Longitudinal relaxation times of 129Xe in rat tissue homogenates at 9.4 T

Hyperpolarized 129 Xe imaging of the brain: Achievements and future challenges

Hyperpolarized (HP) xenon-129 (¹²⁹ Xe) brain MRI is a promising imaging modality currently under extensive development. HP ¹²⁹ Xe is nontoxic, capable of dissolving in pulmonary blood, and is extremely sensitive to the local environment. After dissolution in the pulmonary blood, HP ¹²⁹ Xe travels with the blood flow to the brain and can be used for functional imaging such as perfusion imaging, hemodynamic response detection, and blood-brain barrier permeability assessment. HP ¹²⁹ Xe MRI imaging of the brain has been performed in animals, healthy human subjects, and in patients with Alzheimer's disease and stroke. In this review, the overall progress in the field of HP ¹²⁹ Xe brain imaging is discussed, along with various imaging approaches and pulse sequences used to optimize HP ¹²⁹ Xe brain MRI. In addition, current challenges and limitations of HP ¹²⁹ Xe brain imaging are discussed, as well as possible methods for their mitigation. Finally, potential pathways for further development are also discussed. HP ¹²⁹ Xe MRI of the brain has the potential to become a valuable novel perfusion imaging technique and has the potential to be used in the clinical setting in the future.

Hyperpolarized 129Xe magnetic resonance spectroscopy in a rat model of bronchopulmonary dysplasia

Premature infants often require mechanical ventilation and oxygen therapy, which can result in bronchopulmonary dysplasia (BPD), characterized by developmental arrest and impaired lung function. Conventional clinical methods for assessing the prenatal lung are not adequate for the detection and assessment of long-term health risks in infants with BPD, highlighting the need for a noninvasive tool for the characterization of lung microstructure and function. Theoretical diffusion models, like the model of xenon exchange (MOXE), interrogate alveolar gas exchange by predicting the uptake of inert hyperpolarized (HP) ¹²⁹Xe gas measured with HP ¹²⁹Xe magnetic resonance spectroscopy (MRS). To investigate HP ¹²⁹Xe MRS as a tool for noninvasive characterization of pulmonary microstructural and functional changes in vivo, HP ¹²⁹Xe gas exchange data were acquired in an oxygen exposure rat model of BPD that recapitulates the fewer and larger distal airways and pulmonary vascular stunting characteristics of BPD. Gas exchange parameters from MOXE, including airspace mean chord length (L_m), apparent hematocrit in the pulmonary capillaries (HCT), and pulmonary capillary transit time (t_x), were compared with airspace mean axis length and area density (MAL and ρ_A) and percentage area of tissue and air (PTA and PAA) from histology. L_m was significantly larger in the exposed rats (P = 0.003) and correlated with MAL, ρ_A, PTA, and PAA (0.59<|ρ|<0.66 and P < 0.05). Observed increase in HCT (P = 0.012) and changes in t_x are also discussed. These findings support the use of HP ¹²⁹Xe MRS for detecting fewer, enlarged distal airways in this rat model of BPD, and potentially in humans.

Distribution of hyperpolarized xenon in the brain following sensory stimulation: preliminary MRI findings

In hyperpolarized xenon magnetic resonance imaging (HP (129)Xe MRI), the inhaled spin-1/2 isotope of xenon gas is used to generate the MR signal. Because hyperpolarized xenon is an MR signal source with properties very different from those generated from water-protons, HP (129)Xe MRI may yield structural and functional information not detectable by conventional proton-based MRI methods. Here we demonstrate the differential distribution of HP (129)Xe in the cerebral cortex of the rat following a pain stimulus evoked in the animal's forepaw. Areas of higher HP (129)Xe signal corresponded to those areas previously demonstrated by conventional functional MRI (fMRI) methods as being activated by a forepaw pain stimulus. The percent increase in HP (129)Xe signal over baseline was 13-28%, and was detectable with a single set of pre and post stimulus images. Recent innovations in the production of highly polarized (129)Xe should make feasible the emergence of HP (129)Xe MRI as a viable adjunct method to conventional MRI for the study of brain function and disease.

Measurement of alveolar oxygen partial pressure in the rat lung using Carr-Purcell-Meiboom-Gill spin-spin relaxation times of hyperpolarized 3He and 129Xe at 74 mT

Regional measurement of alveolar oxygen partial pressure can be obtained from the relaxation rates of hyperpolarized noble gases, (3) He and (129) Xe, in the lungs. Recently, it has been demonstrated that measurements of alveolar oxygen partial pressure can be obtained using the spin-spin relaxation rate (R(2) ) of (3) He at low magnetic field strengths (<0.1 T) in vivo. R(2) measurements can be achieved efficiently using the Carr-Purcell-Meiboom-Gill pulse sequence. In this work, alveolar oxygen partial pressure measurements based on Carr-Purcell-Meiboom-Gill R(2) values of hyperpolarized (3) He and (129) Xe in vitro and in vivo in the rat lung at low magnetic field strength (74 mT) are presented. In vitro spin-spin relaxivity constants for (3) He and (129) Xe were determined to be (5.2 ± 0.6) × 10(-6) Pa(-1) sec(-1) and (7.3 ± 0.4) × 10(-6) Pa(-1) s(-1) compared with spin-lattice relaxivity constants of (4.0 ± 0.4) × 10(-6) Pa(-1) s(-1) and (4.3 ± 1.3) × 10(-6) Pa(-1) s(-1), respectively. In vivo experimental measurements of alveolar oxygen partial pressure using (3) He in whole rat lung show good agreement (r(2) = 0.973) with predictions based on lung volumes and ventilation parameters. For (129) Xe, multicomponent relaxation was observed with one component exhibiting an increase in R(2) with decreasing alveolar oxygen partial pressure.

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.

Reinvestigating hyperpolarized (129)Xe longitudinal relaxation time in the rat brain with noise considerations

The longitudinal relaxation time of hyperpolarized (HP) (129)Xe in the brain is a critical parameter for developing HP (129)Xe brain imaging and spectroscopy and optimizing the pulse sequences, especially in the case of cerebral blood flow measurements. Various studies have produced widely varying estimates of HP (129)Xe T(1) in the rat brain. To make improved measurements of HP (129)Xe T(1) in the rat brain and investigate how low signal-to-noise ratio (SNR) contributes to these discrepancies, we developed a multi-pulse protocol during the washout of (129)Xe from the brain. Afterwards, we applied an SNR threshold theory to both the multi-pulse protocol and an existing two-pulse protocol. The two protocols yielded mean +/- SD HP (129)Xe T(1) values in the rat brain of 15.3 +/- 1.2 and 16.2 +/- 0.9 s, suggesting that the low SNR might be a key reason for the wide range of T(1) values published in the literature, a problem that might be easily alleviated by taking SNR levels into account.

A Method for Measuring the Decay Time of Hyperpolarized 129Xe Magnetization in Rat Brain without Estimation of RF Flip Angles

PURPOSE

The decay time of hyperpolarized 129Xe in brain tissue depends on the cerebral blood flow (CBF) as well as the longitudinal relaxation time in the tissue (T(1,tissue)). Therefore, the decay time is an important parameter for investigating the potential of Xe for cerebral studies. Previous attempts to measure the decay time have been performed after correction of the MR signal for the costheta decay induced by multiple radiofrequency (RF) excitation pulses. However, since this method requires accurate knowledge of the RF pulse flip angle, the use of a surface coil is restricted because of its nonuniform RF power, distribution. We present a two-pulse protocol for estimating the decay time without the need for flip-angle estimation and demonstrate it in the rat brain.

METHOD

After rat inhalation of hyperpolarized Xe, two MR spectra of the rat head were obtained at various delay times (4-16 s) and the logarithmic ratio of the two amplitudes was calculated. The decay time was obtained from the slope of the logarithmic ratio against the delay time. The MR measurements were performed with a 4.7T imaging spectrometer with a surface coil located over the head of the anesthetized rat. The gas (25 cc) was smoothly introduced to the lung for 40 s before each measurement began.

RESULT

From 18 experiments on 11 rats, the decay time was estimated to be 17.7+/-1.9 s.

DISCUSSION

Assuming a normal rat CBF value, T(1,tissue) can be estimated from the decay time to be 26+/-4 s.

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.

Improvement of T1 Determination of Hyperpolarized 129Xe in Mouse Brain under Controlled-Flow

The method of determining the longitudinal relaxation time of hyperpolarized 129Xe in the mouse brain has been established in vivo with the ventilation technique under controlled-flow conditions. The uptake and washout processes for nine mice were traced through observation of time-dependent changes in NMR (nuclear magnetic resonance) signal amplitudes and analyzed by means of a two-compartment model, thus providing the quantitative value of 14.1+/-1.6 s as the relaxation time.

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.

Magnetic Resonance Spectra of Hyperpolarized 129Xe in Human Blood and Living Rat Chest

We constructed a gas polarization system to test the feasibility of using hyperpolarized (129)Xe gas as an NMR (nuclear magnetic resonance) probe to explore brain function. Both in vitro and in vivo experiments were performed with a 4.7 T NMR spectrometer. Xenon spectra from human blood confirmed the existence of two peaks corresponding to red blood cells and plasma. In rat studies, three peaks at around 200 ppm were observed. Our results are consistent with previously reported data.

Xenon-129 MR imaging and spectroscopy of rat brain using arterial delivery of hyperpolarized xenon in a lipid emulsion

Hyperpolarized (129)Xe dissolved in a lipid emulsion constitutes an NMR tracer that can be injected into the blood stream, enabling blood-flow measurement and perfusion imaging. A small volume (0.15 ml) of this tracer was injected in 1.5 s in rat carotid and (129)Xe MR spectra and images were acquired at 2.35 T to evaluate the potential of this approach for cerebral studies. Xenon spectra consistently showed two resonances, at 194.5 ppm and 199.0 ppm relative to the gas peak. The signal-to-noise ratio (SNR) obtained for the two peaks was sufficient (ranging from 12 to 90) to follow their time courses. 2D transverse-projection xenon images were obtained with an in-plane resolution of 900 microm per pixel (SNR range 8-15). Histological analysis revealed no brain damage except in two rats that had received three injections.

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.

Distribution and dynamics of laser-polarized (129)Xe magnetization in vivo

The first magnetic resonance imaging studies of laser-polarized (129)Xe, dissolved in the blood and tissue of the lungs and the heart of Sprague-Dawley rats, are described. (129)Xe resonances at 0, 192, 199, and 210 ppm were observed and assigned to xenon in gas, fat, tissue, and blood, respectively. One-dimensional chemical-shift imaging (CSI) reveals xenon magnetization in the brain, kidney, and lungs. Coronal and axial two-dimensional CSI show (129)Xe dissolved in blood and tissue in the thorax. Images of the blood resonance show xenon in the lungs and the heart ventricle. Images of the tissue resonance reveal xenon in lung parenchyma and myocardium. The (129)Xe spectrum from a voxel located in the heart ventricle shows a single blood resonance. Time-resolved spectroscopy shows that the dynamics of the blood resonance match the dynamics of the gas resonance and demonstrates efficient diffusion of xenon gas to the lung parenchyma and then to pulmonary blood. These observations demonstrate the utility of laser-polarized (129)Xe to detect exchange across the gas-blood barrier in the lungs and perfusion into myocardial tissue. Applications to measurement of lung function, kidney perfusion, myocardial perfusion, and regional cerebral blood flow are discussed. Magn Reson Med 42:1137-1145, 1999.