MR ventilation-perfusion imaging of human lung using oxygen-enhanced and arterial spin labeling techniques

Novel Thoracic MRI Approaches for the Assessment of Pulmonary Physiology and Inflammation

Excessive pulmonary inflammation can lead to damage of lung tissue, airway remodelling and established structural lung disease. Novel therapeutics that specifically target inflammatory pathways are becoming increasingly common in clinical practice, but there is yet to be a similar stepwise change in pulmonary diagnostic tools. A variety of thoracic magnetic resonance imaging (MRI) tools are currently in development, which may soon fulfil this emerging clinical need for highly sensitive assessments of lung structure and function. Given conventional MRI techniques are poorly suited to lung imaging, alternate strategies have been developed, including the use of inhaled contrast agents, intravenous contrast and specialized lung MR sequences. In this chapter, we discuss technical challenges of performing MRI of the lungs and how they may be overcome. Key thoracic MRI modalities are reviewed, namely, hyperpolarized noble gas MRI, oxygen-enhanced MRI (OE-MRI), ultrashort echo time (UTE) MRI and dynamic contrast-enhanced (DCE) MRI. Finally, we consider potential clinical applications of these techniques including phenotyping of lung disease, evaluation of novel pulmonary therapeutic efficacy and longitudinal assessment of specific patient groups.

Abnormal pulmonary perfusion heterogeneity in patients with Fontan circulation and pulmonary arterial hypertension

KEY POINTS

The distribution of pulmonary perfusion is affected by gravity, vascular branching structure and active regulatory mechanisms, which may be disrupted by cardiopulmonary disease, but this is not well studied, particularly in rare conditions. We evaluated pulmonary perfusion in patients who had undergone Fontan procedure, patients with pulmonary arterial hypertension (PAH) and two groups of controls using a proton magnetic resonance imaging technique, arterial spin labelling to measure perfusion. Heterogeneity was assessed by the relative dispersion (SD/mean) and gravitational gradients. Gravitational gradients were similar between all groups, but heterogeneity was significantly increased in both patient groups compared to controls and persisted after removing contributions from large blood vessels and gravitational gradients. Patients with Fontan physiology and patients with PAH have increased pulmonary perfusion heterogeneity that is not explainable by differences in mean perfusion, gravitational gradients, or large vessel anatomy. This probably reflects vascular remodelling in PAH and possibly in Fontan physiology.

ABSTRACT

Many factors affect the distribution of pulmonary perfusion, which may be disrupted by cardiopulmonary disease, but this is not well studied, particularly in rare conditions. An example is following the Fontan procedure, where pulmonary perfusion is passive, and heterogeneity may be increased because of the underlying pathophysiology leading to Fontan palliation, remodelling, or increased gravitational gradients from low flow. Another is pulmonary arterial hypertension (PAH), where gravitational gradients may be reduced secondary to high pressures, but remodelling may increase perfusion heterogeneity. We evaluated regional pulmonary perfusion in Fontan patients (n = 5), healthy young controls (Fontan control, n = 5), patients with PAH (n = 6) and healthy older controls (PAH control) using proton magnetic resonance imaging. Regional perfusion was measured using arterial spin labelling. Heterogeneity was assessed by the relative dispersion (SD/mean) and gravitational gradients. Mean perfusion was similar (Fontan = 2.50 ± 1.02 ml min^-1  ml^-1 ; Fontan control = 3.09 ± 0.58, PAH = 3.63 ± 1.95; PAH control = 3.98 ± 0.91, P = 0.26), and the slopes of gravitational gradients were not different (Fontan = -0.23 ± 0.09 ml min^-1  ml^-1  cm^-1 ; Fontan control = -0.29 ± 0.23, PAH = -0.27 ± 0.09, PAH control = -0.25 ± 0.18, P = 0.91) between groups. Perfusion relative dispersion was greater in both Fontan and PAH than controls (Fontan = 1.46 ± 0.18; Fontan control = 0.99 ± 0.21, P = 0.005; PAH = 1.22 ± 0.27, PAH control = 0.91 ± 0.12, P = 0.02) but similar between patient groups (P = 0.13). These findings persisted after removing contributions from large blood vessels and gravitational gradients (all P < 0.05). We conclude that patients with Fontan physiology and PAH have increased pulmonary perfusion heterogeneity that is not explained by differences in mean perfusion, gravitational gradients, or large vessel anatomy. This probably reflects the effects of remodelling in PAH and possibly in Fontan physiology.

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.

Proton MRI of the Lung: How to Tame Scarce Protons and Fast Signal Decay

Pulmonary proton MRI techniques offer the unique possibility of assessing lung function and structure without the requirement for hyperpolarization or dedicated hardware, which is mandatory for multinuclear acquisition. Five popular approaches are presented and discussed in this review: 1) oxygen enhanced (OE)-MRI; 2) arterial spin labeling (ASL); 3) Fourier decomposition (FD) MRI and other related methods including self-gated noncontrast-enhanced functional lung (SENCEFUL) MR and phase-resolved functional lung (PREFUL) imaging; 4) dynamic contrast-enhanced (DCE) MRI; and 5) ultrashort TE (UTE) MRI. While DCE MRI is the most established and well-studied perfusion measurement, FD MRI offers a free-breathing test without any contrast agent and is predestined for application in patients with renal failure or with low compliance. Additionally, FD MRI and related methods like PREFUL and SENCEFUL can act as an ionizing radiation-free V/Q scan, since ventilation and perfusion information is acquired simultaneously during one scan. For OE-MRI, different concentrations of oxygen are applied via a facemask to assess the regional change in T₁ , which is caused by the paramagnetic property of oxygen. Since this change is governed by a combination of ventilation, diffusion, and perfusion, a compound functional measurement can be achieved with OE-MRI. The known problem of fast T₂ * decay of the lung parenchyma leading to a low signal-to-noise ratio is bypassed by the UTE acquisition strategy. Computed tomography (CT)-like images allow the assessment of lung structure with high spatial resolution without ionizing radiation. Despite these different branches of proton MRI, common trends are evident among pulmonary proton MRI: 1) free-breathing acquisition with self-gating; 2) application of UTE to preserve a stronger parenchymal signal; and 3) transition from 2D to 3D acquisition. On that note, there is a visible convergence of the different methods and it is not difficult to imagine that future methods will combine different aspects of the presented methods.

Magnetic resonance elastography of the lungs: A repeatability and reproducibility study

Lung diseases are one of the leading causes of death worldwide, from which four million people die annually. Lung diseases are associated with changes in the mechanical properties of the lungs. Several studies have shown the feasibility of using magnetic resonance elastography (MRE) to quantify the lungs' shear stiffness. The aim of this study is to investigate the reproducibility and repeatability of lung MRE, and its shear stiffness measurements, obtained using a modified spin echo-echo planar imaging (SE-EPI) MRE sequence. In this study, 21 healthy volunteers were scanned twice by repositioning the volunteers to image right lung both at residual volume (RV) and total lung capacity (TLC) to assess the reproducibility of lung shear stiffness measurements. Additionally, 19 out of the 21 volunteers were scanned immediately without moving the volunteers to test the repeatability of the modified SE-EPI MRE sequence. A paired t-test was performed to determine the significant difference between stiffness measurements obtained at RV and TLC. Concordance correlation and Bland-Altman's analysis were performed to determine the reproducibility and repeatability of the SE-EPI MRE-derived shear stiffness measurements. The SE-EPI MRE sequence is highly repeatable with a concordance correlation coefficient (CCC) of 0.95 at RV and 0.96 at TLC. Similarly, the stiffness measurements obtained across all volunteers were highly reproducible with a CCC of 0.95 at RV and 0.92 at TLC. The mean shear stiffness of the lung at RV was 0.93 ± 0.22 kPa and at TLC was 1.41 ± 0.41 kPa. TLC showed a significantly higher mean shear stiffness (P = 0.0004) compared with RV. Lung MRE stiffness measurements obtained using the SE-EPI sequence were reproducible and repeatable, both at RV and TLC. Lung shear stiffness changes across respiratory cycle with significantly higher stiffness at TLC than RV.

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.

Arterial spin labeling-fast imaging with steady-state free precession (ASL-FISP): a rapid and quantitative perfusion technique for high-field MRI

Arterial spin labeling (ASL) is a valuable non-contrast perfusion MRI technique with numerous clinical applications. Many previous ASL MRI studies have utilized either echo-planar imaging (EPI) or true fast imaging with steady-state free precession (true FISP) readouts, which are prone to off-resonance artifacts on high-field MRI scanners. We have developed a rapid ASL-FISP MRI acquisition for high-field preclinical MRI scanners providing perfusion-weighted images with little or no artifacts in less than 2 s. In this initial implementation, a flow-sensitive alternating inversion recovery (FAIR) ASL preparation was combined with a rapid, centrically encoded FISP readout. Validation studies on healthy C57/BL6 mice provided consistent estimation of in vivo mouse brain perfusion at 7 and 9.4 T (249 ± 38 and 241 ± 17 mL/min/100 g, respectively). The utility of this method was further demonstrated in the detection of significant perfusion deficits in a C57/BL6 mouse model of ischemic stroke. Reasonable kidney perfusion estimates were also obtained for a healthy C57/BL6 mouse exhibiting differential perfusion in the renal cortex and medulla. Overall, the ASL-FISP technique provides a rapid and quantitative in vivo assessment of tissue perfusion for high-field MRI scanners with minimal image artifacts.

Assessment of regional lung function with multivolume (1)H MR imaging in health and obstructive lung disease: comparison with (3)He MR imaging

PURPOSE

To introduce a method based on multivolume proton (hydrogen [(1)H]) magnetic resonance (MR) imaging for the regional assessment of lung ventilatory function, investigating its use in healthy volunteers and patients with obstructive lung disease and comparing the outcome with the outcome of the research standard helium 3 ((3)He) MR imaging.

MATERIALS AND METHODS

The institutional review board approved the HIPAA-compliant protocol, and informed written consent was obtained from each subject. Twenty-six subjects, including healthy volunteers (n = 6) and patients with severe asthma (n = 11) and mild (n = 6) and severe (n = 3) emphysema, were imaged with a 1.5-T whole-body MR unit at four lung volumes (residual volume [ RV residual volume ], functional residual capacity [ FRC functional residual capacity ], 1 L above FRC functional residual capacity [ FRC+1 L 1 L above FRC ], total lung capacity [ TLC total lung capacity ]) with breath holds of 10-11 seconds, by using volumetric interpolated breath-hold examination. Each pair of volumes were registered, resulting in maps of (1)H signal change between the two lung volumes. (3)He MR imaging was performed at FRC+1 L 1 L above FRC by using a two-dimensional gradient-echo sequence. (1)H signal change and (3)He signal were measured and compared in corresponding regions of interest selected in ventral, intermediate, and dorsal areas.

RESULTS

In all volunteers and patients combined, proton signal difference between TLC total lung capacity and RV residual volume correlated positively with (3)He signal (correlation coefficient R(2) = 0.64, P < .001). Lower (P < .001) but positive correlation results from (1)H signal difference between FRC functional residual capacity and FRC+1 L 1 L above FRC (R(2) = 0.44, P < .001). In healthy volunteers, (1)H signal changes show a higher median and interquartile range compared with patients with obstructive disease and significant differences between nondependent and dependent regions.

CONCLUSION

Findings in this study demonstrate that multivolume (1)H MR imaging, without contrast material, can be used as a biomarker for regional ventilation, both in healthy volunteers and patients with obstructive lung disease.

Magnetic resonance imaging of pediatric lung parenchyma, airways, vasculature, ventilation, and perfusion: state of the art

Magnetic resonance (MR) imaging is a noninvasive imaging modality, particularly attractive for pediatric patients given its lack of ionizing radiation. Despite many advantages, the physical properties of the lung (inherent low signal-to-noise ratio, magnetic susceptibility differences at lung-air interfaces, and respiratory and cardiac motion) have posed technical challenges that have limited the use of MR imaging in the evaluation of thoracic disease in the past. However, recent advances in MR imaging techniques have overcome many of these challenges. This article discusses these advances in MR imaging techniques and their potential role in the evaluation of thoracic disorders in pediatric patients.

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.

Assessment of pulmonary hypertension what CT and MRI can provide

RATIONALES AND OBJECTIVES

Pulmonary hypertension (PH) is a life-threatening condition, characterized by elevated pulmonary arterial pressure, which is confirmed based on invasive right heart catheterization (RHC). Noninvasive examinations may support diagnosis of PH before proceeding to RHC and play an important role in management and treatment of the disease. Although echocardiography is considered a standard tool in diagnosis, recent advances have made computed tomography (CT) and magnetic resonance (MR) imaging promising tools, which may provide morphologic and functional information. In this article, we review image-based assessment of PH with a focus on CT and MR imaging.

CONCLUSIONS

CT may provide useful morphologic information for depicting PH and seeking for underlying diseases. With the accumulated technological advancement, CT and MRI may provide practical tools for not only morphologic but also functional assessment of patients with PH.

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.

Pulmonary perfusion heterogeneity is increased by sustained, heavy exercise in humans

Exercise presents a considerable stress to the pulmonary system and ventilation-perfusion (Va/Q) heterogeneity increases with exercise, affecting the efficiency of gas exchange. In particular, prolonged heavy exercise and maximal exercise are known to increase Va/Q 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, Vo(2max) = 61 ml. kg(-1).min(-1)) with exercising Va/Q heterogeneity previously characterized by the multiple inert gas elimination technique (MIGET), performed 45 min of cycle exercise at approximately 70% Vo(2max). 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 =or <0.01) but heterogeneity did not (all P >or= 0.18), likely because of perfusion redistribution and vascular recruitment. Density-normalized perfusion was unchanged in the coronal plane postexercise (P = 0.66), however, perfusion heterogeneity was significantly increased as measured by the relative dispersion [RD, pre 0.62(0.07), post 0.82(0.21), P < 0.0001] and geometric standard deviation [GSD, pre 1.74(0.14), post 2.30(0.56), P < 0.005]. These changes in heterogeneity were related to the exercise-induced changes of the log standard deviation of the ventilation distribution, an MIGET index of Va/Q heterogeneity (RD R(2) = 0.68, P < 0.05, GSD, R(2) = 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 Va/Q heterogeneity.

Pulmonary MR angiography techniques and applications

This article discusses the role of magnetic resonance angiography (MRA) in evaluating the pulmonary arterial system. For depiction of pulmonary arterial anatomy and morphology, MRA techniques are compared with CT angiography and digital subtraction x-ray angiography. Perfusion, flow, and function are emphasized, as the integrated MR examination offers a comprehensive assessment of vascular morphology and function. Advances in MR technology that improve spatial and temporal resolution and compensate for potential artifacts are reviewed as they pertain to pulmonary MRA. Current and emerging gadolinium contrast-enhanced and non-contrast-enhanced MRA techniques are discussed. The role of pulmonary MRA, clinical protocols, imaging findings, and interpretation pitfalls are reviewed for clinical indications.

Nonenhanced MR angiography

While nonenhanced magnetic resonance (MR) angiographic methods have been available since the earliest days of MR imaging, prolonged acquisition times and image artifacts have generally limited their use in favor of gadolinium-enhanced MR angiographic techniques. However, the combination of recent technical advances and new concerns about the safety of gadolinium-based contrast agents has spurred a resurgence of interest in methods that do not require exogenous contrast material. After a review of basic considerations in vascular imaging, the established methods for nonenhanced MR angiographic techniques, such as time of flight and phase contrast, are considered and their advantages and disadvantages are discussed. This article then focuses on new techniques that are becoming commercially available, such as electrocardiographically gated partial-Fourier fast spin-echo methods and balanced steady-state free precession imaging both with and without arterial spin labeling. Challenges facing these methods and possible solutions are considered. Since different imaging techniques rely on different mechanisms of image contrast, recommendations are offered for which strategies may work best for specific angiographic applications. Developments on the horizon include techniques that provide time-resolved imaging for assessment of flow dynamics by using nonenhanced approaches.

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.

Oxygen-enhanced MR Imaging: Correlation with Postsurgical Lung Function in Patients with Lung Cancer

PURPOSE

To prospectively determine if lung function as assessed with oxygen-enhanced magnetic resonance (MR) imaging correlates with postsurgical lung function in patients with lung cancer, as compared with quantitative and qualitative findings of computed tomography (CT) and scintigraphy.

MATERIALS AND METHODS

Study received institutional review board approval, and informed patient consent was obtained. Thirty consecutive patients (16 men and 14 women, aged 44-81 years; mean age, 65 years) considered candidates for lung resection underwent oxygen-enhanced MR imaging, CT, perfusion scintigraphy, and measurement of forced expiratory volume in 1 second (FEV1). A respiratory-synchronized inversion-recovery half-Fourier single-shot turbo spin-echo MR sequence was used for data acquisition. Correlation of postsurgical lung function (postsurgical FEV1) as determined with oxygen-enhanced MR imaging (FEV1MR), quantitative assessment with CT (FEV1Quant), qualitative assessment with CT (FEV1Qual), and perfusion scintigraphy (FEV1PS) was conducted with actual postsurgical FEV1, and the limits of agreement of each were determined with Bland-Altman analysis.

RESULTS

Correlation between postsurgical FEV1MR and actual postsurgical FEV1 values was excellent (r2= 0.81, P < .001); it was better than that of FEV1Qual (r2= 0.76) and FEV1PS (r2= 0.77) and similar to that of FEV1Quant (r2= 0.81) values. The limits of agreement of FEV1MR were between -9.9% and 10.9%.

CONCLUSION

Oxygen-enhanced MR imaging can be used to predict posturgical lung function in patients with lung cancer, similar to quantitative CT.

Pulmonary Embolism: Comprehensive Evaluation with MR Ventilation and Perfusion Scanning with Hyperpolarized Helium-3, Arterial Spin Tagging, and Contrast-enhanced MRA

PURPOSE

Development of a comprehensive magnetic resonance (MR) examination consisting of MR angiography (MRA) and MR ventilation and perfusion (MR V/Q) scan for the detection of pulmonary emboli (PE) and assessment of the technique in a rabbit model.

MATERIALS AND METHODS

Reversible PE was induced by inflating a non-detachable silicon balloon in the left pulmonary artery of five New Zealand White rabbits. MR V/Q scans were obtained prior to, during, and after balloon deflation. MRA was performed during balloon inflation. MR ventilation imaging was performed after the inhalation of hyperpolarized helium-3. MR perfusion imaging was performed with Flow-sensitive Alternating Inversion Recovery with an Extra Radiofrequency pulse technique (FAIRER). High-resolution contrast-enhanced MR pulmonary angiography was used to confirm the occlusion of the pulmonary artery. All imaging was performed on a 1.5-T whole body scanner with broadband capabilities.

RESULTS

High-resolution ventilation images of the lungs were obtained. No ventilation defects were detected before, during, or after resolution of simulated PE. FAIRER imaging allowed visualization of pulmonary perfusion. No perfusion defects were detected prior to balloon inflation. During balloon inflation (PE), there was decreased perfusion in the left lower lobe. After reversal of the PE, there was improved perfusion to the left lower lobe. In analogy to nuclear medicine techniques, acute PE produced a mismatched defect in the MR V/Q scan. MRA verified the occlusive filling defect in the left pulmonary artery.

CONCLUSION

High-resolution MRA and MR V/Q imaging of the lung is feasible and allows comprehensive assessment of pulmonary embolism in one imaging session.

Pathophysiology of airflow limitation in chronic obstructive pulmonary disease

The airflow limitation that defines chronic obstructive pulmonary disease (COPD) is the result of a prolonged time constant for lung emptying, caused by increased resistance of the small conducting airways and increased compliance of the lung as a result of emphysematous destruction. These lesions are associated with a chronic innate and adaptive inflammatory immune response of the host to a lifetime exposure to inhaled toxic gases and particles. Processes contributing to obstruction in the small conducting airways include disruption of the epithelial barrier, interference with mucociliary clearance apparatus that results in accumulation of inflammatory mucous exudates in the small airway lumen, infiltration of the airway walls by inflammatory cells, and deposition of connective tissue in the airway wall. This remodelling and repair thickens the airway walls, reduces lumen calibre, and restricts the normal increase in calibre produced by lung inflation. Emphysematous lung destruction is associated with an infiltration of the same type of inflammatory cells found in the airways. The centrilobular pattern of emphysematous destruction is most closely associated with cigarette smoking, and although it is initially focused on respiratory bronchioles, separate lesions coalesce to destroy large volumes of lung tissue. The panacinar pattern of emphysema is characterised by a more even involvement of the acinus and is associated with alpha1 antitrypsin deficiency. The technology needed to diagnose and quantitate the individual small airway and emphysema phenotypes present in people with COPD is being developed, and should prove helpful in the assessment of therapeutic interventions designed to modify the progress of either phenotype.

Perfusion imaging using arterial spin labeling

Arterial spin labeling is a magnetic resonance method for the measurement of cerebral blood flow. In its simplest form, the perfusion contrast in the images gathered by this technique comes from the subtraction of two successively acquired images: one with, and one without, proximal labeling of arterial water spins after a small delay time. Over the last decade, the method has moved from the experimental laboratory to the clinical environment. Furthermore, numerous improvements, ranging from new pulse sequence implementations to extensive theoretical studies, have broadened its reach and extended its potential applications. In this review, the multiple facets of this powerful yet difficult technique are discussed. Different implementations are compared, the theoretical background is summarized, and potential applications of various implementations in research as well as in the daily clinical routine are proposed. Finally, a summary of the new developments and emerging techniques in this field is provided.

Independent cerebral vasoconstrictive effects of hyperoxia and accompanying arterial hypocapnia at 1 ATA

Breathing 100% O2 at 1 atmosphere absolute (ATA) is known to be associated with a decrease in cerebral blood flow (CBF). It is also accompanied by a fall in arterial Pco2 leading to uncertainty as to whether the cerebral vasoconstriction is totally or only in part caused by arterial hypocapnia. We tested the hypothesis that the increase in arterial Po2 while O2 was breathed at 1.0 ATA decreases CBF independently of a concurrent fall in arterial Pco2. CBF was measured in seven healthy men aged 21-62 yr by using noninvasive continuous arterial spin-labeled-perfusion MRI. The tracer in this technique, magnetically labeled protons in blood, has a half-life of seconds, allowing repetitive measurements over short time frames without contamination. CBF and arterial blood gases were measured while breathing air, 100% O2, and 4 and 6% CO2 in air and O2 backgrounds. Arterial Po2 increased from 91.7 +/- 6.8 Torr in air to 576.7 +/- 18.9 Torr in O2. Arterial Pco2 fell from 43.3 +/- 1.8 Torr in air to 40.2 +/- 3.3 Torr in O2. CBF-arterial Pco2 response curves for the air and hyperoxic runs were nearly parallel and separated by a distance representing a 28.7-32.6% decrement in CBF. Regression analysis confirmed the independent cerebral vasoconstrictive effect of increased arterial Po2. The present results also demonstrate that the magnitude of this effect at 1.0 ATA is greater than previously measured.

MRI for the diagnosis of pulmonary embolism

Pulmonary embolism (PE) is one of the most frequently encountered clinical emergencies. The diagnosis often involves multiple diagnostic tests, which need to be carried out rapidly to assist in the safe management of the patient. Recent strides in computed tomography (CT) have made big improvements in patient management and efficiency of diagnostic imaging. This review article describes the developments in magnetic resonance (MR) techniques for the diagnosis of acute PE. Techniques include MR angiography (MRA) and thrombus imaging for direct clot visualization, perfusion MR, and combined perfusion-ventilation MR. As will be demonstrated, some of these techniques are now entering the clinical arena, and it is anticipated that MR imaging (MRI) will have an increasing role in the initial diagnosis and follow-up of patients with acute PE.

Clinical oxygen-enhanced magnetic resonance imaging of the lung

SUMMARY

Oxygen-enhanced magnetic resonance (MR) ventilation imaging is a new technique, and the full extent of its physiologic significance has not been elucidated. This review article includes (1) theory of oxygen enhancement; (2) respiratory physiology; (3) oxygen-enhanced MR imaging (MRI) sequence design; (4) basic study of oxygen-enhanced MRI in animal models and humans; (5) clinical study of oxygen-enhanced MRI; and (6) merits and demerits of the technique in comparison with hyperpolarized noble gas MR ventilation imaging. Oxygen-enhanced MRI provides not only ventilation-related information but also respiration-related information. Although application of oxygen-enhanced MR ventilation imaging to patients with pulmonary diseases has been limited, oxygen-enhanced MRI offers the possibility of demonstrating regional pulmonary function and substituting for nuclear medicine ventilation-perfusion study, when combined with MR perfusion imaging. We believe that further basic studies and clinical applications of this new technique will define the real significance of oxygen-enhanced MR ventilation imaging in the future of pulmonary functional imaging and its usefulness for diagnostic radiology.

Recent advances in magnetic resonance perfusion imaging of the lung

Magnetic resonance imaging has been relatively underused for clinical application in the lung; however, developments in magnetic resonance perfusion imaging using contrast agents and spin labeling techniques have shown significant potential for clinical application in lung perfusion. This article reviews the recent publications on magnetic resonance pulmonary perfusion.

Pulmonary embolism: comparison of angiography with spiral computed tomography, magnetic resonance angiography, and real-time magnetic resonance imaging

In the last decade, spiral computed tomography (CT) and magnetic resonance (MR) angiography (MRA) have become a viable alternative to conventional angiography in the diagnosis of acute pulmonary embolism. However, patients with dyspnea are often unable to hold their breath for a longer time and thus image degradation is frequently observed. Consequently, an imaging sequence that allows free breathing is desirable. The aim of this animal study was to compare contrast-enhanced spiral CT, MRA and a real-time MR sequence, the latter without breath-hold, with pulmonary angiography as reference gold standard. Nine pigs with artificially induced pulmonary embolism underwent this multimodality comparison. All images were independently evaluated for the presence of pulmonary emboli by two reviewers. Forty-three filling defects were detected by conventional angiography on lobar and segmental levels. Sensitivity of CT images was 72.1 and 69.8% for Readers 1 and 2, respectively, and sensitivity of MRA images was 79.1 and 81.4%. With real-time MR imaging, however, the detection rate was 97.7% for both readers. We conclude that, under experimental conditions, real-time MR imaging without the use of radiation or iodinated contrast material is comparable with angiography in the detection of pulmonary emboli.

Computing oxygen-enhanced ventilation maps using correlation analysis

Correlation maps of oxygen-enhanced ventilation were obtained in nine healthy volunteers using complete and selected image series. The complete series included all images acquired with the subjects alternately inhaling room air and 100% oxygen. The selected series were the subsets of the complete series and included only co-registered images that showed matched diaphragmatic position at maximal expiration. Cross-correlation was computed between the time response function of each pixel and the input function representing the alternation between periods of room air and 100% oxygen inhalation. The confidence level for the correlation analysis was set to 0.01. Pulmonary parenchymal anatomy was consistently reproduced throughout the lung, even in anterior slices where published data have reported correlation problems. The overall average correlation coefficient was 0.66 +/- 0.07 for the complete series and 0.75 +/- 0.08 for the selected series. It was concluded that correlation analysis could be used to reconstruct qualitative oxygen-enhanced ventilation maps.

Comparison of first-pass Gd-DOTA and FAIRER MR perfusion imaging in a rabbit model of pulmonary embolism

PURPOSE

To compare the sensitivity of contrast-enhanced magnetic resonance imaging (MRI) and arterial spin labeling to perfusion deficits in the lung.

MATERIALS AND METHODS

A rabbit model of pulmonary embolism was imaged with both flow-sensitive alternating inversion recovery with an extra radiofrequency pulse (FAIRER) arterial spin labeling and Gd-DOTA enhanced MRI. The signal-to-noise ratio (SNR) was measured in the area of the perfusion deficit and the normal lung for both techniques.

RESULTS

The defect was readily visible in all images. The normal lung had an average of 3.8 +/- 1.2 times the SNR of the unperfused lung with the arterial spin labeling technique, and approximately 13.7 +/- 3.3 times the SNR with the contrast-enhanced technique.

CONCLUSION

Gd-DOTA enhanced MRI provides higher SNR in pulmonary perfusion imaging; however, arterial spin labeling is also adequate and may be used when repeated studies are indicated.

Ventilation-perfusion ratio of signal intensity in human lung using oxygen-enhanced and arterial spin labeling techniques

This study investigates the distribution of ventilation-perfusion (V/Q) signal intensity (SI) ratios using oxygen-enhanced and arterial spin labeling (ASL) techniques in the lungs of 10 healthy volunteers. Ventilation and perfusion images were simultaneously acquired using the flow-sensitive alternating inversion recovery (FAIR) method as volunteers alternately inhaled room air and 100% oxygen. Images of the T(1) distribution were calculated for five volunteers for both selective (T(1f)) and nonselective (T(1)) inversion. The average T(1) was 1360 ms +/- 116 ms, and the average T(1f) was 1012 ms +/- 112 ms, yielding a difference that is statistically significant (P < 0.002). Excluding large pulmonary vessels, the average V/Q SI ratios were 0.355 +/- 0.073 for the left lung and 0.371 +/- 0.093 for the right lung, which are in agreement with the theoretical V/Q SI ratio. Plots of the V/Q SI ratio are similar to the logarithmic normal distribution obtained by multiple inert gas elimination techniques, with a range of ratios matching ventilation and perfusion. This MRI V/Q technique is completely noninvasive and does not involve ionized radiation. A limitation of this method is the nonsimultaneous acquisition of perfusion and ventilation data, with oxygen administered only for the ventilation data.

Influence of oxygen flow rate on signal and T(1) changes in oxygen-enhanced ventilation imaging

PURPOSE

To investigate the optimal oxygen flow rate for oxygen-enhanced MR ventilation imaging.

MATERIALS AND METHODS

Using a cardiac-triggered nonselective inversion recovery (IR) half Fourier single-shot fast spin echo sequence, series of images were acquired with the subject alternately inhaling room air and 100% oxygen. Oxygen flow rates of 5 L/min, 10 L/min, 15 L/min, 20 L/min, and 25 L/min were studied, and signal intensity from the oxygen-enhanced ventilation images and T(1) of the lung were measured.

RESULTS

The average signal intensity was 63.0 +/- 21.0 for 5 L/min, 98.7 +/- 26.8 for 10 L/min, 133.8 +/- 20.0 for 15 L/min, 138.7 +/- 19.7 for 20 L/min, and 139.2 +/- 37.9 for 25 L/min. The average T(1)'s of the lung were 1399 msec +/- 130 msec for room air, 1314 msec +/- 101 msec for 5 L/min, 1276 msec +/- 105 msec for 10 L/min, 1207 msec +/- 71 msec for 15 L/min, 1206 msec +/- 90 msec for 20 L/min, and 1207 msec +/- 42 msec for 25 L/min.

CONCLUSION

The optimal flow rate is 15 L/min for oxygen-enhanced ventilation imaging.