Patent ID: 12216187

DETAILED DESCRIPTION

In the following, embodiments of the disclosure will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the disclosure is not intended to be limited by the embodiments described hereinafter or by the drawings, which are to be illustrative only.

The drawings are to be regarded as being schematic representations, and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose becomes apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components of physical or functional units shown in the drawings and described hereinafter may be implemented by an indirect connection or coupling. A coupling between components may be established over a wired or wireless connection. Functional blocks may be implemented in hardware, software, firmware, or a combination thereof.

As will be explained below, a method is provided in which the movement of a marker is monitored and it is checked whether the detected movement of the marker may be really based on a physical movement of the examined object or of the region of interest within the object or whether the detected movement of the marker may be based on a movement of another part of the examined object, to which the marker is attached and which has a negligible influence on the MR image generated from the MR signals, but not of the part of the object for which the MR signals are mainly detected. It is especially checked whether the object is able to carry out the detected movement of the marker taking into account a defined number of degrees of freedom the object is able to use for the movement. If it is detected that the detected movement of a marker cannot originate from the examined object, the corresponding part of the movement is excluded from the correction which might be used to generate a motion corrected MR image by using a description of the movement of the motion model which is only based on the defined number of degrees of freedom. The motion corrected MR image may be generated as known in prospective correction methods.

The disclosure is especially helpful in the MR imaging of the head as the head as rigid object may only carry out certain movements when it is placed in an MR imaging system and when a head coil is used to detect the MR signals. Accordingly, it is possible to differentiate between actual movements of the head and other movements which cannot have the basis in the movement of the head itself.

However, the present application is not restricted to an application in the head, other parts of the body such as the knee, the arm, or the shoulder may be used in a similar way.

FIG.1depicts a schematic view of an MR system1which includes a magnet10generating a polarization field B0. An object under examination12, (e.g., a human being) lies on a table11and is moved into the center of the MR system1where MR signals after RF excitation may be detected by a receiving coil2, which may include different coil sections. Each coil section may be associated with a corresponding detection channel. In the embodiment shown, two different detection channels3are used. By applying RF pulses and magnetic field gradients, the nuclear spins of the object12, especially the part located in the receiving coil2, are excited and the currents induced by the magnetization are detected. The way MR images are generated and how the MR signals are detected using a sequence of RF pulses and a sequence of magnetic field gradients are known in the art so that a detailed explanation thereof is omitted.

The MR system includes a control module13, which is used for controlling the MR system1. The control module13includes a gradient control unit14for controlling and switching the magnetic field gradients, an RF control unit15for controlling and generating the RF pulses for the imaging sequences. An image sequence control unit16is provided which controls the sequence of the applied RF pulses and magnetic field gradients and thus controls the gradient control unit14and the RF control unit15. In a memory17, computer programs needed for operating the MR system and the imaging sequences necessary for generating the MR images may be stored together with the generated MR images. The generated MR images may be displayed on a display18, wherein an input unit19is provided used by a user of the MR system to control the functioning of the MR system. A processing unit20may coordinate the operation of the different functional units shown inFIG.1and may include one or more processors which may carry out instructions stored on the memory17. The memory may include a suitable program code to be executed by the processing unit or by a motion correction device100configured to correct a motion of the object. Furthermore, a camera or image sensor8is shown inFIG.1which is configured to acquire picture data from the object12. The generated picture data may be acquired with a frequency such that a movement of the examined object may be detected in the generated picture frames. By way of example, the frame rate of the generated picture frames may be between one and 100 frames per second. Furthermore, a marker9is attached to the object wherein the movement of the marker is used to deduce the movement of the head itself. In the embodiment shown, an external marker is attached to the examined body. However, it is also possible that a part of the examined body itself is the marker and the position or motion of the marker may be determined with an external image sensor or may be determined based on the generated MR images, e.g. using navigator images generated to detect the motion. The position or movement, determined based on the marker, may also be determined using a certain accuracy, such that possible motions are included with are not possible with the determined motion model. The motion model may then help to only consider motions which may be explained based on the model with the selected model function.

The picture data may be processed either by the processing unit20or by the device100in order to detect a motion of the object12during a time period when the MR signals are detected. In the embodiment shown the processing unit20and the device100for determining and correcting a movement of the object12are separate entities. However, the functions provided by these two entities may be implemented in a single entity or may be implemented by a cloud environment.

FIG.2depicts in more detail how a person as object12is examined wherein the head is examined using a head coil as receiver coil2. In the embodiment shown, the head coil includes three different coil sections. In other embodiments, more or less receiving channels might be used. Attached to the nose of the person12is the marker9. The movement of this marker is detected by the image sensor8. However, the marker may also move when the skin is moving to which the marker is attached, by way of example the nose. The head may not move in all different directions and furthermore it may be assumed that the head may only carry out rigid body transformations. Based on a rotation point of the head on the support structure on which the head is positioned, different nonlinear movements of the head may be expected. The model function may be fitted to the detected motion of the marker in order to determine the motion that may be explained on the used model function, wherein the model function may be any function, e.g., a linear or non-linear function.

All movements of the marker9which do not fit to a common and typical and possible movement of the head may be filtered out and are not considered for the correction of the movement of the head.

One possible implementation is as follows:

In a first act, a motion model is determined which describes the possible movements of the examined object, here the head. A simple model may assume that the head may only rotate around an axis A which is parallel to the spine, which may be considered as a left right rotation from the point of view of the person. Furthermore, it may be assumed that only a nodding rotation is allowed around a pivot point B shown inFIG.2. In addition to these two rotational degrees of freedom, one translational degree of freedom may be considered such as the translation in the cranial caudal direction as indicated by arrow C.

Accordingly, it is assumed that only a defined number of movements are possible, which are a superposition of the possible movements allowed by the different degrees of freedom as specified above. Any other movement which cannot be described with these defined number of degrees of freedom cannot originate from the head itself but may originate from a movement of the part of the body to which the marker9is attached.

The motion model may be determined in a coordinate system of the head or in a coordinate system of the MR imaging system.

The marker may have its own coordinate system. As the movement of the marker is determined to deduce the movement of the head, the movement of the marker and the motion model have to be described in a common coordinate system. The common coordinate system may be any suitable coordinate system, be it the coordinate system of the MR system, the coordinate system of the marker, the coordinate system of the head of the user, or any other coordinate system suited for describing the used motion model. This includes polar, cylindrical, and spherical coordinate systems, such as more complex warped coordinate systems.

In a further act, the motion model is transferred into this common coordinate system. Furthermore, the motion of the marker is determined in the common coordinate system. When both the motion model and the movement of the marker are determined in a common coordinate system, it is possible to determine for each point in time a status of the defined set of the number of degrees of freedom which best describe the determined movement of the marker as determined based on the images generated by camera8. This means that a first motion is determined using only the description of the motion model with the defined number of degrees of freedom. This first motion is a combination of motions defined by the defined set of degrees of freedom. This first motion is the motion which best matches the actual motion of the marker as determined from the images. This is a kind of matching procedure in which different combinations of the different movements allowed by the different degrees of freedom are combined in order to generate a movement which best fits to the detected movement of the marker. This matching may be implemented as a least square fit, however, any other minimization method might be used which minimizes the difference between the motion as determined for the marker based on the images and as determined using the motion model which is only based on a certain number of degrees of freedom in the common coordinate system. When this first motion of the object as described by the motion model is determined, it is possible to use this first motion as the actual motion of the object, here of the head, this first motion is then used for the motion corrected MR images. One possible implementation for generating motion corrected MR images is disclosed in M. Zaitsev, C. Dold, G. Sakas, J. Hennig, and O. Speck “Magnetic resonance imaging of freely moving objects: Prospective real-time motion correction using an external optical motion tracking system”; NeuroImage 31 (2006) 1038-1050.

When the first motion is known in the common coordinate system, it might be necessary to transform this first motion into the coordinate system of the MR system, wherein this first motion is then used to generate motion correct MR images as known in the art. In the present case motion and movement are used interchangeably.

Furthermore, it is possible to consider further degrees of freedom such as all six degrees of freedom. Additionally, is possible to consider a maximum speed that is possible for the movement of the examined object, here the head. Based on the inertia of the head certain movements such as very fast movements may not be possible.

The rotation axis such as the rotation axis A or the pivot point B shown inFIG.2may be determined based on MR images which were generated from the head. Furthermore, it is possible to use additional data which are known from the examined person such as the size, the weight, the age, and/or the type or the position or the inclination of the coil in which the object is located.

FIG.3summarizes some of the main acts carried out by the motion correction device100shown inFIG.1when determining a correction of the motion. In act S31, a motion model is determined using a model function which describes the possible movements of the object based on a defined number of degrees of freedom the object is able to use for the movement, or a more generalized mathematical description thereof. In the example above, only three possible degrees of freedom were considered. However, this may depend on the application and the part of the body for which the imaging system is used. In act S32, the motion of the marker is detected which is provided on the object. The marker may be a marker which is attached to the object such as the marker9shown inFIG.3. However, the marker may also be a grid projected onto the object. In a further act S33, a description of the motion model in a common coordinate system is determined and, in step act S34, the motion of the marker in the common coordinate system. When the motion model and the motion of the marker are known in the common coordinate system, it is possible to determine, in act S35, a first motion of the object in the common coordinate system only using the description of the motion model and which best matches the determined motion of the marker. In act S36, the movement of the object is then used for the correction of the movement of the object, wherein the first motion which only considers certain degrees of freedom is used in order to generate the motion corrected MR images.

FIG.4depicts a schematic architectural view of a motion correction device100which may carry out the above discussed correction of the movement. The device100may be a separate entity or may be implemented as part of a common processing unit within or outside the MR system1. The device100includes an interface or input/output110which is used to receive user data such as MR images or control messages and used to transmit user data or control messages to other entities, such as the motion corrected MR images generated by the device100. The device100furthermore includes a processing unit one120which is responsible for the operation of the motion correction device100as discussed above. The processing unit120may include one or more processors used to carry out instructions stored on a memory130wherein the memory may be a part of the memory17discussed above or may be separate memory. The memory may include a read-only memory, a random-access memory, a mass storage, a hard disk, or the like. The memory may furthermore include a suitable program code to be executed by the processing unit120so as to implement the above-described functionality in which the motion correction device100is involved.

The above-described method provides an improved motion correction as only the possible motions are considered and other motions which cannot be carried out by the corresponding object are filtered out and are not considered for the motion correction.

It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present disclosure. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification.

Although the disclosure has been illustrated and described in detail with the exemplary embodiments, the disclosure is not restricted by the examples disclosed and other variations may be derived therefrom by a person skilled in the art without departing from the protective scope of the disclosure.