Search

US-12625209-B2 - Systems and methods for radio frequency (RF) based spatial encoding in magnetic resonance imaging using frequency-modulated RF pulses

US12625209B2US 12625209 B2US12625209 B2US 12625209B2US-12625209-B2

Abstract

Radio frequency (“RF”) gradient based magnetic resonance imaging (“MRI”) is provided by establishing a gradient in the RF transmit (B1) field using frequency-modulated RF pulses. A difference between the time-bandwidth product of the frequency-modulated RF pulses can be varied to provide impart different phases to magnetic resonance signals, where these different phases provide phase encoding of the acquired data. The time-bandwidth product difference can be created and varied by changing the pulse duration of one frequency-modulated RF pulse relative to the other while keeping the bandwidth of the pulses constant.

Inventors

  • Michael Garwood
  • Efraín Torres
  • Taylor Froelich

Assignees

  • REGENTS OF THE UNIVERSITY OF MINNESOTA

Dates

Publication Date
20260512
Application Date
20220502

Claims (20)

  1. 1 . A method for magnetic resonance imaging, the method comprising: (a) acquiring magnetic resonance data from a subject using a magnetic resonance imaging (MRI) system by: generating a pulse sequence with the MRI system, the pulse sequence comprising: a radio frequency (RF) excitation pulse; a first frequency-modulated RF pulse having a first time-bandwidth product, wherein the first frequency-modulated RF pulse is generated after the RF excitation pulse; a second frequency-modulated RF pulse having a second time-bandwidth product, wherein the second frequency-modulated RF pulse is generated after the first frequency-modulated RF pulse; acquiring magnetic resonance data with the MRI system by sampling magnetic resonance signals formed in response to the RF excitation pulse, wherein a phase whose value is dependent on the first time-bandwidth product and the second time-bandwidth product is imparted to magnetic resonance signals formed in response to the RF excitation pulse, and a difference between the first time-bandwidth product and the second time-bandwidth product is changed at least one of within the pulse sequence or in repetitions of the pulse sequence in order to phase encode the magnetic resonance data; (b) reconstructing an image from the acquired magnetic resonance data.
  2. 2 . The method of claim 1 , wherein the first frequency-modulated RF pulse and the second frequency-modulated RF pulse are adiabatic full-passage RF pulses.
  3. 3 . The method of claim 1 , wherein the first frequency-modulated RF pulse and the second frequency-modulated RF pulse are hyperbolic secant (HS) RF pulses.
  4. 4 . The method of claim 1 , wherein the difference between the first time-bandwidth product and the second time-bandwidth product is changed by changing a pulse duration of at least one of the first frequency-modulated RF pulse or the second frequency-modulated RF pulse in the repetitions of the pulse sequence.
  5. 5 . The method of claim 4 , wherein changing the pulse duration of at least one of the first frequency-modulated RF pulse or the second frequency-modulated RF pulse in the repetitions of the pulse sequence comprises increasing the pulse duration.
  6. 6 . The method of claim 4 , wherein a bandwidth of the first frequency-modulated RF pulse and the second frequency-modulated RF pulse is held constant while changing the pulse duration of at least one of the first frequency-modulated RF pulse or the second frequency-modulated RF pulse in the repetitions of the pulse sequence.
  7. 7 . The method of claim 1 , wherein the first time-bandwidth product is different from the second time-bandwidth product.
  8. 8 . The method of claim 1 , wherein in at least one repetition of the pulse sequence the first time-bandwidth product is set to be equal to the second time-bandwidth product.
  9. 9 . The method according to claim 1 , wherein reconstructing the image from the acquired magnetic resonance data comprises using an iterative model-based image reconstruction that models spatial encoding based on a signal evolution simulation.
  10. 10 . The method of claim 9 , wherein the signal evolution simulation comprises a full Bloch simulation.
  11. 11 . The method of claim 9 , wherein the iterative model-based image reconstruction takes receiver coil sensitivity data as an additional input.
  12. 12 . The method of claim 9 , wherein the iterative model-based image reconstruction comprises iteratively solving a regularized linear inverse problem.
  13. 13 . A method for magnetic resonance imaging, the method comprising: (a) acquiring magnetic resonance data from a subject using a magnetic resonance imaging (MRI) system to generate a pulse sequence that phase encodes the magnetic resonance data by imparting a phase to magnetic resonance signals by applying frequency-modulated radio frequency (RF) pulses to the subject, wherein the frequency-modulated RF pulses have variable time-bandwidth products, thereby defining an R-difference value between the frequency-modulated RF pulses, which causes the phase to be imparted to the magnetic resonance signals; and (b) reconstructing an image from the acquired magnetic resonance data.
  14. 14 . The method of claim 13 , wherein the pulse sequence phase encodes the magnetic resonance data by changing the R-difference value while acquiring the magnetic resonance data, such that a different phase is imparted to the magnetic resonance signals with each different R-difference value.
  15. 15 . The method of claim 14 , wherein the R-difference value is changed by increasing a pulse duration of one of the frequency-modulated RF pulses while keeping a bandwidth of the frequency-modulated RF pulses constant.
  16. 16 . The method of claim 15 , wherein the R-difference value is increased by increasing the pulse duration of one of the frequency-modulated RF pulses while keeping the bandwidth of the frequency-modulated RF pulses constant.
  17. 17 . The method of claim 13 , wherein the pulse sequence is a multi-shot pulse sequence and the R-difference value is changed for each shot of the multi-shot pulse sequence.
  18. 18 . The method of claim 13 , wherein the frequency-modulated RF pulses comprise adiabatic full passage RF pulses.
  19. 19 . The method of claim 18 , wherein the adiabatic full passage RF pulses comprise hyperbolic secant RF pulses.
  20. 20 . The method of claim 13 , wherein the frequency-modulated RF pulses comprise chirp RF pulses.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a U.S. national phase application of pending International Application No. PCT/US2022/027319 filed on May 2, 2022 and now published as WO 2022/232695 A1, and entitled “SYSTEMS AND METHODS FOR RADIO FREQUENCY (RF) BASED SPATIAL ENCODING IN MAGNETIC RESONANCE IMAGING,” which in turn claims the benefit of U.S. Provisional Patent Application Ser. No. 63/182,355, filed on Apr. 30, 2021, and entitled “SYSTEMS AND METHODS FOR RADIO FREQUENCY (RF) GRADIENT ENCODING IN MAGNETIC RESONANCE IMAGING USING FREQUENCY-MODULATED RF PULSES,” and U.S. Provisional Patent Application Ser. No. 63/333,452, filed on Apr. 21, 2022, and entitled “SYSTEMS AND METHODS FOR RADIO FREQUENCY (RF) GRADIENT ENCODING IN MAGNETIC RESONANCE IMAGING USING FREQUENCY-MODULATED RF PULSES,” all of which are herein incorporated by reference in their entirety. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH This invention was made with government support under EB025153 and EB027061 awarded by the National Institutes of Health. The government has certain rights in the invention. BACKGROUND Magnetic resonance imaging (“MRI”) permits robust, high-resolution imaging with tunable image contrast that is free of ionizing radiation, thereby making it an indispensable tool in clinical medicine and biomedical research. However, the financial burden of purchasing and maintaining such a technology is a considerable draw back that limits MRI access to mostly wealthy institutions in developed countries. In recent years, researchers have focused on developing lower cost and/or portable MRI systems to address these challenges. Most of these investigations have focused on redesigning various components of the MRI system. Other approaches have targeted the stringent requirements of the static field B0 gradient systems. Conventional linear B0-gradient coils consume precious space in the magnet bore, require significant electrical power and water cooling to operate, require maintenance, and produce loud acoustic noise, which is a common complaint of patients and potentially damaging to their hearing. Elimination of the B0-gradient system would significantly reduce the infrastructure needs and financial burden of MRI, while simultaneously permitting silent MRI. SUMMARY OF THE DISCLOSURE The present disclosure addresses the aforementioned draw backs by providing a method for magnetic resonance imaging, in which magnetic resonance data are acquired from a subject using an MRI system by generating a pulse sequence with the MRI system, where the pulse sequence includes: a radio frequency (RF) excitation pulse: a first frequency-modulated RF pulse having a first time-bandwidth product: and a second frequency-modulated RF pulse having a second time-bandwidth product. The first frequency-modulated RF pulse is generated after the RF excitation pulse, and the second frequency-modulated RF pulse is generated after the first frequency-modulated RF pulse. Magnetic resonance data are acquired with the MRI system by sampling magnetic resonance signals formed in response to the RF excitation pulse, where a phase whose value is dependent on the first time-bandwidth product and the second time-bandwidth product is imparted to magnetic resonance signals formed in response to the RF excitation pulse. The difference between the first time-bandwidth product and the second time-bandwidth product is changed in repetitions of the pulse sequence in order to phase encode the magnetic resonance data. An image is then reconstructed from the acquired magnetic resonance data. It is another aspect of the present disclosure to provide a method for magnetic resonance imaging, in which magnetic resonance data are acquired from a subject using an MRI system to generate a pulse sequence that phase encodes the magnetic resonance data by imparting a phase to magnetic resonance signals by applying frequency-modulated RF pulses to the subject, where the frequency-modulated RF pulses have different time-bandwidth products, thereby defining an R-difference value between the frequency-modulated RF pulses, which causes the phase to be imparted to the magnetic resonance signals. An image is then reconstructed from the acquired magnetic resonance data. It is still another aspect of the present disclosure to provide a method for magnetic resonance imaging, in which magnetic resonance data are acquired from a subject using an MRI system to generate a pulse sequence that generates a radio frequency (RF) gradient using frequency-modulated RF pulses, wherein the RF gradient provides spatial encoding of the magnetic resonance data. An image is then reconstructed from the acquired magnetic resonance data. It is another aspect of the present disclosure that images can be reconstructed from data acquired using the acquisition techniques described in the present disclosure using a model-based image reconstruction framework. The foregoing and other aspects and advant