# US 12,216,188 B2 · Method and apparatus for limiting radio frequency alternating magnetic field in magnetic resonance imaging

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- Number: US 12,216,188 B2
- Document ID: US12216188B2
- Status: Granted
- Kind: B2
- Application number: 18/126,509
- Filed: 2023-03-27
- Published: 2023-09-28
- Granted: 2025-02-04
- Earliest claimed: 2022-03-28
- Assignees: Siemens Healthineers AG
- Applicants: Siemens Healthcare GmbH
- Inventors: Ying Lun Wang; Qiu Yi Zhang; Zhi Bin Li
- Web page: https://patentgrep.com/patent/US12216188B2

## CPC

- G01R 33/5659: {caused by a distortion of the RF magnetic field, e.g. spatial inhomogeneities of the RF magnetic field (G01R33/56509, G01R33/56518, G01R33/56536 take precedence)}
- G01R 33/288: {Provisions within MR facilities for enhancing safety during MR, e.g. reduction of the specific absorption rate [SAR], detection of ferromagnetic objects in the scanner room}
- G01R 33/34015: {Temperature-controlled RF coils}
- G01R 33/341: comprising surface coils
- A61B 5/055: involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
- G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12)
- A61B: DIAGNOSIS; SURGERY; IDENTIFICATION (usefulness limited to only animals A61D)

## Abstract

A method and apparatus are provided for limiting a B1 field used for magnetic resonance imaging (MRI). The techniques described herein reduce a waste of performance of the B1 field while ensuring patient safety and improving the MR imaging quality.

## Claims

### Claim 1

A method for limiting a B1 field used for magnetic resonance imaging (MRI), comprising:
obtaining a first intensity of the B1 field that is required for a thermal equilibrium temperature of a surface temperature of a local coil to be a maximum safety temperature when the local coil is placed at a predetermined position in a bore of a magnetic resonance (MR) scanner;
obtaining, based on (i) the first intensity of the B1 field, and (ii) a relationship between the surface temperature of the local coil and a scanning duration and an intensity of the B1 field during an MR scanning process, a second intensity of the B1 field that is required for the surface temperature of the local coil to increase by heating to the maximum safety temperature within a predetermined total MR scanning duration when the local coil is placed at the predetermined position; and
determining, based on the second intensity of the B1 field, a third intensity of the B1 field that is required when MR scanning is performed for the predetermined total MR scanning duration,
wherein the third intensity of the B1 field is not greater than the second intensity of the B1 field.

### Claim 2 (depends on 1)

The method according to claim 1, wherein the predetermined position in the bore of the MR scanner is a highest point of an inner bore wall of the MR scanner.

### Claim 3 (depends on 1)

The method according to claim 1, further comprising:
prior to obtaining the first intensity of the B1 field, when the local coil is placed at the predetermined position and the intensity of the B1 field is one μT, obtaining a temperature difference between the thermal equilibrium temperature of the surface temperature of the local coil and an initial temperature of the surface temperature of the local coil, and setting the temperature difference as a first temperature difference,
wherein the obtaining of the first intensity of the B1 field comprises:
when the local coil is placed at the predetermined position in the bore of the MR scanner and the thermal equilibrium temperature of the surface temperature of the local coil is the maximum safety temperature, obtaining a temperature difference between the maximum safety temperature and the initial temperature of the surface temperature of the local coil, and setting the temperature difference as a second temperature difference;
dividing the second temperature difference by the first temperature difference; and
using an obtained quotient as a squared value of the first intensity of the B1 field.

### Claim 4 (depends on 1)

The method according to claim 1, wherein the relationship between the surface temperature of the local coil and the scanning duration and the intensity of the B1 field during the MR scanning process is represented as:
T
⁡
(
t
)
=
T
⁢
0
+
B
⁢
1
2
*
Δ
⁢
T
*
(
1
-
e
-
t
τ
)
,
wherein:
t represents a current scanned duration,
T(t) represents a current temperature of the surface temperature of the local coil,
T0 represents an initial temperature of the surface temperature of the local coil,
B1 represents the intensity of the B1 field,
ΔT represents a temperature difference between the thermal equilibrium temperature of the surface temperature of the local coil and the initial temperature of the surface temperature of the local coil when the local coil is placed at the predetermined position in the bore of the MR scanner and the intensity of the B1 field is one μT, and
τ represents a scanning duration required for the surface temperature of the local coil to reach 0.632ΔT when the local coil is placed at the predetermined position in the bore of the MR scanner and the intensity of the B1 field is one μT.

### Claim 5 (depends on 4)

The method according to claim 4, further comprising:
obtaining the second intensity of the B1 field that is required for the surface temperature of the local coil to increase by heating to the maximum safety temperature within a predetermined total MR scanning duration when the local coil is placed at the predetermined position in accordance with the following relationship:
(
B
⁢
1
short
)
2
=
(
B
⁢
1
infinite
)
2
*
1
1
-
e
-
t
scan
τ
,
wherein:
B1short represents the second intensity of the B1 field,
B1infinite represents the first intensity of the B1 field, and
tscan represents the predetermined total MR scanning duration.

### Claim 6 (depends on 1)

The method according to claim 1, wherein the determining of the third intensity of the B1 field comprises:
when only one scanning protocol is used within the predetermined total MR scanning duration, determining the third intensity of the B1 field as an intensity of the B1 field that corresponds to the scanning protocol.

### Claim 7 (depends on 1)

The method according to claim 1, wherein the determining of the third intensity of the B1 field comprises:
when a plurality of scanning protocols are used within the predetermined total MR scanning duration and a scanning duration for each scanning protocol is the same as one another, determining the third intensity of the B1 field as a sum of intensities of the B1 field from each of the plurality of the scanning protocols,
wherein an intensity of the B1 field that corresponds to an nth scanning protocol from among the plurality of scanning protocols is represented as:
(
B
⁢
1
protocoln
)
2
=
(
1
2
)
n
*
(
B
⁢
1
short
′
)
2
,
and wherein:
B1protocoln represents the intensity of the B1 field that corresponds to the nth scanning protocol,
B1short′ represents the third intensity of the B1 field,
1≤n≤N, and
N represents a total number of the plurality of scanning protocols used within the predetermined total MR scanning duration.

### Claim 8 (depends on 1)

The method according to claim 1, wherein the determining of the third intensity of the B1 field comprises:
when a plurality of scanning protocols are used within the predetermined total MR scanning duration and a scanning duration for each scanning protocol is not the same as one another, determining the third intensity of the B1 field as a sum of intensities of the B1 field of the plurality of scanning protocols,
wherein an intensity of the B1 field that corresponds to an nth scanning protocol of the plurality of scanning protocols is represented as:
(
B
⁢
1
protocoln
)
2
=
(
1
2
)
n
*
(
B
⁢
1
short
′
)
2
*
t
single
⁢
_
⁢
protocol
t
protocoln
,
wherein:
B1protocoln represents the intensity of the B1 field that corresponds to the nth scanning protocol,
B1short′ represents the third intensity of the B1 field,
tsingle_protocol represents a predetermined standard scanning duration for a single scanning protocol,
tprotocoln represents a scanning duration for the nth scanning protocol,
1≤n≤N, and
N represents a total number of the plurality of scanning protocols used within the predetermined total MR scanning duration.

### Claim 9 (depends on 1)

The method according to claim 1, wherein the determining of the third intensity of the B1 field comprises:
when a plurality of different types of scanning sequences are used within the predetermined total MR scanning duration, obtaining, through the following steps A and B, an intensity of a respective B1 field that is used for each one of the plurality of different types of scanning sequences:
A. performing an initialization m=1, and calculating:
(
B
⁢
1_limit
m
)
2
=
min
⁢
{
(
1
2
)
m
*
(
B
⁢
1
B
⁢
1
single
⁢
_
⁢
protocol
)
2
,
(
B
⁢
1
SAR
⁢
_
⁢
limit
)
2
}
wherein:
(
B
⁢
1
single
⁢
_
⁢
protocol
)
2
=
(
B
⁢
1
infinite
)
2
*
1
1
-
e
-
t
single
⁢
_
⁢
protocol
τ
t
allowed
⁢
_
⁢
duration
⁢
_
⁢
m
=
(
B
⁢
1
single
⁢
_
⁢
protocol
)
2
(
B
⁢
1_limit
m
)
2
*
t
single
⁢
_
⁢
protocol
;
a maximum value Pm that satisfies:
∑
p
m
=
1
p
m
=
P
m
t
seq
-
⁢
p
m
≤
t
allowed
-
⁢
duration
-
⁢
m
,
wherein:
the maximum value Pm represents a number of scanning sequences for which B1_limitm is used as the intensity of the B1 field,
B1_limitm represents the mth intensity of the B1 field that is used during a current MR scanning process,
B1single_protocol represents an intensity of the B1 field that is for a single scanning protocol,
B1SAR_limit represents a predetermined limit value of the intensity of the B1 field,
B1infinite represents the first intensity of the B1 field,
tsingle_protocol represents a predetermined standard scanning duration for a single scanning protocol,
τ represents a scanning duration required for the surface temperature of the local coil to reach 0.632*(the thermal equilibrium temperature−an initial temperature of the surface temperature of the local coil) when the local coil is placed at the predetermined position in the bore of the MR scanner and the intensity of the B1 field is one μT,
tallowed_duration_m represents an upper limit of a scanning duration for the mth intensity of the B1 field,
pm represents a sequence number of the pth scanning sequence in all scanning sequences for which B1_limitm is used as the intensity of the B1 field in the current MR scanning process,
Pm represents the number of scanning sequences for which B1_limitm is used as the intensity of the B1 field in the current MR scanning process, and
tseq_pm represents a scanning duration for the pth scanning sequence in all scanning sequences for which B1 _limitm is used as the intensity of the B1 field during the current MR scanning process;
B. letting m=m+1, and returning to step A until:
ΣPm≥P, wherein P represents a total number of scanning sequences used during the current MR scanning process.

### Claim 10

An apparatus for limiting a B1 field used for magnetic resonance imaging (MRI), comprising:
a computer readable medium having instructions stored thereon; and
a processor configured to execute the instructions stored on the computer-readable medium to cause the apparatus to:
obtain a first intensity of a B1 field that is required for a thermal equilibrium temperature of a surface temperature of a local coil to be a maximum safety temperature when the local coil is placed at a predetermined position in a bore of a magnetic resonance (MR) scanner; and
obtain, based on (i) the first intensity of the B1 field, and (ii) a relationship between the surface temperature of the local coil and a scanning duration and an intensity of the B1 field during an MR scanning process, a second intensity of the B1 field that is required for the local coil to be heated to the maximum safety temperature within a predetermined total MR scanning duration when the local coil is placed at the predetermined position; and
determine, based on the second intensity of the B1 field, a third intensity of the B1 field that is required when MR scanning is performed for the predetermined total MR scanning duration,
wherein the third intensity of the B1 field is not greater than the second intensity of the B1 field.