Patent Application: US-201213447325-A

Abstract:
a method of mr imaging and spectroscopy to reduce artifacts occurring due to the motion of an object to be represented , wherein the object position is determined quasi - continuously during the runtime of the mr acquisition , which includes one or more partial acquisitions , and wherein motion correction is performed , which comprises dynamic adaptation of the frequency and phase settings of the rf system of the tomograph and of the orientation and amplitudes of the gradients during the runtime of the mr acquisition according to the current object position . the motion correction is thereby applied during a signal weighting period , during a signal read - out period , or between and / or during the two stated periods and the adaptations for motion correction are performed without interrupting or slowing the temporal progression of the mr acquisition . in this way , artifacts due to motion of the object to be represented can be further reduced .

Description:
fig1 shows a chart of the method for dynamic motion correction during a partial acquisition ta , wherein the partial acquisition is divided into shorter partial acquisition partitions tap ( 1 ), tap ( 2 ), tap ( 3 ), . . . , tap ( m ), wherein if long gradients are necessary they are divided into shorter partial gradients 101 . the figures indicate the times for partial acquisition and for the partial acquisition periods with the symbols t ta , t tap ( 1 ) , t tap ( 2 ) , t tap ( 3 ) and t tap ( m ) , respectively . for comparison , the corresponding partial acquisition without the use of the described method is shown in 102 . a partial gradient is always switched off when the following gradient begins . an uninterrupted transition results from the rise and decay time of a gradient . the measurement system 140 determines position data with which motion correction is performed . the partial acquisition shown is the section of an mr acquisition for dwi . dwi is frequently used in everyday clinical practice to represent microscopic movements in the tissue that are a sensitive marker for many pathological phenomena . for this purpose , so - called diffusion gradients are used during signal encoding . particle motion that occurs during the diffusion gradients required for diffusion weighting results in the spins not being completely refocused at the end of the signal encoding in voxels affected by the motion and in the subsequent measurement signal therefore being attenuated depending on the diffusion . however , sensitization to microscopic movements also makes the dwi sequence very prone to macroscopic movements . a further difficulty is that the resulting phase effects of the macroscopic movements clearly dominate those of the microscope movements . moreover , the typical time requirements for a diffusion - weighted signal encoding is in the range 30 to 400 ms . previous conventional methods corrected movement only once at the beginning and at the end of this long time interval for signal encoding . object movement that occurs during the diffusion gradient and between slice excitation and the refocusing pulses that are typically used could not previously be corrected by known methods . the presented method permits uninterrupted adaptation of the diffusion gradients and the rf pulses to the object motion because of its quasi - continuous motion correction . fig1 shows how gradients are adapted to movement during a partial acquisition without interrupting the signal progression . the rf pulses used for refocusing are also adapted to the current object position during the corresponding partial acquisition without interrupting it . in a further development of this approach , motion and correction progressions are analyzed during a partial acquisition . if motion amplitude or velocity limits are exceeded , one or more partial acquisitions may be rejected and repeated . fig2 shows a chart of a variant of the method according to fig1 , in which correction deviations in previous partial acquisition partitions are detected during the current partial acquisition or partial acquisition partition by analyzing the progression of the movement and are corrected before the next rf pulse by means of additional gradient moments or phase changes . during calculation of the correction parameters of each partial acquisition partition , the best possible assumption about the object position is used for motion correction . if a better assumption about the object position at the beginning or information about movement during the corresponding partial acquisition partition becomes available as the partial acquisition progresses further , the difference between the position used for correction and the real position and the resulting phase errors can be calculated and corrected with additional gradient moments . in the described variant of the inventive method , these additional gradient moments are superposed on the existing gradients . fig3 shows a variant of the configuration of the inventive method for prospective motion correction . the configuration 300 contains a tomograph 310 with a receiver coil 320 and a measurement object 330 to be represented . the configuration also contains a measurement system 140 for sensing the patient position . the configuration 300 also contains a real - time module 350 with a calculation module 351 and a software interface 352 , as well as a signal generator module 360 , an amplifier unit 370 , and a receiver module 380 . in one variant of the inventive method , the calculation module 351 converts the position data of the measurement system 140 into the reference system of the mr system 310 and calculates all sequence parameters from the sequence source code using the software interface 352 . the software interface 352 uses the processed position data for motion correction of all sequence parameters while they are calculated in the calculation module 351 . fig4 shows a further variant of the configuration of the inventive method for prospective motion correction . the configuration 400 contains a tomograph 310 with a receiver coil 320 and a measurement object 330 to be represented . the configuration also contains a measurement system 140 for sensing the patient position . the configuration 300 also contains a real - time module 350 with a calculation module 351 , as well as a signal generator module 360 , an amplifier unit 370 , and a receiver module 380 . a hardware interface 490 is connected upstream of the signal generator module . in one variant of the inventive method , the calculation module 351 converts the position data of the measurement system 140 into the reference system of the mr system 310 and calculates all sequence parameters from the sequence source code . the hardware interface 490 uses the processed position data for motion correction by modifying the digital signals of the real - time module 350 to the signal generator module 360 . fig5 shows a further variant of the configuration of the inventive method for prospective motion correction . the configuration 500 contains a tomograph 310 with a receiver coil 320 and a measurement object 330 to be represented . the configuration further contains a measurement system 140 for sensing the patient position . the configuration 500 also contains a real - time module 350 with a calculation module 351 , as well as a signal generator module 360 , an amplifier unit 370 , and a receiver module 380 . a hardware interface 590 is connected upstream of the amplifier unit 370 . in one variant of the inventive method , the calculation module 351 converts the position data of the measurement system 140 into the reference system of the mr system 310 and calculates all sequence parameters from the sequence source code . the hardware interface 590 uses the processed position data for motion correction by modifying the analog signals of the real - time module 350 to the signal generator module 360 . fig6 shows a further chart of a partial acquisition of an mr acquisition that was corrected using the presented method . the partial acquisition shown is part of a rare sequence . the rare ( rapid acquisition with relaxation enhancement ) sequence , also known as tse or fse ( turbo or fast spin echo ) sequence , is a widespread imaging sequence and a very common read - out module in mr . although its signal behavior is usually regarded as robust , the rare sequence does exhibit a noticeable proneness to artifacts because of movements during its long acquisition time — in the range of approx . 1 second to several minutes — and its application of numerous , partially combined gradients and rf pulses . a rare sequence is either run as a so - called single - shot variant in the form of haste etc . or as a so - called multi - shot variant that is originally also known as hybrid rare . the former has only one excitation pulse with a large number of following refocusing pulses , while the latter repeats the sequence of excitation pulses with refocusing pulses at repetition intervals of the duration tr ( repetition time ), that is typically in the range approx . 0 . 5 seconds to several seconds . previously conventional methods corrected motion in the case of the rare sequence only once at the beginning or end of the rare echo train , typically before slice excitation . in the case of rare - based mr sequences , once - only motion correction per repetition interval is disadvantageous not only because of the long repetition interval , but also extremely problematic because of the essential phase stability of a rare sequence that is widely known as cpmg ( carr - purcell - meiboom - gill ) conditions . each movement within a repetition interval results in sensitive disturbances of the phase stability and therefore very soon in image artifacts in a rare - based mr sequence . because of its quasi - continuous motion correction , the presented method ensures that these cpmg conditions are met even in the presence of object movements and is therefore a precondition for operating a rare sequence with motion correction . looked at in special detail , the following three aspects are required : 1 .) to ensure the cpmg conditions ( phase stability ) are met , a rare - based sequence must keep to the following timing schedule : the first refocusing interval — mid excitation pulse to mid first refocusing pulse — must be half as long as the following refocusing intervals — mid refocusing pulse number n to mid next refocusing pulse number n + 1 . the presented method permits this because it does not require any time within the timing schedule . 2 .) it is essential that the gradient area — also termed zeroth moment — is constant for each encoding axis ( gx , gy and gz ) per refocusing interval , and is exactly half the size in the first refocusing interval as in the following refocusing interval . the presented inventive method enables this by correcting the gradients quasi - continuously in the case of movements and thus achieving and ensuring this necessary phase stability directly in the actual , measured object . in this case , “ gradients ” refers to all different function types of gradients used in a rare , such as read - out encoding , phase encoding , slice selection , crusher , spoiler , etc . in particular , the presented method optionally makes use of the fact that additional gradient moments or phase changes are applied before the end of each refocusing interval . these gradient moments for deviation correction can already be superposed on existing gradients , which does not demand any time within the timing schedule . 3 .) it is also essential that the originally set phase relationships of the rf pulses , the so - called initial phases , are retained . the presented method permits this by adapting the carrier frequencies and phases of the rf pulses according to the motion correction so that the intended , initial phases of the rf pulses are retained in the measured object . the general nature of the proposed principle permits both motion correction on conventional rare types ( rare , tse , fse , haste etc .) with constant flip angles and constant initial phases for the refocusing pulses , and the more modern variants with variable flip angles and constant initial phases for refocusing pulses ( traps , hypertse , space , cube , vista etc .) or variable flip angles and alternating initial phases ( generalized cpmg conditions ). for all types , any excitation pulse can be used , from the flip angle at the same phase or a phase shifted by 90 ° to the first refocusing pulse . moreover , due to the general nature of the approach , rare - like sequences are also possible and included that still meet the cpmg conditions for the gradients but keep both the initial phases and the flip angle for refocusing pulses and excitation pulse variable , such as , for example , rare sequences that follow the hyperecho principle . only a method that performs all these motion corrections achieves artifact - free images in a rare - based imaging sequence or a rare - based read - out module . fig7 shows a chart of a variant of the inventive method , wherein information about the object position is used during the read - out procedure to implement motion correction . for comparison , the corresponding partial acquisition is shown without the use of the described method . the read - out method depicted shows the frequently used echo planar imaging ( epi ). epi permits read - out of a whole mr data set after only one signal encoding and is therefore considered to be particularly resistant to motion artifacts . however , the duration of such a read - out operation is typically 50 - 100ms and motion that occurs in this period results in artifacts . methods that have been used so far only permit once - only motion correction at the beginning of the epi , and motion that may occur during the read - out operation cannot be corrected by these methods . moreover , other read - out methods with a longer duration (& gt ; 4ms ) ( e . g . spiral , rosette , etc .) also benefit from the inventive method . a variant of the method is especially preferred in which the object position is determined in 6 degrees of freedom ( in particular , 3 translations and 3 rotations ) because this permits a complete description of a solid body . in a further variant of the method , an object position in less than 6 degrees of freedom can be used for motion correction . this is possible if , within a partial acquisition partition , all degrees of freedom responsible for possible motion artifacts are known . the motion correction of tap ( 2 ) in fig1 is cited by way of example . for the motion correction to be applied , in this case , it is enough to determine the object position in 2 degrees of freedom ( 2 rotations around the axes orthogonal with respect to the gradient direction ). in a variant of the method , information about the previous progression of the movement is also used in determining the object position . using kinematic models ( e . g . kalman filters ), it is possible to adapt the motion correction accordingly . with this method , it is possible , for example , to reduce errors due to delays in the signal progression or inaccuracies due to signal noise . the best possible correction can also be calculated and applied during detection of motion patterns that do not correspond to those of a solid body . further advantages of the invention result from combining the basic principles explained in fig1 to 7 . thus , the characteristics stated above and described in more detail can be used singly or in any combination according to the invention . the embodiments shown and described are not an exhaustive list but are examples to explain the invention . fig8 shows images of two mr acquisitions ( dwi ), 801 showing an mr acquisition in which the motion correction was performed once before slice excitation and 802 showing an mr acquisition in which the motion correction was applied during diffusion weighting according to the inventive method . comparably large head movements occurred during both acquisitions . whereas in the first acquisition 801 , signal losses can be seen , these signal losses are prevented in the second acquisition 802 . comparison of the two acquisitions clearly shows the advantages of the inventive method . bammer , r ., apparatus and method for real time motion - 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