Klaus Markstaller

Principles of electrical impedance tomography and its clinical application.

Objective:This review outlines the basic principle, in addition to validated and upcoming clinical use of electrical impedance tomography (EIT). EIT generates functional tomograms of the thorax for detection of changes in regional lung aeration. These images allow an intraindividual comparison of changes in regional lung function. Specifically, EIT aims to optimize ventilation therapy in patients with acute lung failure. Data Sources:PubMed: National Library of Medicine and the National Institutes of Health. Study Selection:Studies with the key words “electrical impedance tomography” since 1983. Data Extraction:Qualitative and quantitative results of the studies. Data Synthesis:We summarize basic principles of the technique and subsequent analyzing methods, and give an overview of clinical and scientific questions that can be addressed by EIT. Conclusion:Potential applications in the future as well as limitations of EIT technology are described. In summary, EIT is a promising functional tomography technology on the verge of its clinical application.

Overview of the Pathology of Three Widely Used Animal Models of Acute Lung Injury

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are syndromes of acute diffuse damage to the pulmonary parenchyma by a variety of local or systemic insults. Increased alveolar capillary membrane permeability was recognized as the common end organ injury and a central feature in all forms of ALI/ARDS. Although great strides have been made in understanding the pathogenesis of ALI/ARDS and in intensive care medicine, the treatment approach to ARDS is still relying on ventilatory and cardiovascular support based on the recognition of the clinical picture. In the course of evaluating novel treatment approaches to ARDS, 3 models of ALI induced in different species, i.e. the surfactant washout lavage model, the oleic acid intravenous injection model and the endotoxin injection model, were widely used. This review gives an overview of the pathological characteristics of these models from studies in pigs, dogs or sheep. We believe that a good morphological description of these models, both spatially and temporally, will help us gain a better understanding of the real pathophysiological picture and apply these models more accurately and liberally in evaluating novel treatment approaches to ARDS.

Assessment of a single-acquisition imaging sequence for oxygen-sensitive 3He-MRI

MRI of the lungs using hyperpolarized helium‐3 (3He) allows the determination of intrapulmonary oxygen partial pressures (pO2). The need to separate competing processes of signal loss has hitherto required two different imaging series during two different breathing maneuvers. In this work, a new imaging strategy to measure pO2 by a single series of consecutive scans is presented. The feasibility of the method is demonstrated in three healthy human volunteers. Maps and histograms of intrapulmonary pO2 are calculated. Changes in the oxygen concentration of the inhaled gas mixture are well reproduced in the histograms. Monte Carlo (MC) simulations of the temporal evolution of 3He hyperpolarization within the lungs were performed to evaluate the accuracy of this measurement technique, and its limitations. Magn Reson Med 47:105–114, 2002.

3He-MRI-based measurements of intrapulmonary pO2 and its time course during apnea in healthy volunteers: first results, reproducibility, and technical limitations

We applied a recently developed method of following the time course of the intrapulmonary oxygen partial pressure pO2(t) during apnea by 3He MRI to healthy volunteers. Using two imaging series with different interscan times during two breathholds (double acquisition technique), relaxation of 3He due to paramagnetic oxygen and depolarization by RF pulses were discriminated. In all four subjects, the temporal evolution of pO2 was found to be linear, and was described by an initial partial pressure p0 and a decrease rate R. Also, regional differences of both p0 and R were observed. A correlation between p0 and R was apparent. Finally, we discuss limitations of the double acquisition approach. Copyright

3He MRI in healthy volunteers: preliminary correlation with smoking history and lung volumes

MRI with hyperpolarized helium‐3 (3He) provides high‐resolution imaging of ventilated airspaces. The first aim of this 3He‐study was to compare observations of localized signal defects in healthy smokers and non‐smokers. A second aim was to describe relationships between parameters of lung function, volume of inspired 3He and signal‐to‐noise ratio. With Ethics Committee approval and informed consent, 12 healthy volunteers (seven smokers and five non‐smokers) were studied. Imaging was performed in a 1.5 T scanner using a two‐dimensional FLASH sequence at 30V transmitter amplitude (TR/TE/α = 11 ms/4.2 ms/<10°). Known amounts of 3He were inhaled from a microprocessor‐controlled delivery device and imaged during single breath‐holds. Images were evaluated visually, and scored using a prospectively defined ‘defect‐index’. Signal‐to‐noise ratio of the images were correlated with localization, 3He volumes and static lung volumes. Due to poor image quality studies of two smokers were not eligible for the evaluation. Smokers differed from non‐smokers in total number and size of defects: the ‘defect‐index’ of smokers ranged between 0.8 and 6.0 (median = 1.1), that of non‐smokers between 0.1 and 0.8 (median = 0.4). Intraindividually, an anteroposterior gradient of signal‐to‐noise ratio was apparent. Signal‐to‐noise ratio correlated with the estimated amount of hyperpolarization administered (r = 0.77), but not with static lung volumes. We conclude that 3He MRI is a sensitive measure to detect regional abnormalities in the distribution of ventilation in clinically healthy persons with normal pulmonary function tests. Copyright

Dynamic 19F-MRI of pulmonary ventilation using sulfur hexafluoride (SF6) gas

A new method for dynamic imaging of pulmonary wash‐in and wash‐out kinetics of inhaled sulfur hexafluoride (SF6) gas was developed. Measurements at the fluorine‐19 Larmor frequency were performed in pigs using a gradient echo pulse sequence with 0.5 ms echo time and a measurement time of 9.1 s per image. Dynamic MRI was performed during wash‐in and wash‐out of SF6 gas in mechanically ventilated porcine lungs. A postprocessing strategy was developed for quantitative determination of wash‐out time constants in the presence of noise. Mean wash‐out constants were 4.78 ± 0.48 breaths vs. 4.33 ± 0.76 breaths for left and right lung when ventilation was performed with low tidal volume, and 1.73 ± 0.16 breaths vs. 1.85 ± 0.11 breaths with high tidal volume ventilation. In conclusion, breath‐hold MRI of SF6 gas is feasible in large animals. Moreover, regional wash‐in and wash‐out kinetics of SF6 can be determined noninvasively with this new method. Potential human applications are discussed. Magn Reson Med 45:605–613, 2001.

Spatial and temporal heterogeneity of ventilator-associated lung injury after surfactant depletion

Volutrauma and atelectrauma have been proposed as mechanisms of ventilator-associated lung injury, but few studies have compared their relative importance in mediating lung injury. The objective of our study was to compare the injury produced by stretch (volutrauma) vs. cyclical recruitment (atelectrauma) after surfactant depletion. In saline-lavaged rabbits, we used high tidal volume, low respiratory rate, and low positive end-expiratory pressure to produce stretch injury in nondependent lung regions and cyclical recruitment in dependent lung regions. Tidal changes in shunt fraction were assessed by measuring arterial Po(2) oscillations. After ventilating for times ranging from 0 to 6 h, lungs were excised, sectioned gravitationally, and assessed for regional injury by evaluation of edema formation, chemokine expression, upregulation of inflammatory enzyme activity, and alveolar neutrophil accumulation. Edema formation, lung tissue interleukin-8 expression, and alveolar neutrophil accumulation progressed more rapidly in dependent lung regions, whereas macrophage chemotactic protein-1 expression progressed more rapidly in nondependent lung regions. Temporal and regional heterogeneity of lung injury were substantial. In this surfactant depletion model of acute lung injury, cyclical recruitment produced more injury than stretch.

19F-MRI of perflubron for measurement of oxygen partial pressure in porcine lungs during partial liquid ventilation.

A method for in vivo measurement of oxygen partial pressure (pO2) in porcine lungs during partial liquid ventilation (PLV) with perflubron (PFOB) was developed. A pulse sequence for high‐resolution MRI of the distribution of PFOB in the lung after intratracheal administration was developed as well. Moreover, quantitative measurements of longitudinal relaxation time T1 of 19F resonances for assessment of regional pO2 are described. Due to the need to acquire data during a single expiratory breathhold, only low SNRs were achieved in vivo. Therefore, simulations were performed to investigate the influence of background noise on T1 values calculated from data with low SNR. Based on these simulations, a postprocessing strategy was developed to correct for systematic errors by background noise prior to quantitative analysis. Results of a pilot study in pigs under conditions of PLV are presented. Magn Reson Med 47:82–89, 2002.

Dynamic ventilation 3He-magnetic resonance imaging with lung motion correction: Gas flow distribution analysis

Gast KK, Puderbach MU, Rodriguez I, et al. Dynamic ventilation 3He-MRI with lung motion correction: gas flow distribution analysis. Invest Radiol 2002;37:126–134. rationale and objectives. Software was developed to correct for lung motion to improve the description of hyperpolarized 3He gas distribution in the lung. methods. Five volunteers were studied by dynamic ventilation 3He-MRI using an ultrafast FLASH 2D sequence with a temporal resolution of 128 milliseconds. Signal kinetics were evaluated in the trachea and seven parenchymal Regions of Interest. Reference ranges for healthy subjects were defined for motion-corrected and uncorrected images. results. Motion correction was successfully performed. Reference ranges were 0.11–1.21 seconds for tracheal transit time, 0–0.02 seconds for trachea-alveolar interval, 0.22–0.62 seconds for alveolar rise time and 0–76.6 arbitrary units for alveolar amplitude for motion corrected images, and 0–1.09 seconds, 0–0.11 seconds, 0.26–0.85 seconds, 46.4–99.8 arbitrary units for uncorrected images. conclusions. Evaluation of 3He-distribution in the lung using motion correction of dynamic 3He-ventilation imaging is feasible and gives more narrow reference ranges.

Volumetry of ventilated airspaces by 3He MRI: preliminary results.

Kauczor H-U, Markstaller K, Puderbach M, et al. Volumetry of ventilated airspaces by 3He MRI: Preliminary results. Invest Radiol 2001;36:110–114. rationale and objectives. To develop a validated postprocessing routine for volumetry of the ventilated airspaces by 3He MRI. methods.3Helium MRI and pulmonary function tests were performed in seven healthy volunteers. After segmentation of ventilated airspaces, their volumes were calculated. Functional residual capacity (FRC) was used as a reference. For comparison of absolute volumes, correction factors were evaluated. results.Mean lung volume (± standard deviation) calculated from 3He MRI was 4082 ± 908 mL and mean FRC was 3696 ± 1166 mL, with a mean difference of 386 mL (r = 0.88). After correction for the relative pulmonary air content (factor 0.82), posture (0.72), and the individual tidal volume, 3He MRI volume was 3348 ± 744 mL and mean FRC was 3422 ± 817 mL, with the mean difference down to −74 mL (r = 0.9). Comparison on an individual basis confirmed an improvement in the estimation of absolute lung volume. conclusions.Volumetry of ventilated lung from 3He MRI shows high correlation and good agreement with the results of pulmonary function tests.