Patent Application: US-25505502-A

Abstract:
a multi - slice nmr imaging method for recording n slices from a measuring object , wherein for complete image reconstruction , an acquisition in m repetition steps with different spatial encoding is used , with a magnetization preparation in the form of a saturation pulse which is selective with respect to chemical shift and which is applied p times in each repetition step such that through each of these chemical shift selective saturation pulses the spins to be saturated of one of the n slices in the measuring object are optimally saturated and consequently the signals of n - p slices are saturated only sub - optimally , is characterized in that the temporal sequence of the saturation pulses and / or the sequence of the acquisition of the differently spatially encoded signals of the individual slices is permuted such that after termination of the acquisition , for each slice the signals with identical spatial encoding were recorded in the same fashion also with respect to saturation preparation . this produces uniform fat or water suppression for all slices .

Description:
in a multi - slice experiment with n slices , there are n different acquisition times within a repetition time . with respect to the temporal separation from a chemical shift selective saturation , these acquisition times n correspond to different preparation states p 1 , p 2 , . . . pn . according to the conventional acquisition scheme , all phase encoding steps of a slice have the same preparation state each ( fig1 a ). however , different slices are acquired in different preparation states . each slice therefore has a different contrast with respect to the preparation . the principal idea of hope is the encoding of identically prepared signals for identical phase encoding lines of all slices . in contrast to the conventional method , identical phase encoding steps from different lines must be acquired in different repetitions . all slices have identical image characteristics with respect to fat or water saturation under the following hope condition . all phase encoding steps with the same k y value must have the same preparation state for all slices . phase encoding steps with the desired preparation contrast are encoded in the center of the k space ( fig1 b ). towards this end , the 2 m phase encoding steps − m , . . . , m must be divided into n segments s , s 2 , . . . , sn to which one preparation state is associated in each case . since each preparation state is acquired once in each repetition of a multi - slice recording , all preparation states are acquired equally frequently during the repetitions . the n segments therefore have identical sizes and each contain m / n phase encoding steps . the hope conditions are graphically summarized in fig1 b . all phase encoding steps of a segment have the same preparation state . the phase encoding steps of different segments have different preparation states . all phase encoding steps of a certain k y value have the same preparation state for all slices . the phase encoding steps of the central k - space segment are acquired in the preparation state with the desired contrast . to realize the hope conditions in a multi - slice sequence , two conditions must be met for the acquisition sequence of the phase encoding steps . at first , all slices must be subsequently read out in each repetition and thus be acquired in another preparation state each . secondly , after 2 m repetitions , all phase encoding steps of all slices must be acquired . the first condition may be met e . g . by a trajectory , plotted in fig1 b , which connects phase encoding steps , subsequently acquired in one repetition , diagonally . there are two principally different possibilities of selecting the trajectories . if the chemical shift selective saturation is applied after n slice excitations each at the beginning of a repetition , the sequence of slice excitation within one repetition is permuted for the different trajectories ( fig2 a ). another possibility is to keep the sequence of slice excitations constant within a repetition during the repetitions . in this case , the point in time of application of the preparation must be shifted within a repetition during the repetitions . this produces the trajectories shown in fig2 b . hybrid versions are also feasible which permute the sequence of the slice acquisition and also change the point in time of application time of the preparation . the complete temporal hope sequences used by the two methods a and b shown in fig2 are represented corresponding to the fig3 a and 3b . the time axis of the acquisition extends from the left to the right and from the top to the bottom . in both sequences , the acquisition conditions of the multi - slice technique are met ( all slices are acquired in each repetition and all phase encoding steps of each slice are completely acquired during the repetitions ). the hope conditions are exactly met in method a , and are only approximately met in method b . in variant b , the first phase encoding steps of the segments left of the preparation have a different preparation state than the other phase encoding steps of these segments . the time interval between the chemical shift selective saturation and the acquisition of a phase encoding step differs for these phase encoding steps by the acquisition time ta of a phase encoding step since the time interval refers to the preparation of the previous repetition . the linear allocation of the preparation states to the segments of the k space of fig1 b is only one possibility . in general , each preparation state can be associated with any k - space segment ( fig4 ; left ). contrasts can be produced flexibly through association of a certain preparation state to the central segment of the k space . allocation of the slice counter cs to the geometric positions of the slices may be arbitrary ( fig4 ; right ). all common slice excitation sequences conventionally used can therefore also be used in hope technology . to obtain contrast modification in all slices of a conventional multi - slice recording through chemical shift selective saturation , the relaxation time of the saturated signals must be shorter than the time interval between two preparations . in a t1 time of the fat tissue between 150 ms - 200 ms , the method of single fat preparation per repetition is used only up to repetition times in the same order of magnitude . in spin echo sequences , which typically have a repetition time of approximately 500 ms , in contrast , fat saturation preparation is used directly before each individual spin echo acquisition . in general , this considerably prolongs the total measuring time per slice . to reduce the time between two preparations , several equidistant preparations can be applied within one repetition time [ 8 ]. however , the possibility to apply p ( p & lt ; n ) preparations within a repetition time is limited in the conventional method through periodical signal modulations in the slice direction with a period length of n / p slices . the hope condition for multiple preparations is the same as for individual preparation . all phase encoding steps with the same k y value must be acquired for all slices in the same preparation state . the number of possible preparation states is reduced from n to n / p . the hope condition can be met separately for n / p slices in each case . this traces the problem back to the single preparation case . in each repetition , the same amount of n / p slices is read - out by different preparation states . the solution for these n / p slices can be copied for the other packets of n / p slices , which are acquired in the same repetition time . this is graphically shown in fig5 . the m phase encoding steps are divided into n / p segments each containing steps . each segment is associated with one of the n / p preparation states . for the first n / p slices , the hope condition is met analogously to fig3 . this solution is copied ( p - 1 ) times in the slice direction to obtain the entire sequence of n slices . to minimize artefacts , the signal development in the phase encoding direction must be as smooth as possible . this is true for the signal development on the basis of magnetization preparation as well as for the signal development due to the used repetition times between two excitations of the same slice . the maximum smooth development of prepared signals is produced through linear phase encoding . this minimizes the number of jumps in the signal intensities along the phase encoding direction to n - 1 . these are produced at the transitions between the segments . the size of the jumps is minimized in that neighboring segments are associated with temporally adjacent preparation states . this condition is exactly met through linear allocation . for a conventional multi - slice experiment , the following is true for all phase encoding steps of a slice : tr = n * ta + tp . therein , ta is the time required for acquiring a phase encoding step and tp the time required for preparation . signals from the first excitations of a slice are coded linearly from the edge of the k space to the inside for all slices to minimize artefacts due to initial signal fluctuations . with the new acquisition sequence , the first repetitions include also acquisitions of phase encoding steps close to the center for some slices ( fig3 a and fig3 b ). to avoid artefacts in these slices , some so - called dummy echoes must be produced before the actual acquisition . these are echoes , which are not used for image reconstruction . for repetition times of typically more than 50 ms , the signal for most tissues reaches its equilibrium amplitude after a few tr intervals . this magnetization preparation avoids signal fluctuations in the acquired phase encoding steps . the condition of a constant repetition time cannot be exactly met for the subsequently acquired phase encoding steps either . in case of change of the slice sequence or of the application time of the preparation , the tr intervals change between the two excitations of the same slice ( fig3 a and fig3 b ). with the method of fig3 b , the repetition times for all slices along the phase encoding direction are constant except for each first phase encoding step in the columns below a preparation block . these have a repetition time tr = n * ta which is reduced by tp . they are encoded for all slices for the edge of the k space which minimizes the artefacts caused by the longer tr intervals . with the method of fig3 a , the phase encoding steps have a different repetition time for all slices after each change of the slice sequence than in the other repetitions . the first n - 1 phase encoding steps in the first repetition after a change in the slice sequence have a repetition time tr =( n - 1 )* ta + tp which is reduced by ta . they are encoded each at the edge of a segment . the relative difference in the repetition times δtr / tr between the first and following phase encoding steps of a segment is δ   tr tr = ta ( n * ta ) + tp & lt ; 1 n . for multi - slice experiments with typically n & gt ; 10 slices , this difference can be neglected to a good approximation . the phase encoding step of the — after a change of the slice sequence — right outer slice has a repetition time of tr =( 2n - 1 )* ta + tp . it is encoded for all slices at the edge of the k - space , thereby minimizing artefacts . signal differences , which occur between the first and following echoes of the segments can optionally be additionally reduced by introducing dummy repetitions analogously to the start of the acquisition before the acquisition of phase encoding steps with differing tr intervals . 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