WO2017093090A1

WO2017093090A1 - Rf coil system, computer program product and magnetic resonance imaging system

Info

Publication number: WO2017093090A1
Application number: PCT/EP2016/078496
Authority: WO
Inventors: Michael Günter HELLE; Christoph Leussler; Daniel Wirtz; Peter Mazurkewitz
Original assignee: Koninklijke Philips N.V.
Priority date: 2015-12-01
Filing date: 2016-11-23
Publication date: 2017-06-08

Abstract

It is an object of the invention to provide for an improved coil positioning system. According to a first aspect of the invention, this object is achieved by a coil system for use in magnetic resonance imaging (MRI) system, wherein the coil system is configured to be positioned above a patient and patient table when in use, wherein the coil system comprises: - a coil bridge comprising a first side part, a second side part and an upper part, wherein the coil bridge extends from a first side of the patient to a second side of the patient when in use, such that when in use the first part is located on the first side of the patient, the second side part is located on the second side of the patient and the upper part is located above the patient and; - multiple transmit/and or receive coils connectable to the coil bridge; wherein the coil system is configured such that a number of coils used during MRI scanning is adjustable such that an image volume covered by the coils used can be optimized to patients of different sizes

Description

RF COIL SYSTEM, COMPUTER PROGRAM PRODUCT AND MAGNETIC

RESONANCE IMAGING SYSTEM

FIELD OF THE INVENTION

The invention relates to magnetic resonance imaging and more specifically to the coil positioning. BACKGROUND OF THE INVENTION

Accurate radiation therapy (RT) planning and dose calculation require high quality imaging for tumor delineation and critical organ segmentation. Magnetic resonance imaging (MRI) provides superior soft tissue contrast, therefore allows the generation of precise RT plans in combination with CT images for accurate dose calculations.

For successful treatment and exact dose delivery, images should ideally be acquired with the patient in the exact treatment position. This will also minimize registration errors between MRI and CT images for electron density assignment and subsequent dose calculation.

Nowadays, imaging and treatment of the abdomen is performed with the patient lying on a flat table top in order to facilitate reproducible subject positioning and body contouring for an exact dose calculation. While these requirements can be easily met for CT and linear accelerators or other treatment devices, MRI requires the utilization of dedicated coil arrays (MRI coils) placed in close vicinity to the organ of interest in order to ensure optimal signal receive and high quality images. However, such coils will alter the body contour and shift organs of a patient especially in abdominal MR scans when an anterior coil is positioned on top of the patient. This impedes registration of MR and CT images and represents a major obstacle for MR-only based RT planning approaches.

US 2008/0136412 discloses an MRI apparatus including a flexible upper RF coil unit having a plurality of coils which irradiate RF pulses or detect MR signals to or from an object, and a supporting unit configured to rotatably hold the RF coil unit at an upper end portion of the supporting unit. The coil unit supporting unit has a prescribed height so that the RF coil unit is

US2014/0300355A1 describes a local coil system comprising a plurality of transmission elements. In some cases not all transmission elements are switched on or activated during a measurement based on the magnetic resonance RF field. This selection may depend on the positioning of the examination object in the homogeneity volume. An RF exposure value may be an SAR limit value. A number of active transmission elements in a specific region may be limited due to the exposure value.

US2015/0293188A1 describes a method for positioning at least one local coil system for recording magnetic resonance data with a magnetic resonance device, wherein at least one surface data record, is measured . One position or shape of a local coil is chosen as a function of the surface characteristic in such a way that there is a predetermined distance between the surface of the patient and the surface of the local coil.

SUMMARY OF THE INVENTION

It is an object of the invention to provide for an improved coil positioning system. According to a first aspect of the invention, this object is achieved by a coil system for use in magnetic resonance imaging (MRI) system, wherein the coil system is configured to be positioned above a patient and patient table when in use, wherein the coil system comprises

a coil bridge comprising a first side part, a second side part and an upper part, wherein the coil bridge extends from a first side of the patient to a second side of the patient when in use, such that when in use the first part is located on the first side of the patient, the second side part is located on the second side of the patient and the upper part is located above the patient and;

multiple transmit/and or receive coils connectable to the coil bridge; wherein the coil system is configured such that a number of coils used during MRI scanning is adjustable such that an image volume covered by the coils used can be optimized to patients of different sizes.

Existing coil positioning systems employ mechanical devices - so called coil bridges - to support an MRI coil in such a way that it does not alter the body contour. At the same time coil bridges allow coils to be positioned as close as possible to the anatomic region of interest to ensure sufficient signal to noise ratio (SNR). However, it is an insight of the inventors that for patients presenting large girths the application of coil bridges can result in insufficient covering, especially of lateral located anatomic regions. In order to generate high quality images with homogenous signal, sufficient coil coverage of the patient during pre- therapeutic imaging and therapy control is required. By making the number of coils in the coil system that can be used during MRI scanning adjustable, an image volume covered by the coils used can be optimized to patients of different sizes. For example, dedicated coil elements laterally sided of the body could be used when MRI coil bridges are used for larger patients. The use of such additional coils may lead to improved image quality.

According to embodiments of the invention, the size of the coil bridge is adjustable. Hereby, the coil system according to the invention can be further adapted to patients of different sizes. The distance between the upper part and the patient table can be smaller for smaller patients, or even infants. For larger patients the distance can be made larger. This holds also for the distance between the first side part and the second side part. This distance can be made smaller for smaller patients and larger for larger patients. The adjustment of distances can be achieved in many different ways and the invention is not limited to the ones shown herein. For example one can use extension elements to change the distances or one could use some kind of telescoping mechanism. According to advantageous embodiments of the invention, the coil bridge is configured such that when the distance between the upper part and the patient table and / or the distance between the first side part and / or the second side part is increased, the number of coils used during the MRI scanning can be increased on the parts of the coil bridge causing the increased distance between the upper part and the patient table and / or the distance between the first side part and / or the second side part. According to even more advantageous embodiments of the invention, the number of coils in this case could be increased by adding and / or connecting them to the parts of the coil bridge causing the increased distance between the upper part and the patient table and / or the distance between the first side part and / or the second side part.

According to further embodiments of the invention, the coil system comprises a camera configured for taking at least one image of the patient. The coil system further comprises a processor and a memory for storing machine executable for execution by the processor, wherein execution of the machine executable instructions causes the processor to perform an image analysis on the image in order to determine the size of the patient and further determining one or more favorable coil positions and / or coil number to be used given the determined patient size based on predetermined information about a relation between patient size, coils used, coil position and image quality. This information can be determined before using the coil system, e.g. by means of measurements or simulations. The information can be stored by the coil system for example by means of a lookup table. Image quality could be determined for example based on SNR or homogeneity of the transmit and / or receive field. The coil system further comprises a display configured for displaying the determined one or more favorable coil positions and / or coil number to be used to a user. This embodiment is advantageous because it may improve workflow. Further it may improve image quality, especially in case of inexperienced users.

According to one embodiment of the invention, at least part of the transmit and / or receive coils is integrated in the first and / or second part and / or the upper part of the coil bridge and at least part of the transmit and / or receive coils can be activated by means of switches. This embodiment is advantageous, because it may result in a system with less separate elements. The distance between the upper part and the patient table could for example be increased by adding an extension element. The additional coils could already be integrated in the extension element. Optionally, by connecting the extension element to the rest of the coil bridge, the additional coils could automatically be switched on.

According to another embodiment of the invention, at least part of the transmit and / or receive coils is detachably attachable to the first and / or second part and / or upper part of the coil bridge. This embodiment is advantageous, because it may allow more flexibility as to where the coils can be positioned.

According to another embodiment of the invention, the coil system further comprises coil detection means configured to detect coil presence and / or coil position at the first side part and / or at the second side part and / or at the upper part. This embodiment is advantageous, because the automatic detection can be used for optimization of the homogeneity of an RF transmit and receive field. Also, it can be used to determine whether a desired image quality can be obtained or if additional or other coil positions are needed to obtain a desired transmit and / or receive field and thereby the desired image quality.

According to further embodiments of the invention the coil system further comprises a processor and a memory for storing machine executable for execution by the processor, wherein execution of the machine executable instructions causes the processor to carry out the steps of a method of optimizing the homogeneity of the RF transmit field and / or receive field generated by the coils covering a volume of interest of an object to be scanned based on the coil positions detected by the coil position detection means. For optimization of the RF-fields the technique of RF-shimming is applied. RF shimming is known in the art, for example from Hoult DI, Phil D (2000) Sensitivity and power deposition in a high- field imaging experiment. J Magn Reson Imaging 12:46-67 or Ibrahim TS, Lee R, Baertlein BA, Abduljalil AM, Zhu H, Robitaille PM (2001) Effect of RF coil excitation on field inhomogeneity at ultra high fields: a field optimized TEM resonator. Magn Reson Imaging 19: 1339-1347. For the transmit case amplitudes and phases of the transmitted RF- signal per channel are modulated such, that an optimized excitation e.g. across the selected FOV is guaranteed. For receive, the same coil settings can be applied due to the reciprocity principle. The optimized settings can be generated by acquiring RF field maps in a suitable way and by calculating the RF-parameters per channel. Furthermore model data from RF- simulations can be added to the optimization process (e.g. for meeting local and global specific absorption rate (SAR)-limits). Alternatively, a lookup table can be used to determine shim settings based on coil positions.

According to further embodiment of the invention, the coil system is further configured for notifying a user of sufficient transmit and / or receive homogeneity cannot be achieved given a coil number and coil positions used. These embodiments are advantageous, because they may improve workflow.

According to another embodiment of the invention the coils comprise wires and the coil system comprises means to lead the wires away from a direction in which the coil bridge is adjustable in order to improve the workflow. This is also advantageous, because in this way it is less likely that wires get stuck when the coil bridge is adjusted in size.

According to another aspect the object is also achieved by a computer program product comprising program code means for causing a computer to carry out the steps of a method of:

determining the size of the patient based on an image of the patient and determining one or more favorable coil positions and / or coil number to be used given the determined patient size based on predetermined information about a relation between patient size, coils used, coil position and image quality and

providing the determined one or more favorable coil positions and / or coil number to a user. This is advantageous because hereby, the workflow can be improved.

According to an embodiment of the invention, the computer program product is further configured to determine a favorable distance between the upper part and the patient table and / or the distance between the first side part and / or the second side part.. Favorable in the context of this application could for example refer to sufficient image quality, SNR and / or homogeneity of the transmit and / or receive field

According to another aspect the object is also achieved by a magnetic resonance imaging system comprising the coil system and / or computer program product, which are described above. The use of such an MRI system is especially advantageous in a radiotherapy workflow as by means of an MRI system according to the invention a good image quality can be delivered for patients of different sizes without the need of coils touching the patient. Such images may improve MRI based radiotherapy plans and / or the radiotherapy delivery. Therefore, according to further embodiments of the invention, the MRI system also comprises a radiation delivery system, which is configured to deliver radiation treatment based on the MRI images acquired.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 diagrammatically shows two coil systems according to the invention and

Figure 2 diagrammatically shows a computer program product according to the invention and

Figure 3 illustrates diagrammatically a magnetic resonance guided radiotherapy system in which the invention is used.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 diagrammatically shows two coil systems according to the invention. A patient 107 is positioned on a patient table 101. The coil system 100 comprises a coil bridge 103. The coil bridge comprises a first side part 104, a second side part 105 and an upper part 106. The coil bridge extends from one side of the patient 107 to another side of the patient. The coil bridge in figure 1 shows four angles. However, the coil bridge could have many different shapes and could for example also be half a circle. Figure 1 shows two embodiments of the coil system (A and B). In A the distance between the patient table 101 and the upper part 106 of the coil bridge is adjustable. This is illustrated by means of dashed lines. The dashed lines could for example be extension parts, but it could also be a telescoping mechanism or any other means to adjust the distance. A larger distance can be used when scanning larger patients.

In embodiment B also the distance between the first side part 104 and second side part 105 is adjustable. This is again illustrated by means of dashed lines. Making use of a narrower coil bridge could be useful when scanning smaller patients, e.g. like infants.

In embodiment A, a posterior coil 110 and an anterior coil are used 120. The coil system 100 comprises multiple transmit and / or receive coils 102. The number used during scanning can be adjusted based on a patient's size. For a larger patient the distance between the patient table and the upper part of the coil bridge can be increased. If the coils used do not sufficiently cover an image volume (figure 3, 307), additional coils can be added. For example, in a smaller patient it may be unnecessary to use coils 102b and it may even be unnecessary to use coils 102a, whereas these coils may be needed when scanning a larger patient. The transmit and / or receive coils 102 may be integrated in the system. For example coils 102b could be connected to the dashed extension part. Also the coils could be detachably attachable, which could allow more flexibility as to where to position the coils.

In embodiment B of figure 1 , also the distance between the first side part and the second side part is adjustable. Depending on this distance one could choose whether or not to use coil 102c. Embodiment B also comprises coil detection means 130 for detecting coil presence and coil position. The coil detection means could for example be a sensor, or a camera. Also the coil position could be derived from coil sensitivity measurements. The detected position can be provided to a processor 160, which is configured for carrying out the steps of a method of optimizing the homogeneity of the RF transmit field and / or receive field generated by the coils covering a volume of interest of the patient to be scanned based on the coil positions detected by the coil position detection means. The processor is configured for carrying out those steps based on machine executable instructions stored on a memory 170. For optimization of the homogeneity of RF-fields the technique of RF- shimming is applied. RF shimming is known in the art, for example from Hoult DI, Phil D (2000) Sensitivity and power deposition in a high- field imaging experiment. J Magn Reson Imaging 12:46-67 or Ibrahim TS, Lee R, Baertlein BA, Abduljalil AM, Zhu H, Robitaille PM (2001) Effect of RF coil excitation on field inhomogeneity at ultra high fields: a field optimized TEM resonator. Magn Reson Imaging 19: 1339-1347. For the transmit case amplitudes and phases of the transmitted RF-signal per channel are modulated such, that an optimized excitation e.g. across the selected FOV is guaranteed. For receive, the same coil settings can be applied due to the reciprocity principle. The optimized settings can be generated by acquiring RF field maps in a suitable way and by calculating the RF-parameters per channel. Furthermore model data from RF-simulations can be added to the optimization process (e.g. for meeting local and global specific absorption rate (SAR)-limits). If sufficient homogeneity of the transmit and / or receive field cannot be achieved by the coil number used and the coil positions used the user is notified.

The embodiments shown in figure 1 further comprise means to lead wires 150 away from a direction in which the coil bridge is adjustable. For instance, this can be achieved by clamps 152 or mounts attached at the coil bridge which can employed to keep the coil wires 150 away from the patient and avoid the wires to get stuck when the coil bridge is adapted to a patient. The coil bridge can also comprise one or more cable ducts 154, for example in the extension parts that already include wires and to which the coils can be connected if necessary.

Figure 2 diagrammatically shows a computer program product according to the invention. The computer program product comprises program code means for causing a computer to carry out the steps of a method of:

determining the size of the patient based on an image of the patient and determining one or more favorable coil positions and / or coil number to be used given the determined patient size based on predetermined information about a relation between patient size, coils used, coil position and image quality. Preferably in this step a favorable distance between the upper part and the patient table and / or the distance between the first side part and / or the second side part is determined as well 201 and

providing the determined one or more favorable coil positions and / or coil number to a user 202.

Figure 3 illustrates diagrammatically a magnetic resonance guided

radiotherapy system in which the invention is used. The magnetic resonance imaging system comprises a main magnet 10 which generates a steady homogeneous main magnetic field within the examination zone 14. This main magnetic field causes a partial orientation of the spins in the patient to be examined along the field lines of the main magnetic field. An RF system 12 is provided with one or more RF antennae to emit an RF excitation

electromagnetic field into the examination zone 14 to excite spins in the body of the patient to be examined. The relaxing spins emit magnetic resonance signals in the RF range which are picked up by the RF antennae, notably in the form of RF receiving coils 12. The RF system may be coupled to a Tx/Rx switch (TRSwitch) 11, which in turn is coupled to an RF amplifier (RFamp) 13. Further, gradient coils 16 are provided to generate temporary magnetic gradient fields, notably read gradient pulses and phase encoding gradients. These gradient fields usually are orientated in mutual orthogonal directions and impose spatial encoding on the magnetic resonance signals. Gradient amplifiers 18 (GradAmp) are provided to activate the gradient coils to generate the magnetic gradient encoding fields. The magnetic resonance signals picked up by the RF receiver antennae 12 are applied to an MRI data acquisition system (MRacq) 19. The MRI data acquisition system 19 provides the data to a host computer (HC) 20, which in turn provides it to a reconstructor (Recon) 22, which may reconstruct an image from the data. These data may be displayed on a display (Disp) 17. The radiotherapy system (RT) 32 includes a housing 30 or other support or body supporting a radiation source arranged to move or revolve around the subject. The radiotherapy system 32 may contain a multi-leaf collimator (MLC). The combination of the multi-leaf collimator with the motion of the radiation source around the subject allows the delivery of complex dose distributions by means of for example arc therapy or intensity modulated radiation therapy. The magnetic resonance imaging system is used to determine patient position and / or patient movement. This information can be used for example for (re) positioning the patient, adapting the radiotherapy plan or (temporarily) stop the treatment.

The magnetic resonance guided radiotherapy system comprises a camera 305, which is configured to take at least one image of the patient. This image could for example be taken just after the patient is positioned on the patient table. This image could be used by the computer program product as shown in figure 2 to determine the size of the patient and further determining one or more favorable coil positions and / or coil number to be used given the determined patient size based on predetermined information about a relation between patient size, coils used, coil position and image quality. The computer program product could for example be executed by the host computer of the magnetic resonance guided radiotherapy system. The magnetic resonance guided radiotherapy system further comprises a display 27 configured for displaying the determined one or more favorable coil positions and / or coil number to the user. The use of the computer program product may lead to better quality MRI images. This in turn may lead to more accurate radiotherapy dose delivery.

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

Claims

CLAIMS:

1. A coil system for use in magnetic resonance imaging (MRI) system, wherein the coil system is configured to be positioned above a patient and patient table when in use, wherein the coil system comprises:

multiple transmit/and or receive coils connectable to the coil bridge;

wherein the coil system is configured such that a number of coils used during MRI scanning is adjustable such that an image volume covered by the coils used can be optimized to patients of different sizes.

wherein the coil bridge is configured such that a distance between the upper part and the patient table and / or a distance between the first side part and second side part are adjustable when in use such that the coil system can be optimized to patients of different sizes.

2. A coil system as claimed in any of claims 1 , further comprising:

a camera configured for taking at least one image of the patient and a processor and;

- a memory for storing machine executable for execution by the processor, wherein execution of the machine executable instructions causes the processor to perform an image analysis on the image in order to determine the size of the patient and further determining one or more favorable coil positions and / or coil number to be used given the determined patient size based on predetermined information about a relation between patient size, coils used, coil position and image quality and

a display configured for displaying the determined one or more favorable coil positions and or coil number to be used to a user.

3. A coil system as claimed in any of the preceding claims, wherein at least part of the transmit and / or receive coils is integrated in the first and / or second part and / or the upper part and wherein the at least part of the transmit and / or receive coils can be activated by means of switches.

4. A coil system as claimed in any of the preceding claims, wherein at least part of the transmit and / or receive coils is detachably attachable to the first and / or second part and / or upper part.

5. A coil system as claimed in claim 4 further comprising coil detection means configured to detect coil presence and / or coil position at the first side part and / or at the second side part and / or at the upper part.

6. A coil system as claimed in claim 5, further comprising a processor and a memory for storing machine executable for execution by the processor, wherein execution of the machine executable instructions causes the processor to carry out the steps of a method of optimizing the homogeneity of the RF transmit field and / or receive field generated by the coils covering a volume of interest of an object to be scanned based on the coil positions detected by the coil position detection means.

7. A coil system as claimed in any of the preceding claims, wherein coils comprise wires

and wherein the coil system comprises means to lead the wires away from a direction in which

the coil bridge is adjustable.

8. A coil bridge configured to be used in a coil system according to any of claims 1 -7.

9. Computer program product comprising program code means for causing a computer to carry out the steps of a method of:

providing the determined one or more favorable coil positions and / or coil number to be used to a user.

10. A magnetic resonance imaging system comprising the coil bridge as claimed in any of the preceding claims.

1 1. A magnetic resonance imaging system as claimed in claim 10, further comprising a radiation delivery system