阅读说明：本技术 使用绝热射频脉冲测量射频b1场分布的磁共振成像方法 (Magnetic resonance imaging method using adiabatic radio frequency pulses to measure radio frequency B1 field distribution ) 是由 周欣 袁亚平 郭茜旎 孙献平 叶朝辉 于 2019-11-21 设计创作，主要内容包括：本发明公开了使用绝热射频脉冲测量射频B1场分布的磁共振成像方法,包括以下步骤,设计绝热脉冲,设置频率偏置以及绝热脉冲时间；计算绝热脉冲的K值；不施加绝热脉冲采样获得初始磁化矢量大小；分别施加频率偏置为△ω<Sub>rf</Sub>和-△ω<Sub>rf</Sub>的绝热脉冲以及损毁梯度,采样获得磁化矢量大小M<Sub>z1</Sub>和M<Sub>z2</Sub>；计算B<Sub>1,obs</Sub>,计算由于B0场偏移给B1测量带来的偏差。本发明便于计算与分析；通过施加大△ω<Sub>rf</Sub>值可以很好的避免B0场偏移对B1测量的影响。(The invention discloses a magnetic resonance imaging method for measuring radio frequency B1 field distribution by using adiabatic radio frequency pulse, which comprises the following steps of designing adiabatic pulse, setting frequency offset and adiabatic pulse time, calculating K value of the adiabatic pulse, obtaining initial magnetization vector size without applying adiabatic pulse sampling, and respectively applying frequency offset of △ omega rf And- △ omega rf And the magnitude of the magnetization vector M obtained by sampling z1 And M z2 (ii) a Calculation of B 1,obs And calculating the deviation of B1 measurement caused by B0 field deviation, and applying large △ omega to facilitate calculation and analysis rf The values are good enough to avoid the effect of B0 field offset on the B1 measurement.)

1. A magnetic resonance imaging method using adiabatic radio frequency pulses to measure a radio frequency B1 field distribution, comprising the steps of:

step 1, designing adiabatic pulse, wherein the initial intensity of the adiabatic pulse is zero, and the adiabatic pulse is gradually increased to the maximum intensity B _1maxSetting the frequency offset △ omega _rfAnd adiabatic pulse time;

step 2, calculating the K value of the adiabatic pulse, wherein K is gamma | B _eff|/|dθ/dt|，B _effCharacterization B _effThe intensity of the field, gamma being the gyromagnetic ratio of the nucleus, theta being B _effThe angle between the field and the main magnetic field direction;

step 3, maximum intensity B of adiabatic pulse is not applied or is applied _1maxSet to zero, the sample obtains the initial magnetization vector magnitude M ₀；

Step 4, applying frequency offset of △ omega _rfAnd the magnitude of the magnetization vector M obtained by sampling _z1；

Step 5, applying frequency bias to- △ omega _rfAnd the magnitude of the magnetization vector M obtained by sampling _z2；

Step 6, calculate B using the following formula _1,obs，

Wherein M is ₀In order to be the vector of the initial magnetization,

step 7, according to △ omega ²/(△ω _rf ²-△ω ²) To calculate the deviation of the B1 measurement due to the B0 field offset.

2. A method as claimed in claim 1, wherein the frequency offset △ ω is offset from the frequency of the mr signals used to measure the rf B1 field distribution _rfGreater than the magnitude △ ω of the B0 field offset.

Technical Field

The invention belongs to the technical field of nuclear magnetic resonance imaging, and particularly relates to a magnetic resonance imaging method for measuring radio frequency B1 field distribution by using adiabatic radio frequency pulses. The invention is suitable for measuring the distribution of the radio frequency B1 field in different tissues or samples when a surface coil, a body coil or a phase array coil transmits in magnetic resonance imaging, and is used for image reconstruction, B1 field correction and the like.

Background

Magnetic resonance imaging is an important clinical method for diagnosing and evaluating the progress of diseases, has the advantages of no ionizing radiation, non-invasiveness, high spatial resolution, imaging in any layer, high tissue contrast and the like, and has an irreplaceable effect on the diagnosis of tumors, visceral organs, soft tissue lesions and the like.

The B1 field represents the strength of the radio frequency pulse at the time of transmission by the magnetic resonance coil. In magnetic resonance imaging, the radio frequency B1 field appears unevenly distributed in space due to tissue irregularities and due to the uneven transmission field of the coil. With the increasing magnetic field strength of the current magnetic resonance spectrometer, the problem of nonuniform radio frequency B1 field becomes more serious. In parallel imaging and Chemical Exchange Saturation Transfer (CEST) imaging, which are commonly used in magnetic resonance, image reconstruction and correction of the radio frequency B1 field distribution are also required. The development of a magnetic resonance method for measuring the field distribution of the radio frequency B1 can solve the above problems.

The B0 field indicates the magnitude of the magnetic field induced at a point in space during magnetic resonance imaging, and is closely related to the nuclear spin frequency. The difference between the radio frequency and the nuclear spin frequency at radio frequency excitation is represented by the B0 field offset frequency. The B0 field shift affects the measurement of the B1 field distribution, mainly because the magnetic resonance signal under the off-resonance effect is related to both B0 and B1 when the B0 field shift is large. Therefore, the effect of B0 field shift cannot be neglected when measuring the B1 field distribution.

The existing magnetic resonance methods for measuring the B1 field distribution are mainly based on a dual flip angle method of signal amplitude, such as the B1 field measurement method based on different flip angles proposed by Insko EK et al [ Mapping of the radio frequency field.J.Magn.Reson.A.1993.45: p.82-85 ], and the B1 field measurement method based on different flip angles with higher speed proposed by Cunningham CH et al [ trained double-angle method for rapid B1+ Mapping. Magn.Reson.Med.2006.55: p.1326-1333 ]. However, the excitation waveforms of the pulses with different flip angles are not linear, and the method is sensitive to B0 field shifts.

The AFI (effective flip-angle) method proposed by Yarnykh VL et al [ effective flip-angle imaging in the pulsed step state: for radial-dimensional mapping of the transmitted radio frequency front-end field. Magn. Reson. Med.2007.57: p.192-200.] utilizes the ratio of signal intensities of two different recovery times TR1 and TR2 at the same excitation angle to calculate the B1 field distribution, which is fast but sensitive to B0 field offset and motion.

The method based on the signal phase mainly comprises a measuring method proposed by Sachalick LI et al [ B1 mapping by Bloch-Siegershift.Magn.Reson.Med.2010.63: p.1315-1322 ], wherein the phase change of the Bloch-Siegershift is in direct proportion to the square of a B1 field. The phase reconstruction in the phase method is complicated and may be accompanied by problems of phase convolution (phase exceeding 360 °).

In addition, other methods include the method proposed by PatrickSchuenke et al [ Silvery Mapping of WaterShiftand B1(WASABI) -Application to Field-innovation Correction of CEST MRIData.Magn.Reson.Med.2017.77: p.571-580 ] to simultaneously fit the B0 and B1 Field distributions using the partial resonance effect. The method is applied to chemical exchange saturation transfer imaging, and 10 or more images are required to be acquired for curve fitting.

The invention provides a magnetic resonance imaging method for measuring the field distribution of radio frequency B1 by using adiabatic radio frequency pulses, which is a B1 measuring method which is insensitive to B0 field offset and can be widely applied to different pulse sequences, and provides technical support for measurement of radio frequency B1 fields, image reconstruction correction and the like.

Disclosure of Invention

In order to solve the above problems in the prior art, the present invention provides a magnetic resonance imaging method for measuring the rf B1 field distribution using adiabatic rf pulses, which is based on the following ideas:

in a rotating coordinate system, the magnetization vector can be considered to be along the effective B1 field, B _effThe field precesses. Wherein, B _eff＝[(γB1) ²+(△ω) ²] ^1/2γ represents the gyromagnetic ratio of the nucleus, △ ω represents the magnitude of the B0 field offset, B _effCharacterization B _effThe strength of the field, B1, characterizes the strength of the B1 field. Precession frequency ω ═ γ B _eff。B _effThe field makes an angle θ with the Z axis (i.e., the main magnetic field direction) arctan (γ B1/| △ ω |).

Adiabatic pulsing is a radio frequency pulsing under special conditions. Under adiabatic conditions, with the initial B _effThe magnetization vector parallel to the field will be spin-locked by the adiabatic pulse and always stay with B _effThe field remains collinear.

Adiabatic pulsing requires satisfaction of adiabatic conditions, definition

Then require K>>1 to ensure thermal insulation, i.e. around B _effThe field precession frequency is far greater than B _effVariation of field angle with Z-axis to satisfy adiabatic conditions, a large frequency offset, defined as △ ω, is set for the RF pulses _rfGuarantee △ omega _rf>△ omega. addition of an offset value increases both B and B _effThe value reduces the variation of theta along with the B1 field, and ensures the heat insulation of the radio frequency pulse.

Selecting a radio frequency pulse with initial intensity of 0 and maximum intensity at the end, and setting the maximum intensity of the radio frequency pulse as B _1maxUnder adiabatic conditions, i.e. setting the frequency offset of the RF pulses to △ omega _rfGuarantee △ omega _rf>△ omega, the included angle between the magnetization vector and the Z axis is 0 when the radio frequency pulse starts, and the included angle between the magnetization vector and the Z axis is theta when the radio frequency pulse ends ₁＝arctan[γB _1max/|(△ω+△ω _rf)|]。

Setting the initial magnetization vector to M ₀After the adiabatic pulse is finished, the angle between the magnetization vector and the Z-axis direction is theta ₁＝arctan[γB _1max/|(△ω+△ω _rf)|]Applying a destruction gradient field to destroyBy dropping transverse magnetization vector, only the initial magnetization vector M remains ₀Projection M in Z-axis direction _z1Then M is _z1＝M ₀cos(θ ₁) Similarly, the frequency offset was changed to- △ ω _rfAngle theta between the magnetization vector and the Z axis ₂＝arctan[γB _1max/|(△ω-△ω _rf)|]，M _z2＝M ₀cos(θ ₂)。

Let B _1,obsFor the calculated value of the B1 field, the calculation is disclosed as follows:

in equation 2, △ ω ²/(△ω _rf ²-△ω ²) Term is observed B _1,obsAnd reality B _1maxAt selected △ omega _rfAt 4000Hz, △ omega has a deviation of 1.01% or less in the range-400 to 400Hz, △ omega is insensitive to △ omega in order to ensure the thermal insulation of the pulses and to measure _rfThe larger the selection value, the better, 2000Hz or more can satisfy the experimental conditions.

A magnetic resonance imaging method for measuring a radio frequency B1 field distribution using adiabatic radio frequency pulses, comprising the steps of:

step 1, designing adiabatic pulse, wherein the initial intensity of the adiabatic pulse is zero, and the adiabatic pulse is gradually increased to the maximum intensity B _1maxSet frequency offset △ ω _rfAnd adiabatic pulse time;

and 2, calculating the K value of the adiabatic pulse to ensure that the K value meets the adiabatic condition. The K value is calculated by the formula Give out K>The adiabatic condition is satisfied in case 100. Maximum intensity B _1maxThe larger the K value, the smaller the △ omega _rfThe larger the value of K; the pulse time is as short as possible to reduce the relaxation when K is satisfiedInfluence of Henan wherein B _effCharacterization B _effThe intensity of the field, gamma being the gyromagnetic ratio of the nucleus, theta being B _effThe angle between the field and the Z axis (i.e., the main magnetic field direction), θ ═ arctan (γ B1/| △ ω |), B1 characterizes the strength of the B1 field, and △ ω is the magnitude of the B0 field offset;

step 3, maximum intensity B of adiabatic pulse is not applied or is applied _1maxSet to zero, the sample obtains the initial magnetization vector magnitude M ₀；

Step 4, applying frequency offset of △ omega _rfAnd the magnitude of the magnetization vector M obtained by sampling _z1；

Step 5, applying frequency bias to- △ omega _rfAnd the magnitude of the magnetization vector M obtained by sampling _z2；

Step 6, calculate B using the following formula _1,obs，

Wherein M is ₀Is the initial magnetization vector.

B is to be _1,obsDivided by the maximum intensity B _1maxNormalizing to obtain normalized radio frequency B1 field distribution;

step 7, △ ω is biased according to the selected frequency _rfAccording to △ omega ²/(△ω _rf ²-△ω ²) To calculate the deviation, △ ω, of the B1 measurement due to the B0 field offset _rf>△ω。

Compared with the prior art, the invention has the following beneficial effects:

1. in the method, the adiabatic pulse intensity and the signal intensity under the adiabatic condition present a simple trigonometric function relationship, and are convenient to calculate and analyze.

2. The calculated deviation of the method due to the B0 field deviation △ omega is △ omega ²/(△ω _rf ²-△ω ²) By applying large △ omega _rfThe values are good enough to avoid the effect of B0 field offset on the B1 measurement.

3. In the method, before the adiabatic pulse and the damage gradient are applied to the sampling, the adiabatic pulse and the damage gradient can be integrated into a sub-module without being limited by the sampling method.

Drawings

FIG. 1 is a flow chart of the method of the present invention;

FIG. 2(a) shows a simple linearly increasing pulse shape, B _1max40 muT, a duration of 2ms,

FIG. 2(b) shows the pulse of FIG. 2(a) at a bias △ ω _rfThe K value is changed along with the pulse time at 4000Hz, and the K value is far more than 1 in the whole process, so that the adiabatic condition is met. The pulse used in the present invention is not limited to this pulse shape;

FIG. 3 is a pulse sequence diagram of a B1 field distributed magnetic resonance imaging method using adiabatic pulse measurements; only a pulse sequence diagram for carrying out data acquisition by using spin echo is introduced in the figure, and the data acquisition mode used by the invention is not limited;

FIG. 4 is a graph of the evolution of the magnetization vector with the pulse intensity under adiabatic conditions, the initial magnetization vector M ₀Is pulsed to turn over to B _effDirection, in this case projected in the Z-axis direction as M _z1,2；

FIG. 5 is a normalized B1 field profile of a 3% agarose solution sample measured using the method of the invention;

fig. 6 shows measurement errors due to B0 field offset △ ω in a magnetic resonance imaging method using adiabatic pulses to measure the B1 field distribution.

Detailed Description

The present invention will be described in further detail with reference to examples for the purpose of facilitating understanding and practice of the invention by those of ordinary skill in the art, and it is to be understood that the present invention has been described in the illustrative embodiments and is not to be construed as limited thereto.