Signal dynamics in magnetic resonance imaging of the lung with hyperpolarized noble gases

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.

Improved preclinical hyperpolarized 129 Xe ventilation imaging with constant flip angle 3D radial golden means acquisition and keyhole reconstruction

Hyperpolarized (HP) 129 Xe MRI is increasingly used to noninvasively probe regional lung structure and function in the preclinical setting. As in human imaging, the primary barrier to quantitative imaging with HP gases is nonequilibrium magnetization, which is depleted by T1 relaxation and radio frequency excitation. Preclinical HP gas imaging commonly involves mechanically ventilating small animals and encoding k-space over tens or hundreds of breaths, with small subsets of k-space data collected within each breath. Breath-to-breath magnetization renewal enables the use of large flip angles, but the resulting magnetization decay generates large view-to-view differences in within-breath signal intensity, leading to artifacts and degraded image quality. This deleterious signal decay has motivated the use of variable flip angle (VFA) sampling schemes, in which the flip angle is progressively increased to maintain constant view-to-view signal intensity. However, VFA imaging complicates data acquisition and provides only a global correction that fails to compensate for regional differences in signal dynamics. When constant flip angle (CFA) imaging is used alongside 3D radial golden means acquisition, the center of k-space is sampled with every excitation, thereby encoding signal dynamics alongside imaging data. Here, keyhole reconstruction is used to generate multiple images to capture in-breath HP 129 Xe signal dynamics in mice and thus provide flip angle maps to quantitatively correct images without extra data collection. These CFA images display SNR that is not significantly different from VFA images, and further, high frequency k-space scaling can be used to mitigate decay-induced image artifacts. Results are supported by point spread function calculations and simulations of radial imaging with preclinical signal dynamics. Together, these results show that CFA 3D radial golden means ventilation imaging provides comparable image quality with VFA in small animals and allows for keyhole reconstruction, which can be used to generate flip angle maps and correct images for signal depletion.

Preclinical hyperpolarized 129 Xe MRI: ventilation and T2 * mapping in mouse lungs at 7 T using multi-echo flyback UTE

Fast apparent transverse relaxation (short T2 *) is a common obstacle when attempting to perform quantitative 1 H MRI of the lungs. While T2 * times are longer for pulmonary hyperpolarized (HP) gas functional imaging (in particular for gaseous 129 Xe), T2 * can still lead to quantitative inaccuracies for sequences requiring longer echo times (such as diffusion weighted images) or longer readout duration (such as spiral sequences). This is especially true in preclinical studies, where high magnetic fields lead to shorter relaxation times than are typically seen in human studies. However, the T2 * of HP 129 Xe in the most common animal model of human disease (mice) has not been reported. Herein, we present a multi-echo radial flyback imaging sequence and use it to measure HP 129 Xe T2 * at 7 T under a variety of respiratory conditions. This sequence mitigates the impact of T1 relaxation outside the animal by using multiple gradient-refocused echoes to acquire images at a number of effective echo times for each RF excitation. After validating the sequence using a phantom containing water doped with superparamagnetic iron oxide nanoparticles, we measured the 129 Xe T2 * in vivo for 10 healthy C57Bl/6 J mice and found T2 * ~ 5 ms in the lung airspaces. Interestingly, T2 * was relatively constant over all experimental conditions, and varied significantly with sex, but not age, mass, or the O2 content of the inhaled gas mixture. These results are discussed in the context of T2 * relaxation within porous media.

Mapping and correcting hyperpolarized magnetization decay with radial keyhole imaging

PURPOSE

Hyperpolarized (HP) media enable biomedical imaging applications that cannot be achieved with conventional MRI contrast agents. Unfortunately, quantifying HP images is challenging, because relaxation and radio-frequency pulsing generate spatially varying signal decay during acquisition. We demonstrate that, by combining center-out k-space sampling with postacquisition keyhole reconstruction, voxel-by-voxel maps of regional HP magnetization decay can be generated with no additional data collection.

THEORY AND METHODS

Digital phantom, HP 129 Xe phantom, and in vivo 129 Xe human (N = 4 healthy; N = 2 with cystic fibrosis) imaging was performed using radial sampling. Datasets were reconstructed using a postacquisition keyhole approach in which 2 temporally resolved images were created and used to generate maps of regional magnetization decay following a simple analytical model.

RESULTS

Mean, keyhole-derived decay terms showed excellent agreement with the decay used in simulations (R2 = 0.996) and with global attenuation terms in HP 129 Xe phantom imaging (R2 > 0.97). Mean regional decay from in vivo imaging agreed well with global decay values and displayed spatial heterogeneity that matched expected variations in flip angle and oxygen partial pressure. Moreover, these maps could be used to correct variable signal decay across the image volume.

CONCLUSIONS

We have demonstrated that center-out trajectories combined with keyhole reconstruction can be used to map regional HP signal decay and to quantitatively correct images. This approach may be used to improve the accuracy of quantitative measures obtained from hyperpolarized media. Although validated with gaseous HP 129 Xe in this work, this technique can be generalized to any hyperpolarized agent.

Fast dynamic ventilation MRI of hyperpolarized 129 Xe using spiral imaging

Purpose

To develop and optimize a rapid dynamic hyperpolarized ¹²⁹Xe ventilation (DXeV) MRI protocol and investigate the feasibility of capturing pulmonary signal‐time curves in human lungs.

Theory and Methods

Spiral k‐space trajectories were designed with the number of interleaves N_int = 1, 2, 4, and 8 corresponding to voxel sizes of 8 mm, 5 mm, 4 mm, and 2.5 mm, respectively, for field of view = 15 cm. DXeV images were acquired from a gas‐flow phantom to investigate the ability of N_int = 1, 2, 4, and 8 to capture signal‐time curves. A finite element model was constructed to investigate gas‐flow dynamics corroborating the experimental signal‐time curves. DXeV images were also carried out in six subjects (three healthy and three chronic obstructive pulmonary disease subjects).

Results

DXeV images and numerical modelling of signal‐time curves permitted the quantification of temporal and spatial resolutions for different numbers of spiral interleaves. The two‐interleaved spiral (N_int = 2) was found to be the most time‐efficient to obtain DXeV images and signal‐time curves of whole lungs with a temporal resolution of 624 ms for 13 slices. Signal‐time curves were well matched in three healthy volunteers. The Spearman's correlations of chronic obstructive pulmonary disease subjects were statistically different from three healthy subjects (P < 0.05).

Conclusion

The N_int = 2 spiral demonstrates the successful acquisition of DXeV images and signal‐time curves in healthy subjects and chronic obstructive pulmonary disease patients. Magn Reson Med 79:2597–2606, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Dephasing and diffusion on the alveolar surface

We propose a surface model of spin dephasing in lung tissue that includes both susceptibility and diffusion effects to provide a closed-form solution of the Bloch-Torrey equation on the alveolar surface. The nonlocal susceptibility effects of the model are validated against numerical simulations of spin dephasing in a realistic lung tissue geometry acquired from synchotron-based μCT data sets of mouse lung tissue, and against simulations in the well-known Wigner-Seitz model geometry. The free induction decay is obtained in dependence on microscopic tissue parameters and agrees very well with in vivo lung measurements at 1.5 Tesla to allow a quantification of the local mean alveolar radius. Our results are therefore potentially relevant for the clinical diagnosis and therapy of pulmonary diseases.

Fast Determination of Flip Angle and T1 in Hyperpolarized Gas MRI During a Single Breath-Hold

MRI of hyperpolarized media, such as (129)Xe and (3)He, shows great potential for clinical applications. The optimal use of the available spin polarization requires accurate flip angle calibrations and T1 measurements. Traditional flip angle calibration methods are time-consuming and suffer from polarization losses during T1 relaxation. In this paper, we propose a method to simultaneously calibrate flip angles and measure T1 in vivo during a breath-hold time of less than 4 seconds. We demonstrate the accuracy, robustness and repeatability of this method and contrast it with traditional methods. By measuring the T1 of hyperpolarized gas, the oxygen pressure in vivo can be calibrated during the same breath hold. The results of the calibration have been applied in variable flip angle (VFA) scheme to obtain a stable steady-state transverse magnetization. Coupled with this method, the ultra-short TE (UTE) and constant VFA (CVFA) schemes are expected to give rise to new applications of hyperpolarized media.

(3)He pO2 mapping is limited by delayed-ventilation and diffusion in chronic obstructive pulmonary disease

PURPOSE

Lung pO2 mapping with (3)He MRI assumes that the sources of signal decay with time during a breath-hold are radiofrequency depolarization and oxygen-dependent T1 relaxation, but the method is sensitive to other sources of spatio-temporal signal change such as diffusion. The purpose of this work was to assess the use of (3)He pO2 mapping in patients with chronic obstructive pulmonary disease.

METHODS

Ten patients with moderate to severe chronic obstructive pulmonary disease were scanned with a 3D single breath-hold pO2 mapping sequence.

RESULTS

Images showed signal increasing over time in some lung regions due to delayed ventilation during breath-hold. Regions of physically unrealistic negative pO2 values were seen in all patients, and regional mean pO2 values of -0.3 bar were measured in the two patients most affected by delayed ventilation (where mean time to signal onset was 3-4 s).

CONCLUSIONS

Movement of gas within the lungs during breath-hold causes regional changes in signal over time that are not related to oxygen concentration, leading to erroneous pO2 measurements using the linear oxygen-dependent signal decay model. These spatio-temporal sources of signal change cannot be reliably separated at present, making pO2 mapping using this methodology unreliable in chronic obstructive pulmonary disease patients with significant bullous emphysema or delayed ventilation.

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.

K-space filter deconvolution and flip angle self-calibration in 2D radial hyperpolarised 3He lung MRI

In hyperpolarised (3)He lung MRI with constant flip angles, the transverse magnetisation decays with each RF excitation imposing a k-space filter on the acquired data. For radial data acquired in an angularly-sequential order, this filter causes streaking, angular shading and loss of spatial resolution in the images. The main aim of this work was to reduce the effects of the RF depletion k-space filter in radial acquisitions. Two approaches are presented; (i) retrospective deconvolution of the k-space filter for sequentially-acquired data and (ii) golden angle acquisition order. Radial trajectories sample the centre of k-space with every projection, thereby self-tracking signal decay. The inverse of the signal decay function was used to retrospectively deconvolve RF depolarisation k-space filter effects and the method was demonstrated in 2D radial imaging in phantoms and human lungs. A golden angle radial acquisition was shown to effectively suppress artefacts caused by the RF depletion k-space filter. In addition, the average flip angle per slice was calculated from the signal decay and the values were found to correspond with conventional flip angle maps, providing a means of flip angle self-calibration.

Relaxation of hyperpolarized 129Xe in a deflating polymer bag

In magnetic resonance imaging with hyperpolarized (HP) noble gases, data is often acquired during prolonged gas delivery from a storage reservoir. However, little is known about the extent to which relaxation within the reservoir will limit the useful acquisition time. For quantitative characterization, 129Xe relaxation was studied in a bag made of polyvinyl fluoride (Tedlar). Particular emphasis was on wall relaxation, as this mechanism is expected to dominate. The HP 129Xe magnetization dynamics in the deflating bag were accurately described by a model assuming dissolution of Xe in the polymer matrix and dipolar relaxation with neighboring nuclear spins. In particular, the wall relaxation rate changed linearly with the surface-to-volume ratio and exhibited a relaxivity of κ=0.392±0.008 cm/h, which is in reasonable agreement with κ=0.331±0.051 cm/h measured in a static Tedlar bag. Estimates for the bulk gas-phase 129Xe relaxation yielded T1bulk=2.55±0.22 h, which is dominated by intrinsic Xe-Xe relaxation, with small additional contributions from magnetic field inhomogeneities and oxygen-induced relaxation. Calculations based on these findings indicate that relaxation may limit HP 129Xe experiments when slow gas delivery rates are employed as, for example, in mouse imaging or vascular infusion experiments.

Gradient-induced longitudinal relaxation of hyperpolarized noble gases in the fringe fields of superconducting magnets used for magnetic resonance

When hyperpolarized noble gases are brought into the bore of a superconducting magnet for magnetic resonance imaging (MRI) or spectroscopy studies, the gases must pass through substantial field gradients, which can cause rapid longitudinal relaxation. In this communication, we present a means of calculating this spatially dependent relaxation rate in the fringe field of typical magnets. We then compare these predictions to experimental measurements of (3)He relaxation at various positions near a medium-bore 2-T small animal MRI system. The calculated and measured relaxation rates on the central axis of the magnet agree well and show a maximum (3)He relaxation rate of 3.83×10(-3) s(-1) (T(1)=4.4 min) at a distance of 47 cm from the magnet isocenter. We also show that if this magnet were self-shielded, its minimum T(1) would drop to 1.2 min. In contrast, a typical self-shielded 1.5-T clinical MRI scanner will induce a minimum on-axis T(1) of 12 min. Additionally, we show that the cylindrically symmetric fields of these magnets enable gradient-induced relaxation to be calculated using only knowledge of the on-axis longitudinal field, which can either be measured directly or calculated from a simple field model. Thus, while most MRI magnets employ complex and proprietary current configurations, we show that their fringe fields and the resulting gradient-induced relaxation are well approximated by simple solenoid models. Finally, our modeling also demonstrates that relaxation rates can increase by nearly an order of magnitude at radial distances equivalent to the solenoid radius.

Hyperpolarized Xe MR imaging of alveolar gas uptake in humans

BACKGROUND

One of the central physiological functions of the lungs is to transfer inhaled gases from the alveoli to pulmonary capillary blood. However, current measures of alveolar gas uptake provide only global information and thus lack the sensitivity and specificity needed to account for regional variations in gas exchange.

METHODS AND PRINCIPAL FINDINGS

Here we exploit the solubility, high magnetic resonance (MR) signal intensity, and large chemical shift of hyperpolarized (HP) (129)Xe to probe the regional uptake of alveolar gases by directly imaging HP (129)Xe dissolved in the gas exchange tissues and pulmonary capillary blood of human subjects. The resulting single breath-hold, three-dimensional MR images are optimized using millisecond repetition times and high flip angle radio-frequency pulses, because the dissolved HP (129)Xe magnetization is rapidly replenished by diffusive exchange with alveolar (129)Xe. The dissolved HP (129)Xe MR images display significant, directional heterogeneity, with increased signal intensity observed from the gravity-dependent portions of the lungs.

CONCLUSIONS

The features observed in dissolved-phase (129)Xe MR images are consistent with gravity-dependent lung deformation, which produces increased ventilation, reduced alveolar size (i.e., higher surface-to-volume ratios), higher tissue densities, and increased perfusion in the dependent portions of the lungs. Thus, these results suggest that dissolved HP (129)Xe imaging reports on pulmonary function at a fundamental level.

A short-breath-hold technique for lung pO2 mapping with 3He MRI

A pulse-sequence strategy was developed for generating regional maps of alveolar oxygen partial pressure (pO2) in a single 6-sec breath hold, for use in human subjects with impaired lung function. Like previously described methods, pO2 values are obtained by measuring the oxygen-induced T1 relaxation of inhaled hyperpolarized 3He. Unlike other methods, only two 3He images are acquired: one with reverse-centric and the other with centric phase-encoding order. This phase-encoding arrangement minimizes the effects of regional flip-angle variations, so that an accurate map of instantaneous pO2 can be calculated from two images acquired a few seconds apart. By combining this phase-encoding strategy with variable flip angles, the vast majority of the hyperpolarized magnetization goes directly into the T1 measurement, minimizing noise in the resulting pO2 map. The short-breath-hold pulse sequence was tested in phantoms containing known O2 concentrations. The mean difference between measured and prepared pO2 values was 1 mm Hg. The method was also tested in four healthy volunteers and three lung-transplant patients. Maps of healthy subjects were largely uniform, whereas focal regions of abnormal pO2 were observed in diseased subjects. Mean pO2 values varied with inhaled O2 concentration. Mean pO2 was consistent with normal steady-state values in subjects who inhaled 3He diluted only with room air.

Continuously infusing hyperpolarized 129Xe into flowing aqueous solutions using hydrophobic gas exchange membranes

Hyperpolarized (HP) (129)Xe yields high signal intensities in nuclear magnetic resonance (NMR) and, through its large chemical shift range of approximately 300 ppm, provides detailed information about the local chemical environment. To exploit these properties in aqueous solutions and living tissues requires the development of methods for efficiently dissolving HP (129)Xe over an extended time period. To this end, we have used commercially available gas exchange modules to continuously infuse concentrated HP (129)Xe into flowing liquids, including rat whole blood, for periods as long as one hour and have demonstrated the feasibility of dissolved-phase MR imaging with submillimeter resolution within minutes. These modules, which exchange gases using hydrophobic microporous polymer membranes, are compatible with a variety of liquids and are suitable for infusing HP (129)Xe into the bloodstream in vivo. Additionally, we have developed a detailed mathematical model of the infused HP (129)Xe signal dynamics that should be useful in designing improved infusion systems that yield even higher dissolved HP (129)Xe signal intensities.

Resolution enhancement in MRI of laser polarized 3He by control of diffusion

Diffusion of atoms or molecules in presence of magnetic field gradients not only attenuates the NMR signal but also leads to distortions close to restricting boundaries. This phenomenon is most evident in imaging with laser polarized (LP) noble gases. Diffusion of gases can be manipulated, however, by admixing inert gases of different molecular weight. In this work we analyze the effect of mixing LP-(3)He with SF(6) on the image quality of a phantom consisting of an arrangement of capillaries with different diameters. Admixing buffer gases of higher molecular weight changes the contrast and offers a means to record images with high spatial and time resolution. Additionally we demonstrate how distortions due to edge enhancement can be reduced even for long timed MRI-sequences.

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.

Hyperpolarized (129)Xe dynamic study in mouse lung under spontaneous respiration: application to murine tumor B16BL6 melanoma

This is a study on the analysis of hyperpolarized (HP) (129)Xe dynamics applied in the lung of a pathological model mouse under spontaneous respiration. A novel parameter k(1)k(2) - a product of the rate constant for Xe transfer from gas to dissolved phase (k(1)) and from dissolved to gas phase (k(2)) - was shown to be derived successfully from the analysis of the HP (129)Xe dynamic MR experiment in the mouse lung under spontaneous respiration with the aid of a selective pre-saturation technique. A comparative study using healthy mice and model mice induced with lung cancer (by injection of murine tumor B16BL6 melanoma) was performed and a significant difference was found in the k(1)k(2) values of the two groups, that is, 0.020+/-0.007s(-2) (n=4) for healthy mice and 0.032+/-0.04s(-2) (n=3) for lung cancer model mice (p=0.04). Thus, the parameter obtained by our proposed method is considered useful for detection of lung tumors.

Hyperpolarized 129 Xe MRI of the mouse lung at a low xenon concentration using a continuous flow-type hyperpolarizing system

PURPOSE

To apply a continuous flow-type hyperpolarizing (CF-HP) system to lung imaging and investigate the feasibility of hyperpolarized (129)Xe MRI at a low xenon concentration.

MATERIALS AND METHODS

Under two conditions where a 3% or 70% xenon gas mixture was constantly supplied, gas- and dissolved-phase (129)Xe images and diffusion-weighted (129)Xe-gas images were obtained from the mouse lung. Signal-to-noise ratio (SNR) of the (129)Xe images and the apparent diffusion coefficient (ADC) of xenon were compared between the two gas mixtures.

RESULTS

The SNR of gas- and dissolved-phase images were 28.9 +/- 5.2 and 12.0 +/- 2.0, respectively, using the 70% xenon gas mixture, while they were 22.9 +/- 4.8 and 6.8 +/- 0.6, using the 3% mixture. The ADC of xenon using the 3% xenon gas mixture was approximately 1.5 times higher than that using the 70% one. These results indicated that the high ADC increases the apparent replenishment rate of gas-phase magnetization, thus resulting in a reduction of the SNR loss induced by diluting xenon with quenching gases.

CONCLUSION

The CF-HP system is useful for lung imaging at an extremely low concentration of xenon, which enables one to fully restrain an anesthetic effect of xenon and to reduce consumption of xenon in a measurement.

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.

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.

3D MRI of non-Gaussian (3)He gas diffusion in the rat lung

In (3)He magnetic resonance images of pulmonary air spaces, the confining architecture of the parenchymal tissue results in a non-Gaussian distribution of signal phase that non-exponentially attenuates image intensity as diffusion weighting is increased. Here, two approaches previously used for the analysis of non-Gaussian effects in the lung are compared and related using diffusion-weighted (3)He MR images of mechanically ventilated rats. One approach is model-based and was presented by Yablonskiy et al., while the other approach utilizes the second order decay contribution that is predicted from the cumulant expansion theorem. Total lung coverage is achieved using a hybrid 3D pulse sequence that combines conventional phase encoding with sparse radial sampling for efficient gas usage. This enables the acquisition of nine 3D images using a total of only approximately 1 L of hyperpolarized (3)He gas. Diffusion weighting ranges from 0 s/cm(2) to 40 s/cm(2). Results show that the non-Gaussian effects of (3)He gas diffusion in healthy rat lungs are directly attributed to the anisotropic geometry of lung microstructure as predicted by the Yablonskiy model, and that quantitative analysis over the entire lung can be reliably repeated in time-course studies of the same animal.

Analysis of hyperpolarized 129Xe dynamics in mouse lungs under spontaneous respiration for separate determination of lung functional parameters and relaxation time

Magnetic resonance spectroscopy (MRS) was used to investigate the dynamics of hyperpolarized (HP) 129Xe respiration in the chests of mice under spontaneous respiration. The washout curve was analyzed using Kety's exchange model of inert gases, and the 3 factors that affect the slope of the washout curve, i.e., the RF flip angle, respiratory parameters, and apparent relaxation time (which comprises terms including the relaxation time in alveoli, T1air, and perfusion), were determined separately. Flip angle was determined precisely using the dual flip angle method, and ventilation volume was determined using SF6 gas at thermal equilibrium. Furthermore, an attempt was made to separate out the terms of T1air and perfusion from the apparent relaxation time after exploiting the ventilation model of lungs in steady state. Values of relaxation time T1air=30.5 s and perfusion term lambdaQ/VA=0.016 s-1 were obtained, supporting the applicability of the ventilation model proposed.

Time dependence of 3He diffusion in the human lung: measurement in the long-time regime using stimulated echoes

A stimulated-echo-based technique was developed to measure the long-time-scale apparent diffusion coefficient (ADC) of hyperpolarized 3He during a single breath-hold acquisition. Computer simulations were used to evaluate the performance of the technique and guide the selection of appropriate parameter values for obtaining accurate ADC values. The technique was used in 10 healthy subjects and two subjects with chronic obstructive pulmonary disease (COPD) to measure the global ADC for diffusion times between a few tenths of a second and several seconds, and to acquire spatial maps of the ADC for a diffusion time of 1.5 s. The reproducibility of the technique and its sensitivity to the direction of diffusion sensitization were also investigated. In healthy subjects, global ADC values decreased by severalfold over the range of diffusion times measured (mean values = 0.039 and 0.023 cm2/s at diffusion times of 0.61 and 1.54 s, respectively). ADC maps were generally uniform, with mean values similar to the corresponding global values. For the two COPD subjects, global ADC values were substantially greater than those of every healthy subject at all diffusion times measured. In addition, regional elevations of ADC values were far more conspicuous on long-time-scale ADC maps than on short-time-scale ADC maps.

Phase-contrast velocimetry with hyperpolarized3He for in vitro and in vivo characterization of airflow

This paper describes a technique that combines radial MRI and phase contrast (PC) to map the velocities of hyperpolarized gases ((3)He) in respiratory airways. The method was evaluated on well known geometries (straight and U-shaped pipes) before it was applied in vivo. Dynamic 2D maps of the three velocity components were obtained from a 10-mm slice with an in-plane spatial resolution of 1.6 mm within 1 s. Integration of the in vitro through-plane velocity over the slice matched the input flow within a relative precision of 6.4%. As expected for the given Reynolds number, a parabolic velocity profile was obtained in the straight pipe. In the U-shaped pipe the three velocity components were measured and compared to a fluid-dynamics simulation so the precision was evaluated as fine as 0.025 m s(-1). The technique also demonstrated its ability to visualize vortices and localize characteristic points, such as the maximum velocity and vortex-center positions. Finally, in vivo feasibility was demonstrated in the human trachea during inhalation.

Diffusion in binary gas mixtures studied by NMR of hyperpolarized gases and molecular dynamics simulations

The dependence of the individual mean square displacement of rare gases in binary mixtures is studied by a combined experimental and theoretical approach. We show that the diffusion constant can be varied in a considerable range by changing the molar fractions of the mixtures. On the experimental side, NMR diffusion measurements are done on hyperpolarized 3He and 129Xe, mixed with several inert buffer gases, in the presence of a magnetic field gradient. The results are compared to diffusion coefficients obtained from atomistic molecular dynamics simulations based on Lennard-Jones type potentials of the corresponding gas mixtures, and to appropriate analytical expressions, yielding very good mutual agreement. This study is the first quantitative validation of the effects of the mutual interactions between gas particles on the individual diffusion properties. It is shown that the dependency of gas phase diffusion properties on the local chemical environment may not be neglected, e.g. in diffusion-controlled chemical reactions.

MR Imaging of Mouse Lung Using Hyperpolarized 3He: Image Acquisition and T1 Estimation under Spontaneous Respiration

PURPOSE

The purpose of this study was to acquire a hyperpolarized (HP) (3)He image of the mouse lung and to estimate 3He T1 in the lung from wash-out curve analysis under spontaneous respiration.

MATERIALS AND METHODS

We first tested the K-Rb hybrid method for the spin-exchange optical pumping (SEOP) of 3He using a home-built noble gas polarizer operated at atmospheric pressure and then applied it to MR imaging and spectroscopy of the mouse lung. The longitudinal relaxation time (T1) of 3He in the mouse lung was estimated under spontaneous respiration by exploiting a novel method in which SF6 gas at thermal equilibrium was utilized in combination with the HP 3He gas in the quantitative wash-out curve analysis. This method utilizes the difference in the profile of the wash-out curve of HP 3He and SF6 at thermal equilibrium. That is, the slope of the 3He wash-out curve in the semi-logarithmic plots is affected by 3 factors, including RF pulse angle, respiration, and T1, whereas the slope of the SF6 wash-out curve is only the function of respiratory term.

RESULTS

A 3He lung image was obtained successfully, and we were able to estimate successfully 3He T1 in the mouse lung under spontaneous respiration using a novel method; the estimated T1 value was 68+/-25 s, which was reasonable compared with the value calculated from the literature data measured during breath-hold.

CONCLUSION

We succeeded in acquiring the first 3He image of mouse lung in vivo in this country, and our proposed method of estimating 3He T1 in the lung under spontaneous respiration is noninvasive and readily applied to animals and would be useful to evaluate the alveolar gas exchange function as well as oxygen partial pressure (pO2) in lungs of animals.

Advantages of parallel imaging in conjunction with hyperpolarized helium—A new approach to MRI of the lung

Hyperpolarized helium (3He) gas MRI has the potential to assess pulmonary function. The non-equilibrium state of hyperpolarized 3He results in the continual depletion of the signal level over the course of excitations. Under non-equilibrium conditions the relationship between the signal-to-noise ratio (SNR) and the number of excitations significantly deviates from that established in the equilibrium state. In many circumstances the SNR increases or remains the same when the number of data acquisitions decreases. This provides a unique opportunity for performing parallel MRI in such a way that both the temporal and spatial resolution will increase without the conventional decrease in the SNR. In this study an analytical relationship between the SNR and the number of excitations for any flip angle was developed. Second, the point-spread function (PSF) was utilized to quantitatively demonstrate the unconventional SNR behavior for parallel imaging in hyperpolarized gas MRI. Third, a 24-channel (24ch) receive and two-channel (2ch) transmit phased-array system was developed to experimentally prove the theoretical predictions with 3He MRI. The in vivo experimental results prove that significant temporal resolution can be gained without the usual SNR loss in an equilibrium system, and that the entire lung can be scanned within one breath-hold (approximately 13 s) by applying parallel imaging to 3D data acquisition.

In vivo diffusion weighted19F MRI using SF6

Diffusion weighted 19F images of rat lung in vivo using SF6 are presented. Projection-reconstruction images were acquired by filling the rat lung with a mixture of SF6 and air, during 64 successive apneas. Each apnea lasted for 6 s, the time required to perform 100 accumulations of each k-space radial phase step for the five values of the diffusion gradient (TR = 10 ms). After diffusion images were acquired, an apparent diffusion coefficient (ADC) map was generated, yielding an average value for the ADC of 2.22 x 10(-6) m2/s and SD for ADC values of 1.27 x 10(-6) m2/s. To the best of our knowledge, this is the first in vivo diffusion weighting imaging application and the first ADC map obtained using 19F MRI.

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.

Development of a hyperpolarized 129Xe system on 3T for the rat lungs

MRI (magnetic resonance imaging) with 129Xe has gained much attention as a diagnostic methodology because of its affinity for lipids and possible polarization. The quantitative estimation of net detectability and stability of hyperpolarized 129Xe in the dissolved phase in vivo is valuable to the development of clinical applications. The goal of this study was to develop a stable hyperpolarized 129Xe experimental 3T system to statistically analyze the dissolved-phase 129Xe signal in the rat lungs. The polarization of 129Xe with buffer gases at the optical pumping cell was measured under adiabatic fast passage against the temperature of an oven and laser absorption at the cell. The gases were insufflated into the lungs of Sprague-Dawley rats (n = 15, 400-550 g) through an endotracheal tube under spontaneous respiration. Frequency-selective spectroscopy was performed for the gas phase and dissolved phase. We analyzed the 129Xe signal in the dissolved phase to measure the chemical shift, T2*, delay and its ratio in a rat lungs on 3T. The polarizer was able to produce polarized gas (1.1+/-0.47%, 120 cm3) hundreds of times with the laser absorption ratio (25%) kept constant at the cell. The optimal buffer gas ratio of 25-50% rendered the maximum signal in the dissolved phase. Two dominant peaks of 211.8+/-0.9 and 201.1+/-0.6 ppm were observed with a delay of 0.4+/-0.9 and 0.9+/-1.0 s from the gas phase spectra. The ratios of their average signal to that of the gas phase were 5.6+/-5.2% and 4.4+/-4.7%, respectively. The T2* of the air space in the lungs was 2.5+/-0.5 ms, which was 3.8 times shorter than that in a syringe. We developed a hyperpolarized 129Xe experimental system using a 3T MRI scanner that yields sufficient volume and polarization and quantitatively analyzed the dissolved-phase 129Xe signal in the rat lungs.

Method to determine in vivo the relaxation time T1 of hyperpolarized xenon in rat brain

The magnetic polarization of the stable (129)Xe isotope may be enhanced dramatically by means of optical techniques and, in principle, hyperpolarized (129)Xe MRI should allow quantitative mapping of cerebral blood flow with better spatial resolution than scintigraphic techniques. A parameter necessary for this quantitation, and not previously known, is the longitudinal relaxation time (T(1) (tissue)) of (129)Xe in brain tissue in vivo: a method for determining this is reported. The time course of the MR signal in the brain during arterial injection of hyperpolarized (129)Xe in a lipid emulsion was analyzed using an extended two-compartment model. The model uses experimentally determined values of the RF flip angle and the T(1) of (129)Xe in the lipid emulsion. Measurements on rats, in vivo, at 2.35 T gave T(1) (tissue) = 3.6 +/- 2.1 sec (+/-SD, n = 6). This method enables quantitative mapping of cerebral blood flow.

Distal airways in humans: dynamic hyperpolarized 3He MR imaging--feasibility

Dynamic hyperpolarized helium 3 (3He) magnetic resonance (MR) imaging of the human airways is achieved by using a fast gradient-echo pulse sequence during inhalation. The resulting dynamic images show differential contrast enhancement of both distal airways and the lung periphery, unlike static hyperpolarized 3He MR images on which only the lung periphery is seen. With this technique, up to seventh-generation airway branching can be visualized.

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

Dynamic regional lung function was investigated in rats using a radial acquisition cine (RA-CINE) pulse sequence together with hyperpolarized (HP) (3)He gas delivered by a constant flow ventilator. Based on regional differences in the behavior of inspired air, the lung was conceptually divided into two regions (the major airways and the peripheral airspace) for purposes of functional analysis. To measure regional function in the major airways, a large RF flip angle (24 degrees) was applied to reduce (3)He magnetization in the peripheral airspace, and signal intensity (SI) was normalized with the projected airway diameter to estimate local airflow. Higher normalized signal intensity was observed in the left branch airway as compared to the right branch airway. To determine regional function in the peripheral airspace, a small RF flip angle (6 degrees) was used. Incremental increases of peripheral SI in successive lung images were consistent with the increase in lung volume. A new "skipping" scanning strategy using dummy frames allows a trade-off between the number of frames acquired for dynamic information, the RF flip angle, and the penetration depth of (3)He magnetization into the lung. This work provides a novel approach to simultaneously assess dynamic regional function and morphology.

MRI of the lungs using hyperpolarized noble gases

The nuclear spin polarization of the noble gas isotopes (3)He and (129)Xe can be increased using optical pumping methods by four to five orders of magnitude. This extraordinary gain in polarization translates directly into a gain in signal strength for MRI. The new technology of hyperpolarized (HP) gas MRI holds enormous potential for enhancing sensitivity and contrast in pulmonary imaging. This review outlines the physics underlying the optical pumping process, imaging strategies coping with the nonequilibrium polarization, and effects of the alveolar microstructure on relaxation and diffusion of the noble gases. It presents recent progress in HP gas MRI and applications ranging from MR microscopy of airspaces to imaging pulmonary function in patients and suggests potential directions for future developments.

k-space filtering in 2D gradient-echo breath-hold hyperpolarized 3He MRI: spatial resolution and signal-to-noise ratio considerations

In this work some of the factors that can influence the signal-to-noise ratio (SNR) and spatial resolution in MR images of inhaled hyperpolarized gases are systematically addressed. In particular, the effects of RF depletion of longitudinal polarization and image gradient diffusion dephasing were assessed in terms of their contribution to a k-space filter. By means of theoretical simulations and a novel method of experimental validation using a variable transverse magnetization of the 1H signal, systematic quantitative and qualitative investigations of the effects of k-space filtering intrinsic to imaging of hyperpolarized gas were made. A 2D gradient-echo image is considered for a range of flip angles with centric, sequential, and half-Fourier Cartesian phase-encoding strategies, and the results are assessed in terms of SNR and spatial resolution in the reconstructed images. Centric phase encoding was found to give the best SNR at higher flip angles, with a trade-off in spatial resolution compared to sequential phase encoding. A half-Fourier approach potentially offers increased SNR through the use of higher flip angles without compromising the spatial resolution, which is comparable to that achieved with sequential encoding.

Nuclear magnetic resonance of laser-polarized noble gases in molecules, materials, and organisms

The sensitivity of conventional nuclear magnetic resonance (NMR) techniques is fundamentally limited by the ordinarily low spin polarization achievable in even the strongest NMR magnets. However, by transferring angular momentum from laser light to electronic and nuclear spins, optical pumping methods can increase the nuclear spin polarization of noble gases by several orders of magnitude, thereby greatly enhancing their NMR sensitivity. This review describes the principles and magnetic resonance applications of laser-polarized noble gases. The enormous sensitivity enhancement afforded by optical pumping can be exploited to permit a variety of novel NMR experiments across numerous disciplines. Many such experiments are reviewed, including the void-space imaging of organisms and materials, NMR and MRI of living tissues, probing structure and dynamics of molecules in solution and on surfaces, NMR sensitivity enhancement via polarization transfer, and low-field NMR and MRI.

Relaxation behavior of laser-polarized (129)Xe gas: size dependency and wall effect of the T(1) relaxation time in glass and gelatin bulbs

Size dependency of the relaxation time T(1) was measured for laser-polarized (129)Xe gas encapsulated in different sized cavities made by glass bulbs or gelatin capsules. The use of laser-polarized gas enhances the sensitivity a great deal, making it possible to measure the longer (129)Xe relaxation time in quite a short time. The size dependency is analyzed on the basis of the kinetic theory of gases and a relationship is derived in which the relaxation rate is connected with the square inverse of the diameter of the cavity. Such an analysis provides a novel parameter which denotes the wall effect on the relaxation rate when a gas molecule collides with the surface once in a second. The relaxation time of (129)Xe gas is also dependent on the material which forms the cavity. This dependency is large and the relaxation study using polarized (129)Xe gas is expected to offer important information about the state of the matter of the cavity wall.

Pulmonary ventilation imaged by magnetic resonance: at the doorstep of clinical application

Over the past few years, magnetic resonance imaging (MRI) has emerged as an important instrument for functional ventilation imaging. The aim of this review is to summarize established clinical methods and emerging techniques for research and clinical arenas. Before the advent of MRI, chest radiography and computed tomography (CT) dominated morphological lung imaging, while functional ventilation imaging was accomplished with scintigraphy. Initially, MRI was not used for morphological lung imaging often, due to technical and physical limitations. However, recent developments have considerably improved anatomical MRI, as well as advanced new techniques in functional ventilation imaging, such as inhaled contrast aerosols, oxygen, hyperpolarized noble gases (Helium-3, Xenon-129), and fluorinated gases (sulphur-hexafluoride). Straightforward images demonstrating homogeneity of ventilation and determining ventilated lung volumes can be obtained. Furthermore, new image-derived functional parameters are measurable, such as airspace size, regional oxygen partial pressure, and analysis of ventilation distribution and ventilation/perfusion ratios. There are several advantages to using MRI: lack of radiation, high spatial and temporal resolution and a broad range of functional information. The MRI technique applied in patients with chronic obstructive pulmonary disease, emphysema, cystic fibrosis, asthma, and bronchiolitis obliterans, may yield a higher sensitivity in the detection of ventilation defects than ventilation scintigraphy, CT or standard pulmonary function tests. The next step will be to define the threshold between physiological variation and pathological defects. Using complementary strategies, radiologists will have the tools to characterize the impairment of lung function and to improve specificity.

Registered (1)H and (3)He magnetic resonance microscopy of the lung

Using in vivo magnetic resonance microscopy, registered (1)H and hyperpolarized (3)He images of the rat lung were obtained with a resolution of 0.098 x 0.098 x 0.469 mm (4.5 x 10(-3) mm(3)). The requisite stability and SNR was achieved through an integration of scan-synchronous ventilation, dual-frequency RF coils, anisotropic projection encoding, and variable RF excitation. The total acquisition time was 21 min for the (3)He images and 64 min for the (1)H image. Airways down to the 6th and 7th orders are clearly visible. Magn Reson Med 45:365-370, 2001.

Measurements of hyperpolarized gas properties in the lung. Part III: (3)He T(1)

Hyperpolarized (3)He spin-lattice relaxation was investigated in the guinea pig lung using spectroscopy and imaging techniques with a repetitive RF pulse series. T(1) was dominated by interactions with oxygen and was used to measure the alveolar O(2) partial pressure. In animals ventilated with a mixture of 79% (3)He and 21% O(2), T(1) dropped from 19.6 sec in vivo to 14.6 sec after cardiac arrest, reflecting the termination of the intrapulmonary gas exchange. The initial difference in oxygen concentration between inspired and alveolar air, and the temporal decay during apnea were related to functional parameters. Estimates of oxygen uptake were 29 +/- 11 mL min(-1) kg(-1) under normoxic conditions, and 9.0 +/- 2.0 mL min(-1) kg(-1) under hypoxic conditions. Cardiac output was estimated to be 400 +/- 160 mL min(-1) kg(-1). The functional residual capacity derived from spirometric magnetic resonance experiments varied with body mass between 5.4 +/- 0.3 mL and 10.7 +/- 1.1 mL. Magn Reson Med 45:421-430, 2001.

Laser-polarized (3)He as a probe for dynamic regional measurements of lung perfusion and ventilation using magnetic resonance imaging

Magnetic resonance imaging (MRI) using laser-polarized noble gases, such as (129)Xe and (3)He, allows unparalleled noninvasive information on gas distribution in lung airways and distal spaces. In addition to pulmonary ventilation, lung perfusion assessment is crucial for proper diagnosis of pathological conditions, such as pulmonary embolism. Magnetic resonance perfusion imaging usually can be performed using techniques based on the detection of water protons in tissues. However, lung proton imaging is extremely difficult due to the low proton density and the magnetically inhomogeneous structure of the lung parenchyma. Here we show that laser-polarized (3)He can be used as a noninvasive probe to image, in a single MRI experiment, not only the ventilation but also the perfusion state of the lungs. Blood volume maps of the lungs were generated based on the (3)He signal depletion during the first pass of a superparamagnetic contrast agent bolus. The combined and simultaneous lung ventilation and perfusion assessments are demonstrated in normal rat lungs and are applied to an experimental animal model of pulmonary embolism. Magn Reson Med 44:1-4, 2000.

Dynamic imaging of hyperpolarized (3)He distribution in rat lungs using interleaved-spiral scans

The use of spiral scan techniques is investigated for (3)He lung imaging on small animals. Dynamic series of up to 40 high temporal resolution (3)He ventilation images are obtained using a single bolus of gas. General properties of the spiral technique are discussed and compared to those of standard imaging techniques in relation to the specific case of rare gas imaging. To improve temporal resolution of the image series, the efficiency of a sliding window technique, combining data from two consecutive spiral images, is demonstrated. An example of the typical global (3)He signal variation during the (3)He breathing of the animal is shown. Pixel-by-pixel measurements of the (3)He signal derivative during the gas inspiration are performed. A corresponding lung map of the magnetization per time unit entering the lung during gas inflow is presented.

Intravenous delivery of hyperpolarized (129)Xe: a compartmental model

There is an increasing interest in the use of hyperpolarized 129-xenon (HpXe) NMR for the measurement of tissue perfusion. In this paper we present a theoretical study designed to assess the merit of intravenous HpXe delivery compared with the existing respiration techniques. A compartmental model was created to describe the behavior of the injected bolus in the circulatory system and in the lungs. The dependence of the tissue concentration on the T(1) and solubility of the Xe in the various compartments, and on injection rate, were evaluated. By this process the critical loss mechanisms are identified. It is shown that the predicted tissue concentrations of HpXe in gray and white matter are comparable using respiration or injection techniques.