Active coils used to deflect uniform magnetic field space

Abstract

A method for operating a magnetic resonance imaging (MRI) system and a system for implementing the method are disclosed, the method comprising: operating a gradient coil while turning off an active coil to acquire a first magnetic resonance (MR) signal encoding a first MRI image having a first field of view, the first MR signal being emitted from a first region inside a main magnet of the MRI system, at least a portion of a target body being placed in the first region; and energizing the active coil to shift a magnetic field volume so that a second MR signal is acquired from a second region inside the main magnet of the MRI system, wherein the first region is located within a magnetic field space and the second region is located within the shifted magnetic field space, and wherein the first region and the second region are both sufficiently uniform in magnetic field strength so that the magnetic field inhomogeneity of each region is below a defined threshold.

Description

Technical Field

[0001] The present disclosure relates to magnetic resonance imaging. Background Art

[0002] In a magnetic resonance imaging (MRI) system, the main magnet of the MRI system generates a highly uniform static magnetic field in a specific volume for imaging purposes. Static magnetic field variations can be tolerated in the order of parts per million (ppm); however, MR data cannot be generated where the main field deviation is too large, for example, more than a few hundred parts per million (ppms) over a 20 cm diameter spherical volume. Typically, this uniform volume, also called a uniform magnetic field or main field uniformity, is characterized by a maximum static field deviation over a certain spherical volume. For example, in a 25 cm diameter spherical volume (DSV) with a B0 = 0.5 T static field, 40 ppm may represent a maximum field deviation of ΔB0 = B0max - B0min = 20 μT. The main magnet is designed to achieve a specific uniformity (i.e., the main magnet is designed to have a non-uniformity below a threshold); however, the actual uniformity at the installation site may be affected by objects in or around the MRI scanner. At installation, passive and / or active shimming can be used to improve uniformity to meet the specific uniformity of the main magnet design to achieve the target body before being placed in the inner hole. When a target body (i.e., a person's head) is placed in an MRI scanning instrument, the tissue and any implantable device in this target body can also affect the uniformity of the imaging space, and this uniformity is usually improved again by fine-tuning of active shimming coils, such as by shimming coils, to achieve a specific uniformity. In order to quantify the uniformity of the main magnetic field, some implementations can measure the spectral width of the free induction decay (FID) signal of the region of interest, for example. In this measurement, field uniformity can depend on whether the spectral width of the FID signal is below a determined threshold. More specifically, if the spectral width of the FID signal is narrow enough for the desired imaging application, such as below a determined spectral width value, this shimming process can be considered sufficient. Otherwise, additional shimming processes can be performed to continue to reduce the spectral width of the FID signal.

[0003] In this case, for a whole-body MRI scanner, the aperture is wide enough so that any part of the human body can be moved into the uniform magnetic field space. In such a system, the target subject can be moved so that the body part of interest is located in the center of the uniform region. For a small-aperture MRI scanner designed for imaging the head, which part of the body can be moved into the uniform magnetic field space is also subject to geometric constraints. For example, the geometry of an MRI system used only for the head can be positioned in an average person so that the uniform region usually only extends to the bottom of the chin, but for areas farther away, the inhomogeneity of the main field may exceed a certain threshold and do not allow MR imaging to have the desired quality. The attenuation of image quality can be manifested as a poor signal-to-noise ratio or a low free induction decay (FID) signal peak. In general, it may not be feasible to image the body part below the neck or spine of the person because these areas cannot be further placed in the inner aperture of the scanner. Summary of the invention

[0004] In one aspect, an embodiment of the present invention provides a method for operating a magnetic resonance imaging (MRI) system, comprising: operating a gradient coil to acquire a magnetic resonance (MR) signal having the effect of encoding a first MRI image on a first region in a main magnet in the MRI system, at least a portion of a target body being placed in the MRI system, and the first region being located in a magnetic field space whose inhomogeneity is below a determined threshold; powering an active coil to shift the magnetic field space so that a second region in the main magnet of the MRI system is located in the shifted magnetic field space, at least a portion of the second region is located outside the magnetic field space before the magnetic field space is shifted, and at least a portion of the first region is located outside the magnetic field space after the magnetic field space is shifted; and operating the gradient coil to acquire a magnetic resonance (MR) signal for encoding a second MRI image on the second region.

[0005] Embodiments may include the following functionality. In one example, powering the active coil may include powering the active coil while operating the gradient line to acquire the second MRI image. In another example, powering the active coil may include powering the active coil before operating the gradient coil to acquire the second MRI image. In yet another example, powering the active coil may include driving the active coil using a current of at least 2 amps. In yet another example, powering the active coil may include running a coolant to the active coil to cool the active coil.

[0006] Certain embodiments may further include operating a shim coil to compensate for magnetic susceptibility differences within a portion of the target body located in the offset magnetic field volume, wherein the inhomogeneity of the offset magnetic field volume is below a determined threshold. In this embodiment, operating the shim coil may further include operating the shim coil while operating the gradient coil to acquire MR signals encoding the second MRI image.

[0007] On the other hand, certain embodiments provide a magnetic resonance imaging (MRI) system comprising: a housing having a cavity in which at least a portion of a target body to be imaged is placed; a main magnet contained in the housing and configured to generate a magnetic field space whose inhomogeneity is below a certain threshold value so as to form an MRI image on a region located within the magnetic field space; an active coil that causes the magnetic field space to deflect and deform when powered; a pulse generating coil that sequentially generates and applies radio frequency (RF) pulses to scan a portion of the target body; a gradient coil that provides a disturbance to the magnetic field space so that an MRI signal encoding an MRI image is obtained in response to the applied RF pulse; and a control unit coupled to the MRI system and configured to: operate the gradient coil to acquire MR signals having an encoding of a first MRI image on a first region in the main magnet within the MRI system The invention relates to a method for obtaining a magnetic resonance imaging (MRI) system of the present invention. The method comprises the steps of: placing at least a portion of a target body in the MRI system, and the first region is located in a magnetic field space, and the inhomogeneity of the first region is lower than the determined threshold value, so as to form the first MRI image; powering an active coil to shift the magnetic field space, and the inhomogeneity of the first region is lower than the determined threshold value, so that a second region in the main magnet of the MRI system is located in the shifted magnetic field space, and the inhomogeneity of the second region is lower than the determined threshold value, and at least a portion of the second region is located outside the magnetic field space before the magnetic field space is shifted, and at least a portion of the first region is located outside the magnetic field space after the magnetic field space is shifted; and operating the gradient coil to acquire a magnetic resonance (MR) signal for encoding a second MRI image on the second region, and the second region is located in the shifted magnetic field space, and the inhomogeneity of the first region is lower than the determined threshold value, so as to form the second MRI image.

[0008] Certain embodiments may include one or more of the following features. The active coil and the gradient coil may be integrated into a mechanical assembly. The integrated coil assembly may include a liquid cooling system for both the active coil and the gradient coil. The active coil may be constructed as a removable module configured to be installed with a gradient assembly that houses the gradient coil. The active coil and the gradient coil may be cooled from respective liquid cooling systems. The cooling system may include a reservoir for storing liquid helium or liquid nitrogen.

[0009] The active coil may be constructed on an RF coil assembly that is configured to be mounted within the gradient coil. The active coil and the gradient coil may be cooled from respective liquid cooling systems. The cooling system may include a reservoir storing liquid helium or liquid nitrogen.

[0010] Certain embodiments may include a set of shim coils disposed about the housing and configured to compensate for differences in magnetic susceptibility within a portion of a target body located in the magnetic field space. The control unit may be further configured to power the active coils to offset the magnetic field space wherein a change in magnetic field strength is below the determination threshold. The control unit may be further configured to power the active coils to offset the magnetic field space wherein a free induction decay (FID) signal from the magnet space has a spectral width below the determination threshold.

[0011] The subject matter of this specification may be described in one or more aspects in the following drawings and description. Other features, aspects and advantages of the subject matter will be apparent from the description, drawings and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1A A perspective view of an example magnetic resonance imaging (MRI) system having a solenoidal magnet providing active coils for offsetting a uniform magnetic field space within the solenoidal magnet is shown.

[0013] Figure 1B A cross-sectional view of an example magnetic resonance imaging (MRI) system providing a spatially active coil for deflecting a uniform magnetic field is shown.

[0014] Figure 2 An example of offsetting the uniform magnetic field space within the solenoid magnet of an MRI system is shown.

[0015] Figure 3 An example of operating an MRI system using active coils to vary the uniform magnetic field space while imaging a target subject is shown.

[0016] Like reference symbols in the various drawings indicate like elements. Specific embodiments

[0017] The following reference details discuss and describe various embodiments and aspects of the present disclosure. The following description and drawings are illustrative of the present disclosure and should not be construed as limitations of the present disclosure. A thorough understanding of the various embodiments of the present disclosure is provided in describing a number of specific details. However, in order to provide a brief discussion of the embodiments of the present disclosure, common or customary details will not be described in certain circumstances.

[0018] According to selected embodiments of the present disclosure, a magnetic resonance imaging system may include an active coil, such as an integrated mechanical assembly with a gradient coil, to act as a position-specific static field shimming coil, which, when activated, moves the uniform magnetic field space within the solenoid magnet so that the main field inhomogeneity in the imaging region is below a certain threshold. The active coil is different from the main field shimming coil, which is used to compensate for magnetic susceptibility differences caused by gas and tissue edges or implant devices. Generally, the main field shimming coil cannot shift a uniform magnetic field space suitable for forming an MRI image of sufficient quality. As disclosed in an embodiment, the active coil can use a higher current amperage than used for the shifting procedure. In some cases, the active coil can include active cooling using a coolant, such as liquid nitrogen or liquid helium. The operation of the active coil is different from that of the gradient coil, and when the active coil is activated, the shifted area in the uniform magnetic field remains static, while the gradient coil can cause dynamic fluctuations in the main magnetic field when activated, so that MRI signals encoding magnetization signals can acquire various parts of the target body, from which image reconstruction can be performed to produce MRI images.

[0019] An implementation may allow an operator, such as a clinician, to offset the image coverage area from an initial area, such as the head, to a different area, such as including the neck. More specifically, a patient may be placed in the main magnet and a first image may be collected at the head. Upon determining that the neck requires further investigation, the operator may activate the active coil to offset the area of ​​uniform magnetic field from the initial area to a different area including the neck. The operator may collect MRI images at the neck while in the offset position. This offset approach may be advantageous for smaller magnets where inward movement of the patient's neck may not be feasible due to limitations in magnet size and shape.

[0020] Figure 1A-1B A perspective view and a cross-sectional view of an example magnetic resonance imaging (MRI) system 100 are shown, wherein a solenoid magnet 105 is arranged in a cylindrical shape with an inner cavity 101. A coil assembly 107, which includes a transmitting coil 106 and a gradient coil 104, is arranged in the solenoid magnet 105. Generally, the coil assembly 107 can be formed into an annular structure and housed in the inner cavity of the solenoid magnet 105. In a certain embodiment, the annular coil assembly 107 includes only the gradient coil 104. Generally, the gradient coil 104 provides field gradients in more than one direction, such as all three orthogonal spatial directions. Therefore, this gradient coil 104 can refer to three sets of coils, and each set of coils is configured to generate field fluctuations in a corresponding direction of the main field in the inner cavity of the solenoid magnet 105. This field fluctuation can cause the magnetization from different spatial locations to sense the precessing ions of different frequencies, so that the spatial information of the magnetization can be encoded by the RF excitation pulse.

[0021] In this implementation, the annular coil assembly also does not include a transmit coil 106 or any receiver coils. For this implementation, radio frequency (RF) excitation pulses are transmitted as a local coil to image the head region 102 of the patient 103. In one example, a birdcage-shaped head coil is used to transmit the RF excitation pulses and receive MR signals for imaging the target body. In another example, a curved coil is used to send RF excitation pulses into the target body, and a phased array coil is configured to receive MR signals in response.

[0022] In one embodiment, the shim coil 109 is contained within the cylindrical wall of the solenoid magnet 105. A set of power amplifiers provides power to the shim coil 109. For example, power amplifiers 110A and 110B contained in a control room can be connected to the shim coil 109 to provide shimming for the magnetic field in the inner cavity 101. In driving the shim coil 109, the power amplifier can be controlled by a control unit 111, which typically includes one or more processors and programming logic for configuring the power amplifiers 110A and 110B. In one case, the control unit 111 is contained in a control room separate from the solenoid magnet 105 of the MRI system 100. The drive current of the shim coil 109 can be in the range of hundreds of milliamperes and generally does not exceed 1 ampere. In addition, the shim coil 109 can be actively cooled without the use of a circulating coolant. In this embodiment, a set of shim coil arrays can be used to provide adjustment of the field strength in the inner cavity 101 so that the magnetic field in the inner cavity 101 becomes more uniform.

[0023] The embodiments provided herein may be applicable to intraoperative MRI, as well as MRI systems for emergency room settings. Such MRI systems may include a smaller and more compact bore magnet than conventional whole body scanner magnets. One consequence of using a small bore magnet is that the uniform magnetic field space suitable for imaging (such as magnetic field inhomogeneity below a certain threshold) may not cover all areas of interest. As discussed in further detail below, this certain threshold may represent the degree of magnetic field variation over a spatial length, or the spectral width of a free induction signal (FID) emitted from a particular space. For example, although the head region of the target subject may be within the uniform magnetic field space, the neck region of the target subject may not be. However, this bore size limitation may not allow the target subject to move further into the inner bore of the magnet. A certain embodiment provides an active coil 110 that, when activated, causes the uniform magnetic field space to shift, so as to accommodate imaging of the target subject's neck region, for example, when the target subject cannot be practically moved further into the magnet bore. In other words, this active coil 110 may provide a static shift specific to the uniform magnetic field position suitable for imaging.

[0024] As described above, the active coil 110 can provide a location-specific static offset on the homogenous region when powered. In this example, the active coil 110 can be powered to collect MR signals at locations in the neck and spine when the main magnetic field may be too inhomogeneous to acquire MR signals of sufficient quality. In general, the active coil 110 can change the shape and position of the homogenous region so that the homogenous magnetic field is spatially offset from the head region to encompass portions of the neck and spine.

[0025] Figure 2 An example of a uniform magnetic field space offset within a solenoid magnet of an MRI system 100 is shown. During the imaging process, the head 202 and neck 204 of the target subject are first placed into the inner cavity 101 of the MRI system 100. A first MRI image can be formed at the head 202 and a portion of the neck 204. As shown, the uniform magnetic field space (i.e., the uniformity region 212 when the coil is closed) does not extend from the head region 202 to completely cover the neck region 204. However, as previously described, the shape and size of the inner cavity 101 prevent the target subject from being further placed. In this illustration, the clinician can power the active coil 110 through the control panel on the MRI system 100 to offset the uniform magnetic field space toward the neck 204 so that the offset space of the uniform magnetic field (i.e., the uniformity region of the coil on 214) completely covers the neck 204. When the uniform magnetic field covers the neck 204 so that the main field inhomogeneity in the neck 204 is reduced below a determined threshold, the clinician can start using the gradient coil 104 , the RF coil 106 and the shim coil 109 through the control panel on the MRI system 100 to obtain the desired neck 204 image.

[0026] In some cases, if the offset uniform space encompasses the entirety of the desired imaging space, the active coil 110 may continue to be powered throughout the data collection process. In other cases, if the desired imaging space spans two locations where the active coil 110 needs to be turned off and where the active coil 110 needs to be turned on, the active coil 110 may be turned on and off in a staggered manner. Figure 2 As shown, when the active coil 110 is powered, the main field uniformity near the head can be sacrificed so that the uniformity zone can extend deeper into the neck 204. In a certain configuration, the degree of offset can be adjusted to accurately accommodate the range of the neck 204. For example, the clinician can change the power level of the active coil 110 to a specific level corresponding to the degree of offset to the neck 204 through the control panel of the MRI system 100. This correspondence can be estimated in advance and stored in a lookup table for investigation during the imaging session.

[0027] like Figure 1BAs shown, the active coil 110 is housed within the gradient coil 104 housing. In this configuration, the active coil 110 can share the same active cooling as the gradient coil 104. For example, both the active coil 110 and the gradient coil 104 can be cooled using the same cooling system, which operates a coolant such as liquid nitrogen or liquid helium. This configuration can take advantage of existing cooling systems that already include a reservoir for a liquid coolant, such as liquid nitrogen or liquid helium. The drive current of the active coil 110 can be in the range of 1 to 10 amps, while the gradient coil 104 may require a drive current higher than 10 amps. In other cases, the active coil 110 can be constructed on the RF coil 106 and configured to be mounted inside the gradient coil 104. In other cases, the active coil 110 can be housed in the side wall 105 of the magnet and may require an additional separate cooling system. In the above description, the active coil 110 can be coupled to a control unit on the MRI system 100, for example, to provide the drive current through a power amplifier. The control unit can be housed in a separate control room away from the magnet. The control unit may include a processor or programming logic for configuring a power amplifier that drives the active coil 110 .

[0028] Figure 3 An example of a flow chart 300 for operating the active coil 110 of the MRI system 100 is shown. When the process starts (302), the gradient coil 104 can be operated to acquire a magnetic resonance (MR) signal having the effect of encoding a first MRI image over a first region in a main magnet in the MRI system, wherein at least a portion of a target body is placed in the MRI system, and the first region is located in a uniform magnetic field space, and its inhomogeneity is below a determined threshold (304). The determined threshold can refer to a maximum limit of the main magnetic field double difference within a certain distance range, for example, 20μT / 25cm on a 0.5T main magnet. The determined threshold can also refer to a maximum limit of the spectral width from the desired spatial FID signal. For example, the spectral width can be measured as the full width at half maximum (FWHM) width. The above operation can be initiated by an operator configuring scanning parameters on a control panel. The operation can be initiated by an operator configuring scanning parameters on a control panel. The gradient coil can be coupled to a control unit of the MRI system 100 to receive instructions so that a gradient waveform is played accordingly to provide field fluctuations to the main magnetic field. As previously described, the gradient coils 104 may be combined with more than one gradient coil subset, with each subset being used to implement field fluctuations in one spatial direction in the bore 101 within the MRI system 100. The first image may reveal the head 202 of the target subject, as well as a portion from the neck 204. When acquiring MR signals encoding the first MRI image, the shim coils 109 may be used to compensate for differences in magnetic susceptibility within, for example, the head 202 and the portion from the neck 204.

[0029] It can then be determined whether a new scan is needed (306), for example to display more details of the neck 204 beyond what is depicted on the first image, or to further investigate the neck 204. In some cases, the determination can be made based on input from an operator of the MRI system 100. Other examples can include automatic region offset methods. For example, the MRI system 100 can perform image recognition in the first MRI image and determine that certain body parts in the neck 204 are not fully captured, and therefore it is necessary to offset the uniform magnetic field space. This in turn can determine that the uniform magnetic field space covers the portion of interest in the neck region 204 and does not require further observation. In this case, the MRI system 100 can continue to operate (308) without starting the active coil 110.

[0030] When a shift is necessary, the control unit on the MRI system 100 can power an active coil to shift the uniform magnetic field space so that a second region in the main magnet of the MRI system is located within the shifted uniform magnetic field space, at least a portion of the second region is located outside the uniform magnetic field space before the uniform magnetic field space is shifted, and at least a portion of the first region is located outside the uniform magnetic field space after the uniform magnetic field space is shifted (310). This shifting process can cause the shifted uniform magnetic field space to be moved away so as to cover a portion of interest such as from the neck region 204. As described above, a specific power supply level can result in a specific transition. Powering the active coil 110 can involve an operating current between 1-10 amperes, while the shim coil 109 is typically associated with a drive current of less than one ampere, such as hundreds of milliamperes. At the same time, the gradient coil 104 can typically be driven by a current in the range of 10-100 amperes and may typically require active cooling by circulating coolant.

[0031] When the uniform magnetic field space has been shifted to cover a portion of interest such as the neck 204, the shimming coils are operable to compensate for magnetic susceptibility differences in the target body portion located within the shifted uniform magnetic field space (312). When shimming is performed, the gradient coils 104 are operable to provide field fluctuations to the main magnetic field so that MR signals encoding a second MRI image of a second region located within the shifted uniform magnetic field space (314) can be obtained.

[0032] As used herein, the terms "comprises" and "comprising" should be interpreted as inclusive and open-ended rather than exclusive. Specifically, when used in this specification and claims, the terms "comprises" and "comprising" and their variations mean including the specified features, steps or components. These terms should not be interpreted to exclude the existence of other features, steps or components.

[0033] As used herein, the term "exemplary" means "serving as an example, instance, or illustration," and should not be construed as a preferred or advantageous configuration over other configurations disclosed herein.

[0034] As used herein, the terms "about" and "approximately" are meant to include variations that may exist in the upper and lower limits of a range of values, such as variations in characteristics, parameters, and dimensions. In a non-limiting example, the terms "about" and "approximately" mean ±10% or less.

[0035] The above specific embodiments have been illustrated, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the specific forms disclosed, but to cover all modifications, equivalents and alternatives that fall within the spirit and scope of the present disclosure.

Claims

1. A method for operating a magnetic resonance imaging (MRI) system, the method comprising: include: operating the gradient coils while the active coils are turned off to acquire first magnetic resonance (MR) signals encoding a first MRI image having a first region, the first MR signals being emitted from a first region within a main magnet of the MRI system, at least a portion of a target body being disposed in the first region; and energizing the active coil to shift the magnetic field volume so that a second MR signal is acquired from a second region inside the main magnet of the MRI system, The first region is located in the magnetic field space, and the second region is located in the offset magnetic field space, and the first region and the second region are sufficiently uniform in magnetic field strength so that the magnetic field inhomogeneity of each region is below a defined threshold.

2. The method according to claim 1, further comprising: include: The first MRI image is combined with a second MRI image reconstructed from the second MR signal so that the first region is expanded to include at least a portion of the second region that was outside the magnetic field space before the magnetic field space was shifted.

3. The method according to claim 1, further comprising: include: operating the gradient coil while turning off the active coil to acquire a third magnetic resonance MR signal encoding a third MRI image having a third region; and The active coils are energized to shift the magnetic field space such that fourth MR signals are acquired from a fourth region within the main magnet of the MRI system. 4 . The method of claim 3 , wherein the third region and the first region substantially overlap each other, and the fourth region and the second region substantially overlap each other.

5. The method of claim 1, wherein energizing the active coil include: The active coil is excited while the gradient coil is operated to acquire the second MR signal.

6. The method of claim 1, wherein energizing the active coil include: The active coil is excited before operating the gradient coil to acquire the second MR signal.

7. The method according to claim 1, further comprising: include: Shim coils are operated simultaneously with the gradient coils to compensate for magnetic susceptibility differences within the portion of the target subject within the offset magnetic field space where inhomogeneities are below the defined threshold.

8. A magnetic resonance imaging (MRI) system, include: a housing having an aperture in which at least a portion of a target subject to be imaged is placed; a main magnet accommodated by the housing and configured to generate a magnetic field space; an active coil which, when energized, causes the magnetic field to spatially shift in position and change in shape; a pulse generating coil configured to sequentially generate and apply radio frequency (RF) pulses to scan the portion of the target body; a gradient coil configured to provide a disturbance to the magnetic field space so that a magnetic resonance MR signal encoding an MRI image is acquired in response to the applied RF pulse; and A control unit coupled to the MRI system and configured to: operating the gradient coils while the active coils are turned off to acquire first magnetic resonance (MR) signals encoding a first MRI image having a first region, the first MR signals being emitted from a first region inside a main magnet of the MRI system, at least a portion of a target body being disposed in the first region; and energizing the active coil to shift the magnetic field space so as to acquire a second MR signal from a second region inside the main magnet of the MRI system, The first region is located in the magnetic field space, and the second region is located in the offset magnetic field space, and the first region and the second region are sufficiently uniform in magnetic field strength so that the magnetic field inhomogeneity of each region is below a defined threshold.

9. The MRI system of claim 8, wherein the active coil and the gradient coil are integrated into one mechanical assembly. 10 . The MRI system of claim 8 , wherein the active coil is constructed as a removable module configured to be installed with a gradient assembly that houses the gradient coil.

11. The MRI system of claim 8, wherein the active coil is constructed on an RF coil assembly configured to be mounted inside the gradient coil.

12. The MRI system according to claim 8, further comprising: include: A set of shim coils are positioned around the housing and configured to compensate for differences in magnetic susceptibility within the portion of the target body within the magnetic field volume.

13. The MRI system according to claim 8, wherein the control unit is further configured to: The first MRI image is combined with a second MRI image reconstructed from the second MR signal so that the first region is expanded to include at least a portion of the second region that was outside the magnetic field space before the magnetic field space was shifted.

14. The MRI system according to claim 8, wherein the control unit is further configured to: operating the gradient coils while turning off the active coils to acquire third magnetic resonance MR signals encoding a third MRI image having a third region; and The active coils are energized to shift the magnetic field space such that fourth MR signals are acquired from a fourth region within the main magnet of the MRI system. 15 . The MRI system of claim 14 , wherein the third region and the first region substantially overlap each other, and the fourth region and the second region substantially overlap each other.

16. The MRI system according to claim 14, wherein the control unit is further configured to: The active coil is excited while the gradient coil is operated to acquire the second MR signal.

17. The MRI system according to claim 14, wherein the control unit is further configured to: The active coil is excited before operating the gradient coil to acquire the second MR signal.

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