Patent Application: US-66272310-A

Abstract:
a method for nuclear magnetic resonance or magnetic resonance imaging measurements , includes creation of enhanced polarization of nuclei of a first kind within a sample in a magnetic field at cryogenic temperatures and transfer of the polarized sample to room temperature . the enhanced polarization of nuclei of the first kind is thereby transformed into long - lived states of nuclei of a second kind and these lls are sustained . the lls is at least partially converted into observable magnetization and an nmr or mri measurement is carried out . the method allows one to extend the time needed between hyperpolarized magnetization and nmr detection .

Description:
a method has been designed to convert a small fraction of lls into detectable magnetisation . the dipeptide ala - gly was chosen to demonstrate this method . the j - coupled aliphatic protons h α1 and h α2 of glycine in ala - gly are diastereotopic and therefore apt to sustain long - lived states ( fig1 ). an experiment where an lls comprising the two protons i = h α1 and s = h α2 is created starting from dnp enhanced 13 c polarisation , sustained , and finally converted into detectable magnetisation 17 , 21 yields a proton spectrum ( fig1 ) showing two in - phase doublets with a coupling constant j is =− 17 . 4 , centered at the chemical shifts of the h α1 and h α2 protons . the lifetime of the lls in ala - gly in water at room temperature has been found to be t lls = 42 . 2 ± 0 . 1 s , i . e ., nearly 25 times longer than their spin - lattice relaxation time t 1 ( 1 h )= 1 . 712 ± 0 . 002 s ( i . e ., t lls / t 1 = 24 . 6 ), provided the concentration of paramagnetic oxygen in the sample is reduced by bubbling nitrogen gas . however , in the sample stemming from a dissolution - dnp procedure this lifetime is shortened by dipolar interactions with paramagnetic oxygen and with the tempo radicals required to enhance the polarisation at low temperatures . under the conditions of the experiments described in experimental section , t lls = 16 ± 0 . 6 s and t 1 ( 1 h )= 2 . 3 ± 0 . 2 s , i . e ., t lls / t 1 = 7 have been measured . natural - abundance 13 co of glycine ( fig1 ) can be polarised as previously described 20 ( see experimental section ). the enhancement has been estimated to be at least p =( p α − p β )/( p α + p β )= 4 . 6 %, since the room - temperature signal of the sample after reaching equilibrium was below noise level after 128 scans accumulated using relaxation delays of 200 s . the long spin - lattice relaxation time constant of the quaternary carbons in glycine , t 1 ( 13 co )= 25 . 9 ± 0 . 2 s , can be put to good use to transfer the magnetisation from the polarising to the high - resolution magnet . when used in conjunction with hyperpolarisation , the new experiment comprises four steps : ( i ) create enhanced polarisation in a low magnetic field at cryogenic temperatures ; ( ii ) rapidly transfer the polarised sample to room temperature ; ( iii ) transform the enhanced carbon - 13 magnetisation into long - lived states ( lls ) associated with protons and ( iv ) preserve these lls by suitable means , ( v ) ‘ read out ’ the long - lived states by ( partial ) conversion into observable magnetisation . step ( ii ) typically lasts a few seconds , so it is advantageous to store polarisation on slowly relaxing nuclei ( such as 13 c nuclei ) during this period . due to the large enhancements afforded by dissolution - dnp , isotope labelling of the molecule of interest is not necessary and naturally - occurring isotopes of nitrogen or carbon ( as in the present study ) may be used . step ( v ) may be repeated if only a small fraction of the long - lived states is converted into detectable terms and the rest is sustained as a long - lived state . the pulse sequence denoted ‘ long - lived states ’ in fig2 consists of the generation of lls , sustaining them using rf irradiation , and converting them back to detectable in - phase terms . when the pulse sequence is used in conjunction with dnp , the polarisation is transferred from the carbon to the proton spins in a high - resolution magnet and subsequently converted into lls . the first proton pulses have the role of saturating any unwanted ‘ native ’ proton magnetisation that does not originate in carbon - 13 polarisation . the pumping step consists of a refocused inverse - inept 22 , 23 followed by a spin - echo interval [ τ 3 - π - τ 3 ] to complete the evolution under j ( 1 h , 1 h ) that has started during the [ τ 2 - π - τ 2 ] interval in the preceding inept sequence . a 45 ° pulse converts the antiphase proton coherences into lls , which are sustained by suitable rf irradiation . if only a fraction of the lls is converted back into detectable magnetisation by a small - angle pulse , the remaining part may be sustained during a subsequent period , prior to another detection . two distinct strategies have been explored to this effect : during the interval τ 4 = 1 /( 4δν is ) where δν is = ν ( h α1 )− ν ( h α2 ) the zq x part of q lls evolves into : ( symbols such as zq are universally recognized as product operators ) 17 . these terms are partly converted into detectable single - quantum coherences by a pulse with a phase y and a small flip angle α : zq y →−( i y s z − i z s y ) sin α + zq y cos α , ( 3 ) the leading antiphase single - quantum terms are detected and the remaining zq y is converted back to zq x during the subsequent interval pτ 4 - τ 4 , so that the operator q lls is re - constituted . the pulse sequence in fig2 ( bottom ) is based on an alternative approach . during the short interval τ 0 the zq x part of q lls evolves to : zq x → zq x cos ( 2πτ 0 δν is )+ zq y sin ( 2πτ 0 δν is ) ( 4 ) the second term is entirely converted to detectable single - quantum coherences by a 90 ° pulse : the remaining ( i z s z + zq x ) part , which commutes with the 90 ° pulse , is left intact . the zq x term precesses through an even number of periods while detection is performed and lls may be sustained again after the detection period . dnp experiments have been performed , followed by the transfer of one or two samples of ca . 1 ml each of polarised sample to the high - resolution magnet . for each sample , a dnp - lls experiment was performed with variable times τ m , where the lls is sustained , n = 1 , and converting the entire amount of magnetisation into detectable terms . the resulting intensities were scaled by the intensity of the enhanced 13 c signal , measured in a preliminary experiment using a 13 c pulse with α = 10 °, and the ratios i ( dnp - lls )/ i ( dnp - 13 c ) were fitted to an exponential decay with three parameters ( fig3 , left panel ). the resulting lifetimes were in agreement with the t lls measured for a similar sample starting from equilibrium room - temperature 1 h magnetisation without dnp . in a second series of experiments ( fig3 , right panel ) long - lived states were sustained during n = 5 successive time intervals , each with τ m = 4 s , and a small fraction was converted each time into detectable magnetisation using the method described in fig2 ( top ). the experiments were performed using 0 . 6 m ala - gly mixed with 50 mm tempo and dissolved in 5 ml 35 %/ 65 % v / v glycerol - d8 / d 2 o . the frozen beads were placed in a home - built prepolariser , 20 and irradiated with 30 mw microwaves at 94 ghz for 5 hours at 1 . 2 k and 3 . 35 t . the sample was dissolved in 5 ml d 2 o ( preheated to 190 ° c . at 12 bar ) to a final concentration of ca . 50 mm . the hyperpolarised solution was transferred to 10 mm tubes maintained at a temperature of 25 ° c . in an inverse broadband probe in a high - resolution 7 t wide - bore bruker magnet . the time that elapsed between the start of the transfer from the polarising magnet and detection was ca . 6 s . the room - temperature experiments using nuclear polarisation at boltzmann equilibrium were carried out at 300 mhz ( b 0 = 7 . 05 t ) and 298 k using ˜ 20 mm ala - gly solutions in deuterated water . eight transients were acquired for each lls experiment using a relaxation delay of 30 s . experimental relaxation time constants t 1 and t lls were obtained by fitting normalized spectral intensities to mono - exponentially decaying functions . a waltz sequence with an rf amplitude ν1 = 1 . 5 khz was used to sustain the lls . for dissolution — dnp experiments , the polarisation of electron spins is transferred to nuclei in a polarising magnet prior to nmr detection in a high - resolution magnet 3 , using instrumentation developed at epfl 20 the inventive method allows one to preserve dnp - enhanced polarisation under the form of long - lived spin states and to convert small fractions back to detectable terms . this may open the way to follow metabolic pathways by magnetic resonance and providing the time needed for hyperpolarised magnetisation to reach the target areas in angiography studies . the new experiments may find applications for the study of chemical reactions and slow dynamic phenomena such as flow , diffusion , and exchange . unlike methods that exploit the long lifetimes of nuclei with low gyromagnetic ratios such as carbon - 13 and nitrogen - 15 to increase the spatial and temporal resolution of mri 19 , the methods described in this work do not require any isotopic labelling , and use the advantageous sensitivity of proton nmr . the new hyperpolarised methods may be extended to multiple scans , phase - cycling and even 2d spectroscopy , provided the partial decay of the lls is taken into consideration . 1 . l . r . becerra , g . j . gerfen , r . j . temkin , d . j . singel & amp ; r . g . griffin . dynamic nuclear - 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