阅读说明：本技术 操作mri装置的方法 (Method of operating an MRI apparatus ) 是由 S.比伯 J.尼斯特勒 于 2019-08-19 设计创作，主要内容包括：本发明描述了一种操作MRI装置(1)的方法,所述方法包括以下步骤：激励MRI装置(1)的体线圈(BC)以发射射频信号(f<Sub>BC</Sub>)；确定体线圈(BC)的谐振曲线的中心频率(f<Sub>c</Sub>)；基于确定的中心频率(f<Sub>c</Sub>)计算磁体目标频率(f<Sub>T</Sub>)；以及将磁体(10)斜变到磁体目标频率(f<Sub>T</Sub>)。本发明还描述了一种MRI装置(1)和一种用于执行方法步骤的计算机程序产品。(The invention describes a method of operating an MRI apparatus (1), the method comprising the steps of: exciting a Body Coil (BC) of an MRI apparatus (1) to emit a radio frequency signal (f) BC ) (ii) a Determining the center frequency (f) of the resonance curve of the Body Coil (BC) c ) (ii) a Based on the determined center frequency (f) c ) Calculating the magnet target frequency (f) T ) (ii) a And ramping the magnet (10) to the magnet target frequency (f) T ). The invention also describes an MRI apparatus (1) and a computer program product for performing the method steps.)

1. A method of operating an MRI apparatus (1), the method comprising the steps of:

-exciting a Body Coil (BC) of the MRI apparatus (1) to transmit a radio frequency signal (f)_BC)；

-detecting the reflected radio frequency signal (f)_BC')；

-from the reflected radio frequency signal (f)_BC') determining the center frequency (f) of the resonance curve of the Body Coil (BC)_c)；

-based on the determined center frequency (f)_c) Identifying a magnet target frequency (f)_T) (ii) a And

-ramping the magnet (10) to a magnet target frequency (f)_T)。

2. The method of claim 1, comprising centering the frequency (f)_c) A step of storing in a storage module (13) of the MRI apparatus (1).

3. Method according to claim 1 or 2, wherein the reflected radio frequency signal (f) is averaged_BC') to obtain a center frequency (f)_c)。

4. The method according to any of the preceding claims, wherein the center frequency (f) of the reflected signal is determined by measuring the transmission rate between the Body Coil (BC) and another coil of the MRI apparatus (1)_c)。

5. The method according to any of the preceding claims, wherein the magnet (10) is ramped to include a magnet target frequency (f)_T) And a frequency of a sum of offsets (df), wherein the offsets (df) are determined based on a ramping procedure.

6. The method according to any of the preceding claims, wherein the center frequency (f) of the body coil resonance curve (30) is determined at any time during the manufacture of the MRI apparatus (1) and/or during the lifetime of the MRI apparatus (1)_c)。

7. The method according to any one of the preceding claims, wherein the target frequency (f) of the magnet (10) is determined without a main magnetic field_T)。

8. An MRI apparatus (1) comprising:

a body coil excitation unit (11) for exciting the Body Coil (BC) to transmit a radio frequency signal (f)_BC)；

-a frequency calculation module (14) realized to determine a reflected radio frequency signal (f)_BC') center frequency (f) of the resonance curve (30)_c)；

A target frequency calculation unit (12) realized based on the determined center frequency (f)_c) To calculate the magnet target frequency (f)_T) (ii) a And

-a magnet power supply unit (10P) realized to ramp the magnet (10) to a target frequency (f)_T)。

9. An MRI apparatus as claimed in claim 8, wherein the field strength of the magnet (10) of the MRI apparatus (1) is at most 1.0 tesla, more preferably at most 0.7 tesla, most preferably at most 0.5 tesla.

10. MRI apparatus according to claim 8 or 9, wherein the magnet (10) is a superconducting magnet.

11. MRI apparatus according to any one of claims 8 to 10, wherein the bandwidth of the body coil comprises at most 100kHz, more preferably at most 50 kHz.

12. MRI apparatus according to any one of claims 8 to 11, comprising means for storing the determined centre frequency (f)_c) Is implemented as part of the Body Coil (BC) and/or in a control unit of the MRI apparatus (1).

13. A computer program product for performing the steps of the method according to any one of claims 1 to 7 when the computer program product is loaded into a memory of a programmable device.

14. A computer program product comprising a computer program directly loadable into a memory of a control unit of an MRI apparatus (1) according to any of the claims 8 to 12 and comprising program elements for performing the steps of the method according to any of the claims 1 to 7 when the computer program is executed by the control unit.

15. A computer-readable medium, on which a program element is stored, which program element can be read and executed by a computer unit in order to perform the steps of the method according to any one of claims 1 to 7 when the program element is executed by the computer unit.

Technical Field

A method of operating an MRI apparatus is described. An MRI apparatus is also described.

Background

Developments in the field of Magnetic Resonance Imaging (MRI) systems have led to advances in low-field systems that may be preferred due to their smaller footprint. Low field systems may be open, may allow interventional procedures, and are less expensive. The term "low-field system" generally refers to a system having a magnetic field strength of at most 1.0 tesla. MRI systems with magnetic field strengths in excess of 1.0 tesla are commonly referred to as "high field systems". The magnetic field strength of low-field systems currently under development may even be below 1.0 tesla, and may even be below 0.5 tesla.

When an MRI system is first installed in the field, a ramping procedure is performed to establish the main magnetic field (also referred to as a static background field) in the main coil windings. After the initial installation of the ramping program, the necessary adjustments are made using shim coils to account for the local environment. Usually, the target frequency (i.e. the center frequency of the main magnet) is determined by means of a probe placed at a suitable position in the device. To allow for the inevitable attenuation of the magnetic field due to component aging (typically on the order of hundreds of ppm per year), this target frequency typically exceeds the center frequency of the body coil by an amount sufficient to ensure that the center frequency of the main magnetic field remains above the center frequency of the body coil for as long as possible.

In any superconducting MRI system, decay of the magnetic field is inevitable due to the residual resistance of the magnet. Field decay means that the center frequency of the main magnetic field gradually deviates from the initial setting. Eventually, a re-ramping system is necessary. In high-field systems, the bandwidth of the body coil and the radio frequency system is so large (e.g., body coil bandwidth of ± 100kHz or more) that it may take several years before the field decays beyond specification. Typically, another maintenance procedure, such as a cold head (cold head) exchange, must be scheduled much faster. In order to perform such a maintenance procedure, the magnet must in any case be tilted downwards and upwards, so that there is an opportunity to recalibrate the system.

Prior to ramping the magnet, a target frequency of the main magnetic field is identified. Usually a target frequency is set as high as possible and fine tuning of the main magnetic field is performed using shim coils. By setting the target frequency as high as possible, the attenuation window (the time it takes for the frequency to drift to the lower end of the allowed band) is as long as possible. This scheme is suitable for systems with high bandwidth, as described above. However, the bandwidth of low-field systems is significantly narrower than that of high-field systems (only on the order of 10kHz-25 kHz), so that prior art methods of setting the magnet target frequency are limited to a much shorter "attenuation window". Due to the narrow attenuation window, the magnet frequency of low-field MRI systems decays to an out-of-specification level in a shorter time. This means that low-field MRI systems typically have to be ramped more frequently.

The first superconducting low-field MRI system is typically implemented as a vertical field system, characterized by relatively inefficient body coils. These early low-field vertical systems were known to be less reliable than comparable horizontal-field systems (with birdcage body coils), making maintenance checks must be scheduled relatively frequently. During these maintenance procedures, the frequency of the main magnetic field is checked and the system is ramped back up if necessary. In low field systems, it may be desirable to perform intermittent ramping procedures, such as in the case of infrastructure problems (e.g., outages) or cooling device problems, etc.

Ideally, the center frequency of the body coil will be the same as the frequency of the main magnetic field. However, for low field magnets, the reflection effect can reduce the accuracy of the system, and the center frequency can be lower.

The narrow bandwidth of radio frequency systems, especially the body and receiver coils, means that the reflection coefficient significantly reduces the available power of the Radio Frequency Power Amplifier (RFPA). When the reflection coefficients from the body coil are not equal, the power is partially reflected back to the RFPA, which results in a derating of the RFPA. This reduces the available power of the radio frequency magnetic field generated by the transmit coil or body coil. This magnetic field is commonly referred to as the B1 field. Further, the center frequency of the magnet and the center frequency of the body coil are not necessarily the same. For these reasons, the magnet center frequency must be greater than the body coil center frequency to allow for the inevitable decay of the main magnetic field over time. However, it is difficult to determine a magnet target frequency that works satisfactorily under the constraints of narrow body coil bandwidth and high reflection coefficient.

Disclosure of Invention

It is an object of the present invention to provide a way of ramping an MRI apparatus which overcomes the above mentioned problems.

This object is achieved by a method of operating an MRI apparatus according to claim 1 and by an MRI apparatus according to claim 8.

According to the present invention, a method of operating an MRI apparatus comprises the steps of: energizing a body coil of the MRI apparatus to transmit radio frequency signals; determining a center frequency of the reflected radio frequency signal; calculating a magnet target frequency based on the determined center frequency; and ramping the magnet to the magnet target frequency.

The magnet is ramped at a predetermined time, whereby this should be understood as continuously monitoring the magnetic field during operation of the MRI apparatus, and when it is detected that the magnetic field has decayed to a level close to the lower limit, an alarm may be issued to the operator, who may then schedule the ramping procedure at a next convenient opportunity.

One advantage of the present invention is that the center frequency of the body coil resonance curve is empirically determined and used as the basis for calculating the target frequency of the magnet. This is advantageous in particular in low-field systems where the body coil bandwidth is relatively narrow. Another advantage is that little additional effort is required to implement the method of the invention. Instead of providing a means of tuning the body coil and another means of tuning the magnet, the present invention employs a scheme of using the body coil as it is and tuning the magnet based on the body coil. Any drift in the center frequency of the body coil (e.g., due to aging, varying mechanical load on the patient table of the body coil, etc.) will be accounted for in the target frequency of the magnet. In other words, the magnet will always be ramped to accommodate the transient state of the body coil. It should be appreciated that the body coil will be tuned to within a specified frequency bandwidth.

According to the present invention, an MRI apparatus includes: a body coil excitation unit for exciting the body coil to transmit a radio frequency signal; a calculation module implemented to determine a center frequency of a radio frequency signal transmitted by a body coil; a target frequency determination unit implemented to calculate a magnet target frequency based on the determined center frequency; and a ramping control unit implemented to initiate a ramping procedure to ramp the magnet to a target frequency.

Another advantage of the MRI apparatus of the invention is that it operates significantly more economically than comparable prior art MRI apparatus, since the time interval between successive ramping events can be extended, resulting in reduced down time during which an MRI scan cannot be performed.

The units or modules of the MRI apparatus described above, in particular the frequency determination unit and the selection unit, may be fully or partially implemented as software modules running on a processor of a control unit of the MRI apparatus. Implementation mainly in the form of software modules may have the following advantages: applications already installed on existing MRI systems may be updated with relatively little effort to perform the method steps of the present application. The object of the invention is also achieved by a computer program product with a computer program which is directly loadable into a memory of a control unit of an MRI apparatus and comprises program elements to perform the steps of the method of the invention when the program is executed by the control unit. In addition to computer programs, such computer program products may include other parts such as documents and/or additional components, as well as hardware components such as hardware keys (dongles, etc.) to facilitate access to the software.

A computer readable medium, such as a memory stick, hard disk or other transportable or permanently installed carrier, may be used for transporting and/or storing executable parts of a computer program product such that these can be read from a processor unit of an MRI apparatus. The processor unit may include one or more microprocessors or equivalents thereof.

As disclosed in the following description, the dependent claims present particularly advantageous embodiments and features of the invention. Features from different claim categories may be combined as appropriate to give other embodiments not described herein.

The method of the present invention may be applied to any suitable MRI apparatus, for example an MRI apparatus having a superconducting magnet, a permanent magnet or an electromagnet. However, low-field MRI apparatus with superconducting magnets benefit to a greater extent from the method of the present invention. Thus, in the following, but without limiting the invention in any way, it may be assumed that the magnet of the MRI apparatus is a superconducting magnet.

It may also be assumed that the MRI apparatus is a low-field MRI apparatus. In a particularly preferred embodiment of the invention, the field strength of the magnet is at most 1.0T, more preferably at most 0.7T, most preferably at most 0.5T. As noted above, the RF bandwidth of mid-or high-field MRI devices is typically large (e.g., body coil bandwidth + -100 kHz or more), but the bandwidth of low-field devices is significantly narrower. In another preferred embodiment of the invention, the MRI apparatus has a body coil with an RF bandwidth of no more than 50 kHz.

An advantage of the present invention is that the target frequency of the magnet can be determined independently of the imaging sequence or MR experiment (which of course requires a main magnetic field B0) and is determined based only on the reflection parameters of the body coil. In a particularly preferred embodiment of the invention, the magnet target frequency is determined in the absence of a main magnetic field. In other words, the magnet target frequency can be determined even when the main magnet has been ramped down. The present invention overcomes the problems associated with low-field MRI systems in which it is no longer feasible to specify the magnet frequency independently of the body coil frequency. The bandwidth of the low-field system is very narrow compared to a high-field system where the body coil bandwidth is large enough to include the individually specified magnet frequencies, so that prior art solutions that specify the magnet frequencies independently run the risk of not being within the body coil bandwidth.

To determine the appropriate target frequency for the magnet in the next ramping event, the body coil center frequency is determined by measuring the body coil's reflection coefficient as a function of frequency. To this end, a body coil excitation unit excites the body coil to transmit an RF signal at a set or selected frequency. According to the invention, the center frequency of the body coil is derived from the signal reflected by the body coil. The resonance curve of the reflected signal is detected and averaged in a suitable signal processing module to determine its center frequency. These calculations are preferably performed in the frequency domain.

The resonance curve can be measured by measuring the reflectivity of the body coil using a directional coupler. Such a directional coupler may already be a component of the MRI system. However, some simple MRI systems may not include such a directional coupler, or a directional coupler may be present but not equipped with the necessary detectors. Thus, in another preferred embodiment of the invention, the center frequency of the reflected signal is determined by measuring the transmission rate between the body coil and another coil of the MRI apparatus. This can be achieved in a relatively simple manner by transmitting a signal using a broadband transmit antenna and by detecting the received signal. In one approach, the further coil may be a local coil of the MRI apparatus. Alternatively, the further coil may be a pick-up coil of the MRI apparatus.

Once the body coil center frequency is determined, the magnet target frequency can be calculated. In a particularly preferred embodiment of the invention, the magnet target frequency is adjusted by adding an offset to the determined body coil center frequency.

Drawings

Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.

FIG. 1 shows a simplified circuit diagram of a superconducting low-field MRI apparatus according to an embodiment of the present invention.

Fig. 2 shows a simplified block diagram of an MRI apparatus according to an embodiment of the present invention.

Fig. 3 shows the determination of a magnet target frequency using the method of the invention.

In the drawings, like numerals refer to like elements throughout. The objects in the drawings are not necessarily to scale.

Detailed Description

Fig. 1 shows a greatly simplified circuit diagram of a superconducting low-field MRI apparatus 1. The MRI system 1 comprises various modules and units, most of which will be known to the skilled person and need not be explained here. The apparatus comprises a main magnet 10 which generates a very homogeneous main magnetic field B0. The MPSU 10P is used to supply a current I to the magnet 10 during the ramp-up procedure when the magnet 10 is ramped to a previously determined target frequency₁₀. A switch assembly 17 comprising a superconducting switch in parallel with a shunt resistor is shown connected to the main magnet coils. During the ramp-up procedure the switch is closed so that a small amount of current passes through the shunt resistor. In this exemplary embodiment, an amperometric shunt S is used to measure the magnet current I during ramping₁₀So that the power supply controller 15 can estimate the magnet frequency and compare it with the target frequency f_TMaking a comparison so that when the target frequency f has been obtained_TThe ramp-up procedure may be aborted. At this time, the switch 17 is opened again.

Fig. 2 shows a simplified block diagram of the MRI apparatus 1 indicating the main magnet 10 and the body coil BC. It may be assumed that there is a usual arrangement of additional coils such as shim coils, local coils, pick-up coils and multiple gradient coils. The figure shows a body coil excitation unit 11, which is realized to excite a body coil BC to select a frequency f_BCAn RF signal is transmitted. Detecting the reflected RF signal f_BC', and in the frequency determination module 14Analysing the reflected RF signal f_BC' resonance curve to identify its center frequency f_c. Center frequency f_cStored in the memory module 13, which may be realized as a memory module of the body coil BC or as a memory module of the control unit of the MRI apparatus 1. The reflection coefficient of the system means the reflector coil signal f_BCCentral frequency f of `_cBelow the body coil frequency f_BC。

Target frequency calculation module 12 identifies center frequency f based on_cDetermination of magnet target frequency f_T. Depending on the type of ramp-up sequence to be performed, an offset df may be added to the frequency f_c. In an exemplary process flow, center frequency f_cMay be identified, for example, by the manufacturer or at some point during the life of the MRI apparatus. Either way, the center frequency f_cStored in the memory module 13. Retrieving the center frequency f from the memory module 13 before executing the ramp-up sequence_cAnd adjusted as necessary or desired by an appropriate offset df to give the target frequency f_TAnd the magnet is ramped to the target frequency f_T。

A ramp control module is provided to initiate a subsequent ramp sequence at the appropriate time. Thus, the magnet power supply unit 10P will supply the current I during the ramp-up procedure₁₀Is provided to the magnet 10 in order to ramp the main magnet 10 to the target frequency f_T。

The above units and modules may be implemented as part of a central control system of the MRI apparatus 1.

Fig. 3 shows an exemplary resonance curve of a body coil. The Y-axis shows the reflection coefficient between 0 and 1. The X-axis shows the frequency shift in kHz, with 0 corresponding to the minimum reflection coefficient. Such a curve is obtained by averaging the reflected body coil signals. The shape of the resonance curve 30 is largely determined by the Q factor of the body coil. For the relatively low magnet frequencies of low-field MRI systems, the body coil has a high Q factor during the imaging sequence (i.e., the patient is inside the body coil) due to the low ohmic losses caused by the lower conductivity. Thus, in low-field MRI systems, the quality of the imaging process depends on the magnetThe degree of matching of the frequency with the body coil frequency. Corresponding to the center frequency f of the reflector coil signal_cIs identified and used to reach the magnet target frequency for the subsequent ramp-up sequence.

The target frequency may be set to an identified center frequency f identified in a resonance curve of the body coil reflection_c. However, it is preferable to add an offset to the target frequency. The magnitude of the offset may be selected based on the shape of the resonance curve and/or various parameters of the ramp-up sequence. The resulting offset df can be identified, for example, by identifying the maximum reflection coefficient as shown in fig. 3. In general, it is desirable to set the magnet target frequency higher than the body coil frequency. Thus, the target frequency f_TCan be expressed as

f_T＝f_c+df

Alternatively, a fraction of the offset may be used, such as 25% of the offset. In this case, the target frequency f_TCan be expressed as

For example, the center frequency f of the reflected signal_cCan be determined to be 20.0 MHz. With the addition of a suitable offset (e.g., 50kHz), the target frequency f for the next ramp event is scaled using the above equation_TWas determined to be 20.05 MHz. In this manner, the target frequency f may be identified based on the desired accuracy of the desired ramping procedure_T. The inventive method of determining the target frequency of a magnet using echo experiments is associated with an advantageously high accuracy, i.e. with an error of less than 1.0 kHz. In another example, the center frequency f of the reflected signal_cCan be determined to be 30.1 MHz. With the addition of a suitable offset (e.g., 10kHz), the target frequency f for the next ramp event is scaled using the above equation_TIs 30.11 MHz.

Excitation of the body coil BC, the reflector coil signal f_BC' measurement and center frequency f_cAnd a target frequency f_TCan be performed completely independently of the main magnetic field B0 so that the inventive method can be performed when the magnet 10 is tilted downA method.

Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of "a" or "an" in this application does not exclude a plurality, and "comprising" does not exclude other steps or elements. Reference to "a unit" or "a module" does not preclude the use of more than one unit or module.