Acquisition Duration Optimization Using Visual Grading Regression in [68Ga]FAPI-46 PET Imaging of Oncologic Patients.

Fibroblast activation protein is a promising target for oncologic molecular imaging with radiolabeled fibroblast activation protein inhibitors (FAPI) in a large variety of cancers. However, there are yet no published recommendations on how to set up an optimal imaging protocol for FAPI PET/CT. It is important to optimize the acquisition duration and strive toward an acquisition that is sufficiently short while simultaneously providing sufficient image quality to ensure a reliable diagnosis. The aim of this study was to evaluate the feasibility of reducing the acquisition duration of [68Ga]FAPI-46 imaging while maintaining satisfactory image quality, with certainty that the radiologist's ability to make a clinical diagnosis would not be affected. Methods: [68Ga]FAPI-46 PET/CT imaging was performed on 10 patients scheduled for surgical resection of suspected pancreatic cancer, 60 min after administration of 3.6 ± 0.2 MBq/kg. The acquisition time was 4 min/bed position, and the raw PET data were statistically truncated and reconstructed to represent images with an acquisition duration of 1, 2, and 3 min/bed position, additional to the reference images of 4 min/bed position. Four image quality criteria that focused on the ability to distinguish specific anatomic details, as well as perceived image noise and overall image quality, were scored on a 4-point Likert scale and analyzed with mixed-effects ordinal logistic regression. Results: A trend toward increasing image quality scores with increasing acquisition duration was observed for all criteria. For the overall image quality, there was no significant difference between 3 and 4 min/bed position, whereas 1 and 2 min/bed position were rated significantly (P < 0.05) lower than 4 min/bed position. For the other criteria, all images with a reduced acquisition duration were rated significantly inferior to images obtained at 4 min/bed position. Conclusion: The acquisition duration can be reduced from 4 to 3 min/bed position while maintaining satisfactory image quality. Reducing the acquisition duration to 2 min/bed position or lower is not recommended since it results in inferior-quality images so noisy that clinical interpretation is significantly disrupted.

[ 18 F]-PSMA-1007 PET imaging optimization and inter-rater reliability - a comparison of three different reconstructions read by four radiologists.

OBJECTIVES: To increase understanding of optimal imaging parameters [ 18 F]PSMA-1007 when imaging patients with prostate cancer and to determine interrater agreement using [ 18 F]PSMA-1007. METHODS: In this observational study, four independent physicians read reconstruction sets using bedtimes of 1, 2 and 3 minutes of patients undergoing [ 18 F]PSMA-1007. positron emission topography. Clear and equivocal lesions and their locations were recorded. Image noise was rated on a four-point scale. Lesion counts were compared using inter-class correlation whereas noise ratings were compared using generalized estimating equations. Repeated cases were used to assess intra-rater agreement. RESULTS: Sixty reconstruction sets of 16 consecutively examined participants were included. Participants had a mean age of 71.5 years, six of them were examined prior to any treatment, three had a history of radiotherapy and seven of prostatectomy. Median Gleason score of primary tumors was 7. Imaging was performed after a mean of 132 min using a mean 3.95 MBq/Kg body weight of [ 18 F] PSMA-1007. Neither the total number of lesions per location nor the proportion of equivocal lesions varied consistently between bedtimes. Inter-rater reliability scores varied depending on location from 0.40 to 1.0 and were similar for all bedtimes. Intra-rater reliability varied between 0.70 and 0.76 for the three different bedtimes. Noise ratings were significantly lower for 1 minute than 3 minutes per bed. CONCLUSION: In the setting of [ 18 F]PSMA-1007 PET CT, 1, 2 and 3 minutes per bed produce similar results unlikely to affect clinical interpretation. Image noise ratings favor 2 and 3 minutes per bed.

Tumor Characterization by [68Ga]FAPI-46 PET/CT Can Improve Treatment Selection for Pancreatic Cancer Patients: An Interim Analysis of a Prospective Clinical Trial.

Correct and timely diagnosis of pancreatic cancer (PC) is essential for treatment selection but is still clinically challenging. Standard-of-care imaging methods can sometimes not differentiate malignancies from inflammatory lesions or detect malignant transformation in premalignant lesions. This interim analysis of a prospective clinical trial aimed to evaluate the diagnostic accuracy of [68Ga]fibroblast activation protein inhibitor (FAPI)-46 PET/CT for PC and determine the sample size needed to demonstrate whether this imaging technique improves the characterization of equivocal lesions detected by standard-of-care imaging methods. Methods: [68Ga]FAPI-46 PET/CT imaging was performed on 30 patients scheduled for surgical resection of suspected PC. Target lesions were delineated, SUVmax and SUVmean were determined, and the results were compared with those of standard-of-care imaging. Receiver operating characteristics were calculated for the whole cohort and a subcohort of 11 patients with an equivocal clinical imaging work-up preoperatively. Postoperative histopathologic findings served as a reference standard, and the statistical power was determined. Results: Histopathologic examination revealed malignancy in 20 patients and benign lesions in 10 patients. Significantly elevated [68Ga]FAPI-46 uptake was observed in malignant tumors compared with benign lesions (P < 0.001). Receiver-operating-characteristic analyses established optimal cutoffs for both SUVs for differentiation of malignant from nonmalignant pancreatic tumors. The optimal SUVmax cutoff was 10.2 and showed 95% sensitivity and 80% specificity for the whole cohort, as well as 100% diagnostic accuracy when considering the subcohort with equivocal imaging work-up only. For sufficient statistical power, 38 equivocal observations are needed. Conclusion: We conclude that [68Ga]FAPI-46 PET/CT can accurately differentiate malignant from benign pancreatic lesions deemed equivocal by standard-of-care imaging. This trial will therefore continue to recruit a total of 120 patients to reach those 38 equivocal observations needed for sufficient statistical power. On the basis of our findings, we propose that [68Ga]FAPI-46 PET/CT not only can be clinically applied as a complement but also could become a necessary tool when standard-of-care imaging is inconclusive.

99mTc-Sestamibi SPECT/CT and histopathological features of oncocytic renal neoplasia.

BACKGROUND: 99mTc-Sestamibi Single Photon Emission Computed Tomography/Computed Tomography (SPECT/CT) contributes to the non-invasive differentiation of renal oncocytoma (RO) from renal cell carcinoma (RCC) by characterising renal tumours as Sestamibi positive or Sestamibi negative regarding their 99mTc-Sestamibi uptake compared to the non-tumoral renal parenchyma. PURPOSE: To determine whether 99mTc- Sestamibi uptake in renal tumour and the non-tumoral renal parenchyma measured using Standard Uptake Value (SUV) SPECT, has a beneficial role in differentiating RO from RCC. MATERIAL AND METHODS: Fifty-seven renal tumours from 52 patients were evaluated. In addition to visual evaluation of 99mTc-Sestamibi uptake, SUVmax measurements were performed in the renal tumour and the ipsilateral non-tumoral renal parenchyma. Analysis of the area under the receiver operating characteristic curve identified an optimal cut-off value for detecting RO, based on the relative ratio of 99mTc- Sestamibi uptake. RESULTS: Semiquantitative evaluation of 99mTc-Sestamibi uptake did not improve the performance of 99mTc- Sestamibi SPECT/CT in detecting RO. 99mTc- Sestamibi SPECT/CT identifies a group of mostly indolent Sestamibi-positive tumours with low malignant potential containing RO, Low-Grade Oncocytic Tumours, Hybrid Oncocytic Tumours, and a subset of chromophobe RCCs. CONCLUSION: The imaging limitations for accurate differentiation of Sestamibi-positive renal tumours mirror the recognised diagnostic complexities of the histopathologic evaluation of oncocytic neoplasia. Patients with Sestamibi-positive renal tumours could be better suited for biopsy and follow-up, according to the current active surveillance protocols.

A 68Ga-FAPI-46 PET/CT Imaging Pitfall in Assessing Residual Gastric Cancer Early After Chemotherapy.

ABSTRACT: After neoadjuvant chemotherapy, preoperative imaging with 68Ga-FAPI-46 PET/CT showed a similar level of tracer uptake at the location of the primary tumors in 2 patients with gastric cancer. Postoperative histopathology revealed residual malignant cells only in one of the patients, whereas the elevated FAPI uptake in the other patient correlated to an inflammatory reaction and fibrosis. With this case, we would like to highlight that an increased FAPI uptake in inflammatory and fibrotic tissue early after chemotherapy may represent a potential interpretation pitfall. Further studies evaluating the clinical application of FAPI-PET in assessing residual cancer tissue are warranted.

99mTc-HMPAO-WBC SPECT/CT versus 18F-FDG-WBC PET/CT in chronic prosthetic joint infection: a pilot study.

PURPOSE: The aim of this study was to compare 99mTc-HMPAO-WBC-SPECT/CT combined with 99mTc-nanocollloid SPECT/CT and 18F-FDG-WBC-PET/CT combined with 99mTc-Nanocollloid SPECT/CT for the diagnosis and treatment evaluation of chronic prosthetic joint infection (PJI). METHODS: Patients with suspected chronic PJI were examined with 99mTc-HMPAO-WBC SPECT/CT, 18F-FDG-WBC PET/CT, and 99mTc-nanocolloid SPECT/CT (to visualize bone marrow). The location and patterns of uptake were noted and compared between the two leukocyte examinations. Both leukocyte examinations were evaluated visually for infection. The PET examinations were also evaluated semiquantitatively. Chronic PJI was verified clinically by microbial culture and successfully treated PJI was confirmed by 12 months symptom-free follow-up after cessation of antibiotics. RESULTS: Nineteen patients were included with 10 hip prostheses and nine knee prostheses. Fourteen were diagnosed with chronic PJI and five with successfully treated PJI. The sensitivity of visual evaluation of 99mTc-WBC-HMPAO SPECT/CT for all joints was 0.31 and for 18F-FDG-WBC PET/CT 0.38. The specificity was 0.80 and 0.83, respectively. All patients with a true-positive SPECT examination had a false-negative PET examination and vice versa. Semiquantitative evaluation of the hips gave an area under the curve of 0.905 using the iliac crest as the background. Semiquantitative evaluation of the knees did not produce significant results. CONCLUSION: This pilot study showed no difference in the sensitivity or specificity of 99mTc-HMPAO-WBC SPECT/CT and 18F-FDG-WBC PET/CT when combined with 99mTc-nanocollloid SPECT/CT in the diagnosis or treatment evaluation of suspected late chronic PJI.

Prospective data-driven respiratory gating of [68Ga]Ga-DOTATOC PET/CT.

AIM: The aim of this prospective study was to evaluate a data-driven gating software's performance, in terms of identifying the respiratory signal, comparing [68Ga]Ga-DOTATOC and [18F]FDG examinations. In addition, for the [68Ga]Ga-DOTATOC examinations, tracer uptake quantitation and liver lesion detectability were assessed. METHODS: Twenty-four patients with confirmed or suspected neuroendocrine tumours underwent whole-body [68Ga]Ga-DOTATOC PET/CT examinations. Prospective DDG was applied on all bed positions and respiratory motion correction was triggered automatically when the detected respiratory signal exceeded a certain threshold (R value ≥ 15), at which point the scan time for that bed position was doubled. These bed positions were reconstructed with quiescent period gating (QPG), retaining 50% of the total coincidences. A respiratory signal evaluation regarding the software's efficacy in detecting respiratory motion for [68Ga]Ga-DOTATOC was conducted and compared to [18F]FDG data. Measurements of SUVmax, SUVmean, and tumour volume were performed on [68Ga]Ga-DOTATOC PET and compared between gated and non-gated images. RESULTS: The threshold of R ≥ 15 was exceeded and gating triggered on mean 2.1 bed positions per examination for [68Ga]Ga-DOTATOC as compared to 1.4 for [18F]FDG. In total, 34 tumours were evaluated in a quantitative analysis. An increase of 25.3% and 28.1%, respectively, for SUVmax (P < 0.0001) and SUVmean (P < 0.0001), and decrease of 21.1% in tumour volume (P < 0.0001) was found when DDG was applied. CONCLUSIONS: High respiratory signal was exclusively detected in bed positions where respiratory motion was expected, indicating reliable performance of the DDG software on [68Ga]Ga-DOTATOC PET/CT. DDG yielded significantly higher SUVmax and SUVmean values and smaller tumour volumes, as compared to non-gated images.

Dual-tracer approach vs. dual time-point approach in leukocyte scintigraphy in treatment evaluation of persistent chronic prosthetic joint infection.

BACKGROUND: Both dual time-point 99mTc-hexamethylpropylene amine oxime (HMPAO)-leukocyte scintigraphy and dual-tracer 99mTc-HMPAO-leukocyte scintigraphy (with the addition of 99mTc-nanocolloid bone marrow scintigraphy) have been used to diagnose prosthetic joint infection (PJI). A treatment evaluation of persistent PJI using these imaging protocols has yet to be presented. OBJECTIVE: The purpose of this study was to compare the accuracy of dual time-point 99mTc-HMPAO-leukocyte scintigraphy to the dual-tracer alternative of single time-point 99mTc-HMPAO-leukocyte scintigraphy or single-photon emission computed tomography/computed tomography (SPECT/CT) combined with a 99mTc-nanocolloid bone marrow scintigraphy or SPECT/CT, for treatment evaluation of PJI. MATERIAL AND METHODS: Thirty-one PJI patients under antibiotic treatment were included in this retrospective study. Examinations were organized into three settings. Setting one used dual time-point approach with delayed (2 h) and late (24 h) planar 99mTc-HMPAO-leukocyte scintigraphy, including both visual and semiquantitative analysis. Setting two used delayed (2 h) planar 99mTc-HMPAO-leukocyte scintigraphy combined with 99mTc-nanocolloid bone marrow scintigraphy and for setting three SPECT/CT replaced planar imaging. RESULTS: Accuracy was 0.68 for visual evaluation and 0.55 for semiquantitative evaluation of setting one; 0.71 for setting two; and 0.68 for setting three. Sensitivity was 0.54 for visual evaluation and 0.31 for semiquantitative evaluation of setting one; 0.38 for setting two; and 0.46 for setting three. Specificity was 0.78 for visual evaluation and 0.72 for semiquantitative evaluation of setting one; 0.94 for setting two; and 0.83 for setting three. CONCLUSION: No significant difference in accuracy, sensitivity, or specificity between the approaches for treatment evaluation of suspected persistent PJI in the hip or knee was observed.

Is 99mTc-HMPAO-leukocyte imaging an accurate method in evaluating therapy result in prosthetic joint infection and diagnosing suspected chronic prosthetic joint infection?

BACKGROUND: To investigate the sensitivity and specificity of 99mTc-HMPAO-leukocyte imaging in evaluating therapy result in patients with prosthetic joint infection (PJI) and in diagnosing suspected chronic PJI. METHODS: Sixty-two patients (63 joints) with microbiologically verified PJI were examined by leukocyte imaging to evaluate therapy result during or at the end of antibiotic treatment or if the patient had a chronic PJI after treatment. Group 1 consisted of 49 patients with on-going or within less than 14 days of ending antibiotic treatment examined to evaluate response. Group 2 consisted of 13 patients examined after completed treatment on suspicion of chronic PJI with no or recently initiated renewed antibiotic treatment. This study applied a combination of different imaging approaches of 99mTc-HMPAO-leukocyte scintigraphy: delayed and late planar images, bone marrow imaging and SPECT/CT imaging. All joints were examined with at least two of the approaches and 53 joints with all three approaches. The report was based on the combined results of the approaches used. A chronic PJI was confirmed with a positive microbiological culture. A cured infection was confirmed with either a negative culture or at least 24 months antibiotic-free follow-up with no relapse. RESULTS: In the therapy evaluation group sensitivity was 0.57 and specificity was 0.81. In the suspected chronic infection group sensitivity was 1.00 and specificity 0.91. CONCLUSIONS: 99mTc-HMPAO-leukocyte imaging appears to be an accurate method to diagnose or exclude chronic PJI, but cannot be recommended for therapy evaluation of PJI in patients with on-going antibiotic treatment.

Comparison of acquisition protocols for ventilation/perfusion SPECT-a Monte Carlo study.

One of the most commonly used imaging techniques for diagnosing pulmonary embolism (PE) is ventilation/perfusion (V/P) scintigraphy. The aim of this study was to evaluate the performance of the currently used imaging protocols for V/P single photon emission computed tomography (V/P SPECT) at two nuclear medicine department sites and to investigate the effect of altering important protocol parameters. The Monte Carlo technique was used to simulate 4D digital phantoms with perfusion defects. Six imaging protocols were included in the study and a total of 72 digital patients were simulated. Six dually trained radiologists/nuclear medicine physicians reviewed the images and reported all perfusion mismatch findings. The radiologists also visually graded the image quality. No statistically significant differences in diagnostic performance were found between the studied protocols, but visual grading analysis pointed out one protocol as significantly superior to four of the other protocols. Considering the study results, we have decided to harmonize our clinical protocols for imaging patients with suspected PE. The administered Technegas and macro aggregated albumin activities have been altered, a low energy all purpose collimator is used instead of a low energy high resolution collimator and the acquisition times have been lowered.

Intra- and Interobserver Agreement of SUV SPECT Quantitative SPECT/CT Processing Software, Applied in Clinical Settings for Patients with Solid Renal Tumors.

Quantification tools for SPECT/CT are a field of ongoing research among hybrid imaging techniques. A recent multicenter study evaluating SUV SPECT software on phantoms showed that quantitative SPECT/CT is reproducible in Hybrid Viewer software. The aim of our study was to evaluate the intra- and interobserver agreement of quantitative SUV SPECT measurements in clinical settings for patients with solid renal tumors. Methods: The evaluation was part of a study that examined the role of 99mTc-sestamibi SPECT/CT in the characterization of solid kidney tumors and the differentiation of renal oncocytomas from renal cell carcinomas. Quantitative evaluation of SUV measurements was performed in Hybrid Viewer PDR, version 2.5. Forty-eight renal lesions were identified and examined twice by 2 independent readers. The agreement of the SUV measurements, concerning renal tumors and adjacent renal parenchyma, was evaluated using the intraclass correlation coefficient (ICC). Results: ICC for SUVmax measurements by the same reader was 97%-99% for solid renal tumors and 92%-98% for ipsilateral healthy renal parenchyma. ICC for SUVmax measurements between readers was 87%-89% for solid renal tumors and 72%-73% for ipsilateral healthy renal parenchyma. Estimated ICC for SUVmean measurements of solid renal tumors was 95%-98% for the same reader and 86%-89% between readers. Similar results were found for SUVpeak measurements. Conclusion: The high ICCs indicate a strong agreement among SUV measurements for patients with solid renal lesions undergoing 99mTc-sestamibi SPECT/CT, not only for measurements by the same reader but also for measurements between 2 different readers.

Scatter and crosstalk corrections for (99m)Tc/(123)I dual-radionuclide imaging using a CZT SPECT system with pinhole collimators.

PURPOSE: The energy spectrum for a cadmium zinc telluride (CZT) detector has a low energy tail due to incomplete charge collection and intercrystal scattering. Due to these solid-state detector effects, scatter would be overestimated if the conventional triple-energy window (TEW) method is used for scatter and crosstalk corrections in CZT-based imaging systems. The objective of this work is to develop a scatter and crosstalk correction method for (99m)Tc/(123)I dual-radionuclide imaging for a CZT-based dedicated cardiac SPECT system with pinhole collimators (GE Discovery NM 530c/570c). METHODS: A tailing model was developed to account for the low energy tail effects of the CZT detector. The parameters of the model were obtained using (99m)Tc and (123)I point source measurements. A scatter model was defined to characterize the relationship between down-scatter and self-scatter projections. The parameters for this model were obtained from Monte Carlo simulation using SIMIND. The tailing and scatter models were further incorporated into a projection count model, and the primary and self-scatter projections of each radionuclide were determined with a maximum likelihood expectation maximization (MLEM) iterative estimation approach. The extracted scatter and crosstalk projections were then incorporated into MLEM image reconstruction as an additive term in forward projection to obtain scatter- and crosstalk-corrected images. The proposed method was validated using Monte Carlo simulation, line source experiment, anthropomorphic torso phantom studies, and patient studies. The performance of the proposed method was also compared to that obtained with the conventional TEW method. RESULTS: Monte Carlo simulations and line source experiment demonstrated that the TEW method overestimated scatter while their proposed method provided more accurate scatter estimation by considering the low energy tail effect. In the phantom study, improved defect contrasts were observed with both correction methods compared to no correction, especially for the images of (99m)Tc in dual-radionuclide imaging where there is heavy contamination from (123)I. In this case, the nontransmural defect contrast was improved from 0.39 to 0.47 with the TEW method and to 0.51 with their proposed method and the transmural defect contrast was improved from 0.62 to 0.74 with the TEW method and to 0.73 with their proposed method. In the patient study, the proposed method provided higher myocardium-to-blood pool contrast than that of the TEW method. Similar to the phantom experiment, the improvement was the most substantial for the images of (99m)Tc in dual-radionuclide imaging. In this case, the myocardium-to-blood pool ratio was improved from 7.0 to 38.3 with the TEW method and to 63.6 with their proposed method. Compared to the TEW method, the proposed method also provided higher count levels in the reconstructed images in both phantom and patient studies, indicating reduced overestimation of scatter. Using the proposed method, consistent reconstruction results were obtained for both single-radionuclide data with scatter correction and dual-radionuclide data with scatter and crosstalk corrections, in both phantom and human studies. CONCLUSIONS: The authors demonstrate that the TEW method leads to overestimation in scatter and crosstalk for the CZT-based imaging system while the proposed scatter and crosstalk correction method can provide more accurate self-scatter and down-scatter estimations for quantitative single-radionuclide and dual-radionuclide imaging.

Clinical feasibility study to detect angiogenesis following bone marrow stem cell transplantation in chronic ischaemic heart failure.

BACKGROUND: Bone marrow stem cell (BMSC) therapy for cardiovascular disease has shown considerable preclinical and clinical promise, but there remains a need for mechanistic studies to help bridge the transition from bench to bedside. We have designed a substudy to our REGENERATE-IHD trial (ClinicalTrial.gov Identifier: NCT00747708) to assess the feasibility of a novel imaging technique to detect angiogenesis following BMSC therapy. METHODS AND RESULTS: Nine patients who had been randomized to receive intracoronary injection of G-CSF-mobilized BMSCs or control (serum) were included in this substudy. Patients underwent SPECT imaging using a novel radiolabelled peptide (Tc-NC100692), which has a high affinity for the αvß3 integrin, an angiogenesis-related integrin. This was repeated 4 days after intracoronary injection of BMSCs/control to assess for neoangiogenesis. The imaging study was well tolerated with no adverse effects. Myocardial tracer uptake was detectable at baseline in all nine patients, with no myocardial uptake seen in two control patients used for comparison. Baseline uptake appeared to correlate with baseline ejection fraction but changes with therapy did not reach statistical significance. CONCLUSION: SPECT imaging with a Tc-NC100692 is feasible in patients with heart failure, with baseline activity suggesting persistent angiogenesis in patients with remote myocardial infarction.

The effect of energy and source location on gamma camera intrinsic and extrinsic spatial resolution: an experimental and Monte Carlo study.

Quantification of nuclear medicine image data is a prerequisite for personalized absorbed dose calculations and quantitative biodistribution studies. The spatial response of a detector is a governing factor affecting the accuracy of image quantification, and the aim of this work was to model this impact. To simulate spatial response, a value for the intrinsic spatial resolution (R(intrinsic)) of the gamma camera is needed. R(intrinsic) for (99m)Tc was measured over the field of view (FOV) and an experimental setup was designed to measure R(intrinsic) for radioisotopes with higher photon energies. Monte Carlo (MC) simulations, using the codes SIMIND and GATE, were used to investigate the extrinsic effect of R(intrinsic) as a function of energy and its variation across the FOV. A method was developed to calculate energy-dependent blurring values for input to MC simulations, by separate consideration of the Compton scatter and photoelectric effect in the crystal and statistical variation in the signal. Inclusion of energy-specific blurring values in simulations showed excellent agreement with experimental measurements. The maximum pixel count rate can change by up to 18% when imaged at two different points in the FOV, and errors in the maximum pixel count rate of up to 11% were shown if a blurring value for (99m)Tc was used for simulations of (131)I. We demonstrate that the accuracy of MC simulations of gamma cameras can be significantly improved by accounting for the effect of energy on intrinsic spatial resolution.

Optimization of energy-window settings for scatter correction in quantitative (111)In imaging: comparison of measurements and Monte Carlo simulations.

Activity quantification in nuclear medicine imaging is highly desirable, particularly for dosimetry and biodistribution studies of radiopharmaceuticals. Quantitative (111)In imaging is increasingly important with the current interest in therapy using (90)Y radiolabeled antibodies. One of the major problems in quantification is scatter in the images, which leads to degradation of image quality. The aim of this study was to optimize the energy-window settings for quantitative (111)In imaging with a camera that enabled acquisition in three energy windows. Experimental measurements and Monte Carlo simulations, using the SI-MIND code, were conducted to investigate parameters such as sensitivity, image contrast, and image resolution. Estimated scatter-to-total ratios and distributions, as obtained by the different window settings, were compared with corresponding simulations. Results showed positive agreement between experimental measurements and results from simulations, both quantitatively and qualitatively. We conclude that of the investigated methods, the optimal energy-window setting was two windows centered at 171 and 245 keV, together with a broad scatter window located between the photopeaks.