SYSTEM AND METHOD FOR REMOVING ELECTROMAGNETIC INTERFERENCE FROM LOW-FIELD MAGNETIC RESONANCE IMAGES

The present disclosure provides systems and methods for removing electromagnetic interference from low-field magnetic resonance images. In one aspect, a method can include projecting a low-field strength magnetic field toward an object of interest located within a field of view and transmitting a radio frequency pulse sequence to a radio frequency coil assembly configured to selectively excite magnetization in the object of interest within the field of view. The method can further include receiving an output signal from the radio frequency coil assembly during a signal acquisition period and receiving a sample signal from the radio frequency coil assembly during an interference period. The method can further include comparing the output signal and the sample signal to identify an interference component and adjusting the output signal based on the interference component.

BACKGROUND

The present disclosure relates to magnetic resonance imaging (MRI), medical imaging, medical intervention, and surgical intervention. MRI systems often include large and complex machines that generate significantly high magnetic fields and create significant constraints on the feasibility of certain surgical interventions. Restrictions can include limited physical access to the patient by a surgeon and/or a surgical robot and/or limitations on the usage of certain electrical and mechanical components in the vicinity of the MRI scanner. Such limitations are inherent in the underlying design of many existing systems and are difficult to overcome.

SUMMARY

In one aspect, the present disclosure describes a method. The method includes projecting a low-field strength magnetic field toward an object of interest located within a field of view and transmitting a radio frequency pulse sequence to a radio frequency coil assembly configured to selectively excite magnetization in the object of interest within the field of view. The radio frequency pulse sequence can define a time of repetition and includes a signal acquisition period within each time of repetition. The method can further include receiving an output signal from the radio frequency coil assembly during the signal acquisition period and receiving a sample signal from the radio frequency coil assembly at an interference period within the time of repetition. The interference period can flank the signal acquisition period. The method can further include comparing the output signal and the sample signal to identify an interference component and adjusting the output signal based on the interference component; and recording the adjusted output signal in a k-space matrix.

In another aspect, the present disclosure describes a system. The system includes an array of magnets, a radio frequency coil assembly, and control circuit. The array of magnets can be configured to generate a low-field strength magnetic field toward an object of interest located within a field of view. The radio frequency coil assembly can be configured to selectively excite magnetization in the object of interest in the field of view. The control circuit can include a processor and a memory. The memory can store instructions executable by the processor to transmit a radio frequency pulse sequence to the radio frequency coil assembly. The radio frequency pulse sequence defines a time of repetition, wherein the radio frequency pulse sequence comprises a signal acquisition period and at least one interference period within each time of repetition, wherein each interference period flanks the signal acquisition period. The memory can further store instructions executable by the processor to receive an output signal from the radio frequency coil assembly during the signal acquisition period and receive a sample signal from the radio frequency coil assembly during the interference period within the time of repetition. The memory can further store instructions executable by the processor to compare the output signal and the sample signal to identify an interference component and adjust the output signal based on the interference component and record the adjusted output signal in a k-space matrix.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate various disclosed embodiments, is one form, and such exemplifications are not to be construed as limiting the scope thereof in any manner.

DETAILED DESCRIPTION

Applicant of the present application also owns the following patent applications, which are each herein incorporated by reference in their respective entireties:International Patent Application No. PCT/US2022/72143, titled NEURAL INTERVENTIONAL MAGNETIC RESONANCE IMAGING APPARATUS, filed May 5, 2022.

Before explaining various aspects of neural interventional magnetic resonance imaging devices in detail, it should be noted that the illustrative examples are not limited in application or use to the details of construction and arrangement of parts illustrated in the accompanying drawings and description. The illustrative examples may be implemented or incorporated in other aspects, variations and modifications, and may be practiced or carried out in various ways. Further, unless otherwise indicated, the terms and expressions employed herein have been chosen for the purpose of describing the illustrative examples for the convenience of the reader and are not for the purpose of limitation thereof. Also, it will be appreciated that one or more of the following-described aspects, expressions of aspects, and/or examples, can be combined with any one or more of the other following-described aspects, expressions of aspects and/or examples.

Various aspects are directed to neural interventional magnetic resonance imaging (MRI) devices that allows for the integration of surgical intervention and guidance with an MRI. This includes granting physical access to the area around the patient as well as access to the patient's head with one or more access apertures. In addition, the neural interventional MRI device may allow for the usage of robotic guidance tools and/or traditional surgical implements. In various instances, a neural interventional MRI can be used intraoperatively to obtain scans of a patient's head and/or brain during a surgical intervention, such as a surgical procedure like a biopsy or neural surgery.

FIG.1depicts a MRI scanning system100that includes a dome-shaped housing102configured to receive a patient's head. The dome-shaped housing102can further include at least one access aperture configured to allow access to the patient's head to enable a neural intervention. A space within the dome-shaped housing102forms the region of interest for the MRI scanning system100. Target tissue in the region of interest is subjected to magnetization fields/pulses, as further described herein, to obtain imaging data representative of the target tissue.

For example, referring toFIG.1A, a patient can be positioned such that his/her head is positioned within the region of interest within the dome-shaped housing102. The brain can be positioned entirely within the dome-shaped housing102. In such instances, to facilitate intracranial interventions (e.g. neurosurgery) in concert with MR imaging, the dome-shaped housing102can include one or more apertures that provide access to the brain. Apertures can be spaced apart around the perimeter of the dome-shaped housing.

The MRI scanning system100can include an auxiliary cart (see, e.g. auxiliary cart540inFIG.6) that houses certain conventional MRI electrical and electronic components, such as a computer, programmable logic controller, power distribution unit, and amplifiers, for example. The MRI scanning system100can also include a magnet cart that holds the dome-shaped housing102, gradient coil(s), and/or a transmission coil, as further described herein. Additionally, the magnet cart can be attached to a receive coil in various instances. Referring primarily toFIG.1, the dome-shaped housing102can further include RF transmission coils, gradient coils104(depicted on the exterior thereof), and shim magnets106(depicted on the interior thereof). Alternative configurations for the gradient coil(s)104and/or shim magnets106are also contemplated. In various instances, the shim magnets106can be adjustably positioned in a shim tray within the dome-shaped housing102, which can allow a technician to granularly configure the magnetic flux density of the dome-shaped housing102.

Various structural housings for receiving the patient's head and enabling neural interventions can be utilized with a MRI scanning system, such as the MRI scanning system100. In one aspect, the MRI scanning system100may be outfitted with an alternative housing, such as a dome-shaped housing202(FIG.2) or a two-part housing302(FIG.3) configured to form a dome-shape. The dome-shaped housing202defines a plurality of access apertures203; the two-part housing302also defines a plurality of access apertures303and further includes an adjustable gap305between the two parts of the housing.

In various instances, the housings202and302can include a bonding agent308, such as an epoxy resin, for example, that holds a plurality of magnetic elements310in fixed positions. The plurality of magnetic elements310can be bonded to a structural housing312, such as a plastic substrate, for example. In various aspects, the bonding agent308and structural housing312may be non-conductive or diamagnetic materials. Referring primarily toFIG.3, the two-part housing302comprises two structural housings312. In various aspect, a structural housing for receiving the patient's head can be formed from more than two sub-parts. The access apertures303in the structural housing312provide a passage directly to the patient's head and are not obstructed by the structural housing312, bonding agent308, or magnetic elements310. The access apertures303can be positioned in an open space of the housing302, for example.

There are many possible configurations of neural interventional MRI devices that can achieve improved access for surgical intervention. Many configurations build upon two main designs, commonly known as the Halbach cylinder and the Halbach dome described in the following article: Cooley et al. (e.g. Cooley, C. Z., Haskell, M. W., Cauley, S. F., Sappo, C., Lapierre, C. D., Ha, C. G., Stockmann, J. P., & Wald, L. L. (2018). Design of sparse Halbach magnet arrays for portable MRI using a genetic algorithm.IEEE transactions on magnetics,54(1), 5100112. The article “Design of sparse Halbach magnet arrays for portable MRI using a genetic algorithm” by Cooley et al., published inIEEE transactions on magnetics,54(1), 5100112 in 2018, is incorporated by reference herein in its entirety.

In various instances, a dome-shaped housing for an MRI scanning system, such as the system100, for example, can include a Halbach dome defining a dome shape and configured based on several factors including main magnetic field B0strength, field size, field homogeneity, device size, device weight, and access to the patient for neural intervention. In various aspects, the Halbach dome comprises an exterior radius and interior radius at the base of the dome. The Halbach dome may comprise an elongated cylindrical portion that extends from the base of the dome. In one aspect, the elongated cylindrical portion comprises the same exterior radius and interior radius as the base of the dome and continues from the base of the dome at a predetermined length, at a constant radius. In another aspect, the elongated cylindrical portion comprises a different exterior radius and interior radius than the base of the dome (see e.g.FIGS.2and3). In such instances, the different exterior radius and interior radius of the elongated cylindrical portion can merge with the base radii in a transitional region.

FIG.4illustrates an exemplary Halbach dome400for an MRI scanning system, such as the system100, for example, which defines an access aperture in the form of a hole or access aperture403, where the dome400is configured to receive a head and brain B of the patient P within the region of interest therein, and the access aperture403is configured to allow access to the patient P to enable neural intervention with a medical instrument and/or robotically-controlled surgical tool, in accordance with at least one aspect of the present disclosure. The Halbach dome400can be built with a single access aperture403at the top side418of the dome400, which allows for access to the top of the skull while minimizing the impact to the magnetic field. Additionally or alternatively, the dome300can be configured with multiple access apertures around the structure416of the dome400, as shown inFIGS.2and3.

The diameter pride of the access aperture403may be small (e.g. about 2.54 cm) or very large (substantially the exterior rextdiameter of the dome400). As the access aperture403becomes larger, the dome400begins to resemble a Halbach cylinder, for example. The access aperture403is not limited to being at the apex of the dome400. The access aperture403can be placed anywhere on the surface or structure416of the dome400. In various instances, the entire dome400can be rotated so that the access aperture403can be co-located with a desired physical location on the patient P.

FIG.5depicts relative dimensions of the Halbach dome400, including a diameter Dholeof the access aperture403, a length L of the dome400, and an exterior radius rextand an interior radius rinof the dome400. The Halbach dome400comprises a plurality of magnetic elements that are configured in a Halbach array and make up a magnetic assembly. The plurality of magnetic elements may be enclosed by the exterior radius rextand interior radius rinin the structure416or housing thereof. In one aspect, example dimensions may be defined as: rin=19.3 cm; rext=23.6 cm; L=38.7 cm; and 2.54 cm≤D<19.3 cm.

Based on the above example dimensions, a Halbach dome400with an access aperture403may be configured with a magnetic flux density B0of around 72 mT, and an overall mass of around 35 kg. It will be appreciated that the dimensions may be selected based on particular applications to achieve a desired magnetic flux density B0, total weight of the Halbach dome400and/or magnet cart, and geometry of the neural intervention access aperture403.

In various aspects, the Halbach dome400may be configured to define multiple access apertures403placed around the structure416of the dome400. These multiple access apertures403may be configured to allow for access to the patient's head and brain B using tools (e.g., surgical tools) and/or a surgical robot.

In various aspects, the access aperture403may be adjustable. The adjustable configuration may provide the ability for the access aperture403to be adjusted using either a motor, mechanical assist, or a hand powered system with a mechanical iris configuration, for example, to adjust the diameter D hole of the access aperture403. This would allow for configuration of the dome without an access aperture403, conducting an imaging scan, and then adjusting the configuration of the dome400and mechanical iris thereof to include the access aperture403and, thus, to enable a surgical intervention therethrough.

Halbach domes and magnetic arrays thereof for facilitating neural interventions are further described in International Patent Application No. PCT/US2022/72143, titled NEURAL INTERVENTIONAL MAGNETIC RESONANCE IMAGING APPARATUS, filed May 5, 2022, which is incorporated by reference herein in its entirety.

Referring now toFIG.6, a schematic for an MRI system500is shown. The MRI scanning system100(FIG.1) and the various dome-shaped housings and magnetic arrays therefor, which are further described herein, for example, can be incorporated into the MRI system500, for example. The MRI system500includes a housing502, which can be similar in many aspects to the dome-shaped housings102(FIG.1),202(FIG.2), and/or302(FIG.3), for example. The housing502is dome-shaped and configured to form a region of interest, or field of view,552therein. For example, the housing502can be configured to receive a patient's head in various aspects of the present disclosure.

The housing502includes a magnet assembly548having a plurality of magnets arranged therein (e.g. a Halbach array of magnets). In various aspect, the main magnetic field B0, generated by the magnetic assembly548, extends into the field of view552, which contains an object (e.g. the head of a patient) that is being imaged by the MRI system500.

The MRI system500also includes RF transmit/receive coils550. The RF transmit/receive coils550are combined into integrated transmission-reception (Tx/Rx) coils. In other instances, the RF transmission coil can be separate from the RF reception coil. For example, the RF transmission coil(s) can be incorporated into the housing502and the RF reception coil(s) can be positioned within the housing502to obtain imaging data.

The housing502also includes one or more gradient coils504, which are configured to generate gradient fields to facilitate imaging of the object in the field of view552generated by the magnet assembly548, e.g., enclosed by the dome-shaped housing and dome-shaped array of magnetic elements therein. Shim trays adapted to receive shim magnets506can also be incorporated into the housing502.

During the imaging process, the main magnetic field B0extends into the field of view552. The direction of the effective magnetic field (B1) changes in response to the RF pulses and associated electromagnetic fields transmitted by the RF transmit/receive coils550. For example, the RF transmit/receive coils550may be configured to selectively transmit RF signals or pulses to an object in the field of view552, e.g. tissue of a patient's brain. These RF pulses may alter the effective magnetic field experienced by the spins in the sample tissue.

The housing502is in signal communication with an auxiliary cart530, which is configured to provide power to the housing502and send/receive control signals to/from the housing502. The auxiliary cart530includes a power distribution unit532, a computer542, a spectrometer544, a transmit/receive switch545, an RF amplifier546, and gradient amplifiers558. In various instances, the housing502can be in signal communication with multiple auxiliary carts and each cart can support one or more of the power distribution unit532, the computer542, the spectrometer544, the transmit/receive switch545, the RF amplifier546, and/or the gradient amplifiers558.

The computer542is in signal communication with a spectrometer544and is configured to send and receive signals between the computer542and the spectrometer544. When the object in the field of view552is excited with RF pulses from the RF transmit/receive coils550, the precession of the object results in an induced electric current, or MR current, which is detected by the RF transmit/receive coils550and sent to the RF preamplifier556. The RF preamplifier556is configured to boost or amplify the excitation data signals and send them to the spectrometer544. The spectrometer544is configured to send the excitation data to the computer542for storage, analysis, and image construction. The computer542is configured to combine multiple stored excitation data signals to create an image, for example. In various instances, the computer542is in signal communication with at least one database562that stores reconstruction algorithms564and/or pulse sequences566. The computer542is configured to utilize the reconstruction algorithms to generate an MR image568.

From the spectrometer544, signals can also be relayed to the RF transmit/receive coils550in the housing502via an RF power amplifier546and the transmit/receive switch545positioned between the spectrometer544and the RF power amplifier546. From the spectrometer544, signals can also be relayed to the gradient coils560in the housing502via a gradient power amplifier558. For example, the RF power amplifier546is configured to amplify the signal and send it to RF transmission coils560, and the gradient power amplifier558is configured to amplify the gradient coil signal and send it to the gradient coils560.

In various instances, the MRI system500can include noise cancellation coils554. For example, the auxiliary cart530and/or computer542can be in signal communication with noise cancellation coils554. In other instances, the noise cancellation coils554can be optional. For example, certain MRI systems disclosed herein may not include supplemental/auxiliary RF coils for detecting and canceling electromagnetic interference, i.e. noise.

A flowchart depicting a process570for obtaining an MRI image is shown inFIG.7. The flowchart can be implemented by the MRI system500, for example. In various instances, at block572, the target subject (e.g. a portion of a patient's anatomy), is positioned in a main magnetic field B0in an interest of region (e.g. region of interest552), such as within the dome-shaped housing of the various MRI scanners further described herein (e.g. magnet assembly548). The main magnetic field B0is configured to magnetically polarize the hydrogen protons (1H-protons) of the target subject (e.g. all organs and tissues) and is known as the net longitudinal magnetization M0. It is proportional to the proton density (PD) of the tissue and develops exponentially in time with a time constant known as the longitudinal relaxation time T1 of the tissue. T1 values of individual tissues depend on a number of factors including their microscopic structure, on the water and/or lipid content, and the strength of the polarizing magnetic field, for example. For these reasons, the T1 value of a given tissue sample is dependent on age and state of health.

At block574, a time varying oscillatory magnetic field B1, i.e. an excitation pulse, is applied to the magnetically polarized target subject with a RF coil (e.g. RF transmit/receive coil550). The carrier frequency of the pulsed B1field is set to the resonance frequency of the 1H-proton, which causes the longitudinal magnetization to flip away from its equilibrium longitudinal direction resulting in a rotated magnetization vector, which in general can have transverse as well as longitudinal magnetization components, depending on the flip angle used. Common B1pulses include an inversion pulse, or a 180-degree pulse, and a 90-degree pulse. A 180-degree pulse reverses the direction of the 1H-proton's magnetization in the longitudinal axis. A 90-degree pulse rotates the 1H-proton's magnetization by 90 degrees so that the magnetization is in the transverse plane. The MR signals are proportional to the transverse components of the magnetization and are time varying electrical currents that are detected with suitable RF coils. These MR signals decay exponentially in time with a time constant known as the transverse relaxation time T2, which is also dependent on the microscopic tissue structure, water/lipid content, and the strength of the magnetic field used, for example.

At block576, the MR signals are spatially encoded by exposing the target subject to additional magnetic fields generated by gradient coils (e.g. gradient coils560), which are known as the gradient fields. The gradient fields, which vary linearly in space, are applied for short periods of time in pulsed form and with spatial variations in each direction. The net result is the generation of a plurality of spatially encoded MR signals, which are detected at block577, and which can be reconstructed to form MR images depicting slices of the examination subject. A RF reception coil (e.g. RF transmit/receive coil550) can be configured to detect the spatially-encoded RF signals. Slices may be oriented in the transverse, sagittal, coronal, or any oblique plane.

At block578, the spatially encoded signals of each slice of the scanned region are digitized and spatially decoded mathematically with a computer reconstruction program (e.g. by computer542) in order to generate images depicting the internal anatomy of the examination subject. In various instances, the reconstruction program can utilize an (inverse) Fourier transform to back-transforms the spatially-encoded data (k-space data) into geometrically decoded data.

FIG.8depicts a graphical illustration of a robotic system680that may be used for neural intervention with an MRI scanning system600. The robotic system680includes a computer system696and a surgical robot682. The MRI scanning system600can be similar to the MRI system500and can include the dome-shaped housing and magnetic arrays having access apertures, as further described herein. For example, the MRI system500can include one or more access apertures defined in a Halbach array of magnets in the permanent magnet assembly to provide access to one or more anatomical parts of a patient being imaged during a medical procedure. In various instances, a robotic arm and/or tool of the surgical robot682is configured to extend through an access aperture in the permanent magnet assembly to reach a patient or target site. Each access aperture can provide access to the patient and/or surgical site. For example, in instances of multiple access apertures, the multiple access apertures can allow access from different directions and/or proximal locations.

In accordance with various embodiments, the robotic system680is configured to be placed outside the MRI system600. As shown inFIG.8, the robotic system680can include a robotic arm684that is configured for movements with one or more degrees of freedom. In accordance with various embodiments, the robotic arm684includes one or more mechanical arm portions, including a hollow shaft686and an end effector688. The hollow shaft686and end effector688are configured to be moved, rotated, and/or swiveled through various ranges of motion via one or more motion controllers690. The double-headed curved arrows inFIG.8signify exemplary rotational motions produced by the motion controllers690at the various joints in the robotic arm684.

In accordance with various embodiments, the robotic arm684of the robotic system682is configured for accessing various anatomical parts of interest through or around the MRI scanning system600. In accordance with various embodiments, the access aperture is designed to account for the size of the robotic arm684. For example, the access aperture defines a circumference that is configured to accommodate the robotic arm684, the hollow shaft686, and the end effector688therethrough. In various instances, the robotic arm684is configured for accessing various anatomical parts of the patient from around a side of the magnetic imaging apparatus600. The hollow shaft686and/or end effector688can be adapted to receive a robotic tool692, such as a biopsy needle having a cutting edge694for collecting a biopsy sample from a patient, for example.

The reader will appreciate that the robotic system682can be used in combination with various dome-shaped and/or cylindrical magnetic housings further described herein. Moreover, the robotic system682and robotic tool692inFIG.8are exemplary. Alternative robotic systems can be utilized in connection with the various MRI systems disclosed herein. Moreover, handheld surgical instruments and/or additional imaging devices (e.g. an endoscope) and/or systems can also be utilized in connection with the various MRI systems disclosed herein.

In various aspects of the present disclosure, the MRI systems described herein can comprise low field MRI (LF-MRI) systems. In such instances, the main magnetic field B0generated by the permanent magnet assembly can be between 0.1 T and 1.0 T, for example. In other instances, the MRI systems described herein can comprise ultra-low field MRI (ULF-MRI) systems. In such instances, the main magnetic field B0generated by the permanent magnet assembly can be between 0.03 T and 0.1 T, for example.

Higher magnetic fields, such as magnetic fields above 1.0 T, for example, can preclude the use of certain electrical and mechanical components in the vicinity of the MRI scanner. For example, the existence of surgical instruments and/or surgical robot components comprising metal, specially ferrous metals, can be dangerous in the vicinity of higher magnetic fields because such tools can be drawn toward the source of magnetization. Moreover, higher magnetic fields often require specifically-designed rooms with additional precautions and shielding to limit magnetic interference. Despite the limitations on high field MRI systems, low field and ultra-low field MRI systems present various challenges to the acquisition of high quality images with sufficient resolution for achieving the desired imaging objectives.

LF- and ULF-MRI systems generally define an overall magnetic field homogeneity that is relatively poor in comparison to higher field MRI systems. For example, a dome-shaped housing for an array of magnets, as further described herein, can comprise a Halbach array of permanent magnets, which generate a magnetic field B0having a homogeneity between 1,000 ppm and 10,000 ppm in the region of interest in various aspects of the present disclosure.

In certain instances, MRI systems can include a Faraday shield configured to reject environmental and system sources of electromagnetic interference (EMI). For example, for MRI systems operating at and/or above 0.5 T, the examination room holding the MRI system can be screened with a Faraday shield. Various components in the examination room must be filtered to avoid violating the shield's integrity. For example, the electrical cables in the room for supplying power to the gradient coils may require filters. In various instances, Faraday shielding may also be appropriate for certain LF-MRI systems, such as MRI systems that operate at and/or below 1.0 T and/or ULF-MRI systems, such as MRI systems that operate at and/or below 0.1 T.

Faraday shields reject EMI in the radiofrequency range by as much as 100 dB, yielding MR images with signal-to-noise ratios (SNR) that are limited either by subject body noise (at high fields) or by Johnson noise in the electronics (at low fields), both of which are Gaussian. However, such shields may not be applicable with MRI systems designed for use in a neurosurgical suite or operating room (OR). In such instances, a LF-MRI system may be used and the system may be completely unshielded or have only partial shielding from sources of EMI. Without further mitigation of the EMI, EMI sources can produce artifacts and patterns in the MR images, such as intense stripes, for example.

In certain instances, minimally-shielded and/or unshielded LF-MRI systems can incorporate one or more auxiliary EMI detection coils in concert with the primary imaging RF coil(s). While the addition of a supplemental EMI detection coil may be effective in certain instances, the additional coils increase the complexity of the MRI system and require additional space for the coils and their cables, which may interfere with other systems in the OR, such as life support systems, anesthesia equipment, surgical robots, and imaging/visualization systems like exoscopes and other neuro-navigation tools that require a clear line of sight to the surgical portal, for example.

Instead of relying on auxiliary EMI detection coils, in various aspects of the present disclosure, a MRI system can implement a method for removing EMI from LF-MRI images by sampling additional MR signals within a time of repetition (TR) for a MRI pulse sequence and using the additional sample signal(s) to determine the EMI, which can be subtracted or otherwise removed from the imaging data before the MR image is generated.

In various instances, the EMI detection and removal can be applied on channel-by-channel basis. For example, each RF receiver channel can be sampled to collect reference data for that channel. EMI can be detected and removed from a single channel transmit-and-receive RF coil in certain instances, and, in other instances, from a multi-channel RF receiver, such as a phased array coil, for example.

LF-MRI systems generally define an overall magnetic field homogeneity that is relatively poor in comparison to higher field MRI systems. An increased magnetic field inhomogeneity in the region of interest further corresponds to a shorter MR signal. For example, a dome-shaped housing for an array of magnets, as further described herein, can comprise a Halbach array of permanent magnets, which generate a magnetic field Bo having a homogeneity between 1,000 ppm and 10,000 ppm in the region of interest in various aspects of the present disclosure. For example, a magnet housing that corresponds to a homogeneity at the lower end of the range, i.e. having a homogeneity of 1,000 ppm in the region of interest, and configured to generate a magnetic field of about 80 mT, corresponds to an MR signal linewidth of about 3.5 kHz and a signal time (T2*) of less than 0.3 ms. In such instances, a MR signal would only be detectable for approximately five times the T2*, which corresponds to about 1.5 ms and, in the case of a spin echo, the echo envelope would last for twice this detection time, which corresponds to about 3.0 ms. The foregoing time calculations are based on a homogeneity of 1,000 ppm; however, LF-MRI systems having an increased inhomogeneity, e.g. worse than 1,000 ppm, would produce correspondingly shorter lifetimes. In various instances, the shorter lifespan of the MR signals arising from a relatively heterogeneous LF-MRI systems and array of permanent magnets therefor can provide an opportunity for acquiring additional sample signals before and/or after the desired MR acquisition signal.

For example, in one aspect of the present disclosure, EMI, or signal noise, can be removed from imaging data obtained with a LF-MRI system configured to project a low-field strength magnetic field toward an object of interest (e.g. a patient's brain) located within a field of view. In various instances, a RF pulse sequence can be transmitted to a RF coil assembly configured to selectively excite magnetization in the object of interest within the field of view; the RF pulse sequence can define a time of repetition (TR) and comprise a signal acquisition period within each TR. Moreover, an output signal can be obtained from the RF coil assembly during the signal acquisition period and at least one sample signal can be obtained from the RF coil assembly during an interference period also within the TR. In various instances, the interference period can be temporally adjacent to the signal acquisition period and/or a pair of interference periods can flank either side of the signal acquisition period. The method can further comprise comparing the output signal and the sample signal to identify an interference component, adjusting the output signal based on the interference component, and recording the adjusted output signal in a k-space matrix. The k-space matrix can be used to generate an MR image without (or substantially without) artifacts results from the EMI.

In various instances, mitigation of EMI is essential for successful operation of LF-MRI systems in an unshielded or only partially-shielded environments. Moreover, the foregoing system and method can eliminate or reduce the need forextensive Faraday shielding or auxiliary coils and sensors for detecting the EMI in the system and/or environment. Instead, the properties of the LF-MRI signals and EM interference are exploited in a temporal method using standard RF reception coil(s) and/or coil arrays to collect data for EM interference mitigation during periods when the RF reception coil(s) and/or coil arrays are not typically collecting RF signals and/or when a coherent MR signal is not present.

Referring now toFIG.9, an exemplary pulse sequence diagram1000for an MRI system, such as the MRI scanning system100(FIG.1), is shown. The pulse sequence diagram1000is a multi-echo pulse sequence; however, the reader will appreciate that alternative pulse sequence diagrams are contemplated, such as any type of pulse sequence that includes dead periods between RF pulses and acquisition periods. For example, a multiple gradient echo sequence, or a combined gradient- and spin echo sequence may be implemented. The pulse sequence diagram1000includes a RF transmission waveform1002, a RF reception waveform1004(e.g., RF reception windows1004), a first gradient waveform1006, a second gradient waveform1008, and a third gradient waveform1010. The first gradient waveform1006and the third gradient waveform1010correspond to waveforms for phase encoding, in various aspects of the present disclosure. The second gradient waveform1008corresponds to a waveform for a readout gradient, or frequency encoding gradient, in various aspects of the present disclosure.

As shown, the readout gradient waveform1008is “on” during each MR signal acquisition period S1, S2, . . . Sn, where n is an integer representing the number of echo pulses applied in the pulse sequence and, thus, the number of signal acquisition periods in the pulse sequence. The pulse sequence diagram1000further depicts a pair of interference periods L1and R1, L2and R2, . . . Lnand Rnflanking each signal acquisition period S1, S2, . . . Sn. Referring primarily toFIG.10, the signal acquisition periods S1and S2are shown in relation of the interference periods L1, R1, L2, and R2along a timeline1012representing a portion of the pulse sequence diagram1000.

Referring again toFIG.9, the interference periods L1and R1, L2and R2, Lnand Rnare coincident with gradient switching periods, which are times in which a MR signal is not usually collected. In various instances, the interference period(s) in a pulse sequence can occur whenever (1) a RF pulse is not being applied and (2) a MR signal for imaging data is not being collected. For example, the RF reception coil assembly typically does not collect data while phase encoding gradients are being applied.

In various aspects of the present disclosure, the interference period(s) may not be coincident with gradient switching periods. For example, the interference period(s) may define a “dead period” during which no RF pulse or gradient(s) are being applied. Thus, in various instances, RF signals may be collected during the interference period(s) when no RF pulse or gradient(s) are being applied.

Each pair of interference periods L1and R1, L2and R2, . . . Lnand Rnis positioned adjacent to, or flanking, a signal acquisition period S1, S2, . . . Sn. Stated differently, the pair of interference periods L1and R1, L2and R 2, . . . Lnand Rncan be symmetric about the corresponding signal acquisition period S1, S2, . . . Sn. Each “pair” of interference periods includes a left-side interference period (e.g. L1, L2) and a right-side interference period (e.g. R1, R2); however, the reader will appreciate that a single interference period can be associated with each MR signal acquisition period in alternative aspects of the present disclosure. In other words, the interference periods may not always come in pairs that flank the signal acquisition period. In still other aspects of the present disclosure, a signal acquisition period can be associated with more than two interference periods.

In other aspects of the present disclosure, the RF transmission coil can be operated continuously during an imaging procedure. In such instances, the signal acquisition periods S1, S2, etc. and the interference periods L1, R1, L2, R2, etc. can be determined and defined post-processing by a control circuit or computer associated with the MRI system (e.g. computer542inFIG.6).

In various instances, the relative durations of the signal acquisition period S1, S2, . . . Snand the corresponding interference period(s) L1and R1, L2and R2, Lnand Rncan vary. The relative durations can be determined based on optimal noise reduction performance. In at least one aspect of the present disclosure, each interference period can match the duration of the signal acquisition period. In other instances, the sum of the interference periods flanking each signal acquisition period can match the during of the signal acquisition period. In still other instances, each interference period or the sum of the interference periods flanking each signal acquisition period can be shorter than or longer than the corresponding signal acquisition period. The relative durations of the signal acquisition periods and interference periods can be constant throughout the duration of the pulse sequence, in various aspects of the present disclosure.

Upon collection of the RF signals during the signal acquisition periods S1, S2, . . . Snand the interference periods L1and R1, L2and R2, . . . Lnand Rn, the MRI system and/or a data processing unit coupled thereto is configured to analyze the data to reject the EMI from the MR signal. Referring primarily toFIG.11, a flowchart for processing the signal data collected during a portion of the pulse sequence ofFIG.9is shown. In various instances, the system can collect the RF signal as raw data at block1022for each interference period and signal acquisition period in the pulse sequence. In various instances, because the interference periods and signal acquisition periods are very short (e.g. 1-3 ms each), any RF interference that arises during both the interference periods (e.g. L1and R1) can also be assumed to be present during the signal acquisition period (e.g. S1) therebetween, for example.

The raw data (e.g. for the interference periods L1, R1and the signal acquisition period S1) can then be Fourier transformed to create their corresponding frequency domain spectra depicted for the interference periods Ln(f), Rn(f) and the signal acquisition period Sn(f) at block1024. For example, common peaks in the raw data associated with each signal acquisition period (e.g. the interference period(s) L1, R1flanking the signal acquisition period S1) can be identified to create interference correction spectra Cn(f) for each signal acquisition period Sn. Determination of the common peaks can be determined by correlation, principal component analysis (PCA), or other known analytical techniques used in signal processing.

In various instances, the correction spectrum Cn(f) can be based on either Lnalone, Rnalone, or an average of Lnand Rn, for example. For example, peaks that arise with similar frequency and magnitude in both L1and R1can be more likely to originate from EMI than from the MR signal. Additionally, signals that are detected only during the left-side interference period Lnand not the right-side interference period Rn, or only right-side interference period Rnand not during the left-side interference period Ln, may also be subtracted from the MR signal spectrum Sn(f) the corresponding signal acquisition period Sn, in various instances. In other instances, signals that are detected only during the left-side interference period Lnand not the right-side interference period Rn, or only right-side interference period Rnand not during the left-side interference period Ln, may be retained in the MR signal spectrum Sn(f) the corresponding signal acquisition period Snin instances in which they represent the true MR signal bleeding into one of the interference periods Ln, Rn. For example, a single interference period can be used for each signal acquisition period and spurious signals can be determined with, for example, a thresholding technique.

In various instances, the foregoing EMI mitigation procedure can be applied separately to each line of time domain data collected in the full pulse sequence with each separate line of k-space being cleaned using the adjacent control period pair, for example.

In certain aspects of the present disclosure, reference data from more than one pair of interference periods could be used to determine the EM interference in any one MR signal acquisition period. For example, a model can be built to determine the interference correction spectra Cn(f) using reference data spanning the entire image acquisition time period. In one aspect of the present disclosure, some or all of the reference data spanning the entire image acquisition time period can be averaged together to form a superset of reference data in which persistent sources of EM interference dominate. This superset of reference date may be used to determine the interference correction spectra Cn(f).

Referring now to block1026inFIG.11, the correction spectrum Cn(f) can be subtracted from the corresponding signal acquisition spectrum Sn(f) to produce a cleaned signal spectrum SCn(f). Having subtracted the undesirable frequency components from the MR signal acquisition period spectrum Sn(f), the cleaned signal spectrum SCn(f) from period Snis Fourier transformed back to the time domain at block1028. Upon reverse Fourier transforming the MR signal from the signal acquisition period, the corresponding line of cleaned k-space can be provided to a standard MR image reconstruction algorithm at block1030and/or stored for formation of a 2D- or 3D MR image according to known MR image creation techniques, for example.

Examples

Various additional aspects of the subject matter described herein are set out in the following numbered examples:

Example 1: A method, comprising: projecting a low-field strength magnetic field toward an object of interest located within a field of view; transmitting a radio frequency pulse sequence to a radio frequency coil assembly configured to selectively excite magnetization in the object of interest within the field of view, wherein the radio frequency pulse sequence defines a time of repetition and comprises a signal acquisition period within each time of repetition; receiving an output signal from the radio frequency coil assembly during the signal acquisition period; receiving a sample signal from the radio frequency coil assembly at an interference period within the time of repetition, wherein the interference period flanks the signal acquisition period; comparing the output signal and the sample signal to identify an interference component; adjusting the output signal based on the interference component; and recording the adjusted output signal in a k-space matrix.

Example 2: The method of Example 1, wherein comparing the output signal and the sample signal to identify the interference component comprises performing a Fourier transform computation on the output signal and the sample signal.

Example 3: The method of any one of Example 1 and 2, wherein the interference component comprises undesirable frequency components in the Fourier transformed sample signal.

Example 4: The method of Example 3, wherein adjusting the output signal based on the interference component comprises: subtracting the undesirable frequency components from the Fourier transformed output signal to determine the desirable frequency components; and performing a Fourier transform inversion computation on the desirable frequency components.

Example 5: The method of any one of Examples 1-4, wherein the interference period comprises a pair of interference periods flanking the signal acquisition period, wherein the pair of interference periods comprises a first interference period and a second interference period, and wherein the first interference period and the second interference period are symmetric with respect to the signal acquisition period therebetween.

Example 6: The method of Example 5, wherein the sample signal comprises a first sample signal received in the first interference period and a second sample signal received in the second interference period.

Example 7: The method of Example 6, wherein comparing the output signal to the sample signal to identify the interference component comprises comparing the first sample signal to the second sample signal to identify interference arising during both the first sample signal and the second sample signal.

Example 8: The method of any one of Examples 1 and 5-7, wherein the radio frequency coil assembly comprises a coil array comprising a plurality of radio frequency coil components, wherein the radio frequency pulse sequence further comprises, for each radio frequency coil component, a component signal acquisition period and a component interference period, and wherein the method further comprises, for each component signal acquisition period: receiving the output signal from the radio frequency coil component during the component signal acquisition period; receiving the sample signal from the radio frequency coil component at the component interference period; comparing the output signal and the sample signal for each radio frequency coil component to identify the interference component; adjusting the output signal based on the interference component for each radio frequency coil component; and recording the adjusted output signal in the k-space matrix for each radio frequency coil component.

Example 9: The method of any one of Examples 1 and 5-7, wherein the low-field strength magnetic field comprises a homogeneity between 1000 and 20,000 ppm.

Example 10: The method of any one of Examples 1 and 5-7, wherein the low-field strength magnetic field comprises a heterogeneous field.

Example 11: The method of any one of Examples 1 and 5-10, wherein the low-field strength magnetic field comprises a field strength of less than 1 T.

Example 12: The method of any one of Examples 1 and 5-11, wherein the signal acquisition period is less than 3 ms.

Example 13: The method of any one of Examples 1 and 5-12, wherein the radio frequency pulse sequence comprises a multi-echo pulse sequence.

Example 14: The method of any one of Examples 1 and 5-13, wherein the radio frequency pulse sequence comprises applying a phase encoding gradient along a first gradient within the time of repetition, and wherein the phase encoding gradient temporally overlaps the interference period.

Example 15: The method of any one of Examples 1 and 5-14, wherein the radio frequency pulse sequence comprises applying a phase encoding gradient along a first gradient and a second gradient within the time of repetition, wherein the first gradient and the second gradient are orthogonal, and wherein the phase encoding gradients temporally overlap the interference period.

Example 16: The method of any one of Examples 1 and 5-15, wherein the radio frequency pulse sequence comprises at least one gradient episode coincident with the interference period, and wherein the at least one gradient episode is selected from a list of gradients consisting of a frequency encoding gradient, a prewinding gradient (e.g. rewinding gradient), and a postwinding gradient (e.g., slice selection refocusing gradient).

Example 17: The method of any one of Examples 1 and 5-16, wherein a plurality of gradient episodes are coincident with the interference period.

Example 18: The method of any one of any one of Examples 1 and 5-6, wherein the radio frequency pule sequence comprises applying one or more than one gradient episode within the time of repetition, and wherein the one or more than one gradient episode is not applied while receiving the sample signal.

Example 19: The method of any one of claims1and5-18, further comprising generating a magnetic resonance image from the k-space matrix.

Example 20: The method of any one of claims1-2and5-19, wherein the radio frequency coil assembly comprises multi-channel transmit-and-receive coil array.

Example 21: A system, comprising: an array of magnets configured to generate a low-field strength magnetic field toward an object of interest located within a field of view; a radio frequency coil assembly configured to selectively excite magnetization in the object of interest in the field of view; a control circuit comprising a processor and a memory, wherein the memory stores instructions executable by the processor to: transmit a radio frequency pulse sequence to the radio frequency coil assembly, wherein the radio frequency pulse sequence defines a time of repetition, wherein the radio frequency pulse sequence comprises a signal acquisition period and at least one interference period within each time of repetition, wherein each interference period flanks the signal acquisition period; receive an output signal from the radio frequency coil assembly during the signal acquisition period; receive a sample signal from the radio frequency coil assembly during the interference period within the time of repetition; compare the output signal and the sample signal to identify an interference component; adjust the output signal based on the interference component; and record the adjusted output signal in a k-space matrix.

As used in any aspect herein, the terms “component,” “system,” “module” and the like can refer to a control circuit computer-related entity, either hardware, a combination of hardware and software, software, or software in execution.

A network may include a packet switched network. The communication devices may be capable of communicating with each other using a selected packet switched network communications protocol. One example communications protocol may include an Ethernet communications protocol which may be capable permitting communication using a Transmission Control Protocol/Internet Protocol (TCP/IP). The Ethernet protocol may comply or be compatible with the Ethernet standard published by the Institute of Electrical and Electronics Engineers (IEEE) titled “IEEE 802.3 Standard”, published in December, 2008 and/or later versions of this standard. Alternatively or additionally, the communication devices may be capable of communicating with each other using an X.25 communications protocol. The X.25 communications protocol may comply or be compatible with a standard promulgated by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T). Alternatively or additionally, the communication devices may be capable of communicating with each other using a frame relay communications protocol. The frame relay communications protocol may comply or be compatible with a standard promulgated by Consultative Committee for International Telegraph and Telephone (CCITT) and/or the American National Standards Institute (ANSI). Alternatively or additionally, the transceivers may be capable of communicating with each other using an Asynchronous Transfer Mode (ATM) communications protocol. The ATM communications protocol may comply or be compatible with an ATM standard published by the ATM Forum titled “ATM-MPLS Network Interworking 2.0” published August 2001, and/or later versions of this standard. Of course, different and/or after-developed connection-oriented network communication protocols are equally contemplated herein.