Validation of the standardization framework SSTR-RADS 1.0 for neuroendocrine tumors using the novel SSTR‑targeting peptide [18F]SiTATE

OBJECTIVES

Somatostatin receptor positron emission tomography/computed tomography (SSTR-PET/CT) using [68Ga]-labeled tracers is a widely used imaging modality for neuroendocrine tumors (NET). Recently, [18F]SiTATE, a SiFAlin tagged [Tyr3]-octreotate (TATE) PET tracer, has shown great potential due to favorable clinical characteristics. We aimed to evaluate the reproducibility of Somatostatin Receptor-Reporting and Data System 1.0 (SSTR-RADS 1.0) for structured interpretation and treatment planning of NET using [18F]SiTATE.

METHODS

Four readers assessed [18F]SiTATE-PET/CT of 95 patients according to the SSTR-RADS 1.0 criteria at two different time points. Each reader evaluated up to five target lesions per scan. The overall scan score and the decision on peptide receptor radionuclide therapy (PRRT) were considered. Inter- and intra-reader agreement was determined using the intraclass correlation coefficient (ICC).

RESULTS

The ICC analysis on the inter-reader agreement using SSTR-RADS 1.0 for identical target lesions (ICC ≥ 85%), overall scan score (ICC ≥ 90%), and the decision to recommend PRRT (ICC ≥ 85%) showed excellent agreement. However, significant differences were observed in recommending PRRT among experienced readers (ER) (p = 0.020) and inexperienced readers (IR) (p = 0.004). Compartment-based analysis demonstrated good to excellent inter-reader agreement for most organs (ICC ≥ 74%), except for lymph nodes (ICC ≥ 53%).

CONCLUSION

SSTR-RADS 1.0 represents a highly reproducible and consistent framework system for stratifying SSTR-targeted PET/CT scans, even using the novel SSTR-ligand [18F]SiTATE. Some inter-reader variability was observed regarding the evaluation of uptake intensity prior to PRRT as well as compartment scoring of lymph nodes, indicating that those categories require special attention during further clinical validation and might be refined in a future SSTR-RADS version 1.1.

CLINICAL RELEVANCE STATEMENT

SSTR-RADS 1.0 is a consistent framework for categorizing somatostatin receptor-targeted PET/CT scans when using [18F]SiTATE. The framework serves as a valuable tool for facilitating and improving the management of patients with NET.

KEY POINTS

SSTR-RADS 1.0 is a valuable tool for managing patients with NET. SSTR-RADS 1.0 categorizes patients with showing strong agreement across diverse reader expertise. As an alternative to [68Ga]-labeled PET/CT in neuroendocrine tumor imaging, SSTR-RADS 1.0 reliably classifies [18F]SiTATE-PET/CT.

Imaging-based risk stratification of patients with pulmonary embolism based on dual-energy CT-derived radiomics

BACKGROUND

Technological progress in the acquisition of medical images and the extraction of underlying quantitative imaging data has introduced exciting prospects for the diagnostic assessment of a wide range of conditions. This study aims to investigate the diagnostic utility of a machine learning classifier based on dual-energy computed tomography (DECT) radiomics for classifying pulmonary embolism (PE) severity and assessing the risk for early death.

METHODS

Patients who underwent CT pulmonary angiogram (CTPA) between January 2015 and March 2022 were considered for inclusion in this study. Based on DECT imaging, 107 radiomic features were extracted for each patient using standardized image processing. After dividing the dataset into training and test sets, stepwise feature reduction based on reproducibility, variable importance and correlation analyses were performed to select the most relevant features; these were used to train and validate the gradient-boosted tree models.

RESULTS

The trained machine learning classifier achieved a classification accuracy of .90 for identifying high-risk PE patients with an area under the receiver operating characteristic curve of .59. This CT-based radiomics signature showed good diagnostic accuracy for risk stratification in individuals presenting with central PE, particularly within higher risk groups.

CONCLUSION

Models utilizing DECT-derived radiomics features can accurately stratify patients with pulmonary embolism into established clinical risk scores. This approach holds the potential to enhance patient management and optimize patient flow by assisting in the clinical decision-making process. It also offers the advantage of saving time and resources by leveraging existing imaging to eliminate the necessity for manual clinical scoring.

Indicators for Hospitalization in Acute Pulmonary Embolism: Uncover the Association Between D-dimer Levels, Thrombus Volume and Radiomics

BACKGROUND

The advent of advanced computed tomography (CT) technology and the field of radiomics has opened up new avenues in diagnostic assessments. Increasingly, there is substantial evidence advocating for the incorporation of quantitative imaging biomarkers in the clinical decision-making process. This study aimed to examine the correlation between D-dimer levels and thrombus size in acute pulmonary embolism (PE) combining dual-energy CT (DECT) and radiomics and to investigate the diagnostic utility of a machine learning classifier based on dual-energy computed tomography (DECT) radiomics for identifying patients with a complicated course, defined as at least hospitalization at IMC.

METHODS

The study was conducted including 136 participants who underwent pulmonary artery CT angiography from January 2015 to March 2022. Based on DECT imaging, 107 radiomic features were extracted for each patient using standardized image processing. After dividing the dataset into training and test sets, stepwise feature reduction based on reproducibility, variable importance and correlation analyses were performed to select the most relevant features; these were used to train and validate the gradient-boosted tree models.Receiver operating characteristics (ROC) analysis was utilized to evaluate the association between volumetric, laboratory data and adverse outcomes.

RESULTS

In the central PE group, we observed a significant correlation between thrombus volumetrics and D-dimer levels (p = 0.0037), as well as between thrombus volumetrics and hospitalization at the Intermediate Care Unit (IMC) (p = 0.0001). In contrast, no statistically significant differences were identified in thrombus sizes between patients who experienced complications and those who had a favorable course (p = 0.3162). The trained machine learning classifier achieved an accuracy of 61% and 55% in identifying patients with a complicated course, as indicated by an area under the ROC curve of 0.63 and 0.58.

CONCLUSION

In conclusion, our findings indicate a positive correlation between D-dimer levels and central PE's pulmonary embolic burden. Thrombus volumetrics may serve as an indicator for complications and outcomes in acute PE patients. Thus, thrombus volumetrics, as opposed to D-dimers, could be an additional marker for evaluating embolic disease severity. Moreover, DECT-derived radiomic feature models show promise in identifying patients with a complicated course, such as hospitalization at IMC.

Multiparametric Evaluation of Radiomics Features and Dual-Energy CT Iodine Maps for Discrimination and Outcome Prediction of Thymic Masses

RATIONALE AND OBJECTIVES

To investigate the diagnostic value of radiomics features and dual-source dual-energy CT (DECT) based material decomposition in differentiating low-risk thymomas, high-risk thymomas, and thymic carcinomas.

MATERIALS AND METHODS

This retrospective study included 32 patients (16 males, mean age 66 ± 14 years) with pathologically confirmed thymic masses who underwent contrast-enhanced DECT between 10/2014 and 01/2023. Two experienced readers evaluated all patients regarding conventional radiomics features, as well as DECT-based features, including attenuation (HU), iodine density (mg/mL), and fat fraction (%). Data comparisons were performed using analysis of variance and chi-square statistic tests. Receiver operating characteristic curve analysis and Cox-regression tests were used to discriminate between low-risk/high-risk thymomas and thymic carcinomas.

RESULTS

Of the 32 thymic tumors, 12 (38%) were low-risk thymomas, 11 (34%) were high-risk thymomas, and 9 (28%) were thymic carcinomas. Values differed significantly between low-risk thymoma, high-risk thymoma, and thymic carcinoma regarding DECT-based features (p ≤ 0.023) and 30 radiomics features (p ≤ 0.037). The area under the curve to differentiate between low-risk/high-risk thymomas and thymic cancer was 0.998 (95% CI, 0.915-1.000; p < 0.001) for the combination of DECT imaging parameters and radiomics features, yielding a sensitivity of 100% and specificity of 96%. During a follow-up of 60 months (IQR, 35-60 months), the multiparametric approach including radiomics features, DECT parameters, and clinical parameters showed an excellent prognostic power to predict all-cause mortality (c-index = 0.978 [95% CI, 0.958-0.998], p = 0.003).

CONCLUSION

A multiparametric approach including conventional radiomics features and DECT-based features facilitates accurate, non-invasive discrimination between low-risk/high-risk thymomas and thymic carcinomas.

Cancer patients with venous thromboembolism: Diagnostic and prognostic value of elevated D-dimers

BACKGROUND

D-dimer testing is known to have a high sensitivity at simultaneously low specificity, resulting in nonspecific elevations in a variety of conditions.

METHODS

This retrospective study sought to assess diagnostic and prognostic features of D-dimers in cancer patients referred to the emergency department for suspected pulmonary embolism (PE) and deep vein thrombosis (DVT). In total, 526 patients with a final adjudicated diagnosis of PE (n = 83) and DVT (n = 69) were enrolled, whereas 374 patients served as the comparative group, in which venous thromboembolism (VTE) has been excluded.

RESULTS

For the identification of VTE, D-dimers yielded the highest positive predictive value of 96% (95% confidence interval (CI), 85-99) at concentrations of 9.9 mg/L and a negative predictive value of 100% at .6 mg/L (95% CI, 97-100). At the established rule-out cut-off level of .5 mg/L, D-dimers were found to be very sensitive (100%) at a moderate specificity of nearly 65%. Using an optimised cut-off value of 4.9 mg/L increased the specificity to 95% for the detection of life-threatening VTE at the cost of moderate sensitivities (64%). During a median follow-up of 30 months, D-dimers positively correlated with the reoccurrence of VTE (p = .0299) and mortality in both cancer patients with VTE (p < .0001) and without VTE (p = .0008).

CONCLUSIONS

Although D-dimer testing in cancer patients is discouraged by current guidelines, very high concentrations above the 10-fold upper reference limit contain diagnostic and prognostic information and might be helpful in risk assessment, while low concentrations remain useful for ruling out VTE.

The diagnostic value of contrast-enhanced ultrasound (CEUS) for assessing hepatocellular carcinoma compared to histopathology; a retrospective single-center analysis of 119 patients1

BACKGROUND

HCC as the 6th most common tumor entity with the fourth highest mortality and an increasing prevalence especially due to today's lifestyle acquires a high attention in the clinical setting. Beside CECT and CEMRI, CEUS depicts a dynamic, low-risk and radiation free imaging method that finds its use mainly in screening and active surveillance programs.

PURPOSE

The aim of the retrospective study was to evaluate the diagnostic value of CEUS in correlation to pathologic findings.

MATERIALS AND METHODS

Between 2004 and 2018 a total number of 119 patients were included in this retrospective single-center study. Every patient underwent CEUS in addition to a native B-mode and Color-Doppler scan. After given informed consent SonoVue® (Bracco, Milan, Italy), a second-generation blood-pool agent, was used as contrast medium. Every examination was performed and interpreted by a single experienced radiologist (EFSUMB level 3). A low mechanical index (MI) of <0,2 was chosen to obtain a good imaging quality.

RESULTS

All 119 included patients received CEUS followed by a liver biopsy for inter-modality comparison. In correlation to the pathology results, CEUS showed a diagnostic sensitivity of 96,6%, a specificity of 63,9%, a PPV of 86,7% and a NPV of 88,5% by detecting liver lesions suspicious for HCC. According to the Cohen's Kappa coefficient (k = 0,659) CEUS shows a strong inter-modality agreement in comparison to the histopathological finding.

CONCLUSION

With a high sensitivity and a strong cross-modality comparability to histopathology, the CEUS is highly effective in the detection of suspicious HCC lesions.

Artificial intelligence assistance improves reporting efficiency of thoracic aortic aneurysm CT follow-up

OBJECTIVE

Follow-up of aortic aneurysms by computed tomography (CT) is crucial to balance the risks of treatment and rupture. Artificial intelligence (AI)-assisted radiology reporting promises time savings and reduced inter-reader variabilities.

METHODS

The influence of AI assistance on the efficiency and accuracy of aortic aneurysm reporting according to the AHA / ESC guidelines was quantified based on 324 AI measurements and 1944 radiological measurements: 18 aortic aneurysm patients, each with two CT scans (arterial contrast phase, electrocardiogram-gated) with an interval of at least six months have been included. One board-certified radiologist and two residents (8/4/2 years of experience in vascular imaging) independently assessed aortic diameters at nine landmark positions. Aneurysm extensions were compared with original CT reports. After three weeks washout period, CTs were re-assessed, based on graphically illustrated AI measurements.

RESULTS

Time-consuming guideline-compliant aortic measurements revealed additional affections of the root / arch for 80 % of aneurysms that had initially been reported to be limited to the ascending aorta. AI assistance reduced mean reporting time by 63 % from 13:01 to 04:46 min including manual corrections of AI measurements (performed for 33.6 % of all measurements with predominance at the sinuses of Vasalva). AI assistance reduced total diameter inter-reader variability by 42.5 % (0.42 / 1.16 mm with / without AI assistance, mean of all patients and landmark positions, significant reduction for 6 out of 9 measuring positions). Conventional and AI-assisted quantification aneurysm progress varied to small extent (mean of 0.75 mm over all patients / landmark positions) not significantly exceeding radiologist's inter-reader variabilities.

CONCLUSIONS

Guideline-compliant aorta measurement is crucial to report detailed aneurysm extension which might affect the strategy of interventional repair. AI assistance promises improved reporting efficiency and has high potential to reduce radiologist's inter-reader variabilities that can hamper diagnostic follow-up accuracy.

KEY POINT

The time-consuming guideline-compliant aorta aneurysm assessment is crucial to report aneurysm extension in detail; AI-assisted measurement reduces reporting time, improves extension evaluation and reduces inter-reader variability.

Why does a point of care guided transfusion algorithm not improve blood loss and transfusion practice in patients undergoing high-risk cardiac surgery? A prospective randomized controlled pilot study

Background

Adult cardiac surgery is often complicated by elevated blood losses that account for elevated transfusion requirements. Perioperative bleeding and transfusion of blood products are major risk factors for morbidity and mortality. Timely diagnostic and goal-directed therapies aim at the reduction of bleeding and need for allogeneic transfusions.

Methods

Single-centre, prospective, randomized trial assessing blood loss and transfusion requirements of 26 adult patients undergoing elective cardiac surgery at high risk for perioperative bleeding. Primary endpoint was blood loss at 24 h postoperatively. Random assignment to intra- and postoperative haemostatic management following either an algorithm based on conventional coagulation assays (conventional group: platelet count, aPTT, PT, fibrinogen) or based on point-of-care (PoC-group) monitoring, i.e. activated rotational thromboelastometry (ROTEM®) combined with multiple aggregometry (Multiplate®). Differences between groups were analysed using nonparametric tests for independent samples.

Results

The study was terminated after interim analysis (n = 26). Chest tube drainage volume was 360 ml (IQR 229-599 ml) in the conventional group, and 380 ml (IQR 310-590 ml) in the PoC-group (p = 0.767) after 24 h. Basic patient characteristics, results of PoC coagulation assays, and transfusion requirements of red blood cells and fresh frozen plasma did not differ between groups. Coagulation results were comparable. Platelets were transfused in the PoC group only.

Conclusion

Blood loss via chest tube drainage and transfusion amounts were not different comparing PoC- and central lab-driven transfusion algorithms in subjects that underwent high-risk cardiac surgery. Routine PoC coagulation diagnostics do not seem to be beneficial when actual blood loss is low. High risk procedures might not suffice as a sole risk factor for increased blood loss.

Trial registration

NCT01402739, Date of registration July 26, 2011.

[Simulated Total Wrist Fusion and its Influence on Hand Grip Function]

BACKGROUND

Wrist fusion is still a common treatment for patients with advanced stage arthritis. Since patients are often intimidated by the functional limitations, we intended to evaluate the influence of the lack of wrist motion in different positions on the dynamic grip function and the grip strength of the hand.

METHODS

We simulated wrist fusion in 20° extension and 20° flexion and evaluated the following grip types: fist closure, 2 different power grips, pinch grip and precision grip. A TUB sensor glove was used, which allowed us to dynamically record the range of motion (ROM) of the finger joints as well as grip strength. Nineteen healthy subjects participated and all types of grips were performed using a standardised protocol with and without simulated wrist fusion.

RESULTS

Lack of wrist motion in 20° extension had no relevant effect on the fingers' ROM, grip speed or strength. Simulated fusion in 20° flexion also had no influence on ROM or grip speed, rejecting our hypothesis that a tenodesis effect of the extensors in flexion would decrease ROM in the finger joints and grip speed. However, we were able to show a significant decrease of grip strength in flexion compared with extension or healthy wrists. The decrease averaged between 23 and 42% of healthy values, depending on the grip type. There was no change in strength distribution among the fingers.

CONCLUSION

We didn't find any impact of lack of wrist motion on finger movement during forceful hand grip at normal speed. However, a significant loss of grip strength in flexed position of the wrist joint should be considered in patients with an indication for bilateral wrist fusion.

Evaluation and application of organic micro-pollutants (OMPs) as indicators in karst system characterization

This study presents chances and challenges associated with the application of organic micro-pollutants (OMPs) as indicators in karst system characterization. The methodology and options of possible indications were evaluated based on the interpretation of the spatial distribution of 54 compounds in groundwater in combination with a complex geological setting consisting of multiple aquifer horizons and tectonic faults. A high variety of OMPs are released mainly in an urban area leading to concentrations of several nanograms per liter up to micrograms per liter, which are detectable using a high-performance liquid chromatography with subsequent tandem mass spectrometry (HPLC-MS/MS) method. Since characteristic patterns of spatial distribution were repeatedly observed during a 2-year observation period, important criteria of the aforementioned indicator application are fulfilled. Triazoles, compounds with recent high emission rates, could be successfully applied for the identification of flow directions and the delineation of catchment areas. Concentrations and the number of OMPs are believed to be dependent on properties of covering rock layers. Therefore, OMPs can also be used as a validation tool for vulnerability mapping. Compounds, such as triazines, persistent in the system for more than two decades, demonstrate the interaction between different parts of the aquifer system and the hydraulic characteristics of a tectonic fault zone. Such indicator potentials complement those of artificial tracer tests. Point sources of OMPs and their impact on groundwater could be identified qualitatively. In combination with the interpretation of the geological setting, the distribution of OMPs provides essential information for the development of a conceptual hydrogeological model.

Investigating the dynamics of two herbicides at a karst spring in Germany: Consequences for sustainable raw water management

While karst aquifers are considered as rapid flow and transport systems, their high potential for long-term storage is often ignored. However, to achieve a sustainable raw water quality for drinking water production, the understanding of this potential is highly essential. In this study, the transport dynamics of the two herbicides metazachlor and atrazine as well as a degradation product of the latter (desethylatrazine) were investigated at a karst spring over 1 year. Even 20 years after its ban in Germany, atrazine and its degradation product were almost always detectable in the spring water in the low ng L(-1) range (up to 5.2ng L(-1)). Metazachlor could only be detected after precipitation events, and the observed concentrations (up to 82.9ng L(-1)) are significantly higher than atrazine or desethylatrazine. Comparing the dynamics of the herbicides with the inorganic ions Ca(2+), Mg(2+) and electrical conductivity, a positive correlation of atrazine with these parameters could be observed. From this observation, atrazine is concluded to be located within the aquifer matrix. To achieve a sustainable raw water management at karst springs, the rapidness of these systems needs to be highlighted as well as their long-term storage potential. Persistent substances or transformation products are prone to deteriorate the raw water quality for decades.

Identification of the attenuation potential of a karst aquifer by an artificial dualtracer experiment with caffeine

Little is known with respect to the attenuation capacity of karst aquifers. Even less is known about the risk posed by emerging micropollutants in these systems. In order to identify the attenuation potential of karst aquifers in-situ and to estimate the risk posed by micropollutants, a dualtracer test was conducted in this study in order to investigate differential transport in the subsurface: The reactive compound caffeine was used as a tracer to indicate the attenuation capacity within the aquifer in-situ. Due to the low limit of quantification, only small amounts of caffeine needed to be injected. To calibrate a model and to visualize the attenuation of caffeine a conservative reference tracer (uranine) is injected simultaneously. The methodology is tested in a well-characterised karst system in southwest Germany. The results indicate a significantly higher attenuation rate than was expected for karst aquifers. The attenuation is decribed as a first-order process. The corresponding half-life is 104 h. This low half-life suggests that a generally assumed low natural attenuation capacity of karst aquifers is unjustified. The observed mass loss of caffeine illustrates the potential of caffeine to be used as reactive tracer for indicating in-situ attenuation capacity within highly hydraulically conductive systems, such as karst aquifers. Due to the high attenuation rate of caffeine it does not pose a threat as a long-time contaminant. In combination with a conservative reference tracer an economical and environmentally benign method is presented in this manuscript for the in-situ determination of the attenuation capacity of highly conductive aquifer systems.

[Early results after palmar multi-axial plate osteosynthesis for treatment of distal radius fractures]

In a prospective study a total of 49 fractures in 47 patients were treated with open reduction and palmar plate osteosynthesis. The results were evaluated radiologically, clinically and with the disability of arm-shoulder-hand (DASH) score. The emphasis was on early postoperative results. The patients consisted of 11 men and 36 women aged 21–85 years old. Very good or good results were achieved in 79.6% of the patients, a fair result in 16.3% and a poor result in 4.1% according to the Lidström criteria. In 65.9% of cases very good or good functional results were achieved, in 22.7% fair and in 11.4% only poor results. The DASH score showed high patient satisfaction in 70.2%, moderate satisfaction in 25.5% and only little satisfaction in 4.3% of patients. Complications occurred in 14.3% of patients and the average duration of sick leave was 45.8 days. Open reduction of distal radius fractures using palmar plates with multi-axial angular stability shows good radiological and functional results even shortly after the operation.

Caffeine as an indicator for the quantification of untreated wastewater in karst systems

Contamination from untreated wastewater leakage and related bacterial contamination poses a threat to drinking water quality. However, a quantification of the magnitude of leakage is difficult. The objective of this work is to provide a highly sensitive methodology for the estimation of the mass of untreated wastewater entering karst aquifers with rapid recharge. For this purpose a balance approach is adapted. It is based on the mass flow of caffeine in spring water, the load of caffeine in untreated wastewater and the daily water consumption per person in a spring catchment area. Caffeine is a source-specific indicator for wastewater, consumed and discharged in quantities allowing detection in a karst spring. The methodology was applied to estimate the amount of leaking and infiltrating wastewater to a well investigated karst aquifer on a daily basis. The calculated mean volume of untreated wastewater entering the aquifer was found to be 2.2 ± 0.5 m(3) d(-1) (undiluted wastewater). It corresponds to approximately 0.4% of the total amount of wastewater within the spring catchment.

Multitracer test approach to characterize reactive transport in karst aquifers

A method to estimate reactive transport parameters as well as geometric conduit parameters from a multitracer test in a karst aquifer is provided. For this purpose, a calibration strategy was developed applying the two-region nonequilibrium model CXTFIT. The ambiguity of the model calibration was reduced by first calibrating the model with respect to conservative tracer breakthrough and later transferring conservative transport parameters to the reactive model calibration. The reactive transport parameters were only allowed to be within a defined sensible range to get reasonable calibration values. This calibration strategy was applied to breakthrough curves obtained from a large-scale multitracer test, which was performed in a karst aquifer of the Swabian Alb, Germany. The multitracer test was conducted by the simultaneous injection of uranine, sulforhodamine G, and tinopal CBS-X. The model succeeds to represent the tracer breakthrough curves (TBCs) of uranine and sulforhodamine G and verifies that tracer-rock interactions preferably occur in the immobile fluid region, although the fraction of this region amounts to only 3.5% of the total water. However, the model failed to account for the long tailing observed in the TBC of tinopal CBS-X. Sensitivity analyses reveal that model results for the conservative tracer transport are most sensitive to average velocity and volume fraction of the mobile fluid region, while dispersion and mass transfer coefficients are least influential. Consequently, reactive tracer calibration allows the determination of sorption sites in the mobile and immobile fluid region at small retardation coefficients.

Process-based interpretation of tracer tests in carbonate aquifers

A tracer test in a carbonate aquifer is analyzed using the method of moments and two analytical advection-dispersion models (ADMs) as well as a numerical model. The numerical model is a coupled continuum-pipe flow and transport model that accounts for two different flow components in karstified carbonate aquifers, i.e., rapid and often turbulent conduit flow and Darcian flow in the fissured porous rock. All techniques employed provide reasonable fits to the tracer breakthrough curve (TBC) measured at a spring. The resulting parameter estimates are compared to investigate how each conceptual model of flow and transport processes that forms the basis of the analyses affects the interpretation of the tracer test. Numerical modeling results suggest that the method of moments and the analytical ADMs tend to overestimate the conduit volume because part of the water discharged at the spring is wrongly attributed to the conduit system if flow in the fissured porous rock is ignored. In addition, numerical modeling suggests that mixing of the two flow components accounts for part of the dispersion apparent in the measured TBC, while the remaining part can be attributed to Taylor dispersion. These processes, however, cannot reasonably explain the tail of the TBC. Instead, retention in immobile-fluid regions as included in a nonequilibrium ADM provides a possible explanation.

Interfacing Brownian dynamics simulations

Starting from the flux of particles in a Brownian dynamics simulation we derive boundary conditions, which allow us (i) to couple a Brownian dynamics calculation to a reservoir of particles of a given density, i.e., setting up constant density boundary conditions, and (ii) to build an interface between Brownian dynamics and a diffusional treatment of adjacent simulation volumes. With these algorithms it is sometimes possible to dramatically reduce the system size--and therefore the necessary resources--of multiparticle Brownian dynamics calculations. In this paper we give one-dimensional examples which illustrate potential applications and savings.

Brownian dynamics simulations of simplified cytochrome c molecules in the presence of a charged surface

Simulations were performed for up to 150 simplified spherical horse heart cytochrome c molecules in the presence of a charged surface, which serves as an approximate model for a lipid membrane. Screened electrostatic and short-ranged attractive as well as repulsive van der Waals forces for interparticle and particle-membrane interactions are utilized in the simulations. At a distance from the membrane, where particle-membrane interactions are negligible, the simulation is coupled to a noninteraction continuum analogous to a heat bath [Geyer et al., J. Chem. Phys. 120, 4573 (2004)]. From the particles' density profiles perpendicular to the planar surface binding isotherms are derived and compared to experimental results [Heimburg et al. (1999)]. Using a negatively charged structureless membrane surface a saturation effect was found for relatively large particle concentrations. Since biological membranes often contain membrane proteins, we also studied the influence of additional charges on our model membrane mimicking bacterial reaction centers. We find that the onset of the saturation occurs for much lower concentrations and is sensitive to the detailed implementation. Therefore we suggest that local distortion of membrane planarity (undulation), or lipid demixing, or the presence of charged integral membrane proteins create preferential binding sites on the membrane. Only then do we observe saturation at physiological concentrations.

Negative mass instability for interacting particles in a 1D box: theory and application

We demonstrate that the synchronization effect observed [Pedersen et al., Phys. Rev. Lett. 87, 055001 (2001)]], when a bunch of ions oscillates between two mirrors in an electrostatic ion beam trap, can be explained as a negative mass instability. We derive simple necessary conditions for the existence of a regime in which this dispersionless behavior occurs and demonstrate that in this regime, the ion trap can be used as a high resolution mass spectrometer.