Method and apparatus for limiting radio frequency alternating magnetic fields in magnetic resonance imaging

Abstract

The embodiment of the present application discloses a method and device for limiting radio frequency alternating magnetic field in magnetic resonance imaging. The method comprises: obtaining a first B1 field intensity required when a local coil surface temperature reaches a maximum safe temperature in a thermal equilibrium state when the local coil is placed at a set position in a detection hole of an MR scanner; obtaining a second B1 field intensity required for heating the local coil surface temperature to the maximum safe temperature within a set MR total scanning time when the local coil is placed at the set position according to a relationship among the local coil surface temperature, the scanning time and the B1 field intensity during MR scanning and the first B1 field intensity; and determining a third B1 field intensity required for MR scanning using the set MR total scanning time according to the second B1 field intensity. The embodiment of the present application reduces waste of B1 field performance while ensuring patient safety and improving MR imaging quality.

Description

Technical Field

[0001] This invention relates to the field of MR (Magnetic Resonance) technology, and in particular to a method, apparatus and magnetic resonance scanner for confining radio frequency alternating magnetic fields in MRI (Magnetic Resonance Imaging). Background Technology

[0002] The IEC (Electrical Equipment for Medical Devices) standard stipulates that during MR scanning, the surface temperature of the local coil placed on the scanned human body part must meet the following requirement: maintain below 41°C at radio frequency power to avoid harm to the human body. However, the standard does not specify the method to meet this requirement.

[0003] NEMA (National Electrical Manufacturers Association) MS-14 is a new standard that details radio frequency (RF) heating test instructions for evaluating the safety of MR (Medium-Range) local coils. The new MS-14 test standard requires measuring the surface temperature of the local coil when thermal equilibrium is reached. Therefore, the RF alternating magnetic field generated by the RF coil, i.e., the B1 field, must be more strictly limited and monitored to ensure that older RF coils still comply with the new standard.

[0004] Currently, most MR scanners limit the B1 field based on the patient's Specific Absorption Rate (SAR) or the heating angle of the RF coil surface. In this approach, the B1 field intensity is typically limited to a very low value so that even with a long scan duration, the local coil surface temperature does not exceed 41°C. However, in practical applications, scan durations are limited for most patients, so setting the B1 field intensity too low results in wasted B1 field performance. Tests have shown that if the B1 field intensity is set to the level corresponding to a coil surface temperature not exceeding the regulatory requirement of 41°C, a 20-minute scan duration will result in a B1 field performance loss of 1 - (1 / 1.78) = 43.8%, and a 10-minute scan duration will result in a B1 field performance loss of 66.2%. Summary of the Invention

[0005] In view of this, embodiments of the present invention propose a B1 field limiting method and apparatus in MR imaging, so as to improve MR imaging quality and reduce waste of B1 field performance while ensuring patient safety, given a limited total MR scan time.

[0006] The present invention also proposes an MR scanner to improve MR imaging quality and reduce waste of B1 field performance while ensuring patient safety, given a limited total MR scan time.

[0007] The technical solution of this invention is implemented as follows:

[0008] A radio frequency alternating magnetic B1 field confinement method for magnetic resonance imaging, the method comprising:

[0009] The first intensity of the B1 field required when the thermal equilibrium temperature of the surface temperature of the local coil is at the maximum safe temperature when the local coil is placed at a set position in the detection aperture of the magnetic resonance scanner.

[0010] Based on the relationship between the surface temperature of the local coil and the scanning time and the intensity of the B1 field during the magnetic resonance scanning process, and the first intensity of the B1 field, the second intensity of the B1 field required to heat the surface temperature of the local coil to the maximum safe temperature within the set total magnetic resonance scanning time when the local coil is placed at the set position is obtained.

[0011] Based on the second intensity of field B1, the third intensity of field B1 required for magnetic resonance scanning using the set total magnetic resonance scanning time is determined, wherein the third intensity of field B1 is not greater than the second intensity of field B1.

[0012] The designated location is the highest point of the inner wall of the detection aperture of the magnetic resonance scanner.

[0013] Before obtaining the first intensity of the B1 field required for the thermal equilibrium temperature of the local coil surface to reach the maximum safe temperature when the local coil is placed at a predetermined position within the detection aperture of the magnetic resonance scanner, the process further includes:

[0014] When the local coil is placed at the set position and the B1 field strength is 1 microt, the temperature difference between the thermal equilibrium temperature and the initial temperature of the local coil surface temperature is obtained and set as the first temperature difference.

[0015] The first intensity of the B1 field required for the thermal equilibrium temperature of the surface temperature of the local coil to be at the maximum safe temperature when the local coil is placed at a predetermined position within the detection aperture of the magnetic resonance scanner includes:

[0016] When the local coil is placed at a set position within the detection aperture of the magnetic resonance scanner and the thermal equilibrium temperature of the local coil surface is the maximum safe temperature, the temperature difference between the maximum safe temperature and the initial temperature of the local coil surface is obtained and set as the second temperature difference.

[0017] Divide the second temperature difference by the first temperature difference, and use the resulting quotient as the square of the first intensity of field B1.

[0018] The relationship between the surface temperature of the local coil and the scanning time and B1 field intensity during magnetic resonance scanning is as follows:

[0019]

[0020] Where t is the current scan duration; T(t) is the current temperature of the local coil surface; T0 is the initial temperature of the local coil surface; B1 is the B1 field strength; ΔT is the temperature difference between the thermal equilibrium temperature of the local coil surface and the initial temperature of the local coil surface when the local coil is placed at a set position in the detection aperture of the magnetic resonance scanner and the B1 field strength is 1 microtesla; τ is the scan duration required when the local coil surface temperature reaches 0.632ΔT when the local coil is placed at a set position in the detection aperture of the magnetic resonance scanner and the B1 field strength is 1 microtesla.

[0021] The acquisition of the second intensity of the B1 field, which is required to heat the surface temperature of the local coil to the maximum safe temperature within a set total magnetic resonance scan time when the local coil is placed at the set position, includes:

[0022]

[0023] Among them, B1 short The second strongest in B1; B1 infinite The highest intensity in B1; t scan This is the set total scan time for magnetic resonance imaging.

[0024] The step of determining the third intensity of the B1 field required for magnetic resonance scanning using the set total magnetic resonance scan duration, based on the second intensity of the B1 field, includes:

[0025] When only one scanning protocol is used within the set total magnetic resonance scanning time, the third intensity of the B1 field is taken as the B1 field intensity corresponding to that scanning protocol.

[0026] The step of determining the third intensity of the B1 field required for magnetic resonance scanning using the set total magnetic resonance scan duration, based on the second intensity of the B1 field, includes:

[0027] When multiple scanning protocols are used within a set total magnetic resonance scanning time, and the scanning time of each scanning protocol is the same, the third intensity of the B1 field is taken as the sum of the B1 field intensities corresponding to all scanning protocols; and the B1 field intensity corresponding to the nth scanning protocol is:

[0028]

[0029] Among them, B1 protocoln Let B1 be the field intensity corresponding to the nth scanning protocol. short ′ represents the third intensity of the B1 field, 1≤n≤N, where N is the total number of scanning protocols used within the set total magnetic resonance scanning time.

[0030] The step of determining the third intensity of the B1 field required for magnetic resonance scanning using the set total magnetic resonance scan duration, based on the second intensity of the B1 field, includes:

[0031] When multiple scanning protocols are used within a set total magnetic resonance scanning time, and the scanning time of each scanning protocol is not exactly the same, the third intensity of the B1 field is taken as the sum of the B1 field intensities corresponding to all scanning protocols; and the B1 field intensity corresponding to the nth scanning protocol is:

[0032]

[0033] Among them, B1 protocoln Let B1 be the field intensity corresponding to the nth scanning protocol. short ′ represents the third intensity of field B1, t single_protocol t is the standard scan duration for a single scan protocol. protocoln Let N be the actual scan duration of the nth scan protocol, 1≤n≤N, where N is the total number of scan protocols used within the set total magnetic resonance scan duration.

[0034] The step of determining the third intensity of the B1 field required for magnetic resonance scanning using the set total magnetic resonance scan duration, based on the second intensity of the B1 field, includes:

[0035] When multiple different types of scanning sequences are used within a set total magnetic resonance scan duration, the B1 field intensity for each scanning sequence is obtained through the following steps A and B:

[0036] A. Initialize m = 1 and calculate:

[0037]

[0038] in,

[0039]

[0040]

[0041] Please satisfy:

[0042] Maximum P m If the value is P, then: the maximum P m The value is the value of B1_limit. m The number of scan sequences representing the B1 field intensity;

[0043] in,

[0044] B1_limit mLet B1 be the m-th B1 field intensity used in this magnetic resonance scan. single_protocol B1 field intensity for a single scan protocol; B1 SAR_limit The preset B1 field strength limit value; B1 infinite The highest intensity in B1; t single_protocol τ is the standard scan duration for a single scan protocol; τ is the scan duration required when the surface temperature of the local coil reaches 0.632 * (thermal equilibrium temperature - initial temperature of the local coil surface) with the local coil placed at a set position within the detector aperture of the magnetic resonance scanner and the B1 field strength being 1 microtesla; t allowed_duration_m p is the upper limit of the scan duration for the m-th B1 field intensity; m To use B1_limit during this magnetic resonance scan m P is the seed number of the p-th scan sequence among all the scan sequences of B1 field intensity; m To use B1_limit during this magnetic resonance scan m The number of scan sequences representing the B1 field intensity; To use B1_limit during this magnetic resonance scan m The scan duration of the p-th scan sequence among all scan sequences of B1 field intensity;

[0045] B. Let m = m + 1, return to step A, until...

[0046] ∑P m ≥P

[0047] Where P represents the total number of scanning sequences used in this magnetic resonance scan.

[0048] A radio frequency alternating magnetic field confinement device for magnetic resonance imaging, the device comprising:

[0049] The B1 field first intensity acquisition module is used to acquire the B1 field first intensity required when the thermal equilibrium temperature of the surface temperature of the local coil is at the maximum safe temperature when the local coil is placed at a set position in the detection hole of the MR scanner.

[0050] The B1 field second intensity acquisition module is used to acquire, based on the relationship between the surface temperature of the local coil and the scan duration and the B1 field intensity during the MR scan, and the B1 field first intensity, the second intensity of the B1 field required to heat the local coil to the maximum safe temperature within the set total MR scan duration when the local coil is placed at the set position; and based on the second intensity of the B1 field, to determine the third intensity of the B1 field required for MR scanning using the set total MR scan duration, wherein the third intensity of the B1 field is not greater than the second intensity of the B1 field.

[0051] A magnetic resonance scanner includes a radio frequency alternating magnetic field confinement device for magnetic resonance scanning as described above.

[0052] The embodiments of the present invention improve MR imaging quality and reduce waste of B1 field performance while ensuring patient safety, under the condition of limited total MR scan time. Attached Figure Description

[0053] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which will make the above and other features and advantages of the present invention more apparent to those skilled in the art. In the drawings:

[0054] Figure 1 A schematic diagram showing the change of local coil surface temperature with scanning time for different B1 field intensities;

[0055] Figure 2 This is a flowchart of the B1 field confinement method in MRI provided by an embodiment of the present invention;

[0056] Figure 3 In this embodiment of the invention, MR scanning was performed using different B1 field intensities to obtain the variation curves of the local coil surface temperature under different B1 field intensities and a schematic diagram of the fitting curve of each curve.

[0057] Figure 4 In this embodiment of the invention, when τ = 1454 seconds, the total MR scan time is set to 35 minutes, and the local coil surface temperature reaches the maximum safe temperature of 41°C at the end of the scan, the curve showing the change of the local coil surface temperature; and when the total MR scan time is unlimited, the local coil surface temperature has a thermal equilibrium temperature of 41°C, and the curve showing the change of the local coil surface temperature.

[0058] Figure 5 In this embodiment of the invention, when τ = 1454 seconds, the local coil surface temperature is kept below 41°C within different finite total MR scan durations. The (B1) method is used... short ) 2 With (B1) infinite ) 2 A schematic diagram of the ratio r between them;

[0059] Figure 6 In this embodiment of the invention, when five scanning protocols are used within a set total MR scan duration and the scan duration of each scanning protocol is the same, a schematic diagram of the B1 field intensity corresponding to each scanning protocol is provided.

[0060] Figure 7In this embodiment of the invention, when multiple scanning protocols are used within a set total MR scanning time, and the scanning time of each scanning protocol is not exactly the same, a comparison diagram of the B1 field intensity corresponding to scanning protocol n and scanning protocol q is shown.

[0061] Figure 8 This is a schematic diagram of the B1 field intensity corresponding to each scanning sequence when multiple different types of scanning sequences are used within a set total MR scanning time in an embodiment of the present invention.

[0062] Figure 9 This is a schematic diagram of the structure of the B1 field confinement device in MRI provided in an embodiment of the present invention.

[0063] The reference numerals in the attached figures are as follows:

[0064] Detailed Implementation

[0065] To make the objectives, technical solutions, and advantages of the present invention clearer, the following embodiments are provided to further illustrate the present invention in detail.

[0066] To maximize patient safety, it is generally desirable to find the maximum intensity of the B1 field where the surface temperature of the local coil can never exceed the maximum safe temperature (41°C) when there is no limit to the total MR scan duration.

[0067] Figure 1 A schematic diagram showing the variation of local coil surface temperature with scan duration for different B1 field intensities is presented. It can be seen that, with no limit to the total MR scan duration, the local coil surface temperature gradually increases with increasing scan duration, but eventually reaches thermal equilibrium (i.e., the temperature remains essentially constant). Furthermore, experiments show that, with the same initial local coil surface temperature, a higher B1 field intensity results in a higher final thermal equilibrium temperature for the local coil surface.

[0068] Furthermore, experiments show that, without a limit on the total MR scan time, it typically takes about 100 minutes for the local coil surface temperature to reach thermal equilibrium at 41°C. However, in practical applications, the total MR scan time generally lasts about 20 minutes. Therefore, under this total MR scan time, it is only necessary to ensure that the local coil surface temperature does not exceed the maximum safe temperature at the end of the scan. Thus, the B1 field intensity can be increased.

[0069] Figure 2 The flowchart of the B1 field confinement method in MRI provided by the embodiments of the present invention is as follows:

[0070] Step 201: Obtain the first intensity of the B1 field required when the thermal equilibrium temperature of the surface temperature of the local coil is at the maximum safe temperature when the local coil is placed at the set position in the detection hole of the MR scanner.

[0071] When the MR scanner is working, analysis of the intensity distribution of the B1 field within the detector aperture reveals that the B1 field intensity is non-uniform in the direction perpendicular to the aperture center (i.e., the direction passing through the aperture center and perpendicular to the horizontal plane). Specifically, the B1 field intensity is weakest at the aperture center and increases with distance from the aperture center, reaching its maximum at the aperture apex (i.e., the highest point of the aperture wall). Therefore, the set position in step 201 can be the highest point of the inner aperture wall of the MR scanner.

[0072] In an optional embodiment, before step 201, the method further includes: when the local coil is placed at a set position and the field strength of B1 is 1 microtesla, obtaining the temperature difference between the thermal equilibrium temperature and the initial temperature of the local coil surface temperature, and setting it as the first temperature difference;

[0073] Step 201 specifically includes: when the local coil is placed at a set position in the detection hole of the MR scanner and the thermal equilibrium temperature of the local coil surface temperature is the maximum safe temperature, obtaining the temperature difference between the maximum safe temperature and the initial temperature of the local coil surface temperature, and setting it as the second temperature difference; dividing the second temperature difference by the first temperature difference, and using the resulting quotient as the square value of the first intensity of the B1 field.

[0074] In practical applications, the local coil is placed at a set position on the MR scanner, and scanning is performed using different B1 field intensities. The variation curves of the local coil surface temperature under different B1 field intensities are obtained. By fitting these curves, the relationship between the local coil surface temperature and the scanning time and B1 field intensity during the MR scanning process is obtained as follows:

[0075]

[0076] Where t is the current scan duration, i.e., the duration between the current moment and the start moment of the MR scan; T(t) is the current temperature of the local coil surface; T0 is the initial temperature of the local coil surface; B1 is the B1 field intensity; ΔT is the temperature difference between the thermal equilibrium temperature of the local coil surface and the initial temperature of the local coil surface when the local coil is placed at the set position in the detection aperture of the MR scanner and the B1 field intensity is 1 microt; τ is the scan duration required when the local coil surface temperature reaches 0.632ΔT when the local coil is placed at the set position in the detection aperture of the MR scanner and the B1 field intensity is 1 microt.

[0077] Here, it is assumed that the temperature rise of the local coil surface is linearly related to the square of the B1 field intensity applied to the local coil, and that the thermal resistance of the local coil does not change during heating. Therefore, the temperature rise until the local coil surface temperature reaches thermal equilibrium is determined solely by the power applied to the local coil or the square of the B1 field intensity. Furthermore, it is assumed that different applied power or the square of the B1 field intensity does not change the time constant, but depends only on the heat dissipation conditions.

[0078] Figure 3 To obtain the curves S1 to S5 of the local coil surface temperature under different B1 field intensities and the fitting curves S1' to S5' of each curve by performing MR scanning with different B1 field intensities. Figure 3 The horizontal axis represents the scan duration in hours (h), and the vertical axis represents the local coil surface temperature in degrees Celsius (°C). Since the actual total scan duration of an MR scan is always finite in practical applications, the fitting curve only needs to consider this finite total scan duration. Figure 3 In this process, the fitted curve is only fitted up to 1 hour; the fitted curve after 1 hour is not considered.

[0079] When the total MR scan time is unlimited, the local coil is placed at a set position within the detection aperture of the MR scanner, and the thermal equilibrium temperature of the local coil surface is the maximum safe temperature T. safety (For example, at 41℃), according to formula (1):

[0080]

[0081] Among them, B1 infinite The condition is: when the local coil is placed at a set position within the detection aperture of the MR scanner, and the thermal equilibrium temperature of the local coil surface is the maximum safe temperature T. safety The corresponding B1 field intensity, i.e., the first intensity of the B1 field in step 201.

[0082] According to formula (2):

[0083]

[0084] Step 202: Based on the relationship between the surface temperature of the local coil and the scanning time and the intensity of the B1 field during the MR scanning process, and the first intensity of the B1 field, obtain the second intensity of the B1 field required to heat the surface temperature of the local coil to the maximum safe temperature within the set total MR scanning time when the local coil is placed at the set position.

[0085] When the total MR scan duration is set to t scanTo ensure patient safety, the following conditions must be met: at the end of the scan, the surface temperature of the local coil must not exceed the maximum safe temperature T. safety Then, according to formula (1), we have:

[0086]

[0087] Among them, t scan The set total magnetic resonance scan duration, i.e., B1 short The maximum scanning duration that can be sustained when scanning the B1 field intensity; B1 short For: when the total MR scan duration is t scan Furthermore, the local coil is placed at the set position, and the surface temperature of the local coil reaches the maximum safe temperature T at the end of the scan. safety The corresponding B1 field intensity, i.e., the second intensity of the B1 field in step 202.

[0088] According to formula (4):

[0089]

[0090] According to formulas (3) and (5):

[0091]

[0092] but:

[0093]

[0094] Step 203: Based on the second intensity of field B1, determine the third intensity of field B1 required for MR scanning using the set total MR scan duration, wherein the third intensity of field B1 is not greater than the second intensity of field B1.

[0095] In the above embodiments, the first intensity of the B1 field is first obtained when the thermal equilibrium temperature of the local coil surface is at the maximum safe temperature when the local coil is placed at a set position in the detection aperture of the MR scanner. Then, based on the relationship between the local coil surface temperature, the scan duration, and the B1 field intensity during the MR scan, and the first intensity of the B1 field, the second intensity of the B1 field is obtained when the local coil is placed at the set position and the local coil surface temperature is heated to the maximum safe temperature within a set total MR scan duration. Based on the second intensity of the B1 field, the third intensity of the B1 field required for MR scanning with a set total MR scan duration is determined, wherein the third intensity of the B1 field is not greater than the second intensity of the B1 field. Thus, under the condition of limited total MR scan duration, the MR imaging quality is improved and the waste of B1 field performance is reduced while ensuring patient safety.

[0096] Figure 4Curve 41 shows the temperature change of the local coil surface when τ = 1454 seconds: (i) the total MR scan duration is set to 35 minutes, and the local coil surface temperature reaches the maximum safe temperature of 41℃ at the end of the scan. It can be seen that the local coil surface temperature reaches the maximum safe temperature of 41℃ after 2121 seconds (approximately 35 minutes). (ii) Curve 42 shows the temperature change of the local coil surface temperature when the total MR scan duration is unlimited, with a thermal equilibrium temperature of 41℃. It can be seen that the local coil surface temperature reaches the thermal equilibrium temperature of 41℃ after 11300 seconds (approximately 188 minutes). The horizontal axis represents the scan duration in seconds, and the vertical axis represents the local coil surface temperature in degrees Celsius. The B1 field intensity corresponding to the first curve 41 is greater than that corresponding to the second curve 42; therefore, it is obvious that the MR image corresponding to the first curve 41 has higher quality.

[0097] Figure 5 The method used (B1) to maintain the local coil surface temperature below 41°C within different finite MR total scan durations at τ = 1454 seconds is given. short ) 2 With (B1) infinite ) 2 A schematic diagram of the ratio r between them. The horizontal axis represents the scan duration in minutes, and the vertical axis represents (B1)... short ) 2 With (B1) infinite ) 2 The ratio r between them.

[0098] Table 1 shows the (B1) methods used to maintain the local coil surface temperature below 41°C for different finite total MR scan durations when τ = 1454 seconds. short ) 2 With (B1) infinite ) 2 The specific values ​​that the ratio between them can take.

[0099]

[0100]

[0101] Table 1

[0102] like Figure 5 As shown in Table 1, when the total MR scan time is 5 minutes, to ensure that the surface temperature of the local coil reaches 41°C at the end of the 5-minute scan, the required (B1) value is calculated. short ) 2 =5.363848427*(B1) infinite ) 2 ;

[0103] When the total MR scan duration is 10 minutes, to ensure that the surface temperature of the local coil reaches 41°C at the end of the 10-minute scan, calculate the required (B1) temperature. short ) 2 =2.957624028*(B1) infinite ) 2 .

[0104] In practical applications, a single MR scan may employ one scanning protocol or multiple scanning protocols.

[0105] (i) When only one scanning protocol is used within the set total MR scan time, the third intensity of the B1 field is taken as the B1 field intensity corresponding to that scanning protocol.

[0106] (ii) When multiple scanning protocols are used within the set total MR scan duration, and the scan duration of each scanning protocol is the same, the third intensity of the B1 field is taken as the sum of the B1 field intensities corresponding to all scanning protocols; and the B1 field intensity corresponding to the nth scanning protocol is:

[0107]

[0108] Among them, B1 protocoln Let B1 be the field intensity corresponding to the nth scanning protocol. short ′ represents the third intensity of field B1, 1≤n≤N, where N is the total number of scanning protocols used within the set total MR scan duration.

[0109] Figure 6 This provides the result when five scanning protocols are used within a set total MR scan duration, and the scan duration of each scanning protocol is the same (i.e., t...). protocol1 =t protocol2 =t protocol3 =t protocol4 =t protocol5 This is a schematic diagram of the B1 field intensity corresponding to each scanning protocol. The horizontal axis represents the scan duration, and the vertical axis represents the B1 field intensity.

[0110] B1 field strength corresponding to scanning protocol 1 protocol1 for:

[0111]

[0112] B1 field strength corresponding to scanning protocol 2 protocol2 for:

[0113]

[0114] And so on.

[0115] (iii) When multiple scanning protocols are used within the set total MR scan duration, and the scan duration of each scanning protocol is not exactly the same, the third intensity of the B1 field is taken as the sum of the B1 field intensities corresponding to all scanning protocols; and the B1 field intensity corresponding to the nth scanning protocol is:

[0116]

[0117] Among them, B1 protocoln Let B1 be the field intensity corresponding to the nth scanning protocol. short ′ represents the third intensity of field B1, t single_protocol t is the standard scan duration for a single scan protocol. protocoln Let N be the actual scan duration of the nth scan protocol, 1≤n≤N, where N is the total number of scan protocols used within the set total MR scan duration.

[0118] Figure 7 When multiple scanning protocols are used within a set total MR scan duration, and the scan duration of each scanning protocol is not exactly the same, the scan duration t of a scanning protocol n is defined. protocoln Not equal to t single_protocol The scan duration t of another scanning protocol q protocolq equal to t single_protocol Then the B1 field strength (B1) corresponding to scanning protocol n protocoln ) 2 The B1 field intensity corresponding to the scanning protocol q (B1) protocolq ) 2 A comparison diagram.

[0119] (iv) When multiple different types of scanning sequences are used within the set total MR scan duration, the B1 field intensity used for each scanning sequence is obtained through the following steps A and B:

[0120] A. Initialize m = 1 and calculate:

[0121]

[0122] in,

[0123]

[0124]

[0125] Please satisfy:

[0126]

[0127] Maximum P m If the value is P, then: the maximum P m The value is the value of B1_limit. mThe number of scan sequences representing the B1 field intensity;

[0128] in,

[0129] B1_limit m Let B1 be the m-th B1 field intensity used in this MR scan. single_protocol B1 field intensity for a single scan protocol; B1 SAR_limit The B1 field intensity limit is a preset value based on SAR; B1 infinite The highest intensity in B1; t single_protocol τ is the standard scan duration for a single scan protocol; τ is the scan duration required when the surface temperature of the local coil reaches 0.632 * (thermal equilibrium temperature - initial temperature of the local coil surface) with the local coil placed at a set position within the detection aperture of the MR scanner and the B1 field strength being 1 microtesla; t allowed_duration_m p is the upper limit of the scan duration for the m-th B1 field intensity; m To use B1_limit during this MR scan m P is the seed number of the p-th scan sequence among all the scan sequences of B1 field intensity; m To use B1_limit during this MR scan m The number of scan sequences representing the B1 field intensity; To use B1_limit during this MR scan m The scan duration of the p-th scan sequence among all scan sequences of B1 field intensity;

[0130] B. Let m = m + 1, return to step A, until...

[0131] ∑P m ≥P

[0132] Where P represents the total number of scanning sequences used in this MR scan.

[0133] Figure 8 This is a schematic diagram showing the B1 field intensity for each scanning sequence when using multiple scanning sequences within a set total MR scan duration. (Example:) Figure 8 As shown, scan sequences 1-4 use B1_limit1 as the B1 field intensity, and scan sequences 5-7 use B1_limit2 as the B1 field intensity. The sum of the scan durations for scan sequences 1-4 is: t seq_1 +t seq_2 +t seq_3 +t seq_4 ≤t allowed_duration_1 The sum of scan durations for scan sequences 5 through 7: t seq_5 +tseq_6 +t seq_7 ≤t allowed_duration_2 .

[0134] Figure 9 This is a schematic diagram of the structure of a B1 field confinement device 90 in an MRI provided in the first embodiment of the present invention. The device 90 mainly includes: a B1 field first intensity acquisition module 91 and a B1 field second intensity acquisition module 92, wherein:

[0135] The B1 field first intensity acquisition module 91 is used to acquire the B1 field first intensity required when the thermal equilibrium temperature of the surface temperature of the local coil is at the maximum safe temperature when the local coil is placed at a set position in the detection hole of the MR scanner.

[0136] The B1 field second intensity acquisition module 92 is used to acquire, based on the relationship between the surface temperature of the local coil and the scanning time and the B1 field intensity during the MR scanning process, and the B1 field first intensity, the second intensity of the B1 field required to heat the local coil to the maximum safe temperature within the set total MR scanning time when the local coil is placed at the set position; and to determine, based on the second intensity of the B1 field, the third intensity of the B1 field required for MR scanning using the set total MR scanning time, wherein the third intensity of the B1 field is not greater than the second intensity of the B1 field.

[0137] In one optional embodiment, the set position in the B1 field first intensity acquisition module 91 and the B1 field second intensity acquisition module 92 is: the highest point of the inner wall of the detection hole of the MR scanner.

[0138] In an optional embodiment, before the B1 field first intensity acquisition module 91 acquires the first intensity of the B1 field required when the thermal equilibrium temperature of the surface temperature of the local coil is at the maximum safe temperature at the set position where the local coil is placed in the detection hole of the MR scanner, it further includes: when the local coil is placed at the set position and the B1 field intensity is 1 microt, acquiring the temperature difference between the thermal equilibrium temperature of the surface temperature of the local coil and the initial temperature, and setting it as the first temperature difference;

[0139] The B1 field first intensity acquisition module 91 acquires the required B1 field first intensity when the thermal equilibrium temperature of the surface temperature of the local coil is at the maximum safe temperature at the set position inside the detection hole of the MR scanner, including: when the local coil is placed at the set position inside the detection hole of the MR scanner and the thermal equilibrium temperature of the surface temperature of the local coil is at the maximum safe temperature, acquiring the temperature difference between the maximum safe temperature and the initial temperature of the surface temperature of the local coil, and setting it as the second temperature difference; dividing the second temperature difference by the first temperature difference, and using the quotient as the square value of the B1 field first intensity.

[0140] In one optional embodiment, the relationship between the local coil surface temperature, scanning time, and B1 field intensity during the MR scanning process, as used by the B1 field second intensity acquisition module 92, is as follows:

[0141]

[0142] Where t is the current scan duration; T(t) is the current temperature of the local coil surface; T0 is the initial temperature of the local coil surface; B1 is the B1 field strength; ΔT is the temperature difference between the thermal equilibrium temperature and the initial temperature of the local coil surface when the local coil is placed at a set position in the detection aperture of the MR scanner and the B1 field strength is 1 microt; τ is the scan duration required when the local coil surface temperature reaches 0.632ΔT when the local coil is placed at a set position in the detection aperture of the MR scanner and the B1 field strength is 1 microt.

[0143] In one optional embodiment, the B1 field second intensity acquisition module 92 acquires the B1 field second intensity required to heat the surface temperature of the local coil to the maximum safe temperature within a set total MR scan time when the local coil is placed at the set position, including:

[0144]

[0145] Among them, B1 short The second strongest in B1; B1 infinite The highest intensity in B1; t scan This is the set total MR scan duration.

[0146] In one optional embodiment, the B1 field second intensity acquisition module 92 determines the B1 field third intensity required for MR scanning using the set total MR scan duration based on the B1 field second intensity, including: when only one scanning protocol is used within the set total MR scan duration, the B1 field third intensity is used as the B1 field intensity corresponding to that scanning protocol.

[0147] In one optional embodiment, the B1 field second intensity acquisition module 92 determines the required B1 field third intensity when performing an MR scan using the set total MR scan duration based on the B1 field second intensity, including: when multiple scanning protocols are used within the set total MR scan duration, and the scan duration of each scanning protocol is the same, the B1 field third intensity is taken as the sum of the B1 field intensities corresponding to all scanning protocols; and the B1 field intensity corresponding to the nth scanning protocol is:

[0148]

[0149] Among them, B1 protocolnLet B1 be the field intensity corresponding to the nth scanning protocol. short ′ represents the third intensity of field B1, 1≤n≤N, where N is the total number of scanning protocols used within the set total MR scan duration.

[0150] In one optional embodiment, the B1 field second intensity acquisition module 92 determines the required B1 field third intensity when performing an MR scan using the set total MR scan duration based on the B1 field second intensity, including: when multiple scanning protocols are used within the set total MR scan duration, and the scan duration of each scanning protocol is not exactly the same, the B1 field third intensity is taken as the sum of the B1 field intensities corresponding to all scanning protocols; and the B1 field intensity corresponding to the nth scanning protocol is:

[0151]

[0152] Among them, B1 protocoln Let B1 be the field intensity corresponding to the nth scanning protocol. short ′ represents the third intensity of field B1, t single_protocol t is the standard scan duration for a single scan protocol. protocoln Let N be the actual scan duration of the nth scan protocol, 1≤n≤N, where N is the total number of scan protocols used within the set total MR scan duration.

[0153] In one optional embodiment, the B1 field second intensity acquisition module 92 determines the required B1 field third intensity when performing an MR scan using the set total MR scan duration based on the B1 field second intensity, including: when multiple different types of scan sequences are used within the set total MR scan duration, the B1 field intensity used for each scan sequence is obtained through the following steps A and B:

[0154] A. Initialize m = 1 and calculate:

[0155]

[0156] in,

[0157]

[0158]

[0159] Please satisfy:

[0160]

[0161] Maximum P m If the value is P, then: the maximum P m The value is the value of B1_limit. m The number of scan sequences representing the B1 field intensity;

[0162] in,

[0163] B1_limit m Let B1 be the m-th B1 field intensity used in this MR scan. single_protocol B1 field intensity for a single scan protocol; B1 SAR_limit The B1 field intensity limit is a preset value constrained by SAR; B1 infinite The highest intensity in B1; t single_portocol τ is the standard scan duration for a single scan protocol; τ is the scan duration required when the surface temperature of the local coil reaches 0.632 * (thermal equilibrium temperature - initial temperature of the local coil surface) with the local coil placed at a set position within the detection aperture of the MR scanner and the B1 field strength being 1 microtesla; t allowed_duration_m p is the upper limit of the scan duration for the m-th B1 field intensity; m To use B1_limit during this MR scan m P is the seed number of the p-th scan sequence among all the scan sequences of B1 field intensity; m To use B1_limit during this MR scan m The number of scan sequences representing the B1 field intensity; To use B1_limit during this MR scan m The scan duration of the p-th scan sequence among all scan sequences of B1 field intensity;

[0164] B. Let m = m + 1, return to step A, until...

[0165] ∑P m ≥P

[0166] Where P represents the total number of scanning sequences used in this MR scan.

[0167] This invention also provides an MR scanner, including the B1 field confinement device 90 in MRI as described above.

[0168] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for confining a radio frequency alternating magnetic field in magnetic resonance imaging, characterized in that, The method includes: The first intensity of the radio frequency alternating magnetic field, namely the B1 field, is required when the thermal equilibrium temperature of the surface temperature of the local coil is at the maximum safe temperature at a set position inside the detection aperture of the magnetic resonance scanner. Based on the relationship between the surface temperature of the local coil and the scanning time and the intensity of the B1 field during the magnetic resonance scanning process, and the first intensity of the B1 field, the second intensity of the B1 field required to heat the surface temperature of the local coil to the maximum safe temperature within the set total magnetic resonance scanning time when the local coil is placed at the set position is obtained. Based on the second intensity of field B1, the third intensity of field B1 required for magnetic resonance scanning using the set total magnetic resonance scanning time is determined, wherein the third intensity of field B1 is not greater than the second intensity of field B1.

2. The method according to claim 1, characterized in that, The designated location is the highest point of the inner wall of the detection aperture of the magnetic resonance scanner.

3. The method according to claim 1, characterized in that, Before obtaining the first intensity of the B1 field required for the thermal equilibrium temperature of the local coil surface to reach the maximum safe temperature when the local coil is placed at a predetermined position within the detection aperture of the magnetic resonance scanner, the process further includes: When the local coil is placed at the set position and the B1 field strength is 1 microt, the temperature difference between the thermal equilibrium temperature and the initial temperature of the local coil surface temperature is obtained and set as the first temperature difference. The first intensity of the B1 field required for the thermal equilibrium temperature of the surface temperature of the local coil to be at the maximum safe temperature when the local coil is placed at a predetermined position within the detection aperture of the magnetic resonance scanner includes: When the local coil is placed at a set position within the detection aperture of the magnetic resonance scanner and the thermal equilibrium temperature of the local coil surface is the maximum safe temperature, the temperature difference between the maximum safe temperature and the initial temperature of the local coil surface is obtained and set as the second temperature difference. Divide the second temperature difference by the first temperature difference, and use the resulting quotient as the square of the first intensity of field B1.

4. The method according to claim 1, characterized in that, The relationship between the surface temperature of the local coil and the scanning time and B1 field intensity during magnetic resonance scanning is as follows: Where t is the current scan duration; T(t) is the current temperature of the local coil surface; T0 is the initial temperature of the local coil surface; B1 is the B1 field strength; ΔT is the temperature difference between the thermal equilibrium temperature of the local coil surface and the initial temperature of the local coil surface when the local coil is placed at a set position in the detection aperture of the magnetic resonance scanner and the B1 field strength is 1 microtesla; τ is the scan duration required when the local coil surface temperature reaches 0.632ΔT when the local coil is placed at a set position in the detection aperture of the magnetic resonance scanner and the B1 field strength is 1 microtesla.

5. The method according to claim 4, characterized in that, The acquisition of the second intensity of the B1 field, which is required to heat the surface temperature of the local coil to the maximum safe temperature within a set total magnetic resonance scan time when the local coil is placed at the set position, includes: Among them, B1 short The second strongest in B1; B1 infinite The highest intensity in B1; t scan This is the set total scan time for magnetic resonance imaging.

6. The method according to any one of claims 1 to 5, characterized in that, The step of determining the third intensity of the B1 field required for magnetic resonance scanning using the set total magnetic resonance scan duration, based on the second intensity of the B1 field, includes: When only one scanning protocol is used within the set total magnetic resonance scanning time, the third intensity of the B1 field is taken as the B1 field intensity corresponding to that scanning protocol.

7. The method according to any one of claims 1 to 5, characterized in that, The step of determining the third intensity of the B1 field required for magnetic resonance scanning using the set total magnetic resonance scan duration, based on the second intensity of the B1 field, includes: When multiple scanning protocols are used within a set total magnetic resonance scanning time, and the scanning time of each scanning protocol is the same, the third intensity of the B1 field is taken as the sum of the B1 field intensities corresponding to all scanning protocols; and the B1 field intensity corresponding to the nth scanning protocol is: Among them, B1 protocoln Let B1 be the field intensity corresponding to the nth scanning protocol. short ′ represents the third intensity of the B1 field, 1≤n≤N, where N is the total number of scanning protocols used within the set total magnetic resonance scanning time.

8. The method according to any one of claims 1 to 5, characterized in that, The step of determining the third intensity of the B1 field required for magnetic resonance scanning using the set total magnetic resonance scan duration, based on the second intensity of the B1 field, includes: When multiple scanning protocols are used within a set total magnetic resonance scanning time, and the scanning time of each scanning protocol is not exactly the same, the third intensity of the B1 field is taken as the sum of the B1 field intensities corresponding to all scanning protocols; and the B1 field intensity corresponding to the nth scanning protocol is: Among them, B1 protocoln Let B1 be the field intensity corresponding to the nth scanning protocol. short ′ represents the third intensity of field B1, t sigle_protocol t is the standard scan duration for a single scan protocol. protocoln Let N be the actual scan duration of the nth scan protocol, 1≤n≤N, where N is the total number of scan protocols used within the set total magnetic resonance scan duration.

9. The method according to claim 1, characterized in that, The step of determining the third intensity of the B1 field required for magnetic resonance scanning using the set total magnetic resonance scan duration, based on the second intensity of the B1 field, includes: When multiple different types of scanning sequences are used within a set total magnetic resonance scan duration, the B1 field intensity for each scanning sequence is obtained through the following steps A and B: A. Initialize m = 1 and calculate: in, Please satisfy: Maximum P m If the value is P, then: the maximum P m The value is the value of B1_limit. m The number of scan sequences representing the B1 field intensity; in, B1_limit m Let B1 be the m-th B1 field intensity used in this magnetic resonance scan. single_protocol B1 field intensity for a single scan protocol; B1 SAR_limit The preset B1 field strength limit value; B1 infinite The highest intensity in B1; t single_protocol τ is the standard scan duration for a single scan protocol; τ is the scan duration required when the surface temperature of the local coil reaches 0.632 * (thermal equilibrium temperature - initial temperature of the local coil surface) with the local coil placed at a set position within the detector aperture of the magnetic resonance scanner and the B1 field strength being 1 microtesla; t allowed_duration_m p is the upper limit of the scan duration for the m-th B1 field intensity; m To use B1_limit during this magnetic resonance scan m P is the seed number of the p-th scan sequence among all the scan sequences of B1 field intensity; m To use B1_limit during this magnetic resonance scan m The number of scan sequences representing the B1 field intensity; t seq_pm To use B1_limit during this magnetic resonance scan m The scan duration of the p-th scan sequence among all scan sequences of B1 field intensity; B. Let m = m + 1, return to step A, until... ∑P m ≥P Where P represents the total number of scanning sequences used in this magnetic resonance scan.

10. A radio frequency alternating magnetic field confinement device (90) for magnetic resonance imaging, characterized in that, The device (90) includes: The B1 field first intensity acquisition module (91) is used to acquire the B1 field first intensity required when the thermal equilibrium temperature of the surface temperature of the local coil is at the maximum safe temperature when the local coil is placed at a set position in the detection hole of the MR scanner. The B1 field second intensity acquisition module (92) is used to acquire the B1 field second intensity required to heat the local coil to the maximum safe temperature within the set total MR scan time when the local coil is placed at the set position, based on the relationship between the surface temperature of the local coil and the scan time and the B1 field intensity during the MR scan, as well as the B1 field first intensity. Based on the B1 field second intensity, the B1 field third intensity required for MR scan using the set total MR scan time is determined, wherein the B1 field third intensity is not greater than the B1 field second intensity.

11. A magnetic resonance scanner, characterized in that, Includes the radio frequency alternating magnetic field limiting device (90) for magnetic resonance scanning as described in claim 10.

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