Noninvasive pulmonary perfusion imaging by STAR-HASTE sequence

MR Perfusion Imaging of the Lung

Lung perfusion assessment is critical for diagnosing and monitoring a variety of respiratory conditions. MRI perfusion provides a radiation-free technique, making it an ideal choice for longitudinal imaging in younger populations. This review focuses on the techniques and applications of MRI perfusion, including contrast-enhanced (CE) MRI and non-CE methods such as arterial spin labeling (ASL), fourier decomposition (FD), and hyperpolarized 129-Xenon (129-Xe) MRI. ASL leverages endogenous water protons as tracers for a non-invasive measure of lung perfusion, while FD offers simultaneous measurements of lung perfusion and ventilation, enabling the generation of ventilation/perfusion mapsHyperpolarized 129-Xe MRI emerges as a novel tool for assessing regional gas exchange in the lungs. Despite the promise of MRI perfusion techniques, challenges persist, including competition with other imaging techniques and the need for additional validation and standardization. In conditions such as cystic fibrosis and lung cancer, MRI has displayed encouraging results, whereas in diseases like chronic obstructive pulmonary disease, further validation remains necessary. In conclusion, while MRI perfusion techniques hold immense potential for a comprehensive, non-invasive assessment of lung function and perfusion, their broader clinical adoption hinges on technological advancements, collaborative research, and rigorous validation.

Lung Magnetic Resonance Imaging: Technical Advancements and Clinical Applications

ABSTRACT

Since lung magnetic resonance imaging (MRI) became clinically available, limited clinical utility has been suggested for applying MRI to lung diseases. Moreover, clinical applications of MRI for patients with lung diseases or thoracic oncology may vary from country to country due to clinical indications, type of health insurance, or number of MR units available. Because of this situation, members of the Fleischner Society and of the Japanese Society for Magnetic Resonance in Medicine have published new reports to provide appropriate clinical indications for lung MRI. This review article presents a brief history of lung MRI in terms of its technical aspects and major clinical indications, such as (1) what is currently available, (2) what is promising but requires further validation or evaluation, and (3) which developments warrant research-based evaluations in preclinical or patient studies. We hope this article will provide Investigative Radiology readers with further knowledge of the current status of lung MRI and will assist them with the application of appropriate protocols in routine clinical practice.

Pulmonary Functional Imaging: Part 1-State-of-the-Art Technical and Physiologic Underpinnings

Over the past few decades, pulmonary imaging technologies have advanced from chest radiography and nuclear medicine methods to high-spatial-resolution or low-dose chest CT and MRI. It is currently possible to identify and measure pulmonary pathologic changes before these are obvious even to patients or depicted on conventional morphologic images. Here, key technological advances are described, including multiparametric CT image processing methods, inhaled hyperpolarized and fluorinated gas MRI, and four-dimensional free-breathing CT and MRI methods to measure regional ventilation, perfusion, gas exchange, and biomechanics. The basic anatomic and physiologic underpinnings of these pulmonary functional imaging techniques are explained. In addition, advances in image analysis and computational and artificial intelligence (machine learning) methods pertinent to functional lung imaging are discussed. The clinical applications of pulmonary functional imaging, including both the opportunities and challenges for clinical translation and deployment, will be discussed in part 2 of this review. Given the technical advances in these sophisticated imaging methods and the wealth of information they can provide, it is anticipated that pulmonary functional imaging will be increasingly used in the care of patients with lung disease. © RSNA, 2021 Online supplemental material is available for this article.

Overview of MRI for pulmonary functional imaging

Morphological evaluation of the lung is important in the clinical evaluation of pulmonary diseases. However, the disease process, especially in its early phases, may primarily result in changes in pulmonary function without changing the pulmonary structure. In such cases, the traditional imaging approaches to pulmonary morphology may not provide sufficient insight into the underlying pathophysiology. Pulmonary imaging community has therefore tried to assess pulmonary diseases and functions utilizing not only nuclear medicine, but also CT and MR imaging with various technical approaches. In this review, we overview state-of-the art MR methods and the future direction of: (1) ventilation imaging, (2) perfusion imaging and (3) biomechanical evaluation for pulmonary functional imaging.

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.

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.

Pulmonary perfusion imaging using MRI: clinical application

BACKGROUND

Lung perfusion is one of the key components of oxygenation. It is hampered in pulmonary arterial diseases and secondary due to parenchymal diseases.

METHODS

Assessment is frequently required during the workup of a patient for either of these disease categories.

RESULTS

This review provides insight into imaging techniques, qualitative and quantitative evaluation, and focuses on clinical application of MR perfusion.

CONCLUSION

The two major techniques, non-contrast-enhanced (arterial spin labeling) and contrast-enhanced perfusion techniques, are discussed.

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.

Dual-energy CT for the assessment of contrast material distribution in the pulmonary parenchyma

OBJECTIVE

The purpose of this study was to assess the feasibility and diagnostic value of dual-energy CT iodine mapping at pulmonary CT angiography.

SUBJECTS AND METHODS

Ninety-three patients underwent CT angiography with the dual-energy technique on a dual-source CT scanner. Postprocessing was used to map iodine in the lung parenchyma on the basis of its spectral behavior, and image quality was assessed by two readers. Iodine distribution patterns were rated as homogeneous, patchy, or circumscribed defects. Conventional CT angiographic images reconstructed from the same data sets were reviewed for the presence and localization of pulmonary embolism, whether embolic occlusion was partial or complete, and the presence of changes in the lung parenchyma. Dual-energy perfusion findings were correlated with the CT angiographic and lung-window CT findings in per-patient and per-segment analyses.

RESULTS

Iodine distribution was homogeneous in 49 patients, of whom CT angiography showed no pulmonary embolism in 46 patients and nonocclusive pulmonary emboli in three patients. Images of 29 patients showed a patchy pattern; 24 of these patients had no pulmonary embolism, and five had nonocclusive pulmonary emboli with solely nonocclusive intravascular clots. Images of 15 patients showed segmental or subsegmental defects; four of these patients had evidence of pulmonary embolism, and 11 had occlusive pulmonary emboli with at least one occlusive clot in the pulmonary vasculature.

CONCLUSION

Dual-energy CT is reliable in the detection of defects in pulmonary parenchymal iodine distribution that correspond to embolic vessel occlusion.

Effects of age on pulmonary perfusion heterogeneity measured by magnetic resonance imaging

Normal aging is associated with a decline in pulmonary function and efficiency of gas exchange, although the effects on the spatial distribution of pulmonary perfusion are poorly understood. We hypothesized that spatial pulmonary perfusion heterogeneity would increase with increasing age. Fifty-six healthy, nonsmoking subjects (ages 21-76 yr) underwent magnetic resonance imaging with arterial spin labeling (ASL) using a Vision 1.5-T whole body scanner (Siemens Medical Systems, Erlangen, Germany). ASL uses a magnetically tagged bolus to generate perfusion maps where signal intensity is proportional to regional pulmonary perfusion. The spatial heterogeneity of pulmonary blood flow was quantified by the relative dispersion (RD = SD/mean, a global index of heterogeneity) of signal intensity for voxels within the right lung and by the fractal dimension (D(s)). There were no significant sex differences for RD (P = 0.81) or D(s) (P = 0.43) when age was considered as a covariate. RD increased significantly with increasing age by approximately 0.1/decade until age 50-59 yr, and there was a significant positive relationship between RD and age (R = 0.48, P < 0.0005) and height (R = 0.39, P < 0.01), but not body mass index (R = 0.07, P = 0.67). Age and height combined in a multiple regression were significantly related to RD (R = 0.66, P < 0.0001). There was no significant relationship between RD and spirometry or arterial oxygen saturation. D(s) was not related to age, height, spirometry, or arterial oxygen saturation. The lack of relationship between age and D(s) argues against an intrinsic alteration in the pulmonary vascular branching with age as being responsible for the observed increase in global spatial perfusion heterogeneity measured by the RD.

Quantification of regional pulmonary blood flow using ASL-FAIRER

Pulsed arterial spin labeling (ASL) techniques have been theoretically and experimentally validated for cerebral blood flow (CBF) quantification. In this study ASL-FAIRER was used to measure regional pulmonary blood flow (rPBF) in seven healthy subjects. Two general ASL strategies were investigated: 1) a single-subtraction approach using one tag-control pair acquisition at an inversion time (TI) matched to the RR-interval, and 2) a multiple-subtraction approach using tag-control pairs acquired at various TIs. The mean rPBF averaged 1.70 +/- 0.38 ml/min/ml when measured with the multiple-subtraction approach, and was approximately 2% less when measured with the single-subtraction method (1.66 +/- 0.24 ml/min/ml). Assuming an average lung density of 0.33 g/ml, this translates into a regional perfusion of approximately 5.5 ml/g/min, which is comparable to other measures of pulmonary perfusion. As with other ASL applications, a key problem with quantitative interpretation of the results is the physical gap between the tagging region and imaged slice. Because of the high pulsatility of PBF, ASL acquisition and data analysis differ significantly between the lung and the brain. The advantages and drawbacks of the single- vs. multiple-subtraction approaches are considered within a theoretical framework tailored to PBF.

Quantitative lung perfusion mapping at 0.2 T using FAIR True-FISP MRI

Perfusion measurements in lung tissue using arterial spin labeling (ASL) techniques are hampered by strong microscopic field gradients induced by susceptibility differences between the alveolar air and the lung parenchyma. A true fast imaging with steady precession (True-FISP) sequence was adapted for applications in flow-sensitive alternating inversion recovery (FAIR) lung perfusion imaging at 0.2 Tesla and 1.5 Tesla. Conditions of microscopic static field distribution were assessed in four healthy volunteers at both field strengths using multiecho gradient-echo sequences. The full width at half maximum (FWHM) values of the frequency distribution for 180-277 Hz at 1.5 Tesla were more than threefold higher compared to 39-109 Hz at 0.2 Tesla. The influence of microscopic field inhomogeneities on the True-FISP signal yield was simulated numerically. Conditions allowed for the development of a FAIR True-FISP sequence for lung perfusion measurement at 0.2 Tesla, whereas at 1.5 Tesla microscopic field inhomogeneities appeared too distinct. Perfusion measurements of lung tissue were performed on eight healthy volunteers and two patients at 0.2 Tesla using the optimized FAIR True-FISP sequence. The average perfusion rates in peripheral lung regions in transverse, sagittal, and coronal slices of the left/right lung were 418/400, 398/416, and 370/368 ml/100 g/min, respectively. This work suggests that FAIR True-FISP sequences can be considered appropriate for noninvasive lung perfusion examinations at low field strength.

Single-shot quantitative perfusion imaging of the human lung

The major drawback to quantitative perfusion imaging using arterial spin labeling (ASL) techniques is the need to acquire two images (tag and control), which must be subtracted in order to obtain a perfusion-weighted image. This can potentially result in misregistration artifacts, especially in lung imaging, due to varying lung inflation levels in different breath-holds. In this work a double inversion recovery (DIR) imaging technique that yields perfusion-weighted images of the human lung in a single shot is presented. This technique ensures the complete suppression of background tissue while it preserves signal from the blood. Furthermore, the perfusion-weighted images and an additional (independent) acquired reference scan can be used to obtain quantitative perfusion information from the lungs.

Impact of lung volume on MR signal intensity changes of the lung parenchyma

PURPOSE

To test the hypothesis that, in magnetic resonance (MR) imaging of healthy individuals, equal relative changes in lung volume cause equal relative changes in MR signal intensity of the lung parenchyma.

MATERIALS AND METHODS

In two experimental runs, 10 volunteers underwent spirometrically monitored MR imaging of the lungs, with MR images acquired at 10 incremental lung volumes ranging from total lung capacity to 10% above residual volume. Average signal intensity, signal variability, and signal intensity integrals were calculated for each volunteer and for each lung volume. The effect of lung volume on signal intensity was quantified using linear regression analysis complemented by the runs test. Slopes and intercepts of regression lines were compared with an analysis of covariance. Slopes of the lines of best fit for lung volumes and signal intensities from the two runs were compared to the slope of the line of identity. Comparisons between the two runs were visualized using Bland and Altman plots.

RESULTS

The slopes of the 10 individual regression lines yielded no significant differences (F = 1.703, P = 0.101; F = 1.321, P = 0.239). The common slopes were -0.556 +/- 0.027 (P = 0.0001) for the first and -0.597 +/- 0.0031 (P = 0.0001) for the second experimental run. Both slopes displayed no significant nonlinearity (P = 0.419 and P = 0.067). There was a strong association between changes in lung volumes (rs = 0.991, P = 0.0001) and changes in signal intensity (rs = 0.889, P = 0.0001) in the two experimental runs. Lines of best fit for lung volume and signal intensities were not significantly different from the slope of the line of identity (P = 0.321 and P = 0.212, respectively).

CONCLUSION

Equal changes in lung volume cause equal changes in MR signal intensity of the lung parenchyma. This linear and reproducible phenomenon could be helpful in comparing pulmonary MR signal intensity between individuals.

MR Evaluation of Pulmonary Blood Flow

Pulmonary blood flow is one of the primary determinants of gas exchange. While a number of methods can be used to evaluation pulmonary perfusion, these have substantial limitations. In this paper, we discuss the use of magnetic resonance imaging techniques for the evaluation of pulmonary blood flow. While these methods are not commonly used at present, they have the potential to contribute greatly to the evaluation of suspected pulmonary vascular disease.

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.

Current Concepts and Advances in Clinical Parallel Magnetic Resonance Imaging

Parallel imaging (PI) is one of the most promising recent advances in MRI technology and has, similar to the introduction of multidetector helical scanning in CT, revolutionized MR imaging. The speed of all conventional MRI methods has been limited by either gradient strength or their switching times. The basic idea in PI is to use some of the spatial information contained in the individual elements of a radiofrequency (RF) receiver coil array to increase imaging speed. These PI techniques are removing some of the previous limitations in speed of MRI scanners and set the basis for accelerated image formation. Initially, PI was motivated by the wish to accelerate image acquisition without reducing the spatial resolution of the image. However, depending on the application, it turned out that PI harbors several other advantages. Among those is the possibility for higher spatial resolution, shorter breath-holds or multiple averaging to diminish motion artifacts, reduced image blurring and geometric distortions, better temporal resolution, and means for navigator correction. This overview focuses on technical aspects, clinical applications, and ongoing research in different areas of the human body. The critical review demonstrates PI's great versatility as well as the current trends to use this unique technique in the majority of clinical scan protocols.

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.

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.

CT and MR in pulmonary embolism: A changing role for nuclear medicine in diagnostic strategy

The goal of this article is to summarize current data on computed tomography (CT) and magnetic resonance (MR) in the diagnosis of acute pulmonary embolism (PE) in relation to the radionuclide ventilation perfusion scan. It is important for the nuclear medicine, CT, and MR communities to develop a shared approach to this disorder. Triage using chest radiographs appears to be a practical method for enhancing both nuclear medicine and CT/MR performance. The realization that there is no clinically available gold standard for the diagnosis of PE suggests that the imaging community should replace impractical and idealistic discussions with more realistic outcome-oriented approaches. A simplified one-step evaluation of the pulmonary arteries and the lower extremity veins for deep venous thrombus can provide a comprehensive examination for PE. CT is currently a more practical diagnostic tool, whereas MR offers a scientific probe for pulmonary physiology including the regional mapping of ventilation-perfusion relationships. Nuclear medicine, CT, and MR thus form an imaging triad for the diagnosis of acute PE.

Floww-weighted MRI of the Lungs with the ECG-gated Half-Fourier FSE Technique: Evaluation of the Effect of the Cardiac Cycle

We investigated temporal MR signal changes in the peripheral lung and proximal pulmonary vessels during the entire cardiac cycle in order to evaluate the characteristics of the diastolic-systolic subtraction method in the lung. In eight healthy volunteers free of lung diseases, changes in the MR signal during one breath-hold were investigated with the multiple ECG-triggered half-Fourier single-shot fast-spin echo (SS-FSE) technique. The signal intensity-time course curve in the lung showed that biphasic signals decreased 20% to 47% at systole and 5% to 33% at mid-diastole, measured against the maximum signals at late diastole. This signal decrease in the peripheral lung was correlated to that in the proximal pulmonary vessels during an entire cardiac cycle (r=0.667 to 1.000). The best visualization of the lung was obtained at late diastole, when the intra-vascular flow in the lung was expected to be stagnant. Compared with the late diastolic SS-FSE images, the late diastolic-systolic subtracted SS-FSE images improved the signal-to-noise ratio in the lung as well as the signal-intensity ratio of the peripheral lung to surrounding tissues. Although the flow-induced signal dephasing in the lung was completely unavoidable and its amount was unpredictable even at late diastole, the diastolic-systolic subtracted SS-FSE images showed the relative differences in flow alteration during the cardiac cycle between the images at diastole and those at systole. The main characteristic of diastolic-systolic subtracted SS-FSE was the enhancement of visibility of cardiac-dependent signal changes in the lung due to the alteration in pulsatile flow.

Quantification of pulmonary perfusion with MR imaging: recent advances

Recent advances in magnetic resonance pulmonary perfusion imaging are reviewed, focusing on magnetic resonance perfusion imaging using gadolinium contrasts agents or spin labeling of blood using naturally flowing spins as the source of intravascular signal. These recent developments in magnetic resonance imaging have made it possible to analyze data quantitatively which holds significant potential for clinical imaging of lung perfusion and opens windows to functional MR imaging of the lung. We believe that fast magnetic resonance functional imaging will play an important role in the assessment of pulmonary function and the pulmonary disease process.

Magnetic resonance imaging of the thorax. Past, present, and future

Magnetic resonance imaging is a valuable modality of extreme flexibility for specific problem-solving capability in the thorax. This article reviews MR applications in the imaging of great vessels, which are currently the most important applications in the thorax; other established applications in the thorax; and pulmonary functional MR imaging.

Magnetic resonance imaging of the thorax. Past, present, and future

Magnetic resonance is a valuable modality of extreme flexibility for specific problem-solving capability in the thorax. This article reviews MR applications in the imaging of great vessels, which are currently the most important applications in the thorax; other established applications in the thorax; and pulmonary functional MR imaging.