CN115474921A - Magnetic resonance B0 eddy current digital real-time compensation method and device - Google Patents
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Abstract
The application discloses a method and a device for magnetic resonance B0 eddy current digital real-time compensation. The method and the device reduce the hardware cost of B0 coils and the like required by a B0 simulation compensation mode, can achieve faster B0 eddy current compensation response speed, reduce B0 eddy current compensation error residues, and improve the image quality; the B0 eddy current compensation calculation is processed in real time, so that the dynamic change of parameters influencing the B0 eddy current, such as gradient layer selection, phase encoding, reading and the like, in the scanning process is supported, and the requirement that the correlation value is calculated and then the correlation parameter is locked before scanning is avoided; the post-processing work of B0 eddy current compensation in the image reconstruction stage is reduced, and the complexity of post-image processing is reduced.
Description
Technical Field
The application relates to the technical field of magnetic resonance, in particular to a method and a device for magnetic resonance B0 eddy current digital real-time compensation.
Background
Magnetic Resonance Imaging (MRI) technology has been widely used in medical pathological diagnosis and basic scientific research as a non-invasive diagnostic means capable of reflecting multi-dimensional information.
In an MRI system, different B0 fields correspond to different nuclear resonance frequencies.
When the gradient field is added to X, Y, Z, the dynamically changing gradient field causes an eddy current effect, which causes the B0 field to start to change along with time, further causes the resonance frequency of the atomic nucleus to change, and finally shows that the image has artifacts and black bands, which seriously affects medical diagnosis.
It is therefore necessary to compensate for the B0 eddy current in real time.
At present, many MRI systems cannot compensate for B0 eddy currents or can only perform analog compensation, and need to add a B0 coil and corresponding compensation hardware, and have a poor effect of compensating for rapidly changing eddy currents.
And the individual system adopts a method of calculating a simulation file for compensating the sequence and a variable file of the sequence before scanning and compensating during image reconstruction after scanning, and the method cannot meet the requirement of dynamic switching of scanning parameters and increases the complexity of post-image processing.
It is to be noted that the above information disclosed in this background section is only for background of the inventive concept and may therefore contain information that does not constitute prior art.
Content of application
Aiming at the defects of the prior art, the application discloses a method and a device for magnetic resonance B0 eddy current digital real-time compensation, which can solve the problems that an MRI system cannot compensate the B0 eddy current and cannot meet the requirement of scanning parameter dynamic switching. Under the condition of not increasing hardware cost, the B0 eddy current compensation effect can be improved, the image quality is improved, and the requirement of dynamic switching of various scanning parameters is met.
In order to achieve the above purpose, the present application is implemented by the following technical solutions:
the magnetic resonance B0 eddy current digital real-time compensation method comprises the following steps:
calculating the eddy current value B from the X axis to B0 x (n)：
Wherein, b x And (n) represents the nth eddy current value from the X axis to the B0 at the current moment, m represents the number of terms of an eddy current compensation amplitude constant and a time constant, and k is a term index number, wherein a better compensation effect can be obtained by generally taking more than 6 terms. A. The x ，T x Amplitude constant and time constant from X-axis to B0, respectivelyAnd (4) counting.
Calculating the eddy current value B from Y axis to B0 y (n)：
The sign defines the same formula definition as the calculation of the eddy current value from the X axis to B0, and is omitted here.
Calculating the eddy current value B from the Z axis to B0 z (n)：
The sign defines the eddy current value calculation formula definition from the X axis to the B0, and is omitted here.
X, Y, Z gradient to B0 Total swirl value B0 (n) is:
B0(n)＝b x (n)+b y (n)+b z (n)
where B0 (n) represents the nth swirl value of the gradient from the current time X, Y, Z to B0 total.
The B0 vortex value is converted into a frequency value, and the calculation method comprises the following steps:
f B0 (n)＝(B0(n)/2^N)*V dac *G amp *AL/Ae*γ
wherein f is B0 (N) is the frequency change value of the main magnetic field caused by converting the B0 eddy current value into the B0 eddy current, N is the bit width output to the gradient DAC, and V is the bit width output to the gradient DAC dac Is the voltage value at full output of DAC, G amp The voltage is amplified to the current amplification factor by a gradient power amplifier, AL is the inductance coefficient of a nuclear magnetic resonance system, ae is the effective cross-sectional area, and gamma is the magnetic rotation ratio of atomic nuclei.
The interpolation can be satisfied by various methods, one of which is linear interpolation, and the calculation method is as follows:
f intp (n*I+m)＝f B0 (n-1)+(f B0 (n)-f B0 (n-1))/I*m
wherein f is intp (n + I + m) is a frequency offset value obtained by interpolating a frequency variation value of the main magnetic field of B0 between n-1 and n points, wherein I is an integerAnd m is an interpolation offset sequence number.
The phase accumulation calculation method comprises the following steps:
wherein
Is the accumulated phase error value. For the sake of simplicity, the new sequence number after interpolation is denoted by i. Δ t is the time interval between two points after interpolation, i.e. the working clock period of the radio frequency transmit and receive NCO. mod is the remainder symbol.
The phase correction processing procedure comprises: first the accumulated phase error value
And adding the corrected transmission NCO phase and the corrected receiving NCO phase together with the transmission NCO phase and the receiving NCO phase to obtain the corrected transmission NCO phase and the corrected receiving NCO phase, then comparing the current value with the absolute value of 360, and when the absolute value of the phase correction value exceeds 360, taking the remainder to be within +/-360, otherwise, keeping the remainder unchanged. And finally, converting the transmitted and received phase correction values into respective phase control words respectively, and sending the phase control words to a transmitting NCO phase control interface and a receiving NCO phase control interface.
In addition, this application has still disclosed the digital real-time compensation's of magnetic resonance B0 vortex device, in FPGA, contains: the device comprises a B0 vortex dynamic calculation module, a B0 vortex value conversion frequency value module, a frequency value interpolation module and a phase correction module. The method specifically comprises the following steps:
the B0 vortex dynamic calculation module comprises a floating point multiplier, an adder and a register, receives the digital waveform value of X, Y, Z gradient, latches the digital waveform value into the register, and then calculates the digital waveform value with the preset B0 vortex compensation time constant and amplitude constant parameter value to obtain the vortex value applied to B0 by the gradient of X, Y, Z from the beginning of scanning to the current moment. The working clocks of a floating-point multiplier and an adder in the B0 vortex dynamic calculation module are far higher than the generation speed of the digital waveform with the gradient of X, Y, Z, and the calculation can be completed by a small number of multipliers and adders through a time division multiplexing method.
And the B0 vortex value conversion frequency value module comprises a floating point multiplier, an adder, a floating point-to-fixed point converter and a register. The B0 eddy current value is firstly cached in a register by the B0 eddy current value conversion frequency value module, then the magnetic field change value caused by current change is calculated according to the electromagnetic induction calculation formula, and finally the change frequency value is obtained by multiplying the magnetic field change value and the magnetic rotation ratio of the atomic nucleus. Meanwhile, compared with floating-point calculation, fixed-point calculation saves resources, fixed-point value is converted before the frequency value is output, and fixed-point calculation is adopted in subsequent calculation.
The frequency value interpolation module comprises a fixed point multiplier, an adder, a comparator and a register. The frequency value interpolation module firstly converts the B0 vortex value into the frequency value output by the frequency value module and caches the frequency value in a register, then carries out interpolation calculation by utilizing a fixed-point multiplier and an adder, and the interpolated data rate is matched with the working frequency of the radio frequency transmitting and receiving NCO. And performing accumulation calculation by using the interpolated frequency values to obtain a phase accumulation value at the current moment, comparing the phase accumulation value with an absolute value of 360, and when the absolute value of the phase accumulation value exceeds 360, taking the remainder to be within +/-360.
And the phase correction module comprises a transmitting NCO phase correction interface, a receiving NCO phase correction interface, a fixed-point adder and a comparator. And the phase correction module receives the phase accumulated value of the frequency value interpolation module, and then adds the phase accumulated value with the transmitting NCO phase and the receiving NCO phase to obtain the corrected transmitting NCO phase and receiving NCO phase, compares the phase correction value with an absolute value of 360, and when the absolute value of the phase correction value exceeds 360, the phase correction value is remained within +/-360. And then converting the transmitted and received phase correction values into phase control words respectively, and sending the phase control words to a transmitting NCO phase control interface and a receiving NCO phase control interface.
The application discloses a magnetic resonance B0 eddy current digital real-time compensation method and device, which have the following advantages:
and a full digital calculation method is adopted, so that a B0 eddy current compensation coil and related hardware cost are saved.
The response speed of the simulator element is low, and after the B0 eddy current compensation is performed by adopting digital calculation, the advantages of the FPGA internal hydration and parallel calculation are benefited, the response speed is high, and the residual error of the B0 eddy current is small. Compared with the analog compensation, the compensation effect is better.
The advantages of parallel computation in the FPGA and the full digitalization of the B0 eddy current enable the B0 eddy current compensation to be processed in real time, thereby supporting the dynamic change of parameters influencing the B0 eddy current in the scanning process, such as gradient layer selection, phase encoding, reading and the like, and avoiding the limitation of computing correlation values and then locking the correlation parameters before scanning.
The B0 eddy current compensation value is directly acted on the receiving NCO to compensate to the nuclear magnetic resonance echo signal, so that the post-processing work of B0 eddy current compensation in the image reconstruction stage is avoided, and the complexity of post-image processing is reduced.
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In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below.
It is obvious that the drawings in the following description are only some embodiments of the application, and that for a person skilled in the art, other drawings can be derived from them without inventive effort.
Fig. 1 is a schematic diagram of an embodiment of the present application.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions of the embodiments of the present application will be clearly and completely described below, and it is obvious that the described embodiments are some embodiments of the present application, but not all embodiments.
All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.
As shown in fig. 1, the apparatus for digital real-time compensation of magnetic resonance B0 eddy current according to the embodiment of the present application includes a Field-Programmable gate array (FPGA) of a seling a200 model. The FPGA comprises: the device comprises a B0 vortex dynamic calculation module, a B0 vortex value conversion frequency value module, a frequency value interpolation module and a phase correction module.
In this embodiment, all modules perform a uniform hydration operation, and all initial values of the registers are 0 except for the preset parameter values. The update cycle of X, Y, Z gradient waveform value is 1us, X, Y, Z gradient waveform value enters into B0 vortex dynamic state calculation module. The B0 swirl value is calculated once every time the X, Y, Z gradient waveform value is updated. The B0 vortex dynamic calculation module calculates vortex values from an X axis to B0, from a Y axis to B0 and from a Z axis to B0 in parallel, and then adds the vortex values to obtain a total vortex value from X, Y, Z gradient to B0, wherein the calculation method comprises the following steps:
calculating the eddy current value B from the X axis to B0 x (n)：
Wherein, b x And (n) represents the nth eddy current value from the X axis to the B0 at the current moment, m represents the number of terms of an eddy current compensation amplitude constant and a time constant, and k is a term index number, wherein a better compensation effect can be obtained by generally taking more than 6 terms. A. The x ，T x The amplitude constant and time constant of the X-axis to B0, respectively.
Calculating the eddy current value B from Y axis to B0 y (n)：
The sign defines the eddy current value calculation formula definition from the X axis to the B0, and is omitted here.
Calculating the eddy current value B from the Z axis to B0 z (n)：
The sign defines the same formula definition as the calculation of the eddy current value from the X axis to B0, and is omitted here.
X, Y, Z gradient to B0 Total swirl value B0 (n) is:
B0(n)＝b x (n)+b y (n)+b z (n)
where B0 (n) represents the nth swirl value of the gradient from the current time X, Y, Z to B0 total.
In this embodiment, the floating-point multiplier and adder operation clock is 100MHz. The calculation force requirement can be met by one floating-point multiplier and one floating-point adder in a time division multiplexing mode.
B0 (n) update every 1 us. B0 (n) entering a B0 vortex value conversion frequency value module, wherein the B0 vortex value conversion frequency value module converts B0 (n) into a main magnetic field frequency change value induced by B0 vortex, and the calculation method comprises the following steps:
f B0 (n)＝(B0(n)/2^N)*V dac *G amp *AL/Ae*γ
wherein f is B0 (N) is the frequency variation value of the main magnetic field caused by the conversion of the B0 eddy current value into B0 eddy current, the unit is MHz, N is the bit width output to the Digital to analog converter (DAC), V dac Is the voltage value at full output of DAC, G amp The voltage of a gradient power amplifier is changed to the current amplification factor, AL is the inductance coefficient of the nuclear magnetic resonance system, ae is the effective cross-sectional area, and gamma is the magnetic rotation ratio of the atomic nucleus.
f B0 And (n) updating every 1us, and entering a frequency value interpolation module. Frequency value interpolation module is used for interpolating the frequency value f B0 (n) interpolating to the operating clock frequency of the radio frequency transmit and receive Numerically Controlled Oscillator (NCO). In this embodiment, the operating clock frequency of the radio frequency transmitting and receiving NCO is 100MHz, i.e. the clock interval is 10ns, and the interpolation multiple is 100. In order to simplify the calculation, the present embodiment adopts linear interpolation, and the calculation method thereof is as follows:
f intp (n*I+m)＝f B0 (n-1)+(f B0 (n)-f B0 (n-1))/I*m
wherein f is intp (n x I + m) is a frequency deviation value obtained by interpolating a frequency change value of the main magnetic field B0 between n-1 and n points, wherein I is an interpolation multiple, and m is a frequency deviation valueThe offset order number is interpolated. In other application scenarios, other interpolation methods, such as second-order parabolic fitting, etc., may also be employed.
After frequency values matched with the frequency of the working clock of the radio frequency transmitting and receiving NCO are obtained, the frequency values are accumulated to obtain phase values needing to be compensated, and the calculation method comprises the following steps:
wherein
Is the accumulated phase error value. For the sake of simplicity, the new sequence number after interpolation is denoted by i. Δ t is the time interval between two points after interpolation, i.e. the duty cycle of the radio frequency transmit and receive NCO in this embodiment, Δ t is 10ns.
After continuous accumulation, the value can exceed +/-360 degrees, and the remainder is obtained by mod and then is adjusted to be within +/-360 degrees. In the embodiment, the radio frequency transmitting and receiving NCO are on the same FPGA, so that the radio frequency transmitting and receiving NCO can be shared
The value is compensated. When the radio frequency transmitting and receiving NCO are not on the same FPGA, such as under the application scene of multi-chassis extension or optical fiber remote, the transmission is reduced under the normal condition
The radio frequency transmitting and receiving parts will copy one frequency value interpolation module to obtain low speed f B0 (n) simultaneously to the transmitting and receiving modules, and then simultaneously in the respective modules
And (4) calculating.
The accumulated phase error value is obtained in the calculation
Then, the accumulated phase error value is first
And adding the corrected transmission NCO phase and the corrected receiving NCO phase together with the transmission NCO phase and the receiving NCO phase to obtain the corrected transmission NCO phase and the corrected receiving NCO phase, then comparing the current value with the absolute value of 360, and when the absolute value of the phase correction value exceeds 360, taking the remainder to be within +/-360, otherwise, keeping the remainder unchanged. And finally, converting the transmitted and received phase correction values into respective phase control words respectively, and sending the phase control words to a transmitting NCO phase control interface and a receiving NCO phase control interface. At each 100M operating clock of the transmit NCO and the receive NCO, the corrected phase control word is updated and the B0 eddy current is compensated.
It is noted that relational terms are used herein only to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions.
Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
Without further limitation, an element defined by the phrase "comprising a … …" does not exclude the presence of another identical element in a process, method, article, or apparatus that comprises the element.
It will be apparent to those skilled in the art that the modules or steps of the present application described above can be implemented using a general purpose computing device, which can be centralized on a single computing device or distributed across a network of multiple computing devices.
Alternatively, they may be implemented in program code executable by a computing device such that it is executed by the computing device when stored in a memory device, or separately as individual integrated circuit modules, or as a single integrated circuit module having multiple modules or steps.
The present invention is not limited to any specific combination of hardware and software.
The above embodiments are only used to illustrate the technical solutions of the present application, and not to limit the same; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present application.
Claims (10)
1. A method for digital real-time compensation of magnetic resonance B0 eddy current comprises
Calculating the eddy current value from X axis to B0
Calculating the eddy current value from Y axis to B0
Calculating the eddy current value from Z axis to B0
Wherein, b x (n) represents the nth vortex value from the X axis to B0 at the current moment, m represents the number of terms of the vortex compensation amplitude constant and the time constant, k is the index number of terms, A x Is the amplitude constant from the X-axis to B0, T x Is the time constant from the X axis to B0;
x, Y, Z the total swirl value of the gradient to B0 is set to B0 (n) = B x (n)+b y (n)+b z (n)，
Where B0 (n) represents the nth swirl value of the gradient from the current time X, Y, Z to B0 total.
2. The method for magnetic resonance B0 eddy current digital real-time compensation as claimed in claim 1, wherein the process of converting the B0 eddy current value into a frequency value is set as f B0 (n)＝(B0(n)/2^N)*V dac *G amp *AL/Ae*γ，
Wherein f is B0 (N) is the frequency change value of the main magnetic field caused by converting the B0 eddy current value into the B0 eddy current, N is the bit width output to the gradient DAC, and V is the bit width output to the gradient DAC dac Is the voltage value at full output of DAC, G amp The voltage of a gradient power amplifier is changed to the current amplification factor, AL is the inductance coefficient of the nuclear magnetic resonance system, ae is the effective cross-sectional area, and gamma is the magnetic rotation ratio of the atomic nucleus.
3. The method for magnetic resonance B0 eddy current digital real-time compensation according to claim 1, wherein the process of linear interpolation is set as f intp (n*I+m)＝f B0 (n-1)+(f B0 (n)-f B0 (n-1))/I*m，
Wherein, f intp And (n + I + m) is a frequency offset value obtained by interpolating the frequency change value of the main magnetic field B0 between n-1 and n points, I is an interpolation multiple, and m is an interpolation offset serial number.
4. The method for digital real-time compensation of magnetic resonance B0 eddy current according to claim 1, wherein the process of phase accumulation is set as
Wherein,
for the sake of simplicity, the new sequence number after interpolation is represented by i, Δ t is the time interval between two points after interpolation, i.e. the working clock period of the transmitting and receiving NCO of the radio frequency, mod is the remainder symbol.
5. The method for digital real-time compensation of magnetic resonance B0 eddy currents according to claim 1, wherein a phase correction process is configured,
the accumulated phase error value
Adding the corrected phase of the transmitting NCO phase and the corrected phase of the receiving NCO phase to the phase of the transmitting NCO and the phase of the receiving NCO;
comparing the current value with the absolute value of 360, and when the absolute value of the phase correction value exceeds 360, taking the remainder to be within +/-360, otherwise, keeping the remainder unchanged;
and converting the transmitted and received phase correction values into respective phase control words respectively, and sending the phase control words to the transmitting NCO phase control interface and the receiving NCO phase control interface.
6. The magnetic resonance B0 eddy current digital real-time compensation device comprises
The B0 vortex dynamic calculation module receives X, Y, Z gradient digital waveform information and calculates a vortex value applied to B0 by gradient in real time;
the B0 vortex value conversion frequency value module is used for receiving B0 vortex value information from the B0 vortex dynamic calculation module and converting the B0 vortex value into a main magnetic field frequency change value caused by B0 vortex;
the frequency value interpolation module receives the information of the main magnetic field frequency change value from the B0 vortex value conversion module, interpolates the speed of the main magnetic field frequency change value into the working clock frequency of the radio frequency transmitting and receiving NCO, and then calculates the phase accumulated value of the main magnetic field frequency changing along with the time at the current moment;
and the phase correction module receives the phase accumulation value information from the frequency value interpolation module and corrects the phases of the radio frequency emission signal and the nuclear magnetic resonance signal by using the phase accumulation value information so as to counteract the eddy current-induced phase error caused by the X, Y, Z gradient applied to B0.
7. The apparatus for digital real-time compensation of magnetic resonance B0 eddy currents according to claim 6,
the B0 vortex dynamic calculation module comprises a floating-point multiplier, an adder and a register,
the B0 vortex dynamic calculation module receives the digital waveform value of X, Y, Z gradient, latches the digital waveform value into a register, and then calculates the digital waveform value with a preset B0 vortex compensation time constant and amplitude constant parameter value to obtain a vortex value which is applied to B0 by the gradient of X, Y, Z from the beginning of scanning to the current moment.
8. The apparatus for digital real-time compensation of magnetic resonance B0 eddy currents according to claim 6, wherein,
the B0 vortex value conversion to frequency value module comprises a floating-point multiplier, an adder, a floating-point to fixed-point converter and a register,
the B0 eddy current value is firstly cached in a register by the B0 eddy current value conversion frequency value module, then the magnetic field change value caused by current change is calculated according to the electromagnetic induction calculation formula, and finally the change frequency value is obtained by multiplying the magnetic field change value and the magnetic rotation ratio of the atomic nucleus.
9. The apparatus for digital real-time compensation of magnetic resonance B0 eddy currents according to claim 6,
the frequency value interpolation module comprises a fixed-point multiplier, an adder, a comparator and a register,
the frequency value interpolation module firstly converts the B0 vortex value into a frequency value output by the frequency value module and caches the frequency value in a register, then carries out interpolation calculation by utilizing a fixed-point multiplier and an adder, the interpolated data rate is matched with the working frequency of the radio frequency transmitting and receiving NCO, carries out accumulation calculation by utilizing the interpolated frequency value to obtain the phase accumulated value at the current moment, compares the phase accumulated value with 360 absolute values, and when the absolute value of the phase accumulated value exceeds 360, the phase accumulated value is remained within +/-360.
10. The apparatus for digital real-time compensation of magnetic resonance B0 eddy currents according to claim 6,
the phase correction module comprises a transmitting NCO phase correction interface, a receiving NCO phase correction interface, a fixed-point adder and a comparator,
and the phase correction module receives the phase accumulated value of the frequency value interpolation module, and then adds the phase accumulated value with the transmitting NCO phase and the receiving NCO phase to obtain the corrected transmitting NCO phase and receiving NCO phase, compares the phase correction value with an absolute value of 360, when the absolute value of the phase correction value exceeds 360, the phase correction value is remained within +/-360, then converts the transmitting and receiving phase correction values into phase control words respectively, and sends the phase control words to the transmitting NCO phase control interface and the receiving NCO phase control interface.
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Application publication date: 20221216 |