the nuclear spins of xenon and other noble 
 gases can be hyperpolarized 
 ( hp ) to order unity by the process of spin - exchange optical pumping 
 ( seop ) . in this two - step process
, the electron spins 
 of an alkali metal vapor such as rubidium are first polarized by the 
 absorption of angular momentum from circularly polarized light .
spin - exchange 
 collisions between the alkali metal atoms and xe then 
 transfer the angular momentum to the xe nuclear spins 
 through fermi hyperfine interactions , resulting in a high non - boltzmann 
 distribution of xe spin states that increases the detection 
 sensitivity of nmr / mri .
the two most common approaches to hyperpolarize xe 
 via seop are termed continuous flow(714 ) and stopped flow(1525 ) ( sometimes also referred to as  batch mode  ) with 
 respect to the delivery of xe gas to and from the polarization cell . 

n2 gas is typically added to the gas mixture to quench 
 alkali metal fluorescence .
the batch - mode / stopped - flow systems are attractive not only because 
 of their relative simplicity but also because they can operate in 
 a xenon - rich regime that obviates the need to separate the polarized 
 xe from the n2 ( or he ) via cryocollection , eliminating 
 a potential source of polarization loss as well as facilitating applications using quadrupolar noble gas 
 isotopes .
this production method has also 
 been scaled up for automated production of clinically required quantities .
regardless of the polarization 
 method , hp noble gases have seen 
 wide application varying from fundamental physics experiments to nmr / mri applications including molecular biosensors , probing structural aspects of cage molecules and proteins , and studies of porous materials ( to name only a few ) .
however , it has been biomedical applications that 
 have largely driven the development of hyperpolarized mr techniques 
 over the past decade ; indeed , for gas imaging in particular , hp xe can be used to assess lung function and report on functional 
 and microstructural abnormalities .
a useful figure of merit for xe hyperpolarizers is 
 the total xe magnetization , mxe , delivered in a clinically useful gas volume , typically 0.51 
 l at 760 torr .
mxe is determined by the 
 product of nuclear spin polarization pxe and xe concentration [ xe ] , i.e. , mxe  pxe[xe ] .
it is 
 therefore important to maximize mxe through 
 both pxe and [ xe ] , which is challenging 
 because pxe generally decreases as [ xe ] 
 within the seop - cell increases ( mostly 
 because of increased alkali metal spin - destruction rates from non - spin - conserving 
 collisions with xe ) .
but fundamentally laser photons 
 are the source of xe hyperpolarization ; thus , the decreasing 
 cost of laser diodes narrowly tuned to the alkali metal rubidium d1 wavelength ( 794.8 nm ) has made 
 economically feasible the higher photon fluxes required to improve mxe when xe partial pressures are high . in the present work , a 200 w laser diode array ( lda ) was used in 
 a 3d - printed , automated xe polarizer to study seop dynamics as a function of xenon density , 
 laser power , and seop - cell temperature .
more specifically , the seop 
 polarization conditions at several partial pressures of natural abundance 
 xe ( 26.44% xe isotope enrichment ) were studied : ( i ) 
 275 torr xe and 1725 torr n2 , ( ii ) 515 torr xe and 1485 
 torr n2 , ( iii ) 1000 torr xe and 1000 torr n2 , ( iv ) 1500 torr xe and 500 torr n2 , and ( v ) 2000 torr 
 xe and 200 torr n2 , where the gases were loaded with an 
 accuracy of 25 torr .
the reader is also directed to supporting information for detailed descriptions 
 of the experimental setup , which represents the second - generation 
 device of our hxtc consortium ( figure 1 ) . for each seop cell loading , data were obtained for a range of incident 
 laser power levels ( approximately 100 , 125 , 140 , or 170 w ) and variable 
 seop - cell surface temperatures ranging from 42 to 92 c . for 
 each condition , measurement of xe polarization dynamics 
 allowed the rate constant for pxe accumulation 
 ( seop ) and the maximum attainable steady - state pxe value [ pxe(t 
   ) or xe pmax ] to be determined from exponential fits .
the temperature of the 
 seop cell was monitored by a thermistor mounted directly to its surface ; 
 temperature control allows the rb concentration in the gas phase to 
 be varied .
the 
 seop 3d - printed portable polarizer ( figure 1 ) consists of a 200 w 
 frequency narrowed volume holographic grating ( vhg ) laser diode array 
 ( lda ) , a custom 3d - printed thermoelectric cooling ( tec ) optical pumping 
 ( op ) oven , a 0.5 l seop cell , an electromagnet providing 47 khz xe and h larmor frequencies , in situ nmr polarimetry 
 endowed by a magritek kea2 system , and a magritek 88 mm bore magnet 
 for ex situ nmr polarimetry and mri ( magritek , wellington , new zealand ) . 

the components of the polarizer and 47.5 
 mt mri have been discussed in detail 
 previously and thus are only discussed 
 briefly here .
( a ) spin exchange optical pumping ( seop ) 3d - printed polarizer 
 used 
 to perform all in situ experiments .
( b ) side view of laser aligned 
 to the seop oven , seop cell , and near - ir spectrometer .
( c ) top view , 
 with the oven lid removed to show seop cell . in situ
nmr 
 polarimetry for these experiments was performed via single - shot xe nmr at 47 khz ( figure 2a ) calibrated 
 against h nmr at the same frequency from a sample of thermally 
 polarized water doped with 10 mm cuso4 inside a 0.5 l seop - cell 
 phantom ( 200  000 scans , figure 2b ) . 

the polarizer allows the rb electron spin polarization , prb , to be estimated by comparing the integrated intensities 
 of transmitted laser spectra measured with and without the applied 
 magnetic field ( e.g. , figure 2c ) . for each set of conditions , pxe was 
 sampled every 520 min throughout seop ; the process is repeated 
 by either destroying the xe polarization with a series 
 of  crusher  pulses or allowing it to decay with the 
 laser off .
the time - course examples in figure 2d , e show the excellent reproducibility of pxe , prb , and seop in these experiments ( and those values were not sensitive to the 
 application of the rf pulses ) .
once steady - state xe 
 polarization was achieved , growth curves can be extracted ( e.g. , figure 2f ) and fit to an exponential : pxe(t ) = pmax[1  exp(seopt ) ] . 
 in the absence of seop , in - cell room - temperature ( rt ) measurement 
 of the spin 
lattice relaxation time constant ( t1 ) can be obtained after steady - state pxe has been achieved by quickly bringing the cell to room 
 temperature to minimize the rb gas - phase concentration , turning off 
 the laser , and performing in situ nmr polarimetry while the polarization 
 decays ; for example , the data in figure 2 g 
 were fit to an exponential decay curve : pxe(t ) = pxe(0 )  exp[xet ] , where xe ( = 1/t1 ) is the xe spin - destruction 
 rate , here exhibiting an ultralong in - cell xe t1 of 150.5  2.5 min ( or 2.5 h ) . particularly 
 when optimizing seop under the regimes of high [ xe ] and laser power , 
 it is also important to be observant for the onset of positive feedback 
 effects that give rise to dramatic increases in [ rb ] and laser absorption 
 over time ( and ultimately poorer pxe ) . 

examples showing the manifestation of such  rb - runaway  effects are provided in figure 2h , which shows behavior where relatively small increases 
 in cell surface temperature result not only in reduced peak 
 pxe but also in reduced pxe over time .
( a ) example of an in 
 situ low - field xe nmr spectrum 
 from a seop - cell during seop ( single scan , b0 = 4.00 mt ) .
( b ) corresponding h nmr spectrum 
 from a thermally polarized water reference sample using 200  000 
 scans , b0 = 1.10 mt .
( c ) examples of field - cycled 
 near - ir spectra of laser light transmitted through the seop - cell used 
 to estimate prb : room temperature before 
 seop ( dark gray ) , during seop with b0 electromagnet 
 on ( blue ) , and during seop with b0 electromagnet 
 turned off ( red ) .
( d , e ) examples of data sets for studying time - resolved 
 seop build - up and decay kinetics using a cell containing 1000 torr 
 ( each ) of xe and n2 gas ( 143 w laser power , 65 c ) . 

( d ) plot showing reproducibility of pxe accumulation following the application of > 500 rf  crusher
 
 pulses that nearly zero - out the xe polarization ( time 
 periods marked by vertical green bars ) ; prb ( red circles ) was sampled via field - cycled near - ir spectroscopy 
 ( c ) before and after application of the crusher pulses .
( e ) similar 
 to ( d ) , with xe nmr signals acquired with different 
 interpulse durations and with polarization decay observed after turning 
 the laser off ( times demarked with vertical arrows ) ; here xe decay was observed with the seop cell temperature maintained at 
 65 c .
pulse delay ( pd ) refers to timing between nmr acquisitions 
 during build - up .
( f ) exponential buildup of xe polarization 
 during the seop process for a cell filled with 2000 torr of xe and 
 200 torr of n2 .
( h ) time - course examples showing the temperature - dependent effects 
 of nonequilibrium  rb runaway  in a 1500 torr xe seop cell using only 100 
 w laser power : a normal build - up curve at 72 c ( black squares ) , 
 a mildly distorted build - up curve at 82 c ( red circles ) , and 
 a significantly distorted build - up curve at 92 c ( orange triangles ) . 

all spectra were recorded with a surface coil using small radiofrequency 
 ( rf ) excitation pulses with little to no measurable effect on xe magnetization . except for the fitting curves in ( f ) and 
 ( g ) , connecting lines are meant only to guide the eye .
ex situ pxe at 47.5 mt was calculated 
 by comparing the hp xe signal with the c signal at 508 khz c larmor frequency from a reference 
 sample of thermally polarized sodium 1-c - acetate dissolved 
 in d2o ( figure 4a ) .
the xe t1 relaxation time inside the polypropylene 
 phantom sphere was 9.2 min ( figure 4c ) , which is sufficient for short - term storage of hp xe .
moreover , this xe t1 value was used in parallel experiments to precisely calibrate the 
 rf excitation pulse for the 47.5 mt rf probe shown in figure 5f ; the image signal decay is due to both t1 decay and excitation rf pulses .
a y - slice projection of fast gradient echo ( gre ) imaging with 
 millimeter - scale spatial resolution without slice selection shows 
 the excellent xe signal intensity ( figure 5a ) .
all 20 images ( figure 5b  e , 
 selected images shown ) were acquired identically with te = 4.0 ms , 
 tr  80 ms ( limited by the spectrometer electronics response 
 time ) , 50% k - space sampling , 64  64 imaging 
 matrix with 72  72 mm field of view ( fov ) , and a 
 spectral width of 20 khz . given the relatively long t1 in the phantom ( figure 4c ) , the 
 decay of the hyperpolarized signal was primarily due to rf - pulse - induced 
 polarization loss in figure 5f .
the calibrated 
 rf pulse width for the flip angle ( 2.7  0.1 ) was 
 used for calculation of %  pxe for 
 the hp xe post - transfer in figure 4 ( see supporting information for details ) .
the dependence of xe 
 polarization and its dynamics 
 as functions of temperature , photon flux , and xenon partial pressure 
 was systematically studied under stopped - flow operation in the regimes 
 of high xenon density and photon flux .
results for five xe : n2 seop - cell compositions at four different lda incident powers ( approximately 
 100 , 125 , 140 , and 170 w ) with seop - cell surface temperatures ranging 
 from 42 to 92 c are displayed in figure 3 : figure 3a provides example plots of %  pmax and seop as functions 
 of seop - cell surface temperature for a cell containing 1000 torr of 
 xe and 1000 torr of n2 and illuminated by 100 w of laser 
 power from the lda .
such data were used to create contour plots (  maps  ) 
 of xe %  pmax and seop for each xe density as functions of laser power and seop - cell 
 surface temperature ( figures 3c  l ) ; 
 the highest values achieved for %  pmax for each xe : n2 mix studied are summarized in figure 3b and table 1 ( corresponding 
 numerical values for all data points in figure 3 are tabulated in the supporting information , table s1 ) . the data in figure 3 exhibit 
 several trends . 

first , increasing cell surface temperature gives rise to an exponential 
 increase in seop ( e.g. , figure 3a ) , consistent with the expected exponential increase in the 
 rb gas - phase concentration [ rb ] .
this dependence 
 of seop on [ rb ] arises from the relation1where se and kse are the rb / xe spin - exchange rate and 
 cross - section , respectively .
thus , the behavior of seop mostly reflects the spin - exchange rate , since generally kse[rb ] > xe or kse[rb ]  xe under our 
 conditions . 

( at the highest temperatures studied , kse[rb ]  xe ; at the lowest temperatures kse[rb ] can approach or become less than xe , but xe is expected to have a more mild 
 dependence on surface temperature that trends in the opposite direction . ) however , pmax exhibits significantly 
 different behavior , for example , peaking at 72 c for 
 the data in figure 3a .
pmax is given by2where 
 prb is the spatial average 
 of the local rb electron spin polarization , prb(r ) , which itself is determined 
 by3where 
 op(r ) is the local rb optical pumping 
 rate ( the integrated product of 
 the laser flux at position r and the rb absorption 
 cross section ) and rb is the rb electronic spin 
 destruction rate ( which is essentially proportional to [ xe ] under 
 our conditions ) . intuitively from eq 2 , xe pmax  prb when kse[rb ]  xe , which 
 occurs at higher temperatures .
however , having higher rb densities 
 generally translates into greater optical density , 
 which in turn gives rise to reduced transmittance of the laser light 
 and hence poorer illumination throughout the cell , lower op , and ultimately reduced prb , thereby decreasing pmax .
thus , 
 %  pmax initially grows with increasing 
 temperature as more rb is vaporized ( e.g. , figure 3a ) , but once [ rb ] becomes too high , overall prb decreases in accordance with eq 3 , resulting in lower xe %  pmax at some of the highest temperatures studied .
mapping 
 the conditions of rb / xe seop for five different 
 xe partial pressures .
( a ) example data set showing the dependence 
 of %  pmax and seop on the seop - cell surface temperature , here for a 1000 torr xe pressure 
 cell ( also filled with 1000 torr n2 ) with 100 w incident 
 laser power .
( b ) dependence of %  pmax and seop on xe partial pressure for five cell 
 xe : n2 compositions using 170 w incident laser power and 
 operating at the optimal cell temperature for each xe loading .
( c - l ) 
 %  pmax and seop maps for five cell xe : n2 compositions showing the interdependence 
 on incident laser power and seop - cell surface temperature .
the highest %  pmax values in the 
 contour plots of figure 3c  l were always 
 achieved at the maximum lda power of 170 w. however , as the xe density 
 increased , the optimal temperature decreased from 92 to 62 c , 
 in qualitative agreement with our previous results obtained at a much 
 smaller scale .
this inverse relationship between 
 xe density and optimal cell surface temperature , an effect amplified 
 by the use of frequency - narrowed lasers , may be explained in part by the fact that as [ xe ] rises , xe - induced 
 rb spin - destruction becomes increasingly dominant ; thus , lowering 
 the cell temperature helps maintain a sufficient  photon - to - rb  

ratio to ensure high global prb and hence higher %  pmax ( provided that the cell xe t1 is sufficiently long ) .
the effect 
 may also be exacerbated by greater in - cell temperature gradients caused 
 by ( i ) greater absorption of laser energy and ( ii ) the several - fold 
 lower thermal conductivity of xe compared to that of n2 .
indeed , the effects of differential heating 
 are also manifested in the seop maps : seop ( and hence se ) is not a constant of 
 exterior cell temperature but shows some variation .
for example , the 
 value at 100 w , 82 c for the 2000 torr xe gas composition is 
 nearly twice that for the 275 torr xe gas composition ; overall , apparent 
 seop values tend to increase with increasing laser 
 power and [ xe ] , consistent with higher - than - expected rb vapor densities 
 ( and higher internal temperatures ) under these conditions .
next , 
 there is a clear indication that all the studied xe densities 
 benefited from the increased laser power ( see also figure s1 ) .
consequently , the use of lda power greater than 
 170 w should lead to further increases in %  pmax . furthermore ,
if the increased heat load could be 
 mitigated , greater lda power would allow for operation in the regimes 
 with higher [ rb ] , thereby increasing seop and hp xe production rate .
data are absent from some regions 
 of the seop maps in figures 3c  l .
these 
 regions were avoided because the 
 build - up rate seop was found to be excessively long , 
 the %  pmax values were clearly low , 
 and/or the conditions would render an unfavorably high [ rb ] , resulting 
 in undesirable effects dubbed  rb pre - runaway  or  rb 
 runaway  .
the phenomenon of  rb runaway  takes 
 place when undissipated heat from laser absorption or cell heating 
 rapidly compounds the amount of rb in the vapor phase over a short 
 time ; the increasing [ rb ] results in decreasing prb in more poorly illuminated regions of the 
 cell and hence more laser absorption and heat dissipation from the 
 gas into the inner surface of the cell ( and rb pools ) in a self - reinforcing 
 pattern .
the effect can be characterized by its severity : in full 
 rb runaway , one sees a dramatic decrease in the amount of laser light 
 transmitted through the cell over time which may be followed by deteriorating pxe and even elevated exterior cell surface temperatures . 
 the behavior is also hysteretic , as simple temperature reduction to 
 normal operating regimes generally fails to regain efficient seop . 
 on the other hand , a more mild condition ( here termed  rb pre - runaway  ) 

does not have as pronounced a manifestation in the transmitted laser s 
 near - ir spectroscopy but is readily observed during measurements of 
 the kinetics of pxe build - up .
examples 
 of  rb pre - runaway  can be seen in figure 2h ( orange trace ) , where pxe grows , 
 passes the maximum , and then dips .
the effect is less pronounced in 
 figure 2h ( red trace ) and nonobservable in 
 figure 2h ( black trace ) .
 causes %  pxe to irreversibly 
 deteriorate and requires a restart of the seop procedure from initial 
 conditions to lower [ rb ] ,  rb pre - runaway  is not hysteretic 
 and can be more easily controlled by reducing the cell temperature
. 
 however , it results in lower polarization ( figure 2h ) . in any case , these deleterious effects are more problematic 
 for higher xe densities because of the greater rb spin - destruction 
 rates ( and hence greater light absorption from more poorly polarized 
 rb , as well as any possible contributions from reduced thermal conductivity ) .
apparent or 
 usable xe hyperpolarization %  pmax(app ) 
 is computed according to ref ( 23 ) to reflect that xe is diluted by n2 gas as follows : %  pxe(max , app ) 
 = [ %  pxe(max)](pxe / ptot ) where pxe is partial pressure of xe and ptot is total mixture pressure . the production cycle time ( for 
 producing of 0.8 l of hp xe / n2 gas 
 mix )
is calculated as the sum of 2/seop ( i.e. , when 
 the bulk ( 87% of pmax ) of xe hyperpolarization is established , in addition to a reasonable 
 ( 25 min long ) interval necessary to unload / reload the op cell ( with 
 cool - down / reheating procedure described in ref ( 24 ) ) with xenon mix ; seop value corresponding to a maximum value of %  pmax was used for every gas composition .
the 
 production rate is calculated by dividing 0.8 l gas volume 
 expanded in the tedlar bag during each production cycle by the production 
 cycle time .
table 1 summarizes 
 the maximum achieved xe polarization ( %  pmax ) for every xe : n2 mix studied . 

the results show not only the trend of decreased %  pmax with increasing xe in - cell pressure but also a corresponding 
 decrease in seop measured at these optimal conditions , 
 a finding that predominantly reflects the lower concentration of rb 
 vapor that must be attained to achieve maximal xe polarization 
 at higher xe densities .
nevertheless , the optimization process allows 
 the total magnetization ( mxe ) to continue to grow despite the decrease in %  pmax , as the xe density increases faster than %  pmax decreases . while the xe polarization 
 values ( and amounts ) are significantly higher here than those in ref ( 21 ) , the improvement in mxe from 1000 to 2000 torr of xe is more 
 marginal .
higher laser power may provide further improvements in %  pmax ( and mxe ) at high [ xe ] by allowing operation with higher rb densities and 
 hence higher seop rates .
other useful metrics describing 
 the overall hyperpolarizer performance summarized in table 1 include the apparent %  pxe(max ) due to xe dilution by n2 gas ( %  pxe(max , app ) ) , production cycle time , and apparent 
 production rate of hyperpolarized gas ( l / h ) .
%  pxe(max , app ) is a useful metric because it takes into account hp xe dilution by n2 gas , 
 which has not been eliminated because the hp xe cryocollection step 
 was obviated .
production cycle time corresponds to the time necessary 
 to complete the production of 0.8 l of hp xe / n2 gas composition and return the hyperpolarizer ( i.e. , gas reloading , 
 etc . ) to the same step in the operational cycle .
computed in this 
 fashion production cycle time was used for estimating the apparent 
 production rate of the hyperpolarizer in liters of hyperpolarized 
 xe / n2 mixture per hour .
the production rate in l / h is truly 
 the characteristic of continuous - flow hyperpolarizers , and the apparent 
 production rate values computed in table 1 should 
 be used with care for direct comparison with continuous - flow hyperpolarizers , 
 because the batch - mode method used here produces a single batch per 
 each production cycle , and there is no produced hp xe 
 until the cycle is finished . to validate the in situ nmr results , 

the polarized contents of 
 the seop - cell filled with 1000 torr of xe and 1000 torr of n2 was transferred into an evacuated ( < 10 torr ) 
 0.05 l hollow polypropylene sphere located in a rf probe of a 47.5 
 mt imaging system ( see supporting information for details ) . 
 in - cell
pxe was measured in situ as 54 
  5% before the transfer , and a pxe value of 51  2% was detected in the 47.5 mt preclinical mri 
 scanner ( 558.6 khz xe larmor frequency ) , corresponding 
 to polarization enhancement  >
11  000  000 
 after 
 the gas transfer ( figure 4b ) .
figure 5 also demonstrates the feasibility of millimeter - scale 
 mri of hyperpolarized xe at very low magnetic fields 
 using frequency optimized rf coils .
ex situ 
 47.5 mt nmr spectroscopy of hp xe gas expanded 
 into a phantom .
( a ) c nmr spectroscopy using thermal c polarization ( %  p13c = 
 4.1  10% ) of a 17.5 ml reference sample 
 of 5.2 g of sodium 1-c - acetate dissolved in d2o .
256 averages were acquired at 508 khz resonance frequency with 
 a 90 square excitation rf pulse and a repetition time ( tr ) of 
 200 s. acquisition time was 100 ms .
( b ) ex situ xe nmr 
 spectroscopy of hp xe gas ejected from the polarizer , 
 0.61 mmol of xe spins ( %  pxe= 51  2% ) ; cell loading was 1000 torr of xe and 1000 
 n2 .
the spectrum is acquired at 558.6 khz xe resonance frequency with a single scan ( 2.7 excitation rf 
 pulse ) and tr = 200 ms .
the rf pulse is calibrated by monitoring signal 
 decay in the mri images ( see figure 5 ) and 
 accounting for t1 relaxation of the hp xe in the phantom ( c )
( a ) y - slice / projection across the center of the image shown 
 in ( b ) .
( b  e ) selected mri gradient echo ( gre ) images from 
 a series of 20 images .
all 20 images were acquired identically with 
 te = 4.0 ms , tr  80 ms ( limited by the spectrometer electronics 
 response time ) , 50% k - space sampling , 64  64 
 imaging matrix with 72  72 mm field of view ( fov ) , 
 and a spectral width of 20 khz .
( f ) decay of hp signal primarily due 
 to rf - pulse - induced polarization loss .
the temporal decay of the signal 
 measured between individual images within 20-image series was used 
 to calibrate rf pulse width ( 2.7  0.1 corresponding 
 to 20 s at 80 mw ) using t1 determined by the data shown in figure 4c , 
 because the signal decay in figure 5f is due 
 to both t1 decay and excitation rf - pulse - associated 
 magnetization losses .
simultaneous optimization of various seop conditions 
 ( xe density , 
 cell surface temperature , and photon flux ) combined with previously 
 reported seop hardware improvements yielded greatly improved %  pxe .
indeed , very high values of %  pxe and mxe were demonstrated 
 here for dense ( up to 2000 torr of xe in 2200 torr total ) xe gas mixtures , 
 in part enabled by optimized laser illumination throughout the cell , 
 ultralong in - cell xe relaxation times , and efficient 
 thermal management that also allows for diligent avoidance of  rb 
 runaway  regimes .
the seop condition maps provide guidance 
 for the production of highly polarized xe gas at different 
 xenon densities for a wide variety of applications ranging from materials 
 science to biomedical imaging .
furthermore , our results indicate that 
 the pxe values at higher xe densities 
 are still laser - power - limited .
thus , while the benefit in total xe 
 magnetization was less substantial at the highest xe densities studied , 
 the advantage will likely be improved when more powerful lda instrumentation 
 is available provided that the greater thermal loads can be mitigated . 

finally , the highly reproducible maps of seop build - up 
 rates , combined with automated fine control of cell conditions and 
 real - time spectroscopic feedback , should also allow optimization of 
 multiexponential xe polarization dynamics , pointing the way to multifold 
 improvements in hp xe production efficiency .
 we present a systematic , multiparameter 
 study of rb/129xe spin - exchange optical pumping ( seop ) 
 in the regimes of high xenon 
 pressure and photon flux using a 3d - printed , clinical - scale stopped - flow 
 hyperpolarizer . in situ nmr detection 
 was used to study the dynamics 
 of 129xe polarization as a function of seop - cell operating 
 temperature , photon flux , and xenon partial pressure to maximize 129xe polarization ( pxe ) . 
 pxe values of 95  9% , 73  4% , 60 
  2% , 41  1% , and 31  1% at 275 , 515 , 1000 , 1500 , 
 and 2000 torr xe partial pressure were achieved . 
 these pxe polarization values were separately validated by ejecting 
 the hyperpolarized 129xe gas and performing low - field mri 
 at 47.5 mt . 
 it is shown that pxe in this 
 high - pressure regime can be increased beyond already record levels 
 with higher photon flux and better seop thermal management , as well 
 as optimization of the polarization dynamics , pointing the way to 
 further improvements in hyperpolarized 129xe production 
 efficiency . 
Introduction
Methods
Results and Discussion
Conclusions