Computed Tomography Pulmonary Angiography Scans Research Articles

Ordinal Scoring of Coronary Artery Calcification by Computed Tomography Pulmonary Angiography in Acute Pulmonary Embolism.

To assess coronary artery calcification (CAC) in patients with acute pulmonary embolism and determine whether this correlates with right ventricular dilation (RVD) and mortality. Computed tomography pulmonary angiography scans of 330 patients were used to perform retrospective assessment of the pulmonary artery computed tomography obstruction index (PACTOI), the right/left ventricle diameter ratio, the diameter of the pulmonary trunk, and the ordinal CAC score. Age (P < 0.001), urea level (P < 0.001), D-dimer level (P = 0.006), diameter of the pulmonary trunk (P < 0.001), and PACTOI (P < 0.001) were significantly higher in the RVD-positive patient group. We found a significant relation between increased CAC score and increased mortality (P = 0.038). Left-sided CAC was detected much more often in RVD-positive patients (P = 0.008). Coronary artery calcification is common in patients with acute pulmonary embolism, especially when those patients are also RVD-positive. A significant relation was found between RVD and left-sided CAC.

Sex‐specific differences in clot resolution 3 weeks after acute pulmonary embolism managed with anticoagulants—A substudy of the EINSTEIN‐PE study

BackgroundIt is unknown whether differences in clot structure and resolution contribute to the reported risk differences of recurrent venous thromboembolism (VTE) between men and women. Patients/MethodsWe used data from the EINSTEIN‐PE study, a randomized, multicenter, non‐inferiority study in which patients 18 years and older with acute symptomatic pulmonary embolism (PE) were randomized to rivaroxaban or enoxaparin followed by a vitamin K antagonist. PE was diagnosed by computed tomography pulmonary angiography scan or high‐probability ventilation/perfusion scintigraphy. Three weeks after randomization a follow‐up scan was performed. An independent adjudication committee assessed the degree of vascular obstruction. Results and ConclusionsA total of 371 participants including 174 (46.9%) women and 197 (53.0%) men were included in the present analysis. At 3 weeks, there was no difference between men and women in complete clot resolution: 39.6% and 40.2%, respectively. The absolute reduction in pulmonary vascular obstruction at week 3 was also similar: 12.9% (95% confidence interval [CI]: 11.6–14.2) in men and 12.1% (95% CI: 10.4–13.7) in women, corresponding to a resolution ratio of 0.29 (95% CI: 0.24–0.33) and 0.35 (95% CI: 0.28–0.42), respectively. No differences in clot resolution were observed between men and women diagnosed with acute PE at 3 weeks after start of anticoagulant therapy. These findings suggest that the reported higher rate of VTE recurrence in men cannot be explained by decreased clot resolution.

Performance of an artificial intelligence tool with real-time clinical workflow integration - Detection of intracranial hemorrhage and pulmonary embolism.

Performance of an artificial intelligence tool with real-time clinical workflow integration - Detection of intracranial hemorrhage and pulmonary embolism.

Utilization of serum D-dimer assays prior to computed tomography pulmonary angiography scans in the diagnosis of pulmonary embolism among emergency department physicians: a retrospective observational study

BackgroundA variety of evidence-based algorithms and decision rules using D-Dimer testing have been proposed as instruments to allow physicians to safely rule out a pulmonary embolism (PE) in low-risk patients.ObjectiveTo describe the prevalence of D-Dimer utilization among emergency department (ED) physicians and its impact on positive yields and utilization rates of Computed Tomography Pulmonary Angiography (CTPA).MethodsData was collected on all CTPA studies ordered by ED physicians at three sites during a 2-year period. Using a chi-square test, we compared the diagnostic yield for those patients who had a D-Dimer prior to their CTPA and those who did not. Secondary analysis was done to examine the impact of D-Dimer testing prior to CTPA on individual physician diagnostic yield or utilization rate.ResultsA total of 2811 CTPAs were included in the analysis. Of these, 964 CTPAs (34.3%) were ordered without a D-Dimer, and 343 (18.7%) underwent a CTPA despite a negative D-Dimer. Those CTPAs preceded by a D-Dimer showed no significant difference in positive yields when compared to those ordered without a D-Dimer (9.9% versus 11.3%, p = 0.26). At the individual physician level, no statistically significant relationship was found between D-Dimer utilization and CTPA utilization rate or diagnostic yield.ConclusionThis study provides evidence of suboptimal adherence to guidelines in terms of D-Dimer screening prior to CTPA, and forgoing CTPAs in patients with negative D-Dimers. However, the lack of a positive impact of D-Dimer testing on either CTPA diagnostic yield or utilization rate is indicative of issues relating to the high false-positive rates associated with D-Dimer screening.

EFFECTS OF TEST-BOLUS AND LOW-DOSE SCAN ON CT PULMONARY ANGIOGRAPHY IMAGE QUALITY IN PATIENTS WITH DIFFERENT BODY MASS INDEXES.

This study aimed to investigate different methods of obtaining high-quality Computed Tomography pulmonary angiography (CTPA) images using low-dose scanning in patients with different body mass index (BMI) values. Sixty patients with suspected pulmonary embolism were grouped based on their BMI values (BMI < 25, designated N, and BMI ≥ 25, designated O) and were assigned to receive either test bolus (TB) or bolus tracking (BT) at conventional (C) or low (L) dose. The effective dose (ED) in the N-TB-L group was lower than in the group N-TB-C (0.56±0.05 vs. 3.78±1.16, p<0.001), with similar image quality (4.90±0.31 vs. 4.70±0.47, p=0.120). The ED in the O-TB-L group was lower than in the O-TB-C group (0.54±0.03 vs. 5.14±1.34, p<0.001), but the group O-TB-C's image quality was higher (4.65±0.59 vs. 3.95±0.89, p=0.006). Groups N-TB-L versus O-TB-L, groups N-TB-L versus N-BT-L and groups O-TB-C versus O-BT-C had similar EDs (all ps>0.05), but the image quality was different (all ps<0.05). In conclusion, the results showed that the image quality of low-dose CTPA scanning using TB was similar to that of the conventional-dose CTPA in patients with BMI < 25 but was lower in patients with BMI ≥ 25. TB was better than BT for all patients, regardless of BMI, when receiving the same ED.

Multimodal fusion with deep neural networks for leveraging CT imaging and electronic health record: a case-study in pulmonary embolism detection

Recent advancements in deep learning have led to a resurgence of medical imaging and Electronic Medical Record (EMR) models for a variety of applications, including clinical decision support, automated workflow triage, clinical prediction and more. However, very few models have been developed to integrate both clinical and imaging data, despite that in routine practice clinicians rely on EMR to provide context in medical imaging interpretation. In this study, we developed and compared different multimodal fusion model architectures that are capable of utilizing both pixel data from volumetric Computed Tomography Pulmonary Angiography scans and clinical patient data from the EMR to automatically classify Pulmonary Embolism (PE) cases. The best performing multimodality model is a late fusion model that achieves an AUROC of 0.947 [95% CI: 0.946–0.948] on the entire held-out test set, outperforming imaging-only and EMR-only single modality models.

Abstract 16113: CT Pulmonary Angiography Yield in Patients With an Active Malignancy: Lack of Non-invasive Screening

Patients with an active malignancy are at increased risk of venous thromboembolism due to the production of pro-coagulant factors causing a hypercoagulable state. Providers must keep their index of suspicion high for a pulmonary embolism in these patients because of the high prevalence and often vague symptoms. This is a retrospective study of 2226 patients who underwent CT pulmonary angiography (CTPA) in the emergency department. We compared the diagnosis of PE in patients with and without an active malignancy within the past 6 months. Of those patients without an active cancer, 133/1788 (7.4%) patients were diagnosed with a PE. This is not significantly different from the cohort with cancer where 37/437 (8.5%) patients were diagnosed with a PE (p=0.468). During diagnosis, patients with a history of cancer were less likely to have a chest x-ray, d-dimer or troponin performed prior to the CTPA scan (Table 1). These tests are less invasive and much cheaper than the CTPA and can help identify other pathologies that a patient may be experiencing. Not surprisingly, patients with a malignancy have a higher Wells’ score, revised Geneva score and PERC rule (Table 2), leading providers further down diagnostic algorithms towards CTPA. While many of these high-risk patients should immediately undergo CTPA, patients at low or moderate risk of PE should first be evaluated with less invasive testing such as a chest x-ray or d-dimer. In conclusion, in order to decrease the number of patients unnecessarily undergoing CTPA, patients with an active malignancy should be risk stratified in a similar manner as those patients without a malignancy, including the use of non-invasive testing where appropriate.

Large Granular Lymphocyte Leukemia and Precapillary Pulmonary Hypertension

Large Granular Lymphocyte Leukemia and Precapillary Pulmonary Hypertension

Adherence to the algorithmic approach for diagnosis of pulmonary embolism: A teaching hospital experience, Shiraz, Iran.

BACKGROUNDWe evaluated to see if the algorithmic approach of pulmonary embolism (PE) [Wells’ score, followed by D-dimer test and computed tomography pulmonary angiography (CTPA)] is appropriately followed in teaching hospitals of Shiraz, Iran.METHODSFrom October 2012 to October 2013, we prospectively calculated Wells’ score for all patients who underwent CTPA with clinical suspicion to PE; patients with low probability who had not checked the D-dimer or had low level of D-dimer were considered as non-adherent to the guideline and those with high level of D-dimer or high probability of Wells’ score were labeled as adherent to the PE guideline. CTPA scans were independently reported by two radiologists.RESULTSDuring study period, 364 patients underwent CTPA to rule out PE, of which 125 (34.3%) had Wells’ score > 4 (high probable risk) and 239 had Wells’ score ≤ 4. Amongst low probable risk patients (Wells’ score ≤ 4), only 32 patients had undergone the D-dimer test (23 patients had high level of D-dimer). Based on the algorithmic approach, patients with suspected PE, patients with high probability (125 patients), and patients with low probability with elevated D-dimer level (23 patients) were considered as adherent to the PE guideline; consequently, the total adherence to PE guideline was 148 out of 364 (40.6%).CONCLUSIONWe followed the algorithmic approach guideline in about 40.0% of cases; however, we should pay more attention to the algorithmic approach in patients with suspected PE.

Clinical and imaging profiles of pulmonary embolism: a single-institution experience

BackgroundPulmonary embolism (PE) is a common life-threatening condition with non-specific clinical presentations. The diagnosis of PE depends highly on imaging studies, which may also provide prognostic information. This study aimed to describe the clinical and imaging profiles of patients with PE, emphasizing the differences between central and peripheral PE.MethodsAfter ethics review board approval, this retrospective observational study examined the non-negative results in adult patients who underwent computed tomography pulmonary angiography (CT-PA) at our hospital between May 2016 and December 2019. Demographic and clinical information and imaging findings were collected from the electronic medical records.ResultsThe study included 85 cases that were identified after re-interpreting the 103 non-negative CT-PA scans. Six cases were excluded for incomplete data and 12 cases were false-positive. Central PE was found in 63.5% of the cases. Obesity was the most common risk factor seen in 37.6% of the cases. Furthermore, 9.4% of the patients had sickle cell disease, which tended to be associated with peripheral PE. There was no difference between the peripheral and central PE in most clinical and imaging parameters evaluated (P > 0.05). However, patients with isolated subsegmental PE were more likely to develop hemoptysis (P = 0.04).ConclusionThis study suggests that patients with obesity and sickle cell disease constitute an important proportion of all PE cases. Furthermore, the clinical and imaging profiles in patients with peripheral PE are similar to those in patients with central PE. Future research should focus on the clinical value of peripheral PE in patients with sickle cell disease.

Computed Tomography Angiography-Based Pulmonary Artery Volumetry as a Diagnostic Tool for Pulmonary Hypertension.

We evaluated use of three-dimensional pulmonary artery volumes derived from computed tomography pulmonary angiography (CTPA) in a group with pulmonary hypertension (PH) compared with healthy controls as a tool for the diagnosis of PH. Retrospective analysis was performed of 40 CTPA scans obtained within 90 days of right heart catheterization demonstrating PH. The CTPA scans of 40 age- and sex-matched patients without cardiopulmonary disease were used as comparison. Diameters and volumes of the pulmonary arteries were compared. Adjusted total volume of the main, right, and left proximal pulmonary arteries (PAvol) demonstrated area under the curve of 0.918 (95% confidence interval, 0.860-0.975) for detection of PH, comparable to main pulmonary artery diameter measurement. Area under the curve values for PAvol were higher in subgroups divided by sex and PH severity. Volumetric analysis of the proximal pulmonary arteries using CTPA is a promising diagnostic tool for PH in a real-world cohort.

Additive evidence of the competence of pregnancy-adapted YEARS algorithm in reducing the need for CTPA, Q and/or V/Q scintiscan.

To determine whether a pregnancy-adapted clinical and D-dimer-based algorithm, termed the "YEARS algorithm," can reduce the need for radiological imaging, including lung scintigraphy in pregnant women with suspected pulmonary embolism (PE). This retrospective study included all pregnant women with suspected PE between January 2014 and September 2019 who have undergone D-dimer testing and radiological imaging (computed tomography pulmonary angiography or lung perfusion scans) at presentation. Three criteria from the YEARS algorithm were assessed: clinical signs of deep vein thrombosis, haemoptysis, and whether PE was clinically considered as the most likely diagnosis. Patients who did not have to undergo imaging per the YEARS algorithm were defined as those with no YEARS criteria and a D-dimer of <1μg/mL (group 1) and those with 1-3 YEARS criteria and a D-dimer of <0.5μg/mL (group 2). Patients who had to undergo imaging were those with no YEARS criteria and a D-dimer ≥1μg/mL (group 3) and those with 1-3 YEARS criteria and a D-dimer ≥0.5μg/mL (group 4). Women with symptoms of deep-vein thrombosis had to undergo Doppler ultrasound: If positive, they were anticoagulated and excluded from this analysis, and if negative, they were evaluated further for the need of imaging based on other YEARS criteria and D-dimer level. Of 117 pregnant women with suspected PE analyzed according to the YEARS algorithm five had confirmed deep-vein thrombosis by Doppler ultrasound, were anticoagulated and excluded from the analysis. Of the remaining 112 women (mean age; 30.4±5.7 years), 50 underwent computed tomography pulmonary angiography (CTPA), 54 lung perfusion or ventilation-perfusion (V/Q) scan and eight both; PE was diagnosed in 7 (6.25%), two by CTPA, two by lung perfusion or V/Q scan and three by both. Thirty-three of the 112 women (29.5%) were in groups 1+2 and could, therefore, have avoided CTPA or lung perfusion scans per the YEARS algorithm. None of those 33 women had PE by CTPA or lung perfusion scans vs. 7/79 patients (8.9%) who required CTPA or lung perfusion scans per the YEARS algorithm. The pregnancy-adapted YEARS algorithm can safely rule out PE in about one-third of pregnant women with suspected PE without the need for radiological imaging.

VTE in ICU Patients With COVID-19

VTE in ICU Patients With COVID-19

Acute pulmonary embolism in non-hospitalized COVID-19 patients referred to CTPA by emergency department.

ObjectivesTo evaluate the prevalence of acute pulmonary embolism (APE) in non-hospitalized COVID-19 patients referred to CT pulmonary angiography (CTPA) by the emergency department.MethodsFrom March 14 to April 6, 2020, 72 non-hospitalized patients referred by the emergency department to CTPA for COVID-19 pneumonia were retrospectively identified. Relevant clinical and laboratory data and CT scan findings were collected for each patient. CTPA scans were reviewed by two radiologists to determinate the presence or absence of APE. Clinical classification, lung involvement of COVID-19 pneumonia, and CT total severity score were compared between APE group and non-APE group.ResultsAPE was identified in 13 (18%) CTPA scans. The mean age and D-dimer of patients from the APE group were higher in comparison with those from the non-APE group (74.4 vs. 59.6 years, p = 0.008, and 7.29 vs. 3.29 μg/ml, p = 0.011). There was no significant difference between APE and non-APE groups concerning clinical type, COVID-19 pneumonia lung lesions (ground-glass opacity: 85% vs. 97%; consolidation: 69% vs. 68%; crazy paving: 38% vs. 37%; linear reticulation: 69% vs. 78%), CT severity score (6.3 vs. 7.1, p = 0.365), quality of CTPA (1.8 vs. 2.0, p = 0.518), and pleural effusion (38% vs. 19%, p = 0.146).ConclusionsNon-hospitalized patients with COVID-19 pneumonia referred to CT scan by the emergency departments are at risk of APE. The presence of APE was not limited to severe or critical clinical type of COVID-19 pneumonia.Key Points • Acute pulmonary embolism was found in 18% of non-hospitalized COVID-19 patients referred by the emergency department to CTPA. Two (15%) patients had main, four (30%) lobar, and seven (55%) segmental acute pulmonary embolism. • Five of 13 (38%) patients with acute pulmonary embolism had a moderate clinical type. • Severity and radiological features of COVID-19 pneumonia showed no significant difference between patients with or without acute pulmonary embolism.

Predictors of positive computed tomography pulmonary angiography results.

Computed tomography pulmonary angiography (CT-PA) is the diagnostic modality used for pulmonary embolism (PE). This study aimed to estimate the positive CT-PA rate and identify the predictors of positive CT-PA results. A retrospective observational study was conducted by examining the CT-PA data for ruling out PE in all adult patients who visited the King Fahd Hospital of Imam Abdulrahman Bin Faisal University. The data regarding background demographic information, clinical information, and CT-PA findings were collected from electronic health records. Data were analyzed descriptively using the chi-squared test. Multivariate regression analysis was used to identify the predictors of positive CT-PA results. In total, 548 patients (209 male, 339 female) who underwent CT-PA scans were included. The positive CT-PA rate was 18.8%. The Charlson Comorbidity Index was not significantly associated with positive CT-PA results (p = 0.456). Multivariate logistic regression analysis revealed that smoking (odds ratio [OR] 4.96; 95% confidence interval [95% CI] 2.05-12.02) was an independent factor associated with positive CT-PA results. The CT-PA scans performed in winter and spring were 43% (OR 0.43; 95% CI 0.22-0.84) and 52% (OR 0.52; 95% CI 0.28-0.97) less likely to show positive results compared with those performed in autumn, respectively. The positive CT-PA rate varied significantly between seasons. Smoking was a predictor of positive CT-PA results. These findings may assist in developing interventions for improving the utilization of CT-PA scans to avoid unnecessary exposure of patients to radiation. N/A.

PENet\u2014a scalable deep-learning model for automated diagnosis of pulmonary embolism using volumetric CT imaging

Pulmonary embolism (PE) is a life-threatening clinical problem and computed tomography pulmonary angiography (CTPA) is the gold standard for diagnosis. Prompt diagnosis and immediate treatment are critical to avoid high morbidity and mortality rates, yet PE remains among the diagnoses most frequently missed or delayed. In this study, we developed a deep learning model—PENet, to automatically detect PE on volumetric CTPA scans as an end-to-end solution for this purpose. The PENet is a 77-layer 3D convolutional neural network (CNN) pretrained on the Kinetics-600 dataset and fine-tuned on a retrospective CTPA dataset collected from a single academic institution. The PENet model performance was evaluated in detecting PE on data from two different institutions: one as a hold-out dataset from the same institution as the training data and a second collected from an external institution to evaluate model generalizability to an unrelated population dataset. PENet achieved an AUROC of 0.84 [0.82–0.87] on detecting PE on the hold out internal test set and 0.85 [0.81–0.88] on external dataset. PENet also outperformed current state-of-the-art 3D CNN models. The results represent successful application of an end-to-end 3D CNN model for the complex task of PE diagnosis without requiring computationally intensive and time consuming preprocessing and demonstrates sustained performance on data from an external institution. Our model could be applied as a triage tool to automatically identify clinically important PEs allowing for prioritization for diagnostic radiology interpretation and improved care pathways via more efficient diagnosis.

Assessing Pretest Clinical Risk of Pulmonary Thromboembolism in the Emergency Department: Proposal of a Simple Modification to the Wells' Score.

Clinical scores have been proposed to stratify the risk of pulmonary thromboembolism (PTE), although this approach suffers a low specificity and the unavoidable need for computed tomography pulmonary angiography (CTPA) scans. Our study aimed to investigate a simple modification to the already validated Wells' score to improve its diagnostic accuracy in the emergency department (ED). We retrospectively reviewed all CTPA scans performed in the ED setting to rule out PTE over a 1-year (2017) period. Clinical variables potentially associated with PTE were assessed to improve diagnostic accuracy of the Wells' score, thus introducing a modified Wells' score (mWells). Four thousand four hundred thirteen CTPAs were identified, of which 504 were for suspected PTE. The prevalence of PTE was 23.9%. Among clinical data, only peripheral capillary oxygen saturation was consistently correlated with PTE at univariate (odds ratio 2.75 [95% confidence interval 1.61-4.73]) and multivariate (odds ratio 3.78 [95% confidence interval 2.13-6.72]) logistic regression analysis. The mWells' score had a higher area under the receiver operating characteristic curve compared with the original Wells' score: 0.71 (95% confidence interval 0.67-0.75) vs. 0.65 (95% confidence interval 0.61-0.69) (p<.01) and improved diagnostic accuracy. Current clinical stratification tools for PTE are characterized by low specificity, leading to an overuse of CTPA. mWells', rather than Wells', score showed a better predictive performance of PTE detection. Our results suggest that current diagnostic pathway for PTE may be improved by simple adjustments (i.e., mWells') of clinical prediction scores.

Arterial attenuation in individualized computed tomography pulmonary angiography injection protocol adjusted based on the patient's body mass index

Background:The aim of this study was to optimize computed tomography pulmonary angiography (CTPA) protocols with regard to improve vascular attenuation without increasing contrast media (CM) volumes. Therefore, we compared the standard CTPA protocol to an individualized contrast media injection protocols adjusted for the patient's body mass index (BMI).Materials and Methods:Two groups of 295 patients with suspected pulmonary embolism (PE) have been receiving CTPA. Group 1 received a standard protocol without taking patient's BMI into account. Group 2 received a CTPA scan, where dose and flow rate of CM injections were adjusted for the patient's BMI. Images were retrospectively analyzed by drawing regions of interests in defined positions in the superior vena cava, descending aorta, the pulmonary main trunk as well as the left and right lower lobe arteries. Intravascular attenuation, contrast volumes, and flow rates were compared using unpaired t-tests. Furthermore, a qualitative image analysis was performed by two experienced readers blinded for the protocol used for image acquisition to evaluate the image quality and arterial attenuation.Results:Patient's BMI was similar in both the groups (27.5 ± 1.5 kg/m2 vs. 28.4 ± 2.1 kg/m2; P = 0.67). Contrast volumes were lower (54.2 ± 4.8 ml vs. 55 ml; P < 0.05), and flow rates (4.1 ± 0.3 ml/s vs. 3.5 ml/s; P < 0.05) were significantly higher in the individualized protocol. The qualitative image analysis yielded an agreement on diagnostic interpretability in the individualized and standard group of 49% and 51% (95% Wilson confidence interval for mean), respectively.Conclusion:An individualized CTPA protocol based on the patient's BMI reduced the contrast media volume and led to an increased pulmonary artery enhancement improving image quality, particularly in the evaluation of the peripheral pulmonary arteries. Thus, contrast media volumes in CTPA should be adjusted for the patient's BMI.

Incidental Findings of Computed Tomography Angiography in Patients Suspected to Pulmonary Embolism; a Brief Report.

Computed tomography pulmonary angiography (CTPA) scans are increasingly used in emergency department (ED). Therefore, the observation of incidental findings (IFs) has also increased. This study aimed to evaluate the frequency of IFs in patients who underwent CTPA. All consecutive patients that underwent CTPA scanning for pulmonary embolism (PE) rule out between January 2017 and June 2018 were analysed. Incidental findings were divided into and reported in three categories: group 1 potentially life-threatening, group 2 required follow up, and group 3 with limited clinical significance. 151 cases with the mean age of 61.2 ± 17.6 years were studied (54.3% female). PE was documented in 77 cases (50.9%). 448 IFs were detected (3 IFs were found per patient). 60 (13.3%) IFs were classified as group 1, 180 (40.1%) as group 2, and 208 (46.6%) as group 3. Cardiomegaly was the most frequent finding in group 1 (n=32), followed by aortic aneurysm (n=13). In group 2, pleural effusion (n=58) and pneumonia (n=36) were the most frequent incidental findings. Lung structure changes (n=92) and thoracic bone related findings (n=43) were the most common IFs observed in group 3. IFs were detected in the majority of patients that underwent CTPA. Most of these findings do not require follow-up or treatment. However, more than 50% of cases may require further diagnostic evaluation (40.1%) or immediate treatment (13.3%).

Organ doses from CT localizer radiographs: Development, validation, and application of a Monte Carlo estimation technique.

The purpose of this study was to simulate and validate organ doses from different computed tomography (CT) localizer radiograph geometries using Monte Carlo methods for a population of patients. A Monte Carlo method was developed to estimate organ doses from CT localizer radiographs using PENELOPE. The method was validated by comparing dosimetry estimates with measurements using an anthropomorphic phantom imbedded with thermoluminescent dosimeters (TLDs) scanned on a commercial CT system (Siemens SOMATOM Flash). The Monte Carlo simulation platform was then applied to conduct a population study with 57 adult computational phantoms (XCAT). In the population study, clinically relevant chest localizer protocols were simulated with the x-ray tube in anterior-posterior (AP), right lateral, and PA positions. Mean organ doses and associated standard deviations (in mGy) were then estimated for all simulations. The obtained organ doses were studied as a function of patient chest diameter. Organ doses for breast and lung were compared across different views and represented as a percentage of organ doses from rotational CT scans. The validation study showed an agreement between the Monte Carlo and physical TLD measurements with a maximum percent difference of 15.5% and a mean difference of 3.5% across all organs. The XCAT population study showed that breast dose from AP localizers was the highest with a mean value of 0.24mGy across patients, while the lung dose was relatively consistent across different localizer geometries. The organ dose estimates were found to vary across the patient population, partially explained by the changes in the patient chest diameter. The average effective dose was 0.18mGy for AP, 0.09mGy for lateral, and 0.08mGy for PA localizer. A platform to estimate organ doses in CT localizer scans using Monte Carlo methods was implemented and validated based on comparison with physical dose measurements. The simulation platform was applied to a virtual patient population, where the localizer organ doses were found to range within 0.4%-8.6% of corresponding organ doses for a typical CT scan, 0.2%-3.3% of organ doses for a CT pulmonary angiography scan, and 1.1%-20.8% of organ doses for a low-dose lung cancer screening scan.