Patent Application
20080064951
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.
An embodiment of the present invention will be explained as follows in reference to the drawings.
The MRI apparatus 100 comprises a bed, a static magnetic field generating section, a gradient magnetic field generating section, a transmit and receive section, a control/computing section, a respiration measuring section, and an instructing section. The bed on which a subject 200 is positioned. The static magnetic field generating section generates static magnetic field. The gradient magnetic field generating section adds position information to the static magnetic field. The transmit and receive section transmits and receives radio frequency signals. The control/computing section controls the entire system and assumes image reconstruction. The respiration measuring section measures a respiratory signal synchronized with respiration of the subject 200. The instructing section gives various instructions to the subject 200. As components of each of these sections, the MRI apparatus further comprises a magnet 1, a static power supply 2, a gradient coil unit 3, a gradient power supply 4, sequencer (sequence controller) 5, a host computer 6, an RF coil unit 7, a transmitter 8T, a receiver 8R, a arithmetic unit 10, a memory unit 11, a display unit 12, an input unit 13, a shim coil 14, a shim power supply 15, a voice generator 16, a respiration sensor 17 and a respiration monitor 18.
The static magnetic field generating section includes the magnet 1 and the static power supply 2. For example, a superconducting electromagnet or a normal conducting electromagnet can be used as the magnet 1. The static power supply 2 supplies an electric current to the magnet 1. Hence, the static magnetic field generating section generates a static magnetic field B0 in a cylindrical space (a space for diagnosis) into which the subject 200 is sent. The magnetic field direction of this static magnetic field B0 is approximately consistent with the axial direction (Z axis direction) of the diagnostic space. The static magnetic field generating section is further provided with the shim coil 14. This shim coil 14 is supplied with electric current from the shim power supply 15 under the control of the host computer 6 to generate a correcting magnetic field to homogenize the static magnetic field.
The bed slides the top board onto which the subject 200 is placed into and out of the diagnostic space.
The gradient magnetic field generating section includes the gradient coil unit 3 and the gradient power supply 4. The gradient coil unit 3 is arranged on the inner side of the magnet 1. The gradient coil unit 3 comprises three pairs of coils 3x, 3y and 3z to respectively generate gradient magnetic fields in the directions of an x-axis, y-axis and z-axis, which are mutually orthogonal. The gradient power supply 4 supplies a pulse current to the coils 3x, 3y and 3z under the control of the sequencer 5 to generate the gradient magnetic field. By controlling the pulse current supplied from the gradient power supply 4 to the coils 3x, 3y and 3z in this manner, the gradient magnetic field generating section combines the gradient magnetic fields of physical axes in the three axial (X axis, Y axis and Z axis) directions, and arbitrary sets each gradient magnetic field for each logic axial direction comprised of a slice direction gradient magnetic field Gss, a phase encode direction gradient magnetic field Gpe, and a read out direction (frequency encode direction) gradient magnetic field Gre, which are mutually orthogonal. Each of the gradient magnetic fields Gss, Gpe and Gre in the slice direction, the phase encode direction and the read out direction are superposed on the static magnetic field B0.
The transmit and receive section includes the RF coil unit 7, the transmitter 8T and the receiver 8R. The RF coil unit 7 is arranged near the subject 200 in the diagnostic space. The transmitter 8T and the receiver 8R are connected to the RF coil unit 7. The transmitter 8T and the receiver 8R are operated under the control of the sequencer 5. The transmitter 8T supplies an RF current pulse of Larmor frequency to the RF coil unit 7 to cause nuclear magnetic resonance (NMR). The receiver 8R captures an MR signal (radio frequency signal), such as an echo signal received by the RF coil unit 7, applies various signal processing, such as preamplifying, intermediate frequency conversion, phase detection, low frequency amplification or filtering, thereto, and generates echo data (original data) in a digital amount corresponding to the echo signal by A/D conversion. The RF coil unit 7 is a multi-coil embedded with a plurality of element coils. The receiver 8R is capable of processing the echo signal received by each of the plurality of element coils, in parallel.
The control/computing section includes the sequencer 5, the host computer 6, the arithmetic unit 10, the memory unit 11, the display unit 12 and the input unit 13.
The sequencer 5 comprises a CPU and a memory. The sequencer 5 stores pulse sequence information sent by the host computer 6 in the memory. The CPU of the sequencer 5 controls the operations of the gradient power supply 4, the transmitter 8T and the receiver 8R. The echo data output by the receiver 8R is once input to the sequencer 5, then transferred to the arithmetic unit 10. Here, the sequence information includes all information required to operate the gradient power supply 4, transmitter 8T and receiver 8R in accordance with the series of pulse sequence. Such information includes information related to, for example, strength of the pulse current to be applied to the coils 3x, 3y and 3z, application time and application timing.
The host computer 6 has various functions which can be realized by implementing predetermined software procedures. One of these functions is to give instructions of the pulse sequence information to the sequencer 5 and take control over the operation of the entire system.
Prior to imaging scan, the host computer carries out preparations, such as scanning for positioning. The imaging scan is a three dimensional (3D) scan or a two dimensional (2D) scan, which collects pairs of echo data required for image reconstruction. For the pulse sequence of the imaging scan, methods such as an spin echo (SE) method, fast spin echo (FSE) method, an fast asymmetric spin echo (FASE) method, which is the combination of a high speed method and a half Fourier method, an echo planar imaging (EPI) method and an fast field echo (FFE) method are used.
The echo data output by the receiver 8R is input to the arithmetic unit 10 via the sequencer 5. The arithmetic unit 10 allocates the input echo data to the k-space (also referred to as Fourier space or frequency space) set in the inner memory. The arithmetic unit 10 subjects the echo data allocated to the k-space to two dimensional or three dimensional Fourier conversion to reconstruct image data in real-space. Further, the arithmetic unit 10 is capable of performing, such as, synthesis processing or differential computation processing. Further, the arithmetic unit 10 comprises a function to perform reconstruction processing to realize parallel imaging based on methods such as sensitivity encoding (SENSE) and simultaneous acquisition of spatial harmonics (SMASH).
The synthesis processing includes a processing which adds image data of a plurality of two dimensional frames to each corresponding pixel, and a maximum projection (MIP) processing or a minimum projection processing, which selects a maximum value or a minimum value in the direction of the line of sight with respect to the three dimensional data. In addition, as another example of synthesis processing, the axis of a plurality of frames may be matched on the Fourier space and may be synthesized on an echo data of 1 frame in the form of an echo data. Further, the addition processing includes, for example, a simple addition, an addition averaging and a weighted addition.
The memory unit 11 stores the reconstructed image data as well as the image data having undergone the above synthesis processing and differential processing.
The display unit 12 displays various images to be presented to the user, under the control of the host computer 6. A display device, such as a liquid crystal display unit, can be used as the display unit 12.
The input unit 13 inputs a variety of information, which is related to, for example, imaging conditions, pulse sequence, image synthesis and differential computation required by the operator. The input unit 13 sends the input information to the host computer 6. The input unit 13 is provided arbitrary with a pointing device, such as a mouse or a track ball, a selective device, such as a mode selection switch, or an input device, such as a keyboard.
The instructing section comprises a voice generator 16. The voice generator 16 can set forth various messages by voice under the command of the host computer 6.
The respiration measuring section includes the respiration sensor 17 and the respiration monitor 18. The respiration sensor 17 is attached to the body surface of the subject 200, detects the abdominal movement of the subject 200, and generates a respiration signal showing the respiration condition of the subject 200. The respiration monitor 18 subjects the respiration signal output from the respiration sensor 17 to various processing including digitalization processing, and outputs such signal to the sequencer 5 and the host computer 6. The sequencer 5 uses the respiration signal when carrying out the imaging scan.
An operation of the MRI apparatus 100 configured in the above manner will be explained in detail as follows.
A waveform shown in the uppermost part of
As shown in
In one collecting period PA, a similar sequence is repeated over i times in certain repeating times TR. The sequencer 5 controls the gradient power supply 4 so that a phase encode gradient is varied sequentially in each of the i pieces of periods PR1, PR2, . . . , PRi. However, the amount of change in the phase encode gradient is set so as to skip a part (for instance, ½) of the plurality of phase encode steps which is determined according to the desired field of view (FOV).
As shown in
The collecting operation as mentioned above is carried out in parallel with respect to the echo signals received respectively by at least two element coils among the plurality of element coils embedded in the RF coil 7. The arithmetic unit 10 carries out image reconstruction using an algorithm for parallel imaging, such as the SENSE algorithm, based on the collected data.
Meanwhile, the sequence in each of the periods PR1, PR2, . . . , PRi can be changed to, for example, a sequence according to the FFE method, which simultaneously uses a water excitation method, as shown in
The water excitation method is one of the methods which utilizes the difference of excitation frequency between water and fat and controls the magnetic resonance signal from the fat by mainly exciting the proton of water. In this water excitation method, a binomial RF pulse is used as a flip pulse.
Thus, according to the present embodiment, a slice of data is collected within an exhalation period. Therefore, by allocating the data to be allocated to each phase encode line in the k-space sequentially in the sequence order of the phase encode line, it is possible to arrange the data in the k-space in the order of less phase change caused by respiration, without having to carry out any particular correction or rearrangement as shown in
According to the present embodiment, a part of the plurality of phase encode steps is skipped by carrying out parallel imaging. Therefore, the collecting period PA can be shortened, and the collection of a slice of data can be completed unfailingly within an exhalation period.
According to the present embodiment, the collecting operation starts at time point when after the pass of the delay time TD from the timing of the start of exhalation period. Therefore, the collecting period can be implemented during a period having lesser movement in the exhalation period, which allows for further stabilized imaging. Meanwhile, it is possible to increase the number of collected PE data in a certain imaging time by increasing the number of channels of the coil and improving the increasing rate of imaging speed. As a result, it is possible to seek improvement in resolution.
According to the present embodiment, the simultaneous use of the water excitation method can bring about fat suppression effect without being influenced by a segment division. In the case of a CHESS (chemical shift selective) method, the length of fat control pulse is too long to apply to the present embodiment. Meanwhile, the water excitation method is suitable for the present embodiment since it is capable of keeping down the extension of the repeating time TR.
According to the present embodiment, since there is less influence from movement, a ghost is not emphasized by averaging. Therefore, it becomes easier to control the S/N.
This embodiment is capable of implementing various modified embodiments as follows.
As shown in
The present embodiment can also be adapted to three dimensional imaging carried out by the FFE method (FFE3D). Application of the present invention to the three dimensional imaging will improve slice resolution and expand clinical range of application dramatically. In the case of FFE3D, it is possible to seek reduction in the movement artifact by arranging a trigger for each slice encode and collecting the PE encode portion likewise the above method. Further, it is preferred that each of the plurality of slice encodes in the three dimensional imaging are scanned sequentially in each of the successive respiration cycle.
In the case where the respiration cycle is long, it is also possible to collect data for a plurality of slices within a collecting period PA.
It is possible to collect data for a plurality of slices within a collecting period PA by increasing the speed rate of parallel imaging.
With regard to accumulation method, it is possible to simultaneously use addition in a raw data condition.
With regard to addition method, it is also possible to use real data obtained after fast Fourier transformation (FFT) to average addition.
In abdominal imaging, it is possible to satisfy both an attempt to suppress the blood signal while acquiring a T1 contrast by adjusting the time TI and the flip angle (FA) of the TI pulse. In order to acquire an image as shown in
This is not limited to respiration synchronization, but can also be used with an electrocardiogram (ECG) to collect data for a slice in a time phase with less movement in tissues, such as heart.
Other than using IR, it is also possible to suppress the blood signal by simultaneously using an motion probing gradient (MPG) pulse.
Further, an realtime motion correction (RMC) technique and navigator echo can be used in combination to further suppress movement artifact. This prevents misalignment in the slice-section caused by movement of diaphragm and improves image viewability.
Data may also be collected within the inhalation period.
The increasing rate of parallel imaging speed designated by the user is input to the host computer 6 via the input unit 13. Based on this input increasing rate SR, the host computer 6 is capable of calculating the number of slices SN obtained within a collecting period PA using the following equation (1). Here, PE is the number of matrixes in the encode direction.
SN=PA/(TR×PE/SR) (1)
In the case of varying the collecting period in accordance with the respiration cycle of the subject 200, PA should also be varied in the above equation (1). Here, since the respiration cycle of the subject 200 varies frequently, the collecting period should also be varied in accordance with the respiration cycle. However, if the collecting period varies for each cycle, the collected signal may become unstable. Therefore, it is desirable that the collecting period be determined based on the average value, median value, maximal value or the minimal value of the respiration cycle in the plurality of cycles. The PA in the above equation (1) should also employ a value according to the collecting period determined as above. In the case where the repeating cycle or the number of matrixes in the encode direction is variable, a value which correspond accordingly is employed as TR or PE in the above equation (1).
The host computer 16 is capable of having the display unit 12 display the number of slices calculated by the above equation (1) to notify the user. By doing so, the user is notified of the number of slices which can be collected in a collecting period. In the case where the number of slices to collect in a collecting period is set in accordance with the user's instructions, this enables the user to designate an appropriate number of slices.
Further, the host computer 16 is capable of automatically setting the number of slices to be collected in a collecting period based on the number of slices calculated by the above equation (1).
Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-248423 | Sep 2006 | JP | national |
| 2007-209520 | Aug 2007 | JP | national |
