Iterative shimming for low-field head-optimized MRI
The present disclosure provides various devices, systems, and methods for iterative shimming in magnetic resonance imaging (MRI). In one aspect, a method of iterative shimming can include generating a field map of a magnetic field B 0 from an MRI system. The MRI system can include a shim tray and an array of permanent magnets to generate the magnetic field B 0 . The shim tray can include slots to receive shim magnets. The method can further include shimming the MRI system by iteratively: applying the field map to a genetic algorithm to determine a set of slots to receive a set of shim magnets, installing the set of shim magnets in the set of slots, generating a next field map of the magnetic field B 0 , and either performing a next iteration based on the next field map or determining to not perform the next iteration based on the next field map.
1 . A method, comprising:
generating a first field map of a magnetic field Bo from a magnetic resonance imaging (MRI) system, wherein the MRI system comprises:
an array of permanent magnets in a modified Halbach dome arrangement, wherein the array comprises one open end sized to receive a head of a patient, and wherein the array is configured to generate the magnetic field B 0 ; and
a shim tray comprising slots configured to receive shim magnets;
wherein the first field map corresponds to a magnetic field based on each of the slots being empty;
applying the first field map to a genetic algorithm to determine a first set of slots to receive a first set of shim magnets;
installing the first set of shim magnets in the first set of slots;
generating a second field map of the magnetic field B 0 , wherein the second field map corresponds to a magnetic field based on the first set of shim magnets being installed in the first set of slots;
applying the second field map to the genetic algorithm to determine a second set of slots to receive a second set of shim magnets, wherein the second set of slots is different than the first set of slots; and
installing the second set of shim magnets in the second set of slots.
2 . The method of claim 1 , wherein a size of the first set of shim magnets is the same as a size of the second set of shim magnets.
3 . The method of claim 1 , wherein a size of the first set of shim magnets is different from a size of the second set of shim magnets.
4 . The method of claim 1 , further comprising:
determining a set of empty slots in the shim tray, wherein the set of empty slots consists of the slots excluded from the first set of slots; and
constraining the genetic algorithm to consider only the set of empty slots for inclusion in the second set of slots.
5 . The method of claim 1 , wherein applying the first field map to the genetic algorithm to determine the first set of slots to receive the first set of shim magnets further comprises determining, for each of the first set of slots, a configuration for installing the corresponding one of the shim magnets in the slot, wherein the configuration is selected from a set of possible configurations, and wherein the set of possible configurations comprises a first configuration and a second configuration.
6 . The method of claim 5 , wherein, in the first configuration, the corresponding shim magnet generates a first magnetic field that is substantially aligned with a direction of the magnetic field B 0 , and wherein, in the second configuration, the corresponding shim magnet generates a second magnetic field that is substantially reverse to the direction of the magnetic field B 0 .
7 . The method of claim 6 , wherein the set of possible configurations further comprises a second configuration, a third configuration, a fourth configuration, a fifth configuration, a sixth configuration, a seventh configuration, and an eight configuration.
8 . The method of claim 5 , wherein, in the first configuration, a pole of the shim magnet is oriented in a first direction, and wherein, in the second configuration, the pole of the shim magnet is oriented in a second direction that is opposite the first direction.
9 . The method of claim 5 , wherein applying the second field map to the genetic algorithm to determine the second set of slots to receive the second set of shim magnets further comprises determining, for each of the second set of slots, a configuration for installing the corresponding one of the shim magnets in the slot, wherein the configuration is selected from the set of possible configurations.
10 . The method of claim 1 , wherein each of the slots is configured to receive at least an end portion of a shim magnet, and wherein each of the slots is configured to receive the same size end portion.
11 . The method of claim 1 , wherein applying the first field map to the genetic algorithm to determine the first set of slots to receive the first set of shim magnets comprises determining, for each of the first set of slots, a strength of the corresponding one of the shim magnets.
12 . The method of claim 11 , wherein determining, for each of the first set of shim tray slots, the strength of the corresponding one of the shim magnets comprises determining whether to install either a first shim magnet having a first strength or a second shim magnet having a second strength.
13 . The method of claim 12 , wherein the first shim magnet having the first strength comprises an N42 magnet, and wherein the second shim magnet having the second strength comprises an N52 magnet.
14 . The method of claim 1 , further comprising installing the first set of shim magnets in the first set of slots comprises installing first shim magnets each having a first strength; and
wherein installing the second set of shim magnets in the second set of slots comprises installing second shim magnets having a second strength, wherein the second strength is weaker than the first strength.
15 . A system for magnetic resonance imaging (MRI), comprising:
an array of permanent magnets in a modified Halbach arrangement forming one open end, wherein the open end is sized to receive a head of a patient, wherein the array of permanent magnets is configured to generate a low-field strength magnetic field B 0 toward an object of interest located within a field of view; and
a shim tray positionable within the array of permanent magnets, wherein the shim tray comprises an array of shim tray slots, wherein the shim tray comprises an opening sized to receive the head of the patient, and wherein each of the shim tray slots is configured to receive a shim magnet in a first configuration and a second configuration, wherein, in the first configuration, a polarity of the shim magnet is oriented in a first direction, and wherein, in the second configuration, the polarity of the shim magnet is oriented in a second direction that is opposite the first direction.
16 . The system of claim 15 , wherein the array of permanent magnets comprise primary permanent magnets, wherein the system further comprises a plurality of shim magnets installed in a portion of the shim tray slots, wherein the plurality of shim magnets comprise secondary permanent magnets positioned relative to the primary permanent magnets to increase a homogeneity of the magnetic field B 0 generated by the permanent magnets.
17 . The system of claim 16 , wherein each of the shim tray slots is configured to receive at least an end portion of a shim magnet, and wherein each of the shim tray slots is configured to receive the same size end portion.
18 . The system of claim 17 , wherein the plurality of shim magnets comprises:
a first shim magnet defining a first magnetic field strength; and
a second shim magnet defining a second magnetic field strength.
19 . The system of claim 16 , wherein at least one of the plurality of shim magnets is installed in the second configuration.
20 . The system of claim 16 , wherein the plurality of shim magnets are installed in less than all of the shim tray slots.