System and method for measuring eddy currents with long time constants using pseudo-continuous acquisition

Abstract

A system and method for measuring eddy current having a long time constant includes initiating, via a processor, a calibration scan of a phantom with a magnetic resonance imaging (MRI) scanner. The system and method also include simultaneously generating and measuring, via the processor, the eddy current during the calibration scan using a column of gradient echo sequences. The generation and measurement of the eddy current are both done within each of the column of gradient echo sequences. The system and method also include obtaining, via the processor, k-space data from the column of gradient echo sequences. The system and method also include converting, via the processor, the k-space data into an eddy current gradient field. The system and method even also include estimating, via the processor, a function of the eddy current over time based on the eddy current gradient field.

Description

Background Art

[0001] The subject matter disclosed herein relates to medical imaging, and more particularly, to systems and methods for measuring eddy currents with long time constants using pseudo-continuous acquisition.

[0002] Non-invasive imaging techniques allow images of internal structures or features of a patient / object to be obtained without performing an invasive procedure on the patient / object. Specifically, such non-invasive imaging techniques rely on various physical principles (such as differential transmission of X-rays through a target volume, reflection of acoustic waves within a volume, paramagnetism of different tissues and materials within a volume, decomposition of target radionuclides within the body, etc.) to acquire data and construct an image or otherwise represent the observed internal features of the patient / object.

[0003] During MRI, when a substance such as human tissue is subjected to a uniform magnetic field (polarization field B0), the individual magnetic moments of the spins in the tissue attempt to align with the polarization field, but precess around it in a random order at their characteristic Larmor frequency. If the substance or tissue is subjected to a magnetic field (excitation field B1) that is in the xy plane and close to the Larmor frequency, the net alignment moment or "longitudinal magnetization" M z can be rotated or "tilted" into the xy plane to produce a net transverse magnetic moment M t After the excitation signal B1 is terminated, a signal is emitted by the excited spins and can be received and processed to form an image.

[0004] When these signals are used to generate images, the magnetic field gradient (G x , G y and G z ). Typically, the area to be imaged is scanned in a series of measurement cycles in which these gradient fields are varied according to the particular localization method used. The resulting set of received nuclear magnetic resonance (NMR) signals is digitized and processed to reconstruct an image using one of the well-known reconstruction techniques.

[0005] In MRI, eddy currents are unwanted currents generated in metal structures within the MRI system when the gradient field changes. The magnetic field associated with eddy currents distorts the ideal gradient waveform and causes artifacts in MRI images. Eddy currents can generally be characterized by multiple exponential terms and oscillatory terms. The exponential term is the dominant term. During the installation of the MRI system, eddy currents are measured (e.g., during system calibration). Typically, an MRI sequence for measuring eddy currents (e.g., the setting of a pulse sequence and pulsed field gradients) involves two main steps. The first main step includes starting the eddy current field by applying an excitation gradient pulse. The second main step includes measuring the eddy current field over time using multiple RF pulses. However, this method has limitations. Typically, the duration of the excitation gradient is much longer than the eddy current time constant (e.g., for a very long time constant (e.g., on the order of 10 seconds), the duration is about 45 seconds). This extended gradient duration results in inefficient data collection because no data is obtained during the excitation gradient pulse. Additionally, extended gradient durations introduce susceptibility to magnetic drift effects, such as movement of metal (eg, due to localized areas of ferromagnetic material) or B0 drift of the main magnetic field. Summary of the invention

[0006] The following shows an overview of certain embodiments disclosed herein. It should be understood that these aspects are provided only to provide the reader with a brief overview of these specific embodiments, and these aspects are not intended to limit the scope of the present disclosure. In fact, the present disclosure may cover various aspects that may not be shown below.

[0007] In one embodiment, a computer-implemented method for measuring eddy currents with a long time constant is provided. The computer-implemented method includes initiating a calibration scan of a phantom using a magnetic resonance imaging (MRI) scanner via a processor. The computer-implemented method also includes simultaneously generating and measuring the eddy currents during the calibration scan using a column of gradient echo sequences via the processor, wherein the generation and measurement of the eddy currents are completed within each gradient echo sequence. The computer-implemented method also includes acquiring k-space data from the column of gradient echo sequences via the processor. The computer-implemented method also includes converting the k-space data into an eddy current gradient field via the processor. The computer-implemented method even includes estimating the eddy current as a function of time based on the eddy current gradient field via the processor.

[0008] In another embodiment, a system for measuring eddy currents with long time constants is provided. The system includes a memory that encodes a processor executable routine. The system also includes a processor that is configured to access the memory and execute the processor executable routine, wherein the processor executable routine causes the processor to perform an action when executed by the processor. The action includes initiating a calibration scan of a phantom using a magnetic resonance imaging (MRI) scanner. The action also includes simultaneously generating and measuring the eddy current during the calibration scan using a column of gradient echo sequences, wherein the generation and measurement of the eddy current are completed within each gradient echo sequence in the column of gradient echo sequences. The action also includes acquiring k-space data from the column of gradient echo sequences. The action also includes converting the k-space data into an eddy current gradient field. The action even includes estimating the eddy current as a function of time based on the eddy current gradient field.

[0009] In another embodiment, a non-transitory computer readable medium includes processor executable code, which when executed by a processor causes the processor to perform an action. The action includes initiating a calibration scan of a phantom using a magnetic resonance imaging (MRI) scanner. The action also includes simultaneously generating and measuring the eddy current during the calibration scan using a column of gradient echo sequences, wherein the generation and measurement of the eddy current are completed within each gradient echo sequence in the column of gradient echo sequences. The action also includes acquiring k-space data from the column of gradient echo sequences. The action also includes converting the k-space data into an eddy current gradient field. The action even includes estimating the eddy current as a function of time based on the eddy current gradient field. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] These and other features, aspects, and advantages of the present subject matter will be better understood when the following detailed description is read with reference to the accompanying drawings, in which like characters refer to like parts throughout the several views, and in which:

[0011] Figure 1 A schematic diagram illustrating an embodiment of a magnetic resonance imaging (MRI) system suitable for use with the disclosed technology;

[0012] Figure 2 illustrates a pulse sequence diagram of a gradient echo sequence utilized to measure eddy currents having a long time constant (eg, eddy current excitation gradient having a positive polarity) in accordance with aspects of the present disclosure;

[0013] Figure 3 illustrates a pulse sequence diagram of a gradient echo sequence utilized to measure eddy currents having a long time constant (eg, eddy current excitation gradient having a negative polarity) in accordance with aspects of the present disclosure;

[0014] Figure 4illustrates a pulse sequence diagram of a gradient echo sequence utilized to measure eddy currents having a long time constant (eg, lacking an eddy current excitation gradient) in accordance with aspects of the present disclosure;

[0015] Figure 5 A flow chart illustrating a method for measuring eddy currents having a long time constant according to aspects of the present disclosure;

[0016] Figure 6 is a graph illustrating measured eddy current values ​​fitted as a function of time (e.g., eddy current time constant of 0.01 seconds and amplitude of -0.1%) using the techniques described herein as part of a testing procedure in accordance with aspects of the present disclosure;

[0017] Figure 7 is a graph illustrating measured eddy current values ​​fitted as a function of time (e.g., eddy current time constant of 0.1 seconds and amplitude of -0.1%) using the techniques described herein as part of a testing procedure in accordance with aspects of the present disclosure;

[0018] Figure 8 is a graph illustrating measured eddy current values ​​fitted as a function of time (e.g., eddy current time constant of 1.0 seconds and amplitude of -0.1%) using the techniques described herein as part of a testing procedure in accordance with aspects of the present disclosure;

[0019] Fig. 9 is a graph illustrating measured eddy current values ​​fitted as a function of time (e.g., an eddy current time constant of 10 seconds and an amplitude of -0.1%) using the techniques described herein as part of a testing procedure in accordance with aspects of the present disclosure; and

[0020] Fig.10 A pulse sequence diagram illustrating a free induction decay sequence utilized to measure eddy currents having a long time constant (eg, an eddy current excitation gradient having a positive polarity) in accordance with aspects of the present disclosure is illustrated. DETAILED DESCRIPTION

[0021] One or more specific embodiments will be described below. In order to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be understood that in the development of any such actual implementation, as in any engineering or design project, many implementation-specific decisions must be made to achieve the developer's specific goals, such as complying with system-related and business-related constraints that may vary from implementation to implementation. In addition, it should be understood that such development efforts may be complex and time-consuming, but are still routine tasks for design, fabrication, and manufacturing for ordinary technicians who benefit from this disclosure.

[0022] When introducing elements of various embodiments of the present subject matter, the articles "a," "an," "the," and "said" are intended to indicate that there are one or more elements. The terms "comprising," "including," and "having" are intended to be inclusive, and mean that there may be additional elements in addition to the listed elements. Furthermore, any numerical examples in the following discussion are intended to be non-limiting, and thus the additional numerical values, ranges, and percentages are within the scope of the disclosed embodiments.

[0023] The present disclosure provides a system and method for measuring eddy currents with long time constants. Specifically, the system and method are configured to measure eddy currents with long time constants using a gradient echo sequence. The disclosed embodiment includes initiating a calibration scan of a phantom using a magnetic resonance imaging (MRI) scanner. The disclosed embodiment also includes simultaneously generating and measuring eddy currents during the calibration scan using a column of gradient echo sequences. In the context of this description, the term simultaneously indicates that the generation and measurement of eddy currents are completed within each gradient echo sequence in the column of gradient echo sequences. The disclosed embodiment also includes acquiring k-space data from the column of gradient echo sequences. The disclosed embodiment also includes converting the k-space data into an eddy current gradient field. The disclosed embodiment even includes estimating the function of eddy currents over time based on the eddy current gradient field.

[0024] In some embodiments, the estimated eddy current has a time constant that is at least twice as long as the repetition time of each gradient echo sequence in the column of gradient echo sequences. In some embodiments, the induced eddy current can be approximated as a constant gradient obtained by a duration equal to the time repetition and an amplitude equal to an average gradient, wherein the average gradient is the total gradient area of ​​the corresponding gradient echo sequence in the column of gradient echo sequences divided by the time repetition. In some embodiments, the function of estimating the eddy current over time includes scaling the eddy current gradient field by the average gradient strength and fitting the scaled eddy current gradient field over time. In some embodiments, the function of estimating the eddy current over time includes utilizing Formula 1 as described below.

[0025] In some embodiments, the first part of each gradient echo sequence in the column of gradient echo sequences includes a balanced gradient echo for eddy current measurement (e.g., with a zero gradient area), wherein the balanced gradient echo has a balanced readout gradient and a slice gradient to avoid inducing additional eddy current effects (e.g., from the readout gradient and the slice gradient). In some embodiments, the second part of a group of gradient echo sequences in the column of gradient echo sequences includes an eddy current excitation gradient (or eddy current generating gradient) for generating eddy currents. In some embodiments, the second part of some gradient echo sequences in the group of gradient echo sequences includes a negative eddy current excitation gradient, and the second part of some gradient echo sequences in the group of gradient echo sequences includes a positive eddy current excitation gradient. Using a bipolar (i.e., positive and negative) eddy current excitation gradient, it is possible to distinguish eddy currents from B0 drifts, because eddy currents change direction in response to the bipolar eddy current excitation gradient, while B0 drifts generally do not.

[0026] The disclosed embodiments enable simultaneous generation and measurement of eddy currents. This enables pseudo-continuous scanning (a train of GRE sequences with intermittent repetitions) to be utilized to measure eddy currents with long time constants, taking less time and being more efficient than previous techniques. In addition, the disclosed embodiments are less sensitive to magnetic drift. The disclosed embodiments also improve calibration accuracy for eddy currents with very long time constants. By improving calibration accuracy, the image quality of certain MR images is improved.

[0027] Considering the above, Figure 1 , a magnetic resonance imaging (MRI) system 100 is schematically illustrated as including a scanner 102, scanner control circuitry 104, and system control circuitry 106. According to embodiments described herein, the MRI system 100 is generally configured to perform MR imaging.

[0028] The system 100 also includes a remote access and storage system or device, such as a picture archiving and communication system (PACS) 108, or other devices, such as teleradiology equipment, that enable on-site or off-site access to data acquired by the system 100. In this way, MR data can be acquired and then processed and evaluated on-site or off-site. Although the MRI system 100 can include any suitable scanner or detector, in the illustrated embodiment, the system 100 includes a whole body scanner 102 having a housing 120 through which an aperture 122 is formed. An examination table 124 can be moved into the aperture 122 to allow a patient 126 (e.g., a subject) to be positioned therein to image selected anatomical structures within the patient's body.

[0029] The scanner 102 includes a series of associated coils for generating a controlled magnetic field for exciting gyromagnetic materials within the anatomical structure of the patient being imaged. Specifically, a primary magnetic coil 128 is provided for generating a primary magnetic field B0 that is generally aligned with the orifice 122. A series of gradient coils 130, 132, and 134 allow controlled gradient magnetic fields to be generated during an examination sequence for position encoding of certain gyromagnetic nuclei within the patient 126. A radio frequency (RF) coil 136 (e.g., an RF transmit coil) is configured to generate radio frequency pulses for exciting certain gyromagnetic nuclei within the patient. In addition to the coils that may be located locally at the scanner 102, the system 100 also includes a set of receiving coils or RF receiving coils 138 (e.g., a coil array) configured to be placed proximal to the patient 126 (e.g., against the patient). For example, the receiving coil 138 may include a cervical / thoracic / lumbar (CTL) coil, a head coil, a single-sided spine coil, etc. Generally, the receive coil 138 is placed near or on top of the patient 126 to receive the weak RF signals (weak relative to the transmit pulses generated by the scanner coil) generated by certain gyromagnetic nuclei in the patient's body when the patient 126 returns to his or her relaxed state.

[0030] The various coils of the system 100 are controlled by external circuitry to generate the required fields and pulses and to read the emission from the gyromagnetic material in a controlled manner. In the illustrated embodiment, the main power supply 140 provides power to the primary field coil 128 to generate the primary magnetic field Bo. The power input (e.g., power from a utility or grid), a power distribution unit (PDU), a power supply (PS), and a drive circuit 150 can provide power together to pulse the gradient field coils 130, 132, and 134. The drive circuit 150 may include an amplification and control circuit system for supplying current to the coils as defined by the digitized pulse sequence output by the scanner control circuit system 104.

[0031] Another control circuit 152 is provided for regulating the operation of the RF coil 136. The circuit 152 includes a switching device for alternating between an active mode of operation and a passive mode of operation, wherein the RF coil 136 transmits and does not transmit signals, respectively. The circuit 152 also includes an amplification circuit system configured to generate RF pulses. Similarly, the receiving coil 138 is connected to a switch 154 that is capable of switching the receiving coil 138 between a receiving mode and a non-receiving mode. Thus, in the receiving mode, the receiving coil 138 resonates with the RF signal generated by the release of the gyromagnetic nuclei in the patient 126, and in the non-receiving mode, they do not resonate with the RF energy from the transmitting coil (i.e., the coil 136) to prevent undesired operation. In addition, the receiving circuit 156 is configured to receive data detected by the receiving coil 138, and may include one or more multiplexing and / or amplification circuits.

[0032] It should be noted that although the scanner 102 and control / amplification circuit system described above are illustrated as being coupled by a single line, in actual examples there may be many such lines. For example, separate lines may be used for control, data communication, power transmission, etc. In addition, appropriate hardware may be provided along each type of line for properly processing data and current / voltage. In practice, various filters, digitizers, and processors may be provided between the scanner and either or both of the scanner control circuit 104 and the system control circuit system 106.

[0033] As shown, the scanner control circuit system 104 includes an interface circuit 158 ​​that outputs signals for driving the gradient field coils and the RF coils and for receiving data representing magnetic resonance signals generated in the examination sequence. The interface circuit 158 ​​is coupled to a control and analysis circuit 160. Based on a defined scheme selected via the system control circuit 106, the control and analysis circuit 160 executes commands for driving the circuits 150 and 152.

[0034] The control and analysis circuitry 160 is also used to receive magnetic resonance signals and perform subsequent processing before transmitting the data to the system control circuitry 106. The scanner control circuitry 104 also includes one or more memory circuits 162 that store configuration parameters, pulse sequence descriptions, examination results, etc. during operation.

[0035] Interface circuit 164 is coupled to control and analysis circuit 160 for exchanging data between scanner control circuit system 104 and system control circuit system 106. In some embodiments, control and analysis circuit 160, although shown as a single unit, may include one or more hardware devices. System control circuit 106 includes interface circuit 166, which receives data from scanner control circuit system 104 and transmits data and commands back to scanner control circuit system 104. Control and analysis circuit 168 may include a CPU in a general or special computer or workstation. Control and analysis circuit 168 is coupled to memory circuit 170 to store programming code for operating MRI system 100, and to store processed image data for later reconstruction, display and transmission. The programming code may execute one or more algorithms that are configured to perform reconstruction of acquisition data as described below when executed by a processor. In some embodiments, memory circuit 170 may store one or more neural networks for reconstruction of acquisition data as described below. In some embodiments, image reconstruction may occur on a separate computing device having a processing circuit system and a memory circuit system.

[0036] In certain embodiments, the programming code is configured to measure eddy currents with long time constants. Specifically, the programming code is configured to measure eddy currents with long time constants using MRI sequences such as gradient echo sequences. The programming code is configured to initiate a calibration scan of a phantom using an MRI scanner. The programming code is also configured to repeatedly generate and measure eddy currents in an interlaced manner during the calibration scan using a column of gradient echo sequences. The programming code is also configured to acquire k-space data from the column of gradient echo sequences. The programming code is also configured to convert the k-space data into an eddy current gradient field. The programming code is also configured to estimate the eddy current as a function of time based on the eddy current gradient field.

[0037] In some embodiments, the estimated eddy current has a time constant that is at least twice as long as the time repetition of each gradient echo sequence in the column of gradient echo sequences. In some embodiments, the induced eddy current can be approximated as a constant gradient obtained by a duration equal to the time repetition and an amplitude equal to an average gradient, wherein the average gradient is the total gradient area of ​​the corresponding gradient echo sequence in the column of gradient echo sequences divided by the time repetition. In some embodiments, the programming code is configured to scale the eddy current gradient field by the average gradient intensity and fit the scaled eddy current gradient field as a function of time, estimating the function of the eddy current over time. In some embodiments, the programming code is configured to estimate the function of the eddy current over time including using Formula 1 as described below.

[0038] In some embodiments, the first part of each gradient echo sequence in the column of gradient echo sequences includes a balanced gradient echo for eddy current measurement (e.g., with a zero gradient area), wherein the balanced gradient echo has a balanced readout gradient and a slice gradient to avoid eddy current effects (e.g., from the readout gradient and the slice gradient). In some embodiments, the second part of a group of gradient echo sequences in the column of gradient echo sequences includes an eddy current excitation gradient (or eddy current generating gradient) for generating eddy currents. In some embodiments, the second part of some gradient echo sequences in the group of gradient echo sequences includes a negative eddy current excitation gradient, and the second part of some gradient echo sequences in the group of gradient echo sequences includes a positive eddy current excitation gradient. Using a bipolar (i.e., positive and negative) eddy current excitation gradient, it is possible to distinguish eddy currents from B0 drifts, because eddy currents change direction in response to the bipolar eddy current excitation gradient, while B0 drifts do not.

[0039] Additional interface circuitry 172 may be provided for exchanging image data, configuration parameters, etc. with external system components (such as remote access and storage device 108). Finally, system control and analysis circuitry 168 may be communicatively coupled to various peripheral devices for facilitating operator interface and generating hard copies of reconstructed images. In the illustrated embodiment, these peripheral devices include a printer 174, a monitor 176, and a user interface 178, which includes devices such as a keyboard, a mouse, a touch screen (e.g., integral to the monitor 176), and the like.

[0040] Figure 2 A pulse sequence diagram 180 of a gradient echo sequence 182 utilized to measure eddy currents having a long time constant (e.g., an eddy current excitation gradient having a positive polarity) is illustrated. As described in more detail below, the gradient echo sequence 182 is used as part of a train of gradient echo sequences utilized to measure eddy currents having a long time constant (e.g., where the estimated or measured eddy currents have a time constant that is at least twice longer than the temporal repetition of each gradient echo sequence in the train of gradient echo sequences). The gradient echo sequence 182 is configured to enable simultaneous generation and measurement of eddy currents (e.g., during a calibration scan such as an eddy current calibration scan).

[0041] The four rows 184, 186, 188 and 190 illustrate a gradient echo sequence 182 utilized to measure eddy currents having a long time constant. The first row 184 (e.g., top row) of the pulse sequence diagram 180 illustrates RF (represented along the Y-axis 192) over time (represented along the X-axis 194). As depicted, an RF pulse 196 is generated at approximately 1 millisecond. The second row 186 of the pulse sequence diagram 180 illustrates the gradient applied in the frequency direction (represented as gradient strength along the Y-axis 198) over time (represented along the X-axis 200). The gradient applied in the frequency direction is represented by curve 202. The first portion 204 of the curve 202, located to the left of the dashed line 206, is a balanced gradient echo having a zero gradient area. The second portion 208 of the curve 202, located to the right of the dashed line 206, has an eddy current excitation gradient or eddy current generation gradient 210 having a positive polarity.

[0042] Pulse sequence Figure 1 The third row 188 of 80 illustrates the gradient applied in the phase direction (represented as gradient strength along the Y-axis 212) over time (represented along the X-axis 214). The gradient applied in the phase direction is represented by curve 216. A first portion 218 of the curve 216, located to the left of the dashed line 220, is a balanced gradient echo with zero gradient area. A second portion 222 of the curve 216, located to the right of the dashed line 222, has an eddy current excitation gradient or eddy current generation gradient 224 with positive polarity.

[0043] Pulse sequence Figure 1The fourth row 190 (e.g., bottom row) of the image processing apparatus 80 illustrates the gradient applied in the slice direction (represented as gradient strength along the Y-axis 226) over time (represented along the X-axis 228). The gradient applied in the slice direction is represented by the curve 230. The first portion 232 of the curve 230, located to the left of the dashed line 234, is a balanced gradient echo with zero gradient area. The second portion 236 of the curve 230, located to the right of the dashed line 234, has an eddy current excitation gradient or eddy current generation gradient 238 with positive polarity.

[0044] Eddy current measurements are performed using balanced gradient echoes of the first portion 204, 216, 232 of the applied gradient. The first portions 204 and 232 of the frequency gradient (e.g., readout gradient) and the slice gradient are balanced, respectively, to avoid eddy current effects from the frequency gradient and the slice gradient. Eddy currents are generated using eddy current excitation gradients 210, 224, and 238 of the second portion 208, 222, and 236. The eddy current excitation gradient can be applied once along a single gradient axis. When the eddy current excitation gradient is on the same axis as the readout gradient, the measured eddy current is generally referred to as an on-axis eddy current. When the eddy current excitation gradient is not on the same axis as the readout gradient, the measured eddy current is generally referred to as a cross-term eddy current. It is also possible to apply one excitation gradient and complete sequential measurements along three axes so that both on-axis and cross-term eddy currents are in a single scan. Eddy current excitation gradients 210, 224, and 238 with positive polarity are used in conjunction with a gradient echo sequence (e.g., a gradient echo sequence including eddy current excitation gradients with negative polarity) Figure 3 ) are used in combination with the gradient echo sequence 182 to form a dipole eddy current excitation gradient. These dipole eddy current excitation gradients enable eddy currents to be distinguished from B0 drifts because eddy currents change direction in response to the dipole eddy current excitation gradients, whereas B0 drifts do not. The gradient echo sequence 182 is used in combination with the gradient echo sequence ( Figure 3 ) to form a GRE sequence (e.g., as part of a pseudo-continuous scan) that is repeated intermittently (e.g., every 100 milliseconds (ms)) to measure eddy currents. The GRE sequence is combined with a GRE sequence ( Figure 4 ) described in .

[0045] Figure 3 A pulse sequence diagram 240 of a gradient echo sequence 242 utilized to measure eddy currents having a long time constant (e.g., an eddy current excitation gradient having a negative polarity) is illustrated. As described in more detail below, the gradient echo sequence 242 is used as part of a train of gradient echo sequences utilized to measure eddy currents having a long time constant (e.g., where the estimated or measured eddy currents have a time constant that is at least twice longer than the temporal repetition of each gradient echo sequence in the train of gradient echo sequences). The gradient echo sequence 242 is configured to enable simultaneous generation and measurement of eddy currents (e.g., during a calibration scan such as an eddy current calibration scan).

[0046] The four rows 244, 246, 248 and 250 illustrate a gradient echo sequence 242 utilized to measure eddy currents having a long time constant. The first row 244 (e.g., top row) of the pulse sequence diagram 240 illustrates RF (represented along the Y-axis 252) over time (represented along the X-axis 254). As depicted, an RF pulse 256 is generated at approximately 1 millisecond. The second row 246 of the pulse sequence diagram 240 illustrates the gradient applied in the frequency direction (represented as gradient strength along the Y-axis 258) over time (represented along the X-axis 260). The gradient applied in the frequency direction is represented by curve 262. The first portion 264 of the curve 262, located to the left of the dashed line 266, is a balanced gradient echo having a zero gradient area. The second portion 268 of the curve 262, located to the right of the dashed line 266, has an eddy current excitation gradient or eddy current generation gradient 270 having a negative polarity.

[0047] The third row 248 of the pulse sequence diagram 240 illustrates the gradient applied in the phase direction (represented as gradient strength along the Y-axis 272) over time (represented along the X-axis 274). The gradient applied in the phase direction is represented by the curve 276. The first portion 278 of the curve 276, located to the left of the dashed line 280, is a balanced gradient echo with zero gradient area. The second portion 282 of the curve 276, located to the right of the dashed line 280, has an eddy current excitation gradient or eddy current generation gradient 284 with negative polarity.

[0048] The fourth row 250 (e.g., bottom row) of the pulse sequence diagram 240 illustrates the gradient applied in the slice direction (represented as gradient strength along the Y-axis 286) over time (represented along the X-axis 288). The gradient applied in the slice direction is represented by the curve 290. The first portion 292 of the curve 290, located to the left of the dashed line 294, is a balanced gradient echo with zero gradient area. The second portion 296 of the curve 290, located to the right of the dashed line 294, has an eddy current excitation gradient or eddy current generation gradient 296 with negative polarity.

[0049] Eddy current measurements are performed using balanced gradient echoes of a first portion 264, 278, 292 of applied gradients. The first portions 264 and 292 of the frequency gradient (e.g., readout gradient) and the slice gradient are balanced to avoid eddy current effects from the frequency gradient and the slice gradient, respectively. Eddy currents are generated using eddy current excitation gradients 270, 284, and 298 of a second portion 268, 282, and 296. Eddy current excitation gradients 270, 284, and 298 having negative polarity are used in conjunction with a gradient echo sequence including eddy current excitation gradients having positive polarity (e.g., Figure 2) are used in combination with the gradient echo sequence 242 to form dipole eddy current excitation gradients. These dipole eddy current excitation gradients enable eddy currents to be distinguished from B0 drifts because eddy currents change direction in response to the dipole eddy current excitation gradients, whereas B0 drifts do not. The gradient echo sequence 242 is used in combination with the gradient echo sequence ( Figure 2 ) to form a GRE sequence (e.g., as part of a pseudo-continuous scan) that is repeated intermittently (e.g., every 100 milliseconds (ms)) to measure eddy currents. The GRE sequence is combined with a GRE sequence ( Figure 4 ) described in .

[0050] Figure 4 A pulse sequence diagram 300 is illustrated of a gradient echo sequence 302 utilized to measure eddy currents having a long time constant (eg, lacking an eddy current excitation gradient). As described in more detail below, the gradient echo sequence 302 is similar to a series of gradient echo sequences (eg, Figure 2 and Figure 3 ) to measure eddy currents with a long time constant (e.g., where the estimated or measured eddy currents have a time constant that is at least twice longer than the time repetition of each gradient echo sequence in the train of gradient echo sequences). The gradient echo sequence 302 is configured to remove background fields used to measure eddy currents (e.g., during a calibration scan such as an eddy current calibration scan).

[0051] The four rows 304, 306, 308 and 310 illustrate a gradient echo sequence 302 utilized to measure eddy currents with a long time constant. The first row 304 (e.g., top row) of the pulse sequence diagram 300 illustrates RF (represented along the Y axis 312) over time (represented along the X axis 314). As depicted, the RF pulse 316 is generated at approximately 1 millisecond. The second row 306 of the pulse sequence diagram 302 illustrates the gradient applied in the frequency direction (represented as gradient strength along the Y axis 318) over time (represented along the X axis 320). The gradient applied in the frequency direction is represented by curve 322. The first portion 324 of the curve 322, located to the left of the dashed line 326, is a balanced gradient echo with a zero gradient area. The second portion 328 of the curve 322, located to the right of the dashed line 326, lacks an eddy current excitation gradient or an eddy current generation gradient.

[0052] The third row 308 of the pulse sequence diagram 300 illustrates the gradient applied in the phase direction (represented as gradient strength along the Y-axis 330) over time (represented along the X-axis 332). The gradient applied in the phase direction is represented by curve 334. A first portion 336 of the curve 334, located to the left of the dashed line 338, is a balanced gradient echo with zero gradient area. A second portion 340 of the curve 334, located to the right of the dashed line 338, lacks an eddy current excitation gradient or eddy current generation gradient.

[0053] The fourth row 310 (e.g., bottom row) of the pulse sequence diagram 300 illustrates the gradient applied along the slice direction (represented as gradient strength along the Y-axis 342) over time (represented along the X-axis 344). The gradient applied along the slice direction is represented by curve 346. A first portion 348 of the curve 346, located to the left of the dashed line 350, is a balanced gradient echo with zero gradient area. A second portion 352 of the curve 346, located to the right of the dashed line 350, lacks an eddy current excitation gradient or eddy current generation gradient.

[0054] Eddy current measurements are performed using balanced gradient echoes of the first portion 324, 336, 348 of the applied gradient. The first portions 324 and 348 of the frequency gradient (e.g., readout gradient) and the slice gradient are balanced to avoid eddy current effects from the frequency gradient and the slice gradient, respectively. The lack of eddy current excitation gradients in the second portions 328, 340, and 352 enables the removal of background fields. The gradient echo sequence 302 is similar to a GRE sequence (e.g., repeated intermittently, e.g., every 100 milliseconds (ms)) Figure 2 and Figure 3 ) (eg, as part of a pseudo-continuous scan) to measure eddy currents.

[0055] Figure 5 A flow chart of a method 354 for measuring eddy currents having a long time constant is illustrated. One or more steps of the method 354 may be performed by Figure 1 One or more of the steps of method 354 may be performed simultaneously or in parallel. Figure 5 The order depicted is not the same as the order in which it is executed.

[0056] Method 354 includes utilizing a magnetic resonance imaging system of a magnetic resonance imaging scanner (e.g., Figure 1 The MR scanner 102 in the MRI system 106 initiates a calibration scan (eg, an eddy current calibration scan) of the phantom (block 356). The calibration scan may be performed during installation of the MRI system and / or after maintenance of the MRI system.

[0057] The method 354 also includes using a column of gradient echo (GRE) sequences to simultaneously generate and measure eddy currents during the calibration scan (block 358). The column of gradient echo sequences includes gradient echo sequences, wherein each gradient echo sequence in the gradient echo sequence includes a first portion having a balanced gradient echo sequence (having a zero gradient area) for eddy current measurement. The balanced gradient echoes of each of the gradient echo sequences in the column have a balanced readout gradient and a slice gradient to avoid eddy current effects from the readout gradient and the slice gradient. For example, Figure 2 and Figure 3The gradient echo sequences 182 and 242 respectively have balanced gradient echo sequences and can be used in the series of gradient echo sequences.

[0058] The series of gradient echo sequences also includes a first group of gradient echo sequences, the first group of gradient echo sequences having a second portion, the second portion having an eddy current excitation gradient or an eddy current generation gradient with a positive polarity. Figure 2 The gradient echo sequence 182 in FIG. 1 has an eddy current excitation gradient or an eddy current generation gradient with a positive polarity and can be used in the gradient echo sequence.

[0059] The gradient echo sequence also includes a second group of gradient echo sequences, the second group of gradient echo sequences having a second portion, the second portion having an eddy current excitation gradient or an eddy current generation gradient with a negative polarity. Figure 3 The gradient echo sequence 242 in has an eddy current excitation gradient or eddy current generation gradient with negative polarity and can be used in the column of gradient echo sequences. The first group of gradient echo sequences having a second portion having an eddy current excitation gradient or eddy current generation gradient with positive polarity and the second group of gradient echo sequences having a second portion having an eddy current excitation gradient or eddy current generation gradient with negative polarity are used to form bipolar eddy current excitation gradients. These bipolar eddy current excitation gradients make it possible to distinguish eddy currents from B0 drifts, because eddy currents will change direction in response to the bipolar eddy current excitation gradients, while B0 drifts will not.

[0060] In conjunction with this series of GRE sequences, a GRE sequence having a first portion with a balanced gradient echo sequence for eddy current measurement and a second portion lacking an eddy current excitation gradient or an eddy current generating gradient may be utilized. Figure 4 The gradient echo sequence 302 in the embodiment can be used in combination with the column of GRE sequences. Specifically, the gradient echo sequence 302 can be used to establish a baseline to remove the background field before the column of gradient echo sequences. In addition, the gradient echo sequence 302 can be used after the column of gradient echo sequences.

[0061] The gradient echo sequence utilized in this series of gradient echo sequences (as well as the gradient echo sequence lacking the eddy current excitation gradient or the eddy current generating gradient) is repeated at a regular rate as part of a pseudo-continuous scan (i.e., an eddy current calibration scan). In certain embodiments, the timing may be changed to repeat the gradient echo sequence as long as the pseudo-continuous condition is maintained and the average gradient area remains constant. In certain embodiments, these gradient echo sequences may be utilized in a single channel to measure eddy currents with a long time constant. For example, a gradient echo sequence lacking the eddy current excitation gradient (e.g., the gradient echo sequence 302) may be repeated a set number of times first. Next, a gradient echo sequence having an eddy current excitation gradient with a positive polarity (e.g., the gradient echo sequence 182) may be repeated a set number of times, and then a gradient echo sequence having an eddy current excitation gradient with a negative polarity (e.g., the gradient echo sequence 242) may be repeated a set number of times. Finally, a gradient echo sequence lacking the eddy current excitation gradient (e.g., the gradient echo sequence 302) may then be repeated a set number of times. In certain embodiments, the set number of times a gradient echo sequence having an eddy current excitation gradient with positive polarity and a gradient echo sequence having an eddy current excitation gradient with negative polarity are repeated is the same. In certain embodiments, the set number of times a gradient echo sequence having an eddy current excitation gradient with positive polarity and a gradient echo sequence having an eddy current excitation gradient with negative polarity are repeated is greater than the set number of times a gradient echo sequence lacking an eddy current excitation gradient is repeated. In certain embodiments, the set number of times a gradient echo sequence lacking an eddy current excitation gradient is repeated at the end of a pseudo-continuous scan is greater than the set number of times a gradient echo sequence lacking an eddy current excitation gradient is repeated at the beginning of a pseudo-continuous scan. The set number of times each gradient echo sequence is repeated in a pseudo-continuous scan for simultaneously generating and measuring eddy currents with a long time constant may vary.

[0062] In some embodiments (also for single channels), the order of the gradient echo sequences may vary. For example, a gradient echo sequence lacking an eddy current excitation gradient (e.g., the gradient echo sequence 302) may be repeated a set number of times first. Next, a gradient echo sequence having an eddy current excitation gradient with a negative polarity (e.g., the gradient echo sequence 242) may be repeated a set number of times, and then a gradient echo sequence having an eddy current excitation gradient with a positive polarity (e.g., the gradient echo sequence 182) may be repeated a set number of times. Finally, a gradient echo sequence lacking an eddy current excitation gradient (e.g., the gradient echo sequence 302) may be repeated a set number of times. In some embodiments, the set number of times the gradient echo sequence having an eddy current excitation gradient with a negative polarity and the gradient echo sequence having an eddy current excitation gradient with a positive polarity are repeated is the same. In some embodiments, the set number of times the gradient echo sequence having an eddy current excitation gradient with a negative polarity and the gradient echo sequence having an eddy current excitation gradient with a positive polarity are repeated is greater than the set number of times the gradient echo sequence lacking an eddy current excitation gradient is repeated. In some embodiments, the set number of times the gradient echo sequence lacking the eddy current excitation gradient is repeated at the end of the pseudo-continuous scanning is greater than the set number of times the gradient echo sequence lacking the eddy current excitation gradient is repeated at the beginning of the pseudo-continuous scanning. The set number of times each gradient echo sequence is repeated in the pseudo-continuous scanning for simultaneously generating and measuring eddy currents with a long time constant may vary.

[0063] In certain embodiments, multiple channels (e.g., two channels) may be utilized to measure eddy currents having a long time constant. In a first channel, a gradient echo sequence lacking an eddy current excitation gradient (e.g., the gradient echo sequence 302) may be first repeated a set number of times. Next, a gradient echo sequence having an eddy current excitation gradient having a positive polarity (e.g., the gradient echo sequence 182) may be repeated a set number of times. Finally, a gradient echo sequence lacking an eddy current excitation gradient (e.g., the gradient echo sequence 302) may then be repeated a set number of times. In a second channel, a gradient echo sequence lacking an eddy current excitation gradient (e.g., the gradient echo sequence 302) may first be repeated a set number of times. Next, a gradient echo sequence having an eddy current excitation gradient having a negative polarity (e.g., the gradient echo sequence 242) may be repeated a set number of times. Finally, a gradient echo sequence lacking an eddy current excitation gradient (e.g., the gradient echo sequence 302) may then be repeated a set number of times. In some embodiments, the set number of repetitions of the gradient echo sequence with the eddy current excitation gradient with positive polarity and the gradient echo sequence with the eddy current excitation gradient with negative polarity are the same. In some embodiments, the gradient echo sequence with the eddy current excitation gradient with negative polarity can be used in the first channel, and the gradient echo sequence with the eddy current excitation gradient with positive polarity can be used in the second channel.

[0064] Although previously mentioned using a set number of repetitions of gradient echo sequences, in certain embodiments, the number of times used to repeat each gradient echo sequence may be adaptively varied during a testing session in response to analysis of previously acquired gradient echo sequences.

[0065] Back to Figure 5 , method 354 also includes acquiring k-space data (box 360) from the column of gradient echo sequences (and gradient echo sequences lacking eddy current excitation gradients). The method 354 even includes converting the k-space data into an eddy current gradient field (box 362). Converting the k-space data into an eddy current gradient field may include multiple steps. In some embodiments, each row of the k-space data can be subjected to a one-dimensional Fourier transform to generate complex data. Phase information can be obtained from the complex data, and the phase information and linear regression can be used to obtain a slope and an intercept to determine the eddy current gradient field. The slope provides a spatially linear term for the eddy current. The intercept provides a spatially independent B0 term.

[0066] The method 354 also includes estimating the function of eddy current over time based on the eddy current gradient field (block 364). The induced eddy current can be approximated as being obtained by a constant gradient having a duration equal to the time repetition and an amplitude equal to the average gradient, where the average gradient is the total gradient area of ​​the corresponding gradient echo sequence in the series of gradient echo sequences divided by the time repetition. In some embodiments, estimating the function of eddy current over time includes scaling the eddy current gradient field by the average gradient strength and fitting the scaled eddy current gradient field over time. Specifically, estimating the function of eddy current over time (G ec (t) involves using the following formula:

[0067] G ec(t) =G ave *α*(1-e -t / TC ), (1)

[0068] Among them G ave is the average gradient, α is the eddy current amplitude percentage, t is the time, and TC is the time constant of the corresponding eddy current. As mentioned above, the average gradient G ave is the total gradient area divided by the time repetition of the gradient echo sequence. The average gradient is determined by the gradient echo sequence. The eddy current amplitude α expressed as a percentage is unknown. The time constant is also unknown. Method 354 enables accurate measurement of the time constant and amplitude when the sequence time repetition is much shorter than the time constant (i.e., the time constant is at least twice the time repetition).

[0069] The method 354 even includes storing the estimated eddy currents (block 366). The estimated eddy currents may be utilized to correct the acquired MR scan data.

[0070] As mentioned above, Figure 5Method 354 in enables accurate measurement of the time constant and amplitude of Equation 1 when the sequence time repetition is much shorter than the time constant (i.e., the time constant is at least twice the time repetition). Figures 6 to 9 Verification Figures 6 to 9 is a graph illustrating measured eddy current values ​​fitted as a function of time with various eddy current time constants using the techniques described herein as part of a testing process. It should be noted that Figures 6 to 9 It is proof Figure 5 The injection eddy current test is not a normal part of the method 354.

[0071] The test procedure involved adjusting the linear eddy current in the frequency direction with an amplitude (α) of -1.0% and varying time constants (e.g., 0.01 sec, 0.1 sec, 1 sec, and 10 sec). Figure 5 The method 354 described in the foregoing describes measuring eddy currents. The gradient echo sequence utilized (e.g., Figures 2 to 4 The time repetition of the eddy current excitation gradient (for both positive and negative polarity) was 100 milliseconds. The eddy current excitation gradient (for both positive and negative polarity) was applied only in the frequency direction. The average gradient strength was 1.5 G / cm. The full scan included the following sequences: For a gradient echo sequence lacking an eddy current excitation gradient (e.g., Figure 4 The gradient echo sequence 302 in FIG. 1 is repeated 10 times, and then the gradient echo sequence with the eddy current excitation gradient having the positive polarity (eg, Figure 2 The gradient echo sequence 1 82 in the example was repeated 200 times, and the gradient echo sequence with eddy current excitation gradient with negative polarity (e.g., Figure 3 The gradient echo sequence 242 in ) is repeated 200 times, and the gradient echo sequence with eddy current excitation gradient with positive polarity (for example, Figure 2 A gradient echo sequence 182 in ) is repeated for another 200 times, and a gradient echo sequence with an eddy current excitation gradient with negative polarity (e.g., Figure 3 A gradient echo sequence 242 in ) was repeated for another 200 times, and finally a gradient echo sequence lacking eddy current excitation gradients (e.g., Figure 4 The gradient echo sequence 302 in is repeated for the final 214 times.

[0072] During the test, a gradient echo sequence lacking eddy current excitation gradients (e.g. Figure 4 The first 10 repetitions of the gradient echo sequence 302 in FIG. 3 are used as the baseline. Similarly, a gradient echo sequence with an eddy current excitation gradient having a positive polarity is set (eg, Figure 2 The first repetition time of the gradient echo sequence 1 82 in is 0 seconds. Figure 5The method 354 in converts the measured raw data into an eddy current gradient field. The calculated eddy current gradient is then scaled by the average gradient strength and fitted as a function of time using the above formula 1. This is done for each different eddy current time constant.

[0073] Figure 6 is a graph 368 illustrating measured eddy current values ​​fit as a function of time (eg, an eddy current time constant of 0.01 seconds) using the techniques described herein as part of a testing procedure. Figure 7 is a graph 370 illustrating measured eddy current values ​​fit as a function of time (eg, an eddy current time constant of 0.1 seconds) using the techniques described herein as part of a testing procedure. Figure 8 is a graph 372 illustrating measured eddy current values ​​fit as a function of time (eg, an eddy current time constant of 1 second) using the techniques described herein as part of a testing procedure. Fig. 9 368 , 370 , 372 , 374 represents the eddy current gradient (expressed as a percentage) and the X-axis 378 represents time (expressed in seconds (sec)). Point 380 in each of the graphs 368 , 370 , 372 , 374 represents the first data point at time equal to 0 (the effect of the baseline is removed by subtracting the average value of the eddy current gradient field from 10 repetitions of the gradient echo sequence lacking the eddy current excitation gradient). Segment 382 in each of the graphs 368 , 370 , 372 , 374 represents the data for the first 200 repetitions of the gradient echo sequence having an eddy current excitation gradient with positive polarity. Segment 384 in each of Figures 368, 370, 372, 374 represents data for the first 200 repetitions of a gradient echo sequence with an eddy current excitation gradient having a negative polarity. Segment 386 in each of Figures 368, 370, 372, 374 represents data for the second 200 repetitions of a gradient echo sequence with an eddy current excitation gradient having a positive polarity. Segment 388 in each of Figures 368, 370, 372, 374 represents data for the second 200 repetitions of a gradient echo sequence with an eddy current excitation gradient having a negative polarity. Segment 390 in each of Figures 368, 370, 372, 374 represents data for the last 214 repetitions of a gradient echo sequence lacking an eddy current excitation gradient. In each of Figures 368, 370, 372, 374, the open circles represent measured data, and the line represents a fit of the measured data.

[0074] For the data in Figure 368, the true value of the time constant is 0.01 seconds, and the true value of the amplitude (α) is -1.0%, while the fitting result of the measured time constant is 0.017 seconds, and the fitting result of the amplitude (α) is -0.20%. For the data in Figure 370, the true value of the time constant is 0.1 seconds, and the true value of the amplitude (α) is -1.0%, while the fitting result of the measured time constant is 0.10 seconds, and the fitting result of the amplitude (α) is -0.93%. For the data in Figure 372, the true value of the time constant is 1 second, and the true value of the amplitude (α) is -1.0%, while the fitting result of the measured time constant is 1.01 seconds, and the fitting result of the amplitude (α) is -1.00%. For the data in Figure 374, the true value of the time constant is 10 seconds, and the true value of the amplitude (α) is -1.0%, while the fitting result of the measured time constant is 10.12 seconds, and the fitting result of the amplitude (α) is -1.01%. The data in Figures 372 and 374 illustrate Figure 5 The method 354 described in the above can accurately measure the time constant and amplitude when the time repetition of the gradient echo sequence is much shorter than the time constant (i.e., the time constant is at least twice the time repetition of the gradient echo sequence). The data in Figures 368 and 370 illustrate that when the time repetition of the gradient echo sequence is comparable to or shorter than the time constant, the average gradient hypothesis requires that the time repetition of the gradient echo sequence is much shorter than the time constant, so it cannot be used. Figure 5 The described method 354 accurately quantifies eddy currents. Figure 5 The method 354 described in is used to measure eddy currents with long time constants.

[0075] In certain embodiments, when measuring eddy currents with long time constants, a free induction decay sequence may be used for data collection rather than a gradient echo sequence. Fig.10 A pulse sequence diagram 392 illustrating a free induction decay sequence 394 utilized to measure eddy currents with long time constants (e.g., eddy current excitation gradients with positive polarity) is shown. It should be noted that when measuring eddy currents with long time constants using a free induction decay sequence, a small localized sample (e.g., six samples, two of which are aligned along the X, Y, and Z axes to localize field measurements) with transmit and receive coils (e.g., as part of the GRAFIDY toolkit) is utilized rather than a large phantom.

[0076] The four rows 396, 398, 400 and 402 illustrate a free induction decay sequence 394 utilized to measure eddy currents having a long time constant. The first row 396 (e.g., top row) of the pulse sequence diagram 392 illustrates RF (represented along the Y axis 404) over time (represented along the X axis 406). As depicted, the RF pulse 408 is generated at approximately 1 millisecond. The second row 398 of the pulse sequence diagram 392 illustrates the gradient applied in the frequency direction (represented as gradient strength along the Y axis 410) over time (represented along the X axis 412). The gradient applied in the frequency direction is represented by curve 414. The first portion 416 of the curve 414, located to the left of the dashed line 418, is a free induction decay with a zero gradient area. The second portion 420 of the curve 414, located to the right of the dashed line 418, has an eddy current excitation gradient or eddy current generation gradient 422 with a positive polarity.

[0077] The third row 400 of the pulse sequence diagram 392 illustrates the gradient applied in the phase direction (represented as gradient strength along the Y-axis 424) over time (represented along the X-axis 426). The gradient applied in the phase direction is represented by curve 428. A first portion 430 of the curve 428, located to the left of the dashed line 432, is a free induction decay with zero gradient area. A second portion 434 of the curve 428, located to the right of the dashed line 432, has an eddy current excitation gradient or eddy current generation gradient 436 with positive polarity.

[0078] The fourth row 402 (e.g., bottom row) of the pulse sequence diagram 392 illustrates the gradient applied along the slice direction (represented as gradient strength along the Y-axis 438) over time (represented along the X-axis 440). The gradient applied along the slice direction is represented by curve 442. A first portion 444 of the curve 442, located to the left of the dashed line 446, is a free induction decay with zero gradient area. A second portion 450 of the curve 442, located to the right of the dashed line 446, has an eddy current excitation gradient or eddy current generation gradient 448 with a positive polarity.

[0079] Eddy current measurements are performed using free induction decay of a first portion 416, 430, 444 of the applied gradient. The first portions 204 and 232 of the frequency gradient (e.g., readout gradient) and the slice gradient are balanced, respectively, to avoid eddy current effects from the frequency gradient and the slice gradient. Eddy currents are generated using eddy current excitation gradients 422, 436, and 448 of the second portion 420, 434, and 450. Eddy current excitation gradients 422, 436, and 450 having positive polarity are used in conjunction with a free induction decay sequence including eddy current excitation gradients having negative polarity to form bipolar eddy current excitation gradients. These bipolar eddy current excitation gradients enable eddy currents to be distinguished from B0 drifts because eddy currents will change direction in response to the bipolar eddy current excitation gradients, while B0 drifts will not. The free induction decay sequence 394 is used in conjunction with the free induction decay sequence (to form a train of free induction decay sequences that is repeated intermittently (e.g., every 100 milliseconds (ms)) (e.g., as part of a pseudo-continuous scan) to measure eddy currents. The train of free induction decay sequences is used in conjunction with a free induction decay sequence that lacks an eddy current excitation gradient.

[0080] Technical effects of the disclosed subject matter include enabling the measurement of eddy currents with long time constants using gradient echo sequences. Technical effects of the disclosed subject matter also include enabling the simultaneous generation and measurement of eddy currents. This enables pseudo-continuous scanning (a train of GRE sequences with intermittent repetitions) to be utilized to measure eddy currents with long time constants, taking less time and being more efficient than previous techniques. Additionally, the disclosed subject matter is less sensitive to B0 drift. Technical effects of the disclosed subject matter also include improving the calibration accuracy of eddy currents with very long time constants. By improving the calibration accuracy, the image quality of certain MR images is improved.

[0081] The technology presented and claimed herein is referred to and applied to physical and specific examples of a practical nature that clearly improves upon the present state of the art and, therefore, is not abstract, intangible, or purely theoretical. In addition, if any claim appended to the end of this specification contains one or more elements designated as "means for [performing] the function of ..." or "steps for [performing] the function of ...," it is intended that such elements be interpreted in accordance with 35 USC 112(f). However, for any claim containing elements designated in any other manner, it is not intended that such elements be interpreted in accordance with 35 USC 112(f).

[0082] This written description uses examples to disclose the subject matter, including the best mode, and also to enable those skilled in the art to practice the subject matter, including making and using any devices or systems and performing any included methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insignificant differences from the literal language of the claims.

Claims

1. A method for measuring eddy currents having a long time constant, the method comprising: Initiating a calibration scan of the phantom using the magnetic resonance imaging scanner; Using a series of gradient echo sequences to simultaneously generate and measure the eddy currents during the calibration scan, wherein the generation and measurement of the eddy currents are both performed within each gradient echo sequence in the series of gradient echo sequences; Acquiring k-space data from the gradient echo sequence; converting the k-space data into an eddy current gradient field; and The eddy current as a function of time is estimated based on the eddy current gradient field. 2 . The method according to claim 1 , wherein the first part of each gradient echo sequence in the series of gradient echo sequences comprises a balanced gradient echo for eddy current measurement, wherein the balanced gradient echo has a balanced readout gradient and a slice gradient to avoid eddy current effects. 3 . The method according to claim 2 , wherein the second part of a group of gradient echo sequences in the train of gradient echo sequences comprises an eddy current excitation gradient for generating eddy currents.

4. The method according to claim 3, wherein the second part of some gradient echo sequences in the set of gradient echo sequences includes a negative eddy current excitation gradient, and the second part of some gradient echo sequences in the set of gradient echo sequences includes a positive eddy current excitation gradient. The method according to claim 1 , wherein the estimated eddy current has a time constant at least twice longer than a time repetition of each gradient echo sequence in the train of gradient echo sequences.

6. The method of claim 5, wherein the induced eddy current can be approximated as being obtained by a constant gradient having a duration equal to the time repetition and an amplitude equal to an average gradient, wherein the average gradient is the total gradient area of ​​a corresponding gradient echo sequence in the series of gradient echo sequences divided by the time repetition. 7 . The method of claim 5 , wherein estimating the time function of the eddy current comprises scaling the eddy current gradient field by an average gradient strength and fitting the time function of the scaled eddy current gradient field.

8. The method according to claim 5, wherein the eddy current is estimated as a function of time (G ec (t) including the use of the following formula: G ec(t) =G ave *α*(1-e -t / TC ), where G ave is the average gradient, α is the eddy current amplitude percentage, t is the time, and TC is the time constant of the corresponding eddy current.

9. A system for measuring eddy currents having a long time constant, the system comprising: a memory encoding a processor-executable routine; and A processor configured to access the memory and execute the processor-executable routine, wherein the processor-executable routine, when executed by the processor, causes the processor to implement the method according to any one of claims 1 to 8. 10 . A non-transitory computer-readable medium comprising processor-executable code, which, when executed by a processor, causes the processor to implement the method according to any one of claims 1 to 8.

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