Yulia Lyatskaya

Modeling the phase behavior of polymer-clay composites

To model the phase behavior of polymer-clay composites, we develop a free energy expression for a mixture of polymers and solid, thin disks. The free energy expression is adopted from the Onsager model for the equilibrium behavior of rigid rods. Thus, our theory takes into account the possible nematic ordering of the disks within the polymer matrix. By minimizing this free energy and calculating the chemical potentials, we construct phase diagrams for the polymer-disk mixtures. The findings provide guidelines for tailoring the polymer molecular weight and the volume fraction of the different components to fabricate thermodynamically stable mixtures with the desired morphology.

Theory of polymer chains tethered at interfaces

Abstract Using self-consistent field calculations and scaling analysis, we determined the property of polymers that are tethered onto impenetrable, solid surfaces or adsorbed onto penetrable interfaces. In the case of impenetrable solids, we consider homogeneous walls, as well as surfaces containing chemically distinct patterns. These findings provide guidelines for tailoring the morphology of the polymer layer and thereby controling the interaction between polymer-coated surfaces. In particular, we pinpoint routes for creating ordered arrays of polymer-coated colloidal particles. The results also yield prescriptions for creating laterally patterned polymer films, which are useful in device applications. In the case of penetrable interfaces, we examine the adsorption of polymers into layers of tethered chains. These studies yield design criteria for creating selective filtration and separation systems, and insight into how adsorbing chains interact with a soft, responsive surface.

Evaluation of radiation dose delivered by cone beam CT and tomosynthesis employed for setup of external breast irradiation

A systematic set of measurements is reported for evaluation of doses to critical organs resulting from cone-beam CT (CB-CT) and cone-beam tomosynthesis (CB-TS) as applied to breast setup for external beam irradiation. The specific focus of this study was on evaluation of doses from these modalities in a setting of volumetric breast imaging for target localization in radiotherapy treatments with the goal of minimizing radiation to healthy organs. Ion chamber measurements were performed in an anthropomorphic female thorax phantom at the center of each breast and lung and on the phantom surface at one anterior and two lateral locations (seven points total). The measurements were performed for three different isocenters located at the center of the phantom and at offset locations of the right and left breast. The dependence of the dose on angle selection for the CB-TS arc was also studied. For the most typical situation of centrally located CB-CT isocenter the measured doses ranged between 3 and 7 cGy, in good agreement with previous reports. Dose measurements were performed for a range of start/stop angles commonly used for CB-TS and the impact of direct and scatter dose on organs at risk was analyzed. All measured CB-TS doses were considerably lower than CB-CT doses, with greater decrease in dose for the organs outside of the beam (up to 98% decrease in dose). Remarkably, offsetting the isocenter towards the ipsilateral breast resulted on average to additional 46% dose reduction to organs at risk. The lowest doses to the contralateral breast and lung were less than 0.1 cGy when they were measured for the offset isocenter. The biggest reduction in dose was obtained by using CB-TS beams that completely avoid the critical organ. For points inside the CB-TS beam, the dose was reduced in a linear relation with distance from the center of the imaging arc. The data indicate that it is possible to reduce substantially radiation doses to the contralateral organs by proper selection of CB-TS angles and imaging field sizes. Our results provide the first systematic study on CB-TS doses from setup imaging for external breast irradiation and can be a useful resource for estimating anticipated radiation doses as a function of the conditions chosen for imaging breast setup.

Inter fractional variability of breathing phase definition as determined by fiducial location

Reconstruction of four-dimensional (4D) imaging typically requires an externally measurable surrogate to represent the real-time relative phase of respiration. A common method is to use a reflective marker on the external surface of the patient which moves with respiration and can be tracked in real time. The location of the marker is often chosen to maximize the observable motion, though this location may not be at the region of interest. We evaluate the importance of infrared (IR) marker placement location on breathing phase definition for the purpose of respiratory gating and 4D computed tomography (CT) image reconstruction. Data were collected for ten patients enrolled on an approved IRB protocol. Real-time position data were collected during CT imaging and daily treatments for two external IR reflective markers: one placed near the xyphoid and another at the approximate location of the treatment isocenter. Motion traces from the markers were compared using cross-correlation coefficient and by estimating the relative respiratory phase, based on either marker, as would be used for 4D-CT reconstruction. Cross-correlation analysis revealed differences in the motion waveform, as well as phase differences, both of which were variable between patients as well as day to day for the same patient. Estimated relative phases from each marker were compared by the percentage amount of time the estimated phase for each marker was different, binned based on increments of 10% of a full cycle. For all collected data combined, the frequency with which breathing phase mismatch led to different bin allocation in steps of 10% was as follows: T0%-10% = 65.1%, T10%-20% = 25.3%, T20%-30% = 7.8%, T30%-40% = 1.5% and T40%-50% = 0.4%. Based on ten images per cycle, this indicates that 4D reconstructions would be influenced, depending on which marker was used, by at least 1 bin 34.9% of the time. This number was noticeably higher for some patients; the maximum was 71% of the time for one patient of ten. In conclusion, the respiratory amplitude and relative phase depend significantly on the location of the IR marker used to monitor respiration. For some patients the xyphoid and isocentric markers may be completely out of phase. More importantly, this relationship varies day to day, suggesting that a single marker may be inadequate for the purposes of respiratory gating.

Prospective Assessment of Deep Inspiration Breath Hold Using 3-Dimensional Surface Tracking for Irradiation of Left-Sided Breast Cancer

PURPOSE Deep inspiration breath hold (DIBH) is used to decrease cardiac irradiation during radiation therapy (RT) for breast cancer. The patients most likely to benefit and the impact on treatment time remain largely unknown. We sought to identify predictors for the use of DIBH and to quantify differences in dosimetry and treatment time using a prospective registry. METHODS AND MATERIALS A total of 150 patients with left breast cancer were enrolled. All patients were simulated with both free breathing (FB) and DIBH. RT was delivered by either modality. Alternate scans were planned with use of deformable registration to include identical RT volumes. DIBH patients were monitored by a real-time surface tracking system, AlignRT (Vision RT, Ltd, London, United Kingdom). Baseline characteristics and treatment times were compared by Fisher exact test and Wilcoxon rank sum test. Dosimetric endpoints were analyzed by Wilcoxon signed rank test, and linear regression identified predictors for change in mean heart dose (∆MHD). RESULTS We treated 38 patients with FB and 110 with DIBH. FB patients were older, more likely to have heart and lung disease, and less likely to receive chemotherapy or immediate reconstruction (all P < .05). Treatment times were not significantly different, but DIBH patients had greater variability in times (P = .0002). Of 146 evaluable patients, DIBH resulted in >20 cGy improvement in MHD in 107 patients but a >20 cGy increase in MHD in 14. Both MHD and lung V20 were significantly lower in DIBH than in paired FB plans. On multivariate analysis, younger age (4.18 cGy per year; P < .0001), higher body mass index (6.06 cGy/kg/m(2); P = .0018), and greater change in lung volumes (130 cGy/L; P = .003) were associated with greater ∆MHD. CONCLUSIONS DIBH improves cardiac dosimetry without significantly impacting treatment time in most patients. Greater inspiratory lung volumes augment this benefit. Because the improvement with DIBH was not uniform, patients should be scanned with both FB and DIBH.

Performance and characteristics of an IR localizing system for radiation therapy

We report the development of a new system for interactive patient posture, position, and respiratory control during radiation therapy treatment. The system consists of an infrared (IR) camera, retroreflective markers, and dedicated software that makes it practical to use in the clinic. The system is designed to be used with multiple retroreflective markers to monitor not only the position, but also the posture of the patient in real time. Specific features of the system include the following: (1) The system reports an absolute misalignment at several points on a patient and also provides feedback on any necessary adjustments in terms of site‐specific setup parameters, such as focus‐to‐surface distance (PIN), superior and inferior alignment, and chest‐wall angle. (2) The system is based on a set of predefined templates containing the number and position of control points and feedback parameters developed for different treatment sites. (3) A noninvasive IR‐based “virtual portal vision” procedure projects organ contours in the beams‐eye‐view (BEV) based on the IR marker locations obtained in real time and compares them with digitally reconstructed radiographs (DRRs) from CT simulation. Assuming good correlation between external markers and internal anatomy, the system offers the possibility of mimicking a verification procedure without taking port‐films, which can potentially reduce the setup time. In this paper, we concentrate on the system properties and performance while initial applications on a number of clinical sites are ongoing. Accuracy and precision of this system are evaluated in the context of breast/chest treatments using rigid phantoms. The system has an intrinsic uncertainty of ±1mm. When two systems in different rooms (CT and treatment rooms) are used for correlating positional information, the uncertainty is less than 2 mm. PACS number: 87.56.Da

Evaluation of clip localization for different kilovoltage imaging modalities as applied to partial breast irradiation setup.

Surgical clip localization and image quality were evaluated for different types of kilovoltage cone beam imaging modalities as applied to partial breast irradiation (PBI) setup. These modalities included (i) clinically available radiographs and cone beam CT (CB-CT) and (ii) various alternative modalities based on partial/sparse/truncated CB-CT. An anthropomorphic torso-breast phantom with surgical clips was used for the imaging studies. The torso phantom had artificial lungs, and the attached breast phantom was a mammographic phantom with realistic shape and tissue inhomogeneities. Three types of clips of variable size were used in two orthogonal orientations to assess their in-/cross-plane characteristics for image-guided setup of the torso-breast phantom in supine position. All studies were performed with the Varian on-board imaging (OBI, Varian) system. CT reconstructions were calculated with the standard Feldkamp-Davis-Kress algorithm. First, the radiographs were studied for a wide range of viewing angles to characterize image quality for various types of body anatomy in the foreground/background of the clips. Next, image reconstruction quality was evaluated for partial/sparse/truncated CB-CT. Since these modalities led to reconstructions with strong artifacts due to insufficient input data, a knowledge-based CT reconstruction method was also tested. In this method, the input data to the reconstruction algorithm were modified by combining complementary data sets selected from the treatment and reference projections. Different partial/sparse/truncated CB-CT scan types were studied depending on the total are angle, angular increment between the consequent views (CT projections), orientation of the arc center with respect to the imaged breast and chest wall, and imaging field size. The central angles of the viewing arcs were either tangential or orthogonal to the chest wall. Several offset positions of the phantom with respect to the reference position were studied. The acquired and reconstructed image data sets were analyzed using home-built software focusing on the ability to localize clips in 3D. Streaking and leakage reconstruction artifacts and spatial distortions of breast surface were analyzed as well. Advantages and disadvantages of each kilovoltage CB imaging modality as applied to partial breast setup evaluation based on clips are presented. Because clips were found to be difficult to recognize in radiographs, 3D reconstructions were preferred. Even though it was possible to localize clips with about +/-1 mm accuracy based on reconstructions for short arcs of 40 degrees and incremental angle up to about 5 degrees, identification of clips in such reconstructions is difficult. Reconstructions obtained for arcs of as low as 80 degrees and incremental angle of as high as 3 degrees were suggested for easier clip identification. For more severely undersampled data, iterative CB-CT reconstruction is recommended to decrease the artifacts.

Kilovoltage beam model for flat panel imaging system with bow-tie filter for scatter prediction and correction.

PURPOSE Kilovoltage flat-panel imaging systems are used for cone-beam Computed Tomography (CBCT) and digital Tomosynthesis (DTS). Hereby, the presence of scatter and relatively large dose from imaging are challenging factors. In this study a phenomenological beam model was developed to characterize imager response to imaging beams with a bow-tie filter (Varian OBI system). MATERIALS AND METHOD The kilovoltage beam model was based on dose ratio formalism and thus was using standard concepts of megavoltage dose calculation such as scatter factors, tissue maximum ratio and off-axis ratio. Primary and scatter (head and phantom scatter) were modeled with three Gaussian kernels. Parameters were based on measured transmission images for slabs of solid water of different total thickness and various jaw settings. RESULTS The beam model was used to evaluate contributions from primary, secondary and tertiary contributions for different geometrical objects such as cylinders and step-like phantoms. Theoretical predictions of radiographs using the model for known objects are consistent with the measurements. CONCLUSION Secondary and tertiary contributions were interpreted as scatter and can be subtracted from CBCT projections based on the analytical model. Therefore our model can provide a basis for improvement of image quality (less artifacts due to scatter, better contrast and resolution) in CBCT reconstruction.

Polymorphism in myristoylpalmitoylphosphatidylcholine.

This study focuses on the mixed-chain lipid myristoylpalmitoylphosphatidylcholine (MPPC) near full hydration. The lipid, synthesized according to the procedure of (Mason et al., 1981a, has a low degree of acyl chain migration. When MPPC is temperature-jumped (T-jumped) from the L alpha phase (T = 38 degrees C) to T = 20 degrees C or below, a subgel phase forms; this formation takes less than 1 h at a temperature below T = 12 degrees C. The subgel remains stable up to T = 29 degrees C. When MPPC is T-jumped from the L alpha phase to T = 24 degrees C or above, a ripple phase forms with coexisting ripple wavelengths of 240 A and 130 A. In contrast, when MPPC is melted from the subgel phase, the ripple phase is characterized by bilayers having a single ripple wavelength of 130 A. In agreement with earlier studies (Stumpel et al., 1983; Serrallach et al., 1984. Structure and thermotropic properties of mixed-chain phosphatidylcholine bilayer membranes. Biochemistry 23:713-720.), no stable gel phase was observed. Instead, an ill-defined low-angle X-ray pattern is initially observed, which gradually transforms into the subgel phase below 20 degrees C, or into the ripple phase above 24 degrees C. In the wide-angle X-ray diffraction, a single peak is observed, similar to the ripple phase wide-angle pattern, that either persists above 24 degrees C or transforms into a multi-peaked subgel wide-angle pattern below 20 degrees C. The absence of a gel phase can be understood phenomenologically as the relative dominance of the subgel phase in mixed-chain PCs compared to same-chain PCs. The subgel structure and molecular interactions responsible for this comparative behavior are interesting open issues.

Balancing dose and image registration accuracy for cone beam tomosynthesis (CBTS) for breast patient setup

PURPOSE To balance dose reduction and image registration accuracy in breast setup imaging. In particular, the authors demonstrate the relationship between scan angle and dose delivery for cone beam tomosynthesis (CBTS) when employed for setup verification of breast cancer patients with surgical clips. METHODS The dose measurements were performed in a female torso phantom for varying scan angles of CBTS. Setup accuracy was measured using three registration methods: Clip centroid localization accuracy and the accuracy of two semiautomatic registration algorithms. The dose to the organs outside of the ipsilateral breast and registration accuracy information were compared to determine the optimal scan angle for CBTS for breast patient setup verification. Isocenter positions at the center of the patient and at the breast-chest wall interface were considered. RESULTS Image registration accuracy was within 1 mm for the CBTS scan anglesθ above 20° for some scenarios and as large as 80° for the worst case, depending on the imaged breast and registration algorithm. Registration accuracy was highest based on clip centroid localization. For left and right breast imaging with the isocenter at the chest wall, the dose to the contralateral side of the patient was very low (<0.5cGy) for all scan angles considered. For central isocenter location, the optimal scan angles were 30°-50° for the left breast imaging and 40°-50° for the right breast imaging, with the difference due to the geometric asymmetry of the current clinical imaging system. CONCLUSIONS The optimal scan angles for CBTS imaging were found to be between 10° and 50°, depending on the isocenter location and ipsilateral breast. Use of the isocenter at the breast-chest wall locations always resulted in greater accuracy of image registration(<1mm) at smaller angles (10°-20°) and at lower doses (<0.1cGy) to the contralateral organs. For chest wall isocenters, doses delivered to organs outside of the target breast were much smaller than the scattered and leakage doses of the treatment beams. The complete volumetric information of all clips in the region of interest, combined with the small dose to the contralateral organs and the small scan angle, could result in an advantage for small angle CBTS with off center isocenters over simple orthogonal pairs.