**2. Properties of xenon**

Xenon, with the chemical element symbol Xe and atomic number 54, is a member of the zero-valence elements that are called noble gases or inert gases. Xenon was discovered in the residue left over from evaporating components of liquid air by William Ramsay and Morris Travers in England in 1898, then was named by Ramsay from Greek word *ξένον*, with the meaning 'foreign' and 'strange'. Natural abundant xenon is made of nine stable isotopes, and more than 35 unstable isotopes have been characterized. Nuclei of two isotopes, 129Xe and 131Xe, have non-zero spin quantum number: 26.4% of 129Xe with a nuclear spin I=1/2 ; and 21.2% of 131Xe with a nuclear spin I=3/2 (133Xe is used as a radioisotope in nuclear medicine). These two isotopes are both detectable by NMR with sensitivities of 0.021 (129Xe, per nucleus relative to proton assuming thermal polarization) and 2.710-3 ( 131Xe). The highly enhanced signal of hyperpolarized xenon and extremely long relaxation time greatly simplified and enhanced NMR experiments, and it is the fundamental for possible biological application in MRI.

Xenon, chemically inert with the external electronic orbits fully occupied, is well known as a noble gas at room temperature and an atmospheric pressure. However, the liquid and solid

Hyperpolarized Xenon Brain MRI 129

Owing to the inter-atomic collisions distortion of xenon electron cloud during the interactions with different chemical environment, the chemical shift of 129Xe is extremely sensitive. Total solvent effect on the Xe resonance frequency is over 7500 ppm, very large compared with most other NMR sensitive nuclei ( 1H : 20 ppm、13C : 300 ppm). A few examples are shown in Figure 2, the natural reference point for xenon chemical shift in the gas phase; with respect to the gas resonance (0 ppm), peaks at 70 ppm corresponding to cryptophane-bound xenon; around 197 ppm, five dissolved peaks can be observed, a dominant peak at 194.7 ppm and another discriminable peak at 189 ppm are identified as dissolved hyperpolarized 129Xe in the brain tissue and non-brain tissue, respectively, two small peaks at 191.6 ppm and 197.8 ppm are still unknown, and a smaller broad peak at 209.5 ppm comes from the dissolved hyperpolarized 129Xe in the blood (Zhou et al., 2008).

Fig. 2. Chemical shift of 129Xe in biosensor and biological system (Data taken from Zhou et

As a closed shelled noble gas, xenon has a peculiar large polarizable electron cloud, it can be easily interacted with biological materials, including water 'lipids' and proteins and so on; meanwhile, 129Xe NMR parameters like relaxation time and chemical shift are very sensitive to local chemical environment, these biochemical physiological characters make xenon a very interesting NMR probe for biological applications. Arising from large increase in sensitivity associated with hyperpolarized 129Xe, the biological NMR applications have been dramatically extended. Being Chemically Inert, xenon can be safely delivered into living organisms, associated with another advantage of no background signal, thus *in vivo* MR

Nuclear magnetic resonance (NMR) has been widely used in most fields of natural sciences, such as physics, chemistry, biology, and medicine. Because of the intrinsic low nuclear spin polarization at the thermal equilibrium, NMR is relatively insensitive. The normal 129Xe MRI, i.e., at the thermal equilibrium, is not able to get enough signal to visualize tissues or organs. The polarization can be moderately increased by using lower temperature or higher

imaging and spectroscopy is possible using hyperpolarized 129Xe technique.

**3. Signal enhanced MRI with hyperpolarized 129Xe** 

al., 2011c)

phases of Xenon can be easily obtained within an experimentally accessible range of temperatures and pressures (Cook, 1961) (Fig. 1).

Fig. 1. Phase diagram of xenon. (Figure from: http://science.nasa.gov/science-news/science-at-nasa/2008/25apr\_cvx2/)

Furthermore, the large and highly polarizable electron cloud makes xenon highly lipid soluble in solution, without chemically or structurally damage during interactions with other molecules. Ostwald solubility is defined as the ratio of the volume of the gas absorbed to the volume of the absorbing liquid, measured at same temperature and a pressure of 1 atm (101,325 Pa) (Cherubini, 2003; Oros, 2004) (Table 1).


Table 1. Solubilities of xenon gas in various compounds. (Data taken from Cherubini, 2003)

phases of Xenon can be easily obtained within an experimentally accessible range of

(Figure from: http://science.nasa.gov/science-news/science-at-nasa/2008/25apr\_cvx2/)

Compound Ostwald solubility

Water 0.11 Hexane 4.8 Benzene 3.1 Fluorobenzene 3.3 Carbon disulphide 4.2

Water 0.08 Saline 0.09 Plasma 0.10 Erythrocytes (98%) 0.20 Human albumin (100%,extrapolated) 0.15 Blood 0.14 Oil 1.9 Fat tissue 1.3 DMSO (dimethyl sulfoxide) 0.66 Intralipid (20%) 0.4 PFOB (perflubron) 1.2 PFOB (90% w/v, estimated) 0.62

Furthermore, the large and highly polarizable electron cloud makes xenon highly lipid soluble in solution, without chemically or structurally damage during interactions with other molecules. Ostwald solubility is defined as the ratio of the volume of the gas absorbed to the volume of the absorbing liquid, measured at same temperature and a pressure of 1

T=25 oC

T=37 oC

Table 1. Solubilities of xenon gas in various compounds. (Data taken from Cherubini, 2003)

temperatures and pressures (Cook, 1961) (Fig. 1).

atm (101,325 Pa) (Cherubini, 2003; Oros, 2004) (Table 1).

Fig. 1. Phase diagram of xenon.

Owing to the inter-atomic collisions distortion of xenon electron cloud during the interactions with different chemical environment, the chemical shift of 129Xe is extremely sensitive. Total solvent effect on the Xe resonance frequency is over 7500 ppm, very large compared with most other NMR sensitive nuclei ( 1H : 20 ppm、13C : 300 ppm). A few examples are shown in Figure 2, the natural reference point for xenon chemical shift in the gas phase; with respect to the gas resonance (0 ppm), peaks at 70 ppm corresponding to cryptophane-bound xenon; around 197 ppm, five dissolved peaks can be observed, a dominant peak at 194.7 ppm and another discriminable peak at 189 ppm are identified as dissolved hyperpolarized 129Xe in the brain tissue and non-brain tissue, respectively, two small peaks at 191.6 ppm and 197.8 ppm are still unknown, and a smaller broad peak at 209.5 ppm comes from the dissolved hyperpolarized 129Xe in the blood (Zhou et al., 2008).

Fig. 2. Chemical shift of 129Xe in biosensor and biological system (Data taken from Zhou et al., 2011c)

As a closed shelled noble gas, xenon has a peculiar large polarizable electron cloud, it can be easily interacted with biological materials, including water 'lipids' and proteins and so on; meanwhile, 129Xe NMR parameters like relaxation time and chemical shift are very sensitive to local chemical environment, these biochemical physiological characters make xenon a very interesting NMR probe for biological applications. Arising from large increase in sensitivity associated with hyperpolarized 129Xe, the biological NMR applications have been dramatically extended. Being Chemically Inert, xenon can be safely delivered into living organisms, associated with another advantage of no background signal, thus *in vivo* MR imaging and spectroscopy is possible using hyperpolarized 129Xe technique.

## **3. Signal enhanced MRI with hyperpolarized 129Xe**

Nuclear magnetic resonance (NMR) has been widely used in most fields of natural sciences, such as physics, chemistry, biology, and medicine. Because of the intrinsic low nuclear spin polarization at the thermal equilibrium, NMR is relatively insensitive. The normal 129Xe MRI, i.e., at the thermal equilibrium, is not able to get enough signal to visualize tissues or organs. The polarization can be moderately increased by using lower temperature or higher

Hyperpolarized Xenon Brain MRI 131

Hyperpolarized 129Xe magnetization enhanced by SEOP is non-recoverable, and the T1 of an hyperpolarized gas is the time elapsed for the signal to decay, because its thermal equilibrium polarization is almost zero relative to the hyperpolarized polarization. Usually, small flip angles has to be used to ration the hyperpolarized magnetization, and it is very important to have a T1 as long as possible to ensure that the signal lasts long enough for the acquisition. Therefore, the T1 of hyperpolarized 129Xe in the brain is a critical parameter. This subsection will discuss the aspects that affect T1 of hyperpolarized xenon, and describe the

For conventional MRI, the magnetization at thermal equilibrium is induced by the magnetic field, and the longitudinal relaxation time (T1) is the time for the magnetization, i.e. the magnetic resonance signal, to recover back to the thermal equilibrium. However, hyperpolarized noble gas magnetization produced by SEOP is unable to recover back to the hyperpolarized magnetization by itself, and the T1 of an hyperpolarized noble gas is the decay time of magnetization, because the thermal polarized magnetization is almost zero related to the hyperpolarized magnetization. The longitudinal relaxation time of hyperpolarized 129Xe in the brain is a critical parameter for developing hyperpolarized 129Xe 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 hyperpolarized 129Xe T1 in the rat brain (Choquet, 2003; Wakai, 2005).

The hyperpolarized magnetization is generally tipped by a pulse with a small flip angle, and it is very challenging to make the T1 as long as possible in order to ensure the signal lasts long enough for the acquisition. Therefore, when considering hyperpolarized 129Xe as a marker for brain perfusion by MRI, evaluation of tissue characterization and pulse sequence optimization, the T1 of hyperpolarized 129Xe in the brain is a critical parameter. Previous attempts to measure T1 in the rat brain have yielded strikingly disparate results. Wilson et al. found that T1 measured in rat brain homogenates in vitro ranged from 18±1 to 22±2 s (mean ± SD) (Wilson et. al,2009) depending on the oxygenation level of the tissue, and T1 values from measurements in rat brain *in vivo* have ranged from 3.6±2.1 (Choquet et al, 2003) to 26±4 s (Wakai et. al, 2005). Part of the discrepancy is believed to be due to the protocols used in T1 determination. The attempt of Choquet et al. used a multi-pulse protocol during the uptake and washout process by injecting hyperpolarized 129Xe in a lipid emulsion, whereas the estimation of Wakai et al. used a two-pulse protocol during the washout process after the rat had breathed hyperpolarized 129Xe gas. Under the condition of typically achieved polarizations (5–21%) (Zook et. al, 2001), low signal-to-noise ratio (SNR) due to the low concentration of the dissolved hyperpolarized 129Xe in tissue is an important factor in making T1 measurements in the rat brain (Cherubinia et. al, 2003; Ruppert et. al, 2000). The maximum SNR in the above two measurements in vivo was only 30 (Choquet et. al, 2003) and 46 (Wakai et. al, 2005), and the noise effect was not considered in these studies. When the SNR is low, noise will dominate the measured signal and result in large differences between the true T1 and the measured T1. Thus, low SNR might be a large

**4. Longitudinal relaxation time (T1) of hyperpolarized 129Xe in the brain** 

methods to accurately measure the T1 in the brain (Zhou et. al, 2008).

**4.1 T1 of hyperpolarized 129Xe** 

contributor of error in the published T1 values.

magnetic fields (Navon et al., 1996), whereas the nuclear spin polarizations of noble gases can be increased by four or five orders of magnitude via the spin-exchange optical pumping (SEOP) techniques (Walker, 1997). Therefore, it enables a very high sensitive detection of hyperpolarized 129Xe MRI. This subsection will describe how to produce hyperpolarized 129Xe, which is the basis for the brain imaging.

3He and 129Xe are generally selected for hyperpolarized lung gas imaging, because 3He and 129Xe are the only noble gas nuclei with nuclear spin 1/2, which results in the longitudinal relaxation time of many hours or even days at the standard temperature and pressure. However, hyperpolarized 3He could not be used for the brain study due to the extremely low solubility in blood and tissue, while hyperpolarized 129Xe is a novel contrast agent for the cerebral research. Hyperpolarized 129Xe is generally generated by employing the technique of SEOP.

For SEOP, the first step is to transfer the anglar momentum of the cicularly polarized laser light to the electronic spin, i.e., optical pumping. In principal, any alkali metal vapor can be optically pumped. Rb is normally used as the corresponding pumping laser diode arrays (LDA), which are routinely manufactured in high power configurations. When the Rb vapor experiences an external magnetic field of 20-30 Gauss, the ground state 52S1/2 is split into two Zeeman sublevels, MJ=-1/2 and MJ=1/2, i.e., electronic spin "down" or "up". These two Zeeman states have a nearly equal population at room temperature. After the Rb vapor absorbs the circularly polarized laser light (σ+) centered at 795nm, the Rb D1 transition occurs, i.e., 52S1/2 52P1/2. Accordingly, the ground state with a sublevel MJ=-1/2 is pumped into the excited state, MJ=1/2. The nitrogen gas quenches the excited state back to the ground state. Because the MJ=-1/2 sublevel continues to absorb the circularly polarized light (σ+) an excess of Rb atoms are optically pumped into the Zeeman sublevel MJ=1/2 while the other sublevel MJ=-1/2 is depleted. Therefore, an Rb electronic spin polarization of roughly 100% is able to be achieved.

The second step of SEOP is spin exchange, which occurs between the polarized electronic spins of Rb and the xenon nucleus. The collision between polarized Rb atoms and xenon atoms induces the transfer of angular momentum from the electronic spin to the nuclear spin. During this collision, the electron wave function of the Rb overlaps the nuclear wave function of xenon, which results in the spin exchange between the electronic spin and nuclear spin. Binary collisions dominate the spin exchange at high pressure, while threebody collision (by forming a Rb/Xe van der Waals molecule) dominate at low pressure (a few tens of torr).

Generally, Rb atoms and nitrogen gas are employed for optical pumping, however, Cs may be proposed as a better candidate for spin exchange with 129Xe due to several advantages: the natural abundance of 133Cs is 100% while Rb has two isotopes (85Rb and 87Rb), so that Cs is more convenient than Rb for wide applications of hyperpolarized 129Xe, particularly in the clinic application; optical pumping cells for Cs are operated at lower temperatures with correspondingly fewer chemical corrosion problems; according to the previous experimental results, the spin-exchange rate of Cs-Xe is about 10% higher than the Rb-Xe rate (Zhou et al., 2004a). When the polarization of hyperpolarized 129Xe is high enough, the observed radiation damping has been reported (Zhou, 2004b). The xenon polarizer with a flow feature can be readily extended to produce larger quantities of hyperpolarized 129Xe for not only medical imaging but also materials science and biology (Zhou, 2004c; Zhou, 2009a).

magnetic fields (Navon et al., 1996), whereas the nuclear spin polarizations of noble gases can be increased by four or five orders of magnitude via the spin-exchange optical pumping (SEOP) techniques (Walker, 1997). Therefore, it enables a very high sensitive detection of hyperpolarized 129Xe MRI. This subsection will describe how to produce hyperpolarized

3He and 129Xe are generally selected for hyperpolarized lung gas imaging, because 3He and 129Xe are the only noble gas nuclei with nuclear spin 1/2, which results in the longitudinal relaxation time of many hours or even days at the standard temperature and pressure. However, hyperpolarized 3He could not be used for the brain study due to the extremely low solubility in blood and tissue, while hyperpolarized 129Xe is a novel contrast agent for the cerebral research. Hyperpolarized 129Xe is generally generated by employing the

For SEOP, the first step is to transfer the anglar momentum of the cicularly polarized laser light to the electronic spin, i.e., optical pumping. In principal, any alkali metal vapor can be optically pumped. Rb is normally used as the corresponding pumping laser diode arrays (LDA), which are routinely manufactured in high power configurations. When the Rb vapor experiences an external magnetic field of 20-30 Gauss, the ground state 52S1/2 is split into two Zeeman sublevels, MJ=-1/2 and MJ=1/2, i.e., electronic spin "down" or "up". These two Zeeman states have a nearly equal population at room temperature. After the Rb vapor absorbs the circularly polarized laser light (σ+) centered at 795nm, the Rb D1 transition occurs, i.e., 52S1/2 52P1/2. Accordingly, the ground state with a sublevel MJ=-1/2 is pumped into the excited state, MJ=1/2. The nitrogen gas quenches the excited state back to the ground state. Because the MJ=-1/2 sublevel continues to absorb the circularly polarized light (σ+) an excess of Rb atoms are optically pumped into the Zeeman sublevel MJ=1/2 while the other sublevel MJ=-1/2 is depleted. Therefore, an Rb electronic spin polarization of roughly

The second step of SEOP is spin exchange, which occurs between the polarized electronic spins of Rb and the xenon nucleus. The collision between polarized Rb atoms and xenon atoms induces the transfer of angular momentum from the electronic spin to the nuclear spin. During this collision, the electron wave function of the Rb overlaps the nuclear wave function of xenon, which results in the spin exchange between the electronic spin and nuclear spin. Binary collisions dominate the spin exchange at high pressure, while threebody collision (by forming a Rb/Xe van der Waals molecule) dominate at low pressure (a

Generally, Rb atoms and nitrogen gas are employed for optical pumping, however, Cs may be proposed as a better candidate for spin exchange with 129Xe due to several advantages: the natural abundance of 133Cs is 100% while Rb has two isotopes (85Rb and 87Rb), so that Cs is more convenient than Rb for wide applications of hyperpolarized 129Xe, particularly in the clinic application; optical pumping cells for Cs are operated at lower temperatures with correspondingly fewer chemical corrosion problems; according to the previous experimental results, the spin-exchange rate of Cs-Xe is about 10% higher than the Rb-Xe rate (Zhou et al., 2004a). When the polarization of hyperpolarized 129Xe is high enough, the observed radiation damping has been reported (Zhou, 2004b). The xenon polarizer with a flow feature can be readily extended to produce larger quantities of hyperpolarized 129Xe for not only

medical imaging but also materials science and biology (Zhou, 2004c; Zhou, 2009a).

129Xe, which is the basis for the brain imaging.

technique of SEOP.

100% is able to be achieved.

few tens of torr).
