CN107092002A - A kind of method of change in time and space using the real-time performance electromagnetic field with electric current carrying pathway - Google Patents
A kind of method of change in time and space using the real-time performance electromagnetic field with electric current carrying pathway Download PDFInfo
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- CN107092002A CN107092002A CN201710195424.1A CN201710195424A CN107092002A CN 107092002 A CN107092002 A CN 107092002A CN 201710195424 A CN201710195424 A CN 201710195424A CN 107092002 A CN107092002 A CN 107092002A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
- G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
- G01R33/36—Electrical details, e.g. matching or coupling of the coil to the receiver
- G01R33/3607—RF waveform generators, e.g. frequency generators, amplitude-, frequency- or phase modulators or shifters, pulse programmers, digital to analog converters for the RF signal, means for filtering or attenuating of the RF signal
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
- G01R33/36—Electrical details, e.g. matching or coupling of the coil to the receiver
- G01R33/3621—NMR receivers or demodulators, e.g. preamplifiers, means for frequency modulation of the MR signal using a digital down converter, means for analog to digital conversion [ADC] or for filtering or processing of the MR signal such as bandpass filtering, resampling, decimation or interpolation
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- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Magnetic Resonance Imaging Apparatus (AREA)
Abstract
The invention discloses a kind of method of the change in time and space using the real-time performance electromagnetic field with electric current carrying pathway, comprise the following steps：Step a：Measuring method for measuring radio-wave propagation effect is provided；Step b：Limited measured value is obtained by foregoing measuring method；Step c：A prediction mode is provided based on aforementioned measurements, the radio-wave propagation effect of the arbitrary radio-frequency pulse of prediction mode prediction transmitting, effect includes energy and transmits and dissipate；Step d：Radio-frequency pulse is calculated with foregoing prediction mode, radio-frequency pulse is used to provide energy for the network with electric current carrying pathway；Step e：Radio-frequency pulse provides energy for the network with electric current carrying pathway, realizes electromagnetic field change in time and space；Wherein, prediction mode and calculating radio-frequency pulse manage RF energy transmission and dissipated proactively.It is arranged such, improves the picture quality of magnetic resonance imaging.
Description
The application is the divisional application of original application.The application number of original application：201210491672.8, the application of original application
Day：On November 27th, 2012, the invention and created name of original application：For the radio frequency antenna apparatus in NMR system and side
Method.
Technical field
The present invention relates to magnetic resonance imaging (MRI), more particularly to the parallel radio frequency transmitting of multichannel or multiport and radio frequency
Receive.The invention further relates to a kind of method of the change in time and space using the real-time performance electromagnetic field with electric current carrying pathway, use
In the picture quality of improvement magnetic resonance imaging.
Background technology
Magnetic resonance imaging (MRI) is in transmission/detecting radiofrequency signal, and the nuclear spin of imaging object is and B1+/B1-
, rather than with driving current/induced-current in radio-frequency coil, interaction.At the same time the electric field produced with magnetic field
(concomitant E field) triggers radio frequency loss, shows with radio-frequency power consumption/noise.Electric current in coil has to be weighed very much
The effect wanted, serves as contact tie of the radio-frequency transmissions/reception (Tx/Rx) between system and radiofrequency field, for control magnetic resonance into
Radio frequency electromagnetic field as in.
Feasible program for analyzing radio-frequency transmissions/reception (Tx/Rx) in multiport system is as follows：A. in radio-frequency transmissions
(Tx) during, with predetermined radio-frequency pulse, by one group of radio-frequency power amplifier, the electric current in radio-frequency coil, electric current are driven
B1 are further excited, wherein, B1+Field excitation nuclear spin, and then produce precession.It is related to electric field (E field) to penetrate
Frequency loss causes imaging object to absorb RF energy, and radio frequency loss is weighed by Joule heat and polarization damping force mode, i.e. ∫ (σ+
ωε”)|E|2dv.B. during radio frequency reception (Rx), rotary magnetization caused by rotation precession makes to produce in radio-frequency coil electronic
Gesture, this generates with the B1 in B1-The electric current met.Electric current is detected by radio frequency reception unit, obtains being used to rebuild magnetic
The clock signal of resonance image-forming.The radio frequency loss related to electric field causes the noise in the clock signal that receives.It may be noted that
Be radio-frequency transmissions/radio frequency reception (Tx, Rx) when, magnetic distribution and radio frequency loss are influenceed by imaging object.In radio frequency
During transmitting/reception (Tx/Rx), interaction of the gradient pulse to B1- nuclear spins carries out spatial modulation.This space
Modulate the part control magnetic resonance excitation as radio-frequency transmissions/radio frequency reception scheme and the space orientation of detecting.
Optimize the size and phase (magnitude/phase), temporal modulation and spatial distribution of coil current for improving
Magnetic resonance imaging performance is extremely important.A key factor for promoting optimization is to grasp and configured using coil current pattern or source
(source configuration) is to B1 spatial distributions and the influence of rf power dissipation.Promote optimization another it is crucial because
Element is hardware structure.Except independent driver and receiver, it would be preferable to support the loop construction of the various current path control of Gui work
Also it is extremely important.Present patent application describe consider factor described above be used for improve the new system of magnetic resonance imaging performance
And new method.
Parallel radio frequency transmitting is for control unit radio frequency absorptivity and realizes that high-quality RF excited has outstanding appearance simultaneously
Amount.However, never a kind of practicable method makes full use of foregoing capacity, is implemented as organic life entity living
As performance improvement, and reduce radiofrequency unit absorptivity.In fact, (this is substantially improved in the free degree that parallel radio frequency transmitting band is come
It is the basis of foregoing capacity), usually trigger worry on the contrary, because inappropriate radio-frequency pulse is calculated or radio frequency shimming is calculated, or
Person's multichannel hardware fault, not only anti-that may deteriorate unit radio frequency absorptivity (SAR) for indefinite reduction.There is a kind of power meter scheme
This worry can be tackled to a certain extent.The program monitors the radio-frequency power of different port, with power meter (power
Meters) measurement different port forward and reflected power (forward and reflected power), calculates net transmission in real time
Power, scanning is stopped when overall SAR reaches threshold value.In order to fully tackle safety/performance considerations, it is necessary to further using product
Pole scheme supplements existing real-time monitoring, and aggressive schemes should have following ability：First, predicted before magnetic resonance imaging is carried out
The unit radio frequency absorptivity (SAR consequence) that image sequence is caused to imaging object, next to that further optimizing radio frequency
Pulse, the excitation to be reached (target excitation are realized while unit radio frequency absorptivity (SAR) is reduced
profile)。
Generally speaking, in the urgent need to be practicable SAR quantitative predictions model, model is directed to any B1 shimmings coefficient
(shimming coefficients) or radio-frequency pulse are encouraged, and predict overall unit radio frequency absorptivity (SAR
) and relevant average (volumetric average), and/or the identity morphism in different interest regions consequence
Frequency absorptivity and relevant average.
In principle, (including B1 shimmings and complete parallel are transmitted) is calculated for multi-channel radio frequency pulse, forecast model can lead to
Tracking SAR and guide design are crossed, SAR is clearly reduced, it is possible to based on same model, by presetting a safety
Surplus (margin), tackles hardware fault, it is to avoid unfavorable consequence occurs.In magnetic resonance imaging, these models serve as SAR prisons
Core is controlled, it has the ability predicted and draw SAR, extend the SAR monitoring capacity of power meter scheme.
Existing evidence shows, except pulse is calculated, radio-frequency coil (antenna) geometry influence multichannel or multiport
Tx or Rx performances.Fundamentally, it would be preferable to support the loop construction of the various current path control of Gui work is arrived to lifting Tx or Rx performances
The limit is particularly important.This with it is following identical of views:Radio-frequency pulse adjusts coil current, and the coil current excites radio-frequency electromagnetic
, it is final to determine Tx or Rx performances.What is desired is that a radio-frequency coil structure, the structure one multiport of formation it is intensive
Network structure, can accommodate and realize the radio-frequency current distribution pattern of complexity.
The content of the invention
There is electric current biography it is an object of the invention to provide a kind of utilization to improve the picture quality of magnetic resonance imaging
The method of the change in time and space of the real-time performance electromagnetic field in defeated path.
For achieving the above object, the present invention is adopted the following technical scheme that：
A kind of method, using the change in time and space of the real-time performance electromagnetic field with electric current carrying pathway, comprises the following steps：
Step a：Measuring method for measuring radio-wave propagation effect is provided；
Step b：Limited measured value is obtained by foregoing measuring method；
Step c：A prediction mode is provided based on aforementioned measurements, the arbitrary radio-frequency pulse of prediction mode prediction transmitting
Radio-wave propagation effect, effect includes energy and transmits and dissipate；
Step d：Radio-frequency pulse is calculated with foregoing prediction mode, radio-frequency pulse is used to be the net with electric current carrying pathway
Network provides energy；
Step e：Radio-frequency pulse provides energy for the network with electric current carrying pathway, realizes electromagnetic field change in time and space；Its
In, prediction mode and calculating radio-frequency pulse manage RF energy transmission and dissipated proactively.
As further improved technical scheme of the present invention, the prediction mode is preceding including the use of radio-wave propagation measurement
The numerical model put.
As further improved technical scheme of the present invention, the numerical model includes meeting the function member of quadratic equation
Part, for based on radio-frequency pulse definition prediction RF energy transmission and dissipate, the functional unit have by using with
The information of the power measurement values obtained in advance and predetermined parameter.
As further improved technical scheme of the present invention, the calculating is used to predict that RF energy consumes using at least one
Scattered quadratic equation integrates the prediction mode, the optimization as input by building and solving optimization problem
Output is defined as with radio-frequency pulse.
As further improved technical scheme of the present invention, the conductor part includes high conductivity material.
As further improved technical scheme of the present invention, the conductor portion includes high conductivity material.
It is described to be integrated described by building and solving optimization problem as further improved technical scheme of the present invention
Prediction mode includes：The radio frequency loss measurement based on quadratic function is integrated into the optimization frame of constraint or unconfinement obtained by calibrating
Structure,
Or the measurement of entirety and local SAR, which is integrated into constraint or the optimization of unconfinement, has guide parallel exomonental
The effect of calculating, makes the search to radio-frequency pulse shape tend to reduce SAR direction.
As further improved technical scheme of the present invention, the optimization framework is realized by following process：
By searching for a set of radio-frequency pulse, the radio-frequency pulse produces target excitation spatial form, while minimizing SAR.
As further improved technical scheme of the present invention, the measuring method is to utilize sensor measurement.
As further improved technical scheme of the present invention, the sensor is power sensor, passes through foregoing measurement side
Method, which obtains limited measured value, to be included：Obtain the forward power reading and backward power of multiple ports respectively using power sensor
Reading.
As further improved technical scheme of the present invention, the sensor supports phase sensitivity detecting, and passes through foregoing measurement
Method, which obtains limited measured value, to be included：The noise correlation matrix or many corresponding to multiport system is obtained using the sensor
The collision matrix of port network.
As further improved technical scheme of the present invention, in addition to obtain measured temperature.
The present invention carries out radio-frequency drive for parallel mode and magnetic resonance signal is received, and improves Tx or Rx performances, also may be used
To improve unit radio frequency absorptivity and signal to noise ratio (SNR), and then improve the picture quality of magnetic resonance imaging.
Brief description of the drawings
It is illustrated below to illustrate a preferred embodiment for realizing the present invention.
It is illustrated below to be respectively：
Fig. 1 is the principle schematic for implementing the magnetic resonance imaging system of the present invention.
Fig. 2 sets schematic diagram for the parallel transmission system in one embodiment of the invention.
Fig. 3 A-3B are the schematic diagram of many star coils in another embodiment of the present invention.
Fig. 4 is the schematic diagram of many star coils (constellation coil) with additional port.
Fig. 5 is the schematic diagram of many star coils with N number of port.
Embodiment
It refer to shown in Fig. 1, Fig. 1 illustrates the critical piece of the magnetic resonance system 10 comprising the present invention, and the system is logical
Cross operator's console 12 to be controlled, console 12 includes keyboard or other input equipments 13, control panel 14 and shown
Display screen 16.Console 12 is connected by connection member 18 with an independent computer system 20, and computer system 20 can make operation
Member can control the generation and display of image on display screen 16.The computer system 20 includes carrying out mutual ditch by backboard 20a
Logical some modules, these modules include image processor module 22, CPU modules 24 and memory modules 26, memory modules 26
Can be used for the frame buffer of storage image data group known to industry.Computer system 20 is connected with hard disk 28 and magnetic disc 30,
Storage image data and program, and be connected by high speed serialization connection 34 with an autonomous control system 32.
Control system 32 includes the one group of module interconnected by a backboard 32a, and the module includes CPU moulds
Group 36, the impulse generator module 38 being connected by serial connection (serial link) 40 with console 12.Control system 32 is led to
Cross the instruction that serial connection 40 receives the scanning sequence (scan sequence) performed the need for the operator.Pulse generation
The runtime component of device module 38, the scanning sequence that execution is specified, output data, for example：The meter of the radio-frequency pulse of radio-frequency transmissions
When, intensity, shape, the timing of radio frequency reception and the length of data acquisition window.Impulse generator module 38 is connected to a series of
Gradient amplifier system 42, for the duration of gradient pulse and shape for controlling to produce in (indicate) scanning process.Pulse
Generator module 38 can receive patient information from physiological acquisition controller 44, and the physiological acquisition controller 44 is by being connected to
Some different sensor collection signals of patient, for example, obtain ECG signal by the electrode with patient.Arteries and veins
Rush generator module 38 and be ultimately connected to scan room interface circuit 46, scan room interface circuit 46 is received and the state of an illness and magnetic system phase
The signal that the sensor of pass is produced.By scan room interface circuit 46, patient positioning system 48 receives instruction, removing patient to finger
Positioning, which is put, to be scanned.
The gradient waveform that impulse generator module 38 is produced is applied to the gradient amplifier system 42 with Gx, Gy, Gz,
Each gradient amplifier excites a corresponding gradient coil in gradient coil set 50, produces for generating additional space coding letter
Number magnetic field gradient.Gradient coil set 50 is a part for magnetic assembly 52, and magnetic assembly 52 also includes polarized magnet 54 and body is penetrated
Frequency coil 56.The pulse that transceiver module 58 in control system 32 is produced is amplified by radio frequency amplifier 60, passes through transmission/reception
Switch 62 is coupled with radio-frequency coil 56.The signal that the atomic nucleus being excited in patient body is sent is perceived by radio-frequency coil 56
Arrive, preamplifier 64 is then transferred to by transmission/reception switch 62, the magnetic resonance signal of amplification passes through transceiver module 58
Acceptance division be demodulated, filter, digitized processing.Transmission/reception switch 62 can be by the signal of impulse generator module 38
Control, so that radio frequency amplifier 60 and radio-frequency coil 56 are electrically connected with the transmission mode, in the receiving mode, before electric connection
Put amplifier 64 and radio-frequency coil 56.Transmission/reception switch 62 can make single radio-frequency coil (such as surface coils) in hair
Used under the pattern penetrated and received.
The magnetic resonance signal that radio-frequency coil 56 is collected is digitized processing by transceiver module 58, is then communicated to control
Storage module 66 in system 32 processed.After the one group of original k- spatial data of acquisition of module 66 is stored, the end of scan.Original
K- spatial datas are rearranged into the corresponding single k- space data sets of reconstructed image, each k- spaces with each
Data group is input into array processor 68, carries out combining magnetic resonance signal after Fourier transformation, forms one group of view data, figure
As data by serial connection 34 are transferred to computer system 20, and it is stored in storage device such as hard disk 28.With console 12
The instruction sent is corresponding, and described image data can be got up with longer-term storage, for example, be stored on magnetic disc 30, or pass through image
Processor 22 is further processed and is sent to console 12, and is shown on display screen 16.
The method and system that radio-frequency drive and magnetic resonance signal are received is carried out the present invention be directed to parallel mode, it is this parallel
Excite and reception mode can be by accelerating radio-frequency pulse/accelerating magnetic resonance signal framing to reduce sweep time, can also
Improve unit radio frequency absorptivity and signal to noise ratio (SNR).
Exemplary method described below is on predicting RF power/dissipation and B1 with limited measurement data
Distribution.
, can be by using appropriate system model and from school for any source configuration (source configuration)
The measurement data obtained during standard controls B1 spatial distributions and radio frequency loss.Source configures (source configuration)
Referring to one group, the radio-frequency pulse of setting or one group are used for pair in receiver (Rx case) in the transmitter (Tx case) in advance
Obtain the weight of the image reconstruction of signal.It is highly effective with multiport design analysis, in example as schematically shown in Figure 2, it is imaged mesh
Radio-frequency coil 72 used in mark 78 and magnetic resonance can be considered as a multiport network 90, and the network passes through port 80 and some sources
(source) cooperated.During radio-frequency transmissions (Tx), port is connected to the output end of radio-frequency power amplifier 74.
During radio frequency reception (Rx), port is connected to the input of the prime amplifier of receiver.Obtained by transmission/reception switch 62
Obtain different paths.
The design of core be using appropriate one group of source configuration (source configuration) carry out calibration experiments and
Record the reaction of multiport system.In to radio-frequency transmissions (Tx) calibration process, system response is as a result used for determining and predetermined
Radio-frequency pulse between quantitative relation, the system that prediction can be subsequently used in turn for different predetermined radio-frequency pulses is anti-
Should.
Multiport radio-frequency transmissions/reception (Tx/Rx) system hardware generally ensures the electromagnetic field phase produced in imaging object
It is linear response to source (source).During radio-frequency transmissions (Tx), it is assumed that (they amplify predetermined radio-frequency power amplifier
Radio-frequency pulse and the multiple ports of driving) appropriate linearity specifications (satisfy proper linearity specs) are met, often
Individual amplifier is just shown as the voltage source with output terminal impedance to multiport network, and the voltage source is with being supplied to the defeated of amplifier
Enter the predetermined radio frequency pulsed linear association at end.In this case, the linear of Maxwell's formula is embodied in electromagnetic field to electricity
The reaction of potential source and radio-frequency pulse is linear.Any electromagnetic field component in imaging object as the time function, show as
Lower formula 1：
B (t)=L (v(1)(t),v(2)(t),...,v(N)(t)),
Wherein, L represents linear transformation, and b (t) represents the electromagnetic field component, v(n)(t) predetermined radio-frequency pulse, N generations are represented
The quantity of table Tx ports.
In steady State Analysis, phasor can be used to mark any variations per hour, such as voltage source, the electricity by conductors cross
Stream, a component of electromagnetic field vector.Specifically, variations per hour can be expressed by using complex variable, the variations per hour can be by multiple
Variable and exp (j2 π f0T) multiplied result takes real part to obtain.
Phasor simplifies the description of the radio-frequency information in magnetic resonance.System electronic control is with Δ t durations interval in practice
For unit, Δ t is equivalent to such as 2 microseconds.The realization of one radio-frequency pulse subscribed is the result of a succession of control:In a succession of company
When each interval in the interval of the Δ t durations passed through starts, system electronics are f to a frequency0Sine wave apply one weigh
Weight, the weight is a plural number(n=ports index, p=interval index), corresponding to one size-phase pair.A whole set of weight
From so-called RF pulse designs process.
Phasor mark is adapted to the steady State Analysis at each Δ t durations interval, for example, position r in vivo and in p-th of Δ t
Duration interval, E can be expressed as E
The corresponding phasor mark in field is complex vector：
For the interval, the phasor mark of the predetermined radio frequency pulse of n-th of port is multiple scalar
For a Δ t durations interval, according to formula 1, the linear system behavior marked using phasor can be expressed as square
Configuration, it is specific such as formula 2：
B=A w,
Wherein, b=[b(1)b(2)…b(M)]T, b is an arrow phasor (a vector for collecting variations per hour interested
Collecting phaser representations of the time variables of interest), w=[w(1)
w(2)…w(N)]T, w, which is one, to be used to collect arrow phasor (a vector collecting that predetermined radio frequency pulse is defined
Definitions for the predetermined RF pulses), (its n-th component is a multiple scalar, marks Δ t
Size-phase of duration interval and n-th of port to), A is the M*N matrixes of complex value.
Electromagnetism field calibration sets (i.e. source is configured) to detect multiport networking with one group of meticulously selected input, the electricity measured
Magnetic field reaction is plus a suitable system model and then for predicting the reaction that electromagnetic field is set to any input.
The magnetic resonance system linearity is easy to the calibration of electromagnetic field, and formula 2 is shown, based on linear, is inputted using Line independent
The electromagnetic field reaction that the N number of experiment set is measured can be used to predict the electromagnetic field reaction under any input facilities, because of matrix A
Set and determined by the electromagnetic field reaction and N number of input that measure completely, specifically as shown in Equation 3：
Formula 3 is summarized shown in the index number in formula 2, experiment index ginseng formula.It is pointed out that according to input
The linear independence of setting, the presence of inverse matrix is guaranteed.A simply example is lifted, N number of test inhabitant area Tx can be passed through
Electromagnetic field, wherein n-th of experiment (n=1,2 ... N) is related to n-th of the rectangular radio frequency pulsed drive with unit amplitude and zero phase
Then use the radio-frequency pulse of zero amplitude in port, other ports.In this case [w1w2…wN] it is a unit matrix, so that A=
[b1b2…bN]。
For multiport parallel excitation, this includes this special situation of multiport B1 shimmings, and the principle of formula 3 may be used as
Predict B1+.Predict B1+It is the premise of RF pulse designs process.In this case bNIt is to collect B1 in n-th experiment+Take out in space
The vector of sample.After the A calibration programs represented with formula 3 are obtained, so that it may predict the B1 under any one group of radio-frequency pulse excitation+
Change in time and space, and then, the spatial form of RF excited.This causes the space for entering row energization with appropriate RF pulse designs
Shape control turns into feasible.
When producing the excitation space shape of target, it is necessary to avoid producing excessive radio-frequency power consumption on the object to be imaged
Dissipate.This can follow the trail of radio-frequency power consumption and dissipate by introducing appropriate measurement during RF pulse designs and solve.For example, design is penetrated
Frequency pulse can be considered as an affined minimization problem, and it is when searching radio-frequency pulse, to produce target excitation space shape
Shape is constraint, while minimizing unit radio frequency absorptivity caused by E.
The thermal balance of biological tissue can describe (Pennes Bio-heat equation by Pennes bio-heat equations
describes thermal energy balance for perfused tissue.).Rf power dissipation, when it is present,
It is specific as follows to state formula as the driving force risen with dynamic temperature：
Wherein, ρ, C and k represent tissue density, thermal capacity and thermal conductivity, h respectivelybIt is the heat flow biography between blood and tissue
Defeated rate, heIt is fractional radio frequency's energy absorbing device.The increase dissipated with RF energy, tissue overheat may occur.
It is to be noted that heIt is proportional with local E field intensities caused by Joule heating and polarization decay strength to be
, specific formula for calculation is:he=1/2 σ | E |2, wherein σ=σTissue+ωε”。
Due to interactive between the source under imageable target build and the difference of component, and high-frequency and imageable target plus
Play, electric field and radio-frequency power loss are heavily dependent on imageable target.In order to solve this problem, there is method suggestion base
The test synthesis of the specific absorption ratio result and actual human body profile and field that are obtained in average Human Model Simulation gets up.So
And, what is attracted people's attention is that this integrated approach easily produces big error in high-field magnetic resonance, because between simulation model and reality
Small difference can often cause considerable between SAR results and be difficult to the difference that reconciles.For Accurate Prediction unit radio frequency absorptivity
With improvement magnetic resonance performance, it is necessary to set up SAR forecast models with the measurement based on actual specific imaging object.
,, can not in order to manage SAR particularly in High field strenghth MRI from the perspective of the development of magnetic resonance imaging
What is avoided is needed for organism calibration SAR models living, just as being distributed for managerial Incentives spatial form, it is necessary to calibrate B1
Figure.In order to which the result under specific scan setting is optimal, each imageable target can be based on, weight is used as using B1 and SAR calibration results
Input parameter is wanted to guide optimal pulse calculating or shimming to calculate.And be further to improve setting in itself, extensive school can be integrated
Quasi- result, using it as important input parameter, the optimization of guiding radio system and coil.The present invention is founded and made at two aspects
Predicted with SAR：Overall SAR and any local SAR.This is consistent with following high-field magnetic resonance development trend.
It has been observed that can be from the radio-frequency transmissions in magnetic resonance from the point of view of network point and reception (Tx/Rx), wherein imageable target
It is considered as a multiport networking with radio-frequency coil, the network is cooperated by port with some sources.Work as MR scanner
When maintaining the good linear for fine scanning needs, there is linear relationship between electromagnetic field and source configuration, particularly, with
E fields in p-th of Δ t durations interval can be expressed as the weighted superposition of N number of E corresponding with N number of radio-frequency transmissions port, weight
Respectively wp (1),…wp (N).Therefore calculation formula is as follows for fractional radio frequency's power attenuation:
Wherein, Λ is N row's-N row positive definite Hermite Matrixes,HRepresent conjugate transposition (conjugate transpose), * generations
Table complex conjugate (complex conjugate), bottom line part is row or the row mark of vector or Input matrix component.
Above-mentioned formula shows that fractional radio frequency's power attenuation is w(1)... and w(N)Quadratic equation.Stated with matrix form, it is right
In p-th of Δ t durations interval, he=wHΛ w, wherein w=[w(1)…w(N)]TIt is that a vector is used to collect amplitude-phase pair.
These amplitude-phases are to from definition of N number of parallel radio frequency pulse for Δ t durations interval.Further, in Δ t durations
Overall radio-frequency power loss in interval can also be calculated with quadratic equation, specific as follows：
Overall radio frequency energy loss is ξpTime integral, be specifically expressed as follows:
Wherein wfullIt is a vector, have collected a whole set of weight and (define all amplitude-phases of radio-frequency pulse
It is right).Similarly,
Formula 6 calculates the radio frequency energy loss that whole radio frequency excitation results from local address x.
It is to be noted that description unit radio frequency absorptivity, is coupled even if there be no between element coil, it is contemplated that local and overall
Multiple sources can not be considered as isolated mutually during radio frequency energy loss and treated, this is attributed to E overlapping.This reflects during quantization
The continuous item component of usual non-zero in Λ and Φ, the continuous item component of these non-zeros summarizes the feature that multi-source is interfered,
Contain effectively reduction SAR information needed.Under control, existing shimming calculating/radio-frequency pulse calculate using simple or
The quadratic sum in multiple sources of weighting, the effect that multi-source is interfered when have ignored multiport transmitting is so tracking and managed SAR
Aspect can have severe deviations.
System for being mounted with appropriate sensor, power sensor 82 as shown in Figure 2, Φ can use a calibration process
Infer.When sensor is power sensor and when can provide multiple port forward and reflected power readings, determination Φ's
Method is to set (i.e. source is configured) to test with one group of input suitably preselected, the power reading that then processing experiment is obtained.Root
According to law of conservation of energy, Σ pfwd-Σprfl, total power dissipation of the net radio-frequency power equal to network of input N-port network.If
With wqSet as input, the net rf power dissipation calculated from sensor reading, i.e. Σ pfwd-Σprfl, with wqCorrelation,
It is specific as follows to state formula：
Σpfwd,q-Σprfl,q=wq HΦwq=Σ wq,i *wq,jΦij,
Formula 7
Wherein wq,iAnd wq,jW is represented respectivelyqI-th and j-th of component, formula 7 is ΦijLinear equation, Φ point
Amount is unknown number, product term wq,i *wq,jIt is coefficient.
The radio frequency loss feature of multiport network is easily drawn by some row experiments:The power sensor each tested is read
Number and source set (source configuration) to be combined into the linear equation described in formula 7；Combination is sufficient amount of
Equation, the institute that then just can calculate Φ is important.If with dqRepresent all w collected according to certain orderq,i *wq,j's
Vector is, it is necessary to be important to note that, the selection that input is set should cause one group of obtained dq(q=1,2 ...) linear independence.From
Calculate ΦijStrong anti-interference power from the perspective of, preferably selection input is set in some way, and such mode causes matrix
[d1d2...] conditional number it is smaller (such as so that the matrix is orthogonal).
When sensor supports phase sensitivity detecting, an alternative for determining Φ is to use reciprocity principle (the
Principle of reciprocity), specifically, based on noise correlation matrix (the covariance square corresponding to multiport system
Battle array) Ψ, the matrix can obtain by connecting the receiver of multiport, and Φ calculating can use equation below：
Φ=L1 HΨTL1,
In general, the preamplifier in front of receiver is to the one of multiport network performance during (Rx) is received
Row impedance is different from a series of impedances that power amplifier is showed multiport network during transmitting (Tx).Transformation matrix L1
Look after this factor.In particular cases all of the port is even during Tx and Rx sees 50 Ω, at this moment L1It is unit matrix.
In addition, measurement noise incidence matrix can not be used, and uses the collision matrix S and Bo Simading at multiport networking
Reason calculates Ψ:
Ψ=kT L2 H(I-S SH)L2,
Next, Φ calculating can use equation below：
Φ=L3 H(I-S SH)TL3=L3 H(I-SHS)L3.
L1,L2Or L3It can be determined after different calibrations.
It is feasible to be dissipated using the intravital RF energy of non-intruding mode spatial discrimination.Magnetic resonance temperature measurement is with high accuracy
During progress, temperature can be not only recorded, and the information that can be used for determining Λ (x) is provided and therefore makes local SAR forecast model
It is energetic.This can use Penni (Pennes) bioheat equation to explain, when radio-frequency transmissions experiment be relative to
When being carried out in heat transfer short time enough, the rising of fractional radio frequency's energy dissipation and local temperature is proportional, i.e. Δ T ∝
he=1/2 σ | E |2.Therefore position x in vivo, the Δ T measured values that magnetic resonance temperature measurement is obtained can be used for determining Λ (x):
1/2σ|E|2=wq HΛwq=Σijwq,i *wq,jΛij
Formula 8
As shown in Equation 8, determine that Φ principle may also used to determine Λ (x), difference is that the latter's temperature in use is surveyed
Value Δ T solves the Λ in linear equation as sensing dataij。
One of the obstacle for calibrating local SAR forecast model using magnetic resonance temperature measurement be temperature measurement accuracy in the case of live body/
Dynamic range.But this is not a basic limitation.Different from pursuing diagram and the prediction that the local temperature of pin-point accuracy rises, one
Strategy is the limitation for avoiding accident violation food and drug administration (FDA) to local SAR.It is overall if only relying on
SAR monitorings, prediction and management, such accident violation unlikely 100% are found or eliminated.By to local SAR mould
The grasp of type error, can set up the safe clearance that corresponding radio-frequency pulse is calculated.Overall RF energy is dissipated on live body
Accurate Prediction and the reasonable prediction to heat distribution, an effectively optimization transmitting and reduction live body SAR can be provided by being combined together
New tool.
With the difference for the traditional method for realizing B1 homogenization using body coil in large scale 3d space, giving transmitted in parallel has
The means of power go to improve SAR problems.Intuitively, it is excellent when imaging region is the sub-fraction of the imageable target of transmitting coil covering
Change the utilization to loop construction, for example, making the coiler part close to imaging region more more than other parts contribution, can avoid not
Necessary high rf power dissipation.
In practice, affined it can optimize that framework is clear and definite specifically performs optimization with one.Based on SAR and excitation space shape
The quantitative relationship of shape and radio-frequency pulse, framework reduction SAR is that, by searching for a set of radio-frequency pulse, the set radio-frequency pulse produces mesh
Excitation space shape is marked, while minimizing SAR.The one free framework that optimizes can alternatively, and it is provided more
Fast numerical value solution.In the case of excitation space form accuracy can be reduced moderately, a set of RF pulse-to-pulse is may search for, the set radio frequency
Pulse minimizes form error and SAR weighted sum.The premise of the search of any of the above-described pattern is clearly to know SAR and radio frequency
The quantitative association of pulse.
The invention provides demonstration methodses, the prediction that RF energy is launched/dissipated is incorporated into the calculating of driving pulse
In.
Given prediction model, such as quadratic function wHΦ w (and it is applicable the w of partHΛ w), by SAR value and radio-frequency pulse phase
Association, can be instructed during design radio-frequency pulse from forecast model.It is extra that this allows for that transmitted in parallel system has
The free degree can give play to advantage, accomplish adjustment E and reduction SAR, and following example is illustrated is based on secondary letter by calibration gained
Several radio frequency loss measurements is integrated into the optimization framework of constraint or unconfinement.
In order to reduce overall SAR, the measurement (metric) is the ξ defined in formula 5, described in order to reduce local SAR
Measurement is
ξlocalIt is local rf power dissipation value (formula 6) weighted statistical to all points of interest, nonnegative curvature ηlThen
Specify each regional area SAR of reduction relative importance.The measurement of entirety and local SAR be integrated into constraint or unconfinement it is excellent
Changing (optimization) has the effect of the exomonental calculating of guide parallel, the search to radio-frequency pulse shape is tended to drop
Low SAR direction, because selecting other waveforms, can be increased by metric and pay bigger cost.Design SAR optimizations
Following detailed description is joined in actuated in parallel pulse.
The wide-angle category for encouraging category in low-angle or being amplified out according to it, to a given k- space trackings, can make
Transmitted in parallel pulse is designed with the following method:
Ω be one comprising in visual field weight a little diagonal matrix, these weights reflection important area is (for example, right
1 and 0 are distributed respectively in important area and non-interesting region, and/or pointedly difference in visual field are distributed fractional value to refer to
Relative importance of the fixed accurate excitation to each point).α and β is positive parameter, reduction and exact space shape for balancing SAR
Excitation.β and ηlNumerical value conference limitation point of interest SAR increase.The present embodiment uses power correlation matrix, rather than one
Unit matrix, this has multiple spot advantage, not only helps improve reliability (the regularization effect of Numerical Inversion
(regularization effect)), and accurately follow the trail of important standard when SAR minimizes SAR as RF pulse designs
Then.This is important improvement：For example, for one group of parallel radio frequency pulse, 180 ° of phases can be increased to one of pulse,
Without influenceing wHI w value-for the constructive or destruction interference of E fields by pulsed drive and its changing for caused SAR
Become, the measurement can not be caught.Using unit matrix I it is further assumed that each passage undertakes same contribution-in same RF pulse-to-pulse
Under punching driving, as long as different ports contributes different to SAR, this assumes just invalid.
The affined optimization relative with formula 10 is calculated as follows：
Formula 11 emphasizes that excitation space shape is accurate, but efficiently numerical solution is less.
The technology for optimizing k- spatial samplings (sampling) can be further integrated with minimizing SAR.It is given to one
Transmitting setting and target excitation spatial form, the Orthogonal Subspaces interworking searching method introduced before by selecting a group by a group
Selecting property deletes EfullEach column groups of matrix, dramatically increase k- spaces lack sampling and therefore shorten driving pulse.It is preferably square
Method is to use a criterion changed, and the criterion not only considers contribution of the column groups to generation target excitation spatial form, and
It is also contemplated for its consequence produced to SAR:Computing is iterated at j times and determines G groups and corresponding impulse wave sample w, selection makes following formula most
The G and w of smallization:
This embodies a kind of improvement, because new Pulse Design is with controllable balance, while optimizing the standard of excitation space shape
True property, SAR and pulse length.
For wide-angle transmitted in parallel Pulse Design, a method based on optimum control can be amplified, it is effective to reach
SAR is minimized：
By using control system expression Bloch equations (Block equation), above-mentioned equation is reflected magnetization space
Shape (state vector M (t)) encouraged towards object space shape D (dbjective state) to radio-frequency pulse ripple real part (u (t)) and
The requirement of imaginary part (v (t)).Need the measurement minimized that there is one to be used for end-state at dbjective state
Penalize, it is another that there is an increase being used for SAR to punish.Category, the accurate SAR followed the trail of with specific, concrete are encouraged in wide-angle
Minimum be especially it is desirable that because efficient SAR management, by bigger flip angle/greater compactness of imaging sequence, is
MR application brings new chance.
B1 shimmings can use the formula 10 of reduction version.In order to further improve performance, it can relax completely to phase
It is required that, the uniformity of the amplitude of the spatial form finally motivated is for example pursued, but ignore the spatial variations of phase, the problem
It is expressed as follows:
Wherein w is the Nx1 vectors for collecting shimming coefficient, and E includes B1+Sample (no longer comprising gradient encoding efficiency), ufullGeneration
Entry target Uniform B 1+.
Optimize size/phase of radio-frequency coil electric current, time-modulation and spatial distribution to magnetic resonance imaging performance very
It is important.The important prerequisite of optimization is the radio frequency that pair B1 spatial distribution related to coil current pattern or source configuration and electric field trigger
The understanding of loss.The method of solution is B1 mappings and the calibration of power correlation matrix.Another important prerequisite of optimization, which is to provide, to be had
Beneficial to the hardware structure of optimization.Embodiment provides new radio-frequency coil structure, and this not only supports the various current path control of most Gui work
System, it is allowed to different current-modes are applicable on same conductor structure, and significantly simplify the manufacture of radio-frequency coil.With this
The structure of sample and multichannel or multiport Tx/Rx radio-frequency coil is supported to be referred to as many star coils.
Compared with traditional parallel radio frequency transmitting/receiving coil, the part of correspondence multichannel or multiport on many star coils
It is unnecessary mutually isolated, on the contrary, there is coupling or direct interconnection capacitively or inductively between these parts, these
Decoupling between part is secondary.The structure of many star coils highlights several characteristics, and the characteristic composition is high-quality simultaneously
The basis of row Tx/Rx runnings.This is stated under manufacturing and having far reaching significance using the radio-frequency coil for magnetic resonance imaging
Discussed in bright.
Many star coils of example are grid, and multigroup conductor is located at different physical layers, and in lattice site capacitive character
Coupling.One effective coil manufacture method is that multigroup conductor is disposed in multilayer high dielectric constant substrate, every layer of substrate clamping
Fig. 3 B shows plate sheet of one band, two groups of conductors 101,102 between two groups of conductors, one group of 101 conductor of top layer is etched
In the top surface 104 of substrate 103, one group of conductor 102 of bottom is etched in the bottom surface 105 of substrate 103.Substrate has higher dielectric normal
Number, adds the width and shape of other factors such as substrate thickness and conductor near lattice site, determines that one is larger together
Electric capacity 106 in capacitive couplings, the specific equivalent circuit as in Fig. 3 A is illustrated.
The coil of construction preferably meets following requirement：Electric capacity has low reactance in Rameau frequency, has height in audio
Reactance.Such characteristic allow radio-frequency current pass through capacitive couplings, when the interval between substantially parallel conductor compared with
Hour, dense grid structure provides a kind of continuous conductor structure for radio-frequency current so that such structure is to a certain extent
As radio frequency mask.This continuous performance supports complicated radio-frequency current pattern, including can lift the radio frequency electrical of parallel Tx/Rx performances
Stream mode.Meanwhile, such structure is transparent to audio gradient fields, because such field can not sense big electricity in grid
Stream.
Many star coils are expected to turn into maximally effective parallel radio frequency antenna.The conception be based on electrodynamics principle improve SNR and
SAR performances:It payes attention to the optimization of CURRENT DISTRIBUTION, and determine Tx/Rx performances B1 fields and E from CURRENT DISTRIBUTION.Relative to
Several breakthroughs of conventional coil are summarized as follows：
First, traditional loop construction is excessively discrete and be unable to Gui and live diversely support what high-field magnetic resonance Tx/Rx needed
Complicated current-mode, and decoupling or mode structure add many limitations to electric current.The each several part of many star coils can be good
Coupled well, similar netted radio frequency mask and maximize support complexity/wide band radio-frequency current-mode, not necessarily handle
Decoupling or mode configuration.The current-mode needed can be driven by using the multiple ports (many stars) being distributed in loop construction
Move or receive to realize, this available Kirchhoff's current law (KCL) is analyzed.In fact, the B1+/B1- obtained based on calibration
Distribution map and power/noise correlation matrix, desired current-mode can be by using predetermined transmitted in parallel radio-frequency pulses/simultaneously
Row receives image reconstruction to realize.Specific understanding or special RF pulse designs/image reconstruction to current-mode are not
Need.
Secondly, many star coils can preferably tackle radio frequency loss.The direct-coupling of E and imaging object is that High-Field magnetic is total to
The important radio frequency loss mechanism of center of percussion one, and pass through the big quantity and high density of the distribution capacity on coil, this loss meeting now
Greatly reduce.The conductor cooperation conduction electric current of dense distribution, can help to reduce the impedance suffered by radio-frequency current.Many star coils with it is new
Conductor material, for example, CNT, is also easier to integrate, this provides the chance for further reducing conductor losses.
3rd, conventional coil is effectively and reliably worked, complicated design, manufacture, tuning and test is necessary.It is real
The example with suitable challenge tunes these element coils in the state of being included in the complicated mutual interference of multiple element coils in border, integrates big
Amount parts (capacitor, inductor, and suppress or enable/disable circuit), and failure mode management.Conventional coil also face
To the fact that:A kind of structure can not be enough to be used in radio-frequency transmissions and reception, the different piece of body, multiple frequency and different
Parallel MR accelerates to set.This causes the excessive substantial amounts of demand to coil, and this means the use of costliness to clinical practice
And maintenance cost.
The design of many star coils have followed the SNR/SAR performance improvements optimized based on field.By contrast, attempt to allow multiple lists
Only coil (member) gather together work conventional method it is not only expensive, not to the utmost reliably, application category excessively be limited, in high channel
Especially faced the challenge under the parallel arrangement of situations of number, and it is too constrained so that magnetic resonance signal can not be encouraged and be detected water
It is flat to bring up to new height, realize the limiting performance that electrodynamics is allowed.
As shown in figure 4, when increasing a port 110 to many star coils, if it is desired, the net of a matching can be introduced
Network 111 and realize matching.For radio frequency reception, main target be for receiver before preamplifier desired characteristic impedance is provided,
So that preamplifier is operated in low-noise factor situation.For radio-frequency transmissions, main target is provided for power amplifier
Desired characteristic impedance, can Maximum Power Output.Fig. 5 further illustrates the example with many star coils with N number of port 110
Son, for n-th of port, if it is desired, an extra electric capacity can increase, and parallel with Cn.Electric capacity and inductance Ln
Matching network is composed in series, it has additional characteristic, i.e., can be by low input impedance/power amplifier of preamplifier
Low output impedance change into very high impedance.If needing further exist for, general multi input and multi output interface/matching
Network can be employed so that matching is more facilitated.
The description to system and method includes many characteristics above, and these are not necessarily to be construed as limiting protection domain,
And the only explanation of preferred embodiment.Other conversion and result are also possible by the inspiration of not be the same as Example, for example, field
Foundation and radio frequency loss forecast model can use other sensors data, including field measurement propagates measurement and different magnetic
Resonance signal.Further, many star coils are not limited to shown planar structure or two groups of conductors, same design
Suitable for cylinder, spherical structure and other warp architectures, or meet other complicated profiles of body shape.Build multilayer
Structure can provide additional advantage.Hard or flexible/pliable material can be used, and realize superperformance.Use carbon nanometer
Pipe or superconductor can make Performance optimization.
Claims (10)
1. a kind of method of change in time and space using the real-time performance electromagnetic field with electric current carrying pathway, comprises the following steps：
Step a：Measuring method for measuring radio-wave propagation effect is provided；
Step b：Limited measured value is obtained by foregoing measuring method；
Step c：A prediction mode, the radio frequency of the arbitrary radio-frequency pulse of prediction mode prediction transmitting are provided based on aforementioned measurements
Ripple communication effect, effect includes energy and transmits and dissipate；
Step d：Radio-frequency pulse is calculated with foregoing prediction mode, radio-frequency pulse is used to carry for the network with electric current carrying pathway
Energy supply amount；
Step e：Radio-frequency pulse provides energy for the network with electric current carrying pathway, realizes electromagnetic field change in time and space；Wherein, in advance
Survey mode and calculating radio-frequency pulse manage RF energy transmission and dissipated proactively.
2. a kind of side of change in time and space using the real-time performance electromagnetic field with electric current carrying pathway as claimed in claim 1
Method, it is characterised in that：The prediction mode is measured and preposition numerical model including the use of radio-wave propagation.
3. a kind of side of change in time and space using the real-time performance electromagnetic field with electric current carrying pathway as claimed in claim 2
Method, it is characterised in that：The numerical model includes the functional unit for meeting quadratic equation, for the definition based on radio-frequency pulse
Predict RF energy transmission and dissipate, the functional unit has by using the information with the power measurement values obtained in advance
And predetermined parameter.
4. a kind of side of change in time and space using the real-time performance electromagnetic field with electric current carrying pathway as claimed in claim 1
Method, it is characterised in that：The calculating uses the quadratic equation that at least one is used to predict RF energy dissipation as input, leads to
Cross structure and solve optimization problem and integrate the prediction mode, the optimization is defined as output with radio-frequency pulse.
5. a kind of side of change in time and space using the real-time performance electromagnetic field with electric current carrying pathway as claimed in claim 4
Method, it is characterised in that described integrating the prediction mode includes by structure and solution optimization problem：Will calibration gained base
The optimization framework of constraint or unconfinement is integrated into the radio frequency loss measurement of quadratic function；
Or, the measurement of entirety and local SAR is integrated into constraint or the optimization of unconfinement there is the exomonental meter of guide parallel
The effect of calculation, makes the search to radio-frequency pulse shape tend to reduce SAR direction.
6. a kind of side of change in time and space using the real-time performance electromagnetic field with electric current carrying pathway as claimed in claim 5
Method, it is characterised in that the optimization framework is realized by following process：
By searching for a set of radio-frequency pulse, the radio-frequency pulse produces target excitation spatial form, while minimizing SAR.
7. a kind of side of change in time and space using the real-time performance electromagnetic field with electric current carrying pathway as claimed in claim 1
Method, it is characterised in that the measuring method is to utilize sensor measurement.
8. a kind of side of change in time and space using the real-time performance electromagnetic field with electric current carrying pathway as claimed in claim 7
Method, it is characterised in that the sensor is power sensor, obtaining limited measured value by foregoing measuring method includes：Profit
Obtain the forward power reading and backward power reading of multiple ports respectively with power sensor.
9. a kind of side of change in time and space using the real-time performance electromagnetic field with electric current carrying pathway as claimed in claim 7
Method, it is characterised in that the sensor supports phase sensitivity detecting, and is included by the limited measured value of foregoing measuring method acquisition：
The collision matrix of the noise correlation matrix or multiport network corresponding to multiport system is obtained using the sensor.
10. a kind of change in time and space using the real-time performance electromagnetic field with electric current carrying pathway as claimed in claim 8 or 9
Method, it is characterised in that also including obtain measured temperature.
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Application publication date: 20170825 |