RF amplifier control in parallel RF transmission based on power requirements

Abstract

A magnetic resonance imaging system acquires magnetic resonance data from a target volume in a subject. The magnetic resonance imaging system includes multiple excitation sources for generating a slice-selective or slab-selective spatial radio frequency (RF) excitation magnetic field targeting slice/slab spatial variations in the target volume, and a controller coupled to the excitation sources. The controller is adapted for: determining a power level required by the excitation sources for generating the slice-selective/or slab-selective spatial RF excitation magnetic field, decomposing the slice-selective or slab-selective spatial RF excitation magnetic field into respective RF excitation constituents of the excitation sources, controlling each of the excitation sources to simultaneously generate the respective RF excitation constituent, using the determined power level for acquiring the magnetic resonance data.

Claims

1. A magnetic resonance imaging (MRI) system for acquiring magnetic resonance data from a target volume in a subject, the magnetic resonance imaging system comprising: a plurality of excitation means for generating a-slice-selective or slab-selective spatial radio frequency (RF) excitation magnetic field targeting only slice/slab spatial variations in the target volume, and a controller coupled to the plurality of excitation means, wherein the controller is adapted to control a process comprising: determining a power level that the plurality of excitation means require to generate the slice-selective or slab-selective spatial RF excitation magnetic field for exciting the selected slice or slab, wherein the determining of the power level required by the plurality of excitation means comprises: controlling the MRI system to acquire pre-scan data from a pre-scan of the subject using the slice-selective spatial RF excitation magnetic field and using the pre-scan data to determine the power level; decomposing the slice-selective or slab-selective spatial RF excitation magnetic field into respective RF excitation constituents of the plurality of excitation means, and controlling each of the plurality of excitation means to simultaneously generate the respective RF excitation constituent, using the determined power-level to generate the slice-selective or slab-selective spatial RF excitation magnetic field, wherein the slice-selective or slab-selective spatial RF excitation magnetic field is applied to the selected slice or slab, and the magnetic resonance data is acquired.

2. The magnetic resonance imaging system of claim 1, wherein the determining of the power level required by the plurality of excitation means comprises: receiving data indicative of the power level, the power level being estimated using an RF simulation based on a model of the excitation means for applying the slice-selective or slab-selective spatial radio frequency RF excitation magnetic field on a model of the subject.

3. The magnetic resonance imaging system of claim 1, wherein the controller is further adapted for calculating a specific absorption rate (SAR) value associated with the slice-selective RF excitation magnetic field, wherein controlling each of the plurality of excitation means comprises controlling each of the plurality of excitation means to simultaneously generate the respective RF excitation constituent if the SAR value is below a predetermined SAR level.

4. The magnetic resonance imaging system of claim 1, further comprising: an RF amplifier for supplying current to each of the plurality of excitation means, the RF amplifier output being connected to each excitation means, and an electrical power supply coupled to the RF amplifier to supply a voltage at a first level in accordance with the determined power level to the RF amplifier output to generate a current in the excitation means to emit the slice-selective spatial RF excitation.

5. The magnetic resonance imaging system of claim 4, wherein the controller is further adapted for: controlling generating a two-dimensional spatial selective RF excitation targeting two-dimensional spatial variations in the target volume, and controlling the electrical power supply to adjust the voltage to a second level required for generating the two-dimensional spatial selective RF excitation.

6. The magnetic resonance imaging system of claim 4, wherein the controller is further adapted for: controlling generating a three-dimensional spatial selective RF excitation targeting three-dimensional spatial variations in the target volume, and controlling the electrical power supply to adjust the voltage to a third level required for generating the three-dimensional spatial selective RF excitation.

7. The magnetic resonance imaging system of claim 4, wherein the controller is an add-on module to the electrical power supply and/or the RF amplifier.

8. The magnetic resonance imaging system of claim 1, wherein the determining of the power level required by the plurality of excitation means comprises determining the power level as a sum of the individual power levels required by each of the plurality of excitation means for generating the respective RF excitation constituent.

9. The magnetic resonance imaging system of claim 1, wherein the plurality of excitation means comprises a transmit array coil comprising a plurality of RF transmit coils.

10. A method for acquiring magnetic resonance data, by a magnetic resonance imaging (MRI) system, from a target volume in a subject, the method comprising: generating by a plurality of excitation means a slice-selective or slab-selective spatial radio frequency (RF) excitation magnetic field targeting only slice/slab spatial variations in the target volume; determining by a controller of the magnetic resonance imaging system a power level that the plurality of excitation means-require to generate the slice-selective or slab-selective spatial RF excitation magnetic field for exciting the selected slice or slab, wherein the determining of the power level required by the plurality of excitation means comprises: controlling the MRI system to acquire pre-scan data from a pre-scan of the subject using the slice-selective spatial RF excitation magnetic field and using the pre-scan data to determine the power level; decomposing by the controller the slice-selective or slab-selective spatial RF excitation magnetic field into respective RF excitation constituents of the plurality of excitation means, and controlling by the controller each of the plurality of excitation means to simultaneously generate the respective RF excitation constituent, using the determined power-level to generate the slice-selective or slab-selective spatial RF excitation magnetic field, wherein the slice-selective or slab-selective spatial RF excitation magnetic field is applied to the selected slice or slab, and the magnetic resonance data is acquired.

11. A computer-readable medium that stores computer executable instructions, wherein, when executed by the controller, the computer executable instructions cause the controller of the magnetic resonance imaging system to perform the determining of the power level of claim 10.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following preferred embodiments of the invention will be described, by way of example only, and with reference to the drawings in which:

(2) FIG. 1 shows a cross-sectional and functional view of a medical apparatus,

(3) FIG. 2 shows a radio frequency transmit system,

(4) FIG. 3 is a flowchart of a method for generating a slice-/or slab-selective spatial radio frequency RF excitation magnetic field,

(5) FIG. 4 shows two linear sensitivity profiles for two different through plane variations,

(6) FIG. 5 shows a distribution of the RF power and normalized root mean square for the two-channel scenario,

(7) FIG. 6 shows eight FDTD-simulated sensitivities of the transverse cross-section of an arm, and

(8) FIG. 7 shows a distribution of the RF power and normalized root mean square for the eight-channel scenario.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(9) In the following, like numbered elements in the figures are either similar elements or perform an equivalent function. Elements which have been discussed previously will not necessarily be discussed in later figures if the function is equivalent.

(10) Various structures, systems and devices are schematically depicted in the figures for purposes of explanation only and so as to not obscure the present invention with details that are well known to those skilled in the art. Nevertheless, the attached figures are included to describe and explain illustrative examples of the disclosed subject matter.

(11) FIG. 1 illustrates an exemplary magnetic resonance imaging (MRI) system 100 for generating images of a patient 101. MRI system 100 comprises magnetic assembly 103 to generate magnetic fields that will be applied to patient 101. Magnetic assembly 103 comprises magnet coils 105 adapted to produce a static magnetic field required to perform magnetic resonance imaging and gradient coils 107. The gradient coils 107 are made up of an X-axis gradient coil, Y-axis gradient coil, and Z-axis gradient coil. This enables to image different regions of the patient 101 using the magnetic gradient field being produced by the gradient coils.

(12) MRI system 100 further comprises a gradient amplifier unit 109, and a gradient power controller 111. The gradient amplifier unit 109 includes an X-axis gradient amplifier Gx, Y-axis gradient amplifier Gy, and Z-axis gradient amplifier Gz. The gradient coil 107 is connected with the gradient amplifier 109. The X-axis gradient coil, Y-axis gradient coil, and Z-axis gradient coil of the gradient coil 107 are connected, respectively, with the Gx amplifier, Gy amplifier and Gz amplifier of the gradient amplifier 109. The gradient fields serve to spatially encode the magnetic resonance signals and to span the RF-energy during RF excitation across k-space.

(13) The gradient power controller 111 is connected with the gradient amplifier 109. Gradient power controller 111 generates control signals for controlling the gradient amplifier. In particular, gradient power controller 111 may generate control signals that induce gradient amplifier unit 109 to energize gradient coils 107.

(14) MRI system 100 further comprises an RF transmit coil 113 above the patient 101 for generating the RF excitation pulses. The excitation means 113 include a set of surface coils. The excitation means 113 may be used alternately for transmission of RF pulses as well as for reception of magnetic resonance signals. The RF transmit coil 113 may be implemented as a transmit array coil comprising a plurality of RF transmit coils. The RF transmit coil 113 is connected to an RF amplifier 115. The RF amplifier 115 may be powered by an electrical power supply 117. The electrical power supply 117 supplies a voltage for the RF amplifier 115 to generate a current in the RF transmit coil 113 to produce RF excitation pulses. A RF power controller 119 controls the RF amplifier 115 and the electrical power supply 117.

(15) The gradient power controller 111 and RF power controller 119 are shown as being connected to a hardware interface 154 of a computer system 152. The computer system 152 uses a processor 156 to control the magnetic resonance imaging system 100.

(16) The computer system 152 shown in FIG. 1 is illustrative. Multiple processors and computer systems may be used to represent the functionality illustrated by this single computer system 152. The computer system 152 comprises the hardware interface 154 which allows the processor 156 to send and receive messages to components of the MRI system 100. The processor 156 is also connected to a user interface 158, computer storage 160, and computer memory 162.

(17) The computer storage 160 is shown as containing MRI pre-scan data 168. The pre-scan data is indicative of the required power level by the RF transmit coil array for generating a slice-/or slab-selective spatial RF excitation magnetic field. The RF power controller 119 may read the pre-scan data 168 from computer storage 160 and determine the a power level required by the excitation means for generating the slab-selective spatial RF excitation magnetic field.

(18) The computer storage 160 is further shown as containing a pulse sequence 170. The pulse sequence 170 either contains instructions or it contains a timeline which may be used for constructing instructions which enable the magnetic resonance imaging system 100 to acquire magnetic resonance data 172.

(19) The computer storage 160 is further shown as storing magnetic resonance data 172 acquired by the magnetic resonance imaging system 100.

(20) The computer memory 162 is shown as containing a module 174. The module 174 contains computer-executable code which enables the processor 156 to control the operation and function of the MRI system 100. For example the module 174 may use the pulse sequence 170 to acquire the magnetic resonance data 172.

(21) For the purpose of explanation, the system described in FIG. 2 can be implemented in the MRI system in FIG. 1, but is not limited to this implementation. Therefore, reference numerals from FIG. 1 are not necessarily used in FIG. 2.

(22) FIG. 2 shows an RF transmit system. The RF transmit system comprises a plurality of separate RF transmit coils 201 which are situated within main magnet assembly 203.

(23) The RF transmit system 100 exercises control over the RF transmit coils 201 by means of the control system 205 and the RF power amplifier 215. The control system 205 comprises an RF pulse control module 219 and an electrical power supply 217 which is controlled by a power controller 221 of the RF pulse control module 219. The RF pulse control module 219 is arranged to perform the various functions involved with the RF excitations.

(24) The RF pulse control module 219 uses control signals generated by a computer (not shown) to produce the RF excitation field Bi through power amplifier 215 at a transmit coil i (e.g. 201.1) which emits the Bi field in a target volume of the patient 101. The RF pulse control module 219 comprises a determining unit 225 for determining a power level required by the excitation means for generating a slab-selective spatial RF excitation magnetic field. The RF pulse control module 219 further comprises a decomposing unit 227 for decomposing the slab-selective spatial RF excitation magnetic field into respective RF excitation constituents (Bi) of the excitation means, and a generating unit 229 for controlling each of the excitation means to simultaneously generate the respective RF excitation constituent, using the determined power level for acquiring the magnetic resonance data.

(25) The power rating of the RF power amplifier 215 may depend on the type of spatial-selective RF excitation magnetic field being used by the RF transmit system. The spatial-selective RF excitation magnetic field comprises a slab-selective RF excitation magnetic field and/or 2D/3D-selective RF excitation magnetic field. Depending on the type of the excitation, the power controller 221 may control the electrical power supply 217 to supply a voltage at a level corresponding with the required power by the excitation. For example, the RF power required by a slab-selective RF excitation magnetic field may decrease down to about 0.25% below the RF power required by basic RF shimming (i.e., optimizing amplitude and phase of each Tx channel), and thus the level of the required voltage may be below the standard level.

(26) The RF power amplifier 215 may energize RF coils 201 via the power splitter 223. The power splitter 223 is used for dividing the RF power applied by the RF power amplifier 215 among the RF transmit coils 201 in accordance with RF excitation field Bi of each transmit coil i (e.g. 201.2). In some cases, the ensemble of RF amplifier 215 and power splitter 223 is replaced by an ensemble for separate RF amplifiers 215.1, 215.2, . . . 215.N, thus superseding the power splitter 223 and facilitating independent waveforms for the N different Tx channels.

(27) FIG. 3 is a flowchart of a method for supplying current to a RF Tx coil of a magnetic resonance imaging system by a RF amplifier.

(28) In step 301, the controller determines a power level required by the excitation means for generating the slice-/or slab-selective spatial RF excitation magnetic field. The power level required by the excitation means is determined after receiving data indicative of the power level. The power level is estimated using an RF simulation based on a model of the excitation means for applying the slab-selective spatial radio frequency RF excitation magnetic field on a model of the patient 101. In another example, this power level may be determined by first acquiring by an MRI system pre-scan data from a pre-scan of the subject using the slice-selective spatial RF excitation magnetic field and then using the pre-scan data to determine the power level.

(29) In step 303, the controller decomposes the slab-selective spatial RF excitation magnetic field into respective RF excitation constituents of the excitation means.

(30) In step 305 the controller controls each of the excitation means to simultaneously generate the respective RF excitation constituent, using the determined power level for acquiring the magnetic resonance data.

(31) The slab-selective RF excitations constituents i.e. Bi of each transmit coil and their simultaneous emission, which defines a 1D Transmit SENSE are described in details in the following paragraphs.

(32) The general 3D equation of Transmit SENSE for N independent Tx elements of an array transmit coil which defines a linear superposition of individual pulse profiles is:

(33) $\begin{array}{cc}{P}_{\mathrm{des}}\left(r\right)=\underset{nN}{.Math.}{S}_n\left(r\right)P_n\left(r\right)& \left(1\right)\end{array}$

(34) with P.sub.des is the desired target pattern, S.sub.n the spatial RF-emission profile of Tx element n, and P.sub.n the spatial excitation pattern of Tx element n. Eq. (1) is applied to the through-plane direction assumed to be along z

(35) $\begin{array}{cc}{P}_{\mathrm{des}}\left(z\right)=\underset{nN}{.Math.}{S}_n\left(z\right)P_n\left(z\right).& \left(2\right)\end{array}$

(36) After transformation into k-space (z.fwdarw.k.sub.z), discretization of the time coordinate of the k-space trajectories (k.sub.z(t).fwdarw.k.sub.z(t.sub.k)) on kK time steps, and summarizing s.sub.n(k.sub.z) and p.sub.n(k.sub.z) to s.sub.full(k.sub.z) and p.sub.full(k.sub.z) (as described in U. Katscher et al., MRM 49 (2003) 144-150), respectively, Eq. (2) can be solved by, e.g., regularized pseudo-inversion
p.sub.full(k.sub.z(t.sub.k))=(s.sub.full.sup.Hs.sub.full+).sup.1s.sub.full.sup.Hp.sub.des(k.sub.z(t.sub.k)).(3)

(37) This equation can be used to shorten the duration of the individual pulses (reducing K), which however yields only a negligible effect. Furthermore, the equation is able to slightly reduce the normalized root-mean-square error (NRMSE) between desired and obtained excitation pattern P.sub.des. However, the strongest effect of Eq. (3) is observed reducing the total RF power

(38) $\begin{array}{cc}{P}_{\mathrm{tot}}=\underset{kK}{.Math.}{\underset{\_}{p}}_{\mathrm{full}}^2\left(k_z\left(t_k\right)\right).& \left(4\right)\end{array}$

(39) Similarly, a reduction of the local or global SAR can be achieved if an appropriate SAR model is included in Eqs. 3 and 4. A trade-off between P.sub.tot and NRMSE between desired and obtained excitation pattern is achieved by the regularization parameter in Eq. (3). This freedom can be applied to achieve a better performance where the slab profile is critical (e.g. in 3D imaging) or to emphasize power/SAR reduction (e.g. for REST slabs).

(40) Simulations

(41) Two following simulations demonstrate the feasibility of the present subject matter: a two-channel scenario with synthetic, linear sensitivity profiles, and an eight-channel scenario with simulated sensitivities of the transverse cross-section of an arm. In both scenarios, a constant target profile in the excited slab (P.sub.des=const) was chosen for tailored RF shimming.

(42) Two Channel Scenario:

(43) The two linear sensitivities are shown in FIG. 4 for two different through-plane variations (sensitivity slopes) of 5% and 50%. The trade-off between P.sub.tot and NRMSE using different in Eq. (3) is shown in FIG. 5. The two linear sensitivities allow NRMSE=0 for all possible slopes. Depending on this slope, the required RF power is 10% below the RF power required for basic RF shimming, means the optimal choice of the amplitude and phase in each individual transmit channel, using standard slice-selective excitation. Allowing NRMSE>0, the RF power further decreases down to 25% below basic RF shimming. The power reduction arises from the ability of the RF pulses to excite only the part of the slab with high sensitivity in each individual Tx channel using individually tailored RF waveforms for each channel.

(44) Eight Channel Scenario:

(45) The eight FDTD-simulated sensitivities of the transverse cross-section of an arm are shown in FIG. 6. In this case, the proposed method improves the optimum B1 homogeneity from NRMSE=2.6% for basic RF shimming (amplitude and phase optimised) to NRMSE=1.9% (FIG. 7). More important, for a given NRMSE, the required RF power is up to a factor 10 lower for the proposed 1D tailored shimming compared with basic shimming. For large (i.e., large NRMSE), P.sub.tot is the same for both methods. As expected, both methods outperform quadrature excitation, also shown in FIG. 7.

(46) One Channel Scenario

(47) Using a single Tx channel, the described method is still able to improve the homogeneity of the excited slice/slab, particularly if the sensitivity change inside the slice/slab is in first order oriented perpendicular to the slice/slab.

LIST OF REFERENCE NUMERALS

(48) 100 MRI system 101 patient 103 magnetic assembly 105 magnet coil 107 gradient coil 109 gradient amplifier 111 gradient power controller 113 RF coil 117 electrical power supply 119 RF power controller 152 computer system 154 hardware interface 156 processor 158 user interface 160 computer storage 162 computer memory 168 pre-scan data 170 pulse sequence 172 magnetic resonance data 174 module 200 RF transmit system 201 RF coils 203 magnet system 205 control system 215 RF power amplifier 217 electrical power supply 219 RF power controller 221 power controller 223 power splitter 225 determining unit 227 decomposing unit 229 generating unit