Search

The spatial/temporal dynamics of blood flow in the human lung can be measured noninvasively with magnetic resonance imaging (MRI) using arterial spin labeling (ASL). We report a novel data analysis method using nonlinear prediction to identify dynamic interactions between blood flow units (image voxels), potentially providing a probe of underlying vascular control mechanisms. The approach first estimates the linear relationship (predictability) of one voxel time series with another using correlation analysis, and after removing the linear component, it estimates the nonlinear relationship with a numerical mutual information approach. Dimensionless global metrics for linear prediction (F(L)) and nonlinear prediction (F(NL)) represent the average amplitude of fluctuations in one voxel estimated by another voxel, as a percentage of the global average voxel flow. A proof-of-principle test of this approach analyzed experimental data from a study of high-altitude pulmonary edema (HAPE), providing two groups exhibiting known differences in vascular reactivity. Subjects were mountaineers divided into HAPE-susceptible (S, n = 4) and HAPE-resistant (R, n = 5) groups based on prior history at high altitudes. Dynamic ASL measurements in the lung in normoxia (N, [Formula: see text] = 0.21) and hypoxia (H, [Formula: see text] = 0.13 ± 0.01) were compared. The nonlinear prediction metric F(NL) decreased with hypoxia [7.4 ± 1.3 (N) vs. 6.3 ± 0.7 (H), P = 0.03] and was significantly different between groups [7.4 ± 1.2 (R) vs. 6.2 ± 14.1 (S), P = 0.03]. This proof-of-principle test demonstrates that this nonlinear analysis approach applied to ASL data is sensitive to physiological effects even in small subject cohorts, and it potentially can be used in a wide range of studies in health and disease in the lung and other organs. NEW & NOTEWORTHY Pulmonary blood flow varies both spatially and temporally. We describe a novel technique to uncover nonlinear interactions in dynamic images of pulmonary blood flow measured using MRI, based on mutual information between the flow fluctuations in different imaging voxels. In a proof-of-principle study, we show that the proposed metric of nonlinear interactions was sufficiently sensitive to detect a difference between small cohorts of subjects susceptible to high-altitude pulmonary edema (HAPE) and subjects resistant to HAPE.

Low doses of the carbonic anhydrase inhibitor acetazolamide provides accelerated acclimatization to high-altitude hypoxia and prevention of cerebral and other symptoms of acute mountain sickness. We previously observed increases in cerebral O2 metabolism (CMRO2) during hypoxia. In this study, we investigate whether low-dose oral acetazolamide (250 mg) reduces this elevated CMRO2 and in turn might improve cerebral tissue oxygenation (PtiO2) during acute hypoxia. Six normal human subjects were exposed to 6 h of normobaric hypoxia with and without acetazolamide prophylaxis. We determined CMRO2 and cerebral PtiO2 from MRI measurements of cerebral blood flow (CBF) and cerebral venous O2 saturation. During normoxia, low-dose acetazolamide resulted in no significant change in CBF, CMRO2, or PtiO2. During hypoxia, we observed increases in CBF [48.5 (SD 12.4) (normoxia) to 65.5 (20.4) ml·100 ml−1·min−1 (hypoxia), P < 0.05] and CMRO2 [1.54 (0.19) to 1.79 (0.25) μmol·ml−1·min−1, P < 0.05] and a dramatic decline in PtiO2 [25.0 to 11.4 (2.7) mmHg, P < 0.05]. Acetazolamide prophylaxis mitigated these rises in CBF [53.7 (20.7) ml·100 ml−1·min−1 (hypoxia + acetazolamide)] and CMRO2 [1.41 (0.09) μmol·ml−1·min−1 (hypoxia + acetazolamide)] associated with acute hypoxia but also reduced O2 delivery [6.92 (1.45) (hypoxia) to 5.60 (1.14) mmol/min (hypoxia + acetazolamide), P < 0.05]. The net effect was improved cerebral tissue PtiO2 during acute hypoxia [11.4 (2.7) (hypoxia) to 16.5 (3.0) mmHg (hypoxia + acetazolamide), P < 0.05]. In addition to its renal effect, low-dose acetazolamide is effective at the capillary endothelium, and we hypothesize that local interruption in cerebral CO2 excretion accounts for the improvements in CMRO2 and ultimately in cerebral tissue oxygenation during hypoxia. This study suggests a potentially pivotal role of cerebral CO2 and pH in modulating CMRO2 and PtiO2 during acute hypoxia.

Hypoxic pulmonary vasoconstriction (HPV) is thought to actively regulate ventilation-perfusion (V̇a/Q̇) matching, reducing perfusion in regions of alveolar hypoxia. We assessed the extent of HPV in the healthy human lung using inhaled nitric oxide (iNO) under inspired oxygen fractions (FiO2) of 0.125, 0.21, and 0.30 (a hyperoxic stimulus designed to abolish HPV without the development of atelectasis). Dynamic measures of blood flow were made in a single sagittal slice of the right lung of five healthy male subjects using an arterial spin labeling (ASL) MRI sequence, following a block stimulus pattern (3 × 60 breaths) with 40 ppm iNO administered in the central block. The overall spatial heterogeneity, spatiotemporal variability, and regional pattern of pulmonary blood flow was quantified as a function of condition (FiO2× iNO state). While spatial heterogeneity did not change significantly with iNO administration or FiO2, there were statistically significant increases in Global Fluctuation Dispersion,(a marker of spatiotemporal flow variability) when iNO was administered during hypoxia (5.4 percentage point increase, P = 0.003). iNO had an effect on regional blood flow that was FiO2dependent ( P = 0.02), with regional changes in the pattern of blood flow occurring in hypoxia ( P = 0.007) and normoxia ( P = 0.008) tending to increase flow to dependent lung at the expense of nondependent lung. These findings indicate that inhaled nitric oxide significantly alters the distribution of blood flow in both hypoxic and normoxic healthy subjects, and suggests that some baseline HPV may indeed be present in the normoxic lung.

The temporal dynamics of blood flow in the human lung have been largely unexplored due to the lack of appropriate technology. Using the magnetic resonance imaging method of arterial spin labeling (ASL) with subject-gated breathing, we produced a dynamic series of flow-weighted images in a single sagittal slice of the right lung with a spatial resolution of ∼1 cm3and a temporal resolution of ∼10 s. The mean flow pattern determined from a set of reference images was removed to produce a time series of blood flow fluctuations. The fluctuation dispersion (FD), defined as the spatial standard deviation of each flow fluctuation map, was used to quantify the changes in distribution of flow in six healthy subjects in response to 100 breaths of hypoxia (FiO2= 0.125) or hyperoxia (FiO2= 1.0). Two reference frames were used in calculation, one determined from the initial set of images (FDglobal), and one determined from the mean of each corresponding baseline or challenge period (FDlocal). FDlocalthus represented changes in temporal variability as a result of intervention, whereas FDglobalencompasses both FDlocaland any generalized redistribution of flow associated with switching between two steady-state patterns. Hypoxic challenge resulted in a significant increase (96%, P < 0.001) in FDglobalfrom the normoxic control period and in FDlocal(46%, P = 0.0048), but there was no corresponding increase in spatial relative dispersion (spatial standard deviation of the images divided by the mean; 8%, not significant). There was a smaller increase in FDglobalin response to hyperoxia (47%, P = 0.0015) for the single slice, suggestive of a more general response of the pulmonary circulation to a change from normoxia to hyperoxia. These results clearly demonstrate a temporal change in the sampled distribution of pulmonary blood flow in response to hypoxia, which is not observed when considering only the relative dispersion of the spatial distribution.

MRI images of pulmonary blood flow using arterial spin labeling (ASL) measure the delivery of magnetically tagged blood to an image plane during one systolic ejection period. However, the method potentially suffers from two problems, each of which may depend on the imaging plane location: 1) the inversion plane is thicker than the imaging plane, resulting in a gap that blood must cross to be detected in the image; and 2) ASL includes signal contributions from tagged blood in conduit vessels (arterial and venous). By using an in silico model of the pulmonary circulation we found the gap reduced the ASL signal to 64–74% of that in the absence of a gap in the sagittal plane and 53–84% in the coronal. The contribution of the conduit vessels varied markedly as a function of image plane ranging from ∼90% of the overall signal in image planes that encompass the central hilar vessels to 35% of maximum reduced the conduit vessel contribution to the total ASL signal to ∼20% on average; however, planes with large contributions from conduit vessels underestimate acinar flow due to a high proportion of in-plane flow, making ASL measurements of perfusion impractical. In other image planes, perfusion dominated the resulting ASL images with good agreement between ASL and acinar flow. Similarly, heterogeneity of the ASL signal as measured by relative dispersion is a reliable measure of heterogeneity of the acinar flow distribution in the same image planes.

Specific ventilation (SV) is the ratio of fresh gas entering a lung region divided by its end-expiratory volume. To quantify the vertical (gravitationally dependent) gradient of SV in eight healthy supine subjects, we implemented a novel proton magnetic resonance imaging (MRI) method. Oxygen is used as a contrast agent, which in solution changes the longitudinal relaxation time (T1) in lung tissue. Thus alterations in the MR signal resulting from the regional rise in O2concentration following a sudden change in inspired O2reflect SV—lung units with higher SV reach a new equilibrium faster than those with lower SV. We acquired T1-weighted inversion recovery images of a sagittal slice of the supine right lung with a 1.5-T MRI system. Images were voluntarily respiratory gated at functional residual capacity; 20 images were acquired with the subject breathing air and 20 breathing 100% O2, and this cycle was repeated five times. Expired tidal volume was measured simultaneously. The SV maps presented an average spatial fractal dimension of 1.13 ± 0.03. There was a vertical gradient in SV of 0.029 ± 0.012 cm−1, with SV being highest in the dependent lung. Dividing the lung vertically into thirds showed a statistically significant difference in SV, with SV of 0.42 ± 0.14 (mean ± SD), 0.29 ± 0.10, and 0.24 ± 0.08 in the dependent, intermediate, and nondependent regions, respectively (all differences, P < 0.05). This vertical gradient in SV is consistent with the known gravitationally induced deformation of the lung resulting in greater lung expansion in the dependent lung with inspiration. This SV imaging technique can be used to quantify regional SV in the lung with proton MRI.

Exercise presents a considerable stress to the pulmonary system and ventilation-perfusion (V̇a/Q̇) heterogeneity increases with exercise, affecting the efficiency of gas exchange. In particular, prolonged heavy exercise and maximal exercise are known to increase V̇a/Q̇ heterogeneity and these changes persist into recovery. We hypothesized that the spatial heterogeneity of pulmonary perfusion would be similarly elevated after prolonged exercise. To test this, athletic subjects ( n = 6, V̇o2max = 61 ml· kg−1·min−1) with exercising V̇a/Q̇ heterogeneity previously characterized by the multiple inert gas elimination technique (MIGET), performed 45 min of cycle exercise at ∼70% V̇o2max. MRI arterial spin labeling measures of pulmonary perfusion were acquired pre- and postexercise (at 20, 40, 60 min post) to quantify the spatial distribution in isogravitational (coronal) and gravitationally dependent (sagittal) planes. Regional proton density measurements allowed perfusion to be normalized for density and quantified in milliliters per minute per gram. Mean lung density did not change significantly in either plane after exercise ( P = 0.19). Density-normalized perfusion increased in the sagittal plane postexercise ( P = 2 = 0.68, P < 0.05, GSD, R2 = 0.55, P = 0.09). These data are consistent with but not proof of interstitial pulmonary edema as the mechanism underlying exercise-induced increases in both spatial perfusion heterogeneity and V̇a/Q̇ heterogeneity.

Prone posture increases cardiac output and improves pulmonary gas exchange. We hypothesized that, in the supine posture, greater compression of dependent lung limits regional blood flow. To test this, MRI-based measures of regional lung density, MRI arterial spin labeling quantification of pulmonary perfusion, and density-normalized perfusion were made in six healthy subjects. Measurements were made in both the prone and supine posture at functional residual capacity. Data were acquired in three nonoverlapping 15-mm sagittal slices covering most of the right lung: central, middle, and lateral, which were further divided into vertical zones: anterior, intermediate, and posterior. The density of the entire lung was not different between prone and supine, but the increase in lung density in the anterior lung with prone posture was less than the decrease in the posterior lung (change: +0.07 g/cm3 anterior, −0.11 posterior; P < 0.0001), indicating greater compression of dependent lung in supine posture, principally in the central lung slice ( P < 0.0001). Overall, density-normalized perfusion was significantly greater in prone posture (7.9 ± 3.6 ml·min−1·g−1 prone, 5.1 ± 1.8 supine, a 55% increase; P < 0.05) and showed the largest increase in the posterior lung as it became nondependent (change: +71% posterior, +58% intermediate, +31% anterior; P = 0.08), most marked in the central lung slice ( P < 0.05). These data indicate that central posterior portions of the lung are more compressed in the supine posture, likely by the heart and adjacent structures, than are central anterior portions in the prone and that this limits regional perfusion in the supine posture.

In vivo radioactive tracer and microsphere studies have differing conclusions as to the magnitude of the gravitational effect on the distribution of pulmonary blood flow. We hypothesized that some of the apparent vertical perfusion gradient in vivo is due to compression of dependent lung increasing local lung density and therefore perfusion/volume. To test this, six normal subjects underwent functional magnetic resonance imaging with arterial spin labeling during breath holding at functional residual capacity, and perfusion quantified in nonoverlapping 15 mm sagittal slices covering most of the right lung. Lung proton density was measured in the same slices using a short echo 2D-Fast Low-Angle SHot (FLASH) sequence. Mean perfusion was 1.7 ± 0.6 ml·min−1·cm−3 and was related to vertical height above the dependent lung (slope = −3%/cm, P < 0.0001). Lung density averaged 0.34 ± 0.08 g/cm3 and was also related to vertical height (slope = −4.9%/cm, P < 0.0001). By contrast, when perfusion was normalized for regional lung density, the slope of the height-perfusion relationship was not significantly different from zero ( P = 0.2). This suggests that in vivo variations in regional lung density affect the interpretation of vertical gradients in pulmonary blood flow and is consistent with a simple conceptual model: the lung behaves like a Slinky (Slinky is a registered trademark of Poof-Slinky Incorporated), a deformable spring distorting under its own weight. The greater density of lung tissue in the dependent regions of the lung is analogous to a greater number of coils in the dependent portion of the vertically oriented spring. This implies that measurements of perfusion in vivo will be influenced by density distributions and will differ from excised lungs where density gradients are reduced by processing.

The role of imaging as a tool for investigating lung physiology is growing at an accelerating pace. Looking forward, we wished to identify unresolved issues in lung physiology that might realistically be addressed by imaging methods in development or imaging approaches that could be considered. The role of imaging is framed in terms of the importance of good spatial and temporal resolution and the types of questions that could be addressed as these technical capabilities improve. Recognizing that physiology is fundamentally a quantitative science, a recurring emphasis is on the need for imaging methods that provide reliable measurements of specific physiological parameters. The topics included necessarily reflect our perspective on what are interesting questions and are not meant to be a comprehensive review. Nevertheless, we hope that this essay will be a spur to physiologists to think about how imaging could usefully be applied in their research and to physical scientists developing new imaging methods to attack challenging questions imaging could potentially answer.

To develop a compartmental model for estimating myocardial oxygen consumption rate (MVO2) with [1-11C]acetate, the metabolic fate of radiolabeled acetate was determined in normoxic and ischemic conditions in isolated perfused rat hearts. Glutamate composed 63 +/- 1 and 44 +/- 7% of the total tissue radioactivity 2 min postinjection in normoxic and ischemic myocardium, respectively, and radiolabeled glutamate remained the largest fraction throughout 40 min of perfusion. Based on the biochemical pathway of the tracer and the temporal distribution of 14C-labeled metabolites, a six-compartment model was formulated. Studies using [1-11C]acetate and a pair of NaI detectors were then performed in the same perfused heart system to validate the model. Consistency between the model predictions and biochemical measurements of tissue and effluent metabolites supported the validity of the kinetic model in normoxic and ischemic conditions. Model-estimated MVO2 correlated well with experimentally measured MVO2 for normoxic, hypoxic, and ischemic conditions, with a slope of 0.97 (r = 0.95). In addition, the model-estimated rate constant, k42, which corresponded to the oxidative flux, correlated strongly with the myocardial clearance rate (k1 or kmono) determined from the tissue kinetics. These findings provide a mechanistic basis for the use of k1 or kmono as an index of MVO2 in both normoxic and ischemic myocardium studied with [1-11C]acetate and positron emission tomography.

Radiolabeled monoclonal antibody fragments to myosin, specifically 111In-labeled antimyosin, have been shown to be effective for imaging areas of myocardial infarct. To determine if 111In-labeled antimyosin can be used to assess the extent of necrosis, we compared the tissue retention fraction of 111In-labeled antimyosin with the amount of creatine kinase (CK) released from the isolated, perfused, interventricular rabbit septum after an intervention to induce tissue necrosis. 111In-labeled antimyosin was injected and tissue radioactivity was monitored for a 60-min period under control conditions. Effluent samples were also collected during this period and assayed for CK content. After a period of Ca2+ depletion followed by Ca2+ repletion, 111In-labeled antimyosin was again injected, and washout and CK data were collected. Comparison of the changes in 111In-labeled antimyosin retention fraction from control to intervention with the corresponding increase in CK released during intervention resulted in a correlation coefficient of 0.83. To corroborate the findings of the Ca2+ depletion followed by repletion experiments, further experiments were conducted in which zero-flow ischemia followed by reperfusion was used as a means of introducing necrosis in the septum. The resulting correlation coefficient between CK release and 111In-labeled antimyosin retention fraction was 0.82. The results of these experiments indicate that 111In-labeled antimyosin can be used to quantitatively estimate the extent of necrosis in the rabbit septum, as determined by CK release, and endorse the potential use of tracer kinetics in humans for quantitation of myocardial necrosis in vivo.