Abstract:
An MRI apparatus which obtains a tomogram of an object by utilizing magnetic resonance includes a calibrating device which figures out a relationship between a center frequency and an optimal gain of RF transmission with respect to a predetermined range of central frequencies, a saving device which saves information expressing said relationship, and a setting device which sets the RF transmission gain according to the center frequency during subsequent scanning by utilizing the saved information.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Chinese Patent Application No. 200710110317.5 filed Jun. 15, 2007, which is hereby incorporated by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   The subject matter disclosed herein relates to an MRI apparatus (Magnetic Resonance Imaging) apparatus and an RF (radio frequency) transmission gain setting method, and more particularly to an MRI apparatus which obtains tomograms of the object by utilizing magnetic resonance and an RF transmission gain setting method for MRI apparatuses. 
   In an MRI apparatus, a prescan is done before taking an image, and calibration is performed to optimize the central frequency and the gain of RF transmission by using the imaged data. By the calibration, the center frequency of RF transmission is so adjusted as to be identical with the Larmor frequency of the spin of the object, and the gain of RF transmission is so adjusted as to set the flip angle of spin excitation exactly to a prescribed angle (see U.S. Pat. No. 4,806,866 and/or U.S. Pat. No. 6,025,718, for instance). 
   In an MRI apparatus that generates its main magnetic field with a permanent magnet requires frequent prescanning for calibration because the influence of heating of the gradient magnet field coil causes the magnetic field intensity to drift dependent on the temperature characteristics of the permanent magnet and along with that the Larmor frequency varies. This extends the time required for imaging, with a correspondingly increased load on the patient and inconvenience to the user. 
   BRIEF DESCRIPTION OF THE INVENTION 
   It is desirable that problems described previously are solved. 
   In a first aspect, the invention provides an MRI apparatus which obtains a tomogram of an object by utilizing magnetic resonance, including a calibrating device which figures out a relationship between a center frequency and an optimal gain of RF transmission with respect to a predetermined range of central frequencies, a saving device which saves information expressing the relationship, and a setting device which sets the RF transmission gain according to the center frequency during subsequent scanning by utilizing the saved information. 
   In a second aspect, the invention provides the MRI apparatus according to the first aspect, wherein the information is saved as a mathematical table. 
   In a third aspect, the invention provides the MRI apparatus according to the first aspect, wherein the information is saved as a gain curve. 
   In a fourth aspect, the invention provides the MRI apparatus according to the first aspect, wherein the information is saved as a numerical expression. 
   In a fifth aspect, the invention provides the MRI apparatus according to any of the second aspect through the fourth aspect, wherein the information is saved for each RE coil. 
   In a sixth aspect, the invention provides the MRI apparatus according to the first aspect, wherein the center frequency of RF transmission during the scanning is figured out by calibration. 
   In a seventh aspect, the invention provides the MRI apparatus according to the sixth aspect, wherein the center frequency is corrected according to variations in intensity of a main magnetic field. 
   In an eighth aspect, the invention provides the MRI apparatus according to the seventh aspect, wherein variations in the intensity of the main magnetic field are figured out according to the temperature characteristics of a main magnetic field magnet. 
   In a ninth aspect, the invention provides the MRI apparatus according to the ninth aspect, wherein the temperature of the main magnetic field magnet is predicted according to a calorific value of heat generated by a gradient magnetic field coil. 
   In a tenth aspect, the invention provides the MRI apparatus according to the ninth aspect, wherein the calorific value of heat generated by the gradient magnetic field coil is predicted according to a scan protocol. 
   In an eleventh aspect, the invention provides an RF transmission gain setting method for an MRI apparatus which take a tomogram of an object by utilizing magnetic resonance, including the steps of: figuring out a relationship between a center frequency and an optimal gain of RF transmission with respect to a predetermined range of central frequencies by calibration; saving information expressing the relationship; and setting the RF transmission gain according to the center frequency during subsequent scanning by utilizing the saved information. 
   In a twelfth aspect, the invention provides the RF transmission gain setting method according to the eleventh aspect, wherein the information is saved as a mathematical table. 
   In a thirteenth aspect, the invention provides the RF transmission gain setting method according to the eleventh aspect, wherein the information is saved as a gain curve. 
   In a fourteenth aspect, the invention provides the RF transmission gain setting method according to the eleventh aspect, wherein the information is saved as a numerical expression. 
   In its fifteenth aspect, the invention provides the RF transmission gain setting method according to any of the twelfth aspect through the fourteenth aspect, wherein the information is saved for each RF coil. 
   In its sixteenth aspect, the invention provides the RF transmission gain setting method according to the eleventh aspect, wherein the center frequency of RF transmission during the scanning is figured out by calibration. 
   In a seventeenth aspect, the invention provides the RF transmission gain setting method according to the sixteenth aspect, wherein the center frequency is corrected according to variations in the intensity of the main magnetic field. 
   In its eighteenth aspect, the invention provides the RF transmission gain setting method according to the seventeenth aspect, wherein variations in the intensity of the main magnetic field are figured out according to the temperature characteristics of the main magnetic field magnet. 
   In its nineteenth aspect, the invention provides the RF transmission gain setting method according to the eighteenth aspect, wherein the temperature of the main magnetic field magnet is predicted according to a calorific value of heat generated by a gradient magnetic field coil. 
   In its twentieth aspect, the invention provides the RF transmission gain setting method according to the nineteenth aspect, wherein the calorific value of heat generated by the gradient magnetic field coil is predicted according to a scan protocol. 
   Since the MRI apparatus according to the invention is an MRI apparatus which obtains a tomogram of an object by utilizing magnetic resonance, including a calibrating device which figures out the relationship between the center frequency and the optimal gain of RF transmission with respect to a predetermined range of central frequencies, a saving device which saves information expressing the relationship, and a setting device which sets the RF transmission gain according to the center frequency during subsequent scanning by utilizing the saved information, an MRI apparatus improving the efficiency of calibration can be realized. 
   Further, since the RF transmission gain setting method according to the invention is an RF transmission gain setting method for an MRI apparatus which take a tomogram of an object by utilizing magnetic resonance, wherein the relationship between the center frequency and the optimal gain of RF transmission is figured out in a predetermined range of central frequencies by calibration, information expressing the relationship is saved, and the RF transmission gain is set according to the center frequency during subsequent scanning by utilizing the saved information, an RF transmission gain setting method improving the efficiency of calibration can be realized. 
   As the information may be saved as a mathematical table, gain values can be saved discretely. 
   As the information may be saved as a gain curve, gain values can be saved continuously. 
   As the information may be saved as a numerical expression, gain values can be obtained by calculation. 
   As the information may be saved for each RF coil, adaptation to individual RF coils is made possible. 
   As the center frequency of RF transmission during the scanning may be figured out by calibration, a center frequency based on an actually measured Larmor frequency can be obtained. 
   As the center frequency may be corrected according to variations in the intensity of the main magnetic field, a center frequency matching a variation in the Larmor frequency can be obtained. 
   As variations in the intensity of the main magnetic field may be figured out according to the temperature characteristics of the main magnetic field magnet, adaptation to the temperature characteristics of the main magnetic field magnet is made possible. 
   As the temperature of the main magnetic field magnet may be predicted according to the calorific value of heat generated by a gradient magnetic field coil, the temperature of the main magnetic field magnet can be properly figured out. 
   As the calorific value of heat generated by the gradient magnetic field coil may be predicted according to a scan protocol, the calorific value of heat generated by the gradient magnetic field coil can be accurately figured out. 
   Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an MRI apparatus in one example of best modes for carrying out the invention. 
       FIG. 2  is a flowchart illustrating an exemplary workflow for figuring out the relationship between the center frequency and the optimal gain of RF transmission. 
       FIG. 3  is a diagram showing an example of relationship between the center frequency and the optimal gain. 
       FIG. 4  is a diagram showing one example of mathematical table of the center frequency and the optimal gain. 
       FIG. 5  is a flowchart illustrating an exemplary workflow of imaging by the MRI apparatus in one example of best modes for carrying out the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Best modes for carrying out the invention will be described in detail below with reference to the drawings. Incidentally, the invention is not limited to these best modes for implementing it. A block diagram of an MRI apparatus is shown in  FIG. 1 . The configuration of this apparatus represents one example of best modes for carrying out the invention regarding MRI apparatuses. 
   As shown in  FIG. 1 , this apparatus has a magnetic field generating device  100 . The magnetic field generating device  100  has main magnetic field magnet units  102 , gradient coil units  106  and RF coil units  108 . An object  1 , mounted on a table  500 , is brought in and out of the internal space of the magnetic field generating device  100 . The table  500  is driven by a table driving unit  120 . 
   Each item of these main magnetic field magnet units  102 , gradient coil units  106  and RF coil units  108  is provided in a pair, one piece opposed to the other with a space in-between. Each has a substantially planar shape, and all are arranged around a common center axis. 
   The main magnetic field magnet units  102  generate the main magnetic field in the internal space of the magnetic field generating device  100 . The main magnetic field is a magnetostatic field in which the magnetic field intensity is constant. The direction of the magnetostatic field crosses the body axis direction of the object  1  substantially orthogonally. In other words, it generates a so-called vertical magnetic field. The main magnetic field magnet units  102  are formed of permanent magnets. 
   The gradient coil units  106  generate three gradient magnetic fields to give a gradient in magnetostatic field intensity in the direction of each of mutually normal axes including the slice axis, the phase axis and the frequency axis. The gradient coil units  106  have three gradient coils, not shown, one for each of the three gradient magnetic fields. 
   The RF coil units  108  transmit RF pulses (radio frequency pulses) for exciting spins within the body of the object  1 . The RF coil units  108  also receive magnetic resonance signals that the excited spins give rise to. The RF coil units  108  perform transmission and reception either with the same coils or with different coils. 
   A gradient driving unit  130  is connected to the gradient coil units  106 . The gradient driving unit  130  provides driving signals to the gradient coil units  106  to have gradient magnetic fields generated. The gradient driving unit  130  has three lines of driving circuits, not shown, one for each of the three gradient coils in the gradient coil units  106 . 
   An RF driving unit  140  is connected to the RF coil units  108 . The RF driving unit  140  provides driving signals to the RF coil units  108  to have RF pulses transmitted thereby to excite spins within the body of the object  1 . 
   A data collecting unit  150  is connected to the RF coil units  108 . The data collecting unit  150  captures the receive signals received by the RF coil units  108  by sampling, and collects them as digital data. 
   A control unit  160  is connected to the table driving unit  120 , the gradient driving unit  130 , the RF driving unit  140  and the data collecting unit  150 . The control unit  160  accomplishes imaging by controlling the table driving unit  120  to the data collecting unit  150 . 
   The control unit  160  is configured of, for instance, a computer. The control unit  160  has a memory. The memory stores programs and various data for the control unit  160 . The functions of the control unit  160  are realized by the execution by the computer of programs stored in the memory. 
   The output side of the data collecting unit  150  is connected to a data processing unit  170 . Data collected by the data collecting unit  150  are inputted to the data processing unit  170 . The data processing unit  170  is configured of, for instance, a computer. The data processing unit  170  has a memory. The memory stores programs and various data for the data processing unit  170 . 
   The data processing unit  170  is connected to the control unit  160 . The data processing unit  170  is positioned superior to and supervises the control unit  160 . The functions of this unit are realized by the execution by the data processing unit  170  of programs stored in the memory. 
   The data processing unit  170  stores data collected by the data collecting unit  150  into a memory. A data space is formed in the memory. This data space constitutes a Fourier space. The Fourier space is also referred to as a k-space. The data processing unit  170  reconstructs an image of the object  1  by subjecting data in the k-space to inverse Fourier transform. 
   A display unit  180  and an operating unit  190  are connected to the data processing unit  170 . The display unit  180  is configured of a graphic display or the like. The operating unit  190  is configured of a keyboard or the like provided with a pointing device. 
   The display unit  180  displays reconstructed image and various information outputted from the data processing unit  170 . The operating unit  190 , operated by the user, inputs various instructions, information and the like to the data processing unit  170 . The user can interactively operate this apparatus through the display unit  180  and the operating unit  190 . 
   The method of RF transmission gain setting in this apparatus will be described. This method is one of the best modes for carrying out the invention. This method represents one example of best modes for carrying out the invention regarding RF transmission gain setting methods. 
   For the setting of the RF transmission gain, information expressing the relationship between the center frequency and the optimal gain of RF transmission is utilized. This information is acquired experimentally in advance, and stored into the memory of the data processing unit  170 . The relationship between the center frequency and the optimal gain of RF transmission is unique to the system and the RF coil and, once it is acquired, can be utilized repeatedly. 
     FIG. 2  charts the workflow for figuring out the relationship between the center frequency and the optimal gain of RF transmission. This workflow is accomplished under the control of the data processing unit  170 . As shown in  FIG. 2 , calibration is performed at step  201 . The calibration is performed by a technique similar to usual prescanning. The center frequency f 0  of the RF transmission and the optimal gain G for that frequency are thereby determined. 
   The calibration is carried out in a predetermined range of central frequencies, and the optimal gain for each frequency in the range is definitely determined. The data processing unit  170  that performs calibration at step  201  is one example of a calibrating device according to the invention. 
   The calibration is performed for each RF coil. This results in definite determination of the relationship between the center frequency and the optimal gain for each RF coil in the prescribed frequency range. One example of relationship between the center frequency and the optimal gain is shown in  FIG. 3 . 
   At step  202 , information expressing the relationship between center frequency and the optimal gain is stored into the memory. The data processing unit  170  that performs storing at step  202  is one example of a saving device according to the invention. The information is saved as a mathematical table. One example of mathematical table is shown in  FIG. 4 . Incidentally, the information to be saved is not limited to a mathematical table, but may instead be a gain curve or a numerical expression functionally approximating it. 
     FIG. 5  charts the workflow of imaging utilizing such information. The imaging is accomplished under the control of the data processing unit  170 . As shown in  FIG. 5 , a scan protocol is set at step  501 . The setting of the scan protocol is accomplished by the user through the operating unit  190 . This results in the setting of scanning conditions including the pulse sequence. 
   At step  502 , the center frequency of RF transmission is set. To set the center frequency, first, FID (free induction decay) signals are collected by prescanning, and their center frequency, namely the Larmor frequency is identified. Next, this frequency is modified according to the impact of the heating of the gradient magnetic field coils on the main magnetic field, and the center frequency of RF transmission is so set as to be identical with the modified frequency. 
   The calorific value of the heating of the gradient magnetic field coils is predicted from the scan protocol, the intensity variation of the main magnetic field is predicted from this calorific value and the temperature characteristics of the main magnetic field magnet units  102 , and the variation of the Larmor frequency is predicted from the intensity variation of the main magnetic field. 
   At step  503 , the gain of RF transmission is set. To set the gain, information expressing the relationship between center frequency and the optimal gain saved in the memory in advance, including the mathematical table shown in  FIG. 4 , is utilized. This results in setting of the optimal gain. The data processing unit  170  that performs gain setting at step  503  is one example of a setting device according to the invention. 
   Imaging is accomplished at step  504 . Since RF transmission during imaging is performed according to the set center frequency and gain as described above, spin excitation is properly accomplished. As a result, relatively noise-free data can be collected, and high quality reconstructed images can be obtained. 
   Since gain setting utilizes pre-saved information in this apparatus, no prescan for gain setting is needed. As this feature results in enhanced efficiency of calibration and a reduced length of time required for imaging, the load on the patient is alleviated and the user&#39;s satisfaction is increased. 
   Many widely different embodiments of the invention may be configured without departing from the spirit and the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.