Method for improved simultaneous recording of measurement data from at least two tomographies of an examination object by means of magnetic resonance

Abstract

The method for improved recording of measurement data recorded simultaneously from at least two slices of an examination object by means of a magnetic resonance device according to the application comprises the following steps: - selection of a desired simultaneous recording of measurement data from at least two slices (S1,..., Sn); - determination of a minimum RF pulse duration (d RF ) for avoiding artifacts for the desired recording in consideration of desired recording parameters (PA); - execution of the desired recording with the determined minimum RF pulse duration. By means of the method, a minimum RF pulse duration is determined in consideration of desired recording parameters and parameters inherent to the magnetic resonance device used, which minimum RF pulse duration can be used for a subsequent recording of measurement data. The measurement data thus recorded are less affected by artifacts caused by non-identical phase shifts.

Description

Technical Field

[0001] The present invention relates to a method for improving the simultaneous recording of measurement data from at least two sections of an object under examination by means of magnetic resonance. Background Technology

[0002] Magnetic resonance imaging (MR) is a known technique for producing images of the interior of an object being examined. In short, for this purpose, the object is positioned in an MR apparatus within a relatively strong, static, uniform fundamental magnetic field, also known as a B0 field, with a field strength ranging from 0.2 Tesla to 7 Tesla and greater, such that the nuclear spins of the object are oriented along the fundamental magnetic field. To trigger nuclear spin resonance, which can be measured as a signal, a radio frequency excitation pulse (RF pulse) is incident on the object. The triggered nuclear spin resonance is measured as so-called k-space data, and MR images are reconstructed or spectroscopic data are obtained based on this k-space data. To positionally encode the measurement data, a rapidly switching magnetic gradient field, simply called a gradient, is superimposed on the fundamental magnetic field. The schema used to describe the temporal order of the incident RF pulse and the switching gradient is called a pulse sequence, or simply a sequence. The recorded measurement data is digitized and stored as complex values ​​in a k-space matrix. The corresponding MR image can be reconstructed from the k-space matrix occupied by the values, for example, by means of a multidimensional Fourier transform.

[0003] Perhaps the most commonly used method for generating an echo signal after exciting nuclear spin is the so-called spin echo method. In its simplest case, one can say that the transverse magnetization is "redirected" after the incident RF excitation pulse by the incident of at least one RF refocusing pulse, thereby re-phasing the dephased magnetization and generating a so-called spin echo SE after a time TE called the echo time following the RF excitation pulse.

[0004] The echo signals generated by excitation and measurement are repeated after the repetition time TR (e.g., in the case of switching different gradients for position encoding) until the required number of echo signals are measured and stored in k-space so that the object under inspection can be imaged.

[0005] Under SE sequences, especially the TSE sequence (Turbo Spin Echo) known under the names FSE (Fast Spin Echo) or RARE (Rapid Acquisition with Refocused Echoes) sequences, it is widely used in clinical applications. Compared to the "simple" SE sequence, the advantage of the TSE sequence is that it switches multiple refocusing pulses after the RF excitation pulse, thereby generating multiple spin echo signals (SE) after excitation. This accelerates data recording because fewer repetitions of sequences with different positional encodings are needed to measure all the desired data. Therefore, compared to the conventional SE method, in the case of the TSE sequence, the measurement time for the entire k-space corresponds to a reduction in the number of echo signals refocused and recorded after excitation, the so-called "fast factor."

[0006] The expectation of increasingly faster MR recording in clinical settings has led to a resurgence of methods that simultaneously record multiple images. Typically, these methods are characterized by the targeted simultaneous application of transverse magnetization of at least two slices to the imaging process, at least for a portion of the measurement (“multi-slice imaging”, “schicht-multiplexing”). In contrast, in established “schicht-multiplexing”, signals from at least two slices are recorded alternately, i.e., completely independently of each other, with correspondingly longer measurement times.

[0007] For this purpose, known methods include, for example, so-called Hadamard coding, methods with simultaneous echo refocusing, methods with broadband data recording, and methods using parallel imaging in the tomographic direction. Among the last mentioned methods are, for example, the CAIPIRINHA technique, as described by Breuer et al. in “Controlled Aliasing in Parallel Imaging Results in Hiher Acceleration (CAIPIRINHA) for Multi-Slice Imaging,” Magnetic Resonance in Medicine 53, 2005, pp. 684-691, and the blipped CAIPIRINHA technique, as described by Setsompop et al. in “Blipped-Controlled Aliasing in Parallel Imaging for Simultaneous Multislice Echo Planar Imaging With Reduced g-Factor Penalty,” Magnetic Resonance in Medicine 67, 2012, pp. 1210–1224.

[0008] In this tomographic multiplexing method, so-called multi-band RF pulses are used to simultaneously excite or otherwise manipulate two or more tomes, such as refocusing or saturating them. For example, such a multi-band RF pulse is, in this case, a multiplexing of individualized RF pulses used to manipulate the individual tomes to be manipulated simultaneously. To separate the signals generated by the different tomes, different phases are applied to the individualized RF pulses before multiplexing, for example, by adding a linear phase increase, thereby moving the tomes relative to each other in position space. For example, by multiplexing, a baseband-modulated multi-band RF pulse is obtained from the sum of the pulse shapes of the individualized RF pulses.

[0009] As described, for example, in the paper by Setsompop et al. mentioned above, the g-factor loss caused by the shift between slices can be reduced by, for example, using gradient epochs or correspondingly modulating the phase of individualized RF pulses. As also described in the paper by Setsompop et al., but also in the paper by Breuer et al., signals from simultaneously excited or otherwise manipulated slices can first be combined as if they were from a single slice, so that they can then be separated in post-processing using parallel reconstruction methods, such as the (slice) GRAPPA method ("GeneRalized Autocalibrating Partial Parallel Acquisition"), or another parallel imaging method (PPA), such as the SENSE method (SENSE: Sensitivity encoding).

[0010] To perform this separation of overlapping records, reference data is typically used for each of the records, which is measured, for example, during a pre-scan.

[0011] If a tomographic multiplexing method with the aforementioned gradient spikes and, for example, TSE sequences is used, ghosting artifacts may be observed in the resulting images in records that do not suppress spin signals in at least one tissue type, such as in records that are not fat-saturated. Summary of the Invention

[0012] The purpose of this invention is to reduce artifacts in tomographic reuse methods.

[0013] This invention is based on the understanding that ghosting artifacts can be caused by the different effects of gradient spikes on spins in different tissue types. Due to the difference in the resonant frequencies of spins present in different tissues, such as aqueous and adipose tissues, known as chemical shifts, spins in different tissue types are not excited in the same slice, but rather in slices offset from each other by a slice spacing Δz, wherein:

[0014] Δz=c*B0 / A GS ,

[0015] Where c corresponds to the observed chemical shift of the spin constrained in different tissue types, B0 corresponds to the strength of the fundamental magnetic field, and A GS The intensity (amplitude) of the tomographic selection gradient (GS) applied to the tomographic selection of the RF pulse used.

[0016] Therefore, for spins constrained in different tissue types, the gradient spikes used induce different phase shifts in the tomographic direction, also known as field of view shifts (FOV shifts). These phase shifts are derived from Y*Δz*m0, where m0 corresponds to the zeroth gradient moment of the used gradient spike, and Y corresponds to the gyromagnetic ratio. For example, in the blipped CAIPIRINHA technique mentioned above, the gradient moment m0 ​​changes from one k-spaceline to another, along which measurement data is recorded.

[0017] Gradient spikes are typically chosen such that they cause a desired phase shift for spins constrained in water, but these spikes can cause an additional phase shift, for example, for spins constrained in fat. Consequently, artifacts, particularly so-called fat binarization artifacts, may be included in the image data reconstructed from such recorded measurements.

[0018] The objective is achieved by the method according to the invention for improving the recording of measurement data simultaneously recorded from at least two sections of an examined object by means of a magnetic resonance apparatus, the magnetic resonance apparatus according to the invention, the computer program according to the invention, and the electronically readable data carrier according to the invention.

[0019] The method according to the present invention for improving the recording of measurement data simultaneously recorded from at least two slices of an object under examination by means of a magnetic resonance imaging device comprises the following steps:

[0020] - Select the desired simultaneous recording of measurement data from at least two faults (S1, ..., Sn).

[0021] - Determine the minimum RF pulse duration (d) to avoid artifacts for the desired recording, taking into account the desired recording parameters (PA). RF ),

[0022] - Perform the desired recording using the determined minimum RF pulse duration.

[0023] Using the method according to the invention, a minimum RF pulse duration is (automatically) determined, taking into account desired recording parameters and the inherent parameters of the magnetic resonance imaging equipment used. This minimum RF pulse duration can be used for subsequent recording of measurement data. Measurement data thus recorded is less affected by the different phase shifts between spins in different tissue types, as described above, because the minimized RF pulse duration allows for maximization of the amplitude used for tomographic selection gradients, thereby reducing the offset Δz (see the equation above). Consequently, image data reconstructed from measurement data recorded according to the invention contains fewer artifacts compared to image data reconstructed from conventionally recorded measurement data.

[0024] The magnetic resonance device according to the present invention includes a magnet unit, a gradient unit, a radio frequency unit, and a control device configured to perform the method according to the present invention, the control device having a pulse duration determination unit.

[0025] When a computer program according to the invention is executed on a control device, the computer program implements the method according to the invention on the control device.

[0026] The computer program may also exist in the form of a computer program product, which can be directly loaded into the memory of the control device. The computer program has a program code structure so that when the computer program product is run in the computing unit of the computing system, the method according to the invention is executed.

[0027] The electronically readable data carrier according to the invention includes electronically readable control information stored thereon, the electronically readable control information including at least one computer program according to the invention and designed such that when the data carrier is used in the control device of a magnetic resonance apparatus, the electronically readable control information executes the method according to the invention.

[0028] The advantages and implementation methods described herein are similarly applicable to magnetic resonance imaging devices, computer program products, and electronically readable data carriers. Attached Figure Description

[0029] Other advantages and details of the invention will become apparent from the embodiments described below and from the accompanying drawings. The examples listed are not intended to limit the invention. The drawings show:

[0030] Figure 1 A schematic flowchart of the method according to the present invention is shown.

[0031] Figure 2 The magnetic resonance apparatus according to the invention is shown schematically. Detailed Implementation

[0032] Figure 1 This is a schematic flowchart of a method for improving the recording of measurement data MD according to the present invention, wherein the measurement data MD is simultaneously recorded from at least two slices S1, ..., Sn of the object being examined by means of a magnetic resonance imaging device.

[0033] Here you can load values ​​for the minimum RF pulse duration dmin and the maximum RF pulse duration dmax related to the magnetic resonance imaging (MRI) device used (box 101). To do this, you can first query information about the hardware (HW) of the MRI device used (box 101').

[0034] The minimum RF pulse duration dmin and the maximum RF pulse duration dmax can be determined based on hardware-specific extreme values ​​of the parameters applicable to the magnetic resonance equipment used.

[0035] For example, to determine the minimum RF pulse duration dmin and the maximum RF pulse duration dmax, consider the extreme values ​​of at least one parameter from the following group of parameters of the magnetic resonance device used: maximum RF transmit power (B1max), maximum gradient intensity (Gmax), and minimum tomographic thickness (thmin).

[0036] In particular, the minimum RF pulse duration dmin can be determined as a function F1 of the reciprocals of the maximum gradient strength Gmax and the maximum RF transmit power B1max, thereby applying: dmin = F1(1 / B1max, 1 / Gmax). This approach reflects that a shorter RF pulse duration requires higher RF transmit power and a higher gradient strength.

[0037] The maximum RF pulse duration dmax can be determined, for example, as a function of F2, which is the reciprocal of the maximum gradient intensity Gmax and the minimum fault thickness thmin, thus applying: dmax = F2(1 / thmin, Gmax). This reflects that a longer RF pulse duration increases the achievable minimum fault thickness but reduces the required gradient intensity.

[0038] Here, the values ​​of the minimum RF pulse duration dmin and the maximum RF pulse duration dmax can be obtained, for example, from the manufacturer's specifications of the magnetic resonance imaging (MRI) device, or can be determined once for a given MRI device. However, it is also conceivable that, at least at preset time intervals, the values ​​of the minimum RF pulse duration dmin and the maximum RF pulse duration dmax for the MRI device are re-determined before recording measurement data, either always or after corresponding user input, thereby taking into account possible variations in extreme values.

[0039] The desired recordings of measurement data are selected (box 103), which are simultaneously recorded from at least two fractures S1, ..., Sn of, for example, a total of N fractures N (N≥n) to be recorded from the object under examination. By selecting the desired recordings, the associated recording parameters PA and the feasible RF pulses that can be used in the desired recordings and their associated RF parameters PRF are also determined.

[0040] As a desired recording method, one could choose to use fault reuse methods, particularly by using gradient spikes to apply phase differences to simultaneously record n ​​faults, such as the blipped CAIPIRINHA method.

[0041] If the desired recording permits, the minimum RF pulse duration can be determined here for multi-band RF pulses and / or for VERSE pulses (VERSE: "variable-rate selective excitation"). Multi-band RF pulses are readily used in tomographic multiplexing methods, as described above. VERSE pulses can also be advantageously used due to their inherently low required RF transmit power. Because of the inherently smaller required RF transmit power compared to other RF pulses, the achievable minimum RF pulse duration can be further reduced compared to other RF pulses, and / or the achievable minimum tomographic thickness can be reduced compared to other RF pulses.

[0042] Taking into account the desired recording parameter PA, determine the minimum RF pulse duration d for avoiding artifacts in the desired recording. RF (Box 105)

[0043] Determining the minimum RF pulse duration d RF At the same time, it is also advantageous to consider the values ​​of the minimum RF pulse duration dmin and the maximum RF pulse duration dmax for the magnetic resonance equipment used. In particular, it may be necessary to determine the minimum RF pulse duration dmin. RF It falls within the range defined by the minimum RF pulse duration dmin and the maximum RF pulse duration dmax. In this manner, it is ensured that the determined minimum RF pulse duration can be achieved using a magnetic resonance imaging (MRI) device.

[0044] Determining the minimum RF pulse duration d RF The desired recording parameter PA considered at the time can be one of the following groups: the desired fault thickness thsl and the set gradient intensity G. In this way, for example, it can be ensured that the deviation from the desired fault thickness is not (excessively) excessive.

[0045] Determining the minimum RF pulse duration d RF When determining the minimum RF pulse duration, at least one RF pulse parameter PRF, such as the bandwidth and / or pulse shape, can be considered to characterize the RF pulse to be used. Therefore, minimization can be performed in a pulse type-specific manner.

[0046] Determine the minimum RF pulse duration d RF This may include using the relationship between desired recording parameters and achievable RF pulse duration. If such a relationship between desired recording parameters and achievable RF pulse duration is known, it can be used to determine the minimum RF pulse duration.

[0047] For example, minimum RF pulse duration d RF F3 can be determined as a function related to the desired fault thickness thsl and the assumed gradient intensity G, as follows:

[0048] d RF =F3(Υ / (thsl*G)),

[0049] Where Υ / (thsl*G) reflects the relationship between the desired recording parameters and the RF pulse duration, and the function F3 can also be related to at least one RF pulse parameter, such as the selected pulse shape.

[0050] As mentioned above, in order to ensure the determined minimum RF pulse duration d RF The determination may further include a maximum value function and a minimum value function, wherein the maximum value function returns the largest element among the elements input to it, and the minimum value function returns the smallest element among the elements input to it.

[0051] For example, the minimum RF pulse duration d is first determined by function F3. RF The minimum RF pulse duration dmin can be subjected to a maximum value function to obtain the minimum RF pulse duration d of the first test. RF 1 = max(d RF ,dmin) as the result.

[0052] Minimum RF pulse duration d in the first test RF 1 can be subjected to a minimum function with respect to the maximum RF pulse duration dmax, so as to obtain the minimum RF pulse duration d of the final test as a result. RF 2 = min(d) RF 1,dmax) is used as the result to determine the minimum RF pulse duration.

[0053] Using the determined minimum RF pulse duration d RF To perform the desired recording (block 107) in order to obtain the desired measurement data. The minimum pulse duration d according to the invention is used. RF This can be used to adjust the spin offset in different tissue types along the fault direction, thereby reducing artifacts, especially those caused by chemical offsets.

[0054] Figure 2 A magnetic resonance apparatus 1 according to the present invention is schematically shown. The magnetic resonance apparatus 1 includes a magnet unit 3 for generating a basic magnetic field, a gradient unit 5 for generating a gradient field, a radio frequency unit 7 for incident and received radio frequency signals, and a control device 9 constituting a method according to the present invention.

[0055] exist Figure 2 The sub-units of the magnetic resonance device 1 are shown only schematically. In particular, the radio frequency unit 7 may consist of multiple sub-units, such as multiple coils, coils 7.1 and 7.2 or more as schematically shown, which may be designed to transmit radio frequency signals only, or to receive triggered radio frequency signals only, or both.

[0056] To examine an object U, such as a patient or phantom, the object can be introduced into the magnetic resonance imaging (MRI) device 1 on a bed L within the measurement volume of the MRI device 1. Tomographic slices Sa and Sb exemplarily represent the slices of the object to be simultaneously recorded, from which echo signals should be recorded and detected as measurement data.

[0057] The control device 9 is used to control the magnetic resonance device 1, and in particular, it can control the gradient unit 5 by means of the gradient control device 5' and the radio frequency unit 7 by means of the radio frequency transmit / receive control device 7'. The radio frequency unit 7 may include multiple channels on which signals can be transmitted or received.

[0058] The radio frequency (RF) unit 7, together with its RF transmit / receive control device 7', is responsible for generating and transmitting an incident (transmitted) RF alternating field to manipulate the spin in the region to be manipulated (e.g., in the tomography S to be measured) of the object under inspection U. Here, the center frequency of the RF alternating field, also known as the B1 field, is typically set as close as possible to the resonant frequency of the spin to be manipulated. The deviation between the center frequency and the resonant frequency is called detuning. To generate the B1 field, a current controlled by the RF transmit / receive control device 7' is applied to the HF coil in the RF unit 7.

[0059] Furthermore, the control device 9 includes a pulse duration determination unit 15, by means of which the minimum RF pulse duration can be determined. The control device 9 is configured to perform the method according to the invention.

[0060] The computing unit 13 included in the control device 9 is configured to perform all computational operations required for necessary measurements and determinations. Intermediate and final results required or obtained for this purpose can be stored in the storage unit S of the control device 9. Here, the units shown are not necessarily understood as physically independent units, but merely as subdivided into meaningful units; however, this can be implemented, for example, in fewer units or even in a single, unique physical unit.

[0061] The input / output device E / A of the magnetic resonance device 1 can transmit control commands to the magnetic resonance device by the user and / or display the results of the control device 9, such as image data.

[0062] The methods described herein can also exist in the form of a computer program product, which includes a program and, when the program is run on the control device 9, implements the described methods on the control device 9. Similarly, an electronically readable data carrier 26 may exist with electronically readable control information stored thereon, which includes at least one such computer program product and is designed such that when the data carrier 26 is used in the control device 9 of the magnetic resonance apparatus 1, the electronically readable control information executes the described methods.

Claims

1. A method for improved recording of measurement data (MD) which are recorded simultaneously from at least two slices (S1,..., Sn) of an examination object by means of a magnetic resonance device, the method comprising the following steps: - selecting a desired simultaneous recording of measurement data from at least two slices (S1,..., Sn), - performing the desired recording with the determined minimum RF pulse duration, wherein the minimum RF pulse duration is determined for multi-band RF pulses.

2. The method as claimed in claim 1, wherein the minimum RF pulse duration is also determined for variable-rate selective excitation pulses. - determining the minimum RF pulse duration (d RF ) for the desired recording avoiding artifacts caused by chemical shift in consideration of the desired recording parameters (PA) 3. The method as claimed in claim 1 or 2, wherein the minimum RF pulse duration (dmin) is loaded.

4. The method as claimed in claim 3, wherein the maximum RF pulse duration (dmax) is loaded.

5. The method as claimed in claim 3, wherein for determining the loaded minimum RF pulse duration (dmin) and the loaded maximum RF pulse duration (dmax), the extreme value of at least one parameter from the group consisting of maximum RF transmit power (B1max), maximum gradient strength (Gmax) and minimum slice thickness (thmin) of the magnetic resonance device used is taken into account.

6. The method as claimed in claim 1 or 2, wherein the minimum RF pulse duration (dmin) is loaded.

7. The method as claimed in claim 1 or 2, wherein the maximum RF pulse duration (dmax) is loaded. wherein values for a minimum RF pulse duration (dmin) and a maximum RF pulse duration (dmax) related to the magnetic resonance device used are loaded and the minimum RF pulse duration (d RF ) the loaded values for the minimum RF pulse duration and the maximum RF pulse duration (dmin, dmax) are taken into account.

8. The method as claimed in claim 7, wherein the characteristic RF pulse parameter (PRF) is the bandwidth and / or the pulse shape of the RF pulse.

9. The method as claimed in claim 1 or 2, wherein the minimum RF pulse duration (dmin) is loaded.

10. The method as claimed in claim 3, wherein the maximum RF pulse duration (dmax) is loaded.

11. A magnetic resonance device (1) comprising a magnet unit (3), a gradient unit (5), a radio frequency unit (7) and a control apparatus (9), the control apparatus (9) having a radio frequency transmit / receive control apparatus (7') and a pulse duration determination unit (15), wherein the control apparatus (9) constitutes a device for performing the method as claimed in any one of claims 1 to 10 on the magnetic resonance device (1).

12. A computer program product having a computer program which is directly loadable into the memory of a control apparatus (9) of a magnetic resonance device (1), the computer program having program means for performing the steps of the method as claimed in any one of claims 1 to 10 when the computer program is run in the control apparatus (9) of the magnetic resonance device (1). wherein the desired recording parameters (PA) considered in determining the minimum RF pulse duration (d RF ) are recording parameters (PA) of the group comprising a desired slice thickness (thsl) and a set gradient strength (G). ​ wherein at least one RF pulse parameter (PRF) characteristic for the RF pulse to be used is considered when determining the minimum RF pulse duration (d RF ) of the RF pulse to be used. ​ ​ ​ wherein the minimum RF pulse duration (d RF ) includes using a relationship between desired recording parameters and achievable RF pulse durations. ​ wherein the determined minimum RF pulse duration (d RF ) is in a range defined by the loaded minimum RF pulse duration (dmin) and the loaded maximum RF pulse duration (dmax). ​ ​ 13. An electronically readable data carrier having electronically readable control information stored thereon, which electronically readable control information comprises at least one computer program product according to claim 12 and is designed such that, when the data carrier is used in a control device (9) of a magnetic resonance apparatus (1), the electronically readable control information carries out a method according to any one of claims 1 to 10.

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