EJNMMI Physics最新文献

Background: The novel upright walk-through PET (WT-PET) scanner enhances patient throughput compared to the conventional cylindrical PET systems but introduces unique challenges related to patient motion. This study evaluates the rigid body motion of the head, shoulders, chest, and abdomen of patients standing in a WT-PET mock-up scanner, focusing on ergonomic features, including a headrest and hand supports, designed to minimize motion during upright imaging. To contextualize these findings, a patient study using a conventional PET scanner was conducted, along with a control healthy volunteer study involving both WT-PET and conventional PET systems.

Methods: Motion data were collected from 30 patients positioned on the WT-PET, 13 patients scanned with a conventional cylindrical PET, and 12 healthy volunteers scanned with both systems. Infrared markers placed at anatomical positions tracked three-dimensional marker positions during 30-s periods of normal breathing and breath-hold instructions in the WT-PET mock-up. Conventional PET scans for patients and healthy volunteers involved 8-min acquisitions. Motion was quantified by calculating the Euclidean distance (ED) of the markers' 3D centroids.

Results: In WT-PET patients, breath-holding significantly reduced mean abdominal motion by 24%, with mean ED decreasing from 2.31 ± 1.32 mm during normal breathing to 1.76 ± 0.81 mm during breath-holding. While only 30% of patients completed a full 30-s breath hold, 80% maintained breath holds longer than 20 s. Age was significantly correlated with increased head motion during normal breathing, whereas body mass index and gender showed no significant effects. Compared with WT-PET healthy volunteers, patient motion on the WT-PET was over three times higher for the head (0.47 ± 0.13 mm vs. 1.51 ± 2.32 mm) and 36% higher for the abdomen (1.70 ± 0.63 mm vs. 2.31 ± 1.32 mm). Relative to patients in conventional PET, WT-PET patients showed slightly lower head motion (1.55 ± 1.05 mm vs. 1.51 ± 2.32 mm), but abdominal motion was 44% lower in WT-PET (2.31 ± 1.32 mm vs. 4.12 ± 3.00 mm), underscoring both the distinct motion patterns and the specific challenges of upright imaging.

Conclusions: The upright WT-PET scanner presents distinct motion control challenges in clinical practice. This study demonstrates that combining ergonomic supports with breath-holding protocols can effectively reduce patient motion during upright PET imaging; however, a full 30-s breath-hold is not feasible for 70% of patients. Since 50% of patients were able to perform a moderate breath-hold, a two-step acquisition can be performed, each comprising 15 s. Moreover, including a healthy volunteer control group and comparisons with conventional PET confirm both the feasibility and the current limitations of the WT-PET.

Background: Studies evaluating the impact of advances in CT dosimetry tools on CT organ dose estimations are often limited to a comparison with TLD measurements in anthropomorphic phantoms or a comparison of different dosimetry tools using computational phantoms and CT examinations performed at radiology departments. This study evaluates organ dose estimations obtained using a patient-specific Monte Carlo simulation and a computational phantom-based dosimetry tool for whole-body PET/CT examinations. In addition, the correlation of organ doses with the size-specific dose estimate (SSDE) was investigated.

Methods: Using the Monte Carlo software ImpactMC, patient-specific organ doses were simulated in 100 adult patients using whole-body CT scans acquired on a Siemens Biograph mCT Flow and a GE Discovery MI PET/CT. For each patient, organ doses were also estimated using the computational phantom-based dosimetry tool NCICT. Absolute and normalised to CTDI_vol organ doses and percentage dose differences were assessed for CT acquisitions performed with tube current modulation (TCM). Statistical and regression analysis was performed to evaluate dose differences, their correlation with patient characteristics and the relationship with SSDE.

Results: The average percentage difference of NCICT to ImpactMC organ doses across all organs and BMI categories for whole-body examinations performed with TCM was - 5% and - 22% for the Siemens and GE PET/CT, respectively. Strong variations are observed between patients. Depending on the organ of interest, NCICT under-or overestimates the organ dose. Nevertheless, depending on the PET/CT system, moderate to excellent agreement was found between organ doses estimated with NCICT and ImpactMC. No correlations were observed between the obtained organ dose differences and patient length (R² < 0.1), while weak to no or moderate correlations were found with patient weight (0.2 < R² < 0.6) and BMI (0.2 < R² < 0.7). Very strong correlations (R² > 0.9) were observed between the estimated organ doses and SSDE.

Conclusion: Compared to the patient-specific Monte Carlo CT dosimetry software ImpactMC, the computational phantom-based dosimetry tool NCICT could provide organ dose estimates within ± 22% for whole-body CT scans acquired with TCM. If better accuracies are required, patient-specific Monte Carlo simulations are recommended. Depending on the organ of interest and the specific CT scanner, SSDE may be a good first estimate of the organ dose.

Purpose: This study compares the subcellular dosimetry of ¹⁶¹ Tb and ¹⁷⁷Lu, focusing on β⁻ particles, conversion electrons, and Auger electrons, and their relative contributions to cellular and subcellular damage. We aim to evaluate whether the higher emission yields of ¹⁶¹ Tb provide a therapeutic advantage, particularly for non-internalizing targeting agents in radiopharmaceutical therapy.

Methods: A stochastic radionuclide decay model was implemented in MATLAB, incorporating internal conversion and Auger cascades and validated against ICRP 107. Geant4 track code simulations modeled electron transport in single-cell and voxelized membrane geometries. Energy deposition was assessed in the membrane, cytoplasm, and nucleus for 10,000 decays of each radionuclide.

Results: ¹⁶¹ Tb achieved similar nuclear energy deposition as ¹⁷⁷Lu with about 25% of the decays, due to its higher yield of internal conversion. These conversion electrons contribute to nuclear damage playing a crucial role in cell damage. Auger electrons from ¹⁶¹ Tb additionally produced highly localized energy deposition at the cell membrane, that could also contribute to cell death. However, when normalizing for equivalent radiotoxicity to the bone marrow, around 75% of the ¹⁶¹ Tb decays provide a similar marrow absorbed dose as ¹⁷⁷Lu, while still increasing the absorbed dose to the nucleus by approximately 18%.

Conclusion: ¹⁶¹ Tb offers a more efficient subcellular energy deposition profile than ¹⁷⁷Lu. It enables either similar therapeutic effect with fewer decays or enhanced nuclear absorbed dose under equivalent bone marrow toxicity. These results support the use of ¹⁶¹ Tb in targeted radiopharmaceutical therapy, particularly for isolated tumor cells and micrometastases.

Purpose: Patlak parametric imaging is widely employed for kinetic modeling due to its simplicity and robustness. The time-to-equilibrium (t*), which must be defined to estimate kinetic parameters, is currently set empirically and uniformly across the entire body. In this study, we evaluate the regional impact of varying t* values on kinetic parameter estimates using a multi-tissue segmentation approach at the whole-body level.

Methods: Data from 53 patients who underwent one-hour dynamic 18 F-FDG PET/CT scans were retrospectively analyzed. Parametric maps of the net influx rate (K_i) and blood distribution volume (dv) were calculated for four t* values (10, 20, 30, and 45 min) using in-house software (PET KinetiX). Voxel-wise K_i and dv values were extracted from 10 predefined tissue structures through automated segmentation. Using t* = 30 min as the widely accepted reference, relative mean errors and relative absolute mean errors of K_i and dv estimated at t*shifts = 10, 20 and 45 min were calculated for each tissue. Pearson correlation coefficients between K_i or dv reference values and those estimated at t* shifts = 10, 20, and 45 min were also computed.

Results: Compared to the reference t*30, K_i estimates ranged from - 21.4% (liver) to 7.3% (SAT) at t*10, and from - 13.8% (lungs) to 2.4% (brain) at t*20. Median absolute bias was 12.8% at t*10 (6.5% brain to > 25% liver) and 8.6% at t*20 (3.2% brain to > 15% lungs and liver). At t*45, K_i was consistently overestimated, with a median bias of 19.4% (2.7% brain to > 33% lungs and liver) and median absolute bias of 19.8% (5.5% brain to > 33% lungs and liver). For dv, biases ranged from - 25.2% (brain) to 8.6% (spleen) at t*10; - 13.7% (brain) to 5.7% (lungs) at t*20; - 15.5% (liver) to 8.8% (brain) at t*45. Median absolute biases were 14.0% at t*10 (9.8% heart to 25.2% brain), 9.4% at t*20 (7.7% heart to 14.1% brain), and 15% at t*45 (12.4% skeletal muscle to 18.5% brain). Regardless of t*, K_i values exhibited strong linear correlations (r > 0.7) across all organs, whereas dv correlations showed greater variability, falling below 0.7 in 80% of organs at t*45.

Conclusion: Kinetic parameter sensitivity to time-to-equilibrium (t*) varies across organs in Patlak whole-body parametric imaging, underscoring the necessity of adopting flexible or adaptive t* values at the whole-body level.

The ¹⁷⁷Lu-PSMA therapy is an established treatment for metastatic castration-resistant prostate cancer (mCRPC), targeting the prostate-specific membrane antigen (PSMA). Despite well-established correlations between ⁶⁸Ga-PSMA PET/CT imaging and outcome, predicting individual patient responses remains a significant challenge. This study introduces an automated method for computing the total tumor volume (TTV) from ⁶⁸Ga-PSMA PET/CT imaging and develops predictive models to assess patient biological response via the PSA50 criterion. A retrospective analysis was conducted on a real-world data cohort of 139 mCRPC patients treated in our institution. TTV was automatically extracted from PET/CT images and correlated with treatment response, defined by PSA50 criteria. Machine learning models, including Logictic Regression with L1 (LASSO) and Support Vector Machine (SVM), were developed to predict PSA50 response using imaging and clinical features. The best-performing models achieved F1-scores of 0.68 and 0.67, comparable to existing nomograms. Correlation analysis identified TTV-derived features and time since diagnosis as significant predictors of response. The proposed workflow offers an automated and reproducible approach to predicting treatment response in ¹⁷⁷Lu-PSMA therapy. Limitations remain for lesion segmentation within physiological regions.

Background: The administration of ¹⁷⁷Lu-DOTATATE peptide receptor radionuclide therapy to patients for the treatment of well-differentiated, metastatic neuroendocrine tumours poses an external radiation hazard due to gamma emissions of lutetium-177. Patients are provided with standardised precautions to follow in the first 16 days following therapy, distancing themselves from family members and to limit their radiation exposure to less than 5mSv in 5 years in compliance with UK legislation. The purpose of this study was to measure the radiation exposure of adult family members of patients undergoing ¹⁷⁷Lu-DOTATATE radionuclide therapy using thermoluminescent dosimeters (TLDs) worn continuously for 4 weeks following therapy administration, and to establish whether existing radiation protection precautions are adequately protecting patients' family members.

Results: Participating family members (n = 12) received a median of 0.038 mSv effective dose over 4 weeks from patients administered a median of 7515 MBq ¹⁷⁷Lu-DOTATATE. Patients remained in the Nuclear Medicine department for an average of 6.2 h post-administration and at the time of discharge the median dose rates were 150 µSv/h at 0.1 m and 15 µSv/h at 1 m from the patient's anterior abdomen, corresponding to a median lutetium-177 retention of 35.8% of the administered activity at the time of discharge measured using quantitative SPECT-CT imaging. Family members spent a median total time over the 4 week measurement period of 39.5 h at 1 m and 19.9 h at less than 1 m from the patient.

Conclusion: Implementing standardised contact restrictions for patients and their family members following ¹⁷⁷Lu-DOTATATE PRRT limits the dose received by family members to less than 5 mSv in 5 years, ensuring sufficient protection and compliance with the UK legislation.

Background: This study aimed to evaluate the impact of F-fluorodeoxyglucose (¹⁸F-FDG) uptake outside the field of view on image quality of a silicon photomultiplier tube (SiPM)-based dedicated breast (db) positron emission tomography scanner using a cylindrical breast phantom and simulated myocardium.

Methods: The spheres, arranged in a planar circle inside the phantom, had 3, 5, 7.5, and 10 mm diameters. The cylinder and four spheres were filled with an ¹⁸F-FDG solution to maintain a sphere-to-background (BG) radioactivity concentration ratio of 8:1. The cylindrical breast phantom was positioned so that the centre position of the detector was aligned with the centre of the hot sphere. To simulate ¹⁸F-FDG accumulation in the myocardium, a cylindrical plastic container filled with ¹⁸F-FDG was placed on the cylindrical breast phantom. Scans were performed without and with varying levels of myocardial radioactivity concentration. The myocardial radioactivity concentration was set at ratios of 1, 5, 10, and 15 relative to the BG portion of the cylindrical breast phantom. The coefficient of variation of the BG (COV_BG), contrast recovery coefficient (CRC), and detectability index (DI) were assessed.

Results: The COV_BG tended to increase with higher myocardial radioactivity concentrations, but this trend was not entirely consistent. CRC showed a decreasing tendency for smaller hot spheres as myocardial radioactivity concentrations increased, with greater variability observed. DI tended to decrease as myocardial radioactivity concentrations increased.

Conclusions: Our findings suggested that the presence of radioactivity outside the field of view increases reduces small-lesion contrast at the centre of the detector ring of the SiPM-based db positron emission tomography scanner.

Background: Targeted alpha therapy (TAT) utilizing high-LET alpha particles from radionuclides like ²²⁵Ac and ²²⁷Th shows promise in cancer treatment due to their ability to induce dense, localized DNA damage. This study uses the Geant4-DNA Monte Carlo toolkit to simulate DNA damage and evaluate the radiobiological effects of ²²⁵Ac and ²²⁷Th.

Results: The simulations revealed that both ²²⁵Ac and ²²⁷Th induce significant DNA damage, particularly complex double-strand breaks (DSBs), with ²²⁷Th producing slightly more clustered damage due to its longer decay chain and higher alpha energy. The study also found that both radionuclides exhibited elevated Relative Biological Effectiveness (RBE), especially for complex DSBs. Cell survival analysis showed a sharp decline in viability, consistent with the clustered damage patterns induced by alpha emissions.

Conclusions: These findings underscore the potential of ²²⁵Ac and ²²⁷Th in targeted radionuclide therapy, particularly for tumors resistant to conventional treatments. The high RBE and complex DNA damage induced by these radionuclides suggest they could enhance therapeutic efficacy when combined with other treatment modalities, such as chemotherapy or immunotherapy, and may benefit patients with tumors exhibiting high DNA repair capacity. This study provides valuable insights for optimizing TAT protocols and advancing the clinical translation of ²²⁵Ac and ²²⁷Th therapies.

Background: PET data-driven attenuation correction (AC) methods, including deep learning, are attractive options for quantitative brain imaging on CT-less brain PET systems and low-dose PET/CT. However, current schemes have performance and practical limitations. We previously developed a CT-less transmission-aided AC that combines coincidences from a weak positron source, and the patient, to estimate attenuation with physics alone. In this work, we aim to optimize and assess this new AC method during human [¹⁸F]FDG neuroimaging on whole-body PET/CT.

Methods: Our approach, TRansmission-aided μ-map reconstruction (TRU) AC, includes 1) a low-profile and physically fixed transmission source filled with ~ 14 MBq of ¹⁸F, 2) a modified maximum likelihood reconstruction of attenuation and activity algorithm, and 3) scatter corrections using the exam data alone. We imaged N = 5 patients with the transmission source, immediately after their clinical [¹⁸F]FDG PET/CT. The clinically-consistent protocol included a CT and 10-minute brain-focused PET exam. Using this 10 minutes of patient PET data alone, radiotracer images were reconstructed with the vendor's algorithm and TRU-AC or CT-AC (reference standard), with all else matched. For quantitative analysis, we placed brain-structure volumes of interest with an atlas, and computed error in mean standardized uptake values of TRU-AC relative to CT-AC.

Results: TRU-AC PET showed qualitatively strong agreement with CT-AC. For the VOI analysis, absolute relative error in standardized uptake values for TRU-AC was within 3.6%, across all brain structures and patients. Normalized root mean square error of activity bias for TRU-AC was 1.8%, and voxel-wise noise in the cerebellum showed a very minor increase of 0.2%. Bland-Altman analysis demonstrated that TRU-AC and CT-AC have statistically significant agreement, assuming a maximum allowed difference of ± 5%.

Conclusions: TRU-AC enables quantitative PET for human neuroimaging. This approach may particularly benefit exams where deep learning-based AC schemes show reduced performance, including those focused on radiotracer development, new patient cohorts, and/or pathologies that often lack sufficient training data.