FIELD OF THE INVENTION 
     The present invention pertains to the field of ultrasonic therapy, relates to an ultrasonic therapy system guided by an imaging device and specifically, to an ultrasonic therapy system capable of reducing electromagnetic interference to the imaging device. 
     BACKGROUND OF THE INVENTION 
     The existing ultrasonic therapy system combines an imaging device with an ultrasonic therapy device, thereby a whole diagnosis and treatment plan with higher quality and higher efficiency can be provided to the patients. 
     Because an MRI apparatus can provide safe, fast and accurate imaging, as an imaging device to guide an ultrasonic therapy device, the MRI apparatus has been applied more widely in clinical diagnosis. However, the driving motors in the ultrasonic therapy device and the high-frequency generators in the ultrasonic therapy applicator as main electromagnetic interference sources may cause great electromagnetic interference to the imaging of MRI apparatus. 
     In ultrasonic therapy device, the movement positioning unit of the ultrasonic therapy applicator includes multiple driving motors to drive the focal point of the ultrasonic therapy applicator to move in three-dimensions. 
     As shown in  FIG. 1 , the motor for treatment bed  4  drives the treatment bed (not illustrated in  FIG. 1 ) and the focal point moving board  14 . It drives the treatment bed to go into or withdraw from the bore of the MRI apparatus. Focal point moving board  14  carries X, Y and Z-boards and the X, Y and Z-driving motors to approach or keep away from MRI apparatus. 
     The Z-driving motor  3  for Z-board is fixed on focal point moving board  14 . Z-board  13  is mounted on focal point moving board  14  and it can move in Z-direction relatively. 
     The Y-driving motor  2  for Y-board is fixed on Z-board  13 . Y-board  12  is mounted on Z-board  13  and it can move in Y-direction relatively to Z-board  13 . 
     The X-driving motor  1  for X-board is fixed on Y-board  12 . X-board  11  is mounted on Y-board  12  and it can move in X-direction relatively to Y-board  12 . 
     The ultrasonic therapy applicator  5  is connected to X-board  11  and moves with X-board. 
     The Z-driving motor  3  drives Z-board  13  to move in Z-direction through Z-gearing unit (not illustrated in  FIG. 1 ). Z-board drives Y-board  12 , Y-driving motor  2 , X-board  11 , X-driving motor  1  and ultrasonic therapy applicator  5  to move in Z-direction. The Y-driving motor drives Y-board to move in Y-direction through Y-gearing unit (not illustrated in  FIG. 1 ). Y-board drives X-driving motor  1 , X-board  11  and ultrasonic therapy applicator  5  to move in Y-direction. The X-driving motor  1  drives X-board  11  to move in X-direction through X-gearing unit (not illustrated in  FIG. 1 ). X-board drives ultrasonic therapy applicator  5  to move in X-direction. 
     This shows that the driving motors for X, Y and Z-boards will move correspondingly during the movement of the focal point of the ultrasonic therapy applicator  5 . Because these motors are close to MRI imaging zone, the electromagnetic interference occurred when these driving motors work and the ferromagnetic substances contained in the motors when these motors are moving will cause great interference to the MRI imaging. 
     During diagnosis and treatment, the electric control unit and the high-frequency generator for supplying power to ultrasonic therapy applicator  5  of the ultrasonic therapy system will bring great interference to MRI imaging. 
     During MRI imaging, the factors of electromagnetic interference as above mentioned will seriously influence the accuracy of MRI imaging. Therefore, the effects of diagnosis and treatment will be influenced. 
     So, for an imaging device, especially for an MRI-guided ultrasonic therapy system, to reduce the electromagnetic interference is critical to the whole therapy system. 
     SUMMARY OF THE INVENTION 
     Aiming at the disadvantages of the prior art as mentioned above, the technical problem to be solved in the present invention is to provide an ultrasonic therapy system, which can effectively reduce electromagnetic interference to an imaging device. Accordingly, an ultrasonic system with an electromagnetic compatibility between an ultrasonic therapy device and an imaging device (MRI apparatus) can be realized. 
     The technical solution for the problems proposed by the present invention is as follows: the ultrasonic therapy system capable of reducing electromagnetic interference to the imaging device comprises an imaging device, ultrasonic therapy device and electric control unit. The ultrasonic therapy device comprises an ultrasonic therapy applicator and its movement positioning unit. Said movement positioning unit, comprises driving motors to control the movement of the ultrasonic therapy applicator. Wherein, the driving motor is disposed beyond the area, in which the electromagnetic waves of the motors can bring electromagnetic interference to the imaging device. The driving motor is connected to the ultrasonic therapy applicator through a gearing unit. 
     The impetus of rotation of driving motor may be transmitted to a gearing unit through a long shaft and then it is transmitted to the ultrasonic therapy applicator from the gearing unit. Thus, the driving motor can be arranged far away from the imaging area of the MRI apparatus so that the electromagnetic interference to the imaging device can be avoided. 
     The driving motors are disposed beyond the place, which is 2 meters away from the central point of imaging area of the imaging device. 
     Said movement positioning unit may comprise X-board and the ultrasonic therapy applicator is placed on the X-board. The driving motor is connected to X-board through X-gearing unit and said driving motor is the X-driving motor to drive X-board to move in X-direction. 
     Said movement positioning unit may further comprise Y-board and the Y-driving motor, which is used to drive Y-board to move in Y-direction. Said X-board is placed on the Y-board. The Y-driving motor is placed 2 meters away from the central point of imaging area of the imaging device and it is connected to Y-board through Y-gearing unit. 
     Said movement positioning unit may further comprise Z-board and the Z-driving motor, which is used to drive Z-board to move in Z-direction. Said Y-board is placed on the Z-board. The Z-driving motor is placed 2 meters away from the central point of imaging area of the imaging device and it is connected to Z-board through Z-gearing unit. 
     In order to make ultrasonic therapy applicator enter into the bore of the imaging device easily, Z-board can be placed on focal point moving board and a motor for treatment bed is used to control the movement of the focal point moving board. 
     All driving motors as mentioned above adopt the servo motors. 
     In the present invention, each driving motor is placed far away from the imaging area of the imaging device and the impetus is transmitted to the ultrasonic therapy applicator by each gearing unit. During the position adjusting of focal point of ultrasonic therapy applicator, the positions of all driving motors stay still. Since the driving motors are far away from the imaging area of the imaging device, the electromagnetic interference to the imaging device can be minimized to the utmost. 
     In the present invention, the imaging device mainly adopts MRI apparatus; certainly, other imaging devices can also be used in the present invention. 
     In ultrasonic therapy system, the electric control unit under working status produces electromagnetic interference, which will influence the diagnosis accuracy of imaging device. To improve the diagnosis accuracy of MRI, it is very important to reduce the electromagnetic interference produced by electric control unit in the working MRI apparatus. Therefore, the preferred electric control unit may further comprise an electric control unit for controlling the movement of treatment bed. The electric control unit for controlling the movement of treatment bed provides a position sensor, which will actuate de-energizing the electric control unit for controlling the movement of treatment bed after the treatment bed enters into the imaging area of the imaging device. After the treatment bed of ultrasonic therapy device approaches MRI apparatus, the treatment bed needs to further move into the bore of MRI apparatus for diagnosis and ultrasonic treatment. If the electric control unit for controlling the treatment bed to move into and withdraw from the bore of MRI apparatus is de-energized during the diagnosis and treatment, the electromagnetic interference to MRI apparatus produced by electric control unit under electrifying status can be reduced effectively. When the treatment bed withdraws from the bore of MRI apparatus, the electric control unit can be re-energized. 
     More preferably, the electric control unit further comprises multiple low-pass filters with a high attenuation rate. The filters can filter the electromagnetic waves of the power source of high-frequency generator, electric components, controller for movement system, servo driver and the power source of driving motor respectively so that the electromagnetic interference with the frequency around the working frequency of MRI apparatus can be reduced. 
     The filters in the present invention mainly filter the electromagnetic waves of more than 10 MHz produced by ultrasonic therapy device. The attenuation rate of the filter ranges from 80 dB to 120 dB. 
     For main interference sources (for example, the power source of high-frequency generator, driving motors), shielding measures have been taken. The present invention also applies cutoff wave-guide technique to the place (for example, holes of output shaft of driving motor, the place where the control signal lines entering and exiting the shielding cover) which can not be shielded due to all kinds of reasons. The cutoff wave-guide pipe can adopt simple tubular metal structure and present electric characteristics of high-pass filters. The cutoff wave-guide pipe allows the signals with frequency higher than cutoff frequency to pass and the signals with frequency lower than cutoff frequency will be blocked or attenuated. By use of this feature, the cutoff frequency of wave-guide pipe is so designed that the frequency of interference signals falls into the cutoff area of the wave-guide pipe, thus the interference signals can not go through the wave-guide pipe, in other words, the wave-guide pipe acts as electromagnetic shielding. The cutoff wave-guide pipe of the present invention can filter the interference electromagnetic waves with frequency lower than 100 MHz and its attenuation rate ranges from 80 dB to 120 dB. 
     The outer shell of each driving motor may be covered by a motor shielding cover, which can shield the electromagnetic waves. The cutoff wave-guide, which can bring attenuation of electromagnetic waves, is arranged on the output shaft of each driving motor, and it is connected to the motor shielding cover and earthed. The anode and cathode of each driving motor are respectively connected to filters. 
     Besides, the driving motors and ultrasonic therapy applicator can be placed in the shielding room for shielding the electromagnetic waves where the imaging device is also located in. Firstly, the anode and cathode of each driving motor are respectively connected to the filter located in the shielding room. Then, said filter is connected to another filter located outside of shielding room. The ultrasonic therapy applicator is connected to the high-frequency generator through a filter located outside of shielding room. The motor shielding covers and the outer shell of driving motors are respectively connected to the shielding room by way of single-point earthing. The shielding room is earthed so that the electromagnetic interference can be reduced further. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a structural diagram of movement mechanism in the ultrasonic therapy system of prior art. 
         FIG. 2  is a layout of motors used in the ultrasonic therapy system of the present invention. 
         FIG. 3  is a structural diagram of electric control unit of the present invention. 
         FIG. 4  is a principle diagram of shielding and filtering measures taken in the present invention. 
     
    
    
     Wherein:  1 —X-driving motor  2 —Y-driving motor  3 —Z-driving motor  4 —Motor for treatment bed  5 —Ultrasonic therapy applicator  6 —MRI apparatus  7 —Treatment bed  8 —Treatment bed moving unit  9 —Electric control unit for movement of treatment bed  10 —Movement positioning unit  11 —X-board  12 —Y-board  13 —Z-board  14 —Focal point moving board  15 —Treatment bed stand  16 —Electric control unit for movement positioning  17 —High-frequency generator  18 ,  19 —Driving key  20 —Joining gearing part  21 —X-shaft  22 —Y-shaft  23 —Z-shaft  24 —Shielding cover for motor  25 —Shielding room  26 —DC power  27 —Console  28 —Cutoff guide-wave pipe  29 —Filter  30 —Matching circuit  31 ,  32 —Cutoff guide-wave pipes  33 —Motion controller  34 ,  35 ,  36 ,  37 ,  38 —Filters 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be further explained below in detail with reference to the preferred embodiments and accompanying drawings. 
     The ultrasonic therapy system of the present invention comprises an MRI apparatus  6  (imaging device), ultrasonic therapy device and electric control unit. Wherein, the ultrasonic therapy device comprises a treatment bed  7 , an ultrasonic therapy applicator  5  and its movement positioning unit. 
     As shown in  FIG. 2 , the movement positioning unit of the ultrasonic therapy applicator  5  includes X-board  11 , on which the ultrasonic therapy applicator  5  is placed, X-driving motor  1  connected to X-board  11  through X-gearing unit, Y-board  12 , Y-driving motor  2  connected to Y-board  12  through Y-gearing unit, Z-board  13 , Z-driving motor  3  connected to Z-board  13  through Z-gearing unit, focal point moving board  14  and the motor for treatment bed  4  for driving the focal point moving board  14  in Z-direction. 
     X-board  11  is mounted on Y-board  12  and Y-board  12  is connected to Z-board  13  and Z-board  13  is placed on focal point moving board  14 . 
     In this embodiment, X-driving motor  1 , Y-driving motor  2  and Z-driving motor  3  are placed on treatment bed stand  15  and they are normally 2 meters away from the central point of imaging area of MRI. X-driving motor  1 , Y-driving motor  2  and Z-driving motor  3  are respectively connected to X, Y and Z-gearing units through X-shaft  21 , Y-shaft  22  and Z-shaft  23 . In this embodiment, all the driving motors can adopt servo motors. X-shaft  21 , Y-shaft  22  and Z-shaft  23  all adopt long shafts. When the long shaft rotates, it transfers impetus to the gearing unit and when the long shaft is at rest, the gearing unit may move axially relative to the long shaft. Both sides of X-shaft  21 , Y-shaft  22  and Z-shaft  23  are mounted on treatment bed stand  15 . The ultrasonic therapy applicator  5  is connected to X-board  11  and moves with X-board  11 . 
     The X-gearing unit comprises key gearing mechanism which moves along Z-direction, bevel gear gearing mechanism and screw gearing mechanism. Three gearing mechanisms transfer the impetus from X-driving motor successively. Said key gearing mechanism is connected to X-shaft  21 . There is a key way opened on X-shaft  21 . The length of key way is determined by the travel range of treatment bed and the travel range of focal point of ultrasonic therapy applicator in Z-direction. When Z-board  13  moves in Z-direction (i.e. the direction of bore of MRI apparatus), the key can move in the key way in Z-direction. When X-shaft  21  rotates, the impetus can be transferred to bevel gear gearing mechanism through key gearing mechanism and then transferred from bevel gear gearing mechanism to screw gearing mechanism. Finally the screw gearing mechanism drives X-board  11  to move ultrasonic therapy applicator  5  in X-direction. The screw gearing mechanism includes X-board  11  connected with bevel gear gearing mechanism by screw, a guide rail connected to Y-board  12  along X-direction. X-board  11  can move linearly in X-direction along the guide rail. 
     The structures of Y-gearing unit and Z-gearing unit are similar to that of X-gearing unit and the difference is the different movement directions of them. 
     The motor for treatment bed  4  is placed on end of treatment bed stand  15  which is far away from MRI apparatus. 
     The working process of said ultrasonic therapy device is as follows: 
     The motor for treatment bed  4  drives focal point moving board  14 . The focal point moving board  14  carries Z-gearing unit mounted on it to move axially along Z-shaft  23 . Z-board  13  moves with focal point moving board  14  moves and also carries Y-gearing unit and X-gearing unit to move respectively along Y-shaft  22  and X-shaft  21 . Meanwhile, X-board  11 , Y-board  12  and ultrasonic therapy applicator  5  followed with the treatment bed  7  enter into or withdraw from MRI. 
     The Z-driving motor  3  drives Z-shaft  23  to rotate. The Z-shaft  23  carries Z-board  13  to move in Z-direction through Z-gearing unit (including key gearing mechanism, bevel gear gearing mechanism and screw gearing mechanism). The Z-board  13  carries Y-board  12 , Y-gearing unit, X-board  11  and X-gearing unit to move in Z-direction and accordingly drives ultrasonic therapy applicator  5  to move in Z-direction. 
     The Y-driving motor  2  drives Y-shaft  22  to rotate. The Y-shaft  22  carries Y-board  12  to move in Y-direction through Y-gearing unit (including key gearing mechanism, bevel gear gearing mechanism and screw gearing mechanism). The Y-board carries X-board  11 , X-gearing unit to move in Y-direction and accordingly drives ultrasonic therapy applicator  5  to move in Y-direction. 
     The X-driving motor  1  drives X-shaft  21  to rotate. The X-shaft  21  carries X-board  11  to move in X-direction through X-gearing unit (including key gearing mechanism, bevel gear gearing mechanism and screw gearing mechanism) and accordingly drives ultrasonic therapy applicator  5  to move in X-direction. 
     The X, Y, Z-driving motors drive the focal point of ultrasonic therapy applicator  5  to move respectively in X, Y, Z-dimensional space. During the ultrasonic therapy, the positions of three motors keep unchanged. 
     The driving motors are located far away from the imaging area of MRI apparatus so that the electromagnetic interference to MRI when the driving motors are working can be reduced, and the electromagnetic interference to MRI apparatus during treatment can be avoided when driving motors are moving. 
     As shown in  FIG. 3 , relative to MRI apparatus  6 , the ultrasonic therapy device guided by MRI can be located outside of MRI apparatus or inside the bore of MRI apparatus. 
     Wherein, the electric control unit comprises electric control unit for movement of treatment bed  9  and electric control unit for movement positioning  16  of ultrasonic therapy applicator. Wherein, the electric control unit for movement of treatment bed  9  is used to control treatment bed moving unit  8 . The electric control unit for movement positioning  16  is used to control movement positioning unit  10  of ultrasonic therapy applicator. 
     A position sensor (not illustrated in figures) is installed in an appropriate place in MRI apparatus  6 . This position sensor will actuate de-energizing the electric control unit for movement of treatment bed  9  after the treatment bed  7  enters into the imaging area of MRI apparatus  6 . 
     In the present invention, for main interference sources of ultrasonic therapy system, some measures like shielding and filtering have been taken. As shown in  FIG. 4 , the MRI apparatus is placed in shielding room  25 , which is used to shield the electromagnetic waves. The driving motors and motion controllers are also placed in shielding room  25 . The DC power  26 , console  27  and high-frequency generator  17  are placed outside of shielding room  25 . The shielding room  25  and shielding cover for motor are made of high-permeability magnetic material with high-conductivity, which has a high performance to shield the electromagnetic waves. In this embodiment, the shielding cover for motor  24  is made of stainless steel. In the figure, the X-driving motor  1  is taken as an example. 
     Because the filters are used at both ends of a cable and the performance of shielding electromagnetic waves can be improved, the anode and cathode of DC power  26  are respectively connected to filter  37 , filter  34  and filter  38 , filter  35  for filtering and then respectively connected to the anode and cathode of X-driving motor  1 . The outer shell of X-driving motor  1  is connected to shielding room  25  through filter  36  and the shielding cover for motor  24  is connected to shielding room  25 . Because just in this place, the frequency of interference source to be cleared is lower than 10 MHz, the shielding cover for motor  24  and the outer shell of X-driving motor  1  are respectively connected to shielding room  25  by way of single-point earthing. The shielding room  25  is earthed. 
     A cutoff wave-guide pipe  32  is arranged outside of the output shaft of X-driving motor  1  (by use of electric characteristics of high-pass filters of cutoff wave-guide pipe). The cutoff wave-guide pipe  32  is connected to shielding cover for motor  24 . The output shaft of X-driving motor  1  transfers the impetus by driving key  19  to joining gearing part  20 . Then, the joining gearing part  20  transfers the impetus by driving key  18  to X-shaft  21  (not illustrated in the figure). The joining gearing part  20  is made of nonmetallic material. The motion controller  33  is used to control each movement positioning units and electric control unit of the present ultrasonic therapy system. The console  27  monitors motion control  33  by communication optical fiber. The cutoff wave-guide pipe  31  and cutoff wave-guide pipe  28  are respectively arranged at the places where the communication optical fiber enters into the shielding room  25  and the shielding cover for motor  24 . The output power signals from high-frequency generator  17  will go through filter  29 , matching circuit  30  and then to ultrasonic therapy applicator  5 . Wherein, the outer shell of X-driving motor  1  (via filter  36 ), cutoff wave-guide pipes  28 ,  31  and  32 , shielding cover for motor  24 , shielding room  25  are single-point earthed. 
     In this embodiment, the filters adopt low-pass filters with a high attenuation rate. The filters mainly filter and shield the electromagnetic waves with frequency higher than 10 MHz produced by the power source and high-frequency generator. Its attenuation rate ranges from 80 dB to 120 dB. In this embodiment, filter  34 , filter  35 , filter  36 , filter  37 , and filter  38  adopt rod low-pass filters. 
     In this embodiment, the rod low-pass filters can filter the electromagnetic waves with frequency higher than 10 MHz with an attenuation of 100 dB. Thus, after filtering, the interference of the residual electromagnetic waves to MRI apparatus is controlled within the range, which will not influence the preciseness of MRI diagnosis. Therefore, the interference of the residual electromagnetic waves after filtering to MRI can be reduced effectively. 
     In this embodiment, according to the working frequency of electromagnetic waves of MRI apparatus, the frequency “f” of interference signals ranging from 10 MHz to 100 MHz is set up. According to the cutoff frequency ƒ c (3˜5)×ƒ, the cutoff frequency of cutoff wave-guide pipes  31  and  32  ranging from 300 MHz to 500 MHz is set up. 
     For a cutoff wave-guide pipe with a round section: 
     
       
         
           
             
               
                 
                   
                     f 
                     c 
                   
                   = 
                   
                     
                       175 
                       × 
                       
                         10 
                         3 
                       
                     
                     D 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In formula (1): 
     D—Diameter of cutoff wave-guide pipe (Unit: mm) 
     fc—Cutoff frequency (Unit: MHz) 
     Attenuation “S” for a cutoff wave-guide pipe with a round section: 
     
       
         
           
             
               
                 
                   S 
                   = 
                   
                     32 
                     × 
                     
                       L 
                       D 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In formula (2): 
     S—Attenuation value (Unit: dB) 
     L—Length of cutoff wave-guide pipe (Unit: mm) 
     D—Diameter of cutoff wave-guide pipe (Unit: mm) 
     In this embodiment, f is 100 MHz, fc is 500 MHz, S is 100 dB. 
     Under the premise to meet the relations of 
             D   ≤       175   ×     10   3         f   c             
and
 
             L   ≥       S   ×   D     32           
and with a consideration of spatial structure, the diameter (D) and length (L) of cutoff wave-guide pipe  32  are determined as D=30 mm and L−100 mm.
 
     By arranging cutoff wave-guide pipes on the output shaft of a driving motor, the attenuation ranging from 80 dB to 120 dB for the electromagnetic waves with frequency lower than 100 MHz can be realized and the interference to MRI can be reduced effectively. 
     In the process of ultrasonic diagnosis and treatment, under the control of electric control unit for movement of treatment bed  9 , the treatment bed moving unit  8  drives treatment bed  7  and ultrasonic therapy applicator  5  to move into the bore of MRI apparatus  6 . After the treatment bed  7  contacts with the position sensor, the electric control unit for movement of treatment bed  9  will be de-energized and the treatment bed  7  will keep stable and unchanged during ultrasonic diagnosis and treatment. The electric control unit for movement positioning  16  controls movement positioning unit  10  to drive the focal point of ultrasonic therapy applicator  5  to move in three dimensional space for applying ultrasonic therapy to the diseased part of a patient. 
     In the present invention, after treatment bed  7  and ultrasonic therapy applicator  5  enter into the bore of MRI apparatus  6  and before MRI apparatus  6  starts imaging, the electric control unit for movement of treatment bed  9  is de-energized and therefore, the electromagnetic interference to MRI produced by this kind of electric units under electrifying status can be avoided. 
     After completion of diagnosis and treatment or when the treatment bed  7  needs to be moved, the power supply to electric control unit for movement of treatment bed  9  can be resumed through switches and the treatment bed moving unit  8  is actuated to drive treatment bed  7  to move away from the bore of MRI apparatus  6 . 
     Through cutting off the power supply to electric control unit, the electromagnetic interference to MRI apparatus  6  can be reduced effectively. 
     Thus, the ultrasonic therapy system of the present invention can reduce a lot of electromagnetic interference and the electromagnetic compatibility of ultrasonic therapy device and MRI apparatus can be truly realized.
An ultrasonic therapy system capable of reducing the electromagnetic interference to the imaging device includes an imaging device, an ultrasonic therapy device and an electric controlling unit. The ultrasonic therapy device includes an ultrasonic therapy applicator and the movement positioning units thereof. Driving motors for controlling the movement of the ultrasonic therapy applicator are provided in the movement positioning units, wherein the driving motors are disposed outside the area where the electromagnetic wave of the driving motors can interfere with the imaging device, and the driving motors are connected to the ultrasonic therapy applicator through gearing units.