Radiological Physics and Technology最新文献

{"title":"Development and validation of an automated MRI-based pipeline for temporal classification of intracerebral hemorrhage using U-Net segmentation and machine learning.","authors":"Masayoshi Mori, Takanori Masuda, Toshinori Matsushige, Mikako Takeuchi, Takashi Fujimoto, Masaki Ishikawa","doi":"10.1007/s12194-026-01126-x","DOIUrl":"https://doi.org/10.1007/s12194-026-01126-x","url":null,"abstract":"<p><p>This study aimed to develop and validate an automated magnetic resonance imaging (MRI)-based pipeline for temporal classification of intracerebral hemorrhage (ICH) using clinically feasible multi-sequence MRI, 2.5D U-Net segmentation, and machine learning. This retrospective study included a development cohort of 56 patients and an independent validation cohort of 115 patients with ICH. Five MRI sequences-diffusion-weighted imaging, apparent diffusion coefficient map, fluid-attenuated inversion recovery (FLAIR), time-of-flight magnetic resonance angiography, and susceptibility-weighted angiography-were analyzed. A pontine reference region of interest was automatically placed using FLAIR-based brain segmentation. Hematoma regions were automatically segmented using a 2.5D U-Net trained in the development cohort, and the largest three-dimensional connected component was defined as the representative hematoma region of interest. Pontine ROI-referenced signal features and shape features were extracted, and Random Forest, XGBoost, and multinomial logistic regression classifiers were compared for five-stage temporal classification. The selected final model was applied to the validation cohort. In the validation cohort, automated hematoma segmentation achieved a Dice similarity coefficient of 0.794 ± 0.208, sensitivity of 0.799 ± 0.235, and precision of 0.842 ± 0.151. One case with automated hematoma ROI extraction failure was excluded from temporal classification analysis. In the remaining 114 cases, the final Random Forest model achieved an accuracy of 0.807, balanced accuracy of 0.763, macro F1-score of 0.749, and mean Average Precision of 0.851. The proposed automated MRI-based pipeline showed feasible performance for ICH temporal classification and may support objective MRI-based assessment of hemorrhage stage.</p>","PeriodicalId":46252,"journal":{"name":"Radiological Physics and Technology","volume":" ","pages":""},"PeriodicalIF":1.6,"publicationDate":"2026-08-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148799333","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Comparative dosimetry study of PARaDIM and RT-PHITS with ICRP publication 128: calculation of organ doses from ¹⁸F-FDG radiopharmaceutical.","authors":"Shupti Sarker, Donghee Han, Shingo Baba, Toshioh Fujibuchi","doi":"10.1007/s12194-026-01124-z","DOIUrl":"https://doi.org/10.1007/s12194-026-01124-z","url":null,"abstract":"<p><p>This study evaluates organ doses from ¹⁸F-FDG whole-body PET-CT using the phantom-based software PARaDIM and patient image-based software RT-PHITS and compares with reference data from ICRP Publication 128. Absorbed dose coefficients (mGy/MBq) were calculated using ICRP Publication 145 mesh-type phantoms in PARaDIM and 20 patient datasets (10 male, 10 female) from 'The Cancer Imaging Archives' website in RT-PHITS. Organs were segmented in 3D Slicer, and dose coefficients were derived after physical decay correction of the resultant dose rates (mGy/s). The patient cohort (n = 20; 10 males, 10 females) had an average age of 52.1 years, height of 172.1 cm, weight of 76.65 kg, and total administered activity of 306.55 MBq. Absorbed dose coefficients from PARaDIM and RT-PHITS ranged from 0.01 to 0.07 mGy/MBq. RT-PHITS showed acceptable agreement with ICRP 128 and PARaDIM for most organs, except for the urinary bladder and heart wall. Standard deviations (SD) for RT-PHITS urinary bladder doses were 0.04 relative to ICRP 128 and 0.12 relative to PARaDIM; heart wall SDs were 0.016 and 0.014, respectively. Overall, RT-PHITS organ doses yielded an average SD of 0.0073, with the highest deviation in the heart wall. The results indicate that anatomical differences lead to measurable variations between the generalized and individualized dosimetry methodologies. Because a consistent physical decay model of ¹⁸F was used, these variations primarily reflect structural and geometric factors. This study provides a framework for advancing individualized dosimetry approaches and supports future developments in cumulative exposure assessments and potential theranostic applications.</p>","PeriodicalId":46252,"journal":{"name":"Radiological Physics and Technology","volume":" ","pages":""},"PeriodicalIF":1.6,"publicationDate":"2026-08-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148799300","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Irradiation-induced modulation of T₂ relaxation times in sucrose solutions: a novel externally controlled design strategy for quantitative magnetic resonance imaging phantoms.","authors":"Yasuo Takatsu, Saki Kubota, Naoki Hayashi, Kenmei Mizutani, Mizuki Ito, Akiyoshi Iwase, Masataka Oita, Tosiaki Miyati, Soma Kumasaka","doi":"10.1007/s12194-026-01107-0","DOIUrl":"https://doi.org/10.1007/s12194-026-01107-0","url":null,"abstract":"<p><p>This study investigated whether irradiation-induced chemical changes in sucrose solutions could be utilized to control magnetic resonance imaging (MRI) relaxation properties and to propose a novel design strategy for quantitative MRI phantoms. Aqueous sucrose solutions at concentrations of 5-40% (w/w) containing 3.6 g/L NaCl were irradiated with 6 MV photons at doses of 0-20,000 monitor units. T₁ and T₂ relaxation times were measured at 1.5 T. Hydrogen peroxide (H₂O₂) and glucose concentrations were quantified to assess the radiolytic products. Linear regression and Type III ANOVA were used to evaluate the effects of sucrose concentration and irradiation dose. The T₂ relaxation times increased with increasing irradiation dose (p < 0.001), with a significant interaction between sucrose concentration and dose (p < 0.001). The slope of T₂ monotonically decreased with increasing sucrose concentration. R₂ (defined as 1/T₂) showed a significant linear dose-dependence at all concentrations, with the absolute magnitude of the negative regression coefficients increasing at higher sucrose concentrations, which was consistent with the reciprocal relationship between T₂ and R₂. In contrast, both T₁ and R₁ (defined as 1/T₁) exhibited minimal dose-dependence. The H₂O₂ concentration increased stepwise with the dose and demonstrated a strong positive correlation with T₂ (Spearman's ρ = 1.00), whereas glucose showed no consistent dose-dependence. Irradiation enabled concentration-dependent modulation of T₂ relaxation in sucrose solutions without altering the initial composition. This externally controllable approach provides a simple and flexible framework for designing quantitative MRI phantoms with tunable relaxation properties and offers a new paradigm beyond conventional composition-based phantom fabrication.</p>","PeriodicalId":46252,"journal":{"name":"Radiological Physics and Technology","volume":" ","pages":""},"PeriodicalIF":1.6,"publicationDate":"2026-08-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148799305","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Single-slice ct body-weight prediction with image and metadata fusion: a multi-model comparison.","authors":"Daniel Carrion, M K Badawy","doi":"10.1007/s12194-026-01122-1","DOIUrl":"10.1007/s12194-026-01122-1","url":null,"abstract":"<p><p>A measurable fraction of examinations in retrospective imaging archives lack a recorded body weight, limiting metadata completeness for tasks such as cohort auditing, weight-stratified analysis, and the assembly of machine-learning training datasets. We developed and compared three approaches for body-weight prediction from a single middle-slice CT image: a LightGBM model on 19 CT-derived tabular features (water equivalent diameter, effective diameter, body composition, scanner parameters), an ImageNet-pretrained ResNet-18 on images alone, and a ResNet-18 with late fusion of the same 19 tabular features into the 512-dimensional image embedding. Models were trained with 5-fold patient-disjoint cross-validation on a combined train+validation development pool of 30,857 examinations from 16,169 patients; the best-performing fold of each model family was selected by within-fold validation mean absolute error (MAE) and evaluated, without further tuning, on a held-out test partition of 3,479 examinations from 1,796 patients (patient-disjoint from training and validation). On the test set, the fusion ResNet-18 achieved MAE 4.06 kg (95% CI 3.90-4.23; R² = 0.92), with 69.5% of predictions within ± 5 kg and 93.8% within ± 10 kg. A weighted ensemble of the fusion ResNet and LightGBM produced a small further reduction in MAE to 3.99 kg (paired bootstrap p < .001), but the 0.07 kg difference is below practically meaningful thresholds. The simpler single-model ResNet-18 with metadata fusion may be useful for retrospective missing-weight imputation, with split-conformal 90% and 95% prediction intervals (empirical test coverage 89% and 94%) available for case-level uncertainty quantification.</p>","PeriodicalId":46252,"journal":{"name":"Radiological Physics and Technology","volume":" ","pages":""},"PeriodicalIF":1.6,"publicationDate":"2026-08-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148799283","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Ensemble principal component analysis-based multi-omics for radiation pneumonitis prediction in stage III non-small cell lung cancer: a pipeline comparison.","authors":"Wynn Wingyi Lee, Noriyuki Kadoya, Yoshiyuki Katsuta, Taichi Hoshino, Takaya Yamamoto, Keiichi Jingu","doi":"10.1007/s12194-026-01112-3","DOIUrl":"https://doi.org/10.1007/s12194-026-01112-3","url":null,"abstract":"<p><p>Conventional univariate feature-selection pipelines for radiation pneumonitis (RP) prediction are prone to overfitting in small radiotherapy cohorts, while alternative dimensionality-reduction approaches remain insufficiently evaluated. This study compared a Principal Component Analysis (PCA)-based ensemble pipeline comprising variance pre-filtering, PCA, and LASSO with a conventional pipeline comprising Mann-Whitney U testing, Spearman correlation filtering, and ElasticNet. The analysis included 73 retrospectively analyzed patients with stage III non-small cell lung cancer (training, n = 52; testing, n = 21). A total of 214 features, including 107 radiomic and 107 dosiomic features, were extracted from planning CT images and three-dimensional dose distributions. Both pipelines used five-model ensemble logistic regression with inverse-frequency weighting. Performance was evaluated using testing AUC, the training-to-testing AUC gap, 10-fold nested cross-validation, paired bootstrap comparison of AUC differences with 2,000 resamples, Brier score, and bootstrap feature-selection stability. The PCA-based model achieved a testing AUC of 0.765 with a training-to-testing AUC gap of 0.020, compared with an AUC of 0.684 and a gap of 0.181 for the conventional model (ΔAUC = + 0.081, p = 0.265). The PCA-based model also achieved a nested cross-validation AUC of 0.742. PCA-based dimensionality reduction produced a smaller training-to-testing AUC gap and greater feature-selection stability than conventional univariate filtering, indicating improved internal robustness and reduced overfitting in this small-cohort setting.</p>","PeriodicalId":46252,"journal":{"name":"Radiological Physics and Technology","volume":" ","pages":""},"PeriodicalIF":1.6,"publicationDate":"2026-08-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"148799254","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Impact of surface-guided prepositioning and respiratory coaching on the target localization accuracy in lung stereotactic body radiation therapy.","authors":"Kazuki Onishi, Naoki Hayashi, Tatsunori Saito, Yuta Muraki, Shinya Neri, Masashi Nozue","doi":"10.1007/s12194-026-01008-2","DOIUrl":"10.1007/s12194-026-01008-2","url":null,"abstract":"<p><p>This study assessed the effect of surface-guided radiation therapy (SGRT)-based prepositioning and respiratory coaching on target localization accuracy in lung stereotactic body radiation therapy (SBRT) using deep inspiration breath-holding. Thirty-five patients treated with lung SBRT (September 2022 to March 2025) were classified into three groups: Group A, VOXELAN prepositioning with reproducible respiratory control (≥ 60% setup criteria satisfied); Group B, VOXELAN prepositioning without reproducible control; and Group C, no VOXELAN prepositioning. Cone-beam computed tomography (CBCT) after prepositioning was used to retrospectively assess target localization. The concordance between the VOXELAN setup criteria and CBCT errors (> 5 mm) was analyzed using Fisher's exact test. Positional deviations and rotations were compared among groups using analysis of variance and post-hoc tests. Satisfying the VOXELAN setup criteria significantly correlated with CBCT localization within 5 mm (p = 0.0027). Vertical errors were smaller in Groups A and B than in Group C (p < 0.01), and lateral errors were smaller in Groups A and B than in Group C (p = 0.01 and p < 0.01, respectively). Rotational errors were within ± 1° in all groups, with a significant difference between Groups A and C (p < 0.02). Longitudinal errors were not significantly different between the groups. SGRT-based prepositioning with respiratory coaching improved setup reproducibility and correlation with the internal target position, particularly in the vertical, lateral, and rotational axes. Longitudinal accuracy remained limited, suggesting caution in margin reduction.</p>","PeriodicalId":46252,"journal":{"name":"Radiological Physics and Technology","volume":" ","pages":"471-478"},"PeriodicalIF":1.6,"publicationDate":"2026-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC13253711/pdf/","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147272514","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Evaluation of the clinical validity of fixation accuracy using an open-face black mask in head stereotactic radiotherapy.","authors":"Nanae Takahashi, Yutaka Naoi, Naoko Okano, Yuka Inage, Chie Kurokawa, Naoto Shikama","doi":"10.1007/s12194-026-01041-1","DOIUrl":"10.1007/s12194-026-01041-1","url":null,"abstract":"<p><p>In stereotactic radiotherapy (SRT), precise alignment is crucial for treatment effectiveness and safety and has traditionally been achieved using cone beam computed tomography (CBCT). Recently, radiation-free optimal surface imaging (OSI) systems have gained popularity. Notably, the effectiveness of black face masks for identifying facial surfaces during OSI use has been increasingly reported. This study aimed to determine whether the fixation accuracy of an open-face black mask designed to facilitate detection using the OSI system is clinically acceptable. This study was approved by the institutional review board (protocol code E23-0162) on May 27th, 2023. We analyzed the accuracy and correlation of CBCT- and OSI-based measurements before and after treatment in 20 patients who underwent SRT using open-face black mask fixation. Additionally, intra-fractional motion was monitored in real time using the OSI system. Mean translational deviations measured by CBCT and OSI were 0.1 mm and 0.1 mm (lateral), -0.2 mm and 0.1 mm (longitudinal), and − 0.1 mm and 0.2 mm (vertical), respectively. Mean rotational deviations were 0.1° and − 0.1° (rotation), -0.4° and − 0.1° (roll), and 0.1° and − 0.1° (pitch). All measurements met clinical accuracy criteria. Weak correlations between CBCT and OSI data were observed only in the lateral and roll directions. Several transient outliers were noted in real-time OSI monitoring. The fixation accuracy achieved with the open-face black mask was within clinically acceptable limits based on both CBCT and OSI measurements. Further research is warranted to clarify the source of noise and improve the reliability of real-time OSI-based monitoring.</p>","PeriodicalId":46252,"journal":{"name":"Radiological Physics and Technology","volume":" ","pages":"692-698"},"PeriodicalIF":1.6,"publicationDate":"2026-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC13253583/pdf/","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147515843","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Establishment of local diagnostic reference levels for trunk computed tomography examinations at governmental hospitals in the Gaza Strip: a cross-sectional study.","authors":"Amjad Ayyad, Yasser Alajerami, Ahmed Najim, Husam H Mansour, Muhammad Khalis Abdul Karim, Fahad Alghamdi, Kinan Mokbel","doi":"10.1007/s12194-026-01053-x","DOIUrl":"10.1007/s12194-026-01053-x","url":null,"abstract":"<p><p>To establish the local diagnostic reference levels (LDRLs) for trunk multi-slice CT (MSCT) examinations in the Gaza Strip, Palestine. This cross-sectional study included adult oncology patients undergoing trunk MSCT at two governmental hospitals in Gaza Strip; Al Shifa Medical Complex (SMC) and Al Aqsa Martyrs Hospital (AMH), using an adapted dose survey booklet. Data collected from July 2019 to March 2020 included patient characteristics, volumetric CT dose index (CTDI_vol) and dose length product (DLP). Descriptive, univariate and multivariate analyses identified key factors affecting radiation dose, and the coefficient of variation between scanner- and software-derived dose values was also determined. A total of 170 trunk CT examinations were analysed (57.1% SMC, 42.9% AMH). 72.94% were female; the mean age of the participants is 53.1 ± 15.8 years, and the mean body mass index was 30 ± 6.1. The estimated LDRLs for trunk CT were 13 mGy for CTDI_vol and 1010.4 mGy·cm for DLP. There was notable variation between hospitals in CTDI_vol and DLP (p < 0.001). At SMC, factors such as tube current, peak kilovoltage, scan length, pitch and BMI significantly affected dose indices. In contrast, at AMH, the main influences were tube current and scan length. CTDI_vol had a greater impact on DLP than scan length at both locations. LDRLs for trunk CT scans in the Gaza Strip were established and found to be generally comparable to international benchmarks. Notable variation in doses between hospitals indicates potential for improvement through standardising protocols, managing scan lengths and using techniques tailored to patient size.</p>","PeriodicalId":46252,"journal":{"name":"Radiological Physics and Technology","volume":" ","pages":"782-791"},"PeriodicalIF":1.6,"publicationDate":"2026-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC13253881/pdf/","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147785246","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Technical strategies for radiation dose optimisation in paediatric pelvic radiography: a systematic review.","authors":"Koustubh Kamath, Suresh Sukumar, Leena R David, Rajagopal Kadavigere, Hitesh Hasmukhlal Shah, Nitika C Panakkal, Abhimanyu Pradhan, Winniecia Dkhar, Hari Prakash Planiswamy, Sneha Ravichandran, Dilip Shettigar, Sathya Sabina Muthu","doi":"10.1007/s12194-026-01055-9","DOIUrl":"10.1007/s12194-026-01055-9","url":null,"abstract":"<p><p>Paediatric pelvic radiography is a frequently performed diagnostic procedure that exposes radiosensitive organs to ionising radiation. Optimisation of imaging parameters is essential to minimise radiation dose while preserving diagnostic image quality. This systematic review aimed to evaluate technical strategies for radiation dose optimisation in paediatric pelvic radiography and assess their impact on radiation dose and image quality. A systematic search of PubMed, Scopus, Embase, and Web of Science was conducted for studies published between 2004 and 2025. Experimental and clinical studies assessing radiation dose and image quality in paediatric pelvic radiography were included. Data on exposure parameters, dose metrics, and image quality assessment methods were extracted and synthesised narratively. Nine studies met the inclusion criteria, predominantly comprising phantom-based experimental designs. Increasing tube potential (kVp), extending source-to-image distance (SID), and applying additional aluminium or copper filtration consistently reduced radiation dose. Dose reductions of up to 60-70% were reported without clinically significant loss of image quality. Although objective image quality metrics (e.g., CNR, SNR) occasionally declined, subjective assessments confirmed that images remained diagnostically acceptable. Optimisation of technical parameters in paediatric pelvic radiography can achieve substantial radiation dose reduction while maintaining clinically acceptable image quality. However, the predominance of phantom-based evidence highlights the need for further patient-based clinical studies to support routine implementation.</p>","PeriodicalId":46252,"journal":{"name":"Radiological Physics and Technology","volume":" ","pages":"454-470"},"PeriodicalIF":1.6,"publicationDate":"2026-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.ncbi.nlm.nih.gov/pmc/articles/PMC13253702/pdf/","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147949875","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Understanding nonlinearity in weighted least-squares image reconstruction for nuclear medicine.","authors":"Hiroyuki Shinohara","doi":"10.1007/s12194-026-01021-5","DOIUrl":"10.1007/s12194-026-01021-5","url":null,"abstract":"<p><p>This study investigated a linear approximation of weighted least squares (WLS) and its regularized variants using reductio ad absurdum and numerical simulations. Three phantoms were employed in the simulation: point source 1 represented by a two-dimensional (2D) Gaussian with a full width at half maximum of 3 pixels positioned at the center of the background; point source 2, a 2D Gaussian positioned 24 pixels away from point source 1 along the x-axis; and point source 3 is the sum of them. Ideal 2D parallel-beam projections were used. When the absolute difference of area under curve or the absolute difference of recovery between the reconstructed images of point source 3 and sum of point source 1 and 2, falls within a predetermined threshold, it was defined as linear approximation. The variance of WLS was estimated using either the projections of original image or forward projections. Filtered backprojection and least squares were used as a reference of linear reconstruction. We proved that WLS is nonlinear for small number of iterations but is approximated as linear for large number of iterations by reductio ad absurdum. WLS regularized with a quadratic function (WLS-QU) and a relative difference function (WLS-RD) were both proved to be nonlinear. Simulation results showed that WLS was nonlinear, but it becomes linear approximation when the number of iterations approached 50 with 64 subsets. WLS-QU and WLS-RD was nonlinear.　When the likelihood term itself is nonlinear, the reconstruction becomes nonlinear irrespective of the form of the regularization function.</p>","PeriodicalId":46252,"journal":{"name":"Radiological Physics and Technology","volume":" ","pages":"520-539"},"PeriodicalIF":1.6,"publicationDate":"2026-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"146198053","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}