Imaging in thick samples, a phased Monte Carlo radiation transfer algorithm.

SIGNIFICANCE: Optical microscopy is characterized by the ability to get high resolution, below 1 µm, high contrast, functional and quantitative images. The use of shaped illumination, such as with lightsheet microscopy, has led to greater three-dimensional isotropic resolution with low phototoxicity. However, in most complex samples and tissues, optical imaging is limited by scattering. Many solutions to this issue have been proposed, from using passive approaches such as Bessel beam illumination to active methods incorporating aberration correction, but making fair comparisons between different approaches has proven to be challenging. AIM: We present a phase-encoded Monte Carlo radiation transfer algorithm (φMC) capable of comparing the merits of different illumination strategies or predicting the performance of an individual approach. APPROACH: We show that φMC is capable of modeling interference phenomena such as Gaussian or Bessel beams and compare the model with experiment. RESULTS: Using this verified model, we show that, for a sample with homogeneously distributed scatterers, there is no inherent advantage to illuminating a sample with a conical wave (Bessel beam) instead of a spherical wave (Gaussian beam), except for maintaining a greater depth of focus. CONCLUSION: φMC is adaptable to any illumination geometry, sample property, or beam type (such as fractal or layered scatterer distribution) and as such provides a powerful predictive tool for optical imaging in thick samples.

Validation of a novel system to assess end-expiratory lung volume and alveolar recruitment in an ARDS model.

BACKGROUND: Personalizing mechanical ventilation requires the development of reliable bedside monitoring techniques. The multiple-breaths nitrogen washin-washout (MBNW) technique is currently available to measure end-expiratory lung volume (EELVMBNW), but the precision of the technique may be poor, with percentage errors ranging from 28 to 57%. The primary aim of the study was to evaluate the reliability of a novel MBNW bedside system using fast mainstream sensors to assess EELV in an experimental acute respiratory distress syndrome (ARDS) model, using computed tomography (CT) as the gold standard. The secondary aims of the study were: (1) to evaluate trending ability of the novel system to assess EELV; (2) to evaluate the reliability of estimated alveolar recruitment induced by positive end-expiratory pressure (PEEP) changes computed from EELVMBNW, using CT as the gold standard. RESULTS: Seven pigs were studied in 6 experimental conditions: at baseline, after experimental ARDS and during a decremental PEEP trial at PEEP 16, 12, 6 and 2 cmH2O. EELV was computed at each PEEP step by both the MBNW technique (EELVMBNW) and CT (EELVCT). Repeatability was assessed by performing replicate measurements. Alveolar recruitment between two consecutive PEEP levels after lung injury was measured with CT (VrecCT), and computed from EELV measurements (VrecMBNW) as ΔEELV minus the product of ΔPEEP by static compliance. EELVMBNW and EELVCT were significantly correlated (R2 = 0.97). An acceptable non-constant bias between methods was identified, slightly decreasing toward more negative values as EELV increased. The conversion equation between EELVMBNW and EELVCT was: EELVMBNW = 0.92 × EELVCT + 36. The 95% prediction interval of the bias amounted to ± 86 mL and the percentage error between both methods amounted to 13.7%. The median least significant change between repeated measurements amounted to 8% [CI95%: 4-10%]. EELVMBNW adequately tracked EELVCT changes over time (concordance rate amounting to 100% [CI95%: 87%-100%] and angular bias amounting to - 2° ± 10°). VrecMBNW and VrecCT were significantly correlated (R2 = 0.92). A non-constant bias between methods was identified, slightly increasing toward more positive values as Vrec increased. CONCLUSIONS: We report a new bedside MBNW technique that reliably assesses EELV in an experimental ARDS model with high precision and excellent trending ability.

Hysteresis and Lung Recruitment in Acute Respiratory Distress Syndrome Patients: A CT Scan Study.

OBJECTIVES: Hysteresis of the respiratory system pressure-volume curve is related to alveolar surface forces, lung stress relaxation, and tidal reexpansion/collapse. Hysteresis has been suggested as a means of assessing lung recruitment. The objective of this study was to determine the relationship between hysteresis, mechanical characteristics of the respiratory system, and lung recruitment assessed by a CT scan in mechanically ventilated acute respiratory distress syndrome patients. DESIGN: Prospective observational study. SETTING: General ICU of a university hospital. PATIENTS: Twenty-five consecutive sedated and paralyzed patients with acute respiratory distress syndrome (age 64 ± 15 yr, body mass index 26 ± 6 kg/m, PaO2/FIO2 147 ± 42, and positive end-expiratory pressure 9.3 ± 1.4 cm H2O) were enrolled. INTERVENTIONS: A low-flow inflation and deflation pressure-volume curve (5-45 cm H2O) and a sustained inflation recruitment maneuver (45 cm H2O for 30 s) were performed. A lung CT scan was performed during breath-holding pressure at 5 cm H2O and during the recruitment maneuver at 45 cm H2O. MEASUREMENTS AND MAIN RESULTS: Lung recruitment was computed as the difference in noninflated tissue and in gas volume measured at 5 and at 45 cm H2O. Hysteresis was calculated as the ratio of the area enclosed by the pressure-volume curve and expressed as the hysteresis ratio. Hysteresis was correlated with respiratory system compliance computed at 5 cm H2O and the lung gas volume entering the lung during inflation of the pressure-volume curve (R = 0.749, p < 0.001 and R = 0.851, p < 0.001). The hysteresis ratio was related to both lung tissue and gas recruitment (R = 0.266, p = 0.008, R = 0.357, p = 0.002, respectively). Receiver operating characteristic analysis showed that the optimal cutoff value to predict lung tissue recruitment for the hysteresis ratio was 28% (area under the receiver operating characteristic curve, 0.80; 95% CI, 0.62-0.98), with sensitivity and specificity of 0.75 and 0.77, respectively. CONCLUSIONS: Hysteresis of the respiratory system computed by low-flow pressure-volume curve is related to the anatomical lung characteristics and has an acceptable accuracy to predict lung recruitment.

Polarised light sheet tomography.

The various benefits of light sheet microscopy have made it a widely used modality for capturing three-dimensional images. It is mostly used for fluorescence imaging, but recently another technique called light sheet tomography solely relying on scattering was presented. The method was successfully applied to imaging of plant roots in transparent soil, but is limited when it comes to more turbid samples. This study presents a polarised light sheet tomography system and its advantages when imaging in highly scattering turbid media. The experimental configuration is guided by Monte Carlo radiation transfer methods, which model the propagation of a polarised light sheet in the sample. Images of both reflecting and absorbing phantoms in a complex collagenous matrix were acquired, and the results for different polarisation configurations are compared. Focus scanning methods were then used to reduce noise and produce three-dimensional reconstructions of absorbing targets.

Local Measurement of Flap Oxygen Saturation: An Application of Visible Light Spectroscopy.

The aim was to develop and test a new device (OxyVLS) to measure tissue oxygen saturation by visible light spectroscopy independently of the optical pathlength and scattering. Its local applicability provides the possibility of real time application in flap reconstruction surgery. We tested OxyVLS in a liquid phantom with optical properties similar to human tissue. Our results were in good agreement with a conventional near infrared spectroscopy device.