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EP-4455710-B1 - SHIMMING METHOD AND SHIMMING APPARATUS

EP4455710B1EP 4455710 B1EP4455710 B1EP 4455710B1EP-4455710-B1

Inventors

  • WHEATON, ANDREW JAMES
  • DANNELS, WAYNE R.

Dates

Publication Date
20260506
Application Date
20240426

Claims (14)

  1. A shimming method (400) for performing patient-specific B1 field shimming in a magnetic resonance imaging system, comprising: obtaining (410) patient information of a patient to be imaged by the magnetic resonance imaging system; determining (420) an orientation of a projection based on the obtained patient information; acquiring (430) B1 projection data, using the magnetic resonance imaging system, along the determined orientation of the projection; determining (440) a set of B1 shimming parameters based on the acquired B1 projection data; controlling (450) the magnetic resonance imaging system based on the determined set of B1 shimming parameters.
  2. The shimming method (400) of claim 1 , wherein the step of determining (420) the orientation of the projection further comprises: extracting (622), from the patient information, a specific imaging anatomy, and obtaining (624), from a first look-up table, using the extracted imaging anatomy as a key, the orientation of the projection, and the step of acquiring (430) the B1 projection data further comprises: generating (626) gradient signals based on the obtained orientation of the projection, and applying (628) the generated gradient signals to gradient coil drivers of the magnetic resonance imaging system.
  3. The shimming method (400) of claim 2, further comprising: determining (662) B1 field distributions corresponding to different particular imaging anatomies, respectively; performing (664) principal component analysis on each of the determined B1 field distributions; identifying (666), based on a result of the principal component analysis, a characteristic projection for each of the determined B1 field distributions; and storing (668) the identified characteristic projections and the corresponding imaging anatomies as matched pairs in the first look-up table.
  4. The shimming method (400) of claim 3, wherein the step of determining (662) the B1 field distributions further comprises determining the B1 field distributions based on data collected in a physics simulation, a phantom experiment, an in vivo experiment, and/or a clinical procedure conducted on a patient.
  5. The shimming method (400) of any of claims 2 to 4, wherein the step of determining (440) the set of B1 shimming parameters further comprises: extracting (1024), from the patient information, a specific physical feature; obtaining (1026), from a second look-up table, a B1 field distribution, wherein a particular physical feature related to the B1 field distribution matches the extracted physical feature, and particular projection data related to the B1 field distribution matches the acquired projection data; and determining (1026) a particular set of B1 shimming parameters related to the obtained B1 field distribution to be the determined set of B1 shimming parameters.
  6. The shimming method (400) of claim 5, further comprising: determining (1062) B1 field distributions for different corresponding physical features, respectively; generating (1064) corresponding projection data for each of the determined B1 field distributions; determining (1066) a corresponding set of B1 shimming parameters for each of the determined B1 field distributions; and storing (1068) the determined B1 field distributions in the second look-up table, wherein each of the determined B1 field distributions is stored in association with the corresponding physical feature, the corresponding projection data, and the corresponding set of B1 shimming parameters.
  7. The method (400) of claim 5 or claim 6, wherein the extracted physical feature includes at least one of a dimensional scale of the imaging anatomy, an aspect ratio of the imaging anatomy, a body fat composition of the patient, a gender of the patient, and an age of the patient.
  8. The shimming method (400) of any of claims 2 to 4, wherein the step of determining the set of B1 shimming parameters further comprises: extracting, from the patient information, a specific physical feature; applying the extracted physical feature and the acquired projection data to a trained neural network; and determining the set of B1 shimming parameters from outputs of the neural network.
  9. The shimming method (400) of any of claims 1 to 4, wherein the step of determining (440) the set of B1 shimming parameters further comprises: receiving (1210), along the determined orientation of the projection, B1 projection data corresponding to each of a plurality of sets of B1 shimming parameters, respectively; calculating (1220), based on the received B1 projection data, a cost function score corresponding to each of the plurality of sets of B1 shimming parameters, respectively; identifying (1230) a particular set of B1 shimming parameters which corresponds to a lowest cost function score; and determining (1240) the identified particular set of B1 shimming parameters to be the determined set of B1 shimming parameters.
  10. The shimming method (400) of any of claims 1 to 4, wherein the step of determining (440) the set of B1 shimming parameters further comprises: controlling (1610) a set of B1 shimming parameters to switch ON each of individual transmit channels in an RF transmitter of the magnetic resonance imaging system, with other transmit channels switched OFF; receiving (1620) particular B1 projection data acquired with each of the individual transmit channels switched ON, respectively; analyzing (1630) the received particular B1 projection data to evaluate an effect of each of the individual transmit channels on a symmetry of a profile of the B1 projection data; calculating (1640) a particular set of B1 shimming parameters that maximizes the symmetry of the profile of the B1 projection data; and determining (1640) the calculated particular set of B1 shimming parameters to be the determined set of B1 shimming parameters.
  11. The shimming method (400) of claim 10, wherein the step of determining (420) the orientation of the projection further comprises determining the orientation such that the profile of the B1 projection data acquired along the determined orientation has minimum symmetry.
  12. The shimming method (400) of any preceding claim, wherein the step of acquiring (430) the B1 projection data further comprises at least one of: acquiring the projection data using a Bloch-Siegert Shift method, a Double Angle method, an Actual Flip Angle method, a Dual Refocusing Echo Acquisition Mode method, a Phase Sensitive method, or a Saturation Recovery method; and performing 2D spatial selection to select a portion within a volume of the patient along the determined orientation of the projection.
  13. The shimming method (400) of any preceding claim, wherein the step of acquiring (430) the B1 projection data further comprises performing more than one readout per excitation to acquire more than one projection per repetition time.
  14. A shimming apparatus (300) for performing patient-specific B1 field shimming in a magnetic resonance imaging system, the apparatus comprising: processing circuitry (310, 320, 330) configured to obtain patient information of a patient to be imaged by the magnetic resonance imaging system; determine an orientation of a projection based on the obtained patient information; acquire B1 projection data, using the magnetic resonance imaging system, along the determined orientation of the projection; and determine a set of B1 shimming parameters based on the acquired B1 projection data; control the magnetic resonance imaging system based on the determined set of B1 shimming parameters.

Description

FIELD Embodiments described herein relate generally to a shimming method and shimming apparatus. This disclosure relates to patient-specific shimming of radio frequency (RF) magnetic fields (i.e., "B1 fields") in magnetic resonance imaging (MRI) systems. The B1 field shimming is performed in vivo based on projection data acquired along one or more projections over a patient that are determined in accordance with the anatomy of the patient to be imaged. BACKGROUND The background description provided herein is for the purpose of generally presenting the context of the disclosure. As part of the procedure for producing MRI images within the body of a patient, a static magnetic field (B0) is used by an MRI scanner to align the nuclear spins of atoms. During the scan, RF pulses generated by an RF transmitter cause perturbations to the local magnetic field, and RF signals emitted by the nuclear spins are detected by an RF receiver. In order to achieve diagnostic images with high spatial resolution and high contrast resolution, the strength of B0 fields is increasingly higher (from 1.5T to 3T and above) in clinical practice. Under higher B0 fields, however, RF behavior in the patient becomes more complex. For example, the dielectric properties of the human body can cause local perturbations to the B1 fields, resulting in non-uniform excitation. This can introduce errors in the contrast of resultant diagnostic images, potentially leading to misdiagnosis. To tackle this problem, a common approach is to map the RF transmit magnetic field and perform shimming correction in accordance with the map, so as to obtain a more uniform B1 field distribution. In a spatially resolved B1 map, each pixel represents a measurement of the transmit magnetic field B1 at that location. Besides B1 shimming, the map can also be used for RF transmit calibration (for accurate RF pulse flip angles), parallel transmit (pTx) RF pulse control (pTx is generally necessary for 7T MRIs, for example), and correction of quantitative relaxometry maps (commonly associated with longitudinal relaxation time (T1) mapping). A B1 map can be acquired as a pre-scan procedure, and then the B1 map can be used for calibration, design, or correction of data in subsequent sequence acquisitions during the protocol. US9086446 discloses a method of B1 field mapping relating to Magnetic resonance imaging (MRI). For the sake of saving scan time, it is beneficial for pre-scans to be as short as reasonably possible. Although there exist a number of B1 mapping approaches, a common downside is that the measurement time is impractically prolonged. A B1 shimming method that can be performed to correct the B1 field variation at clinical field strengths (e.g., 1.5T, 3T, etc.) within scan time on the order of one second has yet to be established. Therefore, it is desirable to address these and other deficiencies of current approaches. SUMMARY OF INVENTION The invention is defined by the claims. Certain particular embodiments are presented in the dependent claims. It is noted that B1 projection data is 1D image data obtained using gradient signals along the respective orientation for the projection. BRIEF DESCRIPTION OF THE DRAWINGS Various embodiments of this disclosure that are proposed as examples will be described in detail with reference to the following figures, wherein like numerals reference like elements, and wherein: FIG. 1 shows exemplary dielectric artifacts in MRI images;FIG. 2 shows exemplary B1 field maps acquired in four different imaging anatomies, i.e., head, abdomen, breasts, and thighs;FIG. 3 shows a non-limiting example of a block diagram of a B1 field shimming apparatus according to one embodiment of the present disclosure;FIG. 4 shows a non-limiting example of a flow chart of a B1 field shimming method according to one embodiment of the present disclosure;FIG. 5 shows a non-limiting example of a block diagram of projection determination circuitry that determines one or more projections along which projection data is acquired by the MRI system, according to one embodiment of the present disclosure;FIG. 6 shows a non-limiting example of a flow chart of a process of determining one or more projections along which projection data is acquired by the MRI system, according to one embodiment of the present disclosure;FIG. 7 shows a non-limiting example of different projections and projection data acquired along the projections respectively, according to one embodiment of the present disclosure;FIG. 8 shows a non-limiting example of different projection combinations identified for different imaging anatomy, according to one embodiment of the present disclosure;FIG. 9 shows a non-limiting example of a block diagram of shimming parameter resolving circuitry that resolves a set of shimming parameters based on projection data acquired, according to one embodiment of the present disclosure;FIG. 10 shows a non-limiting example of a flow chart of a process that resolves