Air-tissue Interface Research Articles

Bovine tendon atomization

Thus far, histotripsy has not successfully fractionated tendon tissues, possibly due to bubbles being of insufficient size to create a pressure-release interface. In this study, a planar tissue-air interface was treated with histotripsy to create a micro-fountain and atomization; particulates ejected from the tendon sample were collected and analyzed histologically to determine whether tissue fractionation was achieved. [Work supported by NSF GRFP DGE1255832 and NIH NIBIB EB027886.]

Đánh giá ảnh hưởng của hốc khí và trường chiếu nhỏ tới phân bố liều của kế hoạch JO - IMRT trên bệnh nhân ung thư đầu cổ bằng phương pháp MONTE CARLO

Purposes: The goal of this study was to use Monte Carlo (MC) simulation to examine the dosimetric effects of the air cavity on JO-IMRT dose distribution at air-tissues interfaces in head-and-neck (H&N) patients. Methods: The EGSnrc - MC code system was used to calculate the dose reductions in air-tissue interface region for single field irradiations with 1×1, 2×2, 3×3, 4×4, and 5×5 cm2 in solid acrylic phantoms (30×30×20 cm3) and seven fields in a JO-IMRT plan. With phantom, the PDD values in both with and without an air cavity (15×4×4 cm3) which is 2.5 cm away from the anterior surface of phantom were used to evaluate. With the JO-IMRT plan, the dose-volume histograms (DVH), slice by slice isodose, and the gamma index using global methods implemented in PTW-VeriSoft with 3%/3 mm criteria were used to evaluate. Results: The study results indicate that the dose reductions in the air-tissue interface region of the phantom are strongly dependent on field size. The average percentage dose reductions at 1 mm from the air‑water interface for the field size 1×1, 2×2, 3×3, 4×4, and 5×5 cm2 are 62.04%, 52.34%, 40.71%, 26.72%, and 19.85%, respectively. Additionally, the mean MC dose in the PTV (65.58 Gy) of patients were lower than the TPS predicted dose (71.41 Gy). Conclusions: From this study, we conclude that the dose reduction in near air-tissue interfaces is a significant effect on JO-IMRT dose distribution in head-and-neck (H&N) patients.

Pulmonary MRI: Applications and Use Cases

Magnetic resonance imaging (MRI) has robust soft tissue characterization capability which was previously limited to evaluation of the mediastinum, cardiac, and chest wall imaging. MRI has now progressed from an experimental tool to complementary and alternate radiation free imaging modality for optimal identification and comprehensive evaluation of the lung parenchyma including structural, functional, and real-time imaging covering lung nodules/masses, infections, interstitial lung disease, airway diseases, and vascular and pleural abnormalities. Recent use of fast imaging techniques and respiratory gating has overcome several of the previously reported MRI technical difficulties such as respiratory, cardiac and diaphragmatic motion, as well as susceptibility related to air tissue interface in the lungs. MRI is a viable tool to the imaging armamentarium for the identification and characterization of pulmonary parenchymal abnormalities, providing complementary diagnostic information to computed tomography (CT), improving the non-invasive diagnostic accuracy and problem-solving for indeterminate lesions.

A semi-automated method for unbiased alveolar morphometry: Validation in a bronchopulmonary dysplasia model.

Reproducible and unbiased methods to quantify alveolar structure are important for research on many lung diseases. However, manually estimating alveolar structure through stereology is time consuming and inter-observer variability is high. The objective of this work was to develop and validate a fast, reproducible and accurate (semi-)automatic alternative. A FIJI-macro was designed that automatically segments lung images to binary masks, and counts the number of test points falling on tissue and the number of intersections of the air-tissue interface with a set of test lines. Manual selection remains necessary for the recognition of non-parenchymal tissue and alveolar exudates. Volume density of alveolar septa ([Formula: see text]) and mean linear intercept of the airspaces (Lm) as measured by the macro were compared to theoretical values for 11 artificial test images and to manually counted values for 17 lungs slides using linear regression and Bland-Altman plots. Inter-observer agreement between 3 observers, measuring 8 lungs both manually and automatically, was assessed using intraclass correlation coefficients (ICC). [Formula: see text] and Lm measured by the macro closely approached theoretical values for artificial test images (R2 of 0.9750 and 0.9573 and bias of 0.34% and 8.7%). The macro data in lungs were slightly higher for [Formula: see text] and slightly lower for Lm in comparison to manually counted values (R2 of 0.8262 and 0.8288 and bias of -6.0% and 12.1%). Visually, semi-automatic segmentation was accurate. Most importantly, manually counted [Formula: see text] and Lm had only moderate to good inter-observer agreement (ICC 0.859 and 0.643), but agreements were excellent for semi-automatically counted values (ICC 0.956 and 0.900). This semi-automatic method provides accurate and highly reproducible alveolar morphometry results. Future efforts should focus on refining methods for automatic detection of non-parenchymal tissue or exudates, and for assessment of lung structure on 3D reconstructions of lungs scanned with microCT.

Impact of Magnetic Field on Dose Distribution in MR-Guided Radiotherapy of Head and Neck Cancer.

PurposeThis study investigates the impact of the magnetic field on plan quality and dose at the tissue–air interface in MR-guided radiotherapy of head and neck cancer.Materials and MethodsThe charts of 10 patients with hypopharyngeal carcinoma who were treated with conventional fractionated radiotherapy were collected and reviewed. The skin and tissues containing air cavities were contoured. Three plans using 9 fields of intensity-modulated radiation therapy were generated for each patient in the Monaco treatment planning system of an Elekta Unity MR-linac. The first plan was optimized without the magnetic field (plan0T). The second plan was recalculated in the presence of a 1.5-T magnetic field (plan1.5T_reCal) using the same segment shape and monitor units as the first plan. The third plan was reoptimized in the presence of a 1.5-T magnetic field (plan1.5T_reOpt) using the same cost function as the first plan. The dose to the skin and tissues containing air cavities were compared across the three types of plans. A plan-quality metric method was used to evaluate the plan quality according to the clinical requirements.ResultsThe skin dose was increased in the presence of the 1.5-T magnetic field, and the amplitude increase of plan1.5T_reOpt (ΔDmean 1.30 ± 0.42 Gy, ΔDmax 1.68 ± 1.36 Gy) was smaller than that of plan1.5T_reCal (ΔDmean 1.81 ± 0.79 Gy, ΔDmax 5.43 ± 2.26 Gy). There were no significant differences in terms of the metrics of interfaces of tissues containing air cavities except for an increased maximum dose to the larynx and trachea. The plan quality of plan1.5T_reCal (68.0 ± 9.2) was significantly worse than that of plan0T (82.2 ± 7.0), and the plan quality of plan1.5T_reOpt (80.0 ± 7.0) was similar to that of plan0T.ConclusionThe presence of a 1.5-T magnetic field had an apparent impact on the dose distribution, in particular, a significant increase in the skin dose. The plan quality of plan1.5T_reOpt was similar to that of the original plan0T when the same cost function was used.

Design of Low-SAR Mobile Phone Antenna: Theory and Applications

This article focuses on the design of a low specific absorption rate (SAR) mobile phone antennas. First, the interaction mechanism of the radiation fields of mobile phone antennas with nearby human tissues is analyzed by using boundary conditions of electromagnetics. The relations between SAR and the electric field are constructed. Then, a concept of reverse current is proposed to decrease the electric field radiated by the antenna at the air-tissue interface and, consequently, reduce the SAR of human tissues. Finally, a metal-rimmed mobile phone antenna is designed according to the proposed theory. The ground clearance of the designed antenna is 2 mm. Their measured normalized peak head and body SAR values are lower than 1.0 W/kg, much less than the SAR safety limit of 1.6 W/kg according to the guidelines of IEEE and FCC or 2.0 W/kg according to the guidelines of the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The measured -6 dB impedance bandwidths are 830-1040 and 1670-2730 MHz, respectively. The measured efficiencies are higher than -5 dB at the lower band and -4 dB at the higher band. The measured results prove that the proposed theory can provide guidance for the design of low-SAR mobile phone antennas.

Alveolar mimics with periodic strain and its effect on the cell layer formation.

We report on the development of a new model of alveolar air-tissue interface on a chip. The model consists of an array of suspended hexagonal monolayers of gelatin nanofibers supported by microframes and a microfluidic device for the patch integration. The suspended monolayers are deformed to a central displacement of 40-80 µm at the air-liquid interface by application of air pressure in the range of 200-1,000 Pa. With respect to the diameter of the monolayers, that is, 500 µm, this displacement corresponds to a linear strain of 2-10% in agreement with the physiological strain range in the lung alveoli. The culture of A549 cells on the monolayers for an incubation time of 1-3 days showed viability in the model. We exerted a periodic strain of 5% at a frequency of 0.2 Hz for 1 hr to the cells. We found that the cells were strongly coupled to the nanofibers, but the strain reduced the coupling and induced remodeling of the actin cytoskeleton, which led to a better tissue formation. Our model can serve as a versatile tool in lung investigations such as in inhalation toxicology and therapy.

"Elasto-lung point": A new tool for the sonographic confirmation of pneumothorax.

Standard ultrasound gives the operator a dynamic morphology of the investigated anatomy, whereas ultrasound elastography (USE) provides quantitative and qualitative information about the elastic properties of the tissues. We designed a single-arm prospective study in order to investigate the feasibility of USE in the diagnosis of pneumothorax if a lung point sign is present. Thirty patients were enrolled in this protocol, from January 2017 to December 2018 at the Pneumology Department of the Azienda Socio-Sanitaria Territoriale Spedali Civili (Brescia, Italy). Patients who were suspected of having pneumothorax were previously evaluated with standard ultrasonography, and then, in the presence of lung point, we performed strain elastography. All patients were evaluated in supine and sitting positions with a linear probe (7.5 MHz). USE enhanced the air-tissue interface dividing the normal parenchyma from the air column of pneumothorax with a sharp line. We called this sign "elasto-lung point." The "elasto-lung point" was able to confirm the diagnosis of pneumothorax in every investigated patient. USE is a simple, reproducible and inexpensive technique that can contribute to the diagnosis of pneumothorax, such as the classic "stratosphere" or "Bar Code" sign in M-mode. No false negative cases were observed.

Comparison of 2D BLADE Turbo Gradient- and Spin-Echo and 2D Spin-Echo Echo-Planar Diffusion-Weighted Brain MRI at 3 T: Preliminary Experience in Children

We describe our preliminary experience using a 2D turbo gradient- and spin-echo (TGSE) diffusion-weighted (DW) pulse sequence with non-Cartesian BLADE trajectory at 3 T in pediatric patients. We compared the TGSE BLADE to conventional DW spin-echo echo-planar imaging (SE-EPI) in pediatric brain imaging, assessing the presence of artifacts from signal pile-ups, geometric distortion, motion, susceptibility from air-tissue interface, shunts and orthodontia, and diagnostic image quality. Data were acquired in 53 patients (10.4 ± 7.9 years). All DW imaging data were acquired precontrast, with SE-EPI first. A four-point scale for rating was used-1 (best) and 4 (worst). A neuroradiologist scored the two sequences and further noted whether the TGSE BLADE approach or SE-EPI was preferred in each case. Apparent diffusion coefficients were compared quantitatively between the two sequences in a subset of 16 patients, in 41 separate regions of interests including caudate nucleus, putamen, globus pallidus, thalamus, and pathological areas. In 43.4% of the cases, TGSE BLADE was preferred; in 49.1% of the cases, both sequences were preferred equally. Average scores for SE-EPI were 2.2 ± 0.8 versus TGSE's 1.2 ± 0.4 in assessing diagnostic quality (p < 0.05). Motion artifacts were minimal on both sequences in 92.5% of the cases. In the TGSE BLADE scores, no case received a "4" for significant artifacts with marginally acceptable image quality. Apparent diffusion coefficients values between the two sequences were statistically similar, with a linear regression slope of 0.92 (r2 = 0.97). TGSE BLADE DW imaging exhibited less geometric distortion in the brain and reduced signal pile-ups in areas of high susceptibility than conventional SE-EPI.

Background field removal for susceptibility mapping of human brain with large susceptibility variations.

In quantitative susceptibility mapping (QSM) of human brain, the background field induced by air-tissue interface varies significantly with respect to the rotation angle between the head and the static field, which may result in substantial error in the estimated magnetic susceptibility values. The goal of this study was to develop a strategy to better remove such orientation dependent background field. An improved background field removal method is proposed based on the sophisticated harmonic artifact reduction for phase data using a region adaptive kernel (R-SHARP), named iRSHARP. It uses a spatially weighted spherical Gaussian kernel exploiting the amplitude, gradient, and wrap count of the phase map. The method was validated using both numerical simulations and in vivo human brain data at multiple head orientations. Performance was compared with the variable kernel (V-SHARP) and R-SHARP methods. The proposed iRSHARP method showed improved background removal over R-SHARP while cutting the computational time in half. As compared to V-SHARP and R-SHARP, the iRSHARP generated local field and susceptibility maps showed fewer artifacts in regions of large susceptibility variations, and for the in vivo human brain, the susceptibilities of the deep gray matter nuclei were consistent with the in vivo gold-standard "Calculation of Susceptibility through Multiple Orientation Sampling" (COSMOS) values. iRSHARP can remove the orientation dependent background field effectively. Using iRSHARP, the paranasal sinus regions can be preserved in the brain mask and the brain integrity was conserved, which may facilitate further data analysis and clinical application.

Experimental evaluation of the impact of low tesla transverse magnetic field on dose distribution in presence of tissue interfaces

PurposeAim of this study is to experimental evaluate the impact of a 0.35 T transverse magnetic field on dose distribution in presence of tissue-air and tissue-lung interfaces. MethodsThe investigation was carried out using MRIdian (ViewRay, Cleveland, Ohio) and it consisted of comparing experimental measurements performed by Gafchromic EBT3 film dosimetry, to Montecarlo simulations, carried out in the presence and, as well as, the absence of the magnetic field.A preliminary dose calibration was planned on MRIdian, arranging 3 × 3 cm2 film pieces in a water slab phantom and exposing them at different beam-on times, in a dose range equal to 0.1–12.1 Gy.All experimental measurements were then carried out using the calibrated films and delivering one single beam orthogonally to three different phantoms: without inhomogeneity, with an air gap and with a lung inhomogeneity.The dose distributions measured by EBT3 films in presence of magnetic field were compared to those calculated in the presence and, as well as, the absence of the magnetic field, in terms of gamma analysis. A quantification of electron return effect (ERE) was also performed. ResultsAll the tested plans considering the magnetic field show a gamma-passing rate higher than 98% for 3%/3 mm gamma analysis.In presence of tissue-air interface, the electron return effect causes an over-dosage of +31.9% at the first interface and an under-dosage of −33% at the second interface. The dosimetric variations in presence of tissue-lung interface results to be smaller (+0.8% first interface, −1.3% second interface). ConclusionThe impact of 0.35 T magnetic field is not negligible and it can be effectively modelled by the Montecarlo dose calculation platform available in the MRIdian TPS.

Improved method for MR microscopy of brain tissue cultured with the interface method combined with Lenz lenses

MR in microscopy can non-invasively image the morphology of living tissue, which is of particular interest in studying the mammalian brain. Many studies use live animals for basic research on brain functions, disease pathogenesis, and drug development. However, in vitro systems are on the rise, due to advantages such as the absence of a blood-brain barrier, predictable pharmacokinetics, and reduced ethical restrictions. Hence, they present an inexpensive and adequate technique to answer scientific questions and to perform drug screenings. Some publications report the use of acute brain slices for MR microscopy studies, but these only permit single measurements over several hours. Repetitive MR measurements in longitudinal studies demand an MR-compatible setup which allows cultivation for several days or weeks, and hence properly functioning in vitro systems. Organotypic hippocampal slice cultures (OHSC) are a well-established and robust in vitro system which still exhibits most histological hallmarks of the hippocampal network in vivo. An MR compatible incubation platform is introduced in which OHSC are cultivated according to the interface method following Stoppini et al. In this cultivation method a tissue slice is placed onto a membrane with nutrition medium underneath and a gas atmosphere above, where the air-tissue interface perpendicular to the B0 field induces strong artefacts. We introduce a handling protocol that suppresses these artefacts and increases signal quality significantly to acquire high resolution images of tissue slices. An additional challenge is the lack of available of MR microscopy equipment suitable for small animal scanners. A Lenz lens with an attached capacitor can dramatically increase the SNR in these cases, and wirelessly bring the detection system in close proximity to the sample without compromising the OHSC system through the introduction of wired detectors. The resultant signal gain is demonstrated by imaging a PFA-fixed brain slice with a 72 mm diameter volume coil without a Lenz lens, and with a broadband and a self-resonant Lenz lens. In our setting, the self-resonant Lenz lens increases the SNR 10-fold over using the volume coil only.

Study on the dose modification factor of strut adjusted volume implant (SAVI) with a 169Yb source using MCNP4C.

The SAVI has gained widespread use for accelerated partial breast irradiation (APBI) brachytherapy. Treatments with SAVI produce inherent heterogeneities including variable backscatter due to proximity to the tissue-air interface and variable cavity contents, causing inaccuracy in the dose calculation. In this study, a model of SAVI with sources of 169Yb developed recently was defined and simulations with MCNP4C code of Monte Carlo were performed through different scenarios to assess the effects of these heterogeneities on the dose distribution. The results showed that the dose delivered to target volume may be lower than the planned dose by up to 9-16%. Therefore, the therapy with169Yb must be viewed with caution and a correction factor must be applied in treatment planning systems. It was also observed that the presence of air cavity changed the relative dose 1% for a symmetric plan compared to a water cavity and up to 5% for an asymmetric plan. It was indicated that the dose modification factor (DMF) did not differ in any significant way to that of the symmetric plan. The effect of the composition on the DMF was negligible because the natures of water and breast tissue were approximately similar. All the obtained results indicated that should 169Yb sources be used with SAVI applications, some amendments of treatment planning systems would be employed.

Evaluation of dose profiles using Gafchromic EBT3 films in Leksell Gamma Knife 4C around inhomogeneities in the treatment of pituitary adenoma in anthropomorphic heterogeneous head phantom

PurposeThe Leksell Gamma Knife is a complete system for radiosurgery. Treatment dose planning of Gamma Knife assumes the human head as a homogenous media; thus received dose by target tissue and organs at risk may be lower or higher than predicted dose. Our main interest is to compare measured dose profiles in an anthropomorphic heterogeneous head phantom by EBT3 films with calculated dose by Leksell Gamma Plan. MethodsAn adult head phantom with size, shape and inhomogeneity the same as an adult human head was constructed. Then dose profiles across from isocenter and organs at risk were measured by EBT3 films and compared with those predicted by Leksell Gamma Plan. ResultsOur findings indicate that when isocenter placed at the target tissue near the frontal sinus cavity, overestimation and underestimation of the dose value affect the dose profiles. In some points of organs at risk, measured dose is more than calculated dose. Comparison of isodose curves near air-tissue interface shows that in higher isodose levels, dose perturbation is more seen. Distance to Agreement parameter for some isodose lines exceeds the total positioning error for Gamma Knife (± 2 mm). Also dose differences are more than numerical accuracy in dose delivery for stereotactic radiosurgery (± 5%). ConclusionsInhomogeneities in head impress dose distributions. Ignorance of inhomogeneities can cause the parts of target tissue receive under dosage and also in some points, organs at risk receive more dose. Results of this study are closer to reality because of anthropomorphic heterogeneous head phantom.

Estimating the Dose in Air-Tissue Interface for SAVI-Based HDR Brachytherapy with Geant4 Monte Carlo Simulation

Estimating the Dose in Air-Tissue Interface for SAVI-Based HDR Brachytherapy with Geant4 Monte Carlo Simulation

Background field removal using a region adaptive kernel for quantitative susceptibility mapping of human brain

Background field removal is an important MR phase preprocessing step for quantitative susceptibility mapping (QSM). It separates the local field induced by tissue magnetic susceptibility sources from the background field generated by sources outside a region of interest, e.g. brain, such as air-tissue interface. In the vicinity of air-tissue boundary, e.g. skull and paranasal sinuses, where large susceptibility variations exist, present background field removal methods are usually insufficient and these regions often need to be excluded by brain mask erosion at the expense of losing information of local field and thus susceptibility measures in these regions. In this paper, we propose an extension to the variable-kernel sophisticated harmonic artifact reduction for phase data (V-SHARP) background field removal method using a region adaptive kernel (R-SHARP), in which a scalable spherical Gaussian kernel (SGK) is employed with its kernel radius and weights adjustable according to an energy “functional” reflecting the magnitude of field variation. Such an energy functional is defined in terms of a contour and two fitting functions incorporating regularization terms, from which a curve evolution model in level set formation is derived for energy minimization. We utilize it to detect regions of with a large field gradient caused by strong susceptibility variation. In such regions, the SGK will have a small radius and high weight at the sphere center in a manner adaptive to the voxel energy of the field perturbation. Using the proposed method, the background field generated from external sources can be effectively removed to get a more accurate estimation of the local field and thus of the QSM dipole inversion to map local tissue susceptibility sources. Numerical simulation, phantom and in vivo human brain data demonstrate improved performance of R-SHARP compared to V-SHARP and RESHARP (regularization enabled SHARP) methods, even when the whole paranasal sinus regions are preserved in the brain mask. Shadow artifacts due to strong susceptibility variations in the derived QSM maps could also be largely eliminated using the R-SHARP method, leading to more accurate QSM reconstruction.

Ultrasonographic Detection of Airway Obstruction in a Model of Obstructive Sleep Apnea.

Purpose Obstructive sleep apnea (OSA) is a common clinical disorder characterized by repetitive airway obstruction during sleep. The gold standard for diagnosis of OSA, polysomnogram (PSG), cannot anatomically localize obstruction. Precise identification of obstruction has potential to improve outcomes following surgery. Current diagnostic modalities that provide this information require anesthesia, involve ionizing radiation or disrupt sleep. To mitigate these problems, we conceived that ultrasound (US) technology may be adapted (i) to detect, quantify and localize airway obstruction and (ii) for translational application to home-based testing for OSA. Materials and Methods Segmental airway collapse was induced in 4 fresh cadavers by application of negative pressure. Following visualization of airway obstruction, a rotary US probe was used to acquire transcervical images of the airway before and after induction of obstruction. These images (n=800) were analyzed offline using image processing algorithms. Results Our results show that the non-obstructed airway consistently demonstrated the presence of a US air-tissue interface. Importantly, automated detection of the air-tissue interface strongly correlated with manual measurements. The algorithm correctly detected an air-tissue interface in 90% of the US images while incorrectly detecting it in 20% (area under the curve=0.91). Conclusion The non-invasive detection of airway obstruction using US represents a major step in expanding OSA diagnostics beyond PSG. The preliminary data obtained from our model could spur further research in non-invasive localization of obstruction. US offers the benefit of precise localization of the site of obstruction, with potential for improving outcomes in surgical management.

An Efficient Reconstruction Algorithm Based on the Alternating Direction Method of Multipliers for Joint Estimation of ${R}_{{2}}^{*}$ and Off-Resonance in fMRI.

R*2 mapping is a useful tool in blood-oxygen-level dependent fMRI due to its quantitative-nature. However, like T*2-weighted imaging, standard R*2 mapping based on multi-echo EPI suffers from geometric distortion, due to strong off-resonance near the air-tissue interface. Joint mapping of R*2 and off-resonance can correct the geometric distortion and is less susceptible to motion artifacts. Single-shot joint mapping of R*2 and off-resonance is possible with a rosette trajectory due to its frequent sampling of the k-space center. However, the corresponding reconstruction is nonlinear, ill-conditioned, large-scale, and computationally inefficient with current algorithms. In this paper, we propose a novel algorithm for joint mapping of R*2 and off-resonance, using rosette k-space trajectories. The new algorithm, based on the alternating direction method of multipliers, improves the reconstruction efficiency by simplifying the original complicated cost function into a composition of simpler optimization steps. Compared with a recently developed trust region algorithm, the new algorithm achieves the same accuracy and an acceleration of threefold to sixfold in reconstruction time. Based on the new algorithm, we present simulation and in vivo data from single-shot, double-shot, and quadruple-shot rosettes and demonstrate the improved image quality and reduction of distortions in the reconstructed R*2 map.

Analytic function for predicting light fluence rate of circular fields on a semi-infinite turbid medium.

Accurate determination of in-vivo light fluence rate is critical for preclinical and clinical studies involving photodynamic therapy (PDT). The light fluence distribution in tissue depends on both the tissue optical properties and the incident field size. This study compares the longitudinal light fluence distribution inside biological tissue in the central axis of circular uniform light field with different radii for a range of in-vivo tissue optical properties (absorption coefficients (µa) between 0.01 and 1 cm-1 and reduced scattering coefficients (µs') between 2 and 40 cm-1). This was done using Monte-Carlo simulations for a semi-infinite turbid medium in an air-tissue interface. The end goal is to develop simple analytical expressions that would fit the results from the Monte Carlo simulation for circular beams with different radii. A 6-parameter model (ϕ/ϕair=(1-b⋅e-λ1d)(C2e-λ2d+C3e-λ3d)) can be used to fit MC simulation. Each of these parameters (b, C2, C3, λ1, λ2, and λ3) is expressed as a function of tissue optical properties and beam radius. These results can then be compared against the existing expressions in the literature for broad beam for analysis in both accuracy and applicable range. The analytical function can be used as rapid guide in PDT to calculate in vivo light fluence distribution for known tissue optical properties.

SU-G-JeP2-05: Dose Effects of a 1.5T Magnetic Field On Air-Tissue and Lung-Tissue Interfaces in MRI-Guided Radiotherapy

Purpose: The purpose of the study is to investigate the dose effects of electron-return-effect (ERE) at air-tissue and lung-tissue interfaces under a 1.5T transverse-magnetic-field (TMF). Methods: IMRT and VMAT plans for representative pancreas, lung, breast and head & neck (H&N) cases were generated following clinical dose volume (DV) criteria. The air-cavity walls, as well as the lung wall, were delineated to examine the ERE. In each case, the original plan generated without TMF is compared with the reconstructed plan (generated by recalculating the original plan with the presence of TMF) and the optimized plan (generated by a full optimization with TMF), using a variety of DV parameters, including V100%, D95% and dose heterogeneity index for PTV, Dmax, and D1cc for OARs (organs at risk) and tissue interface. Results: The dose recalculation under TMF showed the presence of the 1.5 T TMF can slightly reduce V100% and D95% for PTV, with the differences being less than 4% for all but lung case studied. The TMF results in considerable increases in Dmax and D1cc on the skin in all cases, mostly between 10-35%. The changes in Dmax and D1cc on air cavity walls are dependent upon site, geometry, and size, with changes ranging up to 15%. In general, the VMAT plans lead to much smaller dose effects from ERE compared to fixed-beam IMRT. When the TMF is considered in the plan optimization, the dose effects of the TMF at tissue interfaces are significantly reduced in most cases. Conclusion: The doses on tissue interfaces can be significantly changed by the presence of a 1.5T TMF during MR-guided RT when the TMF is not included in plan optimization. These changes can be substantially reduced or even removed during VMAT/IMRT optimization that specifically considers the TMF, without deteriorating overall plan quality.