Various systems and methods for T2-weighted and/or diffusion weighted chirped-CPMG sequences are disclosed herein. In one aspect, a method can include projecting a magnetic field along a longitudinal axis and toward an object of interest and transmitting a radio frequency pulse sequence to a radio frequency coil assembly configured to selectively excite magnetization in the object of interest. The radio frequency pulse sequence can include a frequency-swept excitation pulse and a series of frequency-swept refocusing pulses following the excitation pulse. In some aspects, the radio frequency pulse sequence can include a frequency-swept recovery pulse following the series of refocusing pulses. In some aspects, the method can include transmitting a preparation radio frequency pulse sequence to the radio frequency coil assembly.

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

According to one aspect of the present disclosure, a method is disclosed. The method can include projecting a magnetic field along a longitudinal axis and 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 include an excitation pulse, a series of refocusing pulses following the excitation pulse, and a recovery pulse following the series of refocusing pulses. The excitation pulse can be frequency swept across a frequency offset range. Each of the refocusing pulses can be frequency swept across the frequency offset range. Each of the refocusing pulses can be half the duration of the excitation pulse. The recovery pulse can be frequency swept across the frequency offset range. The method can further include receiving an output signal detected by the radio frequency coil assembly intermediate two of the refocusing pulses.

According to another aspect of the present disclosure, a system is disclosed. The system can include an array of magnets, a radio frequency coil assembly, and a 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 preparation radio frequency pulse sequence to the radio frequency coil assembly and transmit a primary radio frequency pulse sequence to the radio frequency coil assembly. The primary radio frequency pulse sequence can include an excitation pulse and a series of refocusing pulses following the excitation pulse. The excitation pulse can be frequency swept across a frequency offset range at a first rate. The excitation pulse is a 90° pulse. Each of the refocusing pulses can be frequency swept across the frequency offset range at a second rate that is twice the first rate. Each of the refocusing pulses are 180° pulses and can be half the duration of the excitation pulse. The memory can further store instructions executable by the processor to receive an output signal detected by the radio frequency coil assembly intermediate two of the refocusing pulses.

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 owns the following patent application that was filed on even date herewith and which is each incorporated by reference herein in its entirety:U.S. Patent Application Attorney Docket No. 220502, titled ACCELERATING MAGNETIC RESONANCE IMAGING USING PARALLEL IMAGING AND ITERATIVE IMAGE RECONSTRUCTION.

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;U.S. patent application Ser. No. 18/057,207, titled SYSTEM AND METHOD FOR REMOVING ELECTROMAGNETIC INTERFERENCE FROM LOW-FIELD MAGNETIC RESONANCE IMAGES, filed Nov. 19, 2022;U.S. patent application Ser. No. 18/147,418, titled MODULARIZED MULTI-PURPOSE MAGNETIC RESONANCE PHANTOM, filed Dec. 28, 2022;U.S. patent application Ser. No. 18/147,542, titled INTRACRANIAL RADIO FREQUENCY COIL FOR INTRAOPERATIVE MAGNETIC RESONANCE IMAGING, filed Dec. 28, 2022; andU.S. patent application Ser. No. 18/147,556, titled DEEP LEARNING SUPER-RESOLUTION TRAINING FOR ULTRA LOW-FIELD MAGNETIC RESONANCE IMAGING, filed Dec. 28, 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 Dholeof 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 text and 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 Dholeof 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 Mo. 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 may 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.

The relatively poor homogeneity (e.g., the inhomogeneity) of the magnetic field B0and/or the effective magnetic field B1generally defined by LF- and ULF-MRI systems can present various challenges. For example, typical RF pulses, such as hard pulses, sinc pulses, and/or fixed-frequency pulses generated using LF- and ULF-MRI systems can fail to excite the entire target subject in the region of interest because of the limited power used to generate the RF pulses. Further, typical RF pulses generated using LF- and ULF-MRI systems may excite only a limited bandwidth of spins. Thus, it can be difficult to produce images with an adequate signal-to-noise (SNR) ratio using LF- and ULF-MRI systems.

Various techniques can be implemented to improve the SNR of images generated by LF- and ULF-MRI systems. One technique involves the generation of frequency-swept pulses (sometimes referred to chirped pulses) as part of a modified Carr-Purcell Meiboom-Gill (CPMG) sequence. Frequency-swept pulses can be generated by modulating from an initial frequency to a final frequency at a particular sweep rate throughout the duration of the RF pulse. As a result, compared to typical RF pulses, frequency swept RF pulses can excite a given bandwidth of spins using a signal that has a lower maximum amplitude. As described in “Quadrupolar nuclear magnetic resonance spectroscopy in solids using frequency-swept echoing pulses” by Bhattacharyya et al., published inThe Journal of Chamical Physics,127, 194503 in 2007, which is incorporated by reference herein in its entirety, combinations of frequency-swept pulses can be used to generate a spin echo. And as described in “Chirped CPMG for well-logging NMR application” by Casabianca et al., published inJournal of Magnetic Resonance,242, 197-202 in 2014, which is incorporated by reference herein in its entirety, frequency-swept pulses can be implemented as part of a “chirped-CPMG sequence.” The chirped-CPMG sequence can be used to achieve improved SNR for proton-density-weighted images and T1-weighted images generated using LF- and ULF-MRI systems.

However, various challenges remain despite the improvements offered by the chirped-CPMG sequence. For example, traditional T2-weighted imaging (T2w) sequences and diffusional-weighted imaging (DWI) sequences typically rely on a long echo time (TE) (e.g., a TE that is in a range of 3 to 5 times greater than T2) and/or a long repetition time (TR) (e.g., a TR that is in a range of 3 to 5 times greater than T1) to achieve the desired image contrast. Because of the long TE, applying T2w or DWI to a chirped-CPMG sequence can result in a poor SNR. Similarly, because of the long TR, applying T2w or DWI to a chirped-CPMG sequence can result in a long acquisition time. Accordingly, a need exists for systems and methods for increasing SNR and/or reducing TR produced by chirped-CPMG sequences, such as T2w or DWI chirped-CPMG sequences.

In various aspects, the present disclosure provides systems and methods for implementing chirped-CPMG sequences that produce a reduced TR compared to traditional chirped-CPMG sequences. The chirped-CPMG sequences provided herein can include a recovery pulse following a spin-echo train. In some aspects, the recovery pulse is a 90° frequency-swept pulse that has the opposite phase (e.g., +/−180º) of an initial 90° excitation pulse of the sequence. The recovery pulse can flip transverse magnetism induced by the initial excitation pulse and subsequent refocusing pulse(s) back to the longitudinal axis, thereby reducing TR. The recovery pulse can achieve a reduced TR without substantially compromising SNR or inducing unwanted T1 weighting. Thus, various chirped-CPMG sequences provided herein can reduce total acquisition time compared to traditional chirped-CPMG sequences, thereby improving efficiency and patient comfort.

In various aspects, the systems and methods provided herein can implement T2w chirped-CPMG sequences and/or DWI chirped-CPMG sequences by implementing a T2w preparation sequence and/or a DWI preparation sequence prior to the primary chirped-CPMG sequence. The T2w preparation sequence can include a first preparation pulse that is a 90° excitation pulse, a second preparation pulse that is a 180° refocusing pulse, and a third preparation pulse that is a 90° recovery pulse. The DWI preparation sequence can be similar to the T2w preparation sequence described immediately above except that the DWI preparation sequence includes (i) a first diffusion gradient intermediate the first preparation pulse and the second preparation pulse and (ii) a second diffusion gradient intermediate the second preparation pulse and the third preparation pulse. The various preparation sequences described herein can be implemented prior to any of the primary chirped-CPMG sequences described herein to support a desired contrast for imaging.

In various aspects, the preparation sequences disclosed herein can be implemented achieve a T2w chirped-CPMG sequence and/or a DWI chirped-CPMG sequence with improved signal quality and/or reduced total acquisition time compared to a T2w chirped-CPMG sequence and/or DWI chirped-CPMG sequence that does not implement the preparation sequence. For example, as noted above, T2 weighting can be achieved by configuring a multi-echo CPMG sequence to generate a long TE. Traditionally, there are two methods of configuring a multi-echo CPMG sequence to generate a long TE. First, while maintaining CPMG sequence conditions, the time interval between 180° refocusing pulses can be increased so that the effective TE (e.g., and the time during which the center of the k-space is collected) becomes longer. However, this method is prone to signal artifacts and unnecessary signal loss because of the longer wait time between echoes. Second, while breaking CPMG sequence conditions, the time interval of the 90° excitation pulse and the first two 180° refocusing pulses can be increased. However, by breaking the CPMG sequence conditions, the acquisition can become prone to signal artifacts. The preparation sequences disclosed herein can induce T2w while enabling the subsequent primary chirped-CPMG sequence to maintain a relatively short interval between the 180° refocusing pulses and maintain the CMPG sequence conditions. For example, the preparation sequences disclosed herein can generate T2 contrast by: (i) implementing a 90° excitation pulse to flip longitudinal magnetization onto the transverse plane; (ii) at first time interval following the 90° excitation pulse (e.g. a time interval long enough to achieve T2 contrast (exp(−t/T2)), implementing a 180° refocusing pulse to invert and rephase the transverse magnetization, thereby adding T2 contrast and generating a spin echo at a second time interval following the refocusing pulse that is equivalent to the first time interval between the excitation pulse and the refocusing pulse; and (3) at the time of the spin echo, implementing 90° recovery pulse to flip transverse magnetism back to the longitudinal plane. A T2w acquisition can then be carried out by implementing the primary chirped-CPMG sequence following the preparation sequence. Accordingly, compared to traditional chirped-CPMG sequences, the systems and methods provided herein can achieve improved SNR and/or reduced TR for T2w chirped-CPMG sequences and DWI chirped-CPMG sequences.

FIG.9is a pulse sequence diagram illustrating a chirped-CPMG sequence1000, according to at least one non-limiting aspect of the present disclosure. In some aspects, the chirped-CPMG sequence1000can be the same or similar to the chirped-CPMG sequences described in the aforementioned publication titled “Chirped CPMG for well-logging NMR application” by Casabianca et al. The various MRI systems described herein can be configured to implement the chirped-CPMG sequence1000. For example, referring toFIGS.6and10, the chirped-CPMG sequence1000can be a pulse sequence566stored in the database562of the MRI system500. The RF transmit/receive coils550can be configured to generate the pulses of the chirped-CPMG sequence1000and/or detect the spin echo signals of the chirped-CPMG sequence1000.

Referring again toFIG.9, the chirped-CPMG sequence1000includes an excitation pulse1002followed by a series of refocusing pulses1004,1006. The excitation pulse1004can be configured to induce a flip angle of 90°. Each of the series of refocusing pulses1004,1006can be configured to induce a flip angle of 180°. An RF pulsed described as being configured to induce a specific flip angle (e.g., 90°, 180°) is sometimes referred to herein as an RF pulse of the specific flip angle (e.g., a 90° RF pulse, a 180° RF pulse). Thus, the excitation pulse1002can be a 90° pulse and each of the series of refocusing pulses1004,1006can be 180° pulses. Accordingly, the chirped-CPMG sequence1000can be configured to induce a spin-echo train.

As illustrated byFIG.9, the series of refocusing pulses1004,1006following the excitation pulse1002can include sequentially repeating first refocusing pulses1004and second refocusing pulses1006. The excitation pulse1002can have a first phase ϕ1, each of the first refocusing pulses1004can have a second phase ϕ2, and each of the each of the second refocusing pulses1006can have a third phase ϕ3. The second phase ϕ2can be shifted 90° (e.g., +/−90°) with respect to the first phase ϕ1, similar to a traditional CPMG sequence. The third phase ϕ3can be shifted 90° (e.g., +/−90°) with respect to the second phase ϕ2.

FIG.9Ais a pulse sequence diagram illustrating a chirped-CPMG sequence1000a. The chirped-CPMG sequence1000aprovides an example implementation of the chirped-CPMG sequence1000ofFIG.9. As shown inFIG.9A, the excitation pulse1002ahas a first phase ϕ1oriented in the +y direction along the transverse axis, the first refocusing pulses1004ahave a second phase ϕ2oriented in the +x direction along the transverse axis (shifted 90° with respect to the first phase ϕ1), and each of the second refocusing pulses1006ahave a third phase ϕ3oriented in the +y direction along the transverse axis (shifted 90° with respect to the second phase ϕ2).

Referring again toFIG.9, the excitation pulse1002, each of the first refocusing pulses1004, and/or each of the second refocusing pulses1006can be frequency-swept pulses. In some aspects, the excitation pulse1002, each of the first refocusing pulses1004, and each of the second refocusing pulses1006are frequency swept across the same frequency offset range. The frequency offset range can be defined by an initial frequency offset Oiand a final frequency offset Of. The initial frequency offset Oiand the final frequency offset Ofcan be selected based on a target resonance frequency. For example, the initial frequency offset Oiand the final frequency offset Ofcan be selected such that the frequency sweep of the pulse is symmetric about the target resonance frequency.

The excitation pulse1002can have an excitation pulse duration τexeand each of the first refocusing pulses1004and the second refocusing pulses1006can have a refocusing pulse duration τref. The excitation pulse1002can be frequency swept at constant excitation sweep rate Rexefrom the initial frequency offset Oito the final frequency offset Ofover the excitation pulse duration τexe. Each of the first refocusing pulses1004and the second refocusing pulses1006can be frequency swept at constant refocusing sweep rate Rreffrom the initial frequency offset Oito the final frequency offset Ofover the refocusing pulse duration τref. In some aspects, refocusing sweep rate Rrefcan be twice the excitation sweep rate Rexeand the refocusing pulse duration τrefcan be half the excitation pulse duration τexe. For example, the chirped-CPMG sequence1000aofFIG.9Aillustrates an example implementation of the chirped-CPMG sequence1000ofFIG.9where each of the first refocusing pulses1004and the second refocusing pulses1006having refocusing pulse duration τrefequal to τexe/2.

Referring again toFIG.9, the excitation pulse1002and the first refocusing pulse1004can cause the generation of an echo signal1008. Further, additional echo signals1008can be generated by the series of refocusing pulses1004,1006. The echo signals1008can be spatially encoded by applying a gradient field, detected by an RF coil assembly, and used for image construction (e.g. according to blocks576,577, and578of method570described with respect toFIG.6). The initial echo signal1008forms at a time τechoafter the first refocusing pulse1004. In some aspects, τechocan be controlled based on the excitation pulse duration τexe, the refocusing pulse duration τref, and/or the time τδbetween the excitation pulse1002and the first refocusing pulse1004. For example, as illustrated by the example chirped-CPMG sequence1000aofFIG.9A, where the refocusing pulse duration τrefis set to half the excitation pulse duration (τref=τexe/2), the echo1008can be formed at a time τechoequal to τδ+τexe/2.

Referring again toFIG.9, the echo signals1008can have an acquisition phase ϕacq. In some aspects, the acquisition phase ϕacqof the echo signals1008are shifted 90° (e.g., +/−90°) with respect to the first phase ϕ1of the excitation pulse1002. For example, as illustrated by the example chirped-CPMG sequence1000aofFIG.9A, the echo signals1008have an acquisition phase ϕacqoriented in the +x direction along the transverse axis (e.g., shifted 90° with respect to the first phase ϕ1, which is oriented in the +y direction).

As noted above, frequency-swept pulses can excite a larger bandwidth of spins compared to traditional hard pulses utilizing the same RF peak power. In some aspects, by applying frequency-swept pulses according to the chirped-CPMG sequence1000, the number of spins contributing to the echo signals1008can be more than 4 times the number of spins contributing to echo signals of a traditional CPMG sequence that utilizes hard pulses. Thus, chirped-CPMG sequence1000can be implemented to achieve a higher SNR compared to those achievable using a traditional CPMG at the same RF peak power. Additional details related to the improved SNR that can be achieved using chirped-CPMG sequences are described the aforementioned publication titled “Chirped CPMG for well-logging NMR application” by Casabianca et al.

Still referring toFIG.9, in some aspects, the chirped-CPMG sequences1000can generate echo signals1010that appear after each of the second refocusing pulses1006. In some aspects, in addition to or in lieu of the echo signals1008, the echo signals1010can be can be spatially encoded by applying a gradient field, detected by an RF coil assembly, and used for image construction (e.g. according to blocks576,577, and578of method570described with respect toFIG.6). In some aspects, the echo signals1008can be characterized as free induction decay (FID) echo signals and the echo signals1010can be characterized as spectral echo signals.

AlthoughFIGS.9and9Ashow the chirped-CPMG sequences1000,1000aas having a series of refocusing pulses1004,1006that includes two (2) first refocusing pulses1004and two (2) second refocusing pulses, the chirped-CPMG sequences1000,1000acan be modified to include any suitable number of first refocusing pulses1004and second refocusing pulses1006, such as any number of first and second refocusing pulses1004,1006that is a positive integer and that results in detectible echo signals1008and/or echo signals1010. Further, persons of ordinary skill in the art will appreciate that the chirped-CPMG sequences1000,1000acan be repeated as desired.

As noted above, traditional T2-weighted imaging (T2w) sequences and diffusional-weighted imaging (DWI) sequences typically rely on a relatively long echo time TE and/or a relatively long repetition time TR to achieve the desired image contrast. Thus, applying T2w or DWI to a chirped-CPMG sequence, such as chirped-CPMG sequences1000,1000a(FIGS.9,9A), can result in poor SNRs and/or a long acquisition times. The chirped-CPMG sequences1100,1100a(FIGS.10,10a) and/or the preparation sequences1114(FIGS.10,10A),1200(FIG.11),1200a(FIG.11A),1300(FIG.12),1300a(FIG.12A) described herein can be implemented to support T2 weighted imaging and/or diffusional weighting imaging. In some aspects, the chirped-CPMG sequences1100,1100a(FIGS.10,10A) and/or the preparation sequences1114(FIGS.10,10A),1200(FIG.11),1200a(FIG.11A),1300(FIG.12),1300a(FIG.12A) can be implemented to achieve shorter TRs and/or improved SNRs compared to the chirped-CPMG sequences1000,1000a(FIGS.9,9A).

FIG.10is a pulse sequence diagram illustrating a chirped-CPMG sequence1100, according to at least one non-limiting aspect of the present disclosure. The chirped-CPMG sequence1100includes an excitation pulse1102, a series of refocusing pulses1104,1106(e.g., first refocusing pulses1104and second refocusing pulses1106), and one or more echo signals1108. In some aspects, the chirped-CPMG sequence1100can include one or more echo signals1110. The chirped-CPMG sequence1100can be similar to the chirped-CPMG sequences1000,1000adescribed above with respect toFIGS.9and9A. For example, the excitation pulse1102(FIG.10) can be similar to the excitation pulse1002(FIG.9), the series of refocusing pulses1104,1106(FIG.10) can be similar to the series of refocusing pulses1004,1006(FIG.11), the echo signals1108(FIG.10) can be similar to the echo signals1008(FIG.9), and/or the echo signals1110(FIG.10) can be similar to the echo signals1010(FIG.9). Various details disclosed above related to the chirped-CPMG sequences1000,1000aofFIGS.9and9Acan similarly apply to the chirped-CPMG sequence1100ofFIG.10. The various MRI systems described herein can be configured to implement the chirped-CPMG sequence1100. For example, referring toFIGS.6and10, the chirped-CPMG sequence1100can be a pulse sequence566stored in the database562of the MRI system500. The RF transmit/receive coils550can be configured to generate the pulses of the chirped-CPMG sequence1100and/or detect the echo signals of the chirped-CPMG sequence1100.

Referring toFIG.10, the chirped-CPMG sequence1100includes a recovery pulse1112. The recovery pulse1112can be a 90° pulse that follows the series of refocusing pulses1104,1106. In some aspects, the recovery pulse1112can flip transverse magnetism induced by the excitation pulse1102and the series of refocusing pulses1104,1106back to the longitudinal axis (e.g., such that the spins are again in alignment with main magnetic field B0). Accordingly, the recovery pulse1112can effectively reduce TR compared to the chirped-CPMG sequence1000(FIG.9). Thus, by including the recovery pulse1112following the series of refocusing pulses1104,1106, total acquisition times associated with the chirped-CPMG sequence1100(FIG.10) may generally be lower than those associated with the chirped-CPMG sequence1000(FIG.9).

Referring toFIG.10, the recovery pulse1112can have a fourth phase ϕ4. As noted above with respect toFIG.9(describing the excitation pulse1002and refocusing pulses1004,1006), the excitation pulse1102can have a first phase ϕ1, each of the first refocusing pulses1104can have a second phase ϕ2, and each of the each of the second refocusing pulses1106can have a third phase ϕ3. The second phase ϕ2can be shifted 90° (e.g., +/−90°) with respect to the first phase ϕ1, the third phase ϕ3can be shifted 90° (e.g., +/−90°) with respect to the second phase ϕ2. The fourth phase ϕ4of the of the recovery pulse1112can be shifted 180° (e.g., +/−180°) with respect to the first phase ϕ1. Thus, the excitation pulse1002can cause spins to flip from alignment with the longitudinal plane (e.g., in the z direction) to the transverse plane (e.g., along the xy plane) and the refocusing pulses1104,1106can invert the spins 180° about the transverse plane to generate spin echoes. Further, the recovery pulse1112can cause spins to flip from alignment with the transverse plane back to the longitudinal plane, thereby effectively reducing TR.

FIG.10Ais a pulse sequence diagram illustrating a chirped-CPMG sequence1100a. The chirped-CPMG sequence1100aprovides an example implementation of the chirped-CPMG sequence1100ofFIG.10. As shown inFIG.10A, the excitation pulse1102ahas a first phase ϕ1oriented in the +y direction along the transverse axis, the first refocusing pulses1104ahave a second phase ϕ2oriented in the +x direction along the transverse axis (shifted 90° with respect to the first phase ϕ1), each of the second refocusing pulses1106have a third phase ϕ3oriented in the +y direction along the transverse axis (shifted 90° with respect to the second phase ϕ2), and the recovery pulse112ahas a fourth phase ϕ4oriented in the −y direction (shifted 90° with respect to the first phase ϕ1).

Referring again toFIG.10, the recovery pulse1112can be frequency swept across a frequency range, such as the same frequency range as that of the excitation pulse1102and the refocusing pulses1104,1106. For example, the excitation pulse1102, the refocusing pulses1104,1106, and the recovery pulse1112can each be frequency swept across a frequency offset range that is symmetric about a target resonance frequency, the frequency offset range having an initial frequency offset Oiand a final frequency offset Of. The recovery pulse1112can have a recovery pulse duration τrec. The recovery pulse1112can be frequency swept at constant recovery sweep rate Rrecfrom the initial frequency offset Oito the final frequency offset Ofover the recovery pulse duration τrec. In some aspects, the recovery sweep rate Rreccan be equal to excitation sweep rate Rexeand the recovery pulse duration τreccan be equal to the excitation pulse duration τexe. For example, the chirped-CPMG sequence1100aofFIG.10Aillustrates an example implementation of the chirped-CPMG sequence1100ofFIG.10where the recovery pulse1112ahas a recovery pulse duration τrecequal to τexe.

AlthoughFIGS.10and10Ashow the chirped-CPMG sequences1100,1100aas having a series of refocusing pulses1104,1106that includes two (2) first refocusing pulses1104and two (2) second refocusing pulses1106, the chirped-CPMG sequences1100,1100acan be modified to include any suitable number of first refocusing pulses1104and second refocusing pulses1106, such as any number of first and second refocusing pulses1104,1106that is a positive integer and that results in detectible echo signals1108and/or echo signals1110. The recovery pulse1112can be implemented after a series of refocusing pulses1104,1106with any suitable number of first refocusing pulses1104and second refocusing pulses1106. Further, persons of ordinary skill in the art will appreciate that the chirped-CPMG sequences1100,1100acan be repeated as desired.

Referring again toFIG.10, in some aspects, a preparation sequence1114may be implemented prior to each chirped-CPMG sequence1100. In various aspects, the preparation sequence1114can be configured to support T2w or DWI without needed to increase the total acquisition time of the primary chirped-CPMG sequence1100. For example, the preparation sequence1114may be configured to achieve T2w or DWI with the chirped-CPMG sequence1100without configuring (e.g., increasing) the time interval between the excitation pulse1102and/or the refocusing pulses1104,1106to generate a TE needed for T2w or DWI. The terms “primary pulse sequence” and “primary RF pulse sequence” are sometimes used herein to refer to a chirped-CPMG sequence that is implemented following a preparation sequence. The various MRI systems described herein can be configured to implement the preparation sequence1114and the chirped-CPMG sequence1100. For example, referring toFIGS.6and10, the preparation sequence1114and the chirped-CPMG sequence1100can be a pulse sequence566stored in the database562of the MRI system500. The RF transmit/receive coils550can be configured to generate the pulses of the preparation sequence1114and the chirped-CPMG sequence1100.

FIG.11is a pulse sequence diagram illustrating a T2w preparation sequence1200. The T2w preparation sequence1200is configured for T2 weighted imaging and may be implemented as the preparation sequence1114(FIG.10). The T2w preparation sequence1200includes a first preparation pulse1202, a second preparation pulse1204, and a third preparation pulse1206. The first preparation pulse1202is a 90° excitation pulse, the second preparation pulse1204is a 180° refocusing pulse, and the third preparation pulse1206is a 90° recovery pulse. The T2w preparation sequence1200can induce T2 weighted imaging while enabling a subsequent primary chirped-CPMG sequence (e.g., chirped-CPMG sequence1000, chirped-CPMG sequence1100) to maintain a relatively short interval between the 180° refocusing pulses and maintain the CMPG sequence condition, thereby reducing acquisition times compared to traditional T2 weighting methods. The T2w preparation sequence1200can generate T2 contrast by: (i) implementing the first preparation pulse1202to flip longitudinal magnetization onto the transverse plane; (ii) implementing the second preparation pulse1204at a first time interval after the first preparation pulse1202that is long enough to achieve T2 contrast (e.g., exp(−t/T2)), thereby generating a spin echo at a second time interval after the second preparation pulse1204that is equivalent to the first time interval between the first preparation pulse1202and the second preparation pulse1204; and (3) implementing the third preparation pulse1206at the time of the generated spin echo to flip transverse magnetism back to the longitudinal plane.

For example, the first preparation pulse1202has a fifth phase ϕ5, the second preparation pulse1204has a sixth phase ϕ6, and the third preparation pulse1206has a seventh phase ϕ7. In some aspects, the fifth phase ϕ5of the first preparation pulse1202can be shifted 90° (e.g., +/−90°) with respect to the first phase ϕ1of the excitation pulse1102(FIG.10), the sixth phase ϕ6of the second preparation pulse1204can be shifted 90° (e.g., +/−90°) with respect to the fifth phase ϕ5, and the seventh phase ϕ7of the third preparation pulse1206can be shifted 180° (e.g., +/−180°) with respect to the fifth phase ϕ5. Thus, the first preparation pulse1202can cause spins to flip from alignment with the longitudinal plane (e.g., in the z direction) to the transverse plane (e.g., along the xy plane), the second preparation pulse1204can flip the spins 180° about the transverse plane, and the third preparation pulse1206cause spins to flip from alignment with the transverse plane back to the longitudinal plane.

FIG.11Ais a pulse sequence diagram illustrating T2w preparation sequence1200a. The T2w preparation sequence1200aprovides an example implementation of the T2w preparation sequence1200ofFIG.11. As shown inFIG.11A, the first preparation pulse1202ahas a fifth phase ϕ5oriented in the +x direction along the transverse axis (shifted −90° with respect to the first phase ϕ1, +y, of the excitation pulse1102aofFIG.10A), the second preparation pulse1204ahas a sixth phase ϕ5oriented in the +y direction along the transverse axis (shifted 90° with respect to the fifth phase ϕ5), and the third preparation pulse1206has a seventh phase ϕ7oriented in the −x direction along the transverse axis (shifted 180° with respect to the fifth phase ϕ5).

Referring again toFIG.11, each of the first preparation pulse1202, the second preparation pulse1204, and the third preparation pulse1206can be frequency swept across a frequency range, such as the same frequency range as that of the excitation pulse1102(FIG.10) and the refocusing pulses1104,1106(FIG.10). For example, the first preparation pulse1202, the second preparation pulse1204, and the third preparation pulse1206can each be frequency swept across a frequency offset range that is symmetric about a target resonance frequency, the frequency offset range having an initial frequency offset Oiand a final frequency offset Of. The first preparation pulse1202can have a first preparation pulse duration τprep1, the second preparation pulse1204can have a second preparation pulse duration τprep2, and the third preparation pulse1206can have a third preparation pulse duration τprep3. The first preparation pulse1202can be frequency swept at constant first preparation pulse sweep rate Rprep1from the initial frequency offset Oito the final frequency offset Ofover the first preparation pulse duration τprep1. The second preparation pulse1204can be frequency swept at constant second preparation pulse sweep rate Rprep2from the initial frequency offset Oito the final frequency offset Ofover the second preparation pulse duration τprep2. The third preparation pulse1206can be frequency swept at constant third preparation pulse sweep rate Rprep3from the initial frequency offset Oito the final frequency offset Ofover the third preparation pulse duration τprep3.

In some aspects, first preparation pulse sweep rate Rprep1and the third preparation pulse sweep rate Rprep3can be equal to the excitation sweep rate Rexe(FIG.10), and first preparation pulse duration τprep1and the third preparation pulse duration τprep3can be equal to the excitation pulse duration τexe(FIG.10). Further, the second preparation pulse sweep rate Rprep2be twice to the excitation sweep rate Rexe(FIG.10), and second preparation pulse duration τprep2can be half the excitation pulse duration τexe(FIG.10). For example, the T2w preparation sequence1200aofFIG.11Aillustrates an example implementation of the T2w preparation sequence1200ofFIG.11where first preparation pulse1202aand the third preparation pulse1206arespectively have a first preparation pulse duration τprep1and a third preparation pulse duration τprep3each equal to the excitation pulse duration τexe(FIG.10). Further, the T2w preparation sequence1200ashows the second preparation pulse1204ahaving a second preparation pulse duration τprep2that is half excitation pulse duration (τexe/2).

FIG.12is a pulse sequence diagram illustrating a DWI preparation sequence1300. The DWI preparation sequence1300is configured for diffusion weighted imaging and may be implemented as the preparation sequence1114(FIG.10). The DWI preparation sequence1300includes a first preparation pulse1302, a second preparation pulse1304, and a third preparation pulse1306. The first preparation pulse1302is a 90° excitation pulse, the second preparation pulse1304is a 180° refocusing pulse, and the third preparation pulse1306is a 90° recovery pulse. The DWI preparation sequence1300further includes a first diffusion gradient1308and a second diffusion gradient1310. The first diffusion gradient1308is applied after the first preparation pulse1302and before the second preparation pulse1304. The second diffusion gradient1310is applied after the second preparation pulse1304and before the third preparation pulse1306. Those of ordinary skill in the art will appreciate that the strength, duration, and time interval of the first diffusion gradient1308and the second diffusion gradient1310can be selected to achieve a desired b-value for diffusion weighting.

The first preparation pulse1302, the second preparation pulse1304, and the third preparation pulse1306of the DWI preparation sequence1300(FIG.12) can be similar to the first preparation pulse1202, the second preparation pulse1204, and the third preparation pulse1206of the T2w preparation sequence1200described above with respect toFIG.11. Various details (e.g., pulse duration, frequency sweeping, frequency sweep rate, phase) disclosed above related to the first preparation pulse1202, the second preparation pulse1204, and the third preparation pulse1206of the T2w preparation sequence1200(FIG.11) can similarly apply to first preparation pulse1302, the second preparation pulse1304, and the third preparation pulse1306(FIG.12). Thus, the DWI preparation sequence1300can induce diffusion weighted imaging while enabling a subsequent primary chirped-CPMG sequence (e.g., chirped-CPMG sequence1000, chirped-CPMG sequence1100) to maintain a relatively short interval between the 180° refocusing pulses and maintain the CMPG sequence condition, thereby reducing acquisition times compared to traditional diffusion weighting methods.

FIG.12Ais a pulse sequence diagram illustrating a DWI preparation sequence1300a. The DWI preparation sequence1300aprovides an example implementation of the DWI preparation sequence1300ofFIG.21. As shown inFIG.12A, the first preparation pulse1302ahas a fifth phase ϕ5oriented in the +x direction along the transverse axis (shifted −90° with respect to the first phase ϕ1, +y, of the excitation pulse1102aofFIG.10A), the second preparation pulse1304ahas a sixth phase ϕ5oriented in the +y direction along the transverse axis (shifted 90° with respect to the fifth phase ϕ5), and the third preparation pulse1306has a seventh phase ϕ7oriented in the −x direction along the transverse axis (shifted 180° with respect to the fifth phase ϕ5). Further, the first preparation pulse1302aand the third preparation pulse1306arespectively have a first preparation pulse duration τprep1and a third preparation pulse duration τprep3each equal to the excitation pulse duration τexe(FIG.10). The second preparation pulse1304ahas a second preparation pulse duration τprep2that is half excitation pulse duration (τexe/2).

The various MRI systems described herein can be configured to implement the DWI preparation sequence1300. For example, referring toFIGS.6and12, the first diffusion gradient1308and the second diffusion gradient1310can be generated by the gradient coils504of the MRI system500. The first preparation pulse1302, the second preparation pulse1304, and the third preparation pulse1306can be generated by the RF transmit/receive coils550of the MRI system500. The DWI preparation sequence1300can be a pulse sequence566stored in the database562of the MRI system500, and may include an RF pulse sequence corresponding to the first preparation pulse1302, the second preparation pulse1304, and the third preparation pulse1306; and a gradient sequence corresponding to the first diffusion gradient1308and the second diffusion gradient1310.

Any of the pulse sequences described herein (chirped-CPMG sequence1000(FIG.9), the chirped-CPMG sequence1100(FIG.10), the preparation sequence1114(FIG.10), the T2w preparation sequence1200(FIG.11), the DWI preparation sequence1300(FIG.12)) can include standard imaging, crusher, and/or spoiler gradients.

EXAMPLES

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

Clause 1: A method, comprising: projecting a magnetic field along a longitudinal axis and 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 comprises: an excitation pulse that is frequency swept across a frequency offset range; a series of refocusing pulses following the excitation pulse, wherein each of the refocusing pulses are frequency swept across the frequency offset range, and wherein each of the refocusing pulses are half the duration of the excitation pulse; and a recovery pulse following the series of refocusing pulses, wherein the recovery pulse is frequency swept across the frequency offset range; and receiving an output signal detected by the radio frequency coil assembly an intermediate two of the refocusing pulses.

Clause 2: The method of Clause 1, wherein the radio frequency pulse sequence is a primary radio frequency pulse sequence, wherein the method further comprises transmitting a preparation radio frequency pulse sequence to the radio frequency coil assembly prior to transmitting the primary radio frequency pulse sequence, and wherein the preparation radio frequency pulse sequence is configured to achieve T2 weighted imaging or diffusion weighted imaging.

Clause 3: The method of claim2, further comprising sequentially repeating the transmission of the preparation radio frequency pulse sequence and the primary radio frequency pulse sequence.

Clause 4: The method of any of Clauses 1-3, wherein the excitation pulse is a 90° pulse, wherein each of the refocusing pulses are 180° pulses, and wherein the recovery pulse is a 90° pulse.

Clause 5: The method of any of Clauses 1-4, wherein the excitation pulse and the recovery pulse are frequency swept across the frequency offset range at a first rate, wherein the refocusing pulses are frequency swept across the frequency offset range at a second rate, and wherein the second rate is twice the first rate.

Clause 6: The method of any of Clauses 1-5, wherein the excitation pulse comprises a first phase ϕ1, wherein the recovery pulse comprises a second phase ϕ2, and wherein the second phase ϕ2 equals the first phase ϕ1 plus 180°.

Clause 7: The method of any of Clauses 1-6, wherein the series of refocusing pulses comprises sequentially repeating first and second refocusing pulses.

Clause 8: The method of Clause 7, wherein receiving an output signal detected by the radio frequency coil assembly intermediate two of the refocusing pulses comprises receiving multiple output signals from the radio frequency coil assembly, wherein each of the multiple output signals are detected intermediate one of the first refocusing pulses and one of the second refocusing pulses.

Clause 9: The method of Clause 8, wherein each of the multiple output signals is detected after one of the first refocusing pulses and prior to a corresponding one of the second refocusing pulses.

Clause 10: The method of any one of Clauses 7-9, wherein the first refocusing pulses comprise a third phase ϕ3, wherein the second refocusing pulses comprise a fourth phase ϕ4, wherein the third phase ϕ3 equals the first phase ϕ1 plus or minus 90°, and wherein the fourth phase ϕ4 equals the third phase ϕ3 plus or minus 90°.

Clause 11: The method of Clause 10, wherein the preparation radio frequency pulse sequence comprises: a first preparation pulse that is frequency swept across the frequency offset range at the first rate, wherein the first preparation pulse is a 90° pulse; a second preparation pulse that is frequency swept across the frequency offset range at the second rate, wherein the second preparation pulse is a 180° pulse; and a third preparation pulse that is frequency swept across the frequency offset range at the first rate, wherein the third preparation pulse is a 90° pulse.

Clause 12: The method of Clause 11, wherein the first preparation pulse comprises a fifth phase ϕ5, wherein the second preparation pulse comprises a sixth phase ϕ6, wherein the third preparation pulse comprises a seventh phase ϕ7, wherein the fifth phase ϕ5 equals the first phase ϕ1 plus or minus 90°, wherein the sixth phase ϕ6 equals the fifth phase ϕ5 plus or minus 90°, and wherein the seventh phase ϕ7equals the fifth phase ϕ5plus 180°.

Clause 13: The method of any of claims10-11, wherein the preparation radio frequency pulse sequence is configured for diffusion weighted imaging, the method further comprising: transmitting a gradient sequence to a gradient coil assembly configured to modify the magnetic field projected along the longitudinal axis, wherein the gradient sequence comprises: a first diffusion gradient after the first preparation pulse and prior to the second preparation pulse of the preparation radio frequency pulse sequence; a second diffusion gradient after the second preparation pulse and prior to the third preparation pulse of the preparation radio frequency pulse sequence.

Clause 14: The method of any of Clauses 1-13, wherein projecting a magnetic field along a longitudinal axis and toward an object of interest located within a field of view comprises projecting a low-field strength magnetic field.

Clause 15: 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 preparation radio frequency pulse sequence to the radio frequency coil assembly; and transmit a primary radio frequency pulse sequence to the radio frequency coil assembly, wherein the primary radio frequency pulse sequence comprises: an excitation pulse that is frequency swept across a frequency offset range at a first rate, wherein the excitation pulse is a 90° pulse; and a series of refocusing pulses following the excitation pulse, wherein each of the refocusing pulses are frequency swept across the frequency offset range at a second rate that is twice the first rate, and wherein each of the refocusing pulses are 180° pulses and are half the duration of the excitation pulse; and receive an output signal detected by the radio frequency coil assembly intermediate two of the refocusing pulses.

Clause 16: The system of Clause 15, wherein the preparation radio frequency pulse sequence comprises: a first preparation pulse that is frequency swept across the frequency offset range at the first rate, wherein the first preparation pulse is a 90° pulse and is the same duration as the excitation pulse; a second preparation pulse that is frequency swept across the frequency offset range at the second rate, wherein the second preparation pulse is a 180° pulse and is half the duration as the excitation pulse; and a third preparation pulse that is frequency swept across the frequency offset range at the first rate, wherein the third preparation pulse is a 90° pulse and is the same duration as the excitation pulse.

Clause 17: The system Clause 16, wherein the primary radio frequency pulse sequence further comprises a recovery pulse following the series of refocusing pulses, wherein the recovery pulse is frequency swept across the frequency offset range at the first rate, and wherein the recovery pulse is a 90° pulse and is the same duration as the excitation pulse.

Clause 18: The system Clause 17, wherein the excitation pulse comprises a first phase ϕ1, wherein the recovery pulse comprises a second phase ϕ2, and wherein the second phase ϕ2 equals the first phase ϕ1 plus 180°.

Clause 19: The system claim18, wherein the series of refocusing pulses comprises sequentially repeating a first refocusing pulse and a second refocusing pulse, wherein the memory stores instructions executable by the processor to receive multiple output signals from the radio frequency coil assembly, and wherein each of the multiple output signals is received intermediate one of the first refocusing pulses and one of the second refocusing pulses.

Clause 20: The system of Clause 19, wherein the first refocusing pulses comprise a third phase ϕ3, wherein the second refocusing pulses comprise a fourth phase ϕ4, wherein the first preparation pulse comprises a fifth phase ϕ5, wherein the second preparation pulse comprises a sixth phase ϕ6, wherein the third preparation pulse comprises a seventh phase ϕ7, wherein the third phase ϕ3 equals the first phase ϕ1 plus or minus 90°, wherein the fourth phase ϕ4 equals the third phase ϕ3 plus or minus 90°, wherein the fifth phase ϕ5 equals the first phase ϕ1 plus or minus 90°, wherein the sixth phase ϕ6 equals the fifth phase ϕ5 plus or minus 90°, and wherein the seventh phase ϕ7 equals the fifth phase ϕ5 plus 180°.

Clause 21: The system of claim20, further comprising: a gradient coil assembly configured to modify the low-field strength magnetic field projected along the longitudinal axis; wherein the memory stores instructions executable by the processor to transmit a gradient sequence to the gradient coil assembly, wherein the gradient sequence comprises: a first diffusion gradient after the first preparation pulse and prior to the second preparation pulse of the preparation radio frequency pulse sequence; a second diffusion gradient after the second preparation pulse and prior to the third preparation pulse of the preparation radio frequency pulse sequence.

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.