Pulmonary perfusion quantified by electron-beam computed tomography: effects of hypoxia and inhaled NO

Assessing the pulmonary vascular responsiveness to oxygen with proton MRI

Ventilation-perfusion matching occurs passively and is also actively regulated through hypoxic pulmonary vasoconstriction (HPV). The extent of HPV activity in humans, particularly normal subjects, is uncertain. Current evaluation of HPV assesses changes in ventilation-perfusion relationships/pulmonary vascular resistance with hypoxia and is invasive, or unsuitable for patients because of safety concerns. We used a noninvasive imaging-based approach to quantify the pulmonary vascular response to oxygen as a metric of HPV by measuring perfusion changes between breathing 21% and 30%O2 using arterial spin labeling (ASL) MRI. We hypothesized that the differences between 21% and 30%O2 images reflecting HPV release would be 1) significantly greater than the differences without [Formula: see text] changes (e.g., 21-21% and 30-30%O2) and 2) negatively associated with ventilation-perfusion mismatch. Perfusion was quantified in the right lung in normoxia (baseline), after 15 min of 30% O2 breathing (hyperoxia) and 15 min normoxic recovery (recovery) in healthy subjects (7 M, 7 F; age = 41.4 ± 19.6 yr). Normalized, smoothed, and registered pairs of perfusion images were subtracted and the mean square difference (MSD) was calculated. Separately, regional alveolar ventilation and perfusion were quantified from specific ventilation, proton density, and ASL imaging; the spatial variance of ventilation-perfusion (σ2V̇a/Q̇) distributions was calculated. The O2-responsive MSD was reproducible (R2 = 0.94, P < 0.0001) and greater (0.16 ± 0.06, P < 0.0001) than that from subtracted images collected under the same [Formula: see text] (baseline = 0.09 ± 0.04, hyperoxia = 0.08 ± 0.04, recovery = 0.08 ± 0.03), which were not different from one another (P = 0.2). The O2-responsive MSD was correlated with σ2V̇a/Q̇ (R2 = 0.47, P = 0.007). These data suggest that active HPV optimizes ventilation-perfusion matching in normal subjects. This noninvasive approach could be applied to patients with different disease phenotypes to assess HPV and ventilation-perfusion mismatch.NEW & NOTEWORTHY We developed a new proton MRI method to noninvasively quantify the pulmonary vascular response to oxygen. Using a hyperoxic stimulus to release HPV, we quantified the resulting redistribution of perfusion. The differences between normoxic and hyperoxic images were greater than those between images without [Formula: see text] changes and negatively correlated with ventilation-perfusion mismatch. This suggests that active HPV optimizes ventilation-perfusion matching in normal subjects. This approach is suitable for assessing patients with different disease phenotypes.

Lung function measurements in preclinical research: What has been done and where is it headed?

Due to the close interaction of lung morphology and functions, repeatable measurements of pulmonary function during longitudinal studies on lung pathophysiology and treatment efficacy have been a great area of interest for lung researchers. Spirometry, as a simple and quick procedure that depends on the maximal inspiration of the patient, is the most common lung function test in clinics that measures lung volumes against time. Similarly, in the preclinical area, plethysmography techniques offer lung functional parameters related to lung volumes. In the past few decades, many innovative techniques have been introduced for in vivo lung function measurements, while each one of these techniques has their own advantages and disadvantages. Before each experiment, depending on the sensitivity of the required pulmonary functional parameters, it should be decided whether an invasive or non-invasive approach is desired. On one hand, invasive techniques offer sensitive and specific readouts related to lung mechanics in anesthetized and tracheotomized animals at endpoints. On the other hand, non-invasive techniques allow repeatable lung function measurements in conscious, free-breathing animals with readouts related to the lung volumes. The biggest disadvantage of these standard techniques for lung function measurements is considering the lung as a single unit and providing only global readouts. However, recent advances in lung imaging modalities such as x-ray computed tomography and magnetic resonance imaging opened new doors toward obtaining both anatomical and functional information from the same scan session, without the requirement for any extra pulmonary functional measurements, in more regional and non-invasive manners. Consequently, a new field of study called pulmonary functional imaging was born which focuses on introducing new techniques for regional quantification of lung function non-invasively using imaging-based techniques. This narrative review provides first an overview of both invasive and non-invasive conventional methods for lung function measurements, mostly focused on small animals for preclinical research, including discussions about their advantages and disadvantages. Then, we focus on those newly developed, non-invasive, imaging-based techniques that can provide either global or regional lung functional readouts at multiple time-points.

Functional lung imaging in thoracic tumor radiotherapy: Application and progress

Radiotherapy plays an irreplaceable and unique role in treating thoracic tumors, but the occurrence of radiation-induced lung injury has limited the increase in tumor target doses and has influenced patients’ quality of life. However, the introduction of functional lung imaging has been incorporating functional lungs into radiotherapy planning. The design of the functional lung protection plan, while meeting the target dose requirements and dose limitations of the organs at risk (OARs), minimizes the radiation dose to the functional lung, thus reducing the occurrence of radiation-induced lung injury. In this manuscript, we mainly reviewed the lung ventilation or/and perfusion functional imaging modalities, application, and progress, as well as the results based on the functional lung protection planning in thoracic tumors. In addition, we also discussed the problems that should be explored and further studied in the practical application based on functional lung radiotherapy planning.

Early detection of pulmonary vascular disease in pulmonary arterial hypertension: time to move forward

Pulmonary arterial hypertension (PAH) can be a rapidly progressive disorder and is associated with high rate of mortality, despite medical intervention. With the availability of effective therapy, early disease detection is an important strategic objective to improve treatment outcomes. Resting echocardiography is currently the recommended screening modality for high-risk population groups. However, it is clear that derangements in resting haemodynamics (and symptoms) are late sequelae of the pathobiological processes that begin in the distal pulmonary arteries. Exercise stress may unmask early pulmonary vascular dysfunction but the definition, clinical significance, and natural history of 'exercise PAH' remain undefined. We will review the currently available and potential future strategies aimed at early disease detection, and propose that ultimately the way forward is to detect disease at a stage prior to the rise in resting pulmonary artery pressure.

Redistribution of pulmonary blood flow impacts thermodilution-based extravascular lung water measurements in a model of acute lung injury

BACKGROUND

Studies using transthoracic thermodilution have demonstrated increased extravascular lung water (EVLW) measurements attributed to progression of edema and flooding during sepsis and acute lung injury. The authors hypothesized that redistribution of pulmonary blood flow can cause increased apparent EVLW secondary to increased perfusion of thermally silent tissue, not increased lung edema.

METHODS

Anesthetized, mechanically ventilated canines were instrumented with PiCCO (Pulsion Medical, Munich, Germany) catheters and underwent lung injury by repetitive saline lavage. Hemodynamic and respiratory physiologic data were recorded. After stabilized lung injury, endotoxin was administered to inactivate hypoxic pulmonary vasoconstriction. Computed tomographic imaging was performed to quantify in vivo lung volume, total tissue (fluid) and air content, and regional distribution of blood flow.

RESULTS

Lavage injury caused an increase in airway pressures and decreased arterial oxygen content with minimal hemodynamic effects. EVLW and shunt fraction increased after injury and then markedly after endotoxin administration. Computed tomographic measurements quantified an endotoxin-induced increase in pulmonary blood flow to poorly aerated regions with no change in total lung tissue volume.

CONCLUSIONS

The abrupt increase in EVLW and shunt fraction after endotoxin administration is consistent with inactivation of hypoxic pulmonary vasoconstriction and increased perfusion to already flooded lung regions that were previously thermally silent. Computed tomographic studies further demonstrate in vivo alterations in regional blood flow (but not lung water) and account for these alterations in shunt fraction and EVLW.

Regional aeration and perfusion distribution in a sheep model of endotoxemic acute lung injury characterized by functional computed tomography imaging

OBJECTIVES

Sepsis-related lung injury is the most common and morbid form of acute lung injury. The objective of this study was to develop an ovine model of septic acute lung injury and characterize its pathophysiology regarding its recruitability and changes in regional aeration and perfusion distributions at injury and during injury evolution.

DESIGN

Experimental animal study.

SETTING

University hospital research laboratory.

SUBJECTS

Adult sheep.

INTERVENTIONS

Twenty-one anesthetized and mechanically ventilated sheep received intravenous Escherichia coli endotoxin infusion until severe hypoxemia was obtained. Inspiratory- and expiratory-gated computed tomography images of the entire lung were acquired in six subjects at baseline, during endotoxin infusion, and at injury. Perfusion images were obtained at apex and base locations at baseline and injury. Computed tomography images were analyzed for total, air, and tissue lung volumes and axial and vertical aeration and perfusion gradients. Lung recruitability was studied in a subgroup of subjects after injury.

MEASUREMENTS AND MAIN RESULTS

Computed tomography imaging showed a patchy, progressive decrease in air volume as injury evolved, partially replaced by an increase in tissue volume. Perfusion showed a nondependent-to-dependent gradient at baseline that remained relatively unchanged with injury. Perfusion to poorly aerated lung regions was unchanged or increased after injury. Aeration and perfusion distributions at baseline were primarily dorsal or dependent. After injury, the heterogeneity of perfusion and aeration increased and the effect of gravity decreased. Recruitment maneuvers and changes in positive end-expiratory pressure resulted in no improvement in aeration or oxygenation.

CONCLUSIONS

The severe hypoxemia, moderate volume loss, and perfusion patterns are consistent with an injury model in which hypoxemia is exacerbated by endotoxin-mediated failure of hypoxic pulmonary vasoconstriction.

Regional pulmonary blood flow in humans and dogs by 4D computed tomography

RATIONALE AND OBJECTIVES

Pulmonary vascular control mechanisms are complex and likely to differ between species. We wish to quantify regional perfusion and the effects of gravity using computed tomography.

MATERIALS AND METHODS

Sequential density measurements following the administration of a bolus of iodinated contrast medium were acquired from four healthy human subjects and four dogs.

RESULTS

In humans, perfusion (Q) was linear throughout most of the range of vertical height, with an overall gradient of -2.6% cm(-1). However, when perfusion was normalized to "tissue" density (blood plus tissue: sQt), maximum perfusion occurred around the mid-range of vertical height, being 9% (range 1-22%) greater than either the dorsal or ventral extreme. Within discrete transverse axial sections, concentric zones of perfusion centered on blood vessels were demonstrated. The relationship between sQt and vertical height in dogs was distinctly linear, with a gradient of -7.2% cm(-1). In dogs, the median gradient of Q was -13.6% cm(-1) (range -9.7 to -17.1%).

CONCLUSIONS

Differences in regional pulmonary perfusion, particularly the vertical gradient observed in humans and dogs, may in part reflect anatomic differences between the symmetric dichotomous branching structure of the human pulmonary vasculature and the more asymmetrical structure found in dogs.

Regional ventilation and lung mechanics using X-Ray CT

Advances in computed tomographic (CT) imaging of the lung in the past decade, particularly with increased speed, resolution, gating capability, and rapidly expanding volumetric image acquisition, along with advances in image processing, have expanded the repertoire of imaging methods beyond anatomic visualization into the noninvasive study of regional lung physiological function. Recognizing that significant local disease or dysfunction can exist before global measures begin to deteriorate, the motivation for the development and application of these regional techniques is to further our understanding of the basic pathophysiological characteristics of evolving lung disease and, ultimately, develop sensitive measures for its early detection. This review emphasizes the key elements of ventilation and lung mechanics relevant for regional approaches and CT measurement principles available for their study. Examples of established and evolving methods for imaging regional ventilation and mechanics, including the xenon CT ventilation method; the relationship between changing regional CT density and air volume change; and registration-based methods for examining regional lung expansion and strain, are presented.

Electron beam CT scan is a valuable and safe imaging tool for the pediatric surgical patient

PURPOSE

Electron beam computed tomography (EBCT) is a relatively new technology that has been used primarily to detect coronary artery calcification in adult patients. EBCT has several potential advantages over traditional CT: (1) fast acquisition times resulting in less need for sedation, (2) decreased radiation exposure, and (3) robust software enabling real-time interactive 3-dimensional visualization of anatomic relationships. In this series of case reports, the authors describe their initial experience with the use of EBCT in pediatric noncardiac imaging.

METHODS

Children with a variety of thoracic and abdominal disease processes were evaluated by EBCT.

RESULTS

All patients underwent EBCT scanning without the need for sedation, and 3-dimensional images of the data sets were rendered in minutes after their acquisition. The diagnostic images provided equivalent spatial resolution to the multislice CT scanner but without motion artifacts and lower radiation exposure.

CONCLUSIONS

EBCT is a safe, effective, and user- and patient-friendly alternative to traditional CT in the care of pediatric surgical diseases. The diagnostic yield of EBCT will continue to improve with new technologic advances and clinical experience.