FIELD OF THE INVENTION

The invention pertains to a method regarding detection of changes of at least one parameter having an influence on acquiring magnetic resonance images, particularly between a preparatory phase and an image acquisition phase of a magnetic resonance imaging system.

BACKGROUND OF THE INVENTION

In the art of magnetic resonance imaging, it is known that changes of parameters having an influence on acquiring magnetic resonance images can affect their quality and can even make acquired magnetic resonance images unusable for further analysis and evaluation. Examples for such parameters include, for instance, a position of a subject of interest within an examination space of a magnetic resonance imaging system. Changes of this parameter due to motion of the subject of interest during acquisition of magnetic resonance images may make the magnetic resonance images useless.

As a remedy, it has been suggested to apply intensity value-based registration techniques to a time-series of magnetic resonance images for spatial alignment. For instance, the paper "Towards Real-Time Multi-Modality 3-D Medical Image Registration" (T. Netsch et al, Computer Vision, 2001, ICCV 2001, Proceedings of the Eighth IEEE International Conference on Computer Vision, p. 718 to 725) describes an application of gray value-based methods and presents a faster method that is based on local cross correlation, also for multi- modality image registration such as registration of computer tomography images and magnetic resonance images. For single-modality image registration of magnetic resonance images, it is described that the image registration technique can be used for motion compensation after image acquisition as well as for detection of large head movements, which limit a post-processing of magnetic resonance image data.

It is further known in the art of magnetic resonance imaging to carry out a preparatory phase so as to generate a pre-defined magnetization state of nuclei of a subject of interest, to conduct magnetic resonance imaging system calibration procedures, or to acquire pre-scans such as a SENSE ^™ pre-scan, before acquiring diagnostic images. The preparatory phase is then followed by an acquisition of magnetic resonance signals from the excited nuclei.

The Japanese patent application JP2005040464 concerns a magnetic resonance imaging method which involves parallel imaging. This known method makes use of a predetermined condition to assess whether previously acquired (coil) sensitivity data is the be re-measured at each image acquisition.

SUMMARY OF THE INVENTION

In the field of magnetic resonance imaging, there are parameters known to have an effect on the acquiring of magnetic resonance images. Among these parameters, but not limiting in any manner, are a position of a subject of interest to be imaged, gradient coil heating, and the homogeneity and field strength of the applied static and radio frequency magnetic field.

Motion of the subject of interest during magnetic resonance imaging may result in magnetic resonance image artifacts and/or may affect a quality of the acquired magnetic resonance images. In particular, motion of the subject of interest occurring in the time span after a preparatory phase and before a phase of acquiring the magnetic resonance imaging signals may affect the calibration of the magnetic resonance imaging system or the accuracy of pre-scans.

It is therefore desirable to have a method for detecting changes of parameters that have an influence on acquiring magnetic resonance images, in particular of parameter changes occurring between the preparatory phase and the image acquisition phase of a magnetic resonance imaging system, in order to improve image quality and to reduce a total scanning time by avoiding the necessity of repeatedly acquiring magnetic resonance images.

It is therefore an object of the invention to provide a method of operating a magnetic resonance imaging system regarding detection of changes of at least one parameter having an effect on acquiring magnetic resonance images. The magnetic resonance imaging system is configured for acquiring magnetic resonance images of at least a portion of the subject of interest and includes

an examination space provided to position at least the portion of the subject of interest within,

a main magnet for generating a static magnetic field in the examination space, a magnetic gradient coil system for generating gradient magnetic fields superimposed to the static magnetic field,

at least one radio frequency antenna device that is provided for applying a radio frequency field to nuclei of or within the subject of interest for magnetic resonance excitation,

at least one radio frequency antenna device that is provided for receiving magnetic resonance signals from the nuclei of or within the subject of interest that have been excited by transmission of the radio frequency field,

a control unit for controlling functions of the magnetic resonance imaging system, and

an image processing unit provided for processing magnetic resonance signals to determine an image of at least the portion of the subject of interest from the received magnetic resonance signals.

An operation of the magnetic resonance imaging system comprises at least one preparatory phase and at least one consecutive phase of acquiring magnetic resonance imaging signals. The phrase "preparatory phase", as used in this application, shall be understood particularly as a time period in which magnetic resonance imaging calibration procedures and/or set ups of radio frequency antenna devices and/or pre-scans for calibration purposes and/or preparation of a magnetization state of spins of nuclei of or within the subject of interest may be carried out while at least the portion of the subject of interest is positioned within the examination space. The nuclei to be excited may be nuclei of the subject of interest's own body, or they may be nuclei that are foreign to the subject of interest's body and have been administered to the subject of interest prior to the acquiring of magnetic resonance imaging signals. The phrase "preparatory phase", as used in this application, shall be understood to be distinct from the phase of acquiring such magnetic resonance imaging signals from the subject of interest which are not meant to serve calibration purposes or other pre-diagnostic information purposes.

The method comprises following steps:

determine a momentary value of the at least one parameter at a first point in time and determine a momentary value of the at least one parameter at a second point in time that is distinct from the first point in time,

determine a difference of the momentary parameter values,

determine whether the difference of the momentary parameter values exceeds at least one predetermined threshold, and repeat at least a part of the preparatory phase if the difference of the momentary parameter values exceeds the at least one predetermined threshold.

It shall be appreciated that the notation of a first point in time and a second point in time is merely for the purpose of distinction and is not meant to imply a

chronological order of occurrences if not explicitly expressed so.

One advantage of the method lies in that magnetic resonance signals can be processed to obtain magnetic resonance images on the basis of a correct calibration and pre- scans, avoiding image artifacts and resulting in an improved image quality.

Another advantage of the method is that the robustness and workflow of magnetic resonance imaging acquisitions can be improved despite the occurrence of changes of parameters having an effect on acquiring magnetic resonance images.

In a preferred embodiment of the method,

the at least one parameter is a position of at least the portion of the subject of interest within the examination space,

- the momentary value of the at least one parameter at the first point in time is determined by acquiring a first magnetic resonance image, and the momentary value of the at least one parameter at the second point in time is determined by acquiring a second magnetic resonance image, the second point in time being distinct from the first point in time,

the difference of the momentary parameter values is determined by applying an image registration algorithm to the first magnetic resonance image and to the second magnetic resonance image, by obtaining a set of alignment parameters and by deriving a transforming measure from the obtained set of alignment parameters, and

determining whether the difference of the momentary parameter values exceeds the at least one predetermined threshold is performed by comparing the transforming measure derived from the obtained set of alignment parameters with a predetermined threshold for the transforming measure.

The phrase "image registration", as used in this application, shall be understood particularly as a technique of transforming two different sets of image data into one coordinate system. Image registration techniques are commonly known in medical imaging and are commercially available (e.g. MATLAB ^® module by Math Works ^® ). The registration transformation is usually determined by optimizing a similarity measure calculated from the different sets of image data. In particular, the phrase "image registration techniques" shall encompass intensity-based and/or feature-based methods, rigid and/or non- rigid image registration as well as local correlation methods and/or registration techniques based on mutual information. Other image registration techniques that appear suitable to the person skilled in the art may as well be applied.

The phrase "acquired at a point in time", as used in this application, shall be understood particularly as the start time of the acquisition period.

The phrase "set of alignment parameter", as used in this application, shall be understood particularly as a set of parameters that characterize the transformation applied to one of the images during image registration. The set of alignment parameters shall in particular encompass an off-center distance and direction, and at least one rotation angle.

In this way, position changes of the portion of the subject of interest within the examination space can be detected at an early stage and image data processing that would turn out useless due to magnetic resonance image artifacts and insufficient quality of acquired magnetic resonance images can be avoided, and the robustness and workflow of magnetic resonance imaging acquisitions can be improved.

In yet another preferred embodiment, the method further comprises a step of checking image registration quality of the first magnetic resonance image and the second magnetic resonance image prior to the step of determining a difference of the momentary parameter values. The step of checking the image registration quality includes a check for at least two local minima of a similarity measure of the image registration algorithm, and further includes eliminating the respective magnetic resonance image from applying the step of determining a difference of the momentary parameter values, if at least two local minima of the similarity measure exist, and the following conditions are satisfied:

the absolute values of the similarity measures of the two local minima do not exceed a pre-determined threshold of a global minimum of the similarity measure, e.g. a threshold of 150% of the global minimum of the similarity measure; and

- a translational distance between the first magnetic resonance image and the second magnetic resonance image exceeds a pre-determined threshold for the translational distance, e.g. of at least three voxels in any direction, or an angular displacement between the first magnetic resonance image and the second magnetic resonance image exceeds a predetermined threshold for the angular displacement, e.g. of at least 5° about any axis.

In this way, unnecessary repetitions of pre-scans caused by insufficient image registration quality of magnetic resonance images can be avoided. Values of the various predetermined thresholds, which are representative of a maximum acceptable motion, can particularly be obtained from the accuracy of the pre-scan. In another preferred embodiment, wherein the at least one parameter is a position of at least the portion of the subject of interest within the examination space,

the momentary value of the at least one parameter at the first point in time and the momentary value of the at least one parameter at the second point in time are determined by an optical tracking device,

the difference of the momentary parameter values is determined by the optical tracking device as a positional change of the subject of interest,

determining whether the difference of the momentary parameter values exceeds the at least one predetermined threshold is performed by comparing the determined positional change of the subject of interest with a predetermined threshold for the positional change of the subject of interest.

Optical tracking devices are commercially available nowadays and therefore need not to be described in more detail herein. Any optical tracking device that appear suitable to the person skilled in the art may be employed.

As an inexpensive camera can be employed, a cost-effective solution for detecting position changes of the portion of the subject of interest within the examination space at an early stage can be provided that can avoid image data processing that would turn out useless due to magnetic resonance image artifacts and insufficient quality of acquired magnetic resonance images, and the robustness and workflow of magnetic resonance imaging acquisitions can be improved.

In yet another preferred embodiment, in which the at least one parameter is a width of a distribution of Larmor frequencies of nuclei of or within the subject of interest, the momentary value of the at least one parameter at the first point in time is determined by acquiring a first magnetic resonance image, and the momentary value of the at least one parameter at the second point in time is determined by acquiring a second magnetic resonance image, the second point in time being distinct from the first point in time,

the difference of the momentary parameter values is determined by calculating the difference of the width of the distribution of Larmor frequencies at the first point in time and the width of the distribution of Larmor frequencies at the second point in time, and - determining whether the difference of the momentary parameter values exceeds the at least one predetermined threshold is performed by comparing the difference of the widths of the distributions of Larmor frequencies at the first point in time and the second point in time with a predetermined threshold for the difference of width of the distribution of Larmor frequencies. The distribution of Larmor frequencies may be determined in a static magnetic field of the main magnet, without activation of the magnetic gradient coil system. As the Larmor frequency of a nucleus of or within the subject of interest depends on a local magnetic field strength at the position of the nucleus, a change of the width of the distribution of Larmor frequencies may be an indication for a drift of magnetic shim hardware, and/or a susceptibility change and/or a motion of the subject of interest within the examination space.

By detecting a change of the width of the distribution of Larmor frequencies, image data processing that would turn out useless due to magnetic resonance image artifacts and insufficient quality of acquired magnetic resonance images can be avoided, and the robustness and workflow of magnetic resonance imaging acquisitions can be improved.

In another embodiment, in which the at least one parameter is a center frequency of a distribution of Larmor frequencies of nuclei of or within the subject of interest,

the momentary value of the at least one parameter at the first point in time is determined by acquiring a first magnetic resonance image, and the momentary value of the at least one parameter at the second point in time is determined by acquiring a second magnetic resonance image, the second point in time being distinct from the first point in time,

the difference of the momentary parameter values is determined by calculating the difference of the center frequency of the distribution of Larmor frequencies at the first point in time and the center frequency of the distribution of Larmor frequencies at the second point in time, and

determining whether the difference of the momentary parameter values exceeds the at least one predetermined threshold is performed by comparing the difference of the center frequencies of the distribution of Larmor frequencies at the first point in time and the second point in time with a predetermined threshold for the difference of center frequencies of the distributions of Larmor frequencies.

The distribution of Larmor frequencies may be determined in a static magnetic field of the main magnet, without activation of the magnetic gradient coil system. As the Larmor frequency of a nucleus of or within the subject of interest depends on a local magnetic field strength at the position of the nucleus, a change of the center frequency of the distribution of Larmor frequencies may be an indication for the occurrence of a sudden change of the magnetic field strength of the main magnet within the examination space.

By detecting a change of the center frequency of the distribution of Larmor frequencies, image data processing that would turn out useless due to magnetic resonance image artifacts and insufficient quality of acquired magnetic resonance images can be avoided, and the robustness and workflow of magnetic resonance imaging acquisitions can be improved.

In a preferred embodiment of the method, the first magnetic resonance image is acquired at the first point in time which lies within the at least one phase of acquiring magnetic resonance imaging signals, and the second acquired magnetic resonance image is a pre-scan image and the second point in time lies within the at least one preparatory phase. By that, changes of the at least one parameter having an effect on acquiring magnetic resonance images in the time span after the preparatory phase and before the phase of acquiring the magnetic resonance imaging signals can be detected. Then, in order to avoid an application of an incorrect calibration to magnetic resonance imaging signals, and in order to avoid image artifacts, the preparatory phase can at least partially be repeated. This is in particular advantageous as pre-scans and calibration measurements carried out in the preparatory phase are frequently re-used for multiple scans to reduce the total examination time.

In another preferred embodiment of the method, both the first point in time and the second first point in time lie within at least one phase of acquiring magnetic resonance imaging signals, such that the first acquired magnetic resonance image and the second acquired magnetic resonance image are part of a time-series or of two consecutively acquired time-series of magnetic resonance images. By that, motion of the subject of interest within a time-series or to consecutively acquired time-series of magnetic resonance images can be detected, and examination time can be saved by rejecting the acquired magnetic resonance signals, and continuing by repeating at least a part of the preparatory phase without processing the magnetic resonance signals.

In yet another preferred embodiment of the method, wherein the at least one parameter is a position of at least the portion of the subject of interest within the examination space,

the momentary value of the at least one parameter at the first point in time is determined by acquiring a magnetic resonance image, and the momentary value of the at least one parameter at the second point in time is determined by acquiring the magnetic resonance image, the second point in time being distinct from the first point in time,

the difference of the momentary parameter values is determined by applying an image analysis algorithm to the magnetic resonance image that is configured for detecting repeated shifted patterns of original patterns in the magnetic resonance image, and determining whether the difference of the momentary parameter values exceeds the at least one predetermined threshold is performed by determining a shift distance between the original pattern and the repeated shifted pattern, and by comparing the determined shift distance with a predetermined threshold for the shift distance.

Software for image analysis is commercially available nowadays and therefore need not to be described in more detail herein. Any image analysis software that appears suitable to the person skilled in the art may be employed.

The phenomenon of shifted repeated patterns in magnetic resonance images is known in the art; e.g. as "ghosting", wherein the original pattern is often an anatomic detail such as a contour of an organ or a bone.

With this method, an efficient and cost-effective solution for detecting position changes of the portion of the subject of interest within the examination space can be provided, and the robustness and workflow of magnetic resonance imaging acquisitions can be improved.

In a further preferred embodiment of the method, the magnetic resonance imaging system comprises a plurality of radio frequency antenna devices provided at least for receiving magnetic resonance signals from the nuclei of or within the subject of interest that have been excited by transmission of the radio frequency field, and the preparatory phase comprises at least one radio frequency receiving coil sensitivity calibration measurement of the plurality of radio frequency antenna devices. In particular, the radio frequency coil calibration measurement may comprise a SENSE ^™ (sensitivity encoding) reference scan which is well known in the art.

The plurality of radio frequency antenna devices may also be provided for transmission of a radio frequency field to the examination space. In this case, the radio frequency coil calibration measurement may also comprise a radio frequency Bi calibration scan that is well known for magnetic resonance imaging systems employing multi-channel radio frequency transmission, using multiple independent radio frequency transmit/receive channels and coil elements in parallel for an improved radio frequency Bi field uniformity in high- field magnetic resonance imaging, wherein the magnetic field strength of the main magnet is typically higher than 1.5 T.

The radio frequency coil calibration measurement may also comprise further pre-scans and reference scans that appear to be suitable to the person skilled in the art. These pre-scans and reference scans are acquired prior to a diagnostic scan and are re-used over multiple diagnostic scans. Another example for such a radio frequency coil calibration measurement is a homogeneity pre-scan for normalizing image intensities (CLEAR). In case of a detection of motion of the subject of interest, the radio frequency coil sensitivity calibration measurement can be repeated for providing a correct calibration basis for processing acquired magnetic resonance signals. This is in particular beneficial if the magnetic resonance imaging system is provided to automatically perform radio frequency coil sensitivity calibration measurements to provide a simple workflow for a user, as in this case a correct calibration basis can be provided without requiring an initiation by the user, which is subject to human error and therefore known to be a recurring issue.

In yet another embodiment, the method further comprises a step of confirmation by the user as a precondition for and prior to the step of repeating at least the part of the preparatory phase. If the user does not confirm, the step of repeating will not be carried out. In one embodiment, the non-confirming or rejecting may require an active action by the user. In another embodiment, the non-confirming or rejecting may be active after a preset time that starts after an input demand has elapsed. By implementing the step of confirmation, a useful option of preventing a continuous repetition of at least the part of the preparatory phase can be provided in case of, for example, an unavoidable involuntary movement of the subject of interest.

In another aspect of the present invention, a magnetic resonance imaging system is provided which is configured for acquiring magnetic resonance signals from at least a portion of a subject of interest. The magnetic resonance imaging system comprises an examination space provided to position the subject of interest within, a main magnet for generating a static magnetic field in the examination space,

a magnetic gradient coil system for generating gradient magnetic fields superimposed to the static magnetic field,

- at least one radio frequency antenna device that is provided for applying a radio frequency field to nuclei of or within the portion of the subject of interest for magnetic resonance excitation,

at least one radio frequency antenna device that is provided for receiving magnetic resonance signals from the nuclei of or within the portion of the subject of interest that have been excited by transmission of the radio frequency field,

a control unit for controlling functions of the magnetic resonance imaging system, and an image processing unit provided for processing magnetic resonance signals to determine an image of at least the portion of the subject of interest from the received magnetic resonance signals.

The control unit is configured to carry out steps of an embodiment of one of the methods disclosed above or a combination thereof.

In yet another aspect of the present invention, a software module is provided for carrying out an embodiment of any of the methods disclosed above or a combination thereof, of operating a magnetic resonance imaging system. The method steps to be conducted are converted into a program code of the software module, wherein the program code is implementable in a memory unit of a control unit of the magnetic resonance imaging system and is executable by a processor unit of the control unit of the magnetic resonance imaging system. The control unit may be the control unit that is customary for controlling functions of a magnetic resonance imaging system. The control unit may alternatively be an additional control unit that is especially assigned to execute the method steps.

The software module can enable a robust and reliable execution of the method and can allow for a fast modification of method steps and/or an adaptation of the image registration algorithm.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.

In the drawings:

Fig. 1 is a schematic illustration of a part of an embodiment of a magnetic resonance imaging system in accordance with the invention,

Fig. 2 shows a flowchart of a method in accordance with the invention of operating the magnetic resonance imaging system pursuant to Fig. 1, and

Fig. 3 is a schematic illustration of two different distributions of Larmor frequencies emitted by nuclei of or within the subject of interest.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a schematic illustration of a part of an embodiment of a magnetic resonance imaging system 10 configured for acquiring magnetic resonance signals from at least a portion of a subject of interest 20. The magnetic resonance imaging system 10 comprises a magnetic resonance scanner 12 having a main magnet 14. The main magnet 14 has a central bore that provides an examination space 16 around a center axis 18 for the subject of interest 20 to be positioned within, and is further provided for generating a substantially static magnetic field at least in the examination space 16. For clarity reasons, a customary table for supporting the subject of interest 20 has been omitted in Fig. 1. The substantially static magnetic field defines an axial direction of the examination space 16, aligned in parallel to the center axis 18. It is appreciated that the invention is also applicable to any other type of magnetic resonance imaging systems providing an examination region within a static magnetic field.

Further, the magnetic resonance imaging system 10 comprises a magnetic gradient coil system 22 provided for generating gradient magnetic fields superimposed to the static magnetic field. The magnetic gradient coil system 22 is concentrically arranged within the bore of the main magnet 14.

The magnetic resonance imaging system 10 comprises a magnetic resonance imaging system control unit 26 provided to control functions of the magnetic resonance scanner 12 and the magnetic gradient coil system 22. The magnetic resonance imaging system control unit 26 includes a human interface device designed as a monitor unit 28 having a touch-sensitive screen.

Furthermore, the magnetic resonance imaging system 10 includes a radio frequency antenna device designed as a whole-body coil 38 that is provided for applying a radio frequency field to nuclei of or within the subject of interest 20 for magnetic resonance excitation during radio frequency transmit time periods to excite the nuclei of or within the subject of interest 20 for the purpose of magnetic resonance imaging. To this end, radio frequency power is fed, controlled by the magnetic resonance imaging system control unit 26, from a radio frequency transmitter 56 to the whole-body coil 38. The whole-body coil 38 has a center axis and, in the operational state, is arranged concentrically within the bore of the main magnet 14 such that the center axis of the whole-body coil 38 and the center axis 18 of the magnetic resonance imaging system 10 coincide. As is well known in the art, a cylindrical metal radio frequency shield 24 is arranged concentrically between the magnetic gradient coil system 22 and the whole-body coil 38.

Moreover, the magnetic resonance imaging system 10 comprises a plurality of radio frequency antenna devices provided for receiving magnetic resonance signals from the nuclei of or within the subject of interest 20 that have been excited by transmission of the radio frequency field. The radio frequency antenna devices of the plurality of radio frequency antenna devices are designed as an array of local coils 40 that are intended to be positioned proximal to a region of the subject of interest 20 to be imaged. The local coils 40 are configured for receiving magnetic resonance signals from the excited nuclei of or within the portion of the subject of interest 20 to be imaged during radio frequency receiving time periods which are distinct from the radio frequency transmit time periods.

Furthermore, the magnetic resonance imaging system 10 comprises an image processing unit 36 provided for processing magnetic resonance signals to determine an image of at least the portion of the subject of interest 20 from the received magnetic resonance signals.

The regular operation of the magnetic resonance imaging system 10 comprises a preparatory phase and consecutive phases of acquiring magnetic resonance imaging signals. The preparatory phase is performed in order to provide proper calibration of components of the magnetic resonance imaging system 10 and comprises a radio frequency coil sensitivity calibration measurement. In order to optimize image quality and to reduce a total scan time, a pre-scan is carried out with the plurality of radio frequency antenna devices that are provided for receiving magnetic resonance signals, i.e. the local coils 40. The pre-scan includes a

SENSE (sensitivity encoding) reference scan. The SENSE method of parallel magnetic resonance imaging is well known in the art and, for instance, described in Pruessmann, K.P. et al, "SENSE: Sensitivity Encoding for Fast MRF, Magnetic Resonance in Medicine

42:952-962 (1999), and shall therefore not be discussed in further detail herein. Since for a typical magnetic resonance imaging examination the subject of interest 20 will lay still, the pre-scan and other calibration results from the preparatory phase are usually re-used for multiple scans to reduce the overall time that the subject of interest 20 has to spend within the examination space 16.

An embodiment of a method of operating the magnetic resonance imaging system 10 pursuant to Fig. 1 regarding detection of changes of a parameter having an effect on acquiring magnetic resonance images which is given by a position of at least the portion of the subject of interest 20 within the examination space 16, will be presented in the following with reference to Fig. 2, showing a flowchart of the embodiment of the method which is in accordance with the invention.

In preparation of operating the magnetic resonance imaging system 10, it shall be understood that all involved units and devices are in an operational state. In order to be able to carry out the method as a specific operation of the magnetic resonance imaging system 10, the magnetic resonance imaging system control unit 26 comprises a software module 34 (Fig. 1). The method steps to be conducted are converted into a program code of the software module 34, wherein the program code is implementable in a memory unit 30 of the magnetic resonance imaging system control unit 26 and is executable by a processor unit 32 of the magnetic resonance imaging system control unit 26.

In a first step 42 of the method, a second magnetic resonance image is acquired at a second point in time to determine the momentary value of the parameter (i.e. the position of at least the portion of the subject of interest 20 within the examination space 16) at the second point in time that lies within the preparatory phase.

Then, in a second step 44 of the method, a first magnetic resonance image is acquired at a first point in time to determine the momentary value of the parameter at the first point in time which lies within the phase of acquiring magnetic resonance imaging signals. It shall be repeated as this point that the notation of a first point in time and a second point in time is merely for the purpose of distinction and is not meant to imply a chronological order of occurrences. As the phase of acquiring magnetic resonance imaging signals is consecutive to the preparatory phase, the second point in time is in fact earlier than the first point in time in a chronological sense.

In a next step 46 of the method, an image registration algorithm is applied to the first magnetic resonance image and the second magnetic resonance image, and the difference of the momentary parameter values, i.e. the change of positions of at least the portion of the subject of interest 20 between the second point in time and the first point in time, is determined. The image registration algorithm is a proper rigid registration algorithm comprising a transformation which preserves distances between every pair of points of an image. Further, from applying the image registration algorithm, a set of alignment parameters is obtained that comprises a translational distance and an angular displacement about an axis that is perpendicular to an image plane of the first magnetic resonance image and the second magnetic resonance image.

Applying the rigid registration algorithm to the first and the second magnetic resonance image provides a value for a similarity measure function that has been minimized in the course of the image registration process. If, in the course of applying the rigid registration algorithm, it turns out that the similarity measure function comprises at least two local minima, that the absolute values of the similarity measures at the two local minima are not exceeding 150% of a global minimum of the similarity measure, and that a translational distance between the first magnetic resonance image and the second magnetic resonance image is at least three voxels in any direction, or an angular displacement between the first magnetic resonance image and the second magnetic resonance image is at least 5° about any axis, then the respective magnetic resonance image can, as part of an optional step 54 of the method, be eliminated from further applying the step 48 of determining an occurrence of motion.

In another step 48 of the method, a transforming measure from the obtained set of alignment parameters is determined which is given as the sum of the magnitude of the translational distance and the magnitude of the circular path length of the angular

displacement at a maximum radius of the field of view. It is then determined whether the difference of the momentary parameter values exceeds at least one predetermined threshold for the transforming measure that resides in the memory unit 30 of the magnetic resonance imaging system control unit 26.

If the predetermined threshold for the transforming measure has been exceeded, this information is signaled to the user by a window having corresponding contents that is popping up on the touch-screen of the monitor unit 28.

A next step 50 of positive confirmation by the user by touching a corresponding first software key provided on the touch-screen of the monitor unit 28 is a precondition for carrying out a next step of the method, which is the step 52 of repeating at least a part of the preparatory phase formed by a SENSE ^™ reference scan. On the touchscreen, a second software key is also provided for an action of rejecting by the user to override the step 52 of repeating and to continue the acquiring of magnetic resonance signals. If, in this embodiment, neither one of the first software key and the second software key is touched by the user within a preset time period of thirty seconds, the magnetic resonance imaging system control unit 26 is configured to continue the acquiring of magnetic resonance signals.

The disclosed method is not meant to be limited to the second point in time lying within the preparatory phase. Rather, the first point in time and the second point in time may lie within at least one phase of acquiring magnetic resonance imaging signals, such that the first acquired magnetic resonance image and the second acquired magnetic resonance image are part of a time-series or of two consecutively acquired time-series of magnetic resonance images. In this case, the steps of the method as disclosed above are also applicable to determine an occurrence of motion of the subject of interest 20 within a time-series of acquired magnetic resonance images or to determine an occurrence of motion of the subject of interest 20 in a time period between two consecutive time-series of acquired magnetic resonance images.

As an alternative, the momentary value of the parameter (i.e. the position of at least the portion of the subject of interest 20 within the examination space 16) at the first point in time and the momentary value of the parameter at the second point in time can be determined by an optical tracking device comprising a camera 58 and an optical tracking processor 60. The camera 58 transmits signals to the optical tracking processor 60 that is configured to generate optical flow data which are transferred to memory unit 30 of the magnetic resonance imaging system control unit 26. Fig. 1 shows the optical tracking device being aligned for a case in which the portion of the subject of interest 20 to be imaged would be the head of the subject of interest 20.

Fig. 3 illustrates an embodiment of the method wherein the at least one parameter is a width 62 of a distribution of Larmor frequencies of nuclei of or within the subject of interest 20. The distribution of Larmor frequencies has been determined in the static magnetic field of the main magnet 14, without activation of the magnetic gradient coil system 22. The momentary value of the width 62 of the distribution of Larmor frequencies at the first point in time, shown in Fig. 3 as a solid line, has been determined by acquiring a first magnetic resonance image formed by a simple scan, and the momentary value of the width 62' of a distribution of Larmor frequencies at the second point in time, shown in Fig. 3 as a dashed line, has been determined by acquiring a second magnetic resonance image formed by another simple scan, the second point in time being earlier than the first point in time. The widths 62, 62' of the distributions of Larmor frequencies are determined as a full width at half minimum (FWHM) of a maximum amplitude.

The difference of the momentary parameter values is determined by calculating the difference of the width 62 of the distribution of Larmor frequencies at the first point in time and the width 62' of the distribution of Larmor frequencies at the second point in time. Then, it is determined whether the difference of the momentary parameter values exceeds the at least one predetermined threshold by comparing the difference of the widths 62, 62' of the distributions of Larmor frequencies at the first point in time and the second point in time with a predetermined threshold for the difference of widths 62, 62' of the distributions of Larmor frequencies that resides in the memory unit 30 of the magnetic resonance imaging system control unit 26.

If the predetermined threshold for the difference of widths 62, 62' of the distributions of Larmor frequencies has been exceeded, this information is signaled to the user by a window having corresponding contents that is popping up on the touch-screen of the monitor unit 28, with a subsequent option of a positive confirmation by the user as a precondition for carrying out a next step 52 of the method, which is repeating at least a part of the preparatory phase, in the same manner as described before.

In an alternative approach, a center frequency 64, 64' of the distribution of Larmor frequencies of nuclei of or within the subject of interest 20 may be selected as parameter, and method steps may be applied in the same way as described for the widths 62, 62' of the distributions of Larmor frequencies.

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. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

REFERENCE SYMBOL LIST

10 magnetic resonance imaging 62 width of Larmor frequency system distribution

12 magnetic resonance scanner 64 center frequency

14 main magnet

16 examination space

18 center axis

20 subject of interest

22 magnetic gradient coil system

24 radio frequency shield

26 magnetic resonance imaging

system control unit

28 monitor unit

30 memory unit

32 processor unit

34 software module

36 image processing unit

38 whole-body coil

40 local coil

42 step of acquiring 2 ^nd MR image

44 step of acquiring 1 ^st MR image

46 step of applying image

registration algorithm

48 step of determining motion

50 step of positive confirmation

52 step of repeating part of

preparatory phase

54 step of checking image

registration quality

56 radio frequency transmitter

58 camera

60 optical tracking processor