Abstract:
A magnetic resonance (MR)-radiotherapy (RT) hybrid system for treating a patient is disclosed. The MR-RT hybrid system comprises: an MR imaging (MRI) apparatus comprising bi-planar magnets configured to generate a magnetic field; a radiation source configured to supply a radiation beam to treat the patient; a gantry configured to couple the MR apparatus at a first end and the radiation source so that they can rotate in unison; a treatment support configured to support the patient; a motor configured to move the treatment support; and a controller. The controller comprises a processor and memory having stored thereon instructions, which when executed by the processor, cause the motor to move the treatment support in order to avoid collision between the MRI apparatus and the patient when the MRI apparatus is rotated. A method for positioning the treatment support within the MR-RT hybrid system is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present invention relates generally to hybrid Magnetic Resonance Imaging-Radiotherapy system and specifically to an apparatus and method for peripheral tumour treatment in such a system. This application claims priority to U.S. Provisional Application No. 62/114,493 filed Feb. 10, 2015. 
     
    
     BACKGROUND 
       [0002]    Most modern radiotherapy (RT) treatments are delivered “isocentrically”, where a target volume in a patient is placed at an isocentre of the radiotherapy apparatus. The target volume can then be irradiated from multiple gantry angles without needing to move the patient in order to realign the target volume to the beam axis. The isocentre is often an intersection of a gantry axis and a beam axis of the radiotherapy apparatus. An example of a radiotherapy apparatus is a linear accelerator. 
         [0003]    More recently, hybrid magnetic resonance (MR)-RT systems have been used to provide MR guided RT treatments. For example, the systems by ViewRay® and Elekta AB both provide MR guided radiotherapy systems. However for these systems it is difficult, if not impossible, to position the patient so that a peripherally located tumour, such as a breast or lung tumour for example, is at the isocentre without coming into contact with the magnet. This, in turn, reduces the gantry angles from which the tumour may be irradiated thereby inhibiting the effectiveness of the treatment. Accordingly, it is an object of the present invention to obviate or mitigate this disadvantage. 
       SUMMARY 
       [0004]    In accordance with an aspect of an embodiment, there is provided a magnetic resonance (MR)-radiotherapy (RT) hybrid system for treating a patient, the MR-RT hybrid system comprising: an MR imaging (MRI) apparatus comprising bi-planar magnets configured to generate a magnetic field; a radiation source configured to supply a radiation beam to treat the patient; a gantry configured to couple the MR apparatus and the radiation source so that they can rotate in unison; a treatment support configured to support the patient; a motor configured to move the treatment support; and a controller comprising: a processor; and memory having stored thereon instructions, which when executed by the processor, cause the motor to move the treatment support in order to avoid collision between the MRI apparatus and the patient when the MRI apparatus is rotated. 
         [0005]    In accordance with another aspect of an embodiment, there is provided a method for positioning a treatment support upon which a patient is positioned within an MR-RT hybrid system, the method comprising: positioning the treatment support at a central location; the central location defined to avoid collision between the patient and the MR-RT hybrid system; rotating a gantry of the MR-RT hybrid system to a gantry angle; moving the treatment support to a treatment position; applying a treatment beam; and moving the treatment support to avoid collision between the MR-RT hybrid system and the patient when the gantry is rotated to a different gantry angle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Embodiments of the invention will now be described by way of example only with reference to the following drawings in which: 
           [0007]      FIG. 1  is a block diagram of a conventional MR-RT hybrid system; 
           [0008]      FIG. 2  is a block diagram of an MR-RT hybrid system in accordance with an embodiment of the present invention; 
           [0009]      FIG. 3 a    is a flow chart illustrating operation of the MR-RT hybrid system; 
           [0010]      FIG. 3 b    is a flow chart illustrating pre-treatment processing; 
           [0011]      FIG. 4  is a block illustrating the MR-RT hybrid system of  FIG. 2  at a different gantry angle; and 
           [0012]      FIG. 5  is is a block diagram of an MR-RT hybrid system in accordance with an alternative embodiment of the present invention;. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    For convenience, like numerals in the description refer to like structures in the drawings. Referring to  FIG. 1 , a block diagram of a cross-section portion of an MR-RT hybrid system is illustrated generally by numeral  100 . The cross-section portion  100  illustrates an image slice of a patient  102  within a magnetic resonance imaging (MRI) apparatus  104 . A target volume  106  is peripherally located within the patient  102 . In an embodiment, the target volume  106  is a tumour. A radiation beam  110  for treating the tumour  106  passes through the centre of the MRI apparatus  104 . Thus, the MR-RT hybrid system  100  has a centrally located isocentre  112 . The MRI apparatus  104  used for the MR-RT hybrid system  100  is typically cylindrical and has a bore of approximately  60  cm. An MR-RT hybrid system  100  using biplanar magnets for the MRI apparatus  104  has a similar pole to pole spacing. For patients that are between  50  and  55  cm wide, of which there are many, there is very little room to laterally move the patient  102 . Thus, it can be difficult, if not impossible to align the tumour  106  to the isocentre  112  of the MR-RT hybrid system. 
         [0014]    In order to allow the MR-RT hybrid system  100  to effectively treat the tumour  106 , it is preferable to align the tumour  106  with the radiation beam  110  at all gantry angles. Such an alignment is straightforward for centrally located tumours, but for peripheral tumors, such as breast tumours and lung tumours for example, this would only be possible for the smallest patients. However, the cost to build a magnet with a larger bore or pole-to-pole spacing becomes prohibitively expensive. 
         [0015]    Accordingly, a peripheral tumour treatment positioning (PTTP) system and method are described herein. The PTTP system and method allow peripheral tumours in larger patients to be placed at, or proximal to, the isocentre  112  of the MR-RT hybrid system  100  without needing a larger bore or larger pole-to-pole spacing. Thus, the PTTP system and method facilitate treating large patients with peripheral tumours in the MR-RT hybrid system  100 . 
         [0016]    Referring to  FIG. 2 , a block diagram of an MRI-RT hybrid system in accordance with an embodiment of the invention is illustrated generally by numeral  200 . The MRI-RT hybrid system  200  includes an MRI apparatus  202 , a rotating gantry (not shown), a radiation source  206 , a treatment support  208 , and a controller  210 . In an embodiment, the treatment support  208  is a couch, table or the like configured to support the patient  102 . The MRI apparatus  202  is coupled to the radiation source  206  via the rotating gantry to enable them to rotate in unison about the treatment support  208 . An example of such an MRI apparatus is described in U.S. Application Publication No. 2009/0149735, titled “Integrated external beam radiotherapy and MRI system” by Fallone et al. The treatment support  208  is movable by a motor (not shown). Different motors capable of moving the treatment support  208  as described below can be used. In an embodiment, the motor is configured to move the treatment support  208  in a direction parallel to a superior-inferior axis of the patient  102  to move the patient  102  into and out of the MRI-RT hybrid system  200 . As is known in the art, the superior-inferior axis runs the length of the patient  102 . Further, the motor is configured to move the treatment support  208  substantially any direction normal to the cranial-caudal axis of the patient to position the patient  102  for treatment, as will be described below. 
         [0017]    The MRI apparatus  202  is a bi-planar MRI apparatus comprising a pair of spaced apart magnets  202   a.  The radiation source  206  is directed at the patient  102  either parallel or antiparallel to the direction of the main magnetic field of the MRI apparatus  202  through a hole  201  in the centre of one of the magnets  202   a.  In the MR-RT hybrid system  200  shown in  FIG. 2 , the patient  102  is  50  cm wide and the bi-planar magnets have a  60  cm pole to pole spacing. 
         [0018]    The bi-planar, space apart, configuration of the magnets  202   a  allows each magnet  202   a  to be individually connected to the gantry at a first end only. Such a configuration allows unrestricted lateral motion of the patient  106  in a direction  212  parallel to a face of the magnets  202   a,  and perpendicular to the radiation beam  110 . Such motion is limited in current cylindrical magnets. The bi-planar configuration of the magnets  202   a  also allows some motion of the patient  102  in a direction  214  parallel to the radiation beam  110 , and perpendicular to the face of the magnets  202   a.    
         [0019]    The controller  210  is a computing device that is configured to control the motion of the treatment support  208 . The controller  210  is programmed to position the MRI apparatus  202 , the radiation source  206 , and the patient  102  so that the target volume  106  is as close to the isocentre of the radiation beam  110  as possible. 
         [0020]    Prior to treating the patient using the MR-RT hybrid system  200 , a patient centre is determined. The patient centre (x c ,y c ) can be calculated based on an analysis of the contours taken during a simulation process. The analysis determines the patient centre (x c ,y c ) such that a distance from the central point to the skin surface is less than the bore diameter or pole-to-pole spacing of the MRI apparatus  202  for all z positions. Although this analysis could be done from a computed tomography (CT) or MR scan as part of the simulation process, it may be inefficient or unethical, in the case of CT, to scan well above and below the treatment area just to get an external contour for this analysis. Therefore, a method of generating the patient contour from head to toe that does not require a CT or MR could also be used. Devices, such as laser contouring devices, are readily available that could do this in a quick and efficient manner. 
         [0021]    Further, a treatment plan is calculated. Specifically, a 3D position of the patient centre (x c ,y c ,z c ) is calculated using contours obtained above. Using techniques similar to conventional isocentric radiotherapy, a 3D location of a pseudo isocentre (x PI ,y PI ,z PI ), and gantry angles for each field are defined. In an embodiment, the centre of the target volume is defined as the pseudo isocentre. Based on these two points, treatment centres (x T (n),y T (n),z T (n)) are calculated for each gantry angle, where n denotes a radiation beam number. As will be appreciated, since the grantry  204  rotates the MRI apparatus  202  and the radiation source  206  about the patient  102 , different gantry angles will likely be associated with different treatment centres. For each of the different gantry angles, the machine isocentre would be relocated to the treatment centre position, and the dose would be calculated. As is well known to those knowledgeable in the art, as the machine isocentre  112  is moved from the pseudo isocentre to the treatment centre, a field size and multileaf collimator (MLC) would need to be adjusted according to divergence. This could be accomplished either manually or through a computerized calculation that adjusted each parameter accordingly. Dose distributions could be calculated and optimized through the various tools normally available in the treatment planning system. If, for any reason, any of the treatment centres needed to be modified as part of the planning process, the system could check that the modified position would be valid and would not cause any collisions. 
         [0022]    Once the treatment plan has been calculated with the different treatment centres for each radiation beam  110 , the patient  102  is ready to be treated with the MR-RT hybrid system  200 . Referring to  FIG. 3 a   , a flow chart illustrating operation of the MR-RT hybrid system  200  to position a patient for treatment is illustrated generally by number  300 . 
         [0023]    At  302 , a pre-treatment process is performed. Referring to  FIG. 3 b   , the pre-treatment process  302  is described in detail. At step  302   a,  a pre-treatment alignment of the patient  102  is performed to align the patient centre with a central location of the MRI-RT hybrid system  200 . In an embodiment, the central location is defined as a position within the MRI-RT hybrid system  200  at which the patient  102  can be placed without fear of contact with the MRI apparatus  202  when the gantry  204  rotates the MRI apparatus  202  and the radiation source  206  about the treatment support  208 . In an embodiment, the central location is the isocentre of the MRI-RT hybrid system  200 . The patient  102  is positioned at the central location by aligning the patient centre as closely with the isocentre of the MRI-RT hybrid system  200  as possible. Specifically, the patient is positioned by aligning the patient centre to a set of external lasers. Optionally, prior to moving the patient into the MR-RT hybrid system  200 , the gantry can be rotated to position the magnets  202   a  vertically. In this position there will be an opening between the magnets  202   a,  up to the ceiling. This may minimize the effect of claustrophobia as the patient  102  is moved into the bore of the MR-RT hybrid system  200 . 
         [0024]    At step  302   b,  the treatment support  208  is translated a predefined distance from the set of external lasers into the MR-RT hybrid system  200 . The predefined distance is configured to correlate the patient centre at the set of external lasers with the isocentre of the MR-RT hybrid system  200 . 
         [0025]    At step  302   c,  high quality MR images of an anatomy of interest are taken to verify that the patient centre is accurately aligned to the isocentre of the MR-RT hybrid system  200 . If the field of view (FOV) of the MR apparatus  202  is insufficient to obtain a high quality image of the entire anatomy of interest of the patient  102 , multiple images can be taken at different treatment support positions and stitched together using known computer graphics techniques. Since most people are wider laterally than they are in the anterior posterior direction, the gantry  204  is rotated to position the magnets  202   a  horizontally. This configuration allows the treatment support  208  to move laterally sufficiently to obtain a full set of images to stitch together. This configuration also allows the treatment support  208  to be moved so that the pseudo isocentre is aligned with a central axis of the radiation beam  110  and the isocentre MR-RT hybrid system  200  is vertically aligned with the pseudo isocentre. 
         [0026]    As a result of the alignment, optimal MR imaging with minimal image distortion is obtained over a central field of view (CFOV) of the MR apparatus  202 . Beyond the CFOV, image distortion increases due to gradient non-linearities and magnetic field inhomogeneity. To provide the best image guidance, image-distortion must be minimized. Therefore, vertically aligning the isocentres facilitates optimum quality pre-treatment imaging of the target volume, with the FOV approximately centred on the target volume. 
         [0027]    Stitching images obtained at the CFOV for multiple treatment support and /or gantry positions would then allow the creation of a composite image over a larger field of view with the geometric accuracy inherent to the CFOV. Those skilled in the art will recognize that this method of producing an image with minimal distortion would be valuable in the treatment simulation process as well as during pretreatment imaging. 
         [0028]    At step  302   d,  once the pre-treatment images are acquired, computer software executing on the controller  210  registers or correlates the pre-treatment images with the MR or CT images used for the treatment planning. This registration could be done using a rigid transformation or a deformable registration, as is known in the art. At step  302   e,  once the two images are registered, the computer software calculates the treatment support  208  shifts, including translations and rotations, needed to align the patient  102  to treatment planning positions. As will be appreciated by a person skilled in the art, in some embodiments the treatment support may be capable of rotating a few degrees to help align the patient  102 . Once the shifts have been calculated the treatment support could be translated and rotated by these known amounts to bring the patient centre to the machine isocentre. 
         [0029]    After the patient  102  has been aligned using to the pre-treatment image guidance procedure above, the radiation delivery phase can be initiated. At  304 , the grantry  204  rotates the MRI apparatus  202  and the radiation source  206  into a first gantry angle for treatment. The initial treatment position is for a first gantry angle, n=1. At  306 , the treatment support is translated along a trajectory that moves the patient  102  parallel to the magnets  202  so that the treatment centre (x T (1),y T (1),z T (1)) becomes aligned with the isocentre  112  along the beam axis at the first gantry angle. By following this trajectory the patient  102  should not collide with the MR-RT apparatus  202 . However, additional known collision avoidance schemes could be used to provide a fail-safe motion trajectory. 
         [0030]    Referring to  FIG. 4 , a block diagram of the MRI-RT hybrid system  200  at a gantry angle  8  is illustrated generally by numeral  400 . As shown, the patient  102  has been translated so that the target region  106  lies along an axis of the radiation beam  110 . Thus, the treatment center (x T ,y T ) is at the intersection of the line from the pseudo isocentre (x pI , y pI ) to the radiation source  206  and the line perpendicular to it that passes through the patient centre (x c ,y c ). When the treatment center is determined for each gantry angle, the treatment will be similar to an isocentric treatment, in that each radiation beam is pointed towards a common point. In the embodiment, the common point is the pseudo isocentre. However, for each angle, there will be different distances to the patent&#39;s skin surface, and from the skin surface to the pseudo isocentre. 
         [0031]    At  308 , the treatment is delivered. This can be done with MR image guidance before, during or after radiation delivery as desired. At step  310 , the treatment support is reversed along the trajectory so that the patient centre is once again aligned with the isocentre of the MR-RT hybrid system  200 . 
         [0032]    The controller returns to  304  and the the grantry  204  rotates the MRI apparatus  202  and the radiation source  206  into a subsequent position, n=2. The process  304  to  310  repeats until all n radiation beams have been delivered. At step  312 , the radiation delivery is complete and the treatment support  208  is translated to remove the patient  102  from the MR-RT hybrid system  200 . 
         [0033]    As will be appreciated, the MR-RT hybrid system  200  described above provides a controller configured to manipulate the treatment support  108  laterally, vertically and in superior-inferior directions such that a target volume  106   s  is substantially aligned to the radiation beam  110 . This may be true even for a peripherally located target volume  106 . 
         [0034]    Thus, the MRI-RT hybrid system  200  can be used in a number of different circumstance but is particularly useful when the target volume  106  cannot be positioned at or near the isocentre of the traditional radiotherapy apparatus and, as such, an isocentric treatment approach is not typically feasible. 
         [0035]    In an alternative embodiment, rather than return the treatment support  208  to the isocentre prior to each rotation of the gantry, the treatment support  208  can be retracted from the MRI apparatus  202 . In yet an alternative embodiment, a trajectory can be devised that allows a treatment support and the gantry to move concurrently. Such a trajectory would not require the patient to be moved to either the central location or to be retracted from the MRI apparatus between gantry angles. 
         [0036]    In the embodiments described above the MRI apparatus  202  comprises a spaced apart bi-planar magnets  202   a.  Depending on the size and configuration of the magnets  202   a , additional features may be necessary to provide structural support. Accordingly, referring to  FIG. 5 , an alternative embodiment of the MRI-RT hybrid system is shown generally by numeral  500 . Only a portion of the MRI-RT hybrid system  500  is illustrated for simplicity. Specifically, the MRI-RT hybrid system  500  is similar to the previous embodiment but the gantry includes a support structure  502  attached to the magnets  202   a  of the MRI apparatus  202  at an end distal to the first end. In an embodiment, the support structure is an annular flange. Since the annular flange  502  primarily provides structural support, it can have a diameter substantially larger than the pole to pole spacing of the magnets  202   a.    
         [0037]    For example, in an embodiment the pole to pole spacing is 60 cm and the diameter of the annular flange  502  is 110 cm. The diameter of 110 cm is selected based on an average patient size. As will be appreciated, the diameter of the annular flange  502  can be larger to accommodate a larger average patient size. Accordingly, although the support structure  502  is described as an annular shaped flange having a particular size, it will be appreciated that other shaped and sized flanges may also be used to provide structural support to the MRI apparatus  202 . 
         [0038]    The annular flanges  502  may inhibit motion of treatment support  208  if a portion of treatment support  208  is positioned outside of the MRI apparatus  202 . However, because the opening of the annular flange  502  is significantly larger than the pole to pole spacing, it will allow substantial motion of the patient support  208 . Further, if the entire treatment support can be positioned within the MRI apparatus  202  then the annular flange  502  may not affect motion of the treatment support  208  at all. 
         [0039]    Although preferred embodiments of the invention have been described herein, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the appended claims.