Regional lung functional impairment in acute airway obstruction and pulmonary embolic dog models assessed with gadolinium-based aerosol ventilation and perfusion magnetic resonance imaging

Magnetic Resonance Imaging of Aerosol Deposition

Nuclear magnetic resonance imaging (MRI) uses non-ionizing radiation and offers a host of contrast mechanisms with the potential to quantify aerosol deposition. This chapter introduces the physics of MRI, its use in lung imaging, and more specifically, the methods that are used for the detection of regional distributions of inhaled particles. The most common implementation of MRI is based on imaging of hydrogen atoms (1H) in water. The regional deposition of aerosol particles can be measured by the perturbation of the acquired 1H signals via labeling of the aerosol with contrast agents. Existing in vitro human and in vivo animal model measurements of regional aerosol deposition in the respiratory tract are described, demonstrating the capability of MRI to assess aerosol deposition in the lung.

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.

Hyperpolarized gas MR Imaging of the lung: current status as a research tool

Hyperpolarized gas magnetic resonance imaging has been explored extensively as a promising tool for the quantitative evaluation of regional pulmonary pathophysiology. This noninvasive technique is capable of providing both structural information down to the level of the alveolar microstructure and functional information, such as dynamic ventilation, intrapulmonary partial pressure of oxygen, and alveolar surface area. This study reviews the role of hyperpolarized 3-helium and 129-xenon magnetic resonance imaging in this research.

Aerosol delivery in ventilated newborn pigs: an MRI evaluation

Pulmonary deposition of inhaled drugs in ventilated neonates has not been studied in vivo. The objective of this study was to evaluate pulmonary delivery of gadopentetate dimeglumine (Gd-DTPA) following nebulization in ventilated piglets using magnetic resonance imaging. Seven ventilated piglets (5 +/- 2 d old, weight 1.8 +/- 0.5 kg) were scanned in the Bruker/Siemens 4T magnetic resonance scanner using T1 weighted spin-echo sequence. Aerosols of Gd-DTPA were generated continuously using the MiniHeart jet nebulizer. Breath-hold coronal images were obtained before and every 10 min during aerosolized Gd-DTPA for 90 min. Signal intensity (SI) changes over the lungs, kidneys, liver, skeletal muscle, and heart were evaluated. A significant increase in SI was observed in the lungs, kidney, and liver at 10, 20, and 40 min respectively after start of aerosol. At the end of 90 min, the SI increased by 95%, 101%, and 426% over the right lung, left lung, and kidney, respectively. A much smaller increase in SI was observed over the liver. In conclusion, we have demonstrated effective pulmonary aerosol delivery within 10 min of contrast nebulization in ventilated piglets. Contrast visualization in the kidneys within 20 min of aerosol initiation reflects alveolar absorption, glomerular filtration and renal concentration.

Advances in magnetic resonance imaging of lung physiology

This review presents an overview of some recent magnetic resonance imaging (MRI) techniques for measuring aspects of local physiology in the lung. MRI is noninvasive, relatively high resolution, and does not expose subjects to ionizing radiation. Conventional MRI of the lung suffers from low signal intensity caused by the low proton density and the large degree of microscopic field inhomogeneity that degrades the magnetic resonance signal and interferes with image acquisition. However, in recent years, there have been rapid advances in both hardware and software design, allowing these difficulties to be minimized. This review focuses on some newer techniques that measure regional perfusion, ventilation, gas diffusion, ventilation-to-perfusion ratio, partial pressure of oxygen, and lung water. These techniques include contrast-enhanced and arterial spin-labeling techniques for measuring perfusion, hyperpolarized gas techniques for measuring regional ventilation, and apparent diffusion coefficient and multiecho and gradient echo techniques for measuring proton density and lung water. Some of the major advantages and disadvantages of each technique are discussed. In addition, some of the physiological issues associated with making measurements are discussed, along with strategies for understanding large and complex data sets.

Accuracy and feasibility of dynamic contrast-enhanced 3D MR imaging in the assessment of lung perfusion: comparison with Tc-99 MAA perfusion scintigraphy

AIM

The aim of this study was to correlate findings of perfusion magnetic resonance imaging (MRI) and perfusion scintigraphy in cases where there was a suspicion of abnormal pulmonary vasculature, and to evaluate the usefulness of MRI in the detection of perfusion deficits of the lung.

METHODS

In all, 17 patients with suspected abnormality of the pulmonary vasculature underwent dynamic contrast-enhanced MRI. T1-weighted 3D fast-field echo pulse sequences were obtained (TR/TE 3.3/1.58 ms; flip angle 30 degrees; slice thickness 12 to 15 mm). The dynamic study was acquired in the coronal plane following administration of 0.1 mmol/kg gadopentetate dimeglumine. A total of 8 to 10 sections repeated 20 to 25 times at intervals of 1s were performed. Perfusion lung scintigraphy was carried out a maximum of 48 h before the MR examination in all cases. Two radiologists, who were blinded to the clinical data and results of other imaging methods, reviewed all coronal sections. MR perfusion images were independently assessed in terms of segmental or lobar perfusion defects in the 85 lobes of the 17 individuals, and the findings were compared with the results of scintigraphy.

RESULTS

Of the 17 patients, 8 were found to have pulmonary emboli, 2 chronic obstructive pulmonary disease with emphysema, 2 bullous emphysema, 2 Takayasu arteritis and 1 had a hypoplastic pulmonary artery. Pulmonary perfusion was completely normal in 2 cases. In 35 lobes, perfusion defects were detected using both methods, in 4 with MR alone and in 9 only with scintigraphy. There was good agreement between MRI and scintigraphy findings (kappa=0.695).

CONCLUSION

Pulmonary perfusion MRI is a new alternative to scintigraphy in the evaluation of pulmonary perfusion for various lung disorders. In addition, this technique allows measurement and quantification of pulmonary perfusion abnormalities.

Feasibility of Pulmonary Ventilation Visualization With Aerosolized Magnetic Resonance Contrast Media

OBJECTIVE

The objective of this study was to evaluate a new approach to noninvasive magnetic resonance assessment of human pulmonary ventilation with aerosolized Gd-DTPA.

MATERIALS AND METHODS

Fifteen experimental procedures were carried out in 15 healthy volunteers on a 1.5-T imager. For a timespan of 10 minutes, the subjects spontaneously inhaled a commercially available Gd-DTPA magnetic resonance contrast agent in aerosolized form through an inflatable facemask. Gd-DTPA was aerosolized by means of a small-particle generator with integrated heater to increase aerosol production. Respiratory gated dynamic T1-weighted turbo spin echo images were obtained before and after contrast agent aerosol administration. After nebulization, homogeneity of aerosol distribution was graded by 2 experienced readers and pulmonary signal intensity (SI) changes were measured in corresponding regions of both lungs.

RESULTS

Pulmonary ventilation visualization, and hence contrast agent delivery, was rated homogeneously distributed by both readers in 14 of 15 cases (93.3%) and slightly inhomogeneous in 1 case (6.7%). Pulmonary SI increased by an average of +37% +/- 8.5 (range, 10-48%). Allergic responses were not noted.

CONCLUSIONS

Human ventilation imaging with aerosolized gadolinium-chelates is viable. The presented modality might evolve as an alternative to current nuclear medicine and magnetic resonance image ventilation imaging procedures, avoiding radiation exposure while offering functional ventilation assessment with an acceptable temporal and spatial resolution. Nevertheless, further evaluation is required to define the potential of gadolinium-based ventilation magnetic resonance imaging in illustrating lung disease.

Assessment of regional lung function impairment in airway obstruction and pulmonary embolic dogs with combined noncontrast electrocardiogram-gated perfusion and gadolinium diethylenetriaminepentaacetic acid aerosol magnetic resonance images

PURPOSE

To define regional function impairment in airway obstruction (AO) and pulmonary embolic (PE) dogs with a combination study of noncontrast electrocardiogram (ECG)-gated perfusion and gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) aerosol magnetic resonance (MR) images.

METHODS

After acquisition of multiphase fast-spin-echo (FSE) MR images during cardiac cycles in 14 AO dogs and 19 PE dogs, ECG-gated perfusion-weighted (PW) images were obtained by subtraction between two-phase images of the minimum lung signal intensity (SI) during systole and maximum SI during diastole. Each dog subsequently inhaled Gd-DTPA aerosol for 20 minutes, and subtracted Gd-DTPA aerosol images were obtained from precontrast and maximally enhanced images. ECG-gated PW images were compared with intravenous Gd-DTPA-enhanced pulmonary arterial perfusion phase (PAPP) images.

RESULTS

ECG-gated PW images were consistent with Gd-DTPA-enhanced PAPP images in all dogs, with significant correlations in the affected-to-unaffected lung perfusion ratios (P < 0.005). Gd-DTPA aerosol images showed sufficient and uniform enhancement in the unaffected lungs. In all the AO areas, these combined images showed the matched perfusion and aerosol deposition defects. These images showed perfusion defects without aerosol deposition defects in the relatively small embolized areas, but showed the matched defects in the widely embolized areas probably due to hypoxic bronchial constriction.

CONCLUSION

The combination MR studies may be acceptable for noninvasively defining regionally impaired lung function in AO and PE.

Contrast-enhanced three-dimensional pulmonary perfusion magnetic resonance imaging: intraindividual comparison of 1.0 M gadobutrol and 0.5 M Gd-DTPA at three dose levels

RATIONALE AND OBJECTIVES

To compare 1.0 M gadobutrol and 0.5 M Gd-DTPA for contrast-enhanced three-dimensional pulmonary perfusion magnetic resonance imaging (3D MRI).

MATERIALS AND METHODS

Ten healthy volunteers (3 females; 7 males; median age, 27 years; age range, 18-31 years) were examined with contrast-enhanced dynamic 3D MRI with parallel acquisition technique (FLASH 3D; reconstruction algorithm: generalized autocalibrating partially parallel acquisitions; acceleration factor: 2; TE/TR/alpha: 0.8/1.9 milliseconds/40 degrees; FOV: 500 x 375 mm; matrix: 256 x 86; slab thickness: 180 mm; 36 partitions; voxel size: 4.4 x 2 x 5 mm; TA: 1.48 seconds). Twenty-five consecutive data sets were acquired after intravenous injection of 0.025, 0.05, and 0.1 mmol/kg body weight of gadobutrol and Gd-DTPA. Quantitative measurements of peak signal-to-noise ratios (SNR) of both lungs were performed independently by 3 readers. Bolus transit times through the lungs were assessed from signal intensity time curves.

RESULTS

The peak SNR in the lungs was comparable between gadobutrol and Gd-DTPA at all dose levels (15.7 vs. 15.5 at 0.1 mmol/kg bw; 12.9 vs. 12.5 at 0.05 mmol/kg bw; 7.6 vs. 8.9 at 0.025 mmol/kg bw). A dose of 0.1 mmol/kg achieved the highest peak SNR compared with all other dose levels (P < 0.05). A higher peak SNR was observed in gravity dependent lung (P < 0.05). Despite different injection volumes, transit times of the contrast bolus did not differ between both agents.

CONCLUSION

Higher concentrated gadolinium chelates offer no advantage over standard 0.5 M Gd-DTPA for contrast-enhanced 3D MRI of lung perfusion.

Assessment of lung perfusion impairment in patients with pulmonary artery-occlusive and chronic obstructive pulmonary diseases with noncontrast electrocardiogram-gated fast-spin-echo perfusion MR imaging

PURPOSE

To evaluate the ability of noncontrast electrocardiogram (ECG)-gated fast-spin-echo (FSE) perfusion MR images for defining regional lung perfusion impairment, as compared with technetium (Tc)-99m macroaggregated albumin (MAA) single-photon emission computed tomography (SPECT) images.

MATERIALS AND METHODS

After acquisition of ECG-gated multiphase FSE MR images during cardiac cycles at selected lung levels in nine healthy volunteers, 11 patients with pulmonary artery-occlusive diseases, and 15 patients with chronic obstructive pulmonary diseases (COPD), the subtracted perfusion-weighted (PW) MR images were obtained from the two-phase images of the minimum lung signal intensity (SI) during systole and the maximum SI during diastole, and were compared with SPECT images.

RESULTS

ECG-gated PW images showed uniform but posture-dependent perfusion gradient in normal lungs and visualized the various sizes of perfusion defects in affected lungs. These defect sites were nearly consistent with those on SPECT images, with a significant correlation for the affected-to-unaffected perfusion contrast (r = 0.753; P < 0.0001). These MR images revealed that the pulmonary arterial blood flow in the affected areas of COPD was relatively preserved as compared with pulmonary artery-occlusive diseases, and also showed significant decrease in blood flow, even in the areas with homogeneous perfusion on SPECT images in patients with focal pulmonary emphysema.

CONCLUSION

This noninvasive MR technique allows qualitative and quantitative assessment of lung perfusion, and may better characterize regional perfusion impairment in pulmonary artery-occlusive diseases and COPD.

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.

Potential of Magnetic Resonance Lymphography With Intrapulmonary Injection of Gadopentetate Dimeglumine for Visualization of the Pulmonary Lymphatic Basin in Dogs

PURPOSE

To evaluate a new approach of magnetic resonance (MR) lymphography with intraalveolar injection of a conventional extracellular contrast agent (gadopentetate dimeglumine) for imaging lymphatic basin draining from specific portions of the lung.

METHODS

Three-dimensional T1-weighted spoiled gradient-recalled echo MR sequence images were acquired serially before and for 40 minutes after intraalveolar injection of gadopentetate dimeglumine in a total of 14 anesthetized beagle dogs. Six of these dogs received 1 mL undiluted and low-concentration (75%) contrast agent into the same portion of the right caudal lobe during a 7-day interval. In all dogs, including these 6 dogs, MR lymphography was repeated with injection of the low-concentration contrast agent into different lung regions at 7-day intervals to evaluate the differences of the visualized draining lymphatic station. Lymphatic enhancement was quantified by percent increases of signal intensity against precontrast. Postmortem examination of the lymphatic anatomy was performed in 7 of these animals.

RESULTS

In all dogs, the lymphatic station draining from the injection sites was visualized within 5 minutes after contrast injection. The maximum percent increase of signal intensity of the same middle tracheobronchial lymph nodes was significantly greater with a low-concentration (75%) contrast agent than with an undiluted one in the same 6 dogs (n = 6, 247.6 +/- 30.5% vs. 204.2 +/- 33.8%; P < 0.01). Different lymphatic stations draining from the different injection sites were visualized in all dogs. In a total of 12 MR studies that showed extended nodal enhancement after injection of the low-concentration contrast agent, the enhancement peak of the most proximal nodes (n = 12) from the injection sites appeared earlier than that of their distant nodes (n = 12), with a maximum percent increase of signal intensity of 249.8 +/- 42.4%. The visualized lymph nodes were found in the appropriate locations postmortem, with significant correlation for nodal sizes (r = 0.965; P < 0.0001).

CONCLUSION

MR lymphography with low-concentration gadopentetate dimeglumine can quickly and sufficiently visualize the drainage lymphatic station from specific lung portions, and may have the potential of sentinel node mapping in lung cancer.

Partially parallel three-dimensional magnetic resonance imaging for the assessment of lung perfusion--initial results

RATIONALE

Contrast-enhanced magnetic resonance imaging (MRI) of lung perfusion requires a high spatial and temporal resolution. Partially parallel MRI offers an improved spatial and temporal resolution.

OBJECTIVE

To assess the feasibility of partially parallel MRI for the assessment of lung perfusion.

METHODS

Two healthy volunteers and 14 patients were examined with a contrast-enhanced 3D gradient-echo pulse sequence with partially parallel image acquisitions (TE/TR/alpha: 0.8/1.9 milliseconds/40 degrees; voxel size 3.6 x 2.0 x 5.0 mm3, TA: 1.5 seconds). The image analysis included an analysis of the signal-to-noise ratio in the lungs in areas with normal and impaired perfusion. 3D MR perfusion image data were analyzed for perfusion defects and compared with radionuclide perfusion scans, which were available for 10 of 14 patients.

RESULTS

The analysis of the 3D perfusion-weighted data allowed a clear differentiation of perfusion abnormalities: MRI showed normal lung perfusion in 9 of 16 cases, whereas perfusion abnormalities were observed in 7 cases. When compared with the radionuclide perfusion scans, a good intermodality agreement was shown (kappa = 0.74). When compared with normally perfused lung a significantly lower signal to noise ratio was observed in hypoperfused lung (7 versus 17; P = 0.02).

CONCLUSION

Partially parallel MRI might be used for the assessment of lung perfusion. Future studies are required to further evaluate the diagnostic impact of this technique.

Altered clearance of gadolinium diethylenetriaminepentaacetic acid aerosol from bleomycin-injured dog lungs: initial observations

To characterize altered alveolar transfer to solute in bleomycin (BLM)-injured lungs, eight dogs underwent a gadolinium diethylenetriaminepentaacetic acid aerosol (Gd-AS) magnetic resonance imaging study before and on Days 7 and 40 after tracheal instillation of BLM (0.75 mg) in the left lungs. Consecutive fast-gradient echo magnetic resonance imaging was acquired during and after spontaneous inhalation of 200-mM Gd-AS. The slope (Kep) and clearance half-time (T1/2) of logarithmic regression lines for clearance curves were estimated. Histology on Day 40 was compared with that on Day 7 in another three dogs. On Days 7 and 40, Gd-AS deposition was heterogeneously reduced in the affected lungs. On Day 7 with multifocal intraalveolar exudative changes, Kep in affected areas was significantly increased compared with baseline (2.5 x 10(-3) minutes(-1) +/- 0.3 versus 1.7 x 10(-3) minutes(-1) +/- 0.2, p < 0.0001), with significant decrease in T1/2 (121.6 +/- 19.7 minutes vs. 170.4 +/- 15.8 minutes, p < 0.001). However, on Day 40 with multifocal interstitial fibrosis, Kep and T1/2 were recovered toward baseline. BLM-injured lungs can be characterized by accelerated Gd-AS clearance during the acute exudative phase and their recovery during the chronic fibrotic phase. This technique is acceptable for monitoring alveolar transfer changes in BLM-injured lungs.

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.