WO2007106360A1

WO2007106360A1 - Real-time shimming of respiration induced polarizing magnetic field changes

Info

Publication number: WO2007106360A1
Application number: PCT/US2007/005826
Authority: WO
Inventors: Jozef H. Duyn; Peter Van Gelderen
Original assignee: The Government Of The United States Of America, As Represented By The Secretary, Department Of Health And Human Services
Priority date: 2006-03-10
Filing date: 2007-03-06
Publication date: 2007-09-20

Abstract

During an MRI scan of the brain, real-time compensation of respiration induced field changes is accomplished by modulating the frequency of applied RF energy, and the intensity of applied spatially varying magnetic fields as a function of respiration state. Prior to scanning, changes to the local magnetic field in the head of the subject are measured as a function of respiration. Changes as a- function of gradient and shim currents are also determined. The changes as a function of respiration are combined with the changes as a function of gradient/shim currents to produce each current as a function of respiration. This result is utilized in real-time to compensate for fluctuations in the spatially varying magnetic field due to respiration.

Description

REAL-TIME COMPENSATION OF RESPIRATION INDUCED FIELD CHANGES

Cross Reference To Related Applications

[0001] The present application claims priority to U.S. Provisional Patent Application No. 60/781,246, entitled "REAL-TIME COMPENSATION OF RESPIRATION INDUCED FIELD CHANGES," filed March 10, 2006, which is hereby incorporated by reference in their entirety.

Technical Field

[0002J The technical field generally relates to magnetic resonance imaging (MRl), and more specifically relates to compensating for MRI image errors resulting from respiratory motion and other respiratory effects.

Background

[0003] When obtaining an image of a brain via magnetic resonance imaging (MRI), respiration can result in image distortion. Distortions such as ghosting, blurring, and shifting have been observed. The distortion is due in part to fluctuations in the polarizing magnetic field (referred to as the Bo field), in the head of a subject caused, in part, by respiratory motion of the subject's chest. [0004] When human tissue is subjected to a polarizing (Bo) field in an MRI scanner, the hydrogen atoms (specifically the protons of the hydrogen atoms) will align with the magnetic field. Radio frequency (RF) pulses are applied, typically via a coil, to the anatomy of interest. The hydrogen atoms in the vicinity of the anatomy of interest absorb the applied energy resulting in the protons to spin, or precess, in a different direction. When the RF pulse ceases, the protons release energy which is received by the coil. This received energy is used to produce an image of the anatomy of interest.

[0005] For optimal results, the Bo field should be uniform. Respiration can cause fluctuations in the B₀ field in the brain. Some causes of this phenomenon include brain motion, blood pulsation, cerebrospinal fluid motion, chest motion, and variations in oxygen content of the blood and the air in proximity to the brain. Previous attempts to correct fluctuations in the Bo field have utilized global correction techniques. The fluctuations in the Bo field due to respiratory motion, however, are spatially varying.

Summary

[0006] Real-time compensation of respiration induced field changes is accomplished by modulating, in real time (z.e., while the subject is being scanned), the frequency of applied RF energy, or the uniform polarizing magnetic field (referred to as the Bo field), and the intensity of spatially varying magnetic fields (gradients and shims, see below) as a function of the respiration state. Prior to scanning, localized changes to the Bo field in the anatomy of interest are measured as a function of respiration. Three dimensional (3D) phase maps are generated indicative of the field change for each image voxel (volume element) in the anatomy of interest as a function of respiration. Phase changes as a function of gradient and shim currents are also determined. Gradient and shim currents are electrical currents applied to various coils of an MRI scanner for producing magnetic fields. The phase changes as a function of respiration are combined with the phase changes as a function of gradient and shim currents to produce each current as a function of respiration. This result is utilized in real time to compensate for fluctuations in the Bo field due to respiration.

[0007] As a subject is being scanned, respiration is measured by a chest position detector. The signals provided by the chest detector are provided to a processor having stored therein the previously determined frequency and gradient/shim current values as a function of respiration. The processor receives the chest position detector signal and provides the appropriate compensation signals to the shim coils, hi an example configuration, RF resonant frequency (f₀), three gradient coils (x, y, and z), and, 5 second-order shim coils are modulated to provide compensation.

Brief Description Of The Drawings

[0008] Figure 1 is a diagram of an example system for providing real-time compensation of respiration induced field changes.

[0009] Figure 2 is an illustration of various functions performed to provide real-time compensation of respiration induced field changes.

[0010] Figure 3 is a flow diagram of an example process for providing real-time compensation of respiration induced field changes.

[0011] ' Figure 4 is a flow diagram of an example process for determining phase change of a magnetic field as a function of electrical current.

[0012] Figure 5 is a flow diagram of an example process for determining phase change of a magnetic field as a function of respiratory motion.

[0013] • Figure 6 shows a respiration waveform and a phase time course from one voxel of a training set.

[0014] Figure 7 shows filtered phase time course with compensation.

[0015] Figure 8 shows gradient echo scans with and without compensation, respectively.

Detailed Description Of Illustrative Embodiments

[0016] Figure 1 is a diagram of an example system 12 for providing real-time compensation of respiration induced field changes. As used herein, the term "real-time" indicates that compensation is provided concurrent with scanning of a subject. Tn a basic configuration, system 12 comprises an MRI scanner 24, a processor 16, an MRI console 14, a chest position detector 32, and an RF coil 34. The MRI console 14 controls the MRI scanner 24. MRI consoles and MRI scanners are known in the art. The MRI console 14 processor receives signals 40 to produce image signals 36, trigger signal 38, gradient signals 48, and shim signals 50. The processor 16 receives signals 36 and 38 from MRI console 14 and receives signal 46 from chest position detector 32. The processor 16 provides correction signals 42 and 44 to be combined with gradient signals 48 and shim signals 50, respectively, by amplifiers/filters 20 and 22, respectively. Compensation signals 56 are provided to gradient coils 26 of MRI scanner 24. Compensation signals 58 are provided to shim coils 28 of MRI scanner 24.

[0017] As described in more detail below, correction signals 42 and 44 facilitate real-time compensation of respiration induced field changes. For the sake of clarity, a single signal line (42) is depicted coupled to a single amplifier/filter 20. Tn an example configuration, signal 42 represents three correction signals: an x-gradient correction signal, a y-gradient correction signal, and a z-gradient correction signal, corresponding to gradient coils 26. Signal 48 represents three gradient signals: an x-gradient signal, a y-gradient signal, and a z-gradient signal, corresponding to gradient coils 26. Amplifier/filter 20 represents three amplifiers/filters, one for combining, amplifying, and/or filtering each of the pair of x- gradient signals, the pair of y-gradient signals, and the pair of z-gradient signals. Signal 56 represents three signals: an x-gradient plus compensation signal, a y-gradient plus compensation signal, and a z-gradient plus compensation signal, for application to gradient coils 26. Also, for the sake of clarity, a single signal line (44) is shown coupled to a single amplifier/filter 22. In an example configuration, signal 44 represents five correction signals, one corresponding to each of the shim coils 28. Amplifier/filter 22 represents five amplifiers/filters: one for combining, amplifying, and/or filtering each of the five pair of shim signals. Signal 58 represents five signals for application to shim coils 28, being the sum of the scanner shim signals from 50 and the compensation signals from 44.

[0018] Processor 16 can comprise any appropriate processor. Processor 16 can be implemented in a single processor, or multiple processors. Multiple processors can be distributed or centrally located. Multiple processors can communicate wirelessly, via hard wire, or a combination thereof. Example processors include, but are not limited to, personal computers, server. computers, hand-held or laptop devices, multiprocessor systems, microprocessor-based systems, set top boxes, programmable consumer electronics, network PCs, minicomputers, mainframe computers, distributed computing environments that include any of the above systems or devices, and the like.

[0019] Chest position detector 32 senses the relative position of the chest of subject 30. Chest position detector 32 can comprise any appropriate sensing device, such as a strain gauge, a device having variable resistance dependent upon the amount of contraction and/or expansion of the device, an elastic strap having resistance proportional to strap length, an optical detector, an ultrasonic detector, a respiratory bellows, or the like, for example. Multiple detectors 32 can be applied to increase the accuracy of the compensation.

[0020] RF coil 34 provides RF energy to the anatomy of interest (e.g., brain) and receives/detects energy from the anatomy of interest. RF coil 34 transduces electrical energy into RF energy and received energy into electrical energy. RF coil 34 functions as a detector for detecting changes in the magnetic field in a predetermined region proximate to RF coil 34. RF coils are known in the art, and RF coil 34 can comprise any appropriate RF coil.

[0021] Figure 2 is an illustration of various functions performed to provide real-time compensation of respiration induced field changes. Block 60 depicts an example training phase of the process of providing real-time compensation of respiration induced field changes. In an example scenario, with respect to Figure 1, subject 30 can be placed within the MRI scanner 24. The chest position detector 32 is placed around the subject's 30 chest. The MRI scanner 24 is activated to provide a polarizing magnetic field (Bo) around subject 30. As subject 30 breathes, the chest position detector 32 detects respiratory motion. For example, as the subject 30 breathes, respiratory motion causes the resistance, or the like, of chest position detector 32 to change accordingly. Signal 46, indicative of respiratory motion, is provided to processor 16. Signal 46 can be optionally amplified and/or filtered by amplifier 52. As subject 30 breathes, the change in phase of the nuclear magnetic resonance (NMR) signals indicative of the changes in the magnetic field, B₀, in the anatomy of interest, e.g., the brain, is measured. The NMR signals are detected by coil 34. It is to be understood that the coil 34 represent any appropriate mechanism configured to detect an NMR signal. Signal 54, the phase of which relates to the changes in the Bo field, is provided to RF converter 18. RF converter 18 processes signal 54 and provides signal 40, indicative of the change in the B₀ field, to MRI console 14. MRI console 14 processes signal 40 and provides signal 36, indicative of image data, and trigger signal 38 to processor 16. The trigger signal 38 provides the processor 16 with timing information indicative of when the image data is acquired, utilized because, the image data provided in signal 36 may not be provided in real time. Processor 16 processes the image data provided by signal 36 with the respiratory motion data provided by signal 46 to determine the change in the Bo field as a function of respiratory motion. The change of phase of the Bo field as a function of respiration can be represented as a phase map, as depicted in block 60 of Figure 2.

[0022J Block 62 of Figure 2 depicts an example reference phase of the process of providing real-time compensation of respiration induced field changes. Separate from the above determination of the change the Bo field as a function of respiratory motion, the change in phase of the Bo field as a function of coil currents is determined. This is accomplished by incrementing the electrical current provided to each of the three gradient coils 26 and each of the five shim coils 28 separately. The resultant change in the B₀ field in the same spatial location as the anatomy of interest is measured for each current increment for each coil individually. MRl console 14 increments the current values and provides signals 48 and 50 indicative thereof. Coil 34 detects the change in phase of the NMR signal due to the change in the Bo field in the anatomy of interest, and provides signal 54 to RF converter 18. As described above, RF converter 18 provides signal 40 to the MRI console 14, which provides respective image data to processor 16 via signal 36. The processor 16 processes the image data provided by signal 36 with the electrical current values provided to gradient coils 26 and shim coils 28 to determine the change in the Bo field as a function of current for each of the three gradient coils and each of the five shim coils. The change of the Bo field as a function of current can be represented as a phase map for each current, as depicted in block 62 of Figure 2. The process need be performed only once, at installation of the scanner, or after a change is made to the hardware or software of the system.

[0023] Block 64 of Figure 2 depicts an example combination phase of the process of providing real-time compensation of respiration induced field changes. At this point, the change in the Bo field as a function of respiratory motion and the change in the B₀ field as a function of coil current are known. The processor 16, utilizing this information, determines the coil current as a function of respiratory motion. [0024] Block 66 of Figure 2 depicts example real-time compensation of respiration induced field changes. In real time, subject 30 is subjected to an MRI scan via the MRI scanner 24. The chest position detector 32 is utilized to determine respiratory motion. As subject 30 breathes, the processor 16, utilizing the information stored therein pertaining to coil current as a function of respiratory motion, applies correction signals 42 and 44, which are combined with gradient signals 58 and shim signals 50, respectively, to provide real time compensated signals 56 and 58 to gradient coils 26 and shim coils 28, respectively. The compensated signals 56 and 58 energize coils 26 and 28, respectively, to provide magnetic fields that are compensated to counteract the respiration induced field changes to the Bo field proximate to the anatomy of interest. The spatially uniform part of the respiratory induced field changes is compensated by modulating the reference frequency 17 of the RF converter 18. Alternatively, a current can be applied to a Bo shim coil present in the system.

[0025] Figure 3 is a flow diagram of an example process for providing real-time compensation of respiration induced field changes. At step 68, changes of the B₀ field in a predetermined region (e.g., in an anatomy of interest, such as the brain) as a function of respiration are determined and recorded. This occurs during the training phase described above, and described in more detail below with reference to Figure 5. At step 70, the changes of the Bo field as a function of each gradient and shim current are determined. The values of current applied to each gradient and shim coil to produce a predetermined amount of change in Bo field current are determined and recorded. This occurs during the reference phase described above, and described in more detail below with reference to Figure 4. In an example embodiment, step 70 is performed after installation of the scanner or after any upgrade to its hardware or software. And, step 68 is performed on the subject 30, at the start of a scan session.

[0026] At step 72, changes in the Bo field as a function of respiration and changes in the B₀ field as a function of each current are utilized to determine each current as a function of respiration. A subject is subjected to an MRI scan and, during the scan, respiratory motion of the subject is monitored at step 74. The monitored real-time respiratory motion is utilized to compensate the appropriate magnetic fields at step 76. Values indicative of motion obtained from the monitored real-time respiration are entered into the formula relating each current value and the frequency reference as a function of respiratory motion. The resulting current values and frequency modulation are applied to the appropriate coils to compensate for the change in magnetic field due to respiratory motion. In an example embodiment, magnetic fields resulting from three gradient coils are compensated, magnetic fields resulting from five shim coils are compensated, and the resonant frequency of the RF pulse is compensated.

[0027] Figure 4 is a flow diagram of an example process for determining change of a magnetic field as a function of electrical current. During the reference phase, the change in the Bo field in a predetermined region, such as in a subject's brain (head), is determined as a function of electrical current applied to a coil causing the magnetic field. In an example embodiment, this is done for each of the three gradient coils and each of the 5 shim coils. Current for each coil is incremented at step 78. This can be accomplished one coil at a time, for example. The resulting changes, or perturbations, in the B₀ field in the predetermined region are measured and recorded at step 80. Three dimensional (3D) maps of the Bo field in the predetermined region are generated at step 82. A 3D phase map is generated for each current, and each phase map typically comprises several thousand voxels. In an example embodiment, to reduce the amount of information to be processed, a second order curve fitting process is applied, which describes the shape and amplitude of each phase map in approximately ten coefficients. A matrix (referred to as a sensitivity matrix) is generated at step 84. The sensitivity matrix comprises the 3D phase map coefficients as a function of current for each of the gradient shim currents and the reference frequency modulation.

[0028] Figure 5 is a flow diagram of an example process for determining change of a magnetic field as a function of respiratory motion. During the training phase, the change in the Bo field in a predetermined region, such as proximate to a subject's brain (head), is determined as a function of respiratory motion. The subject is placed in the MRT scanner with a chest position detector attached. The respiration motion of the subject is measured for a predetermined amount of time at step 86. The predetermined amount of time can be any amount of time appropriate to acquire a full cycle of respiration motion. Several cycles can be measured and averaged to reduce statistical noise. Over the predetermined amount of time, the resulting changes, or perturbations, in the Bo field in the predetermined region are measured and recorded and step 88. The relationship between changes in the B₀ field in the predetermined region as a function of respiration motion {e.g., chest position) is determined for each image voxel at step 90. Three dimensional phase maps of the Bo field in the predetermined region are generated at step 92. Measurements of phase change are taken at discreet spatial locations in the predetermined region. In an example embodiment, a second order curve fitting process is applied across the voxel images, resulting in a set of coefficients (typically ten) describing the amplitude and shape of the phase changes. In another example embodiment, a higher order curve fitting process can be applied. The 3D map represents changes in the Bo field in the predetermined region as a function of respiration motion. The coefficients of 3D map in conjunction with the sensitivity matrix are utilized to determine phase change of the Bo field in the predetermined region as a function of respiratory motion.

[0029] Referring again to Figure 1, an implementation of system 12 was utilized to assess real-time compensation of respiration induced field changes. MRI scanner 24 was implemented with a GE 7T scanner having Resonance Research, Inc. higher order shim amplifiers. Processor 16 was implemented using two computers. Nine digital to analog converters (DAC) were coupled to the implementation of processor 16 to provide analog signals to amplifiers/filters 20 and 22, and to reference synthesizer 17. For the gradient signals provided to gradient amplifiers/filters 20, the gradient amplifiers/filters were modified to have second inputs. For f^"o compensation, the reference synthesizer 17 for RF converter 18 was switched to FM mode, and frequency modulation was controlled by the analog signal provided to frequency synthesizer 17. Respiration signal 46 was provided by MRl scanner 24 using a GE respiratory bellows at a 4 millisecond sample time. Each channel was calibrated by mapping its phase effects in an oil phantom using an Echo Planar Imaging (EPI) sequence (parameters: 96x72 voxels over 24x18 cm², twelve 2mm slices with 8 mm gap, TE 30 ms, TR Is). In a normal scan session, after localization and shimming, the effect of respiration on Bo was measured. For the training session data the respiratory signal, the triggers (for timing) and an EPI time series of 120 seconds were acquired (with the same parameters). The phase changes in this EPI time series were then high pass filtered (>0.11 Hz) and correlated to the respiration signal. Subsequent scans were compensated by calculating the appropriate shim changes as function of the respiratory signal in real time. The shims were updated every 80 milliseconds. One subject was scanned. After the training set, a 60 second EPI series was acquired with compensation, and gradient echo images were acquired with and without compensation. [0030] Figure 6 shows a respiration waveform 96 and a phase ^'time course 98 from one voxel of the training set. A correlation coefficient of 0.98 was calculated between the filtered phase course 98 and the respiratory signal 96. Figure 7 shows the filtered phase time course with compensation. The compensated, filtered phase time course waveform in Figure 7 demonstrates a reduction in fluctuation compared to the uncompensated, filtered phase time course waveform 98 in Figure 6. The compensated filtered phase time course exhibited an average standard deviation that was 2.3 times lower than that of the uncompensated filtered phase time course.

[0031 J Figure 8 shows a gradient echo scan with compensation (100) and a gradient echo scan without compensation (102). A substantial reduction in ghosting is observable in scan 100 as compared to scan 102.

[0032] The various techniques described herein can be implemented in connection with hardware or software or, where appropriate, with a combination of both. Thus, the methods and apparatuses for providing real-time compensation of respiration induced field changes or certain aspects or portions thereof, can take the form of program code (i.e., instructions) embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other machine-readable storage medium, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for providing real-time compensation of respiration induced field changes. In the case of program code execution on programmable computers, the computing device will generally include a processor, a storage medium readable by the processor (including volatile and nonvolatile memory and/or storage elements), at least one input device, and at least one output device. The program(s) can be implemented in assembly or machine language, if desired. Iu any case, the language may be a compiled or inteipreted language, and combined with . hardware implementations.

[0033] The methods and apparatuses for providing real-time compensation of respiration induced field changes also can be practiced via communications embodied in the form of program code that is transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via any other form of transmission, wherein, when the program code is received and loaded into and executed by a machine, such as an EPROM, a gate array, a programmable logic device (PLD), a client computer, or the like, the machine becomes an apparatus for providing real-time compensation of respiration induced field changes. When implemented on a general-purpose processor, the program code combines with the processor to provide a unique apparatus that operates to invoke the functionality of providing real-time compensation of respiration induced field changes. Additionally, any storage techniques used in connection with providing real-time compensation of respiration induced field changes can invariably be a combination of hardware and software.

[0034] While real-time compensation of respiration induced field changes has been described in connection with the example embodiments of the various figures, it is to be understood that other similar embodiments can be used or modifications and additions can be made to the described embodiments for performing the same functions of real-time compensation of respiration induced field changes without deviating therefrom. Therefore, real-time compensation of respiration induced field changes as described herein should not be limited to any single embodiment, but rather should be construed in breadth and scope in accordance with the appended claims.

Claims

What is Claimed:

1. A method for compensating for motion induced magnetic field changes, said method comprising: determining a first function indicative of a first change in a magnetic field as a function of motion of a portion of an object within the magnetic field; determining a second function indicative of a second change in the magnetic field as a function of electrical current applied to a coil for producing the magnetic field; and determining, in accordance with the first function and the second function, a third function indicative of an electrical current applicable to the coil to compensate for the first change in the magnetic field change as a function of the motion.

2. A method in accordance with claim 1, further comprising: placing the object within the magnetic field; and while the object is within the magnetic field: measuring a motion of the portion of the object; determining, in accordance with the third function, an electrical current value corresponding to the measured motion; and providing the corresponding electrical current value to the coil.

3. A method in accordance with claim 2, wherein: the object comprises a chest and a brain; measuring the motion comprises measuring a relative position of the chest of the object; and the first and second changes in the magnetic field are determined for a region of the magnetic field in the brain of the object.

4. A method in accordance with claim 2, further comprising modulating a resonant frequency of RF energy applied to the object to compensate for the first change in the magnetic field as a function of the motion.

5. A method in accordance with claim 1, wherein the magnetic field is generated by a magnetic resonance imaging scanning device.

6. A method in accordance with claim 1, wherein the first and second changes are determined for a predetermined region of the magnetic field.

7. A method in accordance with claim 1, wherein the motion comprises respiratory motion of the object.

8. A method in accordance with claim 1, wherein: the object comprises an anatomy of interest; and the first and second changes are determined for a region of the magnetic field in the anatomy of interest.

9. A method in accordance with claim 1, further comprising: determining the second function as a function of electrical current applicable to a plurality of coils for producing the magnetic field.

10. A method in accordance with claim 9, wherein the plurality of coils comprises at least one gradient coil and at least one shim coil.

1 1. A method in accordance with claim 1 further comprising generating a three dimensional map indicative of the first function.

12. A method in accordance with claim 11, wherein: generating the map comprises fitting image voxels utilizing a polynomial; and the polynomial is of at least a second order.

13. A method in accordance with claim 1 further comprising generating a three dimensional map indicative of the second function.

14. A method in accordance with claim 13, wherein: creating the map comprises fitting image voxels utilizing a polynomial; and the polynomial is of at least a second order.

15. A system for compensating for motion induced magnetic field changes, the system comprising: a coil for producing a magnetic field; a first detector for detecting a change in a predetermined region of the magnetic field; a second detector for detecting a relative position of a portion of an object within the magnetic field; and a processor for: determining a first function indicative of a first change in the predetermined region of the magnetic field as a function of motion of the portion of the object within the magnetic field; determining a second function indicative of a second change in the predetermined region of the magnetic field as a function of electrical current applied to the coil; and determining, in accordance with the first function and the second function, a third function indicative of electrical current applicable to the coil to compensate for the first change in the predetermined region of the magnetic field as a function of the motion.

16. A system in accordance with claim 15, wherein, in determining the first function, the processor further for: receiving a signal from the first detector indicative of the first change in the predetermined region of the magnetic field resulting from the motion of the portion of the object within the magnetic field; and receiving a signal from the second detector indicative of the motion.

17. A system in accordance with claim 15, wherein, in determining the second function, the processor further for: providing a signal to the coil indicative of a value of electrical current applicable to the coil; and receiving a signal from the first detector indicative of the second change in the predetermined region of the magnetic field resulting from the value of electrical current applied to the coil.

18. A system in accordance with claim 15, wherein, while the object is positioned within the magnetic field, the processor further for: receiving a signal indicative of observed motion of the portion of the object; determining, in accordance with the third function, an electrical current value corresponding to the observed motion; and providing the corresponding electrical current value to the coil.

19. A system in accordance with claim 18, wherein: the object comprises a chest and a brain; the observed motion comprises a relative position of the chest of the object; and the predetermined region comprises a region in the brain of the object.

20. A system in accordance with claim 15, the processor further for modulating a resonant frequency of RF energy applied to the object to compensate for the first change in the magnetic field as a function of the motion.

21 . A system in accordance with claim 15, further comprising a magnetic resonance imaging scanning device for generating the magnetic field.

22. A system in accordance with claim 15, wherein the motion comprises respiratory motion of the object.

23. A system in accordance with claim 15, wherein the predetermined region comprises a region in an anatomy of interest of the object.

24. A system in accordance with claim 15, the processor further for deteπnining the second function as a function of electrical current applied to a plurality of coils for producing the magnetic field.

25. A system in accordance with claim 24, wherein the plurality of coils comprises at least one gradient coil and at least one shim coil.

26. A system in accordance with claim 15, the processor further for generating a three dimensional map indicative of the first function.

27. A system in accordance with claim 26, the processor further for interpolating between image voxels utilizing a second order polynomial to generate the map.

28. A system in accordance with claim 15, the processor further for generating a three dimensional map indicative of the second function.

29. A system in accordance with claim 28, the processor further for interpolating between image voxels utilizing a second order polynomial to generate the map.

30. A computer-readable medium having computer-executable instructions stored thereon, the computer-executable instructions for performing acts for compensating for motion induced magnetic field changes comprising: determining a first function indicative a first change in a magnetic field as a function of motion of a portion of an object within the magnetic field; determining a second function indicative of a second change in the magnetic field as a function of electrical current applied to a coil for producing the magnetic field; and determining, in accordance with the first function and the second function, a third function indicative of electrical current applicable to the coil to compensate for the first change in the magnetic field as a function of the motion.