Magnetic resonance imaging method for at least two separate radio-frequency transmit coils with time-delayed slice-selective excitation pulses

Abstract

A method for creating an image data set using a magnetic resonance system including at least two RF transmit coils includes, for each RF transmit coil, calculating a value for a susceptibility magnetic field gradient to be corrected from the G.sub.s map in combination with the B1 map of the RF transmit coil. The method includes, for each RF transmit coil, calculating a time delay of the excitation pulse. The method also includes calculating a complex weighting factor for scaling the pulse profile for each RF transmit coil to achieve an as uniform as possible deflection of the magnetization by the excitation pulse over the area under examination, and passing through the imaging sequence. The RF transmit coils each emit an excitation pulse with the calculated time delay and with a pulse profile scaled according to the calculated complex weighting factors.

Claims

1. A method for creating an image data set of an area under examination using a magnetic resonance system comprising at least two RF transmit coils, wherein at least one slice-selective excitation pulse with a prespecified pulse profile is applied to the area under examination during acquisition of image data by an imaging sequence, the method comprising: accessing B.sub.1 maps that represent a spatial distribution of the sensitivity of the at least two RF transmit coils over the area under examination; accessing a Gs map that represents a spatial distribution of susceptibility magnetic field gradients in the area under examination; for each RF transmit coil of the at least two RF transmit coils, calculating a value for a susceptibility magnetic field gradient to be corrected from the Gs map in combination with the B.sub.1 map of the RF transmit coil; for each RF transmit coil of the at least two RF transmit coils, calculating a time delay of the excitation pulse based on the susceptibility magnetic field gradient to be corrected, a slice gradient to be applied during the excitation pulse, and an echo time of the imaging sequence; calculating a complex weighting factor for scaling a pulse profile for each RF transmit coil of the at least two RF transmit coils to achieve an as uniform as possible deflection of the magnetization by the excitation pulse over the area under examination, wherein the calculation takes into account at least the B.sub.1 maps; and passing through the imaging sequence, wherein, during a simultaneous application of a slice gradient, the at least two RF transmit coils each emit an excitation pulse with the calculated time delay and with a pulse profile scaled according to the calculated complex weighting factors.

2. The method of claim 1, wherein the Gs map is calculated from a B.sub.0 map that represents the spatial distribution of a main magnetic field inhomogeneity in the area under examination.

3. The method of claim 2, wherein for the calculation of the Gs map, a linear dependence of the susceptibility magnetic field gradient on the magnetic field inhomogeneity stored in the B.sub.0 map is assumed.

4. The method of claim 1, wherein accessing the Gs map comprises accessing a standard susceptibility distribution that is stored in a database for a body part to be examined.

5. The method of claim 2, wherein, at the start of the method, the method further comprises: positioning a person to be examined in the area under examination; and determining the B.sub.0 map, the B.sub.1 maps, or the B.sub.0 map and the B.sub.1 maps by B.sub.0 mapping methods, B.sub.1 mapping methods, or B.sub.0 mapping methods and B.sub.1 mapping methods, respectively.

6. The method of claim 1, wherein the value for the susceptibility magnetic field gradient is the highest susceptibility magnetic field gradient within a sensitive region of the RF transmit coil.

7. The method of claim 1, wherein, during the calculation of the value for the susceptibility magnetic field gradient to be corrected, a threshold value is applied to the B.sub.1 map of the RF transmit coil in order to determine a sensitive region of the RF transmit coil.

8. The method of claim 1, wherein the B.sub.1 map is normalized, and during the calculation of the value for the susceptibility magnetic field gradient to be corrected, the B.sub.1 map is applied as spatial weighting to the Gs map.

9. The method of claim 1, wherein during the calculation of the complex weighting factor for scaling the pulse profile, the B.sub.0 map is taken into account.

10. The method of claim 1, wherein, during the calculation of the complex weighting factor for scaling the pulse profile, a k-space trajectory of the excitation pulse and the calculated time delays of the excitation pulses are taken into account.

11. The method of claim 1, wherein calculating the complex weighting factor comprises minimizing the product of a total system matrix with an optimized pulse profile minus a target magnetization.

12. The method of claim 1, wherein, at the start of the method, the method further comprises accessing a user-defined signal-to-noise ration (SNR) value that determines a degree of a possible signal loss due to the time delay, the user-defined SNR value being taken into account during the calculation of the time delay.

13. A magnetic resonance system comprising: at least one gradient coil operable to generate a gradient field; at least two transmit coils, each transmit coil of the at least two transmit coils comprising with an RF transmit channel operable to generate radio-frequency pulses; and a controller configured to create an image data set of an area under examination using the magnetic resonance system, wherein at least one slice-selective excitation pulse with a prespecified pulse profile is applied to the area under examination during acquisition of image data by an imaging sequence, the controller being further configured to: access B.sub.1 maps that represent a spatial distribution of the sensitivity of the at least two RF transmit coils over the area under examination; access a Gs map that represents a spatial distribution of susceptibility magnetic field gradients in the area under examination; for each RF transmit coil of the at least two RF transmit coils, calculate a value for a susceptibility magnetic field gradient to be corrected from the Gs map in combination with the B.sub.1 map of the RF transmit coil; for each RF transmit coil of the at least two RF transmit coils, calculate a time delay of the excitation pulse based on the susceptibility magnetic field gradient to be corrected, a slice gradient to be applied during the excitation pulse, and an echo time of the imaging sequence; calculate a complex weighting factor for scaling a pulse profile for each RF transmit coil of the at least two RF transmit coils to achieve a maximally uniform deflection of the magnetization by the excitation pulse over the area under examination, wherein the calculation takes into account at least the B.sub.1 maps; and pass through the imaging sequence, wherein, during a simultaneous application of a slice gradient, the at least two RF transmit coils each emit an excitation pulse with the calculated time delay and with a pulse profile scaled according to the calculated complex weighting factors.

14. In a non-transitory computer-readable storage medium that stores instructions executable by a magnetic resonance system to create an image data set of an area under examination, the magnetic resonance system comprising at least two RF transmit coils, wherein at least one slice-selective excitation pulse with a prespecified pulse profile is applied to the area under examination during acquisition of image data by an imaging sequence, the instructions comprising: accessing B.sub.1 maps that represent a spatial distribution of the sensitivity of the at least two RF transmit coils over the area under examination; accessing a Gs map that represents a spatial distribution of susceptibility magnetic field gradients in the area under examination; for each RF transmit coil of the at least two RF transmit coils, calculating a value for a susceptibility magnetic field gradient to be corrected from the Gs map in combination with the B.sub.1 map of the RF transmit coil; for each RF transmit coil of the at least two RF transmit coils, calculating a time delay of the excitation pulse based on the susceptibility magnetic field gradient to be corrected, a slice gradient to be applied during the excitation pulse, and an echo time of the imaging sequence; calculating a complex weighting factor for scaling a pulse profile for each RF transmit coil of the at least two RF transmit coils to achieve an as uniform as possible deflection of the magnetization by the excitation pulse over the area under examination, wherein the calculation takes into account at least the B.sub.1 maps; and passing through the imaging sequence, wherein, during a simultaneous application of a slice gradient, the at least two RF transmit coils each emit an excitation pulse with the calculated time delay and with a pulse profile scaled according to the calculated complex weighting factors.

15. The non-transitory computer-readable storage medium of claim 14, wherein the Gs map is calculated from a B.sub.0 map that represents the spatial distribution of a main magnetic field inhomogeneity in the area under examination.

16. The non-transitory computer-readable storage medium of claim 15, wherein for the calculation of the Gs map, a linear dependence of the susceptibility magnetic field gradient on the magnetic field inhomogeneity stored in the B.sub.0 map is assumed.

17. The non-transitory computer-readable storage medium of claim 14, wherein accessing the Gs map comprises accessing a standard susceptibility distribution that is stored in a database for a body part to be examined.

18. The non-transitory computer-readable storage medium of claim 15, wherein a person to be examined is positioned in the area under examination before the accessing of the B.sub.1 maps, and wherein before the accessing of the B.sub.1 maps, the instructions further comprise determining the B.sub.0 map, the B.sub.1 maps, or the B.sub.0 map and the B.sub.1 maps by B.sub.0 mapping methods, B.sub.1 mapping methods, or B.sub.0 mapping methods and B.sub.1 mapping methods, respectively.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a simplified perspective representation of one embodiment of a magnetic resonance system;

(2) FIG. 2 shows a longitudinal section through the magnetic resonance system 1;

(3) FIG. 3 shows a flow diagram of one embodiment of a method;

(4) FIG. 4 shows a sequence diagram of a part of one embodiment of the method.

DETAILED DESCRIPTION

(5) With reference to FIG. 1, in the embodiment shown, a magnetic resonance system 1 includes a main magnet 4 with an inner space 9. A patient 3 lies on a patient bed 2 that may be moved along an axis of the device 20 into the inner space 9.

(6) In the example shown, the head of the patient 3 is placed in a head coil 8, which is, for example, made up of two separate RF transmit coils 8a and 8b. Other exemplary embodiments may be provided. For example, the two RF transmit coils 8a and 8b may also be embodied as whole-body coils and permanently integrated in the magnetic resonance system 1. In one embodiment, more than two RF transmit coils may be provided. For example, head coils including four RF transmit coils each arranged concentrically about an axis may be provided.

(7) The head coil 8 is connected by a cable loom 16 to a connection 17 and, like all other components of the magnetic resonance system 1, is controlled by a control unit 6 (e.g., a controller, a processor). This is typically integrated in an operator console 10. The control unit 6 may be part of a computer (e.g., the central processing unit (CPU). Memory modules (e.g., a hard disc or a RAM or other data storage media for storing predetermined values, pulse profiles etc.) may also be part of the control unit 6. The operator console 10 also includes a screen 5 and optionally an input device such as a keyboard and a mouse (not shown), which enable a user to input data (e.g., an SNR value). A software program including program code portions Prg for carrying out the method according to one or more of the present embodiments may be stored on a digital storage medium 15 (e.g., a digital, optical or magnetic data storage device such as a CD-ROM) and in this way may be uploaded into in the control unit 6.

(8) FIG. 2 shows the bed 2 again in longitudinal section with the patient 3 positioned thereupon. The head of the patient is placed in the head coil 8, which is made up of two independent RF transmit coils 8a and 8b. The head coil 8 is connected to the control device 6 via the cable 16. The control device 6 also controls the other components of the magnetic resonance system 1 via the lines 7. These components include, for example, the main magnet 4. This includes, for example, the coil 12, which generates the main magnetic field and is made of superconducting material. There is also a gradient system 13 including at least one gradient coil to generate a gradient field. Shim coils 11 are also depicted schematically. An RF body coil is designated with the reference number 14. The depiction in FIG. 2 is purely schematic, and the spatial arrangement of these components may be different from that shown. Further components of the magnetic resonance system, such as, for example, ADCs, frequency generators, amplifiers, filters and other converters are not shown in this figure.

(9) The main magnet 4 generates a strong main magnetic field, which, in particular, within the region 18, is sufficiently homogeneous for the acquisition of magnetic resonance data (e.g., image data).

(10) FIG. 3 is a schematic sequence diagram of the method according to one or more of the present embodiments. In act S1, a patient is mounted in the magnetic resonance system, and corresponding RF transmit coils are selected or positioned on the part of the patient's body to be examined if the RF transmit coils are local coils. In act S2, a known B.sub.1 mapping method is used to measure the spatial distribution of the sensitivity of the at least two RF transmit coils and, for example, in the area under examination or over the surface of the slice or slices to be examined. The measured sensitivities are each stored in corresponding image data sets, each corresponding to the individual slices to be examined. In act S3, the inhomogeneities of the main magnetic field B.sub.0 are measured with corresponding B.sub.0 mapping methods. The measured data is stored in corresponding image data sets, each corresponding to a slice to be measured. In one embodiment, only the deviation from the main magnetic field or the mean value of the statistical magnetic field in the area under examination is stored.

(11) In act S4, a G.sub.s map depicting the spatial distribution of the magnetic field gradient in the area under examination is calculated from the B.sub.0 map for the area under examination or the slice to be examined. This is performed with the aid of stored information D1 (e.g., under the assumption of a linear dependence of the magnetic field gradient on the deviation of the magnetic field from the mean value). Corresponding proportionality factors are, for example, values between 1.0 to 2.0 T/m/Hz.

(12) In act S5, for each RF transmit coil, a value for a susceptibility magnetic field gradient to be corrected is calculated from the G.sub.s map in combination with the B.sub.1 map of the RF transmit coil. In one embodiment, a histogram of all magnetic field gradients within a sensitive region of the RF transmit coil, which is determined, for example, using the threshold value method, is formulated. Either the mean value of this statistical distribution or a particularly high value thereof is determined. The mean value or the particularly high value of the statistical distribution is defined as the susceptibility magnetic field gradient, the effect of which on the dephasing of the spins in the direction of the slice thickness is to be compensated by a time delay of the RF excitation pulse.

(13) In act S6, a corresponding time delay .sub.1 . . . .sub.c is calculated based on this susceptibility magnetic field gradient to be corrected and the slice gradient and the echo time of the imaging sequence, and is stored in the data set D2.

(14) Taking into account these time delays and further data stored in D2 (e.g., the B.sub.1 maps), in act S7, a complex weighting factor for scaling the pulse profile for each RF transmit coil is calculated. The complex weighting factors are optimized such that an as uniform as possible (e.g., a maximally uniform) deflection of the magnetization by the excitation pulse or the excitation pulses of all RF transmit coils over the area under examination is achieved.

(15) In act S8, the selected gradient echo imaging sequence is passed through. During the application of a slice gradient, the RF transmit coils each emit an excitation pulse with the calculated time delay and with a pulse profile scaled according to the calculated complex weighting factors.

(16) Acts S4 to S6 are shown more precisely in FIG. 4.

(17) Here, 20 designates the B.sub.0 maps for each slice to be examined. Corresponding G.sub.s maps are calculated therefrom. For example, a G.sub.s map is calculated for each of the slices to be examined 1, . . . , N, by multiplying the slope .

(18) These represent a linear estimation of the susceptibility magnetic field gradient in the direction of the slice thickness z.

(19) In act 22, the B.sub.1 maps are used. Of these, there is a number of C B.sub.1 maps for each slice n, where C indicates the number of RF transmit coils. In FIG. 4, in each case, the sensitivity profiles of the coils 1, 2, 3 and C are only shown schematically for the slice n. In this case, it is evident from the sensitivity profiles that the RF transmit coils are arranged somewhat concentrically about a head. The B.sub.1 maps are then subjected to a threshold value method. This defines a sensitive region or region of interest W.sub.s1, . . . , W.sub.SC for each coil. As shown, the sensitive regions W of the individual RF transmit coils overlap slightly, but not much. This is favorable in order to be able to optimize the RF transmit pulses to the susceptibility magnetic field gradients in each case. However, a suitable value to be corrected for each RF transmit coil is to be found. In act 28, a histogram 25 of all magnetic field gradients within the respective region of interest W stored in the B.sub.1 maps is formulated for each RF transmit coil. In the histograms 25, the maximum value of the magnetic field gradient, the highest value, is in each case marked with an arrow. In this exemplary embodiment, this value is assumed to be a susceptibility magnetic field gradient to be corrected. The value is then multiplied with the echo time TE of the imaging sequence and divided by the slice gradient G.sub.z which then produces the respective time delays .sub.1, . . . , .sub.c. If a plurality of slices are to be measured during the imaging sequence, acts 22, 28, 29 are each also performed for the other slices 1, 2, 3, . . . , N.

Experimental Examples

(20) An embodiment of the method was carried out on a 3 T Magnetom Skyra magnetic resonance system (Siemens, Erlangen, Germany) using a multi-slice gradient-echo-based FLASH sequence. The images were acquired with a field of view of 240240 mm.sup.2, resolution 256256 points, 26 slices, slice thickness 5 mm, TE/TR 20/600 milliseconds, a GRAPPA acceleration factor 2, and a flip angle of 25 degrees.

(21) The complex sensitivity profiles for the RF coils (e.g., B.sub.1 maps) were measured with the aid of presaturation turbo-FLASH-sequence, as described in H. P. Fautz et al B.sub.1-Mapping of Coil Arrays for Parallel Transmission, Proceedings of the 16th Annual Meeting of the ISMRM-Toronto, Canada 2008, 1247.

(22) A B.sub.0 map with fat-water in-phase was calculated on the basis of a multi-echo approach, similarly to as described in the article by J. Dagher et al, Magnetic Resonance in Medicine 71:105-117 (2014). The B.sub.0 map obtained in this way was used during both the calculation of the slice-specific and RF-transmit coil-specific time delays and the calculation of the complex weighting factors.

(23) The time delays were calculated with Mathlab 8.0 (Mathworks, Natick, Mass.) using a magnitude-least-squares approach. Using Hamming-filtered, RF sinc pulse profiles were used as static slice-selective RF wave forms or pulse profiles p and discretized with N.sub.p=200 [scanning] points. In order to avoid an excessive change to the slice profile, the maximum time delay was limited to 50% of the duration of the main sinc lobe. At least one RF channel transmits without a time delay in order to define the standard signal-echo-time relationship (e.g., the RF channel with the originally lowest value for the time delay). The slice gradient was set at 19 mT/m. All pulses were regularized so that the pulses remained within the limits set by the RF Hardware and SAR.

(24) The measurements were performed with two independent and fully integrated whole-body coils.

(25) The time delay was adapted in order to achieve the best compromise between the local compensation of the susceptibility magnetic field gradients and SNR level (=300 s) and to optimize the compensation of the signal loss (e.g., =800 s). The slices were set by the skull.

(26) For comparison, the same pulse sequence was also performed without a time delay (e.g., with the standard RF sinc excitation pulse).

(27) The images acquired with the standard excitation pulse suffered from strong susceptibility artifacts in the frontal-orbital and temporal cortex. In the center of the image, there was also some obscuration due to B.sub.1 effects (e.g., B.sub.1 shading). Contrary to this, the images taken with the time delay of the excitation pulses according to one or more of the present embodiments have significantly less signal loss and less B.sub.1 shading effects. The signal level may be retained, and up to 50% of the signal may be obtained in areas suffering from susceptibility magnetic field gradients in the direction of the slice thickness. The entire signal may be recovered, but at the expense of the SNR, as demonstrated by the measurements with =800 s.

(28) The elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent. Such new combinations are to be understood as forming a part of the present specification.

(29) While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.