Parallel magnetic resonance acquisition technique

Abstract

A method for acquiring magnetic resonance data using a parallel acquisition technique, comprising the steps: a) irradiating an RF excitation pulse (RF1) of a first pulse sequence into a target volume (Z) of a test object (U), b) acquiring echo signals (SE, GRE) generated by the RF excitation pulse (RF1) of the first pulse sequence and storing them in an original measurement data set (Dm), c) following the acquisition of the echo signals (SE, GRE) generated by the RF excitation pulse (RF1) of the first pulse sequence, acquiring reference measurement data using a second pulse sequence and filling a reference measurement data set (Dref) with the acquired reference measurement data, such that high robustness to movements of the test object is achieved, d) if the original measurement data set has not yet achieved the desired completeness,Repeating steps a) to c) until the original measurement data set (Dm) has reached the desired completeness, but is incomplete according to the Nyquist condition; e) Completing the original measurement data set (Dm) using the reference measurement data set (Dref) and a parallel acquisition technique to create a complete measurement data set (D).

Description

[0001] The invention relates to parallel magnetic resonance imaging, in particular a method, a magnetic resonance system, a computer program and an electronically readable data carrier for recording magnetic resonance data using a parallel magnetic resonance technique.

[0002] The magnetic resonance technique (hereinafter the abbreviation MR stands for magnetic resonance) is a known technique with which images of the interior of an examination object can be generated. To put it simply, the examination object is placed in a magnetic resonance device in a comparatively strong, static, homogeneous basic magnetic field, also known as B 0 -field, with field strengths of 0.2 Tesla to 7 Tesla and more positioned so that its nuclear spins are oriented along the basic magnetic field. To trigger nuclear spin resonances, high-frequency excitation pulses (RF pulses) are radiated into the examination object, the triggered nuclear spin resonances are measured as echo signals, which are stored as so-called k-space data, and on the basis of these, MR images are reconstructed, e.g. using a multidimensional Fourier transformation [or spectroscopy data obtained]. Fast-switching magnetic gradient fields are usually superimposed on the basic magnetic field for spatial coding of the measurement data.

[0003] There are basically two ways of generating echo signals after excitation of the nuclear spins. On the one hand, the excited nuclear spins can be manipulated by switching dephasing and rephasing gradients G1, G2 in such a way that the signal decays faster than the T2* decay inherent in the measured tissue, but decays after a certain time, the echo time TE, after the RF excitation pulse RF1 forms a so-called gradient echo GRE to be measured, as is shown schematically in figure 1 is shown. Such sequences are commonly referred to as GRE sequences. On the other hand, a so-called spin echo SE can also be generated by irradiating an RF refocusing pulse RF2 after the irradiation of the RF excitation pulse RF1 after a time TE, again called the echo time, after the RF excitation pulse RF1, which is measured and whose amplitude, however, corresponds to that of the measured tissue inherent T2 decay is reduced. This is schematic in figure 2 shown. Such sequences are commonly referred to as SE sequences. There are also sequences that generate both gradient echoes and spin echoes, e.g. (true)FISP sequences. In any case, the excitation and the measurement of the generated echo signals are repeated for each sequence, if necessary (e.g. by switching different gradients for spatial coding) until the desired number of echo signals has been measured and stored in k-space in order to be able to image the examination object.

[0004] The SE sequences include in particular the TSE sequences (TSE: “Turbo Spin Echo”), which are also known by the names FSE (“Fast Spin Echo”) or RARE (“Rapid Acquisition with Refocussed Echoes”) sequences , widely used in clinical application. The advantage of the TSE sequences over the "simple" SE sequence is that after an RF excitation pulse RF1, multiple refocusing pulses RF2 are switched, and that multiple spin echo signals SE are also generated as a result. This speeds up data acquisition.

[0005] With so-called "single-shot" methods, the entire k-space data to be recorded, e.g. for imaging a slice of an examination object to be imaged, can be recorded after only one RF excitation RF1.

[0006] An example of such a single-shot TSE sequence is the HASTE sequence (“Half-Fourier Acquisition Single-shot Turbo spin Echo imaging”), in which a “partial Fourier” method is also used to reduce the k-space data to be acquired, in particular the Half-Fourier method is used. In this case, the symmetry of k-space compared to complex conjugation is used in order to derive non-measured k-space data from the measured k-space data. In this way, after just one excitation pulse RF1, all the k-space data of a slice to be imaged that are necessary for the method can be recorded. Such HASTE sequences are usually used to record MR images in the abdominal or also in the thoracic area in order to be able to record the largest possible areas of the volume of interest in the examination subject in a breath-hold phase with reduced sensitivity to physiological movements.

[0007] figure 3 is a schematic sequence diagram of such a HASTE sequence. As already mentioned, after an excitation pulse RF1, a number of refocusing pulses RF2 are switched in order to generate a number of spin echo signals, which each arise between the refocusing pulses RF2 (not shown). To limit the excitation to spins of a specific slice, a corresponding slice selection gradient GS can be switched simultaneously with each excitation pulse RF1 and refocusing pulse RF2. Frequency coding gradients GF and phase coding gradients GP can be switched on for further spatial coding. With HASTE, some (usually almost half of) the phase encoding gradients GP can be dispensed with, as is shown in figure 3 is indicated by the asymmetrical arrangement of the phase encoding gradients GP. As mentioned, the corresponding “missing” spin echo signals can be supplemented with a partial Fourier method. The asymmetric arrangement of the phase coding gradients GP shown, in which a larger number of echo signals are measured after the measurement of the echo signals in the k-space center (GP=0) than before the measurement of the k-space center, is advantageous in order to keep the echo times as short as possible hold, for which the region of the k-space center (low amplitudes of the phase encoding gradients GP) are measured. In this way, the echo signals in the region of the k-space center are weakened less strongly in accordance with the T2 decay than in another arrangement which only measures the region of the k-space center at later echo times.

[0008] In addition to the partial Fourier methods mentioned, so-called parallel acquisition techniques (e.g. GRAPPA and SENSE) are also known, with the help of which the acquisition times required for recording the desired data can be shortened. As in the partial Fourier method, only parts of the echo signals actually to be recorded as k-space data according to the Nyquist condition are measured. In contrast to the partial Fourier method, however, the non-measured parts in parallel acquisition techniques are usually distributed more evenly over the k-space to be measured according to Nyquist, so that, for example, only every second k-space line is measured. In addition, the "missing" k-space data are reconstructed with parallel acquisition techniques using coil sensitivity data. This coil sensitivity data is determined from reference measurement data that scan at least one area of ​​k-space to be measured, mostly the central area, completely according to the Nyquist condition.

[0009] If such reference measurement data is recorded with a different sequence type than the measurement data of the actual measurement to be completed, this can lead to artifacts in the reconstruction of the measurement data due to possibly different contrasts and / or different sensitivities to various disturbances. The point in time at which the reference measurement data is recorded compared to the actual measurement data, e.g. due to (unwanted) movements of the examination object, can lead to artifacts in the reconstruction.

[0010] The invention is therefore based on the object of specifying a method, a magnetic resonance system, a computer program product and an electronically readable data carrier which avoid artifacts in the reconstruction of measurement data recorded using parallel acquisition techniques.

[0011] The object is achieved by a method for recording magnetic resonance data using a parallel acquisition technique according to claim 1, a magnetic resonance system according to claim 8, a computer program product according to claim 9 and an electronically readable data carrier according to claim 10.

[0012] A method according to the invention for recording magnetic resonance data using a parallel acquisition technique comprises the steps: a) Radiation of an RF excitation pulse (RF1) of a first pulse sequence into a target volume of an examination subject (U), b) Recording of echo signals (SE, GRE) generated by the RF excitation pulse (RF1) of the first pulse sequence and storing them in an original measurement data record (D m ), c) following the recording of the echo signals (SE, GRE) generated by the RF excitation pulse (RF1) of the first pulse sequence, recording a reference measurement data set (D ref ) by means of a second pulse sequence, d) Repeating steps a) to c) until the original measurement data set (D m ) has reached a desired completeness, but is incomplete according to the Nyquist condition, e) Completion of the original measurement data set (D m ) using the recorded reference measurement data set (D ref ) and a parallel acquisition technique to a complete measurement dataset (D).

[0013] By recording the reference measurement data directly after recording the actual (original) measurement data, i.e. by recording reference and original measurement data alternately, there is no saturation of the magnetization in the recorded echo signals of the original measurement data set. The quality of the MR images obtained can thus be increased since areas with weakened intensity are avoided in this way. No compromises in image contrast have to be accepted by recording the reference measurement data immediately after recording the original measurement data.

[0014] In addition, the recording of the reference measurement data, on the one hand, and the original measurement data, on the other hand, which are close to one another in terms of time, means that the method is highly robust with regard to movements of the examination object.

[0015] A magnetic resonance system according to the invention comprises at least one magnet unit, a gradient unit, a high-frequency unit and a control device, the control device being designed to carry out a method described herein on the magnetic resonance system.

[0016] A computer program product according to the invention comprises a program and can be loaded directly into a memory of a control device of a magnetic resonance system, for example, with program means to carry out the steps of a method described herein when the program is executed in the control device of the magnetic resonance system.

[0017] An electronically readable data carrier according to the invention includes electronically readable control information stored thereon, which includes at least one named computer program product and is designed such that when the data carrier is used in a control device of a magnetic resonance system, it performs a method described herein.

[0018] The advantages and explanations given in relation to the method also apply analogously to the magnetic resonance system, the computer program product and the electronically readable data carrier.

[0019] Further advantages and details of the present invention result from the exemplary embodiments described below and from the drawings. The examples given do not represent any limitation of the invention. They show:

[0020] figure 1 a schematic representation of a gradient echo sequence,

[0021] figure 2 a schematic representation of a spin echo sequence,

[0022] figure 3 a schematic representation of a HASTE sequence,

[0023] figure 4 schematically, a flowchart of a method according to the invention,

[0024] figure 5 shows a schematic representation of an inventive acquisition of reference measurement data and original measurement data, and

[0025] figure 6 shows a schematic representation of a magnetic resonance system according to the invention.

[0026] figure 4 is a schematic flow diagram of a method according to the invention. First, in an original measurement data recording step 101 Spin echoes in a target volume Z of an examination object U are excited by means of an RF excitation pulse of a first pulse sequence PS1 (block 101.1 ). The excited spins are manipulated in a manner inherent in the first pulse sequence PS1, so that echo signals can be measured. These echo signals generated by the RF excitation pulse RF1 of the first pulse sequence PS1 are measured (block 101.2 ) and in an original measurement dataset D m saved. The measurement data recorded in this way of the original measurement data set D m can be reconstructed into MR image data after they have been completed according to the invention using a parallel acquisition technique.

[0027] Following the recording of the echo signals generated by the RF excitation pulse RF1 of the first pulse sequence PS1 and before a possible further RF excitation pulse RF1 of the first pulse sequence PS1 is radiated, a reference measurement data set D ref recorded by means of a second pulse sequence PS2 (block 103 ). This recording of the reference measurement data record includes the RF excitation pulses provided for the second pulse sequence and recording of the echo signals that have arisen. In this case, the central area of ​​the k-space to be filled is filled with reference measurement data in the usual way until a reference measurement data record that is sufficient for the parallel acquisition technique to be used is available. In this case, a certain waiting time, e.g.

[0028] This is also an example in figure 5 shown which waveforms of an example of through the steps 101 and 103 generated echo signals shows. In the example shown, a series of spin echo signals are generated by means of the first pulse sequence PS1, the amplitudes of which decrease over time in accordance with the T2 decay. Directly after recording the last spin echo signal generated by the RF excitation pulse of the first pulse sequence PS1, further echo signals, again spin echo signals in the example shown, are generated by means of a second pulse sequence PS2 and measured as reference measurement data.

[0029] If, after a repetition of the first pulse sequence, i.e. after recording the echo signals generated after an RF excitation pulse RF1, a desired completeness of the original measurement data set D m , which according to the Nyquist condition is to be regarded as incomplete, has not yet been reached, the original measurement data can be recorded 101 and the reference measurement data 103 be repeated until the desired completeness is reached. This can be done, for example, by a query 105 determined and the process controlled accordingly. The desired completeness reflects, for example, the acceleration achieved through the use of the parallel acquisition technique.

[0030] If the original measurement dataset has D m the desired completeness is achieved with the help of the recorded reference measurement data set D ref and a parallel acquisition technique of the original measurement dataset D m completed to a complete measurement data set (D) (block 107 ).

[0031] The target volume Z of the examination object U can be a slice in the examination object U, for example. When recording the original measurement dataset D m for such a layer, the desired completeness can already be achieved after an RF excitation pulse RF1 of the first pulse sequence PS1, so that it is sufficient to carry out the steps 101 and 103 to be performed only once.

[0032] In one embodiment, the first pulse sequence PS1 is a TSE sequence. As already stated, a TSE sequence can be used to generate and measure multiple spin echo signals after an RF excitation pulse. In particular, the first pulse sequence PS1 can be a HASTE sequence.

[0033] In the case of TSE sequences, in particular in the case of HASTE sequences, the use of a parallel acquisition technique is advantageous not only because of the reduction in the (original) measurement data to be recorded and the associated shortening of the measurement time. Because of the long echo trains possible with TSE, i.e. the large number of spin echo signals generated one after the other after an excitation, whose amplitudes decrease according to the T2 decay, an intrinsic k-space filter is created, which reduces the spatial resolution and leads to so-called blurring - Artifacts can result. These can be reduced or even completely avoided by accelerating by means of parallel acquisition.

[0034] The inventive recording of the reference measurement data set using a second pulse sequence PS2 following the recording of original measurement data using a first pulse sequence PS1 does not affect the original measurement data recorded using the first pulse sequence PS1. In particular, in the method according to the invention, there is no saturation of the magnetization for the acquisition of the original measurement data by the acquisition of the reference measurement data. Thus, with the method according to the invention, saturation artefacts, which are represented, for example, as areas with reduced intensity, can be avoided.

[0035] Furthermore, the recording of the original measurement data itself is not influenced by the recording of the reference measurement data, as would be the case, for example, if the reference measurement data, which are completely sampled according to Nyquist, were also recorded as part of the first pulse sequence PS1, or else with an interlaced recording of reference and original measurement data. In particular, the method according to the invention has no (negative) influence on the echo time(s) of the original measurement data, in particular the echo times of the original measurement data located in the k-space center, or the length of the echo train. The method according to the invention thus makes it possible to reduce or even avoid blurring artefacts and comparatively short echo times.

[0036] In addition, the recording of the reference measurement data, on the one hand, and the original measurement data, on the other hand, which are close to one another in terms of time, means that the method is highly robust with regard to movements of the examination object. This is particularly advantageous in the case of so-called breath-hold measurements, in which an examined patient should hold his breath as much as possible during the measurement in order to avoid breathing movements, because it often happens that the breath cannot be held for a sufficiently long time and strong movements occur, especially towards the end of the measurement. Even in this case, the influence of the movement on the quality of the completed measurement data record is at best slight due to the recording of the reference measurement data following the original measurement data.

[0037] Furthermore, the first pulse sequence PS1 can be a TSE-BLADE sequence. In this case, e.g. after excitation by means of an RF excitation pulse RF1 of the first pulse sequence PS1, all of the desired completeness of the original measurement data set D m necessary echo signals of a so-called BLADE blade are measured so that the recording of the original measurement data (block 101 ) and the reference measurement data (block 103 ) is performed once per BLADE.

[0038] The second pulse sequence can be an SE sequence, a TSE sequence, a single-shot TSE sequence or a HASTE sequence. As already shown, the TSE sequences are particularly suitable for recording reference measurement data because of their rapid data acquisition. In addition, SE sequences are generally not sensitive to basic magnetic field inhomogeneities. Using one of the SE sequences mentioned as the second pulse sequence PS2 for recording the reference measurement data can therefore avoid so-called aliasing artefacts, which can arise when the measurement data scan and reference scan have different sensitivities to basic magnetic field inhomogeneities and as a result, for example, parts of the image data are distorted or erased.

[0039] The second pulse sequence PS2 for recording the reference measurement data can have a comparatively low resolution, in particular a lower resolution than the first pulse sequence PS1. As a result, the time between multiple repetitions of the first pulse sequence 101.1 be kept low.

[0040] By selecting the type of the second pulse sequence as the same type of pulse sequence as the first pulse sequence, a particularly high quality of the completed measurement datasets D and thus ultimately of the MR images reconstructed from these can be achieved. Since, e.g. GRE sequences are more susceptible to magnetic field effects, e.g. Measurement data mean that, especially with large FOV and higher magnetic field strengths, peripheral areas of the FOV can no longer be resolved sufficiently well, which leads to so-called aliasing artefacts in the MR images reconstructed at the end.

[0041] For example, when using a GRAPPA technique or a SENSE technique as a parallel acquisition technique, a possibly different contrast in the reference measurement data than in the original measurement data does not affect the reconstruction of the completed measurement data set.

[0042] The first pulse sequence PS1 for recording the original measurement data and the second pulse sequence PS2 for recording the reference measurement data can also have different parameterizations, in particular different bandwidths, e.g. in the excitation, or also different types of gradient switching, e.g. to reduce the overall recording time.

[0043] figure 6 schematically represents a magnetic resonance system according to the invention 1 represents. This includes a magnet unit 3 to generate the basic magnetic field, a gradient unit 5 to generate the gradient fields, a high-frequency unit 7 for irradiating and receiving echo signals and a control device designed for carrying out a method according to the invention 9 . In figure 6 are these sub-units of the magnetic resonance system 1 shown only roughly schematically. In particular, the high-frequency unit 7 consist of a plurality of sub-units, in particular of a plurality of coils, which are designed to receive the triggered echo signals and possibly also to transmit high-frequency signals (RF pulses).

[0044] To examine an examination object U, for example a patient or a phantom, this can be placed on a couch L in the magnetic resonance system 1 are introduced into their measuring volume.

[0045] The control device 9 is used to control the magnetic resonance system and can in particular be the gradient unit 5 using gradient control 5' and the radio frequency unit 7 by means of a radio frequency transmission / reception control 7' Taxes. The high frequency unit 7 may include multiple channels on which signals can be sent or received.

[0046] The high frequency unit 7 is together with their radio frequency transmit / receive control 7' responsible for the generation and irradiation (transmission) of a high-frequency alternating field for manipulating the spins in the examination object U, with spins from at least one specific target volume Z, for example a slice, being able to be excited in a targeted manner, in particular with the aid of the gradient control.

[0047] Furthermore, the control device includes 9 a reconstruction unit 15 , in particular for controlling the recording and processing of the reference measurement data, and is designed to carry out a method according to the invention for recording magnetic resonance data using a parallel acquisition technique. One from the controller 9 included computing unit 13is trained to carry out all arithmetic operations required for the necessary measurements and determinations. Intermediate results and results required for this or determined in this way can be stored in a memory unit S of the control device 9 get saved. The units shown are not necessarily to be understood as physically separate units, but merely represent a subdivision into meaningful units, which can also, for example, be implemented in fewer or in just a single physical unit.

[0048] Via an input / output device I / O of the magnetic resonance system 1 control commands can be sent, e.g. by a user, to the magnetic resonance system and / or results from the control device 9 how, for example, created image data is displayed.

[0049] A method described herein can also be in the form of a computer program product that includes a computer program, the respective method on a control device 9 implemented if it is on the control device 9 is performed. Likewise, an electronically readable data carrier 26 with electronically readable control information stored thereon, which include at least one such computer program as just described and are designed in such a way that when using the data carrier 26 in a control device 9 a magnetic resonance system 1 carry out the procedure described.

Claims

[1] Method for acquiring magnetic resonance data using a parallel acquisition technique comprising the steps: a) Irradiation of an RF excitation pulse (RF1) of a first pulse sequence into a target volume (Z) of a test object (U), b) Recording echo signals (SE, GRE) generated by the RF excitation pulse (RF1) of the first pulse sequence and storing them in an original measurement data set (D m ), c) following the acquisition of the echo signals (SE, GRE) generated by the RF excitation pulse (RF1) of the first pulse sequence, acquisition of a reference measurement data set (D ref ) by means of a second pulse sequence, d) Repeat steps a) to c) until the original measurement data set (D m ) has achieved a desired completeness, but is incomplete according to the Nyquist condition, e) Completing the original measurement data set (D m) using the recorded reference measurement data set (D ref ) and a parallel acquisition technique to a complete measurement data set (D). [2] Method according to claim 1, wherein the first pulse sequence is a TSE sequence. [3] Method according to one of claims 1 or 2, wherein the first pulse sequence is a HASTE sequence. [4] Method according to any one of claims 1 to 3, wherein the target volume of the object under investigation (U) is a layer in the object under investigation (U), and steps a) to c) are performed only once. [5] Method according to one of claims 1 or 2, wherein the first pulse sequence is a TSE-BLADE sequence. [6] Method according to claim 5, wherein steps a) to c) are performed once per BLADE. [7] Method according to any of the preceding claims, wherein the second pulse sequence is an SE sequence, a TSE sequence, a single-shot TSE sequence or a HASTE sequence. [8] Magnetic resonance imaging ( 1 ) comprising a magnetic unit ( 3 ), a gradient unit ( 5 ), a high-frequency unit ( 7 ) and a control unit ( 9 ), wherein the control device ( 9 ) is designed to perform a method according to one of claims 1 to 7 on the magnetic resonance system ( 1 to execute. [9] Computer program product which includes a program and writes directly into a memory of a control device ( 9 ) a magnetic resonance system, with programming means to execute the steps of the method according to any one of claims 1 to 7, when the program is installed in the control unit ( 9 ) the magnetic resonance imaging system is used. [10] Electronically readable data carrier with electronically readable control information stored thereon, which includes at least one program according to claim 21 and is designed such that when the data carrier is used in a control device ( 9 ) of a magnetic resonance system perform the method according to one of claims 1 to 7.