Computing oxygen-enhanced ventilation maps using correlation analysis

Measuring short-term changes in specific ventilation using dynamic specific ventilation imaging

Specific ventilation imaging (SVI) measures the spatial distribution of specific ventilation (SV) in the lung with MRI by using inhaled oxygen as a contrast agent. Because of the inherently low signal to noise ratio in the technique, multiple switches between inspiring air and O₂ are utilized, and the high spatial resolution SV distribution determined as an average over the entire imaging period (~20 minutes). We hypothesized that a trade-off between spatial and temporal resolution could allow imaging at a higher temporal resolution, at the cost of a coarser, yet acceptable, spatial resolution. The appropriate window length and spatial resolution compromise was determined by generating synthetic data with signal- and contrast-to-noise characteristics reflective of that in previously published experimental data, with a known and unchanging distribution of SV, and showed that acceptable results could be obtained in an imaging period of ~7 minutes (80 breaths), with a spatial resolution of ~1cm³. Previously published data were then reanalyzed. The average heterogeneity of the temporally resolved maps of SV were not different to the previous overall analysis, however the temporally resolved maps were less effective at detecting the amount of bronchoconstriction resulting from methacholine administration. The results further indicated that the initial response to inhaled methacholine and subsequent inhalation of albuterol were largely complete within ~22 minutes and ~9 minutes respectively, although there was a tendency for an ongoing developing effect in both cases. These results suggest that it is feasible to use a shortened SVI protocol, with a modest sacrifice in spatial resolution, in order to measure temporally dynamic processes.

Physiological system analysis of the kidney by high-temporal-resolution T 2 ∗ monitoring of an oxygenation step response

PURPOSE

Examine the feasibility of characterizing the regulation of renal oxygenation using high-temporal-resolution monitoring of the T 2 ∗ response to a step-like oxygenation stimulus.

METHODS

For T 2 ∗ mapping, multi-echo gradient-echo imaging was used (temporal resolution = 9 seconds). A step-like renal oxygenation challenge was applied involving sequential exposure to hyperoxia (100% O₂ ), hypoxia (10% O₂ + 90% N₂ ), and hyperoxia (100% O₂ ). In vivo experiments were performed in healthy rats (N = 10) and in rats with bilateral ischemia-reperfusion injury (N = 4). To assess the step response of renal oxygenation, a second-order exponential model was used (model parameters: amplitude [A], time delay [Δt], damping constant [D], and period of the oscillation [T]) for renal cortex, outer stripe of the outer medulla, inner stripe of the outer medulla, and inner medulla.

RESULTS

The second-order exponential model permitted us to model the exponential T 2 ∗ recovery and the superimposed T 2 ∗ oscillation following renal oxygenation stimulus. The in vivo experiments revealed a difference in D_{outer medulla} between healthy controls (D < 1, indicating oscillatory recovery) and ischemia-reperfusion injury (D > 1, reflecting aperiodic recovery). The increase in D_{outer medulla} by a factor of 3.7 (outer stripe of the outer medulla) and 10.0 (inner stripe of the outer medulla) suggests that this parameter might be rather sensitive to (patho)physiological oxygenation changes.

CONCLUSION

This study demonstrates the feasibility of monitoring the dynamic oxygenation response of renal tissues to a step-like oxygenation challenge using high-temporal-resolution T 2 ∗ mapping. Our results suggest that the implemented system analysis approach may help to unlock questions regarding regulation of renal oxygenation, with the ultimate goal of providing imaging means for diagnostics and therapy of renal diseases.

Ventilation/Perfusion Relationships and Gas Exchange: Measurement Approaches

Ventilation-perfusion ( V ˙ A / Q ˙ ) matching, the regional matching of the flow of fresh gas to flow of deoxygenated capillary blood, is the most important mechanism affecting the efficiency of pulmonary gas exchange. This article discusses the measurement of V ˙ A / Q ˙ matching with three broad classes of techniques: (i) those based in gas exchange, such as the multiple inert gas elimination technique (MIGET); (ii) those derived from imaging techniques such as single-photon emission computed tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRI), computed tomography (CT), and electrical impedance tomography (EIT); and (iii) fluorescent and radiolabeled microspheres. The focus is on the physiological basis of these techniques that provide quantitative information for research purposes rather than qualitative measurements that are used clinically. The fundamental equations of pulmonary gas exchange are first reviewed to lay the foundation for the gas exchange techniques and some of the imaging applications. The physiological considerations for each of the techniques along with advantages and disadvantages are briefly discussed. © 2020 American Physiological Society. Compr Physiol 10:1155-1205, 2020.

Functional imaging of the lungs with gas agents

This review focuses on the state-of-the-art of the three major classes of gas contrast agents used in magnetic resonance imaging (MRI)-hyperpolarized (HP) gas, molecular oxygen, and fluorinated gas--and their application to clinical pulmonary research. During the past several years there has been accelerated development of pulmonary MRI. This has been driven in part by concerns regarding ionizing radiation using multidetector computed tomography (CT). However, MRI also offers capabilities for fast multispectral and functional imaging using gas agents that are not technically feasible with CT. Recent improvements in gradient performance and radial acquisition methods using ultrashort echo time (UTE) have contributed to advances in these functional pulmonary MRI techniques. The relative strengths and weaknesses of the main functional imaging methods and gas agents are compared and applications to measures of ventilation, diffusion, and gas exchange are presented. Functional lung MRI methods using these gas agents are improving our understanding of a wide range of chronic lung diseases, including chronic obstructive pulmonary disease, asthma, and cystic fibrosis in both adults and children.

Advances in functional and structural imaging of the human lung using proton MRI

The field of proton lung MRI is advancing on a variety of fronts. In the realm of functional imaging, it is now possible to use arterial spin labeling (ASL) and oxygen-enhanced imaging techniques to quantify regional perfusion and ventilation, respectively, in standard units of measurement. By combining these techniques into a single scan, it is also possible to quantify the local ventilation-perfusion ratio, which is the most important determinant of gas-exchange efficiency in the lung. To demonstrate potential for accurate and meaningful measurements of lung function, this technique was used to study gravitational gradients of ventilation, perfusion, and ventilation-perfusion ratio in healthy subjects, yielding quantitative results consistent with expected regional variations. Such techniques can also be applied in the time domain, providing new tools for studying temporal dynamics of lung function. Temporal ASL measurements showed increased spatial-temporal heterogeneity of pulmonary blood flow in healthy subjects exposed to hypoxia, suggesting sensitivity to active control mechanisms such as hypoxic pulmonary vasoconstriction, and illustrating that to fully examine the factors that govern lung function it is necessary to consider temporal as well as spatial variability. Further development to increase spatial coverage and improve robustness would enhance the clinical applicability of these new functional imaging tools. In the realm of structural imaging, pulse sequence techniques such as ultrashort echo-time radial k-space acquisition, ultrafast steady-state free precession, and imaging-based diaphragm triggering can be combined to overcome the significant challenges associated with proton MRI in the lung, enabling high-quality three-dimensional imaging of the whole lung in a clinically reasonable scan time. Images of healthy and cystic fibrosis subjects using these techniques demonstrate substantial promise for non-contrast pulmonary angiography and detailed depiction of airway disease. Although there is opportunity for further optimization, such approaches to structural lung imaging are ready for clinical testing.

Validating the distribution of specific ventilation in healthy humans measured using proton MR imaging

Specific ventilation imaging (SVI) uses proton MRI to quantitatively map the distribution of specific ventilation (SV) in the human lung, using inhaled oxygen as a contrast agent. To validate this recent technique, we compared the quantitative measures of heterogeneity of the SV distribution in a 15-mm sagittal slice of lung obtained in 10 healthy supine subjects, (age 37 ± 10 yr, forced expiratory volume in 1 s 97 ± 7% predicted) using SVI to those obtained in the whole lung from multiple-breath nitrogen washout (MBW). Using the analysis of Lewis et al. (Lewis SM, Evans JW, Jalowayski AA. J App Physiol 44: 416-423, 1978), the most likely distribution of SV from the MBW data was computed and compared with the distribution of SV obtained from SVI, after normalizing for the difference in tidal volume. The average SV was 0.30 ± 0.10 MBW, compared with 0.36 ± 0.10 SVI (P = 0.01). The width of the distribution, a measure of the heterogeneity, obtained using both methods was comparable: 0.51 ± 0.06 and 0.47 ± 0.08 in MBW and SVI, respectively (P = 0.15). The MBW estimated width of the SV distribution was 0.05 (10.7%) higher than that estimated using SVI, and smaller than the intertest variability of the MBW estimation [inter-MBW (SD) for the width of the SV distribution was 0.08 (15.8)%]. To assess reliability, SVI was performed twice on 13 subjects showing small differences between measurements of SV heterogeneity (typical error 0.05, 12%). In conclusion, quantitative estimations of SV heterogeneity from SVI are reliable and similar to those obtained using MBW, with SVI providing spatial information that is absent in MBW.

Pulmonary magnetic resonance imaging for airway diseases

Pulmonary magnetic resonance (MR) imaging has been put forward as a new research and diagnostic tool mainly to overcome the limitations of computed tomography and nuclear medicine studies. However, pulmonary MR imaging has been difficult to use because of inherently low proton density, a multitude of air-tissue interfaces, which create significant magnetic field distortions and are commonly referred to as susceptibility artifacts; diminishing signal in the lung; and respiratory and/or cardiac motion artifacts. To overcome these drawbacks of pulmonary MR imaging, technical advances made during the last decade in sequencing, scanner and coil, adaptation of parallel imaging techniques, and utilization of contrast media have been reported as being useful for functional and morphologic assessment of various pulmonary diseases including airway diseases. This review article covers (1) pulmonary MR techniques for morphologic and functional assessment of airway diseases, and (2) pulmonary MR imaging for cystic fibrosis, asthma, and chronic obstructive pulmonary disease. Pulmonary MR imaging provides not only morphology-related but also pulmonary function-related information. It has the potential to replace nuclear medicine studies for the identification of regional pulmonary function and may perform a complementary role in airway disease assessment instead of nuclear medicine study. We believe that the findings of further basic studies as well as clinical applications of this new technique will validate the real significance of pulmonary MR imaging for the future of airway disease assessment and its usefulness for diagnostic radiology and pulmonary medicine.

Vertical distribution of specific ventilation in normal supine humans measured by oxygen-enhanced proton MRI

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 O(2) concentration following a sudden change in inspired O(2) reflect 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% O(2), 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.

Analysis of signal dynamics in oxygen-enhanced magnetic resonance imaging

OBJECTIVES

Oxygen-enhanced MRI (O2-MRI) is frequently based on a block paradigm consisting of a series of consecutive T1-weighted scans acquired during alternating blocks with inhalation of room air and of pure oxygen. This design results in a complex signal-time course for each pixel, which displays the oxygen wash-in and wash-out processes and provides spatially resolved information about the lung function. The purpose of the present study was to optimize the signal-time-course analysis to extract (pixelwise) the maximum amount of information from the acquired data, and to introduce an appropriate cross-correlation approach for data sets containing the oxygen wash-in and wash-out periods.

MATERIALS AND METHODS

O2-MRI data of 11 healthy volunteers were acquired with a multislice inversion-recovery single-shot turbo-spin-echo sequence at 1.5 Tesla; lung and spleen were manually segmented on all 44 acquired slices. Six different model functions were pixelwise fitted to the data and compared using the Akaike information criterion. Four different reference functions were compared for cross-correlation analysis.

RESULTS

The optimal model function is a piecewise exponential function (median enhancement in lung/spleen: 16.3%/14.8%) with different time constants for wash-in (29.4 seconds/72.7 seconds) and wash-out (25.1 seconds/29.6 seconds). As a new parameter, it contains the delay between switching the gas supply and the onset of the signal change (4.8 seconds/24.5 seconds). Optimal cross-correlation results were obtained with a piecewise exponential reference function, which was temporally shifted to maximize the correlation, yielding median correlation coefficients of 0.694 and 0.878, median time delays of 7.5 seconds and 38.6 seconds, and median fractions of oxygen-activated pixels of 83.6% and 92.2% in the lung and the spleen, respectively.

CONCLUSIONS

It was demonstrated that the pixelwise assessment of O2-MRI data are optimally performed with piecewise exponential functions. Cross-correlation analysis with a piecewise exponential reference function results in significantly higher fractions of oxygen-activated pixels than with rectangular functions.

Influence of oxygen and carbogen breathing on renal oxygenation measured by T2*-weighted imaging at 3.0 T

The aim of the study was to assess the influence of carbogen (95% O(2), 5% CO(2)) or pure oxygen breathing on renal oxygenation measured by blood oxygenation level dependent (BOLD) magnetic resonance imaging at 3.0 T. Seven healthy young volunteers (median age 25, range 23-35 years) participated in the study. A T2*-weighted fat-saturated spoiled gradient-echo sequence was implemented on a 3.0 T whole-body imager (TE/TR = 27.9 ms/49 ms, excitation angle 20 degrees ) with an acquisition time of approximately 5.3 s. A total of 100 images were acquired during 22 min. A block design was applied for gas administration: 4 min room air, 4 min carbogen/oxygen, 4 min room air, 4 min carbogen/oxygen and 6 min room air. A compartment model was fitted to the data sets accounting for time-dependent increase/decrease of renal oxygenation as well as baseline changes of the scanner. T2*-weighted images showed good image quality without notable artefacts or distortions. Mean relative signal increase due to carbogen breathing was 2.73% (95% confidence interval: 1.34-5.54) in the right kidney and 3.76% (1.53-9.20) in the left kidney, while oxygen breathing led to a signal enhancement of 3.20% (2.57-3.98) in the right kidney and 3.16% (1.83-5.45) in the left kidney. No statistical difference was found between carbogen and oxygen breathing or between the oxygenation of the right and the left kidney. A significant difference was found in the characteristic time constant for the signal increase with a faster saturation taking place for oxygen breathing. Renal tissue oxygenation is clearly influenced by carbogen or oxygen breathing. The changes can be assessed by T2*-weighted MRI at high field strengths. The effects are in the expected range for the BOLD effect of 3-4% at 3.0 T. The proposed technique might be interesting for the assessment of renal tissue oxygenation and its regulation in patients with kidney diseases.

Navigator-triggered oxygen-enhanced MRI with simultaneous cardiac and respiratory synchronization for the assessment of interstitial lung disease

PURPOSE

To evaluate an optimized method for oxygen-enhanced MRI of the lung, using simultaneous electrocardiograph (ECG) and navigator triggering. To correlate oxygen-enhanced MRI with lung function tests assessing alveolar-capillary gas exchange.

MATERIALS AND METHODS

A total of 12 healthy volunteers (aged 20-32 years) and 10 patients (aged 37-87 years) with interstitial lung diseases (ILD) underwent oxygen-enhanced MRI and pulmonary functional tests (PFTs) assessing alveolar-capillary gas exchange. The paradigm room-air-oxygen-room-air was acquired with a nonselective inversion-recovery half-Fourier single-shot turbo spin-echo sequence (inversion time = 1200 msec; acquisition time = 134.5 msec; slice thickness = 20 mm; matrix size = 128 x 128), using simultaneous double triggering (navigator plus ECG trigger). Cross-correlation was performed in regions of interest (ROIs) encompassing both lungs. The number of oxygen-activated pixels over the total number of pixels in the ROIs (OAP%) of volunteers and patients was compared. OAP%s were correlated with PFTs.

RESULTS

The mean OAP% of patients was significantly lower than that of volunteers (36.7 vs. 81.7, P = 0.001). OAP% correlated with the transfer lung factor for carbon monoxide (Tlco) (r = 0.64; P = 0.002), the transfer coefficient (Kco) (r = 0.75; P = 0.001), the arterial partial pressure (r = 0.77; P < 0.001), and the saturation (r = 0.70; P < 0.001) of oxygen.

CONCLUSION

Navigator-triggered oxygen-enhanced MRI of the lung may have a potential role in the quantitative assessment of lung function in ILD.

Basics concepts and clinical applications of oxygen-enhanced MR imaging

Oxygen-enhanced MR imaging is a new technique, and its physiological significance has not yet been fully elucidated. This review article covers (1) the theory of oxygen enhancement and its relationship with respiratory physiology; (2) design for oxygen-enhanced MR imaging sequencing; (3) a basic study of oxygen-enhanced MR imaging in animal models and humans; (4) a clinical study of oxygen-enhanced MR imaging; and (5) a comparison of advantages and disadvantages of this technique with those of hyperpolarized noble gas MR ventilation imaging. Oxygen-enhanced MR imaging provides not only the ventilation-related, but also respiration-related information. Oxygen-enhanced MR imaging has the potential to replace nuclear medicine studies for the identification of regional pulmonary function, and many investigators are now attempting to adapt this technique for routine clinical studies. We believe that further basic studies as well as clinical applications of this new technique will define the real significance of oxygen-enhanced MR imaging for the future of pulmonary functional imaging and its usefulness for diagnostic radiology and pulmonary medicine.

Influence of inversion pulse type in assessing lung-oxygen-enhancement by centrically-reordered non-slice-selective inversion-recovery half-Fourier single-shot turbo spin-echo (HASTE) sequence

PURPOSE

To demonstrate the influence of inversion pulse type and inversion time for assessment of oxygen-enhancement on centrically-reordered non-slice-selective inversion-recovery (IR) half-Fourier single-shot turbo spin-echo (HASTE) sequence.

MATERIAL AND METHODS

Phantoms with and without 100% oxygen and three healthy volunteers were studied with two-dimensional (2D) centrically-reordered non-slice selective IR-HASTE sequence with either composite or block inversion-recovery pulse at increasing inversion times from 200 to 1800 msec. Signal-to-noise ratios (SNRs) of phantom, real signal differences, and relative enhancement ratios of lung parenchyma between oxygen-enhanced and non-oxygen-enhanced MR images on composite and block pulse type were statistically compared at each TI.

RESULTS

SNRs at TIs of 200 and 400 msec using the composite inversion pulse type were significantly lower than those with the block inversion pulse in the in vivo study (P < 0.05), although no significant differences were observed in the phantom study and in the in vivo study at inversion times greater than or equal to 600 msec. Real signal intensity (SI) differences at 400 and 600 msec of the composite inversion pulse type were significantly higher than those with the block inversion pulse type (P < 0.05). Relative enhancement ratio at 800 msec with the composite inversion pulse were significantly lower than that with the block inversion pulse (P < 0.05).

CONCLUSION

IR pulse type and inversion time have influence on assessment of oxygen-enhancement by centrically-reordered non-slice-selective IR-HASTE sequence.

Simultaneous Cardiac and Respiratory Synchronization in Oxygen-Enhanced Magnetic Resonance Imaging of the Lung Using a Pneumotachograph for Respiratory Monitoring

OBJECTIVES

We sought to evaluate an optimized method for oxygen-enhanced magnetic resonance imaging of the lung, using electrocardiogram-trigger and a pneumotachograph for simultaneous cardiac and respiratory synchronization.

MATERIALS AND METHODS

Five series of IR-SSFSE images (echo time = 28.2 milliseconds; inversion time = 1,200 milliseconds) were obtained in 6 volunteers during the ventilation-paradigm room-air/oxygen/room-air: series 1, respiratory-triggered; series 2, cardiac-triggered; series 3, cardiac-triggered and respiratory-synchronized using the signal of the pneumatic belt; series 4, cardiac-triggered and respiratory-synchronized using the external signal of the pneumotachograph; and series 5, not cardiac-triggered and respiratory-synchronized using the signal of the pneumotachograph. Standard deviations of the lung (SI(var)) and diaphragm mismatch (DM) were measured. The relative SI change (DeltaSI) was computed from room-air and oxygen-enhanced images. Parametric maps were obtained from cross-correlation analysis of the ventilation paradigm. Mean correlation coefficients (cc) and the percentage of activated pixels over the lung (Act%) were calculated from these maps. All 5 parameters were compared among the 5 series (Friedman-analysis of variance, Dunn's posthoc test).

RESULTS

In series 4, DM and SI(var) were significantly lower than in respiratory and cardiac-triggered series (DM = 4.7 vs. 14.3 and 18.4; SI(var) = 4.9 vs. 10 and 11). In the same series cc and Act% also were significantly higher than in series 1 and 2 (cc = 0.86 vs. 0.7 and 0.6; Act% = 71.3 vs. 44.7 and 41.2). DeltaSI was not significantly different among all series.

CONCLUSIONS

Effective respiratory and cardiac synchronization can be achieved in oxygen-enhanced magnetic resonance imaging of the lung, using a pneumotachograph for real-time targeting of end-expiration.

Sauerstoff-MRT der Lunge: Optimierte Berechnung von Differenzbildern

BACKGROUND

In oxygen-enhanced lung MRI, difference maps of acquisitions during inhalation of room air and pure oxygen are calculated to assess lung function. The purpose of this study was to analyze how the calculation of these difference maps depends on the delayed signal change after switching the gas supply.

METHODS

Ten healthy volunteers were examined with an ECG and respiratory-triggered T1-weighting inversion recovery HASTE sequence with parallel imaging. Four blocks with 20 repetitions of up to 6 coronal slices were continuously acquired; in blocks 1 and 3 room air was supplied, in blocks 2 and 4 oxygen. Data were postprocessed, discarding between 0 and 19 repetitions after each change of gas supply before calculating the relative signal difference.

RESULTS

The averaged relative signal difference increases from 9.4 to 17.4% when the number of discarded acquisitions increases; the ratio of signal difference and spatial standard deviation reaches a maximum at 5-8 discarded acquisitions.

CONCLUSIONS

An optimized ratio of signal difference and statistical error is found if about 5-8 of 20 respiratory-triggered repetitions are discarded after each change of gas supply for the calculation of difference maps.

Fast oxygen-enhanced multislice imaging of the lung using parallel acquisition techniques

The purpose of this study was to evaluate an optimized multislice acquisition technique for oxygen-enhanced MRI of the lung using slice-selective inversion and refocusing pulses in combination with parallel imaging. An inversion recovery HASTE sequence was implemented with respiratory triggering to perform imaging in end-expiration and with ECG triggering to avoid image acquisition during the systolic phase. Inversion pulses and the readout of echo trains could be interleaved to decrease acquisition time. The sequence was evaluated in 15 healthy volunteers, comparing three acquisition schemes: (1) acquisition of four slices without parallel imaging; (2) acquisition of four slices with parallel imaging; (3) acquisition of six slices with parallel imaging. These multislice acquisitions were repeated 80 times with alternating inhalation of room air and oxygen. The oxygen-induced signal increase showed no significant difference with and without parallel imaging. However, only with parallel imaging did the interleaved acquisition of six or more slices become possible, thus enabling a more complete anatomic coverage of the lung. The average required end-expiration time per repetition to acquire six slices could be significantly reduced from 4112 ms without to 2727 ms with parallel imaging. Total acquisition time varied between 8 and 13 min depending on the respiratory frequency.

Improved quantitative dynamic regional oxygen-enhanced pulmonary imaging using image registration

Oxygen-enhanced MR imaging has been demonstrated in a number of recent studies as a potential method to visualize regional ventilation in the lung. A quantitative pixel-by-pixel analysis is hampered by motion and volume change due to breathing. In this study, image registration via active shape modeling is shown to produce significant improvements in the regional analysis of both static and dynamic oxygen-enhanced pulmonary MRI for five normal volunteers. The method enables the calculation of regional change in relaxation rate between breathing air and 100% oxygen, which is proportional to the change in regional oxygen concentration, and regional oxygen wash-in and wash-out time constants. Registration-corrected mapping of these parameters is likely to provide improved information in the regional assessment of a range of lung diseases.