Abstract:
For the purpose of providing an MRI apparatus which gives rise to no phase deviation due to frequency alteration of the RF magnetic field and signals for detection use, the MRI apparatus includes: signal collecting means which detects and collects magnetic resonance signals generated by applying a magnetostatic field, a gradient magnetic field and an RF magnetic field onto an object; frequency altering means which alters the frequency of the RF magnetic field and the frequency of signals for detection use in accordance with change in the Larmor frequency with time; and image reconstructing means which reconstructs an image on the basis of the collected signals, and further comprises: phase control means which sets as the phase at the time of starting detection of signals for detection use a phase Δθ given by a relational expression Δθ=2π·Δf·T using the difference Δf between the frequency altered by the frequency altering means and a reference frequency, and a time period T from the start of applying the RF magnetic field to the start of detection of magnetic resonance signals.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to an MRI (Magnetic Resonance Imaging) apparatus, and more particularly to an MRI apparatus which alters the frequency of the RF (radio frequency) magnetic field and the frequency signals for detection use in accordance with change in the Larmor frequency with time and collects magnetic resonance signals.  
         [0002]     In an MRI apparatus the Larmor frequency may vary with time as a consequence of a variation in magnetostatic field intensity or the like, and in such a case the frequency of the RF magnetic field and the frequency of signals for detection use are altered in accordance with the variation of the Larmor frequency. The Larmor frequency is measured from FID (Free Induction Decay) signals or the like collected at suitable timings, the frequency of the RF magnetic field and the frequency of signals for detection use are corrected according to the extent of variation from the initial value (see Patent Document 1 for instance).  
         [0003]     [Patent Document 1] Specification of Japanese Patent No. 3513076, pp. 5-7, FIGS. 1 through 5).  
         [0004]     The frequency alteration of the RF magnetic field is performed at the time of starting the transmission of RF pulses, and the frequency alteration of signals for detection use is performed at the time of starting detection. Whereas both frequencies are altered in an initial phase of 0, the signals obtained by detection differ in phase from the detection signals obtained when there is no change in the Larmor frequency. Such a deviation in phase gives rise to an error in differential computation, for instance, of signals of the same region differing in the time of collection at a raw data stage.  
       SUMMARY OF THE INVENTION  
       [0005]     Therefore, an object of the present invention is to provide an MRI apparatus which is free from phase deviation due to the alteration of the frequencies of the RF magnetic field and signals for detection use.  
         [0006]     One aspect of the invention to solve the problem noted above constitutes an MRI apparatus including: signal collecting means which detects and collects magnetic resonance signals generated by applying a magnetostatic field, a gradient magnetic field and an RF magnetic field onto an object; frequency altering means which alters the frequency of the RF magnetic field and the frequency of signals for detection use in accordance with change in the Larmor frequency with time; and image reconstructing means which reconstructs an image on the basis of the collected signals, the MRI apparatus comprising:  
         [0007]     phase control means which sets as the phase at the time of starting detection of signals for detection use a phase Δθ given by a relational expression: 
 
Δθ=2π·Δ f·T  
 
         [0008]     using the difference Δf between the frequency altered by the frequency altering means and a reference frequency, and a time period T from the start of applying the RF magnetic field to the start of detection of magnetic resonance signals.  
         [0009]     Another aspect of the invention to solve the problem noted above constitutes an MRI apparatus including: signal collecting means which detects and collects magnetic resonance signals generated by applying a magnetostatic field, a gradient magnetic field and an RF magnetic field onto an object; frequency altering means which alters the frequency of the RF magnetic field and the frequency of signals for detection use in accordance with change in the Larmor frequency with time; and image reconstructing means which reconstructs an image on the basis of the collected signals, the MRI apparatus comprising frequency alteration control means which so alters the alteration of the frequency of signals for detection use by the frequency altering means as to make the phase at the time of starting detection continuous.  
         [0010]     It is preferable, from the viewpoint of appropriately obtaining a reference frequency, for the reference frequency to be the Larmor frequency before change with time.  
         [0011]     It is preferable, from the viewpoint of appropriately obtaining the initial value of the Larmor frequency, for the Larmor frequency before change with time to be measured from a magnetic resonance signal obtained by pre-scanning by the signal collecting means.  
         [0012]     It is preferable, from the viewpoint of obtaining an appropriate measurement, for the Larmor frequency to be measured from an FID signal.  
         [0013]     It is preferable, from the viewpoint of obtaining an image regarding a function, for the image reconstructing means to reconstruct an image by using signals having gone through mutual computation of signals of the same region differing in the time of collection.  
         [0014]     It is preferable, from the viewpoint of obtaining a differential image, for the computation to be differential computation.  
         [0015]     It is preferable, from the viewpoint of obtaining a contrast image, for one of the signals of the same region differing in the time of collection to be a signal before the injection of a contrast medium and the other to be a signal after the injection of the contrast medium.  
         [0016]     According to one aspect of the present invention, since the MRI apparatus is an MRI apparatus including: signal collecting means which detects and collects magnetic resonance signals generated by applying a magnetostatic field, a gradient magnetic field and an RF magnetic field onto an object; frequency altering means which alters the frequency of the RF magnetic field and the frequency of signals for detection use in accordance with change in the Larmor frequency with time; and image reconstructing means which reconstructs an image on the basis of the collected signals, and comprises phase control means which sets as the phase at the time of starting detection of signals for detection use a phase Δθ given by a relational expression Δθ=2π·Δf·T using the difference Δf between the frequency altered by the frequency altering means and a reference frequency, and a time period T from the start of applying the RF magnetic field to the start of detection of magnetic resonance signals, it can eliminate phase deviation of detection signals due to the alteration of the frequencies of the RF magnetic field and signals for detection use.  
         [0017]     According to another aspect of the present invention, since the MRI apparatus is an MRI apparatus including: signal collecting means which detects and collects magnetic resonance signals generated by applying a magnetostatic field, a gradient magnetic field and an RF magnetic field onto an object; frequency altering means which alters the frequency of the RF magnetic field and the frequency of signals for detection use in accordance with change in the Larmor frequency with time; and image reconstructing means which reconstructs an image on the basis of the collected signals, and comprises frequency alteration control means which so alters the alteration of the frequency of signals for detection use by the frequency altering means as to make the phase at the time of starting detection continuous, it can eliminate phase deviation of detection signals due to the alteration of the frequencies of the RF magnetic field and signals for detection use.  
         [0018]     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  
       [0019]      FIG. 1  is a block diagram of one example of MRI apparatus in the best mode for carrying out the present invention.  
         [0020]      FIG. 2  is a block diagram of the RF drive unit.  
         [0021]      FIG. 3  is a block diagram of the front end part of the data collecting unit.  
         [0022]      FIG. 4  is a chart showing one example of pulse sequence.  
         [0023]      FIG. 5  is a chart showing one example of pulse sequence.  
         [0024]      FIG. 6  is a time chart showing one example of alteration of the excitation frequency and the detection frequency.  
         [0025]      FIG. 7  is a time chart showing another example of alteration of the excitation frequency and the detection frequency. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     The best mode for carrying out the invention will be described in detail below with reference to drawings. Incidentally, the present invention is not confined to the best mode for carrying out the invention. A block diagram of an MRI apparatus is shown in  FIG. 1 . This apparatus is one example of the best mode for carrying out the invention. The configuration of this apparatus represents one example of the best mode for carrying out the present invention regarding an MRI apparatus.  
         [0027]     As shown in the drawing, this apparatus has a magnet system  100 . The magnet system  100  has a main magnetic field coil part  102 , a gradient coil part  106  and an RF coil part  108 . These coil parts are substantially cylindrically shaped, and arranged coaxially with one another.  
         [0028]     An object  1  of imaging, mounted on a cradle  500 , is carried in and out of the substantially columnar bore of the magnet system  100  by carrier means not shown.  
         [0029]     The main magnetic field coil part  102  forms a magnetostatic field in the bore of the magnet system  100 . The direction of the magnetostatic field is substantially parallel to the direction of the body axis of the object  1 . Thus, there is formed a so-called horizontal magnetic field. The main magnetic field coil part  102  is configured of, for instance, superconducting coil. It need not be a superconducting coil, but may as well be configured of a normal conducting coil. Also, a magnet system of a perpendicular magnetic field type, whose magnetostatic field is perpendicular to the body axis of the object  1 , may be used instead of the horizontal magnetic field. In the perpendicular magnetic field type, a permanent magnet, for instance, is used for generating the magnetostatic field.  
         [0030]     The gradient coil part  106  generates, in the directions of three mutually perpendicular axes, namely the slice axis, the phase axis and the frequency axis, three gradient magnetic fields to give gradients to the magnetostatic field intensity.  
         [0031]     Where coordinate axes perpendicular to one another in a magnetostatic field space are represented by x, y and z, any of these axes can be made the slice axis. In that case, one of the remaining two axes is made the phase axis and the other, the frequency axis. It is also possible to give any inclination to the slice axis, the phase axis and the frequency axis relative to the x, y and z axes while maintaining their mutual perpendicularity. In this apparatus, the direction of the body width of the object  1  is supposed to the x direction, that of the body thickness, the y direction and that of the body axis, the z direction.  
         [0032]     The gradient magnetic field in the direction of the slice axis is also referred to as the slice gradient magnetic field. The gradient magnetic field in the direction of the phase axis is also referred to as the encode gradient magnetic field. The gradient magnetic field in the direction of the frequency axis is also referred to as the read out gradient magnetic field. The read out gradient magnetic field is synonymous to the frequency encode gradient magnetic field. To enable such gradient magnetic fields to be generated, the gradient coil part  106  has gradient coils of three lines not shown. Hereinafter, the gradient magnetic field may be simply referred to as the gradient.  
         [0033]     The RF coil part  108  forms an RF magnetic field in the magnetostatic field space to excite spins in the body of the object  1 . Hereinafter, the formation of the RF magnetic field may also be referred to as transmission of an RF excitation signal. The RF excitation signal may also be referred to as the RF pulse.  
         [0034]     The electromagnetic wave generated by the excited spin, namely magnetic resonance signals, is received by the RF coil part  108 . The magnetic resonance signals serve as sampling signals with respect to the frequency domain, namely the Fourier space.  
         [0035]     If magnetic resonance signals are encoded on two axes by the gradients in the direction of the phase axis and in the direction of the frequency axis, the magnetic resonance signals will be obtained as sampling signals with respect to the two-dimensional Fourier space, or if they are encoded on three axes by using the slice gradient as well, they will be obtained as signals with respect to the three-dimensional Fourier space. The gradients determine the sampling positions of signals in the two-dimensional or three-dimensional Fourier space. Hereinafter, the Fourier space may also be referred to as the k-space.  
         [0036]     A gradient drive unit  130  is connected to the gradient coil part  106 . The gradient drive unit  130  gives a drive signal to the gradient coil part  106  to generate a gradient magnetic field. The gradient drive unit  130  has three lines of drive circuits not shown matching the three lines of gradient coils in the gradient coil part  106 .  
         [0037]     An RF drive unit  140  is connected to the RF coil part  108 . The RF drive unit  140  gives a drive signal to the RF coil part  108  to have an RF pulse transmitted to excite a spin in the body of the object  1 .  
         [0038]     A data collecting unit  150  is also connected to the RF coil part  108 . The data collecting unit  150  collects receive signals received by the RF coil part  108  as digital data.  
         [0039]     A sequence control unit  160  is connected to the gradient drive unit  130 , the RF drive unit  140  and the data collecting unit  150 . The sequence control unit  160  controls the gradient drive unit  130  or the data collecting unit  150  to accomplish collection of magnetic resonance signals. Hereinafter, the collection of magnetic resonance signals may as well be referred to as scanning.  
         [0040]     The sequence control unit  160  is configured of a computer or the like. The sequence control unit  160  has a memory not shown. The memory stores programs for the sequence control unit  160  and various sets of data. The function of the sequence control unit  160  is realized by the execution by the computer of a program stored in the memory. The part consisting of the magnet system  100  or the sequence control unit  160  constitutes one example of signal collecting means in the present invention.  
         [0041]     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 or the like. The data processing unit  170  has a memory not shown. The memory stores programs for the data processing unit  170  and various sets of data.  
         [0042]     The data processing unit  170  is connected to the sequence control unit  160 . The data processing unit  170  is positioned superior to and has general control over the sequence control unit  160 . The function of this apparatus is realized by the execution by the data processing unit  170  of a program stored in the memory.  
         [0043]     The data processing unit  170  stores into the memory the data collected by the data collecting unit  150 . A data space is formed within the memory. This data space matches the k-space. The data processing unit  170  reconstructs an image by subjecting the data in the k-space to inverse Fourier transform. The data processing unit  170  represents one example of image reconstructing means in the present invention.  
         [0044]     A display unit  180  and a manipulating unit  190  are connected to the data processing unit  170 . The display unit  180  is configured of a graphic display or the like. The manipulating unit  190  is configured of a keyboard or the like equipped with a pointing device.  
         [0045]     The display unit  180  displays reconstructed images and various items of information outputted from the data processing unit  170 . The manipulating unit  190  is manipulated by the user to input various instructions, information and the like to the data processing unit  170 . The user can manipulate this apparatus interactively through the display unit  180  and the manipulating unit  190 .  
         [0046]      FIG. 2  shows the configuration of the RF drive unit  140 . As shown in this drawing, the RF drive unit  140  mixes the output signal (frequency fc) of a reference transmitter  402  and the output signal (frequency Δfs) of a variable transmitter  404  with a frequency mixer  406  to obtain an RF signal whose frequency is fc+Δfs, inputs this RF signal to an amplitude modulator  410  via a transmission gate  408 , amplifies the RF signal of an amplitude-modulated wave with a power amplifier  412 , and supplies the signal to the RF coil part  108 .  
         [0047]     The frequency fc of the output signal of the reference transmitter  402  is fixed. This frequency fc is equal to the Larmor frequency when the magnetostatic field intensity is at a regular level. Hereinafter, the frequency of the output signal of the reference transmitter may also be referred to as the frequency of the reference transmitter or the reference frequency. The reference frequency may be a pre-measured Larmor frequency before change with time. The Larmor frequency before change with time is measured from, for instance, magnetic resonance signals obtained by pre-scanning.  
         [0048]     The frequency Δfs of the output signal of the variable transmitter  404  is altered under control by the sequence control unit  160 . The variable transmitter  404  is one example of frequency altering means in the present invention. Hereinafter, the frequency of the output signal of the variable transmitter may also be referred to as the frequency of the variable transmitter. The opening and closing of the transmission gate  408  is also controlled by the sequence control unit  160 .  
         [0049]      FIG. 3  shows the configuration of the front end part of the data collecting unit  150 . As shown in the drawing, the data collecting unit  150  amplifies input signals from the RF coil part  108  with a pre-amplifier  512 , and inputs them to two detectors  516  and  516 ′ through a distributor  514  to obtain detected outputs I and Q.  
         [0050]     Regarding signals (carrier) for detection by the detectors  516  and  516 ′, the output signal (frequency fc) of a reference transmitter  502  and the output signal (frequency Δfr) of a variable transmitter  504  are mixed by a frequency mixer  506  to obtain a signal for detection use of fc+Δfr in frequency, and this signal for detection use is inputted to a phase shifter  510  through a reception gate  508  to obtain two signals for detection use, whose phases are 0° and 90°, from the phase shifter  510 .  
         [0051]     While the frequency fc of the reference transmitter  502  is fixed, the frequency Δfr of the variable transmitter  504  is varied under control by the sequence control unit  160 . The variable transmitter  504  is one example of frequency altering means in the present invention. The frequency Δfr of the variable transmitter  504  is the same frequency as or a different frequency from the frequency Δfs of the variable transmitter  404 . The opening and closing of the reception gate  508  is also controlled by the sequence control unit  160 .  
         [0052]      FIG. 4  shows one example of pulse sequence for scanning. This pulse sequence is a pulse sequence by the Gradient Echo method.  
         [0053]     In the drawing, (1) shows the sequence of RF excitation. Every one of (2) through (4) shows the sequence of a gradient magnetic field; (5) shows the sequence of a magnetic resonance signal. Out of the sequences of the gradient magnetic field, (2) is the slice gradient, (3), the frequency encode gradient and (4), the phase encode gradient. Incidentally, the magnetostatic field is constantly applied at a fixed magnetic field intensity. The same applies hereinafter.  
         [0054]     First, spin excitation by an α° takes place. The α° excitation is a selected excitation under the slice gradient Slice. The α° pulse is provided while the transmission gate  408  is open. Hereinafter, this period may also be referred to as the transmission gate period.  
         [0055]     After the α° excitation, a frequency encode gradient Read and a phase encode gradient Phase are applied in a prescribed sequence, and magnetic resonance signals, namely echoes, are read out. The reading of echoes is continued as long as the reception gate  508  is open. Hereinafter, this period may also be referred to as the reception gate period.  
         [0056]     When such a pulse sequence is repeated a prescribed number of times during a time of repetition TR, an echo is read out each time. Each time of repetition, the phase encoding of the echo is altered, and by repeating it a prescribed number of times the collection of echo signals over the whole two-dimensional k-space is accomplished. Incidentally, phase encoding is to be accomplished in the slice direction as well, echo signals are collected in the three-dimensional k-space. By subjecting echo data in the two-dimensional k-space to two-dimensional inverse Fourier transform, a 2D image is reconstructed. By subjecting echo data in the three-dimensional k-space to three-dimensional inverse Fourier transform, a 2D image is reconstructed.  
         [0057]      FIG. 5  shows one example of pulse sequence for FID signal collection. In the drawing, (1) shows the sequence of RF excitation. Every one of (2) through (4) shows the sequence of a gradient magnetic field. (5) shows a sequence of FID signals. As shown in the drawing, collection is carrier in a pulse sequence of which the phase encode gradient Phase is 0.  
         [0058]     Such a pulse sequence is repeated at a prescribed frequency in a form of interrupting into the repetition of the pulse sequence for scanning shown in  FIG. 4 , and every time of interrupt an FID signal is collected. Every time an FID signal is collected, its center frequency is measured. The current value of the Larmor frequency is thereby obtained.  
         [0059]     The frequency of RF excitation in the pulse sequence for scanning is so altered as to become identical with the current value of the Larmor frequency. The alteration of the RF excitation frequency is accomplished by having the sequence control unit  160  control the frequency Δfs of the variable transmitter  404 .  
         [0060]     The carrier frequency for echo signal detection is also altered following the change in the Larmor frequency. The alteration of the carrier frequency is accomplished by having the sequence control unit  160  control the frequency Δfr of the variable transmitter  504 . Hereinafter, the RF excitation frequency may also be referred to as the excitation frequency, and the carrier frequency for echo signal detection, as the detection frequency.  
         [0061]      FIG. 6  shows one example of alteration of the excitation frequency and the detection frequency in a time chart. In the chart, (1) shows the excitation frequency and (2), the detection frequency. However, every frequency is shown in terms of the frequency Δfs of the variable transmitter  402  or the frequency Δfr of the variable transmitter  502 . fc+Δfs and fc+Δfr resulting from the addition of the frequencies fc of the reference transmitters  402  and  502  to these frequencies are the actual excitation frequency and detection frequency, respectively.  
         [0062]     As shown in the chart, the excitation frequency is altered at a point of time t 0  matched with the beginning of the transmission gate period, and the RF excitation is performed at the altered frequency. The amplitude of the RF excitation signal becomes the greatest at a point of time t 1 , and the transmission gate period ends at a point of time t 2 .  
         [0063]     The excited spin rotates at the frequency fc+Δfs. Such a rotation becomes a rotation whose frequency is Δfs on rotational coordinates of the frequency fc. For this reason, the time chart of the excitation frequency Δfs in the drawing shows the behavior of the spin on the rotational coordinates.  
         [0064]     The phase of such a spin on the rotational coordinates is given by:  
               Δ   ⁢           ⁢   θ     =     2   ⁢   π   ⁢       ∫   t0     ⁢     Δ   ⁢           ⁢     fs   ·   t     ⁢     ⅆ   t                   (   1   )             
 
         [0065]     This phase represents a deviation in phase on the rotational coordinates.  
         [0066]     This deviation in phase at a point of time t 3  when the reception gate begins is:  
               Δ   ⁢           ⁢   θ     =       2   ⁢   π   ⁢       ∫   t0   t3     ⁢     Δ   ⁢           ⁢     fs   ·   t     ⁢           ⁢     ⅆ   t           =     2   ⁢     π   ·   Δ     ⁢           ⁢     fs   ·   T                 (   2   )             
 
         [0067]     The reception gate period begins at the point of time t 3 , and detection at the frequency Δfr is performed by a point of time t 4 . Then, signals of the frequency Δfr are so controlled as to make the phase at the point of time t 3  to be identical with the phase given by Equation (2). Under phase control by such signals for detection, output signals after detection involve no deviation in phase on the rotational coordinates. The phase control is performed by the sequence control unit  160  on the variable transmitter  504 . The sequence control unit  160  is one example of phase control means.  
         [0068]      FIG. 7  shows another example of alteration of the excitation frequency and the detection frequency in a time chart. In the chart, (1) shows the excitation frequency and (2), the detection frequency. As shown in the chart, the excitation frequency is altered at the point of time t 0  matched with the beginning of the transmission gate period, and the RF excitation is performed at the altered frequency.  
         [0069]     At the point of time t 0 , the detection frequency Δfr is also altered. This alteration is so accomplished to make Δfr=Δfs hold. However, no detection with this signal is done because it is not yet the reception gate period.  
         [0070]     The reception gate period begins at the point of time t 3 . Since the detection frequency during the reception gate period is determined according to the reception band, Δfr=Δfs does not always hold. When Δfr≠Δfs, the frequency is altered anew at the point of time t 3 . The frequency is then altered on a continuous phase basis. The detection frequency alteration on such a continuous phase basis is accomplished by the sequence control unit  160  upon the variable transmitter  504 . The sequence control unit  160  is an example of frequency alteration n control means.  
         [0071]     Since Δfr=Δfs had held until immediately before the alteration, the phase at the point of time t 3  is the phase given by Equation (2). For this reason, as in the case of  FIG. 6 , output signals after detection involve no deviation in phase on the rotational coordinates.  
         [0072]     In this way, since the signals after detection involve no deviation in phase irrespective of change in the Larmor frequency with time, no error occurs even when the difference between before and after the injection of the contrast medium is to be figured out of signals of the same region differing in the time of collection at a raw data stage. Therefore, a correct contrast image can be obtained. To add, the same applies where some computation, not confined to the difference, is to be done of signals of the same region differing in the time of collection. A correct functional image or the like can be thereby obtained.  
         [0073]     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.