymphatic vessel would be the limited factor of mAb delivery to the tumor at a potentially lethal mAb concentration. When α-emitter-labeled mAbs were used, deeper penetration of α-emitter-labeled mAb inside tumors was more important because of the short range of the α emitter. Therefore, combination therapy strategies aimed at improving mAb tumor penetration and accumulation would be beneficial for maximizing their therapeutic efficacy against solid tumors.

Beyond 18F-FDG: Characterization of PET/CT and PET/MR Scanners for a Comprehensive Set of Positron Emitters of Growing Application--18F, 11C, 89Zr, 124I, 68Ga, and 90Y

This study aimed to investigate image quality for a comprehensive set of isotopes ((18)F, (11)C, (89)Zr, (124)I, (68)Ga, and (90)Y) on 2 clinical scanners: a PET/CT scanner and a PET/MR scanner.

METHODS

Image quality and spatial resolution were tested according to NU 2-2007 of the National Electrical Manufacturers Association. An image-quality phantom was used to measure contrast recovery, residual bias in a cold area, and background variability. Reconstruction methods available on the 2 scanners were compared, including point-spread-function correction for both scanners and time of flight for the PET/CT scanner. Spatial resolution was measured using point sources and filtered backprojection reconstruction.

RESULTS

With the exception of (90)Y, small differences were seen in the hot-sphere contrast recovery of the different isotopes. Cold-sphere contrast recovery was similar across isotopes for all reconstructions, with an improvement seen with time of flight on the PET/CT scanner. The lower-statistic (90)Y scans yielded substantially lower contrast recovery than the other isotopes. When isotopes were compared, there was no difference in measured spatial resolution except for PET/MR axial spatial resolution, which was significantly higher for (124)I and (68)Ga.

CONCLUSION

Overall, both scanners produced good images with (18)F, (11)C, (89)Zr, (124)I, (68)Ga, and (90)Y.

A multicentre comparison of quantitative (90)Y PET/CT for dosimetric purposes after radioembolization with resin microspheres : The QUEST Phantom Study

Purpose

To investigate and compare the quantitative accuracy of ⁹⁰Y imaging across different generation PET/CT scanners, for the purpose of dosimetry after radioembolization with resin microspheres.

Methods

A strict experimental and imaging protocol was followed by 47 international sites using the NEMA 2007/IEC 2008 PET body phantom with an 8-to-1 sphere-to-background ratio of ⁹⁰Y solution. The phantom was imaged over a 7-day period (activity ranging from 0.5 to 3.0 GBq) and all reconstructed data were analysed at a core laboratory for consistent processing. Quantitative accuracy was assessed through measures of total phantom activity, activity concentration in background and hot spheres, misplaced counts in a nonradioactive insert, and background variability.

Results

Of the 69 scanners assessed, 37 had both time-of-flight (ToF) and resolution recovery (RR) capability. These current generation scanners from GE, Philips and Siemens could reconstruct background concentration measures to within 10 % of true values over the evaluated range, with greater deviations on the Philips systems at low count rates, and demonstrated typical partial volume effects on hot sphere recovery, which dominated spheres of diameter <20 mm. For spheres >20 mm in diameter, activity concentrations were consistently underestimated by about 20 %. Non-ToF scanners from GE Healthcare and Siemens were capable of producing accurate measures, but with inferior quantitative recovery compared with ToF systems.

Conclusion

Current generation ToF scanners can consistently reconstruct ⁹⁰Y activity concentrations, but they underestimate activity concentrations in small structures (≤37 mm diameter) within a warm background due to partial volume effects and constraints of the reconstruction algorithm. At the highest count rates investigated, measures of background concentration (about 300 kBq/ml) could be estimated on average to within 1 %, 5 % and 2 % for GE Healthcare (all-pass filter, RR + ToF), Philips (4i8s ToF) and Siemens (2i21s all-pass filter, RR + ToF) ToF systems, respectively. Over the range of activities investigated, comparable performance between GE Healthcare and Siemens ToF systems suggests suitability for quantitative analysis in a scenario analogous to that of postradioembolization imaging for treatment of liver cancer.

Increased absorbed liver dose in Selective Internal Radiation Therapy (SIRT) correlates with increased sphere-cluster frequency and absorbed dose inhomogeneity

Background

The higher tolerated mean absorbed dose for selective internal radiation therapy (SIRT) with intra-arterially infused ⁹⁰Y microspheres compared to external beam therapy is speculated to be caused by absorbed dose inhomogeneity, which allows for liver regeneration. However, the complex liver microanatomy and rheology makes modelling less valuable if the tolerance doses are not based on the actual microsphere distribution. The present study demonstrates the sphere distribution and small-scale absorbed dose inhomogeneity and its correlation with the mean absorbed dose in liver tissue resected after SIRT.

Methods

A patient with marginally resectable cholangiocarcinoma underwent SIRT 9 days prior to resection including adjacent normal liver tissue. The resected specimen was formalin-fixed and sliced into 1 to 2-mm sections. Forty-one normal liver biopsies 6-8 mm in diameter were punched from these sections and the radioactivity measured. Sixteen biopsies were further processed for detailed analyses by consecutive serial sectioning of 15 30-μm sections per biopsy, mounted and stained with haematoxylin-eosin. All sections were scrutinised for isolated or conglomerate spheres. Small-scale dose distributions were obtained by applying a ⁹⁰Y-dose point kernel to the microsphere distributions.

Results

A total of 3888 spheres were found in the 240 sections. Clusters were frequently found as strings in the arterioles and as conglomerates in small arteries, with the largest cluster comprising 453 spheres. An increased mean absorbed dose in the punch biopsies correlated with large clusters and a greater coefficient of variation. In simulations the absorbed dose was 5–1240 Gy; 90% were 10-97 Gy and 45% were <30 Gy, the assumed tolerance in external beam therapy.

Conclusions

Sphere clusters were located in both arterioles and small arteries and increased in size with increasing sphere concentration, resulting in increased absorbed dose inhomogeneity, which contradicts earlier modelling studies.