# US 11,307,271 B2 · MRI method for determining a magnetic field map from a B0 reference scan and a WASSR scan

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- Number: US 11,307,271 B2
- Document ID: US11307271B2
- Status: Granted
- Kind: B2
- Application number: 16/960,412
- Filed: 2019-01-09
- Published: 2020-10-29
- Granted: 2022-04-19
- Earliest claimed: 2018-01-12
- Assignees: Koninklijke Philips N.V.
- Applicants: Koninklijke Philips N.V.
- Inventors: Elwin De Weerdt
- Web page: https://patentgrep.com/patent/US11307271B2

## CPC

- G01R 33/243: {Spatial mapping of the polarizing magnetic field}
- G01R 33/4828: {Resolving the MR signals of different chemical species, e.g. water-fat imaging}
- G01R 33/543: {Control of the operation of the MR system, e.g. setting of acquisition parameters prior to or during MR data acquisition, dynamic shimming, use of one or more scout images for scan plane prescription (G01R33/546 takes precedence)}
- G01R 33/5605: {by transferring coherence or polarization from a spin species to another, e.g. creating magnetization transfer contrast [MTC], polarization transfer using nuclear Overhauser enhancement [NOE]}
- G01R 33/5615: {Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]}
- G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12)

## Abstract

The invention provides for a medical imaging system (100, 300). The medical imaging system (100, 300) comprises a processor (104). Execution of machine executable instructions (120) causes the processor (104) to: receive magnetic resonance data, wherein the magnetic resonance data comprises B0 field data (122) of a reference scan for a plurality of voxels and water saturation data (124) of a WASSR scan for a subset of voxels of the plurality of voxels, the water saturation data (124) comprising data of a limited number of sample points; determine a local absolute water saturation frequency (130) for each voxel of the subset using the water saturation data (124) of the WASSR scan; and reconstruct a field map (132) comprising a local absolute water saturation frequency for each voxel of the plurality of voxels, wherein the reconstruction comprises determining relative frequency differences between the voxels using the B0 field data (122) of the reference scan and adding a frequency offset to the relative frequency differences based on the determined local absolute water saturation frequencies (130) of the subset.

## Claims

### Claim 1

A medical imaging system, comprising:
a memory for storing machine executable instructions; and
a processor for controlling the medical imaging system, wherein execution of the machine executable instructions causes the processor to control the medical imaging system to:
receive magnetic resonance data, wherein the magnetic resonance data comprises B0 field data of a reference scan for a plurality of voxels and water saturation data of a WASSR scan for a subset of voxels of the plurality of voxels, the subset of voxels having fewer voxels than the plurality of voxels, the water saturation data comprising data of a limited number of sample points per voxel of the subset of voxels;
determine a local absolute water saturation frequency for each voxel of the subset of voxels using the water saturation data of the WASSR scan; and
reconstruct a field map comprising a local absolute water saturation frequency for each voxel of the plurality of voxels, wherein the reconstruction of the field map comprises determining relative frequency differences between the voxels using the B0 field data of the reference scan and adding a frequency offset to the relative frequency differences based on the determined local absolute water saturation frequencies of the subset of voxels.

### Claim 2

A method of operating a medical imaging system, the method comprising:
receiving magnetic resonance data, wherein the magnetic resonance data comprises B0 field data of a reference scan for a plurality of voxels and water saturation data of a WASSR scan for a subset of voxels of the plurality of voxels, the subset of voxels having fewer voxels than the plurality of voxels, the water saturation data comprising data of a limited number of sample points per voxel of the subset of voxels;
determining a local absolute water saturation frequency for each voxel of the subset of voxels using the water saturation data of the WASSR scan; and
reconstructing a field map comprising a local absolute water saturation frequency for each voxel of the plurality of voxels, wherein the reconstruction of the field map comprises determining relative frequency differences between the voxels using the B0 field data of the reference scan and adding a frequency offset to the relative frequency differences based on the determined local absolute water saturation frequencies of the subset of voxels.

### Claim 3

A non-transitory computer readable medium storing machine executable instructions for execution by a processor controlling a medical imaging system, wherein execution of the machine executable instructions causes the processor to control the medical imaging system to:
receive magnetic resonance data, wherein the magnetic resonance data comprises B0 field data of a reference scan for a plurality of voxels and water saturation data of a WASSR scan for a subset of voxels of the plurality of voxels, the subset of voxels having fewer voxels than the plurality of voxels, the water saturation data comprising data of a limited number of sample points per voxel of the subset of voxels;
determine a local absolute water saturation frequency for each voxel of the subset of voxels using the water saturation data of the WASSR scan; and
reconstruct a field map comprising a local absolute water saturation frequency for each voxel of the plurality of voxels, wherein the reconstruction of the field map comprises determining relative frequency differences between the voxels using the B0 field data of the reference scan and adding a frequency offset to the relative frequency differences based on the determined local absolute water saturation frequencies of the subset of voxels.

### Claim 4 (depends on 1)

The medical imaging system of claim 1, wherein the limited number of sample points is 2 or 3.

### Claim 5 (depends on 1)

The medical imaging system of claim 1, wherein the subset of voxels comprises voxels with a minimum value of the B0 field data according to the reference scan.

### Claim 6 (depends on 1)

The medical imaging system of claim 1, wherein the subset of voxels is a single voxel.

### Claim 7 (depends on 1)

The medical imaging system of claim 1, further comprising:
a magnetic resonance imaging system configured for acquiring the magnetic resonance data from a subject within an imaging zone, wherein the memory further stores pulse sequence commands, wherein the pulse sequence commands are configured to acquire the B0 field data according to a B0 field mapping protocol and the water saturation data according to a WASSR magnetic resonance imaging protocol, wherein execution of the machine executable instructions causes the processor to control the magnetic resonance imaging system with the pulse sequence commands to acquire the magnetic resonance data.

### Claim 8 (depends on 1)

The medical imaging system of claim 1, wherein the received magnetic resonance data further comprises chemical exchange saturation transfer data of a chemical exchange saturation transfer (CEST) scan, wherein the execution of the machine executable instructions further causes the processor to control the medical imaging system to:
adjust the local absolute water saturation frequency of the chemical exchange saturation transfer data for each voxel of the plurality of voxels using the reconstructed field map; and
reconstruct a CEST map using the adjusted chemical exchange saturation transfer data.

### Claim 9 (depends on 8)

The medical imaging system of claim 8, wherein the memory stores further pulse sequence commands, wherein the further pulse sequence commands are configured to acquire the chemical exchange saturation transfer data according to a CEST magnetic resonance imaging protocol, wherein execution of the machine executable instructions causes the processor to control the magnetic resonance imaging system with the further pulse sequence commands to acquire the chemical exchange saturation transfer data.

### Claim 10 (depends on 8)

The medical imaging system of claim 8, wherein the CEST magnetic resonance imaging protocol is an amide proton transfer weighted magnetic resonance imaging protocol.

### Claim 11 (depends on 1)

The medical imaging system of claim 1, wherein the reconstruction of the field map further comprises reconstructing a B0 field map using the B0 field data of the reference scan comprising data for multiple echo shifts or echo times according to a Dixon-type magnetic resonance imaging protocol.

### Claim 12 (depends on 1)

The medical imaging system of claim 1, wherein the field map is reconstructed using the B0 field data of the reference scan comprising data for multiple echo shifts or echo times with water and fat signals being in-phase.

### Claim 13 (depends on 2)

The method of claim 2, further comprising:
acquiring the magnetic resonance data from a subject within an imaging zone of a magnetic resonance imaging system; and
acquiring the B0 field data according to a B0 field mapping protocol and the water saturation data according to a WASSR magnetic resonance imaging protocol.

### Claim 14 (depends on 3)

The non-transitory computer readable medium of claim 3, further comprising pulse sequence commands, wherein the pulse sequence commands are configured to acquire the B0 field data according to a B0 field mapping protocol and the water saturation data according to a WASSR magnetic resonance imaging protocol, wherein execution of the machine executable instructions further causes the processor to control the medical imaging system to acquire the magnetic resonance data.

### Claim 15 (depends on 2)

The method of claim 2, wherein the limited number of sample points is 2 or 3.

### Claim 16 (depends on 2)

The method of claim 2, wherein the subset of voxels comprises voxels with a minimum value of the B0 field data according to the reference scan.

### Claim 17 (depends on 2)

The method of claim 2, wherein the subset of voxels is a single voxel.

### Claim 18 (depends on 3)

The non-transitory computer readable medium of claim 3, wherein the received magnetic resonance data further comprises chemical exchange saturation transfer data of a chemical exchange saturation transfer (CEST) scan, wherein the execution of the machine executable instructions further causes the processor to control the medical imaging system to:
adjust the local absolute water saturation frequency of the chemical exchange saturation transfer data for each voxel of the plurality of voxels using the reconstructed field map; and
reconstruct a CEST map using the adjusted chemical exchange saturation transfer data.

### Claim 19 (depends on 18)

The non-transitory computer readable medium of claim 18, wherein the execution of the machine executable instructions further causes the processor to control the medical imaging system to acquire the chemical exchange saturation transfer data according to a CEST magnetic resonance imaging protocol.

### Claim 20 (depends on 19)

The non-transitory computer readable medium of claim 19, wherein the CEST magnetic resonance imaging protocol is an amide proton transfer weighted magnetic resonance imaging protocol.