MRI of the lungs using hyperpolarized noble gases

Fast metabolic imaging of systems with sparse spectra: application for hyperpolarized 13C imaging

A fast spiral chemical shift imaging (spCSI) sequence was developed for application to hyperpolarized (13)C imaging. The sequence exploits sparse spectra, which can occur in such applications, and prior knowledge of resonance frequencies to reduce the measurement time by undersampling the data in the spectral domain. As a consequence, multiple reconstructions of a given data set have to be computed in which only components with frequencies within a certain bandwidth are reconstructed "in focus" while others are severely blurred ("spectral tomosynthesis"). The sequence was tested at 3 T on a phantom containing approximately 1.5-M solutions of alanine (Ala), lactate (Lac), and pyruvate-pyruvate hydrate C1-C2 ester (with two resonances, PPE1 and PPE2) at thermal equilibrium polarization, all enriched to 99% (13)C in the C1 carbonyl positions. Results from spCSI with a single spatial interleaf (single-shot spCSI) and three interleaves (three-shot spCSI) were compared with those obtained by phase-encoded free induction decay CSI (FIDCSI). The metabolic maps of all four resonances for three-shot spCSI, and of PPE1 and PPE2 for single-shot spCSI demonstrate resolution and localization properties similar to those of the FIDCSI images. The metabolic maps of Ala and Lac for single-shot spCSI contain minor artifacts due to signal overlap of aliased resonances.

Diffusional kurtosis imaging in the lung using hyperpolarized 3He

Diseases of the small airspaces represent an increasingly important health problem. Asthma is primarily a disease of airway dysfunction, while chronic obstructive pulmonary disease (COPD) is associated with abnormalities in both the small airways and the alveoli. Conventional diffusion magnetic resonance imaging (MRI) of hyperpolarized noble gases, because of the short T(2)* of the gas, is only capable of monitoring diffusion over short times and hence only short distances. Diffusion imaging is therefore only sensitive to changes in small structures of the lung (primarily the alveoli), and will not adequately interrogate diffusion along the longitudinal axes of bronchi and bronchioles. In this communication we present a new method, termed diffusional kurtosis imaging (DKI), that is particularly sensitive to diffusion over longer distances. DKI may therefore be more sensitive to abnormalities in the bronchioles and bronchi than conventional diffusion imaging. Preliminary DKI measurements on healthy human subjects and one patient with symptoms suggestive of small airway disease are presented. Although the apparent diffusion coefficient (ADC) in the patient was similar to that in the normal controls, diffusional kurtosis was markedly reduced. This suggests that DKI measurements may be useful for assessing diseases of the small airways.

XTC MRI: Sensitivity improvement through parameter optimization

Xenon polarization Transfer Contrast (XTC) MRI pulse sequences permit the gas exchange of hyperpolarized xenon-129 in the lung to be measured quantitatively. However, the pulse sequence parameter values employed in previously published work were determined empirically without considering the now-known gas exchange rates and the underlying lung physiology. By using a theoretical model for the consumption of magnetization during data acquisition, the noise intensity in the computed gas-phase depolarization maps was minimized as a function of the gas-phase depolarization rate. With such optimization the theoretical model predicted an up to threefold improvement in precision. Experiments in rabbits demonstrated that for typical imaging parameter values the optimized XTC pulse sequence yielded a median noise intensity of only about 3% in the depolarization maps. Consequently, the reliable detection of variations in the average alveolar wall thickness of as little as 300 nm can be expected. This improvement in the precision of the XTC MRI technique should lead to a substantial increase in its sensitivity for detecting pathological changes in lung function.

Restricted diffusion in a model acinar labyrinth by NMR: theoretical and numerical results

A branched geometrical structure of the mammal lungs is known to be crucial for rapid access of oxygen to blood. But an important pulmonary disease like emphysema results in partial destruction of the alveolar tissue and enlargement of the distal airspaces, which may reduce the total oxygen transfer. This effect has been intensively studied during the last decade by MRI of hyperpolarized gases like helium-3. The relation between geometry and signal attenuation remained obscure due to a lack of realistic geometrical model of the acinar morphology. In this paper, we use Monte Carlo simulations of restricted diffusion in a realistic model acinus to compute the signal attenuation in a diffusion-weighted NMR experiment. We demonstrate that this technique should be sensitive to destruction of the branched structure: partial removal of the interalveolar tissue creates loops in the tree-like acinar architecture that enhance diffusive motion and the consequent signal attenuation. The role of the local geometry and related practical applications are discussed.

Hyperpolarized13C MRI of the pulmonary vasculature and parenchyma

The study of lung perfusion in normal and diseased subjects is of great interest to physiologists and physicians. In this work we demonstrate the application of a liquid-phase hyperpolarized (HP) carbon-13 ((13)C) tracer to magnetic resonance imaging (MRI) of the pulmonary vasculature and pulmonary perfusion in a porcine model. Our results show that high spatial and temporal resolution images of pulmonary perfusion can be obtained with this contrast technique. Traditionally, pulmonary perfusion measurement techniques have been challenging because of insufficient signal for quantitative functional assessments. The use of polarized (13)C in MRI overcomes this limitation and may lead to a viable clinical method for studying the pulmonary vasculature and perfusion.

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.

Advances in magnetic resonance imaging of lung physiology

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

Hyperpolarized 3He diffusion MRI and histology in pulmonary emphysema

Diffusion MRI of hyperpolarized (3)He shows that the apparent diffusion coefficient (ADC) of (3)He gas is highly restricted in the normal lung and becomes nearly unrestricted in severe emphysema. The nature of this restricted diffusion provides information about lung structure; however, no direct comparison with histology in human lungs has been reported. The purpose of this study is to provide information about (3)He gas diffusivity in explanted human lungs, and describe the relationship between (3)He diffusivity and the surface area to lung volume ratio (SA/V) and mean linear intercept (L(m)) measurements--the gold standard for diagnosis of emphysema. Explanted lungs from patients who were undergoing lung transplantation for advanced COPD, and donor lungs that were not used for transplantation were imaged via (3)He diffusion MRI. Histological measurements were made on the same specimens after they were frozen in the position of study. There is an inverse correlation between diffusivity and SA/V (and a positive correlation between diffusivity and L(m)). An important result is that restricted (3)He diffusivity separated normal from emphysematous lung tissue more clearly than the morphometric analyses. This effect may be due to the smaller histologic sampling size compared to the MRI voxel sizes.

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.

Steady-state free precession with hyperpolarized 3He: experiments and theory

The magnetization response of hyperpolarized 3He gas to a steady-state free precession (SSFP) sequence was simulated using matrix product operators. The simulations included the effects of flip angle (alpha), sequence timings, resonant frequency, gas diffusion coefficient, imaging gradients, T1 and T2. Experiments performed at 1.5 T, on gas phantoms and with healthy human subjects, confirm the predicted theory, and indicate increased SNR with SSFP through use of higher flip angles when compared to optimized spoiled gradient echo (SPGR). Simulations and experiments show some compromise to the SNR and some point spread function broadening at high alpha due to the incomplete refocusing of transverse magnetization, caused by diffusion dephasing from the readout gradient. Mixing of gas polarization levels by diffusion between slices is also identified as a source of signal loss in SSFP at higher alpha through incomplete refocusing. Nevertheless, in the sample experiments, a SSFP sequence with an optimized flip angle of alpha=20 degrees, and 128 sequential phase encoding views, showed a higher SNR when compared to SPGR (alpha=7.2 degrees) with the same bandwidth. Some of the gas sample experiments demonstrated a transient signal response that deviates from theory in the initial phase. This was identified as being caused by radiation damping interactions between the large initial transverse magnetization and the high quality factor (Q=250) birdcage resonator. In 3He NMR experiments, performed without imaging gradients, diffusion dephasing can be mitigated, and the effective T2 is relatively long (1 s). Under these circumstances the SSFP sequence behaves like a CPMG sequence with sinalpha/2 weighting of SNR. Experiments and simulations were also performed to characterize the off-resonance behaviour of the SSFP HP 3He signal. Characteristic banding artifacts due to off-resonance harmonic beating were observed in some of the in vivo SSFP images, for instance in axial slices close to the diaphragm where B0 inhomogeneity is highest. Despite these artifacts, a higher SNR was observed with SSFP in vivo when compared to the SPGR sequence. The trends predicted by theory of increasing SSFP SNR with increasing flip angle were observed in the range alpha=10-20 degrees without compromise to image quality through blurring caused by excessive k-space filtering.

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.

[Microstructure of the lung: diffusion measurement of hyperpolarized 3Helium]

Imaging methods to study the lung are traditionally based on x-ray or on radioactive contrast agents. Conventional magnetic resonance imaging (MRI) has only limited applications for lung imaging because of the low tissue density of protons concentration of hydrogen atoms, which are usually the basis for the imaging. The introduction of hyperpolarized noble gases as a contrast agent in MRI has opened new possibilities for lung diagnosis. The present paper describes this new technique. Diffusion-weighted MRI for assessment of the lung microstructure is presented here as an example of the new possibilities of functional imaging. Studies to determine the sensitivity of the diffusion measurement and regarding the correlation with traditionally established methods are also presented, along with results of the measurement of the reproducibility determined in a clinical pilot study on healthy volunteers and patients. Furthermore, a pilot measurement of the 3He diffusion tensor in the lung is presented.

Relaxation and diffusion of perfluorocarbon gas mixtures with oxygen for lung MRI

We report measurements of free diffusivity D(0) and relaxation times T(1) and T(2) for pure C(2)F(6) and C(3)F(8) and their mixtures with oxygen. A simplified relaxation theory is presented and used to fit the data. The results enable spatially localized relaxation time measurements to determine the local gas concentration in lung MR images, so the free diffusivity D(0) is then known. Comparison of the measured diffusion to D(0) will express the extent of diffusion restriction and allow the local surface-to-volume ratio to be found.

State of the Art. A structural and functional assessment of the lung via multidetector-row computed tomography: phenotyping chronic obstructive pulmonary disease

With advances in multidetector-row computed tomography (MDCT), it is now possible to image the lung in 10 s or less and accurately extract the lungs, lobes, and airway tree to the fifth- through seventh-generation bronchi and to regionally characterize lung density, texture, ventilation, and perfusion. These methods are now being used to phenotype the lung in health and disease and to gain insights into the etiology of pathologic processes. This article outlines the application of these methodologies with specific emphasis on chronic obstructive pulmonary disease. We demonstrate the use of our methods for assessing regional ventilation and perfusion and demonstrate early data that show, in a sheep model, a regionally intact hypoxic pulmonary vasoconstrictor (HPV) response with an apparent inhibition of HPV regionally in the presence of inflammation. We present the hypothesis that, in subjects with pulmonary emphysema, one major contributing factor leading to parenchymal destruction is the lack of a regional blunting of HPV when the regional hypoxia is related to regional inflammatory events (bronchiolitis or alveolar flooding). If maintaining adequate blood flow to inflamed lung regions is critical to the nondestructive resolution of inflammatory events, the pathologic condition whereby HPV is sustained in regions of inflammation would likely have its greatest effect in the lung apices where blood flow is already reduced in the upright body posture.

Multiexponential attenuation of the CPMG spin echoes due to a geometrical confinement

The CPMG multi-echo technique is often used to investigate the translational motion of diffusing nuclei in a confining medium. Henceforth, periodically repeated RF pulses with a diffusion-sensitizing gradient yield a formation of spin echoes of gradually decreasing amplitudes. The parameters of their exponential fits may characterize the structure of porous materials or biological tissue. In this paper, a multiexponential character of the CPMG measurements is rigorously demonstrated, once a geometrical confinement is present. Based on the multiple propagator approach, we derived a spectral representation for the echo amplitudes under external magnetic field of an arbitrary gradient profile. The multiple relaxation times and their spectral weights were found in a general form. The study of simple restrictive media allowed to obtain a quantitative condition under which the multiexponential attenuation is reduced to a monoexponential one.

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.

Parallel imaging of hyperpolarized helium-3 with simultaneous slice excitation

Hyperpolarized (HP) gas imaging of the lungs is an ideal potential application for parallel imaging. This is due to the fact that there is limited scan time (breath hold of 20 s) and limited non-renewable polarization. Reduced phase encode parallel imaging is demanding on hardware in that it requires multiple receivers. In this work, simultaneous parallel acquisition of hyperpolarized (HP) 3He images from multiple slices was demonstrated in phantoms and in vivo using a simultaneous slice excitation method, at a field strength of 1.5 T. The pulse sequence allows simultaneous acquisition of n slices per RF excitation, thus reducing the number of RF pulses needed to fully cover a given volume with multi-slicing. Unlike conventional parallel imaging, this method does not require prior reference scan information, which would consume some of the finite longitudinal polarization in lung ventilation studies with HP gas.

q-space analysis of lung morphometry in vivo with hyperpolarized3He spectroscopy

PURPOSE

To examine the utility of a (3)He spectroscopic q-space technique for detecting changes in lung morphometry in vivo.

MATERIALS AND METHODS

A diffusion-weighted spectroscopy sequence was used to collect global diffusion data from healthy adults (N = 11), healthy children (N = 5), and chronic obstructive pulmonary disease (COPD) patients (N = 2) using 40 cc of hyperpolarized (3)He gas within a two second breathhold. Displacement probability profiles (DPP) were obtained by Fourier transformation of diffusion data with respect to q. A bi-Gaussian model was used to decompose the DPPs into narrow and broad components, characterized by root-mean-square displacements X(rms1) and X(rms2), respectively.

RESULTS

In healthy adults, the narrow component (X(rms,1)) of the DPP had a mean displacement of 188 +/- 10 microm, slightly less than the reported average size of the alveoli. The broad component (X(rms,2)) had a mean value of 474 +/- 44 microm, comparable to the diameter of the respiratory bronchioles in the acinus. In children, both X(rms1) (167 +/- 4 microm) and X(rms2) (382 +/- 22 microm) compared to healthy adults (P < 0.01). In COPD patients, the mean displacements were elevated (X(rms1): 265 +/- 71 microm; X(rms2): 530 +/- 109 microm) compared to healthy adults. Excellent correlation was found between rms displacements and age (age vs. X(rms,1): r = 0.78, P < 0.001; age vs. X(rms,2): r = 0.90, P < 0.001).

CONCLUSION

The q-space parameters agreed remarkably well with published alveolar morphometry data. The results suggest that the technique may be sensitive to disease, as evident from the elevated mean displacements in COPD patients compared to healthy volunteers. Detailed lung microstructural information can be obtained using a very low volume of inhaled (3)He.

A System for Open-Access He Human Lung Imaging at Very Low Field

We describe a prototype system built to allow open-access very-low-field MRI of human lungs using laser-polarized (3)He gas. The system employs an open four-coil electromagnet with an operational B(0) field of 4 mT, and planar gradient coils that generate gradient fields up to 0.18 G/cm in the x and y direction and 0.41 G/cm in the z direction. This system was used to obtain (1)H and (3)He phantom images and supine and upright (3)He images of human lungs. We include discussion on challenges unique to imaging at 50 -200 kHz, including noise filtering and compensation for narrow-bandwidth coils.

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.

Hyperpolarized krypton-83 as a contrast agent for magnetic resonance imaging

For the first time, magnetic resonance imaging (MRI) with hyperpolarized (hp) krypton-83 (83Kr) has become available. The relaxation of the nuclear spin of 83Kr atoms (I = 9/2) is driven by quadrupolar interactions during brief adsorption periods on surrounding material interfaces. Experiments in model systems reveal that the longitudinal relaxation of hp 83Kr gas strongly depends on the chemical composition of the materials. The relaxation-weighted contrast in hp 83Kr MRI allows for the distinction between hydrophobic and hydrophilic surfaces. The feasibility of hp 83Kr MRI of airways is tested in canine lung tissue by using krypton gas with natural abundance isotopic distribution. Additionally, the influence of magnetic field strength and the presence of a breathable concentration of molecular oxygen on longitudinal relaxation are investigated.

Potential applications of laser polarised xenon for CBF measurements by NMR

Laser polarised (hyperpolarised) 129Xe offers a new potential way for assessment of cerebral blood flow (CBF) by nuclear magnetic resonance (NMR). We present the basic physical principles underlying noble gases'laser polarisation. Distinguishing characteristics of 129Xe NMR properties as well as theoretical basis for CBF measurement using laser polarised 129Xe are emphasized. Recent results are briefly presented and potential applications as well as current limitations of 129Xe in CBF studies are discussed.

Ultra-fast three dimensional imaging of hyperpolarized 13C in vivo

OBJECTIVE

PASADENA, a chemical method of enhancing nuclear spin polarization has demonstrated 13C polarizations of order unity for the nascent products of molecular addition by parahydrogen. The extreme brevity of signal enhancement obtained by hyperpolarization requires improved 13C MR in vivo imaging techniques for their optimum utility.

MATERIALS AND METHODS

13C imaging sequences, including 13C 3D FIESTA, were compiled for a GE LX 1.5 T clinical MR scanner. Two water soluble 13C imaging agents were hyperpolarized utilizing parahydrogen and an automated polarizer. 13C polarization was quantified in flow phantoms and in rats with jugular vein catheters.

RESULTS

Fast 3D FIESTA 13C MR imaging technique acquired sequential 3D images (3.66 s/acquisition) with superior SNR. Hyperpolarized 13C solutions and vascular phantoms achieved a maximum signal of 26,624+/-593. In vivo 13C MR images of the cardiopulmonary circulation showed maximum 13C signal of 2,402+/-158. 13C images acquired within 3.66 s showed signal enhancement over 10,000 compared to equilibrium polarization.

CONCLUSION

3D-FIESTA was effective for sub-second in vivo imaging of hyperpolarized 13C reagents produced in a custom-built parahydrogen polarizer. Application to 13C hyperpolarized by parahydrogen is demonstrated in vitro and in vivo.

Diffusion-weighted 19F-MRI of lung periphery: Influence of pressure and air-SF6 composition on apparent diffusion coefficients

Lung functional magnetic resonance imaging (MRI) has become a reality using different inert hyperpolarized gases, such as 3He and 129Xe, which have provided an extraordinary boost in lung imaging and has also attracted interest to other chemically inert gaseous contrast agents. In this context, we have recently demonstrated the first diffusion-weighted images using thermally polarized inhaled sulfur hexafluoride (SF6) in small animals. The aim of this study was to evaluate whether or not the diffusion coefficient of this fluorinated gas is sensitive to pulmonary structure, gas concentration and air pressure in the airways. Diffusion coefficients of SF6 (both pure and in air mixtures) measured in vitro at different pressures and 20 degrees C showed an excellent agreement with theoretical values. Measurements of diffusion coefficients were also performed in vivo and post-mortem on healthy rats, achieving satisfactory signal-to-noise ratios (SNRs), and SF6 gas was found to be in an almost completely restricted diffusion regime in the lung, i.e., the transport by molecular diffusion is delayed by collisions with barriers such as the alveolar septa. This observed low diffusivity means that this gas will be less sensitive to structural changes in the lungs than other magnetic resonance sensitive gas such as 3He, particularly at human scale. However, it is still possible that SF6 plays a role since it opens a new structural window. Thus, the interest of researchers in delimiting the important limiting technical factors that makes this process very challenging is obvious. Among them, T2 relaxation is very fast, so gradient systems with very fast switching rate and probably large radiofrequency (RF) power and high field systems will be needed for hexafluoride to be used in human studies.

3He diffusion MRI of the lung

RATIONALE AND OBJECTIVES

MR imaging of the restricted diffusion of laser-polarized 3He gas provides unique insights into the changes in lung microstructure in emphysema.

RESULTS

We discuss measurements of ventilation (spin density), mean diffusivity, and the anisotropy of diffusion, which yields the mean acinar airway radius. In addition, the use of spatially modulated longitudinal magnetization allows diffusion to be measured over longer distances and times, with sensitivity to collateral ventilation paths. Early results are also presented for spin density and diffusivity maps made with a perfluorinated inert gas, C3F8.

METHODS

Techniques for purging and imaging excised lungs are discussed.

Long-lived quantum memory with nuclear atomic spins

We propose to store nonclassical states of light into the macroscopic collective nuclear spin (10(18) atoms) of a 3He vapor, using metastability exchange collisions. These collisions, commonly used to transfer orientation from the metastable state 2 3S1 to the ground state of 3He, can also transfer quantum correlations. This gives a possible experimental scheme to map a squeezed vacuum field state onto a nuclear spin state with very long storage times (hours).

Noninvasive mapping of regional response to segmental allergen challenge using magnetic resonance imaging and [F-18]fluorodeoxyglucose positron emission tomography

Magnetic resonance (MR) and positron emission tomography (PET) imaging techniques were coregistered to demonstrate regional ventilation and inflammation in the lung for in vivo, noninvasive evaluation of regional lung function associated with allergic inflammation. Four Brown Norway rats were imaged pre- and post segmental allergen challenge using respiratory-gated He-3 magnetic resonance imaging (MRI) to visualize ventilation, T(1)-weighted proton MRI to depict inflammatory infiltrate, and [F-18]fluorodeoxyglucose-PET to detect regional glucose metabolism by inflammatory cells. Segmental allergen challenges were delivered and the pre- and postchallenge lung as well as the contralateral lung were compared. Coregistration of the imaging results demonstrated that regions of ventilation defects, inflammatory infiltrate, and increased glucose metabolism correlated well with the site of allergen challenge delivery and inflammatory cell recruitment, as confirmed by histology. This method demonstrates that fusion of functional and anatomic PET and MRI image data may be useful to elucidate the functional correlates of inflammatory processes in the lungs.

3He lung imaging in an open access, very-low-field human magnetic resonance imaging system

The human lung and its functions are extremely sensitive to gravity; however, the conventional high-field magnets used for most laser-polarized (3)He MRI of the human lung restrict subjects to lying horizontally. Imaging of human lungs using inhaled laser-polarized (3)He gas is demonstrated in an open-access very-low-magnetic-field (<5 mT) MRI instrument. This prototype device employs a simple, low-cost electromagnet, with an open geometry that allows variation of the orientation of the imaging subject in a two-dimensional plane. As a demonstration, two-dimensional lung images were acquired with 4-mm in-plane resolution from a subject in two orientations: lying supine and sitting in a vertical position with one arm raised. Experience with this prototype device will guide optimization of a second-generation very-low-field imager to enable studies of human pulmonary physiology as a function of subject orientation.

3D volume-localized pO2 measurement in the human lung with 3He MRI

A method for 3D volume-localized quantification of pO2 in the lungs is presented that uses repetitive frame 3D gradient-echo imaging of (3)He. The method was demonstrated by experiments on (3)He phantoms containing known concentrations of O(2) and in vivo on a group of three healthy human volunteers. The results were compared with those obtained by equivalent 2D thin-slice and 2D projection methodologies, and were found to be consistent with published results from the 2D projection methodologies (pO(2) = 0.09-0.18 bar). Studies performed on the same subject, on three separate occasions, demonstrated a repeatability of pO(2) measurement to within 14% using the 3D technique. Experimental differences between the 2D and 3D methods were substantiated with theoretical and numerical analyses of the signal decay, which took into account the effects of out-of-slice diffusion as a source of error in the thin-slice 2D experiments. It is shown that the 2D thin-slice technique systematically underestimates pO2 when there is significant gas diffusion (factor of 4 underestimate for D = 0.9 cm(2)s(-1) representative of free (3)He in air).

Combined helium-3/proton magnetic resonance imaging measurement of ventilated lung volumes in smokers compared to never-smokers

PURPOSE

To use a combination of helium-3 (3-He) magnetic resonance imaging (MRI) and proton single-shot fast spin echo (SSFSE) to compare ventilated lung volumes in groups of "healthy" smokers, smokers diagnosed with moderate chronic obstructive pulmonary disease (COPD), and never-smokers.

MATERIALS AND METHODS

All study participants were assessed with spirometry prior to imaging. 3-He images were collected during an arrested breath hold, after inhaling a mixture of 200 mL of hyperpolarized 3-He/800 mL of N2. Proton SSFSE images were acquired after inhaling 1 liter of room air. The ventilated volume for each study participant was calculated from the 3-He images, and a ratio was calculated to give a percentage ventilated lung volume.

RESULTS

Never-smokers exhibited a 90% mean ventilated volume. The mean ventilated lung volumes for healthy smokers and smokers diagnosed with COPD were 75.2% and 67.6%, respectively. No correlation with spirometry was demonstrated for either of the smoking groups.

CONCLUSION

Combined 3-He/Proton SSFSE MRI of the lungs is a noninvasive method, using nonionizing radiation, which demonstrates ventilated airspaces and enables the calculation of ventilated lung volumes. This method appears to be sensitive to early obstructive changes in the lungs of smokers.

Long-range diffusion of hyperpolarized 3He in explanted normal and emphysematous human lungs via magnetization tagging

Long-range diffusivity of hyperpolarized 3He gas was measured from the decay rate of sinusoidally modulated longitudinal nuclear magnetization in three normal donor and nine severely emphysematous explanted human lungs. This (long-range) diffusivity, which we call Dsec, is measured over seconds and centimeters and is approximately 10 times smaller in healthy lungs (0.022 cm2/s) than the more traditionally measured Dmsec, which is measured over milliseconds and submillimeters. The increased restriction of Dsec reflects the complex, tortuous paths required to navigate long distances through the maze of branching peripheral airways. In emphysematous lungs, Dsec is substantially increased, with some regions showing nearly the unrestricted value of the self-diffusion coefficient (0.88 cm2/s for dilute 3He in air, a 40-fold increase). This suggests the presence of large collateral pathways opened by alveolar destruction that bypass the airways proper. This destruction was confirmed by comparison with histology in seven lungs and by removal of trapped gas via holes in the pleural surface in five lungs.

Pulmonary Embolism: Comprehensive Evaluation with MR Ventilation and Perfusion Scanning with Hyperpolarized Helium-3, Arterial Spin Tagging, and Contrast-enhanced MRA

PURPOSE

Development of a comprehensive magnetic resonance (MR) examination consisting of MR angiography (MRA) and MR ventilation and perfusion (MR V/Q) scan for the detection of pulmonary emboli (PE) and assessment of the technique in a rabbit model.

MATERIALS AND METHODS

Reversible PE was induced by inflating a non-detachable silicon balloon in the left pulmonary artery of five New Zealand White rabbits. MR V/Q scans were obtained prior to, during, and after balloon deflation. MRA was performed during balloon inflation. MR ventilation imaging was performed after the inhalation of hyperpolarized helium-3. MR perfusion imaging was performed with Flow-sensitive Alternating Inversion Recovery with an Extra Radiofrequency pulse technique (FAIRER). High-resolution contrast-enhanced MR pulmonary angiography was used to confirm the occlusion of the pulmonary artery. All imaging was performed on a 1.5-T whole body scanner with broadband capabilities.

RESULTS

High-resolution ventilation images of the lungs were obtained. No ventilation defects were detected before, during, or after resolution of simulated PE. FAIRER imaging allowed visualization of pulmonary perfusion. No perfusion defects were detected prior to balloon inflation. During balloon inflation (PE), there was decreased perfusion in the left lower lobe. After reversal of the PE, there was improved perfusion to the left lower lobe. In analogy to nuclear medicine techniques, acute PE produced a mismatched defect in the MR V/Q scan. MRA verified the occlusive filling defect in the left pulmonary artery.

CONCLUSION

High-resolution MRA and MR V/Q imaging of the lung is feasible and allows comprehensive assessment of pulmonary embolism in one imaging session.

Image artifacts in very low magnetic field MRI: the role of concomitant gradients

While MRI at very low magnetic fields has certain potential advantages, it may also face problems that are not typical for MRI at conventional and high field (0.1-10T). Major differences arise due to the presence of concomitant components of inhomogeneous magnetic field (gradients) that are transverse to the major B(z) field, B(0). These concomitant transverse field components are inevitably generated by the same gradient coils that generate desired B(z) imaging gradients as routinely used in MRI for spatial encoding. In the hypothetical case (linear spatial variation of B(z) field amplitude due to the imaging gradients, no concomitant transverse field components, no B(0) and B(1) field inhomogeneities, etc.), Fourier transform MRI preserves the shape of the real object being examined. It is demonstrated herein that unavoidable concomitant transverse field gradients, G, result in an image deformation of the object's actual shape by bending straight lines and planes with a characteristic curvature radius R(c)=B(0)/G. For imaging gradients on the order of 10 mT/m and B(0) of 1T, the radius R(c) is about 100 m and image distortions are generally negligible. However, for B(0) of 1 mT, R(c) is 10 cm, which is less than a typical FOV in human studies. This manuscript derives expressions describing geometrical relationships between the imaged object and the obtained MR data. In addition to geometrical distortions, image intensity will be modulated in a complex, spatially dependent manner. Hence, if unaccounted for, corresponding image distortions-geometry and intensity-will create substantial difficulties in very low field image interpretation.

Frequency-narrowed diode array bar

We describe a method to frequency narrow multielement high-power diode bars. Using a commercial 60-W, 49-element, 1-cm-long diode array bar at 795 nm running at 45 W, we narrow the linewidth from 1000 to 64 GHz with only a loss of 33% in output power. The resulting laser light is well suited for spin-exchange optical pumping of noble gas nuclei.

Measurements and modeling of long range 3He diffusion in the lung using a "slice-washout" method

In healthy lung tissue, pulsed-gradient-spin-echo (PGSE) methods reveal apparent diffusion coefficients (ADC) of the order 0.20 cm2 s(-1); for diffusion times of approximately 2 ms. For these short diffusion times the ADC is only sensitive to structures approximately (2Dt)1/2 approximately 0.6mm in size. Recent work, using magnetic tagging of the longitudinal magnetization has revealed much smaller ADC values for longer length scales. In this work, the in vivo ADC from within the air-spaces, was measured using a new technique. The signal from a series of images was analyzed from a slice that was repeatedly imaged. Diffusion tends to "top-up" the non-renewable polarization within the slice, which leads to a non-exponential decay in image signal. Image data were compared to 1D finite-difference simulations of diffusion to calculate a long range ADC value. The results yield values of the order 0.034 cm2 s(-1), which are nearly an order of magnitude smaller than those reported by PGSE measurements at shorter diffusion times.

Functional MRI of the lung using hyperpolarized 3-helium gas

Lung imaging has traditionally relied on x-ray methods, since proton MRI is limited to some extent by low proton density in the lung parenchyma and static field inhomogeneities in the chest. The relatively recent introduction of MRI of hyperpolarized noble gases has led to a rapidly evolving field of pulmonary MRI, revealing functional information of the lungs, which were hitherto unattainable. This review article briefly describes the physical background of the technology, and subsequently focuses on its clinical applications. Four different techniques that have been used in various human investigations are discussed: ventilation distribution, ventilation dynamics, and small airway evaluation using diffusion imaging and oxygen uptake assessment.

Finite-difference simulations of 3He diffusion in 3D alveolar ducts: comparison with the "cylinder model"

Time-dependent measurements of 3He diffusion in the lung could provide an accurate method to quantify alveolar length scales and the progression of diseases such as emphysema. However, the apparent diffusion coefficient (ADC) presents a complex problem to model and solve analytically. Here, finite-difference methods were used to simulate diffusion in 3D alveolar ducts. The results were compared to the only available analytical model--the "cylinder model"--from which it is possible to estimate the average radii of the alveolar ducts from in vivo data. The trend in data observed from simulations was found to agree well with the cylinder model. However, the cylinder model always overestimated the average radii of the simulated alveolar ducts. The simulations also demonstrated that the measurement of the longitudinal ADC (along the alveolar ducts) should be sensitive to early emphysematous changes, whereas the measured radii should be far less sensitive.

Theoretical signal-to-noise ratio and spatial resolution dependence on the magnetic field strength for hyperpolarized noble gas magnetic resonance imaging of human lungs

In hyperpolarized noble gas (HNG) magnetic resonance (MR) imaging, the available polarization is independent of magnetic field strength and for large radiofrequency (rf) coils, such as those used for chest imaging, the body noise becomes the primary noise source making signal-to-noise ratio (SNR) largely frequency independent at intermediate field strengths (0.1-0.5 T). Furthermore, the reduction in the transverse relaxation time, T2, of HNG in lungs with increasing field strength, results in a decrease in the achievable SNR at higher fields. In this work, the optimum field strength for HNG MR imaging was theoretically calculated in terms of both SNR and spatial resolution. SNR calculations used the principle of reciprocity and included contributions to the noise arising from both coil and sample losses in a chest-sized coil for lung imaging. The effects of susceptibility differences, transverse relaxation time, and diffusion were considered in the resolution calculations. The calculations show that the optimum field strength for HNG MR imaging of human lungs is between 0.1 and 0.6 T depending on gas type (helium or xenon) and sample size. At the field strengths currently used by conventional clinical proton MR imaging systems (1-3 T), the predicted SNR are 10%-50% lower than at the optimum field with only slightly worse spatial resolution (10%-20%). At higher fields (>3 T), however, the SNR degrades considerably reducing the achievable spatial resolution. Although HNG of the lung is still feasible at very low field strengths (<50 mT), the available SNR is much lower than at optimum fields and this reduces the achievable spatial resolution. These findings suggest that HNG imaging may be optimally performed at much lower field strengths (0.1-0.6 T) than conventional clinical proton MR imaging systems. This could considerably decrease cost, improve patient access, and reduce chemical shift and susceptibility artifacts and rf heating.

Oxygen-enhanced proton imaging of the human lung usingT2*

Magnetic susceptibility gradients caused by tissue/air interfaces lead to very short T(2)* times in the human lung. These susceptibility gradients are dependent on the magnetic susceptibility of the respiratory gas and therefore should influence T(2)* relaxation. In this work, a technique for quantitative T(2)* mapping of the human lung during one breath hold is presented. Using this method, the lung T(2)* relaxation time was measured under normoxic (room air, 21% O(2)) and hyperoxic (100% O(2)) conditions to verify this assumption. The mean T(2)* difference between room air and 100% O(2) is about 10% and contains ventilation information, since only ventilated regions contribute to signal change due to different susceptibility gradients.

19F MR imaging of ventilation and diffusion in excised lungs

Perfluorinated gases, particularly C2F6, are potentially suitable alternatives to hyperpolarized noble gases for pulmonary airspace spin density and diffusion MRI. This work focuses mainly on 19F imaging of C2F6 gas in healthy and emphysematous explanted lungs, avoiding regulatory issues of human in vivo measurements. Three-dimensional gradient echo and spin echo spin density images of human lungs can be made in 10 s with adequate signal-to-noise, demonstrating the feasibility for breathing dynamics to be captured during a succession of short breath holds. As expected, the spin echo images have much smaller susceptibility artifacts than the gradient echo images. 19F and 3He images of the same lungs are compared. The apparent diffusion coefficient (ADC) of C2F6 is sensitive to restrictions imposed by the lung microstructure: the average ADC is measured to be 0.018 cm2/s in healthy lungs versus 0.031 cm2/s in emphysematous lungs at a diffusion time Delta=2.2 ms. The low free diffusivity of pure C2F6 (D0=0.033 cm2/s) places it in a regime where the ADC measurement allows the surface-to-volume ratio to be determined in each voxel, a potentially valuable quantitative characterization of regional lung tissue destruction in emphysema.

Comparison between 2D and 3D gradient-echo sequences for MRI of human lung ventilation with hyperpolarized 3He

Images of hyperpolarized 3He were acquired during breath-hold in four healthy volunteers with the use of an optimized 3D gradient-echo sequence. The images were compared with existing 2D gradient-echo methods. The average SNR from a 13-mm-thick slice in the peripheral lung was 1.4 times greater with 3D. In the airways the average SNR was 1.7 times greater with 3D. The higher SNR of 3D was particularly evident when regions of unimpeded gas diffusion, such as the major airways, were imaged with thin slices. This is because diffusion dephasing due to the slice-encoding gradient is minimized with a 3D sequence. The in vivo experimental findings were substantiated with experiments on phantoms of free gas, which showed more than four times the SNR with 3D compared to 2D. Theoretical simulations of the 2D and 3D k-space filters were also performed to predict the SNR and spatial resolution observed in the experimental images.

Functional evaluation of emphysema using diffusion-weighted 3Helium-magnetic resonance imaging, high-resolution computed tomography, and lung function tests

PURPOSE

To assess the emphysematous enlargement of distal airspaces and concomitant large and small airway disease using diffusion-weighted Helium-magnetic resonance imaging (MRI), high-resolution computed tomography (HRCT), and lung function tests (LFT).

METHODS

Seven patients were examined after single lung transplantation (LTx) and 1 before double LTx for various forms of emphysema. Five patients after double LTx served as controls. Patients were assessed by Helium-MRI (apparent diffusion coefficient [ADC]), HRCT (mean lung density [MLD], emphysema index [EI]), and LFT.

RESULTS

Transplanted lungs: mean ADC = 0.17 cm/s, MLD = -848 H, EI = 22%. Emphysematous lungs: mean ADC = 0.33 cm/s, MLD = -922 H; EI = 54%. Good correlations were found between ADC and MLD (r = 0.6), EI (r = 0.8), intrathoracic gas volume (r = 0.7), forced expiratory volume in 1 second (r = 0.7), and forced expiratory flows (r = 0.7). In contrast, HRCT only provided moderate correlations with LFT (EI: r = 0.5; MLD: r [le] 0.4).

CONCLUSION

In this initial study, He-MRI yield good correlations with HRCT and agrees better than HRCT with the functional characterization of emphysema regarding hyperinflation, large and small airway disease as provided by LFT.

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.

Imaging of a mixture of hyperpolarized 3He and 129Xe

With the use of hyperpolarized gases, a great number of experiments have been carried out in order to improve the diagnostics of the lung, both from a structural and a functional point of view. 3He is best suited for structural studies, whereas 129Xe gives more detailed information about the functionality of the lung because it enters the bloodstream. In this work, we propose the use of a gas mixture to perform consecutive analysis of lung structure and functionality upon the delivery of a single bolus of gas. We show images of a helium-xenon gas mixture in the presence of a small amount of liquid toluene in order to demonstrate how both nuclei can be detected independently, extracting the spectroscopic information provided by the 129Xe spectra and obtaining an image with high sensitivity for 3He. A second experiment performed on a dissected mouse lung was used to demonstrate how the mixture of gases can enhance sensitivity in the larger airways of the lung.