CN106872920B - Radio frequency calibration method and device for magnetic resonance imaging system - Google Patents
Radio frequency calibration method and device for magnetic resonance imaging system Download PDFInfo
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- CN106872920B CN106872920B CN201710047820.XA CN201710047820A CN106872920B CN 106872920 B CN106872920 B CN 106872920B CN 201710047820 A CN201710047820 A CN 201710047820A CN 106872920 B CN106872920 B CN 106872920B
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- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
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- G—PHYSICS
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- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
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- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
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- G01R33/58—Calibration of imaging systems, e.g. using test probes, Phantoms; Calibration objects or fiducial markers such as active or passive RF coils surrounding an MR active material
- G01R33/583—Calibration of signal excitation or detection systems, e.g. for optimal RF excitation power or frequency
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Abstract
The application provides a radio frequency calibration method and a radio frequency calibration device for a magnetic resonance imaging system, wherein the method comprises the following steps: and in the first search range, carrying out iterative scanning by taking a preset basic amplitude as an initial amplitude of the applied radio frequency pulse, and acquiring a first echo amplitude signal of a designated spin echo in the first correction range. And judging whether a minimum value exists in the first echo amplitude signal or not. And when the first echo amplitude signal has a minimum value, acquiring a radio frequency amplitude value corresponding to the minimum value, and taking the radio frequency amplitude value as a first pulse calibration value for correcting the 90-degree flip angle. According to the method, the basic radio frequency amplitude obtained on the water model is used as an initial value, iterative search in the range of 0-180 degrees is not needed, the iterative range and the calibration time are shortened, and the calibration efficiency is improved.
Description
Technical Field
The present application relates to the field of magnetic resonance imaging systems, and in particular, to a method and an apparatus for calibrating a radio frequency of a magnetic resonance imaging system.
Background
In a magnetic resonance imaging system, in order to obtain an accurate flip angle, a calibration of the radio frequency transmit power is typically performed on the radio frequency system, which calibration is performed during a calibration of the entire system. The specific calibration method is to select a specific radio frequency pulse waveform, apply a radio frequency pulse with a certain frequency and a certain amplitude, continuously increase the amplitude of the radio frequency pulse, and receive a spin echo signal generated in a corresponding signal receiving period. And traversing the whole pulse amplitude interval, and recording the spin echo signal amplitude in the period. Different methods can be employed to obtain the 90 ° flip angle correction energy value using different echo signals. The flip angle corresponding to each correction energy value can be calculated by the acquired echo signal, so that the corresponding relation between the flip angle and the radio frequency amplitude value is obtained, and the radio frequency amplitude value required by a specific flip angle, such as a 90-degree flip angle, can be determined through the corresponding relation. Or the radio frequency amplitude value of the 90 DEG flip angle is determined through the extreme value of the echo signal.
In the process of radio frequency pulse excitation, a scanning protocol generates a corresponding pulse sequence during magnetic resonance scanning imaging, and the pulse sequence is converted into a radio frequency pulse signal and a gradient magnetic field pulse signal. The radio frequency pulse signal is transmitted to act on the imaged object, and then the magnetic resonance signal can be generated. During this excitation, the energy level of the transmitted rf pulses plays a crucial role in the contrast of the finally formed mr image. When a spin echo signal needs to be generated, the excitation angle needs to reach 90 degrees, and the energy of the refocusing pulse needs to reach a flip angle of 180 degrees. In generating field gradient echo signals, the required radio frequency energy also depends on the contrast properties of the image to be formed, so in the magnetic resonance imaging process, it is important to accurately control the radio frequency energy angle.
In magnetic resonance imaging, the loads (imaged objects) during scanning are different, for example, the required excitation radio frequency energy is different for imaged objects with different body weights. To achieve the best signal-to-noise ratio, or to achieve the desired effect in particular clinical applications such as suppression prevention, it is necessary to calibrate the rf energy of the excitation pulses used at each excitation. In the clinical application of magnetic resonance, the conventional method for manually correcting the radio frequency energy is adopted, and the excitation quantity of the radio frequency energy is fixed, so that the defect that the fixed radio frequency energy cannot adapt to each individual when different individuals have large difference exists, the imaging quality is unstable, and the difference between the individuals is large.
Disclosure of Invention
In view of the above, the present application provides a method and an apparatus for calibrating a radio frequency of a magnetic resonance imaging system.
Specifically, the method is realized through the following technical scheme:
a magnetic resonance imaging system radio frequency calibration method, comprising:
in a first search range, carrying out iterative scanning by taking a preset basic amplitude as an initial amplitude of an applied radio frequency pulse, and acquiring a first echo amplitude signal of a designated spin echo in a first correction range;
judging whether a minimum value exists in the first echo amplitude signal or not;
when a minimum value exists in the first echo amplitude signal, acquiring a radio frequency amplitude value corresponding to the minimum value, and taking the radio frequency amplitude value as a first pulse calibration value for correcting a 90-degree flip angle;
or, when the minimum value does not exist in the first echo amplitude signal, acquiring the first echo amplitude signal of the specified spin echo again in a second correction range, acquiring the minimum value in the first echo amplitude signal, and taking the radio frequency amplitude value corresponding to the minimum value as a first pulse calibration value for correcting the 90-degree flip angle.
The radio frequency calibration method of the magnetic resonance imaging system further comprises the following steps:
within a second search range, taking twice of the first pulse calibration value as the initial amplitude of the applied radio frequency pulse to carry out iterative scanning, and acquiring a second echo amplitude signal of the appointed spin echo within the first correction range;
judging whether a minimum value exists in the second echo amplitude signal or not;
when a minimum value exists in the second echo amplitude signal, acquiring a radio frequency amplitude value corresponding to the minimum value, and taking the radio frequency amplitude value as a second pulse calibration value for correcting a 180-degree flip angle;
or, when the minimum value does not exist in the second echo amplitude signal, acquiring the second echo amplitude signal of the specified spin echo again in the second correction range, acquiring the minimum value in the second echo amplitude signal, and taking the radio frequency amplitude value corresponding to the minimum value as a second pulse calibration value for correcting the 180-degree flip angle.
Further, the basic amplitude is the amplitude of the radio frequency pulse obtained when the standard water model is corrected;
the radio frequency pulses comprise a first radio frequency pulse, a second radio frequency pulse and a third radio frequency pulse; the designated spin echo is: and the FID signal generated by the second radio frequency pulse forms a spin echo under the action of the retrofocus of the third radio frequency pulse.
Further, acquiring the specified spin echo within the first correction range and acquiring the specified spin echo again within the second correction range comprises:
taking a preset sampling time domain as a center, applying a sampling window with a fixed width to acquire the spin echo, and acquiring the spin echo according to a formula S-M0cos(α)*sin(α)*sin2(α/2) establishing the corresponding relation between the spin echo and the flip angle, wherein S is the echo amplitude, and M is the echo amplitude0For coefficient, α is the flip angle.
Further, the first correction range includes at least seven different rf energy amplitude points, and the second correction range includes at least thirteen different rf energy amplitude points.
The present application further provides a magnetic resonance imaging system radio frequency calibration apparatus, comprising:
the acquisition module is used for carrying out iterative scanning in a first search range by taking a preset basic amplitude as an initial amplitude of the applied radio frequency pulse and acquiring a first echo amplitude signal of a designated spin echo in a first correction range;
the processing module is used for judging whether a minimum value exists in the echo amplitude signal or not;
the first calibration module is used for acquiring a radio frequency amplitude value corresponding to a minimum value when the minimum value exists in the echo amplitude signal, and taking the radio frequency amplitude value as a first pulse calibration value for correcting a 90-degree flip angle;
and the second calibration module is used for acquiring the first echo amplitude signal of the appointed spin echo again in a second calibration range when the first echo amplitude signal does not have a minimum value, acquiring the minimum value in the first echo amplitude signal, and taking the radio frequency amplitude value corresponding to the minimum value as a first pulse calibration value for correcting a 90-degree flip angle.
Further, the acquisition module is further configured to perform iterative scanning with twice the first pulse calibration value as the initial amplitude of the applied radio frequency pulse within a second search range, and acquire a second echo amplitude signal of the designated spin echo within the first correction range;
the processing module judges whether a minimum value exists in the second echo amplitude signal or not;
when the second echo amplitude signal has a minimum value, the first calibration module acquires a radio frequency amplitude value corresponding to the minimum value, and uses the radio frequency amplitude value as a second pulse calibration value for correcting a 180-degree flip angle;
or, when there is no minimum value in the second echo amplitude signal, the second calibration module acquires the second echo amplitude signal of the specified spin echo again in the second calibration range, acquires the minimum value in the second echo amplitude signal, and takes the radio frequency amplitude value corresponding to the minimum value as a second pulse calibration value for calibrating the 180 ° flip angle.
Further, the basic amplitude is the amplitude of the radio frequency pulse obtained when the standard water model is corrected;
the radio frequency pulses comprise a first radio frequency pulse, a second radio frequency pulse and a third radio frequency pulse; the designated spin echo is: and the FID signal generated by the second radio frequency pulse forms a spin echo under the action of the retrofocus of the third radio frequency pulse.
Further, acquiring the specified spin echo within the first correction range and acquiring the specified spin echo again within the second correction range comprises:
taking a preset sampling time domain as a center, applying a sampling window with a fixed width to acquire the spin echo, and acquiring the spin echo according to a formula S-M0cos(α)*sin(α)*sin2(α/2) establishing the corresponding relation between the spin echo and the flip angle, wherein S is the echo amplitude, and M is the echo amplitude0For coefficient, α is the flip angle.
Further, the first correction range includes at least seven different rf energy amplitude points, and the second correction range includes at least thirteen different rf energy amplitude points.
According to the method, the basic radio frequency amplitude obtained on the water model is used as an initial value, iterative search in the range of 0-180 degrees is not needed, the iterative range and the calibration time are shortened, and the calibration efficiency is improved. And obtaining radio frequency amplitudes corresponding to the flip angles of 90 degrees and 180 degrees through two-step quick iteration by using the appointed spin echo.
Drawings
Fig. 1 is a flowchart illustrating a radio frequency calibration method of a magnetic resonance imaging system according to an embodiment of the present application.
Fig. 2 is a sequence diagram illustrating radio frequency energy calibration in a method for radio frequency calibration of a magnetic resonance imaging system according to an embodiment of the present application.
Fig. 3 is a flowchart illustrating an operation of a radio frequency calibration method of a magnetic resonance imaging system according to an embodiment of the present application.
Fig. 4 is a graph of a calibration result of pulse excitation corresponding to a 90 ° flip angle in a radio frequency calibration method of a magnetic resonance imaging system according to an embodiment of the present application.
Fig. 5 is a graph of a pulse excitation calibration result corresponding to a 180 ° flip angle in a radio frequency calibration method of a magnetic resonance imaging system according to an embodiment of the present application.
Fig. 6 is a block diagram of an rf calibration apparatus of a magnetic resonance imaging system according to an embodiment of the present application.
Fig. 7 is a hardware structure diagram of a radio frequency calibration apparatus of a magnetic resonance imaging system according to an embodiment of the present application.
Detailed Description
Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. When the following description refers to the accompanying drawings, like numbers in different drawings represent the same or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present application. Rather, they are merely examples of apparatus and methods consistent with certain aspects of the present application, as detailed in the appended claims.
It is to be understood that although the terms first, second, third, etc. may be used herein to describe various information, such information should not be limited to these terms. These terms are only used to distinguish one type of information from another. For example, first information may also be referred to as second information, and similarly, second information may also be referred to as first information, without departing from the scope of the present application. The word "if" as used herein may be interpreted as "at … …" or "when … …" or "in response to a determination", depending on the context.
Referring to fig. 1 and 2, a method for radio frequency calibration of a magnetic resonance imaging system is provided. Fig. 2 is a sequence diagram for radio frequency energy calibration, including radio frequency pulses (radio frequency RF), Gradient pulses (Gradient) and radio echoes (Echo). In the magnetic resonance imaging system radio frequency calibration method provided by the application, the radio frequency pulse comprises a first radio frequency pulse, a second radio frequency pulse and a third radio frequency pulse. The three radio-frequency pulses are the same radio-frequency pulse, the amplitude and the duration are the same, the three radio-frequency pulses are sequentially applied on a time sequence, and the optimal radio-frequency amplitude corresponding to the radio-frequency pulse when the corresponding excitation flip angle is 90 degrees is calibrated by utilizing the change of the amplitude of an echo signal generated by the sequential excitation and the back convergence of the three pulses along with the change of the pulse amplitude. The designated spin echo is: and the FID signal generated by the second radio frequency pulse forms a spin echo under the action of the retrofocus of the third radio frequency pulse.
The method is characterized in that the echo generated by the excitation of the three pulses can be used for calculating the flip angle corresponding to the actual excitation or finding an accurate 90 DEG pulse, an echo generated by the pulses in a certain time sequence relation (particularly an echo SE23 generated by the FID signal generated by the excitation of the second pulse under the action of the echo of the third pulse) is used for calibrating the radio frequency pulse (the corresponding required energy at 90 DEG) along with the change of the amplitude of the radio frequency pulse, "tau" and "tau + tm" are time intervals in the figure, and the detailed description is given in the following.
In the conventional radio frequency calibration method, an initial value of the radio frequency pulse is not set to be searched iteratively, the iterative search is required within the range of 0-180 degrees (or a larger range to 240 degrees), and the required correction energy value needs to be large to accurately search the radio frequency energy corresponding to the 90-degree and 180-degree pulses. The method and the device shorten the iterative scanning range by setting an initial value of the radio frequency amplitude input; the method of the application does not need to iterate search in the whole range of 0-180 degrees (or more range to 240 degrees) to realize that the rollover angle of 90 degrees and 180 degrees is determined by iterative scanning in a small range. The invention performs an iterative scan within a first search range to calibrate the required rf amplitude for the 90 ° pulse. I.e. the first correction range is calculated from the initial value RFAmp0 calibrated in water phantom.
The radio frequency calibration method of the magnetic resonance imaging system uses the radio frequency amplitude corresponding to the 90-degree pulse obtained by correction on the water model as a radio frequency amplitude initial value, and calculates a first correction range by using the radio frequency pulse amplitude as a central value. The base amplitude of the RF energy is denoted as RFAmp0, and the RF pulse amplitudes corresponding to the applied first correction range are RFAmp0-150, RFAmp0-100, RFAmp0-50, RFAmp0 (base amplitude), RFAmp0+50, RFAmp0+100, RFAmp0+150, respectively. That is, the rf pulse amplitude is set in steps of 50 to both sides, centered on the base amplitude RFAmp 0. And acquiring the appointed pulse echo in a radio frequency pulse acquisition window based on the radio frequency pulse increment, and acquiring the radio frequency pulse amplitude value corresponding to the flip angle according to whether the appointed pulse echo has a minimum value or not. According to the scheme, on one hand, twice of the radio-frequency pulse corresponding to 90 degrees is used as an initial value for correcting 180 degrees, the search range is shortened, on the other hand, only data of a radio-frequency pulse echo is collected, the data processing is simplified, the collection time is reduced, on the other hand, a large number of correction energy values are not needed, and the calibration efficiency is improved.
The radio frequency calibration method of the magnetic resonance imaging system comprises the following steps:
step S110: and in the first search range, carrying out iterative scanning by taking a preset basic amplitude as an initial amplitude of the applied radio frequency pulse, and acquiring a first echo amplitude signal of a designated spin echo in the first correction range.
The first correction range may be understood as the range of rf energy applied when correcting a 90 ° rf pulse. For example, when the system is installed, the basic amplitude values obtained on a standard water model (described in detail later) are corrected. The step length is the loading radio frequency amplitude quantity separated between two adjacent test points during correction, and the step length is invariable. The iterative scan is performed with a fixed rf pulse step size, for example 50 steps.
The first correction range includes a plurality of correction energy values. The correction energy value can be understood as corresponding different radio frequency energy amplitude points in the set radio frequency pulse energy range, and the step acquires the first echo amplitude signals of the spin echoes corresponding to the plurality of correction energy values. The first echo amplitude signal and the FID signal generated by applying the rf energy in the first search range, i.e. the FID signal generated by the second pulse, form an initial spin echo by the third pulse echo (SE 23).
Step S120: and judging whether a minimum value exists in the first echo amplitude signal or not.
That is, it is determined whether the first echo amplitude signal of the designated spin echo acquired in step S110 has a minimum value within the first calibration range. As can be seen from the functional curve of the given spin echo SE23 (as shown in fig. 4 and described in detail later), if an iterative calibration 90 ° pulse is performed around a set initial value (base amplitude), a very "sharp" minimum appears near the 90 ° pulse. The radio frequency amplitude value corresponding to the 90-degree flip angle can be obtained through the minimum value.
Step S130: and when the first echo amplitude signal has a minimum value, acquiring a radio frequency amplitude value corresponding to the minimum value, and taking the radio frequency amplitude value as a first pulse calibration value for correcting the 90-degree flip angle.
That is, when there is a minimum value in the first echo amplitude signal, the minimum value corresponds to a radio frequency amplitude value, and the first pulse calibration value (first radio frequency amplitude value) is used as a pulse calibration value for correcting a 90 ° flip angle, and the corresponding relationship between the spin echo and the radio frequency amplitude value is established by curve fitting. And the radio frequency amplitude values corresponding to other flip angles are obtained through the corresponding relation between the spin echo and the radio frequency amplitude value.
Alternatively, the spin echo is acquired in a second correction range in which the correction range is expanded with respect to the first correction range. Namely, step S140: when the first echo amplitude signal does not have a minimum value, the first echo amplitude signal of the specified spin echo is collected again in a second correction range, the minimum value in the first echo amplitude signal of the specified spin echo is obtained, and the radio frequency amplitude value corresponding to the minimum value is used as a first pulse calibration value for correcting the 90-degree flip angle.
If the minimum value in the first echo amplitude signal is not obtained in step S130, the corresponding relationship between the spin echo and the radio frequency amplitude value cannot be established, and the first correction range needs to be expanded. The second correction range may be understood as an expanded rf energy range based on the first correction range, the second correction range comprising a greater number of correction energy values than the first correction range. The correction energy value is the correction energy value of the acquired first echo amplitude signal of the acquired spin echo. The radio frequency energy of the second rectification range and the radio frequency energy of the first rectification range are both within a first search range.
And when the second correction range has a minimum value of the first echo amplitude signal, the minimum value corresponds to a radio frequency amplitude value, the first radio frequency amplitude value is used as a pulse calibration value for correcting a 90-degree flip angle, and the corresponding relation between the spin echo and the radio frequency amplitude value is established through curve fitting. And the radio frequency amplitude values corresponding to other flip angles are obtained through the corresponding relation between the spin echo and the radio frequency amplitude value.
Further, the radio frequency calibration method of the magnetic resonance imaging system further comprises a correction step of a flip angle of 180 degrees, and comprises the following steps:
and step S210, in a second search range, performing iterative scanning by taking twice the first pulse calibration value as the initial amplitude of the applied radio frequency pulse, and acquiring a second echo amplitude signal of the appointed spin echo in the first correction range. The second echo amplitude signal and the FID signal generated by applying the rf energy in the second search range, i.e. the FID signal generated by the second pulse under the action of the third pulse echo, form the initial spin echo (SE 23).
This step is different from step S110 in that the base amplitude is changed to twice the determined first pulse calibration value as an initial value, and iterative scanning is performed. Namely, a radio frequency amplitude value corresponding to a 90-degree flip angle is used as an initial value, and a second echo amplitude signal of the appointed spin echo is acquired in iterative scanning of a second scanning range. On the basis of calibrating the 90-degree flip angle, the calibration time is further reduced by using twice of the corresponding radio frequency amplitude of 90 degrees as an initial value. And the second search range is an iteration range further narrowed on the basis of the first search range. I.e. its initial value is twice the first calibrated value for 90 deg., the processing of the radio frequency amplitude value for 180 deg. within the first search range for 90 deg. is eliminated.
In the step, the iteration range is reduced by taking twice of the first calibration value corresponding to 90 degrees as an initial value for calibrating 180 degrees, so that the iteration range is reduced, and the calibration efficiency is improved.
And step S220, judging whether a minimum value exists in the second echo amplitude signal. As can be seen from the functional curve of the given spin echo SE23 (as shown in fig. 5 and described in detail later), if the 180 ° pulse is iteratively calibrated around a set initial value, a very "sharp" minimum appears near the 180 ° pulse. The radio frequency amplitude value corresponding to the 180 DEG flip angle can be obtained through the minimum value.
And step S230, when a minimum value exists in the second echo amplitude signal, acquiring a radio frequency amplitude value corresponding to the minimum value, and taking the radio frequency amplitude value as a second pulse calibration value for correcting the 180-degree flip angle.
Namely, when the second echo amplitude signal has a minimum value corresponding to a radio frequency amplitude value, the second pulse calibration value (second radio frequency amplitude value) is used as the pulse calibration value for correcting the 180-degree flip angle, and the corresponding relation between the spin echo and the radio frequency amplitude value is established through curve fitting. And the radio frequency amplitude values corresponding to other flip angles are obtained through the corresponding relation between the spin echo and the radio frequency amplitude value.
Alternatively, the iterative scanning is performed within a second correction range that is enlarged in correction range with respect to the first correction range. Step S240, when the minimum value does not exist in the second echo amplitude signal, acquiring the second echo amplitude signal of the appointed spin echo again in the second correction range, acquiring the minimum value in the second echo amplitude signal of the appointed spin echo, and taking the radio frequency amplitude value corresponding to the minimum value as a second pulse calibration value for correcting the 180-degree flip angle.
And when the second correction range has a minimum value of the second echo amplitude signal, the minimum value corresponds to a radio frequency amplitude value, the second radio frequency amplitude value is used as a pulse calibration value for correcting a 90-degree flip angle, and the corresponding relation between the spin echo and the radio frequency amplitude value is established through curve fitting. And the radio frequency amplitude values corresponding to other flip angles are obtained through the corresponding relation between the spin echo and the radio frequency amplitude value.
In the above embodiment, calibrating the radio frequency amplitude value corresponding to the 90 ° flip angle may be regarded as a first step of the radio frequency calibration method of the magnetic resonance imaging system, and calibrating the radio frequency amplitude value corresponding to the 180 ° flip angle may be regarded as a second step of the radio frequency calibration method of the magnetic resonance imaging system.
According to the radio frequency calibration method of the magnetic resonance imaging system, the specified spin echo (SE23) is used for calibrating radio frequency energy in two search ranges by adopting a two-step fast iteration method, the iteration range is reduced, and the problem that the radio frequency calibration consumes a long time in clinical application is solved. According to the known basic radio frequency amplitude obtained on the water model as an initial value and twice of the first calibration value corresponding to 90 degrees as the initial value, the radio frequency amplitude corresponding to the accurate 90 degrees and 180 degrees flip angle can be obtained through two steps of quick iteration without setting the initial value for searching to be iteratively searched in the range of 0-180 degrees (or a larger range to 240 degrees).
Referring to fig. 3, the present application will be further described by way of specific embodiments. The specific embodiment of the radio frequency calibration method of the magnetic resonance imaging system comprises the following steps.
Step S310: and starting.
Step S320: inputting the basic amplitude as an initial value into system parameters to be used as an amplitude center for calibrating a 90-degree flip angle; 7 radio frequency amplitude points are calculated, and 50 is used for calibrating and acquiring the pulse.
In one embodiment, the amplitude obtained by correcting the amplitude value obtained on a standard water mold (water mold-Phantom is a scanned mold body used for testing in magnetic resonance imaging, and a certain solution such as CuSO4 solution is poured into a certain mold) during system installation is used as a basic amplitude, and is labeled with RFAmp0 as a system input initial value for calibrating 90 ° pulse. The base amplitude is taken as the center point of the rf amplitude corrected for the 90 flip angle, at which the rf pulse energy is applied (first search range). The basic amplitude is a radio frequency amplitude value which is tested by a standard water model by adopting a conventional radio frequency correction test method when a system is installed, and is equivalent to excitation energy required when the load of human tissues does not exist. And when correcting, calculating a range for correcting test according to the basic amplitude value, the step number and the step length.
The basic amplitude is the radio frequency pulse amplitude obtained when the standard water model is corrected; the radio frequency pulses comprise a first radio frequency pulse, a second radio frequency pulse and a third radio frequency pulse; the designated spin echo is: and the FID signal generated by the second radio frequency pulse forms a spin echo under the action of the retrofocus of the third radio frequency pulse.
Namely, the acquired spin echo is calibrated to be the initial spin echo SE23 formed by the echo of the third pulse under the action of the FID signal generated by the second pulse of the radio frequency pulse. Acquiring a specified spin echo within the first correction range and acquiring the specified spin echo again within the second correction range comprises: and acquiring a first echo amplitude signal of the specified spin echo SE23, taking a preset sampling time domain as a center, applying a sampling window with a fixed width to acquire the spin echo, wherein the spin echo comprises the first echo amplitude signal, and the method further comprises a second echo amplitude signal corresponding to 180 degrees in the following steps. According to the formula S-M0cos(α)*sin(α)*sin2(α/2) establishing the corresponding relation between the spin echo and the flip angle, wherein S is the echo amplitude, and M is the echo amplitude0α is a flip angle for coefficient.
Referring to fig. 2, the isocenter of the first rf pulse (any rf pulse has a certain envelope and time length, and this isocenter can be obtained by considering the actual excitation effect of the rf pulse, and this isocenter can be calculated by using a certain algorithm as long as the rf pulse design is determined) is used as the starting point of the time axis to apply the sampling window, and the time interval between the midpoint of the first pulse and the midpoint of the second pulse is set to tau. The time interval tm between the center of the generated echo SE12 (echo SE12 formed by the refocusing of the first pulse, and the second pulse, at time 2 tau) and the center of the third pulse is the time interval. The preset sampling time domain is set at 3 tau +2 tm as a center, a sampling window is applied, and the duration of the sampling window can be about 5ms or 5 ms. And different iterations of the radio frequency energy are put into the radio frequency pulse event to realize, so that an accurate radio frequency energy correction value can be obtained within the shortest acquisition time.
When acquiring the autorotation loop, 50 is taken as a step length (the step length is a loading radio frequency amplitude quantity separated between two adjacent test points during correction, and the step length is invariable), and 7 correction energy values (7 different radio frequency energy amplitude points corresponding to a set radio frequency pulse energy range) are adopted to correct accurate radio frequency amplitude values corresponding to 90-degree excitation pulses required by different parts during human body imaging. That is, within a first search range, a first echo amplitude signal of the spin echo is acquired, and the first correction range includes at least seven different radio frequency energy amplitude points. And acquiring the corresponding first echo amplitude signals at 7 correction energy values. The 7 collected first echo amplitude signals correspond to 7 applied radio frequency energies (search ranges) and can be recorded as RFAmp0-150, RFAmp0-100, RFAmp0-50, RFAmp0 (basic amplitude), RFAmp0+50, RFAmp0+100 and RFAmp0+ 150.
Step S330: and judging whether a minimum value exists in the iteration range, executing the step S350 if the minimum value exists in the iteration range, and executing the step S340 if the minimum value does not exist in the iteration range.
The iteration range is the first search range, and the search range which takes the basic amplitude as an initial value and can shorten the calibration time is adopted. According to the multiple data (echo amplitudes) in the first echo amplitude signal collected in step S330, it is determined whether there is a minimum value in the multiple data. When there is a minimum, the minimum and its corresponding radio frequency energy (i.e., radio frequency amplitude value) are extracted. The radio frequency energy is a first pulse calibration value that calibrates a 90 ° flip angle.
Referring to fig. 4, a graph of the results of a 90 ° calibration using standard hard pulse excitation, using a calibration run on the abdomen of a human being as an example. According to the function curve of the specified spin echo SE23, i.e. according to the formula S-M0cos(α)*sin(α)*sin2(α/2) if the 90 DEG pulse is iteratively calibrated around the set initial value, a very "sharp" minimum appears at the position close to the 90 DEG pulse, and the radio frequency amplitude value corresponding to the 90 DEG flip angle can be known through the minimum value, wherein the radio frequency pulse amplitude is taken as the abscissa, the echo amplitude is taken as the ordinate, and the radio frequency pulse amplitude corresponding to the minimum value of the first echo amplitude signal is taken as 900.
Step S340: the range of 90 flip angle calibration was extended to 13 amplitude points with the amplitude center still at the base amplitude and 50 steps.
That is, if the minimum value of the spin echo is not obtained in the first correction range, the correction range is expanded to the second correction range. The second correction range includes at least thirteen different rf energy amplitude points. In one embodiment, thirteen correction energy values are used to obtain the echo amplitude value based on the 7 correction energy values expanded correction range centered on the initial value RFAmp0, and using thirteen correction energy values in 50 steps, and then step S350 is performed.
Step S350: after the radio frequency pulse amplitude of 90-degree flip angle is obtained through correction, twice of the radio frequency pulse amplitude is used as an initial value for calibrating 180-degree flip angle, 7 radio frequency amplitude points are calculated, and 50 is used as step length for calibrating and collecting the pulse.
Namely, the iterative search is started within the second search range by taking twice of the first calibration value as the initial value, and the search within a large range (0-180 degrees or 0-240 degrees) is not needed, so that the calibration time can be effectively reduced. And when the 180-degree flip angle is corrected, taking twice of the first pulse calibration value as an initial value for correcting 180 degrees and recording as RFAmp 1. Also within the first correction range, a second echo amplitude signal of the prescribed spin echo is acquired. The first echo amplitude signal and the second echo amplitude signal are used for distinguishing echo amplitude signals under 90-degree and 180-degree calibration scenes.
A second echo amplitude signal specifying a spin echo is acquired within a first correction range including at least seven different radio frequency energy amplitude points (set correction energy values). The second echo amplitude signal is acquired at 7 correction energy values in steps of 50. The 7 second echo amplitude signals corresponding to the 7 applied radio frequency energies (second search ranges) are collected and are respectively marked as RFAmp1-150, RFAmp1-100, RFAmp1-50, RFAmp1 (basic amplitude), RFAmp1+50, RFAmp1+100 and RFAmp1+ 150.
Step S360: and judging whether a minimum value exists in the iteration range, executing the step S380 if the minimum value exists in the iteration range, and executing the step S370 if the minimum value does not exist in the iteration range.
The processing manner of this step is similar to that of step S330, except that the second echo amplitude signal of the spin echo is collected in the second search range by changing the initial value, and whether the second echo amplitude signal has a minimum value is determined.
Referring to fig. 5, fig. 5 is a graph showing the results of calibration of 180 ° using a standard hard pulse during the calibration of abdominal operation. According to the function curve of the specified spin echo SE23, i.e. according to the formula S-M0cos(α)*sin(α)*sin2(α/2) if the calibration is repeated around the initial value, a very "sharp" minimum appears near the 180 DEG pulse, from which the RF amplitude corresponding to the 180 DEG flip angle is known, in the figure, the RF pulse amplitude is plotted on the abscissa, the echo amplitude is plotted on the ordinate, and the RF pulse amplitude corresponding to the minimum of the second echo amplitude signal is plotted at 1850, which is used as the calibration value for the calibration of the 180 DEG flip angle.
Step S370: the range of the 180 flip angle calibration is extended to 13 amplitude points centered at twice the 90 pulse calibration value with 50 being the step size.
Namely, the minimum value of the spin echo can not be acquired in the first search range, the search range is expanded to the second correction range. The second correction range includes at least thirteen different rf energy amplitude points. In one embodiment, thirteen correction energy values are based on 7 correction energy values with an extended correction range, 50 increments of rf energy, and an iterative scan of rf energy centered around the initial value RFAmp 1. When there is a minimum, the minimum and its corresponding radio frequency energy (i.e., radio frequency amplitude value) are extracted. The radio frequency energy is a second pulse calibration value calibrated for a 180 ° flip angle.
Step S380: and outputting the radio frequency pulse amplitudes corresponding to the corrected 90-degree and 180-degree flip angles to system parameters.
In one embodiment, the acquiring step of acquiring the designated spin echo within the first correction range and acquiring the designated spin echo again within the second correction range comprises:
centering on a preset sampling time domain (referring to the step S320, the preset sampling time domain is set at 3 tau +2 tm as a center, a sampling window is applied), applying a sampling window with a fixed width to acquire the spin echo, and acquiring the spin echo according to the formula S-M0cos(α)*sin(α)*sin2(α/2) establishing the corresponding relation between the spin echo and the flip angle.
And acquiring minimum values of spin echoes corresponding to 90-degree and 180-degree flip angles in a first search range and a second search range in two steps, and then taking a first radio frequency amplitude value and a second radio frequency amplitude value corresponding to the minimum values as a first calibration value and a second calibration value for correcting 90-degree and 180-degree respectively. And finally, outputting the first calibration value and the second calibration value to the system.
Step S390: end up
Through two-step quick iteration, the accurate minimum value and the radio frequency amplitude value gain value of the 90-degree pulse corresponding to the minimum value can be obtained in the predicted range around the 90-degree pulse. Then, the gain value of the radio frequency amplitude value corresponding to 180 degrees corresponding to the minimum value can be accurately obtained by fast scanning within the range taking twice of the calibrated 90-degree radio frequency energy as the center. The radio frequency energy is calibrated by adopting a two-step quick iteration method, and the problem that the radio frequency calibration consumes a long time in clinical application is solved.
Referring to fig. 6, the present application further provides a magnetic resonance imaging system rf calibration apparatus 40 corresponding to the above method, including:
the acquisition module 410 is configured to perform iterative scanning within a first search range by using a preset basic amplitude as an initial amplitude for applying a radio frequency pulse, and acquire a first echo amplitude signal of a designated spin echo within a first correction range;
a processing module 420, configured to determine whether a minimum value exists in the echo amplitude signal;
a first calibration module 430, configured to, when a minimum value exists in the echo amplitude signal, obtain a radio frequency amplitude value corresponding to the minimum value, and use the radio frequency amplitude value as a first pulse calibration value for correcting a 90 ° flip angle;
the second calibration module 440 is configured to, when there is no minimum value in the first echo amplitude signal, acquire the first echo amplitude signal of the specified spin echo again within a second correction range, acquire the minimum value in the first echo amplitude signal, and use a radio frequency amplitude value corresponding to the minimum value as a first pulse calibration value for correcting a 90 ° flip angle.
Further, the acquiring module 410 is further configured to perform an iterative scan with twice the first pulse calibration value as the initial amplitude of the applied radio frequency pulse within a second search range, and acquire a second echo amplitude signal of the designated spin echo within the first correction range;
the processing module 420 determines whether a minimum value exists in the second echo amplitude signal;
when a minimum value exists in the second echo amplitude signal, the first calibration module 430 acquires a radio frequency amplitude value corresponding to the minimum value, and uses the radio frequency amplitude value as a second pulse calibration value for correcting a 180-degree flip angle;
or, when there is no minimum value in the second echo amplitude signal, the second calibration module 440 acquires the second echo amplitude signal of the specified spin echo again within the second calibration range, acquires the minimum value in the second echo amplitude signal, and takes the radio frequency amplitude value corresponding to the minimum value as the second pulse calibration value for calibrating the flip angle of 180 °.
Further, the basic amplitude is the amplitude of the radio frequency pulse obtained when the standard water model is corrected;
the radio frequency pulses comprise a first radio frequency pulse, a second radio frequency pulse and a third radio frequency pulse; the designated spin echo is: and the FID signal generated by the second radio frequency pulse forms a spin echo under the action of the retrofocus of the third radio frequency pulse.
Further, acquiring the specified spin echo within the first correction range and acquiring the specified spin echo again within the second correction range comprises:
taking a preset sampling time domain as a center, applying a sampling window with a fixed width to acquire a first echo amplitude signal or a second echo amplitude signal of the spin echo, and acquiring the first echo amplitude signal or the second echo amplitude signal of the spin echo according to a formula S-M0cos(α)*sin(α)*sin2(α/2) establishing the corresponding relation between the spin echo and the flip angle, wherein S is the echo amplitude, and M is the echo amplitude0For coefficient, α is the flip angle.
Further, the first correction range includes at least seven different rf energy amplitude points, and the second correction range includes at least thirteen different rf energy amplitude points.
The implementation process of the functions and actions of each module in the above device is specifically described in the implementation process of the corresponding step in the above method, and is not described herein again.
According to the method and the device for calibrating the radio frequency of the magnetic resonance imaging system, the two times of the radio frequency amplitude value corresponding to 90 degrees are used as initial values according to the currently known basic radio frequency amplitude obtained on a water model, and iterative scanning is carried out in a first search range and a second search range. Iterative search in the whole range of 0-180 degrees is not needed, the iterative range and the calibration time are shortened, and the calibration efficiency is improved. And obtaining radio frequency amplitudes corresponding to the flip angles of 90 degrees and 180 degrees through two-step quick iteration by using the appointed spin echo.
For the device embodiments, since they substantially correspond to the method embodiments, reference may be made to the partial description of the method embodiments for relevant points. The above-described embodiments of the apparatus are merely illustrative, and the units described as separate parts may or may not be physically separate, and the parts calibrated as units may or may not be physical units, may be located in one place, or may be distributed on a plurality of network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the scheme of the invention. One of ordinary skill in the art can understand and implement it without inventive effort.
Depending on the application scenario, the apparatus may be service logic implemented by software, or may be hardware, or a combination of hardware and software. The device of the present application is described below by taking a software implementation as an example. The software is a logical means formed by a processor of the device in which it is located reading corresponding computer program instructions in the non-volatile memory into the memory for execution. Fig. 7 is a hardware block diagram of an example of a magnetic resonance imaging system rf calibration apparatus in which the software apparatus of the present application is located. The magnetic resonance imaging system radio frequency calibration apparatus device may further include other hardware besides the processor, the memory, the IO interface, the network interface, the internal bus, and the nonvolatile memory, which is not described in detail herein. The memory and non-volatile storage store machine-executable instructions corresponding to the calibration logic.
Those skilled in the art will appreciate that all or part of the steps in the above method embodiments may be implemented by a program to instruct relevant hardware to perform the steps, and the program may be stored in a computer-readable storage medium, which is referred to herein as a storage medium, such as: ROM/RAM, magnetic disk, optical disk, etc.
The implementation process of the functions and actions of each unit in the above device is specifically described in the implementation process of the corresponding step in the above method, and is not described herein again.
For the device embodiments, since they substantially correspond to the method embodiments, reference may be made to the partial description of the method embodiments for relevant points. The above-described embodiments of the apparatus are merely illustrative, and the units described as separate parts may or may not be physically separate, and the parts calibrated as units may or may not be physical units, may be located in one place, or may be distributed on a plurality of network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the scheme of the application. One of ordinary skill in the art can understand and implement it without inventive effort.
The above description is only exemplary of the present application and should not be taken as limiting the present application, as any modification, equivalent replacement, or improvement made within the spirit and principle of the present application should be included in the scope of protection of the present application.
Claims (10)
1. A method of radio frequency calibration of a magnetic resonance imaging system, comprising:
in a first search range, carrying out iterative scanning by taking a preset basic amplitude as an initial amplitude of an applied radio frequency pulse, and acquiring a first echo amplitude signal of a designated spin echo in a first correction range;
judging whether a minimum value exists in the first echo amplitude signal or not;
when a minimum value exists in the first echo amplitude signal, acquiring a radio frequency amplitude value corresponding to the minimum value, and taking the radio frequency amplitude value as a first pulse calibration value for correcting a 90-degree flip angle;
when the first echo amplitude signal does not have a minimum value, acquiring the first echo amplitude signal of the specified spin echo again in a second correction range, when the second correction range has the minimum value of the first echo amplitude signal, acquiring the minimum value in the first echo amplitude signal, and taking a radio frequency amplitude value corresponding to the minimum value as a first pulse calibration value for correcting a 90-degree flip angle, wherein the second correction range is an expanded radio frequency energy range based on the first correction range.
2. The magnetic resonance imaging system radio frequency calibration method as set forth in claim 1, further including:
within a second search range, taking twice of the first pulse calibration value as the initial amplitude of the applied radio frequency pulse to carry out iterative scanning, and acquiring a second echo amplitude signal of the appointed spin echo within the first correction range;
judging whether a minimum value exists in the second echo amplitude signal or not;
when a minimum value exists in the second echo amplitude signal, acquiring a radio frequency amplitude value corresponding to the minimum value, and taking the radio frequency amplitude value as a second pulse calibration value for correcting a 180-degree flip angle;
when the second echo amplitude signal does not have a minimum value, acquiring the second echo amplitude signal of the specified spin echo again in the second correction range, acquiring the minimum value in the second echo amplitude signal, and taking the radio frequency amplitude value corresponding to the minimum value as a second pulse calibration value for correcting the 180-degree flip angle.
3. The method of claim 1, wherein the base amplitude is an amplitude of a radio frequency pulse obtained when correcting a standard water phantom;
the radio frequency pulses comprise a first radio frequency pulse, a second radio frequency pulse and a third radio frequency pulse; the designated spin echo is: and the FID signal generated by the second radio frequency pulse forms a spin echo under the action of the retrofocus of the third radio frequency pulse.
4. The magnetic resonance imaging system radio frequency calibration method of claim 2, wherein acquiring the specified spin echo within the first correction range and acquiring the specified spin echo again within the second correction range comprises:
taking a preset sampling time domain as a center, applying a sampling window with a fixed width to acquire the spin echo, and acquiring the spin echo according to a formula S-M0cos(α)*sin(α)*sin2(α/2) establishing the corresponding relation between the spin echo and the flip angle, wherein S is the echo amplitude, and M is the echo amplitude0For coefficient, α is the flip angle.
5. The magnetic resonance imaging system radio frequency calibration method of claim 1, wherein the first correction range includes at least seven different radio frequency energy amplitude points and the second correction range includes at least thirteen different radio frequency energy amplitude points.
6. A magnetic resonance imaging system radio frequency calibration apparatus, comprising:
the acquisition module is used for carrying out iterative scanning in a first search range by taking a preset basic amplitude as an initial amplitude of the applied radio frequency pulse and acquiring a first echo amplitude signal of a designated spin echo in a first correction range;
the processing module is used for judging whether a minimum value exists in the echo amplitude signal or not;
the first calibration module is used for acquiring a radio frequency amplitude value corresponding to a minimum value when the minimum value exists in the echo amplitude signal, and taking the radio frequency amplitude value as a first pulse calibration value for correcting a 90-degree flip angle;
and the second calibration module is used for acquiring the first echo amplitude signal of the specified spin echo again in a second calibration range when the first echo amplitude signal does not have a minimum value, acquiring the minimum value in the first echo amplitude signal when the second calibration range has the minimum value of the first echo amplitude signal, and using a radio frequency amplitude value corresponding to the minimum value as a first pulse calibration value for calibrating a 90-degree flip angle, wherein the second calibration range is an expanded radio frequency energy range based on the first calibration range.
7. The apparatus of claim 6, wherein the acquisition module is further configured to perform an iterative scan with twice the first pulse calibration value as an initial amplitude of the applied rf pulse within a second search range, and acquire a second echo amplitude signal of a specified spin echo within the first correction range;
the processing module judges whether a minimum value exists in the second echo amplitude signal or not;
when the second echo amplitude signal has a minimum value, the first calibration module acquires a radio frequency amplitude value corresponding to the minimum value, and uses the radio frequency amplitude value as a second pulse calibration value for correcting a 180-degree flip angle;
and when the second echo amplitude signal does not have a minimum value, the second calibration module acquires the second echo amplitude signal of the specified spin echo again in the second calibration range, acquires the minimum value in the second echo amplitude signal, and takes the radio frequency amplitude value corresponding to the minimum value as a second pulse calibration value for correcting the 180-degree flip angle.
8. The apparatus according to claim 6, wherein the basic amplitude is an amplitude of a radio frequency pulse obtained when a standard water phantom is corrected;
the radio frequency pulses comprise a first radio frequency pulse, a second radio frequency pulse and a third radio frequency pulse; the designated spin echo is: and the FID signal generated by the second radio frequency pulse forms a spin echo under the action of the retrofocus of the third radio frequency pulse.
9. The magnetic resonance imaging system radio frequency calibration device of claim 7, wherein acquiring the specified spin echo within the first correction range and reacquiring the specified spin echo within the second correction range comprises:
taking a preset sampling time domain as a center, applying a sampling window with a fixed width to acquire the spin echo, and acquiring the spin echo according to a formula S-M0cos(α)*sin(α)*sin2(α/2) establishing a correlation between the spin echo and flip angleWhere S is the echo amplitude, M0For coefficient, α is the flip angle.
10. The magnetic resonance imaging system radio frequency calibration device of claim 6, wherein the first correction range includes at least seven different radio frequency energy amplitude points and the second correction range includes at least thirteen different radio frequency energy amplitude points.
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| CN113093078B (en) * | 2021-03-31 | 2021-11-30 | 无锡鸣石峻致医疗科技有限公司 | Method and device for determining linear relation between radio frequency pulse intensity and flip angle, computer equipment and storage medium |
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