Abstract:
A magnetic resonance transmit and/or receive antenna system configured for being used in combination with a magnetic resonance radiotherapy system. The antenna system can include at least one antenna for transmitting and/or receiving radio frequency signals and a cover enclosing the antenna components. The antenna can include antenna components and the cover can include a spatially varying thickness and/or density towards an outer edge of the surface and/or next to an antenna component as to make the change in radiation attenuation between the enclosing cover compared to the antenna component and/or air more gradual.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a device and computer program product in the field of magnetic resonance imaging. More particularly the inventions finds its application in magnetic resonance imaging guided radiotherapy. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventional radiotherapy is based on CT and cone beam CT imaging for tumor and organ at risk delineation and for treatment guidance. Because of the superior soft tissue contrast of magnetic resonance imaging (MRI), MRI based radiotherapy is becoming increasingly popular. In order to achieve the best possible balance between tumor control and normal tissue complications, a radiation dose should be high in the tumor and as low as possible in the normal tissues. Furthermore, radiation dose delivery needs to be accurate and predictable, while at the same time maintaining sufficient MR image quality. 
       SUMMARY OF THE INVENTION 
       [0003]    In general, embodiments of the disclosure are directed to address at least one of the above mentioned issues related to radiotherapy, while still achieving sufficient MR image quality. 
         [0004]    One aspect of the disclosure is directed to a magnetic resonance transmit and/or receive antenna system configured for being used in combination with a magnetic resonance radiotherapy system. The antenna system comprises at least one antenna for transmitting and/or receiving radiofrequency signals, wherein the antenna comprises antenna components. The antenna system further comprises a cover enclosing the antenna components, wherein the enclosing cover has a spatially varying thickness and/or density towards an outer edge of the surface and/or next to an antenna component as to compensate for differences in radiation attenuation between the enclosing cover compared to the antenna component and/or air. 
         [0005]    Another aspect of the disclosure is directed to a radiotherapy treatment planning computer program product configured for planning a magnetic resonance guided radiotherapy. The computer program product includes a planning module configured for calculating settings for a radiotherapy system such that when these settings are used during radiotherapy a delivered dose distribution resembles a planned fluence distribution within a predetermined range. Further, the computer program product includes an antenna system information module comprising information regarding the position and/or spatial variations in thickness and/or density of the antenna system. The planning module is configured for taking a position or spatial variations in thickness and/or spatial variation in density of a transmit or receive antenna system into account and calculating the settings to compensate for differences in radiation attenuation between an enclosing cover and radiation coefficients of air or an antenna component. The computer program product may be configured to be used in combination with any of the herein disclosed antenna systems. 
         [0006]    During MRI guided radiotherapy transmit and/or receive antenna systems may be placed on or close to a patient to be treated. This positioning of the antenna close to the patient improves the image quality. However, by doing so in an MR guided radiotherapy setting, the antenna system itself is placed directly in the radiation beam path and is exposed to high radiation fluence. Part of the radiation beam may travel through the antenna system, whereas another part of the radiation beam may not travel through the antenna system. Due to differences in radiation attenuation coefficient between air and the antenna system, the antenna system will attenuate the beam more than the surrounding air. Therefore, a presence of the antenna system in the radiation beam may cause sudden changes in a dose profile measured behind the antenna system inside the patient. 
         [0007]    The antenna system comprises at least one antenna. The antenna comprises antenna components such as conductors. The antenna preferably is a coil. The antenna system further comprises a cover enclosing antenna components. Also the radiation attenuation coefficients, and thereby the resulting radiation attenuation, between the enclosing cover and the antenna and its components may differ. Therefore, also the presence of antenna components may cause sudden changes in a dose profile measured behind the antenna system inside the patient. These effects may decrease the accuracy of the dose delivered during radiotherapy, and may make the actual dose delivered more dependent on an exact positioning of the antenna system. These effects can be compensated for by varying the thickness and/or density of the enclosing cover towards an outer edge of a surface formed by the enclosing cover and varying the thickness and/or density of the enclosing cover next to an antenna component. 
         [0008]    According to an embodiment of the disclosure, the thickness and/or density of the enclosing cover decreases in a direction from a center part of the enclosing cover towards an outer edge. The change in attenuation, when the beam crosses the border between air and an edge of the antenna system, can cause a sudden change in fluence profile, resulting in an undesired dose delivery. A decreasing thickness and/or density of the enclosing cover in a direction from the center part towards the edge reduces the undesirable sudden change in fluence profile. In an embodiment, the decrease in thickness and/or density of the enclosing cover is preferably gradual. This makes a delivered dose distribution less sensitive to errors in the placement of the antenna system, which in turn makes radiation dose delivery more accurate and predictable. 
         [0009]    According to an embodiment of the disclosure, the thickness and/or density of the enclosing cover increases substantially near and in a direction towards an antenna component. For example, antenna components likely have a higher radiation attenuation coefficient than the surrounding enclosing cover. Therefore, the presence of antenna components such as conductors may cause sudden density peaks in the antenna system, which in turn may result in a steep increase in radiation attenuation. By increasing the thickness and/or density of the enclosing cover substantially near and in a direction towards the antenna component, the sudden density peak caused by the antenna component can be compensated for. Therefore, the sudden and strong increase in radiation attenuation caused by the antenna system can be made more gradual. 
         [0010]    If for example a radiation beam passing through the antenna system is adjusted to compensate for attenuation of the coil by the radiotherapy treatment planning computer program, the transition between air and the enclosing cover causes less of an undesired behavior in delivered dose. Combining this adjustment of the radiation beam to compensate for attenuation of the antenna system with an antenna system having a decreasing thickness and/or density of the enclosing cover in a direction from the center part towards the edge may further reduce such undesired behaviors. In a similar way, the effects of radiation attenuation caused by the presence of one or more antenna components can be compensated for. 
         [0011]    Additional objects and advantages of the present disclosure will be set forth in part in the following detailed description, and in part will be obvious from the description, or may be learned by practice of the present disclosure. The objects and advantages of the present disclosure will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
         [0012]    It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention, as claimed. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The accompanying drawings, which constitute a part of this specification, illustrate several embodiments and, together with the description, serve to explain the disclosed principles. 
           [0014]      FIG. 1  diagrammatically shows an antenna system for use in magnetic resonance imaging. 
           [0015]      FIG. 2  diagrammatically exemplary shows a transmit and/or a receive antenna system being used during radiotherapy. 
           [0016]      FIGS. 3A and 3B  diagrammatically show the effects of positioning of the antenna system into the radiation beam. 
           [0017]      FIGS. 4A and 4B  diagrammatically show the effects of an antenna system with increasing thickness and density on radiation attenuation for different situations. 
           [0018]      FIGS. 5A and 5B  diagrammatically show the effects of changing the position of an antenna system having an increasing thickness and density on radiation attenuation. 
           [0019]      FIG. 6  diagrammatically shows a transmit and/or receive antenna system with increasing thickness and density of the enclosing cover near and towards an antenna component. 
           [0020]      FIG. 7  diagrammatically shows a radiotherapy treatment planning computer program product configured for planning a magnetic resonance guided radiotherapy. 
           [0021]      FIG. 8  diagrammatically shows an embodiment of a MRI-linear accelerator used with the radiotherapy treatment planning computer program product of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0022]    Exemplary embodiments are described with reference to the accompanying drawings. Wherever convenient, the same reference numbers are used throughout the drawings to refer to the same or like parts. While examples and features of disclosed principles are described herein, modifications, adaptations, and other implementations are possible without departing from the spirit and scope of the disclosed embodiments. Also, the words “comprising,” “having,” “containing,” and “including,” and other similar forms are intended to be equivalent in meaning and be open ended in that an item or items following any one of these words is not meant to be an exhaustive listing of such item or items, or meant to be limited to only the listed item or items. And the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. 
         [0023]      FIG. 1 a    diagrammatically shows antenna system  10  for use in magnetic resonance imaging. The antenna system  10  comprises an antenna  12  and an enclosing cover  14  for protecting and/or supporting the antenna  12 . The antenna system  10  further comprises electronics  16  and a cable  18 , e.g. for powering the antenna system. The enclosing cover  14  as shown in  FIG. 1 b   , comprises a center part  19  and four outer edges  13 . 
         [0024]      FIG. 2  diagrammatically shows how a transmit and/or a receive antenna system  10  could be used during radiotherapy. The antenna system  10  is positioned within the bore of an MRI radiotherapy system on an upper or under side of a patient (not shown). An exemplary MRI radiotherapy system is shown in  FIG. 8 . The radiotherapy system  23  is in this case a linear accelerator, but could for example, also be any other x-ray or gamma ray emitting system. The linear accelerator generates a beam of x-rays  41 , which travels through the antenna system  10  to the patient positioned below the antenna system  10  at position X. The receive coils of the MRI system are placed close to the treated and imaged anatomy to maximize image quality and enable the MRI radiotherapy system to provide efficient MR guidance for the radiation beam. As a consequence the receive coils are located in the radiation beam path, which results in the coils attenuating the beam and also cause non-idealities in the radiation therapy that have to be taken into account during the delivery of the radiation treatment. 
         [0025]      FIG. 3  diagrammatically shows the effects of positioning of the antenna system  10  into the radiation beam. The antenna system  10  is positioned in an x-ray beam  41  as shown in  FIG. 2 . Returning to  FIG. 3 , for example, a constant desired fluence may be desired by the radiation therapy treatment planning system (See  FIG. 7 ). When creating a treatment plan, an assumption is made that the antenna system  10  may be located at a known location during treatment delivery. In practice, when the antenna is placed into the radiotherapy system  23  at treatment delivery time, it may be located several millimeters away from the known location assumed by the treatment plan. Because of this change in location, the treatment planning system is unable to correct for the antenna&#39;s “true” location at treatment delivery time in order to provide the correct level of radiation fluence. 
         [0026]    As shown in  FIG. 3A , the presence of the antenna system  10 , affects the desired fluence  430 . One end of the antenna system  10  is positioned at a first position “A” corresponding to line position I. A corresponding fluence profile (e.g., as known in the art, fluence is an energy density per unit area per unit time) between the antenna system  10  and the patient, when the antenna system  10  is positioned at “A” is illustrated by line  43   a  . The presence of the antenna system  10  causes a dip in the fluence profile at location I, as shown by line  43   a  . The sudden change in the fluence profile at location I (e.g., a step in attenuation profile) is unfavorable.  FIG. 3B  illustrates the movement of the antenna system  10  from the first position “A” (assumed by the treatment planning system) to a second position “B” (e.g.., actual position at treatment time). The change in position of the antenna system  10  results in a change in the fluence profile at location II, as shown by line  43   b  . The presence of the antenna system  10  causes an undesired attenuation of the radiation beam  41 , which needs to be mitigated. The fluence profile is highly dependent on the exact position of the antenna system  10  (e.g., receive coils) that is located in the radiation beam path  41 . As illustrated by the error curve  440 , there is a sudden dip  44 , corresponding to the attenuation of the antenna system  10 , in the fluence that is provided. The attenuation causes non-idealities in the radiation fluence when the “true” location of the coil (e.g., the coil&#39;s location at treatment delivery time), is not known to the radiation therapy planning system. 
         [0027]      FIG. 4  diagrammatically shows the effects of an antenna system  10  with increasing thickness and density on radiation attenuation for different positions of the antenna system  10  in the radiation beam  41 . Similar to  FIG. 3 ,  FIG. 4  exemplary illustrates one embodiment where the fluence desired by the radiation planning system is constant  430 . The error curve  440  from  FIG. 3  is repeatedly illustrated in  FIG. 4  for convenience. What is desired is for the antenna system to decrease the magnitude of the error on the radiation beam  41 . 
         [0028]      FIG. 4 a    shows the antenna system  10  according to an embodiment of the disclosure, where the thickness of the enclosing cover gradually and monotonously decreases in a direction from a center part  19  towards an outer edge  32 . In an embodiment, the enclosing cover has a tapered edge (e.g., the tapering may be on the inside (bottom), outside (top), or both). In a preferred embodiment the taper may be on the top (e.g., outside). As a result, there is still attenuation caused by the antenna system  10  at the location of the coil (see fluence profile  43   c  ). But, because of the varying thickness of the enclosing cover, the degree of attenuation occurs more gradually. Minimizing the undesired behavior due to the antenna system&#39;s actual location at treatment delivery time  10  in the beam path  41  is achieved by making the depth of the Desired Error curve  441  shallower than the depth of the Undesired Error curve  440 . This is achieved by utilizing an antenna system  10  having a change in thickness (or density). 
         [0029]    As discussed above, the antenna system  10  typically has some movement from the assumed first position “A” (e.g., the position assumed by the treatment planning system) to a second position “B” (e.g., the actual position at treatment delivery time), shown in  FIG. 4 b   . Therefore, the fluence profile changes, as shown by lines  43   c  and  43   d  . The advantage, as shown by the desired error curve  441 , is that any attenuation caused by the antenna system is spread out and exhibits a shallower depth. 
         [0030]      FIG. 5  diagrammatically shows in an embodiment an effect to the fluence when the position of the tapered section of the antenna system  10  assumed by the treatment planning system does not overlap with the tapered section of the antenna system when the antenna system is placed at treatment delivery time. As previously illustrated in  FIGS. 3 and 4  and shown for convenience in  FIG. 5 , the desired fluence  430  is constant and the undesired error  440  is a dip in the fluence. Further,  FIG. 5A  illustrates the antenna system  10  positioned at a position “A” assumed by the treatment planning system, where the antenna system  10  has thickness of the enclosing cover gradually decreasing in a direction from a center part  19  towards an outer edge  32 . The resulting fluence is illustrated by curve  43   e  . As shown by curve  43   e  , the fluence drops from point X to a point Y. This drop in fluence has a depth corresponding to the attenuation of the antenna system  10 . 
         [0031]      FIG. 5B  diagrammatically shows an embodiment where the antenna system  10  can be moved far enough to make the position of the tapered section of the antenna system assumed by the treatment planning system not overlap with the tapered section of the antenna system when the antenna system is placed at treatment delivery time. Curve  43   f  illustrates a corresponding fluence. In order to make the Desired Error curve  441  shallower, the length of the tapered section of the antenna system must be greater than or equal to the expected error between the assumed and the actual antenna system locations. 
         [0032]    The effect of any imprecision with regard to the mechanical placement of the antenna system  10  (e.g., 3-5 cm) may be minimized by utilizing the antenna system with a change in thickness or density (e.g., or tapering). In an embodiment, the longer the taper section, the shallower the depth of the error curve  441 . 
         [0033]      FIG. 6  diagrammatically shows a transmit and/or receive antenna system with increasing thickness and density near and towards an antenna component.  FIG. 5 a    shows an antenna system comprising high density conductor material in the form of a PCB trace  51 , and  FIG. 5 b    shows an antenna system comprising high density conductor material in the form of a wire  52 . Because both antenna components (e.g., the PCB trace  51  and the high density wire  52 ) have a higher radiation attenuation coefficient compared to the enclosing cover  14 , a difference between the delivered fluence and the desired fluence planned by the treatment planning system occurs due to the fact that the antenna components locations at treatment delivery time are different from where the treatment planning system assumed they are. Minimizing the effect can be performed by either an increase in the thickness  53  of the enclosing cover substantially near and in a direction towards the antenna component or an increase in density  54  of the enclosing cover substantially near and in a direction towards the antenna component. 
         [0034]      FIG. 7  diagrammatically shows a radiotherapy planning computer program product configured for planning a magnetic resonance guided radiotherapy. The radiotherapy planning computer program product  60  comprises a planning module  61  configured for calculating settings for a radiotherapy system, such that, when these settings are used during radiotherapy a delivered fluence distribution resembles a planned fluence distribution within a predetermined range. The planning module  61  receives information from an antenna system information module  62  regarding the position and/or spatial variations in thickness and/or density of the antenna system. The planning module  61  is configured for taking into account a position and/or spatial variations in thickness and/or density of the antenna system and calculating the settings such as to compensate for differences in radiation attenuation between the enclosing cover and air and/or an antenna component (e.g., at the boundary between air and the edge of the coil, there is a sudden step in attenuation causing inconsistent fluence delivery). For example the calculated settings could be such that a fluence gradient will be placed near the outer edge of the antenna system during treatment. 
         [0035]      FIG. 8  illustrates an embodiment of a radiotherapy device  23 , an MRI-linear accelerator (MRI  85 ) that works with a radiotherapy planning computer program product of  FIG. 7 . In an embodiment by using a linear accelerator  81 , a patient may be positioned on a patient table  82  to receive the radiation fluence determined by the treatment plan. Linear accelerator may include a radiation head  84  that generates a radiation beam. The entire radiation head may be rotatable around a horizontal axis. The intersection of the axis with the center of the beam, produced by the radiation head, is usually referred to as the “isocenter”. The patient table  82  may be motorized so that the patient can be positioned with the tumor site at or close to the isocenter. The radiation head  84  may be mounted on a rotating gantry  83 , to provide patient with a plurality of varying dosages of radiation according to the treatment plan. 
         [0036]    While 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. Moreover, it will be apparent to those skilled in the art to consider the specification and the practice of the present disclosure that various modifications and variations can be made to the disclosed systems, products and methods without departing from the scope of the disclosure, as claimed. Thus, it is intended that the specification and examples be considered as exemplary only, with a true scope of the present disclosure being indicated by that following claims and their equivalents.