Performance assessment of two motion management systems for frameless stereotactic radiosurgery.

BACKGROUND/PURPOSE: Frameless stereotactic radiosurgery (SRS) requires dedicated systems to monitor patient motion in order to avoid inaccurate radiation delivery due to involuntary shifts. The purpose of this study is to assess the accuracy and sensitivity of two distinct motion monitoring systems used for frameless SRS. METHODS: A surface image-guided system known as optical surface monitoring system (OSMS), and a fiducial marker-based system known as high definition motion management (HDMM) as part of the latest Gamma Knife Icon® were compared. A 3D printer-based cranial motion phantom was developed to evaluate the accuracy and sensitivity of these two systems in terms of: (1) the capability to recognize predefined shifts up to 3 cm, and (2) the capability to recognize predefined speeds up to 3 cm/s. The performance of OSMS, in terms of different reference surfaces, was also evaluated. RESULTS: Translational motion could be accurately detected by both systems, with an accuracy of 0.3 mm for displacement up to 1 cm, and 0.5 mm for larger displacements. The reference surface selection had an impact on OSMS performance, with flat surface resulting in less accuracy. HDMM was in general more sensitive when compared with OSMS in capturing the motion, due to its faster frame rate, but a delay in response was observed with faster speeds. Both systems were less sensitive in detection of superior-inferior motion when compared to lateral or vertical displacement directions. CONCLUSION: Translational motion can be accurately and sensitively detected by OSMS and HDMM real-time monitoring systems. However, performance variations were observed along different motion directions, as well as amongst the selection of reference images. Caution is needed when using real-time monitoring systems for frameless SRS treatment.

Impact of small MU/segment and dose rate on delivery accuracy of volumetric-modulated arc therapy (VMAT).

Volumetric-modulated arc therapy (VMAT) plans may require more control points (or segments) than some of fixed-beam IMRT plans that are created with a limited number of segments. Increasing number of control points in a VMAT plan for a given prescription dose could create a large portion of the total number of segments with small number monitor units (MUs) per segment. The purpose of this study is to investigate the impact of the small number MU/segment on the delivery accuracy of VMAT delivered with various dose rates. Ten patient datasets were planned for hippocampus sparing for whole brain irradiation. For each dataset, two VMAT plans were created with maximum dose rates of 600 MU/min (the maximum field size of 21 × 40 cm2) and 1000 MU/min (the maximum field size of 15 × 15 cm2) for a daily dose of 3 Gy. Without reoptimization, the daily dose of these plans was purposely reduced to 1.5 Gy and 1.0 Gy while keeping the same total dose. Using the two dose rates and three different daily doses, six VMAT plans for each dataset were delivered to a physical phantom to investigate how the changes of dose rate and daily doses impact on delivery accuracy. Using the gamma index, we directly compared the delivered planar dose profiles with the reduced daily doses (1.5 Gy and 1.0 Gy) to the delivered planar dose at 3 Gy daily dose, delivered at dose rate of 600 MU/min and 1000 MU/min, respectively. The average numbers of segments with MU/segment ≤ 1 were 35 ± 8, 87 ± 6 for VMAT-600 1.5 Gy, VMAT-600 1 Gy plans, and 30 ± 7 and 42 ± 6 for VMAT-1000 1.5 Gy and VMAT-1000 1 Gy plans, respectively. When delivered at 600 MU/min dose rate, the average gamma index passing rates (1%/1 mm criteria) of comparing delivered 1.5 Gy VMAT planar dose profiles to 3.0 Gy VMAT delivered planar dose profiles was 98.28% ± 1.66%, and the average gamma index passing rate of comparing delivered 1.0 Gy VMAT planar dose to 3.0 Gy VMAT delivered planar dose was 83.75% ± 4.86%. If using 2%/2mm and 3%/3 mm criteria, the gamma index passing rates were greater than 97% for both 1.5 Gy VMAT and 1.0 Gy VMAT delivered planar doses. At 1000MU/min dose rate, the average gamma index passing rates were 96.59% ± 2.70% for 1.5 Gy VMAT planar dose profiles and 79.37% ± 9.96% for 1.0 Gy VMAT planar dose profiles when compared to the 3.0 Gy VMAT planar delivered dose profile. When using 2%/2 mm and 3%/3 mm criteria, the gamma index passing rates were greater than 93% for both 1.5 Gy VMAT and 1.0 Gy VMAT planar delivered dose. Under a stricter gamma index criterion (1%/1 mm), significant differences in delivered planar dose profiles at different daily doses were detected, indicating that the known communication delay between the MU console and MLC console may affect VMAT delivery accuracy.

Radiomics for the Prediction of Pathological Complete Response to Neoadjuvant Chemoradiation in Locally Advanced Rectal Cancer: A Prospective Observational Trial.

(1) Background: An increasing amount of research has supported the role of radiomics for predicting pathological complete response (pCR) to neoadjuvant chemoradiation treatment (nCRT) in order to provide better management of locally advanced rectal cancer (LARC) patients. However, the lack of validation from prospective trials has hindered the clinical adoption of such studies. The purpose of this study is to validate a radiomics model for pCR assessment in a prospective trial to provide informative insight into radiomics validation. (2) Methods: This study involved a retrospective cohort of 147 consecutive patients for the development/validation of a radiomics model, and a prospective cohort of 77 patients from two institutions to test its generalization. The model was constructed using T2-weighted, diffusion-weighted, and dynamic contrast-enhanced MRI to understand the associations with pCR. The consistency of physicians' evaluations and agreement on pathological complete response prediction were also evaluated, with and without the aid of the radiomics model. (3) Results: The radiomics model outperformed both physicians' visual assessments in the prospective test cohort, with an area under the curve (AUC) of 0.84 (95% confidence interval of 0.70-0.94). With the aid of the radiomics model, a junior physician could achieve comparable performance as a senior oncologist. (4) Conclusion: We have built and validated a radiomics model with pretreatment MRI for pCR prediction of LARC patients undergoing nCRT.

Evaluation of Biological Effective Dose in Gamma Knife Staged Stereotactic Radiosurgery for Large Brain Metastases.

Objective: Gamma knife (GK) staged stereotactic radiosurgery (Staged-SRS) has become an effective treatment option for large brain metastases (BMs); however, it has been challenging to evaluate the total dose because of tumor shrinkage between two staged sessions. This study aims to evaluate total biological effective dose (BED) in Staged-SRS, and to compare the BED with those in single-fraction SRS (SF-SRS) and hypo-fractionated SRS (HF-SRS). Methods: Patients treated with GK Staged-SRS at a single institution were retrospectively included. The dose delivered in two sessions of Staged-SRS was summed using the deformable image registration. Each patient was replanned for SF-SRS and HF-SRS. The total BEDs were computed using the linear-quadratic model. Tumor BED98% and brain V84Gy2, equivalent to V12Gy in SF-SRS, were compared between SF-SRS, HF-SRS, and Staged-SRS plans with the Wilcoxon test. Results: Twelve patients with 24 BMs treated with GK Staged-SRS were identified. We observed significant differences (p < 0.05) in tumor BED98% but comparable brain V84Gy2 (p = 0.677) between the Staged-SRS and SF-SRS plans. No dosimetric advantages of Staged-SRS over HF-SRS were observed. Tumor BED98% in the HF-SRS plans were significantly higher than those in the Staged-SRS plans (p < 0.05). Despite the larger PTVs, brain V84Gy2 in the HF-SRS plans remained lower (p < 0.05). Conclusion: We presented an approach to calculate the composite BEDs delivered to both tumor and normal brain tissue in Staged-SRS. Compared to SF-SRS, Staged-SRS delivers a higher dose to tumor but maintains a comparable dose to normal brain tissue. Our results did not show any dosimetric advantages of Staged-SRS over HF-SRS.

Does active coaching reduce actual treatment duration for frameless Gamma Knife stereotactic radiosurgery?

Purpose/Objectives: Frameless Gamma Knife stereotactic radiosurgery (GKSRS) has become an effective supplement to frame-based, which is however sensitive to patient's involuntary motions and prone to prolonged treatment duration. Such delays during treatment inevitably result in patient discomfort and the inability to complete intended treatment. The purpose of this study is to investigate whether active coaching during frameless GKSRS can reduce actual treatment duration. Materials/Methods: Patients treated at a single institution with frameless GKSRS from 2017 to 2020 were retrospectively identified. Beginning in 2019, all patients treated with frameless GKSRS were actively coached to prevent treatment interruptions. Patient characteristics and treatment plans were compared between the cohorts of patients treated with and without active coaching. Linear regressions between the planned and actual treatment duration of treatment sessions were performed on either cohort. ANOVA and Wilcoxon tests were used for statistical analyses with a p-value less than 0.05 considered as significant. Results: Of the total 43 patients and 105 treatment sessions identified, 27 patients underwent 51 treatment sessions of frameless GKSRS with active coaching. There was no significant difference in patient characteristics and treatment plans between the two cohorts. Patients treated with active coaching underwent significantly fewer CBCTs during treatment. The median planned and actual treatment durations were 31.4 and 51.7 min for the non-coached cohort, and 38.6 and 49.8 min for the coached cohort. The results of linear regressions showed that the actual treatment duration was 1.29 and 1.56 times longer with and without active coaching, respectively, which indicated a significant reduction in the actual treatment duration with active coaching. Conclusion: Our results suggest that active coaching was associated with significant reductions of actual treatment duration. This simple intervention can be clinically implemented to prevent unnecessary treatment interruptions, improve patient comfort and ensure completion of treatment as prescribed during frameless GKSRS.

Clinical Evaluation of the Inverse Planning System Utilized in Gamma Knife Lightning.

Objectives: The purpose of this study is to independently compare the performance of the inverse planning algorithm utilized in Gamma Knife (GK) Lightning Treatment Planning System (TPS) to manual forward planning, between experienced and inexperienced users, for different types of targets. Materials and Methods: Forty patients treated with GK stereotactic radiosurgery (SRS) for pituitary adenoma (PA), vestibular schwannoma (VS), post-operative brain metastases (pBM), and intact brain metastases (iBM) were randomly selected, ten for each site. Three inversely optimized plans were generated for each case by two experienced planners (OptExp1 and OptExp2) and a novice planner (OptNov) using GK Lightning TPS. For each treatment site, the Gradient Index (GI), the Paddick Conformity Index (PCI), the prescription percentage, the scaled beam-on time (sBOT), the number of shots used, and dosimetric metrics to OARs were compared first between the inversely optimized plans and the manually generated clinical plans, and then among the inversely optimized plans. Statistical analyses were performed using the Student's t-test and the ANOVA followed by the post-hoc Tukey tests. Results: The GI for the inversely optimized plans significantly outperformed the clinical plans for all sites. PCIs were similar between the inversely optimized and clinical plans for PA and VS, but were significantly improved in the inversely optimized plans for iBM and pBM. There were no significant differences in the sBOT between the inversely optimized and clinical plans, except for the PA cases. No significant differences were observed in dosimetric metrics, except for lower brain V12Gy and PTV D98% in the inversely optimized plans for iBM. There were no noticeable differences in plan qualities among the inversely optimized plans created by the novice and experienced planners. Conclusion: Inverse planning in GK Lightning TPS produces GK SRS plans at least equivalent in plan quality and similar in sBOT compared to manual forward planning in this independent validation study. The automatic workflow of inversed planning ensures a consistent plan quality regardless of a planner's experience.

The Impact of Optic Nerve Movement on Intracranial Radiation Treatment.

Purpose: In radiotherapy, high radiation exposure to optic nerve (ON) can cause optic neuropathy or vision loss. In this study, we evaluated the pattern and extent of the ON movement using MRI, and investigated the potential dosimetric effect of this movement on radiotherapy. Methods: MRI was performed in multiple planes in 5 human subjects without optic pathway abnormalities to determine optic nerve motion in different scenarios. The subjects were requested to gaze toward five directions during MRI acquisitions, including neutral (straight forward), left/right (horizontal movement), and up/down (vertical movement). Subsequently, the measured displacement was applied to patients with peri-optic tumors to evaluate the potential dosimetric effect of this motion. Results: The motion of ON followed a nearly conical shape. By average, the anterior end of ONs moved with 10.8 ± 2.2 mm horizontally and 9.3 ± 0.8 mm vertically, while posterior end has negligible displacement. For patients who underwent stereotactic radiotherapy to a peri-optic tumors, the movement of ON in this measured range introduced non-negligible dosimetric effect. Conclusion: The range of motion of the anterior portions of the optic nerves is on the order of centimeters, which may need to be considered with extra attention during radiation therapy in treating peri-optic lesions.

Direct measurement of the friction between and shear moduli of shells of carbon nanotubes.

We report measurements of the shear modulus of each shell and the friction between the two shells of double-shell carbon nanotubes in single nanotube-based nanoelectromechanical devices operated in a transmission electron microscope. In situ nanobeam electron diffraction is applied to obtain the chiral indices of each shell of the nanotube and it allows us to establish a quantitative correlation between the atomic structure and properties of the nanotube under investigation.

Optimization of treatment isocenter location in single-isocenter LINAC-based stereotactic radiosurgery for management of multiple brain metastases.

PURPOSE: Single-isocenter linear accelerator (LINAC)-based stereotactic radiosurgery (SRS) has become a promising treatment technique for the management of multiple brain metastases. Because of the high prescription dose and steep dose gradient, SRS plans are sensitive to geometric errors, resulting in loss of target coverage and suboptimal local tumor control. Current planning techniques rely on adding a uniform and isotropic setup margin to all gross tumor volumes (GTVs) to account for rotational uncertainties. However, this setup margin may be insufficient, since the magnitude of rotational uncertainties varies and is dependent upon the distance between a GTV and the isocenter. In this study, we designed a framework to determine the optimal isocenter of a single-isocenter SRS plan for multiple brain metastases using stochastic optimization to mitigate potential errors resulting from rotational uncertainties. METHODS: Planning target volumes (PTVs), defined as GTVs plus a 1-mm margin following common SRS planning convention, were assumed to be originally treated with a prescription dose and therefore covered by the prescription isodose cloud. The dose distribution, including the prescription isodose, was considered invariant assuming small rotations throughout the study. A stochastic optimization scheme was developed to determine the location of the optimal isocenter, so that the prescription dose coverage of rotated GTVs, equivalent to the intersecting volumes between the rotated GTVs and original PTVs, was maximized for any random small rotations about the isocenter. To evaluate the coverage of GTVs, the expected V 100 % undergoing random rotations was approximated as the sample average V 100 % undergoing a predetermined number of rotations. The expected V 100 % of each individual GTV and total GTVs was then compared between the plans using the optimal isocenter and the center-of-mass (CoM), respectively. RESULTS: Twenty-two patients previously treated for multiple brain metastases in a single institute were included in this retrospective study. Each patient was initially treated for more than three brain metastases (mean: 7.6; range: 3-15) with the average GTV volume of 0.89 cc (range: 0.03-11.78 cc). The optimal isocenter found for each patient was significantly different from the CoM, with the average Euclidean distance between the optimal isocenter and the CoM being 4.36 ± 2.59 cm. The dose coverage to GTVs was also significantly improved (paired t-test; p < 0.001) when the optimal isocenter was used, with the average V 100 % of total GTVs increasing from 87.1% (standard deviation as std: 11.7%; range: 39.9-98.2%) to 94.2% (std: 5.4%; range: 77.7-99.4%). The volume of a GTV was positively correlated with the expected V 100 % regardless of the isocenter used (Spearman coefficient: ρ = 0.66 ; p < 0.001). The distance between a GTV and the isocenter was negatively correlated with the expected V 100 % when the CoM was used ( ρ = - 0.21 ; p = 0.004), however no significant correlation was found when the optimal isocenter was used ( ρ = - 0.11 ; p = 0.137). CONCLUSION: The proposed framework provides an effective approach to determine the optimal isocenter of single-isocenter LINAC-based SRS plans for multiple brain metastases. The implementation of the optimal isocenter results in SRS plans with consistently higher target coverage despite potential rotational uncertainties, and therefore significantly improves SRS plan robustness against random rotational uncertainties.

Dosimetric Benefits of Omitting Primary Tumor Beds in Postoperative Radiotherapy After Transoral Robotic Surgery Using the Auto-Planning Technique.

INTRODUCTION: It has been suggested that post-transoral robotic surgery (post-TORS) radiotherapy (RT) might reduce the dose to organs at risk (OARs) adjacent to the primary tumor bed; however, the evidence supporting this has yet to be sufficient. This study examined the radiation dose reduction to OARs by omitting the primary tumor bed through the use of an Auto-Planning (AP)-based workflow. METHODS:  Twelve patients were identified who underwent post-TORS RT to the primary tumor bed and the unilateral/bilateral neck lymph nodes. In each patient, two treatment plans were designed: a Comprehensive (Comp)-plan treating the original planning target volume (PTV) including both the primary tumor bed and the lymph nodes, and a Neck-plan treating only the lymph nodes and omitting the primary tumor bed. Both plans were optimized using AP to ensure plan quality consistency. We compared the doses received by 95% of the primary tumor beds and lymph nodes (D95%) and our institutional dose constraints for the OARs between the Comp- and Neck-plans. Statistical analysis was performed using R Statistical Software (R Foundation for Statistical Computing, Vienna, Austria) with a two-tailed paired Wilcoxon signed-rank test. RESULTS:  All plans met target dose coverage requirements with at least 95% of the PTVs covered with the corresponding prescription doses. The primary tumor bed in the Neck-plans was spared with a significantly lower mean D95% (25.9 Gy vs. 60.0 Gy; p < 0.01; Wilcoxon test). The mean dose to the oral cavity (20.9 Gy vs. 28.1 Gy; p < 0.01) and the supraglottis (36.9 Gy vs. 28.2 Gy; p < 0.01) was significantly lower in the Neck-plans. CONCLUSION:  This study suggests that sparing the primary tumor bed during post-TORS RT offers dosimetric benefits to nearby OARs with significant dose reductions to the oral cavity and supraglottis. Further study of the clinical risks and benefits afforded by this strategy is needed.

Comprehensive Output Estimation of Double Scattering Proton System With Analytical and Machine Learning Models.

OBJECTIVES: The beam output of a double scattering proton system varies for each combination of beam option, range, and modulation and therefore is difficult to be accurately modeled by the treatment planning system (TPS). This study aims to design an empirical method using the analytical and machine learning (ML) models to estimate proton output in a double scattering proton system. MATERIALS AND METHODS: Three analytical models using polynomial, linear, and logarithm-polynomial equations were generated on a training dataset consisting of 1,544 clinical measurements to estimate proton output for each option. Meanwhile, three ML models using Gaussian process regression (GPR) with exponential kernel, squared exponential kernel, and rational quadratic kernel were also created for all options combined. The accuracy of each model was validated against 241 additional clinical measurements as the testing dataset. Two most robust models were selected, and the minimum number of samples needed for either model to achieve sufficient accuracy ( ± 3%) was determined by evaluating the mean average percentage error (MAPE) with increasing sample number. The differences between the estimated outputs using the two models were also compared for 1,000 proton beams with a randomly generated range, and modulation for each option. RESULTS: The polynomial model and the ML GPR model with exponential kernel yielded the most accurate estimations with less than 3% deviation from the measured outputs. At least 20 samples of each option were needed to build the polynomial model with less than 1% MAPE, whereas at least a total of 400 samples were needed for all beam options to build the ML GPR model with exponential kernel to achieve comparable accuracy. The two independent models agreed with less than 2% deviation using the testing dataset. CONCLUSION: The polynomial model and the ML GPR model with exponential kernel were built for proton output estimation with less than 3% deviations from the measurements. They can be used as an independent output prediction tool for a double scattering proton beam and a secondary output check tool for a cross check between themselves.

Two-staged stereotactic radiosurgery for the treatment of large brain metastases: Single institution experience and review of literature.

Introduction: Two-staged stereotactic radiosurgery (SRS) has been shown as an effective treatment for brain metastases that are too large for single fraction SRS. Methods: Patients with large brain metastases (>4 cm3) treated with two-staged SRS from January 2017 to December 2019 at our institution were retrospectively identified. Results: There were 23 brain metastases treated. The normal brain volume receiving equivalent 12Gy-in-single-fraction was defined as V12E. The V12E for original single-fraction GKS plan (mean of 41.4 cm3, range 5.6-146.1 cm3) was significantly higher compared to that of the second stage (mean of 23.7 cm3, range 2.8-92.7 cm3). The median tumor volume measured at the second stage (4.30 cm3) was reduced by an average of 52.2% compared to the first stage (9.58 cm3). Three patients (27.3%) showed local tumor progression in 4 tumors (20%). The median time to progression was 152 days. Conclusions: Two-staged SRS is an effective treatment technique for large brain metastasis that results in significant reduction of tumor volume at the second stage SRS. Optimal treatment dose has not yet been defined.

Correlation between plan quality improvements and reduced acute dysphagia and xerostomia in the definitive treatment of oropharyngeal squamous cell carcinoma.

BACKGROUND: To evaluate plan quality using volumetric-modulated arc therapy (VMAT) and step-and-shoot intensity-modulated radiation therapy (SS-IMRT) techniques and for patients treated for oropharyngeal squamous cell carcinoma (OPSCC). METHODS: Treatment plans for patients treated definitively for stages I-IVb, OPSCC between December 2009 and August 2015 were retrospectively reviewed. Dosimetric endpoints of involved organs-at-risk (OARs) were retrieved from clinical plans. Common Terminology Criteria for Adverse Events scores of acute toxicities were compared. RESULTS: Two-hundred twenty-two patients were identified with 134 and 88 receiving SS-IMRT and VMAT with median follow-up time of 23.0 and 7.9 months, respectively. The dosimetric endpoints of the OARs were significantly improved in VMAT cohort, which translated into significantly lower rates of grade 2 or higher acute dysphagia and xerostomia. CONCLUSION: Improvements in stages I-IVb, oropharyngeal cancer plan quality are associated with reduced grade ≥ 2 acute dysphagia and xerostomia.

A hybrid proton and hyperpolarized gas tagging MRI technique for lung respiratory motion imaging: a feasibility study.

The aim of this work was to develop a novel hybrid 3D hyperpolarized (HP) gas tagging MRI (t-MRI) technique and to evaluate it for lung respiratory motion measurement with comparison to deformable image registrations (DIR) methods. Three healthy subjects underwent a hybrid MRI which combines 3D HP gas t-MRI with a low resolution (Low-R, 4.5 mm isotropic voxels) 3D proton MRI (p-MRI), plus a high resolution (High-R, 2.5 mm isotropic voxels) 3D p-MRI, during breath-holds at the end-of-inhalation (EOI) and the end-of-exhalation (EOE). Displacement vector field (DVF) of the lung motion was determined from the t-MRI images by tracking tagging grids and from the High-R p-MRI using three DIR methods (B-spline based method implemented by Velocity, Free Form Deformation by MIM, and B-spline by an open source software Elastix: denoted as A, B, and C, respectively), labeled as tDVF and dDVF, respectively. The tDVF from the HP gas t-MRI was used as ground-truth reference to evaluate performance of the three DIR methods. Differences in both magnitude and angle between the tDVF and dDVFs were analyzed. The mean lung motion of the three subjects was 37.3 mm, 8.9 mm and 12.9 mm, respectively. Relatively large discrepancies were observed between the tDVF and the dDVFs as compared to previously reported DIR errors. The mean ± standard deviation (SD) DVF magnitude difference was 8.3 ± 5.6 mm, 9.2 ± 4.5 mm, and 9.3 ± 6.1 mm, and the mean ± SD DVF angular difference was 29.1 ± 12.1°, 50.1 ± 28.6°, and 39.0 ± 6.3°, for the DIR Methods A, B, and C, respectively. These preliminary results showed that the hybrid HP gas t-MRI technique revealed different lung motion patterns as compared to the DIR methods. It may provide unique perspectives in developing and evaluating DIR of the lungs. Novelty and Significance We designed a MRI protocol that includes a novel hybrid MRI technique (3D HP gas t-MRI with a low resolution 3D p-MRI) plus a high resolution 3D p-MRI. We tested the novel hybrid MRI technique on three healthy subjects for measuring regional lung respiratory motion with comparison to deformable image registrations (DIR) methods, and observed relatively large discrepancies in lung motion between HP gas t-MRI and DIR methods.

An initial investigation of hyperpolarized gas tagging magnetic resonance imaging in evaluating deformable image registration-based lung ventilation.

BACKGROUND: Deformable image registration (DIR)-based lung ventilation mapping is attractive due to its simplicity, and also challenging due to its susceptibility to errors and uncertainties. In this study, we explored the use of 3D Hyperpolarized (HP) gas tagging MRI to evaluate DIR-based lung ventilation. METHOD AND MATERIAL: Three healthy volunteers included in this study underwent both 3D HP gas tagging MRI (t-MRI) and 3D proton MRI (p-MRI) using balanced steady-state free precession pulse sequence at end of inhalation and end of exhalation. We first obtained the reference displacement vector fields (DVFs) from the t-MRIs by tracking the motion of each tagging grid between the exhalation and the inhalation phases. Then, we determined DIR-based DVFs from the p-MRIs by registering the images at the two phases with two commercial DIR algorithms. Lung ventilations were calculated from both the reference DVFs and the DIR-based DVFs using the Jacobian method and then compared using cross correlation and mutual information. RESULTS: The DIR-based lung ventilations calculated using p-MRI varied considerably from the reference lung ventilations based on t-MRI among all three subjects. The lung ventilations generated using Velocity AI were preferable for the better spatial homogeneity and accuracy compared to the ones using MIM, with higher average cross correlation (0.328 vs 0.262) and larger average mutual information (0.528 vs 0.323). CONCLUSION: We demonstrated that different DIR algorithms resulted in different lung ventilation maps due to underlining differences in the DVFs. HP gas tagging MRI provides a unique platform for evaluating DIR-based lung ventilation.

Direct measurement of chiral structure and transport in single- and multi-walled carbon nanotubes.

Electrical devices based on suspended multi-wall carbon nanotubes were constructed and studied. The chiral structure of each shell in a particular nanotube was determined using nanobeam electron diffraction in a transmission electron microscope. The transport properties of the carbon nanotube were also measured. The nanotube device length was short enough that the transport was nearly ballistic, and multiple subbands contributed to the conductance. Thermal excitation of carriers significantly affected nanotube resistance at room temperature.