WO2015197366A1

WO2015197366A1 - Motion correction in magnetic resonance imaging

Info

Publication number: WO2015197366A1
Application number: PCT/EP2015/062996
Authority: WO
Inventors: Tim Nielsen; Peter Koken; Julien Senegas
Original assignee: Koninklijke Philips N.V.
Priority date: 2014-06-23
Filing date: 2015-06-11
Publication date: 2015-12-30

Abstract

The invention provides for a magnetic resonance imaging system (100) for acquiring magnetic resonance data from subject using an imaging pulse sequence instructions (140), a first motion correction pulse sequence instructions (142), and a second motion correction pulse sequence instructions (144). Execution of instructions causes a processor to: acquire (202) the second k-space navigator data using the first motion correction pulse sequence instructions, acquire (204) reference tracker magnetic resonance data (148) using the second motion correction pulse sequence instructions, and reconstruct (206) a predetermined number of reference images (150) using the reference tracker magnetic resonance data. The instructions causes the processor to repeatedly: acquire (208) a portion (152) of the magnetic resonance data using the imaging pulse sequence instructions, detect (210) movement of the subject by comparing the first k-space navigator data to the second k- space navigator data, iteratively (214) correct the scan geometry if the movement of the subject is above a predetermined threshold, correct (216) the portion of magnetic resonance data if the scan geometry has been iteratively corrected. The instructions further causes the processor to reconstruct (220) a magnetic resonance image (168) from the magnetic resonance data after all portions of the magnetic resonance data have been acquired.

Description

Motion correction in Magnetic Resonance Imaj

TECHNICAL FIELD OF THE INVENTION

The invention relates to motion correction during the acquisition of magnetic resonance imaging data, in particular to the use of navigators in both k-space and image space to correction for motion.

BACKGROUND OF THE INVENTION

A large static magnetic field is used by Magnetic Resonance Imaging (MRI) scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This large static magnetic field is referred to as the BO field.

During an MRI scan, Radio Frequency (RF) pulses generated by a transmitter coil cause perturbations to the local magnetic field, and RF signals emitted by the nuclear spins are detected by a receiver coil. These RF signals are used to construct the MRI images. These coils can also be referred to as antennas. Further, the transmitter and receiver coils can also be integrated into a single transceiver coil that performs both functions. It is understood that the use of the term transceiver coil also refers to systems where separate transmitter and receiver coils are used. The transmitted RF field is referred to as the Bl field.

A difficulty in acquiring magnetic resonance images is that the subject needs to remain still during the imaging procedure to prevent artifacts or blurring of the image. In Lin et. al., "Real-Time Motion Correction in Two -Dimensional Multislice Imaging with Through-Plane Navigator," Magnetic Resonance in Medicine 71 : 1995-2005 (2014) a method of motion correction is described. Two through-plane navigators are collected for each imaging slice and are used to reconstruct two orthogonal through-plane navigator projection images both perpendicular to the images slices, within each repetition time. An additional orbital navigator is used to detect rotation within the imaging plane and reject intrarepetition time motion in real time. The European patent application EP 2 626 718 addresses the problem of employing separate navigators for water and fat tissue. Document is not concerned with the acquisition time of the navigators. Several navigator techniques are discussed for prospective motion correction in various anatomical regions. SUMMARY OF THE INVENTION

The invention provides for a magnetic resonance imaging system, a method of operating the magnetic resonance imaging system, and a computer program product in the independent claims. Embodiments are given in the dependent claims.

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as an apparatus, method or computer program product.

Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, microcode, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a "circuit," "module" or "system." Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer executable code embodied thereon.

Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A 'computer-readable storage medium' as used herein encompasses any tangible storage medium which may store instructions which are executable by a processor of a computing device. The computer-readable storage medium may be referred to as a computer-readable non-transitory storage medium. The computer-readable storage medium may also be referred to as a tangible computer readable medium. In some embodiments, a computer-readable storage medium may also be able to store data which is able to be accessed by the processor of the computing device. Examples of computer- readable storage media include, but are not limited to: a floppy disk, a magnetic hard disk drive, a solid state hard disk, flash memory, a USB thumb drive, Random Access Memory (RAM), Read Only Memory (ROM), an optical disk, a magneto-optical disk, and the register file of the processor. Examples of optical disks include Compact Disks (CD) and Digital Versatile Disks (DVD), for example CD-ROM, CD-RW, CD-R, DVD-ROM, DVD-RW, or DVD-R disks. The term computer readable-storage medium also refers to various types of recording media capable of being accessed by the computer device via a network or communication link. For example a data may be retrieved over a modem, over the internet, or over a local area network. Computer executable code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing.

A computer readable signal medium may include a propagated data signal with computer executable code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device.

'Computer memory' or 'memory' is an example of a computer-readable storage medium. Computer memory is any memory which is directly accessible to a processor. 'Computer storage' or 'storage' is a further example of a computer-readable storage medium. Computer storage is any non- volatile computer-readable storage medium. In some embodiments computer storage may also be computer memory or vice versa.

A 'processor' as used herein encompasses an electronic component which is able to execute a program or machine executable instruction or computer executable code. References to the computing device comprising "a processor" should be interpreted as possibly containing more than one processor or processing core. The processor may for instance be a multi-core processor. A processor may also refer to a collection of processors within a single computer system or distributed amongst multiple computer systems. The term computing device should also be interpreted to possibly refer to a collection or network of computing devices each comprising a processor or processors. The computer executable code may be executed by multiple processors that may be within the same computing device or which may even be distributed across multiple computing devices.

Computer executable code may comprise machine executable instructions or a program which causes a processor to perform an aspect of the present invention. Computer executable code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the "C" programming language or similar programming languages and compiled into machine executable instructions. In some instances the computer executable code may be in the form of a high level language or in a pre-compiled form and be used in conjunction with an interpreter which generates the machine executable instructions on the fly.

The computer executable code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider).

Aspects of the present invention are described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block or a portion of the blocks of the flowchart, illustrations, and/or block diagrams, can be implemented by computer program instructions in form of computer executable code when applicable. It is further understood that, when not mutually exclusive, combinations of blocks in different flowcharts, illustrations, and/or block diagrams may be combined. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

A 'user interface' as used herein is an interface which allows a user or operator to interact with a computer or computer system. A 'user interface' may also be referred to as a 'human interface device.' A user interface may provide information or data to the operator and/or receive information or data from the operator. A user interface may enable input from an operator to be received by the computer and may provide output to the user from the computer. In other words, the user interface may allow an operator to control or manipulate a computer and the interface may allow the computer indicate the effects of the operator's control or manipulation. The display of data or information on a display or a graphical user interface is an example of providing information to an operator. The receiving of data through a keyboard, mouse, trackball, touchpad, pointing stick, graphics tablet, joystick, webcam, headset, pedals, wired glove, remote control, and accelerometer are all examples of user interface components which enable the receiving of information or data from an operator.

A 'hardware interface' as used herein encompasses an interface which enables the processor of a computer system to interact with and/or control an external computing device and/or apparatus. A hardware interface may allow a processor to send control signals or instructions to an external computing device and/or apparatus. A hardware interface may also enable a processor to exchange data with an external computing device and/or apparatus. Examples of a hardware interface include, but are not limited to: a universal serial bus, IEEE 1394 port, parallel port, IEEE 1284 port, serial port, RS-232 port, IEEE-488 port, Bluetooth connection, Wireless local area network connection, TCP/IP connection, Ethernet connection, control voltage interface, MIDI interface, analog input interface, and digital input interface.

A 'display' or 'display device' as used herein encompasses an output device or a user interface adapted for displaying images or data. A display may output visual, audio, and or tactile data. Examples of a display include, but are not limited to: a computer monitor, a television screen, a touch screen, tactile electronic display, Braille screen,

Cathode ray tube (CRT), Storage tube, Bistable display, Electronic paper, Vector display, Flat panel display, Vacuum fluorescent display (VF), Light-emitting diode (LED) displays, Electroluminescent display (ELD), Plasma display panels (PDP), Liquid crystal display (LCD), Organic light-emitting diode displays (OLED), a projector, and Head- mounted display.

Magnetic Resonance (MR) data is defined herein as being the recorded measurements of radio frequency signals emitted by atomic spins by the antenna of a

Magnetic resonance apparatus during a magnetic resonance imaging scan. Magnetic resonance data is an example of medical image data. A Magnetic Resonance Imaging (MRI) image is defined herein as being the reconstructed two or three dimensional visualization of anatomic data contained within the magnetic resonance imaging data. This visualization can be performed using a computer.

In one aspect the invention provides for a magnetic resonance imaging system for acquiring magnetic resonance data from a subject for a scan geometry within an imaging zone. A scan geometry as used herein encompasses a portion of an imaging zone for which magnetic resonance data is obtained. For example if a physician or other medical technician is interested in imaging a certain portion of a subject's anatomy they would define planes or volumes from which magnetic resonance data is acquired to reconstruct images. The definition of the volume or planes from which magnetic resonance data is acquired is a scan geometry.

The magnetic resonance imaging system comprises a magnet for generating a main magnetic field in the imaging zone. The magnetic resonance imaging system further comprises a radio-frequency system comprising a magnetic resonance antenna for sending radio-frequency transmissions to the imaging zone and for receiving magnetic resonance signals from the imaging zone. The magnetic resonance antenna may refer to a single antenna or it may refer to multiple antennas. For instance in some magnetic resonance imaging systems a single magnetic resonance antenna or coil is used for both sending and receiving radio-frequency signals. In other magnetic resonance imaging systems there may be separate send and receive coils.

The magnetic resonance imaging system further comprises a gradient coil system for controlling the magnetic field gradient within the imaging zone. The magnetic resonance imaging system further comprises a memory containing machine-executable instructions. The memory further contains imaging pulse sequence instructions. Pulse sequence instructions as used herein either comprise instructions which can be used to control a magnetic resonance imaging system to acquire magnetic resonance data or they may be instructions which may be converted into instructions for controlling the magnetic resonance imaging system to acquire magnetic resonance data. For example the pulse sequence instructions may simply be executable instructions which control the various functions of the magnetic resonance imaging system to execute at various times. In other examples the pulse sequence instructions may for example by a timeline or chart which is programmed by an operator of the magnetic resonance imaging system and then converted into commands which are sequentially executed to acquire the magnetic resonance data.

Herein, a variety of various pulse sequence instructions are referred to. The use of the adjectives or words describing the various pulse sequence instructions are intended to identify specific pulse sequence instructions and to enable the determination of which pulse sequence instructions are being referred to. For instance the term imaging pulse sequence instructions refers to one set of pulse sequence instructions. The term first motion correction pulse sequence instructions refers to another set of pulse sequence instructions. The second motion correction pulse sequence instructions refers to yet another set of pulse sequence instructions. The memory further contains first motion correction pulse sequence

instructions. The memory further contains second motion correction pulse sequence instructions. The imaging pulse sequence instructions comprises additional instructions which cause the magnetic resonance imaging system to sample the magnetic resonance data as portions of k-space for the scan geometry. For example overall the imaging pulse sequence may specify various planes or portions of k-space to sample in order to acquire magnetic resonance data for the scan geometry. This magnetic resonance data can be divided into groups which are then acquired sequentially. These are referred to as portions of k-space.

The imaging pulse sequence instructions further comprise instructions which cause the magnetic resonance imaging system to acquire first k-space navigator data using a k-space navigator technique for every portion of k-space. A k-space navigator technique as used herein encompasses a navigator technique which makes measurements in k-space and then compares the data directly in k-space to determine a motion or movement of the subject. The imaging pulse sequence instructions are constructed such that for every portion of k- space that is acquired there is also acquired first k-space navigator data. In other words each portion of k-space that is acquired has k-space navigator that was acquired also.

The first motion correction pulse sequence instructions further comprises instructions which cause the magnetic resonance imaging system to acquire second k-space navigator data using the k-space navigator technique. The imaging pulse sequence

instructions comprises instructions for acquiring portions of the k-space for the scan geometry and also for acquiring k-space navigator data using the k-space navigator technique. In contrast the first motion correction pulse sequence instructions only acquire the k-space navigator data using the k-space navigator technique. The second motion correction pulse sequence instructions comprise instructions which cause the magnetic resonance imaging system to acquire tracker magnetic resonance data. The tracker magnetic resonance data comprises a predetermined number of slices of the imaging zone. In the tracker method one or more slices of the imaging zone are acquired and then the images that are acquired are compared to determine a motion of the subject.

The tracker method for instance is described in T. Nielsen, P. Bornert, J.

Senegas, "Fast inter-scan motion detection and compensation", Proc. ISMRM 2012, 2472. The tracker method has the benefit of being able to identify out of plane motions of the subject. Depending upon how the subject is constrained and the type of region being imaged one, two, or three slices are typically used. For example for brain imaging the motion model is a rigid body motion in three dimensions. In this case you would use three slices for the tracker method. However, for example correcting for breathing motion in liver imaging you could use only a single two-dimensional slice to track motion in the foot-to-head direction of the subject. This may be sufficient enough to acquire a single sagittal or coronal slice as a tracker.

When comparing the k-space navigator technique and the tracker technique the k-space navigator technique is much more rapid because only a small portion of k-space is compared to determine motion. However, the k-space motion techniques are typically not useful for measuring large motions of the subject or motions where the plane of the subject changes.

There are a number of techniques which are particularly useful in this example.

For instance the so called orbital navigator techniques acquire a circular portion of k-space. An orbital navigator technique is described in Zhuo Wu Fu et. al., "Orbital Navigator Echoes for Motion Measurements in Magnetic Resonance Imaging," Magnetic Resonance in

Medicine, pp. 746-753 (1995).

Another type of k-space navigator technique is the so called clover leaf navigator. Segments of three perpendicular orbital navigators are joined together in a continuous trajectory. This is much faster than three successive orbital navigators but in some cases may be less accurate since only a quarter circle is used from each orbital navigator. A clover leaf navigator technique is described in van der Kouwe et. al, "Real-time rigid body motion correction and shimming using cloverleaf navigators", Magnetic Resonance in Medicin, v. 56, pp. 1019-1032 (2006).

Another example is the so called spherical navigator technique. In this technique a spiral trajectory on a sphere around the k-space center is acquired. This can fully qualify 3D rigid body motion from a single navigator. A drawback is a fairly long trajectory which makes it sensitive to T2* decay. This also may require complicated and potentially slow data analysis. A spherical navigator technique is described in Welch et.al, "Spherical navigator echoes for full 3D rigid body motion measurement in MRI", Magnetic Resonance in Medicine, v. 47, pp. 32-41 (2002).

Another example may be the so called floating navigator. In this technique a straight line through k-space but is off center. Multiple navigator acquisitions may be needed to quantify motion in three dimensions. A floating navigator technique is described in Kadah et al, "Floating navigator echo for in-plane translational motion estimation," Magnetic Resonance in Medicine, v. 51, pp. 403-407 (2004). In Kadah et. al. this method was only used to quantify translations. But more recent work showed that when combined with parallel imaging it can quantify rotations as well. For example, see: Wei Lin et al. "Motion correction using an enhanced floating navigator and GRAPPA operations," Magnetic Resonance in Medicine, v. 63, pp. 339-348 (2010).

The magnetic resonance imaging system further comprises a processor for controlling the magnetic resonance imaging system. For example the processor may execute the machine-executable instructions to operate the magnetic resonance imaging system in a particular manner. Also the processor may use the pulse sequence instructions to control the magnetic resonance imaging system to acquire magnetic resonance data of various types.

Execution of the machine-executable instructions causes the processor to acquire second k-space navigator data using the first motion correction pulse sequence instructions. Execution of the machine-executable instructions further cause the processor to acquire reference tracker magnetic resonance data using the second motion correction pulse sequence instructions .

Execution of the machine-executable instructions further cause the processor to reconstruct a predetermined number of reference images using the reference tracker magnetic resonance data. The predetermined number of reference images corresponds to or is the same as the predetermined number of slices of the imaging zone which are acquired by the tracker. In these first steps initial measurements of the subject using the k-space navigator technique and the tracker magnetic resonance technique have been performed. This will be used in later acquisition of the magnetic resonance data for reference to determine if the subject has moved and if so by how much.

Execution of the machine-executable instructions further causes the processor to repeatedly acquire a portion of the magnetic resonance data using the imaging pulse sequence instructions. The portion of the magnetic resonance data comprises a portion of k- space and the first k-space navigator data.

Execution of the instructions further causes the processor to repeatedly detect movement of the subject by comparing the first k-space navigator data to the second k-space navigator data. The detection of movement of the subject need not be a determination of the absolute motion of the subject. For instance a change between the first k-space navigator data and the second k-space navigator data may be sufficient to detect the movement of the subject. In some cases it could be a geometrical displacement in distance such as millimeters. The displacement could also simply be calculated from the difference of the sample values of both k-space navigator data without any model to transform the signal difference into a geometric displacement. Not calculating a geometric displacement may have the advantage of the technique being extremely fast in detecting the movement of the subject.

Execution of the machine-executable instructions repeatedly causes the processor to iteratively correct the scan geometry if the movement of the subject is above a predetermined threshold.

The scan geometry is iteratively corrected by repeatedly acquiring corrective tracker magnetic resonance data using the second motion correction pulse sequence instructions. The scan geometry is further iteratively corrected by reconstructing a

predetermined number of tracker images from the tracker magnetic resonance data. The predetermined number of tracker images from the tracker magnetic resonance data is the same as the predetermined number of reference images and also the predetermined number of slices of the imaging zone. The scan geometry is further iteratively corrected by correcting the scan geometry by comparing the predetermined number of tracker images with the predetermined number of reference images. The scan geometry is iteratively corrected using the so called tracker method. In the tracker method the reference images are compared to newly acquired tracker images. An image-based analysis is performed to approximate a transformation of the scan geometry which will be correct. However, as only a limited portion of the imaging zone is acquired there may be errors so the method repeats itself first by acquiring tracker images and comparing them to the reference images and then repeating this process until the scan geometry has correctly converged and there are only minor differences between the tracker images and the reference images.

A disadvantage of using the so called tracker method is that it is extremely time consuming. It would be impractical to use the tracker images to check during the acquisition of the magnetic resonance data by the imaging pulse sequence instructions. The technique described above uses the faster k-space navigator technique to detect movement of the subject. If the movement of the subject is sufficiently large then the scan geometry is iteratively corrected using the tracker technique. This may enable more accurate acquisition of the magnetic resonance data for constructing an image.

Execution of the instructions further causes the processor to repeatedly correct the portion of the magnetic resonance data if the scan geometry has been iteratively corrected. Next execution of the instructions further cause the processor to reconstruct a magnetic resonance image from the magnetic resonance data after all portions of the magnetic resonance data have been acquired. This method may have the benefit of providing for more accurate acquisition of magnetic resonance data from the subject when there is subject movement.

The iterative correction of the portion of the magnetic resonance data may be accomplished in several different ways. In one method the portion of the magnetic resonance data is reacquired after the scan geometry has been corrected. In other techniques the previously acquired magnetic resonance data may be recalculated to compensate for motion of the subject so that it may be combined with the previously acquired magnetic resonance data portions.

In some examples the portions of the k-space for the scan geometry may be portions of Cartesian k-space for the scan geometry.

In another embodiment execution of the instructions further cause the processor to acquire a corrective k-space navigator data using the first motion correction pulse sequence instructions after iteratively correcting the scan geometry. Execution of the instructions further cause the processor to correct the scan geometry further by comparing the corrective k-space navigator data with the second k-space navigator data. In this embodiment the k-space navigator technique is used to further correct the scan geometry. This may be beneficial because the k-space navigator techniques are typically very accurate for in plane translations and rotations. The tracker technique may be useful for adjusting the scan geometry so that it has more or less the correct orientation. The scan geometry can then be more finely tuned using the k-space navigator technique.

In another embodiment the second k-space navigator data is replaced with the corrective k-space navigator data. In some instances it may be beneficial to replace the second k-space navigator data with k-space navigator data that represents the most recent scan geometry.

In another embodiment the imaging pulse sequence instructions cause the magnetic resonance imaging system to acquire the magnetic resonance data using a fast spin echo imaging technique. The use of a fast spin echo imaging technique may be beneficial because it may be straight forward or possible to substitute the acquisition of the k-space navigator data during one echo of an acquisition using the fast spin echo imaging technique.

In another embodiment the k-space navigator technique is an orbital navigator technique.

In another embodiment the k-space navigator technique is a clover leaf navigator technique. In another embodiment the k-space navigator technique is a spherical navigator technique.

In another embodiment the k-space navigator technique is a floating navigator technique.

In another embodiment the k-space navigator technique is an orbital navigator technique. The imaging pulse sequence instructions causes the magnetic resonance imaging system to acquire the portion of the magnetic resonance data using multiple echoes. The imaging pulse sequence instructions causes the magnetic resonance imaging system to acquire the magnetic resonance data using phase encoding. The imaging pulse sequence instructions causes the magnetic resonance imaging system to acquire the first k-space navigator data as a navigator echo. The imaging pulse sequence instructions causes the magnetic resonance imaging system to acquire the navigator echo by replacing the phase encoding with a sinusoidal phase encoding for one of the multiple echoes. The technique of acquiring the first k-space navigator data may be the same as acquiring the second k-space navigator data. The use of this example for instance may make the acquisition of the first and second k-space navigator data particularly efficient.

In another embodiment the portion of the magnetic resonance data is corrected by re-acquiring the portion of the magnetic resonance data after correcting the scan geometry. In this example if the movement of the subject is above the predetermined threshold then the scan geometry is corrected iteratively. After the scan geometry has been corrected iteratively then the portion of the magnetic resonance data is reacquired. This example may be beneficial because it ensures that the magnetic resonance data is correct.

In another embodiment the portion of magnetic resonance data is corrected by calculating a corrected portion of the magnetic resonance data using the change in the scan geometry. This example may for instance be beneficial because it is more rapid. The portion of the magnetic resonance data has not been reacquired.

In another embodiment the acquisition of the corrective tracker magnetic resonance data or the reference tracker magnetic resonance data takes longer than 300 ms. The acquisition of the corrective navigator data takes less than 20 ms.

In another embodiment the displacement is determined by calculating a maximum displacement of the subject in the imaging zone. For instance a model could be used to interpret the difference in the k-space navigator data to determine a maximum displacement. In another embodiment the resolution of the magnetic resonance image is higher than the resolution of the determined number of reference images.

In another embodiment the predetermined number of slices of the imaging zone are any one of the following: a single slice of the imaging zone, two slices of the imaging zone, two orthogonal slices of the imaging zone, three slices of the imaging zone, and three orthogonal slices of the imaging zone.

In another embodiment execution of the instructions further causes the processor to replace the predetermined number of reference images with the predetermined number of tracker images if the scan geometry has been iteratively corrected. In some instance it may be beneficial to replace the reference images when the scan geometry has been changed.

In another embodiment the additional instructions are the first motion correction pulse sequence instructions. The acquisition of the second k-space navigator data using the first motion correction pulse sequence instructions is performed by acquiring an initial portion of the magnetic resonance data using the imaging pulse sequence instructions. In this embodiment there are two separate pulse sequences. The imaging pulse sequence instructions and the second motion correction pulse sequence instructions. The second k- space navigator data is acquired the first time the imaging pulse sequence instructions are executed. The portion of the magnetic resonance data acquired during this first execution of the imaging pulse sequence data is also used with the other portions of magnetic resonance data to reconstruct the magnetic resonance image.

As an alternative there are three separate pulse sequences. The imaging pulse sequence instructions, the first motion correction pulse sequence instructions, and the second motion correction pulse sequence instructions. In this alternative, when the second k-space navigator data is acquired there is no magnetic resonance data acquired that will be used in the reconstruction of the magnetic resonance image.

In another aspect the invention provides for a method of operating the magnetic resonance imaging system for acquiring magnetic resonance data from a subject for a scan geometry within an imaging zone. The magnetic resonance imaging system comprises a magnet for generating a main magnetic field in the imaging zone. The magnetic resonance imaging system further comprises a radio-frequency system comprising a magnetic resonance antenna for sending radio -frequency transmissions to the imaging zone and for receiving magnetic resonance signals from the imaging zone. The magnetic resonance imaging system further comprises a gradient coil system for controlling the magnetic field gradient within the imaging zone. The magnetic resonance imaging system further comprises a memory containing imaging pulse sequence instructions. The memory further contains first motion correction pulse sequence instructions. The memory further contains second motion correction pulse sequence instructions. The imaging pulse sequence instructions comprise instructions which cause the magnetic resonance imaging system to sample the magnetic resonance data as portions of k-space for the scan geometry. The imaging pulse sequence instructions further comprise instructions which cause the magnetic resonance imaging system to acquire first k-space navigator data using a k-space navigator technique for every portion of k-space.

The first motion correction pulse sequence instructions further comprises instructions which cause the magnetic resonance imaging system to acquire second k-space navigator data using the k-space navigator technique. The second motion correction pulse sequence instructions comprise instructions which cause the magnetic resonance imaging system to acquire tracker magnetic resonance data. The tracker magnetic resonance data comprises a predetermined number of slices of the imaging zone. The method comprises the step of acquiring second k-space navigator data using the first motion correction pulse sequence instructions. The method further comprises the step of acquiring reference tracker magnetic resonance data using the second motion correction pulse sequence instructions. The method further comprises the step of reconstructing a predetermined number of reference images using the reference tracker magnetic resonance data. The method further comprises the step of repeatedly acquiring a portion of the magnetic resonance data using the imaging pulse sequence instructions. The portion of the magnetic resonance data comprises a portion of k-space and the first k-space navigator data.

The method further comprises the step of repeatedly detecting movement of the subject by comparing the first k-space navigator data to the second k-space navigator data. The method further comprises the step of repeatedly iteratively correcting the scan geometry if the movement of the subject is above a predetermined threshold. The scan geometry is iteratively corrected by repeatedly acquiring the corrective tracker magnetic resonance data using the second motion correction pulse sequence instructions. The scan geometry is further iteratively corrected by reconstructing a predetermined number of tracker images from the tracker magnetic resonance data. The scan geometry is further iteratively corrected by correcting the scan geometry by comparing the predetermined number of tracker images with the predetermined number of reference images. The method further comprises iteratively correcting the portion of the magnetic resonance data if the scan geometry has been iteratively corrected. The method further comprises reconstructing the magnetic resonance image from the magnetic resonance data after all portions of the magnetic resonance data have been acquired.

In another aspect the invention provides for a computer program product containing machine-executable instructions for execution by a processor controlling a magnetic resonance imaging system. The magnetic resonance imaging system may be used for acquiring magnetic resonance data from a subject for a particular scan geometry within an imaging zone. The magnetic resonance imaging system comprises a magnet for generating a main magnetic field in the imaging zone. The magnetic resonance imaging system further comprises a radio-frequency system comprising a magnetic resonance antenna for sending radio-frequency transmissions to the imaging zone and for receiving magnetic resonance signals from the imaging zone. The magnetic resonance imaging system further comprises a gradient coil system for controlling a magnetic field gradient within the imaging zone.

The magnetic resonance imaging system further comprises a memory containing imaging pulse sequence instructions. In some examples the computer program product may also be the same as the memory or may be stored on the memory. The memory further contains first motion correction pulse sequence instructions. The memory further contains second motion correction pulse sequence instructions. The imaging pulse sequence instructions comprise instructions which cause the magnetic resonance imaging system to sample the magnetic resonance data as portions of k-space for the scan geometry.

The imaging pulse sequence instructions further comprise instructions which cause the magnetic resonance imaging system to acquire first k-space navigator data using a k-space navigator technique for every portion of k-space. The first motion correction pulse sequence instructions further comprises instructions which cause the magnetic resonance imaging system to acquire second k-space navigator data using the k-space navigator technique. The second motion correction pulse sequence instructions comprise instructions which cause the magnetic resonance imaging system to acquire tracker magnetic resonance data. The tracker magnetic resonance data comprises a predetermined number of slices of the imaging zone.

Execution of the instructions causes the processor to acquire second k-space navigator data using the first motion correction pulse sequence instructions. Execution of the instructions further cause the processor to acquire reference tracker magnetic resonance data using the second motion correction pulse sequence instructions. Execution of the instructions further cause the processor to reconstruct a predetermined number of reference images using the reference tracker magnetic resonance data. Execution of the instructions causes the processor to repeatedly acquire a portion of the magnetic resonance data using the imaging pulse sequence instructions. The portion of the magnetic resonance data comprises a portion of k-space and the first k-space navigator data. Execution of the instructions further causes the processor to repeatedly detect movement of the subject by comparing the first k-space navigator data to the second k-space navigator data.

Execution of the instructions further causes the processor to repeatedly iteratively correct the scan geometry if the movement of the subject is above a predetermined threshold. The scan geometry is iteratively corrected by repeatedly acquiring corrective tracker magnetic resonance data using the second motion correction pulse sequence. The scan geometry is further iteratively corrected by repeatedly reconstructing a predetermined number of tracker images from the tracker magnetic resonance data. The scan geometry is further iteratively corrected by correcting the scan geometry by comparing the predetermined number of tracker images with the predetermined number of reference images. Execution of the instructions further causes the processor to repeatedly correct the portion of the magnetic resonance data if the scan geometry has been iteratively corrected. Execution of the instructions further cause the processor to reconstruct a magnetic resonance image from the magnetic resonance data after all portions of the magnetic resonance data have been acquired

It is understood that one or more of the aforementioned embodiments of the invention may be combined as long as the combined embodiments are not mutually exclusive.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention will be described, by way of example only, and with reference to the drawings in which:

Fig. 1 Fig. 1 illustrates an example of a magnetic resonance imaging system

100;

Fig. 2 shows a flowchart which illustrates a method of operating the magnetic resonance imaging system of Fig. 1;

Fig. 3 shows an example of imaging pulse sequence;

Fig. 4 shows a timing diagram for the acquisition of magnetic resonance data;

Fig. 5 shows the distribution of data in k-space acquired using the segmented acquisition of Figs. 3 or 4;

Fig. 6 shows a timing diagram for a multi-slice fast spin echo including a navigator from each slice; and Fig. 7 shows an example of an alternative pulse sequence to the pulse sequence shown in Fig. 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Like numbered elements in these figures are either equivalent elements or perform the same function. Elements which have been discussed previously will not necessarily be discussed in later figures if the function is equivalent.

Fig. 1 illustrates an example of a magnetic resonance imaging system 100. The medical instrument 100 has a magnet 104. The magnet 104 is a superconducting cylindrical type magnet 104 with a bore 106 through it. The use of different types of magnets is also possible for instance it is also possible to use both a split cylindrical magnet and a so called open magnet. A split cylindrical magnet is similar to a standard cylindrical magnet, except that the cryostat has been split into two sections to allow access to the iso-plane of the magnet, such magnets may for instance be used in conjunction with charged particle beam therapy. An open magnet has two magnet sections, one above the other with a space in- between that is large enough to receive a subject: the arrangement of the two sections area similar to that of a Helmholtz coil. Open magnets are popular, because the subject is less confined. Inside the cryostat of the cylindrical magnet there is a collection of

superconducting coils. Within the bore 106 of the cylindrical magnet 104 there is an imaging zone 108 where the magnetic field is strong and uniform enough to perform magnetic resonance imaging.

Within the bore 106 of the magnet there is also a set of magnetic field gradient coils 110 which is used for acquisition of magnetic resonance data to spatially encode magnetic spins within the imaging zone 108 of the magnet 104. The magnetic field gradient coils 110 connected to a magnetic field gradient coil power supply 112. The magnetic field gradient coils 110 are intended to be representative. Typically magnetic field gradient coils 110 contain three separate sets of coils for spatially encoding in three orthogonal spatial directions. A magnetic field gradient power supply supplies current to the magnetic field gradient coils. The current supplied to the magnetic field gradient coils 110 is controlled as a function of time and may be ramped or pulsed.

Adjacent to the imaging zone 108 is a radio-frequency coil 114 for

manipulating the orientations of magnetic spins within the imaging zone 108 and for receiving radio transmissions from spins also within the imaging zone 108. The radio frequency antenna may contain multiple coil elements. The radio frequency antenna may also be referred to as a channel or antenna. The radio-frequency coil 114 is connected to a radio frequency transceiver 116. The radio-frequency coil 114 and radio frequency transceiver 116 may be replaced by separate transmit and receive coils and a separate transmitter and receiver. It is understood that the radio -frequency coil 114 and the radio frequency transceiver 116 are representative. The radio -frequency coil 114 is intended to also represent a dedicated transmit antenna and a dedicated receive antenna. Likewise the transceiver 116 may also represent a separate transmitter and receivers. The radio -frequency coil 114 may also have multiple receive/transmit elements and the radio frequency transceiver 116 may have multiple receive/transmit channels.

The magnetic field gradient coil power supply 112 and the transceiver 116 are connected to a hardware interface 128 of computer system 126. The computer system 126 further comprises a processor 130. The processor 130 is connected to the hardware interface 128, a user interface 132, computer storage 134, and computer memory 136. Within the imaging zone 108 can be seen a scan geometry 124. The scan geometry is a portion of the imaging zone 108 which will be imaged by acquiring various points or locations in k-space. The line 125 represents a predetermined number of slices within the imaging zone 108. The slices 125 can be selected to determine the movement of the subject 118.

The computer storage 134 is shown as containing the image pulse sequence instructions. The computer storage 134 is further shown as containing the first motion correction pulse sequence instructions 142 and the second motion correction pulse sequence instructions 144. The computer storage 134 is shown as containing second k-space navigator data 146 that was acquired using the first motion correction pulse sequence instructions 142. The computer storage 134 is further shown as containing reference tracker magnetic resonance data 148 that was acquired using the second motion correction pulse sequence instructions 144. The computer storage 134 is further shown as containing a predetermined number of reference images 150 that was reconstructed from the reference tracker magnetic resonance data 148.

The computer storage 134 is further shown as containing a portion of magnetic resonance data 152 that was acquired using the imaging pulse sequence instructions 140. The portion of the magnetic resonance data 152 is shown as being divided into two parts 154, 156 that are stored in the computer storage 134. The first part 154 is the portion of k-space 154 and the first k-space navigator data 156 that is associated with it. The computer storage 134 is further shown as containing corrective tracker magnetic resonance data 158. The computer storage 134 is further shown as containing a predetermined number of tracker images 160. The corrective tracker magnetic resonance data was acquired using the second motion correction pulse sequence instructions 144.

The computer storage 134 is shown as containing a corrected scan geometry

162 that was calculated using the predetermined number of tracker images 160 and the predetermined number of reference images 150 to correct the scan geometry 124. The computer storage 134 is further shown as containing a reacquired portion of the magnetic resonance data 164. The reacquired portion of the magnetic resonance data 164 is a reacquisition of the portion of magnetic resonance data 152 after a motion has been detected. The computer storage 134 is further shown as containing the complete magnetic resonance data 166 that was acquired using the imaging pulse sequence instructions 140. The complete magnetic resonance data 166 is made up of individual portions of k-space 154. The complete magnetic resonance data 166 is the complete k-space data that was acquired to acquire image data for the scan geometry 124. The computer storage 134 is further shown as containing a magnetic resonance image 168 that was reconstructed from the complete magnetic resonance data 166.

The computer memory 136 is shown as containing control module 170. The control module 170 contains computer executable code which enables the processor 130 to control and operate the magnetic resonance imaging system 100. For instance the control module 170 may contain code for executing a method such as is illustrated in Fig. 2. The computer memory 136 is further shown as containing a motion determination module 172. The motion determination module 172 compares the second k-space navigator data 146 to the first k-space navigator data 156. The computer memory 136 is further shown as containing a scan geometry correction module 174 that uses the predetermined number of tracker images 160 and the predetermined number of reference images 150 to recalculate the corrected scan geometry 162. The computer memory 136 is further shown as containing image processing module 176. The image processing module 176 contains computer executable code which enables the processor 130 to calculate or reconstruct the magnetic resonance image 168 from the magnetic resonance data 166.

Fig. 2 shows a flowchart which illustrates a method of operating the magnetic resonance imaging system 100 of Fig. 1. First in step 200 the method starts. Next in step 202 the processor 130 controls the magnetic resonance imaging system 100 to acquire the second k-space navigator data 146 using the first motion correction pulse sequence instructions 142. Next in step 204 instructions control the processor 130 to control the magnetic resonance imaging system 100 to acquire the reference tracker magnetic resonance data 148 using the second motion correction pulse sequence instructions 144. The order of steps 202 and 204 may be switched. Next in step 206 a predetermined number of reference images 150 is reconstructed using the reference tracker magnetic resonance data 148. Steps 202-206 represent an initial startup portion of the method. In the remainder of the method the complete magnetic resonance data 166 is acquired as portions 152.

Next in step 208 a portion of the magnetic resonance data 152 is acquired using the imaging pulse sequence instructions 140. The portion of the magnetic resonance data contains a portion of the k-space 154 and the first k-space navigator data 156. Next in step 210 the possible movement of the subject is detected by comparing the first k-space navigator data 156 to the second k-space navigator data. Step 212 is a decision box; the question is is the movement of the subject above a predetermined threshold. If the answer is yes then the method proceeds to box 214. If the answer is no then the method proceeds to box 218. In box 214 the scan geometry of the subject is iteratively corrected. This iterative correction is done by repeatedly acquiring the corrective tracker magnetic resonance data 158 and using this to generate a predetermined number of tracker images 160. These are then compared to the predetermined number of reference images 150. The corrected scan geometry 162 is repeatedly calculated in this way until the scan geometry has been corrected sufficiently or within certain bounds. Next in step 216 the portion of the magnetic resonance data 152 is corrected. For instance it may be a reacquisition 164 of the portion of the magnetic resonance data 164 or it may just be a recalculation of the portion of k-space 154.

After step 216 the method then proceeds to box 218. Box 218 is a similar decision box. The question in this box is has the complete magnetic resonance data 166 been acquired. If the answer is yes then the method proceeds to step 220. If the answer is no then the method proceeds back to step 208 where another portion of the magnetic resonance data is acquired using the imaging pulse sequence instructions 140. Steps 208-218 then repeat until the complete magnetic resonance data 166 has been acquired. The method then proceeds to step 220 where the magnetic resonance image 168 is reconstructed from the complete magnetic resonance data 166. The complete magnetic resonance data 166 is assembled from the various portion of k-space 154. Finally in step 222 the method ends.

Motion during an MRI exam can cause serious image quality degradation. This invention proposes a combination of orbital navigator ("onav") or other k-space navigator and a tracker navigator ("tracker") to overcome the disadvantages of the individual techniques and combine their strengths:

1. ) Use onavs or other k-space navigators to detect if motion occurred. This is highly efficient and fast.

2. ) Use trackers to correct for most of the motion up to approximately voxel accuracy. This may result in good through plane correction for subject motion.

3. ) After tracker motion correction, use onavs or other k-space navigators to improve the motion correction to sub-voxel accuracy. This may result in highly accurate correction for in-plane motion.

The proposed combination of onav or other k-space navigators and tracker improves the accuracy of motion correction and improves the efficiency of the motion detection, thus reducing the required time.

Patient motion during an MRI examination can seriously compromise image quality. In the current MR scanners there is no feature that allows detecting motion during an exam. Instead this is detected currently after a scan is completed by either 1.) bad image quality due to motion artifacts or, 2.) wrong anatomy which is imaged. As a consequence the total time of an exam is prolonged because scans need to be repeated.

In interventional imaging it is even more important to correct for motion during the entire procedure.

Several methods for motion tracking have been described in the past. They can be grouped according to different categories, e.g. if they require external markers, are purely MR based, or use non-MR techniques to detect and quantify the motion. The purely MR- based techniques have the advantage that they do not require special patient set-up or additional hardware. They can be divided into image based navigator and k-space navigator techniques.

The MR-based motion detection methods always interfere with the MR imaging process. Therefore, it is one aim to keep the influence on the imaging exam as small as possible.

Two particular methods for compensation of rigid body motion which proved to be effective in the past are orbital navigators and tracker imaging. These methods and their advantages and disadvantages are briefly described below:

Tracker imaging: 1. ) At the beginning of the exam the position of the patient is captured by acquiring three orthogonal slices. (A set of three orthogonal slices is called "tracker" in the following.) These images serve as a position reference for all subsequent scans.

2. ) To check if motion occurred (e.g. between scans or, dynamics, packages, shots, ...) another tracker is acquired and registered (2D) to the slices of the reference tracker by a combination of 2D, slice-by-slice registration steps. If the registration determines a shift or rotation, the scan geometry is corrected accordingly and another tracker is acquired in the new geometry. This process is repeated until it converges.

3. ) Finally, the geometric transformation between reference and current position is applied to correct the scan geometry of the diagnostic scan.

The described method and apparatus may have one or more of the following advantages:

• Can correct for in-plane and through plane motion,

• Can focus motion estimation on region of interest by using a mask in the registration

Disadvantages:

• Accuracy is limited by tracker resolution (resulting in an accuracy of approx. 0.5 mm),

• requires relatively long scan time (-500 ms per tracker iteration)

The orbital navigator (onav) excites a single slice and acquires the signal along a circular trajectory in k-space. From two onav signals of different time- points the in-plane translation and in-plane rotation can be calculated.

Advantages:

Very high accuracy (-40 μιη),

· Fast acquisition (-10 ms)

Disadvantages:

• Only in-plane motion quantification,

• No region of interest capability

Examples may combine of onav or other k-space navigators and tracker to overcome two problems of the tracker approach:

a) Checking if motion occurred using trackers is inefficient because it takes a few hundred milliseconds to acquire a tracker. Depending on how often one wants to check for motion this can slow down the data acquisition considerably. Because of this long time it is also difficult to integrate the tracker into a diagnostic sequence. Instead, the tracker is bound to interrupt the running diagnostic sequence and disrupt its steady state. This problem is overcome by using an onav to detect if motion occurred: This is time efficient and usually can be done without disturbing the imaging sequence.

b) Motion correction using trackers is less accurate than onav based motion correction. In particular the accuracy of tracker-based motion correction is not high enough to perform it within one package of a TSE scan to combine data from before motion and after motion correction in a single dataset/image. The accuracy of onavs is high enough for this [6] but they cannot correct for through plane motion.

This problem is overcome by first performing a tracker based motion correction and then use an onav based motion correction to fine-tune the motion correction to sub-voxel accuracy.

Examples may combine an onav or other k-space navigators and tracker for motion detection and correction:

1.) Use onavs or other k-space navigators to detect if motion occurred. This results in a faster detection and therefore has higher efficiency.

2.) Use trackers to correct for most of the motion up to approximately voxel accuracy. This results in good through plane correction for motion.

3.) After tracker motion correction, use onavs to improve the motion correction to sub-voxel accuracy. This provides for highly accurate in-plane motion correction.

In step 3), depending on the application, signal originating from only one onav slice can be acquired to refine the motion correction in the in-plane directions corresponding to the diagnostic scan. Alternatively, signals originating from a set of two or three orthogonal onav slices can be acquired in order to refine the motion correction in all three spatial directions.

In a further embodiment of the invention, step 3 is applied iteratively until convergence has been reached. This iterative motion correction based on orthogonal onav slices can be done additionally or alternatively to step 2.

The proposed combination of onav and tracker improves the accuracy of motion correction and improves the efficiency of the motion detection, thus reducing the required time.

Fig. 3 shows an example of imaging pulse sequence instructions 140. In Fig. 3 a schematic diagram of a fast spin echo MRI sequence (FSE) includes an orbital navigator as the first echo. The so called fast spin echo technique is also known as rapid acquisition with relaxation enhancement (RARE). This technique is described in section 16.4 of Handbook of MRI Pulse Sequences by Bernstein et al, published in 2004 by Elsevier Inc. Section 16.4 spans pages 774-801. In this timing diagram the line 300 is the Frequency Encoding (FE) gradient. The line 302 represents the Phase Encoding (PE) gradient. Line 304 represents the RF transmitted to the imaging zone and 306 represents when data is acquired. The section of the timing diagram labeled 308 is when the orbital navigator is performed and the sections 310 represent when Cartesian spin echoes are acquired. For the orbital navigator 308 the navigator data 156 is acquired. For the Cartesian spin echoes 310 the portion of k-space 154 is acquired. For the orbital navigator 308 sinusoidal gradients 314 are used.

The navigator echo 308 is acquired by switching special sinusoidal gradients 314 instead of the Cartesian phase encoding and readout gradients. The navigator echo 308 can be acquired at any echo number within the FSE echo train by putting the orbital navigation gradients into the corresponding position. Taking the first or last echo to acquire the navigator data may be advantageous because there is then the same time difference between all imaging echoes in the echo train. This leads to a smooth distribution of T2-decay effects in the acquired data. This makes the first or last echo in the echo train the preferred position for the navigator. Using the last echo is preferred for scans where the echo time of the first imaging echo must be as small as possible. Using the first echo is preferred when echo time is not critical because then the signal strength for the navigator is high.

Fig. 4 shows a timing diagram for the acquisition of magnetic resonance data. The line represents time and various RF pulses 400 are performed. In between these there are a number of orbital navigators 308 or Cartesian spin echoes 310 are performed. The orbital navigator 308 may be performed by switching its position with one of the Cartesian spin echoes 310.

Fig. 5 shows the distribution of data in k-space for the segmented acquisition shown in Figs. 3 or 4. The x-axis is the readout 500 and the y-axis 502 is the phase encoding. The circle labeled 504 represents the k-space navigator data acquired with an orbital navigator. It corresponds to element 156 in Fig. 1. The lines labeled 506 are portions of k- space such as 154 in Fig. 1 that have been acquired using a pulse sequence such as that shown in Fig. 3.

Fig. 6 shows a timing diagram for a multi-slice fast spin echo including a navigator 156 from each slice. The axis 600 represents time. It can be seen over the time that multiple shots 602 are performed. For each slice 604 in a particular shot 602 there is a block of magnetic resonance data. This block of magnetic resonance data comprises the navigator k-space data 156 and the portion k-space 154. Fig. 7 shows an example of an alternative pulse sequence to the pulse sequence 140 shown in Fig. 3. In Fig. 7 the pulse sequence is for a gradient echo magnetic resonance imaging sequence including an orbital navigator 308. The gradient GRE imaging sequence consists of repeating the same sequence building block many times at a preferred repetition rate TR 704, only the phase encoding is varied between different repetitions. The orbital navigator 308 can be inserted into any position within the GRE sequence. It may be advantageous to use the same flip angle a and the same repetition time 704 as for the GRE acquisition. The orbital navigator can be interleaved with magnetization prepared gradient echo TFE sequences in analogy to the FSE sequence. There are also other imaging sequences which have intrinsically long idle times, e.g. inversion recovery, FLAIR, PCASL, and others. Here it is straight forward to acquire the navigator data during the idle time.

Several examples were shown how to integrate a so called orbital navigator into an imaging pulse sequence as shown in Figs. 3 and 7. In addition to using the orbital navigator other k-space navigator techniques may also be used. For instance by analogy the clover leaf navigator, the spherical navigator, or the floating navigator may also be incorporated into these pulse sequences.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope. LIST OF REFERENCE NUMERALS

100 magnetic resonance imaging system

104 magnet

106 bore of magnet

108 imaging zone

110 magnetic field gradient coils

112 magnetic field gradient coil power supply

114 radio-frequency coil

116 transceiver

118 subject

120 subject support

124 scan geometry

125 predetermined slices

126 computer system

128 hardware interface

130 processor

132 user interface

134 computer storage

136 computer memory

140 imaging pulse sequence instructions

142 first motion correction pulse sequence instructions

144 second motion correction pulse sequence instructions

146 second k-space navigator data

148 reference tracker magnetic resonance data

150 predetermined number of reference images

152 portion of magnetic resonance data

154 portion of k-space

156 first k-space navigator data

158 corrective tracker magnetic resonance data

160 predetermined number of tracker images

162 corrected scan geometry

164 re-acquired portion of magnetic resonance data

166 complete magnetic resonance data

168 magnetic resonance image 170 Control module

172 motion determination module

174 scan geometry correction module

176 image processing module

200 start

202 acquire second k-space navigator data using the first motion correction pulse sequence instructions

204 acquire reference tracker magnetic resonance data using the second motion correction pulse sequence instructions

206 reconstruct a predetermined number of reference images using the reference tracker magnetic resonance data

208 acquire a portion of the magnetic resonance data using the imaging pulse sequence instructions

210 detect movement of the subject by comparing the first k-space navigator data to the second k-space navigator data

212 movement detected in k-space navigator data?

214 iteratively correct the scan geometry if the movement of the subject is above a predetermined threshold

216 correct the portion of magnetic resonance data if the scan geometry has been iteratively corrected

218 Acquisition of MR data finished?

220 reconstruct a magnetic resonance image from the magnetic resonance data after all portions of the magnetic resonance data have been acquired

222 end

300 Gradient FE

302 gradient PE

304 RF

306 readout

308 orbital navigator

310 Cartesian spin echo

312 Cartesian spin echo

314 sinuso idal gradients

400 RF pulse

500 readout 502 phase encoding

504 k-space navigator data

506 imaging magnetic resonance data

600 time

602 shot

604 slice

700 imaging pulse sequence

702 cartesian GRE

704 repetion time (TR)

Claims

CLAIMS:

1. A magnetic resonance imaging system (100) for acquiring magnetic resonance data from a subject (118) for a scan geometry (124) within an imaging zone (108), wherein the magnetic resonance imaging system comprises:

a magnet (104) for generating a main magnetic field in the imaging zone; - a radio frequency system (1 14, 1 16) comprising a magnetic resonance antenna

(114) for sending radio frequency transmissions to the imaging zone and for receiving magnetic resonance signals from the imaging zone;

a gradient coil system (110, 112) for generating a magnetic field gradient within the imaging zone;

- a memory (134, 136) containing machine executable instructions (170, 172,

174, 176), wherein the memory further contains imaging pulse sequence instructions (140), wherein the memory further contains first motion correction pulse sequence instructions (142), wherein the memory further contains second motion correction pulse sequence instructions (144), wherein the imaging pulse sequence instructions comprises additional instructions which cause the magnetic resonance imaging system to sample the magnetic resonance data as portions (154) of k-space for the scan geometry, wherein the imaging pulse sequence instructions further comprises instructions which cause the magnetic resonance imaging system to acquire first k-space navigator data (156) using a k-space navigator technique for every portion of k-space, wherein the first motion correction pulse sequence instructions further comprises instructions which cause the magnetic resonance imaging system to acquire second k-space navigator data (146) using the k-space navigator technique, wherein the second motion correction pulse sequence instructions comprise instructions which cause the magnetic resonance imaging system to acquire tracker magnetic resonance data (148, 158), wherein the tracker magnetic resonance data comprises a predetermined number of slices of the imaging zone; and

a processor for controlling the magnetic resonance imaging system, wherein execution of the instructions causes the processor to:

• acquire (202) the second k-space navigator data using the first motion correction pulse sequence instructions, • acquire (204) reference tracker magnetic resonance data (148) using the second motion correction pulse sequence instructions,

• reconstruct (206) a predetermined number of reference images (150) using the reference tracker magnetic resonance data,

wherein execution of the instructions causes the processor to repeatedly:

• acquire (208) a portion (152) of the magnetic resonance data using the imaging pulse sequence instructions, wherein the portion of the magnetic resonance data comprises a portion of k- space and the first k- space navigator data;

• detect (210) movement of the subject by comparing the first k- space navigator data to the second k-space navigator data;

• iteratively (214) correct the scan geometry if the movement of the subject is above a predetermined threshold, wherein the scan geometry is iteratively corrected by repeatedly: acquiring corrective tracker magnetic resonance data using the second motion correction pulse sequence instructions, reconstructing a predetermined number of tracker images from the tracker magnetic resonance data, and correcting the scan geometry by comparing the predetermined number of tracker images with the predetermined number of reference images;

• correct (216) the portion of magnetic resonance data if the scan geometry has been iteratively corrected;

wherein execution of the instructions further cause the processor to reconstruct (220) a magnetic resonance image (168) from the magnetic resonance data after all portions of the magnetic resonance data have been acquired.

2. The magnetic resonance imaging system, wherein execution of the instructions further cause the processor to:

acquire corrective k-space navigator data using the first motion correction pulse sequence instructions after iteratively correcting the scan geometry, and

correct the scan geometry further by comparing the corrective k-space navigator data with the second k-space navigator data.

3. The magnetic resonance imaging system of claim 1 or 2, wherein the imaging pulse sequence instructions cause the magnetic resonance imaging system to acquire the magnetic resonance data using a Fast Spin Echo imaging technique.

4. The magnetic resonance imaging system of claim 1, wherein the k- space navigator technique is any one of the following: an orbital navigator technique, a cloverleaf navigator technique, a spherical navigator technique, and a floating navigator technique.

5. The magnetic resonance imaging system of claim 1, wherein the k- space navigator technique is an orbital navigator technique, wherein the imaging pulse sequence instructions causes the magnetic resonance imaging system to acquire the portion of the magnetic resonance data using multiple echoes (154), wherein the imaging pulse sequence instructions causes the magnetic resonance imaging system to acquire the magnetic resonance data using phase encoding, wherein the imaging pulse sequence instructions causes the magnetic resonance imaging system to acquire the first k-space navigator data as a navigator echo (156), and wherein the imaging pulse sequence instructions causes the magnetic resonance imaging system to acquire the navigator echo by replacing the phase encoding with a sinusoidal phase encoding for one of the multiple echoes.

6. The magnetic resonance imaging system, wherein the portion of the magnetic resonance data is corrected by re-acquiring the portion of the magnetic resonance data after correcting the scan geometry.

7. The magnetic resonance imaging system, wherein the portion of magnetic resonance data is corrected by calculating a corrected portion of magnetic resonance data using the change in the scan geometry.

8. The magnetic resonance imaging system of any one of the preceding claims, wherein acquisition of the corrective tracker magnetic resonance data or the reference tracker magnetic resonance data takes longer than 300 milliseconds; and wherein acquisition of the corrective navigator data takes less than 20 milliseconds.

9. The magnetic resonance imaging system of any one of the preceding claims, wherein the displacement is determined by calculating a maximum displacement of the subject in the imaging zone.

10. The magnetic resonance imaging system of any one of the preceding claims, wherein the resolution of the magnetic resonance image is higher than the resolution of the predetermined number of reference images.

11. The magnetic resonance imaging system of any one of the preceding claims, wherein the predetermined number of slices of the imaging zone are any one of the following: a single slice of the imaging zone, two slices of the imaging zone, two orthogonal slices of the imaging zone, three slices of the imaging zone, and three orthogonal slices of the imaging zone.

12. The magnetic resonance imaging system of any one of the preceding claims, wherein execution of the instructions further cause the processor to replace the predetermined number of reference images with the predetermined number of tracker images if the scan geometry has been iteratively corrected.

13. The magnetic resonance imaging system of any one of the preceding claims, wherein the additional instructions are the first motion correction pulse sequence instructions, and wherein the acquisition of the second k-space navigator data using the first motion correction pulse sequence instructions is performed by acquiring an initial portion of the magnetic resonance data using the imaging pulse sequence instructions.

14. A method of operating a magnetic resonance imaging system (100) for acquiring magnetic resonance data (152) from a subject (118) for a scan geometry (124) within an imaging zone (108), wherein the magnetic resonance imaging system comprises: a magnet (104) for generating a main magnetic field in the imaging zone, a radio frequency system (114, 116) comprising a magnetic resonance antenna (114) for sending radio frequency transmissions to the imaging zone and for receiving magnetic resonance signals from the imaging zone, a gradient coil system (110, 112) for generating a magnetic field gradient within the imaging zone, and a memory (134, 136) containing imaging pulse sequence instructions (140); wherein the memory further contains first motion correction pulse sequence instructions (142); wherein the memory further contains second motion correction pulse sequence instructions (144); wherein the imaging pulse sequence

instructions comprises additional instructions which cause the magnetic resonance imaging system to sample the magnetic resonance data as portions of k-space for the scan geometry; wherein the imaging pulse sequence instructions further comprises instructions which cause the magnetic resonance imaging system to acquire first k- space navigator data (156) using a k- space navigator technique for every portion of k- space; wherein the first motion correction pulse sequence instructions further comprises instructions which cause the magnetic resonance imaging system to acquire second k- space navigator data (146) using the k- space navigator technique; wherein the second motion correction pulse sequence instructions comprise instructions which cause the magnetic resonance imaging system to acquire tracker magnetic resonance data (148, 158); wherein the tracker magnetic resonance data comprises a predetermined number of slices (125) of the imaging zone;

wherein the method comprises the steps of:

acquiring (202) the second k-space navigator data using the first motion correction pulse sequence instructions,

acquiring (204) reference tracker magnetic resonance data (148) using the second motion correction pulse sequence instructions,

- reconstructing (206) a predetermined number of reference images (150) using the reference tracker magnetic resonance data,

wherein the method further comprises the steps of repeatedly:

acquiring (208) a portion (152) of the magnetic resonance data using the imaging pulse sequence instructions, wherein the portion of the magnetic resonance data comprises a portion of k-space (154) and the first k-space navigator data;

detecting (210) movement of the subject by comparing the first k-space navigator data to the second k-space navigator data;

iteratively (214) correcting the scan geometry if the movement of the subject is above a predetermined threshold, wherein the scan geometry is iteratively corrected by repeatedly: acquiring corrective tracker magnetic resonance data using the second motion correction pulse sequence instructions, reconstructing a predetermined number of tracker images from the tracker magnetic resonance data, and correcting the scan geometry by comparing the predetermined number of tracker images with the predetermined number of reference images;

- correcting (216) the portion of magnetic resonance data if the scan geometry has been iteratively corrected; and

wherein the method further comprises reconstructing (220) a magnetic resonance image (168) from the magnetic resonance data after all portions of the magnetic resonance data have been acquired.

15. A computer program product containing machine executable instructions (170,

172, 174, 176) for execution by a processor (130) controlling a magnetic resonance imaging system (100) for acquiring magnetic resonance data from a subject (118) for a scan geometry (124) within an imaging zone (108), wherein the magnetic resonance imaging system comprises:

a magnet (104) for generating a main magnetic field in the imaging zone; a radio frequency system (1 14, 1 16) comprising a magnetic resonance antenna (114) for sending radio frequency transmissions to the imaging zone and for receiving magnetic resonance signals from the imaging zone;

a memory (130) containing imaging pulse sequence instructions, wherein the memory further contains first motion correction pulse sequence instructions (142) , wherein the memory further contains second motion correction pulse sequence instructions (144), wherein the imaging pulse sequence instructions comprises additional instructions which cause the magnetic resonance imaging system to sample the magnetic resonance data as portions of k-space (154) for the scan geometry, wherein the imaging pulse sequence instructions further comprises instructions which cause the magnetic resonance imaging system to acquire first k-space navigator data using a k-space navigator technique for every portion of k-space, wherein the first motion correction pulse sequence instructions further comprises instructions which cause the magnetic resonance imaging system to acquire second k-space navigator data using the k-space navigator technique, wherein the second motion correction pulse sequence instructions comprise instructions which cause the magnetic resonance imaging system to acquire tracker magnetic resonance data (148, 158), wherein the tracker magnetic resonance data comprises a predetermined number of slices (125) of the imaging zone; and

wherein execution of the instructions causes the processor to:

acquire (202) the second k-space navigator data using the first motion correction pulse sequence instructions,

acquire (204) reference tracker magnetic resonance data (148) using the second motion correction pulse sequence instructions,

reconstruct (206) a predetermined number of reference images (150) using the reference tracker magnetic resonance data, wherein execution of the instructions causes the processor to repeatedly:

acquire (208) a portion (152) of the magnetic resonance data using the imaging pulse sequence instructions, wherein the portion of the magnetic resonance data comprises a portion of k-space (154) and the first k-space navigator data;

detect (210) movement of the subject by comparing the first k-space navigator data to the second k-space navigator data;

iteratively (214) correct the scan geometry if the movement of the subject is above a predetermined threshold, wherein the scan geometry is iteratively corrected by repeatedly: acquiring corrective tracker magnetic resonance data using the second motion correction pulse sequence instructions, reconstructing a predetermined number of tracker images from the tracker magnetic resonance data, and correcting the scan geometry by comparing the predetermined number of tracker images with the predetermined number of reference images;

• correct (216) the portion of magnetic resonance data if the scan geometry has been iteratively corrected; and