Measurement of regional lung function in rats using hyperpolarized 3helium dynamic MRI

Quantitative assessment of regional lung ventilation in emphysematous mice using hyperpolarized 129Xe MRI with a continuous flow hyperpolarizing system

BACKGROUND

Lung ventilation function in small animals can be assessed by using hyperpolarized gas MRI. For these experiments a free breathing protocol is generally preferred to mechanical ventilation as mechanical ventilation can often lead to ventilation lung injury, while the need to maintain a gas reservoir may lead to a partial reduction of the polarization.

PURPOSE

To evaluate regional lung ventilation of mice by a simple but fast method under free breathing and give evidence for effectiveness with an elastase instilled emphysematous mice.

ANIMAL MODEL

Emphysematous mice.

MATERIALS AND METHODS

A Look-Locker based saturation recovery sequence was developed for continuous flow hyperpolarized (CF-HP) ¹²⁹Xe gas experiments, and the apparent gas-exchange rate, k', was measured by the analysis of the saturation recovery curve.

RESULTS

In mice with elastase-induced mild emphysema, reductions of 15-30% in k' values were observed as the results of lesion-induced changes in the lung.

DATA CONCLUSION

The proposed method was applied to an emphysematous model mice and ventilation dysfunctions have been approved as a definite decrease in k' values, supporting the usefulness for a non-invasive assessment of the lung functions in preclinical study by the CF-HP ¹²⁹Xe experiments.

Application of a 2D frequency encoding sectoral approach to hyperpolarized 129Xe MRI at low field

Inhaled hyperpolarized ¹²⁹Xe MRI is a non-invasive and radiation risk free lung imaging method, which can directly measure the business unit of the lung where gas exchange occurs: the alveoli and acinar ducts (lung function). Currently, three imaging approaches have been demonstrated to be useful for hyperpolarized ¹²⁹Xe MR in lungs: Fast Gradient Recalled Echo (FGRE), Radial Projection Reconstruction (PR), and spiral/cones. Typically, non-Cartesian acquisitions such as PR and spiral/cones require specific data post-processing, such as interpolating, regridding, and density-weighting procedures for image reconstruction, which often leads to smoothing effects and resolution degradation. On the other hand, Cartesian methods such as FGRE are not short-echo time (TE) methods; they suffer from imaging gradient-induced diffusion-weighting of the k-space center, and employ a significant number of radio-frequency (RF) pulses. Due to the non-renewable magnetization of the hyperpolarized media, the use of a large number of RF pulses (FGRE/PR) required for full k-space coverage is a significant limitation, especially for low field (<0.5 T) hyperpolarized gas MRI. We demonstrate an ultra-fast, purely frequency-encoded, Cartesian pulse sequence called Frequency-Encoding Sectoral (FES), which takes advantage of the long T₂* of hyperpolarized ¹²⁹Xe gas at low field strength (0.074 T). In contrast to PR/FGRE, it uses a much smaller number of RF pulses, and consequently maximizes image Signal-to-Noise Ratio (SNR) while shortening acquisition time. Additionally, FES does not suffer from non-uniform T₂* decay leading to image blurring; a common issue with interleaved spirals/cones. The Cartesian k-space coverage of the proposed FES method does not require specific k-space data post-processing, unlike PR/FGRE and spiral/cones methods. Proton scans were used to compare the FES sequence to both FGRE and Phase Encoding Sectoral, in terms of their SNR values and imaging efficiency estimates. Using FES, proton and hyperpolarized ¹²⁹Xe images were acquired from a custom hollow acrylic phantom (0.04L) and two normal rats (¹²⁹Xe only), utilizing both single-breath and multiple-breath schemes. For the ¹²⁹Xe phantom images, the apparent diffusion coefficient, T₁, and T₂* relaxation maps were acquired and generated. Blurring due to the T₂* decay and B₀ field variation were simulated to estimate dependence of the image resolution on the duration of the data acquisition windows (i.e. sector length), and temperature-induced resonance frequency shift from the low field magnet hardware.

Biomedical imaging with hyperpolarized noble gases

Hyperpolarized noble gases (HNGs), polarized to approximately 50% or higher, have led to major advances in magnetic resonance (MR) imaging of porous structures and air-filled cavities in human subjects, particularly the lung. By boosting the available signal to a level about 100 000 times higher than that at thermal equilibrium, air spaces that would otherwise appear as signal voids in an MR image can be revealed for structural and functional assessments. This review discusses how HNG MR imaging differs from conventional proton MR imaging, how MR pulse sequence design is affected and how the properties of gas imaging can be exploited to obtain hitherto inaccessible information in humans and animals. Current and possible future imaging techniques, and their application in the assessment of normal lung function as well as certain lung diseases, are described.

Preclinical anatomical, molecular, and functional imaging of the lung with multiple modalities

In vivo imaging is an important tool for preclinical studies of lung function and disease. The widespread availability of multimodal animal imaging systems and the rapid rate of diagnostic contrast agent development have empowered researchers to noninvasively study lung function and pulmonary disorders. Investigators can identify, track, and quantify biological processes over time. In this review, we highlight the fundamental principles of bioluminescence, fluorescence, planar X-ray, X-ray computed tomography, magnetic resonance imaging, and nuclear imaging modalities (such as positron emission tomography and single photon emission computed tomography) that have been successfully employed for the study of lung function and pulmonary disorders in a preclinical setting. The major principles, benefits, and applications of each imaging modality and technology are reviewed. Limitations and the future prospective of multimodal imaging in pulmonary physiology are also discussed. In vivo imaging bridges molecular biological studies, drug design and discovery, and the imaging field with modern medical practice, and, as such, will continue to be a mainstay in biomedical research.

Dynamic ventilation 3He MRI for the quantification of disease in the rat lung

Pulmonary diseases are known to be largely inhomogeneous. To evaluate such inhomogeneities, we are testing an image-based method to measure gas flow in the lung regionally. Dynamic, spin-density-weighted hyperpolarized (3)He MR images performed during slow inhalation of this gas were analyzed to quantify regional inflation rate. This parameter was measured in regions of interest (ROIs) that were defined by a rectangular grid that covered the entire rat lung and grew dynamically with it during its inflation. We used regional inflation rate to quantify elastase-induced emphysema and to differentiate healthy (n = 8) from elastase-treated (n = 9) rat lungs as well as healthy from elastase-treated areas of one rat unilaterally treated with elastase in the left lung. Emphysema was also assessed by gold standard morphological and well-established hyperpolarized (3)He MRI diffusion measurements. Mean values of regional inflation rates were significantly different for healthy and elastase-treated animals and correlated well with the apparent diffusion coefficient of (3)He and morphological measurements. The image-based biomarker inflation rate may be useful for the assessment of regional lung ventilation.

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.

A MRI and polarized gases compatible respirator and gas administrator for the study of the small animal lung: volume measurement and control

We have developed over the past years an experimental magnetic resonance imaging (MRI) and polarized gases compatible mechanical respirator for the study of the small experimental animal. The respirator has been successfully used for experiments both in the MRI setting for polarized (3)He, (19)F, and proton imaging as well as for functional measurements of the lungs. The new main pneumatic valve with the two integrated sensors for simultaneous lung pressure and volume measurements and the proportional valve to set the tidal volume of the respiration are described. It is shown how the device measures and controls the tidal volume of the lungs.

1H and hyperpolarized 3He magnetic resonance imaging clearly detect the preventative effect of a glucocorticoid on endotoxin-induced pulmonary inflammation in vivo

Introduction: Proton (1H) magnetic resonance imaging (MRI) can be utilized to quantify pulmonary edema in endotoxin-induced pulmonary inflammation and hyperpolarized (HP) 3He MRI can assess pulmonary ventilation. Neither of the methods has been applied to assess the impact of a drug on endotoxin-induced pulmonary inflammation in vivo. The aim of the current study was to evaluate the capability of 1H and HP 3He MRI to assess the effects of a glucocorticoid on endotoxin-induced pulmonary inflammation in vivo.

Materials and Methods: Mice were exposed to an aerosol of either saline or endotoxin (5 mg/ml) for 10 min. Half of the endotoxin-exposed mice were pretreated with a glucocorticoid (budesonide 3 mg/kg; 2 times/day) and the other half with vehicle p.o. The first budesonide treatment was administered 1 h prior to the aerosol inhalation. Forty-eight hours after the aerosol exposure, the mice were anaesthetized for subsequent imaging. Hyperpolarized 3He was administered and axial MR images of the lungs obtained. Matching 1H MR images were then acquired. The mice were sacrificed and broncho-alveolar lavage (BAL) samples were harvested to determine total and cell differential counts. Results: The lesion volume on both 1H and 3He MRI, were markedly increased by endotoxin exposure (P<0.001). Budesonide strongly reduced lesion volume ( P<0.001). The BAL cell count correlated strongly with both 3He ( P<0.001; r = 0.96) and 1H lesion volumes ( P<0.001; r = 0.97).

Conclusions: Hyperpolarized 3He MRI and 1H MRI clearly visualized the preventative effect of budesonide on the impact of endotoxin on pulmonary ventilation and edema, respectively. The fact that ventilation defects on 3He MRI corresponded to findings from conventional 1H MRI, as well as to counts of BAL inflammatory cells suggests that these imaging techniques constitute promising tools for non-invasive monitoring of pulmonary inflammation in vivo.

A robust protocol for regional evaluation of methacholine challenge in mouse models of allergic asthma using hyperpolarized 3He MRI

Hyperpolarized (HP) (3)He magnetic resonance imaging has been recently used to produce high-resolution images of pulmonary ventilation after methacholine (MCh) challenge in mouse models of allergic inflammation. This capability presents an opportunity to gain new insights about these models and to more sensitively evaluate new drug treatments in the pre-clinical setting. In the current study, we present our initial experience using two-dimensional (2D), time-resolved (3)He MRI of MCh challenge-induced airways hyperreactivity (AHR) to compare ovalbumin-sensitized and challenged (N = 8) mice to controls (N = 8). Imaging demonstrated that ovalbumin-sensitized and challenged animals exhibited many large ventilation defects even prior to MCh challenge (four out of eight) compared to no defects in the control animals. Additionally, the ovalbumin-sensitized and challenged animals experienced a greater number of ventilation defects (4.5 +/- 0.4) following MCh infusion than did controls (3.3 +/- 0.6). However, due to variability in MCh delivery that was specific to the small animal MRI environment, the difference in mean defect number was not statistically significant. These findings are reviewed in detail and a comprehensive solution to the variability problem is presented that has greatly enhanced the magnitude and reproducibility of the MCh response. This has permitted us to develop a new imaging protocol consisting of a baseline 3D image, a time-resolved 2D series during MCh challenge, and a post-MCh 3D image that reveals persistent ventilation defects.

Hyperpolarized 129Xe phase-selective imaging of mouse lung at 9.4T using a continuous-flow hyperpolarizing system

The use of hyperpolarized (HP) 129Xe magnetic resonance (MR) imaging to regionally evaluate gas diffusion and perfusion processes as well as ventilation in the lung has been expected. In this study, we used a continuous-flow hyperpolarizing (CF-HP) system to acquire gas- and dissolved-phase 129Xe images from mouse lung, employing standard gradient echo sequence equipped with chemical shift selective excitation and 90 degrees flip angle. The character of non-recoverable HP magnetization enabled the use of a phase (frequency)-selective 90 degrees pulse for direct visualization of only a given-phase 129Xe magnetization replenished into the slice during repetition time (TR). We combined gas- and dissolved-phase 129Xe images to map the ratio of dissolved- to gas-phase 129Xe replenished into the slice during TR (Mdissolved/Mgas) and found it to be approximately 0.05 to 0.08 in the peripheral regions of mouse lungs. This result suggested that replenishment of dissolved-phase 129Xe magnetization by gas diffusion and pulmonary perfusion would be faster than that of gas-phase by ventilation. The use of a CF-HP system that allows the application of relatively long TR to HP 129Xe imaging using a phase-selective 90 degrees pulse would be useful in evaluating gas transport mechanisms in the lung.

In vivo MRI using real-time production of hyperpolarized 129Xe

MR imaging of hyperpolarized (HP) nuclei is challenging because they are typically delivered in a single dose of nonrenewable magnetization, from which the entire image must be derived. This problem can be overcome with HP (129)Xe, which can be produced sufficiently rapidly to deliver in dilute form (1%) continuously and on-demand. We demonstrate a real-time in vivo delivery of HP (129)Xe mixture to rats, a capability we now routinely use for setting frequency, transmitter gain, shimming, testing pulse sequences, scout imaging, and spectroscopy. Compared to images acquired using conventional fully concentrated (129)Xe, real-time (129)Xe images have 26-fold less signal, but clearly depict ventilation abnormalities. Real-time (129)Xe MRI could be useful for time-course studies involving acute injury or response to treatment. Ultimately, real-time (129)Xe MRI could be done with more highly concentrated (129)Xe, which could increase the signal-to-noise ratio by 100 relative to these results to enable a new class of gas imaging applications.

Effects of ozone exposure in rat lungs investigated with hyperpolarized 3 He MRI

PURPOSE

To investigate the effects of subchronic ozone exposure on rat lung ventilation using hyperpolarized (HP) (3)He MRI.

MATERIALS AND METHODS

A total of 24 Sprague-Dawley rats, distributed in one control group and four groups exposed to 0.5 ppm ozone concentration for two days or six days, either continuously (22 hours/day) or alternatingly (12 hours/day). A three-step MRI protocol was designed and applied to each animal, including: 1) (3)He gas distribution images acquired at inspiratory capacity, 2) measurements of intrapulmonary (3)He diffusion coefficients, and 3) dynamic ventilation acquisitions performed during lung filling with (3)He.

RESULTS

No differentiation between animals exposed to ozone and control animals was observed from the ventilation images obtained at inspiratory capacity. The (3)He diffusion coefficients were not statistically different from one group to another. Ventilation defects, appearing as delayed lung filling regions and heterogeneous lung filling, were observed in the dynamic lung ventilation image series. The percentage of animals with ventilation defects in the control, two-day, and six-day exposed groups were equal to 20%, 43% and 75%, respectively. In the subgroup of the animals exposed six days for 12 hours per day, the percentage of animals exhibiting ventilation defects was equal to 85%.

CONCLUSION

Heterogeneous obstructive patterns in an experimental animal model of subchronic ozone exposure were observed using HP (3)He MRI.

3D 3He diffusion MRI as a local in vivo morphometric tool to evaluate emphysematous rat lungs

In this work, we investigate (3)He magnetic resonance imaging as a noninvasive morphometric tool to assess emphysematous disease state on a local level. Emphysema was induced intratracheally in rats with 25 U/100 g body wt of porcine pancreatic elastase dissolved in 200 microl saline. Rats were then paired with saline-dosed controls. Nine three-dimensional (3D) (3)He diffusion-weighted images were acquired at 1, 2, or 3 wk postdose, after which the lungs were harvested and prepared for histological analysis. Recently introduced indexes sensitive to the heterogeneity of the air space size distribution were calculated. These indexes, D(1) and D(2), were derived from the moments of the mean equivalent airway diameters. Averaged over the entire lung, it is shown that the average (3)He diffusivity (D(ave)) correlates well with histology (R = 0.85, P < 0.0001). By matching small (0.046 cm(2)) regions in (3)He images with corresponding regions in histological slices, D(ave) correlates significantly with both D(1) and D(2) (R = 0.88 and R = 0.90, respectively, with P < 0.0001). It is concluded that (3)He MRI is a viable noninvasive morphometric tool for localized in vivo emphysema assessment.

Investigation of hyperpolarized 3He magnetic resonance imaging utility in examining human airway diameter behavior in asthma through comparison with high-resolution computed tomography

RATIONALE AND OBJECTIVES

Application of a previously developed model-based algorithm on hyperpolarized (HP) (3)He magnetic resonance (MR) dynamic projection images of phantoms was extended to investigate the utility of HP (3)He MR imaging (MRI) in quantifying airway caliber changes associated with asthma.

MATERIALS AND METHODS

Airways of seven volunteers were imaged and measured using HP (3)He MRI and multidetector-row computed tomography (MDCT) before and after a methacholine (MCh) challenge. MDCT data were obtained at functional residual capacity and 1 L above functional residual capacity.

RESULTS

Comparison of the resultant data showed that HP (3)He MRI did not match MDCT in measuring the ratios of airway calibers before and after the MCh challenge in 37% to 43% of the airways from the first six generations at the two lung volumes tested. However, MDCT did yield the observation that 49% to 69% of these airways displayed bronchodilation following MCh challenge.

CONCLUSION

The current implementation of HP (3)He MRI did not match the MCh-induced postchallenge-to-prechallenge airway caliber ratios as measured with MDCT. Elevated parenchymal tethering due to bronchoconstriction-induced hyperinflation was proposed as a possible explanation for this airway dilation.

A high-precision contrast injector for small animal x-ray digital subtraction angiography

The availability of genetically altered animal models of human disease for basic research has generated great interest in new imaging methodologies. Digital subtraction angiography (DSA) offers an appealing approach to functional imaging in small animals because of the high spatial and temporal resolution, and the ability to visualize and measure blood flow. The micro-injector described here meets crucial performance parameters to ensure optimal vessel enhancement without significantly increasing the total blood volume or producing overlap of enhanced structures. The micro-injector can inject small, reproducible volumes of contrast agent at high flow rates with computer-controlled timing synchronized to cardiopulmonary activity. Iterative bench-top and live animal experiments with both rat and mouse have been conducted to evaluate the performance of this computer-controlled micro-injector, a first demonstration of a new device designed explicitly for the unique requirements of DSA in small animals. Injection protocols were optimized and screened for potential physiological impact. For the optimized protocols, we found that changes in the time-density curves for representative regions of interest in the thorax were due primarily to physiological changes, independent of micro-injector parameters.

3He MRI in mouse models of asthma

In the study of asthma, a vital role is played by mouse models, because knockout or transgenic methods can be used to alter disease pathways and identify therapeutic targets that affect lung function. Assessment of lung function in rodents by available methods is insensitive because these techniques lack regional specificity. A more sensitive method for evaluating lung function in human asthma patients uses hyperpolarized (HP) (3)He MRI before and after bronchoconstriction induced by methacholine (MCh). We now report the ability to perform such (3)He imaging of MCh response in mice, where voxels must be approximately 3000 times smaller than in humans and (3)He diffusion becomes an impediment to resolving the airways. We show three-dimensional (3D) images that reveal airway structure down to the fifth branching and visualize ventilation at a resolution of 125 x 125 x 1000 microm(3). Images of ovalbumin (OVA)-sensitized mice acquired after MCh show both airway closure and ventilation loss. To also observe the MCh response in naive mice, we developed a non-slice-selective 2D protocol with 187 x 187 microm(2) resolution that was fast enough to record the MCh response and recovery with 12-s temporal resolution. The extension of (3)He MRI to mouse models should make it a valuable translational tool in asthma research.

Small animal imaging with magnetic resonance microscopy

Small animal magnetic resonance microscopy (MRM) has evolved significantly from testing the boundaries of imaging physics to its expanding use today as a tool in noninvasive biomedical investigations. MRM now increasingly provides functional information about living animals, with images of the beating heart, breathing lung, and functioning brain. Unlike clinical MRI, where the focus is on diagnosis, MRM is used to reveal fundamental biology or to noninvasively measure subtle changes in the structure or function of organs during disease progression or in response to experimental therapies. High-resolution anatomical imaging reveals increasingly exquisite detail in healthy animals and subtle architectural aberrations that occur in genetically altered models. Resolution of 100 mum in all dimensions is now routinely attained in living animals, and (10 mum)(3) is feasible in fixed specimens. Such images almost rival conventional histology while allowing the object to be viewed interactively in any plane. In this review we describe the state of the art in MRM for scientists who may be unfamiliar with this modality but who want to apply its capabilities to their research. We include a brief review of MR concepts and methods of animal handling and support, before covering a range of MRM applications-including the heart, lung, and brain-and the emerging field of MR histology. The ability of MRM to provide a detailed functional and anatomical picture in rats and mice, and to track this picture over time, makes it a promising platform with broad applications in biomedical research.

Comparison of airway diameter measurements from an anthropomorphic airway tree phantom using hyperpolarized 3He MRI and high-resolution computed tomography

An anthropomorphic airway tree phantom was imaged with both hyperpolarized (HP) 3He MRI using a dynamic projection scan and computed tomography (CT). Airway diameter measurements from the HP 3He MR images obtained using a newly developed model-based algorithm were compared against their corresponding CT values quantified with a well-established method. Of the 45 airway segments that could be evaluated with CT, only 14 airway segments (31%) could be evaluated using HP 3He MRI. No airway segments smaller than approximately 4 mm in diameter and distal to the fourth generation were adequate for analysis in MRI. For the 14 airway segments measured, only two airway segments yielded a non-equivalent comparison between the two imaging modalities, while eight more had inconclusive comparison results, leaving only four airway segments (29%) that satisfied the designed equivalence criteria. Some of the potential problems in airway diameter quantification described in the formulation of the model-based algorithm were observed in this study. These results suggest that dynamic projection HP 3He MRI may have limited utility for measuring airway segment diameters, particularly those of the central airways.

Lung MRI for experimental drug research

Current techniques to evaluate the efficacy of potential treatments for airways diseases in preclinical models are generally invasive and terminal. In the past few years, the flexibility of magnetic resonance imaging (MRI) to obtain anatomical and functional information of the lung has been explored with the scope of developing a non-invasive approach for the routine testing of drugs in models of airways diseases in small rodents. With MRI, the disease progression can be followed in the same animal. Thus, a significant reduction in the number of animals used for experimentation is achieved, as well as minimal interference with their well-being and physiological status. In addition, under certain circumstances the duration of the observation period after disease onset can be shortened since the technique is able to detect changes before these are reflected in parameters of inflammation determined using invasive procedures. The objective of this article is to briefly address MRI techniques that are being used in experimental lung research, with special emphasis on applications. Following an introduction on proton techniques and MRI of hyperpolarized gases, the attention is shifted to the MRI analysis of several aspects of lung disease models, including inflammation, ventilation, emphysema, fibrosis and sensory nerve activation. The next subject concerns the use of MRI in pharmacological studies within the context of experimental lung research. A final discussion points towards advantages and limitations of MRI in this area.

Imaging techniques for small animal models of pulmonary disease: MR microscopy

In vivo magnetic resonance microscopy (MRM) of the small animal lung has become a valuable research tool, especially for preclinical studies. MRM offers a noninvasive and nondestructive tool for imaging small animals longitudinally and at high spatial resolution. We summarize some of the technical and biologic problems and solutions associated with imaging the small animal lung and describe several important pulmonary disease applications. A major advantage of MR is direct imaging of the gas spaces of the lung using breathable gases such as helium and xenon. When polarized, these gases become rich MR signal sources. In animals breathing hyperpolarized helium, the dynamics of gas distribution can be followed and airway constrictions and obstructions can be detected. Diffusion coefficients of helium can be calculated from diffusion-sensitive images, which can reveal micro-structural changes in the lungs associated with pathologies such as emphysema and fibrosis. Unlike helium, xenon in the lung is absorbed by blood and exhibits different frequencies in gas, tissue, or erythrocytes. Thus, with MR imaging, the movement of xenon gas can be tracked through pulmonary compartments to detect defects of gas transfer. MRM has become a valuable tool for studying morphologic and functional changes in small animal models of lung diseases.

Retrospective cine 3He ventilation imaging under spontaneous breathing conditions: a non-invasive protocol for small-animal lung function imaging

A non-invasive and free-breathing hyperpolarized (HP) (3)He imaging protocol for small animals was implemented and validated on rats for lung function imaging. Animals were allowed to breathe a mixture of air and (3)He from a mask and a gas reservoir fitted to their heads. Radial imaging sequences were used, and MRI signal intensity changes were monitored for retrospective cine image reconstruction. The ventilation cycle of the animals was imaged with a 100 ms temporal resolution. The sliding window imaging technique was applied to reconstruct 5 ms time-shifted image series covering the complete breathing cycle. Image series were processed to extract quantitative ventilation parameters such as the gas arrival time. The reproducibility and the non-invasiveness of this ventilation imaging protocol were evaluated by multiple acquisitions on the same animals.

Optimized radiographic spectra for small animal digital subtraction angiography

The increasing use of small animals in basic research has spurred interest in new imaging methodologies. Digital subtraction angiography (DSA) offers a particularly appealing approach to functional imaging in the small animal. This study examines the optimal x-ray, molybdenum (Mo) or tungsten (W) target sources, and technique to produce the highest quality small animal functional subtraction angiograms in terms of contrast and signal-difference-to-noise ratio squared (SdNR2). Two limiting conditions were considered-normalization with respect to dose and normalization against tube loading. Image contrast and SdNR2 were simulated using an established x-ray model. DSA images of live rats were taken at two representative tube potentials for the W and Mo sources. Results show that for small animal DSA, the Mo source provides better contrast. However, with digital detectors, SdNR2 is the more relevant figure of merit. The W source operated at kVps >60 achieved a higher SdNR2. The highest SdNR2 was obtained at voltages above 90 kVp. However, operation at the higher potential results in significantly greater dose and tube load and reduced contrast quantization. A reasonable tradeoff can be achieved at tube potentials at the beginning of the performance plateau, around 70 kVp, where the relative gain in SdNR2 is the greatest.

Measurement of xenon diffusing capacity in the rat lung by hyperpolarized 129Xe MRI and dynamic spectroscopy in a single breath-hold

We used the dual capability of hyperpolarized 129Xe for spectroscopy and imaging to develop new measures of xenon diffusing capacity in the rat lung that (analogously to the diffusing capacity of carbon monoxide or DLCO) are calculated as a product of total lung volume and gas transfer rate constants divided by the pressure gradient. Under conditions of known constant pressure breath-hold, the volume is measured by hyperpolarized 129Xe MRI, and the transfer rate is measured by dynamic spectroscopy. The new quantities (xenon diffusing capacity in lung parenchyma (DLXeLP)), xenon diffusing capacity in RBCs (DLXeRBC), and total lung xenon diffusing capacity (DLXe)) were measured in six normal rats and six rats with lung inflammation induced by instillation of fungal spores of Stachybotrys chartarum. DLXeLP, DLXeRBC, and DLXe were 56 +/- 10 ml/min/mmHg, 64 +/- 35 ml/min/mmHg, and 29 +/- 9 ml/min/mmHg, respectively, for normal rats, and 27 +/- 9 ml/min/mmHg, 42 +/- 27 ml/min/mmHg, and 16 +/- 7 ml/min/mmHg, respectively, for diseased rats. Lung volumes and gas transfer times for LP (TtrLP) were 16 +/- 2 ml and 22 +/- 3 ms, respectively, for normal rats and 12 +/- 2 ml and 35 +/- 8 ms, respectively, for diseased rats. Xenon diffusing capacities may be useful for measuring changes in gas exchange associated with inflammation and other lung diseases.

Imaging alveolar-capillary gas transfer using hyperpolarized 129Xe MRI

Effective pulmonary gas exchange relies on the free diffusion of gases across the thin tissue barrier separating airspace from the capillary red blood cells (RBCs). Pulmonary pathologies, such as inflammation, fibrosis, and edema, which cause an increased blood-gas barrier thickness, impair the efficiency of this exchange. However, definitive assessment of such gas-exchange abnormalities is challenging, because no methods currently exist to directly image the gas transfer process. Here we exploit the solubility and chemical shift of (129)Xe, the magnetic resonance signal of which has been enhanced by 10(5) with hyperpolarization, to differentially image its transfer from the airspaces into the tissue barrier spaces and RBCs in the gas exchange regions of the lung. Based on a simple diffusion model, we estimate that this MR imaging method for measuring (129)Xe alveolar-capillary transfer is sensitive to changes in blood-gas barrier thickness of approximately 5 microm. We validate the successful separation of tissue barrier and RBC images and show the utility of this method in a rat model of pulmonary fibrosis where (129)Xe replenishment of the RBCs is severely impaired in regions of lung injury.

Measurement of the internal diameter of plastic tubes from projection MR images using a model-based least-squares fit approach

Hyperpolarized (HP) 3He MRI is an emerging tool in the diagnosis and evaluation of pulmonary diseases involving bronchoconstriction, such as asthma. Previously, airway diameters from dynamic HP 3He MR images of the lung were assessed manually and subjectively, and were thus prone to uncertainties associated with human error and partial volume effects. A model-based algorithm capable of fully utilizing pixel intensity profile information and attaining subpixel resolution has been developed to measure surrogate airway diameters from HP 3He MR static projection images of plastic tubes. This goal was achieved by fitting ideal pixel intensity profiles for various diameter (6.4 to 19.1 mm) circular tubes to actual pixel intensity data. A phantom was constructed from plastic tubes of various diameters connected in series and filled with water mixed with contrast agent. Projection MR images were then taken of the phantom. The favorable performance of the model-based algorithm compared to manual assessment demonstrates the viability of our approach. The manual and algorithm approaches yielded diameter measurements that generally stayed within 1 x the pixel dimension. However, inconsistency of the manual approach can be observed from the larger standard deviations of its measured values. The method was then extended to HP 3He MRI, producing encouraging results at tube diameters characteristic of airways beyond the second generation, thereby justifying their application to lung airway imaging and measurement. Potential obstacles when measuring airway diameters using this method are discussed.

Morphology of the small-animal lung using magnetic resonance microscopy

Small-animal imaging with magnetic resonance microscopy (MRM) has become an important tool in biomedical research. When MRM is used to image perfusion-fixed and "stained" whole mouse specimens, cardiopulmonary morphology can be visualized, nondestructively, in exquisite detail in all three dimensions. This capability can be a valuable tool for morphologic phenotyping of different mouse strains commonly used in genomics research. When these imaging techniques are combined with specialized methods for biological motion control and animal support, the lungs of the live, small animal can be imaged. Although in vivo imaging may not achieve the high resolution possible with a fixed specimen, dynamic functional studies and survival studies that follow the progression of pulmonary change related to disease or environmental exposure are possible. By combining conventional proton imaging with gas imaging, using hyperpolarized 3He, it is possible to image the tissue and gas compartments of the lung. This capability is illustrated in studies on an emphysema model in rats and on radiation damage of the lung. With further improvements in imaging and animal handling technology, we will be able to image faster and at higher resolutions, making MRM an even more valuable research tool.

Quantitative assessment of emphysema using hyperpolarized 3He magnetic resonance imaging

In this experiment, Sprague-Dawley rats with elastase-induced emphysema were imaged using hyperpolarized (3)He MRI. Regional fractional ventilation r, the fraction of gas replaced with a single tidal breath, was calculated from a series of images in a wash-in study of hyperpolarized gas. We compared the regional fractional ventilation in these emphysematous rats to the regional fractional ventilations we calculated from a previous baseline study in healthy Sprague-Dawley rats. We found that there were differences in the maps of fractional ventilation and its associated frequency distribution between the healthy and emphysematous rat lungs. Fractional ventilation tended to be much lower in emphysematous rats than in normal rats. With this information, we can use data on fractional ventilation to regionally distinguish between healthy and emphysematous portions of the lung. The successful implementation of such a technique on a rat model could lead to work toward the future implementation of this technique in human patients.

Quantification of airway diameters and 3D airway tree rendering from dynamic hyperpolarized 3He magnetic resonance imaging

As another step toward extracting quantitative information from hyperpolarized 3He MRI, airway diameters in humans were measured from projection images and multislice images of the lungs. Values obtained were in good agreement with the Weibel lung morphometry model. The measurement of airway caliber can now be achieved without the use of ionizing radiation. Furthermore, it was demonstrated that 3D airway tree renderings could be constructed from the multislice data. Both the measurement of airway diameters and the rendering of 3D airway information hold promise for the clinical assessment of bronchoconstrictive diseases such as asthma and the associated evaluation of treatment effectiveness. Work is being done to address the uncertainties of the manually intensive methods we have developed.

T2-shortening of 3He gas by magnetic microspheres

In a gas-filled material like the lung parenchyma, the transverse relaxation time (T2) for 3He is shortened by the deposition of magnetic microspheres and rapid molecular diffusion through induced field distortions. Here, this unique relaxation process is described theoretically and predicted T2-shortening is validated using pressurized 3He gas in a foam model of alveolar airways. Results demonstrate that: (1) significant T2-shortening is induced by microsphere deposition, (2) shortened 3He T2s are accurately predicted, and (3) measured relaxation times are exploitable for quantifying local deposition patterns. Based on these findings the feasibility of imaging inhaled particulates in vivo with hyperpolarized 3He is examined and performance projections are formulated.

3He MRI-based assessment of posture-dependent regional ventilation gradients in rats

A recently developed method for quantitative assessment of regional lung ventilation was employed for the study of posture-dependent ventilation differences in rats. The measurement employed hyperpolarized (3)He MRI to detect the build-up of the signal intensity after increasing numbers of (3)He breaths, which allowed for computation of a regional ventilation parameter. A group of six anesthetized rats was studied in both supine and prone postures. Three-dimensional maps of the ventilation parameter were obtained with high spatial resolution (voxel volume approximately 2 mm(3)). Vertical (dorsal-ventral) gradients of the ventilation index, defined as the regional ventilation normalized by the average ventilation within the whole lung, were investigated. Variations in the regional distribution of the ventilation parameter, as well as of the ventilation index, could be detected, depending on the posture of the rats. In supine posture, ventilation was elevated in the dependent parts of the lungs, with a linear gradient of the ventilation index of -0.11 +/- 0.03 cm(-1). In prone posture, the distribution of ventilation was more uniform, with a significantly (P < 0.001) smaller gradient of the ventilation index of -0.01 +/- 0.02 cm(-1). It is concluded that the (3)He MRI-based method can detect and quantify regional ventilation gradients in animals as small as the rat and that these gradients depend on prone or supine posture of the animal.

Quantitative analysis of regional airways obstruction using dynamic hyperpolarized3He MRI—Preliminary results in children with cystic fibrosis

PURPOSE

To investigate regional airways obstruction in patients with cystic fibrosis (CF) with quantitative analysis of dynamic hyperpolarized (HP) (3)He MRI.

MATERIALS AND METHODS

Dynamic radial projection MRI of HP (3)He gas was used to study respiratory dynamics in a group of eight children with CF. Signal kinetics in a total of seven regions of interest (ROIs; three in each lung, and one in the trachea) were compared with the results of spirometric pulmonary function tests (PFTs). The tracheal signal intensity was used as a form of "input function" to normalize for input flow effects.

RESULTS

A pattern of low flow rate in the upper lobes was observed. When the flow measurements from the peripheral ROIs were averaged to obtain an index of flow in the peripheral lung, a good correlation was found (P = 3.74 x 10(-5)) with the forced expired volume in one second (FEV1).

CONCLUSION

These results suggest that a quantitative measurement of localized airways obstruction in the early stages of CF may be obtained from dynamic (3)He MRI by using the slope of the signal rise as a measure of air flow into the peripheral lung. This study also demonstrates that children can cooperate well with the (3)He MRI technique.

Dynamic lung morphology of methacholine-induced heterogeneous bronchoconstriction

Hyperpolarized (HP) 3helium (3He) dynamic MRI was used to investigate airway response in rats following intravenous (i.v.) bolus administration of a contractile agent, methacholine (MCh). The method provides direct visualization of the ventilated regions within the lung. Heterogeneous bronchoconstriction following the i.v. MCh injection was evident using this technique. These 3He dynamic lung images revealed that the inspired fresh air was shunted to the less-constricted regions after the MCh challenge in a similar manner as described by Laplace's relationship for the stability between adjacent alveoli. The airways in the more-constricted regions became nearly closed, resulting in air trapping, while the airways in the less-constricted regions remained effectively open, leading to overinflation. These data suggest a lung model of airway constriction partitioned into ventilated and nonventilated regions. These nonventilated regions are heterogeneously distributed in the lung and this distribution cannot be deduced from spirometric measurement of the whole lung. We demonstrate that a combination of functional 3He images and anatomical 1H images provide an effective method to diagnose regional lung abnormalities in rats.

A small animal model of regional alveolar ventilation using HP 3He MRI1

RATIONALE AND OBJECTIVES

The aim of this study was to establish a standardized procedure for the measurement of regional fractional ventilation in a healthy rat model as a baseline for further studies of pulmonary disorder models.

MATERIALS AND METHODS

The lungs of five healthy male Sprague-Dawley rats were imaged using hyperpolarized helium-3 magnetic resonance imaging. From these images, regional fractional ventilation was calculated and maps generated detailing the distribution of fractional ventilation in the lung. The 1.56 mm x 1.56 mm x 4 mm regions of interest were assigned on 5 cm x 5 cm field of view lung maps. Histograms were also generated showing the frequency distribution of fractional ventilation values. To compare fractional ventilation values between animals, the ventilation procedure was standardized to results from individual pulmonary function tests. Each animal's spontaneous tidal volume, respiratory rate, and inspiration percentage (percent of total respiratory cycle in inspiration) were used in their mechanical ventilation settings.

RESULTS

Results were similar among all five healthy rats based on examination of ventilation distribution maps and frequency distribution histograms. Mean (0.13) and standard deviation (0.07) were calculated for fractional ventilation in each animal. However, these values were determined to be influenced by slice selection, and therefore the maps and histograms were favored in analysis of results.

CONCLUSION

This study shows consistent results in healthy rat lungs and will serve as a baseline study for future measurements in emphysematous rat lungs.

Recent advances in imaging the lungs of intact small animals

A new generation of imaging devices now make it possible to generate both structural and functional images for the study of lung biology in small animals, including common laboratory mouse and rat models. "Micro" X-ray computed tomography and positron emission tomography scanners, highly sensitive cooled charge coupled device cameras for bioluminescence and fluorescence imaging, high magnetic field magnetic resonance imaging scanners, and recent advances in ultrasound system technology can be used to study such diverse processes as ventilation, perfusion, pulmonary hypertension, lung inflammation, and gene transfer, among others. Images from more than one modality can also be fused, allowing structure-function and function-function relationships to be studied on a regional basis. These new instruments, part of an emerging suite of techniques collectively known as "molecular imaging," provide an enormous potential for elucidating lung biology in intact animal models and systems.

Ventilation-synchronous magnetic resonance microscopy of pulmonary structure and ventilation in mice

Increasing use of transgenic animal models for pulmonary disease has raised the need for methods to assess pulmonary structure and function in a physiologically stable mouse. We report here an integrated protocol using magnetic resonance microscopy with gadolinium (Gd)-labeled starburst dendrimer (G6-1B4M-Gd, MW = 192 +/- 1 kDa, R(h) = 5.50 +/- 0.04 nm) and hyperpolarized (3)helium ((3)He) gas to acquire images that demonstrate pulmonary vasculature and ventilated airways in live mice (n = 9). Registered three-dimensional images of (1)H and (3)He were acquired during breath-hold at 2.0 T using radial acquisition (total acquisition time of 38 and 25 min, respectively). The macromolecular Gd-labeled dendrimer (a half-life of approximately 80 min) increased the signal-to-noise by 81 +/- 30% in the left ventricle, 43 +/- 22% in the lung periphery, and -4 +/- 5% in the chest wall, thus increasing the contrast of these structures relative to the less vascular surrounding tissues. A constant-flow ventilator was developed for the mouse to deliver varied gas mixtures of O(2) and N(2) (or (3)He) during imaging. To avoid hypoxemia, instrumental dead space was minimized and corrections were made to tidal volume lost due to gas compression. The stability of the physiologic support was assessed by the lack of spontaneous breathing and maintenance of a constant heart rate. We were able to stabilize the mouse for >8 hr using ventilation of 105 breath/min and approximately 0.2 mL/breath. The feasibility of acquiring both pulmonary vasculature and ventilated airways was demonstrated in the mouse lung with in-plane spatial resolution of 70 x 70 microm(2) and slice thickness of 800 microm.