Oxygen-enhanced magnetic resonance ventilation imaging of the human lung at 0.2 and 1.5 T

Review of oxygen-enhanced lung mri: Pulse sequences for image acquisition and T1 measurement

Oxygen-enhanced MR imaging (OE-MRI) is a special proton imaging technique that can be performed without modifying the scanner hardware. Many fundamental studies have been conducted following the initial reporting of this technique in 1996, illustrating the high potential for its clinical application. This review aims to summarise and analyse current pulse sequences and T1 measurement methods for OE-MRI, including fundamental theories, existing pulse sequences applied to OE-MRI acquisition and T1 mapping. Wash-in and wash-out time identify lung function and are sensitive to ventilation; thus, dynamic OE-MRI is also discussed in this review. We compare OE-MRI with the primary competitive technique, hyperpolarised gas MRI. Finally, an overview of lower-field applications of OE-MRI is highlighted, as relatively recent publications demonstrated positive results. Lower-field OE-MRI, which is lower than 1.5 T, could be an alternative modality for detecting lung diseases. This educational review is aimed at researchers who want a quick summary of the steps needed to perform pulmonary OE-MRI with a particular focus on sequence design, settings, and quantification methods.

[Lung imaging in severe asthma]

INTRODUCTION

Asthma is a common disease whose diagnosis does not typically rely on the results of imaging. However, chest CT has gained a key place over the last decade to support the management of patients with difficult to treat and severe asthma.

STATE OF THE ART

Bronchial wall thickening and mild dilatation or narrowing of bronchial lumen are frequently observed on chest CT in people with asthma. Bronchial wall thickening is correlated to the degree of obstruction and to bronchial wall remodeling and inflammation. Diverse conditions which can mimic asthma should be recognized on CT, including endobronchial tumours, interstitial pneumonias, bronchiectasis and bronchiolitis. Ground-glass opacities and consolidation may be related to transient eosinophilic infiltrates, infection or an associated disease (vasculitis, chronic eosinophilic pneumonia). Hyperdense mucous plugging is highly specific for allergic bronchopulmonary aspergillosis.

PERSPECTIVES

Airway morphometry, air trapping and quantitative analysis of ventilatory defects, with CT or MRI, can help to identify different morphological subgroups of patients with different functional or inflammatory characteristics. These imaging tools could emerge as new biomarkers for the evaluation of treatment response.

CONCLUSION

Chest CT is indicated in people with severe asthma to search for additional or alternative diagnoses. Quantitative imaging may contribute to phenotyping this patient group.

3D Oxygen-Enhanced MRI at 3T MR System: Comparison With Thin-Section CT of Quantitative Capability for Pulmonary Functional Loss Assessment and Clinical Stage Classification of COPD in Smokers

BACKGROUND

Oxygen (O₂ )-enhanced MRI is mainly performed by a 2D sequence using 1.5T MR systems but trying to be obtained by a 3D sequence using a 3T MR system.

PURPOSE

To compare the capability of 3D O₂ -enhanced MRI and that of thin-section computed tomography (CT) for pulmonary functional loss assessment and clinical stage classification of chronic obstructive pulmonary disease (COPD) in smokers.

STUDY TYPE

Prospective study.

POPULATION

Fifty six smokers were included.

FIELD STRENGTH/ SEQUENCE

3T, 3D O₂ -enhanced MRIs were performed with a 3D T₁ -weighted fast field echo pulse sequence using the multiple flip angles.

ASSESSMENTS

Smokers were classified into four stages ("Without COPD," "Mild COPD," "Moderate COPD," "Severe or very severe COPD"). Maps of regional changes in T₁ values were generated from O₂ -enhanced MR data. Regions of interest (ROIs) were then placed over the lung on all slices and averaged to determine mean T₁ value change (ΔT₁ ). Quantitative CT used the percentage of low attenuation areas within the entire lung (LAA%).

STATISTICAL TESTS

ΔT₁ and LAA% were correlated with pulmonary functional parameters, and compared for four stages using Tukey's Honestly Significant Difference test. Discrimination analyses were performed and McNemar's test was used for a comparison of the accuracy of the indexes.

RESULTS

There were significantly higher correlations between ΔT₁ and pulmonary functional parameters (-0.83 ≤ r ≤ -0.71, P < 0.05) than between LAA% and the same pulmonary functional parameters (-0.76 ≤ r ≤ -0.69, P < 0.05). ΔT₁ and LAA% of the "Mild COPD" and "Moderate COPD" groups were significantly different from those of the "Severe or Very Severe COPD" group (P < 0.05). Discriminatory accuracy of ΔT₁ (62.5%) and ΔT1 with LAA% (67.9%) was significantly greater than that of LAA% (48.2%, P < 0.05).

DATA CONCLUSION

Compared with thin-section CT, 3D O₂ -enhanced MRI has a similar capability for pulmonary functional assessment but better potential for clinical stage classification in smokers.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 1.

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.

Dynamic oxygen-enhanced MRI of cerebrospinal fluid

Oxygen causes an increase in the longitudinal relaxation rate of tissues through its T1-shortening effect owing to its paramagnetic properties. Due to such effects, MRI has been used to study oxygen-related signal intensity changes in various body parts including cerebrospinal fluid (CSF) space. Oxygen enhancement of CSF has been mainly studied using MRI sequences with relatively longer time resolution such as FLAIR, and T1 value calculation. In this study, fifteen healthy volunteers were scanned using fast advanced spin echo MRI sequence with and without inversion recovery pulse in order to dynamically track oxygen enhancement of CSF. We also focused on the differences of oxygen enhancement at sulcal and ventricular CSF. Our results revealed that CSF signal after administration of oxygen shows rapid signal increase in both sulcal CSF and ventricular CSF on both sequences, with statistically significant predominant increase in sulcal CSF compared with ventricular CSF. CSF is traditionally thought to mainly form from the choroid plexus in the ventricles and is absorbed at the arachnoid villi, however, it is also believed that cerebral arterioles contribute to the production and absorption of CSF, and controversy remains in terms of the precise mechanism. Our results demonstrated rapid oxygen enhancement in sulcal CSF, which may suggest inhaled oxygen may diffuse into sulcal CSF space rapidly probably due to the abundance of pial arterioles on the brain sulci.

Separate pulmonary artery and vein magnetic resonance angiography by use of an arterial spin labeling method

A separate pulmonary vein (PV) is difficult to depict with the commonly used bright-blood magnetic resonance angiography method. Until now, no study has described the depiction of peripheral PVs without the artery. Our purpose in this study was to develop an arterial spin labeling (ASL)-based magnetic resonance angiography sequence to depict the pulmonary artery (PA) and vein separately. We developed such a sequence by using two inversion recovery pulses. The first pulse was non-selective, and the second pulse was selective and was applied to the aorta and heart. All studies were conducted on a 1.5-T clinical magnetic resonance system with six different inversion times for seven healthy volunteers. For evaluation, we categorized the inversion times by using visual scoring. Then, we used the magnitude image to evaluate the PA, and we used the real image to evaluate the PV. For the PA, an inversion time of 300 ms had the lowest score (1.43), and the score changed with increasing times; an inversion time of 1,100 ms had the highest score (3.85). For the PV, an inversion time of 300 ms had the highest score (2.68), and the score decreased with increasing times. The results indicated that the PA and vein could be depicted separately by the use of an ASL-based magnetic resonance angiography method. The optimal inversion times for the PV and artery were 300 and 1,100 ms, respectively.

Lung Parenchymal Signal Intensity in MRI: A Technical Review with Educational Aspirations Regarding Reversible Versus Irreversible Transverse Relaxation Effects in Common Pulse Sequences

Lung parenchyma is challenging to image with proton MRI. The large air space results in ~l/5th as many signal-generating protons compared to other organs. Air/tissue magnetic susceptibility differences lead to strong magnetic field gradients throughout the lungs and to broad frequency distributions, much broader than within other organs. Such distributions have been the subject of experimental and theoretical analyses which may reveal aspects of lung microarchitecture useful for diagnosis. Their most immediate relevance to current imaging practice is to cause rapid signal decays, commonly discussed in terms of short T2* values of 1 ms or lower at typical imaging field strengths. Herein we provide a brief review of previous studies describing and interpreting proton lung spectra. We then link these broad frequency distributions to rapid signal decays, though not necessarily the exponential decays generally used to define T2* values. We examine how these decays influence observed signal intensities and spatial mapping features associated with the most prominent torso imaging sequences, including spoiled gradient and spin echo sequences. Effects of imperfect refocusing pulses on the multiple echo signal decays in single shot fast spin echo (SSFSE) sequences and effects of broad frequency distributions on balanced steady state free precession (bSSFP) sequence signal intensities are also provided. The theoretical analyses are based on the concept of explicitly separating the effects of reversible and irreversible transverse relaxation processes, thus providing a somewhat novel and more general framework from which to estimate lung signal intensity behavior in modern imaging practice.

Pulmonary functional imaging using hyperpolarized noble gas MRI: six years of start-up experience at a single site

RATIONALE AND OBJECTIVES

In this review, we summarize our experience evaluating pulmonary function in 330 different subjects using hyperpolarized noble gas magnetic resonance imaging (MRI) after enrollment and screening of >1100 subjects with and without respiratory disease during the period February 1, 2006, through November 1, 2012.

MATERIALS AND METHODS

We discuss the feasibility of hyperpolarized gas MRI research in a small nonhospital research unit and provide an overview of our experience since we initiated patient-based studies. We also discuss the importance of infrastructure support, collaboration, research trainees, and a large and willing patient population that helped to advance the research and technological deliverables. A summary of patient safety and tolerability, key feasibility, and research milestones is provided, as well as a roadmap for future studies.

RESULTS

Hyperpolarized (3)He and (129)Xe gas MRI is feasible at smaller centers without significant human resources for large and small longitudinal studies by virtue of its excellent patient safety and tolerability, the speed with which images can be acquired and quantitatively analyzed and the high spatial-temporal dynamics of the method that allows for acute and chronic therapy studies.

CONCLUSIONS

The hyperpolarized noble gas MRI community's highly collaborative efforts and motivation to further the development and application of this tool has resulted in a moment-of-opportunity to translate the method clinically to provide an improved understanding of pulmonary disease. There are, as well, new and unprecedented opportunities for the evaluation of disease progression and to help develop the new treatments and interventions critically required for chronic pulmonary 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 functional magnetic resonance imaging for paediatric lung disease

A better understanding of the anatomic structure and physiological function of the lung is fundamental to understanding the pathogenesis of pulmonary disease and how to design and deliver better treatments and measure response to intervention. Magnetic resonance imaging (MRI) with the hyperpolarised noble gases helium-3 ((3)He) and xenon-129 ((129)Xe) provides both structural and functional pulmonary measurements, and because it does not require the use of x-rays or other ionising radiation, offers the potential for intensive serial and longitudinal studies in paediatric patients. These facts are particularly important in the evaluation of chronic lung diseases such as asthma and cystic fibrosis- both of which can be considered paediatric respiratory diseases with unmet therapy needs. This review discusses MRI-based imaging methods with a focus on hyperpolarised gas MRI. We also discuss the strengths and limitations as well as the future work required for clinical translation towards paediatric respiratory disease.

[Oxygen-enhanced functional MR lung imaging]

Current diagnostic tools for the assessment of lung function are limited by global measurements or the need for radioactive tracers. Ideally, these tools should allow quantitative, regional distinct analyses without exposure to radiation. The current paper presents oxygen-enhanced functional MRI for assessment of lung ventilation. First applied in humans in 1996, a considerable amount of experience is now available on 1.5T scanners. The generation of quantitative T1-maps shows a high clinical potential. Low-field MR scanners, which are mostly open-designed, are especially interesting for functional lung imaging. The open design has advantages in respect to patient comfort by lower noise production and easy access to the patients and the costs are lower (no need for helium cooling). Lower signal-to-noise ratios can be overcome by changing the relaxation times. New navigator techniques allow further compensations. This article focuses on the presentation of low-field scanners and the application of T1 and T2(*) maps is described for healthy volunteers and first patients.

Impact of Oxygen Inhalation on the Pulmonary Circulation

PURPOSE

Oxygen-enhanced magnetic resonance (MR)-ventilation imaging of the lung is based on the inhalation of a high concentration of oxygen (hyperoxia). However, the effect of hyperoxia on the pulmonary circulation is not yet fully understood. In this study the impact of hyperoxia on the pulmonary circulation was evaluated.

MATERIALS AND METHODS

Ten healthy volunteers were examined in a 1.5 T MRI system with contrast-enhanced perfusion MRI (saturation recovery 2D turboFLASH) of the lung and phase-contrast flow measurements in the pulmonary trunk. Both measurements were performed breathing room air (RA) and, subsequently, 100% oxygen (15 L/min) (O(2)).

RESULTS

The perfusion measurements showed a significant difference between RA and O(2) for the pulmonary blood flow (181 vs. 257 mL/min/100 mL, P = 0.04) and blood volume (14 vs. 21 mL/100 mL, P = 0.008). The mean transit time of the contrast bolus was not changed (P = 0.4) in the dorsal part of the lung, whereas it was significantly prolonged (P = 0.006) in the central part. The mean heart rate during flow measurements breathing RA (67 +/- 11 beats/min) and O(2) (61 +/- 12 beats/min) were not significantly different (P = 0.055). The average cardiac output (pulmonary trunk) was not significantly lower while breathing O(2) (RA: 5.9 vs. O(2): 5.5 L/min, P = 0.054).

CONCLUSION

Hyperoxia causes a significant increase and redistribution of the pulmonary perfusion, whereas it leads to a not significant decrease in cardiac output. Thus, for MR-perfusion and MR-flow measurements oxygen inhalation should be avoided, if possible. In the context of oxygen-enhanced MR-ventilation imaging of the lung the contribution of this effect needs to be further evaluated.

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.

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.

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.

Proton MRI of lung parenchyma reflects allergen-induced airway remodeling and endotoxin-aroused hyporesponsiveness: A step toward ventilation studies in spontaneously breathing rats

Proton signals from lung parenchyma were detected with the use of a gradient-echo sequence to noninvasively obtain information on pulmonary function in models of airway diseases in rats. Initial measurements carried out in artificially ventilated control rats revealed a highly significant negative correlation between the parenchymal signal and the partial pressure of oxygen (pO2) in the blood, for different amounts of oxygen administered. The magnitude of the signal intensity variations caused by changes in the oxygen concentration was larger than expected solely from the paramagnetic properties of molecular oxygen. Inhomogeneous line-broadening induced by lung inflation may explain the observed signal amplification. Experiments carried out in spontaneously breathing animals challenged with allergen or endotoxin revealed parenchymal signal changes that reflected the oxygenation status of the lungs and were consistent with airway remodeling or hyporesponsiveness. The results suggest that proton MRI of parenchymal tissue is a sensitive tool for probing the functional status of the lung in rat models of respiratory diseases. The method is complementary to the recently described noninvasive assessment by MRI of pulmonary inflammation in small rodents. Overall, these techniques provide invaluable information for profiling anti-inflammatory drugs in models of airway diseases.

Time-resolved three-dimensional pulmonary MR angiography and perfusion imaging with ultrashort repetition time

RATIONALE AND OBJECTIVES

The purpose of this study was to implement ultrafast, multiphase three-dimensional (3D) magnetic resonance (MR) angiography and perfusion imaging after bolus injection of contrast medium to generate preliminary validation of parameters in a pig model and to illustrate potential applications in patients with lung abnormalities.

MATERIALS AND METHODS

Five healthy volunteers, five patients, and three pigs underwent rapid, time-resolved pulmonary MR angiography and perfusion imaging on a 1.5-T MR imager. All patients had undergone correlative computed tomographic or conventional angiography. The pulse sequence was a 3D spin-warp, gradient-echo acquisition with a repetition time of 1.6 msec and an echo time of 0.6 msec. Each 3D acquisition lasted 2-3 seconds, and 8-16 sequential measurements were made in each study. Artificial pulmonary emboli were generated in pigs with gelatin sponge. All patients had diseases of the pulmonary circulation (as confirmed with other studies).

RESULTS

Multiphasic, time-resolved pulmonary parenchymal enhancement was demonstrated in all healthy subjects and animals. All segmental (n = 100) and subsegmental (n = 200) branches were identified in the healthy subjects. Perfusion deficits were clearly demonstrated in all pigs after gelatin embolization. Perfusion defects were identified in two patients with lung disease. Abnormalities of the pulmonary vasculature were clearly identified in the patient group.

CONCLUSION

Dynamic time-resolved 3D pulmonary MR angiography and perfusion imaging is feasible in humans as well as in animals. Induced perfusion deficits are identifiable after artificial embolization in pigs. Combined pulmonary MR angiography and parenchymal (perfusion) imaging may improve evaluation of the pulmonary circulation in a variety of conditions.

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.

Pulmonary diffusing capacity: assessment with oxygen-enhanced lung MR imaging preliminary findings

PURPOSE

To determine differences in the signal intensity (SI) time courses at oxygen-enhanced magnetic resonance (MR) lung imaging in healthy volunteers and patients with pulmonary diseases and to correlate these differences with pulmonary diffusing capacity.

MATERIALS AND METHODS

Seventeen patients with pulmonary diseases and 11 healthy volunteers underwent oxygen-enhanced MR imaging while they breathed room air and 100% oxygen. A turbo spin-echo sequence with global or section-selective inversion pulses was used. For postprocessing, SI slope maps during the breathing of 100% oxygen were calculated. Mean SI slope and SI change values were compared with the diffusing capacity of the lung for carbon monoxide (DLCO).

RESULTS

The SI slopes were significantly different for patients and volunteers (P < or = .05, Mann-Whitney U test). Linear correlations were detected between the DLCO and SI slopes for the section-selective inversion pulse (r(2) = 0.81) and the global inversion pulse (r(2) = 0.74). A lower correlation was associated with the SI change for the section-selective pulse (r(2) = 0.04; global pulse, r(2) = 0.81). Regional differences were seen in the SI slope and SI change maps. These differences correlated with findings on radiographs and computed tomographic scans.

CONCLUSION

The SI slope during the breathing of 100% oxygen allows spatially resolved assessment of the pulmonary diffusion capacity.

Pulmonary ventilation-perfusion MR imaging in clinical patients

The purpose of this study was to evaluate the feasibility of comprehensive magnetic resonance (MR) assessment of pulmonary perfusion and ventilation in patients. Both oxygen-enhanced ventilation MR images and first-pass contrast-enhanced perfusion MR images were obtained in 16 patients with lung diseases, including pulmonary embolism, lung malignancy, and bulla. Inversion recovery single-shot fast spin-echo images were acquired before and after inhalation of 100% oxygen. The overall success rate of perfusion MR imaging and oxygen-enhanced MR imaging was 94% and 80%, respectively. All patients with pulmonary embolism showed regional perfusion deficits without ventilation abnormality on ventilation-perfusion MR imaging. The results of the current study indicate that ventilation-perfusion MR imaging using oxygen inhalation and bolus injection of MR contrast medium is feasible for comprehensive assessment of pulmonary ventilation-perfusion abnormalities in patients with lung diseases.

Low field thoracic MRI--a fast and radiation free routine imaging modality in children

Radiography of the chest is the most frequently performed radiological examination in pediatric imaging. However, it is associated with the application of ionizing radiation. In order to avoid ionizing radiation in children a new and very fast MRI technique has been developed at our center as an alternative to the pediatric chest X-ray. 100 patients who had received a chest X-ray were additionally investigated in a 0.2 T low-field MR-scanner by a modified true FISP sequence with an acquisition time of 3.6-4.6 s for a coronal triple-slice scan. X-ray and MR images were independently evaluated and later compared by two pediatric radiologists. Total investigation times (door-to-door time) for X-ray and MRI were comparable. The signal-to-noise ratio for lung parenchyma was 4.6-7.3. Of 189 pathologic findings 165 were depicted on MR images as well as radiographs, 18 were noted on MRIs only, 6 on X-rays only. Overall kappa was 0.87. True FISP MRI may be a good alternative to conventional chest X-ray. The main advantages are: fast imaging free of ionizing radiation, easy performance, no need for special equipment, optional imaging in all 3 planes, good image quality, and a high diagnostic value.

MR lung imaging at 0.2 T with T1-weighted true FISP: native and oxygen-enhanced

An inversion recovery true fast imaging with steady precession (FISP) pulse sequence was developed to carry out fast imaging of the lungs at 0.2 T. Using this sequence, oxygen-enhanced magnetic resonance (MR) lung imaging was performed on healthy volunteers. The lungs showed signal enhancement (11.7% +/- 3.8%) when breathing 100% oxygen. Using inversion recovery, true FISP at low field may prove promising for MR lung imaging.

Oxygen-enhanced magnetic resonance ventilation imaging of lung

The oxygen-enhanced magnetic resonance (MR) ventilation imaging is a new technique, and the full extent of its physiological significance has not been elucidated. This review article includes background on (1) respiratory physiology; (2) mechanism and optimization of oxygen-enhanced MR imaging technique; (3) recent applications in animal and human models; and (4) merits and demerits of the technique in comparison with hyperpolarized noble gas MR ventilation imaging. Application of oxygen-enhanced MR ventilation imaging to patients with pulmonary diseases has been very limited. However, we believe that further basic studies, as well as 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.

Multiple inversion recovery MR subtraction imaging of human ventilation from inhalation of room air and pure oxygen

The feasibility of MR subtraction imaging of lung ventilation using air against oxygen using a multiple inversion recovery half-Fourier single-shot turbo spin echo (MIR-HASTE) sequence was investigated. Eight healthy, nonsmoking volunteers (3 males, 5 females; from 27 to 48 years of age) were studied on a 1.5 T MR unit. The ventilation image was obtained from the subtraction of the images acquired with the subject inhaling room air and 100% oxygen. By suppressing the signal from subcutaneous fat and thoracic muscle, MIR-HASTE improved the subtraction of signal arising from background tissues. Lung parenchyma, pulmonary veins, descending aorta, spleen, and kidney showed high signal difference, but pulmonary arteries exhibited minimal signal difference. Because of minimal signal change in the pulmonary arteries after inhalation of 100% oxygen, the average signal decreases in the left and right lungs including hilus and periphery amounted to only 19.4+/-4.5 and 20.2+/-3.4%, respectively, compared with regional averages of 23.6+/-5.4 and 24.1+/-3.1% for both lung peripheries alone. Magn Reson Med 43:913-916, 2000.