Patent Application: US-8654906-A

Abstract:
quantitative assessment of haemodynamics by cycled arterial spin labeling that distinguishes between blood magnetization tagged by a specific labeling pulse , using a time series acquisition in which all measured data sets are used for reconstruction of each single time step , thereby reducing measurement time while maintaining signal - to - noise ration compared to conventional asl .

Description:
the goal is to acquire data sets which comprise signal of multiple labeled blood boluses and to extract the signal of each single bolus . since these boluses were labeled at different times before image readout , an asl time series can be extracted . the differentiation requires an encoding of each bolus . as stated above , two phases , a control and label phase , are used to form a bolus of labeled blood . using different combinations of control and label phase for each time step makes it possible to separate the signal of different boluses . in the following , the complete set of the combinations used to separate all boluses will be called a tagging cycle while a single combination will be dubbed a cycle step . the length of the tagging cycle is n . the encoding problem can then be described by a matrix equation : where m =( m ( ti 1 ), m ( ti 2 ), . . . , m ( ti n )) t is the fresh longitudinal magnetization of blood spins at the tagging side at times ti 1 to ti n . the encoding matrix e describes whether a control or a labeling pulse is applied at time ti i for a given cycle step . the elements of e are either + 1 or − 1 for control or labeling phase , with rows representing each tagging cycle step . vector i represents the data sets that are acquired in each cycle step . therefore , the results of the encoding are n image data sets which comprise information from each bolus , either labeled or not . the signal of each bolus can then be reconstructed by inverting eq . 1 if certain restrictions to the encoding matrix e apply : the row vectors of e are independent , i . e . e has to be unitary . since the elements of e are real , matrix e is orthogonal ( i . e . e ′· e = 1 , where e ′ is the transpose of e and 1 is the unitary matrix ). an additional restriction applies : the sum of each row vector of e has to be zero . this restriction is caused by the basic idea of asl to suppress the signal of the stationary tissue by subtracting a control and label phase . to maintain this asl condition the same number of control and label images has to be subtracted to reliably cancel out the signal of the stationary tissue spins . several matrices exist , which fulfill these restrictions . however , it is not a trivial task to construct them for an arbitrary n . a well - known class of matrices , which substantially fulfills the requirements , is called hadamard matrices . hadamard matrices are defined recursively . they exist only for size 2 n , for . h 1 = 1 , h n + 1 = h n h n h n - h n ( eq . ⁢ a1 ) except for the first , all row vectors have the same number of + 1 and − 1 elements , i . e . the sum of each row vector is 0 , except for the first where the sum is n . with hadamard matrices of size n , exactly n − 1 different tis can be encoded . fig3 shows the whole label cycling experiment for n = 8 . here , the first row vector of the hadamard matrix is ignored since it does not fulfill the proper condition of zero summing . therefore , seven different inflow times can be acquired by an eight step cycle . however , since all eight data sets in different combinations are used to reconstruct each image for a certain inflow time ( only consisting of the signal of a single bolus ), the snr of each of the seven images will be as high as for a data set acquired by conventional asl . thus , using cycled asl as proposed in this patent specification , a reduction in measurement time of factor n − 1 ( 7 in this example ) can be achieved by maintaining the same snr compared to standard asl time series experiments . as illustrated in the example of fig3 , instead of using several readouts after one labeling / control pulse as in the its - fair technique , cycled asl ( casl ) in accordance with this patent specification uses in this non - limiting example several labeling pulses before a single readout . in conventional pasl one bolus of labeled blood is produced which is produced at a certain point in time and is acquired at a certain time . in cycled asl several boli of labeled blood are produced at different points in time and those boli are acquired a one point in time . thus , the resulting data will be a mixture of all different boli which already arrived at the imaging site at the time of the acquisition . to be able to distinguish the different boli and reconstruct the time series , control and labeling pulse for each bolus are varied in a certain cycling scheme which allows unique identification of the bolus . the correct order of control and labeling pulses for each phase and bolus can be calculated using a hadamard matrix . since all acquired data sets are used for reconstruction of each bolus the snr of each reconstructed data set at the inflow time ti is maximized . in a practical application of the principles discussed above , five subjects ( 30 - 44y ) were examined on a clinical 1 . 5 t mr - scanner ( magnetom sonata , siemens , erlangen , germany ) with maximum gradients of 40 mt / m and a minimum rise time to full gradient strength of 200 μs . the encoding scheme based on hadamard matrices was used to tag the magnetization of multiple blood boluses within a label cycling step . this mixture of signal tagged at different times before readout were acquired with a fast single shot 3d - grase readout technique ( 9 ) used with the following parameters : echo time te = 36 ms , repetition time tr = 3750 ms , off - resonance fat saturation pulse , 28 interpolated partitions ( 16 acquired , ⅝ fourier ). a substantially isotropic resolution of 4 . 7 mm × 4 . 7 mm × 4 . 5 mm was achieved . two averages of a 16 - step label cycling were used ( total acquisition time : 2 min ), inflow times ranging from ti = 200 ms to 3200 ms , increment 200 ms . to avoid residual signal in the resulting images due to different magnetization transfer effects during control and labeling pulses , a modified tilt ( 13 ) scheme was used as described in ( 14 ). instead of using + 90 °/− 90 ° rf pulse combinations for control and + 90 °/+ 90 ° rf pulses for labeling as in tilt , the modified scheme uses one or two 180 ° adiabatic ( hyperbolic secant ) inversion pulses . thickness of labeling band was 100 mm . a four rf pulse train for saturation was applied in the imaging slab before starting the tagging pulse series . a proper experiment can be ensured when each bolus is clearly defined . to examine the dependency of the time series signal from the distance between successive labeling pulses , an 8 - step label cycling experiment was repeated with different increments of 100 ms , 200 ms , 300 ms and 400 ms starting at ti = 800 ms . thus , the time series ranged from ti = 800 ms to 1500 ms , 800 - 2300 ms , 800 - 3100 ms and 800 - 3900 ms , respectively . tr was adjusted to 4500 ms to allow the proper measurement of the longest ti . all label cycling experiments were performed with and without cardiac triggering . after the trigger signal was received , the slice selective saturation was applied for the imaging slab to erase possible differences in magnetization due to relaxation effects . this was followed by the tagging pulse train . conventional asl time series acquisition was performed without cardiac triggering for inflow times ti = 200 - 3200 ms , equally spaced with increment 200 ms . total measurement time was 32 min . region of interest ( roi ) were hand drawn in both conventional and 16 - step label cycling data set in corresponding locations . snr was estimated by dividing grey matter roi by air roi . the label cycling scheme for pulsed asl disclosed in this patent specification can also be extended to continuous asl . as presented here , the label and control phase of a continuous asl experiment should be combined in an appropriate way to allow distinguishing between different inflow times . again , hadamard encoding can be used to provide the differentiation , thus enabling continuous asl to efficiently acquire time series . testing should be performed to evaluate the behavior of such a technique for different flow scenarios . the thickness of the slab being labeled and images should be selected such that it is not so thick that labeling would interfere with readout to an extent making it difficult to associate boli with readout signals . a way to avoid such potentially undesirable effects is to use continuous cycled asl that is generally according to the principle illustrated in fig3 but differs in implementation in that the different combinations of control and labeling pulses are applied in a sequence followed by a sequence of readouts . this differs from the pcasl illustrated in fig1 c at least because combinations of control and labeling pulses are used before the readout while in fig1 c a readout follows a series of labeling pulseis and another readout follows a series of control pulses . 1 . kim s g . quantification of relative cerebral blood flow change by flow - 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