Del Leary

Hyperpolarized 3He and 129Xe MRI: Differences in asthma before bronchodilation

To compare hyperpolarized helium‐3 (3He) and xenon‐129 (129Xe) MRI in asthmatics before and after salbutamol inhalation.

Modeling stochastic and spatial heterogeneity in a human airway tree to determine variation in respiratory system resistance

Asthma is a variable disease with changes in symptoms and airway function over many time scales. Airway resistance (Raw) is variable and thought to reflect changes in airway smooth muscle activity, but just how variation throughout the airway tree and the influence of gas distribution abnormalities affect Raw is unclear. We used a multibranch airway lung model to evaluate variation in airway diameter size, the role of coherent regional variation, and the role of gas distribution abnormalities on mean Raw (Raw) and variation in Raw as described by the SD (SDRaw). We modified an anatomically correct airway tree, provided by Merryn Tawhai (The University of Auckland, New Zealand), consisting of nearly 4,000 airways, to produce temporal and spatial heterogeneity. As expected, we found that increasing the diameter variation by twofold, with no change in the mean diameter, increased SDRaw more than fourfold. Perhaps surprisingly, Raw was proportional to SDRaw under several conditions-when either mean diameter was fixed, and its SD varied or when mean diameter varied, and SD was fixed. Increasing the size of a regional absence in gas distribution (ventilation defect) also led to a proportionate increase in both Raw and SDRaw. However, introducing regional dependence of connected airways strongly increased SDRaw by as much as sixfold, with little change in Raw. The model was able to predict previously reported Raw distributions and correlation of SDRaw on Raw in healthy and asthmatic subjects. The ratio of SDRaw to Raw depended most strongly on interairway coherent variation and only had a slight dependence on ventilation defect size. These findings may explain the linear correlation between variation and mean values of Raw but also suggest that regional alterations in gas distribution and local coordination in ventilation amplify any underlying variation in airway diameters throughout the airway tree.

Effects of airway tree asymmetry on the emergence and spatial persistence of ventilation defects

Asymmetry and heterogeneity in the branching of the human bronchial tree are well documented, but their effects on bronchoconstriction and ventilation distribution in asthma are unclear. In a series of seminal studies, Venegas et al. have shown that bronchoconstriction may lead to self-organized patterns of patchy ventilation in a computational model that could explain areas of poor ventilation [ventilation defects (VDefs)] observed in positron emission tomography images during induced bronchoconstriction. To investigate effects of anatomic asymmetry on the emergence of VDefs we used the symmetric tree computational model that Venegas and Winkler developed using different trees, including an anatomic human airway tree provided by M. Tawhai (University of Auckland), a symmetric tree, and three trees with intermediate asymmetry (Venegas JG, Winkler T, Musch G, Vidal Melo MF, Layfield D, Tgavalekos N, Fischman AJ, Callahan RJ, Bellani G, Harris RS. Nature 434: 777-782, 2005 and Winkler T, Venegas JG. J Appl Physiol 103: 655-663, 2007). Ventilation patterns, lung resistance (RL), lung elastance (EL), and the entropy of the ventilation distribution were compared at different levels of airway smooth muscle activation. We found VDefs emerging in both symmetric and asymmetric trees, but VDef locations were largely persistent in asymmetric trees, and bronchoconstriction reached steady state sooner than in a symmetric tree. Interestingly, bronchoconstriction in the asymmetric tree resulted in lower RL (∼%50) and greater EL (∼%25). We found that VDefs were universally caused by airway instability, but asymmetry in airway branching led to local triggers for the self-organized patchiness in ventilation and resulted in persistent locations of VDefs. These findings help to explain the emergence and the persistence in location of VDefs found in imaging studies.

Modelling resistance and reactance with heterogeneous airway narrowing in mild to severe asthma.

Ventilation heterogeneity is an important marker of small airway dysfunction in asthma. The frequency dependence of respiratory system resistance (Rrs) from oscillometry is used as a measure of this heterogeneity. However, this has not been quantitatively assessed or compared with other outcomes from oscillometry, including respiratory system reactance (Xrs) and the associated elastance (Ers). Here, we used a multibranch model of the human lung, including an upper airway shunt, to match previously reported respiratory mechanics in mild to severe asthma. We imposed heterogeneity by narrowing a proportion of the peripheral airways to account for patient Ers at 5 Hz, and then narrowed central airways to account for the remaining Rrs at 18 Hz. The model required >75% of the small airways to be occluded to reproduce severe asthma. While the model produced frequency dependence in Rrs, it was upward-shifted below 5 Hz compared with in-vivo results, indicating that other factors, including more distributed airway narrowing or central airway wall compliance, are required. However, Ers quantitatively reflected the imposed heterogeneity better than the frequency dependence of Rrs, independent of the frequency range for the estimation, and thus was a more robust measure of small-airway function. Thus, Ers appears to have greater potential as a clinical measure of early small-airway disease in asthma.

Impedance and Brewster angle measurement for thick porous layers

For thin nonlocally reacting porous layers, a method derived from the work of Chien and Soroka [J. Sound Vib. 43, 9–20 (1975)] has been used to localize the pole of the reflection coefficient located at an angle close to grazing incidence and to measure the surface impedance at this angle. A very simple experimental setup is used to obtain measurements on materials with large flow resistivity, at frequencies larger than 1 kHz, using samples with areas on the order of 1 m2. This method is used in the present work to measure the surface impedance of acoustically thick porous layers of Ottawa sand and glass beads. There is good agreement between the measurements and predicted values. The method is also applied to study the effects of surface sealing. Sealing is modeled as a thin screen of higher flow resistivity at the surface. Reasonable agreement between the measured and predicted additional flow resistance is obtained.

CBCT with specification of imaging dose and CNR by anatomical volume of interest

PURPOSE A novel method has been developed for volume of interest (VOI) cone-beam CT (CBCT) imaging using a 2.35 MV/Carbon target linac imaging beam line combined with dynamic multileaf collimator sequences. METHODS The authors demonstrate the concept of acquisition of multiple, separate imaging volumes, where volumes can be either completely separated or nested, and are associated with predetermined imaging dose and contrast-to-noise ratio (CNR) characteristics. Two individual MLC sequences were established in the planning system (Eclipse, Varian Medical) to collimate the beam according to a defined inner VOI (e.g., containing a target volume under image guidance) and an outer VOI (e.g., including surrounding landmarks or organs-at-risk). MLC sequences were interleaved as a function of gantry angle to produce a reconstructed CBCT image with nested VOIs. By controlling the ratio of inner-to-outer ratio of MLC segments (and thus Monitor Units) during acquisition, the relative dose and CNR in the two volumes can be controlled. Inner-to-outer ratios of 2:1 to 6:1 were examined. RESULTS The concept was explored using an anatomical head phantom to assess image quality. A geometric phantom was used to quantify absolute dose and CNR values for the various sequences. The authors found that the dose in the outer VOI decreased by a functional relationship dependent on the inner-to-outer sequence ratio, while the CNR varied by the square root of dose, as expected. CONCLUSIONS In this study the authors demonstrate flexibility in VOI CBCT by tailoring the imaging dose and CNR distribution in separate volumes within the patient anatomy. This would allow for high quality imaging of a target volume for alignment purposes, with simultaneous low dose imaging of the surrounding anatomy (e.g., for coregistration).

A comparison of methods for the automatic classification of marine mammal vocalizations in the Arctic

JASCO Applied Sciences provided acoustic data collection services to Shell Offshore Incorporated in support of their explorations of the Chukchi and Beaufort Seas during the summer of 2007. A total of 37 ocean bottom hydrophones data recording systems were deployed resulting in a data set in excess of 5 TB. This data equates to almost 5 years of continuous sound recording. This campaign provided information on the migration routes of the marine mammals, which include bowhead, beluga, humpback, gray whales and walruses. Given the large amount of data, manual analysis of the recordings was not feasible. A method to detect and classify the marine mammal vocalizations automatically in a reasonable amount of time had to be developed. The processing is structured in several steps, (1.) the detection of energy events, (2.) the feature extraction, and (3.) the classification into a species variety. This paper focuses on combining the Gaussian mixed models (GMM) algorithm for classification with attributes taken from two different feature extraction algorithms: cepstral coefficients and wavelets. Combinations of these different algorithms are compared using classification operating characteristic (COC) curves for each species tested. This paper compares the performance of these algorithms and their parameters against a large training data set.

Examining surface sealing and crusting using the acoustic to seismic transfer function

Soil sealing is examined by measuring the acoustic to seismic (A/S) transfer function. An A/S transfer function is a swept sine measurement using a suspended loud speaker to impinge acoustic energy from the air onto a soil sample. A laser Doppler vibrometer (LDV) is used to obtain the surface particle velocity as a measure of the seismic energy that has been transferred into the soil. This technique is noncontact and therefore allows successive measurements to be taken in time as the surface crust is formed. Soil samples are rained upon then allowed to dry forming a crust or seal that changes both the stiffness and hydraulic properties of the surface layer. Neshoba soils tested show a quantifiable decrease in the seismic energy transferred, as well as an increase in correlation between successive trials, as the crust forms. Additional measurements done with a submerged transducer show an even greater decrease in surface velocity. Current studies also include an acoustic reflection technique to measure cha...